Showing papers on "K-epsilon turbulence model published in 2017"
TL;DR: Anisotropic particles are common in many industrial and natural turbulent flows as discussed by the authors, and when these particles are small and neutrally buoyant, they follow Lagrangian trajectories while exhibiting rich orientational dynamics from the coupling of their rotation to the velocity gradients of the turbulence field.
Abstract: Anisotropic particles are common in many industrial and natural turbulent flows. When these particles are small and neutrally buoyant, they follow Lagrangian trajectories while exhibiting rich orientational dynamics from the coupling of their rotation to the velocity gradients of the turbulence field. This system has proven to be a fascinating application of the fundamental properties of velocity gradients in turbulence. When particles are not neutrally buoyant, they experience preferential concentration and very different preferential alignment than neutrally buoyant tracer particles. A vast proportion of the parameter range of anisotropic particles in turbulence is still unexplored, with most existing research focusing on the simple foundational cases of axisymmetric ellipsoids at low concentrations in homogeneous isotropic turbulence and in turbulent channel flow. Numerical simulations and experiments have recently developed a fairly comprehensive picture of alignment and rotation in these cases, and t...
257 citations
242 citations
TL;DR: In this article, known structures and self-sustaining mechanisms of wall turbulence are reviewed and explored in the context of the scale interactions implied by the nonlinear advective term in the Navier-Stokes equations.
Abstract: Known structures and self-sustaining mechanisms of wall turbulence are reviewed and explored in the context of the scale interactions implied by the nonlinear advective term in the Navier–Stokes equations. The viewpoint is shaped by the systems approach provided by the resolvent framework for wall turbulence proposed by McKeon & Sharma (J. Fluid Mech., vol. 658, 2010, pp. 336–382), in which the nonlinearity is interpreted as providing the forcing to the linear Navier–Stokes operator (the resolvent). Elements of the structure of wall turbulence that can be uncovered as the treatment of the nonlinearity ranges from data-informed approximation to analysis of exact solutions of the Navier–Stokes equations (so-called exact coherent states) are discussed. The article concludes with an outline of the feasibility of extending this kind of approach to high-Reynolds-number wall turbulence in canonical flows and beyond.
176 citations
TL;DR: In this paper, the authors review the space-time correlation models in both the Eulerian and Lagrangian frames of reference, which include the random sweeping and local straining models for isotropic and homogeneous turbulence, Taylor's frozen-flow model and the elliptic approximation model for turbulent shear flows, and the linear wave propagation model and swept-wave model for compressible turbulence.
Abstract: Space-time correlation is a staple method for investigating the dynamic coupling of spatial and temporal scales of motion in turbulent flows. In this article, we review the space-time correlation models in both the Eulerian and Lagrangian frames of reference, which include the random sweeping and local straining models for isotropic and homogeneous turbulence, Taylor's frozen-flow model and the elliptic approximation model for turbulent shear flows, and the linear-wave propagation model and swept-wave model for compressible turbulence. We then focus on how space-time correlations are used to develop time-accurate turbulence models for the large-eddy simulation of turbulence-generated noise and particle-laden turbulence. We briefly discuss their applications to two-point closures for Kolmogorov's universal scaling of energy spectra and to the reconstruction of space-time energy spectra from a subset of spatial and temporal signals in experimental measurements. Finally, we summarize the current understanding of space-time correlations and conclude with future issues for the field.
133 citations
TL;DR: In this paper, a spectral coherence analysis of the turbulent boundary layer flow structure is carried out through a spectrogram of two-point streamwise velocity signals at a near-wall reference position and a range of wall-normal positions spanning a Reynolds-number range.
Abstract: An assessment of the turbulent boundary layer flow structure, which is coherent with the near-wall region, is carried out through a spectral coherence analysis. This spectral method is applied to datasets comprising synchronized two-point streamwise velocity signals at a near-wall reference position and a range of wall-normal positions spanning a Reynolds-number range . Within each dataset, a self-similar structure is identified from the coherence between the turbulence in the logarithmic region and at the near-wall reference position. This self-similarity is described by a streamwise/wall-normal aspect ratio of , where and are the streamwise wavelength and wall-normal distance respectively.
