Jet noise during the lift-off of a launch vehicle is complicated by the simultaneous flow of multiple jets, and their deflection by jet deflectors. Further, the presence of the launch pedestal, the service structure, and the moving vehicle itself, act as reflecting surfaces, which contribute to the noise environment. The present work involves the suppression of noise as measured at different parts of the launch vehicle in a small-scale replica of a full launch pad, for different locations of the vehicle along its vertical lift-off trajectory. The primary source of noise is the two jets emerging from the base of the launch vehicle at a Mach number of 3.38. Noise is suppressed by water injection at different locations in the launch pad such as the upstream and downstream edges of the jet deflector cover-plate, bottom and top of the launch pedestal, and at two different locations on the service structure. The effect of staged injection of water, i.e., without and with injection at different heights relative to the position of the jets, as the vehicle is at different locations in its trajectory, is examined. It is found that sustained suppression of noise is obtained only with injection of water in successive stages closer to the nozzle exit, as the jet position is moved up. The effect of angle of injection of water is investigated to verify previously reported results that an injection angle of 60°, i.e., along the flow, is superior to injection at 90°to the jet. The effect of injection pressure shows an optimum that supports the effective atomization of the injected water jets into droplets due to shear by the gas jet flow. Investigation with hot jets shows increased noise levels by at least ~2 dB relative to cold jets of nearly the same nozzle-exit Mach number. However, an increased reduction of noise by water injection is apparent with hot jets. The reduction is nearly independent of the jet temperature in the 600-900 K range. The present results collectively confirm the qualitative applicability of different previous studies on single free jets to the complicated launch vehicle noise scenario.

Practical application of active flow control of high speed flows is dependent upon the development of simple and robust actuators that can produce high momentum and are reliable, low cost, and responsive and can be easily integrated. This paper presents an experimental investigation of the characterization and implementation of high bandwidth micro-actuators for the control of supersonic resonant flows. The striking feature of this micro-actuator is its high momentum mean flow along with high amplitude and a tunable frequency unsteady component. First generation micro-actuators are designed and their performance is tested in controlling the highly unsteady impinging jet flow field. The results show that the impinging tones are completely eliminated with the actuation of these micro-actuators, whereas, new peaks at a frequency different from the actuation frequency and its harmonics are observed in the spectra, the occurrence of which need to be further explored. A reduction of 3-4 dB in overall sound pressure levels (OASPL) is achieved over the range of test conditions.

This paper describes an experimental study conducted at the Advanced Aero propulsion Lab (AAPL) for the design and development of actuator systems capable of producing high bandwidth, high momentum microjet arrays for active flow control applications. A systematic approach for designing micro-actuators with high unsteady and mean momentum efflux is followed. Beginning with a simple configuration, i.e., supersonic impinging microjets, we added more geometric complexity to the actuator design to finally arrive at an actuator configuration that provides the desired flow properties. Our first generation actuator design consists of a primary source jet, incident upon a cylindrical cavity. The lower surface of this cavity contains micronozzles through which the unsteady microjets (400ȝm) issue. Results clearly show that microjets produced by this actuator contain very high mean momentum (300-400 m/s) as well as a very significant unsteady component (70-100 m/s). Experiments were conducted over a large range of parameters, in terms of cavity length, source jet NPR and source jet impingement distance. The results unequivocally demonstrate the ability to vary the frequency as well as the amplitude of the mean and unsteady momentum of the microjets issuing from this actuator. By varying the dimensions of the actuator by only few hundred microns, we were able to tune the frequency of the unsteady component over intervals of 10-15 kHz. The ability to produce, unsteady flow with significant mean and unsteady components, where the dynamic range can be easily varied makes these actuators promising for a number of flow control applications.

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