Mach Number

Fluid Mechanics · FE Reference Handbook section

Fluid Mechanics

3 formulas

10 exam-style examples

~51 min

All Fluid Mechanics lectures

Learning objectives

What you must be able to do before leaving this section.

This chapter section covers Mach Number within Fluid Mechanics. Read it the way you would read a textbook chapter: the theory first so the relations mean something, then every equation with its use and its trap, then 10 fully worked examples with the arithmetic shown line by line, and finally a self-check you should be able to answer without notes.

Explain, in your own words, what mach number describes physically and when it applies.
State every one of the 3 relations the handbook lists here and name each symbol with its unit.
Select the correct relation from the wording of an exam stem within 20 seconds.
Carry a complete solution from givens to a "most nearly" answer with the correct unit.
Recognise the distractors generated by the unit trap: γ = 62.4 lb/ft³ or 9.81 kN/m³; convert psi to feet of head early.

Lecture

Why this section exists. Mach Number is the part of Fluid Mechanics that lets you connect a pipeline, jet or submerged surface to a number you can defend. Before any equation is useful you must be able to picture the physical situation it describes; the schematic below is that picture.

How the theory is built. The handbook prints results, not derivations. Each relation in this section comes from one governing principle applied to the idealised system: state the principle, impose the stated assumptions, and the printed equation follows. Knowing which assumption each relation rests on is what lets you reject a wrong answer choice in seconds.

How it is examined. Items from this page are written as continuity plus energy, with one head-loss or force term. Roughly two thirds are direct substitution, one third require one intermediate quantity from a neighbouring relation, and a small number are conceptual — testing whether you know the assumption, not the arithmetic.

The habit that earns the points. Unit discipline. γ = 62.4 lb/ft³ or 9.81 kN/m³; convert psi to feet of head early. Every relation below is dimensionally consistent only when that rule is honoured, and the distractor set is deliberately built from candidates who ignored it. Write the unit next to every number you substitute, every time.

How to study this page. Read the theory, then cover the formula cards and try to reproduce each relation from its description. Then work the examples with the solution hidden, revealing one line at a time. Finish with the self-check questions; if you cannot answer one, return to the matching formula card.

Crane lowering a steel plate girder onto bridge bearings while ironworkers guide it. — Photo 1. Where this shows up in practice: mach number.
Capstone Studio instructional photograph

Fluid Mechanics — Mach Number: reference schematic for orienting the symbols used in this section.

Theory, developed

Read this before the equations — it is what makes them memorable.

The physical situation

Every item from this section describes a pipeline, jet or submerged surface. Sketch it before you compute — a labelled sketch with the givens on it converts a wordy stem into a solvable problem and exposes the quantity the examiner left out on purpose.

The governing principle

The 3 relations on this page are consequences of one principle applied to that idealised system. Identify which quantity is conserved, balanced, or defined, and the correct equation follows without memorisation.

Assumptions and limits of validity

Each printed relation carries silent assumptions — linearity, steady state, uniformity, small deformation, or standard conditions, depending on the subject. Conceptual exam items are written by violating exactly one of these, so read the sentence above the equation as carefully as the equation itself.

Solution procedure you should automate

1) Read the last sentence of the stem to identify the requested quantity. 2) Locate the relation on this page whose left-hand side is that quantity. 3) Tabulate the givens with units and mark the missing symbol. 4) If a symbol is missing, find the one relation that produces it. 5) Rearrange symbolically, substitute once, evaluate, and round only at the end.

Notation used in this section

c	Quantity produced by "c = kRT" — read its definition and unit from the handbook line directly above the equation.

Handbook notes for this section

Definitions and conditions exactly as the handbook states them.

The local speed of sound in an ideal gas is given by:
where
c ≡ local speed of sound
T ≡ absolute temperature
Example: speed of sound in dry air at 1 atm 20°C is 343.2 m/s.
This shows that the acoustic velocity in an ideal gas depends only on its temperature. The Mach number (Ma) is the ratio of the
fluid velocity to the speed of sound.
Ma ≡ Vc
V ≡ mean fluid velocity

Core formulas for this FE topic

Definitions, applicability, units, assumptions and worked examples for each relation.