102 citations
TL;DR: In this article, a buoyancy-modified k-ω SST turbulence model was proposed to suppress the turbulence level at the interface between water and air in a numerical wave flume.
97 citations
17 Oct 2017
TL;DR: In this paper, an established model for such an interaction between near-wall and logarithmic region turbulence is considered that comprises two mechanisms: superposition and modulation, where outer-region motions, of which a fraction is wall-attached, are superimposed onto the near wall dynamics, and concurrently the nearwall motions are modulated by this superimposed signature.
Abstract: The classical view of wall-bounded turbulence considers a near-wall inner region where all velocity statistics are universally dependent on distance from the wall when scaled with friction velocity and the kinematic viscosity of the fluid. This is referred to as an inner scaling and leads to Prandtl's law of the wall. Data from numerical simulations and experiments over the past decade or so, however, have provided compelling evidence that statistics of the fluctuating streamwise velocity do not follow inner scaling in this near-wall region and an interaction of outer and logarithmic regions exists, resulting in a Reynolds number dependence. In this paper we briefly review some of these studies and discuss the Reynolds number dependence of the streamwise turbulence intensity near the wall in terms of an inner-outer interaction. An established model for such an interaction between near-wall and logarithmic region turbulence is considered that comprises two mechanisms: superposition and modulation. Here outer-region motions, of which a fraction is wall-attached, are superimposed onto the near-wall dynamics, and concurrently the near-wall motions are modulated by this superimposed signature. We discuss to what extent the superposition effect can relate changes in the inner-scaled near-wall peak value of streamwise turbulence intensity to logarithmic region turbulence resembling features of attached eddies.
95 citations
TL;DR: In this article, a spatially-averaged two-fluid model (SA-TFM) is derived from ensemble averaging the kinetic-theory based TFM equations, and the residual correlation for the gas-solid drag, which appears due to averaging, is derived by employing a series expansion to the microscopic drag coefficient, while the Reynolds-stress-like contributions are closed similar to the Boussinesq-approximation.
Abstract: We present a spatially-averaged two-fluid model (SA-TFM), which is derived from ensemble averaging the kinetic-theory based TFM equations. The residual correlation for the gas-solid drag, which appears due to averaging, is derived by employing a series expansion to the microscopic drag coefficient, while the Reynolds-stress-like contributions are closed similar to the Boussinesq-approximation. The subsequent averaging of the linearized drag force reveals that averaged interphase momentum exchange is a function of the turbulent kinetic energies of both, the gas and solid phase, and the variance of the solids volume fraction. Closure models for these quantities are derived from first principles. The results show that these new constitutive relations show fairly good agreement with the fine grid data obtained for a wide range of particle properties. Finally, the SA-TFM model is applied to the coarse grid simulation of a bubbling fluidized bed revealing excellent agreement with the reference fine grid solution. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3544–3562, 2017
93 citations
TL;DR: In this article, two-phase and multiphase simulations are conducted in various size industrial hydrocyclones using volume of fluid and modified mixture models coupled with Reynolds Stress Model (RSM), Detached Eddy Simulation and Large Eddy simulation (LES) turbulence models.
91 citations
TL;DR: SedFoam-2.0 as mentioned in this paper is a three-dimensional two-phase flow solver for sediment transport applications, which can deal with complex turbulent sediment transport problems with different combinations of intergranular stress and turbulence models.
