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 ED: Turbulence Simulations I |
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Chair: Changhoon Lee, Yonsei University Room: Long Beach Convention Center 102B |
Sunday, November 21, 2010 4:10PM - 4:23PM |
ED.00001: Effect of Surface Roughness on heat transfer in a Turbulent Channel Flow Stefano Leonardi, Benjamin Cruz Perez, John Lucena DNSs are carried out for passive heat transport in a turbulent channel flow with surface roughness on the wall. The total heat transfer depends on the pitch to height ratio of the roughness. Several configurations have been studied, square bars, circular cylinders, V-shaped turbulators, segmented V-shaped turbulators, with and without fillets. A paramentric study has been performed with the aim of finding the configuration with the largest heat transfer and the minimum drag. The effect of placing the roughness surfaces on both walls or only on one wall is also considered. For transverse square bars, ejections occurring on the leading edge of the roughness elements enhance the heat transfer. On the other hand, when V-shaped turbulators are placed on the wall a secondary motion is induced which is responsible of removing heat from the wall. Applications in turbine engines will be discussed at the conference. [Preview Abstract] |
Sunday, November 21, 2010 4:23PM - 4:36PM |
ED.00002: Turbulence modification by stable stratification in channel flow Manuel Garcia-Villalba, Juan C. del Alamo We study the modification of the turbulent structure of plane channel flow under stable stratification through direct numerical simulations. The simulations are performed at moderate Reynolds numbers (up to Retau=550), in very large computational boxes (up to Lx = 60 h, Lz = 25 h where h is the channel half height) and considering a wide range of stratification levels (up to Ritau=1920). For weak stratification or high Reynolds number, the turbulence is affected by buoyancy in the core of the channel but the near-wall region differs little from the neutral case. With increasing stratification, the near-wall streaks remain essentially unmodified while large-scale super-streaks are damped. In the central region internal gravity waves are dominant. In addition, there is an intermediate outer layer where the dynamics of the turbulent structures is governed by local fluxes. In this region energy spectra collapse when using local Obukhov scaling. With strong stratification, large laminar patches appear in the near wall region, and turbulent momentum and buoyancy fluxes vanish in the core of the channel. Linear transient-growth analysis of the mean velocity and density profiles reproduces the damping of the super-streaks in stratified channels, and predicts maximum energy amplification for infinitely-wide perturbations that are consistent with the internal waves observed in the simulations. [Preview Abstract] |
Sunday, November 21, 2010 4:36PM - 4:49PM |
ED.00003: DNS of multiscale-generated turbulence John Christos Vassilicos, Sylvain Laizet Four spatially evolving turbulent flows, one generated by a regular grid and three generated by fractal square grids of different aspect ratio are studied by means of Direct Numerical Simulations (DNS). An innovative approach which combines high order compact schemes, Immersed Boundary Method and an efficient domain decomposition is used in this study to perform such large simulations. Statistics such as turbulent intensities are investigated with the objective of analysing the two different regions (production and decay regions) downstream from the fractal square grids, as already observed in the experimental results of Mazellier {\&} Vassilicos (PoF, 2010). The main goal of this numerical study is to identify the physical mechanisms implicated in the generation of turbulent flows, especially when generated at different scales, but also to compare the different levels of turbulence intensity generated by each grid. [Preview Abstract] |
Sunday, November 21, 2010 4:49PM - 5:02PM |
ED.00004: Use of POD to Investigate Large-Scale Structures in Turbulent Flow with Rib-Roughness Maziar E. Samani, Donald Bergstrom In this work, Large Eddy Simulation (LES) of turbulent Couette flow is used to study the flow structures associated with the hydrodynamic roughness created by a series of ribs of square cross-section mounted perpendicular to the flow. Due to their geometric simplicity, ribs have been previously used to simulate turbulent flow over a rough surface. Both direct numerical simulation (DNS) (Orlandi et al., 2006) and experimental methods (Krogstad et al., 2005) have been used to study this type of roughness in the context of pressure driven flow in a channel. In this study, LES is used to simulate turbulent Couette flow with rib roughness on one wall. Proper Orthogonal Decomposition (POD) is applied to determine the large-scale flow structures in the wake region of the roughness elements. The spanwise vorticity and both streamwise and wall-normal velocity components are used as the computational variables for the POD. Comparisons are made to a similar POD analysis of the wake structure of an infinite square cylinder mounted in a uniform flow. Of specific interest is the interaction of the rib wakes with the turbulent boundary layer above. [Preview Abstract] |
