If you are simulating a flat plate at zero angle of attack then all the viscous forces will be along X(streamwise direction) and all the pressure forces along Y (normal to wall). But  for an aerofoil, there is a local slope for the wall at every point so in the global coordinates the shear forces will have x and y components. At every local point along the wall the shear force will be tangential to it and pressure forces normal to it. Since the aerofoil, generally. has a curvature with varying slopes there will always be x and y components for both shear and pressure forces.

If you are trying to calculate using viscosity*du/dy (here y is local wall normal coordinate not global y). If you use global coordinates then there is a component of viscous forces from x direction as well not just y.

Carefully accounting for components of forces from different directions at every local point along the aerofoil and then integrating for the whole system will resolve any ambiguities.

Thanks for the response Srisha. I totally agree with you that  X & Y coordinates are to be considered when calculating the shear force on the wall due to the local slope at each point. Applying this principle gave me the viscous force at a particular AOA (15.717069 in my case: AOA 2); which is in global coordinate.

But, here I am trying to figure out how the viscous stress in X & Y coordinates are calculated (highlighted in figure) which when integrated over the surface will give me the local shear stresses (X and Y viscous stresses)

Cheers!

When y+ is less than 11 the velocity profile from the wall to the first grid point is considered linear(viscous sublayer) , if y+ greater than 11 the log law profile is fitted. Use these profiles in -μdu/dx etc and you will find the answer

Hi Markatos,

Thanks for the input; it helped. My y+ falls in the log law layer and I defined the calculation in Fluent using CFF. I tried to validate the calculation with the Fluent method and got XY plot similar to wall shear stress.

But, as mentioned in my initial post; I would be interested in X-wall shear stress and Y-wall shear stress (instead of wall shear stress) as defined in Fluent. Is there any way of getting the same adding vector components?

Cheers!

Best way is to compute it yourself. Take X velocity component gradient (ie  the next to the wall X velocity divided by the first node distance from the wall) and multiply it with viscosity there times area .The same with Y velocity

Markatos, I agree with the approach you mentioned and tried to give it a try. I used the derivative option in the XY plots which gave me a list of options. Aren't these the gradients used in the shear stress calculations. 

I defined a CFF by using the gradient from the derivative option and multiplying with the viscosity. Using this, I should be getting a plot similar to X wall shear stress (Y wall shear stress when using the y gradient). But unfortunately, they do not match. Do you have any idea as to what do these derivative in the XY plot (Fluent) means?

Hi Vinit, have you ever sort this problem out? I'm facing a similar problem which depends on the area of the wall. I want to use the torque (T=P*A*d - P=Pressure / A-Area / d-distance from the axis) as a convergence monitor. But apparently the x-face-area available in CFF (Field Function Mesh -> X FAce Area) is a vector instead of a scalar.. Do you know how to get the surface-cell area of a wall BC tat I could use in Pre-processing?

Thanks in advance.

Regards, Jose