Selecting the Transient Friction Method

The Transient Friction Method option enables you to select the methodology for determining flow resistance and friction losses during calculations. This can be accessed from the Transient Solver calculation options (Analysis > Calculation Options). Available methodologies include:

Steady Friction
Quasi-steady Friction
Unsteady Friction, also known as transient friction

For more information on the theory for each of these friction models, see Friction and Minor Losses.

Steady State Friction Method

In HAMMER, a hydraulic transient analysis usually begins with an Initial Conditions (steady state) calculation, which computes the heads and flows for every pipe in the system. Prior to beginning the transient calculations, HAMMER automatically determines the friction factor based on this information.

If a pipe has zero flow at the initial steady-state, HAMMER use the Friction Coefficient specified in the Pipe Physical properties.

If a pipe has a nonzero flow at the initial steady-state, HAMMER automatically calculates a Darcy-Weisbach friction factor, f, based on the heads at each end of the pipe, the pipe length and diameter, and the flow in the pipe. It uses this calculated value in the transient simulation.

Note: HAMMER always uses the Darcy-Weisbach friction method in performing the hydraulic transient calculations, regardless of which method is specified in the Steady State/EPS Solver Calculation Options. If required, HAMMER will automatically convert the friction factors to the appropriate format.

Quasi-Steady Friction Method

The quasi-steady friction method uses variable Darcy-Weisbach friction factors, f, at each point along the system, so that friction losses for an instantaneous velocity match the friction losses which would occur for fully developed steady flows with the same cross-sectional average velocity. For more information, see Quasi-Steady Friction.

Note: Quasi-steady and unsteady friction models are the result of current research by others. Results should be compared with those obtained with a steady friction model. Always use engineering judgement when interpreting transient simulation results.

Transient or Unsteady Friction

Compared to a steady state, fluid friction increases during hydraulic transient events because rapid changes in transient pressure increase turbulent shear. HAMMER can track the effect of fluid accelerations to estimate the attenuation of transient energy more closely than would be possible with quasi-steady friction.

Computational effort increases significantly if transient friction must be calculated for each time step. This can result in long model calculation times for large systems with hundreds of pipes or more. Typically, transient friction has little or no impact on the initial low and high pressures, and these are usually the largest ever reached in the system (provided the simulation does not involve a vapor pocket collapse).

Note: The steady-state friction method yields conservative estimates of the extreme high and low pressures which usually govern the selection of pipe class and surge-protection equipment. However, if cyclic loading is an important design consideration, the unsteady friction method can yield less-conservative but rigorous estimates of recurring and decaying extremes.

For more information on the implementation of the transient friction method in HAMMER, see Unsteady or Transient Friction.