Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session EC: Turbulence Modeling III |
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Chair: Oleg Vasilyev, University of Colorado Room: Long Beach Convention Center 102A |
Sunday, November 21, 2010 4:10PM - 4:23PM |
EC.00001: Study of the Invariants of the Velocity-Gradient Tensor in Homogeneous Isotropic Turbulence by means of 3C-3D Tomographic PIV Nicolas Buchmann, Sylvain Rouvier, Julio Soria The study of coherent structures (CS) in turbulent flows is essential for understanding turbulence mechanisms in technological and theoretical relevant flows. The recent advent of instantaneous three-component and three-dimensional (3C-3D) measurement techniques now permits detailed experimental investigation into the dynamics and topology of CSs by for example analysis of the invariants of the velocity gradient tensor. For this purpose, the present work presents instantaneous, high-resolution 3C-3D Tomographic Particle Image Velocimetry (TPIV) measurements in a grid-generated, homogeneous isotropic turbulent flow (Re$_{\lambda }\approx $140). The experiments are conducted in a larger water tunnel facility using a passive grid, four high-resolution digital cameras and a pulsed Nd:YAG laser for volume illumination. The invariants of the velocity gradient, rate of strain and rate of rotation tensor are used to characterize the dynamics and topology of the turbulent flow field and in particular its dissipation and vortex structure. Preliminary results are in agreement with previous literature and DNS simulations. The objective of this work is to measure these quantities experimentally and directly without additional assumptions pertaining to the structure and dynamics of the turbulent flow field. [Preview Abstract] |
Sunday, November 21, 2010 4:23PM - 4:36PM |
EC.00002: Investigation of Subgrid-Scale Turbulence in the Atmospheric Surface Layer using AHATS Field Data Khuong Nguyen, Steven Oncley, Tomas Horst, Peter Sullivan, Chenning Tong Data obtained in the atmospheric surface layer during the recent Advection Horizontal Array Turbulence Study (AHATS) field program are used to study issues met in large-eddy simulation (LES) of atmospheric boundary layer. The array technique, which has been successfully employed in several previous programs, is extended to include a second array to measure the advection of the subgrid-scale (SGS) stress. Pressure sensors are also deployed to measure the fluctuating pressure, enabling separation of the resolvable- and subgrid-scale pressure. We analyze the subgrid-scale terms in the joint probability density function (JPDF) of the resolvable-scale velocity, which must be reproduced by the SGS model in order for LES to predict correctly the resolvable-scale velocity JPDF. These terms include the conditional SGS stress (on the resolvable-scale velocity), the conditional SGS stress production rate, the conditional resolvable-scale pressure, and the conditional resolvable-scale pressure-strain rate correlation. We also analyze the advection and pressure terms in the SGS stress budget, which are important for understanding the dynamics of the SGS stress and for modeling the SGS stress using the transport equation. [Preview Abstract] |
Sunday, November 21, 2010 4:36PM - 4:49PM |
EC.00003: Characterization of the flow on the axis of two confined-opposed-jets Jean-Francois Krawczynski, Luminita Danaila, Bruno Renou We document an experimental investigation of a confined-opposed-jets flow, which is the basic spatial periodic pattern of a confined chamber. The question of the nature of the velocity fluctuations which are discussed is addressed. It is shown that the characteristic instabilities of this complex flow, along with the local confinement effects, generate large-scale quasi-organized fluctuations which are superimposed to the random/turbulent fluctuations. Small-scale statistics, such as the kinetic energy dissipation rate, are discussed and estimates based on inertial or large-scales methods are proposed and compared to traditional small-scales estimates. A scale-by-scale energy budget equation, analogous to the famous Yaglom's equation for inhomogeneous and anisotropic turbulence is discussed and partially validated in this complex flow. It is shown that the energy transfer is mainly performed in planes perpendicular to the axisymmetry axis, whereas it is strongly inhibited over the axisymmetry direction. [Preview Abstract] |
Sunday, November 21, 2010 4:49PM - 5:02PM |
