Abstract: The current study couples a quasi-steady Vortex Lattice
Method and a camber correcting technique, ‘Decambering’ for
unsteady post-stall flow prediction. The wake is force-free and
discrete such that the wake lattices move with the free-stream once
shed from the wing. It is observed that the time-averaged unsteady
coefficient of lift sees a relative drop at post-stall angles of attack
in comparison to its steady counterpart for some angles of attack.
Multiple solutions occur at post-stall and three different algorithms
to choose solutions in these regimes show both unsteadiness and
non-convergence of the iterations. The distribution of coefficient of
lift on the wing span also shows sawtooth. Distribution of vorticity
changes both along span and in the direction of the free-stream as
the wake develops over time with distinct roll-up, which increases
with time.
Abstract: This paper is a continuation of the work carried out by various turbulence modelers in Oceanography on the topic of oceanic turbulent mixing. It evaluates the evolution of ocean water temperature and salinity by the appropriate modeling of turbulent mixing utilizing proper prescription of eddy viscosity. Many modelers in past have suggested including terms like shear, buoyancy and vorticity to be the parameters that decide the slow pressure strain correlation. We add to it the fact that dissipation anisotropy also modifies the correlation through eddy viscosity parameterization. This recalibrates the established correlation constants slightly and gives improved results. This anisotropization of dissipation implies that the critical Richardson’s number increases much beyond unity (to 1.66) to accommodate enhanced mixing, as is seen in reality. The model is run for a couple of test cases in the General Ocean Turbulence Model (GOTM) and the results are presented here.
Abstract: Eddy viscosity models in turbulence modeling can be mainly classified as linear and nonlinear models. Linear formulations are simple and require less computational resources but have the disadvantage that they cannot predict actual flow pattern in complex geophysical flows where streamline curvature and swirling motion are predominant. A constitutive equation of Reynolds stress anisotropy is adopted for the formulation of eddy viscosity including all the possible higher order terms quadratic in the mean velocity gradients, and a simplified model is developed for actual oceanic flows where only the vertical velocity gradients are important. The new model is incorporated into the one dimensional General Ocean Turbulence Model (GOTM). Two realistic oceanic test cases (OWS Papa and FLEX' 76) have been investigated. The new model predictions match well with the observational data and are better in comparison to the predictions of the two equation k-epsilon model. The proposed model can be easily incorporated in the three dimensional Princeton Ocean Model (POM) to simulate a wide range of oceanic processes. Practically, this model can be implemented in the coastal regions where trasverse shear induces higher vorticity, and for prediction of flow in estuaries and lakes, where depth is comparatively less. The model predictions of marine turbulence and other related data (e.g. Sea surface temperature, Surface heat flux and vertical temperature profile) can be utilized in short term ocean and climate forecasting and warning systems.
Abstract: A two-dimensional numerical study for flow past a square cylinder in presence of flat plate both at upstream and downstream position is carried out using the single-relaxation-time lattice Boltzmann method for gap spacing 0.5 and 1. We select Reynolds numbers from 80 to 200. The wake structure mechanism within gap spacing and near wake region, vortex structures around and behind the main square cylinder in presence of flat plate are studied and compared with flow pattern around a single square cylinder. The results are obtained in form of vorticity contour, streamlines, power spectra analysis, time trace analysis of drag and lift coefficients. Four different types of flow patterns were observed in both configurations, named as (i) Quasi steady flow (QSF), (ii) steady flow (SF), (iii) shear layer reattachment (SLR), (iv) single bluff body (SBB). It is observed that upstream flat plate plays a vital role in significant drag reduction. On the other hand, rate of suppression of vortex shedding is high for downstream flat plate case at low Reynolds numbers. The reduction in mean drag force and root mean square value of drag force for upstream flat plate case are89.1% and 86.3% at (Re, g) = (80, 0.5d) and (120, 1d) and reduction for downstream flat plate case for mean drag force and root mean square value of drag force are 11.10% and 97.6% obtained at (180, 1d) and (180, 0.5d).
Abstract: In aerovehicles context, the flow around an Ahmed
body profile is simulated using the velocity-vorticity formulation of
the Navier-Stokes equations, associated to a penalization method for
solids and Large Eddy Simulation for turbulence. The study focuses
both on the ground influence on the flow and on the dissymetry of
the wake, observed for a ground clearance greater than 10% of the
body height H. Unsteady and mean flows are presented and analyzed.
POD study completes the analysis and gives information on the most
energetic structures of the flow.