Abstract: . In this paper, a three-dimensional two-phase flow solver, SedFoam-2.0, is presented for sediment transport applications. The solver is extended from twoPhaseEulerFoam available in the 2.1.0 release of the open-source CFD (computational fluid dynamics) toolbox OpenFOAM. In this approach the sediment phase is modeled as a continuum, and constitutive laws have to be prescribed for the sediment stresses. In the proposed solver, two different intergranular stress models are implemented: the kinetic theory of granular flows and the dense granular flow rheology μ(I). For the fluid stress, laminar or turbulent flow regimes can be simulated and three different turbulence models are available for sediment transport: a simple mixing length model (one-dimensional configuration only), a k − e, and a k − ω model. The numerical implementation is demonstrated on four test cases: sedimentation of suspended particles, laminar bed load, sheet flow, and scour at an apron. These test cases illustrate the capabilities of SedFoam-2.0 to deal with complex turbulent sediment transport problems with different combinations of intergranular stress and turbulence models.
89 citations
TL;DR: In this paper, a geometry-resolved full-scale tidal-stream turbine was compared with experimental data from a 1MW machine deployed at the EMEC test site. And the effect of turbulence on loads, with synthetic turbulence prescribed at inlet was employed.
TL;DR: In this article, a multiscale modeling of a diurnal cycle of real-world conditions is presented for the first time, validated using data from the CWEX-13 field experiment.
Abstract: Multiscale modeling of a diurnal cycle of real-world conditions is presented for the first time, validated using data from the CWEX-13 field experiment. Dynamical downscaling from synoptic-scale down to resolved three-dimensional eddies in the atmospheric boundary layer (ABL) was performed, spanning 4 orders of magnitude in horizontal grid resolution: from 111 km down to 8.2 m (30 m) in stable (convective) conditions. Computationally efficient mesoscale-to-microscale transition was made possible by the generalized cell perturbation method with time-varying parameters derived from mesoscale forcing conditions, which substantially reduced the fetch to achieve fully developed turbulence. In addition, careful design of the simulations was made to inhibit the presence of under-resolved convection at convection-resolving mesoscale resolution and to ensure proper turbulence representation in stably-stratified conditions. Comparison to in situ wind-profiling lidar and near-surface sonic anemometer measurements demonstrated the ability to reproduce the ABL structure throughout the entire diurnal cycle with a high degree of fidelity. The multiscale simulations exhibit realistic atmospheric features such as convective rolls and global intermittency. Also, the diurnal evolution of turbulence was accurately simulated, with probability density functions of resolved turbulent velocity fluctuations nearly identical to the lidar measurements. Explicit representation of turbulence in the stably-stratified ABL was found to provide the right balance with larger scales, resulting in the development of intra-hour variability as observed by the wind lidar; this variability was not captured by the mesoscale model. Moreover, multiscale simulations improved mean ABL characteristics such as horizontal velocity, vertical wind shear, and turbulence.
TL;DR: In this article, large-eddy simulations with various constant wind, wave, and surface destabilizing surface buoyancy flux forcing are conducted, with a focus on assessing the impact of Langmuir turbulence on the entrainment buoyance flux at the base of the ocean surface boundary layer.
Abstract: Large-eddy simulations (LESs) with various constant wind, wave, and surface destabilizing surface buoyancy flux forcing are conducted, with a focus on assessing the impact of Langmuir turbulence on the entrainment buoyancy flux at the base of the ocean surface boundary layer. An estimate of the entrainment buoyancy flux scaling is made to best fit the LES results. The presence of Stokes drift forcing and the resulting Langmuir turbulence enhances the entrainment rate significantly under weak surface destabilizing buoyancy flux conditions, that is, weakly convective turbulence. In contrast, Langmuir turbulence effects are moderate when convective turbulence is dominant and appear to be additive rather than multiplicative to the convection-induced mixing. The parameterized unresolved velocity scale in the K-profile parameterization (KPP) is modified to adhere to the new scaling law of the entrainment buoyancy flux and account for the effects of Langmuir turbulence. This modification is targeted on c...
TL;DR: In this article, a graphite and a gritblasting surface were compared with Reynolds numbers for Reynolds numbers Re� = 90, 120, 180, 240, 360, 540, and 720.
Abstract: The database contains representations of the two surfaces studied, a graphite and a gritblasted surface, and the corresponding time-averaged velocity data for Reynolds numbers Re� = 90; 120; 180; 240; 360; 540, and 720.
