Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session GO: Turbulence: Simulations II |
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Chair: Gustaff Jacobs, San Diego State University Room: Salt Palace Convention Center 251 C |
Monday, November 19, 2007 10:30AM - 10:43AM |
GO.00001: DNS of turbulent flow over longitudinally ridged walls. Jony Castagna, Yufeng Yao An in-house DNS code has been developed over the years and validations have been carried out on various flow problems, including boundary-layer, plain channel, bump flow, and many other cases. This finite difference code solves full three-dimensional compressible Navier-Stokes equations using high-order (4th-order) for spatial derivatives and multi-stage Runge-Kutta explicit scheme for time advancement. The code parallelization has also been carried out using the latest MPI library and is portable for various HPC platforms. The unique feature of the code is that it applies the entropy splitting concept for improve the numerical stability, which is one of common problem for DNS code requiring extremely longer run time to get the statistically converged results. In this study, the code has been further extended to include the capability of treating the geometry variation in the spanwise direction by using full 3D grid transformation, similar that done by other researchers. As demonstration, we follow an existing DNS study of turbulent flow over longitudinally ridged walls at the Reynolds number (Re$_{\tau })$ 140, based on the friction velocity, as the first step to validate the new capability of the code. Results from present study have been compared fairly well with available DNS data. In the full version of the paper, detailed analysis will be provided, focusing on three parts: mean propriety, turbulence intensity and turbulent coherent structures. [Preview Abstract] |
Monday, November 19, 2007 10:43AM - 10:56AM |
GO.00002: DNS of Turbulent Channel Flow past Ultrahydrophobic Surfaces with Periodic Microfeatures Michael Martell, Blair Perot, Jonathan Rothstein The results from series of direct numerical simulations for fully-developed pressure-driven turbulent channel flow past ultrahydrophobic surfaces will be presented. We have shown previously that these surfaces can produce significant drag reduction in laminar channel flow by supporting a shear-free air-water interface between hydrophobic microridges or microposts. In this talk, we will focus on the turbulent flow between two parallel plates with a periodic array of longitudinal, ultrahyrdophobic ridges and posts with variable size and spacing. The surfaces of the microfeatures are modeled as no-slip while the air-water interfaces supported between the microfeatures are assumed to be flat and shear free. Relevant Reynolds stresses are calculated and will be compared with previous examples of turbulent channel flows without surface micro-features and the results of our ongoing experiments. Differences in the Reynolds stresses, the implications surface micro-features have on mean flow of the channel, as well as results from instantaneous and statistical turbulent structure analysis will be discussed. [Preview Abstract] |
Monday, November 19, 2007 10:56AM - 11:09AM |
GO.00003: Near-transition dynamics of viscoelastic turbulence and drag reduction in plane Poiseuille flow Li Xi, Wei Li, Michael Graham Nonlinear traveling wave solutions have been found for the Navier-Stokes equations in all canonical parallel flow geometries. These solutions capture the main dynamical features of turbulent flows, especially for near-wall coherent flow structures. Our previous study of the effects of polymer additives on one class of these so-called exact coherent states (ECS) suggests that turbulent drag reduction can be better understood through these traveling waves. Many key aspects of experimental observations can be related with the existence and evolution of ECS solutions in viscoelastic flows. Guided by these results, we conduct direct numerical simulations (DNS) in a minimal flow unit that captures the smallest self-sustaining structure in turbulence. The simulations are performed in a parameter regime close to the laminar-turbulent transition, where our earlier results predict that the laminar-turbulent transition, the onset of drag reduction and the maximum drag reduction (MDR) regime are close to each other in Reynolds number. The connection between these DNS results and traveling waves will be described, and the dynamical structures outside of the existence boundary of ECS will also be investigated, the latter of which could be a good starting point of understanding the nature of MDR. [Preview Abstract] |
