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How to Think About Car Aerodynamics: A Very, Very Basic Overview

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The study of aerodynamics is complicated. If anyone tries to tell you otherwise, run the other direction—it’s a sure sign they don’t know what they’re talking about.   Over the last two years especially, my thinking about aerodynamics and appreciation of its complexity has changed dramatically—a result of my going back to school to get another bachelor’s degree, this time in aerospace engineering where a good working knowledge of airflows is required and education in not just general fluid mechanics but also aircraft aerodynamic design forms a core part of the technical curriculum. I'm in the midst of my last semester now and to clarify my thinking at this point I decided to put some things in writing in the hopes they might help someone else as well as myself, specifically focused on car aerodynamics. A word of warning: I've tried to minimize the amount of math below, but some mathematical relations are unavoidable if you want to build an understanding of fluid flows. If anyth...
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Measuring and Improving Cooling System Performance – Part 5: Fan and Outlet

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Last time, we saw that drag from the heat exchanger varies with the velocity of the flow entering it and measured its loss coefficient. Before that, we measured total pressure losses across the grill and diffuser, as well as other parameters such as static pressure that told us how well these components worked.   Getting air into the cooling system is just one part of the story. Just as important is how we get air out , and what that air does once it has left the cooling system or engine bay. This Fiat 500e has a large heat exchanger package (we learned why at the end of the last post ) with a single, centered fan. Many cars have multiple fans behind the heat exchangers. Fans: State 3 to State 4   The purpose of fans placed in front of (for example, the Tundra cooling system in the first post ) or, more commonly, behind the heat exchangers is to increase mass flow through the cooling system. You can prove this by writing out an energy balance similar to what we did in Par...
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Explainer: Aerodynamic Pressure

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In aerodynamics engineering, we talk about pressure a lot . But what is pressure anyway? It's something we're all familiar with in our everyday lived experience: your ears "popping" when you drive up a mountain due to the pressure change with altitude; feeling drained or tired after a long flight in a cabin at a different pressure than what you're used to; sticking your hand out a car window and feeling the "push" of the air backward. You might even remember from a physics or chemistry course that pressure arises from the molecules in a gas zipping around, occasionally bouncing off a surface, and that the faster they move, the greater is the pressure the gas exerts. "Movement" implies velocity alone, but of course the number and mass of these molecules also matters: more mass, greater pressure. Mass and velocity multiplied give momentum , and pressure arises from the transfer of momentum between molecules in a gas and any surface in contact wit...
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