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  • 1.
    Altimira, Mireia
    et al.
    Mech. Eng. Dept., Tecnun (Univ. of Navarra).
    Rivas, Alejandro
    Mech. Eng. Dept., Tecnun (Univ. of Navarra).
    Sanchez Larraona, Gorka
    Mech. Eng. Dept., Tecnun (Univ. of Navarra).
    Anton, Raul
    Mech. Eng. Dept., Tecnun (Univ. of Navarra).
    Ramos, Juan Carlos
    Mech. Eng. Dept., Tecnun (Univ. of Navarra).
    Characterization of fan spray atomizers through numerical simulation2009In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 30, no 2, p. 339-355Article in journal (Refereed)
    Abstract [en]

    The present paper focuses on the mathematical modeling of industrial fan spray atomizers. The two-phase flow taking place inside the nozzle's tip and the exterior region near the outlet of three different industrial nozzle designs has been modeled and simulated. As a result, valuable information has been obtained regarding the influence of the inner geometry on the flow and also the formation and development of the liquid sheet. Characteristic magnitudes such as the discharge coefficient and the liquid sheet thickness factor have been obtained and validated through experimental measurements. The accumulation of liquid at the border of fan-shaped liquid sheets, also known as rim, has been studied in the analyzed designs, revealing the presence of a tangential velocity component in the liquid sheet and a relationship between the incoming flow rate of the rim and the angle of the liquid sheet. The dependence of the results on turbulence modeling has also been analyzed, drawing interesting conclusions regarding their influence on the liquid sheet mean flow characteristics and on the surrounding gas. Thus, the mathematical model developed has been proved to be a useful tool for nozzle manufacturers; it provides the most important characteristic parameters of the liquid sheet formed given certain nozzle geometry and, additionally, those data necessary to carry out studies of instability, breakup and atomization of the liquid sheet.

  • 2. Battista, F.
    et al.
    Troiani, G.
    Picano, Francesco
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fractal scaling of turbulent premixed flame fronts: Application to LES2015In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 51, p. 78-87Article in journal (Refereed)
    Abstract [en]

    The fractal scaling properties of turbulent premixed flame fronts have been investigated and considered for modeling sub-grid scales in the Large-Eddy-Simulation framework. Since the width of such thin reaction fronts cannot be resolved into the coarse mesh of LES, the extent of wrinkled flame surface contained in a volume is taken into account. The amount of unresolved flame front is estimated via the "wrinkling factor" that depends on the definition of a suitable fractal dimension and the scale at which the fractal scaling is lost, the inner cut-off length e. In this context, the present study considers laboratory experiments and one-step reaction DNS of turbulent premixed jet flames in different regimes of turbulent premixed flames. Fractal dimension is found to be substantially constant and well below that typical of passive scalar fronts. The inner cut-off length shows a clear scaling with the dissipative scale of Kolmogorov for the regimes here considered. These features have been exploited performing Large Eddy Simulations. Good model performance has been found comparing the LES against a corresponding DNS at moderate Reynolds number and experimental data at higher Reynolds numbers.

  • 3. Borg, A.
    et al.
    Fuchs, Laszlo
    LIF study of mixing in a model of a vein punctured by a cannula2002In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 23, no 5, p. 664-670Article in journal (Refereed)
    Abstract [en]

    Steady flow and mixing in a model of an arterialized vein punctured by a cannula as occurs during hemodialysis has been investigated in vitro. The motivation is that a major cause of vascular access dysfunction is the development of venous stenoses. This phenomenon lacks physiological explanation. However, one may attribute this quick process to the chemical content of the dialyzed blood and its flow near the point of infusion. The interest in mixing of chemical compounds in the dialyzed blood supplied through the cannula is, therefore, genuine due to the clinical impacts of haemodyalysis. We are interested in understanding the mixing of the two streams; namely, the untreated blood through the vein and the treated blood through the cannula. This mixing affects the local pH, which in turn can affect the solubility of several salts used for dialysis. In addition, since the blood contains molecules of widely different diffusivity properties, the local composition of blood near the point of injection is of interest. The hypothesis is that concentration non-uniformities may lead to undesired chemical or bio-chemical reactions leading to the pathological processes in the region around the needle. The mixing of a high Schmidt number substance in the stream entering from the cannula with the base flow in the vein is studied by laser induced fluorescence (LIF). The investigations are performed for a range of typical Reynolds numbers in the cannula and the vein found during hemodialysis. The study shows complicated mixing patterns around the cannula, and that non-uniformities in the blood persist over long distances for the lower flow rates found in vivo. For the higher flow rates, the flow loses its stability and mixing is enhanced. The nature of this instability is shown, and quantitative data of concentration fluctuations are given. We have further considered the effect of rotating the cannula. This leads to a significant change in the mixing process. The significance of the non-uniformities in mixing of solvents in the blood for the development of venous stenoses should be further studied not only in fluid dynamical terms but also in terms of endothelial (cellular) effects.

  • 4.
    Canton, Jacopo
    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.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    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.
    Characterisation of the steady, laminar incompressible flow in toroidal pipes covering the entire curvature range2017In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 66, p. 95-107Article in journal (Refereed)
    Abstract [en]

    This work is concerned with a detailed investigation of the steady (laminar), incompressible flow inside bent pipes. In particular, a toroidal pipe is considered in an effort to isolate the effect of the curvature, δ, on the flow features, and to compare the present results to available correlations in the literature. More than 110 000 numerical solutions are computed, without any approximation, spanning the entire curvature range, 0 ≤ δ ≤ 1, and for bulk Reynolds numbers Re up to 7 000, where the flow is known to be unsteady. Results show that the Dean number De provides a meaningful non-dimensional group only below very strict limits on the curvature and the Dean number itself. For δ>10−6 and De > 10, in fact, not a single flow feature is found to scale well with the Dean number. These considerations are also valid for quantities, such as the Fanning friction factor, that were previously considered Dean-number dependent only. The flow is therefore studied as a function of two equally important, independent parameters: the curvature of the pipe and the Reynolds number. The analysis shows that by increasing the curvature the flow is fundamentally changed. Moderate to high curvatures are not only quantitatively, but also qualitatively different from low δ cases. A complete description of some of the most relevant flow quantities is provided. Most notably the friction factor f for laminar flow in curved pipes by Ito [J. Basic Eng. 81:123–134 (1959)] is reproduced, the influence of the curvature on f is quantified and the scaling is discussed. A complete database including all the computed solutions is available at www.flow.kth.se.

  • 5. Conway, S.
    et al.
    Caraeni, D.
    Fuchs, Laszlo
    Large eddy simulation of the flow through the blades of a swirl generator2000In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 21, no 5, p. 664-673Article in journal (Refereed)
    Abstract [en]

    Subsonic turbulent flow between the blades of a swirl generator is considered using large eddy simulations (LES). The boundary layer exhibits transition in the upstream parts of the blade. Further downstream, the Row separates both on the suction and the pressure surfaces. In the boundary layer, one may note the streaky vortices. In addition, due to the blade surface curvature large, time-dependent streamwise vortices are formed. The wake of the blades also contains large scale, time-dependent structures. These phenomena are captured directly by LES. In our study, we use two different subgrid scale (SGS) models: the first model is an implicit model and the second is a novel dynamic model dubbed the dynamic divergence model (DDM). The effects of the SGS model and the grid resolution are also investigated.

