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NASA NTRSDate not supplied

Advances in PSP Testing in LaRC High Reynolds Number Facilities

The use of luminescent coatings for the global measurement of surface aerodynamic properties at full-flight Reynolds numbers has been ongoing at the NASA Langley Research Center since the late 1990s, beginning with Temperature Sensitive Paint (TSP) for boundary layer analysis at the 0.3-m Transonic Cryogenics Tunnel. Since then, significant work has been made to extend these measurements to pressure using Pressure Sensitive Paint (PSP) as well as develop systems for application in larger scale, full-flight Reynolds number facilities, such as the National Transonic Facility (NTF). Recently, a system for the measurement of time-resolved pressure using recently developed unsteady PSP (uPSP) formulations has been designed and implemented in the Transonic Dynamics Tunnel (TDT). The use of PSP (or uPSP) in either of these large scale, full-fight Reynolds number wind tunnels is complicated by the fact that both facilities operate best in oxygen deficient environments; however, the PSP technique relies on the presence of oxygen to work. The NTF is typically operated in cryogenic conditions, which is achieved using liquid nitrogen. In this facility, temperatures can reach as low as 116 K (-250 °F) with nominal oxygen concentrations of less than 50 ppm. This has resulted in the development of specialized PSP formulations that can operate in the cryogenic environment with the introduction of low amounts of oxygen (typically less than 2000 ppm) for PSP response. Likewise, to achieve full-flight Reynolds number conditions in the TDT, the atmosphere in the tunnel is replaced with a “heavy gas” of R-134A (1,1,1,2-tetrafluoroethane), which also has minimal oxygen native in the flow. The use of uPSP in this case will also depend on the introduction of trace amounts of oxygen, with the precise concentrations to be determined in an upcoming test. This presentation will describe some recent advancements that have been made for PSP measurements in both facilities. For the NTF system, there has been significant development of enhanced lighting for use at cryogenic conditions, as well as improvements in the application efficiency of the PSP. Furthermore, there are efforts underway to incorporate advanced data analysis techniques to acquire additional surface aerodynamic properties using the PSP technique that can yield not only improved experimental efficiency, but also provide data needed for next generation vehicle design and development. For the uPSP system in TDT, significant efforts to improve the data transfer rates to a high-performance computing environment for analysis are underway and performance and initial results from the system in the heavy-gas environment will be presented.

Pressure Sensitive Paint↗

NASA NTRS2022

New Insights into the First Two PSP Solar Encounters Enabled by Modeling Analysis with ADAPT-WSA

Parker Solar Probe's (PSP's) unique orbital path allows us to observe the solar wind closer to the Sun than ever before. Essential to advancing our knowledge of solar wind and energetic particle formation is identifying the sources of PSP observations. We report on results for the first two PSP solar encounters derived using the Wang–Sheeley–Arge (WSA) model driven by Air Force Data Assimilative Photospheric Flux Transport (ADAPT) model maps. We derive the coronal magnetic field and the 1 R⊙ source regions of the PSP-observed solar wind. We validate our results with the solar wind speed and magnetic polarity observed at PSP. When modeling results are very reliable, we derive time series of model-derived spacecraft separation from the heliospheric current sheet, magnetic expansion factor, coronal hole boundary distance, and photospheric field strength along the field lines estimated to be connected to the spacecraft. We present new results for Encounter 1, which show time evolution of the far-side mid-latitude coronal hole that PSP corotates with. We discuss how this evolution coincides with solar wind speed, density, and temperature observed at the spacecraft. During Encounter 2, a new active region emerges on the solar far side, making it difficult to model. We show that ADAPT-WSA output agrees well with PSP observations once this active region rotates onto the near side, allowing us to reliably estimate the solar wind sources retrospectively for most of the encounter. We close with ways in which coronal modeling enables scientific interpretation of these encounters that would otherwise not have been possible.

