Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session D20: Turbulence: Compressible Flows |
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Chair: Daniel Livescu, Los Alamos National Laboratory Room: 208 |
Sunday, November 22, 2015 2:10PM - 2:23PM |
D20.00001: Subgrid-scale backscatter after the shock-turbulence interaction Daniel Livescu, Zhaorui Li The interaction of a shock wave with isotropic turbulence (IT) represents a basic problem for studying some of the phenomena associated with high speed flows, such as hypersonic flight, supersonic combustion and Inertial Confinement Fusion (ICF). In many practical applications, the shock width is much smaller than the turbulence scales and the upstream turbulent Mach number is modest. In this case, recent high resolution shock-resolved Direct Numerical Simulations (DNS) (Ryu and Livescu, J. Fluid Mech., 756, R1, 2014) show that the interaction can be described by the Linear Interaction Approximation (LIA). By using LIA to alleviate the need to solve the shock, DNS post-shock data can be generated at much higher Reynolds and shock Mach numbers numbers than previously possible. Here, such results are used to investigate the properties of the subgrid scales (SGS). In particular, it is shown that the shock interaction decreases the asymmetry of the SGS dissipation PDF as the shock Mach number increases, with a significant enhancement in size of the regions and magnitude of backscatter. [Preview Abstract] |
Sunday, November 22, 2015 2:23PM - 2:36PM |
D20.00002: Turbulence generation through intense localized sources of energy Agustin Maqui, Diego Donzis Mechanisms to generate turbulence in controlled conditions have been studied for nearly a century. Most common methods include passive and active grids with a focus on incompressible turbulence. However, little attention has been given to compressible flows, and even less to hypersonic flows, where phenomena such as thermal non-equilibrium can be present. Using intense energy from lasers, extreme molecule velocities can be generated from photo-dissociation. This creates strong localized changes in both the hydrodynamics and thermodynamics of the flow, which may perturb the flow in a way similar to an active grid to generate turbulence in hypersonic flows. A large database of direct numerical simulations (DNS) are used to study the feasibility of such an approach. An extensive analysis of single and two point statistics, as well as spectral dynamics is used to characterize the evolution of the flow towards realistic turbulence. Local measures of enstrophy and dissipation are studied to diagnose the main mechanisms for energy exchange. As commonly done in compressible flows, dilatational and solenoidal components are separated to understand the effect of acoustics on the development of turbulence. Further results for cases that assimilate laboratory conditions will be discussed. [Preview Abstract] |
Sunday, November 22, 2015 2:36PM - 2:49PM |
D20.00003: Shock-resolving direct numerical simulations of strong turbulence interacting with a normal shock wave Chang-Hsin Chen, Diego Donzis In many natural and engineering systems, turbulence is found to interact with shock waves. Thus, canonical interactions between isotropic turbulence and a normal shock have been studied extensively, theoretically and numerically, though theories assume the shock to be a discontinuity and most simulations have used shock-capturing schemes which may miss details of the structure of the shock, especially for weak shocks in relatively strong turbulence. We present results on this regime from shock-resolving direct numerical simulations at a range of Reynolds and Mach numbers. Our focus is on the shock structure and the effect on turbulence downstream of the shock. We study the distribution of velocity gradients, in particular dilatation across the shock and compare with theory available. We characterize turbulent shock jumps which are found to depart from the laminar theory as they depend not only on the mean Mach number but also on the Reynolds and turbulent Mach number. Changes experienced by thermodynamic variables across the shock will also be discussed. [Preview Abstract] |
(Author Not Attending)
