# Pressure Drag

So what is pressure drag? In order to understand

that. We have to understand what drag itself is. This occurs when a body is moving through

a fluid, and when we have drag. We assume that there is not only pressure forces, but

there are viscous forces as well. So the forces that oppose movement. Are what is known as

drag, and we have 2 main components of drag. We have friction drag, which is due to a tangential

force we call tau. We also have pressure drag, and pressure drag is due to the pressure on

the object and it is a normal force. So lets take a look at some examples. If we consider

a flat plate, and we are going to incline this plate, and you have some velocity of

the fluid. Normal to the plate you have your pressure forces. Tangential to the plate you

have your friction forces. So lets take a look at a couple of other situations. What

would happen if you had a plate that was parallel to the direction of the flow. Well there you

will only have tangential forces. We are considering this a template. So there will be no normal

forces. The opposite situation is when you have a thin plate that is perpendicular to

the flow. So again we have a u going this way, and now we only have normal forces or

pressure forces. If you look at a sphere for example, and you look at flow pass the sphere.

So here we have a sphere and we have some velocity of a fluid coming in. You will see

that we not only have normal forces that are pressure. Up here we also have these tangential

forces, which are our friction drag. So as you can see by looking at this the greater

the area of the forces that are acted on. In other words the bigger the object the greater

the drag that is on this. So why would be interested in drag. Well it has a great impact

on things such as acceleration, because it is drag that causes the deceleration of a

drag car, or if you have parachute it is the deceleration of this parachute. That makes

sure you hit the ground at a safe speed. So lets take a look at the pressure drag. So

when we are look at the pressure drag. What we are really look at are these forces that

are normal. So this drag is the integral of the pressure times its position in another

words the angle that it hits at. So its orientation times our differential area, and we define

a dimensionless coefficient of pressure drag as this drag, and here we go d of p because

it is the pressure of drag over 1/2 times rho times u^2 divided by the area. You can

see that this coefficient of drag depends on area and velocity. If we rewrite this,

it is this integral times its pressure times the cosine of theta, da divided by our 1/2,

rho, u^2,A and we let this equal our Cp, which is our coefficient of pressure times the cosine

of theta, da, which is in the integral divided by area, where our Cp equals p- some reference

pressure, divided by rho, u^2, divided by 2.This pressure coefficient is actually a

dimensionless form of the pressure. So you might want to know. What about this reference

pressure? Well the level doesn’t influence the drag directly because what you are really

looking for is the net pressure force on the body, and that is 0 if the pressure is constant

on the entire surface. So what happens if the inertial forces are large. What that means

is that we have a large Re number. If that is the case our pressure coefficient is independent

of the Re. However if we have viscous forces that are large. Now our Re number cannot be

neglected, and the drag coefficient is proportional to 1 over Re. What if the viscosity were 0.

Then you would have no pressure drag in a steady flow at all. There would be large pressure

forces on the front portion, but you would have the opposite and equally large forces

on the rear.

how would you describe the drag if a fluid was flowing over a stationary circular rod? Does drag always happen whenever there is relative motion between the fluid and the solid?

Thanks for the quick response! In my problem, the velocities are so small (of the order 10E-5 ms-1) so is it reasonable to suggest that the effects of both friction drag and pressure drag are sufficiently small that they can be neglected? My problem is basically a flow of a fluid between two plates with two circular rods inserted. The fluid flows because of an imposed pressure difference and I have to analyse the forces affecting the fluid during its flow. I'm having difficulties describing the flow as the fluid encounters the circular rod (where I imagine the friction drag will predominate). I have used a software to numerically compute the velocities at the outlet but describing the theory between the inlet and outlet is proving a bit stubborn…Thanks for the help in advance!

isn't it (Drag coefficient) = Total Drag/(0.5*c*u*u*A) instead of it being (Pressure Drag coefficient) = Pressure Drag/(0.5*c*u*u*A). Or am I missing something here

Pressure drag means Form drag

The value of pressure when plate is held parallel to flow of water is minimum or zero???