Sunday, December 29, 2013

Lotus 72 CFD


For those interested in a bit of retrospective CFD, I've written an aerodynamic analysis of the 1970 Lotus 72, as a contribution to Mark Hughes's latest work, F1 Retro 1970.

We were extremely fortunate here in being able to utilise the CFD resources of Sharc, and in particular the patient and responsive cooperation of Richard Bardwell. Many thanks to all concerned!

You can read the full text in the published work, but there is an omission in that, to avoid deterring the non-technical reader, I decided not to list there the full details of the CFD configuration.

For the technically curious, however, I can now put that to rights: We used a k-epsilon turbulence model, and employed a mesh containing approximately 20 million cells, with a boundary layer mesh comprising 3 layers. A y+1 of around 30 was chosen. The vehicle was simulated with rotating wheels on a rolling road at a freestream airspeed of 50m/s. The sidepod radiator and airbox inlets were treated as outlets (if you see what I mean).

Two ride-height combinations were used: 40mm front/60mm rear, and 60mm front/90mm rear. Runs were conducted for the Straight Ahead case, and with 4 degrees of steering angle. The frontal area used in the lift-coefficient calculations was 1.372msq. For each CFD case, the solver was run for 1000 iterations.

4 comments:

Peter B said...

Merry X-mas and Happy New Year Professor McCabe !!

My present to you:

https://dspace.lboro.ac.uk/dspace-jspui/handle/2134/13646

2013 PhD Thesis on Multi_Channel
Race Car Diffusers

I cannot wait to buy the book you mentioned in your post !!

Please tell me how splitter plates
"constrain" separation in the diffuser. What is the physical mechanism by which this is achieved. The author did not detail it enough in this comprehensive paper.

Cheers !!

Gordon McCabe said...

Cheers Peter. Excellent work.

I'd need to delve into the separation of a 3D-boundary layer to answer your question properly. The vertical fence ('splitter') forms a physical obstruction to crossflow between the different channels, and ensures that each channel has its own pressure gradient. If the boundary layer separates in one channel it therefore doesn't necessarily in the adjacent channel.

Peter B said...

Does that mean that if the flow moves laterally that it in effect has spent kinetic energy in the wrong direction as to get out the diffuser it should flow as straight as possible from front to back ? What is the mechanism by which say a small separation bubble "spreads" into huge bubble without the fences (I mean is it correct to infer that) ? Any suggestions for good literature
on this separation of 3D boundary layers ?

Keep up the good work.

Gordon McCabe said...

I would say separation has a tendency to spread in a lateral direction simply because of the friction between adjacent layers of fluid. However, 3d boundary layers can be complex. Even within each diffuser channel, there will be pressure gradients in a crossflow direction, and the fences separating the channels can also be used to generate vortices. Here's one paper, albeit a few decades old:

http://www.annualreviews.org/doi/abs/10.1146/annurev.fl.14.010182.000425?journalCode=fluid