EC.00004: Scale-by-scale energy budget equations for large-eddy simulations Michael Gauding, Jens Henrik Goebbert, Robert Flesch, Claudia Guenther, Norbert Peters A detailed study of scale-by-scale energy budget equations of homogeneous shear turbulence is performed. The energy budged equations are formulated in terms of structure functions and involve the balance of turbulent kinetic energy production, energy transfer and energy dissipation. In the context of large-eddy simulation (LES) the transfer of energy towards small scales must be correctly satisfied in order to preserve statistically properties of the turbulent flow field. In this work a comparison of filtered DNS data with various LES models is performed in order to assess the performance of the models. Particular emphasis is laid on anisotropic effects. A closure for the subgrid term is proposed based on an eddy-viscosity ansatz. The analysis is performed by means of direct numerical simulations of homogeneous shear turbulence at Taylor-Reynolds number of 120. Homogeneous shear turbulence reveals effects such as large-scale vortex dynamics and anisotropy. [Preview Abstract] |
Sunday, November 21, 2010 5:02PM - 5:15PM |
EC.00005: Streamwise-constant model of intermittent turbulent pipe flow Jean-Loup Bourguignon, Beverley McKeon A streamwise-constant model of intermittent turbulent pipe flow is presented, following the work on Couette flow of Gayme et al. 2010. The model consists of two evolution equations derived from Navier-Stokes, one for the streamfunction describing the in-plane velocities and one for the axial velocity. Under stochastic forcing, the model exhibits a quasi-periodic self-sustaining cycle characterized by the creation and subsequent decay of turbulent clusters of coherent structures remarkably similar to turbulent puffs. The flow structures inside these turbulent clusters correspond to the quasi-streamwise vortices and streaks observed in transition experiments. The time traces of the centerline velocity present numerous puff signatures, i.e. the centerline velocity drops suddenly and then increases smoothly nearly up to its laminar value. Under deterministic forcing, our model shows that the main features of turbulent pipe flow are robust and can easily be reproduced by solving a single momentum balance equation. [Preview Abstract] |
Sunday, November 21, 2010 5:15PM - 5:28PM |
EC.00006: Eddy-Based Model for Wall-Bounded Turbulent Flows Brian Rosenberg, Alexander Smits, Sean Bailey Here we extend the wall-bounded turbulence model of Smits (2008). The original model identifies three eddy motions ubiquitous in wall-bounded flows and captures their energy scaling in wavenumber space. The coherent structures identified are near-wall longitudinal vortex-like streaks, the Large Scale Motions related to packets of hairpin vortices, and the Very Large Scale Motions, interpreted as either outer-layer bulges in boundary layers or meandering superstructures in internal flows. The three eddy functions are summed, neglecting nonlinear interactions, and the Reynolds stress behavior is obtained by integrating over all wavenumbers. While the original model utilizes Gaussian representations of the eddy motions in wavenumber space, we instead construct wavelet-based representations in physical space. The new eddy functions are expected to offer a better physical basis for modeling since the velocity signatures of simple eddies closely resemble wavelets. Simulations at various Reynolds numbers are then compared with the original model and experiments. [Preview Abstract] |
Sunday, November 21, 2010 5:28PM - 5:41PM |
EC.00007: Simulation Wall-Bounded Turbulent Flows with Linear Effective Viscosity Models: Drag Reduction and New Mechanistic Insight R. Wang, C-F. Li, Y.-C. Pan, B. Khomami The efficacy of linear effective viscosity models in predicting the multi-stage transition between the onset of drag reduction and the maximum drag reduction (MDR) asymptote in wall-bounded turbulent flows has been examined. Our Reynolds stress model based computations demonstrate the ability of this class of models to not only predict drag reduction but also capture important characteristics of turbulent drag reduced flows such as the mean velocity profile, root-mean-square velocity fluctuations, Reynolds stress and viscous stress profiles. Specifically, enhancement of the slope of the effective viscosity model gives rise to enhanced drag reduction up to the maximum drag reduction asymptote. Moreover, manipulation of the effective viscosity profile in the channel allows determination of the region of the flow that plays a central role in the multi-stage transition between the onset of drag reduction and MDR. Specifically, we have observed that in the low drag reduction (LDR) regime this region is confined to 100 wall units from the wall, while in high drag reduction (HDR) and MDR regimes this region is enlarged to 200 or more wall units. [Preview Abstract] |