Worked exam-style examples

The four ways this section is written on the real exam — thoughts first, then equations, then substitution.

Example 1

Speed of sound and Mach number in air — Mach Number

Air at 312 K (k = 1.4, R = 287 J/kg·K) flows at 498 m/s. Compute the local speed of sound and the Mach number, and classify the flow regime.

Given

T = 312 K
V = 498 m/s
k = 1.4, R = 287 J/kg·K

Find

c, M and the flow regime

Start with the thinking

The speed of sound depends only on temperature for an ideal gas — pressure does not appear.
M < 0.3 lets you treat the flow as incompressible; M > 1 is supersonic.

Step-by-step solution

Formula
$c = k R T$
Substituting
$c = (1.4 \times 287 \times 312) = 354.1 m / s$
Formula
$M = V / c$
Substituting
$M = 498/354.1 = 1.407$
Classification
$M = 1.41 \to s u p er so ni c$

Answer: c = 354.1 m/s, M = 1.407 (supersonic)

Why the other options are there

c = 299.2 m/s (k omitted)
M = 0.711 (ratio inverted)

Reference: FE Reference Handbook — Fluid Mechanics → Mach Number

Example 2

Speed of sound and Mach number in air — Mach Number (2)

Air at 286 K (k = 1.4, R = 287 J/kg·K) flows at 569 m/s. Compute the local speed of sound and the Mach number, and classify the flow regime.

Given

T = 286 K
V = 569 m/s
k = 1.4, R = 287 J/kg·K

Find

c, M and the flow regime

Start with the thinking

The speed of sound depends only on temperature for an ideal gas — pressure does not appear.
M < 0.3 lets you treat the flow as incompressible; M > 1 is supersonic.

Step-by-step solution

Formula
$c = k R T$
Substituting
$c = (1.4 \times 287 \times 286) = 339.0 m / s$
Formula
$M = V / c$
Substituting
$M = 569/339.0 = 1.679$
Classification
$M = 1.68 \to s u p er so ni c$

Answer: c = 339.0 m/s, M = 1.679 (supersonic)

Why the other options are there

c = 286.5 m/s (k omitted)
M = 0.596 (ratio inverted)

Reference: FE Reference Handbook — Fluid Mechanics → Mach Number

Example 3

Speed of sound and Mach number in air — Mach Number (3)

Air at 267 K (k = 1.4, R = 287 J/kg·K) flows at 201 m/s. Compute the local speed of sound and the Mach number, and classify the flow regime.

Given

T = 267 K
V = 201 m/s
k = 1.4, R = 287 J/kg·K

Find

c, M and the flow regime

Start with the thinking

The speed of sound depends only on temperature for an ideal gas — pressure does not appear.
M < 0.3 lets you treat the flow as incompressible; M > 1 is supersonic.

Step-by-step solution

Formula
$c = k R T$
Substituting
$c = (1.4 \times 287 \times 267) = 327.5 m / s$
Formula
$M = V / c$
Substituting
$M = 201/327.5 = 0.614$
Classification
$M = 0.61 \to s u b so ni cco m p r ess ib l e$

Answer: c = 327.5 m/s, M = 0.614 (subsonic)

Why the other options are there

c = 276.8 m/s (k omitted)
M = 1.630 (ratio inverted)

Reference: FE Reference Handbook — Fluid Mechanics → Mach Number

Example 4

Speed of sound and Mach number in air — Mach Number (4)

Air at 311 K (k = 1.4, R = 287 J/kg·K) flows at 305 m/s. Compute the local speed of sound and the Mach number, and classify the flow regime.

Given

T = 311 K
V = 305 m/s
k = 1.4, R = 287 J/kg·K

Find

c, M and the flow regime

Start with the thinking

The speed of sound depends only on temperature for an ideal gas — pressure does not appear.
M < 0.3 lets you treat the flow as incompressible; M > 1 is supersonic.

