Attenuation of second sound in bulk flowing He II

New measurements of second-sound attenuation in bulk-flowing He II are reported that extend to a region of higher Reynolds number. An expression for the attenuation explicitly containing the quantum vortex line density is developed which allows comparison with vortex-line-density data taken by other means. A bellows-driven experimental apparatus is used to produce bulk flow velocities of 0 to 1 m/sec in a channel of 4.064 mm square internal cross section. Second-sound pulses are produced by applying a square voltage pulse 200 {mu}s width and variable height to a strain-gauge heater. The second-sound pulses are detected with thin-film sensors mounted 56 and 119 mm downstream. The velocity-dependent attenuation, measured as a function of bulk flow velocity at 1.5, 1.8, and 2.0 K, is compared with data from other researchers. The attenuation, and thus the vortex line density, appears to follow a gradual transition from laminar to turbulent behavior. Current theories do not account for the presence of quantized vortices inn bulk flowing He II, where ({nu}{sub n} {minus} {nu}{sub s}) = 0, and thus do not explain the observed second-sound attenuation in this regime.