Abdelkader Frendi

Flow Past a Backward-Facing Step: Comparison of PANS, DES and URANS Results with Experiments

Flow past a backward-facing step is computed using three different turbulence modeling approaches; Detached Eddy Simulation (DES), Unsteady Reynolds Averaged Navier-Stokes (URANS), and the newly developed Partially Averaged Navier-Stokes (PANS) [1, 2] method. PANS is a family of closure models parameterized by different ratios of resolved-to-modeled kinetic energy and dissipation. The corresponding resolution control parameters are f k (the ratio of unresolved to total kinetic energy) and f ϵ (the ratio of unresolved-to-total dissipation). The main objectives of this study are: (i) to investigate the merits of PANS relative to the other two methods by comparing against existing experimental data; and (ii) to examine the effect of the PANS parameter f ϵ on the computed flow field. In addition, we study the sensitivity of PANS results to grid-size and the level of unsteadiness in the inflow. Overall, the results clearly demonstrate the suitability of PANS for computing unsteady turbulent flows at a reasonable computational expense.

Coupling Between a Supersonic Turbulent Boundary Layer and a Flexible Structure

A mathematical model and a computer code have been developed to fully couple the vibration of an aircraft fuselage panel to the surrounding flowfield, turbulent boundary layer, and acoustic fluid. The turbulent boundary-layer model is derived using a triple decomposition of the flow variables and applying a conditional averaging to the resulting equations. Linearized panel and acoustic equations are used. Results from this model are in good agreement with existing experimental and numerical data. It is shown that in the supersonic regime full coupling of the flexible panel leads to lower response and radiation from the panel. This is believed to be due to an increase in acoustic damping on the panel in this regime. Increasing the Mach number increases the acoustic damping, which is in agreement with earlier work.

A jet engine noise measurement and prediction tool

In this paper, the authors describe an innovative jet engine noise measurement and prediction tool. The tool measures sound-pressure levels and frequency spectra in the far field. In addition, the tool provides predicted results while the measurements are being made. The predictions are based on an existing computational fluid dynamics database coupled to an empirical acoustic radiation model based on the far-field approximation to the Lighthill acoustic analogy. Preliminary tests of this acoustic measurement and prediction tool produced very encouraging results.

Computational and Experimental Study of Linear Aerospike Engine Noise

Experimental and computational results of X-33 linear aerospike engine noise tests are presented. Only noise sources external to the nozzle are considered (no combustion noise). The experimental setup is such that the engine, located 18.3 m above the ground, has its exhaust plume diverted from a vertical position to a nearly horizontal position located 3 m above the ground using a J deflector. Experimental measurements are made at several locations in the midfield. A semi-empirical model is derived that accounts for the nonaxisymmetry of the engine exit plane. The model uses mean flow quantities obtained from a computational fluid dynamics computation using a k-e turbulence model. Midfield results obtained using the semi-empirical model are in reasonable agreement with the experimental measurements. The semi-empirical model was able to explain the source of a low-frequency peak observed in the spectra (near 10 Hz). In addition, the midfield directivity pattern and overall sound pressure levels given by the semi-empirical model are in good agreement with the measurements.

Study of the Ignition Overpressure Suppression Technique by Water Addition

The main objective of this study is to gain an understanding of the mechanisms responsible for the suppression of the ignition overpressure observed when water is injected through discrete nozzles into a rocket exhaust. A simplified launch-vehicle/launchpad configuration of relevant importance is selected for this study. This configuration is then numerically modeled using two-phase computational fluid dynamics with a representative motor startup sequence and a series of water addition configurations. The study focuses on the interaction between the ignition overpressure wave and the injected water. Chemical reactions are not included in the model; therefore, the effect of afterburning of fuel-rich exhaust is omitted in this study. A total of 11 water addition configurations were studied. The study demonstrated that ignition overpressure is strongly affected by the cooling of the plume and the amount of obstruction restricting the expansion of the plume. Also, the study suggests the existence of an optimal water addition rate with a weak dependence on the water nozzle pressure drop.

Interaction of Three-Dimensional Protuberances with a Supersonic Turbulent Boundary Layer

A comprehensive computational fluid dynamics study of the surface-pressure fluctuations induced by a cylindrical protuberance in a supersonic turbulent boundary layer is presented. The effects of two important parameters on the surface-pressure fluctuations are investigated: protuberance height to boundary-layer thickness and surface curvature. The turbulent boundary layer is modeled using a hybrid Reynolds-averaged Navier–Stokes and large-eddy-simulation approach known as detached-eddy simulation. At first extensive comparisons to experimental data for the surface-pressure coefficient and the unsteady surface-pressure coefficient were performed. Results from our computational-fluid-dynamics computations compared well to the experimental data in the wake region downstream of the protuberance and in the vicinity of the protuberance at other locations. Increasing the protuberance height relative to the boundary-layer thickness resulted in higher sound-pressure levels on the surface. In addition, the surface...