Solar wind↗

NASA NTRS2016

Experimental Verification of Buffet Calculation Procedure Using Unsteady PSP

Typically a limited number of dynamic pressure sensors are employed to determine the unsteady aerodynamic forces on large, slender aerospace structures. The estimated forces are known to be very sensitive to the number of the dynamic pressure sensors and the details of the integration scheme. This report describes a robust calculation procedure, based on frequency-specific correlation lengths, that is found to produce good estimation of fluctuating forces from a few dynamic pressure sensors. The validation test was conducted on a flat panel, placed on the floor of a wind tunnel, and was subjected to vortex shedding from a rectangular bluff-body. The panel was coated with fast response Pressure Sensitive Paint (PSP), which allowed time-resolved measurements of unsteady pressure fluctuations on a dense grid of spatial points. The first part of the report describes the detail procedure used to analyze the high-speed, PSP camera images. The procedure includes steps to reduce contamination by electronic shot noise, correction for spatial non-uniformities, and lamp brightness variation, and finally conversion of fluctuating light intensity to fluctuating pressure. The latter involved applying calibration constants from a few dynamic pressure sensors placed at selective points on the plate. Excellent comparison in the spectra, coherence and phase, calculated via PSP and dynamic pressure sensors validated the PSP processing steps. The second part of the report describes the buffet validation process, for which the first step was to use pressure histories from all PSP points to determine the "true" force fluctuations. In the next step only a selected number of pixels were chosen as "virtual sensors" and a correlation-length based buffet calculation procedure was applied to determine "modeled" force fluctuations. By progressively decreasing the number of virtual sensors it was observed that the present calculation procedure was able to make a close estimate of the "true" unsteady forces only from four sensors. It is believed that the present work provides the first validation of the buffet calculation procedure which has been used for the development of many space vehicles.

Buffet↗

NASA NTRS1997

PSP Testing at NASA Ames Research Center

Pressure sensitive paints (PSPs) are now used routinely for measuring surface pressures on wind tunnel models at transonic and supersonic Mach numbers. The method utilizes a surface coating containing fluorescent or phosphorescent materials, the brightness of which varies with the local air pressure on the surface. The present paper will summarize PSP activities (in progress and planned) at the NASA Ames Research Center. One of the main accomplishments at NASA Ames has been the development of a PSP measurement system that is production testing capable. This system has been integrated successfully into the large-scale wind tunnel facilities at Ames. There are several problems related to PSP testing which are unique to large-scale wind tunnel testing. The hardware is often difficult to set-up and must operate under harsh conditions (e.g. high pressures and low temperatures). The data acquisition and reduction times need to be kept to a minimum so that the overall wind tunnel productivity is not compromised. The pressure sensitive paints needs to be very robust; the paints must readily adhere to different surfaces with varying geometries and remain functional for long running times. The paint must have well understood, and preferably minimal, temperature sensitivity since fine control of the tunnel temperature is not easily achievable in the larger wind tunnels. In an effort to improve the overall accuracy of the PSP technique, we are currently evaluating some referenced pressure sensitive paints which contain a pressure- independent luminophor in addition to the one which is affected by the surface pressure. The two luminophors are chosen so that their emission wavelengths are somewhat different. Then by taking two 'wind-on' images with either two cameras (with different filters) or one camera with a rotating filter system, the need for 'wind-off' images can be eliminated. The ratio of the two wind-on images accounts for nonuniform lighting and model motion problems (physical movement of the model under aerodynamic loading) are not a concern any more. The paints and data acquisition and reduction software for this type of a PSI system are currently under development and evaluation. Additional information is contained within the original extended abstract.

Bell, J. H.↗

NASA NTRS2011

Deployment of a Pressure Sensitive Paint System for Measuring Global Surface Pressures on Rotorcraft Blades in Simulated Forward Flight: Preliminary PSP Results from Test 581 in the 14- by 22-Foot Subsonic Tunnel

This report will present details of a Pressure Sensitive Paint (PSP) system for measuring global surface pressures on the tips of rotorcraft blades in simulated forward flight at the 14- x 22-Foot Subsonic Tunnel. The system was designed to use a pulsed laser as an excitation source and PSP data was collected using the lifetime-based approach. With the higher intensity of the laser, this allowed PSP images to be acquired during a single laser pulse, resulting in the collection of crisp images that can be used to determine blade pressure at a specific instant in time. This is extremely important in rotorcraft applications as the blades experience dramatically different flow fields depending on their position in the rotor disk. Testing of the system was performed using the U.S. Army General Rotor Model System equipped with four identical blades. Two of the blades were instrumented with pressure transducers to allow for comparison of the results obtained from the PSP. This report will also detail possible improvements to the system.

