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  • 1.
    Ahlman, Daniel
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Brethouwer, Geert
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Johansson, Arne
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Direct numerical simulation of a plane turbulent wall-jet including scalar mixing2007In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 19, no 6, p. 065102-Article in journal (Refereed)
    Abstract [en]

    Direct numerical simulation is used to study a turbulent plane wall-jet including the mixing of a passive scalar. The Reynolds and Mach numbers at the inlet are Re=2000 and M=0.5, respectively, and a constant coflow of 10% of the inlet jet velocity is used. The passive scalar is added at the inlet enabling an investigation of the wall-jet mixing. The self-similarity of the inner and outer shear layers is studied by applying inner and outer scaling. The characteristics of the wall-jet are compared to what is reported for other canonical shear flows. In the inner part, the wall-jet is found to closely resemble a zero pressure gradient boundary layer, and the outer layer is found to resemble a free plane jet. The downstream growth rate of the scalar is approximately equal to that of the streamwise velocity in terms of the growth rate of the half-widths. The scalar fluxes in the streamwise and wall-normal direction are found to be of comparable magnitude. The scalar mixing situation is further studied by evaluating the scalar dissipation rate and the mechanical to mixing time scale ratio.

  • 2.
    Ahlman, Daniel
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brethouwer, Geert
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Johansson, Arne
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
    Direct numerical simulation of non-isothermal turbulent wall-jets2009In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 21, no 3Article in journal (Refereed)
    Abstract [en]

    Direct numerical simulations of plane turbulent nonisothermal wall jets are performed and compared to the isothermal case. This study concerns a cold jet in a warm coflow with an ambient to jet density ratio of ρa/ρj = 0.4, and a warm jet in a cold coflow with a density ratio of ρa/ρj = 1.7. The coflow and wall temperature are equal and a temperature dependent viscosity according to Sutherland’s law is used. The inlet Reynolds and Mach numbers are equal in all these cases. The influence of the varying temperature on the development and jet growth is studied as well as turbulence and scalar statistics. The varying density affects the turbulence structures of the jets. Smaller turbulence scales are present in the warm jet than in the isothermal and cold jet and consequently the scale separation between the inner and outer shear layer is larger. In addition, a cold jet in a warm coflow at a higher inlet Reynolds number was also simulated. Although the domain length is somewhat limited, the growth rate and the turbulence statistics indicate approximate self-similarity in the fully turbulent region. The use of van Driest scaling leads to a collapse of all mean velocity profiles in the near-wall region. Taking into account the varying density by using semilocal scaling of turbulent stresses and fluctuations does not completely eliminate differences, indicating the influence of mean density variations on normalized turbulence statistics. Temperature and passive scalar dissipation rates and time scales have been computed since these are important for combustion models. Except for very near the wall, the dissipation time scales are rather similar in all cases and fairly constant in the outer region.

  • 3.
    Albernaz, Daniel L.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Do-Quang, Minh
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hermanson, J. C.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Thermodynamics of a real fluid near the critical point in numerical simulations of isotropic turbulence2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 12, article id 125105Article in journal (Refereed)
    Abstract [en]

    We investigate the behavior of a fluid near the critical point by using numerical simulations of weakly compressible three-dimensional isotropic turbulence. Much has been done for a turbulent flow with an ideal gas. The primary focus of this work is to analyze fluctuations of thermodynamic variables (pressure, density, and temperature) when a non-ideal Equation Of State (EOS) is considered. In order to do so, a hybrid lattice Boltzmann scheme is applied to solve the momentum and energy equations. Previously unreported phenomena are revealed as the temperature approaches the critical point. Fluctuations in pressure, density, and temperature increase, followed by changes in their respective probability density functions. Due to the non-linearity of the EOS, it is seen that variances of density and temperature and their respective covariance are equally important close to the critical point. Unlike the ideal EOS case, significant differences in the thermodynamic properties are also observed when the Reynolds number is increased. We also address issues related to the spectral behavior and scaling of density, pressure, temperature, and kinetic energy.

  • 4.
    Albernaz, Daniel L.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
    Do-Quang, Minh
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
    Hermanson, Jim C.
    University of Washington, USA.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
    Real fluids near the critical point in isotropic turbulenceIn: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666Article in journal (Refereed)
    Abstract [en]

    We investigate the behavior of a uid near the critical point by using numerical simulations of weakly compressible three-dimensional isotropic turbulence. Much has been done for a turbulent ow with an ideal gas. The primary focus of this work is to analyze uctuations of thermodynamic variables (pressure, density and temperature) when a non-ideal Equation Of State (EOS) is considered. In order to do so, a hybrid lattice Boltzmann scheme is applied to solve the momentum and energy equations. Previously unreported phenomena are revealed as the temperature approaches the critical point. These phenomena include increased uctuations in pressure, density and temperature, followed by changes in their respective probability density functions (PDFs). Unlike the ideal EOS case, signicant dierences in the thermodynamic properties are also observed when the Reynolds number is increased. We also address issues related to the spectral behavior and scaling of density, pressure, temperature and kinetic energy.

  • 5.
    Alfredsson, P. Henrik
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    A new scaling for the streamwise turbulence intensity in wall-bounded turbulent flows and what it tells us about the "outer" peak2011In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, no 4, p. 041702-Article in journal (Refereed)
    Abstract [en]

    One recent focus of experimental studies of turbulence in high Reynolds number wall-bounded flows has been the scaling of the root mean square of the fluctuating streamwise velocity, but progress has largely been impaired by spatial resolution effects of hot-wire sensors. For the near-wall peak, recent results seem to have clarified the controversy; however, one of the remaining issues in this respect is the emergence of a second (so-called outer) peak at high Reynolds numbers. The present letter introduces a new scaling of the local turbulence intensity profile, based on the diagnostic plot by Alfredsson and Orlu [Eur. J. Mech. B/Fluids 42, 403 (2010)], which predicts the location and amplitude of the "outer" peak and suggests its presence as a question of sufficiently large scale separation.

  • 6.
    Altimira, Mireia
    et al.
    Mech. Eng. Dept., Tecnun (Univ. of Navarra).
    Rivas, Alejandro
    Mech. Eng. Dept., Tecnun (Univ. of Navarra).
    Ramos, Juan Carlos
    Mech. Eng. Dept., Tecnun (Univ. of Navarra).
    Anton, Raul
    Mech. Eng. Dept., Tecnun (Univ. of Navarra).
    Linear spatial instability of viscous flow of a liquid sheet through gas2010In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 22, no 7Article in journal (Refereed)
    Abstract [en]

    The present paper focuses on the linear spatial instability of a viscous two-dimensional liquid sheet bounded by two identical viscous gas streams. The Orr–Sommerfeld differential equations and the boundary conditions of the flow configuration are numerically solved using Chebyshev series expansions and the collocation method. The strong dependence of the instability parameters on the velocity profiles is proven by using both quadratic and error functions to define the base flow in the liquid sheet and the gas shear layer. The sensitivity of the spatial instability growth rate to changes in the dimensionless parameters of the problem is assessed. Regarding the liquid sheet Reynolds number, it has been observed that, when this parameter increases, both the most unstable growth rate and the corresponding wavenumber decrease, whereas the cutoff wavenumber increases. The results of this analysis are compared with temporal theory through Gaster transformation. The effects liquid and gas viscosity have on instability are studied by comparing the instability curves given by the presented model with those from an inviscid liquid sheet and a viscous liquid sheet in an inviscid gaseous medium. The model presented in this paper features a variation in the cutoff wavenumber with all the governing parameters of the problem, whereas that provided by cases that account for an inviscid surrounding gas depends only on the liquid sheet Weber number and the ratio of gas to liquid densities. Results provided by the presented model have been experimentally validated and show that quadratic profiles have a greater capacity to predict the disturbance wavelength.

