"Differential models. An Introduction with Mathcad”
Springer Publishing House (Springer-Verlag, 2004, ISBN: 3-540-20852-6)
The book include the engineering-physical projects executed in Mathcad according to identical plan:
q Problem-setting and physical model formulation
q Designing of differential mathematical model, i.e. model in the form of the differential equations (DE).
q Numerical integration of the differential equations
q Visualization of results.
Uniting attribute is the form of a nucleus of projects. In all cases it is the ordinary (ODE) or partial (PDE) differential equations . The purpose of the book is to help students and young engineers to design and solve the differential equations.
Though the projects included in the book, are educational, they contain a research intrigue, have the important practical appendices and are not trivial in computing aspect.
We hope, we could find suitable combination of an engineering significance, on the one hand, and concerning high computing complexity, with another to provide sufficient motivation for the reference to any of modern mathematical software packages.
Chosen for this purpose Mathcad is the effective and rather accessible tool. Probably, as a whole most "democratic" of known mathematical packages. It is especially important today when the computer appears on working tables of the increasing number of young people.
consists of eleven chapters and the appendix on CD
In the first chapter " Differential mathematical models" the origin of the differential equations is discussed. They do not appear at once on a working table of engineer in a ready kind for integration, and more often should be designed by the researcher - as mathematical models of technological processes or devices. It is not unlikely, this stage will be the most difficult in the engineering project, and we considered useful to give examples, how from the indistinct verbal description the differential mathematical models of researched objects are born.
The second chapter " Integrable differential equations " contains the list and brief manual for the equations of the specified type. It is shown, how to apply symbolical Mathcad processor at mathematical calculations in this case.
The third chapter "Dynamic model of the system with heat generating" is devoted to systems which can develop under catastrophic scripts - with ignition and explosion. The concept of catastrophe as some destructive phenomenon is supplemented with a mathematical metaphor, namely, the description of dynamic system in the form of so-called fold catastrophe from the theory of R.Toma. In this chapter are used the elements of the qualitative theory of the differential equations.
In the fourth chapter " Stiff differential equations " the problem of numerical integration of this special type of the equations is in detail considered. Despite of practical importance of a question, it is difficult for student or engineer to find the accessible description of this phenomenon. Mathcad has the effective built - in function for integration of the stiff equations, but its help system also almost nothing contains on a discussed problem. Materials of the given chapter fill somewhat this blank. The reader will find here a plenty of examples with use of the various integrators, discussion of features of explicit and implicit numerical methods.
The fifth chapter " Heat transfer near to the critical point at ñross tube flow " models the following situation, in which the novice engineer can find oneself when solving of a real problem. In educational examples of manuals the right parts of the differential equations are always set by simple analytical expressions. In real problems the right part is almost always represented in the form of complex algorithm, but not as analytical expression, even difficult. Therefore, we show in action the Mathcad structures (the programs, the built - in functions of optimization etc.) which should be used at the decision of real problems with the differential equations, but not only DE solvers as such.
The chapter sixth « Falkner - Skan Equation. Hydrodynamic friction and heat transfer in the boundary layer» is devoted to numerical analysis of a fundamental problem of hydrodynamics and heat transfer. Together with part 1.4 "«Conservation Laws" the sixth chapter forms brief theoretical course of convective heat transfer. From the point of view of operation in Mathcad, main theme is the numerical solution of two-point boundary-value problem for the ordinary differential equations set with application of built-in function permitting to reduce the boundary problem to initial problem.
In the chapter seventh « Rayleigh Equation. Hydrodynamic instability » the stability of free shear flow by a method of small perturbations (linearization method) is parsed. The progressing perturbations, discovered at the analysis, are the predecessors of a turbulence. In the methodical attitude, this chapter can be the elementary introduction in stability theory. The technique of operation with the differential equations in complex area is considered. Is shown how to integrate the equations with complex coefficients by the numerical method and how to interpret the complex-valued solutions.
The eighth chapter «Kinematic waves of concentration in ion-exchange filter» continues the theme of partial equations begun in the first chapter by the problem about shock waves on a motorway. The physical and mathematical model of ion-exchange filter is developed. This devices provide with very pure water the steam plants of thermal and nuclear power stations. The solution of partial differential equation for space-time evolution of impurity concentration is received by characteristics method. Is shown, how the nonlinearity of differential model results in discontinuous nonsingle-valued solution.
The ninth chapter "Kinematic shock waves" continues a theme of the wave equations.
