4-1
ME 306 Fluid Mechanics II
Part 4
Compressible Flow
These presentations are prepared by
Dr. Cรผneyt Sert
Department of Mechanical Engineering
Middle East Technical University
Ankara, Turkey
Please ask for permission before using them. You are NOT allowed to modify them.
4-2
Compressibility and Mach Number
โข Compressibility affects become important when a fluid moves with speeds comparable to the local speed of sound (๐).
โข Mach number is the most important nondimensional number for compressible flows
๐๐ = ๐ / ๐
โข ๐๐ < 0.3 Incompressible flow (density changes are negligible)
โข 0.3 < ๐๐ < 0.9 Subsonic flow (density affects are important, but shock waves
do not develop)
โข 0.9 < ๐๐ < 1.1 Transonic flow (shock waves appear and divide the flow field
into subsonic and supersonic regions)
โข 1.1 < ๐๐ < 5.0 Supersonic flow (shock waves are present and there are no
subsonic regions)
โข ๐๐ > 5.0 Hypersonic flow (very strong shock waves and property
changes)
4-3
Review of Ideal Gas Thermodynamics
โข Ideal gas equation of state is
๐ = ๐๐ ๐
where ๐ is the gas constant.
โข By defining specific volume as ๐ฃ = 1/๐ ideal gas law becomes
๐๐ฃ = ๐ ๐
โข For an ideal gas internal energy (๐ข ) is a function of temperature only.
โข Ideal gas specific heat at constant volume is defined as
๐๐ฃ =๐๐ข
๐๐
โข ๐๐ฃ is also a function of temperature, but for moderate temperature changes it can be taken as constant. In this course weโll take ๐๐ฃ as constant.
โข Change in internal energy between two states is (considering constant ๐๐ฃ)
๐ข 2 โ ๐ข 1 = ๐๐ฃ (๐2 โ ๐1)
4-4
Review of Ideal Gas Thermodynamics (contโd)
โข Enthalpy is defined as
โ = ๐ข +๐
๐ = ๐ข + ๐ ๐
โข For an ideal gas enthalpy is also a function of temperature only.
โข Ideal gas specific heat at constant pressure is defined as
๐๐ =๐โ
๐๐
โข ๐๐ will also be taken as constant in this course. For constant ๐๐ change in enthalpy is
โ2 โ โ1 = ๐๐ (๐2 โ ๐1)
โข Combining the definition of ๐๐ฃ and ๐๐
๐๐ โ ๐๐ฃ = ๐โ
๐๐โ๐๐ข
๐๐ = ๐
4-5
Review of Ideal Gas Thermodynamics (contโd)
โข For air
๐๐ โ ๐๐ฃ = ๐
โข Specific heat ratio is used frequently in compressible flow studies
๐ =๐๐
๐๐ฃ
which has a value of 1.4 for air.
โข Combining the above relations we can also obtain
๐๐ =๐ ๐
๐ โ 1 , ๐๐ฃ =
๐
๐ โ 1
1.005๐๐ฝ
๐๐๐พ
0.718๐๐ฝ
๐๐๐พ
0.287๐๐ฝ
๐๐๐พ
4-6
Review of Ideal Gas Thermodynamics (contโd)
โข Entropy change for an ideal gas are expressed with ๐๐๐ relations
๐๐๐ = ๐๐ข + ๐ ๐1
๐ , ๐๐๐ = ๐โ โ
1
๐ ๐๐
โข Integrating these ๐๐๐ relations for an ideal gas
๐ 2 โ ๐ 1 = ๐๐ฃ ๐๐๐2๐1
+ ๐ ๐๐๐1๐2
, ๐ 2 โ ๐ 1 = ๐๐ ๐๐๐2๐1
โ ๐ ๐๐๐2๐1
โข For an adiabatic (no heat transfer) and frictionless flow, which is known as isentropic flow, entropy remains constant.
Exercise : For isentropic flow of an ideal gas with constant specific heat values, derive the following commonly used relations, known as isentropic relations
๐2๐1
๐/(๐โ1)
=๐2๐1
๐
=๐2๐1
4-7
Speed of Sound (๐)
โข Speed of sound is the rate of propagation of a pressure pulse (wave) of infinitesimal strength through a still medium (a fluid in our case).
โข It is a thermodynamic property of the fluid.
โข For air at standard conditions, sound moves with a speed of ๐ = 343 ๐/๐
http://paws.kettering.edu/~drussell/demos.html
4-8
Speed of Sound (contโd)
โข To obtain a relation for the speed of sound consider the following experiment
โข A duct is initially full of still gas with properties ๐, ๐, ๐ and ๐ = 0
โข Piston is pushed into the fluid with an infinitesimal velocity of ๐๐
โข A pressure wave of infinitesimal strength will form and itโll travel in the gas with the speed of sound ๐.
