Heat & Fluid I Definitions
Chapter 1:
Mechanics – the oldest physical science that deals with both stationary and moving bodies
under the influence of forces
Statics – The branch of mechanics that deals with bodies at rest
Dynamics – The branch that deals with bodies in motion
Fluid Mechanics – The science that deals with the behavior of fluids at rest (fluid statics) or in
motion (fluid dynamics), and the interaction of fluids with solids or other fluids at the
boundaries
Fluid dynamics – Fluid mechanics is also referred to as fluid dynamics by considering fluids at
rest as a special case of motion with zero velocity
Hydrodynamics – The study of motion of fluids that can be approximated as incompressible
(such as liquids, especially water, and gases at low speeds)
Hydraulics – A subcategory of hydrodynamics, which deals with liquid flows in pipes and open
channels
Gas dynamics – Deals with flow of fluids that undergo significant density changes, such as the
flow of gases through nozzles at high speeds
Aerodynamics – Deals with the flow of gases (especially air) over bodies such as aircraft,
rockets, and automobiles at high or low speeds
Meteorology, Oceanography, and Hydrology – Deal with naturally occurring flows
Fluid – A substance in the liquid or gas phase, a fluid deforms continuously under the influence
of a shear stress, no matter how small
Solid – Can resist an applied shear stress by deforming
Stress – Force per unit area
Shear Stress – The tangential component of a force acting on a surface per unit area
Pressure – The normal stress in a fluid at rest
Liquid – Molecules can rotate and translate freely
Gas – Molecules are far apart from each other, and molecular ordering is nonexistent
Vapor – Usually implies that the current phase is not far from a state of condensation
Macroscopic or Classical Approach – Does not require a knowledge of the behavior of individual
molecules and provides a direct and easy way to analyze engineering problems
Microscopic or Statistical Approach – Based on the average behavior of large groups of
individual molecules
No-Slip Condition – The requirement that at the interface between a fluid and a solid surface,
the fluid velocity and surface velocity are equal. Thus if the surface is fixed, the fluid must obey
the boundary condition that fluid velocity = 0 at the surface
Boundary Layer – The flow region adjacent to the wall in which the viscous effects (and thus
the velocity gradients are significant)
Viscous Flows – Flows in which the frictional effects are significant
Inviscid Flow Regions – In many flows of practical interest, there are regions (typically regions
not close to solid surfaces) where viscous forces are negligibly small compared to inertial or
pressure forces
External Flow – The flow of an unbounded fluid over a surface such as a plate, a wire, or a pipe
Internal Flow – The flow in a pipe or duct if the fluid is bounded by solid surfaces
Incompressible Flow – If the density of flowing fluid remains nearly constant throughout (liquid
flow)
Compressible Flow – If the density of fluid changes during flow (high-speed gas flow)
Mach Number – Nondimensional ratio of the characteristic speed of the flow to the speed of
sound, Mach number characterizes the level of compressibility in response to pressure
variations in the flow
Laminar Flow – The highly ordered fluid motion characterized by smooth layers of fluid. The
flow of high-viscosity fluids such as oils at low velocities is typically laminar
Turbulent Flow – The highly disordered fluid motion that typically occurs at high velocities and
is characterized by velocity fluctuations. The flow of low-viscosity fluids such as air at high
velocities is typically turbulent
Transitional Flow – A flow that alternates between being laminar and turbulent
Forced Flow – A fluid is forced to flow over a surface or in a pipe by external means such as a
pump or a fan
Natural Flow – Fluid motion is due to natural means such as the buoyancy effect, which
manifests itself as the rise of warmer (and thus lighter) fluid and the fall of cooler (and thus
denser) fluid
Steady Flow – No change at a point with time
Unsteady Flow – Opposite of unsteady
Transient – Developing flows (typically)
Uniform – No change with location over a specified region
Periodic – The kind of unsteady flow in which the flow oscillates about a steady mean
Steady-Flow Devices – Devices that operate for long periods of time under the same conditions
(turbines, compressors, boilers, condensers, and heat exchangers)
One- Two- Three-Dimensional Flows – If the flow velocity varies in one, two, or three
dimensions, respectively
Uniform Flow – All fluid properties, such as velocity, pressure, temperature… do not vary with
position
System – A quantity of matter or a region in space chosen for study
Surroundings – The mass or region outside the system
Boundary – The real or imaginary surface that separates the system from its surroundings
Closed System (Control Mass) – A fixed amount of mass, and no mass can cross its boundary
Open System (Control Volume) – A properly selected region in space. It usually encloses a