Sunday, November 21, 2010 5:41PM - 5:54PM |
EC.00008: Modeling the convective and pressure terms in finite-volume LES with unresolved wall layers Henry Chang, Robert Moser An incompressible turbulent channel flow is solved using a staggered grid finite volume LES. The grid is uniform with ${\Delta y}^+=50$ and is therefore unresolved near the wall. Our primary focus is on accurately modeling the convective and pressure terms in the LES equations. We use the fractional-step method, along with a pressure model from Harlow and Welch (1965). We have found that the pressure model itself--a discrete divergence-free projection--is sufficiently accurate. It is actually the convective term and it's divergence-free projection that are inadequately modeled. In light of this, we are investigating methods for improving the convective model. For example, we are attempting to construct models which statistically represent both the convective and pressure terms from the Reynolds stress equation. [Preview Abstract] |
Sunday, November 21, 2010 5:54PM - 6:07PM |
EC.00009: The Role of the Algorithm in the Design of LES to Capture Law-of-the-Wall Ganesh Vijayakumar, James Brasseur, Matthew Churchfield, Adam Lavely, Patrick Moriarty, Michael Kinzel, Eric Paterson Large-eddy simulation (LES) has been plagued by the inability to predict law-of-the-wall (LOTW). Brasseur {\&} Wei, (\textit{Phys. Fluids} \textbf{22}) presented a theory that explains the source of the difficulty and a framework within which LES can be designed to rectify the problem. The essential difficulty lies in (1) the nonphysical frictional content within the discretized equations, and (2) the extent to which that frictional content contributes to the inertial scaling underlying LOTW. The latter is determined by the relative contribution of mean resolved to subfilter scale (SFS) stress at the first grid level ($\Re )$. The balance of inertia and friction is represented as an ``LES Reynolds number,'' Re$_{LES}$. $\Re $ and Re$_{LES}$ must exceed critical values to predict LOTW, defining a ``High-Accuracy Zone'' (HAZ) within an $\Re $- Re$_{LES }$parameter space. Frictional content has at least two sources: the model for SFS stress and the numerical algorithm. By comparing the same simulations with a finite volume code (using OpenFOAM) and a spectral code for channel flow and the neutral atmospheric boundary layer, we shall report on the effects of friction in the algorithm on the $\Re $- Re$_{LES }$parameter space and the HAZ, and on the requirements for the LES to capture LOTW. \textit{Support: NSF, DOE.} [Preview Abstract] |
Sunday, November 21, 2010 6:07PM - 6:20PM |
EC.00010: Advances in the Design of LES to Capture Law-of-the-Wall: Role of the SFS Stress Model James Brasseur, Sanjiv Ramachandran, Tie Wei Large-eddy simulation (LES) has been plagued by the inability to capture law-of-the-wall (LOTW). In a recent paper (Brasseur {\&} Wei, \textit{Phys. Fluids} \textbf{22}) we presented a theory that explains the source of the difficulty and a framework within which LES can be designed to rectify the problem. To capture LOTW the LES must reside within the ``High-Accuracy Zone'' (HAZ) of a parameter space that can be adjusted with the model constant and grid. As the simulation penetrates the HAZ, the surface stress model influences LOTW, causing oscillations near the surface. We previously presented the source and a framework to mitigate this problem. These frameworks are independent of specific models. What is the role of the SFS stress model? We compare 3 SFS closures: Smagorinsky, 1-eq. eddy viscosity (EV), and a non EV closure. We show that all SFS models behave similarly and satisfy LOTW only when moved into the HAZ. All develop oscillations near the surface that were mitigated with the proposed adjustment to the surface stress model. The SFS stress model is known to affect (1) the critical parameters that define the HAZ, (2) the details and the path taken as the LES moves into the HAZ, (3) the tradeoff between model constant and grid aspect ratio to reach the HAZ, and (4) the prediction of the von K\'{a}rm\'{a}n constant. \textit{Supported by ARO}. [Preview Abstract] |
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