Sunday, November 21, 2010 5:02PM - 5:15PM |
ED.00005: Coherent Structures in the Flow over Two-Dimensional Dunes Mohammad Omidyeganeh, Ugo Piomelli Dunes are common large-scale bed irregularities found in rivers at high Reynolds numbers. One characteristic feature of the flow over dunes is the presence of boils, upwelling motions observed at the water surface. Understanding the dynamics of the eddies that cause boils is critical, since these eddies lift up sediment from the river bed and carry it to the surface. To analyse the dynamics of these large structures, we performed large eddy simulation of the flow over two-dimensional dunes at laboratory scale (the Reynolds number based on the average channel height and mean velocity is 18,900). Results show that the rollers generated in the separated shear layer interact with wall turbulence to form an inclined horseshoe vortex that reaches the surface; the upwelling is due to the ejection that occurs between the legs of the horseshoe. These vortices are initially vertical, but become inclined when they are advected to the surface; near-wall horseshoe vortices, on the other hand, start from a horizontal orientation. This indicates that these structures are unique to the flow over dunes, in which separation occurs at the crest. While the boil frequency is fairly low (they were found every $40h/U_b$ time, approximately) they are known to affect sediment transport significantly. A more quantitative analysis of the contribution of large vortices to mass and momentum transport is presently being carried out. [Preview Abstract] |
Sunday, November 21, 2010 5:15PM - 5:28PM |
ED.00006: POD Study of Turbulent Boundary Layer Structures Jon Baltzer, Ronald Adrian, Xiaohua Wu 3D POD is performed on a recent DNS turbulent boundary layer simulation (Wu \& Moin, Phys. Fluids, 2010). The modes of various scales are examined. Dominant modes include structures consistent with the inclined ramps commonly observed in turbulent boundary layers. POD modes are examined to interpret the spatial organizations of the structural elements. [Preview Abstract] |
Sunday, November 21, 2010 5:28PM - 5:41PM |
ED.00007: Development and Assessment of Proper Orthogonal Decomposition for Analysis of Turbulent Flow in Piston Engines K. Liu, D.C. Haworth High-resolution optical diagnostics (e.g., particle-image velocimetry -- PIV) and high-resolution numerical models (e.g., large-eddy simulation -- LES) are increasingly being used to develop advanced combustion systems for next-generation piston engines. To date, quantitative comparisons between PIV and LES for engines have been limited mainly to ensemble- (phase-) averaged mean quantities. Proper orthogonal decomposition (POD) has been proposed as an approach for analyzing the dynamics of complex in-cylinder processes, and as a basis for making objective quantitative comparisons between PIV and LES. Here three-dimensional, time-dependent datasets generated by performing multiple-cycle LES of motored flows for simple engine configurations are used to develop and assess the use of POD as a basis for the analysis of turbulent flow in piston engines. We explore POD variants that are required for analysis of statistically nonstationary flows in time-varying domains. We explore sensitivities of mode structure and convergence rate to spatial and temporal resolution, and perform comparisons of two-dimensional and three-dimensional POD analyses. And we explore the use of POD to identify and quantify cycle-to-cycle variations. [Preview Abstract] |
Sunday, November 21, 2010 5:41PM - 5:54PM |
ED.00008: Coherent turbulent motions in a Mach 3 boundary layer Izaak Beekman, Yin-Chiu Kan, Stephan Priebe, Pino Martin We examine coherent structures found in a Mach $3$, compressible, turbulent boundary layer using a new, long domain ($x_{L}=50\delta_{inlet}$), spatial direct numerical simulation (SDNS). Recent studies have shown that certain coherent motions, termed ``superstructures,'' or very large scale motions (VLSM), play an important dynamical role, strongly impacting the near wall cycle.\footnote{I.~Marusic, R.~Mathis~\&~N.~Hutchins. Predictive model for wall-bounded turbulent flow. \emph{Science}, 329(5988):193-6, 2010.} While most previous studies have been performed on incompressible boundary layers and at higher Reynolds numbers, Ringuette, Wu~\&~Martin have shown that these structures are present at the conditions of the current simulation.\footnote{M.~J.~Ringuette, M.~Wu~\&~M.~P.~Martin. Coherent structures in direct numerical simulation of turbulent boundary layers at Mach 3. \emph{J Fluid Mech.}, 594:59-69, 2008.} With this simulation we examine the dynamics and geometry of the large scale turbulence structures using statistical techniques, as well as visualizations. Additionally, we characterize the footprint of these coherent structures and their interaction with the wall. [Preview Abstract] |
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