Orifice Controls (InfoWorks)
An orifice plate in an InfoWorks network controls the discharge from a storage tank or pond. It consists of a plate with a hole, usually circular, for the flow to pass through.
An orifice plate is represented as a link of zero length, forming a head-discharge relationship between two nodes. The boundary condition between the link and a node is that of equal water levels. The orifice invert level determines when the control first comes into operation. The flow characteristics for the calculated flow from headloss through the orifice (before any limit on flow is applied) are identical in both the positive and negative directions.
Flow Characteristics
ICM applies the thin plate rectangular weir equation when the orifice is partially full upstream.
In the case of a full, submerged orifice, ICM calculates flow through the orifice as the lower of the flow values determined using the weir equation and the orifice equations described below.
Orifice equations
For a full, submerged orifice upstream the governing model equation under free discharge conditions (see Wallingford Procedure) is:
|
|
where Ao - orifice cross sectional area (m2) Dcl - height of water level above the centroid of the orifice (m) |
This is modified under drowned conditions, if the downstream water level submerges the orifice, by:
|
|
where Q - discharge (m3/s) Ao - orifice cross sectional area (m2) g - acceleration due to gravity (m/s2) Du - upstream depth above invert (m) Dd - downstream depth above invert (m) |
The basic value of the coefficient of discharge may be determined from:
|
|
where A - area of pipe to which the orifice discharges (m2) |
This basic value should be multiplied by the factor F if there is a significant velocity of the approach flow:
|
|
where r - Ao/Au Au - cross sectional area of flow in chamber (m2) |
Weir equation
If the orifice is partially full upstream the flow characteristics are determined by the thin plate rectangular weir model in all cases. The coefficient of discharge used is the Secondary discharge coefficient (the submerged case using orifice equations described below uses the primary Discharge coefficient), with an effective rectangular width defined by:
|
|
where Ao - orifice cross sectional area (m2) Do - orifice diameter (m) |
Villemonte Equation
If the Use Villemonte equation has been chosen as a Simulation Parameter, when the orifice is under drowned conditions, the Villemonte formula is used to modify the free discharge:
|
|
where: Q is the discharge Q0 is the free discharge Du is the upstream depth with respect to the invert Dd is the downstream depth with respect to the invert |