Step-by-step solution

Formula
$c = k R T$
Substituting
$c = (1.4 \times 287 \times 311) = 353.5 m / s$
Formula
$M = V / c$
Substituting
$M = 305/353.5 = 0.863$
Classification
$M = 0.86 \to s u b so ni cco m p r ess ib l e$

Answer: c = 353.5 m/s, M = 0.863 (subsonic)

Why the other options are there

c = 298.8 m/s (k omitted)
M = 1.159 (ratio inverted)

Reference: FE Reference Handbook — Fluid Mechanics → Mach Number

Example 5

Speed of sound and Mach number in air — Mach Number (5)

Air at 260 K (k = 1.4, R = 287 J/kg·K) flows at 597 m/s. Compute the local speed of sound and the Mach number, and classify the flow regime.

Given

T = 260 K
V = 597 m/s
k = 1.4, R = 287 J/kg·K

Find

c, M and the flow regime

Start with the thinking

The speed of sound depends only on temperature for an ideal gas — pressure does not appear.
M < 0.3 lets you treat the flow as incompressible; M > 1 is supersonic.

Step-by-step solution

Formula
$c = k R T$
Substituting
$c = (1.4 \times 287 \times 260) = 323.2 m / s$
Formula
$M = V / c$
Substituting
$M = 597/323.2 = 1.847$
Classification
$M = 1.85 \to s u p er so ni c$

Answer: c = 323.2 m/s, M = 1.847 (supersonic)

Why the other options are there

c = 273.2 m/s (k omitted)
M = 0.541 (ratio inverted)

Reference: FE Reference Handbook — Fluid Mechanics → Mach Number

Example 6

Speed of sound and Mach number in air — Mach Number (6)

Air at 301 K (k = 1.4, R = 287 J/kg·K) flows at 182 m/s. Compute the local speed of sound and the Mach number, and classify the flow regime.

Given

T = 301 K
V = 182 m/s
k = 1.4, R = 287 J/kg·K

Find

c, M and the flow regime

Start with the thinking

The speed of sound depends only on temperature for an ideal gas — pressure does not appear.
M < 0.3 lets you treat the flow as incompressible; M > 1 is supersonic.

Step-by-step solution

Formula
$c = k R T$
Substituting
$c = (1.4 \times 287 \times 301) = 347.8 m / s$
Formula
$M = V / c$
Substituting
$M = 182/347.8 = 0.523$
Classification
$M = 0.52 \to s u b so ni cco m p r ess ib l e$

Answer: c = 347.8 m/s, M = 0.523 (subsonic)

Why the other options are there

c = 293.9 m/s (k omitted)
M = 1.911 (ratio inverted)

Reference: FE Reference Handbook — Fluid Mechanics → Mach Number

Example 7

Speed of sound and Mach number in air — Mach Number (7)

Air at 248 K (k = 1.4, R = 287 J/kg·K) flows at 656 m/s. Compute the local speed of sound and the Mach number, and classify the flow regime.

Given

T = 248 K
V = 656 m/s
k = 1.4, R = 287 J/kg·K

Find

c, M and the flow regime

Start with the thinking

The speed of sound depends only on temperature for an ideal gas — pressure does not appear.
M < 0.3 lets you treat the flow as incompressible; M > 1 is supersonic.

Step-by-step solution

Formula
$c = k R T$
Substituting
$c = (1.4 \times 287 \times 248) = 315.7 m / s$
Formula
$M = V / c$
Substituting
$M = 656/315.7 = 2.078$
Classification
$M = 2.08 \to s u p er so ni c$

Answer: c = 315.7 m/s, M = 2.078 (supersonic)

Why the other options are there

c = 266.8 m/s (k omitted)
M = 0.481 (ratio inverted)

Reference: FE Reference Handbook — Fluid Mechanics → Mach Number

Example 8

Speed of sound and Mach number in air — Mach Number (8)

Air at 277 K (k = 1.4, R = 287 J/kg·K) flows at 432 m/s. Compute the local speed of sound and the Mach number, and classify the flow regime.

Given

T = 277 K
V = 432 m/s
k = 1.4, R = 287 J/kg·K

Find

c, M and the flow regime

Start with the thinking

The speed of sound depends only on temperature for an ideal gas — pressure does not appear.
M < 0.3 lets you treat the flow as incompressible; M > 1 is supersonic.

