Transition to Turbulence of a Laminar Flow Accelerated to a Statistically Steady Turbulent Flow

This current study investigates the turbulence response in a flow accelerated from laminar to a statistically steady turbulent flow utilising Particle Image Velocimetry (PIV) and Constant Temperature Anemometry (CTA). The dimensions of the rectangular flow facility are 8 m in length, 0.35 m in width, and 0.05 m in height. The flow is increased via the pneumatic control valve from a laminar to a statistically steady turbulent flow, and the laminar-turbulent transition is examined. As the flow accelerates to turbulent from laminar, the friction coefficient increases quickly and approaches its maximum value within a short period. As a result, a boundary layer forms extremely near to the wall, increasing the velocity gradient and viscous force. The friction coefficient and viscous force decrease with increasing boundary layer thickness, and transition occurs as a result of instability of the boundary layer. The friction coefficient is used to specify the beginning and end of the transition. The transition starts when the friction coefficient reaches its minimal value. It increases again, and its maximum value marks the end of the transition to turbulence. The study shows that three stages lead to turbulence near the wall when the flow is accelerated from laminar to turbulent. These phases are similar to the transient turbulent flow reported. The reaction of mean velocity as laminar flow is accelerated to turbulent flow is investigated. The mean velocity behaves like a "plug flow" when the flow accelerates from laminar to turbulent, meaning that everywhere in the flow zone, except for the position extremely near the wall, the flow behaves like a solid body. The changes in the channel flow that accelerates from a laminar to a turbulent condition are presented, together with the turbulence statistics, wall shear stress, bulk velocity, and friction coefficient. Like the boundary layer bypass transition and transient turbulent flows, the transition to turbulence follows a similar process.