Observation of unsteady nonlinear relationship between pressure drop across and flow rate through a restrictor in sinusoidally fluctuating oil hydraulic flows

The present paper provides the discussions on the unsteady characteristics of a restriction flow in a hydraulic line. By ingeniously creating sinusoidal flow at a fixed orifice in a pipe, the unsteady relationships between the pressure drop across and the flow rate through the orifice were properly measured. When being estimated using the experimental results, the unsteady nonlinear pressure loss turned out to change against the flow rate describing a loop with a counter-clockwise hysteresis along the steady-state relation between the two. The finding, probably indicating that the structure of turbulence producing a nonlinear pressure loss in an orifice jet flow changes with a delay behind the change of flow rate, provides an answer to the long pending-question in acoustics; why the acoustic impedance of orifice decreases its reactive part as an amplitude of oscillation increases. A hypothetical mathematical model incorporating the delay into the steady-state relation was proposed to simulate the behavior of an unsteady nonlinear pressure loss in an orifice flow. The impedance analytically obtained by the model explained the decrease of reactance with an increasing amplitude.