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A general analytic method for calculating the scattering of sound by multiple rigid circular cylinders arranged in an arbitrary parallel configuration is presented. The sound scattered by this collection of cylinders is generated by a time-periodic, spatially distributed, axisymmetric source located within the domain of interest. A Hankel transform method is used to calculate the incident field, while separation of variables is used to obtain the scattered fields from each cylinder in the collection. The unknown scattering coefficients are determined through the use of general addition theorems that allows the various fields to be readily transformed between coordinate systems. The method is validated using various two-, three-, and four-cylinder configurations, and the number of coefficients that must be retained in the truncated series is examined. Benchmark configurations consisting of two- and three-cylinder systems with cylinders of varying radii are also presented. These solutions have been used to validate computational aeroacoustic solvers developed for complex geometries.

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In this work, the flow fields associated with two canonical turret geometries, a fully exposed hemisphere on a flat plate and a 50% submerged hemisphere on a flat plate, were simulated using the OVERFLOW 2 flow solver. Both turret geometries utilize a flat-window aperture with an aperture ratio (ratio of the aperture diameter to the turret diameter) of 0.295 and an elevation angle of 57 • . The forward field of regard was the particular focus in this study, and both symmetric (azimuth angle of 0 • ) and asymmetric (azimuth of 45 • ) window orientations were examined. Two flight conditions were also studied; a subsonic case with M = 0.45 and ReD = 6.30 × 10 6 and a transonic case with M = 0.85 and ReD = 9.53 × 10 6 . The flow field was simulated using the Delayed Detached Eddy Simulation capability of OVERFLOW in conjunction with the spatially fifth-order Weighted Essentially Non-Oscillatory (WENO) scheme to capture the off-body vortical structures The incoming boundary layer was set a the same height for both geometries, which corresponded to a quarter of the height of the fully exposed hemisphere and half of the height of the submerged turret. The impact of the turret aerodynamics on the performance of the turrets for directed energy applications is inferred through consideration of the flow features, density and pressure fluctuations, and forces on the turrets.

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SUMMARYA parallel, high-order, overset-grid method is validated for use in large eddy simulation (LES) through its application to fundamental turbulent flow problems. The current method employs a high-order, compact finite-difference approach to evaluate spatial derivatives, with up to tenth-order low-pass filters used to remove high-frequency spurious wave content. These filters have also been found to be effective in modelling the dissipation that occurs at the unresolved scales in the flow for LES simulations. Temporal integration is based on an implicit, approximately factored and diagonalized, second-order algorithm, which reduces the time-step constraints present in explicit time-marching methods for wall-bounded viscous flows. Parallelization, geometric complexity, and local grid refinement are all addressed through the use of an overset-grid approach, with grid communication provided by high-order Lagrangian interpolation. The problems investigated in this work include fully turbulent channel flow at Re = 590 and 1017, and the transitional wake generated by flow over a single circular cylinder at Re D = 3900. The results obtained with the current approach are validated against well-resolved benchmark calculations or experiments and the impact of the order-of-accuracy of the interpolation method is investigated. The benefits obtained by using the general overset-grid technique to reduce grid point requirements compared to single-grid simulations are also examined. It is shown that for the problems considered in this work, substantial grid-point savings may be obtained with an overset-grid approach compared to a single-grid approach, and that the use of high-order interpolation at overset boundaries is important in maintaining overall solution accuracy.

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