Pressure-sensitive Paint Images Research Articles

A deep-learning image registration method for pressure-sensitive paint measurements

Image registration is an essential step in the data processing of pressure-sensitive paint (PSP) measurements. As PSP technology is gradually expanded to increasingly harsh environments, it faces challenges such as severe image misalignment due to model deformations/motions, which pose difficulties to traditional feature-based registration algorithms. To improve registration accuracy and efficiency, we propose an end-to-end image registration method based on deep learning. Initially, a PSP dataset based on PSP images is constructed through data augmentation. Three types of residual network and three training strategies are then adopted to prepare the deep-learning model for automatic image registration. The optimal combination of the residual network and training strategy is selected for validation using fabricated PSP images and outperforms two traditional algorithms (i.e., the Sift and Watershed methods). Finally, the performance of the deep-learning method is compared with that of traditional algorithms adopting a new metric of the overlapping rate for assessing the registration accuracy on experimental PSP images. The results show that the deep learning method outperforms the traditional algorithms in terms of registration accuracy and robustness.

Markerless Image Alignment Method for Pressure-Sensitive Paint Image.

We propose a markerless image alignment method for pressure-sensitive paint measurement data replacing the time-consuming conventional alignment method in which the black markers are placed on the model and are detected manually. In the proposed method, feature points are detected by a boundary detection method, in which the PSP boundary is detected using the Moore-Neighbor tracing algorithm. The performance of the proposed method is compared with the conventional method based on black markers, the difference of Gaussian (DoG) detector, and the Hessian corner detector. The results by the proposed method and the DoG detector are equivalent to each other. On the other hand, the performances of the image alignment using the black marker and the Hessian corner detector are slightly worse compared with the DoG and the proposed method. The computational cost of the proposed method is half of that of the DoG method. The proposed method is a promising for the image alignment in the PSP application in the viewpoint of the alignment precision and computational cost.

Cross-spectral analysis of PSP images for estimation of surface pressure spectra corrupted by the shot noise

We proposed applying the cross-spectrum analysis to the unsteady pressure-sensitive paint (PSP) images to eliminate the electronic shot noise component from the pressure fluctuation spectrum. The data of surface pressure fluctuation behind a square cylinder which measured by means of the polymer/ceramic PSP with a frequency response of approximately 5 kHz which is sufficiently high for studying the Karman vortex shedding was used for the validation of this analysis method. The cross spectrum is compared with the auto-spectrum of PSP images and that of the Kulite pressure transducer that corresponds to the reference signal. The cross spectrum can drastically reduce the electronic shot noise component in the pressure fluctuation spectrum. The second peak of vortex shedding which is not observed in the result of auto-spectrum can be observed thanks to the noise reduction. The convergence of calculation is faster as the size of the spatial area for calculating cross spectrum increases.

Unsteady pressure-sensitive-paint (PSP) measurement in low-speed flow: characteristic mode decomposition and noise floor analysis

Pressure-sensitive-paint (PSP) measurement was conducted for unsteady phenomena at various frequencies up to the order of kHz in low-speed flow to evaluate measurement accuracy of PSP. Pressure fluctuations on the floor surface induced by the Karman vortex were measured by PSP and unsteady pressure transducer. The dominant frequency of the pressure fluctuations is varied from 0.15 to 1.7 kHz by changing the size of the square cylinder. While regions with large pressure fluctuations could be visualized by calculating root mean square of pressure fluctuations from PSP images, the values significantly differed from those measured by pressure transducer. By applying Fast Fourier Transform (FFT), the power spectral density (PSD) at peak frequencies could be obtained within an error of 20%. Singular-value decomposition (SVD) yields a remarkable improvement in signal-to-noise ratio. However, amplitude of pressure fluctuations is changed depending on the way how to select modes. Three mode-selection methods for SVD filtering/reconstruction analysis are proposed in this study which show good improvement compared with convection method and are proved capable of extracting characteristic behaviors of the flow phenomena even below the noise floor.

Pressure-sensitive paint with imprinted pattern for full-field endoscopic measurement using a color camera

An endoscopic system for pressure-sensitive paint (PSP) measurement was presented in the current study, which was composed of a fiberscope, a diode laser and a color camera. The fiberscope was integrated by optical fibers (for transmitting the laser excitation) and a set of lenses (for delivering the PSP images to the camera). Here, a unique PSP with imprinted pattern was developed to address the key issue of limited viewing angle/area encountered during endoscopic measurement. This coating was based on a pressure-sensing luminophore PtTFPP and a recently developed mesoporous-particle binder, but with the addition of a pressure-insensitive reference dye (coumarin 6) forming a random dot pattern. The emission of PtTFPP and reference dye matched with the red and green channels of the color camera, respectively. The randomly distributed dots provided highly distinguishable features required for image stitching operation. This allowed the establishment of image processing algorithms that could combine a series of PSP images (scanned over the measurement field) into a full-field image. Meanwhile, the issue of spectral leakage from the emission of reference dye into the red channel was identified, and a correction method was proposed to remove the related measurement error. This PSP along with the endoscopic system was demonstrated in film cooling measurement on a blade in low-speed flow. The current method provides a practical solution for PSP applications in test environments with limited or even no optical access.