Watkins, Anthony Neal↗

NASA NTRS2020

Possible Evolution of Minifilament-Eruption-Produced Solar Coronal Jets, Jetlets, and Spicules, into Magnetic-Twist-Wave “Switchbacks” Observed by the Parker Solar Probe (PSP)

Many solar coronal jets result from erupting miniature-filament (“minifilament”) magnetic flux ropes that reconnect with encountered surrounding far-reaching field. Many of those minifilament flux ropes are apparently built and triggered to erupt by magnetic flux cancelation. If that cancelation (or some other process) results in the flux rope’s field having twist, then the reconnection with the far-reaching field transfers much of that twist to that reconnected far- reaching field. In cases where that surrounding field is open, the twist can propagate to far distances from the Sun as a magnetic-twist Alfv ́enic pulse. We argue that such pulses from jets could be the kinked-magnetic-field structures known as “switchbacks,” detected in the solar wind during perihelion passages of the Parker Solar Probe (PSP). For typical coronal-jet-generated Alfv ́enic pulses, we expect that the switchbacks would flow past PSP with a duration of several tens of minutes; larger coronal jets might produce switchbacks with passage durations∼1 hr.Smaller-scale jet-like features on the Sun known as “jetlets” may be small-scale versions of coronal jets, produced in a similar manner as the coronal jets. We estimate that switchbacks from jetlets would flow past PSP with a duration of a few minutes. Chromospheric spiculesare jet-like features that are even smaller than jetlets. If some portion of their population are indeed very-small-scale versions of coronal jets, then we speculate that the same processes could result in switchbacks that pass PSP with durations ranging from about∼2 min down to tens of seconds.

Alphonse C Sterling↗

NASA NTRSDate not supplied

A Hinode-IRIS HOP for Searching for Possible Solar Jet Sources for Switchbacks Observed in the Near-Sun Solar Wind by the Parker Solar Probe (PSP)

The Parker Solar Probe (PSP) detects “switchback” magnetic structures in the near-Sun solar wind. A candidate for the source of these is solar coronal jets. Recent studies show that coronal jets are frequently made by eruptions of small-filament-carrying flux ropes, or mini filament flux-rope eruptions, that can inject a magnetic twist wave onto high-reaching ambient magnetic field(Sterling et al. 2015; Moore et al 2015).That is, many coronal jets evidently result from the eruption of a small-scale flux rope of twisted field that holds a mini filament of chromospheric-temperature plasma. The erupting flux rope reconnects with ambient far-reaching field, ejecting both reconnection-heated plasma and cool filament plasma (forming the multi-temperature jet spire), and transferring its twist onto the far-reaching field via a mechanism proposed by Shibata & Uchida(1986).If the far-reaching field opens into the heliosphere, it is plausible that the twist propagates out into the solar wind, possibly appearing as a switchback in the magnetic field in the solar wind (Sterling & Moore 2020).As part of the "Connecting Heliophysics Observatories and Models with PSP" (CHOMP)investigation led by Pete Riley and funded by NASA’s Heliophysics System Observatory Connect (HSOC) Program, we are currently We are currently running a joint Hinode-IRIS HOP, HOP 410, for coordinated observations of the Sun during times of PSP perihelia. An output from this study is a series of soft X-ray movies from Hinode/XRT. If PSP sees a strong switchback during any of its front-side or partly-front-side perihelia, we will examine the corresponding XRT movies to see whether exceptional jet activity occurred at the expected time and location of the potential source of the switchback on the Sun. This work is supported by the NASA HSOC and HGI programs, and by the MSFC Hinode project.

Alphonse C Sterling↗

NASA NTRS2002

Development of Next Generation Lifetime PSP Imaging Systems

This paper describes a lifetime PSP system that has recently been developed using pulsed light-emitting diode (LED) lamps and a new interline transfer CCD camera technology. This system alleviates noise sources associated with lifetime PSP systems that use either flash-lamp or laser excitation sources and intensified CCD cameras for detection. Calibration curves have been acquired for a variety of PSP formulations using this system, and a validation test was recently completed in the Subsonic Aerodynamic Research Laboratory (SARL) at Wright-Patterson Air Force Base (WPAFB). In this test, global surface pressure distributions were recovered using both a standard intensity-based method and the new lifetime system. Results from the lifetime system agree both qualitatively and quantitatively with those measured using the intensity-based method. Finally, an advanced lifetime imaging technique capable of measuring temperature and pressure simultaneously is introduced and initial results are presented.

Watkins, A. Neal↗

NASA NTRS1998

PSP Measurement of Stator Vane Surface Pressures in a High Speed Fan

This paper presents measurements of static pressures on the stator vane suction side of a high-speed single stage fan using the technique of pressure sensitive paint (PSP). The paper illustrates development in application of the relatively new experimental technique to the complex environment of internal flows in turbomachines. First, there is a short explanation of the physics of the PSP technique and a discussion of calibration methods for pressure sensitive paint in the turbomachinery environment. A description of the image conversion process follows. The recorded image of the stator vane pressure field is skewed due to the limited optical access and must be converted to the meridional plane projection for comparison with analytical predictions. The experimental results for seven operating conditions along an off-design rotational speed line are shown in a concise form, including performance map points, mindspan static tap pressure distributions, and vane suction side pressure fields. Then, a comparison between static tap and pressure sensitive paint data is discussed. Finally, the paper lists shortcomings of the pressure sensitive paint technology and lessons learned in this high-speed fan application.

