Amateur Aerodynamics

Wakes are easy to observe in some media. Watch a boat moving through water and the expanding wake is readily apparent; similarly, when you've been swimming and scooped your hand through the water, you have seen the trail of bubbles following the motion. Wakes are harder to see in air—nearly impossible without rain or water spray, or smoke traces in a wind tunnel—but they exist just the same. We're all intuitively familiar with wake behavior and generally know that minimizing the wake tends to correlate with decreased fluid drag (like moving your hand palm-first versus sideways through the pool water). But just what is a wake, and why does it form? Why is wake size important? Or is it? Dragging a chopstick through a bowl of water (with food coloring added for better visibility) creates an expanding wake that looks like a "V" behind the stick. What goes on inside that wake? Wake Formation   Wakes form due to the fact that real fluids are viscous; that is, there are int...

When it comes to cars, "stability" is a loaded term because it is very imprecise. Stability could mean the car's ability to track in a straight line down the highway (which is as much a function of its alignment as anything else), or its ability to maintain its direction and rotation in a constant rate turn, or ability to maintain its composure through a slalom course, or its ability to resist changes in lift, or to not be disrupted as it moves and jostles on its suspension over a washboard road, or any number of other things.   So, to talk about "stability" in any meaningful way, we first have to narrow it down. What sort of stability? Stability with regard to what factor or input and what measurement or output? We'll see in just a minute why these fins might have been a bit misguided. Aerodynamic Stability   Since this is a blog about aerodynamics, let's consider stability as it is influenced by the movement of air around (and through) the car. He...

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...