Synthetic Jets in Crossflow

摘要

Results of an experimental investigation on synthetic jets, with and without crossflow, from orifices of different geometry are presented. Jet Reynolds number up to 2.3 x 10~4 and Stokes number up to 400 are covered. For the round orifice, in the absence of crossflow, the threshold for formation of the synthetic jet is found to be at a stroke length, L_0/D ≈ 0.25. Above L_0/D ≈ 5, the profiles of normalized centerline mean velocity appear to become invariant. It is reasoned that the limiting behavior at high L_0/D is related to the phenomenon of saturation of impulsively generated vortices. For the synthetic jet in crossflow (SJCF), distributions of mean velocity, stream wise vorticity, as well as turbulence intensity are found to be similar to those of a steady jet-in-crossflow (JICF). A pair of counter-rotating streamwise vortices, corresponding to the bound vortex pair of the steady case, is clearly observed. Mean velocity field, for the case of normal injection, exhibits a "dome" of low-momentum fluid pulled up from the boundary layer, and the entire domain is characterized by high turbulence. When the maximum velocity during the discharge stroke of a SJCF equals the exit velocity of a JICF, the penetration of the two are found to be comparable for a given crossflow. Other orifice geometries included a tapered, a pitched, and a cluster of nine, all having the same cross-sectional area at the exit. The flowfield from the cluster, when viewed a few equivalent diameters downstream, resembles that from the single orifice. However, jet penetration is somewhat lower as a result of increased mixing of the distributed jets with the crossflow. Instead of the dome of low-momentum fluid, a pitched SJCF is characterized by a core of high-momentum fluid. Its penetration is the lowest, as expected. The trajectory of the SJCF from a single orifice, normal or pitched, is well represented by the power function correlation available for a JICF.