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  • 101.
    Boij, Susann
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Flow effects on the acoustic end correction of a sudden in-duct area expansion2009In: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524, Vol. 126, no 3, p. 995-1004Article in journal (Refereed)
    Abstract [en]

    For scattering of plane waves at a sudden area expansion in a duct, the presence of flow may significantly alter the reactive properties. This paper studies the influence of a mean flow field and unstable separated flow on the reactive properties of the expansion, formulated as an end correction. Theoretical and experimental results show that the expansion end correction is significantly affected by the flow and hydrodynamic waves excited at the edge of the expansion. The effects are different in three regions where the Strouhal number is small, of order 1, and large. The influence is most significant at Strouhal numbers of the order 1, with specific limiting values for large and small Strouhal numbers, respectively. In the analytic model, an important feature is the shear layer at the edge modeled as a vortex sheet with the unsteady Kutta condition applied at the edge. The influence of Mach number, Helmholtz number, and area expansion ratio is studied, and a quasistationary, small Strouhal number, approximation yields an expression for the end correction. Further, the influence of edge condition is explored, emphasizing the importance of interaction between sound and unsteady vorticity shedding at the edge of the area expansion.

  • 102.
    Bolin, Karl
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prediction Method for Wind-Induced Vegetation Noise2009In: Acta Acoustica united with Acustica, ISSN 1610-1928, E-ISSN 1861-9959, Vol. 95, no 4, p. 607-619Article in journal (Refereed)
    Abstract [en]

    This article examines the sound generated when the wind interacts with vegetation. A wind field model has been coupled to a new method for predicting sound from vegetation. This includes predictions from coniferous, deciduous and leafless trees. The proposed prediction method and an earlier model have been compared with measurements which show improved agreement, in particular in the region below 1 kHz. Comparisons between five measurement sites and predictions show satisfactory agreement for wind speeds up to 8.5 m/s. Fluctuations in the vegetation noise level due to wind turbulence can also be accurately estimated.

  • 103. Borodulin, V. I.
    et al.
    Ivanov, A. V.
    Kachanov, Y. S.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Swedish Defense Research Agency, FOI.
    Laminar-turbulent transition delay on a swept wing2016In: AIP Conference Proceedings, American Institute of Physics (AIP), 2016Conference paper (Refereed)
    Abstract [en]

    The paper describes the results of experiments on robustness of laminar-turbulent transition control on a swept-wing using distributed micro-sized roughness (DMSR) elements. These elements introduce controlled stationary vortices which are able to significantly modify the base flow and its stability characteristics. We have performed parametric study first varying height and period of the DMSR elements in order to find the most stabilizing effect on boundary later flow in compare to uncontrolled reference case without DMSR. Significant downstream shift of laminar-turbulent transition position due to application of DMSR is found and well documented with help of thermography. The robustness of this flow control method was studied by variation of the wind-tunnel flow quality introducing significant sound background or introducing enhanced turbulence level (applying turbulizing grids). The wind-tunnel tests performed with turbulence-generating grids (at enhanced turbulence levels) have shown that laminar-turbulent transition moves upstream in this case, while DMSR-elements loose their effectiveness for transition control (no matter in quiet sound conditions or at elevated sound background). The experiments on acoustic influence have shown that without DMSR acoustic does not effect transition location. However, in case then laminar-turbulent transition is delayed by presence of DMSR, an additional transition delay was observed when harmonic acoustic waves of certain frequency were excited.

  • 104. Borodulin, V. I.
    et al.
    Ivanov, A. V.
    Kachanov, Y. S.
    Mischenko, D. A.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hein, S.
    Excitation of 3D TS-waves in a swept-wing boundary layer by surface vibrations and freestream vortices2018In: AIP Conference Proceedings, American Institute of Physics Inc. , 2018Conference paper (Refereed)
    Abstract [en]

    There are several kinds of velocity disturbances, which may affect the transition to turbulence in a swept wing boundary layer. Tollmien-Schlichting (TS) waves are among most important of them. The properties of TS waves and their potential competition with cross-flow waves on a swept wing are poorly studied in theoretical works and were not studied experimentally at all. This paper presents the method of excitation of fully controlled 3D TS waves via interaction of free-stream vortices and surface vibrations. The experimental approach developed here will be used for investigation of the corresponding receptivity problem.

  • 105.
    Borodulin, V. I.
    et al.
    Khristianovich Inst Theoret & Appl Mech, Novosibirsk 630090, Russia..
    Ivanov, A. V.
    Khristianovich Inst Theoret & Appl Mech, Novosibirsk 630090, Russia..
    Kachanov, Y. S.
    Khristianovich Inst Theoret & Appl Mech, Novosibirsk 630090, Russia..
    Mischenko, D. A.
    Khristianovich Inst Theoret & Appl Mech, Novosibirsk 630090, Russia..
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Hein, S.
    DLR, Inst Aerodynam & Flow Technol, D-37073 Gottingen, Germany..
    Experimental and theoretical study of swept-wing boundary-layer instabilities. Unsteady crossflow instability2019In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 31, no 6, article id 064101Article in journal (Refereed)
    Abstract [en]

    Extensive combined experimental and theoretical investigations of the linear evolution of unsteady (in general) Cross-Flow (CF) and three-dimensional (3D) Tollmien-Schlichting (TS) instability modes of 3D boundary layers developing on a swept airfoil section have been carried out. CF-instability characteristics are investigated in detail at an angle of attack of -5 degrees when this kind of instability dominates in the laminar-turbulent transition process, while the 3D TS-instability characteristics are studied at an angle of attack of +1.5 degrees when this kind of instability is predominant in the transition process. All experimental results are deeply processed and compared with results of calculations based on several theoretical approaches. For the first time, very good quantitative agreement of all measured and calculated stability characteristics of swept-wing boundary layers is achieved both for unsteady CF- and 3D TS-instability modes for the case of a boundary layer developing on a real swept airfoil. The first part of the present study (this paper) is devoted to the description of the case of CF-dominated transition, while the TS-dominated case will be described in detail in a subsequent second part of this investigation.

  • 106.
    Borodulin, V. I.
    et al.
    RAS, SB, ITAM, Novosibirsk 630090, Russia..
    Ivanov, A. V.
    RAS, SB, ITAM, Novosibirsk 630090, Russia..
    Kachanov, Y. S.
    RAS, SB, ITAM, Novosibirsk 630090, Russia..
    Mischenko, D. A.
    RAS, SB, ITAM, Novosibirsk 630090, Russia..
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hein, S.
    DLR, Inst Aerodynam & Flow Technol, D-37073 Gottingen, Germany..
    Quantitative study of localized mechanisms of excitation of cross-flow instability modes in a swept-wing boundary layer2018In: CONFERENCE OF YOUNG SCIENTISTS IN MECHANICS, IOP PUBLISHING LTD , 2018, article id 012008Conference paper (Refereed)
    Abstract [en]

    An experimental study of two efficient receptivity mechanisms of excitation of cross-flow (CF) instability modes is carried out in a boundary layer of a real airfoil section of a swept wing due to: (i) action of localized surface vibrations, and (ii) scattering of 2D freestream vortices on them. It is found that the two mechanisms lead to rather efficient excitation of CF-modes both at surface vibration frequency and at combination 'vortexvibration' frequencies. First estimations of the corresponding localized receptivity coefficients are obtained. Direct comparison of the experimental amplification curves of the excited CF-modes with those calculated based on the linear stability theory (LST) has shown that the experimental data obtained at vibration frequency are in excellent agreement with the LST. At the same time, growth rates of the CF-modes excited at combination frequencies are found to be completely inconsistent with the LST. A possible explanation of this phenomenon via action of a new efficient distributed receptivity mechanism is suggested. This mechanism is associated with scattering of freestream vortices on rather high-amplitude CF-modes excited by surface vibrations.

