KIDNEY AND ANTI-KIDNEY VORTICES IN CROSS-FLOW JETS

Water tunnel experiments were conducted to examine the effect of hole exit geometry on the near-field characteristics of crossflow jets. Hol e 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 ar ound the circumference of the jet was tracked to identify its relative contributions to the nascent streamwise vortices, which evolve eventu ally into kidney vortices downstream. The distinction between sidewall vorticity and that from the leading and trailing edges, though blurre d for a round hole, became clear for a square or a rectangular hole. T he choice of non-circular holes also made it possible to reveal the un expected 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 an d 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 boundar y layer to roll up into nascent kidney vortices. Here, hole width sets the lateral separation of these steady sidewall vortices. The vortice s comprising the upper deck ride intermittently over the top of the 's teady' 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 l ower pair. The origin of the upper deck is the hole leading-edge bound ary layer. This vorticity, initially aligned transverse to the crossfl ow direction, is realigned by the entrainment of crossflow momentum an d thus induces a streamwise component of vorticity. Depending on hole geometry, this induced streamwise vorticity can be opposite to the low er-deck vortex pair. The opposing pair, called the 'anti-kidney' pair, competes with the nascent kidney-vortices and affects the jet lift-of f. 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 o f parameters, all hole boundary layer vorticity, regardless of its ori gin along the hole circumference, is found to influence the kidney vor tices downstream.