A study of turbulence under conditions of transient flow in a pipe

A detailed investigation of fully developed transient flow in a pipe has be en undertaken using water as the working fluid. Linearly increasing or decr easing excursions of flow rate were imposed between steady initial and fina l values. A three-beam, two-component laser Doppler anemometer was used to make simultaneous measurements of either axial and radial, or axial and cir cumferential, components of local velocity. Values of ensemble-averaged mea n velocity, root-mean-square velocity fluctuation and turbulent shear stres s were found from the measurements. Being the first really detailed study of ramp-type transient turbulent flow , the present investigation has yielded new information and valuable insigh t into certain fundamental aspects of turbulence dynamics. Some striking fe atures are evident in the response of the turbulence field to the imposed e xcursions of flow rate. Three different delays have been identified: a dela y in the response of turbulence production; a delay in turbulence energy re distribution among its three components; and a delay associated with the pr opagation of turbulence radially. The last of these is the most pronounced under the conditions of the present study. A dimensionless delay parameter tau(+)[= root 2 tau U-tau 0/D] is proposed to describe it. The first respon se of turbulence is found to occur in the region near the wall where turbul ence production peaks. The axial component of turbulence responds earlier t han the other two components and builds up faster. The response propagates towards the centre of the pipe through the action of turbulent diffusion at a speed which depends on the Reynolds number at the start of the excursion . In the core region, the three components of turbulence energy respond in a similar manner. Turbulence intensity is reduced in the case of accelerati ng flow and increased in decelerating flow. This is mainly as a result of t he delayed response of turbulence. A dimensionless ramp rate parameter gamm a [= (dU(b)/dt)(1/U-b0)(D/U-tau 0)] is proposed, which determines the exten t to which the turbulence energy differs from that of pseudo-steady flow as a result of the delay in the propagation of turbulence.