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Mitch3007

FLC and CFW in GPU

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Question:

I am currently running a GPU model and I am wanting to apply a constriction to represent pier losses from a bridge. My understanding is that within the GPU I can only run a 2d_FLC file which just reads one GIS field for the constriction. I have calculated the constrictions I wish to apply however I am having difficulty finding out how it will be applied within the model. If you could please assist me with the following questions it would be very helpful.

 

-Is the constriction applied to the centre of the cell or at the outer edges.

-When applying the constriction do I apply it per m or by per cell (by using a negative value such as the layered flow constrictions in CPU) similar to what occurs in the CPU flow constrictions. Or do I simply apply the full constriction to the cell centre, if I simply want the constriction to apply to 1 grid cell. 

Is the same process used for the Cell Width Factors?

Answer:

Within the GPU solver,  only the cell centre values are available and used. You can check the FLC or CFW applied to each cell by reviewing the 2d_grd check file. More info is available here: http://wiki.tuflow.com/index.php?title=Check_Files_2d_grd

Regarding how the FLC is applied:

·         You provide the loss value that will be applied to the cell centre/s. It is essentially applied as a minor loss based on the velocity head of that cell centre. FLC * (V^2/2g)

·         If you use a polygon, all the cells centres within the polygon will be selected and the loss applied.

·         You can also use a line that will allow you to select a small row of cells based on the ‘crosshair’ principle.

·         Using the Read GIS FLC == command there is no m by m application of losses that you see when using flow constrictions (which notably are not available in the GPU yet). i.e. you directly apply the loss coefficient to each cell of interest.

Regarding how the CFW is applied:

·         The Read GIS CFW == command is applied in a similar way but acts to reduce the available cross sectional area of the cell. For example, a value of 0.9 will reduce the cell to 90% of full capacity. This factor is applied to all depths of flow through the cell/s.  

Whenever using these approaches we recommend cross checking the losses through your structure with other model outputs such as HECRAS or with reference to Bridge Design Documents such as Bradley: http://www.ciccp.es/ImgWeb/Castilla y Leon/Documentación Técnica/Hydraulics of Bridge Waterways (1978).pdf  to assess the sensitivity of your assumed loss approach. 

Another good post detailing FLCs can be seen here: http://www.tuflow.com/forum/index.php?/topic/1130-flc-values-in-hx-links/

Regards, The TUFLOW Team. 

 

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