Laminar-Turbulent Transition of an Inlet Boundary Layer in a Circular Pipe Induced by Periodic Ejection (Condition for Generating an Isolated Turbulent Patch)

The laminar-turbulent transition of a boundary layer induced by jet flow ejection in the inlet region of a circular pipe was experimentally investigated. The jet flow was periodically inserted radially from a small hole in the inlet region into the pipe flow. Axial velocity was monitored by a hot-wire anemometer. The difference of properties in laminar-turbulent transition from developed Hagen-Poiseuille flow was examined. Isolated turbulence patches were generated by the jets, and then they propagated downstream. The leading edge of the turbulent patch was definite, whereas its trailing edge was not. This characteristic was similar to that of a turbulent spot in a flat-plate boundary layer. The threshold value of the jet flow rate to generate the turbulent patch was then obtained. The threshold value decreased and saturated finally with the increase in jet flow duration. The normalized jet flow duration when the threshold value was saturated increased with the increase in Reynolds number, contrary to the developed region. The normalized threshold flow rate tended to vary with the Reynolds number among three regions. All tendencies were different from those of the developed region. With the increase in jet flow rate beyond the threshold value, the duration of the turbulent patch increased, though the fluctuating velocity within the patch did not. The propagation velocities of the leading and trailing edges, and the duration and fluctuating velocity within the turbulent patch were almost constant irrespective of the jet flow ejection frequency.