DIRECT COMPUTATION OF THE SOUND FROM A COMPRESSIBLE CO-ROTATING VORTEX PAIR

The far-field sound generated by compressible co-rotating vortices is computed by direct computation of the unsteady compressible Navier-Sto kes equations on a computational domain that extends to two acoustic w avelengths in all directions. The vortices undergo a period of co-rota tion followed by a sudden merger. The directly computed far-field soun d is compared to the prediction of the acoustic analogy due to Mohring (1978, 1979), a modified form of the analogy developed by Lighthill ( 1952), and an acoustic analogy derived by Powell (1964). All three pre dictions are in excellent agreement with the simulation. Results of fa r-field pressure fluctuations from an acoustically non-compact, co-rot ating vortex pair are also presented. In this case, the vortex sound t heory over-predicts the sound by 65% in accordance with the analysis o f Yates (1978).