Edward T. Schairer

Planar Velocimetry of Jet/Fin Interaction on a Full-Scale Flight Vehicle Configuration

Stereoscopic particle image velocimetry has been implemented in a production-scale transonic wind tunnel for studying jet/fin interaction created by exhaust plumes from spin rockets on a full-scale model of a finned body of revolution. Data acquired just upstream of the leading edge of the fin root clearly display the counter-rotating vortex pair that dominates the interaction far field and the remnant of the horseshoe vortex near the vehicle surface. The counter-rotating vortex pair is distinctly asymmetric due to originating from a scarfed nozzle and displays some rotation with respect to the model surface. Velocity fields measured over a range of flowfield conditions and model orientations show that the vortex of negative sign is always closer to the fins than its positive counterpart and does not greatly change location as flowfield parameters are altered. The circulation of this vortex correlates with a reduction in the simultaneously measured vehicle roll torque. Further correlations are hindered by untreatable bias errors in the velocimetry. Instead, a model of the vortex structure derived from the velocimetry data reveals that the angle of attack induced upon the fins by the counter-rotating vortex pair correlates with the roll torque loss. Similar correlations suggest that in level flight this effect is dominant, but at angle of attack the horseshoe vortex on the windward side has an additional influence.

Results from the Mars Science Laboratory Parachute Decelerator System Supersonic Qualification Program

In 2010 the Mars Science Laboratory (MSL) Mission will deliver the most massive and scientifically capable rover to the surface of Mars. To deliver this payload, an aerodynamic decelerator is required to decelerate the entry vehicle from supersonic to subsonic speeds, in advance of propulsive descent and touchdown on Mars. The aerodynamic deceleration will be accomplished by a mortar-deployed 21.5-m Viking-type disk-gap-band parachute (DGB), and will be the largest extra-terrestrial decelerator in the history of space exploration [1]. The parachute will deploy at up to Mach 2.2 and 750 Pa, resulting in the highest load and speed experienced by a parachute on Mars. The MSL parachute extends the envelope of the existing heritage deployment space in terms of load, size and Mach number. This has created the challenge of leveraging the existing heritage supersonic- high-altitude database, implementing a ground-based qualification program, and quantifying known aerodynamic instabilities associated with supersonic operation in the Mach regime of the MSL deployment. To address these challenges MSL has embarked upon a physics-based modeling and validation program to explore the fundamental physics associated with DGB-parachute operation in supersonic flow. The functional dependence of parachute performance and stability on Mach number, Reynolds number, parachute size, entry-vehicle size and parachute to entry vehicle proximity, is under investigation. The quantitative understanding garnered from this analytical effort will be used to leverage the existing heritage database of the Viking Lander, Viking Balloon Launched Decelerator Test (BLDT), Mars Pathfinder (MPF) and Mars Exploration Rover (MER) programs for the larger scale, deployment conditions, and modern construction techniques of the MSL parachute system. The physics-based modeling and validation effort includes the development of a coupled fluid and structural solver, i.e. fluid-structure-interaction code, and supersonic wind-tunnel experiments with subscale representations of the flight configuration.

Planning Image-Based Measurements in Wind Tunnels by Virtual Imaging

Virtual imaging is routinely used at NASA Ames Research Center to plan the placement of cameras and light sources for image-based measurements in production wind tunnel tests. Virtual imaging allows users to quickly and comprehensively model a given test situation, well before the test occurs, in order to verify that all optical testing requirements will be met. It allows optimization of the placement of cameras and light sources and leads to faster set-up times, thereby decreasing tunnel occupancy costs. This paper describes how virtual imaging was used to plan optical measurements for three tests in production wind tunnels at NASA Ames.

Photogrammetric recession measurements of ablative materials in arcjets

This paper describes an optical method for measuring the recession time histories of ablative thermal protection system (TPS) materials as they are tested in an arcjet facility. The method is non-intrusive and requires no external light source or modifications to the test article. It does require, first, a test article that exhibits texture as it ablates, and, second, high-resolution video images of the ablating surface from at least two directions. Software automatically reads the sequences of images and, by successive image cross correlation, tracks the deformation of a surface grid that conforms to the shape of the test article. Standard photogrammetric transformations are used to convert image-plane displacements of the surface grid to object-space displacements. The method yields a time history of the displacement of each node of the grid for the full time that the test article is exposed to the arcjet flow. Measurements have been made during many tests in the 60 MW arcjet at NASA Ames Research Center, including tests of TPS materials for the Orion Crew Exploration Vehicle and Mars Science Laboratory. The photogrammetric recession measurements have been in good agreement with post-test measurements of the change in thickness of the test articles.

Predicting Camera Views for Image-Based Measurements in Wind Tunnels

This paper describes a method for creating virtual images of wind-tunnel scenes and shows how these images can be used to plan image-based measurements. The method has been implemented as a stand-alone Windows application and was used extensively to plan Particle Image Velocimetry (PIV) measurements for recent wind-tunnel tests of a 3% scale model of the Space Shuttle. Real and virtual images from these tests are presented to demonstrate the methodology.

