DEVELOPMENT OF A SHOCK NOISE PREDICTION CODE FOR HIGH-SPEED HELICOPTERS — THE SUBSONICALLY MOVING SHOCK

It has been known for over a decade that quadrupole sources contribute significantly to the impulsive noise of high speed helicopter rotors. The quadrupole source has the same directivity and order of magnitude as the thickness noise, i.e. the peak directivity is in the rotor plane in the forward region of flight. In an attempt to develop an efficient quadrupole noise prediction method, Farassat. Brentner, and Myers studied analytically the contribution of regions of high gradients of flow parameters to quadrupole noise. They showed that shock surfaces over high speed rotors can be potent noise generators. Later in 1990, Farassat and Tadghighi showed by a realistic model that rotating shocks at transonic speed generate essentially a negative pressure pulse of the same directivity and magnitude as the thickness noise. Thus, the major component of quadrupole noise is due to shock surfaces on the blades. In addition, a shock noise prediction formula was derived similar to Langley’s formulation 1A for loading and thickness noise calculation. The derivation of this result hinges on mapping the time dependent shock surface to a time independent computational domain. The present paper reports on coding this shock noise prediction formula and incorporating it in Langley’s rotor noise prediction code WOPWOP. The structure and algorithms used in the shock noise prediction code are presented. In the design of the code, special care was exercised to reduce computational time while maintaining high accuracy. Numerical examples of shock noise prediction for hover and forward flight are presented. These examples confirm that shock noise is an important component of the quadrupole source. In addition, previous theoretical analyses of shock noise level and directivity arc confirmed.