Showing papers by "Parviz Moin published in 2000"

Numerical studies of flow over a circular cylinder at ReD=3900

Abstract: Flow over a circular cylinder at Reynolds number 3900 is studied numerically using the technique of large eddy simulation. The computations are carried out with a high-order accurate numerical method based on B-splines and compared with previous upwind-biased and central finite-difference simulations and with the existing experimental data. In the very near wake, all three simulations are in agreement with each other. Farther downstream, the results of the B-spline computations are in better agreement with the hot-wire experiment of Ong and Wallace [Exp. Fluids 20, 441–453 (1996)] than those obtained in the finite-difference simulations. In particular, the power spectra of velocity fluctuations are in excellent agreement with the experimental data. The impact of numerical resolution on the shear layer transition is investigated.

Approximate Wall Boundary Conditions in the Large-Eddy Simulation of High Reynolds Number Flow

Abstract: The near-wall regions of high Reynolds numbers turbulent flows must be modelled to treat many practical engineering and aeronautical applications. In this review we examine results from simulations of both attached and separated flows on coarse grids in which the near-wall regions are not resolved and are instead represented by approximate wall boundary conditions. The simulations use the dynamic Smagorinsky subgrid-scale model and a second-order finite-difference method. Typical results are found to be mixed, with acceptable results found in many cases in the core of the flow far from the walls, provided there is adequate numerical resolution, but with poorer results generally found near the wall. Deficiencies in this approach are caused in part by both inaccuracies in subgrid-scale modelling and numerical errors in the low-order finite-difference method on coarse near-wall grids, which should be taken into account when constructing models and performing large-eddy simulation on coarse grids. A promising new method for developing wall models from optimal control theory is also discussed.

Computation of Trailing-Edge Flow and Noise Using Large-Eddy Simulation

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 e uctuations. Numerically predicting the trailing-edge noise requires that the noise-generating eddies over a wide range of length scales be adequately represented. The large-eddy simulation (LES) technique provides a promising tool for obtaining the unsteady wall-pressure e elds and the acousticsourcefunctions. An LES iscarried out forturbulent boundary-layere ow pastan asymmetrically beveled trailing edge ofa e at strut at a chord Reynolds number of 2 :15 £ 10 6 . The computed velocity and surface pressure statistics compare reasonably well with previous experimental measurements. The far-e eld acoustic calculation is facilitated by the integral solution to the Lighthill equation derived by Ffowcs-Williams and Hall. Computations havebeen carried outto determine thefar-e eld noisespectra,thesource-term characteristics, and therequirement for the integration domain size. It is found that the present LES domain is adequate for predicting noise radiation over a range of frequencies. At the low-frequency end, however, the spanwise source coherenceestimated based on surface pressure e uctuations does not decay sufe ciently, suggesting the need for a wider computational domain.

Compressibility effects in a turbulent annular mixing layer. Part 1. Turbulence and growth rate

Abstract: This work uses direct numerical simulations of time evolving annular mixing layers, which correspond to the early development of round jets, to study compressibility effects on turbulence in free shear flows. Nine cases were considered with convective Mach numbers ranging from Mc = 0.1 to 1.8 and turbulence Mach numbers reaching as high as Mt = 0.8.Growth rates of the simulated mixing layers are suppressed with increasing Mach number as observed experimentally. Also in accord with experiments, the mean velocity difference across the layer is found to be inadequate for scaling most turbulence statistics. An alternative scaling based on the mean velocity difference across a typical large eddy, whose dimension is determined by two-point spatial correlations, is proposed and validated. Analysis of the budget of the streamwise component of Reynolds stress shows how the new scaling is linked to the observed growth rate suppression. Dilatational contributions to the budget of turbulent kinetic energy are found to increase rapidly with Mach number, but remain small even at Mc = 1.8 despite the fact that shocklets are found at high Mach numbers. Flow visualizations show that at low Mach numbers the mixing region is dominated by large azimuthally correlated rollers whereas at high Mach numbers the flow is dominated by small streamwise oriented structures. An acoustic timescale limitation for supersonically deforming eddies is found to be consistent with the observations and scalings and is offered as a possible explanation for the decrease in transverse lengthscale.

Numerical simulation of a Mach 1.92 turbulent jet and its sound field

Abstract: A perfectly expanded turbulent Mach 1.92 jet is simulated by direct numerical solution of the compressible Navier-Stokes equations in a computational domain that includes the near acoustic field. In place of a nozzle, turbulent inflow data were generated in a separate streamwise periodic jet simulation. Reynolds stresses, two-point correlations, and turbulent energy spectra are computed and discussed. The sound field is highly directional and dominated by Mach waves as are commonly observed experimentally. Analysis of the sound using weak-shock theory shows that nonlinear effects are significant away from the jet but that linear theory is sufficient to estimate near-field sound pressure levels. Although no attempt was made to match any particular experiment in detail, sound pressure levels are compared with experimental data at similar flow conditions and are found to agree in general with jets at similar convective Mach numbers.

