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-   -   Length Scale vs. Hydraulic Diameter? (http://www.cfd-online.com/Forums/fluent/33459-length-scale-vs-hydraulic-diameter.html)

Will April 12, 2004 09:52

Length Scale vs. Hydraulic Diameter?
 
I am running a case where I have mass flow inlets. I have 10 large holes and 18 small holes. The ratio between the large hole and small hole is 1.5:1. My domain has periodic boundaries on each end with an outlet opposite and upwards of the holes. My question is should I use the length scale of the domain or hydraulic diameter of each hole? What I mean for length scale is the distance between the periodic boundaries. Currently I have each hole as a mass flow rate inlet, however I created the mesh where all the holes are one mass flow rate, where in Fluent I separated the holes. Which one would create the most accurate result? Since I do not have experimental results I do not know. I currently ran the model and got results for the holes as one entire mass flow rate. The results show that the velocity is the same for all holes. With this case I used the length scale. I know the velocity cannot be true since mass flow is related to the area. Now, I am using each mass flow rate inlet as an indivisual inlet and am getting results where the smaller holes now have the larger velocity. Can anyone give me help?

Dmitry April 13, 2004 06:09

Re: Length Scale vs. Hydraulic Diameter?
 
Hi, I think it's a good idea to create 2 boundary inlets. First for small holes, and second for large holes. In this case you can set hydraulic diameters for both (large and small) holes. Also, i think it's more correct to use hole's diameters as length scale in your problem.


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