Surface Roughness?
 

Hi Dear experts,


I want to simulate an open channel with the given roughness (D50 is known). Based on Flow3D's manual "If the actual surface is uniformly rough, this height can be applied directly". In this way, can I consider the given mean diameter (D50) as surface roughness?



I would be very grateful if you could advise me in this matter.


Thanks beforehand
Hamid

The surface roughness in Flow 3d is a Nikuradse type of roughness (ks), then, if you have values of Manning you can use the following equation:

ks= (n/0.0389)^6

in your case if open channel is concrete lined use n = 0.015 for concrete so ks = 0.00328

REFERENCE: Yen, Ben Chie (1991). "Channel flow resistance: centennial of Manning's formula:. Water Resources Publications

Thanks for your attention & response, Dear Zohran.

Sorry that my explanation was not clear enough. In my case, the experiment has been conducted over a rough fixed bed (D50 is known, uniform sand of median size D50 of 0.71 mm). As far as I know, the relation you pointed out is used for wide open channels, but it is not true in my case.

I would very much appreciate any comments you may have on my issue


Best wishes,
Hamid

Hi Hamid

Is this a scouring application?

Hi Dear Koful,

No, I want only to investigate the flow pattern around bridge pier over a rough fixed bed (D50 is known, uniform sand of median size D50 of 0.71 mm). If you have any experience in this regard, your comments will be highly appreciated.

Best regards,
Hamid

Hi Hamid

I am not sure about using d50 for surface roughness. I did not see any surface rougness reference for sand beds. Packing fractions and shape factors can effect the roughness calculations. Actual roughness value might be smaller than d50. If these are at the same order, yes you can use it.

Hi Dear Koful, thank you for taking the time to explain this.

To wrap up this discussion, I'd be most grateful if other friends/experts would share their own experiences on this matter.

Many thanks,
Hamid