Craig L. Streett

Spectral multi-domain for large-scale fluid dynamic simulations

A number of successful applications of a spectral collocation method extended by a multi-domain patching technique are shown. The multi-domain technique can be used to improve resolution for problems with widely disparate scales, and to reduce the ill-conditioning of the spectral operators for problems in which a large number of points are required for distributed resolution. A new nonreflecting outflow boundary treatment for unsteady transition-to-turbulence simulations is also presented, which relies on the multi-domain technique. The role of multi-domain in improving the efficiency of such calculations is discussed.

A finite Reynolds‐number approach for the prediction of boundary‐layer receptivity in localized regions

Earlier theoretical work on the boundary‐layer receptivity problem utilized the triple‐deck framework, and typically produced only the leading‐order asymptotic result. The applicability of these predictions was limited to the generation of Tollmien–Schlichting‐type (viscosity‐conditioned) instabilities and rather high values of an appropriate Reynolds number. Generalizing the concepts behind the asymptotic theory of Goldstein and Ruban, the classical Orr–Sommerfeld theory is utilized to predict the receptivity due to small‐amplitude surface nonuniformities. This approach accounts for the finite Reynolds‐number effects, and can also be extended easily to problems involving other types of instabilities. It is illustrated here for the case of the Tollmien–Schlichting wave generation in a Blasius boundary layer, due to the interaction of a free‐stream acoustic wave with a region of short‐scale variation in one of the surface boundary conditions. The type of surface disturbances examined include regions of short‐scale variations in wall suction, wall admittance, and wall geometry (roughness). Results from the finite Reynolds‐number approach are compared in detail with previous asymptotic predictions, as well as the available experimental data.

Spatial Direct Numerical Simulation of Boundary-Layer Transition Mechanisms: Validation of PSE Theory

A study of instabilities in incompressible boundary-layer flow on a flat plate is conducted by spatial direct numerical simulation (DNS) of the Navier-Stokes equations. Here, the DNS results are used to evaluate critically the results obtained using parabolized stability equations (PSE) theory and to study mechanisms associated with breakdown from laminar to turbulent flow. Three test cases are considered: two-dimensional Tollmien-Schlichting wave propagation, subharmonic instability breakdown, and oblique-wave breakdown. The instability modes predicted by PSE theory are in good quantitative agreement with the DNS results, except a small discrepancy is evident in the mean-flow distortion component of the two-dimensional test problem. This discrepancy is attributed to far-field boundary-condition differences. Both DNS and PSE theory results show several modal discrepancies when compared with the experiments of subharmonic breakdown. Computations that allow for a small adverse pressure gradient in the basic flow and a variation of the disturbance frequency result in better agreement with the experiments.

Improvements in spectral collocation discretization through a multiple domain technique

Abstract A new parallel solver for ODEs implementing a “parallelism across the steps” has been recently proposed in (Amodio and Brugnano, 1997; Brugnano and Trigiante, 1998), where it is shown that it is able to obtain an almost perfect speed-up on linear problems, and given mesh. A possible way to adapt this algorithm to efficiently handle nonlinear initial value problems has been studied in the companion paper (Brugnano and Trigiante, this issue). The corresponding algorithm is here analyzed in details, in order to show its parallel efficiency. Numerical tests on a distributed memory parallel computer are also included.

Aerodynamic Noise Reduction for High-Lift Devices on a Swept Wing Model

*† † † ‡ § ** †† Aeroacoustic experiments were conducted in the 14-by-22-foot Subsonic Wind Tunnel at NASA Langley Research Center. The test platform was a large, swept wing model with a leading-edge slat, and a trailing-edge flap. The noise sources under investigation were located at the flap side-edges, within the slat cove, and near an inboard slat tip. Acoustic measurements were obtained with a microphone phased array to determine relative source strengths and to assess the effectiveness of noise reduction treatments, which were installed as alterations to the model geometry. The results demonstrate that all three component sources were sufficiently identified and successfully treated with respect to reduction in acoustic radiation. In addition, preliminary aerodynamic results are shown that support future design goals for significant reductions in airframe noise without compromising safety and performance.

A numerical simulation of the appearance of chaos in finite-length Taylor-Couette flow☆

Abstract Taylor-Couette flow, the shear-driven flow between concentric cylinders, exhibits a wide variety of instabilities and modal changes, especially for the case of finite length to gap ratio. The numerical simulations presented here capture many of the experimentally observed features, including the moderately high Reynolds number regime in which temporally aperiodic behavior is seen. The exponential decay of the temporal frequency spectrum of these modes in the simulations indicate such flows possess a low-order chaotic character. In this paper, the spectral collocation methods used in this study are described, select axisymmetric simulations are reviewed, and initial results from three-dimensional unsteady simulations are presented.

Spatial simulation of instability control by periodic suction blowing

The applicability of active control by periodic suction blowing in spatially evolving plane Poiseuille flow is investigated by the direct simulation of the three‐dimensional, incompressible Navier–Stokes equations. The results reveal that significant reductions in perturbation amplitudes can be obtained by a proper choice of the control wave amplitude and phase. The upstream influence of the control wave is shown to be confined to a region in the vicinity of the control slot with no apparent effect on the flow development.

The role of stationary cross‐flow vortices in boundary‐layer transition on swept wings

The spatial evolution of stationary cross‐flow‐vortex packets in a laminar boundary layer on a swept wing is computed by direct numerical simulation with the incompressible Navier–Stokes equations. Steady suction and blowing at the wing surface is used to generate disturbances that are periodic and equally spaced in the spanwise direction. The initiated disturbances become unstable and lead to distinct stages of instability evolution. Initially, each vortex packet undergoes a region of chordwise and spanwise linear independent growth; then the individual packets coalesce downstream and lead to spanwise wave adjustments and linear superposition of adjacent packets; finally, the vortex packets reach sufficiently large amplitudes in later stages of disturbance development to nonlinearly interact, which results in the rapid growth of the disturbances. In this later stage, the low‐velocity fluid near the wing surface is lifted out into the boundary layer and rolled over the high‐speed fluid in the direction of...

On the Computation of Sound by Large-Eddy Simulations

The effect of the small scales on the source term in Lighthills acoustic analogy is investigated, with the objective of determining the accuracy of large-eddy simulations when applied to studies of flow-generated sound. The distribution of the turbulent quadrupole is predicted accurately, if models that take into account the trace of the SGS stresses are used. Its spatial distribution is also correct, indicating that the low-wave-number (or frequency) part of the sound spectrum can be predicted well by LES. Filtering, however, removes the small-scale fluctuations that contribute significantly to the higher derivatives in space and time of Lighthills stress tensor Tij. The rms fluctuations of the filtered derivatives are substantially lower than those of the unfiltered quantities. The small scales, however, are not strongly correlated, and are not expected to contribute significantly to the far-field sound; separate modeling of the subgrid-scale density fluctuations might, however, be required in some configurations.

Integrated Transition Prediction: A Case Study in Supersonic Laminar Flow Control

† Aerospace Technologist, Computational Modeling and Simulation Branch, Senior Member AIAA Email: m.m.choudhari@larc.nasa.gov †† Aerospace Technologist, Computational Modeling and Simulation Branch, Senior Member AIAA ††† Aerospace Technologist, Computational Modeling and Simulation Branch. * Aerospace Technologist, Flow Physics and Control Branch, Senior Member AIAA Copyright