  • 6.
    de Stadler, Matthew B.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Rapaka, Narsimha R.
    Sarkar, Sutanu
    Large eddy simulation of the near to intermediate wake of a heated sphere at Re=10,0002014In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 49, p. 2-10Article in journal (Refereed)
    Abstract [en]

    Large eddy simulation is used to numerically simulate flow past a heated sphere at Re = 10,000. A second order accurate in space and time, semi-implicit finite difference code is used with the immersed boundary to represent the sphere in a Cartesian domain. Visualizations of the vorticity field and temperature field are provided together with profiles of the temperature and velocity fields at various locations in the wake. The laminar separated shear layer was found to efficiently transport heat from the hot sphere surface to the cold fluid in the wake. The thin separated shear layers are susceptible to Kelvin-Helmholtz instability and the pronounced rollers that subsequently form promote entrainment of both cold free-stream fluid and warmer fluid near the back of the sphere. Breakdown of the shear layer into turbulence and subsequent interaction with the recirculation zone results in rapid mixing of the temperature field in the lee of the sphere. The wake dimensions of the velocity field and the temperature field were found to be comparable in the developed flow behind the re-circulating region. Profiles of the mean and fluctuating temperature and velocity in the near wake are provided together with profiles of the Reynolds stresses and thermal fluxes. Similarity was observed for the mean temperature, rms temperature, rms velocity, and the Reynolds stress component < u(x)'u(r)'>, and the thermal fluxes < T'u(x)'> and < T'u(r)'>.

  • 7.
    Eitel-Amor, Georg
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. 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.
    Simulation and validation of a spatially evolving turbulent boundary layer up to Reθ = 83002014In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 47, p. 57-69Article in journal (Refereed)
    Abstract [en]

    Results of a finely resolved large-eddy simulation (LES) of a spatially developing zero-pressure-gradient turbulent boundary layer up to a Reynolds number of Reθ = 8300 are presented. The very long computational domain provides substantial assessment for suggested high Reynolds number (Re) trends. Statistics, integral quantities and spectral data are validated using high quality direct numerical simulation (DNS) ranging up to Reθ = 4300 and hot-wire measurements covering the remaining Re-range. The mean velocity, turbulent fluctuations, skin friction, and shape factor show excellent agreement with the reference data. Through utilisation of filtered DNS, subtle differences between the LES and DNS could to a large extent be explained by the reduced spanwise resolution of the LES. Spectra and correlations for the streamwise velocity and the wall-shear stress evidence a clear scale-separation and a footprint of large outer scales on the near-wall small scales. While the inner peak decreases in importance and reduces to 4% of the total energy at the end of the domain, the energy of the outer peak scales in outer units. In the near-wall region a clear k - 1 region emerges. Consideration of the two-dimensional spectra in time and spanwise space reveals that an outer time scale λt ≈ 10δ99 / U∞, with the boundary layer thickness δ99 and free-stream velocity U∞, is the correct scale throughout the boundary layer rather than the transformed streamwise wavelength multiplied by a (scale independent) convection velocity. Maps for the covariance of small scale energy and large scale motions exhibit a stronger linear Re dependence for the amplitude of the off-diagonal peak compared to the diagonal one, thereby indicating that the strength of the amplitude modulation can only qualitatively be assessed through the diagonal peak. In addition, the magnitude of the wall-pressure fluctuations confirms mixed scaling, and pressure spectra at the highest Re give a first indication of a -7/3 wave number dependence. © 2014 Elsevier Inc.

  • 8.
    Fallenius, Bengt E. G.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Sattari, A.
    Fransson, Jens H. M.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Sandberg, Mats
    University of Gävle.
    Experimental study on the effect of pulsating inflow to an enclosure for improved mixing2013In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 44, p. 108-119Article in journal (Refereed)
    Abstract [en]

    Optimal control of inlet jet flows is of broad interest for enhanced mixing in ventilated rooms. The general approach in mechanical ventilation is forced convection by means of a constant flow rate supply. However, this type of ventilation may cause several problems such as draught and appearance of stagnation zones, which reduces the ventilation efficiency. A potential way to improve the ventilation quality is to apply a pulsating inflow, which has been hypothesised to reduce the stagnation zones due to enhanced mixing. The present study aims at testing this hypothesis, experimentally, in a small-scale two-dimensional water model using Particle Image Velocimetry with an in-house vortex detection program. We are able to show that for an increase in pulsation frequency or alternatively in the flow rate the stagnation zones are reduced in size and the distribution of vortices becomes more homogeneous over the considered domain. The number of vortices (all scales) increases by a factor of four and the swirl-strength by about 50% simply by turning on the inflow pulsation. Furthermore, the vortices are well balanced in terms of their rotational direction, which is validated by the symmetric Probability Density Functions of vortex circulation (Γ) around Γ= 0. There are two dominating vortex length scales in the flow, namely 0.6 and 0.8 inlet diameters and the spectrum of vortex diameters become broader by turning on the inflow pulsation. We conclude that the positive effect for enhanced mixing by increasing the flow rate can equally be accomplished by applying a pulsating inflow.

  • 9.
    Gullman-Strand, Johan
    et al.
    KTH, Superseded Departments, Mechanics.
    Törnblom, Olle
    KTH, Superseded Departments, Mechanics.
    Lindgren, Björn
    KTH, Superseded Departments, Mechanics.
    Amberg, Gustav
    KTH, Superseded Departments, Mechanics.
    Johansson, Arne V.
    KTH, Superseded Departments, Mechanics.
    Numerical and experimental study of separated flow in a plane asymmetric diffuser2004In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 25, no 3, p. 451-460Article in journal (Refereed)
    Abstract [en]

    Computations of the turbulent flow through plane asymmetric diffusers for opening angles from 8degrees to 10degrees have been carried out with the explicit algebraic Reynolds stress model (EARSM) of Wallin and Johansson [J. Fluid Mech. 403 (2000) 89]. It is based on a two-equation platform in the form of a low-Re K - omega formulation, see e.g. Wilcox [Turbulence Modeling for CFD, DCW Industries Inc., 1993]. The flow has also been studied experimentally for the 8.5degrees opening angle using PIV and LDV. The models under-predict the size and magnitude of the recirculation zone. This is, at least partially, attributed to an over-estimation of the wall normal turbulence component in a region close to the diffuser inlet and to the use of damping functions in the near-wall region. By analyzing the balance between the production and dissipation of the turbulence kinetic energy we find that the predicted dissipation is too large. Hence, we can identify a need for improvement of the modeling the transport equation for the turbulence length-scale related quantity.

  • 10.
    Hosseini, Seyed M.
    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.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Swedish Defense Research Agency, FOI, Sweden.
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Direct numerical simulation of the flow around a wing section at moderate Reynolds number2016In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 61, p. 117-128Article in journal (Other academic)
    Abstract [en]

    Abstract A three-dimensional direct numerical simulation has been performed to study the turbulent flow around the asymmetric NACA4412 wing section at a moderate chord Reynolds number of R e c = 400 , 000 , with an angle of attack of A o A = 5 ∘ . The mesh was optimized to properly resolve all relevant scales in the flow, and comprises around 3.2 billion grid points. The incompressible spectral-element Navier–Stokes solver Nek5000 was used to carry out the simulation. An unsteady volume force is used to trip the flow to turbulence on both sides of the wing at 10% of the chord. Full turbulence statistics are computed in addition to collection of time history data in selected regions. The Reynolds numbers on the suction side reach Reτ ≃ 373 and R e Ξ = 2 , 800 with the pressure-gradient parameter ranging from β ≈ 0.0 to β ≈ 85. Similarly, on the pressure side, the Reynolds numbers reach Reτ ≈ 346 and R e Ξ = 818 while β changes from β ≈ 0.0 to β ≈ − 0.25 . The effect of adverse pressure gradients on the mean flow is consistent with previous observations, namely a steeper incipient log law, a more prominent wake region and a lower friction. The turbulence kinetic energy profiles show a progressively larger inner peak for increasing pressure gradient, as well as the emergence and development of an outer peak with stronger APGs. The present simulation shows the potential of high-order (spectral) methods in simulating complex external flows at moderately high Reynolds numbers.

  • 11.
    Ikeya, Yuta
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Keio University, Japan.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fukagata, Koji
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Towards a theoretical model of heat transfer for hot-wire anemometry close to solid walls2017In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 68, p. 248-256Article in journal (Refereed)
    Abstract [en]

    Hot-wire anemometry readings where the sensor is close to a solid wall become erroneous due to additional heat losses to the wall. Here we examine this effect by means of experiments and numerical simulations. Measurements in both quiescent air as well as laminar and turbulent boundary layers confirmed the influences of parameters such as wall conductivity, overheat ratio and probe dimensions on the hot-wire output voltage. Compared to previous studies, the focus lies not only on the streamwise mean velocity, but also on its fluctuations. The accompanying two-dimensional steady numerical simulation allowed a qualitative discussion of the problem and furthermore mapped the temperature field around the wire for different wall materials. Based on these experimental and numerical results, a theoretical model of the heat transfer from a heated wire close to a solid wall is proposed that accounts for the contributions from both convection and conduction.

  • 12.
    Kalpakli, Athanasia
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
    Turbulent pipe flow downstream a 90 degrees pipe bend with and without superimposed swirl2013In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 41, p. 103-111Article in journal (Refereed)
    Abstract [en]

    In the present work, the turbulent flow downstream a 90 degrees pipe bend is investigated by means of stereoscopic particle image velocimetry. In particular, the three dimensional flow field at the exit of the curved pipe is documented for non-swirling and swirling flow conditions, with the latter being generated through a unique axially rotating pipe flow facility. The non-swirling flow was examined through snapshot proper orthogonal decomposition (POD) with the aim to reveal the unsteady behaviour of the Dean vortices under turbulent flow conditions, the so-called "swirl-switching" phenomenon. In respect to the swirling turbulent pipe flow, covering a wide range of swirl strengths, POD has been employed to study the effect of varying strength of swirl on the Dean vortices as well as the interplay of swirling motion and Dean cells. Furthermore, the visualised large scale structures in turbulent swirling flows through the bend are found to incline and tear up with increasing swirl intensity. The present time-resolved, three component, experimental velocity field data will provide a unique and useful database for future studies; in particular for the CFD community.

  • 13.
    Kalpakli Vester, Athanasia
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vortical patterns in turbulent flow downstream a 90° curved pipe at high Womersley numbers2013In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 44, p. 692-699Article in journal (Refereed)
    Abstract [en]

    The present experimental work focuses on highly pulsatile, i.e. inertia dominated, turbulent flow downstream a curved pipe and aims at investigating the vortical characteristics of such a flow. The flow parameters (Dean and Womersley number) investigated are of the same order as those met in the internal combustion engine environment. The technique employed is time-resolved stereoscopic particle image velocimetry at different cross-sections downstream the pipe bend. These measurements allow the large-scale structures that are formed to be analyzed by means of proper orthogonal decomposition. The flow field changes drastically during a pulsatile cycle, varying from a uniform flow direction across the pipe section from the inside to the outside of the bend to vortical patterns consisting of two counter-rotating cells. This study characterizes and describes pulsatile curved pipe flow at Womersley numbers much higher than previously reported in the literature. Furthermore, the oscillatory behaviour of the Dean cells for the steady flow - the so-called 'swirl switching' - is investigated for different downstream stations from the bend exit and it is shown that this motion does not appear in the immediate vicinity of the bend, but only further downstream.

  • 14.
    Kametani, Yukinori
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fukagata, K.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Effect of uniform blowing/suction in a turbulent boundary layer at moderate Reynolds number2015In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 55, p. 132-142Article in journal (Refereed)
    Abstract [en]

    A number of well-resolved large-eddy simulations (LES) of a spatially evolving turbulent boundary layer with uniform blowing or suction is performed in order to investigate the effect on skin friction drag as well as turbulence statistics and spectral composition at moderate Reynolds numbers up to Reθ=2500, based on the free-stream velocity and the momentum-loss thickness. The amplitude of uniform blowing or suction is set to be 0.1% of the free-stream velocity with different streamwise ranges of the controlled region.The boundary layer is thickened by blowing and thinned by suction. The Reynolds shear and normal stresses are increased by blowing and decreased by suction, most prominently, in the outer region. Through spectral analysis of the streamwise velocity and cross-spectra of the Reynolds shear stress, the enhancement and reduction of the fluctuation energy in the outer region by blowing and suction are found, respectively. It is also found that the emergence of a second peak in the outer region is promoted by blowing, while it is inhibited in the case of suction.In spite of the weak amplitude of the control, more than 10% of drag reduction and enhancement are achieved by means of blowing and suction, respectively. In the case of blowing, where drag reduction is achieved, the mean drag reduction rate increases as the blowing region extends because the local reduction rate, i.e.the streamwise gradient of the mean drag reduction rate, grows in the streamwise direction. The net-energy saving rate and the control gain have the same trends. It is found that a more effective skin friction drag reduction and control efficiency can be achieved with a wider control region that starts at a more upstream location.

