NUMERICAL SIMULATIONS OF FLOW MODIFICATION OF SUPERSONIC RECTANGULAR JETS

Numerical simulations have been performed to study the flowfield and n ear-field noise of a supersonic rectangular jet with two paddles inser ted into the how The paddles cause a strong flapping motion to develop that enhances mixing of the jet with the surroundings. These simulati ons have been used to determine the flapping motion's frequency, the m ixing enhancement, the near-field noise, and the thrust loss associate d with the paddles and to study the acoustic feedback mechanism that m odulates the flapping motion. The flapping frequency has been estimate d using the pitot pressure distributions at a sequence of times and Fo urier analysis of the local pressure and z component of velocity. For paddles located x/h = 7.3 from the nozzle, where h is the narrow dimen sion of the nozzle, a frequency of 4700 Hz [St(h) = 0.136] with an amp litude of 157.5 dB at the nozzle lip has been predicted and is in agre ement with experimental results. The pitot pressure drop, the mass, an d the x-momentum fluxes along the flow direction have been used as a m easure of jet mixing for jets with and without paddles inserted into t he flow. In our numerical simulations, a control volume approach was u sed to estimate the thrust loss caused by the insertion of the paddles . The computational value of 13% is close to the experimental value of 14.4%, considering that the physical support for the paddles in the e xperiments is not included in the simulations. A special sequence of l ocal pressure distribution plots, which highlight the acoustic waves, has been used to study the feedback mechanism that modulates the flapp ing motion.