Toasterdynamics Episode 8: Vortex Generators and the Rear Diffuser on the Colt Ralliart

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! :)

[EngineeringSiblings]

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.

[lobofpv]

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.

[alexmat]

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

[driven_nation]

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!

[sphericalsphere]

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.

[CrazyMadElfProd]

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.

[motojojo_]

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.

[ukemike]

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.

[jimastra]

Lower side skirts would be more effective.

[tpike]

This goes to show that on modern day cars with diffusers at rear bottom . Do absolutely nothing

[Charlie]

the music, fucking annoying music made me cut this informative video short!