  • 15. Kim, S. J.
    et al.
    Sung, H. J.
    Wallin, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Design of the centrifugal fan of a belt-driven starter generator with reduced flow noise2019In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 76, p. 72-84Article in journal (Refereed)
    Abstract [en]

    Large eddy simulations based on the explicit algebraic subgrid-scale stress model were carried out to predict the flow-induced noise generated on the centrifugal fan of a belt-driven starter generator using Lighthill's analogy and the method of Ffowcs Williams and Hawkings. The surrounding air was approximated by an ideal gas at fixed room temperature (T in = 300 K), and the rotating velocity of the fan was considered to be 6000 rpm. The blade array angles were designed using the modulation method, and a large blade curvature was adopted. We identified several centrifugal fan design parameters that could minimize the flow-induced noise while also minimizing fan efficiency losses. Three design parameters: the top serrated edge (θ t ), the step leading edge (0.52 H b ) and the tail edge (d b and r b ), played a critical role in preventing vortex generation and collision, significantly weakening the surface pressure fluctuations on the blade. The maximum sound pressure level at 800 Hz at a specific location was reduced by 5.5 dB (at the top serrated edge) and 6.8 dB (at the step leading edge) relative to the baseline case. The sound power, calculated over a hemisphere surface of 950 mm, was reduced by 77.3% (at the top serrated edge) and 61.0% (at the step leading edge) relative to the baseline whereas the mass flow rates were reduced by 5.2% and 10.6%, respectively. Experiments were performed using the optimally designed fan in a semi-anechoic chamber. The predicted sound pressure level and frequency were in good agreement with the experimentally measured values.

  • 16.
    Lazeroms, Werner
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Stockholm University, Sweden .
    Brethouwer, Geert
    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 .
    Johansson, Arne
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Efficient treatment of the nonlinear features in algebraic Reynolds-stress and heat-flux models for stratified and convective flows2015In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 53, p. 15-28Article in journal (Refereed)
    Abstract [en]

    This work discusses a new and efficient method for treating the nonlinearity of algebraic turbulence models in the case of stratified and convective flows, for which the equations for the Reynolds stresses and turbulent heat flux are strongly coupled. In such cases, one finds a quasi-linear set of equations, which can be solved through an appropriate linear expansion in basis tensors and vectors, as discussed in earlier work. However, finding a consistent and truly explicit algebraic turbulence model requires solving an additional equation for the production-to-dissipation ratio (P+G)/ε of turbulent kinetic energy. Due to the nonlinear nature of the problem, the equation for (P+G)/ε is a higher-order polynomial equation for which no analytical solution can be found. Here we provide a new method to approximate the solution of this polynomial equation through an analysis of two special limits (shear-dominated and buoyancy-dominated), in which exact solutions are obtainable. The final result is a model that appropriately combines the two limits in more general cases. The method is tested for turbulent channel flow, both with stable and unstable stratification, and the atmospheric boundary layer with periodic and rapid changes between stable and unstable stratification. In all cases, the model is shown to give consistent results, close to the exact solution of (P+G)/ε. This new method greatly increases the range of applicability of explicit algebraic models, which otherwise would rely on the numerical solution of the polynomial equation.

  • 17.
    Li, Qiang
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    DNS of a spatially developing turbulent boundary layer with passive scalar transport2009In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 30, no 5, p. 916-929Article in journal (Refereed)
    Abstract [en]

    A direct numerical simulation (DNS) of a spatially developing turbulent boundary layer over a flat plate under zero pressure gradient (ZPG) has been carried out. The evolution of several passive scalars with both isoscalar and isoflux wall boundary condition are computed during the simulation. The Navier-Stokes equations as well as the scalar transport equation are solved using a fully spectral method. The highest Reynolds number based on the free-stream velocity U-infinity and momentum thickness 0 is Re-0 = 830, and the molecular Prandtl numbers are 0.2, 0.71 and 2. To the authors' knowledge, this Reynolds number is to date the highest with such a variety of scalars. A large number of turbulence statistics for both flow and scalar fields are obtained and compared when possible to existing experimental and numerical simulations at comparable Reynolds number. The main focus of the present paper is on the statistical behaviour of the scalars in the outer region of the boundary layer, distinctly different from the channel-flow simulations. Agreements as well as discrepancies are discussed while the influence of the molecular Prandtl number and wall boundary conditions is also highlighted. A Pr scaling for various quantities is proposed in outer scalings. In addition, spanwise two-point correlation and instantaneous fields are employed to investigate the near-wall streak spacing and the coherence between the velocity and the scalar fields. Probability density functions (PDF) and joint probability density functions (JPDF) are shown to identify the intermittency both near the wall and in the outer region of the boundary layer. The present simulation data will be available online for the research community.

  • 18.
    Ma, J.M.
    et al.
    Chalmers.
    Peng, Shia Hui
    Chalmers University of Technology; Swedish Defence Research Agency.
    Davidson, L.
    Chalmers.
    Wang, F.
    A low Reynolds number variant of partially-averaged Navier-Stokes model for turbulence2011In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 32, no 3, p. 652-669Article in journal (Refereed)
    Abstract [en]

    A low Reynolds number (LRN) formulation based on the Partially Averaged Navier-Stokes (PANS) modelling method is presented, which incorporates improved asymptotic representation in near-wall turbulence modelling. The effect of near-wall viscous damping can thus be better accounted for in simulations of wall-bounded turbulent flows. The proposed LRN PANS model uses an LRN k-epsilon model as the base model and introduces directly its model functions into the PANS formulation. As a result, the inappropriate wall-limiting behavior inherent in the original PANS model is corrected. An interesting feature of the PANS model is that the turbulent Prandtl numbers in the k and epsilon equations are modified compared to the base model. It is found that this modification has a significant effect on the modelled turbulence. The proposed LRN PANS model is scrutinized in computations of decaying grid turbulence, turbulent channel flow and periodic hill flow, of which the latter has been computed at two different Reynolds numbers of Re = 10,600 and 37,000. In comparison with available DNS, LES or experimental data, the LRN PANS model produces improved predictions over the standard PANS model, particularly in the near-wall region and for resolved turbulence statistics. Furthermore, the LRN PANS model gives similar or better results - at a reduced CPU time - as compared to the Dynamic Smagorinsky model.

  • 19. Maciel, Y.
    et al.
    Facciolo, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Duwig, C.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Near-field dynamics of a turbulent round jet with moderate swirl2008In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 29, no 3, p. 675-686Article in journal (Refereed)
    Abstract [en]

    The near-field characteristics of a turbulent jet with moderate swirl generated by a fully developed, axially rotating pipe flow are investigated with LDV, time-resolved stereoscopic PIV measurements, as well as with large-eddy simulations. Large-scale vortical structures in either double, triple or even quadruple-helix configuration are found at the pipe exit but rapidly break down or amalgamate after two jet diameters. Further downstream, the swirling jet is dominated by large-scale sweeping motions not present at such a scale and strength in the non-swirling case. Of special interest is the recently discovered counter-rotating core (in the mean) which develops about six jet diameters downstream the jet exit. Data for all six Reynolds stresses is reported at this position and it is argued that the counter-rotation is the result of the transport of angular momentum radially outward by the radial-azimuthal Reynolds shear stress. The mechanisms behind this transport are discussed by qualitative analysis of the time-resolved PIV and LES data and comparisons with the non-swirling case are made.

