Toasterdynamics Episode 8: Vortex Generators and the Rear Diffuser on the Colt Ralliart
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- Опубликовано: 30 окт 2020
- Investigation of the aerodynamics of the toaster-shaped Mitsubishi Colt Ralliart Z27AG using basic wool tuft analysis and other techniques.
This episode: I try to improve airflow on the rear diffuser with vortex generators under the body.
Music: "Rev" - Eveningland (Royalty-free), "Flying" - Snowplow
rcoltlyf is not a trained aerodynamicist, mechanic or indeed anything to do with cars. He is a scientist though, likes understanding how things work, and really likes his Colt.
コルトラリーアート! :)
I believe your issue was the holes behind the bolt heads. The bolt heads created a low pressure zone creating suction from those holes and venting in a lot f air. if holes were closed, the flow would most likely re-attach back to the diffusor. And if it was only the holes alone, they would have been only a disturbance. the bolt heads made them effective at letting in the air.
I like your thoughts.
I am curious why you put tufts at the exit from the diffuser? It is unrealistic to expect the airflow to remain laminar beyond the point where the diffuser ends. That region will always be turbulent since it is unrealistic to expect the flow to stay attached where there is a sharp turn. You need to and tufts to the entire diffuser and verify the flow in that region. You are making a big inference that the dirt is deposited because the flow is slower and turbulent. Without actual data you are just speculating about causation. VGs are by their very design turbulent. The swirl of the vortex energizes the boundary layer.
Many good questions here. Why tufts at the ends of the diffuser? Because I was hoping that if there was a significant impact from the VGs then which would reflect in more ordered movement of the tufts - which in fact we may have seen but it is hard to know for sure. Vortices start out at least extremely ordered and not chaotically turbulent - here is a link to some old footage of mine (ruclips.net/video/FeFje7cN5DY/видео.html) where this can be seen on the outside of the splitter endplate. It influenced my decisions and expectations for this video. With respect to putting tufts everywhere I did think about that but I could not work out a way of recording it without the camera influencing flow. With respect to the dirt it is outrageous speculation (I probably should have expressed this better in the video) and difficult to prove, though having plugged the holes and flattened the bolts some time ago I should go back for a look.
I know this has been posted 3 years ago, but have you thought about a transverse recess, perpendicular to the flow? It should reduce some of the boundary layer thickness and it could house the bolts and holes without interrupting the flow
I had not thought of that. I like your thought.
That's an awesome experiment! Do you think style of vortex generator or placing closer to the area of separation might change the results? That dirt is an awesome flow visualization!
Around 10cm prior to separation is (apparently) the butter zone for VG location. And that was pretty much where they were. So I don't think so. But I am no aerodynamicist!
have you ever tried to block the holes you left open? Maybe you have positive pressure in between the undertray and the body of the car. Certainly possible because your cooling inlets exhaust that way. The holes might push low energy air into the boundary layer at that critical spot. Basically the opposite of boundary layer extraction.
Indeed I have blocked the holes but I have not gone back to examine the consequences. I like your thinking though!
@@rcoltlyf I’d be really interested. But I might be wrong of course, maybe it’s just that the screws are too much. Or a combination of both. Or something else. I used to build formula sae cars. We always kinda laughed at teams that put stickers on the suction side of their wings (flaps) as we knew from testing that even those few tenths of a gap caused the flow to separate. But of course the boundary layer on a wing with a very small chord is quite small, and those wings were always on the limit of separation. I think there’s nothing you can do but to test
Need to put you test strings slightly closer to the front not at the edge you will get turbulence from the negative pressure in the back of the car. where you placed them just shows that you have negative pressure and that is a good thing that gives you down force.
Seems like a good idea. 👍
Just a thought, you might want to experiment with the strakes. It looks like tyre squirt (dirty air from the rear tyre/suspension area) might be interfering with flow.
Almost certainly it is tyre squirt. I can't take the strakes lower though and still drive it on the road without scraping - it is borderline as it is.
strakes are mostly for looks. they dont do anything. look at the mercedes EQXX insane diffuser, or light year one/zero car, or any long range production tesla or lucid air. all smooth rear diffuser, no vaping teenager strakes...
what you want to know is whether the flow stays attached along the length of your diffuser not at the back edge. There's a about a 70deg turn there, of course that flow will separate. Put the tufts all over the bottom of the diffuser and figure out how to film that.
Best comment ever? The filming without interfering with the flow is the hard part.
Lower side skirts would be more effective.
Yes they would. But not so good for driving around town which also has to happen.
This goes to show that on modern day cars with diffusers at rear bottom . Do absolutely nothing
That's not true at all. Diffusers work better on modern cars than they did on older ones with "dirtier" under-body areas. But, it's important to distinguish between a real diffuser like this guy has, and a plastic vertical fascia with some cheesy strakes in it. Most "diffusers" on road cars aren't diffusers at all. But even that is better than older cars where the rear bumper was fully parachute-shaped.
Most modern cars with "diffusers" have vaguely diffuser shaped pieces of plastic that are styled to look reminiscent of a diffuser but have no actual aerodynamic effect.
the music, fucking annoying music made me cut this informative video short!