t_vago's compound curve foam board pickup aeroshell

[t vago]

I may have made a mistake with the general shape of the cap.

A couple of nights ago, I was able to view the wake of my truck with the cap installed, in the rain. I noticed what appeared to be a couple of visible counter-rotating vortices right on either side of the back of the truck. From reading Hucho and looking at the pictures that Piwoslaw has in his album, it would appear the the inward-moving air stream at the sides of my aerocap are interacting with the air stream moving rearward on the sides of the truck. I think this is forming the vortices I'm seeing.



This is one of the pictures out of Piwoslaw's album.

I'm going to add material on the rear of my cap to try to straighten out the airflow. That should reduce the strength of the vortices in question. If I see some noticeable increase in FE due to the addition, then I may plan this idea into the version 3 aerocap.

[autogyro]

The VW XL1 seems to have a rear slope of 17 deg., or so.
This design would seem to be some obvious parallels to some of the aero caps in this thread, most notably that rear is also flat sided.
That being said, they're only claiming a CD of 0.186 for the vehicle, which may be due to the steep slope, not to mention a rear cross section similar to the leftmost drawing from Piwoslaw, as shown above .
I would think one would expect more from thse guys, especially since they employed Hucho.

[villain.ind]

contrary to the pictures, i found a CFD pic of an openwheeled indy car and from my understanding vortices are desirable. Wouldnt this mean that the low pressure zone behind the truck is filling with air from the vortex thus reducing drag created by the vacuum?


edit, link to pic since pics wont display properly (could be my browser though i suppose)

http://aprperformance.com/index.php?...=172&Itemid=44

[NeilBlanchard]

Nope, you want as much still air as possible. Vortices are definitely not good.

[t vago]

Vortices are bad for ground vehicles. If you consider the rear of a moving vehicle as having a drag-inducing partial vacuum because of its travel through air, then vortices will form a stronger local vacuum at the point where the vortices meet the rear of the vehicle. This results in more drag than with the general partial vacuum.

I believe you are thinking of small vortices generated by vortex generators placed on the skin of an aerodynamic body, to help boundary layer airflow. This is rather different than the effect I am describing with my aerocap. Besides, these vortex generators do not generally give much of an aerodynamic improvement.

[TEiN]

Formula cars are about as aerodynamically "dirty" or "draggy" as you can get. Many modern designs achieve >1g of braking at 150mph by merely closing the throttle. That's a LOT of drag. They care much more about down-force than about being aerodynamically slick.

[aerohead]

Originally Posted by t vago

I may have made a mistake with the general shape of the cap.

A couple of nights ago, I was able to view the wake of my truck with the cap installed, in the rain. I noticed what appeared to be a couple of visible counter-rotating vortices right on either side of the back of the truck. From reading Hucho and looking at the pictures that Piwoslaw has in his album, it would appear the the inward-moving air stream at the sides of my aerocap are interacting with the air stream moving rearward on the sides of the truck. I think this is forming the vortices I'm seeing.



This is one of the pictures out of Piwoslaw's album.

I'm going to add material on the rear of my cap to try to straighten out the airflow. That should reduce the strength of the vortices in question. If I see some noticeable increase in FE due to the addition, then I may plan this idea into the version 3 aerocap.

It's really tough working without a wind tunnel and models,or having the really sophistcated CFD programs the big-dogs have.
Models can take as many resources as full-scale structures,but if you skip the model and go directly to full-scale;if anything ends up hinky,it takes a lot to do the 'fix'.
Typically,the slower,higher pressure air alongside the bed is going to race towards the lower pressure of the faster moving air up top.The transverse flow whips up into the horizontal tornado attached to each upper transition.
Our challenge,according to Hucho is to get these two airstreams to meet at equal velocity and just merge together.No mean feat without all the big-boy toys!
t vago,I applaud your efforts.Hope you can stomach the work ahead.We'll all benefit and we'll all be indebted.Thanks mucho!

[ChazInMT]

Yeah, what Phil sez.