| Product(s): | HAMMER, WaterGEMS, WaterCAD |
| Version(s): | CONNECT Edition, V8i |
| Area: | Calculations |
How is velocity head handled in WaterGEMS, WaterCAD, and HAMMER? Bernoulli's equation tells us that pressure should increase when velocity decreases.
In WaterGEMS and WaterCAD, as well as the initial conditions calculations for HAMMER, the velocity head is not considered. This is because compared to the elevation head and pressure head, the velocity head is small enough that including it has a negligible impact. This results in the EGL being considered to be identical to the HGL in the system. In most modeling cases, this is suitable situation, even in high velocity cases. There may be cases in Bentley HAMMER and turbine modeling where the velocity head is larger.
During a transient simulation, HAMMER calculates the change in the initial state of the system in response to changes in momentum in the network. The underlying foundation for the program is derived from the partial differential equations for conservation of mass and momentum. By neglecting certain terms, some of which involve velocity and its partial derivative, the solution of the system of equations is amenable to the Method of Characteristics. The question arises whether the aforementioned neglect is equivalent to the omission of velocity head.
HAMMER has been used in a number of high-head hydropower systems using turbines with no impact in results because of the exclusion of velocity head, so a user can feel confident in the results that are generated. During the rare scenario where the velocity is large enough that velocity head is significant, the user can find the velocity head and subtract that out of the hydraulic grade results. The user may need to set "Specify Initial Conditions?" to True in this case and enter the data normally obtained from the initial conditions calculation. To easier add the other initial conditions data, the user can use the Copy Initial Conditions function found in the Tools pulldown menu. In cases near vapor pressure, another workaround would be to increase the vapor pressure by the velocity head in order to simulate the onset and development of cavitation.
With that being said, model results will likely be accurate without these workarounds, given that velocity head is often negligible in results.