Experimental and numerical studies on the starting effect on the secondary flow in a bend

A numerical and experimental modelling study was carried out in a curved tube to analyse the behaviour of unsteady flows in a bend. Based on a test bench, with no mechanical disturbances, the flow behaviour was observed using fluorescein injection. Velocity measurements were performed using hot-film anemometry. In addition, a finite volume method was used to perform three-dimensional unsteady numerical simulations. Womersley parameter values between 8 and 21 and Dean number values between 110 and 420 were used to assess the parameters affecting the flow behaviour. Secondary motions were observed, experimentally and numerically, showing the complexity of the flow patterns. The initiation and subsequent development are explained quantitatively. Based on our analysis of the starting effect, the secondary patterns were found to be highly dependent on both the initial conditions and the flow waveforms.