Quote:
Originally Posted by cr45
I am curious about the reading of 500 pascals at the front of the car as this is greater than the stagnation pressure at 80 km/h.
velocity (m/s) = velocity (km/h) / 3.6 = 80 / 3.6 = 22.222 m/s
air density at 10 C is 1.246 kg/m3 for std pressure
Stagnation Pressure (Pa) = 0.5 * Density * Velocity ^ 2 = 0.5 * 1.246 * 22.222 ^ 2
" " = 307.6 Pascals
This compares with your measurement of 500 Pascals.
What actually causes the measured pressure to be greater than stagnation pressure?
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Glad someone is looking closely at the data! Yes, the measured stagnation pressure is higher than it should be. That's why I always measure stagnation pressure when I am doing car aero pressure measurements. I then usually reference the other pressures against this value. That is, I normally convert stagnation pressure to 100 and then apply the same multiplier to all the other pressures, so the other pressures are relative rather than absolute. (For example, it's then interesting to compare the Insight pressure distribution with the XE Jaguar. - edit: I've now done this in the next post.)
Why is the measured stagnation pressure higher than it should be? I don't know. It could be the gauge, it could be the sensor, or it could be variation in measurement due to atmospheric conditions eg wind. In inches of water, the two stagnation pressure measurements were +1.7 (423 Pa) in one direction and +2.2 (550 Pa) in the other direction. That's a ratio of 1.3. Interestingly, the wake pressures also varied over the same range (+0.15 vs 0.2). In contrast, the other pressures were generally more consistent for the two directions.
For stagnation pressure you could use the calculated value of 310 Pa over the actual measured value (not rounded) of 487 Pa (ie 310 / 487) as a multiplier for the other pressures (ie 0.64) if you wished - of course it wouldn't change the relativity of the readings.
There's also another issue. I was using a 0-3 inches of water gauge and I picked a speed (80 km/h) that gave plenty of deflection (ie 2.2 inches of water) for the max measured pressure. However, that meant that I was working down the very bottom of the scale for measuring the smaller pressures. I could then have gone to one of my more sensitive gauges for these small pressures, but I didn't because I thought the chances of comparative error would be much higher (ie through using two different gauges). But if I were - say - developing a rear spoiler and wanted to measure the pressure change on the rear window, I would use a much more sensitive gauge (eg 120 Pa - 0.5 inches water - full scale deflection) for these measurements.
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