Quote:
Originally Posted by cyclopathic
It is funny how these VG threads degrade after several posts, dog mask, heh.
Back to aero posts: if we take for the basis numbers posted in Mitsubishi article (1.4% Cd reduction and 4.5% in Cl) detecting 1.4% outside of controlled experiment would be difficult. For example on my commute route MPG may vary within 10mpg depending on temperature, lights and traffic situation. 0.7-0.8mpg gain will be lost in the background noise.
I disagree that articles like in OP should be discarded completely. While they cannot claim any numerical accuracy, they still give useful comparison. Even if you can't say by what % A is bigger than B, you can still say that A is bigger than B.
There are numerous credible studies on VG airfoil application which prove that VGs can speed up boundary layer and delay flow separation. With cars having more complex shapes things get more complicated. Finding an optimal location for VGs is fool's errand or it could be.
Cars also need to adhere to aesthetics and safety regulations and should be easily incorporated into manufacturing process. So far the most common VG application is to the rear in attempt to reduce wake trail. I am not convinced they will not be effective in the front but may impose pedestrian crash hazard when applied to bumper, hood or roof leading edge.
But this does not mean that VGs are magical item which only works when placed in right place. There could be still benefits even if they are applied to less than optimal position.
Also last but not least assumptions on point of flow separation could be wrong. Even in relatively simple case (hatchback) you can't just assume it will be roof trailing edge. It could be a gap between the roof and hatch door.
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We'd want to keep an eye on their model scale and tunnel velocities.Gopal & Senthilkumar for instance,used a 1/15-scale model which would require a 300-mph tunnel speed.The 1st problem is,if they actually used 300-mph air,they'd be into transonic flow,with compressibility issues which don't affect real cars.The 2nd issue is that they use only a 16-mph air speed,which locks them into a laminar boundary layer,producing very high drag coefficient.
Their VGs act as dimples on a golfball,in themselves forcing the TBL transition only in the aftbody,resulting in a measured,90% drag reduction @ 15-degrees yaw,and 20% at 10-degrees yaw.The VGs aren't enhancing an ever-present,existing TBL,their affecting it's 'creation.' All real cars are at full TBL by 20-mph with zero enhancement.
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Manikandan et al. used a 1/25-scale model TATA Nano in their CFD work,which requires 500-mph air for TBL.They say that using a 'momentum deficit method' renders acceptable accuracy.When you look at the momentum deficit method,it's predicated upon a max. 0.3-MACH airspeed.They'd be using 0.68-MACH.
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In 'Drag Reduction of a Car Using Vortex Generator,' the authors assume a boundary layer thickness based upon a BL thickness formula for a Laminar Boundary Layer (something which doesn't exist for cars).Their Reynolds number is below 500,000.Their highest airspeed is 4.8 m/s.
They report drag coefficients as high a 6.859 for their car,when a classic parachute is Cd 1.35.
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We just need to careful with these small-scale investigations.