Dimensional Analysis Beyond the Pi Theorem by Bahman Zohuri

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By Bahman Zohuri

Dimensional research and actual Similarity are good understood topics, and the final options of dynamical similarity are defined during this publication. Our exposition is largely various from these to be had within the literature, even though it follows the final rules referred to as Pi Theorem. there are various very good books that you'll check with; even if, dimensional research is going past Pi theorem, that is sometimes called Buckingham’s Pi Theorem. Many concepts through self-similar suggestions can sure recommendations to difficulties that appear intractable.

A time-developing phenomenon is named self-similar if the spatial distributions of its homes at varied deadlines will be received from each other by means of a similarity transformation, and picking one of many self sufficient variables as time. notwithstanding, this can be the place Dimensional research is going past Pi Theorem into self-similarity, which has represented growth for researchers.

In contemporary years there was a surge of curiosity in self-similar recommendations of the 1st and moment type. Such ideas will not be newly found; they've been pointed out and named by means of Zel’dovich, a well-known Russian Mathematician in 1956. they've been utilized in the context of numerous difficulties, similar to surprise waves in gasoline dynamics, and filtration via elasto-plastic materials.

Self-Similarity has simplified computations and the illustration of the homes of phenomena less than research. It handles experimental information, reduces what will be a random cloud of empirical issues to lie on a unmarried curve or floor, and constructs techniques which are self-similar. Variables will be particularly selected for the calculations.

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We could add the mass m to the list, À but if we Á assume that the density is constant as a first approximation, then m ¼ ρ 4πR3 =3 , and the mass is redundant. Therefore, ω is the governed parameter, with dimensions ½ωŠ ¼ T À1 , and (ρ, R, G) are the governing parameters, with dimensions ½ρŠ ¼ MLÀ3 , ½RŠ ¼ L, and ½GŠ ¼ MÀ1 L3 T À2 (check the last one). You can easily check that (ρ, R, G) have independent dimensions; therefore, n ¼ 3and k ¼ 3, so the function Φ is simply a constant in this case.

When all of these approximations hold, then, we expect Ψ to be roughly constant, so ω2 is proportional to gk. In fact, we find ω2 ¼ gk in the limit, but this requires analysis that is more detailed. For very short waves (ripples) in deep water, it seems reasonable to assume that only surface tension is responsible for the wave motion, so that g does not enter the problem. You could do a new dimensional analysis under this assumption, but it is easier to see directly that Ψ must be a linear function of its last argument for g to cancel out.

Summarizing the Buckingham’s Pi theorem, we quote Harald Hance [19]. Any physically meaningful relation ΦðR1 ; Á Á Á; Rn Þ ¼ 0 , with Rj 6¼ 0 , is equivalent to relation of the form Ψ ðπ 1 ; Á Á Á; π nÀr Þ ¼ 0 involving a maximal set of independent dimensionless combinations. The important fact to notice is that the new relation involves r fewer variables than the original relation does; this simplifies the theoretical analysis and experimental design alike. Then he goes on to give a precise meaning to the phrase “physically meaningful,” which was mentioned above.

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