Showing papers by "Parviz Moin published in 1997"
TL;DR: In this paper, a simulation of the Navier-Stokes equations of a backward-facing step flow was performed at a Reynolds number of 5100 based on the step height h and inlet free-stream velocity.
Abstract: Turbulent flow over a backward-facing step is studied by direct numerical solution of the Navier–Stokes equations. The simulation was conducted at a Reynolds number of 5100 based on the step height h and inlet free-stream velocity, and an expansion ratio of 1.20. Temporal behaviour of spanwise-averaged pressure fluctuation contours and reattachment length show evidence of an approximate periodic behaviour of the free shear layer with a Strouhal number of 0.06. The instantaneous velocity fields indicate that the reattachment location varies in the spanwise direction, and oscillates about a mean value of 6.28h. Statistical results show excellent agreement with experimental data by Jovic & Driver (1994). Of interest are two observations not previously reported for the backward-facing step flow: (a) at the relatively low Reynolds number considered, large negative skin friction is seen in the recirculation region; the peak |Cf| is about 2.5 times the value measured in experiments at high Reynolds numbers; (b) the velocity profiles in the recovery region fall below the universal log-law. The deviation of the velocity profile from the log-law indicates that the turbulent boundary layer is not fully recovered at 20 step heights behind the separation.The budgets of all Reynolds stress components have been computed. The turbulent kinetic energy budget in the recirculation region is similar to that of a turbulent mixing layer. The turbulent transport term makes a significant contribution to the budget and the peak dissipation is about 60% of the peak production. The velocity–pressure gradient correlation and viscous diffusion are negligible in the shear layer, but both are significant in the near-wall region. This trend is seen throughout the recirculation and reattachment region. In the recovery region, the budgets show that effects of the free shear layer are still present.
1,076 citations
TL;DR: In this paper, it was shown that discrepancies between the results of dealiased spectral and standard nondialiased finite-difference methods are due to both aliasing and truncation errors with the latter being the leading source of differences.
564 citations
TL;DR: In this paper, the authors simulated flow past a circular cylinder at a Reynolds number of 3.9 X 10 3 using a solver that employs an energy-conservative second-order central difference scheme for spatial discretization.
Abstract: We have simulated flow past a circular cylinder at a Reynolds number of 3.9 X 10 3 using a solver that employs an energy-conservative second-order central difference scheme for spatial discretization. Detailed comparisons of turbulence statistics and energy spectra in the downstream wake region (7.0 < x/D < 10.0) have been made with the results of Beaudan and Moin and with experiments to assess the impact of numerical diffusion on the flowfield. Based on these comparisons, conclusions are drawn on the suitability of higher-order upwind schemes for LES in complex geometries.
397 citations
TL;DR: The sound generated by vortex pairing in a two-dimensional compressible mixing layer is investigated in this article, where direct numerical simulations of the Navier-Stokes equations are used to compute both the near-field region and a portion of the acoustic field.
Abstract: The sound generated by vortex pairing in a two-dimensional compressible mixing layer is investigated Direct numerical simulations (DNS) of the Navier-Stokes equations are used to compute both the near-field region and a portion of the acoustic field The acoustic analogy due to Lilley (1974) is also solved with acoustic sources determined from the near-field data of the DNS It is shown that several commonly made simplifications to the acoustic sources can lead to erroneous predictions for the acoustic field Predictions based on the quadrupole form of the source terms derived by Goldstein (1976a, 1984) are in excellent agreement with the acoustic field from the DNS However, despite the low Mach number of the flow, the acoustic far field generated by the vortex pairings cannot be described by considering compact quadrupole sources The acoustic sources have the form of modulated wave packets and the acoustic far field is described by a superdirective model (Crighton & Huerre 1990) The presence of flow-acoustic interactions in the computed source terms causes the acoustic field predicted by the acoustic analogy to be very sensitive to small changes in the description of the source
368 citations
TL;DR: In this article, a shock-capturing scheme was developed to accurately simulate the unsteady interaction of vortical turbulence with shock waves, and an existing controversy between experiments and theoretical predictions on length scale change was thoroughly investigated through the shockcapturing simulation.
Abstract: As an extension of the authors' work on isotropic vortical turbulence interacting with a shock wave (Lee, Lele & Moin 1993), direct numerical simulation and linear analysis are performed for stronger shock waves to investigate the effects of the upstream shock-normal Mach number (M1). A shock-capturing scheme is developed to accurately simulate the unsteady interaction of turbulence with shock waves. Turbulence kinetic energy is amplified across the shock wave, and this amplification tends to saturate beyond M1 = 3.0. An existing controversy between experiments and theoretical predictions on length scale change is thoroughly investigated through the shock-capturing simulation: most turbulence length scales decrease across the shock, while the dissipation length scale (ρq3/e) increases slightly for shock waves with M1<1.65. Fluctuations in thermodynamic variables behind the shock wave are nearly isentropic for M1<1.2, and deviate significantly from isentropy for the stronger shock waves, due to the entropy fluctuation generated through the interaction.
261 citations
TL;DR: In this article, direct numerical simulation and inviscid linear analysis are used to study the interaction of a normal shock wave with an isotropic turbulent field of vorticity and entropy fluctuations.