Pressure sensitive paint image registration combined with gray level information

Pressure-sensitive paint technology is a wind tunnel pressure measurement frontier technology with high economical efficiency and high speed. In the wind tunnel test, due to the strong wind, the model will be distorted, making the wind image and the windless image difficult to register, which will seriously affect the pressure measurement accuracy. In response to this problem, this paper innovatively applies the two-dimensional non-rigid iterative closest point (ICP) algorithm to solve this problem. The point cloud method is used to make the image detail area to be effectively registered, and it is also conducive to the subsequent three-dimensional reconstruction work. However, due to the two-dimensional non-rigid ICP algorithm, only the two-dimensional coordinate positional relationship is considered. The correlation of the pixel grayscales of the pressure-sensitive paint image is neglected, so that the registration accuracy is not too high. However, if the three-dimensional non-rigid ICP algorithm is directly used, misregistration will occur. Therefore, in order to further improve the registration accuracy, this paper proposes a non-rigid ICP algorithm based on pixel-based search strategy. The algorithm designs a dual-target search strategy that takes 2D coordinates and pixel gray values into consideration and achieves accurate local matching, realizing point search and double goal optimization. The algorithm of this paper is compared with five registration algorithms on multiple sets of pressure sensitive paint images. The experimental results show that the proposed algorithm has the best registration accuracy. Compared to the suboptimal algorithm, the RMSE is improved by more than 15% and the NMI is increased by about 5%.

Experimental analysis of transonic buffet on a 3D swept wing using fast-response pressure-sensitive paint

Transonic buffeting phenomena on a three-dimensional swept wing were experimentally analyzed using a fast-response pressure-sensitive paint (PSP). The experiment was conducted using an 80%-scaled NASA Common Research Model in the Japan Aerospace Exploration Agency (JAXA) 2 m × 2 m Transonic Wind Tunnel at a Mach number of 0.85 and a chord Reynolds number of 1.54 × 106. The angle of attack was varied between 2.82° and 6.52°. The calculation of root-mean-square (RMS) pressure fluctuations and spectral analysis were performed on measured unsteady PSP images to analyze the phenomena under off-design buffet conditions. We found that two types of shock behavior exist. The first is a shock oscillation characterized by the presence of “buffet cells” formed at a bump Strouhal number St of 0.3–0.5, which is observed under all off-design conditions. This phenomenon arises at the mid-span wing and is propagated spanwise from inboard to outboard. The other is a large spatial amplitude shock oscillation characterized by low-frequency broadband components at St < 0.1, which appears at higher angles of attack (α ≥ 6.0°) and behaves more like two-dimensional buffet. The transition between these two shock behaviors correlates well with the rapid increase of the wing-root strain fluctuation RMS.

Detection of small-amplitude periodic surface pressure fluctuation by pressure-sensitive paint measurements using frequency-domain methods

Image measurement using pressure-sensitive paint (PSP) is an effective tool for analyzing the unsteady pressure field on the surface of a body in a low-speed air flow, which is associated with wind noise. In this study, the surface pressure fluctuation due to the tonal trailing edge (TE) noise for a two-dimensional NACA 0012 airfoil was quantitatively detected using a porous anodized aluminum PSP (AA-PSP). The emission from the PSP upon illumination by a blue laser diode was captured using a 12-bit high-speed complementary metal-oxide-semiconductor (CMOS) camera. The intensities of the captured images were converted to pressures using a standard intensity-based method. Three image-processing methods based on the fast Fourier transform (FFT) were tested to determine their efficiency in improving the signal-to-noise ratio (SNR) of the unsteady PSP data. In addition to two fundamental FFT techniques (the full data and ensemble averaging FFTs), a technique using the coherent output power (COP), which involves the cross correlation between the PSP data and the signal measured using a pointwise sound-level meter, was tested. Preliminary tests indicated that random photon shot noise dominates the intensity fluctuations in the captured PSP emissions above 200 Hz. Pressure fluctuations associated with the TE noise, whose dominant frequency is approximately 940 Hz, were successfully measured by analyzing 40,960 sequential PSP images recorded at 10 kfps. Quantitative validation using the power spectrum indicates that the COP technique is the most effective method of identification of the pressure fluctuation directly related to TE noise. It is possible to distinguish power differences with a resolution of 10 Pa $$^2$$ (4 Pa in amplitude) when the COP was employed without use of another wind-off data. This resolution cannot be achieved by the ensemble averaging FFT because of an insufficient elimination of the background noise.

