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A novel pressure-sensitive paint (PSP) system has been developed which uses an embedded organic light-emitting diode (OLED) layer as excitation, and thus eliminates the usual requirement of an external light source. This system consists of an OLED layer, an optical filter layer and a polymer-ceramic (PC) PSP layer with fast response. The OLED provides green excitation light (around 500 nm) for PSP, while the optical filter and (550 nm short-pass) removes unwanted longer wavelength light to prevent possible interference with the PSP signal. A PC-PSP layer is applied on top of the filter with well-controlled basecoat thickness (~ 10 μm) to allow sufficient light transmittance. It is also important that the OLED is modulated with a limited duty cycle and synchronized with the camera exposure to minimize uncertainties mostly due to the inherent heating effect of OLED. This OLED-based PSP has a pressure sensitivity comparable to the regular PC-PSP, and it is well-suited for pressure measurements on moving surfaces since its illumination field is independent of the model motion. Its advantage over a conventional PSP system was demonstrated through applications on a flapping plate under the impingement of a nitrogen jet.

The present work investigates the challenges in measuring the surface pressure distribution using particle image velocimetry (PIV) in comparison with pressure sensitive paint (PSP) measurements. The aim of the investigation is to assess if PIV may be regarded as a complementary method to PSP, or even as a possible replacement at least for 2D flow analysis or at low speed, where PSP fails. The investigation is based on two different test cases: (1) the attached flow around a NACA-0012 airfoil and (2) the separated flow around a backward-facing step (BFS). Both cases are investigated at transonic flow conditions. For the airfoil, both PIV and PSP measurements were conducted for a direct comparison. At such high Reynolds number, the boundary layer thickness is very small, and therefore, the pressure in the flow field outside the boundary layer can be easily determined from the velocity data with an isentropic flow assumption. The results of the PIV-based surface pressure reconstruction and the one of PSP are compared with the static pressure taps installed on the model surface. For the BFS test case, synthetic PIV images are produced from a Large Eddy Simulation (LES). In this case, flow separation as well as strong viscous effects appear which inhibit the use of the isentropic flow assumption. Therefore, the Reynold averaged momentum equation is directly used and the pressure is determined solving the Poisson equation which is obtained by applying the divergence operator to the momentum equation. The results are compared with the pressure distribution on the surface given by the LES simulation. This case aims to analyze the accuracy of PIV-based surface pressure determination in case of massive flow separation. It will be shown that the deviations to the expected results are caused by the spatially discretization as well as by spatially averaging, which occurs in a standard PIV measurement. In addition, the effect of the velocity uncertainty on the estimated surface pressure distribution biases the result significantly. This implies that PSP will be hardly replaced by PIV.

Accepted: 2014-09-21, 資料番号: SA1150051000

Porous pressure-sensitive paint (PSP) is employed as a visualization technique for unsteady flow features on a low-pressure turbine blade. Recognizing that the measurement of high-frequency pressure fluctuations in unsteady flows—especially in turbomachinery—has proven to be difficult, recent advancements in the development of porous PSP have enabled the high-resolution measurement of pressure fields with frequency content of at least 20 kHz. In this work, PSP is applied to an L1A low-pressure turbine blade section (Re = 20,000 based on axial chord) to visualize the surface dynamics of a vortex generator jet (VGJ) pulsed at 10.6 Hz with nitrogen gas. Intensity-based, time-resolved PSP measurements reveal the development and the surface structure of the VGJ as well as the spanwise variation in the blowing profile.

This paper describes the effect of the seams of a rugby ball on the side force and the flight trajectory of the punted kick. Measurement of the aerodynamic force on a non-spinning rugby ball reveals that the side force coefficient depends on the position of the seam as well as the angle of attack. It was found from pressure-sensitive paint measurements that the seam of the ball is the trigger for initiating low pressure when the seam is situated around 60° from the stagnation point. The flight trajectory of the fluctuating ball can be obtained by numerically integrating the six degree-of-freedom non-linear equations of motion. It was shown that a slower spinning ball fluctuates from side to side during flight because of the asymmetrical pressure distribution on the sides of the ball.

