Tech Briefs
Pressure-sensitive paint

Low pressure is indicated by purple and dark blue. As the air hits the front windshield, moves across and then falls off the A pillar, the velocity increases. Higher velocity causes lower pressure. A higher pressure ois indicated (red) at the back edge of the window and the bar in the middle of the window. The units of the y-axis are the dimensionless pressure coefficent, Cp. PSP fundamentals. PSP sprayed on the side glass of a Ford Transit 4/10 scale model. To obtain conventional pressure measurements on this driver's side glass, it took several days to install 120 pressure ports and connecting tubes.

Ford researchers have begun using pressure-sensitive paint (PSP) to measure the external forces exerted on automobiles. PSP is an advanced optical-imaging, absolute-pressure-measuring technique that determines surface pressure distribution on a model. It has been used extensively for measuring airplane loads and for wind-tunnel validation of computational fluid dynamic (CFD) models. Ford worked with one of the first PSP developers, McDonnell Douglas, to adapt PSPs to the automotive industry.

Traditionally in the automotive industry, a large number of pressure taps are used to obtain a pressure distribution across a model surface. Obtaining the side-glass-door pressure distribution of a vehicle is useful in structural and NVH design of the side-glass frame and door seals. However, to install taps on the side glass requires replacing the glass with a metal or plastic plate, drilling many small holes, and plumbing each to pressure transducers; all of which takes several days. Also, the taps are difficult to install on curved surfaces and are located in a grid pattern about 25 to 50 mm (1 to 2 in) apart. Since the only pressure data obtained is at the tap location, obtaining pressures at other locations on the surface requires an interpolation between the measured pressures at each port. Care must also be taken to drill very small, clean holes without burrs in order to maintain the original smooth surface without a disturbance in the airflow, which would cause erroneous values at the pressure taps.

Optical measurements are now achieved by painting a wind-tunnel model with a PSP that fluoresces under short wavelength excitation. PSP fundamentals include a light source shining through a blue filter onto a model. A red (or orange) filter is placed on a camera. The blue light causes the model to emit red (or orange) light, which is detected by the camera and recorded on the computer. Two different images are obtained, one at wind-off or normal room air, and one at wind-on in which the tunnel is running at the desired test velocity. As pressure increases in the wind-on mode, collisions between oxygen and the paint on the model reduce the fluorescence of the paint. The change in fluorescent intensity from wind-off to wind-on is used to calculate surface pressure.

Ford says that PSP has two primary advantages over pressure ports. First, PSP is non-intrusive and can be applied without changing the smooth surface characteristics of the model. PSP can be applied selectively to a model surface, in just a few hours. It can also be easily removed after the wind-tunnel tests are completed. Second, it provides more pressure data from wind-tunnel testing earlier in the vehicle program, resulting in better understanding of critical flow fields that affect various aspects of automotive design. PSP offers a full field of surface data while pressure points only gave information on discreet points.

Ford is currently implementing this technology in-house at its Dearborn Proving Ground.