TL;DR: Analysis of the Vlasov-Maxwell equations from the perspective of turbulence cascade clarifies the role of electromagnetic work, and reveals the importance of the pressure-strain relation in generating internal energy.
Abstract: Analysis of the Vlasov-Maxwell equations from the perspective of turbulence cascade clarifies the role of electromagnetic work, and reveals the importance of the pressure-strain relation in generating internal energy. Particle-in-cell simulation demonstrates the relative importance of the several energy exchange terms, indicating that the traceless pressure-strain interaction ``Pi-D'' is of particular importance for both electrons and protons. The Pi-D interaction and the second tensor invariants of the strain are highly localized in similar spatial regions, indicating that energy transfer occurs preferentially in coherent structures. The collisionless turbulence cascade may be fruitfully explored by study of these energy transfer channels, in addition to examining transfer across spatial scales.
TL;DR: In this paper, uncertainty quantification of turbulence model closure coefficients is performed for transonic flow over an axisymmetric bump and the RAE 2822 transonic airfoil.
Abstract: The goal of this work is to quantify the uncertainty and sensitivity of commonly used turbulence models in Reynolds-averaged Navier–Stokes codes due to uncertainty in the values of closure coefficients for transonic wall-bounded flows and to rank the contribution of each coefficient to uncertainty in various output flow quantities of interest. Specifically, uncertainty quantification of turbulence model closure coefficients is performed for transonic flow over an axisymmetric bump and the RAE 2822 transonic airfoil. Three turbulence models are considered: the Spalart–Allmaras model, Wilcox (2006) k-ω model, and Menter shear-stress transport model. The FUN3D code developed by NASA Langley Research Center is used as the flow solver. The uncertainty quantification analysis employs stochastic expansions based on non-intrusive polynomial chaos for efficient uncertainty propagation. Several integrated and point quantities are considered as uncertain outputs for both computational fluid dynamics problems. Closur...
TL;DR: In this paper, the authors introduce a definition of coherent cluster based on self-similarity, and apply it to distributions of heavy particles in direct numerical simulations of homogeneous isotropic turbulence, with and without gravitational acceleration.
Abstract: Despite the widely acknowledged significance of turbulence-driven clustering, a clear topological definition of particle cluster in turbulent dispersed multiphase flows has been lacking. Here we introduce a definition of coherent cluster based on self-similarity, and apply it to distributions of heavy particles in direct numerical simulations of homogeneous isotropic turbulence, with and without gravitational acceleration. Clusters show self-similarity already at length scales larger than twice the Kolmogorov length, as indicated by the fractal nature of their surface and by the power-law decay of their size distribution. The size of the identified clusters extends to the integral scale, with average concentrations that depend on the Stokes number but not on the cluster dimension. Compared to non-clustered particles, coherent clusters show a stronger tendency to sample regions of high strain and low vorticity. Moreover, we find that the clusters align themselves with the local vorticity vector. In the presence of gravity, they tend to align themselves vertically and their fall speed is significantly different from the average settling velocity: for moderate fall speeds they experience stronger settling enhancement than non-clustered particles, while for large fall speeds they exhibit weakly reduced settling. The proposed approach for cluster identification leverages the Voronoi diagram method, but is also compatible with other tessellation techniques such as the classic box-counting method.
TL;DR: In this paper, the authors proposed a systematic approach to find the optimum values for the closure coefficients of RANS models in order to significantly improve the accuracy of CFD simulations for urban studies.
TL;DR: In this paper, the D3Q27 multiple-relaxation-time lattice Boltzmann method was used to simulate anisotropic porous-walled channel flows.
