Measuring Aerodynamic Pressures
A few weeks ago, I covered the benefits of tuft testing, how to do it and what it can show you about the airflow over your car. You may recall that tufts align themselves to the velocity vector wherever you attach them to a surface. Now we’ll look at pressure testing, which is the other side of that coin since the behaviors of pressure and velocity in a flow are related.
Background and Theory To understand how aerodynamic pressures change, we first
need to understand what a frame of reference is. As you push a car
through air, the car moves relative to the air—moving car, stationary air. In a
wind tunnel, rather than pushing the car through the air, the air is pushed
past the car—moving air, stationary car. However, both of these describe the
same situation: relative motion between car and air. Which one you imagine
“moving” depends on your frame of reference. It's easiest for most people to think of air moving past a
car rather than the other way around. Using the car as the frame of reference,
the air flows backward past it at the same speed as the car drives forward;
this is called the free stream velocity. However, the air’s speed varies
locally from the free stream velocity due to the shape of the car, so that at
any point on the body the speed there may be different from the overall free stream
speed. For example, over the top of the windshield and onto the roof local flow
velocity is usually much higher than free stream velocity:
|As the air bends around the windshield/roof, it speeds up. On cars with a less gradual transition, it can separate here.
|The stagnation point is the spot where all the kinetic energy of the oncoming flow is converted to pressure, i.e. it stops moving. Grill openings are often positioned here to take advantage of that high pressure.
So, the velocity of the air changes over the car. Now, what does this have to do with pressure?
|If you use a Dwyer Magnehelic such as this one, get one that will display both negative and positive pressure—it will be more useful than this one which requires you to predict the sign so the tubes can be connected to the correct port.
|You can see the disk taped in place on the door. Try to position the tubing so it disrupts airflow as little as possible.
|Pressure difference between top and bottom sides, using the bottom as reference, in inches of water.
More recently, after seeing many cars and trucks at auto shows with engine bays vented to the front wheel housings I decided to investigate them on my car.
|These have appeared on a variety of cars, from sedans such as this 2019 Lexus LS500 to body-on-frame SUVs like the Toyota Land Cruiser.
I taped one pressure disk to the inside of the wheel housing, facing the tire, and one to the corresponding spot on the outside of the same panel, in the engine bay. The bay side was 80 Pa higher, which showed that a vent in this location would move air in the direction I wanted, from higher pressure to lower, out of the engine bay. So I installed these: