# Understanding Pressure Generated By Flow Separation

## Main Question or Discussion Point

Hi!

I'm working on a high Reynold's number model, considering flow over an airfoil with a high angle of attack. I know I want to minimize drag. I'm told that flow separation creates pressure drag---relatively low pressure in the wake of my foil.

Q1: Why would flow separation cause lower pressure downstream (relative to the pressure on the upstream surface of an object)?​

Sometimes flow separation causes the fluid to stagnate in the wake, or even reverse in direction.

Q2: Why does flow separation cause an adverse pressure gradient and sometimes a reverse in the direction of flow?​

Many thanks.

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Simon Bridge
Homework Helper
You know about turbulent flow right?
It sounds like you are asking how turbulence happens.

Maybe? If you can tell me how turbulent flow answers my questions, I'll be satifisfied.

Simon Bridge
Homework Helper
You already know how the separation of the flows over the airfoil can create a pressure difference on the different sides - it's the common description of how an airfoil works after all. The trailing edge also redirects the air-flow on both sides. Now - what happens when two, different-speed, different pressure, streams of fluid meet?

Actually could you go into more detail about the pressure difference (that's the heart of my question)? I understand that an airfoil creates a pressure difference above and below the wing. I accept hand-wavy explanations of this with Bernoulli's principle.

However, why does flow separation (specifically) create a pressure difference between the 'front' and the 'back' of the foil?

If you don't know that's okay. I appreciate your help.

PS I do mean http://en.wikipedia.org/wiki/Flow_separation. Not just air flowing in different directions around a foil.

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Simon Bridge
Homework Helper
I am a little uncomfortable with attributing all this directly to "flow separation" - it's a little glib.

iirc: turbulent flow is lower pressure than laminar flow.
The turbulence is caused, in the airfoil case, by having two fluid streams.
You get more turbulence at the trailing edge than the leading edge.

If you hold a flat plane side on to the air flow, the pressure in front of the plane will be higher than behind it right? Same at intermediate angles.

You should google for turbulent flow wrt airfoils.
eg. http://www.aviation-history.com/theory/lam-flow.htm

Gold Member
This phenomenon really doesn't have anything directly to do with turbulence.

Consider first a simple cylinder. You probably know that for inviscid flow, there is predicted to be zero drag on the cylinder since the flow essentially regroups after passing oer the culinder and leaves exactly the same as it came. No work was done on the fluid and thus none on the cylinder. The pressure forces are similarly in balance.

Now throw viscosity into the mix. If the Reynolds number is so low that no separation occurs, then aside from a small skin friction drag, the situation is very similar to that of the inviscid case. However, most cases are much higher Reynolds number an involve separation. In that case, the boundary layer separates from the surface and creates a wry large wake (the so-called Kármán vortex street). The pressure around the downstream side of the cylinder remains nearly constant at the value at the separation point, so is much lower to the stagnation pressure on the front side, leading to massive pressure drag.

An airfoil is similar. At high enough angle of attack, the airfoil ceases being a nice, streamlined shape and the flow separation causes it to resemble an eccentrically-shaped cylinder. The pressure drag then arises the same way.

As for your second question, flow separation doesn't cause reversed flow, but rather the adverse pressure gradient that leads to separation also leads to reversed flow. In essence, the pressure gradient represents a force pushing against the flow. Eventually it pushes enough that the shear stress at the wall is zero and the boundary layer separates. The shape of the body still creates an adverse pressure gradient so the separated region can be pushed backwards (locally) by this same gradient that led to separation in the first place.

I understand that an airfoil creates a pressure difference above and below the wing. I accept hand-wavy explanations of this with Bernoulli's principle.
It's good you question the "Bernoulli explanation" because it's wrong and doesn't really explain anything. You need to understand the Navier Stokes equations and specifically how flow curvature effects the pressure before you can really understand how lift is created. But that is not the topic of this thread so I will leave it at that.

iirc: turbulent flow is lower pressure than laminar flow.
The turbulence is caused, in the airfoil case, by having two fluid streams.
Both of these statements are wrong. Why would a turbulent flow have to be at a lower pressure? That doesn't make any sense. On a flat plate the mean pressure is the same regardless of whether or not the boundary layer is laminar or turbulent. A turbulent boundary layer can affect the pressure distribution on an airfoil but that's only because it can affect the amount of separation as well as the effective shape of the body.

And turbulence has nothing to do with the fact that there are two fluid streams on the airfoil. It is due to instabilities in the boundary layer.