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The effect of non-uniform chordwise stiffness distribution on the self-propulsive performance of three-dimensional flexible plates is studied numerically. Some typical stiffness distributions, including uniform, declining, and growing distribution, are considered. First, the normalized bending stiffness K̃ is derived, which can well represent the overall bending stiffness of the non-uniform plates. For different non-uniformly distributed plates with the same K̃, the maximum displacement difference between the trailing and leading edges of the plate during the flapping is almost identical. There exists a common optimal K̃ at which all the plates achieve their optimal performance, i.e., the highest cruising speed and efficiency. Second, we reveal what kind of non-uniform distribution could be the best at a specific K̃ in terms of the propulsive performance. The force analysis indicates that a larger bending deformation in the anterior part for the growing distribution leads to a larger thrust. Hence, the large local slope along the anterior flexible plate is preferred to enhance the propulsive performance. The results obtained in this study may shed some light on a better understanding of the hydrodynamic effect on the self-propulsion of the non-uniform stiffness wings or fins of animals. [ABSTRACT FROM AUTHOR]

The free locomotion of a two-dimensional flapping flexible plate near the flat ground is studied by the lattice Boltzmann method for fluid flow and a finite-element method for the plate motion. The fluid flow and plate deformation are coupled through the immersed boundary scheme. When the leading edge of the plate is forced to oscillate sinusoidally near the ground, the plate may move freely in the horizontal direction due to the fluid-structure interaction. The mechanisms underlying the ground effect are elucidated. Besides a moderate rigidity, it is found that an appropriate density ratio between the plate and surrounding fluid (M) can improve the propulsive efficiency of the plate. When M is relatively small, the lateral force is enhanced, and the input work is increased when the plate is near the ground; when M is large, the deformation of the plate is inhibited and the input work is decreased when the plate is close to the ground. Usually the closer the plate flapping is to the wall, the more efficient the propulsion is, provided that the tail of the plate would not touch the wall. On the other hand, when the plate is close enough (within a critical lowest distance), the efficiency reaches a plateau with the highest efficiency. The vortices pattern and pressure field are also analyzed to explore the mechanism. This study may shed some light on mechanism for self-propulsion of a flexible plate near the ground. [ABSTRACT FROM AUTHOR]

Interaction between two tandem flexible loops with tension and bending stiffness in a viscous flow is investigated by numerical simulations. In most cases, the heads of the loops facing the oncoming flow are fixed but flapping around the head is allowed. The effect of the gap distance between the two passive flapping loops (G) on the drag coefficient is investigated in detail. Here, for the first time, the sudden drag force reduction at a specific G, i.e., Gc for the downstream loop has been found in the two tandem flexible loops system. It is different from the drag "jump" behavior in the two tandem rigid cylinder system. Although the drag is partially associated with the flapping amplitude, the drag force reduction of the downstream loop may be mainly attributed to flow regimes transition or vortices merging mode transition. The vortices merging is also analysed from a Lagrangian viewpoint, which gives insight into the mechanism. The effects of Reynolds number (20 ≤ Re ≤ 100), bending coefficient (10-4 ≤ K ≤ 2 x 10-2), and tension coefficient (10 ≤ S ≤ 1000) are also investigated and the relevant mechanism is explored. If the head of the downstream loop is set free to move laterally, the critical distance (Gc) where the sudden drag reduction occurs would further decrease compared to the fixed case, which is due to the early shedding regime transition. [ABSTRACT FROM AUTHOR]

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, deflectedflapping, 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. [ABSTRACT FROM AUTHOR]

Sedimentation behaviours of an ellipsoidal particle in narrow and infinitely long tubes are studied by a multi-relaxation-time lattice Boltzmann method (LBM). In the present study, both circular and square tubes with 12/13 ⩽ D/A ⩽ 2.5 are considered with the Galileo number (Ga) up to 150, where D and A are the width of the tube and the length of major axis of the ellipsoid, respectively. Besides three modes of motion mentioned in the literature, two novel modes are found for the narrow tubes in the higher Ga regime: the spiral mode and the vertically inclined mode. Near a transitional regime, in terms of average settling velocity, it is found that a lighter ellipsoid may settle faster than a heavier one. The relevant mechanism is revealed. The behaviour of sedimentation inside the square tubes is similar to that in the circular tubes. One significant difference is that the translation and rotation of ellipsoid are finally constrained to a diagonal plane in the square tubes. The other difference is that the anomalous rolling mode occurs in the square tubes. In this mode, the ellipsoid rotates as if it is contacting and rolling up one corner of the square tube when it settles down. Two critical factors that induce this mode are identified: the geometry of the tube and the inertia of the ellipsoid. [ABSTRACT FROM AUTHOR]

