Enhancing mixing at a very low Reynolds number by a heaving square cylinder

•A CFD/FSI study about mixing fluids at a very low Reynolds has been conducted.•Enhancing mixing by a heaving square cylinder in a microchannel is shown.•Optimum heaving frequency and amplitude have been found.•Mixing efficiency, pressure drop and input works are characterised.

Mixing fluids in a channel under laminar conditions is a challenge for which many options have been proposed to enhance the resulting mixing. Here, an active mixing technique based on the interaction between the fluids and a structure, a square cylinder heaving in the middle of a channel, is studied numerically by means of Finite Volume Methods. For a given cylinder-to-channel width ratio and for a very small Reynolds number, it is shown how the amplitude and the frequency of the heaving oscillation affect the efficiency of the resulting mixing at the outlet section of the channel. After studying 7 different frequencies and 5 different amplitudes, an optimal combination has been found for which the mixing efficiency is the highest, being more than 10 times higher than when the mixing takes place in the same channel but with the cylinder steady. Besides of this, an optimal oscillation frequency has been found for each oscillation amplitude for which the mixing efficiency is the highest. Additionally, the input works needed both to heave the cylinder and to run the channel are also characterised as a function of the heaving frequency and amplitude. Surprisingly, the highest mixing efficiency configuration does not require either the highest input work for heaving the cylinder or for running the channel. Finally, we shall show that the optimum mixing, quantified in terms of a required input energy-efficiency ratio, is achieved with low heaving oscillation frequencies and high heaving amplitudes.