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This paper will present details of a pressure-sensitive paint system for measuring global surface pressures on rotor blades in simulated forward flight at the 14×22 ft subsonic tunnel at the NASA Langley Research Center. The system was designed to use a pulsed laser as an excitation source and pressure-sensitive paint data were collected using the lifetime-based approach. The higher intensity of the laser allowed pressure-sensitive paint images to be acquired using a single laser pulse, resulting in a collection of images that can be used to determine blade pressure at a specific instant in time. This is extremely important in rotorcraft applications because the blades experience dramatically different flowfields depending on their position. In addition, there can be fluctuations on the blade that vary every cycle due to factors such as lead/lag, flapping, and twisting of the blade. These effects generally preclude the use of phase averaging and thus the need for acquiring the data in a single image pair. For this test, the entire upper surface of a blade was painted and imaged. After taking into account temperature effects on the pressure-sensitive paint, the results agree both qualitatively and quantitatively with both expected results as well as with pressure transducers. Several limitations of the technique have been identified and discussion of strategies to overcome them is also presented. [ABSTRACT FROM AUTHOR]

This paper presents an application of the pressure-sensitive paint technique to investigate two-dimensional unsteady flow in a transonic channel. This work is a contribution to the study of the transonic interaction between an oscillating shock wave and a separated boundary layer in a channel flow. The shock-wave oscillation is forced by the periodic variation of the section of a second throat by means of a rotating elliptical shaft located in its section. The channel's lower wall is equipped with a contour profile, or bump, allowing for flow separation. To achieve a reduced response time for surface pressure measurements, we use anodized-aluminum coating as pressure-sensitive paint instead of usual paint. An aluminum insert including the bump was manufactured and coated with anodizedaluminum pressure-sensitive paint. Images were acquired by using a high-speed camera, and pressure-sensitive paint results were compared with pressure tap and Kulite sensor measurements implemented in the insert. Spectral analysis was carried out to assess the ability of anodized-aluminum pressure-sensitive paint for understanding unsteady aspects of such a complex channel flow. [ABSTRACT FROM AUTHOR]

An experimental study was conducted to investigate the pressure fluctuations on the entire sidewall of a rectangular cavity with an L/D of 5.67 using fast-response pressure-sensitive paint. Additionally, the performance of four different passive flow-control devices was quantified. Experiments were conducted in the Trisonic Gasdynamics Facility at the Air Force Research Laboratory at Mach 0.7 and 1.5. The frequency spectrum (including Rossiter tones) and sound pressure levels obtained from the pressure-sensitive paint measurements are validated against data taken with conventional dynamic pressure sensors. The complex flow phenomena over the cavity wall were visualized, and full-wall pressure spectra were calculated. The rod in crossflow showed the best peak suppression, followed closely by the flat spoiler. The large triangular steps showed moderate peak suppression, whereas the ridges did not suppress the peaks at all. High-resolution measurements of both temporal and spatial pressure fluctuations on the wall allowed for the visualization of sound pressure level distribution over the entire cavity wall. This revealed a strong dependence between the Rossiter tone modes and the spatial distribution of sound pressure levels that was not possible to resolve with traditional discrete pressure transducers. [ABSTRACT FROM AUTHOR]

The theoretical foundations are formulated for extraction of skin-friction fields from surface mass-transfer visualizations with pressure-sensitive paint and sublimating coatings. The asymptotic form of the mass transport equation at a wall is expressed as the optical flow equation in the image plane. Then, a relative skin-friction field is obtained by solving the optical flow equation as an inverse problem via the variational method. For steady-state visualizations with pressure-sensitive paint and sublimating coatings, a relative skin-friction field can be obtained from a single normalized intensity image. Further, a superposition procedure of a snapshot solution and a variation solution associated with the unsteady effect is proposed to reconstruct an unsteady skin-friction field from a time sequence of pressure-sensitive paint visualization images. The capability of the proposed method is examined by applying it to pressure-sensitive paint visualizations in impinging nitrogen jets. This method is also used to extract skin-friction fields from sublimation visualization images on a delta wing in a low-speed flow and on a flat surface in fin shock/boundary-layer interaction at Mach 6. The skin-friction topology in these cases is revealed by using this method. In addition, the limitations of this method are discussed. [ABSTRACT FROM AUTHOR]

