Hello guys,
As we know, the shear stress can be calculated by the equation: shear stress=viscosity*du/dy (one dimensional flow) in a laminar flow. However, what the formula looks like in a turbulent flow? I doubt it should be different. In some text book, the concept of turbulent shear stress was proposed, like turbulent shear stress = viscosity*du*(average velocity)/dy + reynolds stress. Although reynolds stress can produce a similar effect like laminar shear stress to inhance mixing, we know that the reynolds stress is not a real stress and originated from u', v', and w' (fluctuation of velocity) in inertia term. In fact, I want to know the fluid stress effect on subjects like animal cells in flow field, so the concept of shear stress in laminar flow is preferred. But the flow in my case includes turbulence, I don't know how to determine this kind of shear stress in Ansys fluent. Any suggestion will be appreciated. Thanks.
Likuan
It depends on the typical length scale of the flow field and of the cells. If you have a macroscopic flow, it may be turbulent. But the flow in the length scale of the animal cell is still laminar. So, if you are looking at both length scales at the same time, you cannot apply a turbulence model with statistics of u'.
Yes, I agree with Mr. Riess. In the length scale you are dealing with flows happens to be laminar, according to Kolmogorov. In this case ANSYS FLUENT provides suitable tools for calculations but biological fluids are mostly non-newtonian, and the issue of a different constitutive equation for stresses and deformations arises. Tipically, standard commercial softwares do not include this situation, so you are forced to produce you own one.
If you considered that the cells are circles in 2D or spheres in 3D, then the wall shear stress on the outside circle's or sphere's wall represents the fluid stress you want to know. You can estimate the wall shear stress on the outer surface of the cell or on a specific part of the outer surface from Reports---> Surface Integrals---> Report type (Area weighted average)---> Field Variable (Wall Fluxes, Wall Shear Stress)--> Surface (you can select the outer surface of the cell or make a new surface for a specific part).
I hope this answer will be useful for you.
First, the statement "As we know, the shear stress can be calculated by the equation: shear stress=viscosity*du/dy (one dimensional flow) in a laminar flow" is not correct, this is the wall stress for a 2D flow aligned along the x direction if the wall is assumed not permeable. Otherwise, the other derivative dv/dx would appear. If the cell membrane has traspiration, that affects the stress.
I think that for this small lenght scale the flow is laminar.
Thanks for all the answers. According to the Kolmogorov theory, the lengh of the smallest eddies is indeed larger than the cells diameter in my flow condition case.
- Badges
- Science topic
- Similar topics
- Engineering
- Thermal Engineering
- Thermofluid
- Fluid Mechanics