Quote:
Originally Posted by basjoos
How does the side taper of the boatail affect the 4 degree limit? The red/blue drawing above shows a 2 dimensional vertical taper, but a boattail that tapers both vertically and horizontally is going to require less airflow to fill the void under the boattail than a boattail that only tapers vertically (as shown in the drawing).
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Hi Mike,
First off, I would like to say that you did a fantastic job with the aerocivic, and in no way am I trying to detract from your design.
With that said, I am trying to present some ideas as to how one *might* get close to the ultimate in performance from a boat tail design. Now I am not an expert by any means, but it does seem that a boat tail that rises from horizontal as sharply as the aerocivic, *may* be compromising what is possible from a boat tail design.
Your comments about air flow in the three dimensions are well taken, and I had this in mind in my discussion shown above. I need to elaborate on this further for clarity. For example:
1) Where the air reaches the stagnation point at the front of the vehicle...
2) The air chooses to go over or under the nose...
3) Air going under the nose is compressed more than the air going over the roof for example, because the air above the car can expand maybe 10 feet away from the roof top to try and alleviate the pressure build-up there. This is why you feel your car move side to side slightly when another car goes by in the opposite direction
4) The compressed air under the car is now squeezed against the car under body and road surface.
5) Depending on where the stagnation point is vertically from the ground, the air could be under much higher pressure here than on let's say the sides for example. (For the sake of simplified explanation, I am purposely avoiding Bernoulli's Equation regarding air velocity versus pressure).
6) In that case the air starts to move laterally from under the car (3D movement) to the sides of the car to escape to a lower pressure zone
7) As the air under the car equalizes more, the lateral movement would slow and ultimately stop
8) Now the boat tail section of the car comes into view, and there is a larger expansive volume there that needs to be filled
9) The air senses this lower pressure area and moves in from the *bottom* of the car AND the *sides* (3D movement again)
10) Since the boat tail underside volume is so large in this area, a local atmospheric depression occurs in the form of a vacuum. It takes more air to fill the void than what comes from the *bottom* of the car. Air can spill in from the sides, but has to travel a minimum of 2 feet laterally just to get to the center of the void. The flow has to *turn* a sharp corner to accelerate from fore-aft movement to lateral movement. Energy is expended. Flow separation is likely here.
11) This lateral air movement under the car takes extra energy from the ICE or electric motor as the case may be, and the result is more air drag. The air is not simply moving from the front of the vehicle, straight to the back and out, but rather side to side under the car as well (3D movement). The air has mass and it takes energy to move it around rather than simply allowing it to go straight underneath.
Side note: if you are moving at 60 mph, then you are also traveling at 90 feet per second. Let's say your car is 14 feet long. It only takes 0.16 seconds for the air to move past your entire car body, let alone, *turn* a corner to fill the void under the boat tail. This acceleration of the air is way more severe than the air flowing smoothly down the TOP side of an 18° elliptical boat tail. This quick 3D air movement will cause flow separation and is the main jest of my discussion here.
12) *If* the boat tail angle from horizontal was matched to what the car can flow *underneath* the body, then the side-to-side air movement would not happen, and maybe, just maybe, the end result would be higher efficiency.
And further food for thought...
Project: removable Geo Metro boat tail
Quote:
Originally Posted by Bicycle Bob
The bottom is not so well supplied with air, even with a great belly pan and rounded. 4 degrees on the belly pan is usually as conservative as 15 deg on the top and sides. If the tapers were continued to a point at the back, it should be only slightly higher than the stagnation point at the front. A bit of rake will help cancel the lift and induced drag if the air goes over the top faster than underneath.
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Hope this elaboration helps, Jim.
