Hydrodynamic Performance of Autonomous Underwater Gliders with Active Twin Undulatory Wings of Different Aspect Ratios

2020 ◽  
Vol 8 (7) ◽  
pp. 476 ◽  
Author(s):  
Yongcheng Li ◽  
Jianxin Hu ◽  
Qiuzhuo Zhao ◽  
Ziying Pan ◽  
Zheng Ma
Keyword(s):  
Linear Growth ◽  
Stokes Equations ◽  
Hydrodynamic Performance ◽  
Navier Stokes ◽  
Immersed Boundary ◽  
Underwater Glider ◽  
Navier Stokes Equations ◽  
Underwater Gliders ◽  
Aspect Ratios ◽  
Autonomous Underwater Glider

The propulsive performance of a bio-inspired autonomous underwater glider (AUG) with active twin undulatory wings undergoing undulatory motion was investigated by numerically solving the viscous incompressible Navier–Stokes equations, coupled with the immersed boundary method. The aspect ratio (AR) effects of the undulatory wings were studied. The simulation results showed that with the increase of AR, the thrust force generated by the active twin undulatory wings showed a linear growth, while the propulsion efficiency of the AUG increased to the peak and then decreased. The optimum magnitude of AR around 2 was obtained in the current study. The vortex structures in the wake of the active twin wings are also presented and discussed. The conclusions acquired here could provide guidance for the new conceptual design of bio-inspired AUGs.

scholarly journals On the Importance of the Influence of both Velocity and Aspect Ratios on the Occurrence of Bifurcation Phenomena within a Two-Sided Lid-Driven Enclosure

2015 ◽  
Vol 55 ◽  
pp. 160-172
Author(s):  
Fayçal Hammami ◽  
Nader Ben Cheikh ◽  
Brahim Ben Beya
Keyword(s):  
Stokes Equations ◽  
Numerical Study ◽  
Numerical Results ◽  
Navier Stokes ◽  
Navier Stokes Equations ◽  
Left Wall ◽  
Driven Cavity ◽  
Aspect Ratios ◽  
The Stability ◽  
The Right

This paper deals with the numerical study of bifurcations in a two-sided lid driven cavity flow. The flow is generated by moving the upper wall to the right while moving the left wall downwards. Numerical simulations are performed by solving the unsteady two dimensional Navier-Stokes equations using the finite volume method and multigrid acceleration. In this problem, the ratio of the height to the width of the cavity are ranged from H/L = 0.25 to 1.5. The code for this cavity is presented using rectangular cavity with the grids 144 × 36, 144 × 72, 144 × 104, 144 × 136, 144 × 176 and 144 × 216. Numerous comparisons with the results available in the literature are given. Very good agreements are found between current numerical results and published numerical results. Various velocity ratios ranged in 0.01≤ α ≤ 0.99 at a fixed aspect ratios (A = 0.5, 0.75, 1.25 and 1.5) were considered. It is observed that the transition to the unsteady regime follows the classical scheme of a Hopf bifurcation. The stability analysis depending on the aspect ratio, velocity ratios α and the Reynolds number when transition phenomenon occurs is considered in this paper.

scholarly journals A Stress Mapping Immersed Boundary Method for Viscous Flows

2021 ◽  
Vol 87 (3) ◽  
pp. 1-20
Author(s):  
Karim M. Ali ◽  
Mohamed Madbouli ◽  
Hany M. Hamouda ◽  
Amr Guaily
Keyword(s):  
Immersed Boundary Method ◽  
Stokes Equations ◽  
Cross Flow ◽  
Navier Stokes ◽  
Immersed Boundary ◽  
Element Formulation ◽  
Navier Stokes Equations ◽  
Boundary Method ◽  
Background Grid ◽  
Dimensional Simulation

This work introduces an immersed boundary method for two-dimensional simulation of incompressible Navier-Stokes equations. The method uses flow field mapping on the immersed boundary and performs a contour integration to calculate immersed boundary forces. This takes into account the relative location of the immersed boundary inside the background grid elements by using inverse distance weights, and also considers the curvature of the immersed boundary edges. The governing equations of the fluid mechanics are solved using a Galerkin-Least squares finite element formulation. The model is validated against a stationary and a vertically oscillating circular cylinder in a cross flow. The results of the model show acceptable accuracy when compared to experimental and numerical results.

On the formation of sediment chains in an oscillatory boundary layer

2016 ◽  
Vol 789 ◽  
pp. 461-480 ◽  
Author(s):  
Marco Mazzuoli ◽  
Aman G. Kidanemariam ◽  
Paolo Blondeaux ◽  
Giovanna Vittori ◽  
Markus Uhlmann
Keyword(s):  
Boundary Layer ◽  
Stokes Equations ◽  
Coupled Model ◽  
Spherical Particles ◽  
Navier Stokes ◽  
Immersed Boundary ◽  
Navier Stokes Equations ◽  
Ripple Formation ◽  
Oscillatory Boundary Layer ◽  
Parameter Ranges

The dynamics of spherical particles resting on a horizontal wall and set into motion by an oscillatory flow is investigated by means of a fully coupled model. Both a smooth wall and a rough wall, the latter being composed of resting particles with a random arrangement and with the same diameter as the moving particles, are considered. The fluid and particle motions are determined by means of direct numerical simulations of Navier–Stokes equations and Newton’s laws, respectively. The immersed boundary approach is used to force the no-slip condition on the surface of the particles. In particular, the process of formation of transverse sediment chains, within the boundary layer but orthogonal to the direction of fluid oscillations, is simulated in parameter ranges matching those of laboratory experiments investigating rolling-grain ripple formation. The numerical results agree with the experimental observations and show that the transverse sediment chains are generated by steady recirculating cells, generated by the interaction of the fluid and particle oscillations.

