# Openflow Water GEMS 2023: Velocity of flow is Zero but it should be 140 m/s

I am trying to create a situation where water is being discharged from a pipe connected to a Reservoir. When I calculate the flow velocity through pipe it is coming 140 m/s, however, In waterGEMs flow velocity is coming Zero. Please guide what should I do?

As We can see through calculation that velocity should be 140 m/s however, Velocity as appearing in WaterGEMS is zero. Can Someone please guide how can I correct the velocity in WaterGEMS.

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• Chandran,

I assume this is for academic purposes since 1000 m of head and 140 m/s are quite extreme.

It looks like the "Flow (typical)" and "Pressure Drop (typical)" were left as zero. These will need to be entered to define the pressure vs. flow relationship of the opening to the atmosphere. Meaning, it will not automatically be based on the pipe diameter but you can use the orifice equation for example to determine the two aforementioned values needed in order to simulate an opening of the same size as the pipe.

The following article explains more about this and how the orifice equation can be used (for example) to accomplish this: Modeling Reference - Discharge To Atmosphere

Regards,

Jesse Dringoli
Technical Support Manager, OpenFlows
Bentley Systems, Inc.

• Sir, I tried to compute Typical pressure for a typical discharge of 211 Liters per second though an pipe of uniform cross sectional diameter of 300 mm.

Sir, there are two articles on how to calculate typical flow and typical pressure for Discharge to Atmosphere element.

For a required typical flow of 2.11 cubic meters per second,

Process-1. If we use method described in "Modeling Reference - Discharge To Atmosphere" under section "Common applications of the D2A acting as an Orifice" undersubsection " 2. Any free discharge point" , We get typical pressure drop of 1.263 meter.

Process-2. If we use method described in "How does WaterGEMS/WaterCAD treat the discharge to atmosphere element?" under section "Method 1: If there is no restriction/contraction at the pipe outlet, then consider using the minor loss equation", We get typical pressure drop of 0.458 meter. Please let me know which is correct Typical pressure drop value.

• Hello Chandan,

The method you use will depend in part on the information that you have available to you. The typical flow and typical pressure drop can be calculated using the the standard headloss equation (referenced in your screenshot above) or the orifice equation. The orifice equation method requires you to assume an orifice coefficient.

The method you use will based on your engineering judgment and the information you have available. Since the methods are different, different results can happen, particularly if you are not sure what the orifice equation is. You will also want to pay attention to the units in the calculation to make sure that you have the right results.

Regards,

Scott

• Hello Chandan,

The method you use will depend in part on the information that you have available to you. The typical flow and typical pressure drop can be calculated using the the standard headloss equation (referenced in your screenshot above) or the orifice equation. The orifice equation method requires you to assume an orifice coefficient.

The method you use will based on your engineering judgment and the information you have available. Since the methods are different, different results can happen, particularly if you are not sure what the orifice equation is. You will also want to pay attention to the units in the calculation to make sure that you have the right results.

Regards,

Scott