  • 7.
    Apazidis, Nicholas
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Numerical investigation of shock induced bubble collapse in water2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 4, article id 046101Article in journal (Refereed)
    Abstract [en]

    A semi-conservative, stable, interphase-capturing numerical scheme for shock propagation in heterogeneous systems is applied to the problem of shock propagation in liquid-gas systems. The scheme is based on the volume-fraction formulation of the equations of motion for liquid and gas phases with separate equations of state. The semi-conservative formulation of the governing equations ensures the absence of spurious pressure oscillations at the material interphases between liquid and gas. Interaction of a planar shock in water with a single spherical bubble as well as twin adjacent bubbles is investigated. Several stages of the interaction process are considered, including focusing of the transmitted shock within the deformed bubble, creation of a water-hammer shock as well as generation of high-speed liquid jet in the later stages of the process.

  • 8.
    Bagheri, Shervin
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Effects of weak noise on oscillating flows: Linking quality factor, Floquet modes, and Koopman spectrum2014In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 26, no 9, p. 094104-Article in journal (Refereed)
    Abstract [en]

    Many fluid flows, such as bluff body wakes, exhibit stable self-sustained oscillations for a wide range of parameters. Here we study the effect of weak noise on such flows. In the presence of noise, a flow with self-sustained oscillations is characterized not only by its period, but also by the quality factor. This measure gives an estimation of the number of oscillations over which periodicity is maintained. Using a recent theory [P. Gaspard, J. Stat. Phys. 106, 57 (2002)], we report on two observations. First, for weak noise the quality factor can be approximated using a linear Floquet analysis of the deterministic system; its size is inversely proportional to the inner-product between first direct and adjoint Floquet vectors. Second, the quality factor can readily be observed from the spectrum of evolution operators. This has consequences for Koopman/Dynamic mode decomposition analyses, which extract coherent structures associated with different frequencies from numerical or experimental flows. In particular, the presence of noise induces a damping on the eigenvalues, which increases quadratically with the frequency and linearly with the noise amplitude. (C) 2014 AIP Publishing LLC.

  • 9.
    Bagheri, Shervin
    et al.
    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.
    The stabilizing effect of streaks on Tollmien-Schlichting and oblique waves: A parametric study2007In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 19, no 7, p. 078103-1-078103-4Article in journal (Refereed)
    Abstract [en]

    The stabilizing effect of finite amplitude streaks on the linear growth of unstable perturbations [Tollmien-Schlichting (TS) and oblique waves] is numerically investigated by means of the nonlinear parabolized stability equations. We have found that for stabilization of a TS-wave, there exists an optimal spanwise spacing of the streaks. These streaks reach their maximum amplitudes close to the first neutral point of the TS-wave and induce the largest distortion of the mean flow in the unstable region of the TS-wave. For such a distribution, the required streak amplitude for complete stabilization of a given TS-wave is considerably lower than for beta=0.45, which is the optimal for streak growth and used in previous studies. We have also observed a damping effect of streaks on the growth rate of oblique waves in Blasius boundary layer and for TS-waves in Falkner-Skan boundary layers.

  • 10.
    Becerra Garcia, Marley
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Frid, Henrik
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Vazquez, Pedro A.
    Self-consistent modeling of laminar electrohydrodynamic plumes from ultra-sharp needles in cyclohexane2017In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 29, no 12, article id 123605Article in journal (Refereed)
    Abstract [en]

    This paper presents a self-consistent model of electrohydrodynamic (EHD) laminar plumes produced by electron injection from ultra-sharp needle tips in cyclohexane. Since the density of electrons injected into the liquid is well described by the Fowler-Nordheim field emission theory, the injection law is not assumed. Furthermore, the generation of electrons in cyclohexane and their conversion into negative ions is included in the analysis. Detailed steady-state characteristics of EHD plumes under weak injection and space-charge limited injection are studied. It is found that the plume characteristics far from both electrodes and under weak injection can be accurately described with an asymptotic simplified solution proposed by Vazquez et al. ["Dynamics of electrohydrodynamic laminar plumes: Scaling analysis and integral model," Phys. Fluids 12, 2809 (2000)] when the correct longitudinal electric field distribution and liquid velocity radial profile are used as input. However, this asymptotic solution deviates from the self-consistently calculated plume parameters under space-charge limited injection since it neglects the radial variations of the electric field produced by a high-density charged core. In addition, no significant differences in the model estimates of the plume are found when the simulations are obtained either with the finite element method or with a diffusion-free particle method. It is shown that the model also enables the calculation of the current-voltage characteristic of EHD laminar plumes produced by electron field emission, with good agreement with measured values reported in the literature.

  • 11. Belson, Brandt A.
    et al.
    Semeraro, Onofrio
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Rowley, Clarence W.
    Henningson, Dan Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Feedback control of instabilities in the two-dimensional Blasius boundary layer: The role of sensors and actuators2013In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 25, no 5, p. 054106-1-054106-17Article in journal (Refereed)
    Abstract [en]

    We analyze the effects of different types and positions of actuators and sensors on controllers' performance and robustness in the linearized 2D Blasius boundary layer. The investigation is carried out using direct numerical simulations (DNS). To facilitate controller design, we find reduced-order models from the DNS data using a system identification procedure called the Eigensystem Realization Algorithm. Due to the highly convective nature of the boundary layer and corresponding time delays, the relative position of the actuator and sensor has a strong influence on the closed-loop dynamics. We address this issue by considering two different configurations. When the sensor is upstream of the actuator, corresponding to disturbance-feedforward control, good performance is observed, as in previous work. However, feedforward control can be degraded by additional disturbances or uncertainties in the plant model, and we demonstrate this. We then examine feedback controllers in which the sensor is a short distance downstream of the actuator. Sensors farther downstream of the actuator cause inherent time delays that limit achievable performance. The performance of the resulting feedback controllers depends strongly on the form of actuation introduced, the quantities sensed, and the observability of the structures deformed by the controller's action. These aspects are addressed by varying the spatial distribution of actuator and sensor. We find an actuator-sensor pair that is well-suited for feedback control, and demonstrate that it has good performance and robustness, even in the presence of unmodeled disturbances.

  • 12. Borg, K I
    et al.
    Soderholm, Lars
    KTH, Superseded Departments, Mechanics.
    Essen, Hanno
    KTH, Superseded Departments, Mechanics.
    Response to "Comment on 'Force on a spinning sphere moving in a rarefied gas' and 'On the inverse Magnus effect in free molecular flow'" [Phys. Fluids 16, 3832 (2004)]2004In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 16, no 10, p. 3833-3833Article in journal (Other academic)
  • 13. Borg, K. I.
    et al.
    Söderholm, Lars H.
    KTH, Superseded Departments, Mechanics.
    Essén, Hanno
    KTH, Superseded Departments, Mechanics.
    Force on a spinning sphere moving in a rarefied gas2003In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 15, no 3, p. 736-741Article in journal (Refereed)
    Abstract [en]

    The force acting on a spinning sphere moving in a rarefied gas is calculated. It is found to have three contributions with different directions. The transversal contribution is of opposite direction compared to the so-called Magnus force normally exerted on a sphere by a dense gas. It is given by F=-alpha(tau)xi2/3piR(3)mnomegaxv, where alpha(tau) is the accommodation coefficient of tangential momentum, R is the radius of the sphere, m is the mass of a gas molecule, n is the number density of the surrounding gas, omega is the angular velocity, and v is the velocity of the center of the sphere relative to the gas. The dimensionless factor xi is close to unity, but depends on omega and kappa, the heat conductivity of the body.

