Adrian Sescu

Multidimensional optimization of finite difference schemes for Computational Aeroacoustics

Because of the long propagation distances, Computational Aeroacoustics schemes must propagate the waves at the correct wave speeds and lower the isotropy error as much as possible. The spatial differencing schemes are most frequently analyzed and optimized for one-dimensional test cases. Therefore, in multidimensional problems such optimized schemes may not have isotropic behavior. In this work, optimized finite difference schemes for multidimensional Computational Aeroacoustics are derived which are designed to have improved isotropy compared to existing schemes. The derivation is performed based on both Taylor series expansion and Fourier analysis. Various explicit centered finite difference schemes and the associated boundary stencils have been derived and analyzed. The isotropy corrector factor, a parameter of the schemes, can be determined by minimizing the integrated error between the phase or group velocities on different spatial directions. The order of accuracy of the optimized schemes is the same as that of the classical schemes, the advantage being in reducing the isotropy error. The present schemes are restricted to equally-spaced Cartesian grids, so the generalized curvilinear transformation method and Cartesian grid methods are good candidates. The optimized schemes are tested by solving various multidimensional problems of Aeroacoustics.

Algebraic/transcendental disturbance growth behind a row of roughness elements

This paper is a continuation of the work begun in Goldstein et al . ( J. Fluid Mech. , vol. 644, 2010, p. 123), who constructed an asymptotic high-Reynolds-number solution for the flow over a spanwise periodic array of relatively small roughness elements with (spanwise) separation and plan form dimensions of the order of the local boundary-layer thickness. While that paper concentrated on the linear problem, here the focus is on the case where the flow is nonlinear in the immediate vicinity of the roughness with emphasis on the intermediate wake region corresponding to streamwise distances that are large in comparison with the roughness dimension, but small in comparison with the distance between the roughness array and the leading edge. An analytical O ( h 2 ) asymptotic solution is obtained for the limiting case of a small roughness height parameter h . These weakly nonlinear results show that the spanwise variable component of the wall-pressure perturbation decays as x −5/3 ln x when x → ∞ (where x denotes the streamwise distance scaled on the roughness dimension), but the corresponding component of the streamwise velocity perturbation (i.e. the wake velocity) exhibits an O ( x 1/3 ln x ) algebraic/transcendental growth in the main boundary layer. Numerical solutions for h = O (1) demonstrate that the wake velocity perturbation for the fully nonlinear case grows in the same manner as the weakly nonlinear prediction – which is considerably different from the strictly linear result obtained in Goldstein et al . (2010).

The Long Range Persistence of Wakes Behind a Row of Roughness Elements

We consider a periodic array of relatively small roughness elements whose spanwise separation is of the order of the local boundary-layer thickness and construct a local asymptotic high-Reynolds-number solution that is valid in the vicinity of the roughness. The resulting flow decays on the very short streamwise length scale of the roughness, but the solution eventually becomes invalid at large downstream distances and a new solution has to be constructed in the downstream region. This latter result shows that the roughness-generated wakes can persist over very long streamwise distances, which are much longer than the distance between the roughness elements and the leading edge. Detailed numerical results are given for the far wake structure.

Boundary-layer transition at high free-stream disturbance levels : beyond Klebanoff modes

We consider a nominally uniform flow over a semi-infinite flat plate and show how a small slowly modulated (predominantly streamwise) disturbance of the upstream flow is amplified by leading-edge bluntness effects and eventually develops into a small-amplitude but nonlinear spanwise motion far downstream from the edge. This motion is then imposed on the viscous boundary layer at the surface of the plate – causing an order-one change in its profile shape, which can reduce the wall shear to zero and thereby causes the boundary layer to separate. The present study is similar to an earlier steady flow analysis, but the unsteady effects now cause the upstream boundary layer to develop inflectional profiles which can support rapidly growing inviscid instabilities that give rise to transition before the separation can occur.

Large-Eddy Simulation and Single-Column Modeling of Thermally Stratified Wind Turbine Arrays for Fully Developed, Stationary Atmospheric Conditions

AbstractEffects of atmospheric thermal stratification on the asymptotic behavior of very large wind farms are studied using large-eddy simulations (LES) and a single-column model for vertical distributions of horizontally averaged field variables. To facilitate comparisons between LES and column modeling based on Monin–Obukhov similarity theory, the LES are performed under idealized conditions of statistical stationarity in time and fully developed conditions in space. A suite of simulations are performed for different thermal stratification levels and the results are used to evaluate horizontally averaged vertical profiles of velocity, potential temperature, vertical turbulent momentum, and heat flux. Both LES and the model show that the stratification significantly affects the atmospheric boundary layer structure, its height, and the surface fluxes. However, the effects of the wind farm on surface heat fluxes are found to be relatively small in both LES and the single-column model. The surface fluxes ar...