  • 20.
    Monokrousos, Antonios
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brandt, Luca
    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.
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    DNS and LES of estimation and control of transition in boundary layers subject to free-stream turbulence2008In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 29, no 3, p. 841-855Article in journal (Refereed)
    Abstract [en]

    Transition to turbulence occurring in a flat-plate boundary-layer flow subjected to high levels of free-stream turbulence is considered. This scenario, denoted bypass transition, is characterised by the non-modal growth of streamwise elongated disturbances. These so-called streaks are regions of positive and negative streamwise velocity alternating in the spanwise direction inside the boundary layer. When they reach large enough amplitudes, breakdown into turbulent spots occurs via their secondary instability. In this work, the bypass-transition process is simulated using direct numerical simulations (DNS) and large-eddy simulations (LES). The ADM-RT subgrid-scale model turned out to be particularly suited for transitional flows after a thorough validation. Linear feedback control is applied in order to reduce the perturbation energy and consequently delay transition. This case represents therefore an extension of the linear approach (Chevalier, M., Hoepffner, J., Åkervik, E., Henningson, D.S., 2007a. Linear feedback control and estimation applied to instabilities in spatially developing boundary layers. J. Fluid Mech. 588, 163-187, 167-187.) to flows characterised by strong nonlinearities. Control is applied by blowing and suction at the wall and it is both based on the full knowledge of the instantaneous velocity field (i.e. full information control) and on the velocity field estimated from wall measurements. The results show that the control is able to delay the growth of the streaks in the region where it is active, which implies a delay of the whole transition process. The flow field can be estimated from wall measurements alone: The structures occurring in the "real" flow are reproduced correctly in the region where the measurements are taken. Downstream of this region the estimated field gradually diverges from the "real" flow, revealing the importance of the continuous excitation of the boundary layer by the external free-stream turbulence. Control based on estimation, termed compensator, is therefore less effective than full information control.

  • 21.
    Negi, Prabal Singh
    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.
    Vinuesa, Ricardo
    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.
    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.
    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.
    Unsteady aerodynamic effects in small-amplitude pitch oscillations of an airfoil2018In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 71, p. 378-391Article in journal (Refereed)
    Abstract [en]

    High-fidelity wall-resolved large-eddy simulations (LES) are utilized to investigate the flow-physics of small-amplitude pitch oscillations of an airfoil at Rec=100,000. The investigation of the unsteady phenomenon is done in the context of natural laminar flow airfoils, which can display sensitive dependence of the aerodynamic forces on the angle of attack in certain “off-design” conditions. The dynamic range of the pitch oscillations is chosen to be in this sensitive region. Large variations of the transition point on the suction-side of the airfoil are observed throughout the pitch cycle resulting in a dynamically rich flow response. Changes in the stability characteristics of a leading-edge laminar separation bubble has a dominating influence on the boundary layer dynamics and causes an abrupt change in the transition location over the airfoil. The LES procedure is based on a relaxation-term which models the dissipation of the smallest unresolved scales. The validation of the procedure is provided for channel flows and for a stationary wing at Rec=400,000.

  • 22.
    Ni, Peiyuan
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Northeastern Univ, Peoples R China.
    Jonsson, Lage Tord Ingemar
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ersson, Mikael
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Jönsson, Pär Göran
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Deposition of particles in liquid flows in horizontal straight channels2016In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 62, p. 166-173Article in journal (Refereed)
    Abstract [en]

    A flow in a horizontal channel is an important method for the transport of materials, products and/or waste gases/liquids. The deposition of particles in a horizontal channel may clog the flow path. The purpose of this paper is to extend the use of a developed Eulerian deposition model to liquid flows in horizontal straight channels to predict the particle deposition rate. For a horizontal pipe, the deposition rates may differ greatly along a cross section, due to the influences of gravity and buoyancy. The current deposition model is first applied to air flows to enable a comparison with available experimental data. Then, the model is applied to liquid flows in horizontal straight pipes. The effects of gravity, buoyancy, water flow rates, wall roughness, particle size and temperature difference in the near-wall boundary layer on the deposition rate have been studied and explained. The results show that the deposition rates of particles increase with an increased flow rate. The gravity separation has a large influence on the deposition of large particle at high and low parts of the horizontal pipe in some flows. Moreover, both the wall roughness and thermophoresis have a significant influence on the deposition rate of small particles. In addition, the roughness also shows an important influence on the large particle deposition at the top of the investigated pipe, due to that a large value of roughness can make the deposition location somewhat far away from the wall, where a stronger turbophoresis exists. The intensity of the turbophoresis relative to the gravity separation before a particle is reaching the deposition location is important for the large particle deposition when the gravity separation play a negative role on the deposition rate. (C) 2016 Elsevier Inc. All rights reserved.

  • 23.
    Niazi Ardekani, Mehdi
    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.
    Asmar, L. A.
    Picano, F.
    Brandt, L.uca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Numerical study of heat transfer in laminar and turbulent pipe flow with finite-size spherical particles2018In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 71, p. 189-199Article in journal (Refereed)
    Abstract [en]

    Controlling heat and mass transfer in particulate suspensions has many applications in fuel combustion, food industry, pollution control and life science. We perform direct numerical simulations (DNS) to study the heat transfer within a suspension of neutrally buoyant, finite-size spherical particles in laminar and turbulent pipe flows, using the immersed boundary method (IBM) to account for the solid fluid interactions and a volume of fluid (VoF) method to resolve the temperature equation both inside and outside the particles. Particle volume fractions up to 40% are simulated for different pipe to particle diameter ratios. We show that a considerable heat transfer enhancement (up to 330%) can be achieved in the laminar regime by adding spherical particles. The heat transfer is observed to increase significantly as the pipe to particle diameter ratio decreases for the parameter range considered here. Larger particles are found to have a greater impact on the heat transfer enhancement than on the wall-drag increase. In the turbulent regime, however, only a transient increase in the heat transfer is observed and the process decelerates in time below the values in single-phase flows as high volume fractions of particles laminarize the core region of the pipe. A heat transfer enhancement, measured with respect to the single phase flow, is only achieved at volume fractions as low as 5% in a turbulent flow.

  • 24.
    Noorani, Azad
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    El Khoury, George. K.
    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.
    Evolution of turbulence characteristics from straight to curved pipes2013In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 41, no SI, p. 16-26Article in journal (Refereed)
    Abstract [en]

    Fully developed, statistically steady turbulent flow in straight and curved pipes at moderate Reynolds numbers is studied in detail using direct numerical simulations (DNS) based on a spectral element discretisation. After the validation of data and setup against existing DNS results, a comparative study of turbulent characteristics at different bulk Reynolds numbers Re-b = 5300 and 11,700, and various curvature parameters kappa = 0, 0.01, 0.1 is presented. In particular, complete Reynolds-stress budgets are reported for the first time. Instantaneous visualisations reveal partial relaminarisation along the inner surface of the curved pipe at the highest curvature, whereas developed turbulence is always maintained at the outer side. The mean flow shows asymmetry in the axial velocity profile and distinct Dean vortices as secondary motions. For strong curvature a distinct bulge appears close to the pipe centre, which has previously been observed in laminar and transitional curved pipes at lower Re-b only. On the other hand, mild curvature allows the interesting observation of a friction factor which is lower than in a straight pipe for the same flow rate. All statistical data, including mean profile, fluctuations and the Reynolds-stress budgets, is available for development and validation of turbulence models in curved geometries.

