Nathan E. Murray

Properties of subsonic open cavity flow fields

Flow over an open cavity was studied for several different subsonic free stream Mach numbers ranging from 0.19 to nearly 0.73. Velocity field information was acquired through an application of particle image velocimetry, while the fluctuating surface pressure was acquired through a linear array of surface pressure sensors. These data were acquired on the centerline of the cavity which had a length to depth ratio of 6 and a turbulent boundary layer upstream of its leading edge. Over the range of free stream Mach numbers the fluctuating surface pressure spectra in the cavity exhibited different behavior ranging from no apparent resonance to resonance being dominated by the second or third Rossiter modes. The broadband levels of surface pressure spectra with strong resonant tones collapse with scaling by the flow dynamic pressure. Velocity measurements reveal that the center of circulation of the flow within the cavity moves from the aft wall towards the center of the cavity with increasing Mach number. The ...

Modified quadratic stochastic estimation of resonating subsonic cavity flow

There is an obvious need for the development of reduced-order time-resolved models of the off-body physics of wall-bounded shear flows. This work represents a preliminary step toward producing such a model for the resonating subsonic cavity flow by presenting the modified quadratic stochastic estimation (mQSE) as a means to estimate flow field quantities using surface pressure measurements. The mQSE estimates these flow field quantities using statistics calculated between surface-pressure fluctuations and the flow fields POD expansion coefficients. In the current work, the mQSE is applied separately to both the velocity and density fields for subsonic flow over a resonating cavity. The velocity data are obtained using particle image velocimetry, while the density gradients are obtained using spark Schlieren photography. The results demonstrate the ability of the mQSE to estimate the structure of the complex cavity flow field using a few wall-pressure measurements. The results also show an interaction bet...

The effect of heat on turbulent mixing noise in supersonic jets

The most prominent component of turbulent mixing noise in jets is associated with Mach wave radiation. Large-turbulence structures radiate Mach waves efficiently when they convect supersonic relative to the ambient. An experimental study is conducted on an unheated (T0 = 286.6K) and heated (T0 = 1020.6K) fully expanded Mach 1.553 jet to investigate the effect of heat on this radiation process. The acoustic near-field was captured using a line array that comprised ten microphones situated in the hydrodynamic periphery of the dominant sound producing region, which is located downstream of the collapse of the potential core. Space-time correlations revealed a convective speed of the pressure signatures that was slightly larger than sonic, relative to the ambient, for the unheated jet, while being around M = 1.48 for the heated jet; Mach wave radiation occurred in both cases. A far-field circular arc array with a radius of 58.5 jet exit diameters was centered at the jet exit and consisted of twelve microphones ranging from 20 to 135 relative to the jet axis. A linear coherence and temporal correlation study unveiled mechanisms by which the nearand far-field pressures are coupled. Mach wave radiation is a fairly linear process. From arrival times of the acoustic disturbances traveling from nearto far-field it was found that propagation speeds were uniform in the heated case, while variations in speed, up to 15% above the ambient sound speed, were found for the unheated case.

Performance efficient jet noise reduction for supersonic nozzles

The variable area, round, axisymmetric, supersonic exhaust nozzle for high performance military aircraft represents a remarkable technological achievement. These variable exhaust nozzles (VEN) utilize conic contraction and expansion sections and provide a simple mechanical means to control the throat-to-exhaust area ratio as a function of throttle input while maintaining structural and thermal integrity. Despite this admirable design, the result is a jet that unavoidably includes shockwaves both inside the nozzle and in the exhaust. The sharp throat radius provides the potential for flow separation which is exacerbated by shock-boundary-layer interactions within the nozzle resulting in reduced pressure recovery on the wall which translates into reduced thrust. A novel design, patented by Dr. John M. Seiner, is shown to eliminate shocks by altering the contour of the VEN seals. This results in the elimination of shock noise in the far-field spectra. The shape of the contoured seals provides the added benefit of generating streamwise vorticity which is evident in the reduction in mixing noise in the far-field spectra. Particle image velocimetry data is presented to demonstrate the enhanced mixing that is generated. Experimental evidence obtained at both model scale and full-scale demonstrates the ability to obtain 2–5 dB noise reduction with an optimized contoured seal design.

Measurements of Store Separation Dynamics

The separation of munitions from payload bays at supersonic speeds was studied experimentally in the tri-sonic wind tunnel at the National Center for Physical Acoustics. A new mechanism for model scale store release was designed and its repeatability evaluated. A 1/15 scale GBU-38 was designed using a “light scaling” approach and the corresponding ejection force was determined. Test drops were performed into a Mach 1.5 free-stream using both the scaled ejection force and a lower ejection force for comparison. An optical tracking method based on PIV was used to measure the trajectory and attitude of the stores. Additionally, the cavity pressure oscillations were measured synchronously with the store release trigger. The results show that the phase relationship between the store release and the cavity pressure oscillations is a key initial condition in determining the resulting flight of the store.

