Tuesday, September 25, 2012

PSP in F1

Hello and thanks for tuning in on this frequency again.

The following article comes exclusively with the help of the kind people from ISSI - Innovative Scientific Solutions Incorporated. Their commercial site is located here.

We are not going to talk about PlayStation Portable, as the headline hints, but for Pressure Sensitive Paint (henceforth abbreviated PSP) - a method which stands between traditional F1 development paths - wind tunnels and CFD (but not a replacement for any of them). This is something that has been in use for quite some time, especially when it comes to NASA and airplanes.


In short, this paint-like coating fluoresces under a specific illumination wavelength of incident light and the fluorescent response is a function of the external air pressure being applied locally to its surface.
A typical PSP is consists of luminescent molecule and a polymer binder which must be permeable to oxygen.

There has been some use and some interest by certain F1 teams in using PSP in their own tunnel testing. 30m/s is typically the lower limit of PSP due to smaller pressure gradients below that speed. Most use PSP to validate CFD results or vice versa.

Imagine that in the end you could receive the same pressure distribution picture as you normally get from a CFD simulation run:
Image credit:  http://www.psp-tsp.com/


A key advantage to traditional experimental techniques like pressure taps and transducers:
  • cost savings 
  • not limited by model geometry
  • provides much higher spatial resolution than traditional methods
Essentially you'll have a "pressure tap" at every pixel of your camera. So if you are using a 1-megapixel camera, that's like having 1 million pressure taps on the surface. Once the experiment is set up, many runs at various conditions can be made rather quickly by comparison to CFD and data turnaround is much quicker as it can be processed on site with some knowledge of the test conditions and local pressure taps on the model if available.
Paint formulations have also been developed recently which allow for unsteady measurements of pressure using a high speed camera. Measurements can be made upwards of 10 kHz on the surface.


The typical process could be described with the following simple steps:
  1. Painting: Whatever the testing object might be, the usual paint gun or airbrush could be used. 
  2. Excitation: The molecules inside the paint have to be excited, so there is a light illumination source applied to the painted surface.
  3. Data gathering: The CCD camera kicks in collecting the fluorescent response from the illuminated surface
  4. Data visualization: Different software packages could be used to visualize what's already being recorded and thus used for analysis of pressure gradients. 
Two immediate questions have arisen, fortunately Steve Palluconi, a research engineer, was available for this short interview: 

Question: Due to its (PSP's) spraying technique - would that be too much disturbance of boundary layer or not at all? Negligible, maybe?
Answer: Generally the layer thickness is 20-30 microns and is very smooth. Negligible for these types of tests. Pressure taps are more invasive as we’ve actually seen flow separation caused by them.

Question: Are there any estimates on cost? For example, would it be too expensive to implement or improper, for example, due to model scales (in F1 - up to 60% of the real car size)
Answer: Costs depend on the scope of the test and size of the model. Large models like the one referenced would usually be imaged with a multi-camera system. Smaller models can be imaged with a single camera. We've done testing on some large aircraft models.

Should you have more in-depth and scientific questions you can always contact Steve through their web site.