Kidney and anti-kidney vortices in crossflow jets

Water tunnel experiments were conducted to examine the effect of hole exit geometry on the near-field characteristics of crossflow jets. Hole shapes investigated were round, elliptical, square, and rectangular, all having the same cross-sectional area. Laser-induced fluorescence (LIF) and particle image velocimetry (PIV) were used. The vorticity around the circumference of the jet was tracked to identify its relative contributions to the nascent streamwise vortices, which evolve eventually into kidney vortices downstream. The distinction between sidewall vorticity and that from the leading and trailing edges, though blurred for a round hole, became clear for a square or a rectangular hole. The choice of non-circular holes also made it possible to reveal the unexpected double-decked structures of streamwise vortices and link them to the vorticity generated along the wall of the hole. The lowermost vortex pair of the double-decked structures, located beneath the jet, is what we call a ‘steady’ vortex pair. This pair is always present and has the same sense of rotation as the kidney vortices. The origin of these lower-deck vortices is the hole sidewall boundary layer: as the jet emanates from the hole, the crossflow forces the sidewall boundary layer to roll up into nascent kidney vortices. Here, hole width sets the lateral separation of these steady sidewall vortices. The vortices comprising the upper deck ride intermittently over the top of the ‘steady’ lower pair. The sense of rotation of these upper-deck vortices depends on hole geometry and can be the same as, or opposite to, the lower pair. The origin of the upper deck is the hole leading-edge boundary layer. This vorticity, initially aligned transverse to the crossflow direction, is realigned by the entrainment of crossflow momentum and thus induces a streamwise component of vorticity. Depending on hole geometry, this induced streamwise vorticity can be opposite to the lower-deck vortex pair. The opposing pair, called the ‘anti-kidney’ pair, competes with the nascent kidney-vortices and affects the jet lift-off. The hole trailing-edge boundary layer can likewise be turned toward the streamwise direction. In this case, the turning is caused by the strong reverse flow just downstream of the jet. In the present range of parameters, all hole boundary layer vorticity, regardless of its origin along the hole circumference, is found to influence the kidney vortices downstream.