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

Study of Supersonic Jet Noise Reduction using LES

Increases in air traffic and denser population around airports have led to stricterregulations on aircraft noise. High noise levels from high-speed aircraft can causehearing damage in pilots and the airfield personnel. The engine is the main sourceof noise of all jet aircraft and is therefore a key component for improvement.Decreasing jet engine noise can in some cases reduce sonic fatigue and therebyincrease the engine lifetime. In this thesis, the response of the radiated noise froma supersonic jet emitted from a converging diverging nozzle to steady-state, pulsedand flapping fluidic injection is studied using Large Eddy Simulation (LES), andcomparisons are made with experimental data. An investigation is also presentedin which actions were taken to reduce the internal shock strength by modifying thenozzle throat, and thereby reduce the radiated noise. The optimized nozzle nearlyeliminates the internal shock, which reduces the double diamond structure in thejet plume but increases the strength of the shock at the nozzle exit. It has lowerturbulence levels at the nozzle exit due to a weaker shock interaction with theshear layer. The optimized nozzle provides equal thrust to the sharp nozzle with4 % less pressure without any acoustic penalty. The pulsed injection showed thatthe radiated noise is sensitive to the pulsation characteristics and the pulsation fre-quency. It was shown that the noise reduction with pulsed injection can equal thenoise reduction of steady-state injection with a lower net mass flow of the pulsedinjection. However, increased noise was noted at the downstream observers. Theflapping injection cases that were investigated did not show improvements overthe corresponding steady injection cases. These are positive findings, since steadyinjection should be simpler and more robust to apply to real jet engines. Theinjection was shown to impact the jet thrust, as expected. The net jet thrust in-creased with increased injection mass flow, whereas the specific thrust decreased.The momentum thrust was shown to decrease with increased injection mass flowwhereas the pressure thrust increased due to a shock shift at the nozzle exit. Thework presented in this thesis adds to the body of knowledge found in the liter-ature about supersonic jet noise generation and its noise reduction using fluidicinjection

Numerical Simulations of Noise Reduction Devices for Aero Engines

Increasing air traffic and denser population around airports have led to stricterregulations on aircraft noise. The engine is the main source of noise of jet aircraft.Decreasing jet engine noise can in some cases reduce sonic fatigue andthereby increase the engine lifetime. In this thesis the performance of a novellow-frequency acoustic liner concept is investigated using unsteady Reynolds-Averaged Navier-Stokes simulations (URANS). The results are compared withthose of an analytical model and experiments. The liner is designed to reduce fannoise upon placement on the outlet guide vanes. Furthermore, the response of theradiated noise from a supersonic jet emitted from a converging diverging nozzleto steady-state and pulsed fluidic injection is tested using Large Eddy Simulation(LES). An investigation is also presented in which actions were taken to reducethe internal shock strength by modifying the nozzle throat, and thereby reduce theresulting noise. The optimized nozzle was evaluated further using LES and experimentaltechniques. The acoustic liner study showed that the resonance frequencyof the liner obtained by the URANS compared within 200Hz to the measuredresonance frequency. It was shown that the analytical model can be tuned with asingle parameter to match the URANS simulations over a wide range of frequencies.Simulations of the sharp throat CD-nozzle with and without fluidic injectioncompared within 2 dB to the measured values of the overall sound pressure level(OASPL) for all observers. The pulsed injection showed that the radiated noiseis sensitive to the pulsation characteristics and the frequency. It was shown thatnoise reduction with pulsed injection can equal the noise reduction of steady-stateinjection with lower net mass flow of the pulsed injection. However, an increasednoise was noted at the downstream observers. The optimized nozzle nearly eliminatesthe internal shock, which reduces the double diamond structure in the jetplume but increases the strength of the shock at the nozzle exit. It has lower turbulencelevels at the nozzle exit due to a weaker shock interaction with the shearlayer. The optimized nozzle provides equal thrust to the sharp nozzle with 4%lesspressure without any acoustic penalty. Good comparison is obtained with RANS,LES and experiments

