Thank you very much Iakshman, I'm smiling when I read this compliment :) Feel free to check out the other aerodynamics videos on our "Aero Theory" playlist: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-DbIIw-WryHY.html
Finally someone tells the people that aerodynamics is not only about cute colorful images, it is still engineering! :) BTW, tricky question Wouter from my side: if you model the wheels as rotating and the ground as moving towards the car's front with the same velocity as the car's then do you also put BL layer cells on the ground wall(surfc) ? I have heard both approaches, with and without BL cells at the ground but in reality such BL does not exist because the wind is still and the car slices through the air. In CFD case, we "hold" the car and move the air, so do you add these cells? It will always give some different EVR values when BL cells added to the ground, isn't it? Cheers!
Hi Eddick, thanks a lot for the nice feedback! At the moment, we don't apply prism layer cells and instead we work with wall functions. We're working on it, but as robustness is the number one goal for us, we first need to make sure it can be done properly, even in difficult locations like the wheel-ground interface. Indeed, the ground air is standing still so in theory there is no need for boundary layer cells. What we are thinking of is to just apply them on the part of the ground that is below the car (plus some margin). Because there, you do have a shear stress towards the ground, as the car starts dragging the air along. And around the wheels, you also have a certain 'sphere of influence', so having prism layer cells there would also be good. But getting the mesh and simulation stable is the primary goal, all is slave to that :)
@@AirShaper Thanks for the answer - it is always about the compromise, as in every branch of engineering :) BTW Do you use also SA turbulence model as it was primarly founded for external aero purposes for aerospace industry or you keep focus on KW-SST with wall funcs ?
Thank you! Are AirsShaper, the preparation time is limited as it works with non watertight 3D models, up to 3GB in size and no need to simplify (up to 10.000 components). But generally yes, you're best to clean up the cad, make it closed, prepare the contact patches of the wheels, simplify radiators, ...
Hi, amazing video. I enjoyed each and everything in the video. However, i just wanna ask why did you get a positive downforce? Fz is 1.02e2 which is positive that means there's a lift. Is that normal or common to see in such simulations? I'm running a cfd analysis and even I'm getting positive downforce which is lift. Thanks
Thanks for the nice video! I have a question on something I always find so little information about. In Aerodynamics we use the total pressure coefficient equal 0 (defined a bit differently than the usual pressure coeff) to highlight separation bubbles and wake. Why is that? I mean, what is the physical meaning behind it? Is it like a parameter indicating that for values below zero, the flow has less energy than the freestream? Indicating hence the losses due to recirculation and turbulent wake?
Hi Raffaele, I recommend reading this article by Willem Toet: www.racetechmag.com/2019/02/willem-toet-explains-cfd-post-processing-part-2/ Indeed, the total pressure is a measure for the energy in the airflow. Keep in mind that the Bernoulli equation is only valid for inviscid flows. In real flows, energy gets lost in the wake for example (as you said), leading to a drop in total pressure (and thus less energy). And loss of energy is a source of aerodynamic drag. So perhaps the volume enclosed by the total pressure below 0 isn't an exact representation of the wake, but rather a representation of energy loss / drag sources. If you want to exactly see the wake, defined as 'the air you drag along in the flow direction', then perhaps you can make an isosurface for the X-component of the velocity equal to perhaps to zero (assuming your object is standing still and the wind flows across the object). That should also be interesting! (We did it before, interesting stuff :) )
I have few doubts.. Please help me ! How can I determine the Characteristic Length of a car with its windows open for external CFD analysis in order to determine the drag force ? Also, what y+ value would you recommend in that case if k-w SST turbulence model is used ?
For a car, the characteristic length is typically the length of the car itself. But keep in mind that this is always subjective and doesn't represent a real physical quantity (see our video on Reynolds Number: airshaper.com/videos/the-relation-between-turbulence-reynolds-and-wind-tunnel-testing/sV8l8xd4MEQ). In terms of y+, when using a turbulence model, it is often advised to have y+ between 30 and 100.
@@AirShaper Thanks Airshaper ! This will really help me to proceed further in my simulation . One more thing I would like to know that how do you determine the appropriate size of a virtual wind tunnel ? Is there any rule of thumb to determine the optimum size of VWT ? Since, as the size of a VWT increases , the number of elements in our mesh also increases , which implies longer computational time . And this becomes important particularly for students like me who don't have access to high performance computers . By the way , thanks one again !