An in-house Pressure Sensitive Paint (PSP) formulation has been developed at the Aero-Physics Laboratory at the University of Manchester. The PSP uses Bathophenanthroline Ruthenium as the luminophore molecule and is incorporated in a sol-gel matrix. Excitation occurs at 400-500 nm and emission at 550-650 nm. The Stern-Volmer plot of the PSP reveals small temperature dependence, which has always been an intrinsic drawback of PSPs. As a baseline experiment the PSP has been applied to examine the side-wall pressure field of the flow through a convergent nozzle with an ejector, at fully expanded Mach numbers in the range M$_{j}$ = 0.52-1.36. Simultaneous static pressure measurements were also conducted to ascertain the accuracy of the PSP results. The paint has demonstrated satisfactory capabilities in not only measuring static pressures but also in visualizing key physical elements of the flow, such as the location of the expansion and oblique shock waves present in such flows.

Micro devices have been widely used in aerospace engineering for years. Engineers are interested in applications of micro devices such as microjets, micro actuators, and micronozzles. The small size nozzles can be used for attitude adjustment and propulsion of micro-satellites or mini-spacecraft. In this paper, convergent-divergent micronozzles have been investigated at supersonic speed with various total pressures and Reynolds numbers. The throat of the micronozzle is 250 micron wide and the nozzle is designed as de Laval type. For the measurements, the Reynolds number at the throat varies from 1200 to 11000 and total pressure varies from 6 psia to 55 psia. Experimental results are obtained with pressure-sensitive paint for pressure measurement and schlieren imaging for flow visualization. Flow visualization is a challenge for conventional techniques due to the small length scales and small depth of the density gradient. A modified schlieren technique is used to increase the sensitivity by taking the ratio of wind-on and wind-off images. Pressure-sensitive paint is also used to obtain global pressure measurement of the flow field and to compare with the schlieren results.

In applying Pressure-Sensitive Paint (PSP) to low-speed flow wind tunnel testing, it is important to minimize any measurement uncertainties. There are various error sources such as camera noise, misalignment of images due to model displacement and temperature distribution over the model. Among these factors, the effects of temperature distribution change during tests on pressure measurement accuracies were studied in the present paper. Pressure and temperature distributions over a simplified car model (1/10 scale Ahmed model) were measured using PSP and Temperature-Sensitive Paint (TSP). Sequential images were acquired at the same intervals over the entire test period, including for the conditions before and after the tunnel run. As a result, it was found that the measurement error caused by temperature distribution over the model could be reduced using a single-point temperature measurement. In addition, by measuring surface temperature distributions on the model using TSP, it was proved that the most accurate pressure measurement could be made by rationing the wind-off image acquired immediately after shutting down the tunnel to the wind-on image acquired immediately before shutting down the tunnel. Using the present measurement technique, complicated pressure fields over the Ahmed model were successfully visualized.

Pressure sensitive paint (PSP) techniques have been used to measure the pressure distribution on a model surface in high-speed flows. In this study, we developed a PSP technique that can be applied to low-speed flows. Four PSP formulations, each comprised of a porphyrin (PtOEP or PtTFPP) and a polymer (Poly(TMSP) or RTV-118), were tested and the performance of each combination was evaluated. In the static calibration, the luminescence intensity of the PSP coatings was measured from 0 kPa to 11 kPa with 0.5, 1, and 2 kPa increments. Among the four PSP formulations tested, the combination of PtOEP and RTV-118 (PSP-3) showed the best performance. In this study, the low-pressure PSP technique using PSP-3 was applied to an oblique impinging subsonic jet to measure pressure field distributions on the impingement plate at various angles of the oblique jet. For comparison, the flow over the impingement plate was visualized using an oil-film method.

The fluidic oscillator is a device that generates an oscillating jet when supplied with fluid at pressure. The oscillator has no moving parts - the creation of the unsteady jet is based solely on fluid-dynamic interactions. Fluidic oscillators can operate at frequencies ranging up to 20 kHz, and are useful for flow control applications. The fluidic oscillator evaluated in the current study is comprised of two fluid jets that interact in an internal mixing chamber, producing the oscillating jet at the exit. Both porous pressure-sensitive paint (PSP) and dye-colored water flow are used to visualize the internal and external fluid dynamics of the oscillator. Porous PSP formulations have been shown to have frequency responses on the order of 100 kHz, which is more than adequate for visualizing the fluidic oscillations. In order to provide high-contrast PSP data in these tests, one of the internal jets of the fluidic oscillator is supplied with oxygen, and the other with nitrogen. Results indicate that two counter-rotating vortices within the mixing chamber drive the oscillations. It is also shown that the fluidic oscillator possesses excellent mixing characteristics.

