  Isolated patches of turbulence in transitional straight pipes are sustained
by a strong instability at their upstream front, where the production of
turbulent kinetic energy (TKE) is up to five times higher than in the core.
Direct numerical simulations presented in this paper show no evidence of such
strong fronts if the pipe is bent. We examine the temporal and spatial
evolution of puffs and slugs in a toroidal pipe with pipe-to-torus diameter
ratio $\delta = D/d = 0.01$ at several subcritical Reynolds numbers. Results
show that the upstream overshoot of TKE production is at most one-and-a-half
times the value in the core and that the average cross-flow fluctuations at the
front are up to three times lower if compared to a straight pipe, while
attaining similar values in the core. Localised turbulence can be sustained at
smaller energies through a redistribution of turbulent fluctuations and
vortical structures by the in-plane Dean motion of the mean flow. This
asymmetry determines a strong localisation of TKE production near the outer
bend, where linear and nonlinear mechanisms optimally amplify perturbations. We
further observe a substantial reduction of the range of Reynolds numbers for
long-lived intermittent turbulence, in agreement with experimental data from
the literature. Moreover, no occurrence of nucleation of spots through
splitting could be detected in the range of parameters considered. Based on the
present results, we argue that this mechanism gradually becomes marginal as the
curvature of the pipe increases and the transition scenario approaches a
dynamical switch from subcritical to supercritical.
