Читать книгу Planning and Executing Credible Experiments - Robert J. Moffat - Страница 32

One of the first problems to be faced in exploratory research, and in development work, is identifying which scalars are significant to the issue at hand. Sometimes a single scalar is sufficient, such as a temperature, pressure, or velocity. Sometimes a compound scalar measure can be put together from a set of simple scalars. One example would be the Reynolds number, used for characterizing the state of the flow in a channel. Sometimes two or more scalars can be combined into one measure to reflect value judgments or “trade‐offs” in desirability. For example, consider the problem of selecting the optimum heat exchanger for an engine application. In even the simplest situation, ignoring such considerations as cost, size, and durability, a heat exchanger for engine service has at least two important scalar descriptors: its effectiveness and its pressure drop. Typically, high effectiveness is “good,” while high‐pressure drop is “bad.” Also typically, pressure drop goes up when effectiveness goes up. Neither by itself is a measure of goodness for the heat exchanger. A weighted sum of the two can be used as a “composite scalar” by finding two weight functions, one for effectiveness and one for pressure drop, which accounts for the effects of both on some single, important parameter – brake‐specific fuel consumption is another example. Such “goodness factors” can be used to account for many factors at a time and to convert a nonmeasurable situation to a measurable one.

The choice of what to measure sets the course of the entire experiment, and that choice should be made with considerable care.