  • 107. Borodulin, V. I.
    et al.
    Ivanov, A. V.
    Kachanov, Y. S.
    Mischenko, D. A.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hein, S.
    Receptivity coefficients of vortex-vibrational type at excitation of 3D Tollmien-Schlichting waves in a boundary layer on a swept wing2019In: HIGH-ENERGY PROCESSES IN CONDENSED MATTER (HEPCM 2019): Proceedings of the XXVI Conference on High-Energy Processes in Condensed Matter, dedicated to the 150th anniversary of the birth of S.A. Chaplygin, American Institute of Physics (AIP), 2019, article id 030044Conference paper (Refereed)
    Abstract [en]

    The paper is devoted to the first results of an experimental quantitative study of the receptivity mechanism of a swept-wing laminar boundary layer related to scattering of 2D freestream vortices (with frequency fv) at 3D local surface vibrations (with frequency fs) resulting in an excitation of Tollmien-Schlichting (TS) waves (having combination frequencies f+ = fs+fv and f- = fs - fv). The experiments were carried out in a low-turbulence level wind tunnel on a high-precision experimental model of long-laminar-run swept airfoil (sweep angle of 35°) at a freestream speed of about 10 m/s. Controlled localized 3D surface vibrations and 2D freestream vortices were generated by special disturbance sources. Quantitative characteristics of the studied receptivity mechanism (receptivity coefficients) were estimated.

  • 108.
    Bouaniche, Alexandre
    et al.
    Normandie Univ, CORIA, CNRS, INSA Rouen Normandie, St Etienne Du Rouvray, France..
    Jaouen, Nicolas
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Normandie Univ, CORIA, CNRS, INSA Rouen Normandie, St Etienne Du Rouvray, France..
    Domingo, Pascale
    Normandie Univ, CORIA, CNRS, INSA Rouen Normandie, St Etienne Du Rouvray, France..
    Vervisch, Luc
    Normandie Univ, CORIA, CNRS, INSA Rouen Normandie, St Etienne Du Rouvray, France..
    Vitiated High Karlovitz n-decane/air Turbulent Flames: Scaling Laws and Micro-mixing Modeling Analysis2019In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 102, no 1, p. 235-252Article in journal (Refereed)
    Abstract [en]

    Turbulent flames with high Karlovitz numbers have deserved further attention in the most recent literature. For a fixed value of the Damkohler number (ratio between an integral mechanical time and a chemical time), the increase of the Karlovitz number (ratio between a chemical time and a micro-mixing time) by an order of magnitude implies the increase of the turbulent Reynolds number by two orders of magnitude (Bray, Symp. (Int.) Combust. 26, 1-26 1996). In the practice of real burners featuring a limited range of variation of their turbulent Reynolds number, high Karlovitz combustion actually goes with a drastic reduction of the Damkohler number. Within this context, the relation between the dilution by burnt gases and the apparition of high Karlovitz flames is discussed. Basic scaling laws are reported which suggest that the overall decrease of the burning rate due to very fast mixing can indeed be compensated by the energy brought to the reaction zone by burnt gases. To estimate the validity of these scaling laws, in particular the response of the quenching Karlovitz versus the dilution level with a vitiated stream, the micro-mixing rate is varied in a multiple-inlet canonical turbulent and reactive micro-mixing problem. A reduced n-decane/air chemical kinetics is used, which has been derived from a more detailed scheme using a combination of a directed relation graphs analysis with a Genetic Algorithm. The multiple-inlet canonical micro-mixing problem includes liquid fuel injection and dilution by burnt gases, both calibrated from conditions representative of an aeronautical combustion chamber. The results confirm the possibility of reaching, with the help of a vitiated mixture, very high Karlovitz combustion before quenching occurs.

  • 109. Boyanova, P.
    et al.
    Do-Quang, Minh
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Neytcheva, M.
    Block-preconditioners for conforming and non-conforming FEM discretizations of the Cahn-Hilliard equation2012In: Large-Scale Scientific Computing, Springer Science+Business Media B.V., 2012, Vol. 7116 LNCS, p. 549-557Conference paper (Refereed)
    Abstract [en]

    We consider preconditioned iterative solution methods to solve the algebraic systems of equations arising from finite element discretizations of multiphase flow problems, based on the phase-field model. The aim is to solve coupled physics problems, where both diffusive and convective processes take place simultaneously in time and space. To model the above, a coupled system of partial differential equations has to be solved, consisting of the Cahn-Hilliard equation to describe the diffusive interface and the time-dependent Navier-Stokes equation, to follow the evolution of the convection field in time. We focus on the construction and efficiency of preconditioned iterative solution methods for the linear systems, arising after conforming and non-conforming finite element discretizations of the Cahn-Hilliard equation in space and implicit discretization schemes in time. The non-linearity of the phase-separation process is treated by Newton's method. The resulting matrices admit a two-by-two block structure, utilized by the preconditioning techniques, proposed in the current work. We discuss approximation estimates of the preconditioners and include numerical experiments to illustrate their behaviour.

  • 110.
    Boyanova, Petia
    et al.
    Department of Information Technology, Uppsala University.
    Do-Quang, Minh
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Neytcheva, Maya
    Department of Information Technology, Uppsala University.
    Efficient Preconditioners for Large Scale Binary Cahn-Hilliard Models2012In: Computational Methods in Applied Mathematics, ISSN 1609-4840, E-ISSN 1609-9389, Vol. 12, no 1, p. 1-22Article in journal (Refereed)
    Abstract [en]

    In this work we consider preconditioned iterative solution methods for numerical simulations of multiphase flow problems, modelled by the Cahn-Hilliard equation. We focus on diphasic flows and the construction and efficiency of a preconditioner for the algebraic systems arising from finite element discretizations in space and the method in time. The preconditioner utilises to a full extent the algebraic structure of the underlying matrices and exhibits optimal convergence and computational complexity properties. Various numerical experiments, including large scale examples, are presented as well as performance compar- isons with other solution methods. 

  • 111.
    Brandefelt, Jenny
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Kjellstrom, E
    Naslund, J O
    Strandberg, G
    Voelker, A H L
    Wohlfarth, B
    A coupled climate model simulation of Marine Isotope Stage 3 stadial climate2011In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 7, no 2, p. 649-670Article in journal (Refereed)
    Abstract [en]

    We present a coupled global climate model (CGCM) simulation, integrated for 1500 yr to quasi-equilibrium, of a stadial (cold period) within Marine Isotope Stage 3 (MIS 3). The simulated Greenland stadial 12 (GS12; similar to 44 ka BP) annual global mean surface temperature (T-s) is 5.5 degrees C lower than in the simulated recent past (RP) climate and 1.3 degrees C higher than in the simulated Last Glacial Maximum (LGM; 21 ka BP) climate. The simulated GS12 is evaluated against proxy data and previous modelling studies of MIS3 stadial climate. We show that the simulated MIS 3 climate, and hence conclusions drawn regarding the dynamics of this climate, is highly model-dependent. The main findings are: (i) Proxy sea surface temperatures (SSTs) are higher than simulated SSTs in the central North Atlantic, in contrast to earlier simulations of MIS 3 stadial climate in which proxy SSTs were found to be lower than simulated SST. (ii) The Atlantic Meridional Overturning Circulation (AMOC) slows down by 50% in the GS12 climate as compared to the RP climate. This slowdown is attained without freshwater forcing in the North Atlantic region, a method used in other studies to force an AMOC shutdown. (iii) El-Nino-Southern Oscillation (ENSO) teleconnections in mean sea level pressure (MSLP) are significantly modified by GS12 and LGM forcing and boundary conditions. (iv) Both the mean state and variability of the simulated GS12 is dependent on the equilibration. The annual global mean T-s only changes by 0.10 degrees C from model years 500-599 to the last century of the simulation, indicating that the climate system may be close to equilibrium already after 500 yr of integration. However, significant regional differences between the last century of the simulation and model years 500-599 exist. Further, the difference between simulated and proxy SST is reduced from model years 500-599 to the last century of the simulation. The results of the ENSO variability analysis is also shown to depend on the equilibration.