  • 25.
    Pouransari, Zeinab
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vervisch, Luc
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Heat release effects on mixing scales of non-premixed turbulent wall-jets: A direct numerical simulation study2013In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 40, p. 65-80Article in journal (Refereed)
    Abstract [en]

    The present study concerns the role of heat release effects on characteristics mixing scales of turbulence in reacting wall-jet flows. Direct numerical simulations of exothermic reacting turbulent wall-jets are performed and compared to the isothermal reacting case. An evaluation of the heat-release effects on the structure of turbulence is given by examining the mixture fraction surface characteristics, diagnosing vortices and exploring the dissipation rate of the fuel and passive scalar concentrations, and moreover by illustration of probability density functions of reacting species and scatter plots of the local temperature against the mixture fraction. Primarily, heat release effects delay the transition, enlarge the fluctuation intensities of density and pressure and also enhance the fluctuation level of the species concentrations. However, it has a damping effect on all velocity fluctuation intensities and the Reynolds shear stress. A key result is that the fine-scale structures of turbulence are damped, the surface wrinkling is diminished and the vortices become larger due to heat-release effects. Taking into account the varying density by using semi-local scaling improves the collapse of the turbulence statistics in the inner region, but does not eliminate heat release induced differences in the outer region. Examining the two-dimensional premultiplied spanwise spectra of the streamwise velocity fluctuations indicates a shifting in the positions of the outer peaks, associated with large energetic structures, toward the inner region.

  • 26.
    Rabault, Jean
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Vernet, Julie A
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lindgren, Björn
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    A study using PIV of the intake flow in a diesel engine cylinder2016In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 62, p. 56-67Article in journal (Refereed)
    Abstract [en]

    The admission flow generated by a parallel valve diesel engine cylinder head was investigated by planar and stereoscopic Particle Image Velocimetry in a steady flow test bench through measurements in the swirl and tumble planes. By combining several sets of measurements a full three-dimensional, three component reconstruction of the flow was made. The flow out of the valves forms a jet which collides with the cylinder wall before flowing down along the wall. Despite the fact that there is no piston a recirculation bubble is formed in the tumble plane. This is due to the entrainment of gas into the jet which needs to be replaced and thereby sets up a counter flow. In the swirl plane complex jet-dominated vortex structures are detected close to the cylinder top. Moving away from the cylinder top, a counter-rotating vortex-pair structure is observed from which a single coherent swirling structure develops further down the cylinder. Some clear differences are observed between the flow at high and moderate valve lifts, which correspond to a distinct change in the swirl intensity. By introducing a strong swirling motion the flow is stabilized which can be seen by tracking the instantaneous position of the swirl centre. For high swirl the variation of the position of the swirl centre decreases substantially. (C) 2016 Elsevier Inc. All rights reserved.

  • 27.
    Rantanen, Pekka
    et al.
    Helsinki University of Technology.
    Valkonen, Antti
    Helsinki University of Technology.
    Cronhjort, Andreas
    KTH. Scania CV AB.
    Measurements of a Diesel Spray with a Normal Size Nozzle and a Large-Scale Model1999In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 20, p. 545-551Article in journal (Refereed)
    Abstract [en]

    Advantages of the large-scale modeling of diesel sprays based on dimensional analysis were studied. Measurements of the spray tip penetration, spray angle, droplet size and velocity in a diesel spray have been made with a small nozzle and a large-scale model of the same nozzle. Measurements were made with image analysis, diffraction drop size analyzer and laser Doppler anemometer. Results show that scaling might give us new possibilities to research diesel sprays.

  • 28.
    Razam, Amin
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Wallin, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    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. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Improving separated-flow predictions using an anisotropy-capturing subgrid-scale model2017In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 65, p. 246-251Article in journal (Refereed)
    Abstract [en]

    The major conclusion of this paper is that resolution requirements for large-eddy simulation (LES) of flow separation and reattachment can be significantly reduced using the anisotropy-capturing explicit algebraic subgrid-scale (SGS) stress model (EASSM) of Marstorp et al. (J. Fluid Mech., vol. 639, 2009, pp. 403–432), instead of the conventional isotropic dynamic eddy-viscosity model (DEVM). LES of flow separation in a channel with streamwise periodic hill-shaped constrictions and spanwise homogeneity is performed at coarse resolutions for which it is observed that flow separation cannot be predicted without a SGS model and cannot be correctly predicted by the DEVM, while reasonable predictions are obtained with the EASSM. It is shown that the lower resolution requirements by the EASSM, compared to the DEVM, is not only due its nonlinear formulation, but also due to the better formulation of its eddy-viscosity part. The improvements obtained with the EASSM have previously been demonstrated using higher-order numerical solvers for channel flows. In this study, it is observed that these improvements still remain using a low-order code with significant inherent numerical dissipation.

  • 29.
    Saglietti, Clio
    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.
    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.
    Wadbro, Eddie
    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.
    Berggren, Martin
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Henningson, Dan S.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Topology optimization of heat sinks in a square differentially heated cavity2018In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 74, p. 36-52Article in journal (Refereed)
    Abstract [en]

    Innovative designs of heat sinks are generated in the present paper through numerical optimization, by applying a material distribution topology optimization approach. The potential of the method is demonstrated in a two-dimensional differentially heated cavity, in which the heat transfer is increased by means of introducing a solid structure that acts as a heat sink. We simulate the heat transfer in the whole system by performing direct numerical simulations of the conjugated problem, i.e. temperature diffusion and convection in the entire domain and momentum conservation in the fluid surrounding the solid. The flow is driven by the buoyancy force, under the Boussinesq approximation, and we describe the presence of solid material as the action of a Brinkman friction force in the Navier–Stokes equations. To obtain a design with a given length scale, we apply regularization techniques by filtering the material distribution. Two different types of filters are applied and compared for obtaining the most realistic solution. Given the large scale of the problem, the optimization is solved with a gradient based method that relies on adjoint sensitivity analysis. The results show the applicability of the method by presenting innovative geometries that are increasing the heat flux. Moreover, the effect of various factors is studied: We investigate the impact of boundary conditions, initial designs, and Rayleigh number. Complex tree-like structures are favored when a horizontal temperature gradient is imposed on the boundary and when we limit the amount of solid volume in the cavity. The choice of the initial design affects the final topology of the generated solid structures, but not their performance for the studied cases. Additionally, when the Rayleigh number increases, the topology of the heat exchanger is able to substantially enhance the convection contribution to the heat transfer. 

