Total Energy Junction

The Total Energy Junction is a unit which provides an alternative to the traditional Open Junction unit. Whereas the Open Junction unit equalises water levels at the junction, the Total Energy Junction equalises the total energy head at all nodes.

Theory and Guidance

Introduction

The Total Energy Junction is a unit which provides an alternative to the traditional Open Junction unit. Whereas the Open Junction unit equalises water levels at the junction, the Total Energy Junction equalises the total energy head at all nodes.

In some instances, particularly those involving high velocities, the Total Energy Junction is a more appropriate tool to use and can provide better representation of the conditions at the junction.

The modeller needs to consider carefully whether the Total Energy Junction is more appropriate; examples of where the Open Junction is inappropriate may result in an energy gain across the junction, or by unrealistic flow splits, e.g. at complex channel confluences/splits..

A conversion utility is provided in the Flood Modeller user interface to enable users to quickly switch between selected Open and Energy Junctions.

Equations

The mass balance (continuity) equation is as per the open junction unit:

Q_in = Q_out

where Q_in is the sum of all inflows at the junction, and Q_out is the total sum of the outflows.

The energy balancing junction is

h_i + v_i²/2g = h_i-1 + v_i-1²/2g (for i = 2, ... n),

where h_i and v_i are the stage and velocity respectively at node i (out of n in total in the junction), and g is the acceleration due to gravity. This is energy-conservative at the junction.

Notes

1. An Energy Junction may only be connected to channel units at present.
2. Existence and uniqueness: there may be certain situations where no solution is possible, or possibly more than one solution. This could lead to convergence problems in such instances.
3. It is noted that physically, the total head at junctions may not be equal. The Total Energy Junction provides an approximation, based on mass and energy conservation at the junction.
4. The Total Energy Junction will not currently work with the Direct Method Transcritical Solver.

Datafile Format

Line 1 - Keyword 'JUNCTION' [comment]

Line 2 - Keyword 'ENERGY'

Line 3 - Labels for the junction (two or more)