THE START-UP RESPONSE OF PIPE FLOW TO A STEP CHANGE IN FLOW RATE

Abstract

The start-up response of pipe flow to a step input of constant flow rate given by an automatic solenoid valve has been studied experimentally by the use of electrochemical technique. The variation of velocity distribution and velocity gradient at the pipe wall with the passage of time has been measured far downstream from the inlet section, where flow becomes fully developed in the steady state. The velocity profiles of the start-up flow development show a trend essentially different from those of steady-state flow development at different distances in the entrance region of a circular pipe: they show a minimum at the axis and a maximum in the intermediate region between the axis and the wall as the result of non-uniformity of acceleration in the central core (annular jet effect). The development of the laminar boundary layer with time could be regarded as that of the constant-stress layer near the wall. Still, the velocity profiles in the laminar boundary layer at different times are similar to each other.