Publications
116 results found
Fouxon I, Schmidt L, Ditlevsen P, et al., 2018, Inhomogeneous growth of fluctuations of concentration of inertial particles in channel turbulence, Physical Review Fluids, Vol: 3, ISSN: 2469-990X
We study the growth of concentration fluctuations of weakly inertial particles in the turbulent channel flow starting with a smooth initial distribution. The steady-state concentration is singular and multifractal so the growth describes the increasingly rugged structure of the distribution. We demonstrate that inhomogeneity influences the growth of concentration fluctuations profoundly. For homogeneous turbulence the growth is exponential and is fully determined by Kolmogorov scale eddies.We derive lognormality of the statistics in this case. The growth exponents of the moments are proportional to the sum of Lyapunov exponents, which is quadratic in the small inertia of the particles. In contrast, for inhomogeneous turbulence the growth is linear in inertia. It involves correlations of inertial range and viscous scale eddies that turn the growth into a stretched exponential law with exponent three halves. We demonstrate using direct numerical simulations that the resulting growth rate can differ by orders of magnitude over channel height. This strong variation might have relevance in the planetary boundary layer.
Alpresa P, Sherwin S, Weinberg P, et al., 2018, Orbitally shaken shallow fluid layers. I. Regime classification, PHYSICS OF FLUIDS, Vol: 30, ISSN: 1070-6631
Orbital shakers are simple devices that provide mixing, aeration, and shear stress at multiple scales and high throughput. For this reason, they are extensively used in a wide range of applications from protein production to bacterial biofilms and endothelial cell experiments. This study focuses on the behaviour of orbitally shaken shallow fluid layers in cylindrical containers. In order to investigate the behaviour over a wide range of different conditions, a significant number of numerical simulations are carried out under different configuration parameters. We demonstrate that potential theory—despite the relatively low Reynolds number of the system—describes the free-surface amplitude well and the velocity field reasonably well, except when the forcing frequency is close to a natural frequency and resonance occurs. By classifying the simulations into non-breaking, breaking, and breaking with part of the bottom uncovered, it is shown that the onset of wave breaking is well described by Δh/(2R) = 0.7Γ, where Δh is the free-surface amplitude, R is the container radius, and Γ is the container aspect ratio; Δh can be well approximated using the potential theory. This result is in agreement with standard wave breaking theories although the significant inertial forcing causes wave breaking at lower amplitudes.
Alpresa P, Sherwin S, Weinberg P, et al., 2018, Orbitally shaken shallow fluid layers. II. An improved wall shear stress model, Physics of Fluids, Vol: 30, ISSN: 1070-6631
A new model for the analytical prediction of wall shear stress distributions at the base of orbitally shaken shallow fluid layers is developed. This model is a generalisation of the classical extended Stokes solution and will be referred to as the potential theory-Stokes model. The model is validated using a large set of numerical simulations covering a wide range of flow regimes representative of those used in laboratory experiments. It is demonstrated that the model is in much better agreement with the simulation data than the classical Stokes solution, improving the prediction in 63% of the studied cases. The central assumption of the model—which is to link the wall shear stress with the surface velocity—is shown to hold remarkably well over all regimes covered.
van Reeuwijk M, Jonker HJJ, 2018, Understanding entrainment processes in the atmosphere: the role of numerical simulation, 10th ERCOFTAC Workshop on Direct and Large Eddy Simulation (DLES), Publisher: SPRINGER INTERNATIONAL PUBLISHING AG, Pages: 53-60, ISSN: 1382-4309
Turbulent entrainment is a process of primary importance in the atmospheric boundary layer; however despite several decades of intense study much remains to be understood. Direct Numerical Simulation (DNS) and Large-Eddy Simulation (LES) have a tremendous potential to improve the understanding of turbulent entrainment, particularly if combined with theory. We discuss a recently developed framework for turbulent jets and plumes to decompose turbulent entrainment in various physical processes, and modify it for use in a stably stratified shear driven (nocturnal) boundary layer. The decomposition shows that inner layer processes become negligible as time progresses and that the entrainment coefficient is determined by turbulence production in the outer layer only.
