UDF定义源项
[ 2006-3-9 20:07:05 | By: caohuali ]
吴子牛《计算流体力学基本原理》
1.在非惯性坐标系中，N-S方程右端需添加反映惯性力影响的源项；
2.考虑重力影响时，需在动量方程中添加重力项；
3.考虑两相流时，在N-S方程右端需条件反映各种流体相互干扰的源项（包括质量交换，动量交换，能量交换）；
4.考虑两方程湍流模型时，湍流模型方程右端含有反映湍流生成与消灭的源项；
5.考虑化学反应时，需在组分浓度方程和能量方程添加由化学反应引起的质量变化与能量变化；
6.将高维问题化为低维问题时，也会出现源项。
陶文铨：数值传热学书中所指的源项是广义量，它代表那些不能包括到控制方程的非稳态项、对流项与扩散项中所有其它各项之和。书中提到将源项局部线形化，S=Sc+SpTp,这种方法比将源项处理为常数会更合理。
查看了一个用UDF自定义的X方向动量源项
/*******************************************************************/
/* UDF for specifying an x-momentum source term in a spatially */
/* dependent porous media */
/*******************************************************************/
＃include "udf.h"
#define C2 100.0
DEFINE_SOURCE(xmom_source, c, t, dS, eqn)
{
real x[ND_ND];
real con, source;
C_CENTROID(x, c, t);
con = C2*0.5*C_R(c, t)*x[1];
source = -con*fabs(C_U(c, t))*C_U(c, t);
dS[eqn] = -2.*con*fabs(C_U(c, t));
return source;
}
自己的体会就是：关键在于找到那些不能包括到控制方程的非稳态项、对流项与扩散项中所有项，写出源项的表达式。局部线形化的时候注意Sp要小于等于0.
2008年5月19日星期一
FLUENT中的MHD模块？
FLUENT也可以计算MHD模块？YES!
先把case文件载入，然后输入如下的命令
/define/models> addon-module
FLUENT Addon Modules:
1. MHD Model
2. Fiber Model
3. PEM Fuel Cell Model
4. SOFC Fuel Cell Model
Enter Module Number: [1] 1
先把case文件载入，然后输入如下的命令
/define/models> addon-module
FLUENT Addon Modules:
1. MHD Model
2. Fiber Model
3. PEM Fuel Cell Model
4. SOFC Fuel Cell Model
Enter Module Number: [1] 1
2008年3月29日星期六
A Brief History of Computational Fluid Dynamics (CFD)
A Brief History of CFD
计算流体力学简史
http://www.fluent.com/about/cfdhistory.htm
class=clearboth>
Speeds', Proc. Royal Society, A141, pp. 651-666, London, 1933
Flow Around a Circular Cylinder at Reynolds Number 40', Journal of Phy. Soc.
Japan, vol. 8, pp. 747-757, 1953.
计算流体力学简史
http://www.fluent.com/about/cfdhistory.htm
A Brief History of CFD
Since the dawn of civilization, mankind has always had a fascination with
fluids; whether it is the flow of water in rivers, the wind and weather in our
atmosphere, the smelting of metals, powerful ocean currents or the flow of blood
around our bodies.
In antiquity, great Greek thinkers like Heraclitus postulated that
"Everything flows" but he was thinking of this in a philosophical sense rather
than in a recognizably scientific way. However, Archimedes initiated the fields
of static mechanics, hydrostatics, and determined how to measure densities and
volumes of objects. The focus at the time was on waterworks: aqueducts, canals,
harbors, and bathhouses, which the ancient Romans perfected to a science.
 width=170> |
 width=170> |
It was not until the Renaissance that these ideas resurfaced again in
Southern Europe when we find great artists cum engineers like Leonardo Da Vinci
starting to examine the natural world of fluids and flow in detail again. He
observed natural phenomena in the visible world, recognizing their form and
structure, and describing them pictorially exactly as they were. He planned and
supervised canal and harbor works over a large part of middle Italy. His
contributions to fluid mechanics are presented in a nine part treatise (Del
moto e misura dell'acqua) that covers water surfaces, movement of water,
water waves, eddies, falling water, free jets, interference of waves, and many
other newly observed phenomena.
src="http://www.fluent.com/about/img/da_vinci_sketch.jpg" width=170 border=0> |
class=clearboth>
Leonardo was followed in the late 17th Century by Isaac Newton in England.
Newton tried to quantify and predict fluid flow phenomena through his elementary
Newtonian physical equations. His contributions to fluid mechanics included his
second law: F=m.a, the concept of Newtonian viscosity in which stress and the
rate of strain vary linearly, the reciprocity principle: the force applied upon
a stationary object by a moving fluid is equal to the change in momentum of the
fluid as it deflects around the front of the object, and the relationship
between the speed of waves at a liquid surface and their wavelength.
 width=170> |
In the 18th and 19th centuries, significant work was done trying to
mathematically describe the motion of fluids. Daniel Bernoulli (1700-1782)
derived Bernoulli's famous equation, and Leonhard Euler (1707-1783) proposed the
Euler equations, which describe the conservation of momentum for an inviscid
fluid, and conservation of mass. He also proposed the velocity potential theory.
