Proximity to the water surface markedly enhances the force production on underwater flapping wings.

The potential application of flapping wings in micro-aerial vehicles is gaining interest due to their ability to generate high lift even in confined spaces. Most studies in the past have investigated hovering wings as well as those flapping near solid surfaces. However, the presence of surface tension at the water-air interface and the ability of the water surface to move might differentiate its response to the proximity of wings, compared to that of solid surfaces.

Learning to school in dense configurations with multi-agent deep reinforcement learning.

Fish are observed to school in different configurations. However, how and why fish maintain a stable schooling formation still remains unclear. This work presents a numerical study of the dense schooling of two free swimmers by a hybrid method of the multi-agent deep reinforcement learning and the immersed boundary-lattice Boltzmann method.

Fast prediction of blood flow in stenosed arteries using machine learning and immersed boundary-lattice Boltzmann method.

A fast prediction of blood flow in stenosed arteries with a hybrid framework of machine learning and immersed boundary-lattice Boltzmann method (IB-LBM) is presented. The integrated framework incorporates the immersed boundary method for its excellent capability in handling complex boundaries, the multi-relaxation-time LBM for its efficient modelling for unsteady flows and the deep neural network (DNN) for its high efficiency in artificial learning. Specifically, the stenosed artery is modelled by a channel for two-dimensional (2D) cases or a tube for three-dimensional (3D) cases with a stenosis approximated by a fifth-order polynomial.

Effects of uniform vertical inflow perturbations on the performance of flapping wings.

Flapping wings have attracted significant interest for use in miniature unmanned flying vehicles. Although numerous studies have investigated the performance of flapping wings under quiescent conditions, effects of freestream disturbances on their performance remain under-explored. In this study, we experimentally investigated the effects of uniform vertical inflows on flapping wings using a Reynolds-scaled apparatus operating in water at Reynolds number ≈ 3600.

Performance of passively pitching flapping wings in the presence of vertical inflows.

The successful implementation of passively pitching flapping wings strongly depends on their ability to operate efficiently in wind disturbances. In this study, we experimentally investigated the interaction between a uniform vertical inflow perturbation and a passive-pitching flapping wing using a Reynolds-scaled apparatus operating in water at Reynolds number ≈3600. A parametric study was performed by systematically varying the Cauchy number () of the wings from 0.

A numerical study of fish adaption behaviors in complex environments with a deep reinforcement learning and immersed boundary-lattice Boltzmann method.

Fish adaption behaviors in complex environments are of great importance in improving the performance of underwater vehicles. This work presents a numerical study of the adaption behaviors of self-propelled fish in complex environments by developing a numerical framework of deep learning and immersed boundary-lattice Boltzmann method (IB-LBM). In this framework, the fish swimming in a viscous incompressible flow is simulated with an IB-LBM which is validated by conducting two benchmark problems including a uniform flow over a stationary cylinder and a self-propelled anguilliform swimming in a quiescent flow.

Numerical simulation of flow over a parallel cantilevered flag in the vicinity of a rigid wall.

Flow over a parallel cantilevered flag in the vicinity of a rigid wall is numerically studied using an immersed boundary-lattice Boltzmann method (IB-LBM) in two-dimensional domain, where the dynamics of the fluid and structure are, respectively, solved by the LBM and a finite-element method (FEM), with a penalty IB to handle the fluid-structure interaction (FSI). Specifically, a benchmark case considering a plate attached to the downstream of a stationary cylinder is first conducted to validate the current solver. Then, the wall effects on the flag are systemically studied, considering the effects of off-wall distance, structure-to-fluid mass ratio, bending rigidity, and Reynolds number.

Dynamic characteristics of a deformable capsule in a simple shear flow.

The dynamic characteristics of a two-dimensional deformable capsule in a simple shear flow are studied with an immersed boundary-lattice Boltzmann method. Simulations are conducted by varying the Reynolds number (Re) from 0.0125 to 2000 and the dimensionless shear rate (G) from 0.

Deformation of a Capsule in a Power-Law Shear Flow.

An immersed boundary-lattice Boltzmann method is developed for fluid-structure interactions involving non-Newtonian fluids (e.g., power-law fluid).