Background-Oriented Schlieren Imaging for Full-Scale and In-Flight Testing

Background-oriented schlieren (BOS) methods suited for large-scale and in-flight testing are presented with special emphasis on the detection and tracing of blade tip vortices in situ. Retroreflective recording and photogrammetric epipolar analysis for the computation of the vortices’ spatial coordinates in the wind tunnel are described. Feasibility and fidelity of referencefree BOS in conjunction with natural formation backgrounds and related evaluation methods are discussed, additionally, illustrating their simplicity and robustness. Results of successful image acquisition from a chaser aircraft are presented allowing vortex wakes to be identified at a wide range of flight attitudes, including complex maneuvers.

Stereoscopic PIV for Jet/Fin interaction measurements on a full-scale flight vehicle configuration.

A stereoscopic particle image velocimeter has been implemented in a production-scale transonic wind tunnel for studying jet/fin interaction created by exhaust plumes from spin rockets on a full-scale model of a finned body of revolution. Data were acquired principally at a measurement plane just upstream of the leading edge of the fin root, which clearly display the counter-rotating vortex pair as well as a smaller vortex pair near the surface believed to be the remnant of the horseshoe vortex. The counter-rotating vortex pair is distinctly asymmetric as a result of having been produced by a jet exiting from a scarfed nozzle and displays some twist with respect to the model surface. The horseshoe vortex is displaced laterally from the nozzle position. Velocities measured over a range of flowfield conditions and model orientations show that the size and position of the vortices chiefly scale with the jet-to-freestream dynamic pressure ratio and that their strength is additionally a function of the crossflow Mach number.

Stereo Photogrammetry Measurements of the Position and Attitude of a Nozzle-Plume/Shock-Wave Interaction Model in the NASA Ames 9- by 7-Ft Supersonic Wind Tunnel

Stereo photogrammetry was used to measure the position and attitude of a slender body of revolution during nozzle-plume/shock-wave interaction tests in the NASA Ames 9by 7-Ft Supersonic Wind Tunnel. The model support system was designed to allow the model to be placed at many locations in the test section relative to a pressure rail on one sidewall. It included a streamwise traverse as well as a thin blade that offset the model axis from the sting axis. With these features the support system was more flexible than usual resulting in higherthan-usual uncertainty in the position and attitude of the model. Also contributing to this uncertainty were the absence of a balance, so corrections for sting deflections could not be applied, and the “wings-vertical” orientation of the model, which precluded using a gravitybased accelerometer to measure pitch angle. Therefore, stereo photogrammetry was chosen to provide independent measures of the model position and orientation. This paper describes the photogrammetry system and presents selected results from the test.

Model Deformation and Optical Angle of Attack Measurement System in the NASA Ames Unitary Plan Wind Tunnel

Both AoA and MDM measurements can be made using an optical system that relies on photogrammetry. Optical measurements are being requested by customers in wind tunnels with increasing frequency due to their non-intrusive nature and recent hardware and software advances that allow measurements to become near real time. The NASA Ames Research Center Unitary Plan Wind Tunnel is currently developing a system based on photogrammetry to measure model deformation and model angle of attack. This paper describes the new system, its development, its use on recent tests and plans to further develop the system.

Model Deformation Measurements of Sonic Boom Models in the NASA Ames 9- by 7-Ft Supersonic Wind Tunnel

The deformations of two sonic-boom models were measured by stereo photogrammetry during tests in the 9- by 7-Ft Supersonic Wind Tunnel at NASA Ames Research Center. The models were geometrically similar but one was 2.75 times as large as the other. Deformation measurements were made by simultaneously imaging the upper surfaces of the models from two directions by calibrated cameras that were mounted behind windows of the test section. Bending and twist were measured at discrete points using conventional circular targets that had been marked along the leading and trailing edges of the wings and tails. In addition, continuous distributions of bending and twist were measured from ink speckles that had been applied to the upper surfaces of the model. Measurements were made at wind-on (M = 1.6) and wind-off conditions over a range of angles of attack between 2.5 deg. and 5.0 deg. At each condition, model deformation was determined by comparing the wind-off and wind-on coordinates of each measurement point after transforming the coordinates to reference coordinates tied to the model. The necessary transformations were determined by measuring the positions of a set of targets on the rigid center-body of the models whose model-axes coordinates were known. Smoothly varying bending and twist measurements were obtained at all conditions. Bending displacements increased in proportion to the square of the distance to the centerline. Maximum deflection of the wingtip of the larger model was about 5 mm (2% of the semispan) and that of the smaller model was 0.9 mm (1% of the semispan). The change in wing twist due to bending increased in direct proportion to distance from the centerline and reached a (absolute) maximum of about -1 at the highest angle of attack for both models. The measurements easily resolved bending displacements as small as 0.05 mm and bending-induced changes in twist as small as 0.05 deg.