Flow Separation Control

What is Flow Separation?

Flow separation is when the boundary layer flowing over the surface of an object detaches (i.e. no longer “follows” the surface of the object) and forms into a “wake”

Boundary Layer = Layer within a fluid near the surface of an object where effects of viscosity are significant. See image below

Flow separation can be visualized in a manner similar to the image below:

The region highlighted by the red circle is where flow separation is occuring, here you can see the “wake” that is being formed

How can Flow Separation be a Bad Thing?

In the region where flow separation has occurred (i.e. where the wake has been formed) this becomes a relatively low pressure region. However, the region of fluid in front of the object will be a higher pressure region with respect to this.

Given that the force exerted by the fluid onto our object is proportional to the pressure at each specific region, we can identify that the Frontal Pressure Force will be greater than the Rear Pressure Force. Adding these two opposing forces up (or really subtracting given that they are in opposite directions) we get the resultant Pressure Drag Force that will be in the same direction as the Frontal Pressure Force. If we use the image below, this force will be pointing Left to Right if our object were travelling Right to Left. Therefore this Pressure Drag Force opposes the direction of motion and will force us to utilize more energy from the battery pack to maintain the same speed and thus decrease our overall range / efficiency.

How can we Mitigate Flow Separation?

Geometry

The most intuitive method to mitigating flow separation is to change the geometry of the object such that it allows for the boundary layer to “continue to follow it” and thus prevent separation. A more technical definition would be to have the geometry of the object be shaped such that it does not cause a pressure differential between the front and the rear where the front has a higher pressure with respect to the rear.

This is the reason why many so called “aerodynamic” shapes are tear-drop shaped (see examples below)

Skin Friction Drag Consideration

With the above information, it may be tempting to create an extremely streamlined tear-drop shape in an attempt to reduce all pressure drag. However, pressure drag is not the only type of drag force, there is another (yoda meme lol) called “Skin Friction Drag”.

Skin Friction Drag is the drag caused by the motion of a fluid along an object’s surface and it is proportional to the area of the object in contact with the fluid during motion. Thus, you can see that as you create more streamlined shapes, you increase this contact area and thus increase the skin friction drag. Therefore it is imperative that a balance between Pressure Drag and Skin Friction Drag is achieved so that the Total Drag is as small as possible.

Boundary Layer Type

Another method to mitigating flow separation is to change the type of Boundary Layer we have. Generally fluids can be classified as being “Laminar” or “Turbulent”. We won’t go into great detail into the definition of this here but below is a quick brief of what they are:

Laminar Flow: When particles within a fluid move in smooth paths in distinct layers. There is little to no mixing between these layers

A good analogy some people have used is imagine the fluid is a stack of papers, each piece of paper can slide along each other. The particles within that fluid “belong to a piece of paper” and can move along that piece of paper but cannot “jump” and move to another piece of paper.

Turbulent Flow: It’s chaos lmao. The fluid does not move in distinct layers and there is mixing happening throughout the fluid.

If we have a “Turbulent” boundary layer, we can actually have less flow separation than if we had a “Laminar” boundary layer. A real life example of this would be golf balls, the dimples they have on the surface are to create a Turbulent boundary layer, thus mitigating flow separation, decreasing pressure drag and therefore increasing the range the golf ball can travel. See the image below: