Supersonic Jet Excitation using Flapping Injection

Supersonic jet noise reduction is important for high speed military aircraft. Lower acoustic levels would reduce structural fatigue leading to longer lifetime of the jet aircraft. It is not solely structural aspects which are of importance, health issues of the pilot and the airfield per- sonnel are also very important, as high acoustic levels may result in severe hearing damage. It remains a major challenge to reduce the overall noise levels of the aircraft, where the supersonic exhaust is the main noise source for near ground operation. Fluidic injection into the supersonic jet at the nozzle exhaust has been shown as a promising method for noise reduction. It has been shown to speed up the mix- ing process of the main jet, hence reducing the kinetic energy level of the jet and the power of the total acoustic radiation. Furthermore, the interaction mechanism between the fluidic injection and the shock structure in the jet exhaust plays a crucial role in the total noise radia- tion. In this study, LES is used to investigate the change in flow struc- tures of a supersonic (M=1.56) jet from a converging-diverging nozzle. Six fluidic actuators, evenly distributed around the nozzle exit, inject air in a radial direction towards the main flow axis with a total mass flow ratio of 3%. Steady injection is compared with flapping injection. With flapping injection turned on, the injection angle of each injector is varied sinusoidally in the nozzle exit plane and the variation is the same for all injectors. This fluid dynamics video is submitted to the APS DFD Gallery of Fluid Motion 2013 at the 66 the Annual Meeting of the American Physical Society, Division of Fluid Dynamics (24-26 November, Pittsburgh, PA, USA).Comment: 3 pages, 2 linked animations/video

Reduction of supersonic jet noise using micro-jets in the diverging part of a conical CD-nozzle

Reducing the radiated noise from supersonic jets with maintainedthrust is a major challenge. An LES/CAA method and experiments are uti-lized to investigate the flow field and the far-field acoustics of a supersonic jetemitted from a circular CD-nozzle with a sharp throat. Air is injected into thedivergent section of the nozzle using 12 evenly distributed micro-jets aroundthe nozzle circumference and the change in total thrust and acoustic signa-ture is investigated. The micro-jets are shown to decrease shock-strength withincreased injection until an optimum point where further increase in injectionmass flow caused increased shock-strength. As a consequence of the reducedshock strength, the far-field shock-noise was decreased. The net thrust wasshown to increase with increased injection mass flow, whereas the specificthrust was slightly decreased. Micro-jet injection into the divergent sectionof a CD-nozzle appears, therefore, to be an effective configuration for reducedshock-noise radiation from supersonic jets

Reduction of supersonic jet noise using micro-jets in the diverging part of a conical CD-nozzle

Reducing the radiated noise from supersonic jets with maintainedthrust is a major challenge. An LES/CAA method and experiments are uti-lized to investigate the flow field and the far-field acoustics of a supersonic jetemitted from a circular CD-nozzle with a sharp throat. Air is injected into thedivergent section of the nozzle using 12 evenly distributed micro-jets aroundthe nozzle circumference and the change in total thrust and acoustic signa-ture is investigated. The micro-jets are shown to decrease shock-strength withincreased injection until an optimum point where further increase in injectionmass flow caused increased shock-strength. As a consequence of the reducedshock strength, the far-field shock-noise was decreased. The net thrust wasshown to increase with increased injection mass flow, whereas the specificthrust was slightly decreased. Micro-jet injection into the divergent sectionof a CD-nozzle appears, therefore, to be an effective configuration for reducedshock-noise radiation from supersonic jets

Elimination of shock associated noise in supersonic jets by destructive wave interference

A novel application of fluidic injection was developed to investigate and understand the effects of discrete fluidic injection internal to the jet nozzle. Various injection locations, angles, and conditions were studied resulting in unique acoustic behavior and flow field modifications. For most conditions the acoustics are relatively unaffected or increased, but for very specific conditions noise was drastically decreased. For optimized conditions the shock noise was completely eliminated and in other cases a jet instability was generated that significantly decreased high frequency noise. Measurements of the velocity field indicated that shock interaction due to shocks from the injection jet interact with the primary jet shocks, and significantly reduce the shock strength, attributing massive shock noise reduction. Validation of the experimental results was achieved with LES, which provided additional insight into the shock suppression due to resolution of the flowfield internal to the nozzle. Optimal injection parameters resulted in reduction of OASPL of -7 dB at the upstream and downstream angles simultaneously through a combination of shock disruption and streamwise vorticity introduction. A new mechanism of supersonic jet noise reduction, destructive interference of the shock structure in the jet is reported

Elimination of Shock-Associated Noise in Supersonic Jets by Destructive Wave Interference

