Fast-responding Pressure-sensitive Paint Research Articles

Dual-luminophore fast-responding pressure-sensitive paint for the simultaneous elimination of motion- and temperature-induced errors

Abstract Motion- and temperature-induced errors are the major sources of error in pressure-sensitive paint (PSP) measurement. In this study, we developed a novel dual-luminophore fast-responding PSP with reference and pressure-sensitive channels that have similar temperature sensitivities, enabling motion- and temperature-induced errors to be simultaneously eliminated by taking the intensity ratio of the two channels. Rhodamine B (RhB), which was loaded on the Mobil Composition of Matter No. 41 (MCM-41) molecular sieve, and platinum tetrakis(pentafluorophenyl) porphyrin (PtTFPP) were chosen as the reference and pressure-sensitive luminophores, respectively. These luminophores were mixed with mesoporous SiO2 particles and a small amount of polymer to form a sprayable motion–temperature cancellation (MTC) PSP. By controlling the concentration of RhB, the temperature sensitivity of the reference channel was adjusted to match that of PtTFPP. To minimize temperature-induced errors, the effect of spectral ranges was also investigated. The lowest temperature sensitivity achieved for the MTC-PSP was 0.025%/℃, yielding an extremely low temperature-induced error of 55 Pa/℃. Its pressure sensitivity and response time were 0.46%/kPa and 145 μs, respectively. In addition, a theoretical model for the MTC-PSP that considers the effect of spectral overlap was proposed. The model accurately predicted the nonlinear relationship between the intensity ratio and pressure. The capability of the MTC-PSP was confirmed in a fast-rotating-disk experiment, and the pressure results agreed well with the theoretical pressure distribution on the disk.

Spanwise unsteadiness in the sidewall-confined shock-wave/boundary-layer interaction

Three-dimensional effects of sidewalls on the low-frequency unsteadiness of the shock-wave/boundary-layer interaction (SBLI) are of academic and practical importance but not yet well understood. Considerable attention has been paid to the viscous effect of sidewalls, whereas the potential inviscid confinement effect of sidewalls has received little attention. The present work provides experimental evidence of multiscale spanwise travelling waves crossing the separation front under the confinement of sidewalls. Global pressure measurements were made for a sidewall-confined 24 $^\circ$ compression ramp interaction in Mach-2.83 flow using fast-responding pressure-sensitive paint. The unsteady pressure in a statistically two-dimensional intermittent region suggests that in addition to the canonical streamwise oscillation, the separation front exhibits significant low-frequency, multiscale spanwise distortion. Modal analysis further reveals that multiscale spanwise unsteadiness has higher intensity and frequency than the streamwise oscillation. Such strong spanwise unsteadiness calls attention to the low-frequency unsteadiness in previous sidewall-confined SBLI experiments and encourages further study on the mechanism of the confinement effect.

Low-frequency large-scale unsteadiness analysis of lateral jet interactions on a slender cylinder in hypersonic flows using fast-responding pressure-sensitive paint

Lateral jet interactions, commonly encountered in high-speed vehicles, are typical complex three-dimensional (3D) shock wave/boundary layer interactions. The spatiotemporal features of surface pressure are crucial for understanding such complex interactions. In this study, fast-responding pressure-sensitive paint measurements are implemented at a sampling rate of 3000 Hz and a spatial resolution of 0.35 mm/pixel to investigate global pressure fields in lateral jet interactions on a slender cylinder under hypersonic flow conditions. Tests are conducted at three angles of attack (0°, 5°, and 10°) and two sideslip angles (0° and 2.5°) at Mach 6 and a Reynolds number of 1.35 × 107/m. The mean pressure results indicate the presence of a 3D high-pressure region beneath the bow shock feet, the area and pressure level of which depend on the cylinder attitude. Moreover, significant low-frequency broadband unsteadiness associated with the large-scale motion of bow shock is found in the high-pressure region. Statistical, coherence, and modal analyses are performed to clarify the characteristics of the large-scale unsteadiness. The results indicate that the dominant pressure mode is induced by streamwise oscillation of the bow shock, which exhibits two subregions with opposite pressure-fluctuation variations. Furthermore, the effects of the cylinder attitude on the unsteadiness extent and intensity are discussed.

