I've done an experiment and there's a problem because of different atmospheric pressure for different days of experiment. It is quite large that effects my analysis. How can I overcome this problem?
Examples: The first day of exp: 101250 atm Pressure. The second day: 101590 atm Pressure. This will effect the total pressure for wind tunnel experiment. How can I overcome this type of problem? Any idea?
You do not mention how these variations actually affect the analysis...
Nontheless, I can not see the problem if the ambient atmospheric (ambient) pressure varies from day to day. If you really want a constant stagnation (total) pressure, for some reason, you can probably adjust for that by fine-tuning the RPM of the fan unit and by recording the stagnation pressure using a Pitot tube. If the problem relates to the air speed the air density can be easiliy corrected for using the ideal gas law. If the fan unit is not speed-controlled you may correct for the variations afterwards; the variations you mention are actually quite tiny, less than +/- 0.2% !
- From an analysis point of view :
Perhaps should you record data during a reference test point without wind, and make your analysis of data-with-wind relatively to it ? However, the variation of atmospheric pressure you mentioned is quite low...
- From a wind tunnel management point of view :
Even if it is not too late, adjusting the speed of the airflow to keep the total pressure constant has a disadvantage: if you study a wing airfoil, your dynamic pressure will change, thus the pressure distribution will change, like the supersonic plateau, the shock location, especially in the transonic domain...
Have you considered converting everything to standard day?
Best regards,
c s a b a
You should realise that the average static pressure prevailing within a test section is quite different for an open jet test facility than for a close loop wind tunnel. Actually in a close loop wind tunnel this average static pressure is a function of the wind speed. Fortunately you only need an accuracy better than 1% for very special experiments. If this is your case, you should describe your experiment and data analysis more carefully in order to allow us to provide a better answer.
Yours sincerely, mico
This is a standard problem for experiments conducted over multiple days/weeks/months/years. The accepted way of dealing with this is to restate your experimental data using standard day conditions as your reference. Most books on experimental aerodynamics have a chapter on how to do this.
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