The flat plate laminar boundary layer functions

that were developed by Blasius can be used to find a number of different characteristics

of a boundary layer. In this screencast, I’ll show you how we use this table to find the

boundary layer thickness at any point along a flat plate and demonstrate it with an example.

By definition, the thickness of the boundary layer, which we call del, is the value of

Y where the velocity in the boundary, u, is 99%, or 0.99, of the free stream velocity.

In other words, U divided by U infinity equals 0.99. So let’s look at this table here, and

if you notice, between these two numbers is 0.99, which means that our eta is going to

equal 5.0. That means that 5.0, if we look at what this column means, equals del, because

that’s where our y is, times the square root of our free stream velocity divided by the

kinematic viscosity of the fluid times x, where x is the length along the plate. Let’s

write this in terms of del. So we’re going to multiply the top and bottom of this by

x. So we get 5.0 times x, divided by the square root of U infinity times x squared, divided

by our kinematic viscosity times x, or we can write this as 5.0 times x divided by the

square root of (U infinity)(x) divided by the kinematic viscosity. And hopefully we

recognize this quantity in the denominator under the square root as the Reynolds Number.

That means, at any x, if we know the fluid property, in particular the kinematic viscosity

and the free stream velocity, we can find the boundary layer thickness, or del. Let’s

look at an example. Let’s say we have water – it’s at 300 K – and our free stream velocity

is 2 m/s. And what we want to do is find the boundary layer thickness at different points

along the plate. So the fist thing that we’re going to need is the kinematic viscosity of

water at 300 K, and that’s equal to 8.6 x 10^-4 m^2/s. Next we find the Reynolds Number,

which is defined as the free stream velocity times x, divided by our kinematic viscosity.

Our Reynolds Number in terms of x equals 2326 m^-1 times x. So then when we put that into

our equation that we found up here for del, we end up with del, which equals 5.0 times

x divided by the square root of the Reynolds Number, or 2326 square root of x, and this

equals 0.104x to the 1/2. Let’s look at this at different points on the plate. So let’s

say we look at 10 mm down our flat plate. Our boundary layer thickness is going to be

0.0104 m. We look at 100 mm and our boundary layer is 0.033 m. And as we expect, as we

go down further on the plate, our boundary layer thickness is gong to increase and in

fact, at 1 m, our boundary layer thickness is 0.104 m. In fact, if we look at a plot

of the boundary layer thickness as we go along the plate, we see the tendency for this thickness

to increase sharply at the beginning, and then plateauing as we continue along the plate.