PERIODIC VELOCITY FLUCTUATIONS IN THE NEAR WAKE OF A ROTATING FLAT-PLATE BLADE AND THEIR ROLE IN THE GENERATION OF BROAD-BAND NOISE

The effect of the shape near the trailing edge of a rotating flat-plat e blade on the periodic velocity fluctuation of the wake of the blade has been experimentally investigated. The results show that the charac teristics of the near wake in the rotating flow field are similar to t hose in the uniform flow field; i.e., periodic velocity fluctuation do es not occur when the suction surface of the blade is beveled near the trailing edge, but occurs if the pressure surface is beveled. In the latter case, two types of trailing edge separation generally occur: th at is, the flow always adheres up to the trailing edge of the suction- side-beveled blade. On the other hand, the flow separation alternately and randomly occurs at two different locations, either the starting p oint of the beveling of the blade or a certain fixed point on the beve led surface in the case of a pressure side beveled blade. The importan t role of the periodic velocity fluctuation in the generation of broad band noise from a rotating flat-plate blade is also verified.