Showing papers by "Parviz Moin published in 1992"

Simulation of spatially evolving turbulence and the applicability of Taylor's hypothesis in compressible flow

Abstract: For the numerical simulation of inhomogeneous turbulent flows, a method is developed for generating stochastic inflow boundary conditions with a prescribed power spectrum. Turbulence statistics from spatial simulations using this method with a low fluctuation Mach number are in excellent agreement with the experimental data, which validates the procedure. Turbulence statistics from spatial simulations are also compared to those from temporal simulations using Taylor’s hypothesis. Statistics such as turbulence intensity, vorticity, and velocity derivative skewness compare favorably with the temporal simulation. However, the statistics of dilatation show a significant departure from those obtained in the temporal simulation. To directly check the applicability of Taylor’s hypothesis, space‐time correlations of fluctuations in velocity, vorticity, and dilatation are investigated. Convection velocities based on vorticity and velocity fluctuations are computed as functions of the spatial and temporal separations. The profile of the space‐time correlation of dilatation fluctuations is explained via a wave propagation model.

The dimension of attractors underlying periodic turbulent Poiseuille flow

Abstract: A lower bound on the Liapunov dimenison, D-lambda, of the attractor underlying turbulent, periodic Poiseuille flow at a pressure-gradient Reynolds number of 3200 is calculated, on the basis of a coarse-grained (16x33x8) numerical solution, to be approximately 352. Comparison of Liapunov exponent spectra from this and a higher-resolution (16x33x16) simulation on the same spatial domain shows these spectra to have a universal shape when properly scaled. On the basis of these scaling properties, and a partial exponent spectrum from a still higher-resolution (32x33x32) simulation, it is argued that the actual dimension of the attractor underlying motion of the given computational domain is approximately 780. It is suggested that this periodic turbulent shear flow is deterministic chaos, and that a strange attractor does underly solutions to the Navier-Stokes equations in such flows.

Interaction of Isotropic Turbulence with a Shock Wave

Abstract: : As a first step to understand the compressibility effects, interaction of isotropic quasi-incompressible turbulence with a weak shock wave was studied by three-dimensional time-dependent direct numerical simulations. In addition, linear analysis was used to study interaction of isotropic turbulence with shock waves of a wide range of strengths. The effects of the fluctuation Mach number M sub t and the average Mach number M sub 1 superscript u of the upstream turbulence on turbulence statistics were investigated. Both numerical simulations and linear analyses of the interaction show that turbulence is enhanced during the interaction with a shock wave. Turbulent kinetic energy (TKE) and transverse vorticity components are amplified, and turbulent length scales are decreased. The predictions of the linear analyses compare favorably with simulation results for flows with M sub t < M sub 1 superscript U-1, which suggests that the amplification mechanism is mainly linear.

Search for subgrid scale parameterization by projection pursuit regression

Abstract: The dependence of subgrid-scale stresses on variables of the resolved field is studied using direct numerical simulations of isotropic turbulence, homogeneous shear flow, and channel flow. The projection pursuit algorithm, a promising new regression tool for high-dimensional data, is used to systematically search through a large collection of resolved variables, such as components of the strain rate, vorticity, velocity gradients at neighboring grid points, etc. For the case of isotropic turbulence, the search algorithm recovers the linear dependence on the rate of strain (which is necessary to transfer energy to subgrid scales) but is unable to determine any other more complex relationship. For shear flows, however, new systematic relations beyond eddy viscosity are found. For the homogeneous shear flow, the results suggest that products of the mean rotation rate tensor with both the fluctuating strain rate and fluctuating rotation rate tensors are important quantities in parameterizing the subgrid-scale stresses. A model incorporating these terms is proposed. When evaluated with direct numerical simulation data, this model significantly increases the correlation between the modeled and exact stresses, as compared with the Smagorinsky model. In the case of channel flow, the stresses are found to correlate with products of the fluctuating strain and rotation rate tensors. The mean rates of rotation or strain do not appear to be important in this case, and the model determined for homogeneous shear flow does not perform well when tested with channel flow data. Many questions remain about the physical mechanisms underlying these findings, about possible Reynolds number dependence, and, given the low level of correlations, about their impact on modeling. Nevertheless, demonstration of the existence of causal relations between sgs stresses and large-scale characteristics of turbulent shear flows, in addition to those necessary for energy transfer, provides important insight into the relation between scales in turbulent flows.

