Well... actually, not really. The reason people talk about attached and separated flow is because of Bernoulli's Principle. Separated flow does not apply to it outside of the separated area.
It states that ..::IMPORTANT::.. in a
streamline, faster flow has a lower pressure than slower flow. Here is the equation to further analyze what is happening:
P1 + .5*[rho]*V1^2 = P2 + .5*[rho]*V2^2
P1 = (static) Pressure at point one in the flow
P2 = (static) Pressure at point two in the flow
rho = density of the air (sea level density is roughly 0.0023 slugs/ft^3 english units)
V1 = velocity of air at point one
V2 = velocity of air at point two
Lets examine flow on the front of the car first. The air isn't really traveling at a certain velocity but relative to the vehicle it is. Lets say we're traveling at 102.6 ft/s (70 mph), the air in front of the vehicle does two things. In the middle of the grill, the air gets pushed on by the car and is stopped, around the sides of the grill the air gets pushed away. **Check out the FloWorks model on the main page of the colorful velocity field.. the blue means stagnant or stopped air** If the atmospheric pressure that day was roughly 2116 lbf/ft^2, plugging into Bernoulli's equation we come up with what is called a Total Pressure which amounts to:
2116 lbf/ft^2 + .5*(0.0023 slug/ft^3)*(102.6 ft/s)^2 = 2128 lbf/ft^2 + 0
As the air travels up and over the hood, it is being accelerated (ignoring the stagnant pocket at the base of the windshield). Keeping the front half of this equation, we alter the velocity from 0 where it stagnates at the grill to some value higher than 102.6 ft/s.. w/o calculating it you can see that P2 must be lower to keep both sides equal.
As flow slows down (as with attached flow over the
AFT section of an airfoil or any streamlined body) the V2 value slows to close to the V1 value thus bringing P2 back to something close to P1. In a perfect world, the pressure on the back of the vehicle would be the same as the pressure in the front and there wouldn't be any drag!
Where separation comes into effect is when the air is getting decelerated too much coming around the aft section and the pressure behind the car is greater than the pressure on top of the car. Since flow goes from high to low pressure, the air doesn't want to suck down behind the vehicle and separates from the car. This 'high' pressure behind the vehicle is however, not as high as we would like and compared to the pressure on the front of the car it is low. High Pressure in the front + Low Pressure in the back = Poor Aerodynamics.
Putting a cone in the front of the vehicle minimizes the stagnant air region (Total Pressure area). This is why you see rockets, birds, even tuna with pointed front ends.
SORRY SO LONG BUT I HOPE IT HELPS!! Have a great night!
