Aerodynamics Fundamentals: Ground Effect
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Ground Effect
06.14
| 00:00 | Ground effect is a mechanism by which our aerodynamic devices get more powerful as they approach the ground. |
| 00:06 | However, it's poorly understood in club circles. |
| 00:09 | Ground effect is essentially the result of an aerodynamic symmetry that takes place across the ground plane. |
| 00:16 | Imagine you have two wings near each other, one above the other, but upside down, with an air gap in between them. |
| 00:23 | If we were to draw a horizontal line exactly between these two wings, the flow would travel straight along this line, as both wings are pulling it with the same strength. |
| 00:32 | This horizontal line is exactly the same as what the ground plane does. |
| 00:36 | So, you can imagine that when we have a wing near the ground plane, it's like we have two wings pulling against each other. |
| 00:42 | This generates a lot more suction on each wing than if they were to be alone in the freestream, as they can essentially pull against each other. |
| 00:49 | This is the fundamental mechanism of ground effect. |
| 00:53 | We're powering up a wing as it approaches the ground. |
| 00:56 | Another way to think of it is in terms of Bernoulli's Principle. |
| 01:00 | Let's start with a channel with an expansion on top and a flat surface on the bottom. |
| 01:06 | We'll give this channel an expansion ratio of 2 to 1, with an 100mm throat and a 200mm exit. |
| 01:13 | It's a situation very similar to a floor and diffuser setup on a race car. |
| 01:17 | Currently, under our floor, let's say we have a velocity of 100km per hour, and then at the exit it would be 50km per hour, which we'll say is our exit condition that's being driven by our system. |
| 01:28 | If we bring our lower flat surface up to 50mm, this changes our distances to 50mm in the throat, and 150mm at the exit, for an expansion ratio of 3 to 1. |
| 01:41 | This would increase the speed in our throat to 150km per hour if the exit speed was constant, which, by Bernoulli's principle, significantly increases our suction. |
| 01:51 | We only care about the load on our car, which would be the top surface in this example, so a large increase in suction is beneficial. |
| 01:59 | We don't care how much the ground is pulled up as a result, because it's not going anywhere and it doesn't affect our tyre vertical load. |
| 02:06 | Now, it's easy to take this approach and just assume that what we always want in a car is to have a floor that's as close to the ground as possible and a big expansion behind it. |
| 02:14 | But real world effects get in the way here. |
| 02:17 | Firstly, any large increases in suction with no corresponding decrease in exit pressure means that the adverse pressure gradient is substantially larger. |
| 02:26 | The pressure gets lower under the floor, but not further back. |
| 02:30 | This means that as we move closer to the ground, the ground effect will make it more likely that we suffer from flow separation, so there's a limit to how close we can run to the ground without starting to lose performance. |
| 02:42 | In more extreme cases, if we end up with flow separation as we approach the ground on something like a front splitter, the splitter will depower and lose downforce. |
| 02:51 | This will cause the force on the suspension to reduce, and the suspension will push the car upwards. |
| 02:56 | With the ride height higher, the ground effect is reduced, the splitter will no longer have flow separation, and will start to make more downforce again. |
| 03:04 | This will compress the suspension back to the low ride heights, and we end up with a loop situation of up and down bouncing that you may be familiar with, known as porpoising. |
| 03:14 | This isn't the only mechanism that causes porpoising, but is one of the key factors on club level cars which suffer from it. |
| 03:21 | The other concern is when we generate more suction near the ground, we tend to suck in air from the sides of the car as it's sealed. |
| 03:29 | This diminishes our performance, as our effective channel area below the car is larger than we'd expect just from taking a slice through the gap between the floor and the ground. |
| 03:39 | I've seen quite a few amateurs try to solve this problem with skirts that go between the car and the ground, but this usually isn't a good idea. |
| 03:47 | If we're making good suction and you put on skirts, what happens is that the car gets extremely ride sensitive, as the sealing of the skirt is massively dependent on how close it is to the ground. |
| 03:58 | 5mm can make a very large difference. |
| 04:02 | This is one of the main reasons that skirts were banned in Formula 1 back in 1981. |
| 04:07 | They provided huge amounts of grip until they suddenly didn't, due to a disturbance in the track or skirt damage, which is extremely dangerous, especially at the cornering speeds these cars were capable of. |
| 04:19 | Ground effect error was subsequently banned completely by 1983. |
| 04:23 | F1 car geometry in the last few years has also brought back the ability to run close to the ground with the outer edges of the floor, and this has resulted in teams struggling with downforce inconsistency and ride height control on these cars, despite having huge amounts of resources available to them to try and fix these issues. |
| 04:41 | It's worth noting that the previous generation of Formula 1 cars, from 2017 to 2021, did not have these issues, and had much higher floor edges, and even with those higher floor edges they were making more downforce and producing faster lap times than their ground-hugging descendants. |
| 04:58 | Club cars, generally speaking, don't have great ride height control compared to professional cars. |
| 05:04 | This results in large downforce fluctuations at different phases of the corner, and when going over bumps, which makes the car very difficult to drive, and in my experience, slower than cars without skirts. |
| 05:16 | The other scenario is that if we aren't making great suction alongside the skirt and throw the skirt on, this typically results in the skirt making the downforce worse, as it can introduce separated and untidy flows into the underfloor. |
| 05:28 | I've seen plenty of skirts that have made performance considerably worse once they're fitted to the car. |
| 05:34 | With that covered, let's run through the main takeaways before moving on. |
| 05:38 | Ground effect enhances the power of aerodynamic devices as they approach the ground by creating a suction effect similar to two wings pulling against each other, amplifying downforce. |
| 05:49 | However, bringing the car too close to the ground can increase the risk of flow separation and performance issues, particularly with adverse pressure gradients and airflow leakage from the side. |
| 05:59 | Skirts to seal the gap between the car and the ground can worsen performance, especially in club level cars without precise ride height control, leading to instability and inconsistent downforce. |
