What is a Car Diffuser?
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- Опубліковано 28 вер 2024
- In this video, we will be discussing car diffusers. How do they work? Why do car designers install them? How can you make one for your design?
Theory - the Bernoulli effect
The Bernoulli effect states that if you follow an air particle along its path within a flow the energy density remains constant. This means that if one term goes up, another must go down. This means that for example, if the velocity increases, the pressure has to go down.
Relation to the car underbody
The car underbody is the bottom side of a car. At the front of the car, you have high pressure because the air collides with the front of the car. At the back of the car, the pressure is lower because the wake wants to pull the car backward. This pressure difference between front and back of the car is the driving force of the airflow beneath the car which is called the car underbody or car underbody aerodynamics.
Down force
Because this section is quite narrow, the air has to speed up and this is where the Bernuit effect comes in:
higher velocity, lower pressure, creating a suction effect pulling the car downwards.
For a sports car that's fantastic, it means more grip.
Diffuser
This effect of accelerating the air below the car can be much improved by shaping the rear of the car underbody, by giving it a slight upward angle.
This improves the transition to the normal velocity of the surrounding air and it helps to fill the wake behind the car.
The result is an even bigger acceleration underneath the car with more downforce as a result.
Design
So if you want to design a diffuser, the question is, how large should the angle be?
Too low and the effect will be too limited. Too high and the air will separate and not follow the geometry of the diffuser.
So much depends on local dimensions and aspect ratios.
An angle between 7 and 10° is considered quite a good starting point for your design.
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We then use those basics to explain the aerodynamics of (race) cars (aerodynamic efficiency of electric vehicles, aerodynamic drag, downforce, aero maps, formula one aerodynamics, ...), drones and airplanes (propellers, airfoils, electric aviation, eVTOLS, ...), motorcycles (wind buffeting, motogp aerodynamics, ...) and more!
For more information, visit www.airshaper.com
Link to sample report: airshaper.com/...
Would you be able to inform about if side skirts help/hurt or have no affect on a cars aero?
Hi!! Good question :) To generate down force on a car, the pressure at the top part has to be higher than the pressure at the bottom. Especially when a diffuser is helping to lower the pressure below the car. To prevent spill-over from the high pressure top to the low pressure bottom, side skirts are sometimes used. They prevent this and thus help to maintain down force!
Hi Lewis, it depends on the car: side skirts can help to "seal" the airflow underneath the car, which in some cases can help to create a better acceleration there. Or, if the edge is designed well (sharp enough), it can help to create a vortex for the air that enters from the sides, which generates downforce as well. And last but not least, if the side skirts extend the width of the underfloor, they increase the bottom surface and which, in case you have low pressure there, also increases the down force.
These explanations on the role of the car diffusers are helpful.
Thanks for the feedback José! If you have any specific questions on aero, make sure to ask them, we would be glad to help and maybe we can make a video on it! :) - Kind regards, the AirShaper Team
Explained in a very simple way. Loved it. Thanks for this!
Thank you very much!
Thanks for the simple explanation. I recently started playing a racing game called Need for Speed: Payback and you can modify the diffuser on the cars. I had a rough idea of what it did, but thanks for explaining it.
Now that's nostalgia! Played NFS3 when I was a kid :) Did the modified diffuser make the car faster in the game?
@@AirShaper I'm not sure yet, I've only just started experimenting with them. I think they do, though.
What a presentation! 9.5/10.
Almost perfect
Click here to download the Aquilo simulation report! airshaper.com/assets/reports/aquilo.pdf
Please try to make a video on how to build and design a fixed wing drones or guide us from where to start to build one
Hi Moustapha, ah, that would indeed be very cool! At a previous company where I worked, we did so: we milled our own wings, laminated them with carbon, fitted it with controller, props, battery, and off we went! At AirShaper, the focus is on the simulations part, so likely, we won't make a movie on making the drone itself (but from a personal point of view, I'd have great fun doing so :) ).
Hi Moustapha, in the meantime we have done so:
ua-cam.com/video/kAXN3MlQxxc/v-deo.html
ua-cam.com/video/FZHYcqRcDJM/v-deo.html
I hope you like the videos! Good luck with your design :)
Could a well-designed diffuser significantly reduce aerodynamic drag on a road car/SUV/pickup truck?
The main purpose of the diffuser is to help the flow underneath the car and connect it to the wake behind. That acceleration leads to a lower pressure and thus improved down force (or less lift). The diffuser can add more drag though: you can see on some supercars that they have different settings for the diffuser: a high-down force setting with a larger upward angle and a high-top speed setting with lower angle. I would guess that if the angle is not too agressive, it might help to reduce the size of the wake behind the car and thus reduce drag. But it would require a smooth underfloor to start, otherwise the flow is already "messed up" by the time it reaches the diffuser and so the effect would be gone.
