An Experimental Investigation of Initiation and Cessation of Flow in a Tube for Newtonian Fluids

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An experimental investigation of sudden initiation and cessation of laminar flow in a tube following the opening and closure of a direct acting solenoid valve is reported for Newtonian fluids in the viscosity range 0.9 ≤ μ ≤ 39.5 centipoise. A non-contact probing Laser Doppler Anemometer and a high speed data logging system are used to make local axial velocity measurements at 85 tube diameters from the entrance to a test section, 1.36 cm I.D. and 1.4 meters long. Some qualitative measurements of the pressure gradient over the test section are also made. The results show that for Newtonian fluids, with μ ≤ 27.1 cp, the sudden opening or closure of the solenoid valve is not equivalent to a step input in either pressure drop or flow rate. Both show overshoots, the magnitude of which decreases with increasing viscosity of the fluid. The normalized time for a 90% development or decay increases with the viscosity of the fluid. For fluids with μ ≥ 39.5 cp, the behavior appears to approach that due to a step input in pressure gradient.