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.

 
…while at the very front of the car local flow velocity is much lower than free stream velocity—close to zero (relative to the car), in fact!

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?
 
Well, velocity and pressure are negatively correlated; that is, as one goes up, the other goes down and vice versa. So, the change in velocity somewhere on the car will change the pressure at that spot, and we can measure that pressure pretty easily on the road.
 
Pressure Testing Toolkit
 
To do this, you will need some small-diameter tubing (available in the plumbing section of most hardware stores), a manometer with two ports that can measure differences in pressure, a pitot tube and tall pole and a way to mount it at the front of your car so it isn’t subject to interference from the body, and at least one pressure sensing disk. You can use either a mechanical or electronic manometer; I have both a Dwyer Magnehelic (mechanical) and Perfect Prime (electronic).

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.

Disks are available commercially, from Scanivalve in Liberty Lake, WA, and from the Automotive Research Center in Indianapolis, IN. You can also find out how to make your own in Modifying the Aerodynamics of Your Road Car by Julian Edgar.

You can see the disk taped in place on the door. Try to position the tubing so it disrupts airflow as little as possible.

The pitot tube will give your manometer a static pressure (or atmospheric pressure) reading for comparison to the panel pressure. Basically, this shows the difference in pressure/air speed at that spot as compared to the free stream pressure/speed. The difference between those two is what we call gauge pressure. To see what effect changes to your car have, you’ll be measuring gauge pressures.
 
When you’re ready to test, find a stretch of road that is relatively free of traffic and, preferably, changes in terrain and foliage. Fit your pole with the pitot tube at the front of the car, tape the disk on the spot where you want to measure pressure, connect them both to the manometer (I run the tubing through a cracked window on the opposite side of the car from my measurement location) and drive at a set speed; don’t make it too slow (I’ve done most of my pressure testing at 50 mph/80 kph, although sometimes you may want to measure at different speeds). I like to use a digital manometer, since it has an averaging function and I can choose the display units, and drive over a mile-long road near my home that is lightly traveled. Note the gauge pressure, then drive in the opposite direction and average the two measurements; make the change to the body and do it again. Then compare the two averaged pressure measurements—did it get larger? Smaller? No change?

Centerline gauge pressures as measured on the front of my 2013 Prius. You can use stagnation pressure—the pressure at the very front of the car—as a check; it should be close to atmospheric density (in kg/m³ if you’re using metric) multiplied by your car’s speed (in m/s) squared, all divided by two. Here the measured 265 Pa is very close
to the predicted 271 Pa.

What We Learn from Measuring Pressure
 
Now that you know what the pressure is at any spot on your car, you can try modifying it to change those pressures in ways that will be beneficial—usually, trying to reduce drag and/or lift. Aerodynamic force is generated in large part by the pressures acting on the surface of your car—pressures that you can now measure!—and altering them will change that force and its effects. To think about how to do this, look carefully at the spot on the car you’re investigating: which direction does it point? If the panel faces backward or up—say, the back window or roof—increasing pressure will usually help reduce drag, lift, or both, for example by fitting a spoiler. If it’s pointing forward—say, the front window or the front bumper cover—reducing pressure will generally help reduce drag (but at the expense of increasing lift if it’s angled like the windshield). If it’s pointing down—basically anything underneath the car—reducing pressure will help reduce lift, for example by attaching smooth panels to the underside. As always, you’ll have to balance the characteristics of your car with your goals to figure out what it is you want to achieve and how best to do it.
 
Of course, those are generalizations only; you may, for example, find that fitting a front air dam increases pressure on the front bumper but reduces drag if your car has a very rough underside, or that a spoiler increases pressure on the back window but also increases drag from a larger wake or a change in base pressure. These sorts of interactions are why it’s best to use a variety of test methods to figure out what any single modification or combination of mods does. Pressure testing can and should be one of those methods in your toolbox.
 
Your reference pressure does not need to be atmospheric, either, depending on what you want to learn or investigate. Sometimes you will want to see what the pressure difference is before and after some feature on the car body, or between opposite sides of the same panel. For example, years ago I measured the pressure difference between the top and underside of the hood on my 2013 Prius when I was thinking about fitting hood vents:

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:

Without measuring the pressure I would have had no way of knowing if this location was a good one for a vent on my car to do what I wanted. Now I know; no guessing required!