Image blurring is a problem encountered when pressure-sensitive paint is applied to rotating surfaces such as rotorcraft blades. The issue is particularly problematic near the leading and trailing edges of the blade; these are the regions where the impact of blurring is the most significant, yet they also contain the most valuable pressure information. Recent work has developed image-deblurring techniques based on deconvolution of the image with a point-spread function based on the known lifetime decay of pressure-sensitive paint and the rotation speed of the blade. This deblurring technique is effective in recovering information at the blade edges when the amount of blurring is not too high. However, the existing deblurring algorithm assumes rectilinear motion and uniform distribution of luminophore lifetime (i.e., constant pressure distribution). The objective of this work is to relax the rectilinear assumption by allowing for rotational blur and to assess the impact of strong pressure gradients on the resulting deblurred pressure distribution. The new deblurring scheme is evaluated by experiments on a spinning disk with a known pressure field and a corotating, grazing nitrogen jet. The sensitivity of the deblurred results to various input parameters is evaluated, and recommendations for further algorithm development are provided. [ABSTRACT FROM AUTHOR]

The article focuses on a study which systematically investigated the temperature variations on the model made of three different materials with two different thicknesses of 2 and 10 millimeter (mm) and two ways of coating, with the single layer of pressure-sensitive paint (PSP) and with the double layers of a primary layer (PL) and PSP. The thermal properties of the materials are noted. It also examines the pressure sensitivity of high thermal-conductive PSP.

An experimental investigation was conducted to study the effects of passive splitter plates placed in the recirculation region behind a blunt-based axisymmetric body aligned in supersonic flow (Mach 2.49). The goals of this research were to obtain a better understanding of the physical phenomena that govern these massively separated high-speed flows and to determine the flow-control authority of these passive devices. Triangular splitter plates dividing the near wake into one-half-, one-third-, and one-fourth-cylindrical regions were designed to exploit specific stability characteristics of this flow, to affect the near-wake flow, to alter the base pressure, and to ultimately affect base drag. Mean and high-frequency static-pressure measurements were acquired on the base to assess the influence of these plates. Schlieren imaging, surface flow visualization, and pressure-sensitive paint measurements were also employed to document the near-wake flowfield, surface flow structure, and surface pressure, respectively. The time-averaged base pressure distribution, time-series pressure fluctuations, and presumably the stability characteristics were altered by the spatial division of the near wake. The area-integrated pressure was only slightly affected. Normalized root-mean-square levels indicate pressure fluctuations were significantly reduced (as much as 39%) with the addition of the splitter plates. Power-spectral-density estimates revealed a spectral broadening of fluctuating energy for the one-half-cylinder configuration and a bimodai distribution for the one-third- and one-fourth-cylinder configurations. [ABSTRACT FROM AUTHOR]

This paper presents an application of the pressure-sensitive-paint technique under unsteady flow conditions. Anodized-aluminum pressure-sensitive-paint coating was used to investigate the flowfield inside a nozzle during the transient phase and to assess the pressure gradient at the outlet region during the steady-state phase. The anodization procedure was adjusted to apply anodized-aluminum pressure-sensitive-paint coating on a large three-dimensional model. The images were acquired using a fast-frame-rate camera up to 5000 images per second, which allows accurate imaging of the paint, for which the response time is below 200 ms. Discrepancies between pressure-sensitive- paint results and pressure measurements obtained with Kulite sensors are due to a temperature effect and to condensation of the exhaust air, which modifies the optical path between the observed surface and the camera. [ABSTRACT FROM AUTHOR]

