3,971 research outputs found
A full Eulerian finite difference approach for solving fluid-structure coupling problems
A new simulation method for solving fluid-structure coupling problems has
been developed. All the basic equations are numerically solved on a fixed
Cartesian grid using a finite difference scheme. A volume-of-fluid formulation
(Hirt and Nichols (1981, J. Comput. Phys., 39, 201)), which has been widely
used for multiphase flow simulations, is applied to describing the
multi-component geometry. The temporal change in the solid deformation is
described in the Eulerian frame by updating a left Cauchy-Green deformation
tensor, which is used to express constitutive equations for nonlinear
Mooney-Rivlin materials. In this paper, various verifications and validations
of the present full Eulerian method, which solves the fluid and solid motions
on a fixed grid, are demonstrated, and the numerical accuracy involved in the
fluid-structure coupling problems is examined.Comment: 38 pages, 27 figures, accepted for publication in J. Comput. Phy
ChainQueen: A Real-Time Differentiable Physical Simulator for Soft Robotics
Physical simulators have been widely used in robot planning and control.
Among them, differentiable simulators are particularly favored, as they can be
incorporated into gradient-based optimization algorithms that are efficient in
solving inverse problems such as optimal control and motion planning.
Simulating deformable objects is, however, more challenging compared to rigid
body dynamics. The underlying physical laws of deformable objects are more
complex, and the resulting systems have orders of magnitude more degrees of
freedom and therefore they are significantly more computationally expensive to
simulate. Computing gradients with respect to physical design or controller
parameters is typically even more computationally challenging. In this paper,
we propose a real-time, differentiable hybrid Lagrangian-Eulerian physical
simulator for deformable objects, ChainQueen, based on the Moving Least Squares
Material Point Method (MLS-MPM). MLS-MPM can simulate deformable objects
including contact and can be seamlessly incorporated into inference, control
and co-design systems. We demonstrate that our simulator achieves high
precision in both forward simulation and backward gradient computation. We have
successfully employed it in a diverse set of control tasks for soft robots,
including problems with nearly 3,000 decision variables.Comment: In submission to ICRA 2019. Supplemental Video:
https://www.youtube.com/watch?v=4IWD4iGIsB4 Project Page:
https://github.com/yuanming-hu/ChainQuee
Deformable ellipsoidal bubbles in Taylor-Couette flow with enhanced Euler-Lagrange tracking
In this work we present numerical simulations of sub-Kolmogorov
deformable bubbles dispersed in Taylor-Couette flow (a wall-bounded shear
system) with rotating inner cylinder and outer cylinder at rest. We study the
effect of deformability of the bubbles on the overall drag induced by the
carrier fluid in the two-phase system. We find that an increase in
deformability of the bubbles results in enhanced drag reduction due to a more
pronounced accumulation of the deformed bubbles near the driving inner wall.
This preferential accumulation is induced by an increase in the resistance on
the motion of the bubbles in the wall-normal direction. The increased
resistance is linked to the strong deformation of the bubbles near the wall
which makes them prolate (stretched along one axes) and orient along the
stream-wise direction. A larger concentration of the bubbles near the driving
wall implies that they are more effective in weakening the plume ejections
which results in stronger drag reduction effects. These simulations which are
practically impossible with fully resolved techniques are made possible by
coupling a sub-grid deformation model with two-way coupled Euler-Lagrangian
tracking of sub-Kolmogorov bubbles dispersed in a turbulent flow field which is
solved through direct numerical simulations. The bubbles are considered to be
ellipsoidal in shape and their deformation is governed by an evolution equation
which depends on the local flow conditions and their surface tension
Conditional stability of particle alignment in finite-Reynolds-number channel flow
Finite-size neutrally buoyant particles in a channel flow are known to
accumulate at specific equilibrium positions or spots in the channel
cross-section if the flow inertia is finite at the particle scale. Experiments
in different conduit geometries have shown that while reaching equilibrium
locations, particles tend also to align regularly in the streamwise direction.
In this paper, the Force Coupling Method was used to numerically investigate
the inertia-induced particle alignment, using square channel geometry. The
method was first shown to be suitable to capture the quasi-steady lift force
that leads to particle cross-streamline migration in channel flow. Then the
particle alignment in the flow direction was investigated by calculating the
particle relative trajectories as a function of flow inertia and of the ratio
between the particle size and channel hydraulic diameter. The flow streamlines
were examined around the freely rotating particles at equilibrium, revealing
stable small-scale vortices between aligned particles. The streamwise
inter-particle spacing between aligned particles at equilibrium was calculated
and compared to available experimental data in square channel flow (Gao {\it et
al.} Microfluidics and Nanofluidics {\bf 21}, 154 (2017)). The new result
highlighted by our numerical simulations is that the inter-particle spacing is
unconditionally stable only for a limited number of aligned particles in a
single train, the threshold number being dependent on the confinement
(particle-to-channel size ratio) and on the Reynolds number. For instance, when
the particle Reynolds number is and the particle-to-channel height
size ratio is , the maximum number of stable aligned particles per
train is equal to 3. This agrees with statistics realized on the experiments of
(Gao {\it et al.} Microfluidics and Nanofluidics {\bf 21}, 154 (2017)).Comment: 13 pages, 13 figure
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