Finding the coefficient of discharge, C, is one of the most common studies in hydraulic engineering laboratories. The factor is proportional to the effective water discharge, Q, in the spillway formula below:
B is the width of the spilway and H is the elevation difference between the top of the spillway and the water level behind the spillway. The C coefficient will vary with the geometry of the structure. A large number of dams have non-standard spillways where the coefficient of discharge can not be taken from tables or literature. The hydraulic engineer is therefore often faced with the problem of finding the parameter. A physical model study is costly. Over the last years, it has been increasingly popular to use a CFD program for this purpose. A large number of CFD studies have been carried out where the results have been in good agreement with physical model studies.
OpenFOAM is one of the most popular open source general purpose CFD programs today. It is possible to use its solver interFoam to compute a mixture of water and air, to model the free surface of the flow over a spillway. With the user-specified inflow and outflow velocity and the given areas, the user is defining a water discharge. The program will compute the water level behind the spillway, and it is thereby possible to find the coefficient of discharge for the spillway.
The broad-crested weir has also been computed with Flow-3D and SSIIM. More information is given here
PDF file of OpenFOAM homework for the course TVM 4155 Numerical Modelling and Hydraulics
Link to NTNU OpenFOAM Spillway Tutorial The tutorial shows how to make a grid using the OpenFOAM tools. The spillway geometry is defined with an STL file. Note that SSIIM 2 is able to make an STL file of the bed of the SSIIM grid.
Some animations of the flow are given below:
Broad-crested weir 1 (71 MB) (F 64 11)
Broad-crested weir 2 (17 MB)
Broad-crested weir with wedge-cells (19 MB)
Plate (16 MB) (F 64 0)
Weir with vertical upstream side (43 MB) (F 64 2)
Weir with ugly grid (32 MB) causing strange results
Back to CFD page