DISCRETE FREQUENCY SOUND RADIATION DUE TOFLUID FLOW OVER FLAT PLATES WITHACOUSTICALLY NON-COMPACT CHORD

摘要

The intensity and the directivity of discreternfrequency sound generated from flat plates withrnvarious chord-to-thickness ratios and two typesrnof edge shapes were investigated by wind tunnelrnexperiments and numerical analyses. Thernexperimental data indicated that as either arnchord length becomes longer or wind velocityrnbecomes larger, the intensity of aerodynamicrnsound is not simply proportional to the flowrnvelocity to the sixth power (sixth power law)rnand the dipole type directivity pattern is biasedrntoward upper stream direction. That is, the flatrnplate becomes acoustically non-compact evenrnwhen the wind velocity is at very low Machrnnumber. Numerical analyses for some platesrnwere performed in order to be compared withrnthe experimental data. A combined method wasrnapplied. That consists of a two-dimensionalrncomputational fluid dynamics (CFD) codernbased on the incompressible Navier-Stokesrnequations and a two-dimensional aero-acousticsrncode based on Lighthill’s equation employing arnGreen's function tailored to the geometry ofrneach flat plate. The calculated sound fieldsrnqualitatively agree with the experimental ones.rnMoreover, identification of sound sourcernlocation was carried out by the numericalrnmethod. It was found that the effectivernLighthill's sound source exists near the trailingrnedge, whatever the edge shape is. On the basisrnof these results, a simplified model to presentrnthe sound field around a non-compact plate isrnintroduced. Furthermore, a practical criterionrnfor compactness of a flat plate is proposed thatrnthe ratio of the chord to the acoustic wavelengthrnis less than about 0.3.