Numerical Simulations of Noise Reduction Devices for Aero Engines

Increasing air traffic and denser population around airports have led to stricterregulations on aircraft noise. The engine is the main source of noise of jet aircraft.Decreasing jet engine noise can in some cases reduce sonic fatigue andthereby increase the engine lifetime. In this thesis the performance of a novellow-frequency acoustic liner concept is investigated using unsteady Reynolds-Averaged Navier-Stokes simulations (URANS). The results are compared withthose of an analytical model and experiments. The liner is designed to reduce fannoise upon placement on the outlet guide vanes. Furthermore, the response of theradiated noise from a supersonic jet emitted from a converging diverging nozzleto steady-state and pulsed fluidic injection is tested using Large Eddy Simulation(LES). An investigation is also presented in which actions were taken to reducethe internal shock strength by modifying the nozzle throat, and thereby reduce theresulting noise. The optimized nozzle was evaluated further using LES and experimentaltechniques. The acoustic liner study showed that the resonance frequencyof the liner obtained by the URANS compared within 200Hz to the measuredresonance frequency. It was shown that the analytical model can be tuned with asingle parameter to match the URANS simulations over a wide range of frequencies.Simulations of the sharp throat CD-nozzle with and without fluidic injectioncompared within 2 dB to the measured values of the overall sound pressure level(OASPL) for all observers. The pulsed injection showed that the radiated noiseis sensitive to the pulsation characteristics and the frequency. It was shown thatnoise reduction with pulsed injection can equal the noise reduction of steady-stateinjection with lower net mass flow of the pulsed injection. However, an increasednoise was noted at the downstream observers. The optimized nozzle nearly eliminatesthe internal shock, which reduces the double diamond structure in the jetplume but increases the strength of the shock at the nozzle exit. It has lower turbulencelevels at the nozzle exit due to a weaker shock interaction with the shearlayer. The optimized nozzle provides equal thrust to the sharp nozzle with 4%lesspressure without any acoustic penalty. Good comparison is obtained with RANS,LES and experiments

Aerodynamic Shape Optimization of High-Speed Trains

The paper presents a new, fully automatic multi-objective shape optimization method for improving the aerodynamic properties of trains. The optimization method was applied to a multi-objective optimization problem of crosswind stability of a train placed on an embankment. The train was optimized with two objective functions and the geometry was changed according to two design parameters. Furthermore, two flow scenarios were used in the optimization where the train was placed either on the windward or the leeward side of the two-track embankment. The optimization resulted in an optimal shape of the train which was the same regardless of the train's location on the embankment. The present approach is shown to be robust and capable of obtaining an optimal design of the train without the influence of the user during the optimization process. The example of the optimization problem presented in this paper was multi-objective but one objective was chosen to be the dominant one. Although there are no limitations in the number of design parameters or objective functions in the method developed, an increase in the number of parameters will result in an increase in the computational effort required for the optimization. An interesting result of the present work is that almost identical optimal shapes for the train were obtained for both trains traveling on the windward and the leeward sides of the embankment. This is a desirable outcome of the optimization as it does not require selecting the shape that is optimal for only one operational condition of the train

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

Acoustic Signature of a Supersonic Jet Emanating from a Rectangular C-D Nozzle

We live in a world with ever increasing air traffic and the demand for fuel efficient low noise emitting aircraft is high. The use of blended wing bodies (BWB) has gained interests within the aerospace industry due to its potential for reduced fuel consumption. These type of aircraft are generally equipped with rectangular nozzles. The drawback of such nozzles is increased instability of the emanating jet which increases the risk of higher noise radiation. Understanding the instability patterns and the underlying flow physics is therefore the key to improved stability and reduced noise. In the presented paper, an LES/CAA approach is utilized to predict the flow dynamics and the radiated noise from a rectangular nozzle. The nozzle is operated at underexpanded conditions. The simulations are compared with experiments and are used as a complement to the experimental data for improved understanding of the flow physics. The supersonic jet is found to exhibit an intense flapping motion followed by a large jet spreading in the minor-axis plane. In general, the prediction of the most amplified frequency and higher harmonics observed in the near-field and far-field spectra is in agreement with the experiment. Two types of flow events associated with the generation of high amplitude acoustic waves are detected. These events are identified as vortex-collision and shock-leakage through the shear layer

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