Plotting Shear Stress

michael.collins · January 21, 2016, 07:16

Hi,

I'm modelling a cup and bob viscometer. This consists of two concentric cylinders - a hollow stationary outer cylinder with an inner rotating cylinder. A gap exists between the cylinders which is filled with fluid (let's say water). For the rotational speeds I'm looking at, the flow is laminar, incompressible and viscous. I have the no-slip condition applied to all surfaces so this should create Couette flow (i.e. constant shear stress) at all points across the gap.

I have plotted the velocity profile across the gap and am seeing a linear velocity gradient (consistent with Couette flow and a constant shear stress). I want to plot the shear stress across the gap to verify this constant shear stress. However, when I use the Wall Shear Stress function to plot this I always just get zero shear stress at every point in the flow. Does anybody know why this is happening? Am I using the wrong function?

Thanks!
Michael

fluid23 · January 21, 2016, 11:39

According to the help files, wall shear stress is only defined in cells that exist on a wall face (aka a cell with a wall boundary). Therefore I would venture to guess that is why you see zero everywhere (although you should see something at the walls).

If you want to verify the shear stress in the whole domain you will need to write a field function. It should be fairly straight forward if you work in a cylindrical coordinate system. However, if you have already verified that your velocity gradient is correct then your shear stress field function will also be correct since du/dy (or in cylindrical coordinates dVtheta/dR) is really what drives the stress calculation.

January 21, 2016, 07:16	Plotting Shear Stress	#1
michael.collins New Member Michael C Join Date: Jan 2016 Posts: 1 Rep Power: 0	Hi, I'm modelling a cup and bob viscometer. This consists of two concentric cylinders - a hollow stationary outer cylinder with an inner rotating cylinder. A gap exists between the cylinders which is filled with fluid (let's say water). For the rotational speeds I'm looking at, the flow is laminar, incompressible and viscous. I have the no-slip condition applied to all surfaces so this should create Couette flow (i.e. constant shear stress) at all points across the gap. I have plotted the velocity profile across the gap and am seeing a linear velocity gradient (consistent with Couette flow and a constant shear stress). I want to plot the shear stress across the gap to verify this constant shear stress. However, when I use the Wall Shear Stress function to plot this I always just get zero shear stress at every point in the flow. Does anybody know why this is happening? Am I using the wrong function? Thanks! Michael

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January 21, 2016, 11:39		#2
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 17	According to the help files, wall shear stress is only defined in cells that exist on a wall face (aka a cell with a wall boundary). Therefore I would venture to guess that is why you see zero everywhere (although you should see something at the walls). If you want to verify the shear stress in the whole domain you will need to write a field function. It should be fairly straight forward if you work in a cylindrical coordinate system. However, if you have already verified that your velocity gradient is correct then your shear stress field function will also be correct since du/dy (or in cylindrical coordinates dVtheta/dR) is really what drives the stress calculation.