Abstract: Direct numerical simulation and inviscid linear analysis are used to study the interaction of a normal shock wave with an isotropic turbulent field of vorticity and entropy fluctuations. The role of the upstream entropy fluctuations is emphasized. The upstream correlation between the vorticity and entropy fluctuations is shown to strongly influence the evolution of the turbulence across the shock. Negative upstream correlation between u′ and T′ is seen to enhance the amplification of the turbulence kinetic energy, vorticity and thermodynamic fluctuations across the shock wave. Positive upstream correlation has a suppressing effect. An explanation based on the relative effects of bulk compression and baroclinic torque is proposed, and a scaling law is derived for the evolution of vorticity fluctuations across the shock. The validity of Morkovin's hypothesis across a shock wave is examined. Linear analysis is used to suggest that shock-front oscillation would invalidate the relation between urms and Trms, as expressed by the hypothesis.
207 citations
193 citations
Journal Article•
TL;DR: In this paper, direct numerical simulation and inviscid linear analysis are used to study the interaction of a normal shock wave with an isotropic turbulent field of vorticity and entropy fluctuations.
Abstract: Direct numerical simulation and inviscid linear analysis are used to study the interaction of a normal shock wave with an isotropic turbulent field of vorticity and entropy fluctuations. The role of the upstream entropy fluctuations is emphasized. The upstream correlation between the vorticity and entropy fluctuations is shown to strongly influence the evolution of the turbulence across the shock. Negative upstream correlation between u′ and T′ is seen to enhance the amplification of the turbulence kinetic energy, vorticity and thermodynamic fluctuations across the shock wave. Positive upstream correlation has a suppressing effect. An explanation based on the relative effects of bulk compression and baroclinic torque is proposed, and a scaling law is derived for the evolution of vorticity fluctuations across the shock. The validity of Morkovin's hypothesis across a shock wave is examined. Linear analysis is used to suggest that shock-front oscillation would invalidate the relation between urms and Trms, as expressed by the hypothesis.
177 citations
06 Jan 1997
64 citations
TL;DR: In this paper, it is shown that linear stability theory can be used to specify the source terms in Lighthill's equation; the resulting predictions are also in good agreement with the direct computations.
Abstract: The intense Mach waves radiated by the growth and decay of linear instability waves in the shear layer of a perfectly expanded, axisymmetric jet with an initial centerline Mach number of Mj = 2.0 are directly computed by solution of the compressible Navier-Stokes equations on a computational domain that includes both the near and far fields. The directly computed far-field sound is compared to predictions obtained using an analysis based on linear stability theory, Lighthill's equation, and the Kirchhoff surface method. All of the predictions are in good agreement with the direct computations. Using Lighthill's equation, we demonstrate that it is essential to properly address the acoustical noncompactness of the sources. It is also shown that linear stability theory can be used to specify the source terms in Lighthill's equation; the resulting predictions are also in good agreement with the computations.
61 citations
06 Jan 1997
TL;DR: In this paper, a numerical technique for computations of turbulent flows is described, which is based on B-spline and allows grid embedding in physically significant flow regions, and the results indicate that the method is accurate and efficient.
Abstract: A numerical technique for computations of turbulent flows is described. The technique is based on B-splines and allows grid embedding in physically significant flow regions. Numerical tests, which include solutions of nonlinear advection-diffusion equations and computations of flow over a circular cylinder at Reynolds numbers up to 300, indicate that the method is accurate and efficient. In computations of flow over a cylinder, the lift, drag, and base suction coefficients agree well with existing experimental data and previous numerical simulations.
29 Jun 1997
TL;DR: In this article, robust control theory is examined as a technique by which effective control algorithms based on limited noisy observations might be developed for turbulent flows and other nonlinear phenomena subjected to external disturbances.
Abstract: Recent work by the authors in the application of optimal control theory to turbulence have been quite successful when full state information is provided to the control algorithm. However, this approach has not yet been successful for the development of algorithms which depend on wall information only. For this reason, robust control theory, which is currently well developed only for linear problems, is now examined as a technique by which effective control algorithms based on limited noisy observations might be developed for turbulent flows and other nonlinear phenomena subjected to external disturbances.
01 Jan 1997
TL;DR: In this paper, a temporally developing turbulent round mixing layer at Mach number M(j) = 1.92 has been simulated, and results compared to a similar nearly incompressible simulation at M(k) = 0.4.
Abstract: A temporally developing turbulent round mixing layer at Mach number M(j) = 1.92 has been simulated, and results compared to a similar nearly incompressible simulation at M(j) = 0.4. The supersonic mixing layer growth rate, defined with a delta99 thickness parameter, is found to be suppressed relative to its low Mach number counterpart. The Reynolds stresses in the supersonic flow are also suppressed in a manner similar to experimental results. Flow visualization, 1D energy spectra, and two point correlations indicate that a realistic turbulent flow is being simulated. At a given thickness in the development of the two mixing layers, the mean velocity, mean temperature, Reynolds stresses, Reynolds shear stress anisotropy, pressure fluctuations, budgets of Reynolds stresses, and turbulent kinetic energy are compared. It is found that even though maximum turbulent Mach number reaches 0.43 in the compressible case, dilatational effects are minimal, and the total viscous dissipation is nearly the same fraction of the production of turbulent kinetic energy for both flows. (Author)
29 Jun 1997
TL;DR: The results indicate that signiicant drag reduction can be achieved using wall information only and the potential for optimization of feedback control algorithms, using neu-rocomputing methodologies is outlined.
Abstract: Some recent developments on the feedback control of turbulent ows are presented. Physical mechanisms associated with opposition control algorithms are investigated. A new control method based on the sensing and manipulation of vorticity creation at the wall is presented. The results indicate that signiicant drag reduction can be achieved using wall information only. The potential for optimization of feedback control algorithms, using neu-rocomputing methodologies is outlined.
01 Nov 1997