| Applies To | ||
| Product(s): | HAMMER | |
| Version(s): | CONNECT Edition, V8i | |
| Area: | Modeling | |
| Original Author: | Scott Kampa, Bentley Technical Support Group |
How are demands treated during the transient simulation in HAMMER?
Nodal demands will be satisfied in the initial conditions calculation and the resulting flow and head results will be passed to the transient engine to use as the starting conditions. This applies to "regular" demands as well as other demands such as emitter coefficients - HAMMER looks at the total calculated demand (outflow) for a given node and its resulting pressure. During the transient simulation, the demand will be treated as an orifice to atmosphere based on that pressure and flow (Note: It will not allow air into the system during subatmospheric pressures).
The end result of this is that the demand node will always be pressure dependent, based on the orifice coefficient calculated from the initial flow and head, using the following equation:
Q = C A (2 g P)^0.5
Q - Discharge (cfs, cms)
C - A 'discharge coefficient' (distinct from CV used elsewhere in HAMMER)
A - The cross-sectional area of the opening (sq. ft, sq. m)
g - gravitational acceleration
P - Pressure head (ft, m)
The C coefficient is calculated using the initial flow and pressure head from the initial conditions calculation. So, if the pressure drops, the flow will drop appropriately. If the pressure is negative, the flow will be zero since HAMMER assumes a backflow preventer exists. If the pressure drops below vapor pressure it may appear that reverse flow occurs, but this is due to the vapor pocket volume increasing.
By default, HAMMER assumes that any air allowed into the system at the individual demand points cannot reach the aggregate demand location. If this is not the case and you need to allow air into the system, use the Discharge to Atmosphere element.
To control exactly how an inflow or outflow changes over time during a transient simulation, use the Periodic Head-Flow element. See: Modeling a change in HGL or flow during a transient simulation
What kind of demands should be used as the initial conditions of a transient simulation?