Nan-Sheng Liu

Locomotion of a passively flapping flat plate

Locomotion of a passively flapping flat plate has been studied numerically by means of a multiblock lattice Boltzmann method. A flexible plate is modelled by a rigid plate with a torsion spring acting about the pivot at the leading edge of the plate. A dynamic model of this kind is called a lumped-torsional-flexibility model. When the leading edge is forced to heave sinusoidally, the plate pitches passively and propels itself in the horizontal direction as a result of the fluid–plate interaction. We have investigated various aspects of the mechanics behind the behaviour of the flapping plate, including the periodic- and non-periodic-flow states, the spontaneous motion of the plate, vortical structure and how they compare to similar propulsion systems in animals. In the periodic-flow regime, two dynamical responses of the passively pitching plate (forward and backward movements) are observed. Which movement will occur depends only on the frequency ratio F of the natural frequency of the system and the heaving frequency associated with the lumped torsional flexibility. It is found that the plate will select the forward movement when F > 1 and the backward movement when F ≤ 1. In the forward-movement regime, analysis of the dynamical behaviours and propulsive properties of the passively pitching plate indicates that the torsional flexibility can remarkably improve the propulsive performance. In addition, four kinds of vortex structures in the near wake are identified, which mainly depend on the forward speed of the plate. Finally the forward movement is compared to the flapping-based locomotion of swimming and flying animals. The results obtained in this study are consistent with the observations and measurements of swimming and flying animals; thus, they may provide physical insights into understanding of the propulsive mechanisms of the flapping wings and fins of animals.

Numerical Analysis of the Ground Effect on Insect Hovering

The ground effect on insect hovering is investigated using an immersed boundary-lattice Boltzmann method to solve the two-dimensional incompressible Navier-Stokes equations. A virtual model of an elliptic foil with oscillating translation and rotation near a ground is used. The objective of this study is to deal with the ground effect on the unsteady forces and vortical structures and to get the physical insights in the relevant mechanisms. Two typical insect hovering modes, i.e., normal and dragonfly hovering mode, are examined. Systematic computations have been carried out for some parameters, and the ground effect on the unsteady forces and vortical structures is analyzed.

Dynamics of an inverted flexible plate in a uniform flow

The dynamics of an inverted flexible plate with a free leading-edge and a fixed trailing-edge in a uniform flow has been studied numerically by an immersed boundary-lattice Boltzmann method for the fluid flow and a finite element method for the plate deformation. Mechanisms underlying the dynamics of the fluid-plate system are elucidated systematically. A series of distinct states of the plate deformation and motion are identified and can be described as straight, flapping, deflected, deflected-flapping, and asymmetric-flapping states. Which state to occur depends mainly on the bending stiffness and aspect ratio of the plate. The forces exerted on the plate and the elastic strain energy of the plate are analyzed. It is found that the flapping state can improve the conversion of fluid kinetic energy to elastic strain energy. In addition, the effects of the mass ratio of the plate and the fluid, the Reynolds number, and the angle of attack of the uniform flow on the dynamics and the elastic strain energy of flexible plate are also investigated in detail. The vortical structures around the plate are given to discuss the connection of the evolution of vortices with the plate deformation and motion. The results obtained in this study provide physical insight into the understanding of the mechanisms on the dynamics of the fluid-plate system.

DIRECT NUMERICAL SIMULATIONS OF TURBULENT CHANNEL FLOWS WITH CONSIDERATION OF THE BUOYANCY EFFECT OF THE BUBBLE PHASE

Turbulent channel flows with consideration of the buoyancy effect of the bubble phase is investigated by means of the Direct Numerical Simulation (DNS). This two-phase system is solved by a two-way coupling Lagrangian-Eulerian approach. The Reynolds number based on the friction velocity and the half-width of the channel is 194, and the gravitational acceleration varies from −0.5 to 0.5, ranging from the upflow to the downflow cases. This study aims to reveal the influence of buoyancy on the turbulence behavior and the bubble motion. Some typical statistical quantities, including the averaged velocities and velocity fluctuations for the fluid and bubble phases, as well as the flow structures of the turbulence fluctuations, are analyzed.

Simulation of compressible two-phase flows with topology change of fluid-fluid interface by a robust cut-cell method

We develop a robust cut-cell method for numerical simulation of compressible two-phase flows with topology change of the fluid-fluid interface. In cut cell methods the flows can be solved in the finite volume framework and the jump conditions at the interface are resolved by solving a local Riemann problem. Therefore, cut cell methods can obtain interface evolution with high resolution, and at the same time satisfactorily maintain the conservation of flow quantities. However, it remains a challenge for the cut cell methods to handle interfaces with topology change or very high curvature, where the mesh is not sufficiently fine to resolve the interface. Inappropriate treatment could give rise to either distorted interface advection or unphysical oscillation of flow variables, especially when the regularization process (e.g. reinitialization in the level set methods) is implemented. A robust cut-cell method is proposed here, with the interface being tracked by a level set function. The local unphysical oscillation of flow variables in the presence of topology change is shown to be greatly suppressed by using a delayed reinitialization. The method can achieve second-order accuracy with respect to the interface position in the absence of topology changes of interface, while locally degrading to first-order at the interface region where topology change occurs. Its performance is examined through a variety of numerical tests, such as Rayleigh collapse, shock-bubble interaction, and shock-induced bubble collapse in water. Numerical results are compared against either benchmark solutions or experimental observations, and good agreement has been achieved qualitatively and/or quantitatively. Finally, we apply the method to investigating the collapse process of two tandem bubbles in water.

