Applications for PSP
Measurements of surface pressure in low-speed tunnels using PSP has been an area of significant interest for several years. Generally, the effects of temperature and model deformation/displacement have significantly degraded the quality of the data.
Automotive: A one twenty-fourth scale car model was painted with BF405 (Binary FIB PSP) and placed in the Air Force Institute of Technology (AFIT) low speed wind tunnel in Dayton Ohio. This is an open circuit wind tunnel with a maximum speed of Mach 0.2 and a 3 ft. by 4 ft. test section. The model was illuminated with four LM2X-DM-400 arrays and imaged using a PCO-1600 CCD camera through a FS-3 filter switch. Tests were performed at 50 m/s and 20 m/s at approximately 0 and 10 degrees yaw. At 50 m/s the dynamic pressure is approximately 0.2 psi, this would correspond to a change in intensity from the PSP of about 1%. To demonstrate the effects of model movement, the ratio of the wind-off to wind-on data that was acquired through the reference channel is presented here.
Reference Channel, 1:24 scale car model painted with Binary FIB PSP
Note here that the signal from the reference channel, which is not sensitive to pressure, has changed by as much as 3% and varies by over 1% for different regions of the model. These variations in intensity are attributed to movement and deformation of the model at the wind-on condition. Temperature effects are minimized in this experiment since this is an open circuit wind tunnel, they are not however negligible. Consider that the total anticipated signal change due to pressure is about 1%, however the errors due to illumination are larger than this.
Next the data was reduced using both the signal and reference images and the resulting pressure distribution is shown here.
Pressure distribution, 1:24 scale car model painted with Binary FIB PSP
Note that the maximum pressure is about equal to the tunnel dynamic pressure. Regions of high pressure are evident on the front bumper and the windshield. As the flow accelerates around the front bumper and down the sides of the car, a region of low pressure is evident. Compensation for model movement is essential for general application of PSP to low speed tests.
Another demonstration of PSP for low speed wind tunnels is the pressure distribution on an automotive mirror. Here, a mirror was mounted onto the floor of the AFIT low-speed tunnel, painted with Binary FIB, and instrumented with 6 pressure taps. Data was acquired at Mach 0.2 and the PSP data was compared to the taps. In this case the deviation between the taps and PSP was less than 200-Pa, demonstrating that PSP can be used in this low speed regime.
Supersonic Flame Holders: An investigation of pylon geometry for supersonic flame holders was conducted in a direct-connect supersonic facility located at Wright Patterson Air Force Base. The test facility included excellent optical access as windows are located on both side walls and top wall. Two illumination sources (LM2XX-400) and a CCD camera (PCO.1600 with filter switch) were positioned to obtain the side view and top view of the pylon. Various struts were painted with Binary FIB and installed on the bottom wall of a constant-area (13.1 x 15.24 ) test section. Examples of three strut models coated with PSP are shown below, along with the experimental setup.
Experimental Setup and Painted Pylons
The surface pressure distributions on a pylon in Mach 2 flow is illustrated in the following figure. A high pressure zone at the thin leading edge of the strut due to the flow impingement and a low pressure region behind the expansion is clearly observed in the Figure. As these pylons are under investigation for supersonic flame holders, there are several fuel injection ports evident on the surface.
Pressure distributions on the pylon models
An interesting use of PSP is as a flow visualization tool. Here Nitrogen () was injected through the injection ports and the resulting pressure field is indicative of both pressure and mass fraction of the injected gas. A pair of bow shocks are formed in front of the injection ports as would be expected. The trace of the injected near the pylon surface is identified by the low pressure region behind the injection ports. These regions are not really low pressure but the result of the replacement of oxygen by the injected . The presence of the gas on the surface would indicate that some of the injected gas should enter the stagnant sub-sonic zone behind the strut, and therefore, possibly be available for combustion and flame holding.
Transverse Jet Injection: Another example of the use of Binary FIB for measurements of pressure in a wind tunnel is transverse jet injection in a supersonic flow. Here two test plates were inserted into the TGF at Wright Patterson Air Force Base. The data shown here was acquired at Mach 2 for both transverse and 45 degree injection of the jets. This is another good example of the utility of PSP for capturing high-resolution pressure distributions.
Aerospace: In some situations, PSP can serve as a flow visualization tool. In this example, a re-entry vehicle was tested in the SARL at Wright Patterson Air Force Base. This vehicle had detachable vertical winglets that were relatively thin, and therefore, these winglets would be difficult to tap. Furthermore, it is often difficult to decide where to locate the pressure taps. The winglets were painted with Binary FIB and data was acquired at Mach 0.4 and several angles of attack. Note the formation of the multi-vortex structure at the low angles of attack, transitioning to a single vortex at the high angle of attack. It would be difficult to locate pressure taps to investigate this complex pressure field as the multi-vortex structure is not the anticipated result.
An example of the use of Binary FIB for measurements of pressure in a wind tunnel is shown here. A basic wing section was painted with Binary FIB and experiments were performed at Aircraft Research Association at a range of angles of attack and Mach numbers. The ARA tunnel is a 9-foot by 8-foot closed circuit tunnel located in Bedford, England. The data shown here was acquired at Mach 0.7 and 3 degrees angle of attack. A comparison of the PSP and pressure tap data is also shown here. In this case, the mean squared deviation between the taps and PSP was about 0.03 Cp (about 400-Pa).
Lifetime PSP Measurements: An example of the use of Lifetime PSP measurements in the Aircraft Research Association wind tunnel is shown here. In this case, a basic wing section was painted with UniFIB and experiments were performed at a range of angles of attack and mach numbers. This experiment was conducted simultaneously with the Binary PSP experiment described previously. The data shown here was acquired at Mach 0.7 and 3 degrees angle of attack. A comparison of the PSP and pressure tap data is also shown here. In this case, the mean squared deviation between the taps and PSP was again about 0.03 Cp (about 400-Pa).