Ric, this is wrong in my opinion: shear stress and viscous forces are parallel to flow (this results from shear stress equation).

# Ahmed Body Drag Validation Problem

**Ricardopg**#43

Hey Retsam!

Yes, I agree with you. I was referring especifically to the y component (y wallshearstress that was plotted on the post above). Sorry, Iāll edit this sentence to make it more clear.

**Retsam**#44

Ok Ric: odds are also that low Re are not relevant fort he case we try to solve. We are possibly in a regime called ādrag crisisā , explained a bit on sphere geometry in different documentation:

This happens to sphere, which is ābluntā shape, I expect its extends much more for streamlined objects, like airfoil profiles. Anyway, this is a very interesting topic. Some guys where inventing golf ball, where pimples are adding kind of viscous layer and air is āfeelingā much less resistanceā¦

**DaleKramer**#45

What is best practice āreference lengthā for RE calcs in internal pipe flow?

What is the āreference lengthā for RE calcs and BL layer calcs for that sim run?

EDIT: I took the time to do a yPlus Histogram for that sim run:

I really think that we should try to make sure we are using a AvgY+=1 Full Resolution Wall BC for this example.

I am not sure it will make a difference but I think we should cover all bases.

I might be able to get some time to re-mesh and run tomorrow, or maybe you could try thatā¦

I am even thinking we should try running prism layers right to pipe center, what do you think?

That is a GREAT presentation you linked to, it will take me a bit to absorb it

**Ricardopg**#46

For flow inside a round pipe, the characteristic length is the hydraulic diameter (dh):

dh = 4*cross sectional area/wetted perimeter = internal diameter.

And yes, we can do that, just to make sure and make some further comparisons between hex and tet

**DaleKramer**#47

Ric has given us a simple explanation and a possible reason for the apparent discrepancy between OpenFoam integration of the Wall Shear Stress vector into viscous lift and drag forces and my ParaView integrationā¦

So, before going down too many rabbit holes trying to solve this riddle, I decided to concentrate on trying to find out what Frame of Reference that OpenFoam actually uses for its calculation of the Wall Shear Stress vectorā¦

Consequentially, I researched Wall Shear Stress (WSS), in relation to CFD, and with a new goal of finding out OpenFoamsā Frame of Reference for itā¦

I found nothing related specifically to OpenFoam, but I did find this statement in a ScienceDirect article:

" In arterial blood flow, the wall shear stress expresses the force per unit area exerted by the wall on the fluid in a direction on the local tangent plane."

I am pretty sure that is NOT a reference to the WSS vector which we are given by OpenFoam because, **our WSS vector is NOT "in a direction on the local tangent plane"**

Putting that difference aside, it does appear that it may be standard practice to **define wall shear stress** vector in the frame of reference where **forces are exerted by a wall on the fluid**ā¦

This **seems contrary** to the way we are used to having a Frame of Reference defined for our aerodynamic forces, where **Drag is defined as**](https://www.sciencelearn.org.nz/resources/1346-causes-of-aerodynamic-drag) **āpushing in the opposite direction to the motion of the objectā**.

So, I am now willing to agree that the āproperā way to integrate the WSS vector components in ParaView, is by adding a multiplication factor of -1 on the WSS vector components, during integrationā¦

By doing that, ParaView integration agrees with OpenFoam integration and everyone should be happy now.

For those that follow, I will soon create a Forum topic on how to integrate WSS into viscous forces using ParaView ā¦

PS. Many thanks to Ric and Retsam for putting up with me in a detailed RocketChat discussion today, which led to this post