Vortex dynamics in a normally impinging planar jet

The flow development of a jet issued from a high-aspect ratio slot nozzle impinging on a flat plate is investigated experimentally. Time-resolved, two-component particle image velocimetry is used to characterize the flow for Reynolds numbers based on slot width and streamwise jet centerline velocity of 3000 and 6000, and impingement height ratios of 2 and 4. A quantitative description of the vortex dynamics is provided and the effects of Reynolds number and impingement height on the vortex evolution are characterized for the experimental conditions investigated. Primary vortices are shed in a highly periodic manner with strengths that scale with Reynolds number. Primary vortex merging is observed in the wall-jet region for all test conditions while increasing the impingement height ratio from 2 to 4 causes vortex merging to also occur in the free-jet region. Passage of single and merged primary vortices induces the formation of secondary vortices along the impingement surface, though the shedding frequencies of these secondary structures exhibit higher variability than that of the primary vortices. The secondary vortices are similar in strength to the primary vortices for a Reynolds number of 3000, but their relative circulation is decreased at a Reynolds number of 6000. Secondary vortex shedding at the lower Reynolds number leads to a higher growth rate of the wall-jet half-width due to pairing with primary vortices and subsequent ejection away from the surface, which is not found at the higher Reynolds number investigated.