Roughness and friction coefficient

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Jerome Morrow

Feb 2, 2023, 5:22:11 AMFeb 2
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Dear all,

I'm modelling a fire inside a tunnel, based on a 1D simulation. There's something that I don't quite understand: In the 1D simulation, a dimensionless friction coefficient is specified (lambda, value = 0.035). How can I get the same friction coefficient in FDS? 

As I understand it from the user guide and technical reference guide, FDS is using a log law for the wall model, based on a sand grain roughness (s). What is the default roughness value in FDS, ans how is it related to the friction coefficient iy 1D simulation? Is it by the moody diagram?



Feb 2, 2023, 6:00:26 AMFeb 2
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The User's Guide gives the default values for all input parameters in Chapter 22

Moody diagrams plot curves for various alues of lamda on a chart where the x-axis is Re and the y-axis is relative roughness (sand grain / diameter).  So with your tunnel diameter, desired lambda, and the Re of the flow in the tunnel you can determine what relative roughness gives that point and use that to find the sand grain value.

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