Quote:
Originally Posted by Sven7
Okay, so can we say turbulence acts similarly to wake? Are they different words for the same thing?
I didn't look close enough to recognize PRK  Nice job- I thought it was out of a textbook. It's nice to see that (if I'm reading this right) as long as the trailer form fits inside the "template", it will reduce drag, without factoring in RR and such. |
'Wake' is short for 'turbulent wake' or 'wake turbulence' and many use the two words interchangeably.
Whatever we want to call it,it is the region of separated flow behind the vehicle which possesses a 'base pressure' which is equal to the pressure at the point of separation.
A proper boat tail allows for a deceleration and pressure rise of the flow while preventing the separation.
A full boat tail allows for zero separation and maximum pressure recovery,swapping static pressure for velocity(dynamic)pressure.
The truly low drag vehicle has zero separation and full pressure regain and no kinetic energy is lost to eddies and turbulence which cannot be converted back to static pressure.
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From the table which has the bus dragging the boat-tailed railroad tail car,you can see that with the 'trailer' at about the same length as the bus,the drag for the bus/trailer is almost the same as for the solo bus; Cd 0.51 vs Cd 0.50.
As the trailer is shortened from about 12-meters in length,surface friction drag begins to fall and the drag of the bus/trailer is actually LESS than for the bus by itself.
When all the trailer is erased with exception to the boat tail itself,the Cd 0.05 surface friction of the tail car has been eliminated completely,leaving only 0.05 for the tail,its own friction,and any remnant of base pressure acting upon it.
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So actually,you could pull a rather long trailer and still have lower overall drag than of the 'tug' by itself.
The drag minimum does occur with just the tail,and it offers the greatest promise in a range-extender trailer application.