Calculation of shear rate/film thickness

[blackmot]

I am trying to calculate the shear rate on the surface of a metal coupon being moved at a constant rate in a Newtonian fluid. In order to do this, I need to know the thickness of the boundary film, and I have not been able to find a means of calculating or estimating the boundary film thickness? I would appreciate any help, or references to good reference material that will provide what I need.

Much appreciated

 

[Latexman]

Try this:

http://www.brookfieldengineering.com/support/documentation/solutions-to-sticky-problems.asp

Good luck,
Latexman

 

[blackmot]

While this is an exellent reference to the operation and theory of rotational viscometers, it contains no information applicable to my problem. The formulas presented herein for calculation of shear rate are solely for rotational systems, with sufficiently small gaps that the shear distance is readily calculable. My problem is that I am trying to calculate shear on a plate suspended in a flowing liquid, where the "gap" is large (approximating infinite). I know the speed of flow, the product viscosity, and the material of the plate, but need to calculate the shear rate and the anticipated shear stress.

 

[Latexman]

By the "thickness of the boundary film" do you mean the thickness of the boundary layer? If so and I recollect correctly, you do not need the thickness of the boundary layer. You need the boundary conditions at the surface of the metal coupon. Unless your fluid is in turbulent flow, in which case Newton's Law of Viscosity does not apply, and you are approximating the results by treating the boundary layer as being in laminar flow. Is that what is going on?

Good luck,
Latexman

 

[blackmot]

Thanks for the replies Latexman.

I am tryng to calculate the shear rate on a piece of metal with a given flow moving past it. I am rotating a coupon in an autoclave enviroment, with the large surfaces oriented parallel to rotation so that the flow runs smoothly over the coupon in a high Re laminar flow (Re = 1800 to 2000). It is simple enough to calculate a min/max/ave velocity through the test liquid; however, calculation of shear implies a specific distance over which the velocity gradient occurs. In a pipe or duct this is obvious, but in a my scenario it seems more complicated. I also was considering testing at turbulent conditions (Re > 4000) as well, which would imply a boundary layer, with essentially all of the delta V occuring in the boundary layer.

 

[Latexman]

blackmot,

What is generating this "flow moving past" the rotating coupon in the autoclave?

If this flow is being induced by the rotating coupon, that suggests you should treat the coupon like an agitator. If this is so, does the autoclave have baffles to hinder rotational motion? It'll need them to more closely approximate a mixer. If what you have does closely approximate a mixer, the following post will help:

thread798-185449

Good luck,
Latexman