โข As it passes over the gas particles it will create infinitesimal property changes.
๐ ๐
๐ ๐ = 0
๐๐ ๐ + ๐๐ ๐ + ๐๐
๐ ๐ ๐
๐ + ๐๐ ๐๐
๐ ๐ = 0
Moving wave front
4-9
Speed of Sound (contโd)
โข For an observer moving with the wave front with speed ๐, wave front will be stationary and the fluid on the left and the right would move with relative speeds
โข Consider a control volume enclosing the stationary wave front. The flow is one dimensional and steady.
๐ ๐
๐ ๐ = ๐
Stationary wave front
๐ + ๐๐ ๐ + ๐๐
๐ + ๐๐ ๐ = ๐ โ ๐๐
in out
Cross sectional area ๐ด
4-10
Speed of Sound (contโd)
โข Continuity equation for the control volume
๐ ๐๐ = ๐ ๐๐ข๐ก
๐๐ด๐ = ๐ + ๐๐ ๐ด(๐ โ ๐๐)
๐๐ด๐ = ๐๐ด๐ โ ๐ ๐ด ๐๐ + ๐ด ๐ ๐๐ โ ๐ด ๐๐ ๐๐
๐๐ =๐
๐ ๐๐
โข Linear momentum equation in the flow direction is (consider only pressure forces, but no viscous forces since they are negligibly small for the process of interest)
๐น = ๐ + ๐๐ ๐ด โ ๐๐ด = ๐ ๐๐ ๐ โ ๐ ๐๐ข๐ก (๐ โ ๐๐)
Negligibly small term
๐ ๐๐ = ๐ ๐๐ข๐ก = ๐๐ด๐
4-11
Speed of Sound (contโd)
โข Momentum equation simplifies to
๐๐ =1
๐๐ ๐๐
โข Combining continuity and momentum equation results
๐ =๐๐
๐๐๐
Exercise : In deriving speed of sound equation, we did not make use of the energy equation. Show that it gives the same result.
Exercise : What is the speed of sound for a perfectly incompressible fluid.
Exercise : Show that speed of sound for an ideal gas is equal to
๐ = ๐๐ ๐
Propagation of a sound wave is an isentropic process
4-12
Wave Propagation in a Compressible Fluid
โข Consider a point source generating small pressure pulses (sound waves) at regular intervals.
โข Case 1 : Stationary source
โข Waves travel in all directions symmetrically.
โข The same sound frequency will be heard everywhere around the source.
4-13
Wave Propagation in a Compressible Fluid (contโd)
โข Case 2 : Source moving with less than the speed of sound (๐๐ < 1)
โข Waves are not symmetric anymore.
โข An observer will hear different sound frequencies depending on his/her location.
โข This asymmetry is the cause of the Doppler effect.
4-14
Wave Propagation in a Compressible Fluid (contโd)
โข Case 3 : Source moving the speed of sound (๐๐ = 1)
โข The source moves with the same speed as the sound waves it generates.
โข All waves concentrate on a plane passing through the moving source creating a Mach wave, across which there is a significant pressure change.
โข Mach wave separates the filed into two as zone of silence and zone of action.
Zone of
action
Zone of silence
๐ = ๐
โข First aircraft exceeding the speed of sound : http://en.wikipedia.org/wiki/Bell_X-1
4-15
Wave Propagation in a Compressible Fluid (contโd)
โข Case 4 : Source moving with more than the speed of sound (๐๐ > 1)
โข The source travels faster than the sound it generates.
โข Mach cone divides the field into zones of action and silence.
โข Half angle of the Mach cone is called the Mach angle ๐.
Zone of action
Zone of silence
๐ > ๐
๐
4-16
Wave Propagation in a Compressible Fluid (contโd)
Exercise : For โโCase 4โโ described in the previous slide show that
sin ๐ = 1/๐๐
Exercise : A supersonic airplane is traveling at an altitude of 4 ๐๐. The noise generated by the plane at point A reached the observer on the ground at point B after 20 ๐ . Assuming isothermal atmosphere, determine
a) Mach number of the airplane b) velocity of the airplane c) distance traveled by the airplane before the observer hears the noise d) temperature of the atmosphere
A
B
๐ป = 4 ๐๐
๐ฟ = 5 ๐๐ F/A-18 breaking
the sound barrier
http://en.wikipedia.org
4-17
One Dimensional, Isentropic, Compressible Flow
โข Consider an internal compressible flow, such as the one in a duct of variable cross sectional area
โข Flow and fluid properties inside this nozzle change due to
โข Cross sectional area change
โข Frictional effects
โข Heat transfer effects
โข In ME 306 weโll only study these flows to be one dimensional and consider only the effect of area change, i.e. assume isentropic flow.
