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Aerodynamic shear stress over a sphere
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Source：Internet Author：Unknow Pubdate：2010-02-19
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robinfisichella (Automotive)
10 Jan 10 12:35
Hi everyone, im having some probelms visulising the levels of shear stess over a sphere.
I want to know a few things;
.does a turbulent boundary layer have a greater shear stress compared to a laminar one
.once flow separation occurs would there intially be high levels of shear stress inside a forming wake behind a sphere?
.in a low pressure region such as the wake behind a sphere are there higher or lower levels of shear stress?
Many thanks.
GregLocock (Automotive)
10 Jan 10 18:35
does a turbulent boundary layer have a greater shear stress compared to a laminar one
which has more drag?
etc Cheers
Greg Locock
I rarely exceed 1.79 x 10^12 furlongs per fortnight
字串5
robinfisichella (Automotive)
10 Jan 10 18:39
Yes it does, a turbulent BL will usually create a high skin friction drag, as long as both BL's remain attached.
GregLocock (Automotive)
10 Jan 10 18:59
So what is the answer to your question? Cheers
Greg Locock
I rarely exceed 1.79 x 10^12 furlongs per fortnight
robinfisichella (Automotive)
10 Jan 10 20:13
Yes true, though im still not sure about the other two....
gerritgroot (Aerospace)
11 Jan 10 8:19
---.does a turbulent boundary layer have a greater shear stress compared to a laminar one.
Yes, because the velocity gradient near the surface increases (see BL profiles). It increases becuase there's more interchange of kinetic energy between different "sliding" layers of air in the BL. So slow layers go faster and fast layers go slower, this is why delta increases as well 字串3
---once flow separation occurs would there intially be high levels of shear stress inside a forming wake behind a sphere?
At the point of separation, the shear stress is zero (see agian something on BL velocity profiles), after that it can be slightly negative, positvie, negative... chaotic and relatively low. Depending on the vortex structure in the wake behind. (see "von Karman shedding" on cylinders, a sphere will be something similar)
---.in a low pressure region such as the wake behind a sphere are there higher or lower levels of shear stress?
I guess lower (I guessed that in your previous question), but that's a guess. You may look for oilflow visualisation experiments and look at the streaklines in a wake compared to streaklines in attached flow. Don't trust ANY CFD to get results here, all good CFD results in wakes are fake and have been tinkeres by experienced users who knew the outcome beforehand.
robinfisichella (Automotive)
11 Jan 10 10:32
字串2
Thanks gerritgroot i agree with your explanation of shear stress increase in turbulent BLs. I also agree that the shear stress between 'wall and surface' becomes zero at point of separation.
However my PIV results unfortunatly suggest that the wake region behind the sphere are of high reynolds shear stress uv bar and also high normal stress vv.
I am thinking that since the tubulent kinetic energy is high the normal stresses must also be high since tke=.5(uu+vv+ww).
maybe the high turbulence intensity of counter rotating eddies creates some shear force between particles???
gerritgroot (Aerospace)
11 Jan 10 11:11
That's interesting. Nevertheless, even though the local shear forces can be high, seeing your PIV results, my common sense tells me that the netto shear force on the sphere must be zero because of the symmetry.
And between air particles mutually? Yeah, why not? I suppose there will be quite some shear force, but from your first question I understood that you were talking about the shear force on the sphere's surface. 字串4
Anyway, if it's about the sphere, I suppose the pressure forces are much higher with such a huge wake behind it. Why bother about the internal shear force in the vortex?
(Your problem is extremely Reynolds (and surface roughness) dependent BTW. Did you measure the forces as well or do you have some clue about the flow by means of look up tables in Hoerner's book or something?)
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