The Mathcad-program for numerical integration of the partial differential equations with effective reproduction of shock waves is developed. Thanks to this program, research of problems about shock waves on motorways and shock waves in filters is completed. The problem about gravitational bubble flows such as floating bubbles in a glass with champagne either with beer or in two-phase contours of evaporator is in detail considered. As well as in problems about traffic and about the filter, formation of shock waves will be observed, but already of steam content shock waves.
In tenth chapter « Numerical modelling of the CPU-board temperature field» on an example of a thermal conduction the numerical methods for partial equations of the second order are considered. To the standard Mathcad-tools the new program is supplemented, which permits to simulate stationary two-dimensional temperature fields for a dilated circle of problems and to describe correctly the thermal interaction with a surrounding .
The eleventh chapter " Temperature waves " is devoted to non-stationary temperature fields. Engineering appendices of this part of thermal physics are very various. For example, in power engineering for optimum control of starting procedure it is necessary to forecast non-stationary temperature fields in elements of machines and the equipment, with the purpose to exclude infringements of backlashes in moving elements because of unequal expansion or to prevent occurrence of destroying thermal stress in massive parts.
Interesting problem is modelling of action of super-power energy fluxes to constructions. Almost always high-power actions have pulsing, periodic character, and in solid bodies arise the temperature waves. The one-dimensional non-stationary problem with interior heat generating is surveyed as model of the circumscribed above processes and the corresponding Mathcad-program is designed on the basis of sweep method.
Authors aspired to show on real examples how to use effectively Mathcad at all development stage of an engineering project:
At analytical preprocessing the mathematical description (during normalization, research of the special points, identical transformations, etc.)
At analytical decisions where it is possible (or where it the opportunities of symbolical Mathcad-processor allows)
At the numerical decision when analytical decisions are impossible or inefficient
At results presentation and visualization.
We followed belief, that the basic modern tendency in engineering researches and designing, as well as in engineering education - more and more wide use of computer models to take into account essentially important effects and to work with full mathematical models of objects. The engineer - researcher should not be under pressing fears, that the developed model is too difficult for computing and that therefore it is necessary to simplify a problem more and more. Can be, up to such degree, that the model absolutely will cease to be similar to real object.
At the end of the book the list of the literature is given. In addition to direct references to sources, it includes some general literature about numerical analysis and computer modelling, apparently, not full. The index contains those books concerning our theme which were read by authors, have seemed to them interesting and, hence, in the explicit or implicit form were used during our work. From R.Hemming's book [Îøèáêà! Èñòî÷íèê ññûëêè íå íàéäåí.] we would like to use the following motto for our book too:
" The Purpose of calculations - not numbers, but understanding "
In power engineering and thermal technology for optimum control of starting or transition procedure it is necessary to compute the time-dependent temperature field in elements of machines and equipment. Forecasting of a temperature field allows to avoid inadmissible temperature rise or too big temperature drops. A characteristic example is the start control of the powerful steam turbine on a thermal power station.
Aspiration to set in operation rapidly the stand-by capacity, encounters the restrictions since inadmissible changes of axial backlashes in a moving elements of the turbine because of unequal expansion or breaking temperature stresses in massive details of a rotor and stator turbines can arise.
More and more actual there are the problems of influence of super-power energy fluxes on construction elements. For example, in the problem of Controlled Thermonuclear Synthesis the heat-flux density on walls can reach 108 W/m2.
In heavy conditions the graphite plasmatron electrodes are working. This devices are used for high-temperature material processing. The major heat fluxes and high temperatures arise during laser or electron-beam processing with the purpose of surface hardening. The similar processes take place at manufacture of chips.
Almost always high-power actions have pulsing, periodic character, and in solid bodies arise the temperature waves. The one-dimensional non-stationary problem with interior heat generating is surveyed as model of the circumscribed above processes (Fig. 11.1).
Fig. 11.1. One-dimensional non-stationary thermal conduction problem
It is supposed, that the spatial changes happen only along an axis of coordinates x. The lateral area is considered adiabatically isolated. If it is necessary, the heat exchange on a lateral area should be imitated by an negative internal heat source, is exact just as in the previous chapter.
To provide universality of model, we shall take advantage of the numerical method and we shall develop for this purpose the Mathcad-program on the basis of sweep method, famous in a numerical analysis as very-high-speed algorithm for the solution of greet system with diagonal structure.