  • 112.
    Brandefelt, Jenny
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Otto-Bliesner, B. L.
    Equilibration and variability in a Last Glacial Maximum climate simulation with CCSM32009In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 36Article in journal (Refereed)
    Abstract [en]

    We present results from a 1862 year simulation of the Last Glacial Maximum (LGM) with the Community Climate System Model version 3 (CCSM3). A quasi steady state is reached after approximately 100 years of integration when the initial cooling trend in the annual global mean atmospheric surface temperature (T-s) levels off and even reverses. After another 150 years of integration the climate continues to cool and reaches a new equilibrium after a total of 800 years of integration. The cause of the continued adjustment of the climate to LGM forcing and boundary conditions is found in the abyssal ocean which is cooling at a rate decreasing from 0.15 degrees C per century until the new equilibrium is reached. The new equilibrium differs substantially from the first quasi steady state with 1.1 degrees C colder global mean Ts and regional differences of 5-15 degrees C in the North Atlantic region and a 30% reduction of the strength of the Atlantic meridional overturning circulation (AMOC). Further, the variability in the global mean Ts is significantly larger in the new equilibrium. This variability is associated with coupled ocean-atmosphere-sea ice variations in the North Atlantic region. Citation: Brandefelt, J., and B. L. Otto-Bliesner (2009), Equilibration and variability in a Last Glacial Maximum climate simulation with CCSM3, Geophys. Res. Lett., 36, L19712, doi: 10.1029/2009GL040364.

  • 113.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    The lift-up effect: The linear mechanism behind transition and turbulence in shear flows2014In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 47, p. 80-96Article in journal (Refereed)
    Abstract [en]

    The formation and amplification of streamwise velocity perturbations induced by cross-stream disturbances is ubiquitous in shear flows. This disturbance growth mechanism, so neatly identified by Ellingsen and Palm in 1975, is a key process in transition to turbulence and self-sustained turbulence. In this review, we first present the original derivation and early studies and then discuss the non-modal growth of streaks, the result of the lift-up process, in transitional and turbulent shear flows. In the second part, the effects on the lift-up process of additives in the fluid and of a second phase are discussed and new results presented with emphasis on particle-laden shear flows. For all cases considered, we see the lift-up process to be a very robust process, always present as a first step in subcritical transition.

  • 114.
    Brandt, L.uca
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Ardekani, Mehdi Niazi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Picano, F.
    Costa, P.
    Breugem, W. -P
    Numerical study of turbulent channel flow laden with finite-size non-spherical particles2017In: 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017, International Symposium on Turbulence and Shear Flow Phenomena, TSFP10 , 2017, Vol. 4Conference paper (Refereed)
    Abstract [en]

    We present interface-resolved numerical simulations of turbulent channel flow laden with non-spherical rigid and neutrally-buoyant particles. We first focus on the case of oblate particles of aspect ratio 1/3 at volume fractions up to 15% and show that the turbulent drag is decreasing when increasing the particle volume fraction although the effective viscosity of the suspension actually increases. We relate the observed drag reduction to turbulence attenuation and to particle migration away from the near-wall region. Particles tend to align parallel to the wall with rotation rates significantly lower than those reported for spheres. In the second part of the study, we examine the effect of the particle slenderness on the observed drag reduction and show that the drag increases for flatter particles.

  • 115.
    Brandt, Luca
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    de lange, H. C.
    Interactions between finite-length streaks and breakdown to turbulence2007In: ADVANCES IN TURBULENCE XI / [ed] Palma, JMLM; Lopes, AS, BERLIN: SPRINGER-VERLAG BERLIN , 2007, Vol. 117, p. 133-135Conference paper (Refereed)
  • 116.
    Brandt, Luca
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Duguet, Yohann
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Larsson, Robin
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Nonlinear optimal perturbations in plane Couette flow2009In: ADVANCES IN TURBULENCE XII: PROCEEDINGS OF THE 12TH EUROMECH EUROPEAN TURBULENCE CONFERENCE / [ed] Eckhardt, B., 2009, Vol. 132, p. 85-88Conference paper (Refereed)
  • 117.
    Brandt, Luca
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Picano, F.
    Breugem, W. -P
    Turbulent flow of a suspension of rigid spherical particles in plane channels2016In: Springer Proceedings in Physics, 2016, p. 311-315Conference paper (Refereed)
    Abstract [en]

    Suspensions of solid particles are frequently found in applications and environmental flows. Several studies concern the rheological properties of suspensions in laminar flows, but much less is known of turbulent suspensions. The present work fills this gap providing DNS data on dense suspensions of neutrally-buoyant rigid sphere in a turbulent channel flow at the bulk Reynolds number of Re = U0h/ν = 2800. We show that considering volume fractions Φ ≤ 0.1 the turbulent flow is similar to the unladen case with higher turbulence intensities. On the contrary, the flow behavior strongly changes at Φ = 0.2where the turbulence appears to be attenuated. © Springer International Publishing Switzerland 2016.

  • 118.
    Brandt, Luca
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Sipp, Denis
    Pralits, Jan O.
    Marquet, Olivier
    Effect of base-flow variation in noise amplifiers: the flat-plate boundary layer2011In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 687, p. 503-528Article in journal (Refereed)
    Abstract [en]

    Non-modal analysis determines the potential for energy amplification in stable flows. The latter is quantified in the frequency domain by the singular values of the resolvent operator. The present work extends previous analysis on the effect of base-flow modifications on flow stability by considering the sensitivity of the flow non-modal behaviour. Using a variational technique, we derive an analytical expression for the gradient of a singular value with respect to base-flow modifications and show how it depends on the singular vectors of the resolvent operator, also denoted the optimal forcing and optimal response of the flow. As an application, we examine zero-pressure-gradient boundary layers where the different instability mechanisms of wall-bounded shear flows are all at work. The effect of the component-type non-normality of the linearized Navier-Stokes operator, which concentrates the optimal forcing and response on different components, is first studied in the case of a parallel boundary layer. The effect of the convective-type non-normality of the linearized Navier-Stokes operator, which separates the spatial support of the structures of the optimal forcing and response, is studied in the case of a spatially evolving boundary layer. The results clearly indicate that base-flow modifications have a strong impact on the Tollmien-Schlichting (TS) instability mechanism whereas the amplification of streamwise streaks is a very robust process. This is explained by simply examining the expression for the gradient of the resolvent norm. It is shown that the sensitive region of the lift-up (LU) instability spreads out all over the flat plate and even upstream of it, whereas it is reduced to the region between branch I and branch II for the TS waves.

  • 119.
    Brandt, Luca
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tao, Jianjun
    Peking University, China.
    Editorial: Recent advances in hydrodynamic instability and transition to turbulence2015In: Theoretical and Applied Mechanics Letters, ISSN 2095-0349, Vol. 5, no 3, p. 101-102Article in journal (Refereed)
  • 120.
    Braunbehrens, Robert
    et al.
    Innogy SE, Wind Onshore, Kapstadtring 7, D-22297 Hamburg, Germany..
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    A statistical model for wake meandering behind wind turbines2019In: Journal of Wind Engineering and Industrial Aerodynamics, ISSN 0167-6105, E-ISSN 1872-8197, Vol. 193, article id UNSP 103954Article in journal (Refereed)
    Abstract [en]

    A new wake model is proposed to account for wake meandering in simulations of wind-turbine wakes performed on steady solvers, through a wake-meandering description based on the dispersion theory of Taylor (1921, P. Lond. Math Soc., vol. 20, pp. 196-211). Single-turbine simulations were performed by means of the linearised solver ORFEUS. By analysing the steady wake behind a turbine, a set of parameters describing the wake was first obtained and synthesised into a look-up table. The proposed meandering model extended the simulation results by superimposing the lateral and vertical meandering motions to the steady wake. As a result, the time-averaged velocity distribution of the wake was increased in width and reduced in intensity. Through this combination, the model provides rationale for the wake-deficit decrease and for the power underestimation effects of several wake models. The new wake model is validated against the Lillgrund and Horns Rev data sets.

