Gregory A. Blaisdell

Coupling of integral acoustics methods with LES for jet noise prediction

This study is focused on developing a Computational Aeroacoustics (CAA) methodology that couples the near field unsteady flow field data computed by a 3-D Large Eddy Simulation (LES) code with various integral acoustic formulations for the far field noise prediction of turbulent jets. The LES code employs state-of-the-art numerical schemes and a localized version of the dynamic Smagorinsky subgrid-scale (SGS) model. The code also has the capability to turn off the SGS model and treat the spatial filter that is needed for numerical stability as an implicit SGS model. Noise computations performed for a Mach 0.9, Reynolds number 400,000 jet using various integral acoustic results are presented and the results are compared against each other as well with those from experiments at similar flow conditions. Our results show that the surface integral acoustics methods (Kirchhoff and Ffowcs Williams – Hawkings) give similar results to the volume integral method (Lighthills acoustic analogy) at a much lower cost. To the best of our knowledge, Lighthills acoustic analogy is applied to a Reynolds number 400,000 jet at Mach 0.9 for the first time in this study. The distribution of Lighthill sources that radiate noise in the direction of various observer locations is evaluated. A source decomposition shows significant cancellations among the individual components of the Lighthill source.

3-D large eddy simulation for jet aeroacoustics

of the unresolved scales on the resolved scales. A computational grid consisting of 12 million points was used in the present simulation. Mean flow results obtained in our simulation are found to be in excellent agreement with the available experimental data of jets at similar flow conditions. Furthermore, the near field data provided by LES is coupled with the Ffowcs Williams-Hawkings method to compute the far field noise. Far field aeroacoustics results are also presented and comparisons are made with another computational study.

Evaluation of the dynamic model for simulations of compressible decaying isotropic turbulence

Several issues involving the use of the dynamic subgrid-scale model in large-eddy simulations of compressible turbulent flows are investigated. The model is employed in simulations of compressible decaying isotropic turbulence, and its performance is compared against results from direct numerical simulations and experiments. Results from a parametric study suggest the model captures compressibility effects well. Use of the dynamic model in simulations of inhomogeneous flows requires filtering of the flowfield in physical space rather than Fourier wave space. The use of spatial filters is examined by conducting simulations of isotropic turbulence. Several implicit filters are found to perform extremely well and similar to the sharp cutoff filter. One explicit filter performed well, but all others provided excessive dissipation at higher modes. Two formulations of the dynamic model, proposed by Moin et al. and Lilly, perform well, with Lillys being more accurate. Results suggest also a great insensitivity of the model on the filter width ratio. A modification of the convective terms in the momentum and energy equations is found to reduce the effects of aliasing errors. Finally, different formulations of the energy equation are examined.

Application of Compact Schemes to Large Eddy Simulation of Turbulent Jets

We present 3-D large eddy simulation (LES) results for a turbulent Mach 0.9 isothermal round jet at a Reynolds number of 100,000 (based on jet nozzle exit conditions and nozzle diameter). Our LES code is part of a Computational Aeroacoustics (CAA) methodology that couples surface integral acoustics techniques such as Kirchhoffs method and the Ffowcs Williams– Hawkings method with LES for the far field noise estimation of turbulent jets. The LES code employs high-order accurate compact differencing together with implicit spatial filtering and state-of-the-art non-reflecting boundary conditions. A localized dynamic Smagorinsky subgrid-scale (SGS) model is used for representing the effects of the unresolved scales on the resolved scales. A computational grid consisting of 12 million points was used in the present simulation. Mean flow results obtained in our simulation are found to be in very good agreement with the available experimental data of jets at similar flow conditions. Furthermore, the near field data provided by the LES is coupled with the Ffowcs Williams–Hawkings method to compute the far field noise. Far field aeroacoustics results are also presented and comparisons are made with experimental measurements of jets at similar flow conditions. The aeroacoustics results are encouraging and suggest further investigation of the effects of inflow conditions on the jet acoustic field.

Large-Eddy Simulation of a Spatially Evolving Supersonic Turbulent Boundary-Layer Flow

Several issues involving the large-eddy simulation of wall-bounded compressible turbulent flows are investigated. A spatially evolving supersonic boundary layer is simulated using a high-order-accurate finite difference scheme and the dynamic subgrid-scale model. A parametric study suggests that the finite difference scheme has a detrimental effect on the resolution of the smaller scales due to excessive numerical dissipation from the spatial differencing. Also, because the dynamic model uses the smaller resolved-scale eddies to determine the model coefficients, the predicted coefficients do not have the appropriate values. The use of higher-order schemes is found to better capture the smaller resolved scales and to improve substantially the quality of the results. The effect of descretization errors on large-eddy simulation needs to be addressed further before proceeding with large-eddy simulation of flows of engineering interest

