A. Vortex flow model in twisted tube bundles

In bundles of twisted tubes, approximate hydrodynamic similarity may be ensured even when geometrical similarity is absent, if the maximum acceleration value (g_e.f.) of the centrifugal force field is used as a parameter that characterizes the specific features of flow in those bundles.

\[\label{eq1} g_{e.f.}=2u^{2}_{\tau\;\rm max}/d,\tag{1}\]

where d is the outer diameter of the twisted tube and

\[\label{eq2} u_{\tau\;\rm max}=\pi du_{mean}/s.\tag{2}\]