@@hemantpandey9676 increasing the size of the tunnel helps to keep the blockage factor & wall effects to a minimum (see this video: airshaper.com/videos/scale-model-wind-tunnel-testing/qqYiz1eo-F4). As it's a virtual tunnel, the cost of increasing the size is quite low, especially because you can use coarse cells for the outer parts of the tunnel. We typically aim for a blockage factor of 1% at AirShaper. But a VAWT is a very "airy" design, so additionally I would also aim to stay well away from the walls. Perhaps this paper can be interesting: pdf.sciencedirectassets.com/271422/1-s2.0-S0167610518X00063/1-s2.0-S016761051730702X/am.pdf?X-Amz-Security-Token=IQoJb3JpZ2luX2VjEO%2F%2F%2F%2F%2F%2F%2F%2F%2F%2F%2FwEaCXVzLWVhc3QtMSJHMEUCIQCNBx%2B%2FjQmBf6py3uvwDhNo9iXCupLn%2F%2Bcy%2FfYGdjJ64AIgdb7723fv15OvzfBLnkqxXYy6utHTLYjkSrVbd8qDxtEq%2BgMIWBAEGgwwNTkwMDM1NDY4NjUiDGpn6aMOKOJwJGti5CrXA%2FK48kspBMu72CgrT5MhOmJxHjoRWjoR548YEkFnRuqnmRXblAo9S3Ikcd3%2FLlVOg0Ce5AmVxDhu20X0P7f5cdn%2BPfxzBbbIeZWXduzlmowImfgJVWmH%2F32wnl0sW1moeXUnVirggxU1Lu7DkTKGLXasXsYgp2tSQpMdbdoo3qPli%2BfeQMaSOYaQ%2FbUzKllaS%2F0M5vfq8O5PrHpmIzo0KAnlP65wyWD6d4azVqzys0KvYv%2BToS4RZvVvxLtZc73W56wIHtMyQIr5xfEo4NQVO3fPmJBKZF4AbHrRZHpOUHK7BQqyXfpkuRQEx1Ds3rbyQHmvkAUHS7MImGkLnDTcmBQMgokxIMIipdiouCA2LcYUhsiKz6XrcuaBQERKKYDcL4NCKbwhmUraf0m42WazB57k9HvBedSMNK1JpSfMPoMugYgUvi20OtQg7Zq14cHs3P59Ao8KQX5Im9h6h5PtylAO5kIZDc2JF1wq9EQ8F2R3KsRUJcT9e7ASVco5Rt8wG16fBNrs7kMhQko2LHAqs%2FsUVFJrPmgnA%2BoStVAz5L9FSjVWuNqb2XsHagiFw3sXPrXrVKpz4YyxX8xB8g3ja7i9Hg6%2FH6RUZgXHnT%2BWP5UxLmHbH5uCsDDV6YaRBjqlAXiA84cRn5HS%2FX%2BKiFslE4jPO0%2BJSpld%2FxZBe%2FbrBGK4MhCL6XTLqohZaz2NRXC3bZNApAStRjA6x1X22bNTPRfLcZ9ufzenP0Z1LOp6%2FvrQSKY9HHYtL4RCO%2BsQk6HZGCP7LEdX2LvAe3j%2BPAOtg14J1cOo4NnbMTWHy26kSMAMYGtLdAmC1rlNpEOujl%2BGXdsP%2FfVS5uowCyjgol0eYn33CTePiQ%3D%3D&X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Date=20220304T074000Z&X-Amz-SignedHeaders=host&X-Amz-Expires=300&X-Amz-Credential=ASIAQ3PHCVTYVY6LGHGJ%2F20220304%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Signature=6b9f7abd07fd79a3332e6f99f27c9a36dec4fd6b8da851bf25e528c748f59c08&hash=43c5be083dc84b60e55481de3f2c48206a4efe0a6168ce86d3d100303da452c3&host=68042c943591013ac2b2430a89b270f6af2c76d8dfd086a07176afe7c76c2c61&pii=S016761051730702X&tid=pdf-802dc23c-e5b6-4a9d-93a8-c37c6d7cfc10&sid=9acaff934353b74e3368f5a46ff93a4f80cbgxrqb&type=client
*Thank you, this is very detailed and easily understandable* ! - By the way, the interpretation of the aero analysis of a teardrop car with 3 covered wheels and extremely low to the ground would show extremely low surface pressures right ?
