Historical Remarks

The working principles of PSP are based on the oxygen quenching of luminescence that was first discovered by H. Kautsky and H. Hirsch (1935). The quenching effect of luminescence by oxygen was used to detect small quantities of oxygen in medical applications (Gewehr and Delpy 1993) and analytical chemistry (Lakowicz 1991, 1999) before experimental aerodynamicists realized its utility as an optical sensor for measuring air pressure on a surface. J. Peterson and V. Fitzgerald (1980) demonstrated a surface flow visualization technique based on the oxygen quenching of dye fluorescence and revealed the possibility of using oxygen sensors for surface pressure measurements. Pioneering studies of applying oxygen sensors to aerodynamic experiments were initiated independently by scientists at the Central Aero-Hydrodynamic Institute (TsAGI) in Russia and the University of Washington in collaboration with the Boeing Company and the NASA Ames Research Center in the United States. The conceptual transformation from oxygen concentration measurement to surface pressure measurement was really a critical step for aerodynamic applications of PSP, signifying a paradigm shift from conventional point-based pressure measurement to global pressure mapping.

G. Pervushin and L. Nevsky (1981) of TsAGI, inspired by the work of I. Zakharov et al. (1964, 1974) on oxygen measurement, suggested the use of the oxygen quenching phenomenon for pressure measurements in aerodynamic experiments. The first PSP measurements at TsAGI were conducted at Mach 3 on a sphere, a half-cone and a flat plate with an upright block that were coated with a long-lifetime luminescent paint excited by a flash lamp. Their PSP was acriflavine or beta-aminoanthraquinone in a matrix consisting of silichrome, starch, sugar and polyvinylpyrrolidone. A photographic film camera was used for imaging the luminescent intensity field. The results obtained in these tests were in reasonable agreement with the known theoretical solution and pressure tap data (Ardasheva et al. 1982, 1985). Another TsAGI group consisting of A. Orlov, V.

Mosharov, S. Fonov and V. Radchenko started their research in 1983 to improve the accuracy of PSP by measuring the lifetime (the decay time). In their first tests on a cone-cylinder model at Mach 2.5 and 3.0, they used a photomultiplier tube as a detector and a pulsed Argon laser mechanically scanned over a surface to excite a newly developed PSP (Radchenko 1985). Unfortunately, they found that the lifetime measurements suffered from very strong temperature sensitivity (about 7%/°C) and a very long lifetime (about several minuets) of their first PSP. As a result, their effort has been exclusively focused on the development of intensity – based techniques since 1985. At TsAGI, a number of proprietary PSP formulations have been developed and applied to various subsonic, transonic, supersonic, shock, dynamic tunnels, and rotating machinery (Bukov et al. 1992, 1993; Troyanovsky et al. 1993; Mosharov et al. 1997). The imaging devices used at TsAGI covered a range of photographic film cameras, TV cameras, scientific grade CCD cameras and photomultiplier tubes with laser scanning systems. In the later 1980s, TsAGI marketed its PSP technology through the Italian firm INTECO and issued a one-page advertisement in the magazine ‘Aviation Week & Space Technology’ in February 12, 1990. Interestingly, scientists in the Western World were not aware of Russia’s work on PSP until reading the advertisement. Then, TsAGI’s PSP system was demonstrated in several wind tunnel tests at the Boeing Company in 1990 and Deutsche Forschungsanstalt fur Luft – und Raumfahrt (DLR) in Germany in 1991, which attracted widespread attention of researchers in the aerospace community (Volan and Alati 1991).

PSP was independently developed by a group of chemists led by M. Gouterman and J. Callis at the University of Washington (UW) in the late 1980s (Gouterman et al. 1990; Kavandi et al. 1990). The chemists at UW were initially interested in use of porphyrin compounds as an oxygen sensor for biomedical applications. After stimulating discussions with experimental aerodynamicists J. Crowder of the Boeing Company and B. McLachlan of NASA Ames, Gouterman and Callis understood the important implication of oxygen sensors in aerodynamic testing and started to develop a luminescent coating applied to surface for pressure measurements. Their classical PSP used platinum-octaethylporphorin (PtOEP) as a luminescent probe molecule in a proprietary commercial polymer mixture called GP-197 made by the Genesee Company. In 1989, using PtOEP in GP-197, M. Gouterman and J. Kavandi conducted PSP measurements on a NACA 0012 airfoil model (3-in chord and 9-in span) in the 25×25 cm wind tunnel at NASA Ames Fluid Dynamics Laboratory. The model was spray coated with a commercial white epoxy Krylon base-coat and then sprayed with PtOEP in GP-197. An UV lamp was used for excitation, and an analog camera interfaced to an IBM-AT computer with an 8-bit frame grabber for image acquisition. The model was set at the angle-of-attack of 5o and the Mach numbers ranged from 0.3 to 0.66. Their data showed very favorable agreement with pressure tap data, clearly indicating the formation of a shock on the upper surface of the model as the Mach number increases (Kavandi et al. 1990; McLachlan et al. 1993a). More importantly, this work established the basic procedures for intensity-based PSP measurements such as image ratioing and in-situ calibration. Following the tests at NASA Ames, Kavandi demonstrated the same PSP system in the Boeing Transonic Wind

Tunnel on various commercial airplane models, which was briefly discussed by Crowder (1990). Several proprietary paint formulations have been developed at UW, and successfully applied to wind tunnel testing at the Boeing Company and NASA Ames (McLachlan et al. 1993a, 1993b, 1995; McLachlan and Bell 1995; Bell and McLachlan 1993, 1996; Gouterman 1997).