Prediction of Noise from Realistic Rotor-Wake/Stator-Row Interaction Using Computational Aeroacoustics

Progress towards the computational prediction of the turbulent flow and broadband noise generated due to the interaction of rotor wakes and stator blades is presented. An inflow boundary treatment is developed which allows arbitrary incoming unsteady flow disturbances to be imposed while preserving the desired mean flow and generating minimal reflections. The boundary condition was implemented in a nonlinear Euler CAA code and validated on a 2D CAA benchmark problem. Preliminary results are shown for a loaded 2D cascade with realistic stator wakes specified from experimental data.

Hybrid Discontinuous Galerkin and Finite Volume Method for Launch Environment Acoustics Prediction

Launch vehicles experience extreme acoustic loads during liftoff driven by the interaction of rocket plumes and plume-generated acoustic waves with ground structures. Currently employed predictive capabilities to model the complex turbulent plume physics are too dissipative to accurately resolve the propagation of acoustic waves throughout the launch environment. Higher fidelity liftoff acoustic analysis tools to design mitigation measures are critically needed to optimize launch pads for the Space Launch System and commercial launch vehicles. To this end, a new coupled two-field simulation capability has been developed to enable accurate prediction of liftoff acoustic physics. Established unstructured computational fluid dynamics algorithms are used for simulation of acoustic generation physics and a high-order-accurate discontinuous Galerkin nonlinear Euler solver is employed to accurately propagate acoustic waves across large distances. An innovative hybrid computational fluid dynamics/computational ae...

Coupled Overset Unstructured Discontinuous Galerkin Method for Launch Environment Acoustics Prediction

A novel approach for the accurate prediction of launch environment acoustic physics is presented. Launch vehicles experience extreme acoustic loads during liftoff, driven by the interaction of rocket plumes and plume-generated acoustic waves with ground structures. In this work, a well-established hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation unstructured mesh solver is used to efficiently model the complex turbulent plume physics, and a high-order accurate discontinuous Galerkin solver is used to accurately propagate acoustic waves across large distances throughout the launch environment. The two solvers operate on separate overlapping meshes, and an innovative overset coupling approach is used to transmit the plume-generated acoustics to the far field in a one-way manner in which the turbulent plume prediction is unaffected by the outer acoustic propagation physics. The framework upon which the solvers are developed is described along with details outlining the overset domain connectivity...

A Study of the Steady-State Performance of a Pressurized Air Wave Bearing at Concentric Position

The steady-state performance of a pressurized air wave bearing in concentric position is predicted using a commercial computational fluid dynamics (CFD) code. This code solves the three-dimensional compressible Navier-Stokes equations in the turbulent regime, taking into account the real geometry of both the bearing fluid film and the supply regions. The code can provide detailed information about the flow (pressure, turbulent kinetic energy distributions, velocity profiles, etc.) in all bearing regions including the supply holes. This approach does not involve a correction of the flow rate with an empirical discharge coefficient. The predicted values of the supply flow rates are compared to the experimental values obtained with a dedicated wave bearing test rig located at NASA Glenn Research Center in Cleveland, Ohio.

Numerical Study of Boundary Layer Receptivity to Free-stream Disturbances and Surface Excrescences

Numerous studies conducted over the years have shown that the transition onset in boundary layer flows is strongly dependent on the receptivity to various environmental disturbances. The objective of this paper is to study the mechanism by which free-stream acoustic and vorticity disturbances interact with a boundary layer flow developing over a flat-plate featuring a small excrescence located at a certain distance from a blunt leading edge. The numerical tool is a high-fidelity implicit numerical algorithm solving for the unsteady, compressible form of the Navier-Stokes equations in a body-fitted curvilinear coordinates, and employing high accurate compact differencing schemes with Pade type filters. Acoustic and vorticity waves are generated using a source term in the momentum and energy equations, as opposed to using inflow boundary conditions, to avoid spurious waves that may propagate from boundaries. The results show that the receptivity to surface excrescences is largely the result of an overall adverse pressure gradient posed by the step, and that the free-stream disturbances accelerate the generation of instabilities in the downstream. As expected, it is found that the acoustic disturbance interacting with the surface imperfection is more efficient in exciting the Tollmien-Schlichting waves than the vorticity disturbance. The latter generates TS waves that are grouped in wave packets with the length consistent with the wavelength of the free-stream disturbance.