Quote:
Originally Posted by 3-Wheeler
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.
Theory:
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.
Jim. |
Thanks Jim.
With respect to these automotive pressure histories,we're encouraged to focus on boundary layer phenomena.
*In this 1st wind tunnel photo of spheres,we see the non-dimpled flow separation before the max camber position on a smooth,laminar boundary layer golf ball.The air from the back of the ball is traveling against the flow towards the high-velocity/low-pressure 'windshield area' of the golf ball,triggering separation and high drag.
*In the second image,we've dimpled the ball (driven our car beyond 20-mph) and the inviscid laminar outer flow field is transferring momentum energy into the Reynolds number-induced turbulent boundary layer,allowing flow attachment back to 4-seconds after 12:00 on 'top',and 4-seconds before 6:00 on 'bottom.'
*If you measure the local tangent angle at the separation point you get 22-degrees.
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*All the air around the sphere still wants to flow to the original laminar boundary layer separation line,but it is prevented by the momentum interchange into the turbulent boundary layer AS LONG AS the body doesn't reach a 'fast' contour (pseudo-fastback).
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*The yellow spike at the windshield top in all the pressure tables is a reminder of where this 'low' exists on the car,and if we compromise the shape of the car,this is where the air in the turbulent boundary layer is going to attempt to get to.
*If we keep increasing the radius of the aft-body contour,we eventually reach a crude streamline body of revolution,and the wake is eliminated.
*This is what Hucho's books are all about,and underlies all automotive streamlining.
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*In your wind tunnel,the air mover,or vacuum vessel which drives the supersonic flow,is powering the delta-P which enables the flow.Work is added to the system just as the brake horsepower of the cars engine.
*The wind tunnel is also a closed system and you have conservation of air mass volume.
*Whatever passes P1 must equal P2 and P3,by definition.
*The total energy of any given imaginary streamline filament will be equal to any other streamline element under steady-state conditions.
*This is the isoenergetic concept established by Bernoulli.
*P1 is accelerated from a low velocity/high-pressure,to mostly velocity at P2,then decelerated to a lower velocity/higher pressure again at P3.
*This is exactly what Daniel Bernoulli demonstrated.
*You have lost some energy to surface friction and viscous friction.This energy is lost forever (2nd-law of Thermo).
*In your transonic and supersonic work you've got to deal with shockwave losses as well.
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*Our engine or motor power induces motion.
*Motion displaces the air mass.
*Displacement creates acceleration
*Acceleration creates velocity
*Velocity affects pressure
*The difference between the forward stagnation point and base pressure of the wake is the pressure drag which is the major portion of the total drag,of which streamlining (by definition) attempts to reduce or eliminate.
*If the aft-body is correct,there'll be zero separation,almost total pressure recovery,and the base pressure will be close to that of the forward stagnation point,killing pressure drag.