Identification of Discrete Tonal Noise in Inclined Jet Impingement Using Pressure-Sensitive Paint

This paper presents the utilization of fast-responding pressure-sensitive paint (fast PSP) to identify the discrete tonal noise generated in 55 deg inclined jet impingement. Cases of subsonic () and supersonic () jet impingement with nozzle-to-plate distances of , and 8 are investigated. The fluctuating wall-pressure field measured using fast PSP at high spatiotemporal resolutions provides unique insights into the discrete tonal noises associated with inclined jet impingement. A far-field microphone is used to validate the frequency of each tonal noise. The fast PSP data are subjected to spectral proper orthogonal decomposition, and the results demonstrate wave propagation along the wall surface. At , the circumferential propagation characteristics of the identified discrete tones correspond to the toroidal mode of the shear layer instability wave. As increases, the dominant tone decays, and several secondary tones disappear. Screech tones are identified at and and 8. Two perpendicular flapping modes are identified at the discrete tone for each , and the dominant flapping direction varies with .

Temperature sensitivity elimination in sprayable fast-responding pressure-sensitive paint

Temperature-induced error is a major issue in pressure-sensitive paint (PSP) measurement, which usually comes from the inherent thermal quenching effect of luminophores. Herein, we successfully eliminate the temperature sensitivity in a sprayable fast-responding PSP. This PSP contains two luminophores (coumarin 6 and PtTFPP) with opposite temperature sensitivities, which cancel the temperature effect in the 515–800 nm spectral range, making it a temperature-canceled PSP (TC-PSP). The sensing properties of the TC-PSP (pressure sensitivity, temperature sensitivity, signal level, and response time) are characterized and compared with those of a single-luminophore PSP with the same mesoporous particle binder (MP-PSP). The TC-PSP exhibits an extremely low temperature sensitivity of 0.015 %/℃ in the 20–60 ℃ range, a value approximately 1 % that of the MP-PSP (1.35 %/℃). Its temperature sensitivity and corresponding temperature-induced error are significantly lower than the previously reported temperature-insensitive fast PSPs. In a jet impingement experiment conducted for validation, the TC-PSP showed excellent performance in eliminating the temperature-induced error. Overall, the developed TC-PSP holds great potential for high-speed aerodynamic testing applications. • We developed a sprayable fast PSP with extremely low temperature sensitivity. • The PSP contains two luminophores with opposite temperature sensitivities. • Temperature-induced error is completely removed in a jet impingement experiment.

Resolving dynamic features of kilohertz pressure fluctuations using fast-responding pressure-sensitive paint: measurement of inclined jet impingement

Resolving dynamic features of kilohertz pressure fluctuations using fast-responding pressure-sensitive paint: measurement of inclined jet impingement

Ruthenium-based fast-responding pressure-sensitive paint for measuring small pressure fluctuation in low-speed flow field

A sprayable fast-responding pressure-sensitive paint (fast-PSP) has been developed to measure time-resolved small pressure fluctuation on model surfaces. To realize PSP with fast response, high robustness, and high luminescence intensity, we have developed a novel ruthenium complex-based fast-PSP. The binder layers of the proposed fast-PSP were prepared by mixing room-temperature-vulcanizing silicone (silicone) and titanium dioxide (TiO2) particles with different sizes and their effects on the properties including the time response, pressure sensitivity, and luminescence intensity were investigated. The effects of the hydrophilic and hydrophobic surface treatments of TiO2 particle on the structure and mechanical robustness of the binder layer were also studied. The developed fast-PSP was applied to the flow field measurements behind a square cylinder and was evaluated. We have succeeded in obtaining time-series data of pressure fluctuation with high accuracy even at a mean velocity of 20 m s−1.

