Concept of Computational Fluid Dynamics Material

Module 1: Introduction to Finite Difference Method and Fundamentals of CFD
Lecture 1: Finite Difference Method
The Lecture deals with:
Classification of Partial Differential Equations
Boundary and Initial Conditions
Finite Differences
Objectives_template file:///D:/chitra/nptel_phase2/mechanical/cfd/lecture1/1_1.htm
1 of 1 6/19/2012 4:29 PM

For analyzing the equations for fluid flow problems, it is convenient to consider the case of
a second-order differential equation given in the general form as
(1.1)
If the coefficients A, B, C, D, E, and F are either constants or functions of only (x, y) (do
not contain or its derivatives), it is said to be a linear equation; otherwise it is a
non-linear equation.
An important subclass of non-linear equations is quasilinear equations.
In this case, the coefficients may contain or its first derivative but not the second
(highest) derivative.
If the aforesaid equation is homogeneous, otherwise it is non-homogeneous.
1 of 1 6/19/2012 4:30 PM

Refer to eqiation 1.1
if the equation is Parabolic
if the equation is Elliptic
if the equation is Hyperbolic
Unsteady Navier-Strokes equations are elliptic in space and parabolic in time.
At steady-state, the Navier-Strokes equations are elliptic.
In Elliptic problems, the boundary conditions must be applied on all confining surfaces.
These are Boundary Value Problems.
A Physical Problem may be Steady or Unsteady.
1 of 1 6/19/2012 4:30 PM

In this slide we'll discuss Mathematical aspects of the equations that describe fluid flow
and heat transfer problems.
Laplace equations:
(1.2)
Poisson equations:
(1.3)
Laplace equations and Poisson equations are elliptic equations and generally
associated with the steady-state problems.
The velocity potential in steady, inviscid, incompressible, and irrotational flows satisfies
the Laplace equation.
The temperature distribution for steady-state, constant-property, two-dimensional
condition satisfies the Laplace equation if no volumetric heat source is present in the
domain of interest and the Poisson equation if a volumetric heat source is present.
1 of 1 6/19/2012 4:30 PM

The parabolic equation in conduction heat transfer is of the form
(1.4)
The one-dimensional unsteady conduction problem is governed by this equation when
and are identified as the time and space variables respectively, denotes the
temperature and B is the thermal diffusivity. The boundary conditions at the two ends an
initial condition are needed to solve such equations.
The unsteady conduction problem in two-dimension is governed by an equation of the
form
(1.5)
Here denotes the time variable, and a souce term S is included. By comparing the
highest derivatives in any two of the independent variables, with the help of the
conditions given earlier, it can be concluded that Eq. (1.5) is parabolic in time and elliptic
in space. An initial condition and two conditions for the extreme ends in each special
coordinates is required to solve this equation.
Fluid flow problems generally have nonlinear terms due to the inertia or acceleration
component in the momentum equation. These terms are called advection terms. The
energy equation has nearly similar terms, usually called the convection terms, which
involve the motion of the flow field. For unsteady two-dimensional problems, the
appropriate equation can be represented as
(1.6)
denotes velocity, temperature or some other transported property,
and are velocity components,
B is the diffusivity for momentum or heat, and
S is a source term.
The pressure gradients in the momentum or the volumetric heating in the energy
equation can be appropriately substituted in S. Eq. (1.6) is parabolic in time and elliptic in
space.
For very high-speed flows, the terms on the left side dominate, the second-order terms on
the right hand side become trivial, and the equation become hyperbolic in time and space.
1 of 1 6/19/2012 4:31 PM

Boundary & Initial Conditions
Formulation of the problem requires a complete specification of the geometry of interest
and appropriate boundary conditions. An arbitrary domain and bounding surfaces are
sketched in Fig. 1.1.
Figure 1.1: Schematic sketch of an arbitrary Domain
The conservation equations are to be applied within the domain. The number of boundary
conditions required is generally determined by the order of the highest derivatives
appearing in each independent variable in the governing differential equations.
The unsteady problems governed by a first derivative in time will require initial condition
in order to carry out the time integration. The diffusion terms require two spatial
boundary conditions for each coordinate in which a second derivative appears.
1 of 1 6/19/2012 4:31 PM

Boundary & Initial Conditions
The spatial boundary conditions in flow and heat transfer problems are of three general
types. They may stated as
(1.7)
(1.8)
(1.9)
and denote three separate zones on the bounding surface in Fig. 1.1.
The boundary conditions in Eqns. (1.7) to (1.9) are usually referred to as Dirchlet,
Neumann and mixed boundary conditions, respectively. The boundary conditions are
linear in the dependant variable .
In Eqns. (1.7) to (1.9), is a vector denoting position on the boundary, is
the directional derivative normal to the boundary, and and are arbitrary
functions. The normal derivative may be expressed as
(1.10)
Here is the unit vector normal to the boundary, is the nabla operator, [.] denotes
the dot product, are the direction-cosine components of and are the
unit vectors aligned with the coordinates.
1 of 1 6/19/2012 4:31 PM

Finite Differences
Analytical solutions of partial differential equations provide us with closed-form
expressions which depict the variation of the dependent variable in the domain.
The numerical solutions, based on finite differences, provide us with the values at
discrete points in the domain which are known as grid points.
Consider Fig. 1.2, which shows a domain of calculation in the plane.
Figure 1.2; discrete Grid Points
Let us assume that the spacing of the grid points in the direction is uniform, and given
by . Likewise, the spacing of the points in the direction is also uniform, and given
by
It is not necessary that or be uniform. We could imagine unequal spacing in both
directions, where different values of between each successive pairs of grid points are
used. The same could be presumed for as well.
However, often, problems are solved on a grid which involves uniform spacing in each
direction, because this simplifies the programming, and often result in higher accuracy.
In some class of problems, the numerical calculations are performed on a transformed
1 of 2 6/19/2012 4:31 PM

computational plane which has uniform spacing in the transformed-independent-variables
but non-uniform spacing in the physical plane.
These typical aspects will be discussed later.
At present let us consider uniform spacing in each coordinate direction. According to our
consideration, and are constants, but it is not mandatory that be equal to
2 of 2 6/19/2012 4:31 PM

Let us refer to Fig. 1.2.
The grid points are identified by an index which increases in the positive - direction,
and an index which increases in the positive -direction. If is the index of
point in Fig.1.2, then the point immediately to the right is designated as and
the point immediately to the left is and the point directly below is
The basic philosophy of finite difference method is to replace the derivatives of the
governing equations with algebraic difference quotients. This will result in a system of
algebraic equations which can be solved for the dependent variables at the discrete grid
points in the flow field.
In the next lecture we'll look at some of the common algebraic difference quotients in
order to be acquainted with the methods related to discretization of the partial differential
equations.
Congratulations, you have finished Lecture 1. To view the next lecture select it from the left hand side
menu of the page or click the next button.
1 of 1 6/19/2012 4:32 PM

Concept of Computational Fluid Dynamics Material

More Related Content

Similar to Concept of Computational Fluid Dynamics Material (20)

More from Balaji Ganesh (6)

Recently uploaded (20)

Concept of Computational Fluid Dynamics Material