Greg Elliott

Microramp Flow Control of Normal Shock/Boundary-Layer Interactions

Boundary-layer bleed has conventionally been used to control separation due to shock wave/boundary-layer interactions within supersonic engine inlets. However, bleed systems result in a loss of captured mass flow, incurring higher drag and, ultimately, lower propulsion system efficiency. Microramp sub-boundary-layer vortex generators arranged in a spanwise array have been proposed in the past as a form of flow-control methodology for shock wave/ boundary-layer interactions. Experiments have been conducted herein at Mach 1.4 to characterize flow details of such devices and obtain quantitative measurements of their ability to control the interaction of a normal shock with a turbulent boundary layer. The flowfield was analyzed using schlieren photography, surface oil flow visualization, pressure-sensitive paint, and particle image velocimetry. An array of three microramps, for which the height was scaled to 36% of the incoming boundary-layer thickness, was placed ahead of the normal shock interaction. It was demonstrated that the microramps did entrain higher-momentum fluid into the boundary layer, which improved boundary-layer health. Specifically, the incompressible displacement thickness, momentum thickness, and shape factor were decreased, and the skin friction coefficient was increased, for the shock wave/boundary-layer interaction with the microramp array relative to the no-array case.

Characterization of Schlieren Light Source Using Laser -Induced Optical Breakdown in Argon

Introduction S CHLIEREN imaging systems are widely used for both qualitative and quantitative flow visualization in compressible flows and active index of refraction flowfields such as two-index mixing problems or combustion flowfields. A common implementation of schlieren imaging uses a pulsed light source to provide instantaneous measurements in unsteady flowfields. A variety of pulsed light sources has been used in the past with schlieren imaging systems, including arc lamps, incandescent bulbs, flash tubes, spark gaps, and light-emitting diodes.1 Lasers have been used to provide a narrow linewidth illumination source, which is useful for filtering broad spectral emissions from plasmas or flames, but at the cost of image degradation due to laser speckle. A recent technical note described the use of a laser-induced spark as a point source, which could be inserted in the flowfield avoiding the need to integrate through the density fluctuations associated with the boundary layers on the wind-tunnel walls.2 Recent papers have also described the use of a laser-induced spark as a light source for schlieren imaging in a plasma flow3 and an exploding wire bridge,4 both applications that benefit from a very high-intensity schlieren light source. Our objective in this Technical Note is to characterize a laserinduced spark schlieren imaging technique that provides a very highintensity light source, with short time duration and with repeatable temporal and spatial characteristics. Spatial and temporal variations in intensity are reported for this light source, as well as a comparable light source using a laser discharge in air. This light source has

Pulsed-laser vibrometer using photoelectromotive-force sensors

We demonstrate experimentally significant improvement in the sensitivity of photoelectromotive-force (photo-EMF) laser vibrometers using pulsed-light sources. The vibrating surface is discretely sampled by individual laser pulses and recorded by the photo-EMF sensor via the generation of photocurrent pulses whose magnitudes are proportional to the instantaneous surface displacements. With a sufficiently high sampling rate, reconstruction of the vibration wave form can be achieved by conducting envelope (or peak) detection of the resultant series of photocurrent pulses. Significantly higher peak optical power levels of the probe laser pulses, which can be orders of magnitude greater than those of continuous-wave interrogation lasers with the same average power, lead to proportional enhancement in the photo-EMF response and remarkable improvement in detection sensitivity when the photodetection process is initially amplifier noise current limited.

High-precision measurement of optical frequency differences between Q-switched laser pulses using photo-electromotive-force sensors

We demonstrate the measurement of optical frequency differences between Q-switched laser pulses by using photo-electromotive-force (photo-EMF) optical frequency sensors. The presence of high-peak-power laser pulses affords the photo-EMF frequency sensor with ultrafast response times limited only by the free-carrier lifetime of the sensor material. Such fast response times lead to a broad dynamic range for optical difference frequency measurements with an experimentally demonstrated frequency detection bandwidth in excess of 50 MHz, limited only by the capability of our test equipment. Such a large dynamic range for optical frequency detection makes photo-EMF sensors ideal candidates for adaptive remote sensing of high-speed objects.

