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Old 08-05-2019, 08:33 AM   #1 (permalink)
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dragging dust

Noticed on my Kia Niro Suv a large amount of dust on my rear window after driving about 15 min on gravel road. What brought it to my attention was 2 other suvs a Rav4 and a Suzuki x7 which had little dust driving over the same road. So is my ecco niro having drag issues? Feel free to correct my grammar!

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Old 08-05-2019, 12:22 PM   #2 (permalink)
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I'm thinking that accumulation is not by itself an indicator of bad aero other than where turbulence is being picked up into the wake from. The Rav4 has a higher drag coefficient of 0.31 to your 0.30 from what I found. Not sure on the Suzuki.

Perhaps you just drive faster?
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Old 08-05-2019, 05:54 PM   #3 (permalink)
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As Snax intimated, I wouldn't read anything into this.
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Old 08-06-2019, 10:26 AM   #4 (permalink)
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My Insight gets virtually no dust on the hatch top window, which should have nearly laminar flow, but gets a lot of dirt on the vertical window at the back, where there is a lot of turbulence.
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Old 08-06-2019, 12:35 PM   #5 (permalink)
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Some vehicles get less dirt on the back glass because they have a diffuser or slots through the rear roof spoiler to push clean air off the roof across the back window. My Forester had a plastic diffuser and my Minivan has a slot in the spoiler.
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Old 08-06-2019, 03:59 PM   #6 (permalink)
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since it is a MK6 Golf pet peeve, does those rice racer vortex generators do anything for the vortex calming?
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Old 08-06-2019, 05:44 PM   #7 (permalink)
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Quote:
Originally Posted by Ecky View Post
My Insight gets virtually no dust on the hatch top window, which should have nearly laminar flow, but gets a lot of dirt on the vertical window at the back, where there is a lot of turbulence.
Just a terminology note: "laminar" =/= "attached." That far back on any car at any appreciable speed*, the flow is turbulent--but attached over the rear window as you observed.

*Flow characteristics are related to the Reynolds number, which is the ratio of the inertial forces acting on the fluid particles to the viscous forces. When the inertial forces outweigh the viscous forces, i.e. Reynolds number is large, the flow will be turbulent. And since the inertial forces are the product of fluid velocity and test length, as speed increases or the point of investigation moves further along the body, Reynolds number increases. Since the kinematic viscosity of air is very small, 1.46x10^-5 m^2/s, it doesn't take much speed or much length for the flow over a car to transition from laminar to turbulent.
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Old 08-06-2019, 06:54 PM   #8 (permalink)
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Quote:
Originally Posted by Vman455 View Post
Just a terminology note: "laminar" =/= "attached." That far back on any car at any appreciable speed*, the flow is turbulent--but attached over the rear window as you observed.

*Flow characteristics are related to the Reynolds number, which is the ratio of the inertial forces acting on the fluid particles to the viscous forces. When the inertial forces outweigh the viscous forces, i.e. Reynolds number is large, the flow will be turbulent. And since the inertial forces are the product of fluid velocity and test length, as speed increases or the point of investigation moves further along the body, Reynolds number increases. Since the kinematic viscosity of air is very small, 1.46x10^-5 m^2/s, it doesn't take much speed or much length for the flow over a car to transition from laminar to turbulent.
I stand corrected.

Regardless it's pretty cool to be able to drive through a rainstorm and have a nearly horizontal window at the back of the car remain completely dry.
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Old 08-07-2019, 08:37 AM   #9 (permalink)
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Tires, engine fan, underbody.
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Old 08-07-2019, 11:09 AM   #10 (permalink)
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transition length

Quote:
Originally Posted by Vman455 View Post
Just a terminology note: "laminar" =/= "attached." That far back on any car at any appreciable speed*, the flow is turbulent--but attached over the rear window as you observed.

*Flow characteristics are related to the Reynolds number, which is the ratio of the inertial forces acting on the fluid particles to the viscous forces. When the inertial forces outweigh the viscous forces, i.e. Reynolds number is large, the flow will be turbulent. And since the inertial forces are the product of fluid velocity and test length, as speed increases or the point of investigation moves further along the body, Reynolds number increases. Since the kinematic viscosity of air is very small, 1.46x10^-5 m^2/s, it doesn't take much speed or much length for the flow over a car to transition from laminar to turbulent.
There was a Master's thesis on a RAM pickup,which included a CFD study.According to the numerical model,at supercritical Reynolds number,the pickup had 30mm of laminar boundary layer at the nose.The rest was turbulent.

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