Verso L, van Reeuwijk M, Liberzon A, 2017, Steady state model and experiment for an oscillating grid turbulent two-layer stratified flow, PHYSICAL REVIEW FLUIDS, Vol: 2, ISSN: 2469-990X
Turbulence generated by an oscillating grid in a two-layer stably stratified system is a classical flow utilized to study various aspects of turbulence in presence of stratification without mean shear. This flow evolves in a quasisteady state, in which the layer thickness and density difference evolves in a quasisteady manner due to the large separation of timescales between the turbulence and the setup. We present an extension of the classical setup that enables full steady state conditions and in which the entrainment velocity can be prescribed separately from the Richardson number. We develop a theoretical box-model and show that the model is in good agreement with the experiments. The model allows to predict the transient response of the system for a variety of initial conditions and the imposed steady state. The new setup is necessary to obtain the steady position of the density interface, for example, when using advanced optical techniques to measure the small-scale features of turbulence near the interface.
Ghim M, Alpresa P, Yang S, et al., 2017, Visualization of three pathways for macromolecule transport across cultured endothelium and their modification by flow., AJP - Heart and Circulatory Physiology, Vol: 313, Pages: H959-H973, ISSN: 1522-1539
Transport of macromolecules across vascular endothelium and its modification by fluid mechanical forces are important for normal tissue function and in the development of atherosclerosis. However, the routes by which macromolecules cross endothelium, the hemodynamic stresses that maintain endothelial physiology or trigger arterial disease, and the dependence of transendothelial transport on hemodynamic stresses are controversial. Here we visualised pathways for macromolecule transport and determined the effect on these pathways of different types of flow. Endothelial monolayers were cultured under static conditions or on an orbital shaker producing different flow profiles in different parts of the wells. Fluorescent tracers that bound to the substrate after crossing the endothelium were used to identify transport pathways. Maps of tracer distribution were compared with numerical simulations of flow to determine effects of different shear stress metrics on permeability. Albumin-sized tracers dominantly crossed the cultured endothelium via junctions between neighbouring cells, high-density-lipoprotein-sized tracers crossed at tricelluar junctions whilst low-density-lipoprotein-sized tracers crossed through cells. Cells aligned close to the angle that minimised shear stresses across their long axis. The rate of paracellular transport under flow correlated with the magnitude of these minimised transverse stresses, whereas transport across cells was uniformly reduced by all types of flow. These results contradict the long-standing two-pore theory of solute transport across microvessel walls and the consensus view that endothelial cells align with the mean shear vector. They suggest that endothelial cells minimise transverse shear, supporting its postulated pro-atherogenic role. Preliminary data show that similar tracer techniques are practicable in vivo.
Craske J, Salizzoni P, van Reeuwijk M, 2017, The turbulent Prandtl number in a pure plume is 3/5, Journal of Fluid Mechanics, Vol: 822, Pages: 774-790, ISSN: 0022-1120
We derive a new expression for the entrainment coefficient in a turbulent plume usingan equation for the squared mean buoyancy. Consistency of the resulting expressionwith previous relations for the entrainment coefficient implies that the turbulent Prandtlnumber in a pure plume is equal to 3/5 when the mean profiles of velocity and buoyancyhave a Gaussian form of equal width. Entrainment can be understood in terms of thevolume flux, the production of turbulence kinetic energy or the production of scalarvariance for either active or passive variables. The equivalence of these points of viewindicates how the entrainment coefficient and the turbulent Prandtl and Schmidt numbersdepend on the Richardson number of the flow, the ambient stratification and the relativewidths of the velocity and scalar profiles. The general framework is valid for self-similarplumes, which are characterised by a power-law scaling. For jets and pure plumes it isshown that the derived relations are in reasonably good agreement with results fromdirect numerical simulations and experiments.
Verso L, van Reeuwijk M, Gurka R, et al., 2017, Experimental study of the initial growth of a localized turbulent patch in a stably stratified fluid, International Journal of Heat and Fluid Flow, Vol: 66, Pages: 127-136, ISSN: 0142-727X
We present a laboratory experiment of the initial growth of a turbulent patch in a stably stratified fluid. The patch is created due to a localized source of turbulence, generated by a horizontally oriented and vertically oscillating grid much smaller than the tank size and far from solid boundaries. Synchronized and overlapping particle image velocimetry(PIV) and planar laser induced fluorescence (PLIF) measurements capture the evolution of the patch through its initial growth until it reached a maximum size. The simultaneous measurements of density and velocity fields allow for a direct quantification of the distribution of kinetic energy, buoyancy and degree of mixing within the patch. We can also relate the propagation speed of the turbulent/non-turbulent interface and its thickness to the properties of the turbulent fluid inside the evolving patch. The velocity measurements in this setup indicate significant transient effects inside the patch during its growth. A local analysis of the turbulent/non-turbulent interface provides direct measurements of the entrainment velocity we as compared to the local vertical velocity and turbulent intensity at the proximity of the interface. The detailed information about the growth of localized sources of turbulence in stratified environment might be of use in stealth design of autonomous underwater vehicles.
