Objectives This study focuses on the influence and drag force of tandem square cylinders of variable size (with a side length ratio of 1.5) in order to determine the flow past tandem square cylinders in a constrained boundary under different gap ratios at a Reynolds number of 100.

Methods Based on the lattice Boltzmann method with the bounce-back boundary condition, comparisons of the effects of tandem cylinders of equal and variable size are made under different gap ratios.

Results The results indicate that cylinders of variable size can accelerate the change of the flow field pattern to a certain extent. However, they can also delay the change of the flow pattern at a gap ratio of 6. In addition, they can achieve a resistance reduction of 9.16% at most compared with the flow past a single square cylinder. However, tandem cylinders of equal size can achieve 7.76% at most. On the other hand, compared with tandem cylinders of equal size, those of variable size can achieve a resistance reduction of 67.79%. Thus, tandem cylinders of variable size can obtain a better reduction in drag force, making them a feasible way to achieve flow control and drag reduction.

Conclusions The results of this study can provide basic guidance for the optimization of drag and flow control for flow past square cylinders in a constrained boundary.