Abstract: To investigate which component of the anisotropic permeability tensor of porous media influences turbulence over porous walls, direct numerical simulation of anisotropic porous-walled channel flows is performed by the D3Q27 multiple-relaxation-time lattice Boltzmann method. The presently considered anisotropic permeable walls have square pore arrays aligned with the Cartesian axes. Vertical, streamwise and spanwise pore arrays are systematically introduced to the walls to impose anisotropic permeability. Simulations are carried out at a friction Reynolds number of 111 and 230, which is based on the averaged friction velocity of the porous bottom and the smooth top walls. It is found that streamwise and spanwise permeabilities enhance turbulence whilst vertical permeability itself does not. In particular, the enhancement of turbulence is remarkable over porous walls with streamwise permeability. Over streamwise permeable walls, development of high- and low-speed streaks is prevented whilst large-scale intermittent patched patterns of ejection motions are induced. It is revealed by two-point correlation analysis that streamwise permeability allows the development of streamwise large-scale perturbations induced by Kelvin–Helmholtz instability. Spectral analysis reveals that this perturbation contributes to the enhancement of the Reynolds shear stress, leading to significant skin friction of the porous interface. Through the comparison between the two different Reynolds-number cases, it is found that, as the Reynolds number increases, the streamwise perturbation becomes larger and more organized. Consequently, owing to the enhancement of the large-scale perturbation, a significant Reynolds-number dependence of the skin friction of the porous interface can be observed over the streamwise permeable wall. It is also implied that the wavelength of the perturbation can be reasonably scaled by the outer-layer length scale.
TL;DR: It is shown that both longitudinal and transverse velocity increments scale on locally averaged dissipation rate, just as postulated by Kolmogorov's refined similarity hypothesis, and that, in isotropic turbulence, a single independent scaling adequately describes fluid turbulence in the inertial range.
Abstract: Using the largest database of isotropic turbulence available to date, generated by the direct numerical simulation (DNS) of the Navier-Stokes equations on an 8192^{3} periodic box, we show that the longitudinal and transverse velocity increments scale identically in the inertial range. By examining the DNS data at several Reynolds numbers, we infer that the contradictory results of the past on the inertial-range universality are artifacts of low Reynolds number and residual anisotropy. We further show that both longitudinal and transverse velocity increments scale on locally averaged dissipation rate, just as postulated by Kolmogorov's refined similarity hypothesis, and that, in isotropic turbulence, a single independent scaling adequately describes fluid turbulence in the inertial range.
01 Jan 2017
TL;DR: In this article, the bending effect in turbulent premixed flames has been investigated using Direct Numerical Simulation (DNS) to investigate the bending effects of high-intensity turbulence.
Abstract: In turbulent premixed flames, much experimental evidence points to a strong influence of pre-mixture turbulence intensity on the turbulent burning velocity. The linear enhancement of turbulent burning velocity in low-intensity turbulence is predicted accurately by current models. In contrast, the deviation from linearity in high-intensity turbulence, known as the “bending effect,” remains to be explained. The present work has employed Direct Numerical Simulation (DNS) to investigate the bending effect. An initially laminar methane-air premixed flame was subjected to increasing levels of turbulence across five different simulations which maintained all parameters except the turbulence intensity constant. The bending effect was captured within these simulations. Subsequently, plausible explanations were investigated using the framework of the Flame Surface Density (FSD) approach. From the ensuing analysis, it is evident that flame surface area reflects distinctly the variation of turbulent burning velocity with turbulence intensity. Local flame quenching does not appear to be the primary mechanism behind the bending effect. Instead, the observed bending effect results from a shift in balance, under high-intensity turbulence, towards mechanisms that favour destruction of flame surface area. These mechanisms tend to preserve the reaction layer and, thereby, ensure the validity of Damkohler’s hypothesis and flamelet models in conditions that cause the bending effect that is observed here to occur.
TL;DR: In this article, the effect of large-scale motions (LSMs) on the turbulent/non-turbulent (T/NT) interface is examined in a turbulent boundary layer.