The current work focuses on the development and application of fast-responding polymer/ceramic pressure-sensitive paint (PSP) as an advanced optical surface pressure measurement technique for unsteady flowfields in large-scale wind tunnels. Three different data acquisition methods for PSP are evaluated and compared: phase averaging, real-time (high-speed) imaging, and single-shot lifetime imaging. All three techniques were applied to the measurement of the time-resolved, global surface pressure distribution on a hemispherical dome in Mach 0.6 freestream flow at a total pressure of 71.8 kPa, where the Reynolds number based on dome diameter (0.254 m) was 2.4 x 106. At this flow condition, a predominant shear layer oscillating at 400 Hz over the test model was detected by pressure transducer measurement, providing an appropriate trigger for the PSP optical instrumentation. In the phase-averaging technique, pulsed LED arrays were phase-locked to the shear-layer frequency, and a long camera exposure time captured the pressure distribution over many phase-averaged cycles. In the real-time approach, high-intensity continuous LED illumination was paired with a high-speed camera operating at a frame rate of 10 kHz to capture the pressure distribution without any image averaging. In the lifetime-based single-shot approach, instantaneous pressure information was acquired by exciting the paint with pulsed laser energy and ratioing two exposure gates, which covered the luminescent lifetime decay of the luminophore molecules resulting from each excitation pulse. An assessment of the three methods is presented, with the benefits and disadvantages of each technique evaluated through the case study of the hemispherical dome. [ABSTRACT FROM AUTHOR]

We consider the question of how to reconcile integrated forces and moments from a pressure-sensitive paint measurement with measured forces and moments from a balance. We show that it is possible to compute the smallest change in pressure distribution that would be required to bring the two sets of data into agreement. We refer to this as the gap distribution and show that it can be expressed in terms of a set of basis functions that are determined by the geometry of the test article. The use of these gap basis functions allows discrepancies in forces and moments to be expressed in terms of a common unit of measure, namely the magnitude of the implied gap distribution. We apply this gap analysis to data from a wind-tunnel test of the NASA Orion command module, for which both pressure-sensitive paint data and balance data are available. Results of the analysis confirm earlier suspicions that there was a problem with the normal component of force of the balance. Still, it is shown that application of the gap correction leads to improved pressure-sensitive paint data, as determined by the level of agreement with pressure tap data. The analysis procedure involving a pressure-gap distribution should be applicable to the comparison of balance data to integrated forces and moments from computational fluid dynamics calculations and other techniques. [ABSTRACT FROM AUTHOR]

A lifetime imaging system to measure simultaneously pressure and temperature images from a luminescent lifetime decay of pressure-sensitive paint (PSP) has been developed. The dependence of the luminescent lifetime decays on pressure and temperature was clarified by analysis of PtTFPP PSP using a streak camera. The method is proposed to acquire pressure and temperature images from three luminescent images. The three gated times of an intensified charge-coupled device camera were determined, referring to the analysis of streak camera data. The performance of the measurement system was evaluated over the wide range of pressures and temperatures using a PSP-coated coupon. The ratioed lifetime images could be fitted with a smooth function of pressure and temperature. This allowed reconstruction of pressure and temperature images from three luminescent lifetime images. As a verification test, pressure and temperature distributions induced by a sonic-jet impingement flow were visualized. PSP data agreed with pressure tap data, indicating that this lifetime imaging system was a useful tool to measure pressure and temperature fields on an aerodynamic model surface. [ABSTRACT FROM AUTHOR]

Pressure-sensitive paints (PSP) have recently been extended to high-frequency flowfields. Paint formulations have been used effectively to characterize pressure fluctuations on the order of 100 kHz. As the limits of PSP are extended, various experimental results indicate that the unsteady response characteristics are nonlinear. A thorough understanding of the photophysical mechanisms in paint response is needed. Gas transport properties, coupled with the nonlinear nature of the Stern-Volmer relationship have an effect on the paint response. This work discusses the full implications of a diffusion-based model for the unsteady response of pressure-sensitive paint. Based on this model, it is shown that the indicated pressure response of PSP is faster for a decrease in pressure, and slower for a pressure increase. Effects of other factors, such as pressure-jump magnitude, pressure-jump range, and Stern-Volmer nonlinearity, are evaluated. Furthermore, a fluidic oscillator is used to demonstrate experimentally the quenching kinetics of two types of PSP—polymer/ceramic and fast FIB. Results from the oscillator operated with argon, nitrogen, and oxygen gases at 1.59 kHz demonstrate behavior that agrees with the diffusion model. The polymer/ceramic PSP exhibited no delay between different test gases, indicating a flat frequency response of at least 1.59 kHz. Fast FIB, on the other hand, demonstrated a significant delay in rise time between the nitrogen and oxygen cases. Both the diffusion model and the experimental results demonstrate that the different responses to nitrogen and oxygen only become critical when the period of the flowfield oscillations is shorter than the response time of the paint formulation. [ABSTRACT FROM AUTHOR]