  • 14.
    Brandt, Luca
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    de Lange, H. C.
    Streak interactions and breakdown in boundary layer flows2008In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 20, no 2Article in journal (Refereed)
    Abstract [en]

    The objective of this paper is to show that the interaction of streamwise velocity streaks of finite length can lead to turbulent breakdown in the flat-plate boundary layer flow. The work is motivated by previous numerical and experimental studies of transitional flows where the high-frequency oscillations leading to turbulence are seen to form in the region of strongest shear induced by streaks in relative motion. Therefore, a model for the interaction of steady and unsteady (i.e., slowly moving in the spanwise direction) spanwise periodic streaks is proposed. The interaction of two subsequent streaks is investigated for varying collision parameters. In particular, the relative spanwise position and angle are considered. The results show that the interaction is able to produce both a symmetric and asymmetric breakdown without the need for additional random noise from the main stream. Velocity structures characteristic of both scenarios are analyzed. Hairpin and A vortices are found in the case of symmetric collision between a low-speed region and an incoming high-speed streak, when a region of strong wall-normal shear is induced. Alternatively, when the incoming high-momentum fluid is misaligned with the low-speed streak in front, single quasi-streamwise vortices are identified. Despite the different symmetry at the breakdown, the detrimental interaction involves for both cases the tail of a low-speed region and the head of a high-speed streak. Further, the breakdown appears in both scenarios as an instability of three-dimensional shear layers formed between the two streaks. The streak interaction scenario is suggested to be of relevance for turbulence production in wall-bounded flows.

  • 15.
    Brandt, Luca
    et al.
    KTH, Superseded Departments, Mechanics.
    Henningson, Dan Stefan
    KTH, Superseded Departments, Mechanics.
    Ponziani, D
    Weakly nonlinear analysis of boundary layer receptivity to free-stream disturbances2002In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 14, no 4, p. 1426-1441Article in journal (Refereed)
    Abstract [en]

    The intent of the present paper is to study the receptivity of a zero pressure gradient boundary layer to free-stream disturbances with the aim to isolate the essential features involved in the generation of streamwise streaks. A weakly nonlinear formulation based on a perturbation expansion in the amplitude of the disturbance truncated at second order is used. It is shown that the perturbation model provide an efficient tool able to disentangle the sequence of events in the receptivity process. Two types of solutions are investigated: the first case amounts to the receptivity to oblique waves generated by a wave-like external forcing term oscillating in the free stream, the second the receptivity to free-stream turbulence-like disturbances, represented as a superposition of modes of the continuous spectrum of the Orr-Sommerfeld and Squire operators. A scaling property of the governing equations with the Reynolds number is also shown to be valid. The relation between this nonlinear receptivity process and previously investigated linear ones is also discussed.

  • 16. Brunet, P.
    et al.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Control of thermocapillary instabilities far from threshold2005In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 17, no 10Article in journal (Refereed)
    Abstract [en]

    We report experiments on control of thermocapillary instabilities at high temperature differences, in an annular geometry. Previous studies [Phys. Fluids 14, 3039 (2002)] showed that a reasonable control of oscillatory instability could be achieved by optimizing a local heating feedback process. We conducted experiments with a basic flow converging from periphery to center. This constitutes a more unstable configuration than previously, and enables appearance of higher-order instabilities and chaos. Applying successfully local feedback control to the periodic state close to the threshold, we extend the process to higher temperature differences, where nonlinear as well as proportional/derivative control laws are necessary to obtain a significant decrease of the temperature fluctuations. Finally, proportional control allows us to synchronize a chaotic state, to a periodic one.

  • 17.
    Bäbler, Matthäus
    et al.
    ETH, Inst Chem & Bioengn, Dept Chem & Appl Biosci.
    Sefcik, J
    Morbidelli, M
    Baldyga, J
    Hydrodynamic interactions and orthokinetic collisions of porous aggregates in the Stokes regime2006In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 18, no 1, p. 013302-Article in journal (Refereed)
    Abstract [en]

    The hydrodynamic interaction of two neutrally buoyant porous aggregates is investigated under creeping flow conditions for the case where the undisturbed velocity of the surrounding flow field is a linear function of position. In this framework, the relative velocity between two aggregates is given by the deformation of the undisturbed flow expressed through the rate of strain and the angular velocity of the flow field, and by two flow-independent hydrodynamic functions, typically referred to as A and B, which account for the disturbance of the flow field due to the presence of the particles [G. K. Batchelor and J. T. Green, J. Fluid Mech. 56, 375 (1972)]. In the present paper, the analysis of thehydrodynamic interaction that is known for the case of two impermeable, solid particles is extended to the case of porous aggregates by applying Brinkman's equation to describe the flow within the aggregates. A reflection scheme is applied to calculate A and B and the obtained expressions are applied to interpret the orthokinetic aggregation ofaggregates in diluted suspensions, where the collision frequency is computed using the method of relative trajectories of a pair of aggregates.

  • 18.
    Carlson, Andreas
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Do-Quang, Minh
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Modeling of dynamic wetting far from equilibrium2009In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 21, no 12Article in journal (Refereed)
    Abstract [en]

    In this paper we present simulations of dynamic wetting far from equilibrium based on phase field theory. In direct simulations of recent experiments [J. C. Bird, S. Mandre, and H. A. Stone, Phys. Rev. Lett. 100, 234501 (2008)], we show that in order to correctly capture the dynamics of rapid wetting, it is crucial to account for nonequilibrium at the contact line, where the gas, liquid, and solid meet. A term in the boundary condition at the solid surface that naturally arises in the phase field theory is interpreted as allowing for the establishment of a local structure in the immediate vicinity of the contact line. A direct qualitative and quantitative match with experimental data of spontaneously wetting liquid droplets is shown.

  • 19.
    Carlsson, C.
    et al.
    Division of Fluid Mechanics, Department of Energy Sciences, Lund University, Sweden.
    Alenius, Emma
    Division of Fluid Mechanics, Department of Energy Sciences, Lund University, Sweden.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. Lund University, Sweden.
    Swirl switching in turbulent flow through 90 degrees pipe bends2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 8, article id 085112Article in journal (Refereed)
    Abstract [en]

    Turbulent flow through 90 degrees pipe bends, for four different curvatures, has been investigated using large eddy simulations. In particular, the origin of the so-called swirl switching phenomenon, which is a large scale oscillation of the flow after the bend, has been studied for different bend curvature ratios. A classification of the phenomenon into a high and a low frequency switching, with two distinct physical origins, is proposed. While the high frequency switching stems from modes formed at the bend, and becomes increasingly important for sharp curvatures, the low frequency switching originates from very-large-scale motions created in the upstream pipe flow.

  • 20. Coppola, Gennaro
    et al.
    Semeraro, Onofrio
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Interfacial instability of two rotating viscous immiscible fluids in a cylinder2011In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, no 6, p. 064105-Article in journal (Refereed)
    Abstract [en]

    A complete original study of the linear temporal instability analysis of two viscous and immiscible fluids enclosed in a rigid cylinder rotating about its axis and separated by a cylindrical interface is performed for the case of higher density fluid located in the annulus. The results of the present contribution fill the lack of an overall assessment of the system behavior due to the increase of both the analytical difficulties and the number of the governing parameters when the several physical effects are all included. The analysis is carried out numerically by discretizing the equations of the evolution of disturbances separately in the two phases formulated in a rotating reference frame. Normal mode analysis leads to a generalized eigenvalue problem which is solved by means of a Chebyshev collocation spectral method. The investigation of the preferred modes of instability is carried out over wide ranges of the parameters space. The behavior of the system is physically discussed and is compared to inviscid asymptotic limits and to viscous approximate solutions of the previous literature.

