Exit Topology Effects on Low‑Mach Jet Impingement

Planar PIV characterizes a low‑Mach (M = 0.05) air jet impinging normally on a plate at fixed spacing H/D = 4.0. Four nozzles of equal hydraulic diameter (D = 50.8 mm) are compared: circular, rectangular (AR 3), diamond, and elliptical (AR 2); asymmetric exits are tested in major and minor orientations. Mean velocity, azimuthal vorticity, and turbulence kinetic energy reveal that exit topology strongly modulates the stagnation footprint and shear‑layer growth. The circular jet forms the canonical toroidal wall jet and a narrow stagnation core (dₛ = 0.9D). Orienting the rectangular major axis toward the plate elongates the core to 1.8D x 0.6D and suppresses peak TKE by 30 %, whereas the minor‑axis jet concentrates momentum, doubling local vorticity. Diamond and elliptical exits generate four- and two‑lobe wall jets, broadening the high‑TKE annulus and displacing turbulence production away from the centerline. All non‑circular cases lower the maximum wall shear stress and near‑wall velocity fluctuations by up to 28 % yet conserve area‑integrated TKE, indicating redistribution rather than dissipation of momentum. The results demonstrate that simple geometric tailoring can passively attenuate erosion, acoustic loading, and recirculation beneath launch and test stands without compromising thrust efficiency.