Abstract: In many practical situations, bubbles are dispersed in a
liquid phase. Understanding these complex bubbly flows is therefore
a key issue for applications such as shell and tube heat exchangers,
mineral flotation and oxidation in water treatment. Although a large
body of work exists for bubbles rising in an unbounded medium,
that of bubbles rising in constricted geometries has received less
attention. The particular case of a bubble sliding underneath an
inclined surface is common to two-phase flow systems. The current
study intends to expand this knowledge by performing experiments
to quantify the streamwise flow structures associated with a single
sliding air bubble under an inclined surface in quiescent water. This
is achieved by means of two-dimensional, two-component particle
image velocimetry (PIV), performed with a continuous wave laser
and high-speed camera. PIV vorticity fields obtained in a plane
perpendicular to the sliding surface show that there is significant bulk
fluid motion away from the surface. The associated momentum of the
bubble means that this wake motion persists for a significant time
before viscous dissipation. The magnitude and direction of the flow
structures in the streamwise measurement plane are found to depend
on the point on its path through which the bubble enters the plane.
This entry point, represented by a phase angle, affects the nature and
strength of the vortical structures. This study reconstructs the vorticity
field in the wake of the bubble, converting the field at different
instances in time to slices of a large-scale wake structure. This is, in
essence, Taylor’s ”frozen turbulence” hypothesis. Applying this to the
vorticity fields provides a pseudo three-dimensional representation
from 2-D data, allowing for a more intuitive understanding of the
bubble wake. This study provides insights into the complex dynamics
of a situation common to many engineering applications, particularly
shell and tube heat exchangers in the nucleate boiling regime.
Abstract: In this numerical study, effects of using Al2O3-water
nanofluid on the rate of heat transfer have been investigated. Physical
model is a square enclosure with insulated top and bottom horizontal
walls, while the vertical walls are kept at different constant
temperatures. Two appropriate models are used to evaluate the
viscosity and thermal conductivity of nanofluid. The governing
stream-vorticity equations are solved using a second order central
finite difference scheme, coupled to the conservation of mass and
energy. The study has been carried out for the nanoparticle diameter
30, 60 and 90 nm and the solid volume fraction 0 to 0.04. Results are
presented by average Nusselt number and normalized Nusselt number
in different range of φ and D for mixed convection dominated
regime. It is found that different heat transfer rate is predicted when
the effect of nanoparticle diameter is taken into account.
Abstract: In this research numerical simulations are performed,
using the multi-relaxation-time lattice Boltzmann method, in the
range 3 ≤ β = w[d] ≤ 30 at Re = 100, 200 and 300, where β the
blockage ratio, w is the equispaced distance between centers of
cylinders, d is the diameter of the cylinder and Re is the Reynolds
number, respectively. Special attention is paid to the effect of the
equispaced distance between centers of cylinders. Visualization of
the vorticity contour visualization are presented for some simulation
showing the flow dynamics and patterns for blockage effect. Results
show that the drag and mean drag coefficients, and Strouhal number,
in general, decrease with the increase of β for fixed Re. It is found
that the decreasing rate of drag and mean drag coefficients and
Strouhal number is more distinct in the range 3 ≤ β ≤ 15. We found
that when β > 15, the blockage effect almost diminishes. Our results
further indicate that the drag and mean drag coefficients, peak value
of the lift coefficient, root-mean-square value of the lift and drag
coefficients and the ratio between lift and drag coefficients decrease
with the increase of Re. The results indicate that symmetry boundary
condition have more blockage effect as compared to periodic
boundary condition.
Abstract: The thermal control in many systems is widely
accomplished applying mixed convection process due to its low cost,
reliability and easy maintenance. Typical applications include the
aircraft electronic equipment, rotating-disc heat exchangers, turbo
machinery, and nuclear reactors, etc. Natural convection in an inclined
square enclosure heated via wall heater has been studied numerically.
Finite volume method is used for solving momentum and energy
equations in the form of stream function–vorticity. The right and left
walls are kept at a constant temperature, while the other parts are
adiabatic. The range of the inclination angle covers a whole revolution.
The method is validated for a vertical cavity. A general power law
dependence of the Nusselt number with respect to the Rayleigh
number with the coefficient and exponent as functions of the
inclination angle is presented. For a fixed Rayleigh number, the
inclination angle increases or decreases is found.
