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Boundary Layer Separation

Updated: Apr 20

Why and how they get separated?


Table of Contents

  1. Introduction

  2. Equation of Continuity & Bernoulli's Principle

  3. Boundary Layer Intact

  4. Boundary Layer Separation

  5. At the point of Separation

  6. Definition

  7. Summary

  8. Short Video

Isn't it sad that even boundary layers get separated from the solid body? LOL!

This is a continuation of the Boundary Layer Topic, which is the separation of the boundary layer. Let's learn about why, when, and how does boundary a layer separate. As usual, let's understand few basics before actually jumping into the topic.

Equation of Continuity:

Continuity equation states that the product of cross-sectional area and the velocity is constant through-out the fluid flow.

Additionally, by Bernoulli's Principle Pressure and velocity are inversely proportional. These two equations combined to give a very important conclusion that is: as area increases, pressure increases, and velocity decreases.

Let's consider this figure below:

The points A, B, C, D, and E are lying on a curved plate. The X-axis lies on this plate, and similarly, Y-axis lies perpendicular to the plate. Let's assume, that air (or any fluid) is flowing over the curved plate. When the fluid is at position (0, 0), i.e. at the starting point at the plate, the boundary layer starts to form. As you can see in the figure above, the area of the plate decreases initially then goes on increasing.


Boundary Layer Intact:

The area of the solid plate is decreasing through points A, B, up to C. Initially when the flowing air touches the tip of the solid plate, a Boundary layer is formed. As the area decreases in this section the velocity increases. The fluid flow velocity goes on increasing till point C. Also at the same time, the pressure goes on decreasing. Fluid always flows from high to low pressure. The fluid flows from high pressure to low pressure from point A, B to C.

Boundary Layer Intact
Boundary Layer Intact

In this section, the boundary layer is in contact with the solid plate. The boundary layer is intact.


Boundary Layer Separation:

The area of the solid plate is increasing through points C, D, and E. As the area increases, the velocity of the fluid decreases. At the same time, the pressure goes on increasing. But, we do know that fluid always flows from high to low pressure. So in this case the opposite is happening. You can imagine it easily as, there is some sort of vacuum formed at point C, which will make it easier to imagine.

Boundary Layer Separated
Boundary Layer Separated

At the point of Separation of Boundary Layer:

As you can see in the figure, at point C, the Boundary Layer got separated with respect to the curved plate. As we have seen before, after point C, the velocity got decreased as the area of the plate increased. At point C, the velocity got reduced to zero (0).

Beyond point C, the velocity goes on reducing and becomes more and more negative. That means the direction of fluid flow reverses. The thickness of the separation to goes on increasing. This is how the boundary layer is separated!

The boundary layer separation phenomenon is seen when the pressure in the fluid starts to increase after reaching the highest point, due to reduction in velocity and increase in the pressure.

So, is this separation desirable? You guessed it right! No! It's not desirable, because the negative velocity formed is pushing getting pushed in the opposite direction. Imagine, you are driving a car and the boundary layer that is formed got separated. As the fluid will change its flow direction, the velocity of the car will decrease or it may also have a skidding effect.

How to avoid the separation of boundary? There are different techniques to avoid separation. Like out the part before the point of separation occurs, or adding a few slots or holes.

Summary:

  1. Continuity equation states that the product of cross-sectional area and the velocity is constant throughout the fluid flow.

  2. By Bernoulli's Principle, Pressure and velocity are inversely proportional.

  3. As area increases, pressure increases, and velocity decreases.

  4. Beyond the point of separation, the velocity goes on reducing and becomes more and more negative. This means the direction of fluid flow reverses.

  5. The boundary layer separation phenomenon is seen when the pressure in the fluid starts to increase after reaching the highest point, due to a reduction in velocity and increase in the pressure.

Do follow our page to know how to avoid this separation and their solutions! Do comment if you want to know what exactly you want to know about the same!

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