Krug D, Holzner M, Marusic I, et al., 2017, Fractal scaling of the turbulence interface in gravity currents, Journal of Fluid Mechanics, Vol: 820, Pages: R3-1-R3-12, ISSN: 0022-1120
It was previously observed by Krug et al. (J. Fluid Mech., vol. 765, 2015, pp. 303–324) that the surface area 𝐴𝜂 of the turbulent/non-turbulent interface (TNTI) in gravity currents decreases with increasing stratification, significantly reducing the entrainment rate. Here, we consider the multiscale properties of this effect using direct numerical simulations of temporal gravity currents with different gradient Richardson numbers 𝑅𝑖𝑔 . Our results indicate that the reduction of 𝐴𝜂 is caused by a decrease of the fractal scaling exponent 𝛽 , while the scaling range remains largely unaffected. We further find that convolutions of the TNTI are characterized by different length scales in the streamwise and wall-normal directions, namely the integral scale ℎ and the shear scale 𝑙𝑆𝑘=𝑘1/2/𝑆 (formed using the mean shear 𝑆 and the turbulent kinetic energy 𝑘 ) respectively. By recognizing that the anisotropy implied by the different scaling relations increases with increasing 𝑅𝑖𝑔 , we are able to model the 𝑅𝑖𝑔 dependence of 𝛽 in good agreement with the data.
Suter I, Maksimovic C, Van Reeuwijk M, 2017, A neighbourhood-scale estimate for the cooling potential of green roofs, Urban Climate, Vol: 20, Pages: 33-45, ISSN: 2212-0955
Green roofs offer the possibility to mitigate multiple environmental issues in an urban environment. A common benefit attributed to green roofs is the temperature reduction through evaporation. This study focuses on evaluating the effect that evaporative cooling has on outdoor air temperatures in an urban environment. An established urban energy balance model was modified to quantify the cooling potential of green roofs and study the scalability of this mitigation strategy. Simulations were performed for different climates and urban geometries, with varying soil moisture content, green roof fraction and urban surface layer thickness. All simulations show a linear relationship between surface layer temperature reduction ΔTs and domain averaged evaporation rates from vegetation mmW, i.e. ΔTs = eW ⋅ mmW, where eW is the evaporative cooling potential with a value of ∼ −0.35 Kdaymm−1. This relationship is independent of the method by which water is supplied. We also derive a simple algebraic relation for eW using a Taylor series expansion.
Bozovic R, Maksimovic C, Mijic A, et al., 2017, Blue Green Solutions. A Systems Approach to Sustainable and Cost-Effective Urban Development
This guide presents an innovative framework to systematically unlock the multiple benefits of city natural infrastructure; thus producing resilient, sustainable and cost-effective solutions. The framework is applicable at a building, neighbourhood and city-scale and is suitable both for new and retrofit developments.
Van Reeuwijk M, Krug D, Holzner M, 2017, Small-scale entrainment in inclined gravity currents, Environmental Fluid Mechanics, Vol: 18, Pages: 225-239, ISSN: 1573-1510
We investigate the effect of buoyancy on the small-scale aspects ofturbulent entrainment by performing direct numerical simulation of a gravity cur-rent and a wall jet. In both flows, we detect the turbulent/nonturbulent interfaceseparating turbulent from irrotational ambient flow regions using a range of en-strophy iso-levels spanning many orders of magnitude. Conform to expectation,the relative enstrophy isosurface velocityvnin the viscous superlayer scales withthe Kolmogorov velocity for both flow cases. We connect the integral entrainmentcoefficientEto the small-scale entrainment and observe excellent agreement be-tween the two estimates throughout the viscous superlayer.The contribution ofbaroclinic torque tovnis negligible, and we show that the primary reason forreduced entrainment in the gravity current as compared to the wall-jet is thereduction in the surface area of the isosurfaces.
