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MIT 12 000 - A Brief Philosophy of Laboratory Experimentation

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I6The Originand Developmentof LaboratoryModels andAnalogues of theOcean CirculationAlan . Faller16.1 A Brief Philosophy of LaboratoryExperimentation"No one believes a theory, except the theorist. Every-one believes an experiment--except the experimen-ter." This often used adage, although not aways accu-rate in detail, carries a certain element of truth. Herewe consider certain experiments-analogs, models, orfundamental studies of basic fluid dynamics-that areintended to be relevant to one or another aspect of theocean circulation. These are physical, fluid models-asopposed to numerical, analytical, abstract, or conceptualmodels-that make use of a real fluid. When the fluidin a container is subjected to some driving force, thefluid moves. It is observed by the experimenter. It isthere. It is a real fluid circulation. Apart from its rele-vance, why should it not be believed?The experienced experimenter is well aware of thepitfalls of his trade. Aside from the accuracy of hisreported observations there are many questions. Werethe boundary conditions well controlled? Were thephysical properties of the materials and their variationsduring the experiment known? Did the methods ofobservation (probes, dyes, tracers, lighting) influencethe results? Were the reported observations complete,or at least representative? or were they filtered, mas-saged, interpreted, selectively reported, etc.? In factthere is always an element of judgment, selectivity,and interpretation concerning what to observe, andhaving observed, what to record, and what to report.Thus, despite the apparent confidence that may be ex-uded in publication, the cautious experimenter main-tains a restraint born of continual reflection on theextensive preparations necessary for an apparently sim-ple experiment and the many opportunities for errorand misinterpretation.We can broadly classify experiments into four cate-gories, according to their intent, under these headings:(1) simulation, (2) abstraction, (3) verification, and (4)extension. In the first category the experimenter at-tempts to represent nature in miniature in so far aspossible. An effort is made to include all of the relevantdriving mechanisms, and the geometry is scaled as innature, although some distortion may be necessary.Using theoretical guides such as the matching of theappropriate nondimensional ratios, the intent is tolearn by trial and error to what extent the ocean cir-culation can be reproduced as a scale model. If it werepossible to reproduce known features of the North At-lantic circulation, for example, such a model could beused to predict similarly scaled features of the oceancirculation in less accessible regions of the world. Thepredictions would serve as a guide to further explora-tion and would be compared with observations as theybecame available. The simulation mode of experimen-462Alan J. Fallertation is appealing to the eye of the layman who canrather easily be convinced of its possible applicationand relevance.The second mode of experimentation is rather likeabstract art. The artist draws out (abstracts) from somenatural subject those features that he imagines to be ofsignificance, and he displays his interpretation of thosefeatures on canvas or in stone for the reaction of hispeers and his public. The experimental scientist con-ceptually isolates one or more processes that he be-lieves to be significant in nature, and he displays andtests them in the form of an operational laboratoryexperiment. Just as it may be difficult for the artist topersuade his lay audience of the sincerity of his efforts(although his fellow artists understand), so also thescientist may have difficulty convincing his nonprofes-sional audience that his experiment is relevant to thegrand scheme of things past, present, and future.Abstract experiments may be particularly successfulin systems that can be decomposed linearly withoutdoing violence to the essential dynamics, i.e., systemsin which the abstracted phenomenon can be isolatedby virtue of a lack of coupling with other processes.This may be possible because of a smallness of ampli-tude of potentially interactive processes, or because ofmismatch in the temporal and spatial scales of thevarious processes. But even in those situations wheredecomposition is not warranted, one can say, "If ....Perhaps a planet will be found where these conditionsprevail; or perhaps some machinery in a chemicalplant, somewhere, generates the conditions that I amstudying; or perhaps I can build upon this experimentto incorporate the interactive processes necessary fora more realistic representation of the oceanic circula-tion (simulation?). In any case I will publish my ab-stract results for the benefit of posterity."Abstraction experiments, in contrast to direct sim-ulation, are more readily subject to a posteriori math-ematical analysis because of their relative simplicity.They may more directly lead to the advancement oftheoretical aspects of the problem. Either mode maybe regarded as exploratory, for it is likely that certainnew aspects of the fluid circulations will emerge thatwere not anticipated and that will require rationaliza-tion. Here again the abstract experiment has the ad-vantage of its simplicity, for deviations from the antic-ipated behavior will be more clearly recognized. Thesimulation experiment, however, will generally pose alarger variety of unanticipated phenomena because ofits inherently greater complexity.The verification mode of laboratory experiment im-plies an apparatus designed to test and verify) a specificanalytical or numerical model. A certain theoreticalmodel predicts a steady-state circulation, the temporaldevelopment of a flow, or perhaps an instability. Ap-paratus is designed to match the conditions of the the-ory in so far as possible, and, as often as not, the theoryis modified to conform to the limitations of the exper-iment. But in all cases under this category there is adetailed theory capable of a priori predictions and thephysical conditions of the


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MIT 12 000 - A Brief Philosophy of Laboratory Experimentation

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