# Fluid Mechanics - Dimensional Analysis Complete Study Notes

By Sidharth Jain|Updated : February 11th, 2022

Complete coverage of the APPSC AE Exam syllabus is a very important aspect for any competitive examination but before that important subjects and their concept must be covered thoroughly. In this article, we are going to discuss the Dimensional Analysis topic which is very useful for APPSC AE Exams.

## Dimensional Analysis

Dimensional analysis is a mathematical technique that uses the study of dimensions as an aid to the solution of several engineering problems. It deals with the dimensions of the physical quantities is measured by comparison, which is made concerning an arbitrarily fixed value.

Length L, mass M and Time T are three fixed dimensions that are important in fluid mechanics. If heat is involved in any fluid mechanics problem, then the temperature is also taken as a fixed dimension. These fixed dimensions are called fundamental dimensions or fundamental quantity.

Secondary or Derived quantities are those quantities that possess more than one fundamental dimension. For example, velocity is defined by distance per unit time (L/T), density by mass per unit volume (M/L3), and acceleration by distance per second square (L/T2). Then the velocity, density, and acceleration become secondary or derived quantities. The expressions (L/T), (M/L3), and (L/T2) are called the dimensions of velocity, density, and acceleration, respectively.

### Dimensional homogeneity

• Dimensional homogeneity means the dimensions of each term in an equation on both sides are equal. Thus, if each term's dimensions on both sides of an equation are the same, the equation is known as the dimensionally homogeneous equation.

• The powers of fundamental dimensions, i.e., L, M, T on both sides of the equation, will be identical for a dimensionally homogeneous equation. Such equations are independent of the system of units.

• Let us consider the equation V = u + at

Dimensions of L.H.S = V= L/T = LT-1
Dimensions of R.H.S = LT-1 + (LT-2) (T)
= LT-1 + LT-1
= LT-1
Dimensions of L.H.S = Dimensions of R.H.S = LT-1

Therefore, equation V = u + at is dimensionally homogeneous

### Uses of Dimensional Analysis

• It is used to test the dimensional homogeneity of any derived equation.
• It is used to derive the equation.
• Dimensional analysis helps in planning model tests.

### Methods of Dimensional Analysis

• If the number of variables involved in a physical phenomenon is known, then the relationship among the variables can be determined by the following two methods.

### Rayleigh's Method

• Rayleigh‟s method of analysis is adopted when the number of parameters or variables is less (3 or 4 or 5).
• If the number of independent variables becomes more than four, then it is very difficult to find the expression for the dependent variable

### Buckingham's (Π– theorem) Method

• If there are n – variables in a physical phenomenon and those n-variables contain 'm' dimensions, then the variables can be arranged into (n-m) dimensionless groups called Π terms.
• If f (X1, X2, X3, ……… Xn) = 0 and variables can be expressed using m dimensions then. f (Π1, Π2,Π3, ……… Πn - m) = 0 Where, Π1, Π2, Π3, ……… are dimensionless groups.
• Each Π term contains (m + 1) variables out of which m are of repeating type and one is of non-repeating type.
• Each Π term being dimensionless, the dimensional homogeneity can be used to get each Π term.

### Method of Selecting Repeating Variables

• Avoid taking the quantity required as the repeating variable.
• Repeating variables put together should not form a dimensionless group.
• No two repeating variables should have the same dimensions.
• Repeating variables can be selected from each of the following properties
• Geometric property - Length, Height, Width, Area
• Flow property - Velocity, Acceleration, Discharge
• Fluid property – Mass Density, Viscosity, Surface Tension

## Model Studies

• Before constructing or manufacturing hydraulics structures or hydraulics machines, tests are performed on their models to obtain desired information about their performance.
• Models are small-scale replicas of actual structures or machines.
• The actual structure is called the prototype.

Similitude

• It is defined as the similarity between the prototype and its model. It is also known as a similarity. These three types of similarities are as follows. 

Geometric similarity

• Geometric similarity exists between the model and prototype if the ratio of corresponding linear dimensions between model and prototype are equal. i.e.

Where Lr is known as scale ratio or linear ratio.

Vm/Vp= √(Lr) .... (Velocity Scale Ratio)

tm/tp= √(Lr) .....(Time Scale Ratio)

Hence, am/ap= 1 .....(Acceleration Scale Ratio)

Kinematic Similarity

• Kinematic similarity exists between prototype and model if quantities such as velocity and acceleration at corresponding points on model and prototype are the same.

Where Vr  is known as velocity ratio

Dynamic Similarity

• Dynamic similarity exists between model and prototype if the ratio of forces at corresponding points of model and prototype is constant.

Where Fr is known as force ratio.

### Dimensionless Numbers

Following dimensionless numbers are used in fluid mechanics.

• Reynolds's number
• Froude's number
• Euler's number
• Weber's number
• Mach number

Reynolds’s number

• It is defined as the ratio of inertia force of the fluid to viscous force.

NRe= Fi/Fv

Froude’s Number (Fr)

• It is defined as the square root of inertia force to gravity force ratio.

Fr=√Fi/Fg

### Model Laws (Similarity laws)

Reynolds’s Model Law

• For the flows with inertia force, the similarity of flow in the model and predominant force, the similarity of flow in model and prototype can be established if Re is the same for both systems.
• This is known as Reynolds's Model Law.
• Re for model = Re for prototype
• (NRe)m = (NRe)p

Applications

• In the flow of incompressible fluids in closed pipes.
• The motion of the submarine completely underwater.
• Motion of airplanes.

Froude’s Model Law

• When the force of gravity is predominant in addition to inertia force, the similarity can be established by Froude's number.
• This is known as Froude's model law.
• (Fr)m = (Fr)p

Applications:

• Flow over spillways
• Channels, rivers (free surface flows).
• Waves on the surface.
• Flow of different density fluids one above the other

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