Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session A27: Stratified Turbulent Flows 
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Chair: Elias Balaras, George Washington Univesrity Room: 2009 
Sunday, November 23, 2014 8:00AM  8:13AM 
A27.00001: Probing the effect of buoyancy on secondorder statistics in stablystratified boundary layers Elie BouZeid, Stimit Shah Staticallystable turbulent boundary layer flows are particularly challenging due to the potential breakdown of Kolmogorov's theory and to the emergence of laminar regions, gravity waves, and other complicating flow patterns. To develop a more fundamental understanding of how buoyancy influences turbulence in such flows, direct numerical simulations and large eddy simulations of turbulent boundary layers with rotation are performed. Under the highest stabilities, global intermittency (the almost compete decay of turbulence and then its regeneration) is observed, but could be the result of initial and boundary conditions rather than flow dynamics. Under more moderate stabilities, continuous turbulence is maintained, but it is significantly damped compared to neutral flows. This reduction of the TKE under stable conditions is very well known; however, here we show that it is mainly triggered by reduced mechanical production associated with reduced transport of Reynolds stresses from aloft toward the surface, rather than by direct destruction of TKE by buoyancy. This raises questions about the suitability of some conventional stability parameters, such as the flux Richardson number, in describing the influence of buoyancy in such flows. [Preview Abstract] 
Sunday, November 23, 2014 8:13AM  8:26AM 
A27.00002: Energetics of vertical fluid particle dispersion in stably stratified turbulence James Rottman, Seungbum Jo, Keiko Nomura The vertical dispersion of fluid particles in stably stratified turbulence is investigated. We present an analysis framework which describes the associated flow energetics in the Lagrangian frame. The available potential energy (APE) density is a locally defined quantity associated with nonequlibrium displacement. The equilibrium potential energy (EPE) density is defined accordingly and represents the minimum energy required to change the particle equilibrium height. The corresponding evolution equations elucidate the key sequence of processes and clarify previous interpretations of the transport mechanisms. The analysis shows that in the case of stationary flow, the rate of mean square displacement is equal to the rate of mean square equilibrium displacement which is given by the scalar dissipation rate. A dispersion model is developed and compared with previous models. [Preview Abstract] 
Sunday, November 23, 2014 8:26AM  8:39AM 
A27.00003: Turbulent transport across an interface between dry and humid air in a stratified environment Daniela Tordella, Luca Gallana, Francesca De Santi, Silvio Di Savino, Renzo Ricchiardone, Michele Iovieno The transport of energy and water vapor across a thin layer which separates two decaying isotropic turbulent flows with different kinetic energy and humidity is considered. The interface is placed in a shearless stratified environment in temporal decay. This system reproduces a few aspects of small scale turbulent transport across a dry air/moist air interface in an atmospheric like context. In our incompressible DNS at $Re_\lambda=250$, Boussinesq's approximation is used for momentum and energy transport while the vapor is modeled as a passive scalar (Kumar, Schumacher \& Shaw 2014). We investigated different stratification levels with an initial $Fr$ between 0.8 and 8 in presence of a kinetic energy ratio equal to 7. As the buoyancy term becomes of the same order of the inertial ones, a spatial redistribution of kinetic energy, dissipation and vapor concentration is observed. This eventually leads to the onset of a well of kinetic energy in the low energy side of the mixing layer which blocks the entrainment of dry air. Results are discussed and compared with laboratory and numerical experiments. {\it A posteriori} estimates of the eventual compression/expansion of fluid particles inside the interfacial mixing layer are given (Nance \& Durran 1994). [Preview Abstract] 
Sunday, November 23, 2014 8:39AM  8:52AM 
A27.00004: Intermittent dynamics in stably stratified plane Couette flows Enrico Deusebio, John R. Taylor, Colmcille Caulfield, Rich R. Kerswell Turbulence in a stratified fluid is a fundamental process in the atmosphere and oceans, responsible for mixing density and various tracers and dissipating kinetic and potential energy. Although turbulence is generally suppressed in very statically stable conditions, intermittent bursts of turbulence are still seen when the Reynolds number is sufficiently large. In this work, we study stratified turbulence in plane Couette flow using direct numerical simulations, focusing on the complexity arising from the spatiotemporal intermittency of the flow as the stabilizing stratification increases. Two external dimensionless parameters control the dynamics: the Reynolds number $Re$ and the bulk Richardson number $Ri_b$. We trace the boundary between laminar and turbulent states in the $Re$$Ri_b$ plane and discuss the relevant dynamical quantities involved in the relaminarization process. We analyze the structures populating the intermittent regime and the coexistence between laminar and turbulent patches, focusing on similarities and differences between small$Re$small$Ri_b$ and large$Re$large$Ri_b$ intermittent dynamics. We conclude by discussing the applicability and breakdown of existing stratified turbulence theories, including the MoninObukhov selfsimilarity theory. [Preview Abstract] 
Sunday, November 23, 2014 8:52AM  9:05AM 
