Showing papers by "Parviz Moin published in 1991"

A dynamic subgrid‐scale eddy viscosity model

Abstract: One major drawback of the eddy viscosity subgrid‐scale stress models used in large‐eddy simulations is their inability to represent correctly with a single universal constant different turbulent fields in rotating or sheared flows, near solid walls, or in transitional regimes. In the present work a new eddy viscosity model is presented which alleviates many of these drawbacks. The model coefficient is computed dynamically as the calculation progresses rather than input a priori. The model is based on an algebraic identity between the subgrid‐scale stresses at two different filtered levels and the resolved turbulent stresses. The subgrid‐scale stresses obtained using the proposed model vanish in laminar flow and at a solid boundary, and have the correct asymptotic behavior in the near‐wall region of a turbulent boundary layer. The results of large‐eddy simulations of transitional and turbulent channel flow that use the proposed model are in good agreement with the direct simulation data.

A dynamic subgrid‐scale model for compressible turbulence and scalar transport

Abstract: The dynamic subgrid-scale (SGS) model of Germano et al. (1991) is generalized for the large eddy simulation (LES) of compressible flows and transport of a scalar. The model was applied to the LES of decaying isotropic turbulence, and the results are in excellent agreement with experimental data and direct numerical simulations. The expression for the SGS turbulent Prandtl number was evaluated using direct numerical simulation (DNS) data in isotropic turbulence, homogeneous shear flow, and turbulent channel flow. The qualitative behavior of the model for turbulent Prandtl number and its dependence on molecular Prandtl number, direction of scalar gradient, and distance from the wall are in accordance with the total turbulent Prandtl number from the DNS data.

The minimal flow unit in near-wall turbulence

Abstract: Direct numerical simulations of unsteady channel flow were performed at low to moderate Reynolds numbers on computational boxes chosen small enough so that the flow consists of a doubly periodic (in x and z) array of identical structures. The goal is to isolate the basic flow unit, to study its morphology and dynamics, and to evaluate its contribution to turbulence in fully developed channels. For boxes wider than approximately 100 wall units in the spanwise direction, the flow is turbulent and the low-order turbulence statistics are in good agreement with experiments in the near-wall region. For a narrow range of widths below that threshold, the flow near only one wall remains turbulent, but its statistics are still in fairly good agreement with experimental data when scaled with the local wall stress. For narrower boxes only laminar solutions are found. In all cases, the elementary box contains a single low-velocity streak, consisting of a longitudinal strip on which a thin layer of spanwise vorticity is lifted away from the wall. A fundamental period of intermittency for the regeneration of turbulence is identified, and that process is observed to consist of the wrapping of the wall-layer vorticity around a single inclined longitudinal vortex.

Subgrid-scale backscatter in turbulent and transitional flows

Abstract: Most subgrid‐scale (SGS) models for large‐eddy simulations (LES) are absolutely dissipative (that is, they remove energy from the large scales at each point in the physical space). The actual SGS stresses, however, may transfer energy to the large scales (backscatter) at a given location. Recent work on the LES of transitional flows [Piomelli et al., Phys. Fluids A 2, 257 (1990)] has shown that failure to account for this phenomenon can cause inaccurate prediction of the growth of the perturbations. Direct numerical simulations of transitional and turbulent channel flow and compressible isotropic turbulence are used to study the backscatter phenomenon. In all flows considered roughly 50% of the grid points were experiencing backscatter when a Fourier cutoff filter was used. The backscatter fraction was less with a Gaussian filter, and intermediate with a box filter in physical space. Moreover, the backscatter and forward scatter contributions to the SGS dissipation were comparable, and each was often much...

Eddy shocklets in decaying compressible turbulence

Abstract: The existence of eddy shocklets in three‐dimensional compressible turbulence is controversial. To investigate the occurrence of eddy shocklets, numerical simulations of temporally decaying isotropic turbulence are conducted. Dilatation statistics from simulations with different initial fluctuation Mach numbers, Mt, show that dilatation is more intermittent and more negatively skewed for higher Mt. By studying instantaneous flow fields, shocklets are found and verified to have all the characteristics of a typical shock wave, such as proper jumps in pressure and density along with a local entropy peak inside the high‐compression zone. Although overall compressible dissipation contributes to less than one‐tenth of the total dissipation, compressible dissipation around shocklets is about an order of magnitude larger than typical values of incompressible dissipation. In the zones of eddy shocklets, pressure is highly correlated with dilatation to convert kinetic energy into internal energy. These mechanisms ne...

The free compressible viscous vortex

Abstract: The present study investigates the effects of compressibility on free (unsteady) viscous heat-conducting vortices. Analytical solutions are found in the limit of large but finite Reynolds number and small but finite Mach number. It is shown that the spreading of the vortex causes a radial flow. This flow is given by the solution of an ordinary differential equation, which gives the dependence of the radial velocity on the tangential velocity, density, and temperature profiles of the vortex. Estimates of the radial velocity found by solving this equation are found to be in good agreement with numerical solutions of the full equations. The equations for the viscous evolution are expanded in powers of Mach number to obtain detailed analytical solutions. It is shown that swirling axisymmetric compressible flows generate negative radial velocities far from the vortex core owing to viscous effects, regardless of the initial distributions of vorticity, density, and entropy.

Direct numerical simulation of transition and turbulence in a spatially evolving boundary layer

Abstract: A high-order-accurate finite-difference approach to direct simulations of transition and turbulence in compressible flows is described. Attention is given to the high-free-stream disturbance case in which transition to turbulence occurs close to the leading edge. In effect, computation requirements are reduced. A method for numerically generating free-stream disturbances is presented.

On the effect of riblets in fully developed laminar channel flows

Abstract: The effect of longitudinal riblet surfaces on viscous drag in fully developed laminar channel flows was investigated. Unlike turbulent flows, drag reduction was not obtained in the laminar flows. Results were independent of Reynolds number. Wall‐shear rates on most regions of the cross‐sectional perimeter of riblets were smaller than that of corresponding plane channel flow even though the net drag was increased.

Direct Finite-Difference Simulations Of Turbulent Flow

Abstract: Report discusses use of upwind-biased finite-difference numerical-integration scheme to simulate evolution of small disturbances and fully developed turbulence in three-dimensional flow of viscous, incompressible fluid in channel. Involves use of computational grid sufficiently fine to resolve motion of fluid at all relevant length scales.

Direct numerical simulation and analysis of shock turbulence interaction

Abstract: Two kinds of linear analysis, rapid distortion theory (RDT) and linear interaction analysis (LIA), were used to investigate the effects of a shock wave on turbulence. Direct numerical simulations of two-dimensional isotropic turbulence interaction with a normal shock were also performed. The results from RDT and LIA are in good agreement for weak shock waves, where the effects of shock front curvature and shock front unsteadiness are not significant in producing vorticity. The linear analyses predict wavenumber-dependent amplification of the upstream one-dimensional energy spectrum, leading to turbulence scale length scale decrease through the interaction. Instantaneous vorticity fields show that vortical structures are enhanced while they are compressed in the shock normal direction. Entrophy amplfication through the shock wave compares favorably with the results of linear analyses.

Large eddy simulation of the flow in a transpired channel

Abstract: The flow in a transpired channel has been computed by large eddy simulation. The numerical results compare very well with experimental data. Blowing decreases the wall shear stress and enhances turbulent fluctuations, while suction has the opposite effect. The wall layer thickness normalized by the local wall shear velocity and kinematic viscosity increases on the blowing side of the channel and decreases on the suction side. Suction causes more rapid decay of the spectra, larger mean streak spacing and higher two-point correlations. On the blowing side, the wall layer structures lie at a steeper angle to the wall, whereas on the suction side this angle is shallower.

Scattering of sound waves by a compressible vortex

Abstract: Scattering of plane sound waves by a compressible vortex is investigated by direct computation of the two-dimensional Navier-Stokes equations. Nonreflecting boundary conditions are utilized, and their accuracy is established by comparing results on different sized domains. Scattered waves are directly measured from the computations. The resulting amplitude and directivity pattern of the scattered waves is discussed, and compared to various theoretical predictions. For compact vortices (zero circulation), the scattered waves directly computed are in good agreement with predictions based on an acoustic analogy. Strong scattering at about + or - 30 degrees from the direction of incident wave propagation is observed. Back scattering is an order of magnitude smaller than forward scattering. For vortices with finite circulation refraction of the sound by the mean flow field outside the vortex core is found to be important in determining the amplitude and directivity of the scattered wave field.

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

Advances and Some Novel Experiments Using Direct Numerical Simulation of Turbulence

Abstract: Recent advances in direct numerical simulation methodology are reviewed. These include numerical methods for complex geometries, boundary conditions at the open boundaries, and applications to compressible flows. Some instructive numerical experiments that would be difficult to perform in the laboratory are described. The primary example is the isolation of the minimal flow unit near the wall and its application to the study of the flow over riblets, three-dimensional boundary layers, and active control of turbulent boundary layers.