The surface pressure doesn't need to be low everywhere to have low drag. It's the 'nett' pressure sum across the entire surface that determines the total drag. That means you can still have some pressure increase at the front qs long as you 'recover' that pressure at the back (which requires nicely attached flow, no separation). Even the most streamlined shapes have a stagnation point somewhere at the leading edge.
5:57 would having only one front wheel help with that? I am trying to build kinda small 3 wheel ev so that's why i am asking (speed about 25-50kph) also does aerodynamics matter at these speeds ?
Yes, aerodynamics definitely matter at those speeds! When cycling at30 kp/h for example, already more than half of the energy is spent on aerodynamics. We did a video on the aerodynamics of a low-speed city vehicle: airshaper.com/videos/smart-mobility-city-vehicle-aerodynamics/video/mp-yl8wwEz0 Having just one front wheel could indeed reduce aerodynamic drag. But it would influence the entire underfloor aerodynamics of the vehicle, so you may need to rethink the bottom floor as well (to cope with the flow coming off the front wheel). Feel free to drop me a line at wouter@airshaper.com if you want to run a simulation on your model (if you have a 3D model already)!
@@oplkfdhgk Ah, then be sure to check our interview with Lowie Vermeersch on how they helped design the Lightyear One Solar Powered Car: airshaper.com/videos/lowie-vermeersch-2-the-lightyear-one-solar-car/video/TKHjyC_L0OM
Hello Wouter! For this simulation, which turbulence model did you use? How many layers of elements were placed in the vehicle's boundary layer? What is the maximum Y+ of the first layer of elements surrounding the vehicle? If a plane of symmetry was used, could one get the same results? Why did you choose hexahedra instead of polyhedra for the mesh? Thank you
Hi Henrique, we used a k-omega SST turbulence model to run a steady state simulation. We didn't apply prism layers and used wall functions instead. I don't have info on the Y+ values (they vary across the surface), but you can check this sample project and then download the sample data and check it yourself if you like: app.airshaper.com/projects/sample/sports-car We use hexa cells for the "main" mesh as this limits some of the discretization errors, and close to the surface, the mesh is cut & snapped to the surface (we use snappyHexMesh). Hope that answers your questions!
Cp = 0 does not mean there is no pressure - this is the total pressure in a simulation where the atmospheric pressure is not included (so the "reference pressure" is zero). Cp = 0 is an interesting iso-surface, as it shows the 3D volumes inside which you are loosing energy (because total pressure in a way represents this).
sir i have one question what is the range value that aerodynamics or cfd actually need ....like if my vehicle aero resistance is 90N means then we have to put cfd or not??? velocity of our vehicle is 16.67m/s ( its an electric atv sir ) so how we should decide that ?
Hi Pablo, do you lean the small flaps, or air deflectors, just ahead of the tires? If yes, then indeed, on their own they create extra drag. But they create a wake that shields the tires, reducibg the drag on those. If the reduction in drag on the tires is higher than the extra drag on the deflectors then the nett result is lower drag.
As @AirShaper stated, in the automotive sector (expecially motorsport) it's always a trade off, for example in F1 they use a big vortex line y=250 to shield the undertray
Hello. Can you help settle a bet? My friend said his roof racks on his car generated enough drag to noticeably affect his mpg on the highway. He estimated 3 to 5 mpg less than without racks. I told him I thought that sounded reasonable. Another friend thought this seemed unlikely. Now we have ourselves a wager. Is it reasonable to assume that normal type roof racks could have that much affect?
That sounds realistic! Depends on loading, position, size, ... of the rack, but this could perhaps increase drag by 10%, which would increase fuel consumption by 5-7% depending on the driving speed. That could indeed translate into a 3-5mpg difference. You can do a top speed test with and without the rack on a track. With some calculations, you can then estimate the drag difference!
@@AirShaper I have learned quite a bit from your channel. (I earned a dollar from winning the bet) However at the moment I am building small fixed wing drones from scratch. Attempting to maximize efficiency and flight characteristics using Vehicle Sketch Pad (VSP). Hopefully someday I'll have the resources available to use AirShaper during design!
@@AirShaper thank you I will try it, looks amazing. And I have a question, did you engineered the vents of the car in this video? Like by applying formulas of Fluid dynamics or you simply designed it for sort of fun without getting into FD and engineering side. Just curious coz simulation was amazing.
