Pressure Sensitive Paint

The basic concepts of pressure sensitive paint (PSP) are simple. After a photon of radiation with a certain frequency is absorbed to excite the luminophore from the ground electronic state to the excited electronic state, the excited electron returns to the unexcited ground state through radiative and radiationless processes. The radiative emission is called luminescence (a general term for both fluorescence and phosphorescence). The excited state can be deactivated by interaction of the excited luminophore molecules with oxygen molecules in a radiationless process; that is, oxygen molecules quench the luminescent emission. According to Henry’s law, the concentration of oxygen in a PSP polymer is proportional to the partial pressure of oxygen in gas above the polymer. For air, pressure is proportional to the oxygen partial pressure. So, for higher air pressure, more oxygen molecules exist in the PSP layer and as a result more luminescent molecules are quenched. Hence, the luminescent intensity is a decreasing function of air pressure.

The relationship between the luminescent intensity and oxygen concentration can be described by the Stern-Volmer relation. For experimental aerodynamicists, a convenient form of the Stern-Volmer relation between the luminescent intensity I and air pressure p is

— = A + B-^-, (1.1)

1 Pref

where Iref and pref are the luminescent intensity and air pressure at a reference

condition, respectively. The Stern-Volmer coefficients A and B, which are temperature-dependent due to the thermal quenching, are experimentally determined by calibration. Theoretically speaking, the intensity ratio Iref/I can

eliminate the effects of non-uniform illumination, uneven coating and non­homogenous luminophore concentration in PSP. In typical tests in a wind tunnel, I ref is taken when the tunnel is turned off and hence it is often called the wind-off

intensity (or image); likewise, I is called the wind-on intensity (or image). Figures 1.2 and 1.3 show, respectively, the luminescent intensity as a function of pressure at the ambient temperature and the corresponding Stern-Volmer plots for three PSPs: Ru(ph2-phen) in GE RTV 118, Pyrene in GE RTV 118 and PtOEP in GP 197.

A measurement system for PSP or TSP is generally composed of paint, illumination light, photodetector, and data acquisition/processing unit. Figure 1.4 shows a generic CCD camera system for both PSP and TSP. Many light sources are available for illuminating PSP/TSP, including lasers, ultraviolet (UV) lamps, xenon lamps, and light-emitting-diode (LED) arrays. Scientific-grade charge – coupled device (CCD) cameras are often used as detectors because of their good linear response, high dynamic range and low noise. Other commonly-used photodetectors are photomultiplier tubes (PMT) and photodiodes (PD). A generic laser-scanning system, as shown in Fig. 1.5, typically uses a laser with a computer-controlled scanning mirror as an illumination source and a PMT as a detector along with a lock-in amplifier for both intensity and phase measurements. Optical filters are used in both systems to separate the luminescent emission from the excitation light.

Pressure Sensitive Paint

Fig. 1.2. The luminescent intensity as a function of pressure for three PSPs at the ambient temperature, where pref is the ambient pressure and Iref is the luminescence intensity at the ambient conditions.

 

Pressure Sensitive Paint

Fig. 1.3. The Stern-Volmer plots for three PSPs at the ambient temperature, where prf is the ambient pressure and Iref is the luminescence intensity at the ambient conditions.

 

Computer

 

CCD camera

 

Lummophore molecule

 

Fi ter

 

Polymer

 

Illumination light

 

Pressure Sensitive Paint

Pressure Sensitive Paint

Basecoat

Mode surface

Painted mode

Target

Fig. 1.4. Generic CCD camera system for PSP and TSP

Once PSP is calibrated, in principle, pressure can be directly calculated from the luminescent intensity using the Stern-Volmer relation. Nevertheless, practical data processing is more elaborate in order to suppress the error sources and improve the measurement accuracy of PSP. For an intensity-based CCD camera system, the wind-on image often does not align with the wind-off reference image due to aeroelastic deformation of a model in wind tunnel testing. Therefore, the image registration technique must be used to re-align the wind-on image to the wind-off image before taking a ratio between those images. Also, since the Stern – Volmer coefficients A and B are temperature-dependent, temperature correction is certainly required since the temperature effect of PSP is the most dominant error source in PSP measurements. In wind tunnel testing, the temperature effect of PSP is to a great extent compensated by the in-situ calibration procedure that directly correlates the luminescent intensity to pressure tap data obtained at well – distributed locations on a model during tests. To further reduce the measurement uncertainty, additional data processing procedures are applied, including image
summation, dark-current correction, flat-field correction, illumination compensation, and self-illumination correction. After a pressure image is obtained, to make pressure data more useful to aircraft design engineers, data in the image plane should be mapped onto a model surface grid in the 3D object space. Therefore, geometric camera calibration and image resection are necessary to establish the relationship between the image plane and the 3D object space.

