Daniel R. Cuppoletti

A Comprehensive Investigation of Pulsed Fluidic Injection for Active Control of Supersonic Jet Noise

Fluidic injection for noise control of high Reynolds number jets has shown promise and recent tests have demonstrated improved noise reduction while decreasing the injection mass flow required. This investigation was an experimental and numerical study on the capability of pulsed fluidic injection to reduce noise on a Md = 1.56 supersonic jet. The effect of pulse frequency, duty cycle, injector phasing, and injection angle on the noise components were studied. The pulsed injectors were characterized with hot-wire measurements. Far-fleld acoustics was used to survey the noise reduction of pulsed injection (up to 400 Hz) in comparison to the baseline and steady injection cases. Injection angles θinj = 30° to 90° with respect to the primary jet axis were investigated. High-speed shadowgraph was used to quantify the time scales involved in response of the shock train and screech instabilities with pulsed fluidic injection. LES and CAA were compared with measurements to evaluate the capability of numerical simulation of the pulsed injection configurations. It was shown that reduction of turbulent mixing noise generally scales with the actual duty cycle of applied injection. For 30 Hz injection at 20% mass flow up to up to 80% of the steady flow {increment}OASPL is achieved, demonstrating that low frequency injection is capable of enhanced noise reduction at certain conditions. The shocks in the jet potential core respond in 1 ms when injection is removed, while the jet column instability requires up to 7 ms to redevelop after injection is removed. The results demonstrate the feasibility of using active control with pulsed fluidic actuators to provide at least steady flow noise reduction with significantly reduced injection mass flow.

Nozzle throat optimization for supersonic jet noise reduction

Noise from engines that operate at supersonic conditions, especially high performance military aircraft, often utilize a converging-diverging nozzle with variable area control. This design usually includes a sharp nozzle throat which creates internal shock formation. Turbulent structure interaction with these shocks results in additional noise components other than turbulent mixing noise to be introduced to the jet noise spectrum. The present study investigates how weakening the internal shocks affects the flow and acoustics of a Mach 1.6 jet. RANS simulations were used to minimize internal shock formation and optimize the flow contours of the converging portion and throat of a C-D nozzle. A response surface methodology was used to evaluate 3000 possible designs using the RANS results as model inputs. An experimental investigation was conducted with a splined nozzle design that is virtually free of internal shocks. The flow field was measured using PIV for comparison with RANS and LES. Mean velocity and turbulence was captured well by the computations for the sharp throat and splined nozzles. Although the throat shocks were nearly eliminated, the overall shock strength was relatively unchanged. Far-field acoustic results showed little difference at thrust matched conditions since the overall shock strength was unchanged. The nozzle performance is greatly improved through throat optimization, providing equivalent thrust with 4% less pressure with no acoustic penalty.

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.

Fluid Injection Effects on Acoustics of a Supersonic Jet at Various Mach Numbers

The effect of fluidic injection on a convergent-divergent supersonic jet operating at design and imperfectly expanded conditions was investigated. The nozzle had flow contours simulating those of tactical military jet engines and twelve pairs of fluidic injectors were incorporated into the nozzle lip. The effect of fluidic injection was studied at the design Mach number of 1.56, under-expanded, and over-expanded conditions. Extensive far-field acoustic measurements were conducted to determine the optimum injection momentum ratios and the effect on imperfectly expanded jets. SPIV was used to resolve three velocity components and compute the mean and turbulent flow field. Modification of the turbulence distribution was related to the acoustic results. At all conditions, increasing the injector momentum flux ratio resulted in greater reduction of OASPL. At over-expanded conditions, OASPL of 4 dB and 3 dB were observed at forward and downstream angles, respectively. At nozzle design conditions, up to 15 dB reduction in BBSN was observed in the narrowband spectra. OASPL reduction of up to 3.5 dB and 2 dB was achieved at the upstream and sideline angles, respectively. A study of the number of injection points determined that optimum noise reduction could be achieved with less injection points. The interaction of the fluidic injectors with the shock structure was the main noise reduction mechanism observed. Shear layer spreading rate was increased, but peak turbulence levels were effectively the same downstream.

