Aerodynamics Fundamentals: Compressibility & Speed Effects
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Compressibility & Speed Effects
05.43
| 00:00 | Some of the most common misconceptions around car aerodynamics are centered around the compressibility of air, and in this module we'll discuss why a lot of that isn't relevant for the sort of aerodynamic work we do on race cars. |
| 00:12 | Air is what is known as a compressible fluid, which means that as we increase the pressure applied on the fluid, the volume and consequently the density of the fluid can change substantially. |
| 00:23 | You would have seen this effect before on items like tires and bike pumps, or you may be aware of it from the fundamentals of engine operation and turbocharging. |
| 00:33 | We can contrast that against incompressible fluids like water or brake fluid, which don't substantially change volume even when we apply significant amounts of pressure. |
| 00:43 | Now, where a lot of race car misunderstandings come from is the fact that at velocities below around 350 kilometers per hour, the suction in pressure introduced by aerodynamic devices is too low to cause substantial changes in air density. |
| 00:59 | If we consider a wing on a car making a substantial amount of downforce, this wing may only be producing, say, 6 kPa or 0.06 bar of suction at a speed of 250 k's per hour. |
| 01:11 | This is equivalent to roughly a six percent change in air density. |
| 01:16 | These changes are so small they don't significantly change the flow field around the car when compared to a completely incompressible flow field. |
| 01:24 | And of course at lower speeds, the effects are even less, and you won't find many club level cars that are cornering at more than 300 k's per hour. |
| 01:32 | As such, for most race car aerodynamics, we assume air to be incompressible, as it only really becomes compressible at very high end-of-straight speeds. |
| 01:42 | So, why does this matter? Well, a large part of this is due to a lot of people learning their base understanding of aerodynamics from aircraft, and aircraft, particularly passenger and fighter jets, operate at much higher speeds than race cars. |
| 01:56 | Fighter jets, for example, can travel faster than the speed of sound, which is known as supersonic. |
| 02:02 | Conceptually, the speed of sound is the speed at which any disturbance in the air can travel through the air. |
| 02:08 | This is often referred to as the speed that sound waves travel through the air, but it also affects the speed at which air upstream of an object is moved by the object. |
| 02:18 | To clarify, an object will build up pressure in front of it. |
| 02:22 | This pressure will cause the air far further in front of it to slow and change direction, or move around the object. |
| 02:28 | The speed at which these effects propagate upstream is given by the speed of sound. |
| 02:33 | We can see that once we reach the speed of sound, the object is moving faster than the pressure wave in front of it, so the particles upstream can no longer move in response to the downstream object, and only move as a result of direct influence of the object. |
| 02:49 | This causes abrupt changes in direction as the air interacts with the object, and means that we end up with a bunch of pressure waves along a single line. |
| 02:57 | This creates what's known as a shock wave. |
| 03:00 | Now, why is this important? A huge amount of the reason behind swept aircraft wings is minimizing the strength of shock waves, in addition to some handling and stability characteristics. |
| 03:11 | Neither of these requirements apply to race cars, which is a huge reason as to why aircraft in general shouldn't be an inspiration for race car aerodynamic designs. |
| 03:21 | Another common compressibility misunderstanding is related to the flow "choking". |
| 03:26 | This is a phenomenon that occurs when we have a closed duct, like an engine intake, and the mass flow flowing through the duct causes the speed to be supersonic at the duct's narrowest point. |
| 03:37 | Once a flow goes supersonic, it doesn't obey the duct area rules that we discussed earlier in the Bernoulli's Principle Module. |
| 03:44 | If we expand a supersonic flow in a duct, we end up with the velocity increasing and the density decreasing without any additional mass flow. |
| 03:53 | Essentially, we have subsonic air before the restriction in the duct, then supersonic air after the throat across the narrowest part of the duct, which experiences what's known as a normal shock wave. |
| 04:04 | This particular configuration means that the engine cannot draw any more air through this duct, and this results in the engine being choked. |
| 04:12 | Many people think this same phenomenon occurs in the underfloor of cars, which have a narrow throat and then an expansion in the form of the diffuser. |
| 04:20 | However, this isn't the case. |
| 04:22 | When it comes to the car's underbody, there are insufficiently aerodynamic pressures to achieve a choked flow, and there are too many opportunities for airflow to come in from the side skirts of the car, which aren't perfectly sealed to the ground, to consider the floor a closed duct. |
| 04:37 | As such, a car will never experience choked flow, and supersonic flows will never occur externally on a car unless we're running a land speed record vehicle. |
| 04:47 | This brings us to the conclusion of our section covering fundamentals behind aerodynamics. |
| 04:52 | But before we move on to real-world aerodynamic influences, let's summarise the main points to remember from this module covering speed effects and compressibility. |
| 05:00 | Compressibility misconceptions in race car aerodynamics arise from comparison to aircraft, as air behaves close to an incompressible fluid at speeds below 350km per hour, which is what we'd see in nearly all types of motorsport bar land speed racing. |
| 05:15 | At these speeds, the pressure changes caused by aerodynamic devices are too small to significantly affect air density, unlike in supersonic aircraft where shock waves fall. |
| 05:24 | While phenomena like choked flow occur in engine intakes, they don't apply to car underbodies due to insufficient aerodynamic pressures and mass flow conditions. |
| 05:34 | Therefore, race car designs should not be influenced by compressibility considerations from aviation. |