Effect of fluid wall shear stress on non-linear beam vibration

In this paper, the effects of tension and fluid wall shear stress on non-linear structural vibrations are investigated. Results show that fluid wall shear stress both suppresses and increases the non-linear response given by the large deflection response depending on its value. An in-depth analysis of the non-linear vibration response shows vibration energy flowing from the fundamental to the harmonics and then subharmonics as the excitation level increases. Various techniques to suppress non-linear vibrations were examined and most gave the same result, which is suppression of the subhramonics and harmonics.

On the Propagation of Plane Acoustic Waves in a Duct With Flexible and Impedance Walls

This Technical Memorandum (TM) discusses the harmonic and random plane acoustic waves propagating from inside a duct to its surroundings. Various duct surfaces are considered, such as rigid, flexible, and impedance. In addition, the effects of a mean flow are studied when the duct alone is considered. Results show a significant reduction in overall sound pressure levels downstream of the impedance wall for both mean flow and no mean flow cases and for a narrow duct. When a wider duct is used, the overall sound pressure level (OSPL) reduction downstream of the impedance wall is much smaller. In the far field, the directivity is such that the overall sound pressure level is reduced by about 5 decibels (dB) on the side of the impedance wall. When a flexible surface is used, the far field directivity becomes asymmetric with an increase in the OSPL on the side of the flexible surface of about 7 dB.

Control of Combustion-Instabilities Through Various Passive Devices

It is well known that under some operating conditions, rocket engines (using solid or liquid fuels) exhibit unstable modes of operation that can lead to engine malfunction and shutdown. The sources of these instabilities are diverse and are dependent on fuel, chamber geometry and various upstream sources such as pumps, valves and injection mechanism. It is believed that combustion-acoustic instabilities occur when the acoustic energy increase due to the unsteady heat release of the flame is greater than the losses of acoustic energy from the system [1, 2]. Giammar and Putnam [3] performed a comprehensive study of noise generated by gasfired industrial burners and made several key observations; flow noise was sometimes more intense than combustion roar, which tended to have a characteristic frequency spectrum. Turbulence was amplified by the flame. The noise power varied directly with combustion intensity and also with the product of pressure drop and heat release rate. Karchmer [4] correlated the noise emitted from a turbofan jet engine with that in the combustion chamber. This is important, since it quantified how much of the noise from an engine originates in the combustor. A physical interpretation of the interchange of energy between sound waves and unsteady heat release rates was given by Rayleigh [5] for inviscid, linear perturbations. Bloxidge et al [6] extended Rayleigh s criterion to describe the interaction of unsteady combustion with one-dimensional acoustic waves in a duct. Solutions to the mass, momentum and energy conservation equations in the pre- and post-flame zones were matched by making several assumptions about the combustion process. They concluded that changes in boundary conditions affect the energy balance of acoustic waves in the combustor. Abouseif et al [7] also solved the one-dimensional flow equations, but they used a onestep reaction to evaluate the unsteady heat release rate by relating it to temperature and velocity perturbations. Their analysis showed that oscillations arise from coupling between entropy waves produced at the flame and pressure waves originating from the nozzle. Yang and Culick [8] assumed a thin flame sheet, which is distorted by velocity and pressure oscillations. Conservation equations were expressed in integral form and solutions for the acoustic wave equations and complex frequencies were obtained. The imaginary part of the frequency indicated stability regions of the flame. Activation energy asymptotics together with a one-step reaction were used by McIntosh [9] to study the effects of acoustic forcing and feedback on unsteady, one-dimensional flames. He found that the flame stability was altered by the upstream acoustic feedback. Shyy et al [10] used a high-accuracy TVD scheme to simulate unsteady, one-dimensional longitudinal, combustion instabilities. However, numerical diffusion was not completely eliminated. Recently, Prasad [11] investigated numerically the interactions of pressure perturbations with premixed flames. He used complex chemistry to study responses of pressure perturbations in one-dimensional combustors. His results indicated that reflected and transmitted waves differed significantly from incident waves.

Effect of airfoil leading edge waviness on flow structures and noise

Purpose – The tubercles at the leading edge of Humpback Whale flippers have been shown to increase aerodynamic efficiency. The purpose of this paper is to compute the flow structures and noise signature of a NACA0012 airfoil with and without leading edge waviness, and located in the wake of a cylinder using the hybrid RANS-LES method. Design/methodology/approach – The mean flow Mach number is 0.2 and the angle of attack used is 2°. After benchmarking the method using existing experimental results, unsteady computations were then carried-out on both airfoil geometries and for a 2° angle of attack. Findings – Results from these computations confirmed the aerodynamic benefits of the leading edge waviness. Moreover, the wavy leading edge airfoil was found to be at least 4 dB quieter than its non-wavy counterpart. In-depth analysis of the computational results revealed that the wavy leading edge airfoil breaks up the large coherent structures which are then convected at higher speeds down the trough region of ...