Lepicovsky, Jan↗

NASA NTRS2000

Intensity Biased PSP Measurement

The current pressure sensitive paint (PSP) technique assumes a linear relationship (Stern-Volmer Equation) between intensity ratio (I(sub 0)/I) and pressure ratio (P/P(sub 0)) over a wide range of pressures (vacuum to ambient or higher). Although this may be valid for some PSPs, in most PSPs the relationship is nonlinear, particularly at low pressures (less than 0.2 psia when the oxygen level is low). This non-linearity can be attributed to variations in the oxygen quenching (de-activation) rates (which otherwise is assumed constant) at these pressures. Other studies suggest that some paints also have non-linear calibrations at high pressures; because of heterogeneous (non-uniform) oxygen diffusion and c quenching. Moreover, pressure sensitive paints require correction for the output intensity due to light intensity variation, paint coating variation, model dynamics, wind-off reference pressure variation, and temperature sensitivity. Therefore to minimize the measurement uncertainties due to these causes, an in- situ intensity correction method was developed. A non-oxygen quenched paint (which provides a constant intensity at all pressures, called non-pressure sensitive paint, NPSP) was used for the reference intensity (I(sub NPSP)) with respect to which all the PSP intensities (I) were measured. The results of this study show that in order to fully reap the benefits of this technique, a totally oxygen impermeable NPSP must be available.

Subramanian, Chelakara S.↗

NASA NTRS2000

Intensity Biased PSP Measurement

The current pressure sensitive paint (PSP) technique assumes a linear relationship (Stern-Volmer Equation) between intensity ratio (I(sub o)/I) and pressure ratio (P/P(sub o)) over a wide range of pressures (vacuum to ambient or higher). Although this may be valid for some PSPs, in most PSPs the relationship is nonlinear, particularly at low pressures (less than 0.2 psia when the oxygen level is low). This non-linearity can be attributed to variations in the oxygen quenching (de-activation) rates (which otherwise is assumed constant) at these pressures. Other studies suggest that some paints also have non-linear calibrations at high pressures; because of heterogeneous (non-uniform) oxygen diffusion and quenching. Moreover, pressure sensitive paints require correction for the output intensity due to light intensity variation, paint coating variation, model dynamics, wind-off reference pressure variation, and temperature sensitivity. Therefore to minimize the measurement uncertainties due to these causes, an insitu intensity correction method was developed. A non-oxygen quenched paint (which provides a constant intensity at all pressures, called non-pressure sensitive paint, NPSP) was used for the reference intensity (I(sub NPSP) with respect to which all the PSP intensities (I) were measured. The results of this study show that in order to fully reap the benefits of this technique, a totally oxygen impermeable NPSP must be available.

Subramanian, Chelakara S.↗

NASA NTRS1979

PSP resins, new materials which can be hardened by thermal treatment for use in composite materials resistant to heat and fire

A class of easy-to-prepare heterocyclic-aromatic polymers which can be used for matrices in reinforced laminates is described. These polymers can be cured after B-staging with very little evolution of volatile materials, and they retain a low melt-viscosity which leads to low-void laminates. Resins are stable at temperatures below 150 C. Properties of composites with various reinforcements, in particular carbon-fiber unidirectional laminates, are described, and the fire behavior of PSP-glass laminates is reported.

Ropars, M.↗

NASA NTRS1981

Graphite polystyryl pyridine (PSP) structural composites

PSP6022 M resin, PSP 6024 M resin and W 133 Thormel T 300 graphite fabric reinforced panels were fabricated and provided to NASA Ames Research Center. PSP6022 and PSP6024 characteristics, process specifications for the fabriation of prepregs and of laminates are detailed. Mechanical properties, thermomechanical properties and moisture resistance were evaluated. PSP6022 and PSP6024 appear as high performance thermostable systems, very easy to process, being soluble in MEK for prepregging and being cured at no more than 250C, and even 200C.

