  This study investigates drag reduction capability of
naturally-occurring-oscillating axial secondary flow(ASF) induced by
helical-corrugated surface in Taylor Couette flow(TCFHelical) for three values
of pitch to wavelength-ratios(P* =1,2,3) and amplitude to wavelength-ratio(A*)
of 0.25. As reported in Razzak et al. (2020), emergence of
naturally-occurring-oscillating ASF induced by longitudinal-corrugated surface
in TCF(TCFLongitudinal) and increasing trend on its magnitude with Reynolds
number (Re) results in the occurrence of drag reduction. This has motivated us
to study the possibility of enhancing drag reduction by maintaining a
consistently increasing trend with Re in the magnitude of
naturally-occurring-oscillating ASF induced by the helical-corrugated surface
on the stationary outer cylinder in TCF. From flow structures, steady ASF with
non-zero mean is observed at Re=60 which suppresses the strength of azimuthal
vorticities for Re>85 and contributed to occurrence of drag reduction. As Re is
increased to 100,90 and 85 for P* =1,2, and 3, respectively, formation of
periodic oscillating ASF with non-zero mean and its increasing trend in
magnitude with Re suppresses azimuthal vorticities further which contributes to
the maximum drag reduction of 13%. For Re>165,145 and 140 for P*=1,2 and 3,
respectively, non-periodic oscillating ASF is observed, and its magnitude
remains nearly unchanged or decreases slightly with Re which results in the
suppression effect of azimuthal vortices to be weaker. This results in decrease
in drag reduction. Oscillating ASF observed in TCFHelical is found to occur at
earlier Re and it is stronger than that of TCFLongitudinal which contributes to
occurrence of higher drag reduction in TCFHelical.
