### Transient Friction Panel

Impulse provides several options for modeling the pipe friction during the transient simulation:

• Constant - The pipe frictional resistance (shear stress) obtained in the steady-state part of the simulation will be used as a constant in the transient part of the simulation

• Time Varying - The pipe friction will differ for each pipe section because the flowrate (and hence Reynolds number) is varying along the pipe due to the fluid transient. Further, the resistance will vary with time as the flowrate changes.

• Simple: A steady state approximation of the pipe frictional resistance (shear stress) will be calculated as the fluid velocity for each pipe station changes by more than 1%. In AFT Impulse 7 and earlier, this model was included as the "Use Variable Pipe Resistance" option.

Which Friction Model to Use

The three friction models available in AFT Impulse are each intended for different use cases.

The Constant friction model is best used when flow in the simulation stays in the turbulent or highly turbulent regimes. Here, pipe friction is either weakly dependent on flowrate (turbulent flow) or independent of flowrate (highly turbulent flow in the Fully Rough zone of the Moody chart). As a result, the friction factors determined in the steady state system likely remain valid throughout the transient simulation.

The Time Varying - Simple friction model is best used when zero or low flow is seen in some pipes either in the steady state system or in the transient simulation. This situation could arise when events like pump startups or valve closures are being modeled. Using the steady state friction factor throughout the transient simulation could lead to incorrect estimations of pressure drops in the system, especially when there is zero flow in a pipe during steady state.

The Time Varying - Advanced friction model is best used when it is important to account for the effects that a change in velocity have on pipe friction. The Time Varying - Simple model assumes that each velocity pipe friction factors are calculated for is a 'steady state' velocity, and that a change in fluid velocity does not lead to any additional friction losses other than the friction factor changing. The Time Varying - Advanced model adds in an unsteady shear stress term that helps to account for velocity changing. In models that see flow in a pipe change direction, this term can have a large impact on the transient results.

Note: Both Time Varying friction models require additional calculations and run time when compared to the Constant friction model. Experience suggests that the Time Varying - Simple model increases run time by 20% while the Time Varying - Advanced model  increases run time by roughly a factor of three.

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