We examine the response of an underexpanded jet to a depleting, finite reservoir withudexperiments and simulations. An open-ended shock tube facility with variable reservoirudlength is used to obtain images of nitrogen and helium jet structures at successive instancesudduring the blowdown from initial pressure ratios of up to 250. The reservoir and ambientudpressures are simultaneously measured to obtain the instantaneous pressure ratio. Weudestimate the time-scales for jet formation and reservoir depletion as a function of the specificudheat ratio of the gas and the initial pressure ratio. The jet structure formation time-scaleudis found to become approximately independent of pressure ratio for ratios greater thanud50. In the present work, no evidence of time-dependence in the Mach disk shock locationudis observed for rates of pressure decrease associated with isentropic blowdown of a finiteudreservoir while the pressure ratio is greater than 15. The shock location in the finite-udreservoir jet can be calculated from an existing empirical fit to infinite-reservoir jet dataudevaluated at the instantaneous reservoir pressure. For pressure ratios below 15, however,udthe present data deviate from a compilation of data for infinite-reservoir jets. A new fitudis obtained to data in the lower pressure regime. The self-similarity of the jet structureudis quantified and departure from similarity is noted to begin at pressure ratios lower thanudabout 15, approximately the same ratio which limits existing empirical fits.ud
展开▼
