Grid Design and Computational Scheme
All the computations are carried out in the x — r plane. The size of the computational domain is 35D x 17D. By extending the computational domain to 35D downstream, all the screech tone noise sources are effectively included in the computation. Also, the amplitude of the excited instability waves would have decayed significantly at the outflow boundary, thus lessening the likelihood of reflection of unsteady disturbances back into the computational domain. By extending the computational domain to 17D in the radial direction, it is believed that the outer boundary is approximately in the far field. By measuring the radiation angle from the location in the jet where tone emission is the strongest, accurate screech tone directivity can be determined.
The computational domain is divided into four subdomains each having a different mesh size as shown in Figure 15.54. The subdomain immediately downstream of the nozzle exit, where the jet mixing layer is thin and the jet plume contains a shock cell structure, has the finest mesh: Ax = Д r = D/64. The mesh size of the next subdomain increases by a factor of 2. This continues on so that, outside the jet where
the mesh is the coarsest, has a mesh size of D/8. The governing equations in cylindrical coordinates are discretized by the multi-size-mesh multi-time-step method (see Chapter 12). The use of multiple time steps is crucial to the computational effort. It greatly reduces the run time of the computer code.