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.