Step-by-step solution

Formula
$c = k R T$
Substituting
$c = (1.4 \times 287 \times 277) = 333.6 m / s$
Formula
$M = V / c$
Substituting
$M = 432/333.6 = 1.295$
Classification
$M = 1.29 \to s u p er so ni c$

Answer: c = 333.6 m/s, M = 1.295 (supersonic)

Why the other options are there

c = 282.0 m/s (k omitted)
M = 0.772 (ratio inverted)

Reference: FE Reference Handbook — Fluid Mechanics → Mach Number

Example 9

Speed of sound and Mach number in air — Mach Number (9)

Air at 271 K (k = 1.4, R = 287 J/kg·K) flows at 496 m/s. Compute the local speed of sound and the Mach number, and classify the flow regime.

Given

T = 271 K
V = 496 m/s
k = 1.4, R = 287 J/kg·K

Find

c, M and the flow regime

Start with the thinking

The speed of sound depends only on temperature for an ideal gas — pressure does not appear.
M < 0.3 lets you treat the flow as incompressible; M > 1 is supersonic.

Step-by-step solution

Formula
$c = k R T$
Substituting
$c = (1.4 \times 287 \times 271) = 330.0 m / s$
Formula
$M = V / c$
Substituting
$M = 496/330.0 = 1.503$
Classification
$M = 1.50 \to s u p er so ni c$

Answer: c = 330.0 m/s, M = 1.503 (supersonic)

Why the other options are there

c = 278.9 m/s (k omitted)
M = 0.665 (ratio inverted)

Reference: FE Reference Handbook — Fluid Mechanics → Mach Number

Example 10

Speed of sound and Mach number in air — Mach Number (10)

Air at 260 K (k = 1.4, R = 287 J/kg·K) flows at 517 m/s. Compute the local speed of sound and the Mach number, and classify the flow regime.

Given

T = 260 K
V = 517 m/s
k = 1.4, R = 287 J/kg·K

Find

c, M and the flow regime

Start with the thinking

The speed of sound depends only on temperature for an ideal gas — pressure does not appear.
M < 0.3 lets you treat the flow as incompressible; M > 1 is supersonic.

Step-by-step solution

Formula
$c = k R T$
Substituting
$c = (1.4 \times 287 \times 260) = 323.2 m / s$
Formula
$M = V / c$
Substituting
$M = 517/323.2 = 1.600$
Classification
$M = 1.60 \to s u p er so ni c$

Answer: c = 323.2 m/s, M = 1.600 (supersonic)

Why the other options are there

c = 273.2 m/s (k omitted)
M = 0.625 (ratio inverted)

Reference: FE Reference Handbook — Fluid Mechanics → Mach Number

Self-check

Answer these without notes before moving on.

Without looking, state the relation on this page whose left-hand side is the quantity most often requested, and name every symbol in it.
Which assumption, if violated, makes the main relation of this section invalid?
Given a pipeline, jet or submerged surface, what is the first quantity you would compute, and why that one first?
Which unit conversion in this subject most often produces a wrong answer choice, and what is its numerical factor?
Rework Example 1 above from the givens alone, without reading the solution lines.

Chapter summary

Mach Number contains 3 relations; you must be able to find this page in under 15 seconds.
Exam style: continuity plus energy, with one head-loss or force term.
Unit rule: γ = 62.4 lb/ft³ or 9.81 kN/m³; convert psi to feet of head early.
Work the 10 examples until the solution path, not the answer, is automatic.

Common traps in this section

γ = 62.4 lb/ft³ or 9.81 kN/m³; convert psi to feet of head early
Answering the intermediate quantity instead of the quantity requested.
Rounding intermediate values before the final step.
Using a relation from an adjacent handbook section that shares a symbol.
Skipping the sketch — most lost points on this page start with a misread geometry.

Mach Number

Learning objectives

Lecture

Theory, developed

Notation used in this section

Handbook notes for this section

Core formulas for this FE topic

c=kRT

k≡ratioofspecificheats=c

R≡specificgasconstant=R/(molecularweight)

Worked exam-style examples

Self-check

Chapter summary

Common traps in this section