Surface Dipole Strength Generated by Cylindrical Struts

Résumé

The purpose of this work was to further establish the viability of the Causality Correlation Technique as a diagnostic tool for the treatment of noise problems. Acoustical dipole radiation can be generated by obstructing a subsonic flow with a rigid strut, if the strut exerts fluctuating forces on the fluid. Such forces would be the forces of reaction arising from the unsteadiness in the local flow and would form a distribution of acoustical dipole sources over the surface of the strut. For the experiments reported herein, a subsonic flow issues from a circular nozzle which is 3.8×10^-2 m in diameter. The ‘quiet’ air jet operates at an exit Mach number of .217. The exit velocity is 72 m/s and is approximately uniform over the exit plane. The cylinder model is stationed at the potential core of the jet the Reynolds number is 6.3×10⁴ (based on cylinder diameter and exit velocity). The ‘Dipole Radiation Intensity (DRI)’ is a uniquely defined and measurable quantity that is intimately related to the classical dipole. The ‘spatial distribution’ of the DRI can be constructed on a surface using the Causality Correlation Technique (see Siddon). The ‘DRI distribution’ is constructed on the surface of the rigid cylindrical strut. A diagnosis is made of the aerodynamic noise generation mechanisms using the said distribution. The far field SPL originating from the surface exclusively is predicted from the integrated DRI distribution. For laminar incident flow the predicted SPL is (69.3 ± 2.3 dB). This may be compared with an overall SPL of (70.1±.5 dB) which was directly measured.