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D20.00004: Lagrangian Statistics of Velocity-gradient in Compressible Turbulence Mohammad Danish, Sawan Suman, Balaji Srinivasan The Lagrangian-based analysis of various flow quantities, in particular the velocity-gradient tensor, has been a cornerstone in the study of turbulence. The physics of many important turbulence processes such as cascading, scalar mixing, material element deformation etc. can be explained in terms of the dynamics of velocity-gradient tensor itself. In this context, we present the Lagrangian statistics of the invariants of the velocity-gradient tensor over a wide range of Mach and Reynolds numbers in compressible turbulence. For this purpose, we track a large number of fluid particles in well resolved direct Navier-Stokes (DNS) simulations of decaying compressible turbulence. We show that these statistics significantly depend on the existence of shocklets. Specifically, we observe that the presence of shocklets tends to increase the decorrelation rate of Lagrangian autocorrelations of velocity-gradient tensor. [Preview Abstract] |
Sunday, November 22, 2015 3:02PM - 3:15PM |
D20.00005: Bulk viscosity effect on freely decaying compressible homogeneous isotropic turbulence Shaowu Pan, Eric Johnsen Despite growing interests in compressible turbulence, the effect of bulk viscosity has been long ignored. For certain gases, the bulk viscosity may be 1000 times greater than the shear viscosity and thus modify energy transfer and dissipation mechanisms. In this study, we use direct numerical simulations to investigate the role of bulk viscosity on decaying isotropic compressible turbulence. Our results show that bulk viscosity exhibits a negligible decrease on enstrophy, but moderate and significant increases on the turbulent kinetic energy and Taylor-scale Reynolds number, respectively. A Helmholtz decomposition of the velocity field indicates that the bulk viscosity has a negligible effect on the solenoidal part, but exhibits a cross-scale effect on the dilatational component. [Preview Abstract] |
Sunday, November 22, 2015 3:15PM - 3:28PM |
D20.00006: A Mesoscopic Model for the Description of Small-scale Inhomogeneity in Turbulent Flows with Thermal Nonequilibrium Venkat Raman, Romain Fievet, Peter Clarke, Philip Varghese Turbulent mixing of nonequilibrium flows exhibit certain anamolous properties as compared to equilibrium flows. In high-speed flows where the vibrational or rotational relaxation occurs over time and length-scales comparable to flow-through times, there a unique non-homogeneity introduced at the small-scales: The molecules are spatially homogeneous but their internal motions still exhibit inhomogeneity. This differential relaxation rate poses hurdles in the direct numerical simulation of turbulent nonequilibrium flows. In this work, a mesoscopic description of this homogeneity is introduced. Using a probabilistic model, the inhomogeneous vibrational modes are tracked as a function of space and time. Further, direct numerical solutions of the extended Boltzmann equation are used to develop mesoscopic closure models for this stochastic description. Simulations of representative scramjet isolators are used to illustrate the applicability and relevance of this mathematical description. [Preview Abstract] |
Sunday, November 22, 2015 3:28PM - 3:41PM |
D20.00007: Compressible turbulent mixing: Effects of compressibility and Schmidt number Qionglin Ni Effects of compressibility and Schmidt number on passive scalar in compressible turbulence were studied. On the effect of compressibility, the scalar spectrum followed the k$^{\mathrm{-5/3}}$ inertial-range scaling and suffered negligible influence from compressibility. The transfer of scalar flux was reduced by the transition from incompressible to compressible flows, however, was enhanced by the growth of Mach number. The intermittency parameter was increased by the growth of Mach number, and was decreased by the growth of the compressive mode of driven forcing. The dependency of the mixing timescale on compressibility showed that for the driven forcing, the compressive mode was less efficient in enhancing scalar mixing. On the effect of Schmidt number (Sc), in the inertial-convective range the scalar spectrum obeyed the k$^{\mathrm{-5/3}}$ scaling. For Sc\textgreater \textgreater 1, a k$^{\mathrm{-1}}$ power law appeared in the viscous-convective range, while for Sc\textless \textless 1, a k$^{\mathrm{-17/3}}$ power law was identified in the inertial-diffusive range. The transfer of scalar flux grew over Sc. In the Sc\textgreater \textgreater 1 flow the scalar field rolled up and mixed sufficiently, while in the Sc\textless \textless 1 flow that only had the large-scale, cloudlike structures. In Sc\textgreater \textgreater 1 and Sc\textless \textless 1 flows, the spectral densities of scalar advection and dissipation followed the k$^{\mathrm{-5/3}}$ scaling, indicating that in compressible turbulence the processes of advection and dissipation might deferring to the Kolmogorov picture. Finally, the comparison with incompressible results showed that the scalar in compressible turbulence lacked a conspicuous bump structure in its spectrum, and was more intermittent in the dissipative range. [Preview Abstract] |
