Let's Talk Aero

Discussion in 'Road Racing Forum' started by antman450, Oct 21, 2017.

  1. Olaaf

    Olaaf TMO Intermediate

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    It's not validation, it's correlation. I said i don't know what the impact would be. OK, just checked my mustang. No plastic push pins. 10/32 screws and rivits from the aluminum skirt to the plastic rocker, and then to the steel jacking rail welded to the pinch/rocker.

    [​IMG]

    Most definitely. When i was an MX-5 cup engineer, we raced with IMSA as a support series. I chatted with the head Porsche GTLM engineer for awhile and he told me that they wax their cars before qualifying for this very reason. As you know, drag increases with the square of speed, so the average speed around the track isn't the important bit for factoring skin friction drag.
     
  2. Busdriver

    Busdriver TMO Intermediate

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    Air passing under the outboard portion of the splitter exhausts into the wheel well, which is what would be venting from the fender.
    The vents would ensure a clean exit of air, so the mass flow rate under the splitter could be maximized, which is where the negative pressure comes from.
    The "normal" path for air to exit the engine area is under the car, so venting high pressure air to the relative low under the car. Which contributes to the lift producing shape of the car. A big hole in the hood allows a path to the top of the car for that high pressure air, which helps kill that lift. But there's still a path to the bottom as well. "Closing" the path to the underside maximizes the pressure "exchange" with the low pressure area above the hood.

    Any incidental increase in pressure under the hood wouldn't really matter since it would be pushing on the top of the splitter as much as the bottom of the hood. Given the size of the trackspec louvers, I don't think there would be much pressure increase anyway.

    I'm thinking of running the splitter back to the front edge of the K member. I would think actually sealing the splitter to the firewall would require a side pipe exhaust.
     
  3. VoodooBoss

    VoodooBoss Rick Moderator

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    ACLU is one of your sponsors?
     
  4. Busdriver

    Busdriver TMO Intermediate

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    ????
     
  5. bpracer

    bpracer Mark

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    ACLU sticker on the side of Olaaf's car photo above.
     
  6. Busdriver

    Busdriver TMO Intermediate

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    Ahh. Missed that, poor connection. Thought I was missing a good zinger.
     
  7. Grant 302

    Grant 302 OPM Spent: $665,833 Moderator

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    But impact for removing:
    So in conclusion: No down force, but you don't know.

    Sure, yes, it's relevant for the whole car. No, it's not relevant when talking about the skin friction effects or differential for just the rocker panels or side skirts. You're making the parts of parts that should be irrelevant to a discussion to be bigger than they are.

    Oh, they're there. Just look under the stuff you bolted on. You didn't bolt on that plastic rocker panel.
     
  8. Grant 302

    Grant 302 OPM Spent: $665,833 Moderator

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    Not really. Most of that would contribute to tire 'squish', which would get mostly pushed laterally at ground level.

    Yes, but not from under splitter air. That wouldn't affect 'flow rate', mass or velocity under the splitter.

    EDIT: Depending on the placement, it might even raise the stagnation point at the front of the car, not a good thing IMO.

    I understood what you're getting at, but the way it 'nets out' or sums up is that there will be a higher pressure on the remaining hood surface. That surface is still larger than any extension of the splitter under the engine toward the fire wall. Seems to me that it would net little and possibly some lift, while creating very high potential for thermal management issues.
     
  9. Grant 302

    Grant 302 OPM Spent: $665,833 Moderator

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    I think that makes sense. And agreed about issues running it to the firewall.

    And FWIW, I'm not against fender well vents, just skeptical about the 'very good upgrade' comment from earlier. JMO, but I think you want them generally positioned as high or higher than your front grille opening, if possible.
     
  10. Busdriver

    Busdriver TMO Intermediate

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    I seriously doubt their position relative to the front grille matters. The vents up high have more to do with tire rotation pumping air up in the aft portion of the wheel well. At the end of the day, cutting a big ass hole in the fender is fairly easy and there isn't a decent space to mount another set of louvers on the 11-14 front fender.

