Aerodynamics Fundamentals: Viscosity, Reynolds Number & Speed Effects
Watch This Course
$199 USD
-OR-
Or
8 easy payments of only $24.88
Instant access. Easy checkout. No fees.
Viscosity, Reynolds Number & Speed Effects
04.33
| 00:00 | As you're no doubt already aware, not all fluids are the same. |
| 00:03 | For example, water flows differently to air, which flows differently to honey. |
| 00:08 | One of the key parameters that drives these differences is known as viscosity. |
| 00:13 | This is essentially a fluid's resistance to any shearing motion across the fluid. |
| 00:19 | If we imagine the particles of the fluid being pushed and sliding with respect to each other as we, say, move a spoon through a jar of honey, we're essentially shearing the fluid as we move through it, and we feel resistance on the spoon as a result. |
| 00:31 | Air has very low viscosity, while something like honey has very high viscosity. |
| 00:36 | The viscosity dictates many things, but primarily it will determine the amount of shearing force on a surface and the behavior of the fluid as the speed increases. |
| 00:47 | This brings us to a phenomenon heavily related to viscosity, which is turbulence. |
| 00:52 | This is essentially small-scale, chaotic, and randomly swirling eddies within the fluid flow. |
| 00:58 | These swirls can be of varying sizes, and we refer to these as turbulent length scales. |
| 01:04 | If we imagine a consistent, uniform flow moving in one direction, it will be smooth with no eddies in it. |
| 01:12 | This is called laminar flow. |
| 01:14 | However, if we disturb this flow by having it interact with the surface above a certain level of interaction, we'll introduce instabilities within the flow, which will result in chaotic eddies of turbulence. |
| 01:28 | The flow is more likely to be turbulent the more we interact with it, so having an object of longer length or moving the flow at a faster speed will generally increase the amount of turbulence within a flow. |
| 01:38 | We characterize the level of turbulence within a flow using what is known as the Reynolds number, which is given by Re, Reynolds number, equals rho times v times d over mu, where rho is the density of the fluid, v is the velocity of the fluid, d is the characteristic length of the object, and v is the velocity of the fluid, d is the characteristic length of the object. |
| 01:58 | We are flowing the fluid over, and mu is the dynamic viscosity of the fluid. |
| 02:02 | You'll note that the top of this number is related to the velocity, while the bottom is related to the viscosity. |
| 02:08 | This is because the effects of viscosity dominate more at lower speeds and lower Reynolds numbers. |
| 02:14 | What this means is that if Reynolds numbers are low, viscosity suppresses the instabilities that cause turbulence, while when the velocities are high, or there's been a large distance for instabilities to grow over, viscosity becomes effective at suppressing instabilities and eddies, and so the flow becomes more turbulent. |
| 02:34 | Flow being more turbulent isn't necessarily a bad thing, as much the internet would have us believe. |
| 02:40 | However, low energy air with very large turbulent length scales, like what's shed off the wake of a car, is generally speaking poor for the performance of aerodynamic devices. |
| 02:49 | With the Reynolds number and turbulence now defined, we can talk about the effects of speed. |
| 02:55 | As discussed earlier, aerodynamic systems can go up with velocity squared. |
| 02:59 | So, that aspect of the car's aerodynamics changes with respect to speed. |
| 03:03 | However, the fundamental flow features don't change significantly over the range of speeds that a car is seeing. |
| 03:10 | People will often ask me whether or not wing will perform much better at low speed or if there's a big downforce decrease at a high speed. |
| 03:17 | The reality is that the downforce coefficients just won't change that much. |
| 03:21 | What will change is the Reynolds number with the speed, and generally speaking, the run-on effects of this are that at higher speeds, the higher Reynolds numbers and relatively decreased viscosity effects result in more performance and higher aerodynamic coefficients, but only by a small amount, typically a few percent over the range of speeds that a race car would typically see. |
| 03:42 | With that covered, let's quickly go over some takeaways from this module. |
| 03:46 | Viscosity is defined as a fluid's resistance to shearing motion and plays a significant role in how different fluids flow, with air having low viscosity and substances like honey having high viscosity. |
| 03:56 | Viscosity impacts turbulence, which involves chaotic swirls in fluid flow, and is more likely to occur when the fluid interacts with the surface, increasing with speed or object length. |
| 04:08 | The level of turbulence is characterized by the Reynolds number, which gives the ratio between velocity and viscosity-based forces, with higher numbers indicating more turbulence. |
| 04:18 | Although aerodynamic forces increase with velocity squared, fundamental flow characteristics remain stable across a race car's speed range, with only small changes in aerodynamic performance at higher speeds. |