  • 30.
    Sakowitz, Alexander
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Turbulent flow mechanisms in mixing T-junctions by Large Eddy Simulations2014In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 45, no 1, p. 135-146Article in journal (Refereed)
    Abstract [en]

    We consider the turbulent mixing process in two T-junction geometries as simplified models for mixing in the intake manifolds of Internal Combustion (IC) engines. These junctions have square and circular cross-sections, respectively. The turbulent flow structures and modes are analyzed by Large Eddy Simulations (LES). A grid sensitivity study is performed and the velocity field and the mixing scalar are compared to experimental data. The agreement is good for high enough mesh resolutions. Furthermore, the LES results are compared to unsteady Reynolds averaged Navier-Stokes (URANS) results, in order to gain an understanding of the shortcomings associated with URANS. The secondary structures found in both geometries include Dean-like vortices due to flow curvature in the region of the junction. Further downstream of the junction, these vortices are dissipated and due to an upward motion of the bulk flow, new vortical structures are generated. These downstream vortical structures rotate in the opposite direction relative to the upstream ones and govern the mean scalar distribution far downstream of the junction. We find also that the URANS results show qualitatively different flow structures leading to different scalar distributions as compared to experimental and LES results. The mixing quality is studied using a uniformity index showing a more uniform and faster mixing in the circular cross-section case. Spectral analysis of the LES data show for both geometries a shear layer instability with a dimensionless frequency in the order of unity. Additionally to that, vortex-shedding phenomena are observed in the circular case at St approximate to 0.5.

  • 31.
    Schlatter, Philipp
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Li, Qiang
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Brethouwer, Geert
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Simulations of spatially evolving turbulent boundary layers up to Re-theta=43002010In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 31, no 3, p. 251-261Article in journal (Refereed)
    Abstract [en]

    A well-resolved large-eddy simulation (LES) of a spatially developing turbulent boundary layer under zero-pressure-gradient up to comparably high Reynolds numbers (Re-theta = 4300) is performed. The laminar inflow is located at Re-delta = 450 (Re-theta approximate to 1180), a position where natural transition to turbulence can be expected. The simulation is validated and compared extensively to both numerical data sets, i.e. a recent spatial direct numerical simulation (DNS) up to Re-theta = 2500 (Schlatter et al., 2009) and available experimental measurements, e.g. the ones obtained by Osterlund (1999). The goal is to provide the research community with reliable numerical data for high Reynolds-number wall-bounded turbulence, which can in turn be employed for further model development and validation, but also to contribute to the characterisation and understanding of various aspects of wall turbulence. The results obtained via LES show that good agreement with DNS data at lower Reynolds numbers and experimental data can be obtained for both mean and fluctuating quantities. In addition, turbulence spectra characterising large-scale organisation in the flow have been computed and compared to literature results with good agreement. In particular, the near-wall streaks scaling in inner units and the outer layer large-scale structures can clearly be identified in both spanwise and temporal spectra. (C) 2010 Elsevier Inc. All rights reserved.

  • 32. Schlatter, Philipp
    et al.
    Stolz, S.
    Kleiser, L.
    LES of transitional flows using the approximate deconvolution model2004In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 25, no 3, p. 549-558Article in journal (Refereed)
    Abstract [en]

    Large-eddy simulations of transitional incompressible channel flow on rather coarse grids are performed. The standard approximate deconvolution model (ADM) as well as two modifications are compared to fully resolved direct numerical simulation (DNS) calculations. The results demonstrate that it is well possible to simulate transitional flows on the basis of ADM. During the initial phase of transition, the models remain inactive and do not disturb the flow development as long as it is still sufficiently resolved on the coarse large-eddy simulation (LES) grid. During the later stages of transition the model contributions provide necessary additional dissipation. Due to the dynamic determination of the model coefficient also employed for the standard ADM, no ad hoc constants or adjustments are needed. The results of the modified ADM show excellent agreement with DNS already on coarser meshes than the standard ADM, e.g. in the skin friction throughout the transitional phase, while preserving the accuracy for the fully developed turbulent channel flow. A grid-resolution study demonstrates convergence of LES to the DNS results. Results of the dynamic Smagorinsky model are included for comparison.

  • 33. Tsuji, Y.
    et al.
    Marusic, I.
    Johansson, Arne V.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Amplitude modulation of pressure in turbulent boundary layer2016In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278Article in journal (Refereed)
    Abstract [en]

    The interaction between pressure fluctuations associated with large- and small-scale motions is studied. Using a small pressure probe, both static pressure and wall pressure fluctuations were measured inside the zero-pressure gradient boundary layer at relatively high Reynolds numbers. How the large scales in the log-region affect the small scales near the wall is analyzed by means of an amplitude modulation procedure. Analyzing the wall pressure fluctuations, it was found that the large scale interacts with the small scale but there is a time-lag between them. © 2016 Elsevier Inc.

  • 34.
    Vester, A. Kalpakli
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Nishio, Y.
    Tohoku Univ, Grad Sch Engn, Aoba Ku, 6-6-01 Aramaki Aoba, Sendai, Miyagi 9808579, Japan..
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Investigating swirl and tumble using two prototype inlet port designs by means of multi-planar PIV2019In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 75, p. 61-76Article in journal (Refereed)
    Abstract [en]

    Flow structures created during the intake stroke of an engine were investigated by means of multi-planar particle image velocimetry (PIV). A unique water-analogue engine model has been developed, where all essential parts and parameters, such as the cylinder head, valve timing, piston geometry and motion, etc. can easily be modified. Two cylinder heads with geometrically different inlet ports were investigated and experiments were performed with both moving and fixed valves. Three-dimensional visualisations of the flow field, mode decomposition through proper orthogonal decomposition, circulation as well as classical statistics were obtained and evaluated in order to gain an understanding of the flow structures, i.e. tumble and swirl, created by the two cylinder heads. It was clearly shown that one of the cylinder heads created a strong swirling motion in the cylinder. Three different fixed valve positions were investigated and the fully opened valve gave the strongest large-scale structures, whereas with smaller openings a larger amount of the kinetic energy was converted into small-scale turbulence. Results showed a more organised and stable flow field consisting of a well-defined swirl motion occupying the whole cylinder at the end of the intake stroke when the valves were fixed at the highest position. The moving valve case gave results similar to the fully open case but with slightly higher turbulence. Cycle-to-cycle variations were found to be less pronounced for these two cases as compared to the smaller fixed valve lifts. The second cylinder head showed a flow field that was more turbulent and much less coherent. Statistical analysis showed that this had a direct effect on cyclic variations in the flow where this head showed more profound variations.

  • 35. Vidal, A.
    et al.
    Vinuesa, Ricardo
    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.
    Nagib, H. M.
    Influence of corner geometry on the secondary flow in turbulent square ducts2017In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 67, p. 69-78Article in journal (Refereed)
    Abstract [en]

    Direct numerical simulations of fully-developed turbulent flow through a straight square duct with increasing corner rounding radius r were performed to study the influence of corner geometry on the secondary flow. Unexpectedly, the increased rounding of the corners from r=0 to 0.75 does not lead to a monotonic trend towards the pipe case of r=1. Instead, the secondary vortices relocate close to the region of wall-curvature change. This behavior is connected to the inhomogeneous interaction between near-wall bursting events, which are further characterized in this work with the definition of their local preferential direction. We compare our results with those obtained for the flow through a square duct (which corresponds to r=0) and through a round pipe (r=1), focusing on the influence of r on the wall-shear stress distribution and the turbulence statistics along the centerplane and the corner bisector. The former shows that high-speed streaks are preferentially located near the transition between straight and curved surfaces. The Reynolds numbers based on the centerplane friction velocity and duct half-height are Reτ, c ≃ 180 and 350 for the cases under study.

