A method to evaluate the effect of unsteady pipe friction and unsteady orifice resistance is presented. Velocity dependent resistance is added to the standard TMM modelling approach. Following [S] the resistance of orifices can be divided into three velocity ranges. For very low orifice velocities only the unsteady pipe friction in the orifice is important and the resistance is therefore frequency dependent. For high orifice velocities the resistance seems to depend only on the velocity. In the medium range the influence of the frequency dependent friction decreases while the velocity dependent resistance becomes more important.
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