Pressure drag and the Reynolds number

[LT72884]

Ok, I am kind of lost in my fluid dynamics course on a specific topic. I understand the reynolds number and how it works. What i do not get::

if Re = VD/v, and if we increase V(velocity) which in turn will increase the inertial effects of the number, why is it called a pressure drag? pressure DECREASES with velocity, hence stagnation points on an airfoil...

thanks

 

[sophiecentaur]

why is it called a pressure drag

You really should avoid getting hung up on the terms you can find in Science. They are often based on historical use and we can't always justify them. It would be worse if those terms were suddenly to change to something more 'acceptable'. You'd have to check on the publishing date of any textbook before you read it!

If you understand the equation and the variables involved then just treat the names as nonsense words. Over history, there have been struggles with what to call things and the 'true nature' of quantities. Look at Momentum and Kinetic Energy; for a long time they were regarded as 'the same thing' with two different definitions.

Having said all that, I realize that many terms are perfectly matched to what they describe - so you just have to learn the rules of the game, I'm afraid.

 

[russ_watters]

why is it called a pressure drag? pressure DECREASES with velocity, hence stagnation points on an airfoil...

That's only true in the context of static pressure in bernoullis equation. You don't think higher velocity means lower stagnation pressure, do you?

 

[boneh3ad]

There are two ways a fluid can exert a force on the surface of an airfoil: through pressure (i.e. a force normal to the surface) and through viscosity (i.e. due to shear stress). Pressure drag is a component resulting from the pressure acting on the surface, hence the name pressure drag. Viscous drag or skin friction drag would be the other.

I should note that these two are not entirely decoupled. The viscous behavior of the fluid moving around an airfoil can directly alter the pressure drag, e.g. if separation occurs.

I have no idea why Reynolds number was brought up here. It is not in any way related to the discussion of this terminology. I would surmise that @LT72884 does not, in fact, understand the Reynolds number and "how it works."

 

[cjl]

Ok, I am kind of lost in my fluid dynamics course on a specific topic. I understand the reynolds number and how it works. What i do not get::

if Re = VD/v, and if we increase V(velocity) which in turn will increase the inertial effects of the number, why is it called a pressure drag? pressure DECREASES with velocity, hence stagnation points on an airfoil...

thanks

A couple things here...

First off, as boneh3ad stated, it's called pressure drag because the way the fluid is interacting with the object is through normal forces to the object's surface, otherwise known as pressure. Viscous drag similarly is named because it originates from forces parallel to the interacting surface, via viscosity in the boundary layer.

I am a bit worried about this statement though:

pressure DECREASES with velocity, hence stagnation points on an airfoil...

This shows a bit of a misunderstanding here. It seems like the reason you're confused is that you (correctly) know that pressure drag (along with viscous drag) increases with higher velocity through the fluid. You have also heard the bernoulli relation, which states that pressure drops with velocity, but it's important to note that the bernoulli relation only actually applies along a streamline in a given flow. It is not generally true that faster flows are lower pressure, and in fact, in a high velocity flow causing greater drag, the total pressure of the flow (and thus the pressure at the stagnation point) is higher than it is in a slower flow, since in a typical example, the static pressure of the flow is held constant while comparing the effects of different velocities.

Bernoulli is a very useful relation, but remember that at it's core, it's a statement of conservation of energy. The pressure decreases as velocity increases because the energy to speed the flow up had to come from somewhere, but this only applies if the same bit of fluid was formerly moving slower and now had to speed up with no energy addition. If the fluid flows through a fan? Bernoulli doesn't apply, since energy was added. Want to compare the air pressure on the outside of your car window where the air is going by at 60mph to the pressure on the inside where it's still (relative to the car)? Bernoulli doesn't apply, since there's no reason to believe the air outside the car and the air inside the car have the same energy per unit mass. Want to compare the pressure just above a wing to the ambient pressure well in front of the wing? Now you can use Bernoulli, since the same air goes from freestream well in front of the wing to the conditions next to the wing with no addition or loss of energy (I'm assuming we're outside of the boundary layer here).

 

[cjl]

That's only true in the context of static pressure in bernoullis equation. You don't think higher velocity means lower stagnation pressure, do you?

More importantly, it's only true along a streamline, where the stagnation pressure is constant no matter what the velocity is (I'm assuming the standard simplifying assumptions apply here). When comparing two entirely separate scenarios, Bernoulli can't say anything about static, dynamic, or total pressure, since you can define your new flow however you'd like.