  • 21. Cossu, C.
    et al.
    Brandt, Luca
    KTH, Superseded Departments, Mechanics.
    Stabilization of Tollmien-Schlichting waves by finite amplitude optimal streaks in the Blasius boundary layer2002In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 14, no 8, p. L57-L60Article in journal (Refereed)
    Abstract [en]

    In this Letter we show by numerical simulation that streamwise streaks of sufficiently large amplitude are able to stabilize Tollmien-Schlichting waves in zero pressure gradient boundary layers at least up to Re=1000. This stabilization is due to the spanwise averaged part of the nonlinear basic flow distortion induced by the streaks and occurs for streak amplitudes lower than the critical threshold beyond which secondary inflectional instability is observed. A new control strategy is implemented using optimal perturbations in order to generate the streaks.

  • 22. Cossu, Carlo
    et al.
    Brandt, Luca
    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.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Secondary threshold amplitudes for sinuous streak breakdown2011In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, no 7, p. 074103-Article in journal (Refereed)
    Abstract [en]

    The nonlinear stability of laminar sinuously bent streaks is studied for the plane Couette flow at Re = 500 in a nearly minimal box and for the Blasius boundary layer at Re(delta)(*)=700. The initial perturbations are nonlinearly saturated streamwise streaks of amplitude A(U) perturbed with sinuous perturbations of amplitude A(W). The local boundary of the basin of attraction of the linearly stable laminar flow is computed by bisection and projected in the A(U) - A(W) plane providing a well defined critical curve. Different streak transition scenarios are seen to correspond to different regions of the critical curve. The modal instability of the streaks is responsible for transition for A(U) = 25%-27% for the considered flows, where sinuous perturbations of amplitude below A(W) approximate to 1%-2% are sufficient to counteract the streak viscous dissipation and induce breakdown. The critical amplitude of the sinuous perturbations increases when the streamwise streak amplitude is decreased. With secondary perturbations amplitude A(W) approximate to 4%, breakdown is induced on stable streamwise streaks with A(U) approximate to 13%, following the secondary transient growth scenario first examined by Schoppa and Hussain [J. Fluid Mech. 453, 57 (2002)]. A cross-over, where the critical amplitude of the sinuous perturbation becomes larger than the amplitude of streamwise streaks, is observed for streaks of small amplitude A(U) < 5%-6%. In this case, the transition is induced by an initial transient amplification of streamwise vortices, forced by the decaying sinuous mode. This is followed by the growth of the streaks and final breakdown. The shape of the critical A(U) - A(W) curve is very similar for Couette and boundary layer flows and seems to be relatively insensitive to the nature of the edge states on the basin boundary. The shape of this critical curve indicates that the stability of streamwise streaks should always be assessed in terms of both the streak amplitude and the amplitude of spanwise velocity perturbations.

  • 23. Cossu, Carlo
    et al.
    Chevalier, Mattias
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Optimal secondary energy growth in a plane channel flow2007In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 19, no 5Article in journal (Refereed)
    Abstract [en]

    The optimal growth of perturbations to transiently growing streaks is studied in Poiseuille flow. Basic flows are generated by direct numerical simulation giving primary optimal spanwise periodic vortices of finite amplitude as the initial condition. They evolve into finite amplitude primary transiently growing streaks. Linear secondary optimal energy growth supported by these primary flows are computed using an adjoint technique which takes into full account the unsteadiness of the basic flows. Qualitative differences between primary and secondary optimal growths are found only when the primary streaks are locally unstable. For locally stable primary streaks, the secondary optimal growth has the same scalings with Reynolds number R as the primary optimal growth and the maximum growth is attained by streamwise uniform vortices, suggesting that the primary and secondary optimal growth are based on the same physical mechanisms. When the primary streaks are locally unstable the secondary optimal growth of unstable wavenumbers scale differently with R and the maximum growth is attained for streamwise nonuniform sinuous perturbations, indicating the prevalence of the inflectional instability mechanism.

  • 24.
    Cvetkovic, Vladimir
    KTH, Superseded Departments, Land and Water Resources Engineering.
    Colloid-facilitated tracer transport by steady random ground-water flow2000In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 12, no 9, p. 2279-2294Article in journal (Refereed)
    Abstract [en]

    We study the transport of reactive solute in a three-phase system (water-solid matrix-colloids) in natural porous media. Semianalytical (integral) solutions are derived for the first time, which can be used for computing expected concentration, mass flux, or discharge for the dissolved as well as for colloid-bounded tracer. The results are based on a few simplifying assumptions: advection-dominated transport, linear mass transfer reactions, and steady-state colloidal concentration. Derived semianalytical expressions capture the main features of colloid-facilitated transport (the reversible-equilibrium and irreversible-kinetic sorption of tracers on colloids), and are applicable for the general class of linear sorption processes on the porous matrix. Derived solutions account for spatial variability of flow and sorption parameters, which is relevant for field-scale applications. We apply the theoretical results to the transport of neptunium and plutonium, using flow and transport data from the alluvial aquifer near Yucca Mountain, Nevada. Based on the zeroth and first temporal moment, dimensionless indicators are proposed for assessing the potential impact of colloid-facilitated tracer transport in aquifers. Generic sensitivity curves show the importance of tracer-colloid kinetic rates. Even very low irreversible rates (which will generally be difficult to determine in the laboratory) may yield observable effects for sufficiently long transport times. The obtained results can be used for assessing the significance of colloid-facilitated tracer transport under field conditions, as well as for setting further constraints on relevant parameters which need to be estimated in the field.

  • 25.
    Do-Quang, Minh
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    The splash of a solid sphere impacting on a liquid surface: Numerical simulation of the influence of wetting2009In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 21, no 2Article in journal (Refereed)
    Abstract [en]

    The impact of a solid sphere on a liquid surface has challenged researchers for centuries and remains of interest today. Recently, Duez [Nat. Phys. 3, 180 (2007)] published experimental results of the splash generated when a solid sphere enters water. Interestingly, the microscopic properties of the solid surface control the nature of the macroscopic behavior of the splash. So by a change in the surface chemistry of the solid sphere, a big splash can be turned into an inconspicuous disappearance and vice versa. This problem was investigated by numerical simulations based on the Navier-Stokes equations coupled with the Cahn-Hilliard equations. This system allows us to simulate the motion of an air-water interface as a solid sphere impacts the liquid pond. The inclusion of the surface energies of the solid surface in the formulation gives a reasonably quantitative description of the dynamic wetting. Numerical results with different wetting properties and impact speed are presented and directly compared with the recent experimental results from Duez.

  • 26. Duguet, Yohann
    et al.
    Monokrousos, Antonios
    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.
    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.
    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.
    Minimal transition thresholds in plane Couette flow2013In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 25, no 8, p. 084103-Article in journal (Refereed)
    Abstract [en]

    Subcritical transition to turbulence requires finite-amplitude perturbations. Using a nonlinear optimisation technique in a periodic computational domain, we identify the perturbations of plane Couette flow transitioning with least initial kinetic energy for Re <= 3000. We suggest a new scaling law E-c = O(Re-2.7) for the energy threshold vs. the Reynolds number, in quantitative agreement with experimental estimates for pipe flow. The route to turbulence associated with such spatially localised perturbations is analysed in detail for Re = 1500. Several known mechanisms are found to occur one after the other: Orr mechanism, oblique wave interaction, lift-up, streak bending, streak breakdown, and spanwise spreading. The phenomenon of streak breakdown is analysed in terms of leading finite-time Lyapunov exponents of the associated edge trajectory.

  • 27. Duguet, Yohann
    et al.
    Pringle, Chris C. T.
    Kerswell, Rich R.
    Relative periodic orbits in transitional pipe flow2008In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 20, no 11, p. 114102-Article in journal (Refereed)
    Abstract [en]

    A dynamical system description of the transition process in shear flows with no linear instability starts with knowledge of exact coherent solutions, among them traveling waves (TWs) and relative periodic orbits (RPOs). We describe a numerical method to find such solutions in pipe flow and apply it in the vicinity of a Hopf bifurcation from a TW which looks to be especially relevant for transition. The dominant structural feature of the RPO solution is the presence of weakly modulated streaks. This RPO, like the TW from which it bifurcates, sits on the laminar-turbulent boundary separating initial conditions which lead to turbulence from those which immediately relaminarize.

  • 28.
    Duguet, Yohann
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Localized edge states in plane Couette flow2009In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 21, no 11Article in journal (Refereed)
    Abstract [en]

    The dynamics at the threshold of transition in plane Couette flow is Investigated numerically in a large spatial domain for a certain type of localized initial perturbation, for Re between 350 and 1000 The corresponding edge state is all unsteady spotlike Structure, localized in both streamwise and spanwise directions, which neither grows nor decays in size. We show that the localized nature of the edge state is numerically robust. and IS not Influenced by the size of the computational domain The edge trajectory appeals to transiently visit localized steady states This suggests that basic spatiotemporally intermittent features of transition to turbulence. such as the growth Of it turbulent spot, call be described as a dynamical system.

  • 29. Duwig, Christophe
    et al.
    Fuchs, Laszlo
    Large eddy simulation of vortex breakdown/flame interaction2007In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 19, no 7Article in journal (Refereed)
    Abstract [en]

    The dynamics of a swirl-stabilized premixed flame is studied using large eddy simulation (LES). A filtered flamelet model is used to account for the subgrid combustion. The model provides a consistent and robust reaction-diffusion expression for simulating the propagation of turbulent premixed flames correctly. The numerical results were found to be relatively insensitive to small changes in the inflow boundary conditions and to the numerical mesh employed. Furthermore, the results were found to agree well with the available experimental data both for velocity and scalar fields. In addition, unsteady flame features [i.e., precessing vortex core (PVC)] were identified and compared with experimental data. The agreement between LES results and experimental data, in terms of flame dynamics, was also good. Increasing swirl did not affect the flame strongly but a decrease of swirl number was shown to change the flame shape and suppress the PVC. The PVC and flame dynamics were studied using proper orthogonal decomposition (POD) allowing us to identify and isolate the PVC from smaller-scale turbulence. The POD results indicate that the PVC corresponds to a helical wave consisting of two counter-rotating helices. A dynamical reduced model was also derived do describe the flame response to the PVC.

  • 30. Einarsson, J.
    et al.
    Candelier, F.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Angilella, J. R.
    Mehlig, B.
    Rotation of a spheroid in a simple shear at small Reynolds number2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 6, article id 063301Article in journal (Refereed)
    Abstract [en]

    We derive an effective equation of motion for the orientational dynamics of a neutrally buoyant spheroid suspended in a simple shear flow, valid for arbitrary particle aspect ratios and to linear order in the shear Reynolds number. We show how inertial effects lift the degeneracy of the Jeffery orbits and determine the stabilities of the log-rolling and tumbling orbits at infinitesimal shear Reynolds numbers. For prolate spheroids, we find stable tumbling in the shear plane and log-rolling is unstable. For oblate spheroids, by contrast, log-rolling is stable and tumbling is unstable provided that the particle is not too disk-like (moderate asphericity). For very flat oblate spheroids, both log-rolling and tumbling are stable, separated by an unstable limit cycle.

  • 31.
    Eitel-Amor, Georg
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Örlü, Ramis
    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. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Flores, O.
    Hairpin vortices in turbulent boundary layers2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 2, article id 025108Article in journal (Refereed)
    Abstract [en]

    The present work presents a number of parallel and spatially developing simulations of boundary layers to address the question of whether hairpin vortices are a dominant feature of near-wall turbulence, and which role they play during transition. In the first part, the parent-offspring regeneration mechanism is investigated in parallel (temporal) simulations of a single hairpin vortex introduced in a mean shear flow corresponding to either turbulent channels or boundary layers (Re-tau less than or similar to 590). The effect of a turbulent background superimposed on the mean flow is considered by using an eddy viscosity computed from resolved simulations. Tracking the vortical structure downstream, it is found that secondary hairpins are only created shortly after initialization, with all rotational structures decaying for later times. For hairpins in a clean (laminar) environment, the decay is relatively slow, while hairpins in weak turbulent environments (10% of nu(t)) dissipate after a couple of eddy turnover times. In the second part, the role of hairpin vortices in laminar-turbulent transition is studied using simulations of spatial boundary layers tripped by hairpin vortices. These vortices are generated by means of specific volumetric forces representing an ejection event, creating a synthetic turbulent boundary layer initially dominated by hairpin-like vortices. These hairpins are advected towards the wake region of the boundary layer, while a sinusoidal instability of the streaks near the wall results in rapid development of a turbulent boundary layer. For Re-theta > 400, the boundary layer is fully developed, with no evidence of hairpin vortices reaching into the wall region. The results from both the parallel and spatial simulations strongly suggest that the regeneration process is rather short-lived and may not sustain once a turbulent background is developed. From the transitional flow simulations, it is conjectured that the forest of hairpins reported in former direct numerical simulation studies is reminiscent of the transitional boundary layer and may not be connected to some aspects of the dynamics of the fully developed wall-bounded turbulence.

  • 32.
    Eliasson, Veronica
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Tillmark, Nils
    Szeri, Andrew
    University of California, Department of Mechanical Engineering, Berkeley.
    Apazidis, Nicholas
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Light emission during shock wave focusing in air and argon2007In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 19, no 10, p. 106106-1-106106-17Article in journal (Refereed)
    Abstract [en]

    The light emission from a converging shock wave was investigated experimentally. Results show that the shape of the shock wave close to the center of convergence has a large influence on the amount of emitted light. It was found that a symmetrical polygonal shock front produced more light than an asymmetrical shape. The light emission appears as the shock wave collapses. The full width at half maximum of the light pulse is about 200 ns for all geometrical shapes. It was also found that argon as a test gas produces more light than air. Numerical simulations showed good agreement with experimental results regarding the shape of the shock and the flow field behind the shock. The temperature field from the numerical simulations was investigated and shows that the triple points behind the shock front are hot spots that increase the temperature at the center as they arrive there.

