Farrukh S. Alvi

Supersonic Cavity Flows and Their Control

A detailed experimental study of supersonic, Mach 2, flow over a three-dimensional cavity was conducted using shadowgraph visualization, unsteady surface pressure measurements, and particle image velocimetry. Large-scale structures in the cavity shear layer and visible disturbances inside the cavity were clearly observed. A large recirculation zone and high-speed reverse flow was revealed in the cavity. In addition, supersonic microjets were used at the leading edge to suppress flow unsteadiness within the cavity. With a minimal mass flux (blowing coefficient B c = 0.0015), the activation of microjets led to reductions of up to 20 dB in the amplitudes of cavity tones and of more than 9 dB in the overall sound pressure levels. The microjet injection also modified the cavity mixing layer and resulted in a significant reduction in the flow unsteadiness inside the cavity as revealed by the shadowgraphs and the velocity-field measurements.

Control of Supersonic Impinging Jet Flows Using Supersonic Microjets

Supersonic impinging jets, such as those occurring in the next generation of short takeoff and vertical landing aircraft, generate a highly oscillatory e ow with very high unsteady loads on the nearby aircraft structures and the landing surfaces. These high-pressure and acoustic loads are also accompanied by a dramatic loss in lift during hover.Previousstudies of supersonic impinging jets suggestthatthehighly unsteady behavioroftheimpinging jets is due to a feedback loop between the e uid and acoustic e elds, which leads to these adverse effects. A unique active control technique was attempted with the aim of disrupting the feedback loop, diminishing the e ow unsteadiness, and ultimatelyreducing theadverseeffectsofthise ow.Flowcontrolwasimplementedbyplacingacirculararray of 400-πm-diamsupersonicmicrojetsaroundtheperipheryofthemainjet.Thiscontrolapproachwasverysuccessful in disrupting the feedback loop in that the activation of the microjets led to dramatic reductions in the lift loss (40%), unsteady pressure loads (11 dB), and near-e eld noise (8 dB). This relatively simple and highly effective control technique makes it a suitable candidate for implementation in practical aircraft systems. NUNDERSTANDINGoftheimpingingjete owe eld isnecessary for the design of efe cient short takeoff and vertical landing (STOVL) aircraft. When such STOVL aircraft are operating in hovermode,thatis,in closeproximityto theground,thedownwardpointing lift jets produce high-speed, hot e ow that impinges on the landing surface and generates the direct lift force. It is well known that in this cone guration several e ow-induced effects can emerge, which substantially diminish the performance of the aircraft. In particular, a signie cant lift loss can be induced due to e ow entrainment bytheliftingjetsfromtheambientenvironmentinthe vicinityofthe airframe. Other adverse phenomena include severe ground erosion on the landing surface and hot gas ingestion into the engine inlets. In addition, the impinging e owe eld usually generates signie cantly highernoiselevelsrelativetothatofafreejetoperatingundersimilar conditions. Increased overall sound pressure levels (OASPL) associated with the high-speed impinging jets can pose an environment pollution problem and adversely affect the integrity of structural elements in the vicinity of the nozzle exhaust due to acoustic loading. Moreover, the noise and the highly unsteady pressure e eld are frequently dominated by high-amplitude discrete tones, which may match the resonant frequencies of the aircraft panels, thus further exacerbating the sonic fatigue problem. These problems become more pronounced when the impinging jets are supersonic, the operating regime of the STOVL version of the future joint strike e ghter. In addition, the presence of multiple impinging jets can potentially further aggravate these effects due to the strong coupling between the jets and the emergence of an upward-moving fountain e ow e owing opposite to the lift jets. 1 A

Use of High-Speed Microjets for Active Separation Control in Diffusers

Inlets to aircraft propulsion systems must supply flow to the compressor with minimal pressure loss, flow distortion, or unsteadiness. Flow separation in internal flows such as inlets and ducts in aircraft propulsion systems and external flows such as over aircraft wings is undesirable because it reduces the overall system performance. An experimental investigation is described that was carried out to study the feasibility of using high-speed microjets, supersonic for most cases, to control boundary-layer separation in an adverse pressure gradient. The geometry used is a simple diverging Stratford ramp equipped with arrays of 400-μm-diam microjets. Measurements include detailed surface flow visualizations, mean surface pressure distributions, and velocity field measurements using particle image velocimetry. The results clearly indicate that by activating these microjets the separated flow regions were eliminated. This led to a significant increase in the momentum of the flow near the surface where the gain in momentum was at least an order of magnitude higher than the momentum injected by the microjets. Given the simplicity of the system and its low mass flow requirements, combined with the benefits achieved by this approach, microjets appear to be promising actuators for efficient separation control for internal and external flow applications.

Active and Passive Control of Supersonic Impinging Jets

The behavior of supersonic impinging jets is dominated by a feedback loop due to the coupling between the fluid and acoustic fields. This leads to many adverse effects when such flows occur in short takeoff and vertical landing aircraft, such as a significant increase in the noise level, very high unsteady loads on the nearby structures, and an appreciable loss in lifting during hover. In earlier studies, it was demonstrated that by using supersonic microjets one could disrupt the feedback loop that leads to substantial reductions in the aforementioned adverse effects. However, the effectiveness of control was found to be strongly dependent on the ground plane distances and the jet-operating conditions. The effect of various microjet control parameters are investigated in some detail to identify their influence on control efficiency and additional insight is provided on the physical mechanism behind this control method. Parameters studied include microjet angle, microjet pressure, and the use of microtabs instead of microjets. These results indicate that by choosing appropriate control parameters it should be possible to devise a control strategy that produces optimal control for the entire operating range of conditions of the supersonic impinging jet. Moreover, the experimental results provide convincing evidence of the generation of significant streamwise vorticity by the activation microjets. It is postulated that the generation of streamwise vorticity and its evolution in the jet flow might be one of the main physical phenomena responsible for the reduction of flow unsteadiness in impinging jets.

Experimental and computational investigation of supersonic impinging jets

The results of an experimental and computational study of a moderately underexpanded axisymmetric supersonicjetissuingfroma convergingnozzleand impingingonagroundplanearepresented.Thegoalofthisworkisto develop a better understanding oftheimpinging jet e owe eld, which is of signie cantpracticalinterestbecause of its presence in short takeoff and vertical landing (STOVL)aircraft during hover as well as in other aerospace-related and industrial applications. Theexperimental measurementsinclude e ow visualization, surface-pressuredistributions,and velocitye elddataobtained using particleimagevelocimetry (PIV).Theexperimentaldata,especially the velocity e eld measurements, were used to verify theaccuracy of computational predictions. Computational results obtained using two differentturbulencemodels produced almost identical results. Comparisons with experimental results reveal that both models capture the signie cant features of this complex e ow and were in remarkably good agreement with the experimental data for the primary test case. The experiments and computations both revealed the presence of the impingement zone stagnation bubble, which contains low velocity recirculating e ow. Other features, including the complex shock structure and the high-speed radial wall jet, were also found to be very similar. The ability to measure and predictaccurately theimpinging jetbehavior, especially neartheground plane, is critical because these are regions with very high mean shear, thermal loads, and unsteady pressure forces, which contribute directly to the problem of ground erosion in STOVL applications.

Experimental study of physical mechanisms in the control of supersonic impinging jets using microjets

Supersonic impinging jet(s) inherently produce a highly unsteady flow field. The occurrence of such flows leads to many adverse effects for short take-off and vertical landing (STOVL) aircraft such as: a significant increase in the noise level, very high unsteady loads on nearby structures and an appreciable loss in lift during hover. In prior studies, we have demonstrated that arrays of microjets, appropriately placed near the nozzle exit, effectively disrupt the feedback loop inherent in impinging jet flows. In these studies, the effectiveness of the control was found to be strongly dependent on a number of geometric and flow parameters, such as the impingement plane distance, microjet orientation and jet operating conditions. In this paper, the effects of some of these parameters that appear to determine control efficiency are examined and some of the fundamental mechanisms behind this control approach are explored. Through comprehensive two- and three-component velocity (and vorticity) field measurements it has been clearly demonstrated that the activation of microjets leads to a local thickening of the jet shear layer, near the nozzle exit, making it more stable and less receptive to disturbances. Furthermore, microjets generate strong streamwise vorticity in the form of well-organized, counter-rotating vortex pairs. This increase in streamwise vorticity is concomitant with a reduction in the azimuthal vorticity of the primary jet. Based on these results and a simplified analysis of vorticity transport, it is suggested that the generation of these streamwise vortices is mainly a result of the redirection of the azimuthal vorticity by vorticity tilting and stretching mechanisms. The emergence of these longitudinal structures weakens the large-scale axisymmetric structures in the jet shear layer while introducing substantial three-dimensionality into the flow. Together, these factors lead to the attenuation of the feedback loop and a significant reduction of flow unsteadiness.

