Showing papers by "Meng Wang published in 2003"

Analysis of Flow Conditions in Freejet Experiments for Studying Airfoil Self-Noise

Abstract: A new set of mean wall pressure data has been collected on a controlled diffusion airfoil at a chord Reynolds number of 1.2 £ £105 in a freejet anechoic wind tunnel. Comparisons of the experimental data with Reynoldsaveraged Navier‐ Stokes (RANS) simulationsin freeairshow signie cant e owe eld and pressure loading differences, indicatingsubstantialjetinterferenceeffects.Toanalyzetheseeffects,asystematicRANS-basedcomputationale uid dynamicsstudyoftheexperimentale owconditionshasbeencarriedout,whichquantie esthestrongine uenceofthe e nite jet (nozzle) width on the aerodynamic loading and e ow characteristics. When the jet width is not sufe ciently large compared to the frontal wetted area of the airfoil, the airfoil pressure distribution is found to be closer to the distribution on a cascade than that of an isolated proe le. The airfoil lift is signie cantly reduced. Accounting for the actual wind-tunnel setup recovers the wall pressure distribution on the airfoil without further empirical angle-of-attack corrections. These jet interference effects could be responsible for the discrepancies among some earlier experimental and computational studies of airfoil self-noise. They should be accounted for in future noise computations to ensure that the experimental e ow conditions are simulated accurately.

Optimal aeroacoustic shape design using the surrogate management framework

Abstract: Shape optimization is applied to time-dependent trailing-edge flow in order to minimize aerodynamic noise. Optimization is performed using the surrogate management framework (SMF), a non-gradient based pattern search method chosen for its efficiency and rigorous convergence properties. Using SMF, design space exploration is performed not with the expensive actual function but with an inexpensive surrogate function. The use of a polling step in the SMF guarantees that the algorithm generates a convergent subsequence of mesh points in the parameter space. Each term of this subsequence is a weak local minimizer of the cost function on the mesh in a sense to be made precise later. We will discuss necessary optimality conditions for the design problem that are satisfied by the limit of this subsequence. Results are presented for an unsteady laminar flow past an acoustically compact airfoil. Constraints on lift and drag are handled within SMF by applying the filter pattern search method of Audet and Dennis, within which a penalty function is used to form and optimize a surrogate function. Optimal shapes that minimize noise have been identified for the trailing-edge problem in constrained and unconstrained cases. Results show a significant reduction (as much as 80%) in acoustic power with reasonable computational cost using several shape parameters. Physical mechanisms for noise reduction are discussed.

Combustion instability due to the nonlinear interaction between sound and flame

Abstract: It has been observed in experiments that significant levels of sound may be produced when a curved flame propagates downwards along a tube in a gravity field In this paper, we present a mathematical description of this acoustic amplification process, which represents a simple form of combustion instability First, based on the large-activation-energy and small-Mach-number assumptions, a general asymptotic formulation is derived, in which the nature of flame-sound coupling is brought out explicitly This framework is then employed to study the weakly nonlinear coupling between a Darrieus-Landau (D-L) instability mode of the flame and an acoustic mode of the tube, which is the main mechanism for sound generation in the experiments In order to provide a somewhat unified description, the linear coupling via the direct pressure effect has also been included in our analysis A set of coupled equations which govern the evolution of the acoustic and D-L modes was derived The solutions show that the nonlinear coupling leads to very rapid amplification of sound After reaching an appreciable level, the sound inhibits the flame, causing the latter to flatten The sound then saturates at an almost constant level, or continues to grow at a smaller rate owing to the pressure effect The above theoretical predictions are in good qualitative agreement with experiments The present study also considered the influence of weak vortical disturbances in the oncoming flow It is shown that certain components in these perturbations may form resonant triads with the acoustic and D-L modes, thereby providing an additional coupling mechanism

Study of Rotor Tip-Clearance Flow Using Large-Eddy Simulation

Abstract: A large-eddy simulation has been performed to study the temporal and spatial dynamics of a rotor tip-clearance flow, with the objective of determining the underlying mechanisms for low pressure fluctuations downstream of the tip-gap. Simulation results are compared with experimental measurements, and favorable agreements are observed in both qualitative and quantitative sense. Typical vortical structures such as the tip-leakage vortex and tip-separation vortices are revealed, and their evolution is shown to be strongly influenced by the moving endwall and the blade wake. These vortical structures are the main sources of turbulence energy and Reynolds stresses as well as low-pressure fluctuations. Cavitation-inception analysis shows a high correlation between cavitation and the tip-leakage vortex.

