Boxster 986 Aerodynamics @180km/hr

[Nine8Six]

Originally Posted by The Radium King

w the roof the air is forced to follow the roofline which is a longer distance than the air travelling under the car (a curve vs a straight line). conservation of mass and all that says that the air traveling over the roof has to get to the back of the car the same time as the air under the car. since it is a longer distance over the top it has to travel faster (red vs green) to get to the back at the same time as the underneath air. the faster air is less dense, which creates a pressure differential btw top and bottom of the car - this is lift. the purpose of the rear spoiler is not downforce (otherwise it would be a wing) but rather to 'spoil' the airflow over the top to kill the lifting effect (the spoiler creates a pocket of turbulent air which causes the airflow to delaminate from the roof further forward than it would otherwise, forcing it to travel straight back instead of along the car - shortening the path).

w/o the roof then the curved path is gone and the spoiler not as necessary. replaced by a big pocket of turbulent air - less lift, but it takes work to make air turbulent, so increased air resistance.

the soln is to keep the roof (less air resistance) but kill airflow under the car to reduce lifting effect - front lip, side skirts. add a rear diffuser which effectively extends the length of the underside of the car so that the travel distance is the same under as over and no lift effect. stop those frigging radiators from venting air down and under the car. all that kills lift.

next step is to add downforce - wings, front canards, vent the radiators upwards. vent the wheel wells (wheel wells accumulate air with a high pressure point at the top of the wheel just forward of the wheel centre line - vent the wheel wells up and you convert lift to downforce). beware downforce - the faster you go the better it works - chicken out and slow down and you lose downforce which means you lose traction ...

Thanks for that TRK. Knew you'd save the show

What is this? dual-purpose? (serious question)

[The Radium King]

i think so, yes. the tailbase spoils lift (and grabs air to cool the engine) while the wing sits up in the clean air to create downforce.

[Nine8Six]

TRK - I place 1mm plates under the tires to measure lift and downforce. They are essentially sensor preset at -1N/mm2 and I then solve those transient with accelerating air flow. How legal is it to publish/plot this online? let's say I'd do the full analysis of the Porsche Boxster 986 (as per model you've seen here). To your knowledge is this scope for potential attacks?!

To me this is what you guys would need, not the effect of a windy day in BoxsterTown lollll Although you must agree it does great wallpaper and t-shirts!

Also, your take on Ram Air Intake. Given that amazing dynamic pressure over the 986, any scope for a hard roof retrofit? instead of wasting our time doing bigger plenums and all the fuss behind them. No?

[The Radium King]

no idea on legality, but the pressure info would be excellent. even just seeing the airspeed info is interesting. in your initial post, the blue air behind the cockpit is slower than the incoming air so is the turbulent area i was speaking of. am i correct in assuming that the model with the car on an angle has the spoiler up, while the top/side models do not? if so, note how the red 'lift' area is reduced, replaced by the blue turbulent flow.

re ram air, i think grab the fastest air you can - somewhere on the roof in this case? this appears to reflect real world practice as well.

[Nine8Six]

Originally Posted by The Radium King

am i correct in assuming that the model with the car on an angle has the spoiler up, while the top/side models do not?

re ram air, i think grab the fastest air you can - leading edge of the windshield in this case? this appears to reflect real world practice as well.

I'll be honest I have no recollection of what model/angle was what. I have about 30 more screen grabs here with and without the spoiler, wings etc and just picked up the best looking one I had (my current mobby's wallpaper!).

I'll see if I can get an answer from the ppl I know at P but doubt they will be helpful 'anything legal', hence why I was asking you. The problem is the model is so bloody realistic (and scaled also) that I'm sure someone will hit me behind the head if I go too far. I'll ask, never know. In a meantime there is no problem to PM if you need any data on whatever.... given of course it doesn't take me more than a day or two to solve

ps. I had the TMG solvers included in the advanced sim package we got for the Co back in 2016 (mainly for structural). So going through all the online training that actually came with the deal. We only have 4 months left on the maintenance and tough Mrs Boxster was a good candidate to get tings squared for CFD. Let me know if you need anything bud

[Nine8Six]

Oh anyone reading and have a spare hard top, please retrofit and go kill some 911 cars with it

Good car shapes... natural air compressor lolll

[steved0x]

Originally Posted by Nine8Six

Oh anyone reading and have a spare hard top, please retrofit and go kill some 911 cars with it

Good car shapes... natural air compressor lolll

I have a hardtop but I haven't been running it on track because of the extra 55 pounds...

I'll take it with me next time, and hopefully the temp and other conditions will be similar, and I'll post a comparison.

Unfortunately there will be another change at the same time between the two runs, I have sold my Beluga Exhaust and putting my original exhaust back on, but with the Fister mod. With the Beluga I was consistently getting to 126/127 with two times of 128 on the front straight at RRR (vs 122 with the stocker). Some of this is technique but I am not coming through turn 9 6 mph faster so some is the car. Will be interesting to see what happens with the Fister. I may do some runs with the hard top off and then add it back on for a comparison, that way we can do both on the same day.

