The Autopian wants me to write about how a wind tunnel works, and I like the idea. We always see nice wind-tunnel shots of cars, but how do you actually get data from that? I’d love to tell you, as such a story would allow me to touch on many aspects of aerodynamic design, but here’s the hiccup: If I cram all that info in a single article it will turn into a textbook! So in order to protect our collective sanity, I’ll take you for a ride in theory land for a few articles. Don’t leave! I’ll keep this digestible and entertaining! And by the end of today’s post you’ll even understand how a carburetor works.
[Editor’s Note: It’s Manuel, the aerodynamicist from France here to tell you about how aerodynamics works! Last time, he told us about drag coefficients, and now here’s a discussion about how fluids flow, particularly in the context of pressure, velocity, density, and more! -DT]
Lifting The Universe’s Hood Reveals: Thermodynamics
When we talked about drag coefficients, I threw a lot of terms around without giving much explanation. Today we’ll take things from the very start and talk about energy. Yes guys and gals, we’re about to do thermodynamics. Breathe, breathe it’s going to be fine.
Thermodynamics (or energetics as my teacher used to say) is the science of movement and what creates it. It basically covers the entire realm of energy except for subatomic dark magic. But what is energy exactly? Energy is a quantity of stuff that allows movement, and is measured in Joules. That’s it. It can take a few extra steps, but if your “energy” can’t move something eventually, it’s not energy. Poltergeists? Energy. BDE? Not energy.
Let’s see the different types we’re likely to encounter on a car website:
- Kinetic energy: if something is moving, it’s got energy. It’s written E=.m.v (m=mass; v=velocity)
- Potential energy: if you lift something, it can gain speed while falling. Height is energy. The equation is E=m.g.z (m=mass, g=gravity, z=height)
- Electrical energy: it can create magnetic fields that move crap. Electricity is energy. I know nothing about that shit so let’s skip the math and just trust me on this
- Thermal energy: heat can make water boil, shaking a cooking pot’s lid. Heat is energy. E=C. T (C=heat capacity, T=temperature difference)
- Chemical energy: when you burn gasoline it creates heat, which is energy. Gasoline is energy. So is Start Ya Bastard! E=m.LHV (m=mass, LHV=Lower Heating Value)
Now, what’s really amazing is that you can turn one type of energy into another because of THE FIRST LAW OF THERMODYNAMICS: No energy shalt be created, nor shalt energy be destroyed. The number of Joules shalt remain constant for all of eternity or the universe shalt explode like a Holy Hand Grenade.
I know it seems oh so far away from designing race car spoilers, but trust me, thermodynamics can be used for everything and can change your entire view on the world, including driving. You don’t trust me? Here, let me demonstrate via something we all know and love: a road trip.
A Trip To Thermodynamicstan
It is 6:00AM; you sit in your ultimate driving machine, half asleep. You turn the key, allowing the flow of current from your battery to power an electric pump, which makes fuel flow from the tank to the engine. You just converted electricity into kinetic energy in your pump’s motor, which transferred its own kinetic energy to a fluid.
You turn the key one more click, allowing the battery to power a starter motor that gives your inline six its first revs of the day. You’ve converted electrical energy into kinetic energy again.
As the pistons move down, they draw fresh air mixed with a fine mist of gasoline and compress it. Your battery then fuels a spark plug, creating the hot spot that will ignite the mixture, increasing pressure in the cylinder. That pressure pushes against the piston which in turn makes your crankshaft spin. You’ve converted mechanical kinetic energy into fluid kinetic energy and then fluid potential energy (pressure). You then used electricity again to create heat that allowed the conversion of chemical energy into more thermal energy. That heat then expanded a fluid, which pushed against a surface, creating more mechanical kinetic energy from the chemical one. And you’re now ready to go!
9:00 A.M.: After the first part of your trip spent in plains with the gas pedal at the exact same position for hours, the terrain starts showing small elevation changes. You don’t feel like moving your foot, so you lose speed while gaining height, and regain speed as you descend. You’ve traded kinetic energy for gravitational potential energy while climbing hills, and done the contrary when descending.
11:15 A.M.: You’ve now reached the mountains, requiring you to give it the beans. More throttle input on the way up means some brake action on the way down. You’ve increased the car’s gravitational potential energy while maintaining its kinetic energy, meaning you had a surplus of total mechanical energy in the slopes, so you used your brake pads which converted kinetic energy into thermal energy (your discs heated up) that got flung away from your vehicle, decreasing its overall energy level.
1:00PM: Time for lunch! The burger you’re about to swallow whole is also chemical energy!
See? It works with everything.
Now, most of you are probably able to explain that your car slows down on an incline using good old mechanics equations. A smaller group of you (fluid dynamics people) are typing right now that they can compute how much power they need for a pump to move a given flow of fuel without all this energy fancy talk. You may be thinking that thermodynamics is useless as you can demonstrate everything I just explained with another set of equations you already know. To you I say: Its not redundant, its complementary. It applies to all these different fields (mechanics, fluid dynamics ) that look so different from one another!
Thermodynamics covers all the aforementioned fields. It’s a powerful tool that can give you a holistic understanding of any system, no matter the physics involved. (Except for electromagnetism, which is the work of the devil as far as I know. [Editor’s Note: I got a 62.5% on my electricity and magnetism exam in physics in college. The curve brought that test up to a B. I aced the course despite knowing very little about E&M. -DT]).
Lets put all this into equations! Our little roadster’s energy, when no throttle is applied and no loss is considered can be written like so:
Indeed, mechanical energy for a solid is nothing but the sum of its kinetic energy (1/2*m*v) and its gravitational energy (m*g*z) and it remains constant because the first law says so!