Sunday, November 22, 2015 3:41PM - 3:54PM |
D20.00008: On the effect of finite-time correlations on the turbulent mixing in smooth chaotic compressible velocity fields Siim Ainsaar, Jaan Kalda For incompressible flows, most theoretical studies about turbulent mixing have used the Kraichnan model where the velocity field has zero correlation time. Most of their predictions are derived through (the ratios of) two sets of parameters: Lyapunov exponents (LEs), and their ``diffusivities'' (defined as the asymptotic values of $t{\rm Var}(\Lambda)$; $\Lambda$ is a finite-time LE for time $t$). However, for compressible flows, there is a serious mismatch between the theoretical predictions for these parameters, and both simulations and experiments. We present a simple theoretical model that derives the LEs and their ``diffusivities'' from basic statistics of the velocity gradient tensor $\nabla\bf v$. For finite correlation times, there is a breakdown of universality: the ratios of these parameters do not depend only on the flow compressibility and the correlation time, but also on the determinant of $\nabla\bf v$ - a parameter discussed very sparsely, so far. Our model is in a good agreement with previously unexplained studies regarding the role of finite time correlations [G. Boffetta et al, 2004]. Our mapping from the statistics of $\nabla\bf v$ to the LEs and their ``diffusivities'' extends a wide range of existing analytical ``Kraichnanian'' results to real time-correlated flows. [Preview Abstract] |
Sunday, November 22, 2015 3:54PM - 4:07PM |
D20.00009: Dynamics of Strongly Compressible Turbulence Colin Towery, Alexei Poludnenko, Peter Hamlington Strongly compressible turbulence, wherein the turbulent velocity fluctuations directly generate compression effects, plays a critical role in many important scientific and engineering problems of interest today, for instance in the processes of stellar formation and also hypersonic vehicle design. This turbulence is very unusual in comparison to ``normal,'' weakly compressible and incompressible turbulence, which is relatively well understood. Strongly compressible turbulence is characterized by large variations in the thermodynamic state of the fluid in space and time, including excited acoustic modes, strong, localized shock and rarefaction structures, and rapid heating due to viscous dissipation. The exact nature of these thermo-fluid dynamics has yet to be discerned, which greatly limits the ability of current computational engineering models to successfully treat these problems. New direct numerical simulation (DNS) results of strongly compressible isotropic turbulence will be presented along with a framework for characterizing and evaluating compressible turbulence dynamics and a connection will be made between the present diagnostic analysis and the validation of engineering turbulence models. [Preview Abstract] |
Sunday, November 22, 2015 4:07PM - 4:20PM |
D20.00010: LES prediction and analysis of the aero-optical environment around a 3-D turret Edwin Mathews, Kan Wang, Meng Wang, Eric Jumper Using wall-modeled large-eddy simulation, a Mach 0.4 flow over a hemisphere-on-cylinder turret at the experimental Reynolds number of $Re_D = 2.3 \times 10^6$ is simulated to study the aero-optical distortions caused by turbulent density fluctuations. The optical distortions are calculated at over 250 viewing angles during the simulation to thoroughly investigate the optical environment around the turret. Flow field and optical results show good comparisons with experimental measurements. A large database of three-dimensional velocity and density fields is generated for study of the connection between global flow dynamics and local optical distortions. Proper orthogonal decomposition and dynamic mode decomposition are applied to both the distorted wavefronts and the flow-field database. A method of reconstructing the optical wavefronts from the density field modes is investigated. Relations between prominent flow features and wavefront components including tip/tilt and higher-order effects will be discussed. [Preview Abstract] |
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