  • 33. Elperin, T.
    et al.
    Kleeorin, Nathan
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Liberman, Mikhail
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Rogachevskii, Igor
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Tangling clustering instability for small particles in temperature stratified turbulence2013In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 25, no 8, p. 085104-Article in journal (Refereed)
    Abstract [en]

    We study tangling clustering instability of inertial particles in a temperature stratified turbulence with small finite correlation time. It is shown that the tangling mechanism in the temperature stratified turbulence strongly increases the degree of compressibility of particle velocity field. This results in the strong decrease of the threshold for the excitation of the tangling clustering instability even for small particles. The tangling clustering instability in the temperature stratified turbulence is essentially different from the inertial clustering instability that occurs in non-stratified isotropic and homogeneous turbulence. While the inertial clustering instability is caused by the centrifugal effect of the turbulent eddies, the mechanism of the tangling clustering instability is related to the temperature fluctuations generated by the tangling of the mean temperature gradient by the velocity fluctuations. Temperature fluctuations produce pressure fluctuations and cause particle accumulations in regions with increased instantaneous pressure. It is shown that the growth rate of the tangling clustering instability is root Re (l(0)/L-T)(2)/(3Ma)(4) times larger than that of the inertial clustering instability, where Re is the Reynolds number, Ma is the Mach number, l(0) is the integral turbulence scale, and L-T is the characteristic scale of the mean temperature variations. It is found that depending on the parameters of the turbulence and the mean temperature gradient there is a preferential particle size at which the particle clustering due to the tangling clustering instability is more effective. The particle number density inside the cluster after the saturation of this instability can be by several orders of magnitude larger than the mean particle number density. It is also demonstrated that the evaporation of droplets drastically changes the tangling clustering instability, e. g., it increases the instability threshold in the droplet radius. The tangling clustering instability is of a great importance, e. g., in atmospheric turbulence with temperature inversions.

  • 34.
    Facciolo, Luca
    et al.
    KTH, Superseded Departments, Mechanics.
    Alfredsson, P. Henrik
    KTH, Superseded Departments, Mechanics.
    The counter-rotating core of a swirling turbulent jet issued from a rotating pipe flow2004In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 16, no 9, p. L71-L73Article in journal (Refereed)
    Abstract [en]

    Axially rotating turbulent pipe flow is an example where rotation strongly affects the turbulence and thereby the Reynolds stresses and mean flow properties. The present Letter reports new measurements where a rotating pipe flow is used to establish a swirling jet. The measurements in the jet show that at some distance downstream (approximately 6 nozzle diameters) the central part of the jet starts to rotate in the opposite direction as compared to the rotation of the pipe. This effect is explained by the influence of the cross flow Reynolds stress originating in the pipe flow.

  • 35. Facciolo, Luca
    et al.
    Tillmark, Nils
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Talamelli, Alessandro
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    A study of swirling turbulent pipe and jet flows2007In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 19, no 3Article in journal (Refereed)
    Abstract [en]

    Axially rotating turbulent pipe flow is an example in which the rotation strongly affects the turbulence, which then also influences the mean flow properties. For instance, in the fully developed flow as well, the fluid is not in solid body rotation due to the influence of the cross-stream Reynolds stress. The present paper reports new measurements from a rotating pipe flow and the streamwise mean velocity distribution is compared with recent scaling ideas of Oberlack [J. Fluid Mech. 379, 1 (1999)] and good agreement is found. A second part of the paper deals with the initial stages when the flow leaves the pipe and forms a swirling jet. The measurements in the jet show that at some distance downstream (approximately five jet diameters) the central part of the jet actually rotates in the opposite direction as compared to the rotation of the pipe. This effect is explained by the influence of the cross-stream Reynolds shear stress.

  • 36.
    Fornari, Walter
    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.
    Formenti, A.
    Picano, F.
    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 effect of particle density in turbulent channel flow laden with finite size particles in semi-dilute conditions2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 3, article id 033301Article in journal (Refereed)
    Abstract [en]

    We study the effect of varying the mass and volume fraction of a suspension of rigid spheres dispersed in a turbulent channel flow. We performed several direct numerical simulations using an immersed boundary method for finite-size particles changing the solid to fluid density ratio R, the mass fraction χ, and the volume fraction φ. We find that varying the density ratio R between 1 and 10 at constant volume fraction does not alter the flow statistics as much as when varying the volume fraction φ at constant R and at constant mass fraction. Interestingly, the increase in overall drag found when varying the volume fraction is considerably higher than that obtained for increasing density ratios at same volume fraction. The main effect at density ratios R of the order of 10 is a strong shear-induced migration towards the centerline of the channel. When the density ratio R is further increased up to 1000, the particle dynamics decouple from that of the fluid. The solid phase behaves as a dense gas and the fluid and solid phase statistics drastically change. In this regime, the collision rate is high and dominated by the normal relative velocity among particles.

  • 37.
    Fransson, Jens H. M.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Talamelli, A.
    Cossu, C.
    Experimental study of the stabilization of Tollmien-Schlichting waves by finite amplitude streaks2005In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 17, no 5Article in journal (Refereed)
    Abstract [en]

    It has recently been found by using temporal and spatial numerical simulations that steady optimal streaks of moderate amplitude, i.e., sufficiently large but not exceeding the critical amplitude for the inflectional instability, are able to reduce the growth of Tollmien-Schlichting (TS) waves up to their complete suppression. This investigation aims at experimentally verifying this stabilizing effect by generating stable and symmetric, close to sinusoidal, streaks of moderate amplitudes (similar to 12% of the free-stream velocity) by means of a spanwise array of cylindrical roughness elements. The three-dimensional (3D) streaky base flow is then subjected to a secondary instability generated through a spanwise slot in the plate by means of regulated blowing and suction. In this study the stabilizing role of the streaks on TS waves is unambiguously confirmed and by increasing the height of the roughness elements, thus inducing larger amplitude streaks, we are also able to show that the stabilizing action on the TS waves increases with the streak amplitude. These results are the first to confirm the numerical predictions reported in earlier works. The full cross-stream plane has been measured at different downstream positions allowing a complete evaluation and comparison of the different amplitude measures used in previous experimental works. Furthermore, theoretical impulse response analysis and stability calculations are applied to the present experimental streaky base flow enabling a qualitative comparison of the 3D modulated TS wave distribution.

  • 38.
    Fransson, Jens H. M.
    et al.
    KTH, Superseded Departments, Mechanics.
    Brandt, Luca
    KTH, Superseded Departments, Mechanics.
    Talamelli, Alessandro
    KTH, Superseded Departments, Mechanics.
    Cossu, C.
    Experimental and theoretical investigation of the nonmodal growth of steady streaks in a flat plate boundary layer2004In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 16, no 10, p. 3627-3638Article in journal (Refereed)
    Abstract [en]

    An experimental and theoretical investigation aimed at describing the nonmodal growth of steady and spanwise periodic streamwise streaks in a flat plate boundary layer is presented. Stable laminar streaks are experimentally generated by means of a spanwise periodic array of small cylindrical roughness elements fixed on the plate. The streamwise evolution of the streaks is measured and it is proved that, except in a small region near the roughness elements, they obey the boundary layer scalings. The maximum achievable amplitude is mainly determined by the relative height of the roughness elements. Results are compared with numerical simulations of optimal and suboptimal boundary layer streaks. The theory is able to elucidate some of the discrepancies recently noticed between experimentally realizable nonmodal growth and optimal perturbation theory. The key factor is found to be the wall normal location and the extension of the laminar standing streamwise vortices inducing the streaks. The differences among previous experimental works can be explained by different dominating streak generation mechanisms which can be linked to the geometry and to the ratio between the roughness height and the boundary layer scale.

