On the vortex structures and flow characteristics of circular and elliptic jet impingement upon convex cylinders
Date of Issue2016
School of Mechanical and Aerospace Engineering
An experimental investigation based on laser induced fluorescence (LIF) and digital particle image velocimetry (DPIV) technique was conducted to provide a better understanding into the effects of jet-to-cylinder separation distance, cylinder-to-jet relative curvature and jet configuration on the vortical structures and dynamic behaviours resulting from circular and AR = 3 elliptic jet impingement upon both flat-plate and convex cylinders. Due to their different orientations with respect to the convex cylinders, two elliptic jet configurations (i.e. EJ1- and EJ2-configured) were studied. In particular, EJ1-configured represents that the elliptic jet major-axis is aligned with the cylinder axis, while EJ2-configured indicates the opposite case where the minor-axis is aligned with the cylinder axis. Separation distances of 1, 2 and 4 jet diameters were investigated, relative curvatures varied between D/d = 1, 2 and 4 for circular jet configuration and D/dh = 1.15, 2.3 and 4.6 for elliptic jet configurations. In all cases, the jet flow was maintained in a laminar condition with Reynolds number of Re = 2200 and 2100 for circular jet and elliptic jet configurations, respectively. LIF and DPIV results show that circular jet impingements produce key vortical behaviours reported previously (i.e. jet ring-vortex initiation, wall-separated vortex initiation, vortex-dipole formation and vortex-separation). For flat-plate impingements, these vortical behaviours always occur further away from the impingement point when the separation distance is smaller. The size of the vortex-dipoles and the spacing between consecutive vortex-dipoles decrease when the separation distance is reduced as well. When the flat-plate is replaced by convex cylinders, these trends persist and the impingement cylinders lead to non-uniform flow developments and vorticity distributions where the ring-vortex core size is reduced drastically along the cylinder straight-edges as compared to that along convex surfaces due to vortex-stretching effects. Moreover, as the relative curvature decreases, the locations for the previous key vortical behaviours are delayed further along the cylinder convex surfaces, but closer to the impingement point along the cylinder straight-edges. Interestingly, vortex-dipoles along the cylinder straight-edges demonstrate a strong tendency to travel upstream and interact with other upstream neighbours at separation distance H/d = 4. However, smaller separation distances introduce another flow mode of vortex-dipole pairing. In addition, varying the separation distance also influences the recirculating wake region size at the cylinder lee-sides. Elliptic jet-cylinder impingements produce similar effects caused by separation distance and relative curvature, as well as demonstrating sensitivity towards the jet orientation with respect to the cylinder axis. In particular, EJ1-cylinder impingements increase the non-uniform flow distributions over circular jet configuration, while EJ2-configuration produces comparable flow developments and hence leads to the least non-uniform flow distributions. The resulting flows of both EJ1 and EJ2 configurations remain attached to the impingement surfaces further downstream as compared to circular jet configuration. Apart from this, some unique vortical behaviours occur resulting from elliptic jets. For instance, vortex engulfment behaviours occur between adjacent ring-vortices prior to impingement at a separation distance of four jet diameters along the elliptic jet minor-plane and subsequently lead to the resulting different flow modes along the cylinder straight-edges. The braid vortices interact with adjacent ring-vortices and rib structures prior to impingement along the major-plane and they introduce intriguing impingement behaviour thereafter. Mean and instantaneous skin friction coefficient distributions determined from DPIV results not only showcase the different effects of separation distance, relative curvature and jet configurations on the wall shear stress, but also illustrate how the flow dynamics and unique vortical behaviours influence the local wall shear stress levels. On the other hand, wall-separated vortex initiation, vortex-dipole formation and vortex-separation events incur little distinctive changes upon the local wall shear stress distributions that may ease their unique identification. Lastly, possible impacts on heat transfer performances are discussed for different jet configurations. More details on the above-mentioned jet impingement flow scenarios and behaviour will be elaborated in the thesis.