# How does a shear layer make vortices?

How does a shear layer make vortices? If you have 2 streams of air that are moving at different speeds or opposite directions right beside each other, how would that make a vortex? What causes the rotation, in other words.

A shear layer is no different than a boundary layer. Except both sides are a fluid vs. fluid/solid. And usually we choose a reference frame that has the solid frame at zero velocity -- and frequently, both sides of a shear layer have non-zero velocity (but we could choose a frame such that one side is zero and it would be the same flow).

Put your hands together flat like you are clapping. Move your hands -- they can only move in the same direction (or they pass through each other, or you have a void between them, which we disallow). If they move at different velocities, there is shear (friction) between them. The fast one tries to speed up the slow one - and the slow one tries to slow down the fast one. However, if they move the same speed, there is no friction.

Think about an element (tiny square) of fluid. Pressure acts perpendicular to the faces of the square. Shear acts parallel to the faces of the square.

Shear force is proportional to a change in velocity -- if velocity of two streamlines is the same, there is no shear between them. No viscous force.

Shear in a fluid causes vorticity.

You should check out these videos. In particular, watch this one.

• oh okay, thanks for your help! So as you said, shear force acts parallel. If it acts parallel, what force is causing the rotation/curve needed for a vortex? Sorry if I'm not picking up on something obvious. Also, interesting video! I find it interesting how such an old video can still be straight forward and helpful. Mar 10 at 20:25
• The Shapiro videos are great. Some of the best discussions of vorticity and other phenomena. There are two on Vorticity, (I linked to the first), watch them both. The shear forces cause the fluid element to deform. Rotation is a part of that deformation. This is similar to solid mechanics and should be covered in any textbook. Mar 10 at 22:13
• Ah okay. I watched the first one, and if you look at time 12:50, you'll see a little sketch he did. Is the sideways x denoting the flow encountering a disturbance and turning into a vortex? It goes from a + to a sideways x, then to a vortex. I assumed the sideways x was when the flow encountered something which then caused a rotation. Mar 11 at 0:24
• I believe the X is what happens to the flow marked + when it encounters a vortex. Mar 11 at 5:32
• That is pretty much it. Mar 12 at 0:13