Abstract: The effect of large-scale motions (LSMs) on the turbulent/non-turbulent (T/NT) interface is examined in a turbulent boundary layer. Using flow fields from direct numerical simulation, the shape of the interface and near-interface statistics are evaluated conditional on the position of the LSM. The T/NT interface is identified using the vorticity magnitude and a streak detection algorithm is adopted to identify and track the LSMs. Two-point correlation and spectral analysis of variations in the interface height show that the spatial undulation of the interface is longer in streamwise wavelength than the boundary-layer thickness, and grows with the Reynolds number in a similar manner to the LSMs. The average variation in the interface height was evaluated conditional on the position of the LSMs. The result provides statistical evidence that the interface is locally modulated by the LSMs in both the streamwise and spanwise directions. The modulation is different when the coherent structure is high- versus low-speed motion: high-speed structures lead to a wedge-shaped deformation of the T/NT interface, which causes an anti-correlation between the angles of the interface and the internal shear layer. On the other hand, low-speed structures are correlated with crests in the interface. Finally, the sudden changes in turbulence statistics across the interface are in line with the changes in the population of low-speed structures, which consist of slower mean streamwise velocity and stronger turbulence than the high-speed counterparts.
TL;DR: In this article, the aerodynamic characteristics of a high-speed train in an atmospheric boundary layer are investigated in a wind tunnel, and different atmospheric boundary layers simulations are performed by changing the turbulence intensity generated by a collection of spires.
TL;DR: In this paper, a three-dimensional micro combustor model was built to investigate impact of various turbulence models on combustion and emission behavior of studied hydrogen/air flames, and performance evaluation of these models was executed by examining combustor outer wall temperature distribution; combustor centerline temperature, velocity, pressure, species and NOx profiles.
TL;DR: Bachman et al. as mentioned in this paper introduced two methods for dynamically prescribing eddy-induced diffusivity, advection, and vis6 cosity appropriate for primitive equation models with resolutions permitting the forward potential enstrophy 7 cascade of quasigeostrophic dynamics.
Abstract: 5 This paper introduces two methods for dynamically prescribing eddy-induced diffusivity, advection, and vis6 cosity appropriate for primitive equation models with resolutions permitting the forward potential enstrophy 7 cascade of quasigeostrophic dynamics, such as operational ocean models and high-resolution climate mod8 els with O(25)km horizontal resolution and finer. Where quasigeostrophic dynamics fail (e.g., the equator, 9 boundary layers, deep convection), the method reverts to scalings based on a matched two-dimensional en10 strophy cascade. A principle advantage is that these subgrid models are scale-aware, meaning that the model 11 is suitable over a range of grid resolutions: from mesoscale grids that just permit baroclinic instabilities to 12 grids below the submesoscale where ageostrophic effects dominate. 13 Two approaches are presented here using Large Eddy Simulation (LES) techniques adapted for three14 dimensional rotating, stratified turbulence. The simpler approach has one non-dimensional parameter, Λ, 15 which has an optimal value near 1. The second approach dynamically optimizes Λ during simulation using 16 a test filter. The new methods are tested in an idealized scenario by varying the grid resolution, and their use 17 improves the spectra of potential enstrophy and energy in comparison to extant schemes. The new meth18 ods keep the gridscale Reynolds and Péclet numbers near one throughout the domain, which confers robust 19 numerical stability and minimal spurious diapycnal mixing. Although there are no explicit parameters in 20 the dynamic approach, there is strong sensitivity to the choice of test filter. Designing test filters for hetero21 geneous ocean turbulence adds cost and uncertainty, and we find the dynamic method does not noticeably 22 improve over setting Λ = 1. 23 ∗Corresponding Author address: Scott Bachman, DAMTP, Centre for Mathematical Sciences, Wilberforce Road, Cambridge CB3 Preprint submitted to Elsevier November 4, 2016
27 Jan 2017
TL;DR: In this article, the pseudosound mode dominates over the acoustic mode at relatively small scales, and the spectrum of pseudosounds velocity exhibits a power-law scaling with the exponent -3.
Abstract: Spectra and statistics in compressible isotropic turbulence are studied. In weakly compressible turbulence, the pseudosound mode dominates over the acoustic mode at relatively small scales, and the spectrum of pseudosound velocity exhibits a power-law scaling with the exponent -3.
TL;DR: In this paper, the authors used a Stream Line pulsed coherent Doppler lidar (PCDL) with the use of conical scanning by a probing beam around the vertical axis.
