Showing papers by "Parviz Moin published in 1999"

Study of flow in a planar asymmetric diffuser using large-eddy simulation

Abstract: Large-eddy simulation (LES) has been used to study the flow in a planar asymmetric diffuser The wide range of spatial and temporal scales, the presence of an adverse pressure gradient, and the formation of an unsteady separation bubble in the rear part of the diffuser make this flow a challenging test case for assessing the predictive capability of LES Simulation results for mean flow, pressure recovery and skin friction are in excellent agreement with data from two recent experiments The inflow consists of a fully developed turbulent channel flow at a Reynolds number based on shear velocity, Reτ = 500 It is found that accurate representation of the inflow velocity field is critical for accurate prediction of the flow in the diffuser Although the simulation in the diffuser is well resolved, the subgrid-scale model plays a significant role for both mean momentum and turbulent kinetic energy balances Subgrid-scale stresses contribute a maximum of 8% to the local value of the total shear stress with the maximum values found in the inlet duct and along the flat wall where the flow remains attached The subgrid-scale model adapts to the enhanced turbulence levels in the rear part of the diffuser by providing more than 80% of the dissipation rate for turbulent kinetic energy The unsteady separation excites large scales of motion which extend over the major part of the duct cross-section and penetrate deeply into the core of the flow Instantaneous flow reversal is observed along both walls immediately behind the diffuser throat which is far upstream of the location of main separation While the mean flow profile changes gradually as the flow enters the expansion, turbulent stresses undergo rapid changes over a short streamwise distance along the deflected wall An explanation is offered which considers the strain field as well as the influence of geometry changes The effect of grid resolution and spanwise domain size on the flow field prediction has been documented and this allows an assessment of the computational requirements for carrying out such simulations

Direct computation of the sound generated by vortex pairing in an axisymmetric jet

Abstract: The sound generated by vortex pairing in axisymmetric jets is determined by direct solution of the compressible Navier–Stokes equations on a computational grid that includes both the near field and a portion of the acoustic far field. At low Mach number, the far-field sound has distinct angles of extinction in the range of 60°–70° from the jet's downstream axis which can be understood by analogy to axisymmetric, compact quadrupoles. As the Mach number is increased, the far-field sound takes on a superdirective character with the dominant sound directed at shallow angles to the jet's downstream axis. The directly computed sound is compared to predictions obtained from Lighthill's equation and the Kirchhoff surface method. These predictions are in good agreement with the directly computed data. The Lighthill source terms have a large spatial distribution in the axial direction necessitating the introduction of a model to describe the source terms in the region downstream of the last vortex pairing. The axial non-compactness of the quadrupole sources must be adequately treated in the prediction method.

Space–time characteristics of the wall shear-stress fluctuations in a low-Reynolds-number channel flow

Abstract: A database obtained by direct numerical simulation of turbulent channel flow is used to compute the frequency/wave number spectra of the wall shear-stress fluctuations. Auto- and cross-correlations of the wall shear-stress and pressure fluctuations are obtained in space and time domains. The convection velocities as a function of frequency, wave number, and spatial and temporal separations are calculated. The overall convection velocities of wall shear-stress fluctuations for the most energetic structures are obtained and the validity of Taylor’s hypothesis is confirmed.

Algorithms for shear flow control and optimization

Abstract: We present an outline of our research efforts in the area of flow control and optimization We focus on identifying low order models that can be implemented in active control strategies using (i) our understanding of fundamental physical processes such as vorticity generation and (ii) machine learning and optimization algorithms such as neural networks and evolution strategies The results presented encompass a wide variety of problems, such as drag minimization, neural net modeling of the near wall structures, enhanced jet mixing and parameter optimization in turbine blade film cooling

Large Eddy Simulation of a Coaxial Jet Combustor.

Abstract: : A review of progress in large eddy simulation of complex reacting flows is presented. Topics include: the governing equations for low Mach number combustion, assumed PDF subgrid-scale models, subgrid-scale modeling for the variance and dissipation rate of a conserved scalar, inflow and exit boundary conditions for confined swirling flows, simulation results for isothermal swirling flow with experimental validation, and results from a preliminary reacting flow simulation.

Turbulence simulations on parallel computers

Abstract: Publisher Summary This paper presents a review of the recent turbulent-flow computations on parallel computers performed at the Center for Turbulence Research. Examples of these computations include direct numerical simulation of time-evolving annular mixing layer, a jet diffusion flame, and large eddy simulations of flow over a circular cylinder and flow in a coaxial jet combustor. Parallel algorithms used in the turbulence simulation codes are described. Parallel processing has also been used to enhance the statistical sample available at each time step in turbulence simulations.

Application of large-eddy simulation for trailing-edge noise prediction

Abstract: Turbulent boundary layers near the trailing-edge of a lifting surface are known to generate intense, broadband scattering noise as well as surface pressure fluctuations. To numerically predict the trailing-edge noise requires that the noise-generating eddies over a wide range of length scales be adequately represented. Large-eddy simulation technique provides a promising tool for obtaining the unsteady wall-pressure fields and the acoustic source functions. In the present work, a large-eddy simulation is carried out for turbulent boundary layer flow past an asymmetrically beveled trailing-edge of a flat strut at a chord Reynolds number of 2.15 × 106. The computed velocity and surface pressure statistics compare reasonably well with the experimental measurements of Blake. The far-field acoustic calculation is facilitated by the integral solution to the Lighthill equation derived by Ffowcs Williams & Hall. Computations have been carried out to determine the far-field noise spectra, the source-term characteristics, and the requirement for the integration domain size. It is found that the present LES is adequate for predicting noise radiation over a wide frequency range. At the low frequency end, however, the spanwise source coherence estimated based on surface pressure fluctuations does not decay sufficiently, suggesting the need for a wider computational domain.