Showing papers by "Parviz Moin published in 1996"

Large-eddy simulation of turbulent confined coannular jets

Abstract: Large-eddy simulation (LES) was used to study mixing of turbulent, coannular jets discharging into a sudden expansion. This geometry resembles that of a coaxial jet-combustor, and the goal of the calculation was to gain some insight into the phenomena leading to lean blow-out (LBO) in such combustion devices. This is a first step in a series of calculations, where the focus is on the fluid dynamical aspects of the mixing process in the combustion chamber. The effects of swirl, chemical reactions and heat release were not taken into account. Mixing of fuel and oxidizer was studied by tracking a passive scalar introduced in the central jet. The dynamic subgrid-scale (DM) model was used to model both the subgrid-scale stresses and the subgrid-scale scalar flux. The Reynolds number was 38000, based on the bulk velocity and diameter of the combustion chamber. Mean velocities and Reynolds stresses are in good agreement with experimental data. Animated results clearly show that intermittent pockets of fuel-rich fluid (from the central jet) are able to cross the annular jet, virtually undiluted, into the recirculation zone. Most of the fuel-rich fluid is, however, entrained into the recirculation zone near the instantaneous reattachment point. Fuel trapped in the recirculation zone is, for the most part, entrained back into the step shear layer close to the base of the burner.

Computation of quadrupole noise using acoustic analogy

Abstract: Acoustic analogy computations of vortex shedding noise were carried out in the context of a two-dimensional, low-Mach-number laminar flow past a NACA 0012 airfoil at chord Reynolds number of 10 4 . The incompressible Navier-Stokes equations were solved numerically to give an approximate description of the near-field flow dynamics and the acoustic source functions. The radiated far-field noise was computed based on Curle's extension to the Lighthill analogy. This study emphasizes an accurate evaluation of the Reynolds stress quadrupoles in the presence of an extensive wake. An effective method for separating the physical noise source from spurious boundary contributions caused by eddies crossing a permeable computational boundary is presented. The effect of retarded-time variations across the source region is also examined. Computational solutions confirm that the quadrupole noise is weak compared with the noise due to lift and drag dipoles when the freestream Mach number is small. The techniques developed in this study are equally applicable to flows in which the volume quadrupoles act as a prominent noise source.

Calculation of the radiated sound field using an open Kirchhoff surface

Abstract: Means of improving the accuracy of Kirchhoff integral solutions for sound fields in cases where the surface may not be completely closed are investigated. Asymptotic analysis for large temporal wave number is used to analyze time harmonic integral forms. Extension to time dependent equations is discussed briefly. Applicability to the "moderate" temporal wave numbers of real problems is discussed. Stationary phase arguments are used to show geometrically where good results are expected from a Kirchhoff integral . on an open surface. A similar asymptotic analysis is used to provide correction terms to partially account for the missing portion of the integral surface. The present study is restricted to the case where the mean flow is parallel to the open surface, but results are applicable to arbitrary flow situations in any number of dimensions. Two dimensional numerical examples are given to demonstrate and evaluate the method. It is found that the correction terms can dramatically reduce the error in an open surface calculation of the radiated sound field.

Sound radiation during local laminar breakdown in a low-Mach-number boundary layer

Abstract: The far-field sound of an unstable wave packet undergoing transition in a low-Mach-number, flat-plate boundary layer is investigated in the framework of Lighthill's acoustic analogy. Detailed accounts of the wave packet evolution are obtained by solving the full incompressible Navier-Stokes equations at Re δ . = 1000. The numerically simulated flow structures show qualitative agreement with experimental observations of the fundamental breakdown type. The acoustic calculations are focused on the quadrupole source functions arising from Reynolds stress fluctuations. The wave packet is shown to produce negligible sound throughout the primary and secondary instability stages. Dramatic amplification of the Reynolds stress quadrupoles occurs as a result of the disintegration of the detached high-shear layer and the associated vortex shedding near the boundary layer edge. The dominant frequency of source oscillations coincides with that of vortex shedding.

A new approach to turbulence modeling

Abstract: A new approach to Reynolds averaged turbulence modeling is proposed which has a computational cost comparable to two equation models but a predictive capability approaching that of Reynolds stress transport models. This approach isolates the crucial information contained within the Reynolds stress tensor, and solves transport equations only for a set of 'reduced' variables. In this work, Direct Numerical Simulation (DNS) data is used to analyze the nature of these newly proposed turbulence quantities and the source terms which appear in their respective transport equations. The physical relevance of these quantities is discussed and some initial modeling results for turbulent channel flow are presented.