Holzner M, van Reeuwijk M, 2017, The turbulent/nonturbulent interface in penetrative convection, Journal of Turbulence, Vol: 18, Pages: 260-270, ISSN: 1468-5248
The effect of buoyancy on the turbulent/nonturbulent interface (TNTI) and viscous superlayer are studied by performing direct numerical simulation of penetrative convection. In this flow, rising turbulent thermals alternate with unmixed fluid entrained from above, forming a TNTI between the turbulent and irrotational flow regions. We detect the TNTI using a broad range of enstrophy iso-levels, from the very low levels of the outer fringes of the turbulent flow region to high levels located in the turbulent flow region. We study the local entrainment velocity vn by which the TNTI propagates outwards relative to the fluid flow while entraining unmixed fluid into the turbulent region. The relative entrainment velocity is decomposed into a viscous, an inertial and a baroclinic torque term, respectively. For low enstrophy levels we find a viscous superlayer (VSL) where viscous diffusion dominates, while inertial and baroclinic torque terms are small. It is only for higher iso-levels in the buffer region of the TNTI, which extends from the edge of the VSL to the threshold for which vn = 0, that the inertial enstrophy production term plays a significant role. Penetrative convection does not feature a turbulent core where vn > 0 (i.e. inward moving enstrophy isosurfaces) that has been previously identified in other entraining flows such as jets or gravity currents. Surprisingly, the baroclinic torque remains inactive throughout the whole range of enstrophy iso-levels. The smallness of the baroclinic torque against viscous effects in the TNTI is supported by a dimensional argument which predicts that at high Reynolds number the baroclinic torque term will be negligible.
Van Reeuwijk M, sallizoni P, Hunt GR, et al., 2016, Turbulent transport and entrainment in jets and plumes: A DNS study, Physical Review Fluids, Vol: 1, ISSN: 2469-990X
We present a direct numerical simulation (DNS) data set for a statistically axisymmetric turbulent jet, plume, and forced plume in a domain of size 40r0×40r0×60r0, where r0 is the source diameter. The data set supports the validity of the Priestley-Ball entrainment model in unstratified environments (excluding the region near the source) [Priestley and Ball, Q. J. R. Meteor. Soc. 81, 144 (1955)], which is corroborated further by the Wang-Law and Ezzamel et al. experimental data sets [Wang and Law, J. Fluid Mech. 459, 397 (2002); Ezzamel et al., J. Fluid Mech. 765, 576 (2015)], the latter being corrected for a small but influential coflow that affected the statistics. We show that the second-order turbulence statistics in the core region of the jet and the plume are practically indistinguishable from each other, although there are significant differences near the plume edge. The DNS data indicate that the turbulent Prandtl number is about 0.7 for both jets and plumes. For plumes, this value is a result of the difference in the ratio of the radial turbulent transport of radial momentum and buoyancy. For jets, however, the value originates from a different spread of the buoyancy and velocity profiles, in spite of the fact that the ratio of radial turbulent transport terms is approximately unity. The DNS data do not show any evidence of similarity drift associated with gradual variations in the ratio of buoyancy profile to velocity profile widths.
Schmidt L, Fouxon I, Krug D, et al., 2016, Clustering of particles in turbulence due to phoresis, Physical Review E, Vol: 93, ISSN: 1539-3755
We demonstrate that diffusiophoretic, thermophoretic, and chemotactic phenomena in turbulence lead to clustering of particles on multifractal sets that can be described using one single framework, valid when the particle size is much smaller than the smallest length scale of turbulence l0. To quantify the clustering, we derive positive pair correlations and fractal dimensions that hold for scales smaller than l0. For scales larger than l0 the pair-correlation function is predicted to show a stretched exponential decay towards 1. In the case of inhomogeneous turbulence we find that the fractal dimension depends on the direction of inhomogeneity. By performing experiments with particles in a turbulent gravity current we demonstrate clustering induced by salinity gradients in conformity to the theory. The particle size in the experiment is comparable to l0, outside the strict validity region of the theory, suggesting that the theoretical predictions transfer to this practically relevant regime. This clustering mechanism may provide the key to the understanding of a multitude of processes such as formation of marine snow in the ocean and population dynamics of chemotactic bacteria.