As an initial statement we shall accept the energy equation in general enunciation (see Chapter 1), in which the convective transport must be eliminated and one-dimensionality, t = t(x, τ), must be accepted. After that we shall receive:
At the left-hand (x = 0) and at the right butt-end (x = L) of object (see. Fig. 11.1) there is a thermal interaction to environment, and here the suitable boundary conditions should be set. Universal way to describe the various interactions with surroundings is to set the boundary conditions of third kind (mixed-boundary conditions):
where two expressions for heat flux are compared, namely
incoming from surroundings (Newton law at right hand side)
and conducting inwards (Fourier law at left hand side).
This equality is entirely correct only in case of absence of phase transformation on the boundary. Generally the difference of heat fluxes on both boundary sides will be spent for a melting, solidification, evaporation etc. But we shall not solve here such composite problems with phase changes.
To solve the partial differential equation (11.1), his finite-difference approximation must be prepared. We receive the necessary result, noting the energy conservation law for the small, but finite control volume. Here it would be useful to see once again the similar calculations which already were carried out earlier (see Chapter 1, Chapter 9).
Fig. 11.2. Control volume and energy fluxes
So, we want to make up the energy balance for control volume (Fig. 11.2), formed in a neighbourhood of node P. The last is now in focus of our attention, but the derived further relations will suit any interior node.
From west W and east E nodes the heat fluxes owing to thermal conduction arrive. Inside of volume the heat source qV operates. As a result the thermal energy will grow. And we shall detect it on rise in temperature - from T0P up to TP in time Δτ:
The quantity of balance should be zero. As capital Greek "«lambda" the relative values of thermal conductivity are designated, which assigned to the on Fig. 11.2 specified control intersurfaces. For example, for east surface (between P and E) :
were λ is the reference thermal conductivity.
Is heat conductivity constant, the single characteristic value λ can be specified and all Λ must be as unit (1) assigned. If the thermal conductivity greatly depends on temperature or even has disruptures because of layer material structure , harmonic mean (11.4) ensures the heat flux evaluation with good precision.
Equation (11.3) is given for interne nodes . Futher the similar equation (11.7) for boundary nodes will be derived.
Implicit scheme. The temperatures in neighboring nodes, i.e. TW and TE are unknowns from "future", same as TP. Therefore, relationship (11.3) is equation with three unknowns. The set of such equations for all nodes with unknown temperature must be solved as system of linear equations.
In other words: there is no explicit expression for each unknown. Such schemes in a numerical analysis is termed implicit. They have the important property of numerical stability, though are complicated because of necessity to solve an equations set.
Explicit scheme. It is also possible, to attach all temperatures in the second and third members (11.3) to "past" and to supply they with label "0". The explicit scheme in this case is gained: each equation contains a unique unknown value TP (in the first member of (11.3) ). The program and the evaluations will be very simple. But if the time step will exceed some value, parasitic oscillations will be progressing . The restrictions on a step are rather burdensome, therefore today prefer the implicit schemes.
Detailed arguing of the explicit and implicit schemes and the computing example are given in Chapter 1.
Let's go back to the analysis of the equation (11.3). Two variants of record further are given. First - in the mnemonic shape, second - in index, necessary for programming. Unknowns in these equations are temperature from "future". The coefficients A, B, C, D are collected from known quantities including temperature T0P, taken from the previous time step, or from the initial conditions.
Comparing (11.3) and (11.5) yields the formulas for A, B, C, D. This operations make Mathcad:
is Fourier-Number, the dimensionless time step.
So we receive the linear equations set with three-diagonal matrix as shown below in the demonstration example for the grid from five nodes :
The matrix of big system will appear almost empty. For hundred nodes only three percents of meshes will be engaged, the others will be zero. Therefore at Gaussian elimination in basic the zeros will be handled.
However there is an express method of elimination taking into account three-diagonal matrix structure and termed by a "sweep" method [Îøèáêà! Èñòî÷íèê ññûëêè íå íàéäåí., Îøèáêà! Èñòî÷íèê ññûëêè íå íàéäåí., Îøèáêà! Èñòî÷íèê ññûëêè íå íàéäåí., Îøèáêà! Èñòî÷íèê ññûëêè íå íàéäåí., Îøèáêà! Èñòî÷íèê ññûëêè íå íàéäåí.]. Mathcad-program of this method is given on Ðèñ. 11.5 .