Abstract: In this paper, effects of using Alumina-water
nanofluid on the rate of heat transfer have been investigated
numerically. Physical model is a square enclosure with insulated top
and bottom horizontal walls, while the vertical walls are kept at
different constant temperatures. Two appropriate models are used to
evaluate the viscosity and thermal conductivity of nanofluid. The
governing stream-vorticity equations are solved using a second order
central finite difference scheme, coupled to the conservation of mass
and energy. The study has been carried out for the Richardson
number 0.1 to 10 and the solid volume fraction 0 to 0.04. Results are
presented by isotherms lines, average Nusselt number and normalized
Nusselt number in different range of φ and Ri for forced, combined
and natural convection dominated regime. It is found that higher heat
transfer rate is predicted when the effects of nanoparticle is taken into
account.
Abstract: Thewake flow behind two yawed side-by-sidecircular
cylinders is investigated using athree-dimensional vorticity probe.
Four yaw angles (α), namely, 0°, 15°, 30° and 45° and twocylinder
spacing ratios T*
of 1.7 and 3.0 were tested. For T*
= 3.0, there exist
two vortex streets and the cylinders behave as independent and
isolated ones. The maximum contour value of the coherent streamwise
vorticity ~* ωx
is only about 10% of that of the spanwise vorticity ~* ωz
.
With the increase of α,
~* ωx
increases whereas ~* ωz
decreases. At α =
45°, ~* ωx
is about 67% of ~* ωz
.For T* = 1.7, only a single peak is
detected in the energy spectrum. The spanwise vorticity contours have
an organized pattern only at α = 0°. The maximum coherent vorticity
contours of ~* ω x
and ~* ωz
for T*
= 1.7 are about 30% and 7% of those
for T*
= 3.0.The independence principle (IP)in terms of Strouhal
numbers is applicable in both wakes when α< 40°.
Abstract: The effect of an axial electric field on the capillary instability of a cylindrical interface in the presence of heat and mass transfer has been investigated using viscous potential flow theory. In viscous potential flow, the viscous term in Navier-Stokes equation vanishes as vorticity is zero but viscosity is not zero. Viscosity enters through normal stress balance in the viscous potential flow theory and tangential stresses are not considered. A dispersion relation that accounts for the growth of axisymmetric waves is derived and stability is discussed theoretically as well as numerically. Stability criterion is given by critical value of applied electric field as well as critical wave number. Various graphs have been drawn to show the effect of various physical parameters such as electric field, heat transfer capillary number, conductivity ratio, permittivity ratio on the stability of the system. It has been observed that the axial electric field and heat and mass transfer both have stabilizing effect on the stability of the system.
Abstract: A method for simulating flow around the solid bodies has been presented using hybrid meshfree and mesh-based schemes. The presented scheme optimizes the computational efficiency by combining the advantages of both meshfree and mesh-based methods. In this approach, a cloud of meshfree nodes has been used in the domain around the solid body. These meshfree nodes have the ability to efficiently adapt to complex geometrical shapes. In the rest of the domain, conventional Cartesian grid has been used beyond the meshfree cloud. Complex geometrical shapes can therefore be dealt efficiently by using meshfree nodal cloud and computational efficiency is maintained through the use of conventional mesh-based scheme on Cartesian grid in the larger part of the domain. Spatial discretization of meshfree nodes has been achieved through local radial basis functions in finite difference mode (RBF-FD). Conventional finite difference scheme has been used in the Cartesian ‘meshed’ domain. Accuracy tests of the hybrid scheme have been conducted to establish the order of accuracy. Numerical tests have been performed by simulating two dimensional steady and unsteady incompressible flows around cylindrical object. Steady flow cases have been run at Reynolds numbers of 10, 20 and 40 and unsteady flow problems have been studied at Reynolds numbers of 100 and 200. Flow Parameters including lift, drag, vortex shedding, and vorticity contours are calculated. Numerical results have been found to be in good agreement with computational and experimental results available in the literature.
Abstract: The motion of a sphere moving along the axis of a
rotating viscous fluid is studied at high Reynolds numbers and
moderate values of Taylor number. The Higher Order Compact
Scheme is used to solve the governing Navier-Stokes equations. The
equations are written in the form of Stream function, Vorticity
function and angular velocity which are highly non-linear, coupled
and elliptic partial differential equations. The flow is governed by
two parameters Reynolds number (Re) and Taylor number (T). For
very low values of Re and T, the results agree with the available
experimental and theoretical results in the literature. The results are
obtained at higher values of Re and moderate values of T and
compared with the experimental results. The results are fourth order
accurate.