Craske J, van Reeuwijk M, 2016, Generalised unsteady plume theory, Journal of Fluid Mechanics, Vol: 792, Pages: 1013-1052, ISSN: 0022-1120
We develop a generalised unsteady plume theory and compare it with a new direct numerical simulation (DNS) dataset for an ensemble of statistically unsteady turbulent plumes. The theoretical framework described in this paper generalises previous models and exposes several fundamental aspects of the physics of unsteady plumes. The framework allows one to understand how the structure of the governing integral equations depends on the assumptions one makes about the radial dependence of the longitudinal velocity, turbulence and pressure. Consequently, the ill-posed models identified by Scase & Hewitt (J. Fluid Mech., vol. 697, 2012, p. 455) are shown to be the result of anon-physical assumption regarding the velocity profile. The framework reveals that these ill-posed unsteady plume models are degenerate cases amongst a comparatively large set of well-posed models that can be derived from the generalised unsteady plume equations that we obtain. Drawing on the results of DNS of a plume subjected to an instantaneous step change in its source buoyancy flux, we use the framework in a diagnostic capacityto investigate the properties of the resulting travelling wave. In general, the governing integral equations are hyperbolic, becoming parabolic in the limiting case of a `top-hat' model, and the travelling wave can be classified as lazy, pure or forced according to the particular assumptions that are invoked to close the integral equations. Guided by observations from the DNS data, we use the framework in a prognostic capacity to develop a relatively simple, accurate and well-posed model of unsteady plumes that is based on the assumption of a Gaussian velocity profile. An analytical solution is presented for a pure straight-sided plume that is consistent with the key features observed from the DNS.
Holzner M, van Reeuwijk M, Jonker H, 2016, Turbulent entrainment in a gravity current, 4th International-Association-for-Hydro-Environment-Engineering-and-Research (IAHR) Congress, Publisher: CRC PRESS-BALKEMA, Pages: 1025-1031
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- Citations: 1
Morel CRG, van Reeuwijk M, Graf T, 2015, Systematic investigation of non-Boussinesq effects in variable-density groundwater flow simulations, JOURNAL OF CONTAMINANT HYDROLOGY, Vol: 183, Pages: 82-98, ISSN: 0169-7722
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- Citations: 14
Van Reeuwijk M, Craske J, 2015, Energy-consistent entrainment relations for jets and plumes, Journal of Fluid Mechanics, Vol: 782, Pages: 333-355, ISSN: 0022-1120
We discuss energetic restrictions on the entrainment coefficient α for axisymmetric jets and plumes. The resulting entrainment relation includes contributions from the mean flow, turbulence and pressure, fundamentally linking α to the production of turbulence kinetic energy, the plume Richardson number Ri and the profile coefficients associated with the shape of the buoyancy and velocity profiles. This entrainment relation generalises the work by Kaminski et al. (J. Fluid Mech., vol. 526, 2005, pp. 361–376) and Fox (J. Geophys. Res., vol. 75, 1970, pp. 6818–6835). The energetic viewpoint provides a unified framework with which to analyse the classical entrainment models implied by the plume theories of Morton et al. (Proc. R. Soc. Lond. A, vol. 234, 1955, pp. 1–23) and Priestley & Ball (Q. J. R. Meteorol. Soc., vol. 81, 1954, pp. 144–157). Data for pure jets and plumes in unstratified environments indicate that to first order the physics is captured by the Priestley and Ball entrainment model, implying that (1) the profile coefficient associated with the production of turbulence kinetic energy has approximately the same value for pure plumes and jets, (2) the value of α for a pure plume is roughly a factor of 5/3 larger than for a jet and (3) the enhanced entrainment coefficient in plumes is primarily associated with the behaviour of the mean flow and not with buoyancy-enhanced turbulence. Theoretical suggestions are made on how entrainment can be systematically studied by creating constant- Ri flows in a numerical simulation or laboratory experiment.