    I seriously doubt the longer splitter will create much in the way of lift problems when combined with the already large holes in my hood. The hood vents would be large enough to put your head through if the louvers weren't there, and you can feel the air blasting out of them when the fan kicks on. Thermal management is something I've been thinking about however, especially since Terry Fair's crew already figured out how to move all the junk out of the way. But I figure it would require a new oil cooler setup to do right.
     
  11. Grant 302

    Grant 302 OPM Spent: $665,833 Moderator

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    It does depend where you're drawing the air from. My comment was for engine bay air. If the supply for the vents is from the rear of the fender liner like the GT350, it won't matter as much. I should have clarified.

    I don't mean that it would 'create problems', but I do think it could make a small net lift contribution if extending it to the firewall. Even as large at the vents are, there is still hood remaining toward the rear bulb seal. The only way to know for sure would be to take pressure readings on a car built that way.

    A little off-topic, but I'd put an oil cooler right under a hood vent if/when possible. Again, JMO.
     
  12. Busdriver

    Busdriver TMO Intermediate

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    Fair enough.
    Assuming it actually increased under-hood pressure at all, sure. But I would think the larger splitter surface area (bottom side) of low pressure would more than offset.
    Only a little, managing air flow is aero, right? It's an interesting thought, what I really want to do is duct the backside of the radiator to the hood. But I'm not sure there's enough space to get the ducting (let alone an oil cooler) in there with the Boss manifold even after moving the intake/coolant reservoir.
     
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  13. Olaaf

    Olaaf TMO Intermediate

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    I can't throw out numbers without knowing the car, wheel width, brake rotor size, ride height, camber, splitter, canards, etc. Then i would need to model it. I don't like guessing that much. 10/lbs of what? DF? DF net? or DF over the front axle? drag? force is force.

    I can say that when looking at TP slices of a production type car, there is alot of pressure build up in the wheel wells. This is a TP slice of my s197 with no fender venting and an incomplete floor model (no strut tower, tie rods modeled yet). Total pressure is also know as stagnation pressure, where the flow is stopped. tp=p+q, where p is atmospheric and q is dynamic pressure.

    Where do you think this flow will go? Guess correctly and i'll post the next slice 1 foot further down the body. It's currently pressing inside the boundaries of the wheel well in the Z+ and X- direction, which means it's exerting both a drag and a lifting force.

    A fender vent gives this pressure build up an exit path that is not a hard perpendicular path, which creates undesirable flow structures.

    Ignore the flow structure I've crossed out, that is caused by an element on my splitter (not shown on the model below the pressure slice)

    [​IMG]

    [​IMG]
     
  14. Olaaf

    Olaaf TMO Intermediate

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    It's immaterial, but no, i removed them when building the flat floor and side skirts, replaced with 10/32 hardware and rivits.
     
  15. Olaaf

    Olaaf TMO Intermediate

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    haha, i wish. kind of the opposite.
     
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  16. Olaaf

    Olaaf TMO Intermediate

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    Correct- i don't know.

    Not quite true, ever buy a real carbon race splitter or diffusor, or seen one on a pro car? The molded side is the "shiny smooth side" and is always presented upside down, that is, the smoothest side of the part is presented to the underside (in the case of a splitter or diffusor) because it matters the most. RHR does this on their parts, and this was stressed to me and AJ by RHR's aero consultant before he went to F1. It doesn't look sexy looking at a rough carbon where it's visible when standing around the car, but a company like RHR does produces racing parts, not show parts, it matters to them.

    If, as you say, the GT350 side skirts are aerodynamically important, than their surface area (not inconsiderable given their length, i think) ought to be a factor?

