 So, the Reynolds number varies within a flow. If you have the same fluid, for example, water, the density and viscosity remain constant, and so those aren't likely to flow, those don't change within, say, for example, a channel. However, the flow speed and the flow depth vary within a water flow, both as the geometry of the flow changes, but also with depth into the flow. So what I want to talk about in this video is a boundary layer that is present at the edge of any water flow, or ice flow or air flow, for that matter. So if we look at a water, we're going to have the flow generally to the right, and then we have our solid surface down at the bottom, and this will be our water here. One of the things that happens at the base of the flow, if we look at this right here, there's friction between the surface and the solid, and that's because the water molecules flowing or interacting with the bottom of the flow, which is stationary, and you have again those hydrogen bonds create a boundary layer. And so right at this solid surface here, the flow speed right above it is zero. And so our flow speed is u, so right here, u equals zero because of that friction. But the rest of the bulk of the water is still flowing downstream. So what we can do is we can make a plot of the flow speed versus water. So I've now moved my diagram over so I can plot flow speed with depth in the flow. So this is our flow, it goes to zero, and then there'll be some average flow. I'm going to say u bar, so u bar equals the average flow speed. And then this is our flow depth, or the height above, this will be the height above the surface here. So this is, and I'm going to call this variable z. And at the top of the flow here, it goes, the height above the flow surface goes from zero to L. So this L is our flow depth for the Reynolds number, and that's equal to the surface of the flow. So we talked about the friction, so the flow speed right at the bottom here is zero, and I will use a different color. And then most of the flow is going at about the average flow speed. So most of the flow is going up like this. So we have a flow here that's going fast and here that's going zero. So what happens is this flow is going slowly, it exerts a little bit, or it's not moving at all, it exerts a little bit of friction on the flow that's above it, that exerts a little friction on the flow above that. And then as you come down, the fast flow here is pulling the water below it downstream. So the flow goes a little bit higher and it increases until you hit the average flow speed here. So if we think about it in terms of the Reynolds number, if we look at where the flow speed is zero right at the surface here, our density of viscosity are the same, and so we have our Reynolds number equals a flow speed of zero times our density over the viscosity, these don't matter, so our Reynolds number is actually zero. So if we look up here, our Reynolds number is the average flow speed, or u bar, times the flow depth, times the density divided by the viscosity up in this zone here. So then there's this transition from the Reynolds number of zero, and if the Reynolds number is less than 500, it's a laminar flow. So we have this zone in here that's laminar, once the flow is just infinitesimally above zero, and this for water is almost always turbulent. So we have a turbulent flow here, and then in the middle here somewhere we go from a laminar flow to a turbulent flow. And so what we do is we define the flow speed here, depending on the flow depth where the Reynolds number is 500. Everything below, with a z lower than this point, has a laminar flow. And then in between the two, so this is the zone where Reynolds number is less than 500. So in this zone here, you then have an interval where the flow speed is influenced by the bottom of the surface. And so we have different names for these three components. So this one where the Reynolds number is zero is called, because the flow is laminar, it's called the laminar sublayer. And then this whole zone that's influenced by the boundary in here is called the boundary layer. So the laminar sublayer is part of the boundary layer because it's influenced by this bottom part. Then you have the main flow up here. So in summary, what we have is a solid surface that creates friction. That friction slows down the flow that's in touch with it. That friction is long as transmitted upward in the flow to the top of the boundary layer. And then you have the rest of the flow going at an average flow speed downstream. The laminar sublayer is this part at the bottom where the Reynolds number is less than 500, and that's why it's called the laminar flow. It's sometimes also called the viscous sublayer, and that's because the force of viscosity has a larger influence on the behavior of the fluid than the kinetic forces related to the flow speed. So we're going to use the term laminar sublayer, but if you want you can also use viscous sublayer for that. Thanks for watching.