Experimental Verification of Buffet Calculation Procedure Using Unsteady Pressure-Sensitive Paint

Typically, a limited number of dynamic pressure sensors is employed to determine the unsteady aerodynamic forces on large, slender aerospace structures. This paper describes a robust calculation procedure based on frequency-specific correlation lengths and validation from an experiment conducted on a flat panel coated with fast-response pressure-sensitive paint. The first part of the paper describes the procedure used to analyze the pressure-sensitive paint images and a calibration method using dynamic pressure transducers. Excellent comparison in spectra, coherence, and phase, measured via pressure-sensitive paint and dynamic pressure sensors, validates the pressure-sensitive paint data. The second part of the paper describes the buffet validation process, the first step of which was to use pressure histories from all pixels to determine the true force fluctuations. In the next step, only a selected number of pixels was 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 eight sensors. It is believed that the present work provides the first validation of a buffet calculation procedure.

Global surface pressure measurements of static and dynamic stall on a wind turbine airfoil at low Reynolds number

Recognizing the need for global surface measurement techniques to characterize the time-varying, three-dimensional loading encountered on rotating wind turbine blades, fast-responding pressure-sensitive paint (PSP) has been evaluated for resolving unsteady aerodynamic effects in incompressible flow. Results of a study aimed at demonstrating the laser-based, single-shot PSP technique on a low Reynolds number wind turbine airfoil in static and dynamic stall are reported. PSP was applied to the suction side of a Delft DU97-W-300 airfoil (maximum thickness-to-chord ratio of 30 %) at a chord Reynolds number of 225,000 in the University of Wyoming open-return wind tunnel. Static and dynamic stall behaviors are presented using instantaneous and phase-averaged global pressure maps. In particular, a three-dimensional pressure topology driven by a stall cell pattern is detected near the maximum lift condition on the steady airfoil. Trends in the PSP-measured pressure topology on the steady airfoil were confirmed using surface oil visualization. The dynamic stall case was characterized by a sinusoidal pitching motion with mean angle of 15.7°, amplitude of 11.2°, and reduced frequency of 0.106 based on semichord. PSP images were acquired at selected phase positions, capturing the breakdown of nominally two-dimensional flow near lift stall, development of post-stall suction near the trailing edge, and a highly three-dimensional topology as the flow reattaches. Structural patterns in the surface pressure topologies are considered from the analysis of the individual PSP snapshots, enabled by a laser-based excitation system that achieves sufficient signal-to-noise ratio in the single-shot images. The PSP results are found to be in general agreement with observations about the steady and unsteady stall characteristics expected for the airfoil.

Measuring Surface Pressures on Rotor Blades Using Pressure-Sensitive Paint

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 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.

1501 カルマン渦が誘起する非定常圧力場の感圧塗料計測(OS15-1 先端的熱流体計測法,オーガナイズドセッション)

To understand unsteady flow phenomena around a body, it is important to measure time-resolved surface pressure distributions with high spatial resolution. Pressure Sensitive Paint (PSP) has a capability of providing high-resolution pressure images. In this study, we propose a new PSP technique to obtain time-resolved surface pressure distributions by using conditional sampling of time-series PSP images. To obtain high SNR data, the high-speed camera images are phase-averaged after image acquisition, using a simultaneously-measured pressure transducer data to detect a phase of each image. This method allows us to obtain high SNR images at arbitrary phase of the periodic phenomena using a simplify measurement system and image processing. The effectiveness of the present method was evaluated by applying it to a 3-D square cylinder in low-speed wind tunnel testing. As a result of this experiment, we could obtain time-resolved unsteady pressure images around the square cylinder caused by periodic vortex shedding.

Measurement of Oxygen Distribution underneath Ribs with Pressure Sensitive Paint

Pressure Sensitive Paint (PSP) is useful measuring method to analyze oxygen distribution underneath a rib in a fuel cell. High spatial resolution of the PSP measurement is made by creating the uniformly thin PSP film. By using spin coating, which is effective implement to make a thin film, the spatial resolution is improved to 150 µm. PSP phosphorescence intensity depends on oxygen partial pressure. An accurate image of oxygen partial pressure is obtained from calibration curve of each pixel of a PSP image, which is captured by a CCD camera. By calculating each oxygen partial pressure from each calibration curve of its pixels, the error of oxygen partial pressure is decreased to 10%.

