Ryan Schmit

Investigation of Cavity Flow Using Fast-Response Pressure-Sensitive Paint

An experimental study was conducted to investigate the pressure fluctuations on the entire sidewall of a rectangular cavity with an L/D of 5.67 using fast-response pressure-sensitive paint. Additionally, the performance of four different passive flow-control devices was quantified. Experiments were conducted in the Trisonic Gasdynamics Facility at the Air Force Research Laboratory at Mach 0.7 and 1.5. The frequency spectrum (including Rossiter tones) and sound pressure levels obtained from the pressure-sensitive paint measurements are validated against data taken with conventional dynamic pressure sensors. The complex flow phenomena over the cavity wall were visualized, and full-wall pressure spectra were calculated. The rod in crossflow showed the best peak suppression, followed closely by the flat spoiler. The large triangular steps showed moderate peak suppression, whereas the ridges did not suppress the peaks at all. High-resolution measurements of both temporal and spatial pressure fluctuations on the wall allowed for the visualization of sound pressure level distribution over the entire cavity wall. This revealed a strong dependence between the Rossiter tone modes and the spatial distribution of sound pressure levels that was not possible to resolve with traditional discrete pressure transducers.

Boundary Feedback Flow Control: Proportional Control with Potential Application to Aero-Optics

A large percentage of the losses in performance and effectiveness of airborne optical systems are caused by turbulence. In an effort to reduce these adverse effects in airborne optical systems, we are exploring the use of both openand closed-loop flow control over a cylindrical turret. A series of experiments were performed at a Reynolds number of 2 10, based on the turret’s diameter and freestream velocity, which corresponds to aMach number of 0.3. The three-dimensional turret contained an actuation system that consists of 17 synthetic jets placed upstream from the leading edge of the aperture. Initially, a large database containing no control and open-loop control was obtained. These data sets provide a rich ensemble for the development and application of a simple proportional closed-loop control with the use of proper orthogonal decomposition. Surface pressuremeasurements were acquired across the aperture region for all cases studied. Results from the open-loop test demonstrate a reduction of 19.6% in the root-mean-square values when compared to the baseline case. The closed-loop flow control results show that the root-mean-square pressure fluctuations are reduced by 25.7%, the integral scales are significantly reduced, and the flow is driven toward homogeneity.

Flow and Aero-Optics Around a Turret Part II: Surface Pressure Based Proportional Closed Loop Flow Control

As focused light passes through turbulent ∞ow the light is distorted and the intensity is reduced. An extended study using active ∞ow control to afiect the turbulent region over the a ∞at aperture of a 3-D hemispheric turret was conducted in the Air Force Research Laboratory’s Subsonic Aerodynamic Research Laboratory (SARL) wind tunnel at WrightPatterson Air Force Base. The SARL experiments were performed at a Mach number of :3, which gives Reynolds number on the order of 2;000;000. At these Reynolds numbers the ∞ow becomes highly complex and more challenging to study. A large database from previous work containing no control and open loop control cases provided a rich ensemble for plant model development based on low dimensional techniques such as the split-POD method of Camphouse (2007). PIV velocity data was acquired along with simultaneously sampled surface pressure data at various planes across the turret with and without control. Control authority was acquire by actuators mounted upstream of the aperture that generated a momentum ∞ux in the ∞ow around the turret. Simple proportional closed-loop control was performed using the bandpass flltered temporal POD mode coe‐cients of the surface pressure as the feedback signal. This paper shows that the active control reduced the root mean squared of the pressure ∞uctuations, shrunk the integral scales, and drove the ∞ow towards homogeneity.

A SNAPSHOT DECOMPOSITION METHOD FOR REDUCED ORDER MODELING AND BOUNDARY FEEDBACK CONTROL.

Abstract : In this paper, we develop a reduced basis construction method that allows for separate consideration of baseline and actuated dynamics in the reduced modeling process. A prototype initial boundary value problem, governed by the two-dimensional Burgers equation, is formulated to demonstrate the utility of the method in a boundary control setting. Comparisons are done between reduced and full order solutions under open-loop boundary actuation to illustrate advantages gained by separate consideration of actuated dynamics. A tracking control problem is specified using a linear quadratic regulator formulation. Comparisons of feedback control effectiveness are done to demonstrate benefits in control effectiveness obtained from separate consideration of actuated dynamics during model reduction.

High and low frequency actuation comparison for a weapons bay cavity

This paper compares the effectiveness of zero- low- and high-frequency flow control methodologies when applied to a generic weapons bay cavity. Using the High Frequency Excitation Active Flow Control for Supersonic Weapons Release (HIFEX) generic weapons bay model the actuators were tested at Mach 0.85 and 1.19. The zero-frequency actuator tested was a conventional one-delta “sawtooth” spoiler, while the low-frequency device was a pulse-blowing actuator. The sawtooth spoiler performed as expected by reducing the Overall Spectrum Level (OASL) slightly in both the subsonic and supersonic cases. The pulsed-blowing actuator reduced the OASL as much as 9.6dB for both the subsonic and supersonic cases. The high-frequency devices tested were the Powered Resonance Tube (PRT) and a Splash Jet actuator. The PRT and Splash Jet reduced the (OASL) equally well at Mach 0.85 for all the mass flow rates tested. The PRT outperformed all other actuators at Mach 1.19 except for the highest mass flow rate of 0.227 kg/s, where the Splash Jet performed equally as well the PRT.

