Abstract
Abstract
A force balance can provide limited information on the loads generated by the flow through a nozzle, such as the magnitude and direction of time average loads, but the flow features responsible for generating them remain elusive. In this study, a comparison of steady axial thrust measurements is carried out between a conventional load cell and a flow survey by using particle image velocimetry (PIV) and pressure probes to validate that the latter method can be used for thrust measurements as well as to investigate the flow field. A Mach 4 convergent-divergent nozzle with 12.7-mm throat diameter was tested by using compressed air at a range of substantially over-expanded conditions using jet stagnation pressures of 4 to 7.5 times the nozzle exit ambient pressure. The nozzle supply line was fitted with an electric heater generating stagnation temperatures in the range of 300 to 450 K. Using a load cell, the thrust produced by the nozzle was measured at each condition. The flow field downstream of the nozzle was surveyed by using a pressure rake and 2-dimensional PIV. Combining the data from both flow surveys, static pressure, and axial flow velocity, momentum flux as well as static pressure contribution to thrust were calculated. A comparison between the thrust obtained by the 2 methods verified that PIV and pressure surveys could be used to determine the time-averaged thrust of a nozzle to within 6% of the load cell reading.
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