How are demands treated during the transient simulation in HAMMER?

Applies To 
Product(s): HAMMER
Version(s): CONNECT Edition, V8i
Area:  Modeling
Original Author: Scott Kampa, Bentley Technical Support Group


Problem

How are demands treated during the transient simulation in HAMMER?

Solution

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.

What if I want to directly control the demand or inflow change over time during the transient simulation?

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


See Also

What kind of demands should be used as the initial conditions of a transient simulation?

Modeling demand patterns in Bentley HAMMER

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