Besides the intensity-ratio method for a single-luminophore PSP, lifetime measurement systems and multi-luminophore PSP systems have also been developed. Theoretically speaking, the luminescent lifetime is independent of the luminophore concentration, illumination level and coating thickness. Hence, the lifetime method does not require the reference intensity (or image) and it is ideally immune from the troublesome ratioing process in the intensity-ratio method for a deformed model. Similarly, one of the purposes of developing the multiple – luminophore PSP system is to eliminate the need of the wind-off reference image and reduce the error associated with model deformation. Another goal of using the multiple-luminophore PSP system is to compensate the temperature effect of PSP.

Pressure Sensitive Paint

Fig. 1.5. Generic laser scanning lifetime system for PSP and TSP

Most PSP measurements have been conducted in high subsonic, transonic and supersonic flows on various aerodynamic models in both large production wind tunnels and small research wind tunnels. PSP is particularly effective in a range of Mach numbers from 0.3 to 3.0. Figure 1.6 shows a typical PSP-derived pressure field on the F-16C model at Mach 0.9 and the angle-of-attack of 4 degrees, which was obtained by Sellers and his colleagues (Sellers 1998a, 1998b, 2000; Sellers and Brill 1994) at the Arnold Engineering Development Center (AEDC). For PSP measurements in large wind tunnels, the accuracy of PSP is typically 0.02-0.03 in the pressure coefficient, while in well-controlled experiments the absolute pressure accuracy of 1 mbar (0.0145 psi) can be achieved. In short-duration hypersonic tunnels (Mach 6-10), measurements require very fast time response of PSP and minimization of the temperature effect of PSP. Binder-free, porous anodized aluminum (AA) PSP has been used in hypersonic flows and rotating machinery since it has a very short response time of 30-100 in comparison with a timescale of about 0.5 s for a conventional polymer-based PSP. Furthermore, because AA-PSP is a part of an aluminum model, an increase of the surface temperature in a short duration is relatively small due to the high thermal conductivity of aluminum. Since a porous PSP usually exhibits the pressure sensitivity at cryogenic temperatures, AA-PSP and polymer – based porous PSP have been used for pressure measurements in cryogenic wind tunnels where the oxygen concentration is extremely low and the total temperature is as low as 90 K.

Pressure Sensitive Paint

Fig. 1.6. PSP image for the F-16C model at Mach 0.9 and the angle-of-attack of 4 degrees. From Sellers (2000)

PSP measurements in low-speed flows are difficult since a very small change in pressure must be resolved and the major error sources must be minimized to obtain acceptable quantitative pressure results. Some low-speed PSP measurements were conducted on delta wings where upper surface pressure exhibited a relatively large change induced by the leading-edge vortices. In addition, experiments were conducted on airfoils, car models and impinging jets at speeds as low as 20 m/s. The pressure resolution of PSP in low-speed flows is ultimately limited by the photon shot noise of a CCD camera. Instead of pushing PSP instrumentation to the limit in low-speed flows, the pressure-correction method was proposed to recover the incompressible pressure coefficient from PSP results obtained in subsonic flows at suitably higher Mach numbers by removing the compressibility effect.

Since PSP is a non-contact technique, it is particularly suitable to pressure measurements on high-speed rotating blades in rotating machinery where conventional techniques are difficult to use. Both CCD camera and laser scanning systems have been used for PSP measurements on rotating blades in turbine engines and helicopters. Impinging jets were used in some studies as a canonical flow for testing the performance of PSP systems. Flight test is a challenging area where PSP has showed its advantages as a non-contact, optical pressure measurement technique. The pressure distributions on wings and parts of aircrafts have been measured using film-based camera systems in early in-flight experiments and a laser scanning system in recent flight tests.

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