Craske J, Debugne ALR, van Reeuwijk M, 2015, Shear-flow dispersion in turbulent jets, Journal of Fluid Mechanics, Vol: 781, Pages: 28-51, ISSN: 0022-1120
We investigate the transport of a passive scalar in a fully developed turbulent axisymmetric jet at a Reynolds number of Re = 4815 using data from direct numerical simulation. In particular, we simulate the response of the concentration field to an instantaneous variation of the scalar flux at the source. To analyse the time evolution of this statisticallyunsteady process we take an ensemble average over 16 independent simulations. We find that the evolution of Cm(z, t), the radial integral of the ensemble-averaged concentration, is a self-similar process, with front position and spread both scaling as √t. The longitudinal mixing of Cm is shown to be primarily caused by shear-flow dispersion.Using the approach developed by Craske & van Reeuwijk (J. Fluid Mech., vol. 763, 2014,pp. 538–566), the classical theory for shear-flow dispersion is applied to turbulent jets to obtain a closure that couples the integral scalar flux to the integral concentration Cm. Model predictions using the dispersion closure are in good agreement with the simulation data. Application of the dispersion closure to a two-dimensional jet results in an integraltransport equation that is fully consistent with that of Landel et al. (J. Fluid Mech., vol.711, 2012, pp. 212–258)
van Reeuwijk M, Hadziabdic M, 2015, Modelling high Schmidt number turbulent mass transfer, INTERNATIONAL JOURNAL OF HEAT AND FLUID FLOW, Vol: 51, Pages: 42-49, ISSN: 0142-727X
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- Citations: 16
Craske J, van Reeuwijk M, 2015, Energy dispersion in turbulent jets. Part 2. A robust model for unsteady jets, Journal of Fluid Mechanics, Vol: 763, Pages: 538-566, ISSN: 0022-1120
In this paper we develop an integral model for an unsteady turbulent jet that incorporates longitudinal dispersion of two distinct types. The model accounts for the difference in the rate at which momentum and energy are advected (type I dispersion) and for the local deformation of velocity profiles that occurs in the vicinity of a sudden change in the momentum flux (type II dispersion). We adapt the description of dispersion in pipe flow by Taylor (Proc. R. Soc. Lond. A, vol. 219, 1953, pp. 186–203) to develop a dispersion closure for the longitudinal transportation of energy in unsteady jets. We compare our model’s predictions to results from direct numerical simulation and find a good agreement. The model described in this paper is robust and can be solved numerically using a simple central differencing scheme. Using the assumption that the longitudinal velocity profile in a jet has an approximately Gaussian form, we show that unsteady jets remain approximately straight-sided when their source area is fixed. Straight-sidedness provides an algebraic means of reducing the order of the governing equations and leads to a simple advection–dispersion relation. The physical process responsible for straight-sidedness is type I dispersion, which, in addition to determining the local response of the area of the jet, determines the growth rate of source perturbations. In this regard the Gaussian profile has the special feature of ensuring straight-sidedness and being insensitive to source perturbations. Profiles that are more peaked than the Gaussian profile attenuate perturbations and, following an increase (decrease) in the source momentum flux, lead to a local decrease (increase) in the area of the jet. Conversely, profiles that are flatter than the Gaussian amplify perturbations and lead to a local increase (decrease) in the area of the jet.
Van Reeuwijk M, Salizzoni P, Craske J, 2015, Turbulent entrainment in jets and plumes, Pages: 175-178
We perform direct simulation of a statistically steady jets, forced plume and pure plume in a neutral environment and present the value of the entrainment coefficient decomposed into 1) turbulence production; 2) buoyancy effects; and 3) deviations from self-similarity. The value of the decomposed entrainment coefficient is in excellent agreement with a direct estimate from the volume conservation equation. It is shown that the Priestley and Ball entrainment model describes the entrainment physics reasonably well.
Verso L, van Reeuwijk M, Gurka R, et al., 2015, Entrainment of a turbulent patch in a stratified fluid
Turbulent patches are localized events of turbulence, typically characterized by sharp differences between the flow characteristics across their interfaces. These localized events might add to the global mixing, heat exchange and mass transfer, playing a non-negligible role in the total energy balance in lakes or the ocean. This study takes a detailed look at the inner structure of a localized, mechanically forced patch in a linearly stratified ambient using laboratory experiments utilizing synchronized PIV and PLIF. The results point out that the role of the turbulent/non-turbulent interface at the edge of the patch could be significant in determining the growth rate and the maximum size of the patch.
