Chiang Shih

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

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 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.

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.

Investigation of flow at leading and trailing edges of pitching-up airfoil

The dynamic stall process of an NACA 0012 airfoil undergoing a constant-rate pitching-up motion is studied experimentally in a water towing tank facility. This study focuses on the detailed measurement of the unsteady separated flow in the vicinity of the leading and trailing edges of the airfoil. The measurements are carried out using the particle image velocimetry technique. This technique provides the two-dimensional velocity and associated vorticity fields, at various instants in time, in the midspan of the airfoil. Near the leading edge, large vortical structures emerge as a consequence of van Dommelen and Shen type separation and a local vorticity accumulation. The interaction of these vortices with the reversing boundary-layer vorticity initiates a secondary flow separation and the formation of a secondary vortex. The mutual induction of this counter-rotating vortex pair eventually leads to the ejection process of the dynamic stall vortex from the leading-edge region. It is found that the trailing-edge flowfield only plays a secondary role on the dynamic stall process.

Aeroacoustic Properties of Supersonic Cavity Flows and Their Control

A detailed experimental study of supersonic, Mach 2, flow over a 3D cavity was conducted using shadowgraph, particle image velocimetry (PIV), and unsteady surface pressure measurements. Large-scale structures in the cavity shear layer and visible disturbances inside the cavity were observed. The PIV data reveals the highly unsteady nature of the entire flowfield and the presence of a large recirculation zone with reverse velocities as high as 40 % of the freestream velocity. Supersonic microjets at the leading edge are used to control the cavity flow and suppress resonance in the cavity. Using minimal mass flux through the microjets, overall sound pressure level (OASPL) was reduced by greater than 9 dB with tonal reductions greater than 20 dB. The PIV data reveals that microjet injection modifies the cavity shear layer and results in a significant reduction in the unsteadiness of the cavity velocity-field.

Another Look at Supersonic Cavity Flows and Their Control

A Mach 2 supersonic cavity flow, with L/D between 1 and 5, was investigated with velocity field measurements, acoustic/unsteady pressure measurements and flow visualization. The evolution in the flowfield between deep (L/D=1) and relatively shallow (L/D~5) was examined both in terms of mean and unsteady L/D flow properties. By implementing microjet-based actuators, the flow induced resonance and the accompanying high unsteady pressure loads inside the cavity are significantly reduced, both in terms of the overall sound pressure level (OASPL) and the dominant cavity tone. The required mass flux of the microjet actuator is very low, depends on the L/D: Bc between 0.1% to 0.5% is sufficient for 5 to 11 dB reduction in OASPL and 13 to 28 dB on dominant cavity tones.

A PIV Study of Supersonic Impinging Jet

An experimental investigation of the flow and acoustic properties of an axisymmetric impinging jet, with and without microjet control, has been performed. The near-field acoustic measurements clearly show that the use of microjet control eliminates or significantly suppresses the impinging tones, and reduces broadband spectral amplitudes. To understand the physical mechanism responsible for the control effectiveness, the flowfields with and without control are examined using Particle Image Velocimetry Technique (PIV). The detailed PIV measurements reveal that the activation of microjets introduces strong streamwise vorticity in the form of well-organized, counter-rotating vortex pairs. The generation of these streamwise structures is speculated as the result of the tilting and stretching of the shear layer vortices when the microjets interact with primary jet flow. Due to this redistribution, the peak value of azimuthal vorticity is significantly reduced leading to a subsequent weakening of the primary jet instabilities. On the other hand, near the nozzle exit, the use of Microjets control increases the thickness of the shear layer further limiting the number of the unstable modes. The combined effect of an increase in thickness of the shear layer and a decrease of the peak azimuthal vorticity suppresses the instabilities in the primary shear layer. This sequence of events leads to the weakening of the feedback loop and the subsequent reduction in the flow unsteadiness of the supersonic impinging jet flow.

Experimental Observations of Instability Modes in a Rectangular Jet

The instability modes of a jet issuing from a rectangular nozzle of aspect ratio 4 have been studied experimentally at exit Mach numbers ranging from 0.03 to 1.5. Depending on the exit Mach number, several distinct characteristics are identified according to the arrangement of the flow structure with respect to the jet centerline. In the very low velocity range, U ≤ 20 m/s, a symmetric mode prevails. An antisymmetric mode dominates at all other Mach numbers, except in the range 0.6 ≤ M ≤ 0.85, where both symmetric and antisymmetric modes exist and there is a continuous switching between them