The flow field and axial thrust generated by a rotating rigid helix at low Reynolds numbers

Here we report an experimental study of the flow field and axial thrust generated by a single rigid helix rotating around its axis at Reynolds numbers based on the tangential velocity and the helix diameter between 0.045 and 0.18. Particle image velocimetry measurements are carried out on helices with pitch angles of 30°, 45° and 60°, at three different rotating frequencies. It is demonstrated that the helix produces a helical flow pattern about its axis that is 180° out of phase to the shape of the helix. An axial flow is generated which is in the opposite direction to the thrust produced by the helix. At a fixed pitch angle, the magnitude of this axial velocity increases linearly with the frequency of rotation, whereas at a fixed frequency the helix with a pitch angle of 45° produces the highest axial velocity. The same trend is observed in the axial thrust, which is measured using a force sensor. The measured thrust coefficient is compared with the thrust coefficient predicted by resistive-force theory. The thrust coefficient agrees well with that predicted by resistive-force theory for lower pitch angles, but resistive-force theory breaks down for pitch angles <45°, which is attributed to hydrodynamic interactions among helical loops at higher pitch angles.