Amateur Aerodynamics

One of the unfortunate realities of cooling system modification and testing is that we can't visually observe what goes on under the hood while the car is driving. Building a physical model of the cooling system may shed some light on what the real system is doing and can be designed for easy observation if you make a window one side of the duct. The air filter here functions the same as a heat exchanger in that it restricts flow and dissipates energy in the form of total pressure loss. Similar to the real cooling system as I decided to analyze it in the previous post, this model has no nozzle outlet. Instead, we have the same as we get in an engine bay: a pressure boundary , meaning an enforced static pressure behind the heat exchanger. Here, that is simply ambient pressure ( C P = 0). While it is not possible to vary atmospheric pressure here, we saw in Part 5 (and will revisit later on) that modifications such as vents can change engine bay static pressure and thus the bound...

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...

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...