An evaluation of the assumed beta probability density function subgrid-scale model for large eddy simulation of nonpremixed, turbulent combustion with heat release

Abstract: The assumed beta distribution model for the subgrid-scale probability density function (PDF) of the mixture fraction in large eddy simulation of nonpremixed, turbulent combustion is tested, a priori, for a reacting jet having significant heat release (density ratio of 5). The assumed beta distribution is tested as a model for both the subgrid-scale PDF and the subgrid-scale Favre PDF of the mixture fraction. The beta model is successful in approximating both types of PDF but is slightly more accurate in approximating the normal (non-Favre) PDF. To estimate the subgrid-scale variance of mixture fraction, which is required by the beta model, both a scale similarity model and a dynamic model are used. Predictions using the dynamic model are found to be more accurate. The beta model is used to predict the filtered value of a function chosen to resemble the reaction rate. When no model is used, errors in the predicted value are of the same order as the actual value. The beta model is found to reduce this error by about a factor of two, providing a significant improvement.

Jet Mixing Enhancement by High-Amplitude Fluidic Actuation

Abstract: Recent experiments have shown that properly designed high-amplitude, low mass flux pulsed slot jets blowing normal to a jet's shear layer near the nozzle can significantly alter the jet's development. In contrast to commonly used low-amplitude forcing, this strong excitation appears to overwhelm the turbulence, having nearly the same effect at high and low Reynolds numbers. It can, therefore, be studied in detail by direct numerical simulation. Direct numerical simulations of Mach 0.9, Reynolds number 3.6 × 10 3 jets exhausting into quiescent fluid are conducted. Physically realistic slot jet actuators are included in the simulation by adding localized body-force terms to the governing equations. Three cases are considered in detail: a baseline unforced case and two cases that are forced with flapping modes at Strouhal numbers 0.2 and 0.4. (Sr = 0.4 was found to be the most amplified frequency in the unforced case.) Forcing at either frequency causes the jet to expand rapidly in the plane parallel with the actuators and to contract in the plane perpendicular to the actuators, as observed experimentally

Compressibility effects in a turbulent annular mixing layer. Part 2. Mixing of a passive scalar

Abstract: The mixing of fuel and oxidizer in a mixing layer between high-speed streams is important in many applications, especially air-breathing propulsion systems. The details of this process in a turbulent annular mixing layer are studied with direct numerical simulation. Convective Mach numbers of the simulations range from M c = 0.1 to M c = 1.8. Visualizations of the scalar field show that at low Mach numbers large intrusions of nearly pure ambient or core fluid span the mixing region, whereas at higher Mach numbers these intrusions are suppressed. Increasing the Mach number is found to change the mixture fraction probability density function from non-marching to marching and the mixing efficiency from 0.5 at M c = 0.1 to 0.67 at M c = 1.5. Scalar concentration fluctuations and the axial velocity fluctuations become highly correlated as the Mach number increases and a suppressed role of pressure in the axial momentum equation is found to be responsible for this. Anisotropy of scalar flux increases with M c , and is explained via the suppression of transverse turbulence lengthscale

Numerical Issues in Large Eddy Simulation of Complex Turbulent Flows and Application to Aeroacoustics

Abstract: This paper reports on numerical issues in large eddy simulations that have been identified in recent computations of complex flows and theoretical studies. These issues include the effects of inherent numerical dissipation in upwind spatial finitedifference schemes on turbulent power spectra, and the sensitivity of separated and vortex dominated flow solutions to inflow and outflow boundary conditions. We also describe the results from several large eddy simulations of complex turbulent flows, including those performed with a novel numerical technique based on B-splines and a simulation of turbulent flow past an asymmetric trailing-edge with the objective of predicting the far-field noise.

Construction of Commutative Filters for LES on Unstructured Meshes

Abstract: A method of constructing discrete filters for large eddy simulation of turbulent flows on unstructured meshes is presented. The commutation error between differentiation and filtering can be made arbitrarily small with these filters. The filtering method is applied to various test cases to demonstrate commutation. An extension to three dimensions and implementation into an unstructured solver for LES are discussed.

Real-Time Feedback Control of Mixing in a Heated Jet

Abstract: : This report describes work performed to investigate real-time feedback control of mixing in a heated jet and to demonstrate the resulting technique in a laboratory environment. Large- scale direct numerical simulations were performed for jets with various types of actuation and were useful for optimization of the actuators with regard to Strouhal number and amplitude. The integral of the squared velocity and the integral of the concentration were found to be appropriate objective functions for evaluating the performance of a given actuator. In the simulation effort evolution strategies and simulated annealing were used for optimization of the jet actuators. In an experimental study of a jet using a miniature (RAM 750) engine, use of a genetic algorithm showed that it could drive the system to undesirable states during optimization. A classical proportional-integral controller was used to provide real-time feedback control of plume temperature.