Abstract: . The method and results of lidar studies of spatiotemporal variability of wind turbulence in the atmospheric boundary layer are reported. The measurements were conducted by a Stream Line pulsed coherent Doppler lidar (PCDL) with the use of conical scanning by a probing beam around the vertical axis. Lidar data are used to estimate the kinetic energy of turbulence, turbulent energy dissipation rate, integral scale of turbulence, and momentum fluxes. The dissipation rate was determined from the azimuth structure function of radial velocity within the inertial subrange of turbulence. When estimating the kinetic energy of turbulence from lidar data, we took into account the averaging of radial velocity over the sensing volume. The integral scale of turbulence was determined on the assumption that the structure of random irregularities of the wind field is described by the von Karman model. The domain of applicability of the used method and the accuracy of the estimation of turbulence parameters were determined. Turbulence parameters estimated from Stream Line lidar measurement data and from data of a sonic anemometer were compared.
TL;DR: In this paper, an attempt has been made to find out the better mixing of fuel and air in the supersonic ramjet engines (SCRAMJET) by considering the DLR scramjet model as a standard and reference model for the validation of results.
TL;DR: It is observed that weak spatial pairing between vortices and antivortices develops in the turbulent BEC, which corresponds to the vortex-dipole gas regime predicted for high dissipation, which provides a direct quantitative marker for the survey of various 2D turbulence regimes in the BEC system.
Abstract: In a two-dimensional (2D) classical fluid, a large-scale flow structure emerges out of turbulence, which is known as the inverse energy cascade where energy flows from small to large length scales. An interesting question is whether this phenomenon can occur in a superfluid, which is inviscid and irrotational by nature. Atomic Bose-Einstein condensates (BECs) of highly oblate geometry provide an experimental venue for studying 2D superfluid turbulence, but their full investigation has been hindered due to a lack of the circulation sign information of individual quantum vortices in a turbulent sample. Here, we demonstrate a vortex sign detection method by using Bragg scattering, and we investigate decaying turbulence in a highly oblate BEC at low temperatures, with our lowest being ~0.5T
c
, where T
c
is the superfluid critical temperature. We observe that weak spatial pairing between vortices and antivortices develops in the turbulent BEC, which corresponds to the vortex-dipole gas regime predicted for high dissipation. Our results provide a direct quantitative marker for the survey of various 2D turbulence regimes in the BEC system.
TL;DR: In this paper, a large-eddy simulation (LES) study of wave breaking processes over a near-prototype scale barred beach was performed using the open-source CFD toolbox OpenFOAM.
Abstract: To understand the interaction between wave-breaking induced turbulent coherent structures and suspended sediment transport, we report a Large-Eddy Simulation (LES) study of wave breaking processes over a near-prototype scale barred beach. The numerical model is implemented using the open-source CFD toolbox, OpenFOAM®, in which the incompressible three-dimensional filtered Navier-Stokes equations for the water and air phases are solved with a finite volume scheme. A Volume of Fluid (VOF) method is used to capture the evolution of the water-air interface. The numerical model is validated with measured free surface elevation, turbulence averaged flow velocity, turbulent intensity, and for the first time, the intermittency of breaking wave turbulence. Simulation results confirm that as the obliquely descending eddies (ODEs) approach the bottom, significant bottom shear stress is generated. Remarkably, the collapse of ODEs onto the bed can also cause drastic spatial and temporal changes of dynamic pressure on the bottom. By allowing sediment to be suspended from the bar crest, intermittently high sediment suspension events and their correlation with high turbulence and/or high bottom shear stress events are investigated. The simulated intermittency of sediment suspension is similar to previous field and large wave flume observations. Coherent suspension events account for only 10% of the record but account for about 50% of the sediment load. Model results suggest that about 60∼70% of coherent bottom stress events are associated with surface-generated turbulence. Nearly all the coherent sand suspension events are associated with coherent turbulence events due to wave-breaking turbulence approaching the bed. This article is protected by copyright. All rights reserved.