Malassine, B.↗

NASA NTRS1998

Low Speed PSP Testing in Production Wind Tunnels

The brightness signal from a pressure-sensitive paint varies inversely with absolute pressure. Consequently high signal-to-noise ratios are required to resolve aerodynamic pressure fields at low speeds, where the pressure variation around an object might only be a few percent of the mean pressure. This requirement is unavoidable, and implies that care must be taken to minimize noise sources present in the measurement. This paper discusses and compares the main noise sources in low speed PSP testing using the "classical" intensity-based single-luminophore technique. These are: temperature variation, model deformation, and lamp drift/paint degradation. Minimization of these error sources from the point of view of operation in production wind tunnels is discussed, with some examples from recent tests in NASA Ames facilities.

Bell, James↗

NASA NTRSDate not supplied

Boundary layer transition measured by DIT on the PSP rotor in forward flight

A well-defined reference set of data for CFD and comprehensive code validation for a scaled helicopter main rotor with boundary layer transition in forward flight is presented (Fig. 1). The boundary layer transition was measured using differential infrared thermography (DIT) on the top (suction) side of the three-bladed NASA/Army “PSP rotor”, in the NASA Langley 14- by 22-Foot Subsonic Tunnel at an advance ratio of 0.3 (115kt).

Boundary layer transistion↗

NASA NTRSDate not supplied

Effect of Electronic Shot Noise on Dynamic Measurements using Optical Techniques: Examples from Rayleigh Scattering and Unsteady PSP

Electronic shot noise is an unavoidable reality in all optical techniques that depend on measuring light intensity. For steady-state, time-averaged measurements the impact of shot noise can be easily reduced by increasing the exposure time, or by averaging over multiple-exposures. That is not the case for unsteady measurements, where time histories of light intensity variations need to be created either by high-speed photography (unsteady PSP) or via photoelectron counting (spectrally resolved Rayleigh scattering) over short time durations using a photo-multiplier tube (PMT) and photon counting electronics. Electronic shot noise introduces a fixed amount of random error, which can overwhelm the light intensity variation caused by turbulent fluctuations. Spectrum computed from such time series shows a fixed noise floor that is independent of the number of data points in the time series. For a fixed optical system, where the collected power of the luminescent light (uPSP), or the scattered light (Rayleigh) is fixed, one needs to resort to special techniques to obtain enough signal-to noise ratio (SNR). For the uPSP application it is shown that averaging of the adjacent pixels improves SNR; although this may lead to a sacrifice of spatial resolution. A second means is to increase the exposure time, which leads to a lowering of the frequency range. For the Rayleigh application, improvements of SNR can be achieved via two different cross-correlation based approaches. The first involves measuring the light intensity using two PMTs using short, contiguous gates; thereby, creating two time-series of data. The second one involves collecting one long time-series of data using one set of measurement device, followed by an odd-even splitting into two time series. When the two time-series are cross-correlated, and a power spectrum is calculated, a significant reduction in the shot noise floor can be achieved. Examples from measurements of density and velocity fluctuations spectra from two different Rayleigh setup are presented to demonstrate the process.

uPSP↗

NASA NTRSDate not supplied

Implementation of an Unsteady PSP System in the NASA Transonic Dynamics Tunnel

An unsteady pressure-sensitive paint (uPSP) system has been developed to provide time-resolved pressure measurements in the NASA Langley Transonic Dynamics Tunnel (TDT). Obtaining these measurements necessitated the development of environmental enclosures to protect the high-speed camera and ultraviolet lights required for uPSP from the harsh environment present during tunnel operation. Since the facility main drive was non-functioning during the testing window, performance of the uPSP system was demonstrated using an impinging jet with a passive oscillator attachment to provide unsteady flow with a known frequency independent of amplitude. Measurements were obtained for tunnel pressures ranging from 565 to 2116 psf, and model angles of attack between -4 degrees and 4 degrees. Results indicate that the system is capable of measuring surface pressure differentials on the order of 0.01 psi at full scale with a camera frame rate of at least 10 kHz. Spectral analysis shows that the fundamental frequency of the oscillating jet is captured by the uPSP system, as are the second and third harmonics. Dynamic mode decomposition highlights the dominant coherent spatial structures of the surface pressure, along with the associated frequency and growth rate of each mode, allowing for a de-noised reconstruction of the uPSP measurements. The experimental campaign outlined within this report also confirmed compatibility of the uPSP system with the TDT facility data acquisition system, and verified the successful integration with existing processing capabilities within the NASA advanced supercomputing environment.

unsteady pressure-sensitive paint↗

NASA NTRSDate not supplied

Implementation of an Unsteady PSP System in the NASA Transonic Dynamics Tunnel

unsteady pressure-sensitive paint↗