a) Damping time, normalised to the full tube damping time, as a function of the length of the thread, for ρ_ev = ρ_c. b) Damping time, normalised to the full tube damping time, as a function of the density in the evacuated part of the tube, for L_thread = 0.2L. In both figures lines correspond to the numerically computed damping times and the symbols are the values obtained through the energy analysis described in Sect. 3.3.