“…Clearly, such descriptions are mostly qualitative and sometimes even speculative, but nevertheless, we are making an attempt to provide some more insight into this question. First we note that results obtained for instabilities of inviscid vortices (Godeferd et al, 2001;Chomaz et al, 2010;Carnevale et al, 2016), as well as for inviscid vortex pairs (Billant, 1999;Roy et al, 2008;Leweke et al, 2016), in an unbounded domain, cannot be applied for the bounded viscous flow considered here. Moreover, in the present helical pipe configuration, the flow through the pipe and the secondary Dean vortices are already interconnected in the base flow, contrarily to inviscid vortices superimposed with axial flow (e.g., Roy et al, 2008) or with background rotation (e.g., Godeferd et al, 2001;Gargan-Shingles, 2016).…”