Velocity Distributions in Laminar Phases for Oscillatory Rectangular Duct Flows with Occurrence of Turbulent Transition

概要

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Velocity measurement are made for oscillatory air flows in a square duct and a rectangular duct with an aspect ratio γ=10. The flow investigated here consists of two phases in each half cycle, namely laminar, or, in a strict sense, quasi-laminar phase and turbulent one. the flow in the latter phase can be considered to be in turbulent quasi-steady state as shown in a previous paper. But a flow in laminar phase is quite different from one which is described by the solution for a temporally fully developed oscillatory flow. Through analysis of hot-wire signals, it is found that the flow reverses its direction at almost the same instant over a whole cross-section, due to a strong mixing of turbulent motion. Taking note of this fact, the flow after reversal may possibly be represented by the case when a fluid suddenly begins laminar motion following prescribed variation of a flow rate. So the analytical solutions are derived for laminar transient flows in a rectangular duct and a parallel-plates channel. The measured velocity distributions in laminar phases are described by these solutions with sufficient accuracy.
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