Forced laminar-to-turbulent transition of pipe flows

This paper presents the results of investigations into particular features of laminar-to-turbulent transition of pipe flows. The first part considers transitional flows that occur 'naturally', i.e. without any forcing, when a critical Reynolds number is reached. Measurements are reported that were carried out to study the intermittent nature of pipe flows before they become fully turbulent. The second part of the paper concentrates on forced laminar-to-turbuient transition where the forcing was achieved by ring-type obstacles introduced into the flow close to the pipe inlet. The influence of the ring height was investigated and the results showed a dependence of the critical Reynolds number on the normalized height of the disturbances. The laminar-to-turbulent transition was also investigated when caused by partially closing an iris diaphragm that permitted the flow to be forced to turbulence over short time intervals. Investigations of controlled intermittency became possible in this way and corresponding results are presented.