Control of High-Temperature Supersonic Impinging Jets Using Microjets

The flowfield associated with supersonic impinging jets has been of interest to both engineers and researchers for some time due to its wide range of practical applications and its complex nature from a fundamental fluid dynamic point of view. An example of supersonic impinging jets occurs in short takeoff and vertical landing aircraft, for which the highly oscillatory flowfield and the associated acoustic loads are also accompanied by a dramatic loss in lift during hover, severe ground erosion of the landing surface, and hot gas ingestion into the engine inlets. Another characteristic feature of this flowfield is an intensive heat transfer between the jet and the impingement surface. In the past we have examined impinging jets and their control using microjets at cold conditions; the present study is a step toward examining this flowfield and the effectiveness of microjet control at increasingly realistic thermal conditions. An ideally expanded, Mach 1.5 primary jet issuing from an axisymmetric nozzle was heated up to a stagnation temperature of ∼500 K. Mean and unsteady temperature and pressure measurements were obtained on a lift plate representative of the undersurface of an aircraft and on the ground plane over a range of nozzle-to-plate distances (representing aircraft hover conditions). In addition, near-field noise was also measured using a microphone. The velocity field of the impinging jet for both cold and hot conditions was mapped using particle image velocimetry. Our results show that the temperature recovery factor at the stagnation point on the ground plane is strongly dependent on the temperature ratio and nozzle-to-plate distance, similar to observations in subsonic impinging jets. The hover lift loss for hot jets is much higher than for cold jets, nearly 75% of the primary jet thrust at small nozzle-to-plate distances. The pressure fluctuations generated by hot impinging jets are also substantially higher than their cold counterparts. As in cold jets, pressure and noise spectra for hot jets show discrete, high-amplitude acoustic tones (generally known as impinging tones) at frequencies varying with jet temperature. The activation of microjet control shows a substantial reduction in pressure fluctuations both in terms of overall sound pressure levels (up to 20 dB on the ground plane and 15 dB on the lift plate) and the attenuation of discrete, high-amplitude impinging tones (up to 32 dB). High-temperature peaks were observed in the temperature spectra at frequencies corresponding to impingement tones in the pressure and noise spectra; these were also substantially attenuated with microjet control. As much as 50% of the lift loss was recovered by using control for hot jets at smaller nozzle-to-plate distances. In general, the results provide evidence of the feasibility of using this active control approach under increasingly realistic conditions to achieve desired reductions in noise, unsteady pressures, and thermal loads.

Effects of Counterflow on the Aeroacoustic Properties of a Supersonic Jet

The influence of counterflow on the mixing and acoustic characteristics of a Mach 1.4 rectangular jet operated at on- and off-design conditions were studied experimentally for different levels of counterflow. The results show that counterflow significantly enhances shear-layer mixing and reduces the jet potential core length under all operating conditions. Significant changes in both shock-cell spacing and strength were ohserved when counterflow was applied to nonideally expanded jets. Consequently, screech tones were either reduced or totally eliminated, and broadband shock-associated noise was shifted to higher frequencies. In the underexpanded mode, a Mach disk was formed at certain levels of counterflow, which substantially weakened the subsequent periodic shock-cell structure and reduced the broadband shock-associated noise and the overall sound pressure level (OASPL) by as much as 3 dB. Interestingly, it was also discovered that a jet operating at overexpanded conditions could be decelerated nearly isentropically by applying the proper amount of counterflow. This modification led to a 4 dB reduction in OASPL. Based on the present study, it is suggested that counterflow warrants further investigation as a potential noise reduction technique.

Control of Pressure Loads in Geometrically Complex Cavities

The need to reduce the fluctuating surface pressure loads in realistic three-dimensional cavity configurations is clear for many applications. In this paper, we describe the results of an experimental study that examined the properties of flow over a highly three-dimensional cavity, which included angled side walls and a sloped floor. The unsteady pressure measurements revealed that the primary spectral properties, such as the frequencies of the cavity tones, are very similar to those of simpler, rectangular cavities. The study also explored the effects of different active fluidic injection methods, at the cavity leading edge, on the unsteady loads generated in the cavity. Specifically, the two active suppression concepts examined were microjets and rectangular slots at the leading edge. Both concepts showed significant reductions in the fluctuating surface pressures, upwards of 50% on the cavity aft wall, with very modest amounts of mass flowing through the injectors. When appropriately scaled for full-scale applications, the actuator mass flux required falls well within the practical range for most aircraft. Different angles for the fluidic Injection were also examined and maximum reductions were observed when injection was perpendicular to the approaching freestream flow. Additionally, the best blowing configurations were found when the injectors did not fully span the leading edge of the cavity. The reductions observed in the fluctuating surface pressure levels resulted from decreases in both the broadband and resonant features of the surface pressures. By conducting these experiments at two different facilities and over a range offreestream dynamic pressures and temperatures, this study also demonstrated that appropriate scaling of the spectral features can be achieved. This allows for the expansion of the results presented here to larger (and different) scale studies and ultimately to full-scale applications.

Suppression of Cavity Loads Using Leading-Edge Blowing

We present hybrid Reynolds-averaged Navier-Stokes/large eddy simulation-based analysis of the suppression of fluctuating pressure loads on the walls of a complex nonrectangular cavity using leading-edge mass blowing. The unique aspect of the concepts discussed here is the very low mass flow rates used to achieve significant suppression. The simulation results are used to gain insight into the mechanism governing the effectiveness of these jets. The jets are applied to an L/D = 5.6 cavity at supersonic conditions of Mach 1.5. The simulation results show excellent agreement with experiments demonstrating an overall reduction in fluctuating pressure levels on the order of 50% with the control concepts. The primary mechanism of reduction is the break up of the spanwise coherence in the shear layer into smaller vortical structures thus reducing the shear layer flapping and leading to a smaller imprint on the wall pressures.