  • 121.
    Brethouwer, Geert
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Influence of spanwise rotation and scalar boundary conditions on passive scalar transport in turbulent channel flow2019In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, no 1, article id 014602Article in journal (Refereed)
    Abstract [en]

    Direct numerical simulations of passive scalar transport in turbulent channel flow subject to spanwise rotation are carried out with two different boundary conditions for the scalar. In the first case the scalar transport is driven by an assigned scalar difference at the walls and in the second case by a constant mean streamwise scalar gradient. The Reynolds number Re = U(b)h/nu is fixed at 14 000 and the rotation number Ro = 2 Omega h/U-b is varied from 0 to 0.75, where U-b is the mean bulk velocity, h half the channel gap width, and Omega the rotation rate. This work is a continuation of Brethouwer [J. Fluid Mech. 844, 297 ( 2018)] to further study the influence of rotation and also the influence of scalar boundary conditions on scalar transport in channel flow. Mean scalar profiles and other scalar statistics differ in the two cases with different boundary conditions but are similar in the near-wall region in terms of local wall units. The conclusion of Brethouwer that the Reynolds analogy for scalar-momentum transfer does not apply to rotating channel flow is independent of scalar boundary conditions. Rotation influences the turbulent scalar flux differently than the Reynolds shear stress and strongly reduces the turbulent Prandtl number on the unstable channel side, irrespective of the scalar boundary conditions. Scalar structures are larger than the turbulence structures in rotating channel flow, in contrast to nonrotating channel flow where these are similar.

  • 122.
    Brethouwer, Geert
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Duguet, Yohann
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Numerical study of turbulent-laminar patterns in MHD, rotating and stratified shear flows2011In: Direct and Large-Eddy Simulation VIII, 2011, p. 125-130Conference paper (Refereed)
    Abstract [en]

    Coexisting laminar and turbulent regions have been observed in several types of wall bounded flows. In Taylor Couette flow, for example, alternating helical shaped laminar and turbulent regions have been observed within a limited Reynolds number range (Prigent et al., 2002) and oblique laminar and turbulent bands have been seen in experiments (Prigent et al., 2002) and simulations (Barkley and Tuckerman, 2005), (Duguet et al., 2010) of plane Couette flow for Reynolds numbers Re=U w h/ν between about 320 and 380. Here ±U w is the velocity of the two walls, h is the half width of the wall gap and ν is the viscosity. In this Reynolds number range the turbulent-laminar patterns seem to sustain while at lower Re the flow becomes fully laminar and at higher Re no clear laminar patterns can be distinguished and the flow eventually becomes fully turbulent. Similar oblique laminar-turbulent bands appeared as well in direct numerical simulations (DNS) of plane channel flow for friction Reynolds numbers Re τ =u τ h/ν=60 and 80 (Fukudome et al., 2009), (Tsukahara, 2010), where u τ is the friction velocity and h is again the gap half width.

  • 123.
    Brethouwer, Geert
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Lindborg, Erik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Numerical study of vertical dispersion by stratified turbulence2009In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 631, p. 149-163Article in journal (Refereed)
    Abstract [en]

    Numerical simulations are carried Out to investigate vertical fluid particle dispersion in uniformly stratified stationary turbulent flows. The results are compared with the analysis of Lindborg & Brethouwer (J. Fluid Mech., vol. 614, 2008, pp. 303-314), who derived long- and short-time relations for the mean square vertical displacement of fluid particles. Several direct numerical simulations (DNSs) with different degrees of stratification and different buoyancy Reynolds numbers are carried out to test the long-time relation = 2 epsilon(P)t/N-2. Here, epsilon(P) is the mean dissipation of turbulent potential energy; N is the Brunt-Vaisala frequency; and t is time. The DNSs show good agreement with this relation, with a weak dependence on the buoyancy Reynolds number. Simulations with hyperviscosity are carried out to test the relation = (1 + pi C-PL)2 epsilon(P)t/N-2, which should be valid for shorter time scales in the range N-1 << t << T, where T is the turbulent eddy turnover time. The results of the hyperviscosity simulations come closer to this prediction with C-PL about 3 with increasing stratification. However, even in the simulation with the strongest stratification the growth of is somewhat slower than linear in this regime. Based on the simulation results it is argued that the time scale determining the evolution Of is the eddy turnover time, T, rather than the buoyancy time scale N-1, as suggested in previous studies. The simulation results are also consistent with the prediction of Lindborg & Brethouwer (2008) that the nearly flat plateau Of observed at t similar to T should scale as 4E(P)/N-2, where E-P is the mean turbulent potential energy.

  • 124.
    Brethouwer, Geert
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lindborg, Erik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Particle Diffusion in Stably Stratified Flows2010In: PROGRESS IN TURBULENCE III / [ed] Peinke, J.; Oberlack, M.; Talamelli, A., 2010, Vol. 131, p. 163-166Conference paper (Refereed)
    Abstract [en]

    Numerical simulations are used to study the vertical dispersion of fluid particles in homogeneous turbulent flows with a stable stratification. The results of direct numerical simulations are in good agreement with the relation for the long time fluid particle dispersion, = 2 epsilon(P)t / N-2, derived by [6], though with a small dependence on the buoyancy Reynolds number. Here, is the mean square vertical particle displacement, epsilon p is the dissipation of potential energy, t is time and N is the Brunt-Vaisala frequency. A simulation with hyperviscosicity is performed to verify the relation = (1 + pi C-PL)2 epsilon(P)t / N-2 for shorter times, also derived by [6]. The agreement is reasonable and we find that C-PL similar to 3. The onset of a plateau in is observed in the simulations at t similar to E-P / epsilon(P) which scales as 4E(P) / N-2, where E-P is the potential energy.

  • 125.
    Brethouwer, Geert
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Duguet, Yohann
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Recurrent Bursts via Linear Processes in Turbulent Environments2014In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 112, no 14, p. 144502-Article in journal (Refereed)
    Abstract [en]

    Large-scale instabilities occurring in the presence of small-scale turbulent fluctuations are frequently observed in geophysical or astrophysical contexts but are difficult to reproduce in the laboratory. Using extensive numerical simulations, we report here on intense recurrent bursts of turbulence in plane Poiseuille flow rotating about a spanwise axis. A simple model based on the linear instability of the mean flow can predict the structure and time scale of the nearly periodic and self-sustained burst cycles. Poiseuille flow is suggested as a prototype for future studies of low-dimensional dynamics embedded in strongly turbulent environments.

  • 126.
    Brethouwer, Geert
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Johansson, Arne
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Effects of rapid spanwise rotation on turbulent channel flow with a passive scalar2011In: Proc. 7th International Symposium on Turbulence and Shear Flow Phenomena, 2011Conference paper (Refereed)
    Abstract [en]

    Direct numerical simulations of fully developed turbulentchannel flow including a passive scalar rotating about thespanwise axis have been performed. The mean bulk Reynoldsnumber, Reb = Ubh/n ≥ 20000, where Ub is the bulk meanvelocity and h the channel half width, is higher than in previoussimulations and the rotation rate covers a wide range.At moderate rotation rates, turbulence on the stable channelside is significantly less damped than in DNS at lower Reb. Athigh rotation rates we observe re-occurring, quasi-periodic instabilitieson the stable channel side. Between these events theturbulence is weak, but during the instability events the wallshear stress and turbulence intensity are much stronger. Theinstabilities are caused by structures resembling Tollmien-Schlichting (TS) waves that at some instant rapidly grow, thenbecome unstable and finally break down into intense turbulence.After some time the TS waves form again and the processrepeats itself in a periodic-like manner.Mean scalar profiles are also strongly affected by rotationand large scalar fluctuations are found on the border of the stableand unstable channel side. The turbulent Prandtl/Schmidtnumber of the scalar is much less than unity if there is rotation.Predicting scalar transport in rotating channel flow willtherefore pose a challenge to turbulence models.

  • 127.
    Brethouwer, Geert
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Johansson, Arne
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Turbulence instabilities and passive scalars in rotating channel flow2011In: 13th European Turbulence Conference (ETC13): Instability, Transition, Grid Turbulence And Jets / [ed] K. Bajer, Institute of Physics Publishing (IOPP), 2011, p. 032025-Conference paper (Refereed)
    Abstract [en]

    Fully developed channel flow with a passive scalar rotating about the spanwise axis is studied by direct numerical simulations. The Reynolds number based on the bulk mean velocity Re-b is up to 30000, substantially higher than in previous studies, and the rotation rates cover a broad range. Turbulence on the stable channel side is less strongly damped at moderate rotation rates than in channel flow at lower Re-b. At high rotation rates and sufficiently high Re-b, intermittent strong instabilities occur on the stable side caused by rapidly growing modes resembling two-dimensional Tollmien-Schlichting waves which at some instant become unstable and break down into intense turbulence. The turbulence decays and after some time the waves form again and the process is repeated in a cyclic manner. Rotation also strongly affects the mean passive scalar profiles and turbulent scalar fluxes. Large scalar fluctuations are observed on the border between the stable and unstable channel sides. While in non-rotating channel flow the turbulent Prandtl number of the passive scalar is about one like in other shear flows, it is much smaller in the rotating cases.