A novel application of fluidic injection was developed to investigate and understand the effects of discrete fluidic injection internal to the jet nozzle. Various injection locations, angles, and conditions were studied, resulting in unique acoustic behavior and flowfield modifications. For most conditions, the acoustics are relatively unaffected or increased, but for very specific conditions, noise was drastically decreased. For optimized conditions, the shock noise was completely eliminated, and in other cases, a jet instability was generated that significantly decreased high-frequency noise. Measurements of the velocity field indicated that shock interaction, due to shocks from the injection jets, interacts with the primary jet shocks and significantly reduces the shock strength, attributing massive shock noise reduction. Validation of the experimental results was achieved with large-eddy simulation, which provided additional insight into the shock suppression due to resolution of the flowfield internal to the nozzle. Optimal injection parameters resulted in reduction of overall sound pressure level of -7 d B at the upstream and downstream angles simultaneously through a combination of shock disruption and streamwise vorticity introduction. A new mechanism of supersonic jet noise reduction and destructive interference of the shock structure in the jet is reported

Analysis of Supersonic Jet Thrust with Fluidic Injection

Considerable focus on noise abatement for aircraft has spawned various noise control devices, passive and active. Aircraft and propulsion system design now has the additional criteria of acoustic performance to consider among many other criteria in advanced flight vehicle design. It is essential to consider the effect that noise control methods have on the performance of the propulsion device and overall effect on system performance. Thrust calculated from measurements and LES are compared for a Md = 1.56 jet at various operating conditions for validation. Experimental measurements on the baseline supersonic jet are used to validate computational results for the pressure and momentum thrust components. Thrust for various fluidic injection configurations are evaluated using computational results from the highly three dimensional flowfield. Analysis and discussion of requirements for fluidic injection air are provided to develop a complete system approach to aid design of fluidic injection systems. Fluidic injection decreases momentum thrust by creating axial velocity deficits in the region of injection. Pressure thrust is increased from local pressure rise from the injectors and area control at the nozzle exit. Fluidic injection increases total thrust as the pressure thrust gains are greater than the momentum thrust deficits. Specific thrust is reduced slightly with 6 injectors being a more efficient use of the injection air with greater noise reduction

Active Suppression of Supersonic Jet Noise Using Pulsating Micro-Jets

Noise suppression devices on military jet engines are motivated by the need to reduce community noise aswell as the acoustic load on airfield personnel during peacetime operation. They may also reduce problemswith sonic fatigue on the aircraft. Micro-jets have previously been shown as a promising tool for active noisesuppression. In the work presented here, compressible LES simulations have been done for slightly overexpandedconical C-D nozzle with a Mach number of 1.58 at NPR = 4.0 and a free stream flow Mach numberof 0.1. Two microjet configurations have been simulated. One with steady-state injection and an other withpulsating trailing-edge injection having a maximum mass flow-rate of mi/mj = 1.6%. The acoustic field isexpanded to the far field using the Kirchhoff integral method. The effect of injection frequency and pulsationcharacteristics on the flow-field and the radiated sound is investigated. Comparison is made between the LESand simulations and experiments for the steady-state and no injection cases and shows excellent agreement forthe screech tone frequency and the predictided OASPL is within 2 dB deviation from the measurements. Thepulsating injection cases investigated show that the frequency spectrum and the noise levels are sensitive to theinjection frequency as well as pulsation characteristics. It is shown that steady-state injection and pulsating injectionof equal max mass flow result in comparable reduction in terms of OASPL. The latter, however, comeswith the penalty of increased noise for the upstream observers

Noise Control of Supersonic Jet with Steady and Flapping Fluidic Injection

Large-eddy simulation is used to investigate steady-state mass flow injection into a supersonic jet stream with and without flapping motion of the microjets. The results are validated with particle image velocimetry and acoustic measurements. The effect of microjet penetration on the far-field acoustics is studied by altering the number of injectors, the cross-sectional area of each injector, and the injection mass flow. The injectors are evenly distributed around the nozzle exit. The injection angle is 90 deg relative to the main jet flow. This research is a continuation of a previous large-eddy simulation study of pulsed injection that showed that the unsteady injection-induced pressure pulses in the jet caused increased tonal noise for far-field observers at low angles. Flapping jet injection was shown to minimize the creation of the pressure pulses, except for high-amplitude flapping angles and high injection mass flows, where the injections divert out of the shear layer and introduce periodic superposition of the double shock-cell structure. Furthermore, the flapping injection did not show improved noise reduction compared with the steady injection, which is essentially promising because steady injection proves to be a more practical solution for implementation in real jet engine applications