Simulation of phenomena

The researchers, the engineers, the soldiers and many other professionals often put the question: which is the result which I obtain if I exert such action on an element?

The simplest means would be to try the experiment , i.e. to exert the action desired on the element in question to be able to observe or measure the result. In many cases the experiment is unrealizable, too expensive or opposite with ethics. One has then recourse to the simulation : to seek an element which reacts in a way similar to that one wants to study and which will make it possible to deduce the results.

The simulation is a tool used by the researcher, the engineer, soldier etc to study the results of an action on an element without carrying out the experiment on the real element.

When the tool for simulation uses a computer one speaks about Digital simulation.

Real phenomenon

The real phenomenon to study can belong to many branches and in particular:

physics (mechanical, optical, thermodynamic, electronic, etc):

simple example: movement of a mass suspended with a spring and subjected to an impulse.
more complex example: movement of the case of a car in displacement on a road.

economy:

simple example: refunding of a loan with interest.
more complex example: income of a tax which one varies the rate.

biology:

simple example: diffusion of a drug in blood according to time.
more complex example: evolution of an epidemic in a population according to the rate of vaccination and time.

the reasoning:

simple example: artificial player of play of failure.
more complex example: decision-making aid in a military engagement (War game).

The question

Through the examples quoted above, certain professionals can question themselves:

the engineer on the influence of a change of shock absorbers on the behavior of a vehicle in displacement on a road.

the Minister for the Budget on the report/ratio of the tax to the added-value on a product when the rate is modified by it.

the doctor on the influence of a vaccine on the eradication of a disease in a population.

the soldier on the tactics to be used in an engagement of air forces.

The answer

In all the cases above the answer could be obtained by trying the experiment .

the engineer can build new shock absorbers, to integrate them on the vehicle, to make it roll while having in the cockpit of the sensors movement (accelerometers) which will make known to him the forces undergone by the driver and the passengers.

the minister can issue the increase or the fall of the VAT on a product and raise, at the end of the year, the results on the payments of the tradesmen.

the doctor can practice the vaccination of the population and measure the effects during the years.

the soldier can engage of the forces against the enemy and measure the results.

But all these experiments have one or more disadvantages:

they can be expensive: the construction of a new car is relatively expensive.

they can be long: to measure the impact of a vaccination during the years takes… years.

they can be against ethics: one does not test a new vaccine on a population without a minimum of guarantees on the results, one does not explode a bomb on a population only to measure of them the effects, one does not carry out a test of accident on a vehicle with human passengers on board.

they can be " politically incorrectes" : one cannot increase or decrease a tax without envisaging the consequences of them before.

they can be difficult, even impossible to implement: the material does not exist or the population of reference does not exist.

the results cannot be measured with certainty: the experiment cannot be realized several times under identical conditions.

Alternative solutions

The experiment posing various problems of realization, one for a long time called upon very many means and tools to try to envisage the results:

prototypes and the models: one builds a specimen, possibly on reduced scale, material and one carries out on him the tests. Simulation is very close to the experiment and one thus has part of the disadvantages (costs, duration).

one replaces the human one by an animal: it is necessary to find populations animal to which the behaviors are close to the man with respect to a given phenomenon. Many lobbies fight against this practice.

one represents the phenomenon by an equation: the examples abound and were used by all the pupils and students in the courses of physics, chemistry, etc Seuls the simplest phenomena are likely of this type of simulation.

operations: the soldiers make clash two opposite troops (oranges against the blue ones) on a true ground with material truths but without using real ammunition. Referees decide inflicted damage.

All these tools are simulations. They are more or less close to the experiment and more or less easy to implement.

For a few years a new tool has made its appearance: the computer and the Digital simulation. The basic principle is that of the representation of the phenomenon by an equation. The computer makes it possible however to be freed from the principal limitation: the representation of the simplest phenomena. Thanks to a computing power always increasing and with the increase in volumes of data storable it is possible to cut out a complex phenomenon in thousands, even in million, simple phenomena and thus to calculate the results on the complex phenomenon.