[Nine8Six]

Originally Posted by steved0x

I have a hardtop but I haven't been running it on track because of the extra 55 pounds...

Gut it man, toss the useless weight away and install this with ducts running to your air intake (testing your fabrication skills haha). You'll get something like +35% compressed air on the 986. See first few plots.

Always wondered why the roof area was used by the performance guys (ruff, mirage cgt, etc). It just never made any sense to me (roof?!?) until I saw why like you guys did.

[steved0x]

Originally Posted by Nine8Six

Gut it man, toss the useless weight away and install this with ducts running to your air intake (testing your fabrication skills haha). You'll get something like +35% compressed air on the 986. See first few plots.

Always wondered why the roof area was used by the performance guys (ruff, mirage cgt, etc). It just never made any sense to me (roof?!?) until I saw why like you guys did.

I want it! Maybe with two channels - the other to feed an air to oil cooler

I saw a 2nd gen MR Turbo with a roof scoop like this and it was sweet!

[BIGJake111]

Originally Posted by steved0x

I have a hardtop but I haven't been running it on track because of the extra 55 pounds...



I'll take it with me next time, and hopefully the temp and other conditions will be similar, and I'll post a comparison.



Unfortunately there will be another change at the same time between the two runs, I have sold my Beluga Exhaust and putting my original exhaust back on, but with the Fister mod. With the Beluga I was consistently getting to 126/127 with two times of 128 on the front straight at RRR (vs 122 with the stocker). Some of this is technique but I am not coming through turn 9 6 mph faster so some is the car. Will be interesting to see what happens with the Fister. I may do some runs with the hard top off and then add it back on for a comparison, that way we can do both on the same day.

Butt dyno says hardtop off is faster.

[BRAN]

Just a tiny little change --> 986 (printed or even better as a badge, would need a little scare box around it) on the arm side.
Right in the center is a little too prominent for my taste

[eicheldp]

Originally Posted by BRAN

Just a tiny little change --> 986 (printed or even better as a badge, would need a little scare box around it) on the arm side.
Right in the center is a little too prominent for my taste

Wow,
What a great design. Put me down for one. Size = XL.

@Fred - I wish I had your toys. I always enjoyed CFD when I was modelling furnaces for heat treatment. Sadly retirement took those toys away. Great rasters Fred!

[Nine8Six]

Ok, I want a t-shirt too. Who ever put this up please link up the "Buy Now" button (Jon?)

Originally Posted by eicheldp

Wow,
What a great design. Put me down for one. Size = XL.

@Fred - I wish I had your toys. I always enjoyed CFD when I was modelling furnaces for heat treatment. Sadly retirement took those toys away. Great rasters Fred!

Wow, surprised, me did not know that, Don!

Yea you should see what's available to us today in terms of problem solving you'd freak out. 'Dynamic-anything' throughout the whole study domain. Toys indeed

The only difference today (from your time I'm guessing) is the abilities to coupled any of the CFD data fields with thermal and/or structural problem solving (PLM suite). Friggin scary when you start thinking about it (lets just hope only kind & honest ppl have access to these tools I tell ya).

Hope you are well there buddy, my regards to you and your family as always

[MarkAP]

Originally Posted by eicheldp

Wow,
What a great design. Put me down for one. Size = XL.

@Fred - I wish I had your toys. I always enjoyed CFD when I was modelling furnaces for heat treatment. Sadly retirement took those toys away. Great rasters Fred!

I'm in for one in XL too.

[Racer Boy]

One thing to keep in mind are that most functioning spoilers don't create downforce, they kill lift. The retractable spoilers found on our 986s and Audi TTs are there for a reason, the factory did air tunnel tests and actual road tests, and found that they made the car more stable by killing rear-lift. The factory wants a car that has more lift at the front of the car than the rear, because that makes for a much more stable and forgiving car.

The software shown in this thread is amazing, but it doesn't replace air tunnel testing. If you were to see photos from air tunnel testing, the smoke plumes would show that there is a dramatic increase in turbulence around the top of the car with the top down. Turbulence = drag. My guess is that a 986 is faster with the top up than down, because of the increased drag. The top speed on my Miata was about 5 - 8 mph slower with the top down. The car shapes are different, but the top shape is pretty similar. A Boxster is about 30 mph + faster than a first generation Miata, so the drag differences between top up and top down would be even more dramatic.

[The Radium King]

here you go ...



or



or just build your own ...

GTM Roof Scoop - Factory Five Parts Catalog

[BRAN]

CTR3...

[JayG]

Submitted it to Blipshift for you Fred

[Nine8Six]

Originally Posted by JayG

Submitted it to Blipshift for you Fred

and then what, they offer the designing work and then they'll email you back the link for us buy them? Let us see let us see

[JayG]

If they accept the artwork, they will create the shirt with lots of options (short sleeve, long sleeve, sweatshirt, etc) and post it on their website. They make them all limited editions and are only available for a few days.

They have great quality and good pricing. If they accept it, Ill order one for you