Chapter 14 Molecular Tagging Velocimetry in Gases
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Velocimetry↗
Engineering topics
Paul M. Danehy
Publications and source records attributed to Paul M. Danehy.
10-kHz Rate Tomographic Rayleigh Scattering Imaging
This presentation gives an overview of tomographic Rayleigh scattering imaging in ground test facilities.
particle image velocimetry↗
Seeding Mechanism for High-Pressure Nozzles
NASA tests new launch and entry vehicle configurations in wind tunnels, where flow visualizations and quantitative flow field measurements are often desired. Some of these vehicles have rocket motors for propulsion, retro-propulsion, or reaction control. Unseeded,high-pressure air is often used to simulate these rocket motor plumes. However, it is difficult to make off-body measurements in these regions, for several reasons. First, the plumes themselves are usually unseeded and at such low pressures that the flow velocity cannot currently be measured. Second, the plumes displace seeded tunnel air, preventing measurements such as laser sheet visualizations. Third, they force shock waves ahead of the vehicle, which melts the ice crystal fog commonly used for many visualization and measurement techniques. The Virginia Tech Advanced Propulsion and Power Lab has designed and tested a novel method for seeding the flow in these small-scale, high-pressure nozzles to allow quantitative measurement with a particle-based measurement such as particle image velocimetry (PIV). The method involves a Venturi contraction to draw the seed liquid out of a reservoir and into the nozzle channel, wherein shearing forces atomize the seed into particles. The concept was tested with a laser sheet visualization, which showed the nozzle spray pattern and the controllability of the seeding flow rate. This relatively inexpensive and simple technique may prove useful in wind tunnel experiments involving particle-based laser diagnostics and small, high-pressure nozzles.
particle image velocimetry↗
Testing of Mars Retropropulsion Concepts in the Langley Unitary Plan Wind Tunnel
Future Mars human lander missions will require using retrorockets beginning at supersonic conditions, something which has never been done before at Mars. Significant aerosciences challenges exist due to interactions between the retrorocket exhaust plumes, the freestream flow, and the vehicle that alter the aerodynamic behavior. Historical retropropulsion ground and flight data do not provide a basis for calibrating the computational uncertainties for aerodynamic interference forces and moments on proposed Mars descent vehicles, either because of insufficient data or dissimilar vehicles geometries and/or conditions. A new retropropulsion wind tunnel test was conducted in the NASA Langley Unitary Plan Wind Tunnel in order to address some of the deficiencies of previous tests and to provide a basis for computational model comparisons on relevant geometries. The test included improvements in model design and measured data. Two sub-scale model geometries derived from full-scale vehicle concepts were tested: a blunt low lift-to-drag vehicle and a more slender geometry. The first model includes variations in nozzle parameters that were estimated to have first-order effects on the measurements: location, pointing direction, size, and exit-to-throat area ratio. Both models were instrumented with discrete steady and unsteady pressure sensors on the aerodynamic and interior flow path surfaces. The heats shields of both models also were covered with pressure sensitive paint that effectively measures a steady pressure distribution in an otherwise unsteady flow field. This pressure distribution was integrated over the surface geometry in order to estimate the steady aerodynamic force and moment coefficient contribution from the painted surfaces; this approach was used as a replacement for using a flow-through balance. This paper will cover the test objectives, model design, test matrix, and available test results for each model configuration at different freestream Mach numbers, model attitude, and non-dimensional thrust magnitude. A subsequent publication will cover uncertainty quantification of the test data and comparisons to computational fluid dynamics model results.
Powered Descent↗
Velocity Measurements Across an Oblique Shock Using Pulse-burst Cross-correlation DGV
Pulse-burst cross-correlation Doppler global velocimetry is performed in the NASA Langley Unitary Plan Wind Tunnel. The technique is used to make planar velocity measurements across an oblique shockwave, which is generated by a large splitter plate set at a -2° angle of attack in a Mach 2.4 flow. Assessment of the velocimetry indicates agreement with theoretical velocity values to within 0.6 percent on average. The single-scan spatial velocity precision is measured to be between 0.3 and 0.5 m/s, while the ensemble (1σ) precision of the measurements was within 2.2 percent of the measured velocities. The promising results encountered in this testing campaign indicate that the technique could be useful for studying more complex flow fields.