A Computational Methodology for Large-Eddy Simulation of Tip-Clearance Flows

Abstract: A large-eddy simulation (LES) solver which combines an immersed-boundary technique with a curvilinear structured grid has been developed to study the temporal and spatial dynamics of an incompressible rotor tip-clearance flow. The overall objective of these simulations is to determine the underlying mechanisms for low-pressure fluctuations downstream of the rotor near the endwall. Salient features of the numerical methodology, including the mesh topology, the immersed boundary method, the treatment of numerical instability for non-dissipative schemes on highly skewed meshes, and the parallelization of the code for shared memory platforms are discussed. The computational approach is shown to be capable of capturing the evolution of the highly complicated flowfield characterized by the interaction of distinct blade-associated vortical structures with the turbulent endwall boundary layer. Simulation results are compared with experiments and qualitative as well as quantitative agreement is observed.Copyright © 2003 by ASME

Constrained Aeroacoustic Shape Optimization Using the Surrogate Management Framework

Abstract: Reduction of noise generated by turbulent flow past the trailing-edge of a lifting surface is a challenge in many aeronautical and naval applications. Numerical predictions of trailing-edge noise necessitate the use of advanced simulation techniques such as large-eddy simulation (LES) in order to capture a wide range of turbulence scales which are the source of broadband noise. Aeroacoustic calculations of the flow over a model airfoil trailing edge using LES and aeroacoustic theory have been presented in Wang and Moin and were shown to agree favorably with experiments. The goal of the present work is to apply shape optimization to the trailing edge flow previously studied, in order to control aerodynamic noise.

Large-eddy simulation and analysis of tip-clearance flows in turbomachinery applications

Abstract: The tip-leakage flow in axial turbomachines is studied using large-eddy simulation with an emphasis on understanding the underlying mechanisms for low-pressure fluctuations and cavitations downstream of the tip-gap. Simulation results are validated against experimental measurements, and reasonable agreements are obtained. Dominant vortical structures such as the tip-leakage vortex and tip-separation vortices are examined, and their effects on the turbulent flow characteristics as well as on the low-pressure statistics are investigated. Analysis of the velocity and pressure fields suggests a high correlation between cavitations inception and the tip-leakage vortex. To suppress the tip-leakage vortex and the associated low-pressure events, the use of a grooved casing wall is explored, and preliminary simulation results show good promise. Additional simulations to investigate the effects of inflow vortices and tip-gap size are also discussed.

On the Computation of Space-Time Correlations by Large-Eddy Simulation

Abstract: Numerical comparisons in decaying isotropic turbulence suggest that there exist discrepancies in time correlations evaluated by DNS and LES using eddy-viscosity-type SGS models. This is consistent with the previous observations in forced isotropic turbulence. Therefore, forcing is not the main cause of the discrepancies. Comparisons among different SGS models in the LES also indicate that the model choice affects the time correlations in the LES. The multi-scale LES method using the dynamic Smagorinsky model on the small scale equation is the most accurate of the all models, the classic Smagorinsky model is the least accurate and the dynamic Smagorinsky model and spectral eddy viscosity model give intermediate results with small differences. The generalized sweeping hypothesis implies that time correlations in decaying isotropic turbulence are mainly determined by the instantaneous energy spectra and sweeping velocities. The analysis based on the sweeping hypothesis explains the discrepancies in our numerical simulations: the LES overpredicts the decorrelation time scales because the sweeping velocities are smaller than the DNS values, and underpredicts the magnitudes of time correlations because the energy spectrum levels are lower than the DNS ones. Since the sweeping velocity is determined by the energy spectra, one concludes that an accurate prediction of the instantaneous energy spectra guarantees the accuracy of time correlations. An analytical expression of sound power spectra based on Lighthill's theory and the quasi-normal closure assumption suggests that the sound power spectra are sensitive to errors in time correlations. Small errors in time correlations can cause significant errors in the sound power spectra, which exhibit a sizable drop at moderate to high frequencies accompanied by a shift of the peaks to lower frequencies. Based on the above analysis, two possible ways to improve the acoustic power spectrum predictions can be considered. The first is to construct better SGS models to improve the LES accuracy for time correlations. The second is to remedy the temporal statistics of the Lighthill stress tensor in order to "recover" the contribution from the unresolved scales in LES to time correlations.