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Old 12-24-2014, 06:22 PM   #5 (permalink)
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Originally Posted by aerohead View Post
.....This is one of the most important things to know as an aero-modder.
All the air around your vehicle wants to flow to this area of lowest pressure!
Let me repeat that.
All the air around your vehicle wants to flow to the area around your windshield header!
Hi Phil,

I read through this particular post several times, and have to mention something.

When you said, "All the air around your vehicle wants to flow to the area around your windshield header!", I have a simple comment.

I work in a Flow Lab, and have worked with sonic nozzles and subsonic orifices for many years.


Any time one wants to get air molecules to move, a differential pressure (dP) is required.

In this graphic, the molecular flow is from left to right, and
- P1 is inlet pressure (usually gage + baro)
- P2 is throat pressure (or lack of, and can be vacuum)
-- an extremely high vacuum for sonic nozzles
-- can be close to 28 inHg vacuum or higher
- P3 is exhaust pressure (typically at baro or ambient)

Sonic nozzles are typically used for controlling bulk or mass flow, depending on the application. We use them to calibrate mass flow meters in the Lab.

The only reason that flow transfers from P1 past P2, to P3 is dP or delta pressure. The flow at the throat (P2) can be almost 100% kinetic energy, with no pressure component, and P1 and P3 are mostly potential energy, with no kinetic.

When it is stated that the air molecules "want" to move from P1 to P2 due to the low pressure there, then why would the flow at P2 want to progress towards P3? Location P3 is at higher pressure, and based on your observation, P3 flow would want to reverse and flow towards P2.

This does not happen is life so another mechanism causes the general flow from P1 past P2 to P3. It is dP or delta pressure that causes generalized flow across any flow device, be it a flow orifice, nozzle, or any other flow restrictor.

Summing up, I would say that any dP around our vehicle moving down the road causes a generalize flow around the vehicle. The pressure at the front of the vehicle has to be higher at the front, than at the rear for the dP to be enough to cause flow around the object.

Phil, thanks for all the nice graphics in your post.

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Last edited by 3-Wheeler; 12-24-2014 at 07:15 PM..
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