  • 128.
    Brethouwer, Gert
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Passive scalar transport in rotating turbulent channel flow2018In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 844, p. 297-322Article in journal (Refereed)
    Abstract [en]

    Passive scalar transport in turbulent channel flow subject to spanwise system rotation is studied by direct numerical simulations. The Reynolds number R-e= U(b)h/nu is fixed at 20 000 and the rotation number R-o= 2 Omega h/U-b is varied from 0 to 1.2, where U-b is the bulk mean velocity, h the half channel gap width and Omega the rotation rate. The scalar is constant but different at the two walls, leading to steady scalar transport across the channel. The rotation causes an unstable channel side with relatively strong turbulence and turbulent scalar transport, and a stable channel side with relatively weak turbulence or laminar-like flow, weak turbulent scalar transport but large scalar fluctuations and steep mean scalar gradients. The distinct turbulent-laminar patterns observed at certain Ro on the stable channel side induce similar patterns in the scalar field. The main conclusions of the study are that rotation reduces the similarity between the scalar and velocity field and that the Reynolds analogy for scalar-momentum transport does not hold for rotating turbulent channel flow. This is shown by a reduced correlation between velocity and scalar fluctuations, and a strongly reduced turbulent Prandtl number of less than 0.2 on the unstable channel side away from the wall at higher Ro. On the unstable channel side, scalar scales become larger than turbulence scales according to spectra and the turbulent scalar flux vector becomes more aligned with the mean scalar gradient owing to rotation. Budgets in the governing equations of the scalar energy and scalar fluxes are presented and discussed as well as other statistics relevant for turbulence modelling.

  • 129.
    Brethouwer, Gert
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Duguet, Y.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Turbulent-laminar coexistence in wall flows with Coriolis, buoyancy or Lorentz forces2012In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 704, p. 137-172Article in journal (Refereed)
    Abstract [en]

    Direct numerical simulations of subcritical rotating, stratified and magnetohydrodynamic wall-bounded flows are performed in large computational domains, focusing on parameters where laminar and turbulent flow can stably coexist. In most cases, a regime of large-scale oblique laminar-turbulent patterns is identified at the onset of transition, as in the case of pure shear flows. The current study indicates that this oblique regime can be shifted up to large values of the Reynolds number R e by increasing the damping by the Coriolis, buoyancy or Lorentz force. We show evidence for this phenomenon in three distinct flow cases: plane Couette flow with spanwise cyclonic rotation, plane magnetohydrodynamic channel flow with a spanwise or wall-normal magnetic field, and open channel flow under stable stratification. Near-wall turbulence structures inside the turbulent patterns are invariably found to scale in terms of viscous wall units as in the fully turbulent case, while the patterns themselves remain large-scale with a trend towards shorter wavelength for increasing Re. Two distinct regimes are identified: at low Reynolds numbers the patterns extend from one wall to the other, while at large Reynolds number they are confined to the near-wall regions and the patterns on both channel sides are uncorrelated, the core of the flow being highly turbulent without any dominant large-scale structure.

  • 130.
    Brethouwer, Gert
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lindborg, Anders V.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Investigation of fluid particle dispersion in stably stratified turbulence2009In: 6th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2009, International Symposium on Turbulence and Shear Flow Phenomena, TSFP , 2009, p. 1160-1163Conference paper (Refereed)
    Abstract [en]

    Numerical simulations are used to study vertical dispersion of fluid particles in homogeneous turbulent flows with a stable stratification (Brethouwer and Lindborg, 2009). The results of direct numerical simulations are in good agreement with the relation for the long time fluid particle dispersion, δz2 = 2εP t/N2, derived by Lindborg and Brethouwer (2008), though with a small dependence on the buoyancy Reynolds number. Here, δz2 is the mean square vertical particle displacement, εP is the dissipation of potential energy, t is time and N is the Brunt-Väisälä frequency. Simulations with hyperviscosicity are performed to verify the relation δz2 = (1 + πCP L)2εP t/N2 for N−1 t T, where N is the Brunt-Väisälä frequency and T is the turbulent eddy turnover time. The simulation results approach the relation for increasing stratification and we find that CP L is about 3 in strongly stratified fluids. The onset of a plateau in δz2 is observed in the simulations at t ∼ T . 

  • 131.
    Brethouwer, Gert
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lindborg, Erik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Numerical simulations of particle dispersion in stratified flows2009In: ADVANCES IN TURBULENCE XII: PROCEEDINGS OF THE 12TH EUROMECH EUROPEAN TURBULENCE CONFERENCE / [ed] Eckhardt, B., 2009, Vol. 132, p. 51-55Conference paper (Refereed)
  • 132.
    Brett, Calvin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Mittal, Nitesh
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Ohm, Wiebke
    DESY, Hamburg, Germany..
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Roth, Stephan V.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology, Biocomposites. DESY, Hamburg, Germany..
    GISAS study of spray deposited metal precursor ink on a cellulose template2019In: Abstracts of Papers of the American Chemical Society, ISSN 0065-7727, Vol. 257Article in journal (Other academic)
  • 133.
    Broman, Lars Mikael
    et al.
    Karolinska Univ Hosp, ECMO Ctr Karolinska, Dept Pediat Perioperat Med & Intens Care, Eugeniavagen 23, S-17176 Stockholm, Sweden.;Karolinska Inst, Dept Physiol & Pharmacol, Stockholm, Sweden.;EuroElso, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England..
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Centres, BioMEx.
    Westlund, C. Jerker
    Karolinska Univ Hosp, ECMO Ctr Karolinska, Dept Pediat Perioperat Med & Intens Care, Eugeniavagen 23, S-17176 Stockholm, Sweden..
    Gilbers, Martijn
    Maastricht Univ, Dept Cardiothorac Surg, Heart & Vasc Ctr, Cardiovasc Res Inst Maastricht CARIM,Med Hosp, Maastricht, Netherlands.;Maastricht Univ, Dept Physiol, Maastricht, Netherlands..
    da Camara, Luisa Perry
    Hosp Curry Cabral, Ctr Hosp Lisboa Cent, Lisbon, Portugal..
    Swol, Justyna
    EuroElso, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;Paracelsus Med Univ, Dept Pulmonol, Intens Care Med, Nurnberg, Germany..
    Taccone, Fabio S.
    EuroElso, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;ULB, Dept Intens Care, Hop Erasme, Brussels, Belgium..
    Malfertheiner, Maximilian V.
    EuroElso, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;Univ Med Ctr Regensburg, Dept Internal Med Cardiol & Pneumol 2, Regensburg, Germany..
    Di Nardo, Matteo
    EuroElso, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;Childrens Hosp Bambino Gesu, IRCCS, Pediat Intens Care Unit, Rome, Italy..
    Vercaemst, Leen
    EuroElso, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;Univ Hosp Gasthuisberg, Dept Perfus, Leuven, Belgium..
    Barrett, Nicholas A.
    EuroElso, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;Guys & St Thomas NHS Fdn Trust, Dept Crit Care, London, England.;Guys & St Thomas NHS Fdn Trust, Severe Resp Failure Serv, London, England..
    Pappalardo, Federico
    EuroElso, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;Univ Vita Salute San Raffaele, Adv Heart Failure & Mech Circulatory Support Prog, Hosp San Raffaele, Milan, Italy..
    Belohlavek, Jan
    EuroElso, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;Charles Univ Prague, Gen Univ Hosp Prague, Dept Cardiovasc Med, Dept Med 2, Prague, Czech Republic.;Charles Univ Prague, Fac Med 1, Prague, Czech Republic..
    Mueller, Thomas
    EuroElso, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;Univ Med Ctr Regensburg, Dept Internal Med Cardiol & Pneumol 2, Regensburg, Germany..
    Belliato, Mirko
    EuroElso, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;Fdn IRCCS Policlin San Matteo, UOC Anestesia & Rianimaz 1, Pavia, Italy..
    Lorusso, Roberto
    EuroElso, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;Maastricht Univ, Dept Cardiothorac Surg, Heart & Vasc Ctr, Cardiovasc Res Inst Maastricht CARIM,Med Hosp, Maastricht, Netherlands..
    Pressure and flow properties of cannulae for extracorporeal membrane oxygenation I: return (arterial) cannulae2019In: Perfusion, ISSN 0267-6591, E-ISSN 1477-111X, Vol. 34, p. 58-64Article in journal (Refereed)
    Abstract [en]