Example: one knows, in aerodynamics, to represent by an equation the forces (bearing pressure, trail) which result from the action of a draft on a plane plate. One cannot represent by an equation these same forces when the action is exerted on a complex surface such as the wing of a plane. The digital simulation makes it possible to cut out the wing in several million small elements which one regards as being plane plates. One can then calculate the forces which are exerted on each one among it and to combine them to calculate the forces on the complete wing.

Limits and advantages of simulation

The computer makes it possible today to simulate very complex phenomena such as a complete plane but the power remains still insufficient to represent the whole of the weather phenomena: the simulation of the evolution of time still remains very difficult beyond a few hours.

Simulation makes it possible to carry out research on an isolated system, while varying the parameters one by one and while starting again with the same conditions initiales.
The experimentation, except for the simple phenomena, always does not make it possible to isolate the system to be studied of its environment; the control of the initial conditions can be complicated and the experiment can destroy the studied system or modify it sufficiently to prevent from starting again.

Simulation is often less expensive than the experimentation and involves much less risks when the man belongs to the studied system. The results can be obtained much more quickly.

Simulation (especially numerical) is based on a knowledge of the phenomena which can be obtained only by the experimentation. A simulation can thus be carried out only if one has a sufficient asset of knowledge obtained by experiments on former and similar phenomena. Whatever the quality of simulation, it does not replace the experimentation completely.

Certain simulations have a very high cost (even if there remains weak in front of that of the experimentation). This explains why the users of simulation, in particular when it uses exceptional means of calculation, are industries with strong added value (aeronautical and spaces, nuclear) or at the high risk (military).

Various types of simulation

One calls model an element, analogical or numerical, whose behavior with respect to a phenomenon is similar to that of the element to study. The left are the elements which one wants to study. The entered , forced parameters and are the elements whose variation influences the behavior of the model; one calls entered those which are ordered by the experimenter, parameters those which the operator chooses to fix and forced those which depend on external elements. One calls simulation the unit consisted a model, the orders of entry, the parameters and constraints, and the results obtained.

As higher indicated the models, prototypes, etc can be regarded as analog models and the tests, tests, operations, etc like analogical simulations.

The equations are digital simulations. Today this term applies primarily to the models and simulations realized to computer.

In certain cases one can carry out simulations hybrid, analog-to-digital, which integrate various elements of which some only are represented by equations.

When the calculator is sufficiently fast to provide a result at the same speed, even more quickly, that the real phenomenon one speaks about simulation in real-time . One can then carry out analog-to-digital simulations where one of the analogical elements is the man: it is about simulation with the man in the loop . A Simulateur of piloting in is an good example: the pilot (analogical) sat in a cockpit quasi-real (analogical) and pilot his plane. The orders which it gives are read by a computer which calculates the movements of the plane (numerical). These movements are restored (analogical) on the cabin and the screens what makes it possible to the pilot to feel and see the effects of the orders that it gave.

The Jeu of simulation is a recent application of the same principle. The difference between the play and the simulator in study resides is in:

the cost of the analogical elements: a simulator of piloting to play replaces the real cabin by a keyboard of computer and uses one screen to show the instruments and the landscape. On the other hand, at least in the case of a private plane, the equations of flight are the same ones as those of the simulator of study which is used by the engineer.

in the introduction of fantastic parameters: the simulator to play introduced phenomena which do not exist in reality such as supernatural capacities, new weapons, etc

Use of simulation

Simulations are used by the professionals (enquiring, engineers, economists, doctors, etc) in all the phases of search or study for a phenomenon or to conceive and improve the systems.

The analog-to-digital hybrid simulators with man in the loop, are used more and more for teaching or the drive. Their relatively high cost initially held them for the professions more at a peak or the risk (piloting of an aircraft, orders of a nuclear plant, armed engagement, etc). The reduction in the price of the video systems makes it possible to even consider today applications to the control of the trucks of the cars.

Finally of many video games are users of the same techniques is while allowing the player to be in an exciting or dangerous situation (fighter pilot…) maybe in a phantasmagoric situation.

Simple: Simulation

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