# Dimensional Analysis and Similitude: Dimensional Analysis

## Dimensionless parameters are obtained using a method called dimensional analysis. It is based on the idea of dimensional homogeneity: all terms in an equation must have the same dimensions. By simply using this idea, we can minimize the number of parameters needed in an experimental or analytical analysis, as will be shown. Any equation can be expressed in terms of dimensionless parameters simply by dividing each term by one of the other terms. For example, consider Bernoulli’s equation,

Note the dimensionless parameters, V 2/gz and p/γ z.

## Let’s use an example to demonstrate the usefulness of dimensional analysis. Suppose the drag force is desired on an object with a spherical front that is shaped as shown in Fig. 6.1. A study could be performed, the drag force measured for a

Figure 6.1 Flow around an object.

## particular radius R and length L in a ﬂuid with velocity V, viscosity m, and density r.Gravity is expected to not inﬂuence the force. This dependence of the drag force on the other variables would be written as

To present the results of an experimental study, the drag force could be plotted as a function of V for various values of the radius R holding all other variables ﬁxed. Then a second plot could show the drag force for various values of L holding all other variables ﬁxed, and so forth. The plots may resemble those of Fig. 6.2. To vary the viscosity holding the density ﬁxed and then the density holding the viscos- ity ﬁxed, would require a variety of ﬂuids leading to a very complicated study, and perhaps an impossible study.

## The actual relationship that would relate the drag force to the other variables could be expressed as a set of dimensionless parameters, much like those of Eq. (6.2), as

(The procedure to do this will be presented next.) The results of a study using the above relationship would be much more organized than the study suggested by the curves of Fig. 6.2. An experimental study would require only several different models, each with different R/L ratios, and only one ﬂuid, either air or water. Varying the velocity of the ﬂuid approaching the model, a rather simple task, could vary the other two dimensionless parameters. A plot of FD /(ρV R ) versus ρVR/μ for the

## sveral values of R/L would then provide the results of the study.

Figure 6.2 Drag force versus velocity: (a) L, m, r ﬁxed; (b) R, m, r ﬁxed.

## Before we present the details of forming the dimensionless parameters of Eq. (6.4), let’s review the dimensions on quantities of interest in ﬂuid mechanics. Many quantities have obvious dimensions but for some, the dimensions are not so obvious. There are only three basic dimensions, since Newton’s second law can be used to relate the basic dimensions. Using F, M, L, and T as the dimensions on force, mass, length, and time, we see that F = ma demands that the dimensions are related by

We choose to select the M-L-T system1 and use Eq. (6.5) to relate F to M, L, and T. If temperature is needed, as with the ﬂow of a compressible gas, an equation of state, such as

where the brackets mean “the dimensions of.” Note that the product RT does not introduce additional dimensions.

## a functional relationship such as that of Eq. (6.3). Write the primary variable of interest as a general function, such as

where n is the total number of variables. If m is the number of basic dimensions, usually 3, the Buckingham p theorem demands that (n − m) dimensionless groups of variables, the p terms, are related by

Table 6.1 Symbols and Dimensions of Quantities of Interest Using the M-L-T System

The p term p1 is selected to contain the dependent variable [it would be FD of Eq. (6.3)] and the remaining p terms contain the independent variables. It should be noted that a functional relationship cannot contain a particular dimension in only one variable; for example, in the relationship v = f (d, t, ρ) , the density r cannot occur since it is the only variable that contains the dimension M, and M would not have the possibility of canceling out to form a dimensionless p term.

## Step 3 is carried out by either inspection or by an algebraic procedure. The method of inspection will be used in an example. To demonstrate the algebraic procedure, let’s form a p term of the variables V, R, r, and m. This is written as

This p term is dimensionless regardless of the value of d. If we desire V to be in the denominator, select d = 1; if we desire V to be in the numerator, select d = −1. Select d = −1 so that

Suppose that only one p term results from an analysis. That p term would then be equal to a constant which could be determined by a single experiment.

## may not inﬂuence a particular problem, but it is interesting to observe the ﬁnal relationship of dimensionless terms. Dimensional analysis, using V, l, and r as repeating variables provides the relationship

Each term that appears in this relationship is an important parameter in certain ﬂow situations. The dimensionless term with its common name is listed as follows:

Not all of the above numbers would be of interest in a particular ﬂow; it is highly unlikely that both compressibility effects and surface tension would inﬂuence the same ﬂow. These are, however, the primary dimensionless parameters in our study

of ﬂuid mechanics. The Euler number is of interest in most ﬂows, the Froude number in ﬂows with free surfaces in which gravity is signiﬁcant (e.g., wave motion), the Reynolds number in ﬂows in which viscous effects are important, the Mach number in compressible ﬂows, the Weber number in ﬂows affected by surface tension (e.g., sprays with droplets), and the Strouhal number in ﬂows in which rotation or a periodic motion plays a role. Each of these numbers, with the exception of the Weber number (surface tension effects are of little engineering importance), will appear in ﬂows studied in subsequent chapters. Note: The Froude number is often deﬁned as V 2/ lg; this would not inﬂuence the solution to problems.

## First, select the repeating variables. Do not select Δ p since that is the dependent variable. Select only one D, L, and e since they all have the dimensions of length. Select the variables that are thought2 to most inﬂuence the pressure drop: V, D, and r. Now, list the dimensions on each variable (refer to Table 6.1)

First, combine Δ p, V, D, and m into a p term. Since only Δ p and r have M as a dimension, they must occur as a ratio Δ p/r. That places T in the denominator so that V must be in the numerator so the T ’s cancel out. Finally, check out the L’s: there is L2 in the numerator so D2 must be in the denominator providing

The second p term is found by combining L with the three repeating variables

## V, D, and r. Since both L and D have the dimension of length, the second p term is

The third p term results from combining e with the repeating variables. It has the dimension of length so the third p term is

The last p term is found by combining m with V, D, and r. Both m and r contain the dimension M demanding that they form the ratio r/m. This puts T in the numerator demanding that V goes in the numerator. This puts L in the denominator so that D must appear in the numerator. The last p term is then

The ﬁnal expression relates the p terms as π1 = f (π 2 , π3 , π 4 ) or, using the variables,

If L had been chosen as a repeating variable, it would simply change places with D since it has the same dimension.

## Since M occurs in only one variable, that variable r cannot be included in the relationship. The remaining three terms are combined to form a single p term; it is formed by observing that T occurs in only two of the variables, thus V 2 is in the numerator and g is in the denominator. The length dimension is then canceled by placing h in the denominator. The single p term is

Since this p term depends on all other p terms and there are none, it must be at most a constant. Hence, we conclude that

A simple experiment would show that C = 2. We see that dimensional analysis rules out the possibility that the speed of free fall, neglecting viscous effects (e.g., drag), depends on the density of the material (or the weight).