Two other very important contributors to the field of fluid flow emerged at this
time; the Frenchman, Claude Louis Marie Henry Navier (1785-1836) and the
Irishman, George Gabriel Stokes (1819-1903) who introduced viscous transport
into the Euler equations, which resulted in the now famous Navier-Stokes
equation. These forms of the differential mathematical equations that they
proposed nearly 200 years ago are the basis of the modern day computational
fluid dynamics (CFD) industry, and they include expressions for the conservation
of mass, momentum, pressure, species and turbulence. Indeed, the equations are
so closely coupled and difficult to solve that it was not until the advent of
modern digital computers in the 1960s and 1970s that they could be resolved for
real flow problems within reasonable timescales. Other key figures who developed
theories related to fluid flow in the 19th century were Jean Le Rond d'Alembert,
Siméon-Denis Poisson, Joseph Louis Lagrange, Jean Louis Marie Poiseuille, John
William Rayleigh, M. Maurice Couette, Osborne Reynolds, and Pierre Simon de
Laplace.
 width=170> |
 width=170> |
In the early 20th Century, much work was done on refining theories of
boundary layers and turbulence in fluid flow. Ludwig Prandtl (1875-1953)
proposed a boundary layer theory, the mixing length concept, compressible flows,
the Prandtl number, and much more that we take for granted today. Theodore von
Karman (1881-1963) analyzed what is now known as the von Karman vortex street.
Geoffrey Ingram Taylor (1886-1975) proposed a statistical theory of turbulence
and the Taylor microscale. Andrey Nikolaevich Kolmogorov (1903-1987) introduced
the concept of Kolmogorov scales and the universal energy spectrum for
turbulence, and George Keith Batchelor (1920-2000) made contributions to the
theory of homogeneous turbulence.
It is debatable as to who did the earliest CFD calculations (in a modern
sense) although Lewis Fry Richardson in England (1881-1953) developed the first
numerical weather prediction system when he divided physical space into grid
cells and used the finite difference approximations of Bjerknes's "primitive
differential equations". His own attempt to calculate weather for a single
eight-hour period took six weeks of real time and ended in failure! His model's
enormous calculation requirements led Richardson to propose a solution he called
the "forecast-factory". The "factory" would have involved filling a vast stadium
with 64,000 people. Each one, armed with a mechanical calculator, would perform
part of the flow calculation. A leader in the center, using colored signal
lights and telegraph communication, would coordinate the forecast. What he was
proposing would have been a very rudimentary CFD calculation. The earliest
numerical solution for flow past a cylinder was carried out in 1933 by Thom and
reported in England:
Speeds', Proc. Royal Society, A141, pp. 651-666, London, 1933
Kawaguti in Japan obtained a similar solution for flow around a cylinder in
1953 by using a mechanical desk calculator, working 20 hours per week for 18
months!
Flow Around a Circular Cylinder at Reynolds Number 40', Journal of Phy. Soc.
Japan, vol. 8, pp. 747-757, 1953.
During the 1960s, the theoretical division of NASA at Los Alamos in the U.S.
contributed many numerical methods that are still in use in CFD today, such as
the following methods: Particle-In-Cell (PIC), Marker-and-Cell (MAC),
Vorticity-Stream function methods, Arbitrary Lagrangian-Eulerian (ALE) methods,
and the ubiquitous k - e turbulence model. In the 1970s, a group
working under D. Brian Spalding, at Imperial College, London, developed
Parabolic flow codes (GENMIX), Vorticity-Stream function based codes, the SIMPLE
algorithm and the TEACH code, as well as the form of the k - e
equations that are used today (Spalding & Launder, 1972). They went on to
develop Upwind differencing, 'Eddy break-up' and 'presumed pdf' combustion
models. Another key event in CFD industry was in 1980 when Suhas V. Patankar
published " Numerical Heat Transfer and Fluid Flow", probably the most
influential book on CFD to date, and the one that spawned a thousand CFD codes.
It was in the early 1980s that commercial CFD codes came into the open market
place in a big way. The use of commercial CFD software started to become
accepted by major companies around the world rather than their continuing to
develop in-house CFD codes. Commercial CFD software is therefore based on sets
of very complex non-linear mathematical expressions that define the fundamental
equations of fluid flow, heat and materials transport. These equations are
solved iteratively using complex computer algorithms embedded within CFD
software. The net effect of such software is to allow the user to
computationally model any flow field provided the geometry of the object being
modeled is known, the physics and chemistry are identified, and some initial
flow conditions are prescribed. Outputs from CFD software can be viewed
graphically in color plots of velocity vectors, contours of pressure, lines of
constant flow field properties, or as "hard" numerical data and X-Y plots.
CFD is now recognized to be a part of the computer-aided engineering (CAE)
spectrum of tools used extensively today in all industries, and its approach to
modeling fluid flow phenomena allows equipment designers and technical analysts
to have the power of a virtual wind tunnel on their desktop computer. CFD
software has evolved far beyond what Navier, Stokes or Da Vinci could ever have
imagined. CFD has become an indispensable part of the aerodynamic and
hydrodynamic design process for planes, trains, automobiles, rockets, ships,
submarines; and indeed any moving craft or manufacturing process that mankind
has devised.
订阅：
博文 (Atom)