Recent Progress of Hot Jet Aeroacoustics Using 3-D Large-Eddy Simulation

Improvements in computing speed over the past decade have made Large Eddy Simulation (LES) an attractive tool to study jet noise. In addition, the study of turbulent hot jets for noise prediction is desirable compared to cold/isothermal jets since all jet engines fltted on aircraft operate at hot exhaust conditions. In this regard, we present results for two heated jets with temperature ratios of Tj=T1 = 1:76 and Tj=T1 = 2:70, respectively. A computational grid with approximately 4.8 million grid points is used the simulation. Spatial flltering is used as an implicit subgrid scale SGS model in place of the classical Smagorinsky and Dynamic Smagorinsky models. To study the far-fleld noise, the porous Ffowcs Williams-Hawkings (FWH) surface integral acoustic formulation is employed. The jet development results obtained using our LES methodology are consistent with other LES data and experimental results. The predicted OASPL values for our heated jets follow the trend measured by experiments though our results over-predict by approximately 3dB. Overall, our LES methodology coupled with the Ffowcs Williams-Hawkings aeroacoustics methodology provide satisfactory results.

ON THE CONSISTENCY OF REYNOLDS STRESS TURBULENCE CLOSURES WITH HYDRODYNAMIC STABILITY THEORY

The consistency of second‐order closure models with results from hydrodynamic stability theory is analyzed for the simplified case of homogeneous turbulence. In a recent study, Speziale, Gatski, and Mac Giolla Mhuiris [Phys. Fluids A 2, 1678 (1990)] showed that second‐order closures are capable of yielding results that are consistent with linear stability theory for the case of homogeneous shear flow in a rotating frame. It is demonstrated in this paper that this success is due to the fact that the stability boundaries for rotating homogeneous shear flow are not dependent on the details of the spatial structure of the disturbances. For those instances where they are—such as in the case of elliptical flows where the instability mechanism is more subtle—the results are not so favorable. The origins and extent of this modeling problem are examined in detail along with a possible resolution based on Rapid Distortion Theory (RDT) and its implications for turbulence modeling.

Numerical Investigation of 3-D Supersonic Jet Flows using Large Eddy Simulation

The farfield noise generated by supersonic jets is investigated by a computational aeroacoustics methodology that couples 3-D large-eddy simulation (LES) near field data with the Ffowcs Williams- Hawkings method for farfield noise prediction. In order to accurately simulate jets at off-design supersonic conditions, we employ LES with characteristic filters for shock-capturing. This approach limits the dissipation of noise-producing turbulent fluctuations, and is suitable for incorporation into existing solvers. To further limit dissipation, a shock detector is used to determine shock locations and characteristic filters are applied locally. In this study, both perfectly-expanded and under-expanded unheated jets are investigated, with and without using characteristic filters. Comparisons with similar numerical and experimental data show reasonable agreement of the jet mean flow, turbulent statistics, and acoustics results. Preliminary grid-refinement shows improvement in these results.

Effects of Inflow Forcing on Jet Noise Using Large Eddy Simulation

Recent discoveries have shown that by adjusting selected in∞ow forcing parameters, properties such as turbulent ∞ow development and most importantly jet noise are in∞uenced to some extent. To implement fully a nozzle structure in a high-end simulation like Large Eddy Simulation (LES) would require a prohibitive number of grid points to resolve the boundary layer for realistic Reynolds numbers. Thus, in∞ow forcing currently seems to be a reasonable substitute for a nozzle geometry. However, the drawback of this approach is that the ∞ow fleld results are sensitive to in∞ow forcing parameters used. With LES as an investigative tool, this paper studies the efiects of in∞ow forcing with particular emphasis on the number of azimuthal modes. We flnd that by removing the flrst few modes results in the jet developing slower, i.e. longer potential core. Furthermore, the peak turbulence intensities increase when we remove the flrst 6 and 8 modes of forcing. Due to this high peak turbulence intensities we found that the overall sound pressure level (OASPL) also increases at all observation angles for a closed control surface using the Ffowcs Williams-Hawkings method.

Assessment of Turbulent Shock-Boundary Layer Interaction Computations Using the OVERFLOW Code

The performance of two popular turbulence models, the Spalart-Allmaras model and Menter s SST model, and one relatively new model, Olsen & Coakley s Lag model, are evaluated using the OVERFLOWcode. Turbulent shock-boundary layer interaction predictions are evaluated with three different experimental datasets: a series of 2D compression ramps at Mach 2.87, a series of 2D compression ramps at Mach 2.94, and an axisymmetric coneflare at Mach 11. The experimental datasets include flows with no separation, moderate separation, and significant separation, and use several different experimental measurement techniques (including laser doppler velocimetry (LDV), pitot-probe measurement, inclined hot-wire probe measurement, preston tube skin friction measurement, and surface pressure measurement). Additionally, the OVERFLOW solutions are compared to the solutions of a second CFD code, DPLR. The predictions for weak shock-boundary layer interactions are in reasonable agreement with the experimental data. For strong shock-boundary layer interactions, all of the turbulence models overpredict the separation size and fail to predict the correct skin friction recovery distribution. In most cases, surface pressure predictions show too much upstream influence, however including the tunnel side-wall boundary layers in the computation improves the separation predictions.