U-shaped swimmer for hydrodynamic capture and delivery of cargo at low Reynolds numbers

Using the two-dimensional lattice Boltzmann method, this study systematically investigates the cargo capture and delivery characteristics of a U-shaped swimmer composed of two pullers and two pushers, each with diameter D, within the swimming Reynolds number range of 0.01 ≤ Res ≤ 0.5. The U-shaped swimmer has both pushing and pulling modes. In the pushing mode, the delivery pattern depends on the cargo size: small cargo (d 0.6D) exhibits central, offset, and dynamic delivery; medium cargo (0.6D–1.4D) exhibits central and offset delivery; and large cargo (d 1.4D) exhibits only central delivery. The capture range of the pulling mode is significantly smaller because the high-pressure zone ahead forms an unstable equilibrium. In the pulling mode, cargo with diameters of 0.5D–1.5D mainly exhibits central and dynamic delivery. In both modes, the capture range gradually decreases as Res increases. Larger cargo diameters yield larger capture ranges and exhibit greater sensitivity to changes in Res. The squirmer parameter β regulates the intensity and spatial extent of the flow field, thereby controlling the capture range and the dominance of delivery modes without altering the underlying delivery mechanisms. The U-shaped swimmer can deliver multiple cargoes simultaneously; the cargo it carries can alter the overall hydrodynamic structure and even improve propulsion efficiency.