For boundary nodes, the individual expression for the heat balance should be derived. The control volumes here appear of the half size, as shown in Ðèñ. 11.3. The express formulation for a heat flux through a boundary edge should be applied (see the second member on the right hand side) :
This equation is discrete analog of boundary conditions, given above by relations (11.2). The index "inner" means the proximate interior node: (n-1) of right butt-end, 2 - for the left-hand butt-end. We shall not repeat evaluation for surface nodes. They are completely similar to that are carried out for interior nodes. The final expressions can be read in the text of the program Coef (Fig. 11.4).
This subroutine (Fig. 11.4) compute the coefficients A, B, C, D for system like (11.6).
Vector T0 in the formal arguments list contain the initial values of temperature. The indexing starts with unity, i.e. the value Origin of Mathcad environment should be installed in unity. The dimensionality of vector T0 is equal to number of mesh nodes.
Quantities Tf and Bi are two-elements vectors specifying temperatures of medium and Bi-numbers at the left-hand and right butt of object respectively (Fig. 11.1). The Bi- number is a dimensionless heat-transfer coefficient: Bi = α·Δx/λ.
The parameter iTime informs the subroutine, on what time step there is a process of calculations. It is important for calculation of temperature of the environment contacting to the butt. Setting in vector Pulse nonzero amplitude Ampl and some value of frequency ν , we can simulate periodic thermal influences.
Fig. 11.4. The subroutine for calculation of the coefficients matrix
A, B, C, D
for three-diagonal system are
prepared by subroutine Coef. In a body of procedure SYSTRD D-values will be replaced by the
calculated values of temperature. So, the procedure SYSTRD returns a vector of decisions.
Program TimeHistory (Fig. 11.6) organizes calculations on time steps. The parameter nTime sets number of time steps. Function TimeHistory returns the temperature distributions on coordinate x for the consecutive time moments iTime as columns of F - matrix.
Fig. 11.6. The main program
On each time step the procedure Coef is called to calculate the coefficients A, B, C, D. Then the solver SYSTRD will be activate. With resulting T-vector the old T0-vector of temperatures on the previous time step will is updated and the new column to matrix F will is added.
The preparation of the basic data is shown on Fig. 11.7. It is supposed to investigate the temperature fluctuations in a brass core length 39 mm when at the right butt-end the temperature pulsing about average value 800ºC with amplitude 320ºC is given, and at the left butt-end the constant zero temperature is supported.
These boundary conditions are simulated by indicating of the requisites liquid temperatures and such values of Bi-number which correspond to very big heat transfer coefficients at right and left butts.
Preparatory calculations on Fig. 11.7 are clear without comments. At the end of this fragment the T0-vector of inital temperatures (100ºC) and also the Λ-vector of relative heat conductivity are formed.
In the given example Λ is accepted as constant. If heat conductivity depends on coordinates and/or temperature an additional procedure or an insert in program Coef is required. The best way of averaging of heat conductivity will be calculation of harmonic mean values under the formula (11.4). We leave these improvements of the program as exercise for readers.
Fig. 11.7. Example of calculation: the basic data
Fig. 11.8. Results of calculation: temperature waves
Results of calculations are shown on Fig. 11.8. The right diagram represents a series of temperature distributions along axis x for the various time moments. More evident is three-dimensional representation on the left diagram. The plane in the basis of the diagram is constructed on longitudinal coordinate x (marks 1..40) and time coordinate (marks of time step number within the limits 1..200). On a vertical axis the temperature is postponed. The compelled temperature fluctuations on the one of butt-ends (mark 40 on coordinate axis) and reduction of amplitude by approaching to opposite butt-end are clearly visible.
Showy representation of a non-stationary temperature field can be received with the animation as it is described in Chapter 9.
We have gived only one example of application of the here developed program. Numerical experiment can be continued in the following directions:
q To investigate the influence of heat-transfer coefficient on hot butt-end temperature
q To investigate the influence of material properties on the maximal surface temperature and on the heat sink through the core
q To investigate thr penetration of temperature waves, having taken the long core and having set adiabatic conditions at the left butt-end
q To investigate the heat transmission through the wall when the heat-transfer coefficient on ones side will pulse in time (for that it is required to modify slightly the program Coef, having provided the variations of Bi-number, the same as now it was made with temperature of surrounding)
q To investigate the temperature modes of cylinder walls in combustion engine with air cooling
q To investigate the temperature modes of building walls at weather and seasonal changes of temperature
It is possible to analyse the majority of classical heat conduction problems considered in educational courses, making numerical experiment on the developed computer model. With method of computing to steady state many stationary problems can be solved, e.g. the heat conduction in finned surface and so on. The whole complex of educational researches may be executed for stationary and non-stationary problems with internal sources as imitatio