Incompressible Flow Simulation Over a Half Cylinder with Results Used to Compute Associated Acoustic Radiation

Computation of flow generated noise is increasingly possible due to the availability of increasingly detailed numerical solutions to the flow. The most general way to compute the noise radiation by a turbulent flow is to numerically solve the Navier-Stokes equation. The computation needs to be performed over a large spatial domain for long time intervals, simultaneously, with the ability to resolve small scales. This requirement overwhelms present day computing power. However, depending on the speed and nature of the flow, certain simplifications can be used to make the computations feasible. For low Mach number flows in which there is no significant back reaction from the acoustics on the flow, the noise calculations can be reduced to a two step procedure: (1) Computation of the underlying flow. (2) Acoustic computations. Since Mach number of the underlying flow is small, incompressibility is still a valid assumption for the flow simulation. The second step can be accomplished by Acoustic Analogy or Kirchhoff's method with the sources calculated from the solution available from the first step. The present work uses the two step procedure just outlined, with the numerical solution to the flow problem obtained by solving the incompressible, time dependent Reynolds Averaged Navier Stokes equation. Experiments were conducted at the Glenn L. Martin Wind Tunnel to ascertain the accuracy of the computational simulation. The acoustic solution is obtained by solving the two dimensional Lighthill's wave equation which is transformed to the Helmholtz equation for the generalized body fitted coordinate system and reduced to a finite number of algebraic equations using the Finite Analytic technique. The two step procedure is demonstrated for the case of a half cylinder in and out of ground effect by enforcing the Dirichlet boundary condition derived from the computational simulations. The computed far field noise radiation shows dipole patterns for both the lift and drag type for the free stream case with a considerable change in the directivity pattern when the ground is present.