Pressure drag and friction drag

Anna Tian · April 4, 2014, 10:16

Hi,

I'm wondering, for external flow, if there is no boundary layer separation happening, will pressure drag still exist?

Let's take airfoil as an example. If there is no boundary layer separation, will all airfoil drag be from friction drag?

lovecraft22 · April 5, 2014, 19:05

A good part of your airfoil drag will be from the viscous component, yes.

As for the pressure component, you'll still have it regardless of your flow being separated or not as you'll still have a stagnation point on the leading edge which generates pressure drag along with the pressure gradient along your airfoil.

In theory, only a flat plate with no thickness will have only viscous drag.

Also, potential flow will result in no pressure drag (or no drag at all) for some bodies (as a 2D cylinder).

Anna Tian · April 6, 2014, 00:32

Quote:

Originally Posted by lovecraft22

A good part of your airfoil drag will be from the viscous component, yes.

As for the pressure component, you'll still have it regardless of your flow being separated or not as you'll still have a stagnation point on the leading edge which generates pressure drag along with the pressure gradient along your airfoil.

In theory, only a flat plate with no thickness will have only viscous drag.

Also, potential flow will result in no pressure drag (or no drag at all) for some bodies (as a 2D cylinder).

What do you mean by good part? Is that over 50% or not? Let's take airfoil NACA2412 as an example, at the AoA of 0 degree, will friction drag be the main part of the whole drag?

Joachim · April 6, 2014, 10:43

You should have at least 90% viscous drag at 0 deg for an airfoil I guess (that will depend on the airfoil though).

If you really want to go in the details, your boundary layer growing on the airfoil will change the effective change of the airfoil (boundary layer displacement thickness), so that the Kutta condition will not be exactly satisfied. That typically results in some small additional pressure drag, even at low angles of attack.

If the wing is finite, you will also have some lift induced drag, which is basically pressure drag (influence of the tip vortex). This will depend mostly on the aspect ratio of your wing.

April 4, 2014, 10:16	Pressure drag and friction drag	#1
Anna Tian Senior Member Meimei Wang Join Date: Jul 2012 Posts: 494 Rep Power: 16	Hi, I'm wondering, for external flow, if there is no boundary layer separation happening, will pressure drag still exist? Let's take airfoil as an example. If there is no boundary layer separation, will all airfoil drag be from friction drag? __________________ Best regards, Meimei

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April 5, 2014, 19:05		#2
lovecraft22 Senior Member lore Join Date: Mar 2010 Location: Italy Posts: 460 Rep Power: 18	A good part of your airfoil drag will be from the viscous component, yes. As for the pressure component, you'll still have it regardless of your flow being separated or not as you'll still have a stagnation point on the leading edge which generates pressure drag along with the pressure gradient along your airfoil. In theory, only a flat plate with no thickness will have only viscous drag. Also, potential flow will result in no pressure drag (or no drag at all) for some bodies (as a 2D cylinder).

April 6, 2014, 10:43		#4
Joachim Senior Member Joachim Join Date: Mar 2012 Location: Paris, France Posts: 145 Rep Power: 15	You should have at least 90% viscous drag at 0 deg for an airfoil I guess (that will depend on the airfoil though). If you really want to go in the details, your boundary layer growing on the airfoil will change the effective change of the airfoil (boundary layer displacement thickness), so that the Kutta condition will not be exactly satisfied. That typically results in some small additional pressure drag, even at low angles of attack. If the wing is finite, you will also have some lift induced drag, which is basically pressure drag (influence of the tip vortex). This will depend mostly on the aspect ratio of your wing.