Fourier Analysis of High Speed Shadowgraph Images around a Mach 1.5 Cavity Flow Field

An examination of a rectangular cavity with an L/D of 5.67 was tested at Mach 0.7 and 1.5 with corresponding Reynolds numbers of 2x10 6 /ft and 2.3x10 6 /ft, respectively. High speed shadowgraph movies were simultaneously sampled with the dynamic pressure sensors at 75 kHz. Fourier analysis was performed on the high speed movies as well as the dynamic pressure data which resulted in determining the locations of dominant cavity frequencies in the flow field. From the high speed shadowgraph movies, observations of the in the cavity flow physics are discussed. Several cavity related issues are examined e.g. How do vortices form in the shear layer? What is the actual starting mechanism for these cavity acoustic tones? How do the cavity acoustic tones affect the shear layer?

Flow characteristics of active control around a 3D turret

At high speeds the wake of a hemisphere turret is completely separated and fully turbulent. Within this region of separated flow large density fluctuations develop. The problem comes in with attempting to propagate light through the turbulent wake of the turret. As the a light beam passes through the wake and the shear layer of the turret, the beam becomes distorted. Distortion of the beam causes the intensity of the light to be reduced making a laser less effective. By manipulating the flow around the turret, in particular the region just over the laser aperture, the loss of light intensity can be minimized. Active flow control has been shown in pervious work to alter the flow characteristics around an object. In the effort to delay the onset of stall around an airfoil at high angles of attack Glauser et al (2004) and Ausseur et al (2005) were successful at showing that utilizing an active flow control system was a very effective means of control. The active flow control included open and closed loop control. For the closed loop control, a low dimensional analysis of the velocity field around the airfoil and the surface pressure along the airfoil, was fed back into a simple proportional controller. Closed loop flow control reduced the onset of stall as the angle of attack was increased. and the system also consumed less power to prevent stall than the open loop control. A pervious experiment perform under similar conditions and procedures was performed in the Syracuse University windtunnel. In this experiment open loop flow control was test on a quarter sized turret at a low freestream speed. After a large database of surface pressure and velocity measurements where obtain it was seen that there was a reduction in the unsteady fluctuating characteristics. In an effort to take this a step further and improve on the open loop control, the next step was to investigate the effects of open loop and closed loop control upon the flow and aero-optics of the system at a much higher Reynolds number. Like the pervious work before, the surface pressure based proportional closed loop control was found to have a positive effect upon the wake of the turret.

Analysis of Cavity Passive Flow Control using High Speed Shadowgraph Images

An examination of a rectangular cavity with an L/D of 5.67 was tested at Mach 0.7 with a corresponding Reynolds number of 2x10 6 /ft. High speed shadowgraph movies were simultaneously sampled with dynamic pressure sensors at 75 kHz. Fourier analysis was performed on the high speed movies as well as the dynamic pressure data, which resulted in determining the locations of dominant cavity frequencies in the flow field. Five passive flow control devices were tested, three of which have historically preformed well, while two neither reduced the main acoustic tones nor reduced the broadband levels. From the high speed shadowgraph movies, observations are made in the changes in the cavity flow physics when the passive flow control devices are used, and will be discussed.

Flow and aero-optics around a turret. Part 1. Open loop flow control

A large percentage of the losses in performance and eectiveness of airborne optical systems are caused by turbulence. In particular, separated turbulent flow phenomena is present in several aero-optics applications. In an eort to reduce the adverse eects of turbulence in airborne optical systems, we are exploring the use of both open and closedloop flow control over a cylindrical turret. A series of experiments were performed at Reynolds number of 2;000;000 that corresponds to a Mach number of 0:3 using a half scale test model. The 3D turret contained an actuation system that consists of 17 synthetic jets placed upstream from the leading edge of the aperture. Multiple actuation cases were tested to evaluate the eects of active flow control over the aperture area and their control authority. Simultaneous surface pressure and velocity measurements were acquired in the separated region for both with and without flow control. Pressure results from the open loop test presents reduction of 10 percent in the root-mean-square values when compare to the baseline case. Two-point statistics showed that the forcing is driving the flow towards homogeneity across the surface of turret.

Suction Flow Control at High Reynolds Number

Flow control has been shown to reduce the aero-optic distortion due to density ∞uctuations within a wake of an airborne turret. By manipulating the ∞ow over the turret directly afiects the density ∞uctuation that incur aero-optic distortions. This paper focuses on suction ∞ow control over a three-dimensional turret at a freestream velocity of Mach 0.3. Aero-optic measurement were taken over the ∞at aperture of the turret with and without open loop suction control clearly showed a decrease in the distortions as the amount of suction increased. Fluctuating surface pressure simultaneously sampled with the aero-optic measurements showed the inverse where as the suction increased the surface pressure rms decreased. Another set of tests were taken as the hemisphere dynamically pitching with open-loop and closed-loop suction ∞ow control. The closed-loop control system employed a low dimensional measurement based proportional controller. Both the open-loop and closed loop control test showed a change in ∞ow over the aperture as compared to the baseline case.