    Adequate extracorporeal membrane oxygenation support in the adult requires cannulae permitting blood flows up to 6-8 L/minute. In accordance with Poiseuille's law, flow is proportional to the fourth power of cannula inner diameter and inversely proportional to its length. Poiseuille's law can be applied to obtain the pressure drop of an incompressible, Newtonian fluid (such as water) flowing in a cylindrical tube. However, as blood is a pseudoplastic non-Newtonian fluid, the validity of Poiseuille's law is questionable for prediction of cannula properties in clinical practice. Pressure-flow charts with non-Newtonian fluids, such as blood, are typically not provided by the manufacturers. A standardized laboratory test of return (arterial) cannulae for extracorporeal membrane oxygenation was performed. The aim was to determine pressure-flow data with human whole blood in addition to manufacturers' water tests to facilitate an appropriate choice of cannula for the desired flow range. In total, 14 cannulae from three manufacturers were tested. Data concerning design, characteristics, and performance were graphically presented for each tested cannula. Measured blood flows were in most cases 3-21% lower than those provided by manufacturers. This was most pronounced in the narrow cannulae (15-17 Fr) where the reduction ranged from 27% to 40% at low flows and 5-15% in the upper flow range. These differences were less apparent with increasing cannula diameter. There was a marked disparity between manufacturers. Based on the measured results, testing of cannulae including whole blood flows in a standardized bench test would be recommended.

  • 134.
    Broman, Lars Mikael
    et al.
    Karolinska Univ Hosp, Dept Pediat Perioperat Med & Intens Care, ECMO Ctr Karolinska, S-17176 Stockholm, Sweden.;Karolinska Inst, Dept Physiol & Pharmacol, Stockholm, Sweden.;EuroELSO, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England..
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Centres, BioMEx.
    Westlund, C. Jerker
    Karolinska Univ Hosp, Dept Pediat Perioperat Med & Intens Care, ECMO Ctr Karolinska, S-17176 Stockholm, Sweden..
    Gilbers, Martijn
    Maastricht Univ, Hosp Med, Cardiovasc Res Inst Maastricht CARIM, Heart & Vasc Ctr,Dept Cardiothorac Surg, Maastricht, Netherlands.;Maastricht Univ, Dept Physiol, Maastricht, Netherlands..
    da Camara, Luisa Perry
    Hosp Curry Cabral, Ctr Hosp Lisboa Cent, Lisbon, Portugal..
    Westin, Jan
    Karolinska Univ Hosp, Dept Med Technol, Stockholm, Sweden..
    Taccone, Fabio Silvio
    EuroELSO, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;ULB, Dept Intens Care, Hop Erasme, Brussels, Belgium..
    Malfertheiner, Maximilian Valentin
    EuroELSO, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;Univ Med Ctr Regensburg, Dept Internal Med Cardiol & Pneumol 2, Regensburg, Germany..
    Di Nardo, Matteo
    EuroELSO, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;Childrens Hosp Bambino Gesu, IRCCS, Pediat Intens Care Unit, Rome, Italy..
    Swol, Justyna
    EuroELSO, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;Paracelsus Med Univ, Dept Pulmonol, Intens Care Med, Nurnberg, Germany..
    Vercaemst, Leen
    EuroELSO, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;Univ Hosp Gasthuisberg, Dept Perfus, Louven, Belgium..
    Barrett, Nicholas A.
    EuroELSO, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;Guys & St Thomas NHS Fdn Trust, Dept Crit Care, London, England.;Guys & St Thomas NHS Fdn Trust, Severe Resp Failure Serv, London, England..
    Pappalardo, Federico
    EuroELSO, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;Univ Vita Salute San Raffaele, Hosp San Raffaele, Adv Heart Failure & Mech Circulatory Support Prog, Milan, Italy..
    Belohlavek, Jan
    EuroELSO, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;Charles Univ Prague, Dept Med 2, Dept Cardiovasc Med, Gen Univ Hosp Prague, Prague, Czech Republic.;Charles Univ Prague, Fac Med 1, Prague, Czech Republic..
    Mueller, Thomas
    EuroELSO, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;Univ Med Ctr Regensburg, Dept Internal Med Cardiol & Pneumol 2, Regensburg, Germany..
    Belliato, Mirko
    EuroELSO, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.;Fdn IRCCS Policlin San Matteo, UOC Anestesia & Rianimaz 1, Pavia, Italy..
    Lorusso, Roberto
    KTH, School of Engineering Sciences (SCI), Centres, BioMEx. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. EuroELSO, Working Grp Innovat & Technol, Newcastle Upon Tyne, Tyne & Wear, England.
    Pressure and flow properties of cannulae for extracorporeal membrane oxygenation II: drainage (venous) cannulae2019In: Perfusion, ISSN 0267-6591, E-ISSN 1477-111X, Vol. 34, p. 65-73Article in journal (Refereed)
    Abstract [en]

    The use of extracorporeal life support devices such as extracorporeal membrane oxygenation in adults requires cannulation of the patient's vessels with comparatively large diameter cannulae to allow circulation of large volumes of blood (>5 L/min). The cannula diameter and length are the major determinants for extracorporeal membrane oxygenation flow. Manufacturing companies present pressure-flow charts for the cannulae; however, these tests are performed with water. Aims of this study were 1. to investigate the specified pressure-flow charts obtained when using human blood as the circulating medium and 2. to support extracorporeal membrane oxygenation providers with pressure-flow data for correct choice of the cannula to reach an optimal flow with optimal hydrodynamic performance. Eighteen extracorporeal membrane oxygenation drainage cannulae, donated by the manufacturers (n = 6), were studied in a centrifugal pump driven mock loop. Pressure-flow properties and cannula features were described. The results showed that when blood with a hematocrit of 27% was used, the drainage pressure was consistently higher for a given flow (range 10%-350%) than when water was used (data from each respective manufacturer's product information). It is concluded that the information provided by manufacturers in line with regulatory guidelines does not correspond to clinical performance and therefore may not provide the best guidance for clinicians.

  • 135.
    Brosse, Nicolas
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Finmo, Carl
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bagheri, Shervin
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Experimental study of a three-dimensional cylinder–filament system2015In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 56, no 6, article id 130Article in journal (Refereed)
    Abstract [en]

    This experimental study reports on the behavior of a filament attached to the rear of a three-dimensional cylinder. The axis of the cylinder is placed normal to a uniform incoming flow, and the filament is free to move in the cylinder wake. The mean position of the filament is studied as a function of the filament length L. It is found that for long (L/D > 6.5, where D is the cylinder diameter) and short (L/D < 2) filaments, the mean position of the filament tends to align with the incoming flow, whereas for intermediate filament lengths (2 < L/D < 6.5), the filament lies down on the cylinder and tends to align with the cylinder axis. The underlying mechanism of the bifurcations is discussed and related to buckling and inverted-pendulum-like instabilities.

