# water film thickness

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 July 18, 2002, 05:09 water film thickness #1 david Guest   Posts: n/a If a moving plane surface is being moved up (e.g., @ 1 m/s) from a liquid water bath, what should be the typical water film. Any liquid film modeler out there ? To model a y mm thick of water film, how many cells should be used along the depth of the film? Any references, suggestions, comments are welcome. DC

 July 18, 2002, 14:48 Re: water film thickness #2 Lanre Guest   Posts: n/a Perform a 1D force balance. The film thickness would be governed by the viscosity of water. Estimate the film thickness. Reference Bird, Stewart & Lightfoot for solution tips (They solve many similar problems to yours though not exactly the one you describe). The grid resolution is up to you. What kind of accuracy do you want? If you had a 10 mm film height, a cell height of 1 mm might be reasonable. The rate of change of aspect ratio of the cells in the direction of your flowing film is also quite important. Use the guideline of no more than 20% growth.

 July 18, 2002, 20:57 Re: water film thickness #3 david Guest   Posts: n/a Lanre, where did you find "no more than 20% growth" concept? Is this guideline appropriate even near the wall solid surface when the motion is driven by the wall ? DC

 July 19, 2002, 06:27 Re: water film thickness #4 Lanre Guest   Posts: n/a I cannot recollect the origin of the 20% growth rule of thumb, i.e. a geometric growth rate of 1.2 for cell heights in the boundary layer. The height of the first cell dictates the resolution and you cannot maintain that grid spacing throughout the domain. Hence, it is an appropriate technique for use with the finite volume method employed by FLUENT to expand the grid away from the high gradient BL while minimizing errors associated with spatial discretization. This guideline is quite general and can be applied to your case...imagine it is not the wall but the fluid moving, i.e. put yourself in the reference frame of the wall. Keep in mind that you will require appropriate grid resolution in regions of high gradients anyway. You may want to keep a uniform grid height in the film region since you do not know the height a priori.