Refuging rainbow trout selectively exploit flows behind tandem cylinders.

Fishes may exploit environmental vortices to save in the cost of locomotion. Previous work has investigated fish refuging behind a single cylinder in current, a behavior termed the Kármán gait. However, current-swept habitats often contain aggregations of physical objects, and it is unclear how the complex hydrodynamics shed from multiple structures affect refuging in fish.

Fluid-structure interaction involving large deformations: 3D simulations and applications to biological systems.

Three-dimensional fluid-structure interaction (FSI) involving large deformations of flexible bodies is common in biological systems, but accurate and efficient numerical approaches for modeling such systems are still scarce. In this work, we report a successful case of combining an existing immersed-boundary flow solver with a nonlinear finite-element solid-mechanics solver specifically for three-dimensional FSI simulations. This method represents a significant enhancement from the similar methods that are previously available.

IB-LBM simulation on blood cell sorting with a micro-fence structure.

A size-based blood cell sorting model with a micro-fence structure is proposed in the frame of immersed boundary and lattice Boltzmann method (IB-LBM). The fluid dynamics is obtained by solving the discrete lattice Boltzmann equation, and the cells motion and deformation are handled by the immersed boundary method. A micro-fence consists of two parallel slope post rows which are adopted to separate red blood cells (RBCs) from white blood cells (WBCs), in which the cells to be separated are transported one after another by the flow into the passageway between the two post rows.

The role of finite displacements in vocal fold modeling.

Human vocal folds experience flow-induced vibrations during phonation. In previous computational models, the vocal fold dynamics has been treated with linear elasticity theory in which both the strain and the displacement of the tissue are assumed to be infinitesimal (referred to as model I). The effect of the nonlinear strain, or geometric nonlinearity, caused by finite displacements is yet not clear.

Simulation of a pulsatile non-Newtonian flow past a stenosed 2D artery with atherosclerosis.

Atherosclerotic plaque can cause severe stenosis in the artery lumen. Blood flow through a substantially narrowed artery may have different flow characteristics and produce different forces acting on the plaque surface and artery wall. The disturbed flow and force fields in the lumen may have serious implications on vascular endothelial cells, smooth muscle cells, and circulating blood cells.

IB-LBM simulation of the haemocyte dynamics in a stenotic capillary.

To study the behaviour of a haemocyte when crossing a stenotic capillary, the immersed boundary-lattice Boltzmann method was used to establish a quantitative analysis model. The haemocyte was assumed to be spherical and to have an elastic cell membrane, which can be driven by blood flow to adopt a highly deformable character. In the stenotic capillary, the spherical blood cell was stressed both by the flow and the wall dimension, and the cell shape was forced to be stretched to cross the stenosis.

An efficient immersed boundary-lattice Boltzmann method for the hydrodynamic interaction of elastic filaments.

We have introduced a modified penalty approach into the flow-structure interaction solver that combines an immersed boundary method (IBM) and a multi-block lattice Boltzmann method (LBM) to model an incompressible flow and elastic boundaries with finite mass. The effect of the solid structure is handled by the IBM in which the stress exerted by the structure on the fluid is spread onto the collocated grid points near the boundary. The fluid motion is obtained by solving the discrete lattice Boltzmann equation.

Interaction between a flexible filament and a downstream rigid body.

A filament flapping in the bow wake of a rigid body is considered in order to study the hydrodynamic interaction between flexible and rigid bodies in tandem arrangement. Both numerical and experimental methods are adopted to analyze the motion of the filament, and the drag force on both bodies is computed. It is shown that the results largely depend on the gap between the two objects and the Reynolds number.

Secondary vortex street in the wake of two tandem circular cylinders at low Reynolds number.

The experiments on two tandem circular cylinders were conducted in a horizontal soap film tunnel for the Reynolds number Re=60 , 80, and 100 and the nondimensional center-to-center spacing Gamma ranging in 1 approximately 12. The flow patterns were recorded by a high-speed camera and the vortex shedding frequency was obtained by a spatiotemporal evolution method. The secondary vortex formation (SVF) mode characterized by the formation of a secondary vortex street in the wake of the downstream cylinder was found at large gamma.