    And, yes, skin friction drag is calculated in CFD when post processed, it's referred to as wallshearstress- the equation is (2*tauw/(rho*V^2)). I'll post some examples from my S197 when I'm a bit further down the development road.

    Edited to fix quote markers G-
     
    Last edited by a moderator: May 15, 2018
  17. Olaaf

    Olaaf TMO Intermediate

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    [/QUOTE]

    Only in conversation with RHR's aero consultant- he wanted us to close out the AI spec crusher splitter to the firewall to prevent air reversal into the engine bay. NASA AI rules wouldn't allow that (they have strict aero rules), however. The TT/ST splitter was designed for a flat floor and this wasn't a factor.

    I've placed a MAP sensor in the engine bay of my car and was surprised with my results which were negligible with minimal hood venting (this is when i was transitioning my car from a NASA Spec Iron car to an AER endurance car with unlimited rules). I don't think it was the appropriate sensor, or my sampling rate was lower than needed to capture pressure fluctuations and reversals. I think a good real life test would be to place tufts and a go pro on the end of the splitter. I'll see if i can do that (without melting the go-pro). In simulation i can run some streamlines and glyphs on my next CFD run with the splitter and better detailed floor.

    Yeah, you'd have to be clever with the exhaust.
     
  18. Olaaf

    Olaaf TMO Intermediate

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    Yes, very very true- many people ignore what happens to the air after the inlet- for example, once it's ducted to the radiator or cooler, what then? What is the exit plan? You still have the same air mass as you did before the inlet, so the outlet is just as important.

    A waterfall style radiator exit is the most ideal situation. If you are willing to cut out the front/top radiator support, lean the rad forward, weld in a support, and make a "waterfall duct" up to the hood, get a small fan or two, some sheet metal fab, you can ensure the rad air gets ducted out of the hood- all GT3 and GTLM cars do this.

    I haven't done the lean forward bit myself (yet), but i moved the water and PS tanks to where the battery used to be located, and re-located the battery to the passenger compartment. Still not much room with the intake elbow and stock radiator position for a waterfall exit, so the next step is cutting and welding.

    As for an oil cooler, you want to place that in front of the radiator if possible, not behind. I personally use an RX-7 oil cooler- easy to get on ebay, and super low profile but really thick and handles alot of cooling load- been running on my 4.6 3v for 4 years with oil temps never above 260, even in very hot ambient conditions and 8 hour long endurance races.
     
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  19. Grant 302

    Grant 302 OPM Spent: $665,833 Moderator

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    [​IMG]

    [​IMG]
    [​IMG]

    First thing I notice is that the slice doesn't match the iso view...

    The iso view is of a stockish early S197 with a massive 'box' for the engine. And where are all the elements that are sliced-through in the iso view? And is there a strake-like element behind the rocker panel or not?

    No scale or legend on the pressure plot/slice? I haven't usually seen plots where atmospheric/free stream is red. Low pressures are usually *blue*.

    "a lot" of fender well pressure is relative. A stockish car like shown will certainly have higher pressure at the lower front face of the back of the fender liner.

    Despite the inconsistencies of the presented problem and lack of test parameters, my guess is that 12" downstream most of this flow structure will begin dissipating and its cross sectional 'size' will be smaller and somewhat attached to both the ground and the body panel/door and rocker. It would look like it's tapering down if plotted as an 'isosurface' at the border between say the green/yellow or yellow/red pressure values from your slice. Just my guess, and assuming you aren't using v=mach 1 ;)
     
  20. Grant 302

    Grant 302 OPM Spent: $665,833 Moderator

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    Really? I don't see how you got the nut on the back side of the 5/16" hardware in there, but O.K. :rolleyes:

    Have you seen the surface of the GT350 plastic parts? It's not smooth, and I think that's actually a design element included for its aero properties...

    But either way: No, I don't think it's (skin friction on the skirt/rocker panel) a relevant or significant factor to this discussion regarding downforce and comparing to their removal.
     

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