device that involves mass flow such as a compressor, turbine, or nozzle. Both mass and energy
can cross the boundary of a control volume
Control Surface – The boundaries of a control volume, may be real or imaginary
Dimensions – Used to characterize a physical quantity
Units – The magnitudes assigned to the dimensions
Primary or Fundamental Dimensions – Basic dimensions, measurable
Secondary or Derived Dimensions – Expressed in terms of the primary dimensions
Metric SI System – A simple and logical system based on a decimal relationship between the
various units
English System – It has no apparent systematic numerical base, and various units in this system
are related to each other rather arbitrarily
Dimensional Homogeneity – All equations must be
Unity Conversion Ratios – identically equal to 1 and are unitless, thus such ratios (or their
inverses) can be inserted conveniently into any calculation to properly convert units
Chapter 2:
Property – Any characteristic of a system. Some familiar properties are pressure P, temperature
T, volume V, and mass m
Intensive properties – Those that are independent of the mass of a system, such as
temperature, pressure, and density
Extensive properties – Those whose values depend on the size- or extent- of the system
Specific properties – Extensive properties per unit mass
Continuum – view a substance as continuous, homogenous matter with no holes, allows us to
treat properties as point functions and to assume properties vary continually in space with no
jump discontinuities (valid as long as the size of the system we deal with is large relative to the
space between the molecules)
Specific gravity – The ratio of the density of a substance to the density of some standard
substance at a specified temperature (usually water at 4 degrees C)
Specific weight – The weight unit volume of a substance
Equation of state – Any equation that relates the pressure, temperature, and density (or specific
volume) of a substance
Ideal-gas equation of state – The simplest and best-known equation of state for substances in
the gas phase
Kelvin scale – The thermodynamic temperature scale in the SI system
Rankine scale – The thermodynamic temperature scale in the English system
Ideal gas – A hypothetical substance that obeys the relation Pv=RT, it closely approximates the
P-v-T behavior of real gases at low densities
Saturation temperature – The temperature at which a pure substance changes phase at a given
pressure
Saturation pressure – The pressure at which a pure substance changes phase at a given
temperature
Vapor pressure – The pressure exerted by its vapor in phase equilibrium with its liquid at a
given temperature. It is identical to the saturation pressure of the liquid
Partial pressure – The pressure of a gas or vapor in a mixture with other gases. For example,
atmospheric air is a mixture of dry air and water vapor, and atmospheric pressure is the sum of
the partial pressure of dry air and the partial pressure of water vapor
Cavitation bubbles – vapor bubbles that form cavities in the liquid, caused by liquid pressure in
liquid-flow systems dropping below the vapor pressure at some locations
Cavitation - a mechanism in which vapor bubbles (or cavities) in a fluid grow and collapse due
to local pressure fluctuations
Total Energy – Sum of system energies in several forms- thermal, mechanical, kinetic, potential,
electric, magnetic, chemical, and nuclear
Macroscopic forms of energy – Those a system possesses as a whole with respect to some
outside reference frame, such as kinetic and potential energies
Microscopic forms of energy – Those related to the molecular structure of a system and the
degree of the molecular activity
Internal energy – The sum of all the microscopic forms of energy.
Kinetic Energy – The energy that a system possesses as a result of its motion relative to some
reference frame
Potential Energy – The energy that a system possesses as a result of its elevation in a
gravitational field
Flow energy/ flow work – The energy per unit mass needed to move the fluid and maintain flow
Enthalpy - a thermodynamic quantity equivalent to the total heat content of a system. It is
equal to the internal energy of the system plus the product of pressure and volume
Water Hammer – Characterized by a sound that resembles the sound produced when a pipe is
“hammered”. This occurs when a liquid in a piping network encounters an abrupt flow
restriction (such as a closing valve) and is locally compressed
Isothermal Compressibility – The inverse of the coefficient of compressibility
Coefficient of Volume Expansion – The variation of the density of a fluid with temperature at
constant pressure
Speed of sound (sonic speed) – The speed at which an infinitesimally small pressure wave
travels through a medium
Mach number Ma – The ratio of the actual speed of the fluid (or an object in still fluid) to the
speed of sound in the same fluid at the same state
Viscosity – A property that represents the internal resistance of a fluid to motion or the “fluidity”
Drag Force – The force a flowing fluid exerts on a body in the flow direction. The magnitude of
this force depends, in part, on viscosity