Influence of upstream surface roughness on a jet in crossflow

Résumé fourni par la source

The flow behaviour of an inclined jet in crossflow (JICF) subjected to different upstream roughness regimes – smooth, transitionally rough ( ${k}_{s}/{D}=0.129$ , where $k_{s}$ is the equivalent roughness height and $D$ is the jet hole diameter) and fully rough ( ${k}_{s}/{D}=0.782$ ) – is investigated using refractive-index-matching time-resolved particle image velocimetry for velocity ratios VR = 0.33, 0.67 and 1.0. The transitionally rough regime produces intensified, unbroken large-scale vortices that induce strong intermittency in the boundary-layer thickness. Its energy spectrum retains the original multi-scale coherence, but exhibits an additional wavelength associated with this intermittency. This intermittent state imposes the strongest unsteady influence on the downstream JICF, manifested as a binary flow pattern consisting of alternating jet lift-off and regular shedding. Time-averaged fields show a vertically elongated counter-rotating vortex pair (CRVP), while extended spectral proper orthogonal decomposition (ESPOD) indicates that the binary state drives modal bifurcation and distinct pathways associated with prematurely initiated centreline disturbance amplification. In contrast, the fully rough regime promotes rapid breakdown into small-scale turbulence. The ESPOD confirms that multi-scale frequency coherence decays rapidly, whereas low-frequency coherence experiences a brief amplification due to upstream signal injection. Additionally, secondary flows are observed to exert a localised, asymmetric modulation of the CRVP at VR ≤ 0.67: on the high-momentum pathway (downwash) side, one CRVP lobe is vertically compressed and its decay is delayed, while on the low-momentum pathway (upwash) side, the opposite lobe is vertically stretched and its decay is accelerated.