I've been looking at passive solar collector designs on the web that use columns of soda cans as the collectors. Most videos show the cans have a large hole cut on each end. A few show a large hole cut at one end of the can. At the other end, a small hole is cut and then slits are cut radiating outward from the hole. The fins formed by these slits are bent to direct the air in a swirling pattern.

Are there empirical or engineering guidelines about which way of cutting the cans is best?