Learning Forum

[taifour]

how wall shear stress is calculated in a turbulent flow in CFX? In the guide it is said that is depend on the turbulence model but no formulae is provided. nIs wall shear stress calculated as conservative variable and then corrected into a hybrid variable? and how CFX performs the conversion from conservative to hybrid variable? in other words, what is the correction method used to take into account the BC. nIn CFD-post the wall shear stress is a hybrid variable, should we understand that it is plotted exactly at the wall nodes?n

[rfblumen]

1.) Wall shear stress is defined in the Ansys CFX Theory Guide by Eq 2-224, where wall shear stress is the product of fluid density, near-wall velocity and friction velocity. The definitions for near-wall velocity and friction velocity are provided in this section.n2.) A discussion of hybrid vs. conservative values for variables can be found in 16.1 of the CFX Reference Guide. The difference between conservative values of a variable and hybrid values is that conservative values represent an 'average' value of the variable throughout the control volume while hybrid values represent the value of the variable at the boundary node itself. Therefore, for nodes on the boundary, many variables will have both a conservative value and a hybrid value (e.g. pressure, velocity, temperature, etc.) whereas other variables (e.g. wall shear stress) only have a hybrid value.n3.) The Wall Shear variable is only defined as a hybrid variable because wall shear is defined as acting on the boundary.nn

[taifour]

Thank you rfblumen for your detailed response. nUnfortunatley, I am still a bit confused about the position where the sall shear is calculated. Is it exactly at the wall (node 1 below) or as an average value on the first control volume (hilighted in red in the following figure form CFX guide). nThank youn

[YasserSelima]

To find the shear stress, you need the velocity gradient. How can you find a gradient with one point where the velocity is equal to zero? n

[taifour]

YasserSelima , you can read comment 1 of rfblumen. n