Craske J, van Reeuwijk M, 2015, Dispersion in unsteady jets and plumes
We investigate the transport of both passive and active scalars in fully developed turbulent axisymmetric jets and plumes using data from direct numerical simulation. In both cases we simulate the response of the flow to an instantaneous increase in the scalar flux at the source and our focus is on the determination of the rate at which the resulting disturbance propagates and spreads in the longitudinal direction. We apply Taylor’s theory of shear-flow dispersion [9] to free-shear flows and therefore model the way in which departures from self-similarity result in the longitudinal mixing of integral quantities. The resulting integral models exhibit a good agreement with the simulation data and, in the case of passive scalar transport, admit an analytical similarity solution. For the case of active scalar transport we examine the buoyancy flux in an unsteady plume and show that the momentum–energy framework [7], rather than the classical volume–momentum framework [6], provides the natural setting from which to view the effects of dispersion. Consequently, we demonstrate the effect that dispersion has on turbulent entrainment and the way in which a plume responds to source perturbations in its buoyancy flux.
Craske J, van Reeuwijk M, 2015, Energy dispersion in turbulent jets. Part 1. Direct simulation of steady and unsteady jets, JOURNAL OF FLUID MECHANICS, Vol: 763, Pages: 500-537, ISSN: 0022-1120
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- Citations: 55
van Reeuwijk M, Salizzoni P, Craske J, 2015, Turbulent entrainment in jets and plumes, 8th International Symposium On Turbulence Heat and Mass Transfer (THMT), Publisher: BEGELL HOUSE, INC, Pages: 175-178, ISSN: 2377-2816
Suter I, Maksimovic C, van Reeuwijk M, 2015, LES study of a mixed layer above urban street canyons, 8th International Symposium On Turbulence Heat and Mass Transfer (THMT), Publisher: BEGELL HOUSE, INC, Pages: 667-670, ISSN: 2377-2816
van Reeuwijk M, Holzner M, 2014, The turbulence boundary of a temporal jet, Journal of Fluid Mechanics, Vol: 739, Pages: 254-275, ISSN: 0022-1120
We examine the structure of the turbulence boundary of a temporal plane jet at Re=5000 using statistics conditioned on the enstrophy. The data is obtained by direct numerical simulation and threshold values span 24 orders of magnitude, ranging from essentially irrotational fluid outside the jet to fully turbulent fluid in the jet core. We use two independent estimators for the local entrainment velocity vn based on the enstrophy budget. The data show clear evidence for the existence of a viscous superlayer (VSL) that envelopes the turbulence. The VSL is a nearly one-dimensional layer with low surface curvature. We find that both its area and viscous transport velocity adjust to the imposed rate of entrainment so that the integral entrainment flux is independent of threshold, although low-Reynolds-number effects play a role for the case under consideration. This threshold independence is consistent with the inviscid nature of the integral rate of entrainment. A theoretical model of the VSL is developed that is in reasonably good agreement with the data and predicts that the contribution of viscous transport and dissipation to interface propagation have magnitude 2vn and −vn , respectively. We further identify a turbulent core region (TC) and a buffer region (BR) connecting the VSL and the TC. The BR grows in time and inviscid enstrophy production is important in this region. The BR shows many similarities with the turbulent–non-turbulent interface (TNTI), although the TNTI seems to extend into the TC. The average distance between the TC and the VSL, i.e. the BR thickness is about 10 Kolmogorov length scales or half a Taylor length scale, indicating that intense turbulent flow regions and viscosity-dominated regions are in close proximity.
Jonker HJJ, van Reeuwijk M, Sullivan PP, et al., 2013, On the scaling of shear-driven entrainment: a DNS study, Journal of Fluid Mechanics, Vol: 732, Pages: 150-165, ISSN: 0022-1120
The deepening of a shear-driven turbulent layer penetrating into stably stratified quiescent layer is studied using Direct Numerical Simulation (DNS). The simulation design mimics the classical laboratory experiments by Kato and Phillips (J. Fluid Mech. 37, 643–655, 1969) in that it starts with linear stratification and applies a constant shear stress at the lower boundary, but avoids inherent side wall and rotation effects of that experiment. It is found that the layers universally deepen as a function of the square root of time, independent of the initial stratification and the Reynolds number of the simulations, provided that the Reynolds number is large enough. Consistent with this finding, the dimensionless entrainment velocity varies with the bulk Richardson number as Ri−1/2 . In addition it is observed that all cases evolve in a self-similar fashion. A selfsimilarity analysis of the conservation equations shows that only a square root growth law is consistent with self-similar behaviour.
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