lasers↗
Stereo Camera Simulation for Lunar Surface Photogrammetry
During the rocket-powered landing of a vehicle on a planetary body, the interaction between the rocket plume and the surface material beneath the vehicle plays a significant role in the descent dynamics and the safety of powered descent. However, in-situ data taken to investigate plume-surface interaction has been limited. The upcoming flight and lunar landing of the Intuitive Machines Nova-C will be equipped with the Stereo Cameras for Lunar Plume-Surface Studies (SCALPSS) science package, enabling the direct measurement of plume-induced surface cratering during powered descent. In this work, we present the engineering constraints, design iteration process, and simulation results that drove the design selection for the SCALPSS camera system. The chosen design will provide 3D-imaging coverage of 84 percent of the lunar surface directly under the landed Nova-C, in addition to some coverage of the neighboring surface. SCALPSS will provide a total 3D-imaging coverage area of approximately 13 square meters. With the anticipated Nova-C landing in October of 2021, SCALPSS will provide the first dedicated in-situ measurement of plume-induced surface cratering.
Ryan J. Thompson↗
A comparison of NO laser-induced fluorescence models at conditions relevant to supersonic and hypersonic flows
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laser-induced fluorescence↗
Hybrid pulse-burst / cross-correlation DGV for high-speed flow measurements at 100 kHz
Hybrid pulse-burst Doppler global velocimetry (PB-DGV)/pulse-burst, cross-correlation DGV (PB-CC-DGV) was demonstrated for the first time in the NASA Langley 4-foot Supersonic Unitary Plan Wind Tunnel, allowing simultaneous acquisition of mean and instantaneous high-repetition-rate velocity fields. The technique was used to make planar velocity measurements across an oblique shockwave generated by a large splitter plate set at a -2° angle of attack in a Mach 2.4 flow. Sequences of more than 100 consecutive instantaneous velocity images were obtained at a rate of 100 kHz. Velocity fields from both aspects of the hybrid measurement indicated errors less than 1-percent of anticipated velocities. Likewise, all assessments of measurement precision were approximately 1.6-percent of the local velocities. Additional demonstrations in different flowfields including a subsonic axi-symmetric jet and a supersonic Mars reentry vehicle model further illustrate the utility of the new hybrid method.
laser↗
Single-Component Average Velocity Profiles in the Wake of the Orion Crew Capsule at the National Transonic Facility
A minimally intrusive molecular tagging instrument measured single-component average velocity profiles in two planes in the wake of the Orion capsule with different heat shield configurations in the National Transonic Facility at the NASA Langley Research Center. Reynolds number effects at subsonic conditions have proven difficult to predict due to the largely separated wake flow. Therefore two measurement planes in the wake of the model were probed to measure the wake profile for different heat shield configurations and act as a validation reference for computational tools. Air testing included Mach numbers of 0.3, 0.5, and 0.7 at Reynolds numbers of 5.3 and 7.5 million. In cryogenic nitrogen, the instrument was employed under transient conditions during the facility warm up at M = 0.3 with decreasing Reynolds numbers from 16 million. An exhaustive list of the results is shown and discussed here. For free transition heat shield configurations, the size of the wake was found to increase with Mach number, yet remain constant with Reynolds number for low (M = 0.3) and high (M = 0.7) subsonic Mach numbers. However, intermediate Mach numbers (M = 0.5) showed that the wake was smaller at higher Reynolds numbers for the IDAT heat shield. The addition of surface roughness in the form of grit, known as the fixed transition cases, negated any Mach number dependence to the wake profile and increased the size of the wake for all cases.
Velocimetry↗
Advancements in Burst-Mode Filtered Rayleigh Scattering for High-Speed Gas-Phase Multi-Parameter Measurements
Filtered Rayleigh scattering (FRS) can be used for velocity, density, temperature, and/or pressure measurements over a wide range of non-reacting flow conditions as well as more limited reacting flow conditions. However, the application of FRS to gas-phase measurements in large-scale test facilities can be limited due to low signal levels, strong background interferences, or the requirement for multiple cameras and/or molecular filters to spectrally resolve the signal. This work focuses on the spectral characteristics and wavelength-agility of burst-mode lasers for use in FRS systems. These advancements could improve measurement capability and data quality in applications with high levels of stray background scattering as well as where increased temporal resolution is desirable, such as high-speed flows.