Thermal stability improvement of sprayable fast-responding pressure-sensitive paint for measurement above 100 °C

The thermal stability of sprayable fast-responding Pressure-Sensitive Paint (fast PSP) was investigated to explore the possibility for application in turbomachinery and hypersonic research with temperature above 100 °C. The first part of the study focused on a widely-used Polymer Ceramic PSP (PC-PSP). The effects of thermal degradation on its key sensing properties, including luminescent intensity, pressure sensitivity and response time, were examined for a temperature range from 60 to 100 °C. Severe degradation in intensity and pressure sensitivity was found as temperature reached 70 °C or higher, which would cause failure of PSP application in these conditions. Subsequently, a fast-responding Mesoporous-Particle PSP (MP-PSP) was developed which did not show degradation effects until 140 °C. The greatly improved thermal stability of MP-PSP was attributed to: selection of polymer with higher glass transition temperature (polystyrene) to delay the saturation effect of oxygen quenching as temperature increased; porous and hollow structure of particles for luminophore deposition that minimizes polymer–luminophore interaction. This new paint formulation has significantly raised the upper temperature limit of fast PSP and offers more opportunities for applications in harsh environment.

Fast pressure-sensitive paint for understanding complex flows: from regular to harsh environments

This review summarizes state-of-the-art knowledge on fast-responding pressure-sensitive paint (fast PSP), which has evolved into a powerful experimental tool for studying complex flow problems. As the formulation of porous paint with kilohertz response is now well established, full-field pressure measurements with high spatial and temporal resolution have been achieved on both stationary and moving targets. Recent studies have significantly advanced every aspect of this technology, including paint development, theoretical modeling, system integration and data processing. Novel paint formulations with superior sensing properties and additional functions for multi-physical measurements are being continually developed. The dynamic response mechanism is better understood through analysis and modeling considering the processes of photoluminescence, gas diffusion and light transmission. More importantly, applications of fast PSP are being expanded from regular wind-tunnel tests to more challenging conditions featuring hypervelocity, fast rotation and high temperature. Interdisciplinary research has played a key role in these development processes, and will remain vital for future breakthroughs in PSP technology.

Suppression of thermal quenching in fast-responding pressure-sensitive paint by restricting lattice relaxation of luminescent molecules

The thermal quenching of fast-responding pressure-sensitive paint (Fast PSP) has been effectively suppressed by restricting the lattice relaxation of luminescent molecules based on Franck-Condon principle. This is significant for reducing the temperature sensitivity while improving the signal level of Fast PSP. This approach used a self-assembled monolayer (SAM) technique to process PSP samples based on anodized aluminum oxide membrane. Due to the aluminum oxide's capability of forming a strong bond with fatty acid, the monomolecular coating formed by SAM on the PSP membrane was highly organized and dense which restricted the lattice relaxation of luminescent molecule. The sensing performance of PSP samples, including temperature sensitivity, pressure sensitivity, signal level, and response time, was evaluated based on static and dynamic calibrations. Significant reduction in temperature sensitivity (up to 95.7% in a temperature range of 273–323 K) was achieved after the samples were coated by SAM. The effects of geometric parameters (of the porous structure) on temperature sensitivity were investigated in detail. The luminescent intensities of the samples with or without SAM coating were also compared, which showed an improvement of up to 3 times after the samples were coated by SAM. In addition, the effect of temperature in SAM process on the reduction of PSP's temperature sensitivity was evaluated to determine the optimal treatment temperature.

