Reynolds number

[aerohead]

Reynolds number determines whether a structure will have a laminar or turbulent boundary layer.
At 20-mph and above,because of their length,automobiles will transition to turbulent boundary layer (TBL),which helps the air to stay attached (like a dimpled golf ball).
If the TBL does stay attached,then the outer,inviscid flow will be laminar all the way across the car,and the only penalty will be the bit of TBL in the wake which cannot be converted to a pressure rise.
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Reynolds number Rn = Length (feet) X Velocity (feet per second)/ v (kinematic viscosity [ feet-squared per second])

or, Rn = L X V X 1/v

Standard kinematic viscosity is 0.00015 ft-square per second

1/v = 6380

So,simplifying, Rn = L X V X 6380
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Due to legal constraints on camera systems and interest in the drag of smaller structures on a car (mirrors/tires),knowing the Rn may mean a better understanding of whether or not,these smaller structures might benefit from some modification for boundary layer control (dimples/VGs/grooves/stubs/jets/etc.)
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*With no outside turbulence the TBL transition occurs at Rn = 500,000.
*Earth's TBL can have a thickness of 5-kilometers on any windy day.
*Typically,with any wind at all,automobiles will be fully immersed in the Earth's TBL and have no chance of any LBL and are at full TBL by Rn = 75,000.
*A Prius is at full TBL by 4-mph.
*One MIRA aerodynamicist reported that in any given year,they might have only 7-calm days in which they could conduct outdoor testing in England.
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NACA 6-Series airfoils on aircraft experience minimum-drag flight Rns of 20,000,000-40,000,000.

[chillsworld]

Would a person need to understand the relationship between the body of the car and smaller object, say a mirror, before doing calculations? That might be poorly worded... Can one treat the smaller object as a stand alone unit when considering boundary layer manipulation, or does one also have to consider the larger objects (Car) impact on the smaller objects boundary layer? Or does it really matter either way?


~C

[aerohead]

Originally Posted by chillsworld

Would a person need to understand the relationship between the body of the car and smaller object, say a mirror, before doing calculations? That might be poorly worded... Can one treat the smaller object as a stand alone unit when considering boundary layer manipulation, or does one also have to consider the larger objects (Car) impact on the smaller objects boundary layer? Or does it really matter either way?


~C

*This is the best question!
*It's only been recently that carmakers have had to consider these small points,driven by the upcoming CAFE standards of 2025.
*And we have no current scientific (a Priori ) knowledge of these interactions.
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*The CFD/Small-scale wind tunnel studies on mirrors which have been shared so far are flawed,in the sense that,when completed,the investigators concluded that only full-scale wind tunnel testing or Direct Numerical Simulation would yield accurate results.
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*Earlier testing in the late fifties was on Cd 0.55 cars with so much turbulence,that a small side mirrors drag would only be 1% of the overall drag.
*Cd 0.30 cars are reported to see 2-7% drag associated with mirrors.
*The mirrors on the 'Spirit of Ecomodder.com' added 10% drag.
*The Cd 0.154 Ford Probe-IV was one of the last concept cars which actually had side mirrors.
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*As to the mirrors,I would analyze them as a small structure in free flight,then add in some interference drag component as a start,when integrated onto the car.
*We don't yet know what the interactions are for sure.And they'll be on a case-specific-basis.
*But there are drag coefficients available for just about any 'components' we might choose to create small structures.
*Local airflow will be a supervelocity compared to the road speed and needs to be guestimated.