Store-cavity interaction was investigated by using fast-response pressure transducers, high-speed schlieren photography, and pressure-sensitive paint (PSP) at freestream Mach numbers of 1.8 and 2.9. The influence of Mach number on the interaction of a cavity (L/D = 3.6, W/D = 3.8) and a modeled store was characterized. High- speed schlieren photography illustrated the real-time motion of coherent spanwise structures in the Mach 1.8 flow and revealed the absence of these structures in the Mach 2.9 flow. The spectra measured in the Mach 1.8 flow exhibited clear resonant peaks consistent with Rossiter modes, whereas the Mach 2.9 flow did not. The mean floor pressure increased as the store was positioned nearer to the free shear layer for both Mach number conditions. Interestingly, the level of the pressure fluctuations measured on the cavity floor decreased for the Mach 1.8 ease when the store was positioned in the freestream but increased for the Mach 2.9 case. PSP was applied to the cavity floor and to the modeled store. Integration of the measured pressure field on the store yielded information on the forces and pitching moment. Schlieren imaging of a modeled store exiting the cavity indicated that the free shear layer is slightly displaced by the moving store. [ABSTRACT FROM AUTHOR]

Pressure-sensitive paint (PSP) is used to acquire high-fidelity images of steady pressure distributions across surfaces. The PSP luminescence emission response to excitation light depends in a straightforward way on the oxygen concentration (hence air pressure) and the irradiance incident on the PSP lumiphores. Often, however, the phenomenon of interest is unsteady. During a rapid change in pressure, the oxygen concentration within a PSP layer varies with depth. Moreover, the excitation irradiance varies with depth as a result of attenuation, and this affects signal generation when the spatial distribution of oxygen is also changing. Prior studies have been reported on the transient response of PSP using the radiometric technique to detect pressure; we have extended the study by applying a luminescence lifetime technique. Oxygen concentration as a function of time and depth are obtained by solving the mass diffusion equation, with appropriate initial and boundary conditions. We develop a luminescence emission model that takes the spatially varying oxygen concentration and excitation absorption profile into account and relate pressure to the lifetime of the luminescence emission. Oxygen concentration is governed by mass diffusion of oxygen through a PSP layer during a rapid change in pressure, and the absorption of excitation light is governed by Beer's law. The significance of both the mass diffusivity and the optical depth of PSP are assessed. A numerical method was used to solve the mass diffusion equation that allows modeling of the oxygen concentration within the PSP given an arbitrary pressure function. We have used a double-image digital camera to acquire PSP emission lifetime data from several paint thicknesses during a rapid change in pressure. These data were compared to the results predicted by the model. [ABSTRACT FROM AUTHOR]

Image alignment plays an important role in pressure-sensitive-paint (PSP) data processing. A systematic comparative study of two commonly used approaches, namely, image transformation using image-warping transforms before ratioing and, alternatively, image mapping on to three-dimensional model space (using resection methodology prior to ratioing) prior to ratioing, is attempted here. PSP results along with conventional static port data as on a generic aircraft model at a freestream Mach number of 0.6 and incidence of 10 deg (taken from earlier work) are utilized to critically assess the merit of the preceding two approaches. The analyses show that image-warping methods can result in larger errors in regions of strong spatial gradients of pressure and on bodies that are significantly three dimensional. Further, the resolution obtained from the resection approach is significantly greater (of the order of less than a pixel) compared to the transform methods. [ABSTRACT FROM AUTHOR]

An iterative pressure-correction method is developed that can be used to recover the incompressible pressurecoefficient distribution from pressure-sensitive paint (PSP) results obtained at two suitably higher subsonic Mach numbers. The pressure-correction method provides an alternative approach to overcome the well-known difficulty of using PSP in low-speed flows. For validation, this method is applied to flows over a circular cylinder, sphere, prolate spheroid, transonic body, and delta wing. Limitations and uncertainty of the pressure-correction method are discussed. [ABSTRACT FROM AUTHOR]