  • 36.
    Vidal, A.
    et al.
    IIT, Dept MMAE, Chicago, IL 60616 USA..
    Vinuesa, Ricardo
    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.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes.
    Nagib, H. M.
    IIT, Dept MMAE, Chicago, IL 60616 USA..
    Turbulent rectangular ducts with minimum secondary flow2018In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 72, p. 317-328Article in journal (Refereed)
    Abstract [en]

    In the present study we perform direct numerical simulations (DNSs) of fully-developed turbulent rectangular ducts with semi-cylindrical side-walls at Re-t,Re- c similar or equal to 180 with width-to-height ratios of 3 and 5. The friction Reynolds number Re-tau,Re- (c) is based on the centerplane friction velocity and the half-height of the duct. The results are compared with the corresponding duct cases with straight side-walls (Vinuesa et al., 2014), and also with spanwise-periodic channel and pipe flows. We focus on the influence of the semi-cylindrical side-walls on the mean cross-stream secondary flow and on further characterizing the mechanisms that produce it. The role of the secondary and primary Reynolds-shear stresses in the production of the secondary flow is analyzed by means of quadrant analysis and conditional averaging. Unexpectedly, the ducts with semi-cylindrical side-walls exhibit higher cross-flow rates and their secondary vortices relocate near the transition point between the straight and curved walls. This behavior is associated to the statistically preferential arrangement of sweeping events entering through the curved wall and ejections arising from the adjacent straight wall. Therefore, the configuration with minimum secondary flow corresponds to the duct with straight side-walls and sharp corners. Consequences on experimental facilities and comparisons between experiments and various numerical and theoretical models are discussed revealing the uniqueness of pipe flow.

  • 37.
    Vinuesa, Ricardo
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Negi, Prabal Singh
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Atzori, M.
    Hanifi, Ardeshir
    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.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Turbulent boundary layers around wing sections up to Re-c=1, 000, 0002018In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 72, p. 86-99Article in journal (Refereed)
    Abstract [en]

    Reynolds-number effects in the adverse-pressure-gradient (APG) turbulent boundary layer (TBL) developing on the suction side of a NACA4412 wing section are assessed in the present work. To this end, we analyze four cases at Reynolds numbers based on freestream velocity and chord length ranging from Re-c = 100, 000 to 1,000,000, all of them with 5 degrees angle of attack. The results of four well-resolved large-eddy simulations (LESs) are used to characterize the effect of Reynolds number on APG TBLs subjected to approximately the same pressure-gradient distribution (defined by the Clauser pressure-gradient parameter beta). Comparisons of the wing profiles with zero pressure-gradient (ZPG) data at matched friction Reynolds numbers reveal that, for approximately the same beta distribution, the lower-Reynolds-number boundary layers are more sensitive to pressure-gradient effects. This is reflected in the values of the inner-scaled edge velocity U-e(+), the shape factor H, the components of the Reynolds-stress tensor in the outer region and the outer-region production of turbulent kinetic energy. This conclusion is supported by the larger wall-normal velocities and outer-scaled fluctuations observed in the lower-Re-c cases. Thus, our results suggest that two complementing mechanisms contribute to the development of the outer region in TBLs and the formation of large-scale energetic structures: one mechanism associated with the increase in Reynolds number, and another one connected to the APG. Future extensions of the present work will be aimed at studying the differences in the outer-region energizing mechanisms due to APGs and increasing Reynolds number.

  • 38. Wallin, Stefan
    et al.
    Johansson, Arne V.
    Modelling streamline curvature effects in explicit algebraic Reynolds stress turbulence models2002In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 23, no 5, p. 721-730Article in journal (Refereed)
    Abstract [en]

    A curvature correction for explicit algebraic Reynolds stress models (EARSMs), based on a formal derivation of the weak equilibrium assumption in a streamline oriented curvilinear co-ordinate system is presented. The curvature correction is given from the rotation rate of the curvilinear co-ordinate system following the mean flow. Two methods for defining that rotation rate are proposed, one is derived from the strain-rate tensor, and the other is derived from the local mean acceleration vector. Both methods are fully three-dimensional and Galilean invariant and the correction vanishes in cases without curvature or rotation effects. The EARSM proposed by Wallin and Johansson (J. Fluid Mech. 403 (2000) 89) was extended with the proposed curvature corrections and recalibrated in such a way that the original model was retrieved in cases without curvature or rotation effects. Rotating homo homogeneous turbulent shear flows with vanishing mean vorticity should be close to neutral stability according to linear stability theory, also observed from large eddy simulations. This was used for the recalibration. The importance of the curvature correction and the proposed recalibration is shown for rotating homogeneous shear and rotating channel flows.

  • 39. Wang, Z.
    et al.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Chung, Y. M.
    Direct numerical simulation of a turbulent 90° bend pipe flow2018In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 73, p. 199-208Article in journal (Refereed)
    Abstract [en]

    Direct numerical simulation (DNS) has been performed for a spatially developing 90° bend pipe flow to investigate the unsteady flow motions downstream of the bend. A recycling method is implemented to generate a fully-developed turbulent inflow condition. The Reynolds number of the pipe flow is ReD=5300 and the bend curvature is γ=0.4. A long straight pipe section (40D) is attached in the downstream of the bend to allow the flow to develop. Flow oscillations downstream of the bend are measured using several methods, and the corresponding oscillation frequencies are estimated. It is found that different characteristic frequencies are obtained from various flow measurements. The stagnation point movement and single-point velocity measurements may not be good measures to determine the swirl-switching frequency. The oscillations of the lateral pressure force on the pipe wall and half-sided mass flow rate are proposed to be a more unambiguous measure of the unsteady flow motions downstream of the bend. 

  • 40.
    Wikström, Petra M.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Hallbäck, Magnus
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Johansson, Arne, V.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Measurements and heat-flux transport modelling in a heated cylinder wake1998In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, no 19, p. 556-562Article in journal (Refereed)
    Abstract [en]

    Hot-wire measurements of velocity and temperature fluctuations have been made in the self-preserving turbulent wake region of a heated cylinder. Second order statistics including Reynolds fluxes, , are determined along with relevant triple correlations appearing in the Reynolds stress and Reynolds flux transport equations. The primary aim with these measurements is to study different modelling levels for passive scalar quantities. Models for the pressure scalar-gradient correlation, appearing in the transport equation of the Reynolds fluxes, are compared to measured data. A significant improvement of the simplest model, , is achieved by including a model for the rapid term that is linear in the mean velocity gradients. The mixed timescale, , seems to be an appropriate choice for τ. Also models for the triple correlations, , are compared with the experiments.

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