Micro-vortex generators and recirculating flow control of normal shock stability and position sensitivity

Experiments have been conducted herein at Mach 1.4 to determine the effect on normal shock stability of a micro-ramp array, ramped vane arrays, porous plates over a cavity, and an open cavity as compared to no control (solid wall) in the region of a 5 o diffuser. For each of the control devices, the mean position and standard deviation in position of the normal shock were determined for a set stagnation pressure over the range of tunnel stagnation pressures from 134.4 to 148.2 kPa using schlieren photography. For each of the set stagnation pressures, the boundary layer static pressure fluctuations downstream of the diffuser entrance were also obtained for each control device. An array of three micro-ramps and an array of three ramped vanes, whose heights were scaled to 40% of the incoming boundary layer thickness, were placed ahead of the normal shock, as well as an array of two ramped vanes with a height of 0.6 the incoming boundary layer thickness. Three porous plate configurations with a porosity of 5% were also examined with varying lengths and positions. The open cavity used was 13.5 and 5 incoming boundary layer thicknesses long and deep, respectively. It was demonstrated that the control devices did affect shock stability in the region of the diffuser, both increasing and decreasing shock stability depending on shock position. Schlieren photography showed the micro-vortex generators reduced fluctuations of normal shock position when the shock moved into the diffuser, while slightly increasing fluctuations in shock position when the mean shock position was upstream of the diffuser entrance. The porous plate that terminated 3.5 incoming boundary layer thicknesses upstream of the diffuser entrance reduced the shock position fluctuations near the diffuser entrance. High frequency wall pressure measurements showed that the open cavity should not be used to improve stability. Also, the standard deviation of the wall static pressure fluctuations inside the diffuser was improved with the micro-vortex generators when the mean shock position was near and downstream of the diffuser shoulder.

Micro-Ramp Flow Control of Normal Shock/Boundary Layer Interactions

Boundary layer bleed has conventionally been used to control separation due to shock wave / boundary layer interactions (SBLIs) within supersonic engine inlets. However, bleed systems result in a loss of captured mass flow, incurring higher drag and ultimately lower propulsion system efficiency. Micro-ramp sub-boundary layer vortex generators (SBVGs) arranged in a spanwise array have been proposed in the past as a form of flow control methodology for shock wave/boundary layer interactions. Experiments have been conducted at Mach 1.4 to characterize flow details of such devices and obtain quantitative measurements of their ability to control the interaction of a normal shock and a turbulent boundary layer. The flow field was analyzed using Schlieren photography, surface oil flow visualization, and particle image velocimetry. An array of three micro-ramps, whose height was scaled to 40% of the incoming boundary layer thickness, was placed ahead of the shock interaction. It was demonstrated that the micro-ramps did entrain higher momentum fluid into the boundary layer which could improve boundary layer health. Specifically, the incompressible displacement thickness, momentum thickness and shape factor were decreased, and the skin friction coefficient was increased, for the SBLI with the micro-ramp array relative to the no-array case.

Multiple computational simulations performed for comparison with planar doppler velocimetry measurements

FD has evolved into an integral part of aircraft design methodology. Once a particular mesh, technique, and turbulence model is validated, it may be used to scale models and to pinpoint potential areas of design improvement. Nonetheless, despite important advances in turbulence modeling and mesh generation, attaining a level of confidence in the validity of the computational simulations still requires experimental ground testing. For the effort presented in this paper, experimental testing and computational analyses were performed integrally such that the strengths of both methods could be capitalized. Initially, a grid and subsequent solution were generated to provide needed information for the fabrication and stress analysis of the model using a stereolith rapid prototyping manufacturing technique. Three-dimensional Euler solutions were obtained for the UCAV X-45A using AVUS

Photo-electromotive-force sensor-based laser vibrometer with gain-switched laser diode

We demonstrated vibration measurement using photo-electromotive-force detector and gain-switched laser diode. This configuration can lead to laser vibrometer with easy-to-setup optical train, low average optical power, compact design, and higher vibration frequency.

Numerical Study of Control of Normal Shock by Energy Pulse

of an expansion wave originating from the compressor face and moving the normal shock downstream. First, a two dimensional viscous simulation was performed to capture the normal shock formed in a Mach 1.5 nozzle. Then a 2D energy spot was instantaneously added half way through the cross section upstream of the normal shock. This energy spot was characterized by a Gaussian profile of temperature and pressure (density is assumed constant when the energy is deposited). The Mach 1.5 normal shock causes the separation of the boundary layer and the increase of the boundary layer thickness. Three dierent dimensionless energy levels ( =1, 10 and 100) were considered. The interaction at = 100 demonstrated a prominent upstream movement of the normal shock, and a significant change of the separation region due to interaction of the energy spot induced compression wave with the separated boundary layer.

Air jet characterization using photo-EMF sensors and Q-switched laser pulses

Summary from only given. The experimental setup adopted to characterize subsonic air jets and scattering rotating wheels using photo-EMF sensors and pulsed laser sources is presented. The Q-switched, frequency-doubled Nd:YAG laser had the pulse width of 10 ns and 10 Hz repetition rate. Neutral density (ND) filters were used to attenuate the laser power to avoid saturating the detection electronic circuit.