  • 39.
    Fransson, Jens H.M.
    et al.
    KTH, Superseded Departments, Mechanics.
    Alfredsson, P.H.
    KTH, Superseded Departments, Mechanics.
    On the hydrodynamic stability of channel flow with cross flow2003In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 15, no 2, p. 436-441Article in journal (Refereed)
    Abstract [en]

    We study plane channel flow, with a homogeneous cross flow through porous walls, mainly with respect to the stability to two-dimensional wave disturbances. Since the stability of a shear flow depends both on the velocity distribution and the Reynolds number we partly investigated this flow under the conditions that the flow Reynolds number was constant. The flow exhibits some interesting and unexpected stability characteristics. The effect of the cross flow was for certain parameter regions stabilizing and for others destabilizing. The latter result is in contrast to previous studies.

  • 40. Frisch, U.
    et al.
    Bec, J.
    Aurell, Erik
    KTH, School of Engineering Sciences (SCI), Physics.
    "Locally homogeneous turbulence": Is it an inconsistent framework?2005In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 17, no 8Article in journal (Refereed)
    Abstract [en]

    In his first 1941 paper Kolmogorov assumed that the velocity has increments that are homogeneous and independent of the velocity at a suitable reference point. This assumption of local homogeneity is consistent with the nonlinear dynamics only in an asymptotic sense when the reference point is far away. This inconsistency is illustrated numerically using the Burgers equation. Kolmogorov's derivation of the four-fifths law for the third-order structure function and its anisotropic generalization are actually valid only for homogeneous turbulence, but a local version due to Duchon and Robert still holds. A Kolmogorov-Landau approach is proposed to handle the effect of fluctuations in the large-scale velocity on small-scale statistical properties; it is only a mild extension of the 1941 theory and does not incorporate intermittency effects.

  • 41. Garrett, S. J.
    et al.
    Cooper, A. J.
    Harris, J. H.
    Özkan, M.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Thomas, P. J.
    On the stability of von Kármán rotating-disk boundary layers with radial anisotropic surface roughness2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 1, article id 014104Article in journal (Refereed)
    Abstract [en]

    We summarise results of a theoretical study investigating the distinct convective instability properties of steady boundary-layer flow over rough rotating disks. A generic roughness pattern of concentric circles with sinusoidal surface undulations in the radial direction is considered. The goal is to compare predictions obtained by means of two alternative, and fundamentally different, modelling approaches for surface roughness for the first time. The motivating rationale is to identify commonalities and isolate results that might potentially represent artefacts associated with the particular methodologies underlying one of the two modelling approaches. The most significant result of practical relevance obtained is that both approaches predict overall stabilising effects on type I instability mode of rotating disk flow. This mode leads to transition of the rotating-disk boundary layer and, more generally, the transition of boundary-layers with a cross-flow profile. Stabilisation of the type 1 mode means that it may be possible to exploit surface roughness for laminar-flow control in boundary layers with a cross-flow component. However, we also find differences between the two sets of model predictions, some subtle and some substantial. These will represent criteria for establishing which of the two alternative approaches is more suitable to correctly describe experimental data when these become available. © 2016 AIP Publishing LLC.

  • 42.
    Grigoriev, Igor A.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Wallin, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brethouwer, Gert
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    A realizable explicit algebraic Reynolds stress model for compressible turbulent flow with significant mean dilatation2013In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 25, no 10, p. 105112-Article in journal (Refereed)
    Abstract [en]

    The explicit algebraic Reynolds stress model of Wallin and Johansson [J. Fluid Mech. 403, 89 (2000)] is extended to compressible and variable-density turbulent flows. This is achieved by correctly taking into account the influence of the mean dilatation on the rapid pressure-strain correlation. The resulting model is formally identical to the original model in the limit of constant density. For two-dimensional mean flows the model is analyzed and the physical root of the resulting quartic equation is identified. Using a fixed-point analysis of homogeneously sheared and strained compressible flows, we show that the new model is realizable, unlike the previous model. Application of the model together with a K - omega model to quasi one-dimensional plane nozzle flow, transcending from subsonic to supersonic regime, also demonstrates realizability. Negative "dilatational" production of turbulence kinetic energy competes with positive "incompressible" production, eventually making the total production negative during the spatial evolution of the nozzle flow. Finally, an approach to include the baroclinic effect into the dissipation equation is proposed and an algebraic model for density-velocity correlations is outlined to estimate the corrections associated with density fluctuations. All in all, the new model can become a significant tool for CFD (computational fluid dynamics) of compressible flows.

  • 43.
    Grigoriev, Igor A.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Wallin, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Swedish Defence Research Agency (FOI), Sweden.
    Brethouwer, Geert
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Capturing turbulent density flux effects in variable density flow by an explicit algebraic model2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 4, article id 1.4917278Article in journal (Refereed)
    Abstract [en]

    The explicit algebraic Reynolds stress model of Grigoriev et al. ["A realizable explicit algebraic Reynolds stress model for compressible turbulent flow with significant mean dilatation," Phys. Fluids 25, 105112 (2013)] is extended to account for the turbulent density flux in variable density flows. The influence of the mean dilatation and the variation of mean density on the rapid pressure-strain correlation are properly accounted for introducing terms balancing a so-called "baroclinic" production in the Reynolds stress tensor equation. Applying the weak-equilibrium assumption leads to a self-consistent formulation of the model. The model together with a K - ω model is applied to a quasi-one-dimensional plane nozzle flow transcending from subsonic to supersonic regimes. The model remains realizable with constraints put on the model parameters. When density fluxes are taken into account, the model is less likely to become unrealizable. The density variance coupled with a "local mean acceleration" also can influence the model acting to increase anisotropy. The general trends of the behaviour of the anisotropy and production components under the variation of model parameters are assessed. We show how the explicit model can be applied to two- and three-dimensional mean flows without previous knowledge of a tensor basis to obtain the general solution. Approaches are proposed in order to achieve an approximate solution to the consistency equation in cases when analytic solution is missing. In summary, the proposed model has the potential to significantly improve simulations of variable-density flows.

  • 44.
    Grigoriev, Igor
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Wallin, Stefan
    Swedish Defence Research Agency (FOI), Stockholm, Sweden.
    Brethouwer, Geert
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Grundestam, Olof
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Algebraic Reynolds stress modeling of turbulence subject to rapid homogeneous and non-homogeneous compression or expansion2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 2, p. 026101-Article in journal (Refereed)
    Abstract [en]

    A recently developed explicit algebraic Reynolds stress model (EARSM) by Grigoriev et al. ["A realizable explicit algebraic Reynolds stress model for compressible turbulent flow with significant mean dilatation," Phys. Fluids 25(10), 105112 (2013)] and the related differential Reynolds stress model (DRSM) are used to investigate the influence of homogeneous shear and compression on the evolution of turbulence in the limit of rapid distortion theory (RDT). The DRSM predictions of the turbulence kinetic energy evolution are in reasonable agreement with RDT while the evolution of diagonal components of anisotropy correctly captures the essential features, which is not the case for standard compressible extensions of DRSMs. The EARSM is shown to give a realizable anisotropy tensor and a correct trend of the growth of turbulence kinetic energy K, which saturates at a power law growth versus compression ratio, as well as retaining a normalized strain in the RDT regime. In contrast, an eddy-viscosity model results in a rapid exponential growth of K and excludes both realizability and high magnitude of the strain rate. We illustrate the importance of using a proper algebraic treatment of EARSM in systems with high values of dilatation and vorticity but low shear. A homogeneously compressed and rotating gas cloud with cylindrical symmetry, related to astrophysical flows and swirling supercritical flows, was investigated too. We also outline the extension of DRSM and EARSM to include the effect of non-homogeneous density coupled with "local mean acceleration" which can be important for, e.g., stratified flows or flows with heat release. A fixed-point analysis of direct numerical simulation data of combustion in a wall-jet flow demonstrates that our model gives quantitatively correct predictions of both streamwise and cross-stream components of turbulent density flux as well as their influence on the anisotropies. In summary, we believe that our approach, based on a proper formulation of the rapid pressure-strain correlation and accounting for the coupling with turbulent density flux, can be an important element in CFD tools for compressible flows.