Pressure sensitive paint measurements on a delta-wing in supersonic flow

AbstractExperiments have been performed documenting the pressure field on the lee-side of a delta-wing at three incidence angles (5°, 10°, and 15°) and at Mach 1·8 using a PSP (Pressure Sensitive Paint) technique. The delta-wing model having a leading edge sweep of 60° was instrumented with 31 spanwise pressure ports at 68% of mean chord location. The Optrod-B1 binary paint was utilised and the PSP images were processed employing a resection based methodology. The comparisons of PSP results with those measured employing pressure taps show good agreement at different incidence angles.

B311 低温風洞へのLifetime Imaging法の適用可能性

The intensity-based method is a most common technique by which Pressure-Sensitive Paint images can be converted to the surface pressure. However, this method cannot be applied to a cryogenic wind tunnel because the thermal equilibrium condition cannot be achieved when stopping the tunnel to take the wind-off reference image. In the present experiment, we applied the lifetime-based method to a cryogenic wind tunnel to calculate surface pressure from two images obtained during the wind-on condition. In this experiment, the circular-arc bump model sprayed with PtTFPP/PTMSP paint was tested at the JAXA 0.1m Cryogenic Transonic Wind Tunnel using a UV-LED pulse illuminator and a multi-gated CCD camera. As a result, we could succeed in visualizing surface pressure distribution induced by a shock wave on the model by using lifetime PSP imaging method. This result has demonstrated a feasibility of using lifetime imaging method to a cryogenic wind tunnel.

Lifetime measurement and calibration from pressure-sensitive paint luminescence images

A dual-image lifetime technique for acquiring surface pressure measurements from pressure-sensitive paint image data has been developed. This technique eliminates the need to acquire a “wind-off” reference image as required by the traditional radiometric technique, which is known to corrupt results. Here a luminescence lifetime-versus-pressure calibration experiment was conducted. Uncertainty was nominally less than ±4% and decreased as signal level increased. The nominal sensitivity was less than ±5 Torr at 100 Torr, less than ±50 Torr at 700 Torr, and improved with signal level.

垂直着陸する宇宙機の噴流と地面の干渉に関する感圧塗料計測

The ground effect of the impinging jet is experimentally investigated by using the pressure sensitive paint (PSP). Two cases, that is, a vertical landing rocket with a plug nozzle and a spacecraft landing on a planet without atmosphere are considered. These experiments are conducted in the atmosphere and in the vacuum chamber, respectively. The pressure distribution on the ground and the base of the model is successfully measured through the ground plate made from transparent material. In the former case, the PSP images reveal the characteristics of the ground effect of the impinging jet on the pressure distribution over the vehicle surface, for example, the cell structure in the pressure distribution on the nozzle surface and the pressure drop near the nozzle exit on the vehicle base. In the latter case, the results show that the PSP method is also promising in the research of low pressure rarefied flows.

The Application of Pressure- and Temperature-Sensitive Paints to an Advanced Compressor

An innovative pressure-measurement technique that employs the tools of molecular spectroscopy has been widely investigated by the aerospace community. Measurements are made via oxygen-sensitive molecules attached to the surface of interest as a coating, or paint. The pressure-sensitive-paint (PSP) technique is now commonly used in stationary wind-tunnel tests; this paper presents the use of this technique in advanced turbomachinery applications. New pressure- and temperature-sensitive paints (P/TSPs) have been developed for application to a state-of-the-art transonic compressor where pressures up to 1.4 atm and surface temperatures to 90°C are expected for the suction surface of the first-stage rotor. PSP and TSP data images have been acquired from the suction surface of the first-stage rotor at 85 percent of the corrected design speed for the compressor near-stall condition. A comparison of experimental results with CFD calculations is discussed.

Image registration for pressure-sensitive paint applications

Pressure-sensitive paint (PSP) data are generally derived by ratioing two images of a PSP-coated wind tunnel model. One image is taken at a known “wind-off” reference pressure while the other is taken at the test condition, in order to factor out the effects of nonuniform illumination and paint thickness. However, airloads on the model cause it to move during testing with respect to both the camera and the illumination source used to excite the P & P. Thus, the raw PSP images must be aligned, or registered, by means of a spatial transform. Once a proper ratioed image is obtained, image coordinates must be related to model coordinates so that pressure data can be obtained at the desired points on the model. This article compares several transforms used for image registration, and demonstrates the use of the projective equation of photogrammetry to relate model to image coordinates. An application in a small-scale wind tunnel test is shown, demonstrating mapping of pressure data to the three-dimensional model surface.