39 research outputs found
Computational Study of Bouncing and Non-bouncing Droplets Impacting on Superhydrophobic Surfaces
We numerically investigate bouncing and non-bouncing of droplets during
isothermal impact on superhydrophobic surfaces. An in-house,
experimentally-validated, finite-element method based computational model is
employed to simulate the droplet impact dynamics and transient fluid flow
within the droplet. The liquid-gas interface is tracked accurately in
Lagrangian framework with dynamic wetting boundary condition at three-phase
contact line. The interplay of kinetic, surface and gravitational energies is
investigated via systematic variation of impact velocity and equilibrium
contact angle. The numerical simulations demonstrate that the droplet bounces
off the surface if the total droplet energy at the instance of maximum
recoiling exceeds the initial surface and gravitational energy, otherwise not.
The non-bouncing droplet is characterized by the oscillations on the free
surface due to competition between the kinetic and surface energy. The droplet
dimensions and shapes obtained at different times by the simulations are
compared with the respective measurements available in the literature.
Comparisons show good agreement of numerical data with measurements and the
computational model is able to reconstruct the bouncing and non-bouncing of the
droplet as seen in the measurements. The simulated internal flow helps to
understand the impact dynamics as well as the interplay of the associated
energies during the bouncing and non-bouncing.Comment: Theoretical and Computational Fluid Dynamics, 201
Recent Developments on Colloidal Deposits Obtained by Evaporation of Sessile Droplets on a Solid Surface
Understanding flow patterns and coupled transport phenomena during
evaporation of droplets loaded with colloidal particles is central to design
technical applications such as organizing proteins/DNA on a solid surface. We
review recent reports on evaporating sessile droplets of colloidal suspensions
on a solid surface. Starting from the classical mechanism of formation of a
ring-like deposit, we discuss the influence of several problem parameters.
Notably, thermal or solutal Marangoni effect, particle size, particle
concentration, particle shape, substrate wettability, pH of the suspension etc
have been found important in controlling the deposition pattern. The deposit
pattern complexity and shape have been attributed to the underlying coupled
transport phenomena during the evaporation. We discuss important regimes maps
reported for different types of deposit, which allow us to classify the
deposits and coupled physics. We also present studies that have demonstrated
particles sorting in an evaporating bi-dispersed colloidal suspensions on a
solid surface. Finally, some remarks for the future research opportunities in
this arena are presented
Harnessing Electrical Power from Vortex-Induced Vibration of a Circular Cylinder
The generation of electrical power from Vortex-Induced Vibration (VIV) of a
cylinder is investigated numerically. The cylinder is free to oscillate in the
direction transverse to the incoming flow. The cylinder is attached to a magnet
that can move along the axis of a coil made from conducting wire. The magnet
and the coil together constitute a basic electrical generator. When the
cylinder undergoes VIV, the motion of the magnet creates a voltage across the
coil, which is connected to a resistive load. By Lenz's law, induced current in
the coil applies a retarding force to the magnet. Effectively, the electrical
generator applies a damping force on the cylinder with a spatially varying
damping coefficient. For the initial investigation reported here, the Reynolds
number is restricted to Re < 200, so that the flow is laminar and
two-dimensional (2D). The incompressible 2D Navier-Stokes equations are solved
using an extensively validated spectral-element based solver. The effects of
the electromagnetic (EM) damping constant xi_m, coil dimensions (radius a,
length L), and mass ratio on the electrical power extracted are quantified. It
is found that there is an optimal value of xi_m (xi_opt) at which maximum
electrical power is generated. As the radius or length of the coil is
increased, the value of xi_opt is observed to increase. Although the maximum
average power remains the same, a larger coil radius or length results in a
more robust system in the sense that a relatively large amount of power can be
extracted when xi_m is far from xi_opt, unlike the constant damping ratio case.
The average power output is also a function of Reynolds number, primarily
through the increased maximum oscillation amplitude that occurs with increased
Reynolds number at least within the laminar range, although the general
qualitative findings seem likely to carry across to high Reynolds number VIV
Flow-Induced Deformation of a Flexible Thin Structure as Manifestation of Heat Transfer Enhancement
Flow-induced deformation of thin structures coupled with convective heat
transfer has potential applications in energy harvesting and is important for
understanding functioning of several biological systems. We numerically
demonstrate large-scale flow-induced deformation as an effective passive heat
transfer enhancement technique. An in-house, strongly-coupled fluid-structure
interaction (FSI) solver is employed in which flow and structure solvers are
based on sharp-interface immersed boundary and finite element method,
respectively. In the present work, we validate convective heat transfer module
of the in-house FSI solver against several benchmark examples of conduction and
convective heat transfer including moving structure boundaries. The thermal
augmentation is investigated as well as quantified for the flow-induced
deformation of an elastic thin plate attached to lee side of a rigid cylinder
in a heated channel laminar flow. We show that the wake vortices past the plate
sweep higher sources of vorticity generated on the channel walls out into the
high velocity regions, promoting the mixing of the fluid. The self-sustained
motion of the plate assists in convective mixing, augmenting convection in bulk
and near the walls, and thereby reducing thermal boundary layer thickness as
well as improving Nusselt number at the channel walls. We quantify the thermal
improvement with respect to channel flow without any bluff body and analyze the
role of Reynolds number, Prandtl number and material properties of the plate in
the thermal augmentation