Dynamic models of fluid power systems require accurate descriptions of hydraulic pipeline systems. For laminar flow in a rigid pipeline, modal approximations of transcendental transfer functions lead to a multi-degrees-of-freedom description. This suggests the application of experimental modal analysis to investigate fluid dynamics in hydraulic pipeline systems. The concept of modal testing is adapted accordingly and demonstrated for a straight pipeline, the same pipeline with a single side branch, and a pipeline system with three side branches. Frequency response functions are determined by injecting a defined flow rate excitation and measuring pressure responses along the pipelines. The underlying theory is confirmed by comparisons between calculated transcendental, measured, and estimated rational frequency response functions. Natural frequencies, damping ratios, and pressure mode shapes are identified. Although the experiments are made for low flow rates and stiff pipeline walls, they indicate the way to perform modal testing in practical applications of fluid power.
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