Wall-Pressure Modes in Subsonic Cavity Flows

Subsonic ∞ow over an open cavity with a length-to-depth ratio of 6 was studied using 11 ∞oor mounted pressure transducers evenly spaced along the centerline of the cavity in the streamwise direction. Thirteen free-stream Mach numbers were tested between 0.2 and 0.8 in 0.05 increments. The transducers allowed the dynamic wall-pressure loads to be recorded as a function of time at each of the 11 streamwise locations. Using Fourier decomposition in time and the Proper Orthogonal Decomposition in space, the streamwise organization of the energy associated with a particular Rossiter mode was able to be established. The results show that this energy distribution has a consistent spatial organization that is independent of Mach number.

Effect of Door Configuration on Cavity Flow Modulation Process

Supersonic ow over a high-aspect-ratio cavity was studied with variations on the conguration of the cavity doors. The rectangular cavity had a length of 9 inches and a depth of 1 inch giving an aspect ratio of L=D = 9. The cavity width was 2 inches. Flat panels were attached to the longitudinal edges of the cavity opening to represent doors. In the open position the panels protruded into the oncoming ow with a height of 1=2 of the cavity width, or 1 inch. Wall-pressure signals were measured at 9 locations along the centerline of the cavity for three door con gurations: no doors (N-N), both doors open (O-O), and one door open with one door closed (O-C). The O-O con guration did not exhibit resonant tones in the spectra. Compared to N-N, the O-C spectra exhibited sharper resonant peaks that were higher in amplitude and shifted to slightly higher frequency. The modulation model of Delprat was used to evaluate the shift in tonal frequencies. The results suggest that the presence of the closed door spanning half the cavity opening resulted in a higher shear layer convection velocity and a generally stronger uid/acoustic coupling.

Dynamic Surface Pressure Based Estimation for Flow Control

The need for adaptive control methodologies which involve realistically obtainable information is driving the direction of active flow control research. In this work, time resolved estimates of the velocity field using a mean-square estimation procedure with the unsteady surface pressure as the condition are presented which highlight the relationships between these quantities. Understanding the causal relationships between the velocity field and the surface pressure, an adaptive control strategy solely based on surface pressure can be developed. To this end dynamic stochastic estimation and system identification approaches are shown to accurately predict future surface pressures based on their time histories which can then be used to form the basis of a closed loop control strategy.

The Effects of Gappy POD on Higher-Order Turbulence Quantities

Velocity data obtained using particle image velocimetry is often marred by missing data in various spatial locations due to inconsistent seeding, variations in illumination, and other factors. In order to examine instantaneous or time-varying quantities, it is often necessary to provide as estimate for the missing data. The Gappy POD method provides a means to acquire this estimate using the proper orthogonal decomposition which takes into account the statistics of the data set along with all the valid data in a given snapshot to estimate the missing data in that snapshot. This provides a higher level of fidelity compared to interpolating values based only on neighboring points. The Gappy POD method was evaluated by applying it to 2-D PIV data of a subsonic cold jet with a jet Mach number of 0.85. The estimated velocity was used to compute instantaneous Reynold’s stresses and vorticity which were compared to that obtained from the available data. The results demonstrate that the Gappy POD can provide an estimate that is acurate to within the experimental uncertainty of the measured data. One drawback is observed in that the estimate is optimized using the measured values; therefore, at best it will inherit the experimental uncertainty from the available data.

Flow and Acoustics of Clustered Rockets During Startup

The plume produced by a cluster of two large-area-ratio thrust-optimized parabolic contour nozzles is visualized over a range of nozzle pressure ratios by way of retroreflective shadowgraphy. Both nozzles exhibit free-shock separated flow, restricted-shock separated flow, and an end-effects regime before flowing full. Transient (startup) operations of the nozzles are studied, with the primary focus being the pulsations that form during the end-effects regime. This occurs at a pressure ratio of 37 for these nozzles and is associated with elevated sound levels in the immediate vicinity of the nozzles and vehicle. The shadowgraphy images reveal the formation of turbulent large-scale structures, on the order of the nozzle diameter, during the end-effects regime. These large-scale structures are driven by the intermittent opening of the last trapped annular separation bubble to the ambient and grow rapidly within the first two nozzle diameters.