  • 136.
    Brouzet, Christophe
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Mittal, Nitesh
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Characterizing the Orientational and Network Dynamics of Polydisperse Nanofibers on the Nanoscale2019In: Macromolecules, ISSN 0024-9297, E-ISSN 1520-5835, Vol. 52, no 6, p. 2286-2295Article in journal (Refereed)
    Abstract [en]

    Polydisperse fiber networks are the basis of many natural and manufactured structures, ranging from high-performance biobased materials to components of living cells and tissues. The formation and behavior of such networks are given by fiber properties such as length and stiffness as well as the number density and fiber-fiber interactions. Studies of fiber network behavior, such as connectivity or rigidity thresholds, typically assume monodisperse fiber lengths and isotropic fiber orientation distributions, specifically for nano scale fibers, where the methods providing time-resolved measurements are limited. Using birefringence measurements in a microfluidic flow-focusing channel combined with a flow stop procedure, we here propose a methodology allowing investigations of length-dependent rotational dynamics of nanoscale polydisperse fiber suspensions, including the effects of initial nonisotropic orientation distributions. Transition from rotational mobility to rigidity at entanglement thresholds is specifically addressed for a number of nanocellulose suspensions, which are used as model nanofiber systems. The results show that the proposed method allows the characterization of the subtle interplay between Brownian diffusion and nanoparticle alignment on network dynamics.

  • 137.
    Brouzet, Christophe
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Mittal, Nitesh
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, Daniel
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Lundell, Fredrik
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Size-Dependent Orientational Dynamics of Brownian Nanorods2018In: ACS Macro Letters, E-ISSN 2161-1653, Vol. 7, no 8, p. 1022-1027Article in journal (Refereed)
    Abstract [en]

    Successful assembly of suspended nanoscale rod-like particles depends on fundamental phenomena controlling rotational and translational diffusion. Despite the significant developments in fluidic fabrication of nanostructured materials, the ability to quantify the dynamics in processing systems remains challenging. Here we demonstrate an experimental method for characterization of the orientation dynamics of nanorod suspensions in assembly flows using orientation relaxation. This relaxation, measured by birefringence and obtained after rapidly stopping the flow, is deconvoluted with an inverse Laplace transform to extract a length distribution of aligned nanorods. The methodology is illustrated using nanocelluloses as model systems, where the coupling of rotational diffusion coefficients to particle size distributions as well as flow-induced orientation mechanisms are elucidated. 

  • 138.
    Brynjell-Rahkola, Mattias
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Studies on instability and optimal forcing of incompressible flows2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis considers the hydrodynamic instability and optimal forcing of a number of incompressible flow cases. In the first part, the instabilities of three problems that are of great interest in energy and aerospace applications are studied, namely a Blasius boundary layer subject to localized wall-suction, a Falkner–Skan–Cooke boundary layer with a localized surface roughness, and a pair of helical vortices. The two boundary layer flows are studied through spectral element simulations and eigenvalue computations, which enable their long-term behavior as well as the mechanisms causing transition to be determined. The emergence of transition in these cases is found to originate from a linear flow instability, but whereas the onset of this instability in the Blasius flow can be associated with a localized region in the vicinity of the suction orifice, the instability in the Falkner–Skan–Cooke flow involves the entire flow field. Due to this difference, the results of the eigenvalue analysis in the former case are found to be robust with respect to numerical parameters and domain size, whereas the results in the latter case exhibit an extreme sensitivity that prevents domain independent critical parameters from being determined. The instability of the two helices is primarily addressed through experiments and analytic theory. It is shown that the well known pairing instability of neighboring vortex filaments is responsible for transition, and careful measurements enable growth rates of the instabilities to be obtained that are in close agreement with theoretical predictions. Using the experimental baseflow data, a successful attempt is subsequently also made to reproduce this experiment numerically.

    In the second part of the thesis, a novel method for computing the optimal forcing of a dynamical system is developed. The method is based on an application of the inverse power method preconditioned by the Laplace preconditioner to the direct and adjoint resolvent operators. The method is analyzed for the Ginzburg–Landau equation and afterwards the Navier–Stokes equations, where it is implemented in the spectral element method and validated on the two-dimensional lid-driven cavity flow and the flow around a cylinder.

  • 139.
    Brynjell-Rahkola, Mattias
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Barman, Emelie
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    On the stability of a Blasius boundary layer subject to localized suction2017Report (Other academic)
    Abstract [en]

    In this work the problem of premature transition in boundary layers due to localized suction is revisited. A thorough study involving nonlinear direct numerical simulations, a three-dimensional linear stability analysis, a sensitivity study and a Koopman analysis is presented. The ensemble of these different techniques enables the origins of oversuction to be studied in great detail and provides new insight into the transition process of the flow. The configuration considered consists of an infinite row of widely separated suction pipes that are mounted to the plate at right angles. For the parameter range investigated, the flow inside the pipe is seen to bifurcate at a lower suction ratio than the boundary layer and thus act as an oscillator that forces the external flow over the plate. At low levels of suction, this forcing is not enough to cause transition in the boundary layer, but as the suction level is increased beyond criticality, modes originating from the pipe and extending into the boundary layer are seen to destabilize as well. These modes enable the perturbations forced in the pipe to also amplify in the boundary layer, which leads to a rapid breakdown to turbulence in the wake of the suction hole.

  • 140.
    Brynjell-Rahkola, Mattias
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    A note on the numerical realization of helical vortices: application to vortex instability2017Report (Other academic)
    Abstract [en]

    The need to numerically represent a free vortex system arises frequently in fundamental and applied research. Many possible techniques for realizing this vortex system exist but most tend to prioritize accuracy either inside or outside of the vortex core, which therefore makes them unsuitable to for a stability analysis considering the entire flow field. In this article, a simple method is presented that is shown to yield an accurate representation of the flow inside and outside of the vortex core. The method is readily implemented in any incompressible Navier–Stokes solver using primitive variables and Cartesian coordinates. It can potentially be used to model a wide range of vortices but is here applied to reproduce a recent experiment by Quaranta et al. (2017) considering two helices. A three-dimensional stability analysis is performed and yields an eigenvalue spectrum that features both long- and short-wave instabilities.

  • 141.
    Brynjell-Rahkola, Mattias
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Modal analysis of roughness-induced crossflow vortices in a Falkner-Skan-Cooke boundary layer2013In: International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2013, TSFP-8 , 2013Conference paper (Refereed)
    Abstract [en]

    A three-dimensional global stability analysis using high-order direct numerical simulations is performed to investigate the effect of surface roughness with Reynolds number (based on roughness height) Rek above and below the critical value for transition, on the eigenmodes of a Falkner-Skan-Cooke boundary layer. The surface roughness is introduced with the immersed boundary method and the eigenvalues and eigenfunctions are solved using an iterative time-stepper method. The study reveals a global instability for the case with higher Reynolds number that causes the flow in the non-linear simulations to break down to turbulence shortly downstream of the roughness. Examination of the unstable linear global modes show that these are the same modes that are observed in experiments immediately before breakdown due to secondary instability, which emphasizes the importance of these modes in transition.

  • 142.
    Brynjell-Rahkola, Mattias
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Mechanics.
    Shahriari, Nima
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Mechanics.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Mechanics.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Mechanics.
    Stability and sensitivity of a cross-flow-dominated Falkner-Skan-Cooke boundary layer with discrete surface roughness2017In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 826, p. 830-850Article in journal (Refereed)
    Abstract [en]

    With the motivation of determining the critical roughness size, a global stability and sensitivity analysis of a three-dimensional Falkner-Skan-Cooke (FSC) boundary layer with a cylindrical surface roughness is performed. The roughness size is chosen such that breakdown to turbulence is initiated by a global version of traditional secondary instabilities of the cross-flow (CF) vortices instead of an immediate flow tripping at the roughness. The resulting global eigenvalue spectra of the systems are found to be very sensitive to numerical parameters and domain size. This sensitivity to numerical parameters is quantified using the epsilon-pseudospectrum, and the dependency on the domain is analysed through an impulse response, structural sensitivity analysis and an energy budget. It is shown that while the frequencies remain relatively unchanged, the growth rates increase with domain size, which originates from the inclusion of stronger CF vortices in the baseflow. This is reflected in a change in the rate of advective energy transport by the baseflow. It is concluded that the onset of global instability in a FSC boundary layer as the roughness height is increased does not correspond to an immediate flow tripping behind the roughness, but occurs for lower roughness heights if sufficiently long domains are considered. However, the great sensitivity results in an inability to accurately pinpoint the exact parameter values for the bifurcation, and the large spatial growth of the disturbances in the long domains eventually becomes larger than can be resolved using finite-precision arithmetic.