Optimization of Propulsion Kinematics of a Flexible Foil Using Integrated CFD-CSD Simulations

2012 ◽  
Author(s):  
Jiho You ◽  
Jinmo Lee ◽  
Donghyun You
Keyword(s):  
Structural Dynamics ◽  
Immersed Boundary Method ◽  
Stokes Equations ◽  
Computational Simulation ◽  
Moving Boundaries ◽  
Navier Stokes ◽  
Immersed Boundary ◽  
Navier Stokes Equations ◽  
Simulation Methodology ◽  
Computational Structural Dynamics

A computational simulation methodology, which combines a computational fluid dynamics technique and a computational structural dynamics technique, is employed to design a deformable foil of which kinematics is inspired by the propulsive motion of a fin or a tail of fish and cetacean. The unsteady incompressible Navier-Stokes equations are solved using a second-order accurate finite-difference method and an immersed-boundary method to effectively impose boundary conditions on complex moving boundaries. A finite-element-based structural dynamics solver is employed to compute the deformation of the foil due to interaction with fluid. A phase angle between pitching and heaving motions as well as the flexibility of the foil, which is represented by the Youngs modulus are varied to find out how these factors affect the propulsion efficiency.

Centrifugal instabilities during spin-down to rest in finite cylinders. Numerical experiments

1981 ◽  
Vol 102 ◽  
pp. 329-352 ◽  
Author(s):  
G. P. Neitzel ◽  
Stephen H. Davis
Keyword(s):  
Stokes Equations ◽  
Viscous Incompressible Fluid ◽  
Onset Time ◽  
Reynolds Numbers ◽  
Navier Stokes ◽  
Initial State ◽  
Navier Stokes Equations ◽  
Aspect Ratios ◽  
Gradient Forces ◽  
Axis Of Symmetry

A cylinder filled with a viscous, incompressible fluid is in an initial state of rigid-body rotation about its axis of symmetry. If the container is brought to rest impulsively, the resulting unsteady spin-down flow may be subject to sidewall instabilities due to an imbalance between centrifugal and pressure gradient forces. These instabilities are examined numerically using a finite-difference simulation to integrate the axisymmetric Navier–Stokes equations for a variety of aspect ratios and Reynolds numbers. The Taylor–Görtler vortex-wavelength spectrum, the torque and the angular momentum histories are calculated. Criteria for the onset time for instability and the spin-down time are given. The effects of the enhanced mixing due to instability on the spin-down characteristics and torque are discussed. The results are compared with experiment.

On the Immersed Boundary Method: Finite Element Versus Finite Volume Approach

2012 ◽  
Author(s):  
Angelo Frisani ◽  
Yassin A. Hassan
Keyword(s):  
Finite Element ◽  
Analytical Solution ◽  
Finite Volume ◽  
Stokes Equations ◽  
Numerical Results ◽  
Navier Stokes ◽  
Immersed Boundary ◽  
Order Accuracy ◽  
Navier Stokes Equations ◽  
Two Dimensional Flow

A projection approach is presented for the coupled system of time-dependent incompressible Navier-Stokes equations in conjunction with the Immersed Boundary Method (IBM) for solving fluid flow problems in the presence of rigid objects not represented by the underlying mesh. The IBM allows solving the flow for geometries with complex objects without the need of generating a body fitted mesh. The no-slip boundary constraint is satisfied applying a boundary force at the immersed body surface. Using projection and interpolation operators from the fluid volume mesh to the solid surface mesh (i.e., the “immersed” boundary) and vice versa, it is possible to impose the extra constraint to the incompressible Navier-Stokes equations as a Lagrange multiplier in a fashion very similar to the effect pressure has on the momentum equations to satisfy the divergence-free constraint. The projection operation removes the immersed boundary surface slip and non-divergence-free components of the velocity field. The boundary force is determined implicitly at the inner iterations of the fractional step method implemented. No constitutive relations for the immersed boundary objects fluid interaction are required, allowing the formulation introduced to use larger CFL numbers compared to previous methodologies. An overview of the immersed boundary approach is presented showing third order accuracy in space and second order accuracy in time when the simulation results for the Taylor-Green decaying vortex are compared to the analytical solution using the Immersed Finite Element Method (IFEM). For the Immersed Finite Volume Method (IFVM) a ghost-cell approach is used. Second order accuracy in space and first order accuracy in time are obtained when the Taylor-Green decaying vortex test case is compared to the analytical solution. The numerical results are compared with the analytical solution also for adaptive mesh refinement (for the IFEM) showing an excellent error reduction. Computations were performed using IFEM and IFVM approaches for the time-dependent incompressible Navier-Stokes equations in a two-dimensional flow past a stationary circular cylinder at Re = 20, and 40, where shedding effects are not present. The drag coefficient and the recirculation length error compared to the experimental data is less than 3–4%. Simulations for the two-dimensional flow past a stationary circular cylinder at Re = 100 were also performed. For Re numbers above 46, unsteadiness generates vortex shedding, and an unsteady flow regime is present. The results shown are in excellent quantitative and qualitative agreement with the flow pattern expected. The numerical results obtained with the discussed IFEM and IFVM were also compared against other immersed boundary methodologies available in literature and simulation performed with the commercial computational fluid dynamics code STAR-CCM+/V5.02.009 for which a body fitted finite volume numerical discretization was used. The benchmark showed that the numerical results obtained with the implemented immersed boundary methods are very close to those obtained from STAR-CCM+ with a very fine mesh and in a good agreement with the other IBM techniques. The IBM based of finite element approach is numerically more accurate than the IBM based on finite volume discretization. In contrast, the latter is computationally more efficient than the former.

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