MIKE+ models outlet head losses at nodes. The general flow equations are valid only for continuous conduits, where the only resistance to flow is bottom and sidewall friction. Hydraulic conditions in nodes, i.e. at manholes and structures, serve as boundary conditions in the computation of flows in conduits.
In turn, hydraulic conditions in a node depend on the flows in the inlet and outlet conduits. These hydraulic conditions, expressed in terms of the energy conservation principle, are calculated as water levels and velocity heads. The calculation is based on the mass continuity and formulation of more or less advanced energy relation between the node and the neighboring links, including energy losses caused by local flow disturbances at different locations in the node.
Head losses are calculated at manholes/basins structures in MIKE+. The MIKE 1D engine calculates following losses:
- Inlet expansion loss
- Direction or bend loss
- Drop loss
- Outlet pipe contraction loss
The head loss formulation in MIKE+ is quite flexible and different head loss formulations can be combined into individual Head Loss IDs. A Head Loss ID is basically, made up by
- Effective Node Area
- Loss Coefficient
- Max Loss Limits
The Method represents the ‘Classic’, ‘Mean Energy Approach’ and ‘No Head Losses’. ‘Classic’ corresponds to the former MOUSE head loss options, Round Edged, Sharp Edged, Outlet, Orifice Outlet embedded in MIKE 1D engine. The inlet expansion loss is included in this equation. The different option uses the same equation, but different energy loss coefficients.
‘Mean Energy Approach’ corresponds to the MOUSE head loss option, ‘Mean Energy Approach’ and it does not include inlet expansion loss. It assumes that the energy level in the manhole is the same as in the inlet pipe. This means that the inlet expansion loss is ignored, and the outlet losses are applied across the manhole.
‘No Head Losses’ is equivalent to MOUSE head loss option, ‘No Cross-Section Changes’. This option provides the option of applying no head losses at all. This option is often used when connecting open cross sections, fictitious nodes i.e. underground connection of pipes. The volume of these nodes is still included so the diameter of the node should be considered carefully. In MIKE+ a new type of node has been introduced, the junction type, this node does not account for head losses.
Effective Node Area
The effective area is used when applied to manholes with one ingoing pipe and one out going pipe. The calculation of the inlet and the outlet contraction loss involves the cross-sectional area of the manhole. There are two options either the flow expands into the full cross-sectional area of the manhole or passes through the manhole as a type of submerged jet. The submerged jet implementation is only valid for one ingoing pipe and one out going pipe. If the option with submerged jet is applied to manholes with more than one ingoing pipe or outgoing pipe, then the full cross-sectional area of the manhole will be used.
Loss Coefficient (Km, Contraction HLC or Total HLC) - see Fig. 1
The loss coefficient is applied to the calculation of the head loss at the outlet of the manhole. These factors do not influence the calculation of the inlet expansion loss.
- Contraction HLC
- Total HLC
Fig. 1 - Head loss definition tab
The Km value is only used in the calculation of the outlet contraction loss. Km is an intermediate head loss coefficient and is scaled by the velocity in the manhole and in the outlet pipe.
The Contraction HLC is only used in the calculation of the outlet contraction loss but is not scaled by the velocity in the manhole and outlet pipe. Using the Contraction HLC effectively fixes the value of the outlet contraction loss coefficient.
The Total HLC not only overrides the outlet contraction loss coefficient, but it also overwrites the calculation of the bend loss coefficient and the drop loss coefficient.
Max Loss Limits
The upper limit of the maximum head loss can be applied to any head loss combination. There is two options available either based on the flow depth or the velocity on the outlet pipe
Project common practices while choosing the Head Loss Formulation
Some applications of these formulation are described below from a compilation of different DHI project works:
- Nodes connected to catchments are commonly set to “MOUSE Classic (Engelund)” - see Fig. 2
Fig. 2 - Manholes connected to catchments are usually modelled using the MOUSE Classic head loss formulation
- Nodes set to ‘Tail Node’ type with one inflow and one outflow pipe are assumed to still be part of force-main systems, hence they are expected not to suffer energy losses and are set to ‘No Cross Section Change.’ - see Fig. 3
Fig. 3 - Tail Node between a gravity main and a pressure main branch
- Nodes with more than two connected pipes are normally set to ‘MOUSE Classic (Engelund)’, the other methods are not the most stable under these conditions - see Fig. 4
Fig. 4 - Manhole connected to more than two pipes.
- Nodes downstream of weirs, orifices, and valves are dummy nodes and shall be set to ‘No Cross Section Changes’ - see Fig. 5
- Nodes identified as ‘Manholes’, located along a Force-main are commonly assumed to be ‘pressure hatchs’ nodes without maintenance access thus it is suitable to set the head loss formulation to ‘No Cross Section Change’ (i.e., no losses).
- Model nodes assumed to be network fittings or junctions are set to “No Cross Section Change” (i.e., no losses). In MIKE+ a junction type of node has been introduced; these nodes are particularly intended to be used as fiction nodes.
This type of node has already disabled the Head Loss Formulation - see Fig. 6:
Fig. 6 - Junction node with disabled option for Head Loss computation.
- Nodes identified as manhole structures and with one inlet pipe and one outlet pipe are commonly set to “Weighted Inlet Energy” (i.e., ignore inlet loss) - see Fig. 7
Fig. 7 - Manhole structure with one inlet and one outlet set to use Weighted Inlet Energy
There is no recipe when it comes to set up the head loss formulation in all models however some common practices have been highlighted to achieve better calibration results of hydraulic models. The most important thing is to be consistent throughout the model.
FURTHER INFORMATION & USEFUL LINKS
MIKEPlus Release Notes
MIKE+ CS - Getting started with urban drainage modelling