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

Proper Orthogonal Decomposition on a Subsonic Jet Exhausted from an Axisymmetric Nozzle and Chevron Nozzles with Varying Penetration

This paper utilizes Proper Orthogonal Decomposition (POD) to study a jet engine exhaust to identify differences between a baseline axisymmetric jet and one with chevrons. The analysis is done using flow velocity data obtained from Particle Image Velocimetry and performed for the flow exhausting through a baseline conical nozzle, low penetration chevron nozzle, and high penetration chevron nozzle. The mean velocity and TKE results show the chevron configurations reduce the potential core length, increase the jet spread, and reduce TKE levels near the end of the potential core. POD was then used to further quantify these results. The POD modes for the chevron flows and baseline flows are very similar but the energy within the POD modes is lower for the chevron cases which are consistent with the TKE results. Projection of the PIV images from the chevron configurations onto the baseline POD modes is used to directly quantify changes to the energy distribution of the largescale structures. It is seen that both near the nozzle and downstream, the highest energy containing modes are attenuated for the chevron cases. The reduction in growth of the large-scale structures near the jet exit is due to the increased spread of the jet and ultimately the stabilization of the jet column mode instabilities.

High Frequency Combustion Instabilities with Radial V-Gutter Flameholders

tested over a range of overall equivalence ratios. The temperature and velocity profiles were mapped at varying locations downstream of the V-gutter system with and without secondary combustion. Dynamic pressure measurements were performed downstream of the V-gutter to characterize the amplitude and frequency of the instabilities and high speed imaging was performed to identify flame structure during the instability cycle. An overall equivalence ratio of 0.5 was found to have the strongest screech instability at 425 Hz at an inlet Mach number of 0.20 and inlet temperature of 750 o F for continuous V-gutter configuration. It was shown that the strongest screech instability occurs when vortex shedding from the V-gutter tips occurs at the same frequency as acoustic modes in the facility. Nomenclature M = Mach number p = pressure T = temperature φ = equivalence ratio D,L = V-gutter width f = frequency c = sonic velocity m,n = lateral acoustic mode number

Optimization of Stochastic Estimation Techniques in a High-Speed, Axisymmetric Jet

Stochastic Estimation (SE) is utilized to investigate the large-scale structure of a jet exhausted through a baseline axisymmetric nozzle and two chevron nozzles with different penetrations. Particle image velocimetry (PIV) was used to measure the streamwise and radial velocity components on a 2D streamwise plane over the first 15 jet equivalent diameters. Two 16 microphone, linear, near-field arrays separated by 180° in the azimuthal plane were simultaneously acquired with the PIV measurements. The analysis included investigation of the near-field pressure spectra and flow field mean velocity and Turbulent Kinetic Energy. SE was used to reconstruct the velocity field from the acquired pressure data. The optimal number and optimal placement of sensors was investigated to obtain the best reconstructed velocity. Near the nozzle, the optimal sensor location was near the jet while further downstream, SE was better when the microphones were slightly farther away. This is directly related to the wavelength of the large-scale structures which increased downstream. SE was successful at reconstructing the TKE profiles in shape but significantly under predicted the amplitudes. It was also shown that SE was not as successful reconstructing flow exiting the chevron nozzles due to a drop in correlation between the flow velocity and near-field pressure. Even with the lower correlations, the shape of the TKE profiles were successfully reconstructed using SE for the chevron flow fields.

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.

Experimental Analysis of Screech Tone Effects for a V-Gutter Stabilized Flame

The objective of this work is to experimentally test and verify the instability and vortex shedding effects resulting in combustion instabilities within a combustion wind tunnel facility (CWTF). While bluff bodies set within a combustible high-speed flow are known to stabilize flames, the effects that screech tones have on these bodies’ flame holding stability is unknown. Due to the highly destructive nature of instabilities, (high amplitude noise, strong pressure oscillations, deterioration, and failure of hardware) a more intimate knowledge of the relation between instabilities, flame holders, and the flow dimensions must be understood. In order to investigate these effects, instabilities were excited within the CWTF and investigated with high speed and phase locked ICCD imagery.