Resolving vortex-induced pressure fluctuations on a cylinder in rotor wake using fast-responding pressure-sensitive paint

The interaction between rotor wake and a cylinder has been studied experimentally in the current work. The cylinder was placed in close proximity to the rotor plane, and the pressure fluctuations induced by the rotor wake on the cylinder surface were measured by microphones and fast-responding pressure-sensitive paint. Based on the developed data processing methods, challenges such as the low signal-to-noise ratio were resolved and small pressure fluctuations (less than 100 Pa) during the interaction were successfully extracted. The high-resolution vortex-induced pressure field under different blade-cylinder separation distances and rotor collective pitches were compared and analyzed, which clearly showed the effects of tip vortex strength and its evolution. More importantly, for cylinders with different cross section shapes, the pressure footprints left on the surface showed significant distinction in both pressure patterns and overall fluctuation levels. The flat surface would break the structure of the tip vortex and lead to both pressure rise and drop on the surface, while wedge-shaped obstacles would cut the vortex in half and result in two strong pressure drops on both sides. The square cylinder with a 0° installation angle (parallel to the blade) generated the least amount of pressure fluctuation due to its capability of fully breaking the vortex structure during the interaction.

Development of one-component fast-responding pressure-sensitive paint with a response time of O(10μs)

Development of one-component fast-responding pressure-sensitive paint with a response time of O(10μs)

Development of fast-responding Pressure-Sensitive Paint with low temperature sensitive using poly(trimethylsilyl)propyne

Development of fast-responding Pressure-Sensitive Paint with low temperature sensitive using poly(trimethylsilyl)propyne

Effect of dye solvents of fast-responding pressure-sensitive paint on the binder structure and the uniformity of luminescence lifetime

Effect of dye solvents of fast-responding pressure-sensitive paint on the binder structure and the uniformity of luminescence lifetime

Data mining of a clean signal from highly noisy data based on compressed data fusion: A fast-responding pressure-sensitive paint application

A data mining approach based on compressed data fusion is developed to extract a clean signal from highly noisy data and it has been successfully applied to flow measurement using fast-responding pressure-sensitive paint (fast PSP). In this approach, spatially resolved but noisy full-field data are fused with clean but scattered data to reconstruct full-field clean data. The fusion process is accomplished based on a compressed sensing algorithm, which has shown significantly improved performance compared with low-dimensional analysis. This is because, in low-dimensional analysis such as proper orthogonal decomposition (POD), the selection criteria of proper POD modes for reconstruction are usually based on subjective observation and the mode coefficients can be severely distorted by noise, which restricts the applications of this method to complicated flow phenomena and leads to a low-quality reconstruction. The solutions to these two problems can be expressed via mathematical optimization by determining the optimal coefficients to reconstruct clean data using the most relevant POD modes. Here, compressed sensing is used as a suitable solution to explore the sparse representation of scattered clean data based on the POD modes obtained from noisy full-field data. A high-quality reconstruction can be obtained using the optimized coefficients. The new method is first demonstrated by using fabricated patterns, demonstrating a reduction of 75% in the reconstruction error compared with POD analysis. It is thereafter successfully applied to recover the unsteady pressure field induced by a cylinder wake flow at low speed. Fast PSP measurement and microphones are used to obtain full-field but noisy pressure field data and scattered but clean data, respectively. In the cases of single and step cylinders, the reconstruction errors are approximately 5% and 25%, respectively, and the accuracy of reconstruction depends on the low dimensionality of the flow phenomena and the total number of microphone sensors. The current technique provides a reliable method to recover clean signals from strong noise, with significant potential for applications to flow measurement, control, and monitoring.

A novel sprayable fast-responding pressure-sensitive paint based on mesoporous silicone dioxide particles

A novel formulation of sprayable fast-responding pressure-sensitive paint (PSP) has been developed which uses mesoporous, hollow silicone dioxide particles as hosts for luminescent molecules (PtTFPP). The mesoporous particles are formed by nano-scale particles through Van del Waals force and chemical bonds, providing favorable environment for luminophore deposition. The resulting highly porous structures facilitate oxygen diffusion within the PSP binder which leads to a response time as low as 50 μs. More importantly, the current formulation can resolve the conflict between dynamic response and paint durability (in high-speed flows). The mesoporous-particle-based PSP (MP-PSP) prepared by mix-and-spray method features both fast response (about 100 μs) and high paint durability due to its highly porous structure and uniform luminophore distribution throughout the binder. Meanwhile, other negative effects of bonding polymer on PSP’s sensing properties are reduced, which results in increased pressure sensitivity and greatly improved photostability. The influence of particle size, paint thickness and fabrication method on sensing performance is also discussed in detail. This novel MP-PSP has shown great potential for applications in high-speed, unsteady aerodynamic testing.

