[Danial1992]

dear friends,
I am going to define shear stress on surface of an open channel water flow.
how can I do it?
regards.

[ghorrocks]

If you are using a free surface model then you will need a function which finds the surface and then applies a source term to it.

[Danial1992]

Originally Posted by ghorrocks

If you are using a free surface model then you will need a function which finds the surface and then applies a source term to it.

Thanks for ur response , may u give me more informations , I am going to define shear sterss made by wind.instead of modeling wind to reduce solve costs.
how can I do it?
The free surface has a stable level and doesnt chand by time, could u give some example or introduce me a source.
Best regards.

[ghorrocks]

It depends on what you are trying to do. I would do a literature review on this topic before proceeding. I suspect the interaction between wind and fluid is quite complex due to the waves changing the air and fluid flow conditions, so simple shear stress analogies are unlikely to be accurate.

[Danial1992]

Originally Posted by ghorrocks

It depends on what you are trying to do. I would do a literature review on this topic before proceeding. I suspect the interaction between wind and fluid is quite complex due to the waves changing the air and fluid flow conditions, so simple shear stress analogies are unlikely to be accurate.

the wind velocity is so slow (like 0.5 m/s) then it is not going to make big wave and the fluid surface will be stable. so I think shear stress could be defined instead of wind.

[ghorrocks]

If you think a shear stress is OK, then simply apply a wall boundary with a shear stress.

But I would check you are in a regime where surface waves do not develop.

[Danial1992]

Originally Posted by ghorrocks

If you think a shear stress is OK, then simply apply a wall boundary with a shear stress.

But I would check you are in a regime where surface waves do not develop.

yeah , I considered a moving wall on fluid surface which will make a shear stress as wind flows.
thanks in advanced.

[ghorrocks]

You can do it using many different methods. But you use the method which you can describe mathematically what is happening. So if you know the shear stress, use that, if you know the wall velocity (or the equivalent) then use that.

[Danial1992]

Originally Posted by ghorrocks

You can do it using many different methods. But you use the method which you can describe mathematically what is happening. So if you know the shear stress, use that, if you know the wall velocity (or the equivalent) then use that.

I just know the shear stress but how can I find equivalent wall velocity which can make the same shear stress? I didn't find any thing in theory documents about moving wall shear stress.

[ghorrocks]

If you know the shear stress then just use the shear stress. Why convert it to a moving wall velocity?

[Danial1992]

Originally Posted by ghorrocks

If you know the shear stress then just use the shear stress. Why convert it to a moving wall velocity?

How can I define shear stress on water surface (in cfx or fluent)?

[ghorrocks]

In the boundary condition tab, under boundary details. Choose the "Specified Shear" option under Mass and Momentum.