Large-Scale Investigation of a Flat-Plate Pulsed Vortex Generator Jet in Crossflow Using PIV

摘要

The use of small jets of air has proved to be an effective means of flow control on low Reynolds number turbine blades. Pulsing of these jets has also shown benefits in reducing the amount of air needed to achieve the same level of flow control. An experiment using Particle Image Velocimetry (PIV) has been set up to investigate how these pulsed jets interact with the boundary layer to help stabilize it and keep the flow attached. A 25x scaled jet in a plate has been utilized. The 25.4 mm diameter jet has a pitch angle of 30 degrees and a skew angle of 90 degrees. Pitch angle is defined as the angle the jet makes with the surface of the plate, and the skew angle is the angle that the jet makes with the cross flow. The jet was pulsed at a frequency of f = 0.5 Hz with duty cycle (pulse duration of the total period T) of Delta = 50%. Blowing ratios pjVj/pinfinityUinfinity, or ratios of jet velocity, Vj, to freestream velocity, Uinfinity, when the densities pj and pinfinity are equal of B = 0, 0.5, 1, 2, and 4 were studied. A reduced frequency parameter was defined as F+ = fl/Uinfinity, with the reference length l equal to the jet diameter, d, resulting in F+ = 0.004 with Uinfinity = 3.15 m/s. Velocity and vorticity planes were obtained at different spanwise locations and used in the characterization of the flow. Based on previous research, the starting vortex, which develops at the beginning of every cycle for each pulsed case, is the critical flow characteristic. The current study shows that both the starting and ending of every duty cycle are keys to obtaining attached flow, and that attachment is improved with larger in-plane vorticity, omegaj.