filtered Rayleigh scattering↗
Advances in Stereo Photogrammetry Capabilities for in-Situ Ejecta Measurements in Lunar Plume-Surface Interactions
Plume-surface interaction is a critical topic of interest as human spaceflight programs establish a renewed presence on the Moon and push onwards to Mars. The erosion of regolith and high-speed ejection of soil and rocks which occurs during a powered spacecraft landing on lunar or terrestrial bodies can pose serious risks to the vehicle during landing and to existing ground assets. Ground testing campaigns have provided data sets in relevant environments (e.g., low pressure) in tandem with predictive computational model validation. To-date, limited in-situ flight data exists to inform the study of the key phenomena of plume-surface interaction: plume-flow physics, soil erosion, and ejecta dynamics. In two upcoming lunar missions, erosion of the lunar regolith will be measured in-situ using the stereo photogrammetry technique as part of NASA’s Commercial Lunar Payload Services (CLPS) Program. The Stereo Cameras for Lunar Plume Surface Studies (SCALPSS) will use a multiple-camera system to measure the erosion of the lunar regolith due to the vehicles’ rocket engine plumes during descent and after landing. The application of stereo photogrammetry to accurately measure a realistic eroded shape has been validated in laboratory work ahead of the scheduled flights. In preparation for future CLPS, Artemis, or other lunar science missions, recent advancements have been made to measure the ejecta structure and particle dynamics in addition to the erosion of regolith using a SCALPSS-like photogrammetry system. The thickness and ejection angle of entrained soil and dust produced during plume-induced erosion can be used to determine how far debris will travel, and thus how it may impact nearby infrastructure or the landing vehicle itself. The present work highlights the development of this new flight instrumentation capability and its value to advancements in the plume-surface interaction measurement topic for lunar or planetary landings.
Olivia K. Tyrrell↗
Demonstration of Frequency-Scanning Burst-mode Filtered Rayleigh Scattering for Multi-Parameter Gas-Phase Measurements
Filtered Rayleigh scattering (FRS) is a diagnostic technique used for measurements of macroscopic properties of a gas. The FRS signal is passed through a narrow bandwidth molecular filter, commonly gaseous iodine, to measure the broadening and Doppler-shift of the scattering light from the molecules of the gas flow. However, in single-frequency, intensity-based approaches, the effects of pressure, temperature, and velocity can be difficult to deconvolve. To overcome this challenge, frequency-scanning FRS has been used to quasi-spectrally resolve the signal. In this work, a frequency-scanned, burst-mode laser was used to perform scanning FRS in an underexpanded jet to measure temperature, pressure, and radial velocity at measurement rates up to 1 kHz which is ~100,000 times faster than prior implementations of scanning FRS.
filtered Rayleigh scattering↗
Flow Visualization of Intrusive and Non-Intrusive Configurations for Lunar- and Martian-Relevant Plume-Surface Interaction
Flow visualization of a heated, inert-gas plume impinging onto a rigid surface was performed in lunar- and Martian-relevant pressure conditions. The experimental campaign was part of a broader effort to improve predictive models and capabilities for plume-surface interactions in spacecraft landing environments relevant to the Moon and Mars. The experiments used the planar laser induced fluorescence (PLIF) technique to visualize the flow of the jet over both a full-plane configuration using a flat impingement plate and a half-plane configuration where the jet flow was bisected by a splitter edge mounted to the impingement plate. The latter configuration has been previously used to study erosion mechanisms in plume-surface interactions because the technique enables cross-sectional optical access for visualizing the plume-induced crater. However, this approach has some uncertainty regarding the influence of the splitter edge on the flow field. The present work evaluates the differences in flow structures and characteristics between the flat plate and splitter plate experimental configurations at eight unique test conditions with and without the splitter edge where the vacuum chamber pressure, nozzle mass flow rate, and height of the nozzle were varied. Several features are identified which differ between the flat plate and splitter plate comparison cases, and these are summarized in this paper. The results presented provide insights to the differences between intrusive and non-intrusive experimental configurations for plume-surface interaction studies that can be used to further validate predictive models and inform future ground and flight test results.