Direct computation of the sound from a compressible co-rotating vortex pair

Abstract: The far-field sound from corotating vortices is computed by direct computation of the unsteady, compressible Navier-Stokes equations on a computational mesh that extends to two acoustic wavelengths in all directions. The vortices undergo a period of corotation followed by a sudden merger. A 2D version of Moehring's equation is developed and used in conjunction with source terms computed in the simulation to predict the far-field sound. The prediction agrees with the simulation to within 3 percent. Results of far-field pressure fluctuations for an acoustically noncompact case are also presented for which the prediction is 66 percent too high. Results also indicate that the monopole contribution of 'viscous sound' is negligible for this flow.

Numerical study of axial turbulent flow over long cylinders

Abstract: The effects of transverse curvature are investigated by means of direct numerical simulations of turbulent axial flow over cylinders. Two cases of Reynolds number of about 3400 and layer-thickness-to-cylinder-radius ratios of 5 and 11 were simulated. All essential turbulence scales were resolved in both calculations, and a large number of turbulence statistics were computed. The results are compared with the plane channel results of Kim et al. (1987) and with experiments. With transverse curvature the skin friction coefficient increases and the turbulence statistics, when scaled with wall units, are lower than in the plane channel. The momentum equation provides a scaling that collapses the cylinder statistics, and allows the results to be interpreted in light of the plane channel flow. The azimuthal and radial length scales of the structures in the flow are of the order of the cylinder diameter. Boomerang-shaped structures with large spanwise length scales were observed in the flow.

Compressible Turbulence and Shock Waves

Abstract: If the velocity scale associated with the turbulent fluctuations is a substantial fraction of the mean speed of sound, compressibility effects are expected to arise. This direct effect of compressibility, which is also the most well documented, appears to be responsible for the reduced spreading rate of turbulent mixing layers under compressible conditions, and the slower decay of hypersonic wakes. A different circumstance where the compressibility effects can become significant occurs when an otherwise quasi-incompressible turbulent flow is subjected to a rapidly changing environment. For example a turbulent flow passing through a shock wave or a steep expansion wave may suddenly enter in a state of disequilibrium. The sudden change may also generate significant fluctuations of acoustic and entropy modes. Thus the recovery of the turbulence from this disequilibrium may also exhibit compressibility effects. Other phenomena where the global behavior of the flow is strongly affected by compressibility include shock induced separations of a turbulent flow, phenomena involving the Ranque-Hilsh effect and other effects requiring chemical or thermodynamic nonequilibrium. The problem of shock wave boundary layer interaction in a hypersonic free-stream involves all of the above processes.

Active Turbulence Control in Wall Bounded Flows Using Direct Numerical Simulation

Abstract: : The objective of the study is to explore concepts for control of turbulent boundary layers leading to skin-friction reduction using the direct numerical simulation technique. This report is divided into three parts where different control methods are investigated: an active control by sensing and perturbing structures near the wall, a feedback control procedure guided by control theory, and a passive control by longitudinal riblets. Part I: significant drag reduction is achieved when the surface boundary condition is modified to suppress the dynamically significant coherent structures present in the wall region. Part II: mathematical methods of control theory are applied to the problem of control of fluid flow. Part III: direct numerical simulation is performed to analyze turbulent flow over longitudinal riblets, and to educe the mechanism of drag reduction by riblets. TF-55 'Turbulent Drag Reduction: Studies of Feedback Control and Flow Over Riblets' by Haecheon Choi, Parviz Moin and John Kim.... Turbulent drag reduction, Feedback control, Passive control (Riblet).