[Moreza7]

Hello,

I was reading some papers about laminar separation bubbles over the airfoils.
As you can see in the picture, the pressure coefficient is constant along with the separation bubble (the plateau part) :


Aslo the friction coefficient has the same behavior.

Is it because the flow is approximately static in that region?

[aerosayan]

Originally Posted by Moreza7

Hello,

I was reading some papers about laminar separation bubbles over the airfoils.
As you can see in the picture, the pressure coefficient is constant along with the separation bubble (the plateau part) :

Aslo the friction coefficient has the same behavior.

Is it because the flow is approximately static in that region?

This paper ( https://www.longdom.org/open-access/...73.1000131.pdf ) states that the separation bubble is essentially a "dead air region" thus, this causes a sudden rise in Cp and since the flow is energetic enough, it reattaches downstream.


This is stated in the second paragraph.


I'm not an expert on the matter, you might want to consult others too.

[Moreza7]

Originally Posted by aerosayan

This paper ( https://www.longdom.org/open-access/...73.1000131.pdf ) states that the separation bubble is essentially a "dead air region" thus, this causes a sudden rise in Cp and since the flow is energetic enough, it reattaches downstream.


This is stated in the second paragraph.


I'm not an expert on the matter, you might want to consult others too.

Thank you.

I know the cause of the increase in pressure coefficient, but I don't know why the pressure coefficient is constant in separation bubble. I think it is related to dead air. Then what is the definition of dead air? Semi-static air, quasi-static air or what?

[aerosayan]

Originally Posted by Moreza7

Thank you.

I know the cause of the increase in pressure coefficient, but I don't know why the pressure coefficient is constant in separation bubble. I think it is related to dead air. Then what is the definition of dead air? Semi-static air, quasi-static air or what?

Dead air probably means that the fluid stops moving at the original high velocity inside the separation bubble. I don't know if the fluid stops completely, but it might be approximately taken as a flow with zero velocity.


From the wikipedia ( https://en.wikipedia.org/wiki/Pressure_coefficient ) Cp = (p-p_inf) / (p0 - p_inf)


Since the flow is said to stop/stagnate inside the separation bubble, maybe p becomes approximately equal to p0? Since p0 is the stagnation pressure.


This would return a constant Cp value even if it's not exactly equal to 1.

[FMDenaro]

Originally Posted by aerosayan

Dead air probably means that the fluid stops moving at the original high velocity inside the separation bubble. I don't know if the fluid stops completely, but it might be approximately taken as a flow with zero velocity.


From the wikipedia ( https://en.wikipedia.org/wiki/Pressure_coefficient ) Cp = (p-p_inf) / (p0 - p_inf)


Since the flow is said to stop/stagnate inside the separation bubble, maybe p becomes approximately equal to p0? Since p0 is the stagnation pressure.


This would return a constant Cp value even if it's not exactly equal to 1.

The discussion is about a constant Cp, but you cannot reach p=p0 (that would lead to Cp=1). In the bubble the viscous effects are relevant

[Gerry Kan]

Dear Moreza:

If you think about this physically, when flow separates, it creates a low pressure wake region. Due to shear interaction between the freestream flow and the outer edge of this wake region, flow in this wake region must recirculate over time. This usually means the near-wall flow in the wake will be in the opposite direction than the near-wall flow outside the wake region. Your attached figure shows this.

We know that pressure gradient must be favorable (-ve) for the flow to travel in the same direction of the freestream, prior to the wake, and adverse inside the wake region. So there must exist a point where the pressure gradient is zero, and that is the start of the wake region, also known as the separation point.

The pressure coefficient is (more or less) constant in the wake region because the pressure gradient, albeit non-zero, is very small compared to the incoming flow. And this pressure rise in the region is very small compared to the pressure drop upstream.

Gerry.

[LuckyTran]

When flow separates, it ignores the changes in the solid boundary and continues to advect along its initial trajectory. It maintains the characteristics of the flow at the point of separation and therefore continues along a constant Cp line. This is just an approximation/asymptotic behavior, there are still viscous effects and body forces at play for strong separation. You can sort of visualize this better if you replace the boundary of the separation bubble with a hypothetical solid wall, you would get the same flow field forgetting about the separation bubble.