Luminescent Method for Pressure Measurement

Pressure-sensitive paint (PSP) is an image-based technology that provides a convenient method for continuous and nonintrusive global mapping of pressure distributions on aerodynamic surfaces. Its development offers a feasible alternative to pressure measurements obtained from point-to-point instruments such as pressure taps or pressure transducers, which were often costly to utilize and intrusive to the flow being studied. The paints are composed of pressure-sensitive dye, which is made of an oxygen-sensitive luminescent dye, that is incorporated into an oxygen-permeable polymer binder and dissolved in a volatile solvent to form a paint that can be easily applied to surfaces using a spray can or airbrush. The emitted intensity of the pressure-sensitive paint is quenched by collisions with oxygen molecules; the intensity of the emitted light is inversely proportional to the pressure at the surface. The measurement is accomplished by applying the paint to the surface of interest and illuminating the surface with blue or UV light to excite the dye. The surface is imaged through a filter that isolates the excitation light from the pressure-sensitive luminescence of the paint. Each pixel on the camera then acts as a nonintrusive pressure tap, and therefore, continuous distributions of the pressure on the painted surface are acquired. Standard pressure paints can be used for mean measurements with response times of about 1 Hz, and fast response systems can be used for high frequency measurements, with a bandwidth of over 100 kHz. The use of PSP is becoming more common in large transonic tunnels, with production systems in use in several facilities such as AEDC, ARA, DLR, ADD, JAXA, and AVIC. These measurements have been conducted using both binary and lifetime-based systems. While PSP has proven effective in transonic tunnels, PSP measurements in large low-speed tunnels have proven particularly challenging. A significant reason for this difficulty is that in low-speed tunnels the dynamic pressures are small (∼1 kPa at 40 m/s), and therefore, the dynamic range of the signal to be detected in a low-speed tunnel is often 1% or less. Recent advances in cameras, illumination sources, and paint technology have extended the operating range of PSP systems to as low as 40 m/s. This chapter will cover in detail the following: principles and theoretical treatment of pressure-sensitive paints, pressure-sensitive dyes, polymers, measurement methods, and practical implementations of PSP to a wide series of wind tunnel applications. Keywords: CCD camera; dye; imaging; luminescence; polymer; pressure-sensitive paint (PSP); quenching; wind tunnel