@@JayTheMachine Hi Jay, I designed those channels to generate lateral forces. I actually didn't know CFD when I did that, but it's the reason I got into it later on. You can read more about it on this page: www.airshaper.com/cases/aquilo-patented-aerodynamic-concept-car
thanks a lot for this informative video, i appreciate it if you would tell the mesh cell type you used in your simulation (tetra or Hexa or mixed between them?
Hi Mohamed, thanks a lot! It's a hex dominant mesh with non-hexagonal cells close to the surface. We use snappyHexMesh (the mesher provided in OpenFOAM).
@@AirShaper but why non hexagonal cells close to surface specifically? I would suggest if you can make a video on meshing best practices for aerodynamics simulations Thanks again for your help
@@mohamedabdelmaksod660 snappyHexMesh cuts them around the surface and then snaps the "remaining" parts to the surface. More info here: cfd.direct/openfoam/user-guide/v6-snappyhexmesh/ As we have automated the meshing at AirShaper, we don't provide options for custom meshing. But we do have a video on how to "chose" between different mesh resolutions - I hope that one is at least partially useful for you: airshaper.com/videos/choosing-your-simulation-accuracy-vs-speed-vs-cost/video/Evxpkwjb5dQ
We use a highly modified OpenFOAM environment. If you want to learn OpenFOAM, feel free to check out our old course material at www.airshaper.com/courses Good luck!
Can I ask question about condition velocity at 50 m/s ? What is reason or some idea that why we should compare each model with V=50 m/s . Thanks for advanced .
The choice for 50 m/s was completely arbitrary for this simulation. Typically, you would select either the top speed (when optimizing for top speed), the average speed around a track (when optimizing for lap times) or the average driving speed across for example the WLTP (when optimizing for vehicle emissions).
@@paulstephen7225 It's often a stand-aline part that is added to the car. But in terms of design & aerodynamics, it indeed interacts with many other aspects of the car.
For pressure we often use the stagnation pressure as a limit for the scale (0.5 * rho * v²). But in general, it depends greatly on which aspect you want to analyze (lift, drag, efficiency, ...). Perhaps you can check our videos on drone design as well: i.ytimg.com/an_webp/kAXN3MlQxxc/mqdefault_6s.webp?du=3000&sqp=CPqs6JMG&rs=AOn4CLDSLfaEPxCTY5Tr-p7LRdZMqdpuig ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-FZHYcqRcDJM.html
Hi sir. Firs of all thank you for the presentation it is pretty illustrative. I have a questions, how do you transform TKE to noise energy? Which model are you using?
@@AirShaper Thank you very much. I am beginning on noise modeling for industrial flows this far I was know that just LES or DES are used to middling but I was not aware of those analogies that may fit better in terms of simulation time. Thank very much.
You're welcome Cesar. Keep in mind that this analogy is a simplifief approach which won't provide frequency information or perceived noise at a certain location. But it's very good as a first assessment on the location of noise sources. Good luck!
Good comment! Yes, that would have been possible here. But it is not exactly the same as a full model (you eliminate lateral glow oscillations) and when we open/close internal channels asymmetrically, we'd need a full model again and we'd be comparing different meshes.
@@ahmetkirhan5896 a lot of the air around the car is affected, but the wake is that part where air is being dragged forward (opposed to the rest of the air passing without too much change in momentum)
Not in this simulation if I remember correctly, but we do have that functionality (both tangential velocity and MRF). Are you looking to run such a simulation?
@@sebastienricciardi883 Hi Sebastien, ok cool I'm curious to see the model! Keep in mind that AirShaper can work directly with open surface models (non-watertight geometry). So if you're referring to that issue, there is no need to prepare models for AirShaper. Just drop me an email at wouter@airshaper.com if you need more info!
You doing videos like this are so much lighter and more interesting to watch compared to you reading of a script with a bunch of info in it, it makes it very heavy, dense and tbh at the end of the day bland and boring. It is much more interesting you just being you trying to explain things.
Hi Nesmio, do you mean it's better in this video (me talking while showing stuff) or better of I stand in front to explain stuff? Thx for the honest feedback!
@@AirShaper It is better in this video when you are just free flowing and being natural instead of in my personal opinion the super forced in-front of camera presentations.
@@AirShaper No problem, I love your videos and presentations with the information in them, they are just a bit awkward to watch a lot of the time, this though was perfect! Keep it up.
