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Dr Faustus of Modern Physics - Department of Speech, Music and ...

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8.4. SMALL CAUSE - BIG EFFECT: ILLPOSED 43<br />

8.4 Small Cause - Big Effect: Illposed<br />

Pr<strong>and</strong>tl said that a small cause (an arbitrarily thin boundary layer) can<br />

change the global features <strong>of</strong> a flow around a body <strong>and</strong> cause substantial<br />

drag. Pr<strong>and</strong>tl thus claimed that his flow problem was illposed, but insisted<br />

that his model anyway was scientifically acceptable. This was breach <strong>of</strong><br />

the fundamental principle <strong>of</strong> wellposedness. Pr<strong>and</strong>tl commited the crime in<br />

order to resolve d’Alembert’s paradox, because it appeared that it could not<br />

be solved within the laws <strong>of</strong> rational wellposed science.<br />

At the end <strong>of</strong> the book you will be invited to a wellposed scientifically<br />

lawful resolution <strong>and</strong> discover that Pr<strong>and</strong>tl’s sacrifice was meaningless. But<br />

to get there you have to browse the book.<br />

Figure 8.2: Wellposed Newtonian models.

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