## Fluids

#### Error in computing shear stress

• clabr
Subscriber
Hi everyone nI have got a fluid flowing inside a cylindrical pipe and I want to compute shear stresses in the entire fluid domain (not only at the wall). I've tried calculating them using this formula: viscosity*shear rate and I've used the shear rate computed by Fluent.nIf I consider the walls, as a result I get a value of WSS equal to the one computed by Fluent at the outlet but different at the inlet. It seems that at the inlet the values that I've calculated are halved with respect to the WSS computed by Fluent.nDoes anybody know why this happens?nThanks in advance!n
• Karthik R
Hi, nShear stress at the wall generally gives you an indication of the overall viscous losses in the pipe. As I'm trying to understand your question, could you please elaborate on why you are estimating shear stress at the inlet and outlet boundary? Also, are you estimating this value at the edge where the inlet boundary meets the wall? nThank you.nKarthikn
• clabr
Subscriber
Hi Karthik,nThank you for your help. I have blood flowing and I want to estimate shear stress inside the blood, not only at the wall. So I made a UDF to calculate them by using C_STRAIN_RATE_MAG(c,t), then what I do is check that the values that I've obtained at the wall are equal to the ones computed by Fluent, in order to do this I make a xy plot of these quantities along a line at the inlet and one at the outlet. Then I compare the values I have at the wall (when the line meets the wall) with the one I've computed. nI just use the line at the inlet and the one at the outlet to check if my calculation is ok but I'm interested in the shear stress in the all domain.nn
• Rob
Ansys Employee
Check a little into the domain, computed values on the boundaries can be misleading: especially on inlets where the flow is sorting itself out from the boundary condition to the developed flow field. n
• clabr
Subscriber
I checked inside the domain and this is the result: nwhile if I make the contour, values are quite different in the first part:nnIn the xy plot of the inlet the difference is even more evident. I know that the fluid is still developing but shouldn't I get the same results since I am using the same formula that Ansys is using?nThis is the udf I used to compute the SS:nThank you in advance.n
• Rob
Ansys Employee
Why would the wall shear stress give a meaningful result away from the wall? n
• clabr
Subscriber
I want to check the shear stress profile on the bloodn
• Rob
Ansys Employee
Wall shear stress isn't the same as bulk flow shear stress. Strain Rate and other derivatives are available via the Derivatives section of the post processing. n