manometer usage

[NeilRoshier]

I would like to use a manometer to measure changes in pressures on a car following modifications to the bodywork. Are there any guidelines as to the appropriate use of such a tool?

 

[GregLocock]

(grin) put a tap in the line that you leave shut until you are ready to measure.

Use one reference pressure for all your measurements.

Measure the airspeed witha pitot tube for each run.

max pressure expected is of the order of .06*kph^2, in Pa. 1000 mm water= 9810 Pa

Good luck

Cheers

Greg Locock

 

[NeilRoshier]

Ah Greg I was tempted to just ask you rather than post! Happy new year etc.

My thinking was that I should be able to measure laminar flow in a very basic way by using wool tufts and a digital camera, which should be enough for my basic needs.
I was however interested in pressure differentials b/w the top and bottom surfaces of the car and diffuser effectiveness etc.

I had not thought of an airspeed indicator....more damned expense! I will have to investigate this aspect too I guess.

 

[fredt]

Greg

A small point: The relationship between velocity pressure (pv)and velocity is pv = 1/2 x rho x V^2. Where V is velocity in m/s and rho is the density of atmospheric air in kg/m^3.

If we assume standard air (rho = 1.2) and working with kph I get a constant of 0.0463 instead of .06 but maybe you are using different conditions.

In order to answer the question it would be necessary to know what you were trying to achieve. If this is the surface pressures on the car bodywork a single point reading would not be very meaningful.

 

[NeilRoshier]

Fredt I agree that a single point would not be very meaningful. I think that I would be able to tap into multiple points (say 6 in one region)with a single manometer and as two local tracks have 1/2 mile straights I may be able to get some (very) basic data. The data is for my own education more than anything else hence my reluctance to part with large sums of money!. I would like to try some different chin spoiler designs and perhaps even undertrays and diffusers if all goes well. I just don't like the idea of trying all of this 'blind' as it were.

 

[GregLocock]

0.046 is fine, I was doing it in my head.

You won't find laminar flow with a manometer.

There are three sorts of flow (I think) of practical importance for a car, in their usual order -

laminar, attached
turbulent, attached (you need a microphone or a stethoscope)
turbulent, separated (wool tufts or smoke show this)

Under the right conditions you could have separated laminar flow, but I haven't seen that in the tiny amount of aero I've done. My reference book here (Kay and Nedderman) agrees with the above, but doesn't specify how likely sep/lam is in practice, implication is that it is a bit odd.

I was unable to detect the attached laminar/turbulent transition using wool tufts on our full size testing of the solar car.


Cheers

Greg Locock

 

[i278]

A couple details to add:

Yes, laminar separation is rare; it requires (as you can imagine) very low Re flow. The only place I've run into it is the design of airfoils for small, slow model airplanes and human-powered aircraft. Google 'laminar separation bubble' if you're interested in examples.

A classic wind tunnel method for finding laminar/turbulent transition is to simply paint the test article with kerosene or a similar 'heavy' solvent. The article is tested while still wet. Regions with a laminar boundary layer will remain wet for a much longer time than regions with a turbulent boundary layer--transition is thus marked by the boundary of wet and dry regions on the article. Simple, but a little tricky, subjective, and time-dependent.

A slightly improved method (that I've seen, but never had to use) is to add a dye (a fine pigment would probably work as well) to the kerosene. Once the turbulent regions have dried, the solvent in the laminar region continues to flow aft to the wet/dry transition and accumulate. The test is run until all solvent has evaporated, and you should be left with lines of dried crud to indicate transition. This would probably be more practical for full-scale testing.

For interest's sake, one other way to find the transition point would be to take advantage of the fact that a turbulent boundary layer has a much higher convective heat transfer rate than a corresponding laminar boundary layer. So, you could heat the test article, put it into the flow, and measure the stream-wise surface temperature distribution. The sharp stream-wise drop in temp would indicate transition. I have it in my head that I've seen this before, but I can't find any reference to it, so I may have just dreamed it; either way, I think it's an interesting idea. Probably wouldn't be too practical for one-off testing, though.

Regards

 

[NeilRoshier]

Greg, as above the manometer is for pressure and the wool tufts are for flows, laminar or not. I have read a text on flow visualisation and have details on the fluid/pigment methods and also about the various ways to use thread to visualise flows. Since my needs are very basic I think that simply knowing if the flows are attached/turbulent-seperated will be fine. As I have said this is really just for my own development of knowledge.

 

[GregLocock]

Hey, as soon as you do an experiment you will learn 10x as much as anyone who hasn't. Go for it. Keep asking questions.

When you are done look for MicroCFD - a brilliant 2D windtunnel. The demo license lasts 3 days, use it wisely.




Cheers

Greg Locock