The authors have carried out a study to investigate and clarify the characteristics of purely oscillating pipe flows over the developing region. The main objective of this study is to establish the method of time-dependent velocity profiles obtained by the ultrasonic velocity profile (UVP) measurement method. First, the relationship between the test fluids and the micro-particles, as reflectors of ultrasonic pulses, was investigated, and also the relationship between the sound speeds of the test fluids and wall materials was studied. Second, the UVP was used to obtain the instantaneous velocity profiles in oscillating pipe flows, and the developing characteristics of the flows were analyzed. Finally, the "entrance length" (analogy with a unidirectional pipe flow) required for oscillating pipe flows were analyzed examining the amplitude of the harmonic spectral components of the oscillating frequency. The fast Fourier transform (FFT) is proposed as the applicable method to estimate the "entrance length". From the Fourier transform of the velocity on the centerline, nonlinear oscillation of fluid occurs in the "entrance length" of the oscillating flows, and the viscous dissipation of the higher order velocity harmonics determines the entrance region. The "entrance length" can be obtained from the dissipation length of the third order harmonic. These results prove that the UVP method is highly applicable for carrying out flow measurements in the "entrance length" of the oscillating pipe flow.
Experiments in Fluids – Springer Journals
Published: Mar 1, 2003
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