Abstract: Laminar natural-convective heat transfer from a
horizontal cylinder is studied by solving the Navier-Stokes and
energy equations using higher order compact scheme in cylindrical
polar coordinates. Results are obtained for Rayleigh numbers of 1,
10, 100 and 1000 for a Prandtl number of 0.7. The local Nusselt
number and mean Nusselt number are calculated and compared with
available experimental and theoretical results. Streamlines, vorticity -
lines and isotherms are plotted.
Abstract: This work is to study a roll of the fluctuating density
gradient in the compressible flows for the computational fluid dynamics
(CFD). A new anisotropy tensor with the fluctuating density
gradient is introduced, and is used for an invariant modeling technique
to model the turbulent density gradient correlation equation derived
from the continuity equation. The modeling equation is decomposed
into three groups: group proportional to the mean velocity, and that
proportional to the mean strain rate, and that proportional to the mean
density. The characteristics of the correlation in a wake are extracted
from the results by the two dimensional direct simulation, and shows
the strong correlation with the vorticity in the wake near the body.
Thus, it can be concluded that the correlation of the density gradient
is a significant parameter to describe the quick generation of the
turbulent property in the compressible flows.
Abstract: A numerical study is made of laminar, unsteady flow
behind a rotationally oscillating circular cylinder using a recently
developed higher order compact (HOC) scheme. The stream function
vorticity formulation of Navier-Stokes (N-S) equations in cylindrical
polar coordinates are considered as the governing equations. The
temporal behaviour of vortex formation and relevant streamline
patterns of the flow are scrutinized over broad ranges of two
externally specified parameters namely dimensionless forced
oscillating frequency Sf and dimensionless peak rotation rate αm for
the Reynolds-s number Re = 200. Excellent agreements are found
both qualitatively and quantitatively with the existing experimental
and standard numerical results.
Abstract: The development of biomimetic micro-aerial-vehicles
(MAVs) with flapping wings is the future trend in military/domestic
field. The successful flight of MAVs is strongly related to the
understanding of unsteady aerodynamic performance of low Reynolds
number airfoils under dynamic flapping motion. This study explored
the effects of flapping frequency, stroke amplitude, and the inclined
angle of stroke plane on lift force and thrust force of a bio-inspiration
corrugated airfoil with 33 full factorial design of experiment and
ANOVA analysis. Unsteady vorticity flows over a corrugated thin
airfoil executing flapping motion are computed with time-dependent
two-dimensional laminar incompressible Reynolds-averaged
Navier-Stokes equations with the conformal hybrid mesh. The tested
freestream Reynolds number based on the chord length of airfoil as
characteristic length is fixed of 103. The dynamic mesh technique is
applied to model the flapping motion of a corrugated airfoil. Instant
vorticity contours over a complete flapping cycle clearly reveals the
flow mechanisms for lift force generation are dynamic stall, rotational
circulation, and wake capture. The thrust force is produced as the
leading edge vortex shedding from the trailing edge of airfoil to form a
reverse von Karman vortex. Results also indicated that the inclined
angle is the most significant factor on both the lift force and thrust
force. There are strong interactions between tested factors which mean
an optimization study on parameters should be conducted in further
runs.
Abstract: Numerical calculations of flow around a square cylinder are presented using the multi-relaxation-time lattice Boltzmann method at Reynolds number 150. The effects of upstream locations, downstream locations and blockage are investigated systematically. A detail analysis are given in terms of time-trace analysis of drag and lift coefficients, power spectra analysis of lift coefficient, vorticity contours visualizations and phase diagrams. A number of physical quantities mean drag coefficient, drag coefficient, Strouhal number and root-mean-square values of drag and lift coefficients are calculated and compared with the well resolved experimental data and numerical results available in open literature. The results had shown that the upstream, downstream and height of the computational domain are at least 7.5, 37.5 and 12 diameters of the cylinder, respectively.
Abstract: Smoke discharging is a main reason of air pollution
problem from industrial plants. The obstacle of a building has an
affect with the air pollutant discharge. In this research, a mathematical
model of the smoke dispersion from two sources and one source with
a structural obstacle is considered. The governing equation of the
model is an isothermal mass transfer model in a viscous fluid. The
finite element method is used to approximate the solutions of the
model. The triangular linear elements have been used for discretising
the domain, and time integration has been carried out by semi-implicit
finite difference method. The simulations of smoke dispersion in
cases of one chimney and two chimneys are presented. The maximum
calculated smoke concentration of both cases are compared. It is then
used to make the decision for smoke discharging and air pollutant
control problems on industrial area.