Pitot pressure measurements and flow visualizations were used to investigate streamwise vortices previously observed in underexpanded jets. A simple model was developed, which gives reasonable agreement with the pressure measurements. A converging nozzle and converging-diverging nozzle of design Mach number 1.5 were used to generate jet flows of equivalent Mach numbers up to 2.5 (stagnation to ambient pressure ratios up to 17.1). By operating the nozzles fully expanded, overexpanded, and underexpanded, insight was gained into both the occurrence and cause for formation of the vortices. Spatially stationary streamwise vortices were found to exist in the near-field region around the circumference of underexpanded jets in the vicinity of the jet boundary. Short exposure visualizations show the vortices persist much farther downstream with a loss of spatial organization. Visualizations suggest adjacent vortices have streamwise vorticity of opposite sign, so the action of adjacent vortices is to either pump jet fluid radially outward or entrain ambient fluid radially inward toward the jet. The downstream extent, strength, and number of vortices around the jet circumference increase with degree of underexpansion. A large number of vortices is found near the nozzle exit. Fewer vortices of larger scale are found farther downstream, indicative of a merging process. The absence of the vortices in fully expanded and overexpanded jets suggests the vortices are a consequence of a Taylor-Goertler-type instability.
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