Vlog 012 - Do You Need a Flat Underbody for a Diffuser to Work?

[@octurbojoe]

I love you guys.

[@marioelburro1492]

Trying to learn as much as i can about aero so i can one day modify my car with the purpose of functionality as well over looks. Thx for this video👍🏼

[@VerusEngineering]

Of course! Always happy to spread the knowledge.

[@user-gk2cg2th9h]

Same man these videos are great

[@CountChokcula]

I've been using ansys static structural, and my current machine component design class has me presenting tables like these in a very similar format. hahaha it's so crazy to see that it's what engineers really do in the field

[@VerusEngineering]

Happy to share what we do.

[@mas921]

What are the tweaks you mentioned that would reduce the lift of a diffuser when used with a smooth underbody?

[@VerusEngineering]

mas921 - changing design of the diffuser.

[@VerusEngineering]

mas921 using ANSYS Fluent to validate the changes.

[@rolandotillit2867]

Question, in normal air handling systems and even with air planes, diffusers increase the pressure of the air by slowing it down, yet on a car, a diffuser has the opposite effect. Normally air behaves this way when it's density is affected, such as when air is compressed, or it's local velocity exceeds M = .3. Would it be a stretch to assume that there are compressibility effects at play in diffusers? If there weren't wouldn't it be better to nozzle the flow at the rear, that is instead of a diffuser expanding, why don't people put nozzles at the rear? In subsonic incompressible flow, constricting flow increases its velocity, yet the airflow that passes out of the diffuser doesn't slow down right away. As your CFD example shows with the pressure equalizing behind the diffuser(maybe reducing drag due to a small amount of thrust by the diffuser upwash and bumper recirculation?). This leads me to believe that compressibility effects are more prevalent, and that local air flow velocities in the underfloor magnify this effect, and that this can be exploited for massive downforce, or drag if done wrong. I believe, what we see with under body diffusers are a type of expansion fan phenomena, that accelerates airflow locally to the point that we have trans-sonic flow.

[@VerusEngineering]

We are not looking at compressibility effects during most car analyses. In some cases, it might make sense. One such case would be top fuel dragsters which would have localized flow that would see compressibility effects. You aren't looking at what is going on in the right context. First, ignore pressure and velocity for a moment. Look at the system in terms of mass flow. With a diffuser expanding towards the rear, it allows more mass flow. Now bring velocity and pressure into it. If more mass flow is able to flow under the car but area/volume is limited, velocity will increase. This will decrease pressure. The diffuser itself isn't physically what is creating most of the downforce, it is the throat and before the throat. So a floor is not needed but can help. A properly designed diffuser with a floor has a possibility to make more downforce than a properly designed diffuser with no floor. However, that doesn't change the fact that a flat floor is required. For drag, a diffuser certainly doesn't need a flat floor as it is filling the deadwater region behind the vehicle and decreases overall pressure drag on the car.

[@rolandotillit2867]

@@VerusEngineering Mass flow means compressibility though doesn't it? Pressure is a result of mass flow, compressed air has more mass. So increasing static pressure at the inlet by raising the mass flow, means the airflow at the throat will be faster. In fact, if inlet pressure is roughly twice the pressure at the throat you have choked flow as the flow has reached the speed of sound at the throat.

[@michaelw6277]

Great video! I noticed you guys were selling a diffuser for the Mk6 GTI, and after experiencing some high speed instability during hard braking at 120 MPH at the track I’ve added it to the short list of things to buy at some point over the next season. Question: given that my car is turbocharged and will be running higher than stock boost, I have to consider maximizing airflow through the cooling stack at the front of the car. I do not wish to cut holes in the hood so I intend to continue to vent beneath the car. Assuming a properly sealed engine bay will a diffuser have enough of an impact on airflow beneath the car to allow for more airflow through the cooling stack? Thanks for the good information, I look forward to trying out your products.

[@VerusEngineering]

Can you please rephrase your question?

[@tetraktys6540]

@@VerusEngineering I believe the above Q was: Is there an air flow impact through the engine bay from a diffuser? They are generating more heat from a bigger turbo, and want engine bay extraction via underbody.

[@VerusEngineering]

@@tetraktys6540 No. Since the rear diffuser is at the rear of the car, it would have no effect on the engine bay.

[@marcusr3666]

May I cite your work for my research essay? Is there perhaps an article I could cite or something that you made?

[@VerusEngineering]

We have a blog on the website you can take a look at.

[@grips2k845]

Not sure if I missed it in the video but at what vehicle speed are all these numbers calculated at?

[@VerusEngineering]

80 mph

[@GP-fc7jv]

Should this also make a difference in gas mileage?

[@VerusEngineering]

Yes, flat underbodies and rear diffusers can increase gas mileage.