Diffuser VS Boat tail

[Kevin Cooper]

A rear diffuser can reduce drag when it has a small area ratio (equals small angle). It works the same way as a boattail, increasing base pressure. However, it is not effective if the underbody is smooth. See: "The Aerodynamic Performance of Automotive Underbody Diffusers" SAE 980030 and "Selecting Automotive Diffusers to Maximise Underbody Downforce" SAE 2000-01-0345.

[aerohead]

Originally Posted by MazdaMatt

2.5 degrees steady? could it not curve upwards? ie, if i did make it curve upwards, would flow disconnect at the 2.5* point of the curve?

Sorry MazdaMatt for late response! In Hucho's work,he cites R&D done with diffusers,and the research spells out that if the diffuser upsweep angle exceeds 2.5-degrees,that you'll experience flow separation,losing any static regain benefit of the diffuser.---------- MechEngVT and Kevin Cooper may be more current than Hucho,as Kevin is or has been doing work at Canada's National research tunnel I think.Virginia Tech may be doing some current stuff too.I'm not current with SAE so maybe use Hucho as a rule of thumb,with an expectation that diffuser knowledge is at a higher state-of-the-art.We ought to see if SAE is on-line now,and if we can purchase papers as could be done in the past.They used to cost $3.50 per paper( a great bargain!) and SAE needs to be supported for all the good they do.

[tasdrouille]

Originally Posted by Kevin Cooper

[...]However, it is not effective if the underbody is smooth.[...]

Now that's of much interest to me. I would honestly have thought otherwise. Why is this so? Could you please elaborate a bit for those of us who do not have free access to the mentionned SAE papers.

[donee]

Hi All,

The Basjoos boat tail is designed so the point is above the center line. This is creating a upward motion of the air from underneath, and around the lower half of the car. This is good for stability, and pulling stagnant air along the sides to fill in the rear flow.

A diffuser is limited in the amount of air it can direct, as the air into it is only from beneath the car. The BT can pull air from aside the car upward and backward too.

[Kevin Cooper]

I would like to make a correction to my post on diffusers. I stated that the diffuser is not effective if the underbody is smooth. It should read unless the underbody is smooth. The diffuser can be viewed as a rather weak pump that does not work well if the flow path has high drag due to roughness typical of car underbodies. You will note that all race car underbodies are very smooth.

Also, on the topic of diffusers, if the diffuser angle is small a small drag reduction can occur. Certainly a boattail on the top and sides is better, but the diffuser is out of sight and can add to the boattail. The same comment applies to needing a smooth underbody to get the most from the lower panael of a boattail, which is effectively a diffuser.