Transonic Blade-Vortex Interactions: The Far Field

A study is made of mid-field and far-field noise generated by transonic Blade-Vortex Interactions {BVI), typical of helicopter main rotor noise. The VTRAN2 small disturbance, two-dimensional, transonic flow code, which includes convected vorticity, is used to compute near-and mid-field flow information. Because of mesh limitations this information is restricted to the mid-field and does not give the desired far-field and three-dimensional information. A method of extending the solutions to the far field in three dimensions is developed based on Kirchhoffs solution to the linear flow outside a surface S enclosing the nonlinear near field. A spherical wave is first used as a test case for the method. A number of the directivity aspects of BVI noise are explained in terms of the propagation characteristics of the waves. The relation of unsteady type "C" shock motion on the airfoil to far-field sound is shown. The far-field radiation is affected by Mach number, airfoil thickness and shape. Vortex strength and miss distance are also found to be important. Finally, some results for three-dimensional interactions with swept and curved vortices are also obtained and discussed.