  • 143.
    Brynjell-Rahkola, Mattias
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Shahriari, Nima
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. Swedish Defence Research Agency, Sweden.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Stability and sensitivity of a crossflow-dominated Falkner–Skan–Cooke boundary layer with discrete surface roughness2016In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645Article in journal (Refereed)
    Abstract [en]

    With the motivation of determining the critical roughness size, a global stability and sensitivity analysis of a three-dimensional Falkner–Skan–Cooke (FSC) boundary layer with a cylindrical surface roughness is performed. The roughness size is chosen such that breakdown to turbulence is initiated by a global version of traditional secondary instabilities of the crossflow (CF) vortices, instead of an immediate flow tripping at the roughness. The resulting global eigenvalue spectra of the systems are found to be very sensitive to numerical parameters and domain size. This sensitivity to numerical parameters is quantified using the "-pseudospectrum, and the dependency on the domain is analysed through an impulse response and an energy budget. It is shown that the growth rates increase with domain size, which originates from the inclusion of stronger CF vortices in the baseflow. This is reflected in a change in the rate of advective energy transport by the baseflow. It is concluded that the onset of global instability in a FSC boundary layer as the roughness height is increased does not correspond to an immediate flow tripping behind the roughness, but occurs for lower roughness heights if su ciently long domains are considered. However, the great sensitivity results in an inability to accurately pinpoint the exact parameter values for the bifurcation, and the large spatial growth of the disturbances in the long domains eventually becomes larger than what can be resolved using finite precision arithmetics. 

  • 144.
    Brynjell-Rahkola, Mattias
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Tuckerman, L. S.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Computing Optimal Forcing Using Laplace Preconditioning2017In: Communications in Computational Physics, ISSN 1815-2406, E-ISSN 1991-7120, Vol. 22, no 5, p. 1508-1532Article in journal (Refereed)
    Abstract [en]

    For problems governed by a non-normal operator, the leading eigenvalue of the operator is of limited interest and a more relevant measure of the stability is obtained by considering the harmonic forcing causing the largest system response. Various methods for determining this so-called optimal forcing exist, but they all suffer from great computational expense and are hence not practical for large-scale problems. In the present paper a new method is presented, which is applicable to problems of arbitrary size. The method does not rely on timestepping, but on the solution of linear systems, in which the inverse Laplacian acts as a preconditioner. By formulating the search for the optimal forcing as an eigenvalue problem based on the resolvent operator, repeated system solves amount to power iterations, in which the dominant eigenvalue is seen to correspond to the energy amplification in a system for a given frequency, and the eigenfunction to the corresponding forcing function. Implementation of the method requires only minor modifications of an existing timestepping code, and is applicable to any partial differential equation containing the Laplacian, such as the Navier-Stokes equations. We discuss the method, first, in the context of the linear Ginzburg-Landau equation and then, the two-dimensional lid-driven cavity flow governed by the Navier-Stokes equations. Most importantly, we demonstrate that for the lid-driven cavity, the optimal forcing can be computed using a factor of up to 500 times fewer operator evaluations than the standard method based on exponential timestepping.

  • 145.
    Byström, Martin G.
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Pralits, Jan O.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Heninngson, Dan S.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Luchini, Paolo
    University of Salerno.
    Optimal Disturbances in Three-dimensional Boundary-Layer Flows2007Conference paper (Refereed)
    Abstract [en]

    In the present paper,  two di!erent approaches tocompute the optimal disturbances in the quasi three-dimensional flows are presented. One of the approachesis based on the Multiple Scales method and the otherone utilises the Parabolised Stability Equations.

  • 146.
    Bäbler, Matthäus
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Energy Processes.
    Biferale, Luca
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Feudel, Ulrike
    Guseva, Ksenia
    Lanotte, Alessandra S.
    Marchioli, Cristian
    Picano, Francesco
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. University of Padua, Italy.
    Sardina, Gaetano
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Soldati, Alfredo
    Toschi, Federico
    Numerical simulations of aggregate breakup in bounded and unbounded turbulent flows2015In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 766Article in journal (Refereed)
    Abstract [en]

    Breakup of small aggregates in fully developed turbulence is studied by means of direct numerical simulations in a series of typical bounded and unbounded flow configurations, such as a turbulent channel flow, a developing boundary layer and homogeneous isotropic turbulence. The simplest criterion for breakup is adopted, whereby aggregate breakup occurs when the local hydrodynamic stress sigma similar to epsilon(1/2), with epsilon being the energy dissipation at the position of the aggregate, overcomes a given threshold sigma(cr), which is characteristic for a given type of aggregate. Results show that the breakup rate decreases with increasing threshold. For small thresholds, it develops a scaling behaviour among the different flows. For high thresholds, the breakup rates show strong differences between the different flow configurations, highlighting the importance of non-universal mean-flow properties. To further assess the effects of flow inhomogeneity and turbulent fluctuations, the results are compared with those obtained in a smooth stochastic flow. Furthermore, we discuss the limitations and applicability of a set of independent proxies.

  • 147. Camarri, S.
    et al.
    Fallenius, Bengt E. G.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Stability and sensitivity analysis of experimental flow fields measured past a porous cylinder2011Conference paper (Other academic)
  • 148. Camarri, S.
    et al.
    Trip, Renzo
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Investigation of passive control of the wake past a thick plate by stability and sensitivity analysis of experimental data2017In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 828, p. 753-778Article in journal (Refereed)
    Abstract [en]

    In this paper we propose a strategy, entirely relying on available experimental data, to estimate the effect of a small control rod on the frequency of vortex shedding in the wake past a thick perforated plate. The considered values of the flow Reynolds number range between Re similar or equal to 6.6 x 10(3) and Re = 5.3 x 10(4). By means of particle image velocimetry, an experimental database consisting of instantaneous flow fields is collected for different values of suction through the body surface. The strategy proposed here is based on classical stability and sensitivity analysis applied to mean flow fields and on the formulation of an original ad hoc model for the mean flow. The mean flow model is obtained by calibrating the closure of the Reynolds averaged Navier-Stokes equations on the basis of the available experimental data through an optimisation algorithm. As a result, it is shown that the predicted control map agrees reasonably well with the equivalent one measured experimentally. Moreover, it is shown that even when turbulence effects are neglected, the stability analysis applied to the mean flow fields provides a reasonable estimation of the vortex shedding frequency, confirming what is known in the literature and extending it up to Re = 5.3 x 10(4). It is also shown that, when turbulence is taken into account in the stability analysis using the same closure that is calibrated for the corresponding mean flow model, the prediction of the vortex shedding frequency is systematically improved.

  • 149. Camarri, Simone
    et al.
    Fallenius, Bengt E. G.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Stability analysis of experimental flow fields behind a porous cylinder for the investigation of the large-scale wake vortices2013In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 715, p. 499-536Article in journal (Refereed)
    Abstract [en]

    When the linear stability analysis is applied to the time-averaged flow past a circular cylinder after the primary instability of the wake, a nearly marginally stable global mode is predicted with a frequency in time equal to that of the saturated vortex shedding. This behaviour has recently been shown to hold up to Reynolds number Re = 600 by direct numerical simulations. In the present work we verify that the global stability analysis provides reasonable estimation also when applied to experimental velocity fields measured in the wake past a porous circular cylinder at Re similar or equal to 3.5 x 10(3). Different intensities of continuous suction and blowing through the entire surface of the cylinder are considered. The global direct and adjoint stability modes, derived from the experimental data, are used to sort the random instantaneous snapshots of the velocity field in phase. The proposed method is remarkable, sorting the snapshots in phase with respect to the vortex shedding, allowing phase-averaged velocity fields to be extracted from the experimental database. The phase-averaged flow fields are analysed in order to study the effect of the transpiration on the kinematical characteristics of the large-scale wake vortices.

  • 150. Camarri, Simone
    et al.
    Fallenius, Bengt E. G.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Stability analysis of experimental flow fields behind aporous cylinder for the investigation of the large-scale wake vorticesReport (Other academic)
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