Excellent work on PSP was also made at the former McDonnell Douglas (MD, now the Boeing Company at St. Louis) (Morris et al. 1993a, 1993b; Morris 1995; Morris and Donovan 1994; Donovan et al. 1993; Dowgwillo et al. 1994, 1996; Crites 1993; Crites and Benne 1995). MD PSPs were mainly based on Ruthenium compounds that were successfully used in subsonic, transonic and supersonic flows for a generic wing-body model, a full-span ramp, F-15 model, and a converging-diverging nozzle. Other major PSP research groups in the United States include NASA Langley, NASA Glenn, Arnold Engineering Development Center (AEDC), United States Air Force Wright-Patterson Laboratory, Purdue University, and University of Florida. European researchers in DLR (Germany), British Aerospace (BAe, UK), British Defense Evaluation and Research Agency (DERA, UK), and Office National d’Etudes et de Recherches Aerospatiales (ONERA, France) have been active in the field of PSP (Engler et al. 1991, 1992; Engler and Klein 1997a, 1997b; Engler 1995; Davies et al. 1995; Lyonnet et al. 1997). In Japan, the National Aerospace Laboratory (NAL), in collaboration with Purdue and a number of Japanese universities, developed cryogenic and fast – responding PSPs (Asai 1999; Asai et al. 2001, 2003). More and more research institutions all over the world are becoming interested in developing PSP technology because of its obvious advantages over conventional techniques. Brown (2000) gave a historical review with personal notes and recollections from some pioneers on early PSP development.

Before the advent of polymer-based luminescent TSPs, thermographic phosphors and thermochromic liquid crystals have been used for measuring the surface temperature distributions in heat transfer and aerothermodynamic experiments. Thermographic phosphors are usually applied to a surface in the form of insoluble powder or crystal in contrast to polymer-based luminescent TSPs although both techniques utilize the temperature dependence of luminescence. A family of thermographic phosphors can cover a temperature range from room temperature (293 K) to 1600 K, which overlaps with the temperature range of polymer-based TSPs from cryogenic temperature (about 100 K) to 423 K. In this sense, thermographic phosphors and polymer-based luminescent TSPs are complementary to cover a broader range from cryogenic to high temperatures. L. Bradley (1953) explored aerodynamic application of thermographic phosphors mixed with binders and ceramic materials to measure surface temperature. Then, thermographic phosphors were used for temperature measurements in high-speed wind tunnels (Czysz and Dixon 1969; Buck 1988, 1989, 1991; Merski 1998, 1999), gas turbine engines (Noel et al. 1985, 1986, 1987; Tobin et al. 1990; Alaruri et al. 1995), and fiber-optic thermometry systems (Wickersheim and Sun 1985). Allison and Gillies (1997) gave a comprehensive review on thermographic phosphors. Thermochromic liquid crystals applied to a black surface selectively reflect light and hue varies depending on the temperature of the surface, which allows measurement of the surface temperature in a relatively narrow range from 25 to 45oC. After E. Klein (1968) used liquid crystals in aerodynamic testing, this technique for global temperature measurement has been used in turbine machinery (Jones and Hippensteele 1988; Hippensteele and Russell 1988; Ireland and Jones 1986), hypersonic tunnels (Babinsky and Edwards 1996), and turbulent flows (Smith et al. 2000).

Polymer-based TSPs are relatively new compared to thermographic phosphors and thermochromic liquid crystals. P. Kolodner and A. Tyson (1982, 1983a, 1983b) of the Bell Laboratory used a Europium-based TSP in a polymer binder to measure the surface temperature distribution of an operating integrated circuit. A family of TSPs have been developed at Purdue University and used in low-speed, supersonic and hypersonic aerodynamic experiments (Campbell et al. 1992, 1994; Campbell 1994; Liu et al. 1992b, 1994a, 1994b, 1995a, 1995b, 1996, 1997a, 1997b). Two typical TSPs are EuTTA in model airplane dope (-20 to 100oC) and Ru(bpy) in Shellac (0 to 90oC). Several cryogenic TSPs (-175 to 0oC) were first discovered at Purdue University (Campbell et al. 1994) and used for transition detection in cryogenic flows (Asai et al. 1997c; Popernack et al. 1997). Further development of cryogenic TSPs was made at Purdue (Eransquin 1998a, 1998b), NAL in Japan (Asai et al. 1997c; Asai and Sullivan 1998) and NASA Langley. TSP formulations were also studied at the University of Washington (Gallery 1993) and one of the paints was used for boundary-layer transition detection at NASA Ames (McLachlan et al. 1993b).

PSP and TSP have become an active and growing interdisciplinary research area, offering the promise of quantitative pressure and temperature mapping on the one hand and giving new technical challenges on the other hand. Useful reviews were given by Crites (1993), McLachlan and Bell (1995a), Crites and Benne (1995), Liu et al. (1997b), Mosharov et al. (1997), Bell et al. (2001), and Sullivan (2001). This book provides a systematic and detailed description of all the technical aspects of PSP and TSP, including basic photophysics, paint formulations and their physical properties, radiative energy transport, measurement methods and systems, uncertainty, time response, image and data analysis techniques, and various applications in aerodynamics and fluid mechanics.

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