Putting The pressure On
Remember how we said pressure is equivalent to energy per unit of volume? I’d like to expand on that as pressure is the clay us aero people build cars, planes and hand dryers with. First, what is pressure? I mean, its weird right? Theres air pushing on my skin the whole time, yet I dont really feel it? My ears pop when driving up mountains? Cest quoi ce bordel ?
Unlike in solids, molecules are not neatly arranged in crystals and immobile relative to each other when they are in their fluid form. If you were to look at the air around you, using the best microscope ever, youd be able to see the NO2, O2, CO2 and their buddies moving around and hitting each other like drunken sailors on cocaine dancing to Dragon Force. They move around like crazy, and as a consequence, bounce on each other as in the messiest game of snooker ever. Thats called the Brownian motion.
There are two ways to control the intensity of those shocks happening in a given volume of air. You either compress it, pushing the molecules closer to one another (sort of like packing more sailors on the dance floor), or you heat them up (turn the music up), increasing the sailors’ (or molecules, whatever) energy and therefore the number of impacts.
If these moving particles encounter a solid, they will impact it too and push it away, creating a force. Thats what we called static pressure (noted “p”) last time, and it is whats keeping your tires in a nice donut shape instead of a deflated mess. I said pressure is energy per volume as itll take you more sweat (i.e. energy) to inflate you cars tire to 2 bars (29 psi) than it will to do the same thing to your bikes because the first one has a more important volume than the second.
That gives us this equation:
E=p*V (p=pressure, V=volume)
The Bernoulli Principle
At this point you are probably wondering where I’m going with all of this physics talk. Rest assured, now is the time where it’s all coming together!
So, we know what pressure is, and that it is energy if we multiply it by a volume.We also learned about the first law of thermodynamics and how it applies to solids (the sum of kinetic energy and potential energy remain constant). Some Swiss dude named Bernoulli put all this together and defined the conservation of mechanical energy for fluids in 1738. It goes like this:
This is the total energy of a fluid, and that last “p*V” term is what some call “pressure energy.” It is the only difference between a BMW 2-seater’s energy and that of the wind. (In other words, if you remove “p*V” from this equation, you get the kinetic energy equation you’d consider for a solid in this example, a car).
If we divide everything by the volume of fluid we’re analyzing, as aerodynamicists love to do, we get this:
m/V, that rings a bell
1/2*rho*v+rho*g*z + p=constant
Hey look! It’s our pal density! And his buddy dynamic pressure! Man, I’m so happy to see them again that I’m gonna make this equation even simpler, neglecting the gravity stuff because our systems are not big enough for it to matter in aerodynamics:
Hey that’s rather easy to understand now! This is the fluid version of the conservation of mechanical energy, but expressed in terms of pressure. Indeed, the first term is what we called “dynamic pressure” and the second one “static pressure.” The sum of these is called “total pressure” and just like before, it implies that if we gain speed (*rho*v), we lose pressure (p), and vice versa because the total pressure remains constant. And that is the principle behind
The Venturi Effect
This is probably something you already have heard of, but can you explain precisely how the venturi effect works? And were you aware it’s the foundation on which carburetors are built? Please answer “no,” otherwise there’s no point reading this article!
Now that you know about the first law of thermodynamics, Bernoulli and all that jazz, I’ll throw you one last life-altering information: the conservation of mass. It’s simple; you can’t create nor destroy the amount of merde the big bang threw around. You can rearrange atoms or whatever, but you cannot pull mass out of your derrire.
That is relevant to aerodynamics as it applies to pipes. If a quantity of stuff gets in a pipe, the same quantity goes out. Let’s consider Italian plumbers! If four a-plumbers! enter a pipe every minute, four of them have to come out in the same interval. This is our Mario flow rate (MFR), measured in Mario per minute (mpm) and has to remain constant throughout the pipe, because we can’t create nor destroy a Mario in the ducting. There are several ways to achieve this MFR; you can either push a group of 2 Marios in every 30 seconds, or push them one by one every 15s. Now, a couple of video game heroes needs a bigger pipe to pass together compared to a single mustachioed mushroom enthusiast, but the latter will have to run faster than the Peach pursuing pair to achieve the same MFR of four mpm. So if you want your red sporting character to enter the duct walking with his doppelganger, but leaving running, the pipe needs a bigger cross section at the intake than the exhaust. This is probably intuitive for those of you who have put your fingers over the end of a hose to restrict the outlet area to increase the outlet velocity.
Managing sections in ducting is how engineers manipulate air speed, and therefore air pressure, and that is precisely how a carburetor works. The amount of air going through the carb at any second is dictated by the engine’s displacement and rotational speed, and remains constant throughout the engine’s induction system. At the intake of the carb (1), the pressure is the same as the atmospheric one, but there’s a restriction in the middle of the carburetor (2), accelerating the air speed and conversely decreasing the air pressure. Since the fuel in the carb tank (3) is at 1 bar thanks to a vent, that pressure difference will suck the fuel through the nozzle (6), allowing your Atlantique-300 to fire right up!
Using a restriction in a duct to accelerate a driving fluid in order to pump a driven fluid thanks to the pressure difference is the Venturi effect. A simple application of the Bernoulli principle and the conservation of mass allowing the existence of amazing things such as paint guns, F1 under body down force or, more importantly to a Frenchman, wine aerators.
Conclusion (It’s French for “Conclusion”)
Pfew! Done! No more science today! Light up a Gauloise or a dumpster to celebrate, because we have just covered a majority of the basic concepts of aerodynamics: pressure, Bernoulli and the conservation of mass. All that theory will be put to good use later when we try and design wind tunnels for the next JAM 808.
But before that, our next episode will have to deal with the evil twin of the first law of thermodynamics: the second law of thermodynamics. Stay tuned for your upcoming hit of weird equations, approximate analogies and half-assed schematics!
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