In numerical simulations, to our knowledge, usually two methods have been used to generate bands at low Reynolds numbers ($Re\lesssim 1000$). The one is to start from a fully turbulent flow field at a higher Reynolds number (above $Re\simeq 1500$) or random noise, and wait for the flow to form discrete bands [@Tsukahara2005; @TsukaharaTakeda2014; @Tuckerman2014]. The other is to start from a localised vortical perturbation such as that proposed by @Henningson1991. As @Tao2018 pointed out that random noise cannot trigger turbulent bands at $Re<800$, no matter being localised or not, and that only perturbations with effective structures can work. In our simulations, we also found that both methods cannot assure the generation of bands at $Re\lesssim 750$, regardless of the amplitude of the initial perturbations. At higher Reynolds numbers, even if bands can be triggered by noisy fields or localised vortical perturbations, the tilt direction of the bands are not known a priori using both methods and the first method even cannot predict the number and positions of the generated bands.

In this work, we propose a method that can effectively trigger turbulent bands directly at low Reynolds numbers and more importantly, allows a precise control on the position and 