Laminar-turbulent transition of an inlet boundary layer in a circular pipe induced by periodic injection (Turbulence within isolated turbulent patches and its growth mechanism)

The laminar-turbulent transition of a boundary layer induced by a jet injection in the inlet region of a circular pipe was experimentally investigated. The jet was periodically injected radially from a small hole in the inlet region into the pipe flow. Axial velocity was measured by a hot-wire anemometer. The turbulence induced by the jet within the boundary layer developed into turbulent patches which grew in the axial, circumferential and radial directions downstream. Turbulent fluctuations within the patch were maximum a little inside the leading edge in the axial direction and slightly inside the circumferential interface in the circumferential direction. Non-turbulent fluid was entrained into the patch through the leading and trailing edges. This was the main reason for the axial growth of the patch downstream. If the entrainment is suppressed, the axial growth is inhibited. The axial distance between the leading edge and maximum fluctuation position varied little downstream. On the other hand, the distance between the maximum fluctuation position and the trailing edge increased downstream due to the increase in the entrainment of non-turbulent fluid across the trailing edge.