SBIR-STTR Award

The technical objectives of the present effort, over three SBIR phases, are to develop a far field wave propagation tool that will accurately predict the aeroacoustic characteristics of rotorcraft in the far field, a number of kilometers away, while accounting for the presence of irregular terrain, atmospheric and (approximate) ground absorption, temperature and density gradients in the atmosphere, and prevailing winds. This work is divided into three phases. Phase I focuses on implementing the present far field method and validating it for well documented thickness noise and BVI noise simulations for wind tunnel conditions. The Phase I option period involves extending the methodology to wave length dependent effects such as nonlinear wave steepening and accurate ground absorption. Phase II focuses on further validation of this methodology for rotorcraft for a variety of steady, descent, and maneuver operations while accounting for terrain and atmospheric effects. Phase III focuses on incorporating industry requirements and feedback into an industry standard methodology that includes simulation and visualization of rotorcraft acoustic propagation phenomena of interest to helicopter industries.

Keywords:
Wave Confinement, Rotorcraft Acoustics, Nonlinear Solitary Waves, Finite Difference Methods, Acoustic Based Position Location, Detectibility.

The purpose of this SBIR project is to develop a rotorcraft acoustic solver that can treat long distance propagation through general, realistic environments. Only a Finite Difference Time Domain approach is capable of treating propagation through general inhomogeneous media, and only be considered outside the “inner” region (near the rotorcraft). However, the range limitation precludes the use of conventional formulations. This problem can easily be overcome by a new method, Wave Confinement (WC), developed by the proposed principal investigator. The basic idea of WC is first to extend the basic linearized Euler pde, by adding a nonlinear “confinement” term. This term results in thin stable, propagating structures, whose centroids propagate exactly as the pulses of the original “unenhanced” pde’s. Also, the total amplitude, integrated along a ray, with geometric (azimuthal) spreading taken into account, is conserved. The difference is that pulse computed with WC has a profile defined by the computational method and not by the original, physical pulse profile. With WC (as with shock capturing methods), the important quantities involve conserved integrals through the thin shock, or pulse and the centroid location, but not profile. In addition, WC can treat reflections from obstacles in a very simple way.

Keywords:
rotorcraft Acoustics, Long Range Propagation, Discrete Methods, Wave Confinement, Wave Equation