Wind tunnels, wakes, and low pressure

37:32tunnel tests why this is happening. So, this is at NASA Ames Research Center. NASA loves aerodynamically testing these balls. So, here you've got a soccer ball in a wind tunnel. The air is coming in from the left, and as you're spinning, what's happening is your laminar flow of the really nice air is getting broken up at the surface of the ball, and it's creating turbulent flow behind it, right? >> That has a very weird effect on the drag. >> Mhm. >> Okay? Here's what's going to happen. The wake behind that ball is going to create a region of low pressure. You can imagine like here the ball is stationary and the wind is going through, right? But in real soccer play,

38:13the ball is going to be moving through like in towards the right in this direction, right? Um now, on the right-hand side, the the ball is hitting the air particles, so there's high pressure, right? Because it's like a piston that's like sort of compressing air. And on the behind it, there's going to be low pressure. So, there's going to be a force backwards that is not just the particles hitting, but there's like a pressure difference. >> Yeah, yeah, yeah. >> And that's going to cause some weird stuff. >> Okay, okay. >> Okay? >> Yep. >> And it turns out that the faster you move, that that low pressure effect becomes less and less. >> Interesting. >> Which is a little bit non-trivial. So, let's go to the next

38:54photo, which is photo number 20. Yeah, here's what we're doing. The ball is moving to the right hand side, okay? On the left panel, the ball is not moving that fast. When the ball is not moving that fast, the wake is actually a lot bigger. So, the region of low pressure is a lot more, which means that pressure difference is bigger. And if the pressure difference is bigger, the drag force is going to be bigger. There's going to be a bigger contribution due to that drag pressure difference. On the right hand side, now you've got the ball moving faster. When you go faster, the laminar flow disconnects from the ball faster. And when that happens, you can actually like flow through the ball.

39:35Like you can you can follow the curve of the ball more, which is a little bit non-intuitive. Like the faster you move, the more the laminar flow hugs the ball. And so, your low pressure region is actually smaller, meaning that the pressure difference is not that much, which means your drag has actually gone down. The faster you move, the drag actually goes down. >> This makes me think of all the science to F1 cars, which deal with a variety of concepts very heavily but >> That This is a big part of it. You want to You want to make that low pressure region behind you not that big, so that you have like a lot >> Which is why the cars are designed the way they are. >> Yeah. And so, the wake can be different at different speeds, right? And if you go to um

40:16the photo number 21,