94,137 research outputs found
Computation of unsteady momentum and heat transfer from a fixed circular cylinder in laminar flow
This paper presents a finite difference solution for 2D, low Reynolds number, unsteady flow around and heat transfer from a stationary circular cylinder placed in a uniform flow. The fluid is assumed to be incompressible and of constant property. The governing equations are the Navier-Stokes equations, the continuity equation, a Poisson equation for pressure and the energy equation. The temperature of the cylinder wall is kept constant and the viscous energy dissipation term is neglected in the energy equation. The computed Strouhal numbers, time-mean drag and base pressure coefficients, as well as the average Nusselt numbers compare well with existing experimental results
Stability and Vortex Shedding of Bluff Body Arrays
The primary purpose of this study was to develop an understanding of the stability of laminar
flow through bluff body arrays, and investigate the nature of the unsteady vortex shedding regime
that follows. The flow was numerically investigated using a specially developed multi-domain
spectral element solver. Important criteria in the solver development were flexibility, efficiency, and
accuracy. Flexibility was critical to the functionality of the code, as arrays of varying geometry
were investigated. Efficiency with a high degree of accuracy was also of primary importance, with
the code implemented to run efficiently on today's massively parallel architectures.
Numerical two-dimensional stability analysis of the flow in several configurations of inline and
staggered array geometries was performed. The growth rate, eigenfunction, and frequency of the
disturbances were determined. The critical Reynolds number for flow transition in each case was
identified and compared to that of flow over a single body. Based on the solutions of the laminar
flow, a one-dimensional analytical analysis was performed on selected velocity profiles in the wake
region. The results of this analysis were used to guide the interpretation of the two dimensional
results and formulate a general theory of stability of inline and staggered bluff body arrays. The
nature of the flow in the unsteady regime following the onset of instability was examined for an
inline and a staggered arrangement. Particular attention was focused on the vortex shedding which
was visualized and quantified through computation of the flow swirl, a quantity which identifies
regions of rotary motion. The conditions required for the generation of leading edge vortex shedding
were identified and discussed. Finally, a third geometry related to the inline and staggered arrays
was considered. Flow solution data for this geometry is presented and its suitability as a model for
louvered arrays was discussed.Air Conditioning and Refrigeration Project 11
A "poor man's" approach to topology optimization of natural convection problems
Topology optimization of natural convection problems is computationally
expensive, due to the large number of degrees of freedom (DOFs) in the model
and its two-way coupled nature. Herein, a method is presented to reduce the
computational effort by use of a reduced-order model governed by simplified
physics. The proposed method models the fluid flow using a potential flow
model, which introduces an additional fluid property. This material property
currently requires tuning of the model by comparison to numerical Navier-Stokes
based solutions. Topology optimization based on the reduced-order model is
shown to provide qualitatively similar designs, as those obtained using a full
Navier-Stokes based model. The number of DOFs is reduced by 50% in two
dimensions and the computational complexity is evaluated to be approximately
12.5% of the full model. We further compare to optimized designs obtained
utilizing Newton's convection law.Comment: Preprint version. Please refer to final version in Structural
Multidisciplinary Optimization https://doi.org/10.1007/s00158-019-02215-
Thermal receptivity of free convective flow from a heated vertical surface: linear waves
Numerical techniques are used to study the receptivity to small-amplitude thermal disturbances of the boundary layer flow of air which is induced by a heated vertical flat plate. The fully elliptic nonlinear, time-dependent Navier–Stokes and energy equations are first solved to determine the steady state boundary-layer flow, while a linearised version of the same code is used to determine the stability characteristics. In particular we investigate (i) the ultimate fate of a localised thermal disturbance placed in the region near the leading edge and (ii) the effect of small-scale surface temperature oscillations as means of understanding the stability characteristics of the boundary layer. We show that there is a favoured frequency of excitation for the time-periodic disturbance which maximises the local response in terms of the local rate of heat transfer. However the magnitude of the favoured frequency depends on precisely how far from the leading edge the local response is measured. We also find that the instability is advective in nature and that the response of the boundary layer consists of a starting transient which eventually leaves the computational domain, leaving behind the large-time time-periodic asymptotic state. Our detailed numerical results are compared with those obtained using parallel flow theory
A Method for Geometry Optimization in a Simple Model of Two-Dimensional Heat Transfer
This investigation is motivated by the problem of optimal design of cooling
elements in modern battery systems. We consider a simple model of
two-dimensional steady-state heat conduction described by elliptic partial
differential equations and involving a one-dimensional cooling element
represented by a contour on which interface boundary conditions are specified.
The problem consists in finding an optimal shape of the cooling element which
will ensure that the solution in a given region is close (in the least squares
sense) to some prescribed target distribution. We formulate this problem as
PDE-constrained optimization and the locally optimal contour shapes are found
using a gradient-based descent algorithm in which the Sobolev shape gradients
are obtained using methods of the shape-differential calculus. The main novelty
of this work is an accurate and efficient approach to the evaluation of the
shape gradients based on a boundary-integral formulation which exploits certain
analytical properties of the solution and does not require grids adapted to the
contour. This approach is thoroughly validated and optimization results
obtained in different test problems exhibit nontrivial shapes of the computed
optimal contours.Comment: Accepted for publication in "SIAM Journal on Scientific Computing"
(31 pages, 9 figures
On deterministic approximation of the Boltzmann equation in a bounded domain
In this paper we present a fully deterministic method for the numerical
solution to the Boltzmann equation of rarefied gas dynamics in a bounded domain
for multi-scale problems. Periodic, specular reflection and diffusive boundary
conditions are discussed and investigated numerically. The collision operator
is treated by a Fourier approximation of the collision integral, which
guarantees spectral accuracy in velocity with a computational cost of
, where is the number of degree of freedom in velocity space.
This algorithm is coupled with a second order finite volume scheme in space and
a time discretization allowing to deal for rarefied regimes as well as their
hydrodynamic limit. Finally, several numerical tests illustrate the efficiency
and accuracy of the method for unsteady flows (Poiseuille flows, ghost effects,
trend to equilibrium)
Invariant Discretization Schemes Using Evolution-Projection Techniques
Finite difference discretization schemes preserving a subgroup of the maximal
Lie invariance group of the one-dimensional linear heat equation are
determined. These invariant schemes are constructed using the invariantization
procedure for non-invariant schemes of the heat equation in computational
coordinates. We propose a new methodology for handling moving discretization
grids which are generally indispensable for invariant numerical schemes. The
idea is to use the invariant grid equation, which determines the locations of
the grid point at the next time level only for a single integration step and
then to project the obtained solution to the regular grid using invariant
interpolation schemes. This guarantees that the scheme is invariant and allows
one to work on the simpler stationary grids. The discretization errors of the
invariant schemes are established and their convergence rates are estimated.
Numerical tests are carried out to shed some light on the numerical properties
of invariant discretization schemes using the proposed evolution-projection
strategy
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