The study of fluid–structure interactions (FSI) is an extremely broad and important topic, with research spanning orders-of-magnitude in length scale of the structure and involving various types of fluids. The well-known problem of vortex-induced vibration (where the frequency of vortex-shedding at a high Reynolds number, Re, matches the natural frequency of the structure) must be considered in the engineering of bridges, aeroplanes, tall buildings, and underwater pipes and cables. FSI are also vital in many green energy harvesting systems (e.g., wind and tidal power generation). In the case of such large structures, the fluids involved are generally Newtonian and either compressible (air) or incompressible (water). At smaller length scales, the motions of and interactions between colloidal particles in suspensions impact the bulk flow properties (rheology) of the fluid as a whole. Furthermore, the motions of active structures such as cilia and flagella drive flows that enable the low Re swimming of micro-organisms and the clearance of mucus from the lungs. In these cases of microscopic structures, the fluids involved are frequently viscoelastic, which introduces an additional timescale (the fluid relaxation time) into an already complex problem.
This special issue entitled Fluid–Structure Interactions: From Engineering to Biomimetic Systems comprises proceeding papers from a mini-symposium that was held at the Okinawa Institute of Science and Technology Graduate University (OIST), Okinawa, Japan, in January 2020. The mini-symposium hosted 12 international invited speakers (from institutions in the U.S., Europe, and Asia) and 4 internal OIST speakers for 2.5 days of talks and informal discussions. Presentations spanned experimental, theoretical, and numerical approaches to understanding fluid–structure interactions over a range of length scales (from environmental to lab-on-a-chip) and in a range of fluids (from compressible and incompressible Newtonian fluids to complex viscoelastic and biological samples). Below, we give a brief overview of the papers submitted to this special issue.
Several contributors report research on microswimmers.1–3 Matsui et al.1 theoretically examined the rheology of dilute suspensions of deformable ciliate swimmers in a shear flow. They showed how the microswimmers gradually orient toward the shear plane or vorticity axis, resulting in shear-thinning properties and impacting the first and second normal stress differences. Taketoshi et al.3 numerically investigated elasto-hydrodynamic interactions between swimming spermatozoa. Compared to a solitary sperm, their results show that when two sperms swim in line, the front sperm swims faster and the rear sperm swims slower. On the other hand, two side-by-side sperms may both swim up to 16% faster than an individual sperm due to the elasto-hydrodynamic synchronization of the flagella. Collective swimming of spermatozoa is thus affected by fluid–structure interactions between flagella, which may be of importance in the fertilization process. Finally, Yasuda et al.2 reported on simple models of microswimmers able to translate themselves through viscoelastic fluids by reciprocal motion. By considering three different model swimmers based on spheres connected by extendible/retractable arms, they demonstrated that a micromachine must be structurally asymmetric in order to swim in a viscoelastic fluid using only reciprocal body motions.
Two authors provided contributions related to colloidal suspensions and dynamics. A review of recent advances in colloid separation by diffusiophoresis in microfluidics was presented by Shin.4 The review provided a pedagogical explanation of diffusiophoresis followed by a survey of recent literature related to diffusiophoresis colloid separation in cross-gradient and counter-gradient flows. A theoretical study of the low-Re dynamics of microscopic objects with discrete rotational symmetry was presented by Ishimoto.5 It is shown that if an object has fourfold (or greater) rotational symmetry, two parameters are sufficient to determine the dynamics: (1) the chirality and (2) the shape or Bretherton parameter. However, for objects with threefold rotational symmetry, an additional new “triangularity” parameter is required. In simple shear flows, we learn that triangularity can lead to chaotic angular dynamics.
Relevant to understanding low-Re viscoelastic flows past flexible filamentous structures, Varchanis et al.6 used a combination of numerical simulation and experimental measurements to study the onset of viscoelastic flow instabilities around microscopic cylinders. They reported the onset of steady asymmetric flow states that develop due to the combination of elasticity in the downstream wake of the cylinder and shear-thinning at its sides.
Finally, numerical simulations were performed by Rosti and Brandt7 to study the fluid–structure interaction between elastic walls and laminar and turbulent flows through channels. They found that wall oscillations, driven by the flow, result in chaotic flow states and that the turbulent-like state can be sustained at any Reynolds number, providing that the wall elastic modulus is chosen appropriately. This effect could be exploited for the passive enhancement of mixing in microfluidic flows at low Re.
In conclusion, fluid–structure interaction problems impact large scale, high-Re engineering projects as well as microscale, low-Re systems. Although the papers in this special issue on Fluid–Structure Interactions: From Engineering to Biomimetic Systems are focused on microscopic flows, the breadth of the topics covered is striking, with contributions made to the understanding of viscoelastic flow instabilities, colloidal particle dynamics, enhanced microfluidic mixing, hydrodynamic synchronization of flagella, and cell motility. The continued exploration of these subjects by experimental, numerical, and theoretical researchers from a variety of disciplines will certainly lead to further advances in diverse areas from healthcare and biomedical engineering to fluid formulation and process control.