Synchronous Surface Pressure and Velocity Measurements of standard model in hypersonic flow

Experiments in the Hypersonic Wind tunnel of NUAA(NHW) present synchronous measurements of bow shockwave and surface pressure of a standard blunt rotary model (AGARD HB-2), which was carried out in order to measure the Mach-5-flow above a blunt body by PIV (Particle Image Velocimetry) as well as unsteady pressure around the rotary body. Titanium dioxide (Al2O3) Nano particles were seeded into the flow by a tailor-made container. With meticulous care designed optical path, the laser was guided into the vacuum experimental section. The transient pressure was obtained around model by using fast-responding pressure-sensitive paint (PSP)sprayed on the model. All the experimental facilities were controlled by Series Pulse Generator to ensure that the data was time related. The PIV measurements of velocities in front of the detached bow shock agreed very well with the calculated value, with less than 3% difference compared to Pitot-pressure recordings. The velocity gradient contour described in accord with the detached bow shock that showed on schlieren. The PSP results presented good agreement with the reference data from previous studies. Our work involving studies of synchronous shock-wave and pressure measurements proved to be encouraging.

Single-shot lifetime-based PSP and TSP measurements on turbocharger compressor blades

Fast-responding pressure-sensitive paint (Fast PSP) and temperature-sensitive paint (TSP) measurements were conducted on two turbocharger compressors using a single-shot lifetime-based technique. The fast PSP and TSP were applied on separate blades of one compressor, and both paints were excited by a pulsed 532 nm Nd:YAG laser. The luminescent decay signals following the laser pulse were recorded by a CCD camera in a double-exposure mode. Instantaneous pressure and temperature fields on compressor blades were obtained simultaneously, for rotation speeds up to 150,000 rpm. The variations in pressure and temperature fields with rotation speed, flow rate and runtime were clearly visualized, showing the advantage of high spatial resolution. Severe image blurring problems and significant temperature-induced errors in the PSP results were found at high rotation speeds. The first issue was addressed by incorporating a deconvolution-based deblurring algorithm to recover the clear image from the blurred image using the combination of luminescent lifetime and rotation speed. The second issue was resolved by applying a pixel-by-pixel temperature correction based on the TSP results. The current technique has shown great capabilities in flow diagnostics of turbomachinery and can serve as a powerful tool for CFD validations and design optimizations.

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.

Investigation of transverse jet injections in a supersonic crossflow using fast-responding pressure-sensitive paint

Traditional pressure-sensitive paint (PSP) systems can provide data with high spatial resolution; however, the bandwidth is limited to a few Hz by the response time of the paint. Fast-responding paints have demonstrated response times of up to 100 kHz. Ultra-bright LEDs and fast-framing cameras combined with a porous polymer PSP can be used to produce a system capable of both high spatial resolution and high temporal bandwidth. Measurements of mean and unsteady pressure have been acquired on an experimental setup composed of a Mach-2 channel flow with transverse jet injection. The unsteady pressure data clearly resolve structures not present in the mean pressure data, including multiple lambda shocks upstream of a strong bow shock, high-frequency perturbations in the location of these shocks, and significant deformations of the bow shock structure. Time series of data can be extracted at each pixel, and the spectral content and phase relationship of the flow can be presented as maps of pressure fluctuations at specific frequencies or as correlation coefficients between a control point and the remaining flow. This type of map can be created using arrays of fast pressure transducers; here, we present data representing an array of over 26,000 fast pressure transducers.