TURBULENT OPEN CHANNEL FLOW SUBJECTED TO THE CONTROL OF A SPANWISE TRAVELING WAVE

Turbulent open channel flows subjected to the control of a spanwise traveling wave have been investigated by means of Direct Numerical Simulation (DNS). The objective of this study is to reveal the response of the near-wall and surface-influenced turbulence to the spanwise traveling wave control. Three typical frequencies of the spanwise traveling wave, i.e., high-, middle- and low-frequency, corresponding to the exciting periods at 25, 50 and 100, are considered to study the turbulence dynamics in the wall and surface regions. To elucidate the behaviors of turbulence statistics, some typical quantities, including the mean velocity, velocity fluctuations and the structures of turbulence fluctuations, are exhibited and analyzed.

A deformable plate interacting with a non-Newtonian fluid in three dimensions

We consider a deformable plate interacting with a non-Newtonian fluid flow in three dimensions as a simple model problem for fluid-structure-interaction phenomena in life sciences (e.g., red blood cell interacting with blood flow). A power-law function is used for the constitutive equation of the non-Newtonian fluid. The lattice Boltzmann equation (the D3Q19 model) is used for modeling the fluid flow. The immersed boundary (IB) method is used for modeling the flexible plate and handling the fluid-plate interaction. The plate drag and its scaling are studied; the influences of three dimensionless parameters (power-law exponent, bending modulus, and generalized Reynolds number) are investigated.

Direct numerical simulation of Taylor-Couette flow subjected to a radial temperature gradient

Direct numerical simulations have been performed to study the Taylor-Couette (TC) flow between two rotating, coaxial cylinders in the presence of a radial temperature gradient. Specifically, the influence of the buoyant force and the outer cylinder rotation on the turbulent TC flow system with the radius ratio η = 0.912 was examined. For the co-rotating TC flows with Rei (inner cylinder) =1000 and Reo (outer cylinder) =100, a transition pathway to highly turbulentflows is realized by increasing σ, a parameter signifying the ratio of buoyant to inertial force. This nonlinear flow transition involves four intriguing states that emerge in sequence as chaotic wavy vortexflow for σ = 0, wavy interpenetrating spiral flows for σ = 0.02 and 0.05, intermittent turbulent spirals for σ = 0.1 and 0.2, and turbulent spirals for σ = 0.4. Overall, the fluid motion changes from a centrifugally driven flow regime characterized by large-scale wavy Taylor vortices (TVs) to a buoyancy-dominated flow regime characterized by small-scale turbulentvortices. Commensurate changes in turbulence statistics and heat transfer are seen as a result of the weakening of large-scale TV circulations and enhancement of turbulentmotions. Additionally, the influence of variation of the outer cylinder rotation, −500 < Reo < 500 in presence of buoyancy (σ = 0.1) with Rei = 1000, has been considered. Specifically, it is demonstrated that this variation strongly influences the azimuthal and axial mean flows with a weaker influence on the fluctuating fluid motions. Of special interest, here are the turbulent dynamics near the outer wall where a marked decrease of turbulence intensity and a sign inversion of the Reynolds stressRrz are observed for the strongly counter-rotating regimes (Reo = − 300 and −500). To this end, it has been shown that the underlying flow physics for this drastic modification are associated with the modification of the correlation between the radial and axial fluctuating motions. In turn, the intriguing effects of this modification on the mean axial flow,turbulent statistics, force balance, and dynamic processes such as turbulence production and dissipation are discussed.

Lattice Boltzmann simulations of turbulent shear flow between parallel porous walls

The effects of two parallel porous walls are investigated, consisting of the Darcy number and the porosity of a porous medium, on the behavior of turbulent shear flows as well as skin-friction drag. The turbulent channel flow with a porous surface is directly simulated by the lattice Boltzmann method (LBM). The Darcy-Brinkman-Forcheimer (DBF) acting force term is added in the lattice Boltzmann equation to simulate the turbulent flow bounded by porous walls. It is found that there are two opposite trends (enhancement or reduction) for the porous medium to modify the intensities of the velocity fluctuations and the Reynolds stresses in the near wall region. The parametric study shows that flow modification depends on the Darcy number and the porosity of the porous medium. The results show that, with respect to the conventional impermeable wall, the degree of turbulence modification does not depend on any simple set of parameters obviously. Moreover, the drag in porous wall-bounded turbulent flow decreases if the Darcy number is smaller than the order of O(10−4) and the porosity of porous walls is up to 0.4.

DIRECT NUMERICAL SIMULATION OF TURBULENT FLOWS IN A VERTICAL ROTATING OPEN-CHANNEL

Direct numerical simulation (DNS) is carried out to study turbulence characteristics in a vertical rotating open-channel with the rotation number Nτ = 0-0.12 and the Reynolds number Reτ = 180 based on the wall friction velocity of non-rotating case and the channel depth. Here, two typical rotation regimes are identified. As 0 0.06, the turbulence statistics are suppressed significantly because the effect of Coriolis force plays as a dominant role.