UBC Theses and Dissertations
Critical velocities and generation of vorticity in liquid helium II in large pipes of rectangular cross-section at 1.40 K Slater, Williams James
Critical velocities have been measured in liquid helium at 1.40 K for counterflow, superflow and counterflow superimposed on superflow. The measurements were made in five pipes having rectangular cross-sections and ranging in size nominally from 0.2 x 1.2 x 10 cm to 1.2 x .1.2 x 10 cm. The turbulence was detected by means of negative ion currents perpendicular to the flow. The results are interpreted in terms of creation of quantized vortex lines. For superflow, the critical velocities were slightly higher but within the experimental range of those measured for counterflow. For the case of superflow-plus-counterflow a second critical point was observed, in addition to the one corresponding to superflow only. Considerable data was also collected regarding the nature of the ionic current attenuation effects for supercritical flow. The delay times between when the flow rate was raised to or above the critical velocity and when the current attenuations were first observed ranged from more than five minutes for the smallest pipe to three-quarters of a minute or less for the largest one. When the pipe width is greater than the ion source width the attenuations may be explained by Vinen's theory of vortex generation and decay but for smaller pipe widths the attenuation buildup and decay times are greatly increased. A method for increasing the sensitivity of the detection method for large pipes is also outlined.
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