Monday, November 19, 2007 11:09AM - 11:22AM |
GO.00004: Direct Numerical Simulation of Superfluid Turbulence Karla Morris, Damian Rouson, Joel Koplik At low temperatures, as quantum effects become increasingly apparent, helium (He$^{4})$ transforms into a superfluid.~ The motion of superfluid helium (He II) can be decomposed into two interpenetrating components: a superfluid, which is an inviscid liquid comprised of line vortices with quantized circulation, and a normal fluid, which is a gas of elementary thermal excitations. At sufficiently high driving velocities, the motion of He II becomes unstable and makes a transition to turbulence, commonly termed superfluid turbulence or quantum turbulence.~ A growing body of empirical evidence suggests that the macroscopic statistical behavior of quantum turbulence closely matches that of classical turbulence despite considerable differences in the physics at the mesoscopic and microscopic scales.~ This paradoxical similarity has been attributed to a phenomenology involving quantum-vortex/normal-vortex locking [1]. Results will be presented from direct numerical simulations (DNS) of superfluid vortices driven by homogeneous normal fluid turbulence. ~Wavelet transform techniques inspired by Farge et al. [2] will be used to compare the location and alignment of normal fluid vortices with those of the quantum vortices. [1] Stalp S.R et al. (2002) Physics of Fluids, Vol. 14, Number 4 [2] Farge M. et al. (2003) Physics of Fluids, Vol 15, Number 10 [Preview Abstract] |
Monday, November 19, 2007 11:22AM - 11:35AM |
GO.00005: Simulation of turbulent MHD channel flow with spanwise magnetic field Thomas Boeck, Dmitry Krasnov, Oleg Zikanov We study the effect of a homogeneous magnetic field on turbulent plane Poiseuille flow of an electrically conducting fluid. The field is oriented in the spanwise direction and does not modify the critical Reynolds number for linear instability of Poiseuille flow. Simulations are performed for super-critical Reynolds numbers with a pseudospectral Fourier-Chebyshev method either as direct simulations or as LES. As the Lorentz force tends to suppress motion in the spanwise direction, the flow eventually becomes two-dimensional when the applied magnetic field is sufficiently strong. For weaker magnetic fields three-dimensional turbulence can be sustained, but the anisotropy due to the Lorentz force leads to qualitative changes in the velocity and Reynolds stress profiles. As in the case of homogeneous MHD turbulence, the essential features of this anisotropic turbulence are well predicted by the LES with either classical or dynamic Smagorinsky model. [Preview Abstract] |
Monday, November 19, 2007 11:35AM - 11:48AM |
GO.00006: Interactions of eddies and waves in magnetohydrodynamic turbulence Annick Pouquet, Pablo Mininni Direct numerical simulations of three-dimensional magnetohydrodynamic turbulence at a Taylor Reynolds number of 1100 on a grid of $1536^3$ points are reported (arXiv:0707.3620 astro-ph, submitted to {\it Phys. Rev. Lett.}). The flow is incompressible and decaying in time, and the initial condition is a superposition of large scale ABC flows for wavenumbers $k \le 4$ and random noise at small scales with a $k^{-3}$ spectrum, with negligible correlation between velocity and the magnetic field ($\rho_C\sim 10^{-4}$) and equal kinetic and magnetic energies; finally, no uniform magnetic field is imposed. At peak of dissipation, the resulting energy spectrum is a combination of two components, each moderately resolved. Isotropy obtains in the large scales, with a spectrum compatible with the Iroshnikov-Kraichnan theory stemming from the weakening of nonlinear interactions due to Alfv\'en waves and leading to a $\sim k^{-3/2}$ law; scaling of structure functions confirms the non-Kolmogorovian nature of the flow in this range. At small scales, weak turbulence emerges with a $k_{\perp}^{-2}$ spectrum, the perpendicular direction referring to the local quasi-uniform magnetic field. Whether such results are universal is not clear, and several parameters may play a role, such as $\rho_C$ or the amount of magnetic helicity in the flow. Thus, high-resolution parametric studies are needed in order to understand in detail the interactions of turbulent eddies and Alfv\'en waves. [Preview Abstract] |
Monday, November 19, 2007 11:48AM - 12:01PM |
GO.00007: Unsteady effects in normal shock wave/turbulent boundary layer interaction Matteo Bernardini, Sergio Pirozzoli, Francesco Grasso The interaction of a spatially developing supersonic turbulent boundary layer with a normal shock wave is analyzed by means of direct numerical simulation of the compressible Navier-Stokes equations. At the selected flow conditions, corresponding to a mild shock, no mean flow separation is observed. However, the flow is strongly unsteady, and intermittent regions of flow reversal are found near the wall, while large vortical structures are observed away from it. Such structures are mainly responsible for the amplification of noise and turbulence across the interaction zone. In particular, the sound field attains very large values (up to $162 \mathrm{dB}$) near the nominal impingement point. The intense acoustic loads occurring in the interaction zone are found to be strictly related to the Reynolds shear stress distribution. The analysis of the pressure energy spectra shows a behavior consistent with that observed in incompressible boundary layers in adverse pressure gradient. In particular, a power-law scaling is recovered: at low frequencies the spectra scale as $St^{0.4}$, while at high frequencies they decay as $St^{-5}$. The results show that the interacting shock primarily acts as a low-pass filter for the turbulence spectra. The main effect is to enhance the low-frequency components while inhibiting the higher ones. We acknowledge the CASPUR computing consortium (University of Rome `La Sapienza') for providing the computational resources to perform the numerical simulation. [Preview Abstract] |
Monday, November 19, 2007 12:01PM - 12:14PM |
GO.00008: Lattice Boltzman Simulations for 2D Turbulence with Passive and Active Particles Yuehong Qian, Lubing Wang, Wanheng He, Howard Hu We consider in this presentation the studies of two dimensional turbulence using the lattice Boltzmann approach. There has been a constant interest in 2D experimental investigations, and some interest is focused on the compressibility effect and air resistance mechanism. We try to address these issues by using the lattice Boltzmann approach in addition to simple and idealized 2D turbulence simulations. Passive particles (without feedback) and active particles (with feedback) will be studies as potential applications in geophysical flows. Some comparisons with experiments and finite element method will be also presented. [Preview Abstract] |
Monday, November 19, 2007 12:14PM - 12:27PM |
GO.00009: Numerical simulation of laser--induced breakdown of air Shankar Ghosh, Krishnan Mahesh The laser--induced breakdown of air is studied using numerical simulation. When focused onto a small volume of air, a laser beam heats and ionizes the air, causing a plasma to form. Three models of air with varing levels of physical complexity are considered. The simulations are challenging due to presence of very strong shock waves and very low densities in the plasma core. These challenges are addressed. The time evolution of the flow field resulting from laser energy deposition is compared to experiment. The flow field is classified into three phases: shock formation, shock propagation and subsequent collapse of the plasma core. Each phase is studied in detail. Vorticity generation in the flow is described for short and long times. Scaling analysis is performed for different amounts of deposited laser energy. [Preview Abstract] |
Monday, November 19, 2007 12:27PM - 12:40PM |
GO.00010: Large-Eddy Simulation of a Transverse Jet in a Supersonic Crossflow using High-Order Compact Scheme with Artificial Fluid Properties Soshi Kawai, Sanjiva K. Lele Transverse jet injected into a supersonic crossflow is studied numerically by non-reactive compressible large-eddy simulation. High-order compact differencing scheme is used to resolve the scales of turbulence through the entire computational region and coupled with artificial nonlinear fluid properties to capture the complex flow discontinuities which include unsteady shock waves, entropy discontinuities between the jet and crossflow and its interactions. The method of artificial fluid properties which is based on the approach proposed by Cook and Cabot [JCP 195 (2004) 594-601], Fiorina and Lele [JCP 222 (2007) 246-264] and Cook [POF 19 055103 (2007)] is extended to a generalized coordinate framework. Simulated flow properties are compared with the available experimental data. Preliminary results show qualitative agreement with the experiment. Important flow features such as the bow shock, the barrel shock and Mach disk within the jet, the jet penetration and its mixing are well captured. [Preview Abstract] |
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