A27.00005: Largeeddy simulations of stratification layer erosion by a jet Aleksandr Obabko, Elia Merzari, Ananias Tomboulides, Shashi Aithal, Paul Fischer Following Fukushima disaster, the OECD/NEA has chosen the PANDA experiment for 2014 benchmark exercise where predictive capabilities of computational fluid dynamics (CFD) tools are tested for multispecies convection in notorious regime of transition from turbulent to laminar flow and from forced to natural convection. Accurate prediction of these phenomena will beneficial for a range of applications including reactor thermalhydraulics where it will further our understanding of reactor behavior during accidents and help design safer and more efficient reactors for a carbonfree energy option. In fact, the convection and mixing flow in the containment played an important role in the Fukushima accident as the buoyant hydrogen gas mixed with oxygen and detonated resulting in significant destruction and radioactive pollution. Here we present the threedimensional largeeddy (LES) simulations of the PANDA experiment with the spectralelement opensource code Nek5000. The results are compared and contrasted for a range of parameters using Boussinesq and lowMach number approximations. [Preview Abstract] 

A27.00006: ABSTRACT WITHDRAWN 
Sunday, November 23, 2014 9:18AM  9:31AM 
A27.00007: The turbulent, stratified near wake of a sphere at Re = 3700 and Fr = 3 Anikesh Pal, Antonio Posa, Elias Balaras, Sutanu Sarkar Direct numerical simulation of flow past a sphere in a stratified fluid has been carried out at a subcritical Reynolds number of 3700 and Froude number of 3. The choice of Re = 3700 allows validation against previous unstratified wake simulation including the recent DNS of Rodriguez et al. (2011). The conservation equations are solved in a cylindrical coordinate system and an immersed boundary method is employed to represent the sphere. The primary focus of this study is to understand buoyancy effects on near wake characteristics. The separated shear layer from the surface of the sphere becomes unstable resulting in transition to turbulence. The recirculation region is found to be affected by buoyancy. The turbulent stratified wake experiences a substantial suppression in the vertical direction in comparison to the corresponding unstratified case. Nevertheless, in the horizontal direction, the turbulent wake expands significantly more than in the unstratified case. Changes in the intensity, spectral content and structure of nearwake fluctuations in the wake are assessed. The momentum and energy transported by the internal gravity waves generated by the turbulent wake are also quantified. [Preview Abstract] 
Sunday, November 23, 2014 9:31AM  9:44AM 
A27.00008: Direct simulations of unstratified and stratified turbulent flow past a sphere using a bodyconforming grid Karu Chongsiripinyo, Sutanu Sarkar Direct numerical simulations of unstratified and stratified flows past a sphere are conducted in the subcritical regime at Re = 3700. The objective is to investigate the flow at and near the body. This study takes advantage of a bodyconforming grid which provides better accuracy and boundary layer representation than possible with the immersed boundary method. The bodyfitted grid is generated by creating half of a Ctype grid using a hyperbolic grid generation method and then rotating it around the wakecut axis. The incompressible NavierStokes equations are solved using a semiimplicit scheme with the CrankNicholson method and the low storage RKW3ADI. The Poisson equation for pressure correction is solved using semicoarsening multigrid (SMG) from the HYPRE library. The singularity problem at the wakecut is resolved by rewriting the discretized governing equation in finitevolume formulation and then set all fluxes across the cut to zero. For flux terms at nodes in the proximity of the wake cut, onesided finite difference is used to avoid crossing the wake cut. Boundary layer separation and vortical structures immediately behind the sphere are examined. Turbulence statistics in the near wake region for unstratified and stratified flows are also compared. [Preview Abstract] 
Sunday, November 23, 2014 9:44AM  9:57AM 
A27.00009: High Reynolds Number NearField Stratified Wake Measurements behind a Sphere Kenneth Kalumuck, Alan Brandt, Kara Shipley, Michael Jozkowski To characterize the nearfield of a stratified wake at Reynolds numbers, Re $\sim$ 2 x 10$^{5}$  10$^{6}$, experiments are being conducted in a thermally stratified fresh water lake with large diameter (D $\sim$ 0.5 m) spheres. The submerged sphere and associated instrumentation are affixed to a frame that is towed through the lake at velocities U $\sim$ 0.5  2 m/s. Measurements of three components of the turbulent fluctuating and mean wake velocities are being made with Acoustic Doppler Velocimeters (ADVs), while density fluctuations (inferred from temperatures) are being made with an array of fast response thermistors. Stratification is such that BV frequencies, N, up to 50 cph (0.09 /s) can be achieved, enabling Froude numbers F$=$U/ND $\ge $ 10. Existing stratified nearfield wake data for spheres are for Re $\sim$ 10$^{4}$ and less, while only a very limited set of data under simpler unstratified conditions exists at these large Re, primarily measurements along the sphere (drag, pressure, separation) rather than wake data. Advances in CFD have enabled simulations at these high Reynolds numbers without quantitative data available for validation despite the existence of many natural and manmade systems that operate in these ranges. Here, experimental system design, results of a preliminary data set, and plans for ongoing and future work are presented. [Preview Abstract] 
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