When pressure-sensitive paint (PSP) is used to measure unsteady pressures, the thickness of the paint layer critically affects both the PSP signal-to-noise ratio (SNR) and frequency response. As the thickness of a paint layer increases, the brightness of the signal, and thus the SNR, increases, whereas the frequency response decreases, resulting in attenuation of the unsteady component of the signal. In addition, if pressure fluctuations and frequencies are large and the paint layer is thick, the unsteady PSP signal may be distorted and the steady component may be offset from its true mean value. These observations suggest that, for each application, there is an optimum paint thickness where the "unsteady SNR" (the ratio of unsteady signal amplitude to steady noise) is a maximum and where the offset and distortion are small. This hypothesis is explored by numerically evaluating published analytic solutions to a simplified model of the unsteady PSP problem. It was found that over a wide range of parameters the unsteady SNR is a maximum when the paint thickness corresponds to a 1.25 dB attenuation of the unsteady signal. At this optimum, the nondimensional time constant (h²ω/D) is approximately 1.5. [ABSTRACT FROM AUTHOR]

Recent experiences with pressure-sensitive paint have shown that very thin paint layers on wind-tunnel models tested at high Reynolds numbers can significantly after the pressure distributions, and thus the forces and moments, on the models. This was observed during two tests of transport-like wings: a "clean" supercritical wing at transonic cruise conditions in the NASA Ames High Reynolds Number Channel 2 and a high-lift wing, complete with slats and flaps, at landing conditions in the NASA Ames 12-Foot Pressure Wind Tunnel. The effect of paint on the cruise wing was to displace the shock wave slightly upstream from its no-paint position. Smoothing the paint, and even removing it entirely from the leading edge, decreased this displacement slightly. Applying paint to only the slats of the high-lift wing caused the wind to stall prematurely at the highest Reynolds number. This effect could be eliminated by smoothing the paint. Adding paint to other parts of the model had little effect. Paint intrusiveness was much smaller and more ambiguous at lower Reynolds numbers. In both tests, paint intrusiveness occurred because the model surfaces were rougher when they were painted than when they were not, and this additional roughness altered the development of the turbulent boundary layers. [ABSTRACT FROM AUTHOR]

Presents a study which investigated the application to unsteady flowfield measurement of fast response time characteristics of porous pressure-sensitive paint, which measures surface pressure distributions by the processes of luminescence and oxygen quenching. Theoretical background; Computational approach; Results.

Focuses on the development of two models for the dynamic behavior of pressure-sensitive paints. Empirical models; Diffusion models; Diffusion model with linear Stern-Volmer calibration.

Measures the surface temperatures and pressures on an elliptic cone lifting body in a hypersonic flowfield using thin-film temperature- and pressure-sensitive paints. Three-dimensional characteristics of laminar, transitional and turbulent flow; Test description; Feasibility of measuring multiple full-field surface temperatures over a 3-minute duration.

Focuses on a study which investigated the uncertainty in pressure-sensitive paint (PSP) measurements from a standpoint of system modeling. Correlation of the luminescent radiation and photodetector response; Modeling of PSP measurement system; Allowable upper bounds of elemental errors; PSP uncertainty estimates on a Joukowsky airfoil in subsonic flows.

Studies the in situ calibration uncertainty of pressure-sensitive paint (PSP). Simulation of PSP calibration in subsonic Joukowsky airfoil flows; Elastic wing deformation under aerodynamic loads; Reference luminescence intensities.

Provides a paint measurement system that would not require the wind-off calibration and would correct for differences in illumination intensity over the model surface. Background on pressure sensitive paints (PSP) and nonpressure sensitive paints (NPSP); List of paint and binder combinations that were tested; Result on the variation of PSP and NPSP intensities with respect to pressure, temperature, paint type and time or age.

Derives the expressions for the effective diffusivity and the diffusion response time of a porous polymer layer from a standpoint of phenomenology to understand the time response of a porous pressure-sensitive paint (PSP). Definition of PSP; Exponent of a power-law relation between timescale and coating thickness for the polymer poly(TMSP) as a function of temperature; Element of a porous polymer layer.