PLIF↗
Development of a fs/Ps CARS System for Temperature Measurements Within Supersonic and Hypersonic Environments
A hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) system was developed for quantitative measurements of temperature in a laboratory-scale supersonic jet facility. Measurements were recorded at low pressures and densities relevant for supersonic and hypersonic environments, with special interest in exploring the feasibility of deploying this technique in the 20-inch Mach 6 and 31-inch Mach 10 wind tunnels located at NASA Langley Research Center. Modifications to the existing supersonic jet facility were made to simulate a test section with a width of 31 inches, so that the size of the test section is relevant for either wind tunnel. The CARS system was designed such that a similar beam geometry can be used in the laboratory to acquire point-based fs/ps CARS measurements along two axes of translation. Rotational Raman transitions of O 2 and N 2 were targeted.
CARS↗
Characterization of Naturally-Occurring Particles in the Nasa Langley 0.3-m Transonic Cryogenic Tunnel Using PTV
Naturally-occurring particles within the NASA Langley 0.3-m Transonic Cryogenic Tunnel are characterized for their aerodynamic performance using particle tracking velocimetry. Two sets of experiments were conducted to observe different behaviors of the particles. The normal shockwave emanating from the top surface of a supercritical airfoil was used to induce velocity lag in the particles, and the subsequent spatial decay of the velocity was used to estimate the effective diameter of the particles. Mean particle diameters between 1.6 and 1.9 𝝁m were measured, with sizes ranging from 0.2 to 3.5 𝝁m over the entire ensemble. The response of particles to separated flow was investigated. By operating in “high-lift” (low Mach number, high angle of attack) conditions with a semi-span airfoil, the ability of particles to detect separated flow on the upper surface of the airfoil was assessed. Transition from fully attached flow to fully separated flow was observed on the top surface of the airfoil accompanying a variation of angle of attack from 8° to 12°. Examination of velocity distributions indicates less than 10 percent of particle trajectories did not respond to the regions of separated flow. These results are promising, but further facility-specific work is needed to answer the broader question of particle tracking reliability.
Transonic↗
Self-Aligned Focusing Schlieren at the 0.3-M Transonic Cryogenic Tunnel and the National Transonic Facility
The implementation of a self-aligned focusing schlieren (SAFS) system at two cryogenic wind tunnels at NASA Langley Research Center is discussed. Risk-reduction testing of the SAFS system was first performed at the 0.3-M Transonic Cryogenic Tunnel to evaluate the system’s operation in a small-scale characteristic cryogenic facility. Testing was conducted with three models: a three-quarter span 25.4-mm-diameter cylinder, a semi-span 65A006 tapered unswept airfoil, and a full-span SC(3)-0712 airfoil. Testing with the cylinder and semi-span airfoil revealed a highly dynamic shock environment, whereas the shock on the full-span airfoil was stationary, solidifying the usage of this model for a pre-/post-shock particle tracking velocimetry measurement. Temperature-induced polarization-altering window stresses were encountered during low-temperature testing, and were mitigated using a “non-ideal” quartz/quartz Rochon prism that had largely been neglected since the SAFS system’s first introduction in favor of the more favorable “ideal” glass/quartz Rochon prism. The size of the SAFS system was then decreased in order to fit inside an environmentally-controlled camera can enclosure at the National Transonic Facility (NTF) for testing of a sting-mounted aircraft model. The SAFS system was demonstrated to be effective at filtering out the large density gradient flow in the 0.3-M plenum, and the thick, high density turbulent boundary layers on the wind tunnel walls at the NTF. Results of the testing campaigns and improvements to future systems are discussed.
Joshua M. Weisberger↗
Evaluation of a Laser-Dot Grid-Projection System for Lunar Lander Crater-Shape and Ejecta-Sheet Measurements
Development, testing, and evaluation of an active-illumination laser-dot grid-projection system was performed to evaluate its potential inclusion on an upcoming uncrewed lunar lander mission to the south pole region of the moon. Active illumination may be required for this mission due to the shallow angle of the sun at the landing location, which may cast shadows in the landing region, hindering the stereo camera imaging system’s ability to determine the pre-and post-landing lunar surface/crater shape. Testing includes characterization of the diffractive optical elements used for the grid projection, single-camera tests to determine the imaged laser dot intensity as a function of camera exposure and imaging angle relative to the laser grid projection, laser dot brightness/saturation impact on depth map calculations, angle of imaged laser grid impact on depth map calculations for steep crater edges, impact of density gradients on the depth map calculations, and the feasibility of making measurements of the ejecta sheet.
Joshua M. Weisberger↗