So well explained.. Thank you! More presentations like this 👍
Thank you very much!
Some details of the working principle of the diffusor - as you explain it - are kind of confusing for me. I somehow like the explanation that the diffusor helps to improve the transition of the high velocity below the car to the free stream velocity in the wake of the car, thus reducing drag and reducing the size of the low pressure zone behind the car (as compared to a situation without a diffusor). That seems somewhat intuitive. One thing that particular disturbs me is the fact that the diffusor will generate a reduction in velocity and pressure increase (as long as the flow stays unseperated). So when the flow arrives at the diffusor region you will have an adverse pressure gradient that slows the flow down, recovering potential energy and reducing kinetic energy. How can this generate a situation where the velocity beneath the car will even speed up more, when the wind sees a larger resistance (higher pressure) at the end of the tunnel? And the increase in pressure beneath the rear car part in the diffusor section should actually increase lift, shouldn't it? Increase in pressure beneath the car == lift. Can you maybe explain my confusion? What is wrong with my argumentation?
I explain the diffusor principle to myselve differently, and this is the only way it feels intuitive for me:
1. As long as there's no separation in the diffusor section the diffusor helps to guide the streamlines upwards in a way that the wake gets smaller (like for a falling water droplet). It is easier for the streamlines to normalize behind the car. Or in other words: the distance behind the car, which is needed until the flow is "undisturbed" again, gets probably shorter and therefore less energy loss is created - drag improves.
2. The diffusor helps to guide the flow beneath the car UPWARDS, instead of just releasing it straight to the rear end (like without diffusor). This is possible for diffusor designs, which don't trigger sepration (e.g., moderate diffusor angles). If the car (or the car's shape) generates a force, which makes the flow turn more upwards, the car must obviously receive a force that points downwards - downforce (actio = reactio). So to speak: the diffusor's principle is not so different from the rear spoiler, which also deflects a more or less road parallel streamlined airflow a little bit upwards and therefore generates downforce.
I don't know if my two arguments can be considered correct or if they contain a physical mistake, but this is the way I explain it to myself and somehow I am missing this in your argumentation. Do you habe anything to add to your explanation?
The upwash from the diffuser is essentially blowing the rear recirculation region behind the car. Even though a diffuser volume as a whole slows down air and raises its pressure, the throat before the diffuser produces a high speed jet that lowers the back pressure of the turbulently mixing air behind the car. Gentle curves accelerate flow due to the Coanda effect, aggressive curves cause recirculation zones, now those recirculation zones can be used as conveyor belts, or air bearings. As the re-circulating vortices can help entrain air and attach to a more gentle curve downstream, assuming of course you haven't reached transonic velocities ~M.6. At that point you really need smooth curves and avoid recirculation zones or sonic shocks can create serious instabilities. Expansion fans require very smooth curves, and is the only way to accelerate flows once flow is choked. The beauty is theta angle for expansion fans is relatively easy to calculate since expansion fans are isentropic.
Exactly what I was gonna say
Hey mate. Loved your insights. Could you possibly recommend me some books on this. I would appreciate any suggestions you have. And thank you
''yes''
Immense thanks for the download link . the real stuff lies in the sample report. awesome
Thanks a lot! I'm glad you liked it :) Are you running aero simulations yourself?
@@AirShaper I usually run the simulations in Solidworks flow simulation module. I have also used Openfoam for an unsteady flow through a tidal turbine.
@@TheOnlyRaceEngineer Cool! Then you know your way around CFD code :)
@@AirShaper Well I would say there is no end to learning and your videos are a great refresher for anyone who admires the wonders of CFD. Thank you once again.
Does the vehicles uses different shapes of diffusers such as elliptical, circular, plane diffuser or just a plane diffuser for better aerodynamics?
Nice explanation. Thank you
Thanks Vamsee you're welcome!
What are the vertical fins on the diffuser for?
Those can sometimes help to prevent lateral flows on the diffuser (which could perhaps lower the velocity and reduce the suction effect due to air speed up). The outer ones sometimes also help to prevent the wake of the tires from destroying the airflow to the diffuser.
@@AirShaper got it! Thanks for the answer!
How to find the report which you mentioned that you have added as bonus? Thanks in advance
Hi Paul, it's in the description of the video!
Wouter, there is no link to download the sample report. Could you please add it?
Ah, it was in the comments. I added it to the description now as well - hope it works!
Yes. Thanks a lot. Came here from LinkedIn. Your videos are very informative. Waiting for next one.
Thanks! The next one is probably about the Gordon Murray T50!
Thanks a lot.. Make more videos like this
Thanks Sky, working on it!! If you want, you can check out our "Race car aerodynamics" playlist: ua-cam.com/video/GApwSjKYUpQ/v-deo.html
I thought it was for chopping up people you run over.
Perhaps that was the original intent until they found out it was actually useful to generate downforcz