  • 45.
    Gustavsson, Katarina
    et al.
    KTH, School of Computer Science and Communication (CSC), Numerical Analysis, NA. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tornberg, Anna-Karin
    KTH, School of Computer Science and Communication (CSC), Numerical Analysis, NA. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Gravity induced sedimentation of slender fibers2009In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 21, no 12Article in journal (Refereed)
    Abstract [en]

    Gravity induced sedimentation of slender, rigid fibers in a highly viscous fluid is investigated by large scale numerical simulations. The fiber suspension is considered on a microscopic level and the flow is described by the Stokes equations in a three dimensional periodic domain. Numerical simulations are performed to study in great detail the complex dynamics of a cluster of fibers. A repeating cycle is identified. It consists of two main phases: a densification phase, where the cluster densifies and grows, and a coarsening phase, during which the cluster becomes smaller and less dense. The dynamics of these phases and their relation to fluctuations in the sedimentation velocity are analyzed. Data from the simulations are also used to investigate how average fiber orientations and sedimentation velocities are influenced by the microstructure in the suspension. The dynamical behavior of the fiber suspension is very sensitive to small random differences in the initial configuration and a number of realizations of each numerical experiment are performed. Ensemble averages of the sedimentation velocity and fiber orientation are presented for different values of the effective concentration of fibers and the results are compared to experimental data. The numerical code is parallelized using the Message Passing Instructions (MPI) library and numerical simulations with up 800 fibers can be run for very long times which is crucial to reach steady levels of the averaged quantities. The influence of the periodic boundary conditions on the process is also carefully investigated.

  • 46. Haugen, Nils Erland L.
    et al.
    Kleeorin, Nathan
    Rogachevskii, Igor
    Brandenburg, Axel
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Detection of turbulent thermal diffusion of particles in numerical simulations2012In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 24, no 7, p. 075106-Article in journal (Refereed)
    Abstract [en]

    The phenomenon of turbulent thermal diffusion in temperature-stratified turbulence causing a non-diffusive turbulent flux (i.e., non-counter-gradient transport) of inertial and non-inertial particles in the direction of the turbulent heat flux is found using direct numerical simulations (DNS). In simulations with and without gravity, this phenomenon is found to cause a peak in the particle number density around the minimum of the mean fluid temperature for Stokes numbers less than 1, where the Stokes number is the ratio of particle Stokes time to turbulent Kolmogorov time at the viscous scale. Turbulent thermal diffusion causes the formation of inhomogeneities in the spatial distribution of inertial particles whose scale is large in comparison with the integral scale of the turbulence. The strength of this effect is maximum for Stokes numbers around unity, and decreases again for larger values. The dynamics of inertial particles is studied using Lagrangian modelling in forced temperature-stratified turbulence, whereas non-inertial particles and the fluid are described using DNS in an Eulerian framework.

  • 47.
    Henningson, Dan S.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Åkervik, Espen
    The use of global modes to understand transition and perform flow control2008In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 20, no 3Article in journal (Refereed)
    Abstract [en]

    The stability of nonparallel flows is considered using superposition of global modes. When perturbed by the worst case initial condition, these flows often exhibit a large transient growth associated with the development of wave packets. The global modes of the systems also provide a good starting point for the design of reduced order models used to control the growing disturbances. Three recent investigations are reviewed. The first example is the growth of a wave packet on a falling liquid sheet. The optimal perturbation analysis shows that the worst case initial condition is a localized disturbance that creates a propagating wave packet that hits the downstream end, regenerating a wave packet upstream through a global pressure pulse. Second, we consider two-dimensional disturbances in the Blasius boundary layer. It is found that a wave packet is optimally excited by an initial condition consisting of localized backward leaning Orr structures. Finally, the control of a globally unstable boundary-layer flow along a shallow cavity is considered. The disturbance propagation is associated with the development of a wave packet along the cavity shear layer, unstable to the Kelvin-Helmholtz mechanism, followed by a global cycle related to the two unstable global modes. Direct numerical simulations of this flow are coupled to a measurement feedback controller, which senses the wall shear stress at the downstream lip of the cavity and provides the actuation at the upstream lip. A reduced order model for the control is obtained by a projection on the least stable global eigenmodes. The linear-quadratic-Gaussian controller is run in parallel to the Navier-Stokes time integration and it is shown to damp out the global oscillations.

  • 48. Hiwatashi, Kazuaki
    et al.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Tillmark, Nils
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Nagata, M.
    Experimental observations of instabilities in rotating plane Couette flow2007In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 19, no 4Article in journal (Refereed)
    Abstract [en]

    The transition from the two-dimensional (2D) longitudinal roll cell state to 3D flows in the rotating plane Couette system, predicted by the theoretical investigation [M. Nagata, J. Fluid Mech. 358, 357 (1998)], is examined experimentally. The streamwise and spanwise wave numbers of observed steady 3D flows seem to agree with those predicted by the theory when the rotation rate is relatively large. However, we observe unsteady 3D states in the region where the theory predicts stable steady 3D flows when the rotation rate is small.

  • 49. Hoepffner, Jerome
    et al.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Stochastic approach to the receptivity problem applied to bypass transition in boundary layers2008In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 20, no 2Article in journal (Refereed)
    Abstract [en]

    To study the flow behavior in the presence of external disturbances of chaotic nature, a stochastic approach is pursued. In particular, transition to turbulence in boundary layers exposed to high levels of free-stream turbulence is considered. The late stages of this transition scenario, characterized by the growth and breakdown of streamwise-elongated streaks, are examined by considering the linear evolution of perturbations to a base flow consisting of the Blasius profile and the streaks. A stochastic initial condition is considered where the free-stream perturbations are described by the correlations of isotropic homogeneous turbulence. The spatial correlation of the excited flow at later times can be computed by the numerical solution of a Lyapunov equation. It is shown that free-stream turbulence has the necessary features to excite secondary energy growth, thus playing a central role in the transition to turbulence. The method proposed here can be used to examine the receptivity of other flows to external noise whose statistical properties are known or can be modeled.

  • 50. Hogberg, M.
    et al.
    Bewley, T. R.
    Henningson, Dan S.
    KTH, Superseded Departments, Mechanics.
    Relaminarization of Re-tau=100 turbulence using gain scheduling and linear state-feedback control2003In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 15, no 11, p. 3572-3575Article in journal (Refereed)
    Abstract [en]

    The first successful application of linear full-state feedback optimal control theory to consistently relaminarize turbulent channel flow at Re-tau=100 with full state information and gain scheduling is reported. The actuation is zero-net mass-flux blowing and suction on the channel walls. Two key issues central to the success of this strategy are: (a) the choice of the mean-flow profile about which the equations are linearized for the computation of the linear feedback gains, and (b) the choice of an objective function which targets the control effort on the flow perturbations of interest. A range of mean-flow profiles between the laminar and fully turbulent profiles and a weighted energy measure which targets flow perturbations in the near-wall region were found to provide effective feedback gains. A gain-scheduling strategy to tune the feedback gains to the nonstationary mean-flow profile is introduced, resulting in consistent relaminarization of the turbulent flow in all realizations tested.

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