Killing Laminar Flow: Post 56

When you hear people describe flow over baseballs, there is frequent reference to the seams causing Laminar flow to become Turbulent. To start, I’d better explain what those terms mean.

Laminar flow is what Juniors in Physics or Engineering learn about in their fluid dynamics class. It is usually steady and can often be described analytically. It seldom exists in a practical circumstance unless the fluid is very viscous or the length scales (e.g. the size of the ball) is very small.

Laminar flow does not generate much skin friction, so it is desirable. But, for reasons I won’t get into here, Laminar flow is less able to remain attached to a surface. It is prone to form a wake early, and this can cause drag, which is the sum of the skin friction and wake effects, to become much larger. Golf balls are dimpled to cause the flow on that small object to be turbulent and, in turn, reduce their drag.

One example of important laminar flow that may surprise you a lot was the US Space Shuttle. Re-entry flow along the heat shield of the shuttle was laminar, and it was critical that it remained that way. Turbulent flow generates heat, and that extra heating would destroy the shuttle. You can find reference to that issue here if you search for “turbulent.” The shuttle was obviously very large, and the upper atmosphere is not very viscous, but low density also helps keep flow laminar, so there you are.

The images below were acquired using Particle Image Velocimetry. If you are new to our measurements and how to interpret them, we have a primer here.

Back to baseballs. I’ve been watching them carefully for about 18 months, and early on, we tried to develop some rules, such as the flow on the front of the ball is laminar unless it crosses a seam. Then it is turbulent. And I’ll tell you something about baseballs: they don’t follow rules very well. I want to show two very good examples.

This baseball is traveling 80 mph to the left. The separation points (where the blue streak leaves the top of the ball and the red streak leaves the bottom) indicate turbulent flow since they are on the rear of the ball and are messy. But, on the center plane of the ball where this picture was acquired, a seam is never encountered on the front of the ball. We find that this result is quite consistent for this configuration.

In general, it is hard to find a laminar boundary layer separation from a baseball at relevant speeds. The only time we see them consistently is when there is a seam just past the top of the ball. Like this:

In this case, the flow over the top of the ball DOES encounter a seam (although near the nose where the pressure gradients are forgiving) and then separates on the FRONT of the ball while laminar. Notice how tight the blue streak is.

This is one reason why the Looper works well. The seam causes separation even when it is on the back.

Looking at the raw PIV data for this case, one can easily see the laminar separation and the strong shear layer that it forms. PIV processing smears that line out quite a bit, but it is very thin.

Looking back at a previous post, we’ve shown this before at much higher resolution. It’s not in the picture, but like any MLB ball, there is a seam just to the right of that logo. We are not entirely sure why the downstream seam promotes laminar separation.

So, laminar separation can happen. But it’s rare. And I think baseball people talk about it more than they should.