In this paper, the viscous coupling effects for immiscible two-phase (gas-liquid) flow in porous media were studied using the Shan–Chen-type single-component multiphase lattice Boltzmann model. Using the model, the two-phase flows in porous media with density ratio as high as 56 could be simulated and the contact angle of the gas-liquid interface at a solid wall is adjustable. To investigate viscous coupling effects, the co- and countercurrent steady-state two-phase flow patterns and relative permeabilities as a function of wetting saturation were obtained for different capillary numbers, wettabilities, and viscosity ratios. The cocurrent relative permeabilities seem usually larger than the countercurrent ones. The opposing drag-force effect and different pore-level saturation distributions in co- and countercurrent flows may contribute to this difference. It is found that for both co- and countercurrent flows, for strongly wet cases and viscosity ratio M>1, knw increase with the driving force and the viscosity ratio. However, for neutrally wet cases, the variations of knw and kw are more complex. It is also observed that different initial pore-level saturation distributions may affect final steady-state distribution, and hence the relative permeabilities. Using the cocurrent and countercurrent steady flow experiments to determine the generalized relative permeabilities seems not correct. [ABSTRACT FROM AUTHOR]

The ground effect on insect normal 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 body surface or ground is used. Computations have been carried out for some parameters including the distance between the foil and the surface, phase difference between the rotation and translation, and amplitude of oscillating rotation. The ground effect on the unsteady forces and vortical structures is analyzed. In particular, three typical regimes of force behavior due to the ground effect, i.e., force enhancement, force reduction, and force recovery regime, are identified and closely associated with the evolution of vortex structures. The results obtained in this study provide physical insight into the understanding of aerodynamics and flow structures for insect normal hovering flight with a ground effect and flying mechanisms relevant to insect perching on body. [ABSTRACT FROM AUTHOR]

Flow over traveling wavy foils in a side-by-side arrangement has been numerically investigated using the space-time finite element method to solve the two-dimensional incompressible Navier-Stokes equations. The midline of each foil undergoes lateral motion in the form of a streamwise traveling wave, which is similar to the backbone undulation of swimming fish. Based on the phase difference between the adjacent undulating foils, two typical cases, i.e., in-phase and anti-phase traveling wavy movements, are considered in the present study. The effects of lateral interference among the foils on the forces, power consumption, propeller efficiency, and flow structures are analyzed. It is revealed that the lateral interference is of benefit to saving the swimming power in the in-phase case and enhancing the forces in the anti-phase case. Some typical vortex structures, e.g., vortex-pair row, single vortex row, and in-phase and anti-phase synchronized vortex-street, are observed in the wake of the traveling wavy foils. The results obtained in this study provide physical insight into the understanding of hydrodynamics and flow structures for flow over the traveling wavy foils and swimming mechanisms relevant to fish schooling. [ABSTRACT FROM AUTHOR]

A large eddy simulation (LES) approach of compressible turbulent flow without density weighting (or Favre averaging) is proposed and examined based on compressible turbulent channel flow. In this Brief Communication, we attempt to remove an inconsistent treatment including both the resolved and Favre-averaged variables and to provide a possible approach to deal with the flows with rapid property variation. The subgrid-scale (SGS) terms in the resolved equations of mass, momentum, and energy conservation are modeled reasonably and the relevant coefficients in the SGS models are computed dynamically. The present LES approach is verified to be reliable and effective by comparing with direct numerical simulation results of compressible turbulent channel flow. [ABSTRACT FROM AUTHOR]

The stability of channel flow modulated by oscillatory wall suction/blowing is investigated using linear stability analysis together with Floquet theory based on numerical calculation and asymptotic expansion. Two typical flows with either the driven pressure gradient or the flow rate constant are considered. The basic flows subject to the oscillatory wall suction/blowing are time periodic with multiple frequency components. The stability problem is formulated into a time-dependent eigenvalue problem, and the Floquet exponents are obtained using a spectral collocation method. It is revealed that the periodic wall suction/blowing induces the Stokes layer, which interacts with the disturbance shear wave and eventually affects the disturbance growth. Results show that the modulations of the oscillatory wall suction/blowing to the channel flows have a destabilizing effect and the similar stability characteristics of both the typical flows occur. Critical Reynolds numbers and wave numbers are predicted for a wide range of parameters. Asymptotic expansions of the growth rate at small amplitude Δ of the oscillatory wall suction/blowing are developed. The correction terms for the growth rate occur in O(Δ2) and are positive, indicating that the flow is destabilized. It is found that the destabilizing effect is mainly connected to the steady corrections of the mean flow profile in the O(Δ2) terms. [ABSTRACT FROM AUTHOR]

In experimental aerodynamics (and hydrodynamics) it is well known that, if the flow past a solid body is steady, then the total force on the body can be conveniently estimated by the measured flow data on an appropriate control surface alone. We now show that, for the first time, the steady-flow condition can be removed provided that the flow is incompressible: two innovative formulas for the total force acting on any solid body that moves and deforms arbitrarily in a viscous incompressible fluid, solely in terms of control-surface integrals, are derived based on derivative-moment transformations. The formulas are verified by a numerical test for flow over a two-dimensional fishlike swimming body. [ABSTRACT FROM AUTHOR]