Quote:
Originally Posted by freebeard
A vertical Coanda nozzle fed by engine cooling air and/or exhaust. On the left.
It's easy when the engine is at the right end of the car. 
Maybe I should revisit this. in 2014 I couldn't merge the boat tail and car body. |
Hey freebeard! Is that your Superbeetle?
I didn't understand what you were trying to do the first time you showed it. If you are trying to make an integrated boat tail / radiator exhaust, then the optimal design is to start with that right pic. Then figure out what your radiator inlet and outlet areas are going to be. Find where the cross section of tail is equal to the outlet area and then truncate it there. Run a duct with that cross sectional area from the radiator to that new opening. Put on a ribbed rim with a flame and call it the Beetlemobile.
By the way, you want the tail to be symmetric from above but asymmetric from the side. The Beetle has most air go over the whole upper body, leaving the underside with less airflow. If the radiator intake is from underneath the vehicle, that further lowers where the exit plane should be. With certainty the optimal height is somewhere between a) at most the halfway point of the car body excluding wheels (i.e. the undertray and top of car) and b) at least the halfway point of the nose (same undertray plane and where hood meets windshield).
Do you have access to pressure measuring equipment? If so, then we could precisely determine where the exit plane should be. If you don't mind doing a mockup first, you could also tuft test. Start at the midpoint between a) and b) and then look at the tufts. Then tweak the exit plane up or down until you get the smoothest most attached flow.
You could also get some real gains by rerouting the radiator flow. I don't know how the Superbeetle is set up, but if those lines under the rear window are the exhaust, it's likely that air has to take a sharp turn to get up into the radiator inlet, then another sharp turn to get to the outlet. The optimal airflow is to gently deflect from underneath (or the sides), then take the straightest path to the outlet, with as gradual an angle change as possible. So if the radiator is at the bottom of the boot (say), then have the duct run along the bottom of the inside of the tail. The less we force air to zig zag around, the less drag we get.
As for fastening, my newest love are Rivnuts. If the body metal is still solid, you could easily Rivnut the tail onto the body with a good seam. If you have a tow hitch (or can put one on), then just engineer things so that most of the tail weight lies on the hitch. That will further reduce the stress on the tail / body fasteners. Otherwise, put the main weight on the back bumper / something else reasonably solid.
Hope this helps!