Measurements of the surface pressure on a simplified automobile model have been performed using pressure-sensitive paint (PSP). The program was undertaken to investigate the use of commercial and in-house PSP at low speed (between 11 m/s and 92 m/s) and to find techniques to improve the measurement accuracy. A comparison of a priori and in situ calibration methods was also conducted. A linear in situ calibration or a priori methods combined with one tap pressure, correcting for bias errors, were found to provide the best accuracy.

A pressure sensitive paint (PSP) measurement has been known as a pressure field measurement technique based on the oxygen quenching phenomenon of luminescence of specific luminophores. A PSP measurement was applied for pressure field measurement in a low-solidity circular cascade diffuser of a single-stage transonic centrifugal compressor with 5 in pressure ratio for HFC134a gas. The oxygen concentration was about 500 ppm. Ru (bath-phen) was adsorbed on a silica-gel thin-layer chromatography sheet, and the sheet was pasted onto the side-wall between the cascade vanes. A drastic change in luminescent intensity was recognized during a surge condition. Also the pressure variations based on luminescent intensity agreed well with the pressure fluctuations measured using a semiconductor pressure sensor with high-frequency-response. It was shown that a PSP measurement worked well to investigate the unsteady pressure fields in a circular cascade diffuser of a transonic centrifugal compressor. Moreover, the time response of PSP becomes clear as a problem to be overcome for the present.

A comparative wind tunnel test of various Pressure Sensitive Paint (PSP) formulations has been performed at the Institute for Aerospace Research (IAR) 1.5 m × 1.5 m Trisonic Blowdown Wind Tunnel. The model under study is a prototype supercritical wing of a half model. The results are presented at the cruise Mach number: M = 0.74. The effect of the Reynolds number on the pressure distribution is assessed by varying the stagnation pressure of the flow. The evaluated paints all use the same porphyrin molecule as the luminescent sensor and the differences in sensitivity to pressure and temperature are a result of the PSP binder, which differs for each formulation. Examples of the processed results are given and the accuracy of the different PSP formulations is also discussed.

The mixing fields within a SCRAM-jet combustion chamber are visualized using pressure-sensitive paint (PSP) as an oxygen sensor. The experiments are performed in a small supersonic wind tunnel at the National Aerospace Laboratory - Kakuda Research Center (NAL-KRC). The main stream Mach number is 2.4, and the dynamic pressure ratios between the injected gas and the main flow are 0.3, 0.7, 1.1 and 1.5. Three fuel injection nozzles are used; oxygen is injected from the central nozzle and air from the two nozzles at either side. The spread of the injected gas is measured to observe the effects of placing the nozzles in different positions. The results show that the jet has its own independent flow structure, and that little mixing of gases occurs between the flow structures created by each nozzle. When the injection dynamic pressure ratio is increased, the oxygen fraction rises in the recirculation zone and falls in the separation zone downstream of the injection.

The flow field structures of low density supersonic free jets impinging on a tilt plate are studied by hybrid use of LIF (Laser Induced Fluorescence) and PSP (Pressure Sensitive Paint). The jet through an orifice flows into a low pressure chamber and impinges on the tilt plate with angle from jet axis 45, 60 or 90 degrees. A plane including the jet axis and the normal of the plate is visualized by LIF of seeded iodine molecules, scanning a laser beam along the jet axis. On the other hand, the pressure distribution on the tilt plate is visualized by PSP. In comparing the results of the two methods, the complicated shock wave system is analyzed. Deformations of the Mach disk and the barrel shock are also confirmed.

Visualization of aerodynamic effects on a three-dimensional double-delta wing aircraft model was conducted using an optical pressure measurement system, based on the Pressure Sensitive Paint (PSP) technique, and in addition a laser-light sheet method. The combination of PSP technology with the laser-light sheet method, provides a good understanding of the flow around the wind tunnel model. In recent years, this novel PSP-technology has attracted considerable attention in the aerospace community. The PSP technique can be used to realize absolute pressure measurements on a surface of a model and in addition to evaluate quantitative aerodynamic flow phenomena using a scientific grade camera and image processing techniques. The PSP system was tested in the Transonic Wind Tunnel of the German Aerospace Center in Gottingen (TWG) under real flow conditions. Instantaneous pressure distributions are recorded in almost real-time so that the recognition and analysis of the vortex dynamics on the model surface is possible. Even the vortex breakdown process, as well as the fine structured Kelvin-Helmholtz instabilities and secondary vortex structures, can be detected by this measurement technique.