NOT the subject of ME 306
โข Energy balance between sections 1 and 2 is
โ1 +๐12
2+ ๐๐ง1 = โ2 +
๐22
2+ ๐๐ง2 โ ๐ + ๐ค๐
โข For gas flows potential energy change is negligibly small compared to kinetic energy change. Energy equation reduces to
โ1 +๐12
2= โ2 +
๐22
2
4-18
1D, Isentropic Flow (contโd)
1 2
Heat transfer and friction work is neglected for
isentropic flow
โข The sum โ +๐2
2 is known as stagnation enthalpy and it is constant inside the duct.
โ0 = โ +๐2
2 = constant
โข It is called โโstagnationโโ enthalpy because a stagnation point has zero velocity and the enthalpy of the gas is equal to โ0 at such a point.
4-19
Stagnation Enthalpy
stagnation enthalpy
Fluid in this large reservoir is almost stagnant. This reservoir is said to be at stagnation state.
โข Stgnation state is a reference state used in compressible flow calculations.
โข It is the state achieved if a fluid at any other state is brought to rest isentropically.
โข For an isentropic flow there will a unique stagnation state.
4-20
Stagnation State
State 1 โ1, ๐1, ๐1, ๐1, etc.
Isentropic deceleration
Isentropic deceleration
State 2 โ2, ๐2, ๐2, ๐2, etc.
Unique stagnation state โ0, ๐0 = 0, ๐0, ๐0, etc.
โข Isentropic deceleration can be shown on a โ โ ๐ diagram as follows
4-21
Stagnation State (contโd)
โข During isentropic deceleraion entropy remains constant.
โข Energy conservation: โ0 +02
2= โ +
๐2
2 โ โโ = โ0 โ โ =
๐2
2
Isentropic deceleration
โ0
โ
โ
๐
๐0
๐
๐2/2
Any state โ, ๐, ๐, ๐, ๐ , etc.
Stagnation state โ0, ๐0 = 0, ๐0, ๐0, ๐ 0, etc.
โข For an ideal gas this enthalpy change can be expressed as a temperature change
๐๐โ๐ = โโ
๐๐(๐0 โ ๐) =๐2
2
๐0 = ๐ +๐2
2๐๐
โข During the isentropic deceleration temperature of the gas increases by ๐2
2๐๐.
Exercise : An airplane is crusing at a speed of 900 ๐๐/โ at an altitude of 10 ๐๐. Atmospheric air at โ60 โ comes to rest at the tip of its pitot tube. Determine the temperature rise of air.
Read about heating of space shuttle during its reentry to the earthโs atmosphere.
http://en.wikipedia.org/wiki/Space_Shuttle_thermal_protection_system 4-22
Stagnation State (contโd)
Exercise : For the flow of air as an ideal gas, express the following ratios as a function of Mach number and generate the following plot.
๐
๐0
๐
๐0
๐
๐0
๐
๐0
4-23
Stagnation State (contโd)
1.0
0.5
0 1 2 3 4 5
Adapted from Whiteโs Fluid Mechanics book
๐๐
๐
๐0 ๐
๐0
๐
๐0 ๐
๐0
Exercise : Using Bernoulliโs equation, derive an expression for ๐0/๐ for incompressible flows. Compare it with the one derived in the previous exercise and determine the Mach number below which two equations agree within engineering accuracy.
4-24
Stagnation State (contโd)
2.0
1.4
0 0.2 0.4 0.6 0.8 1
Adapted from Fox, Pritchard & McDonaldโs book
๐๐
1.8
1.6
1.2
1.0
Compressible
Incompressible ๐0๐
Exercise : An aircraft cruises at 12 km altitude. A pitot-static tube on the nose of the aircraft measures stagnation and static pressures of 2.6 kPa and 19.4 kPa. Calculate
a) the flight Mach number of the aircraft
b) the speed of the aircraft
c) the stagnation temperature that would be sensed by a probe on the aircraft.
Exercise : Consider the differential control volume shown below for 1D, isentropic flow of an ideal gas through a variable area duct. Using conservation of mass, linear momentum and energy, determine the
a) Change of pressure with area
b) Change of velocity with area
4-25
Simple Area Change Flows (1D Isentropic Flows)
๐ ๐ ๐ โ ๐ด
๐ + ๐๐ ๐ + ๐๐ ๐ + ๐๐ โ + ๐โ ๐ด + ๐๐ด
๐๐ฅ
๐ฅ
4-26
Simple Area Change Flows (contโd)
โข Results of the previous exercise are
๐๐
๐๐2=๐๐ด
๐ด
1
1 โ ๐๐2 ,
๐๐
๐= โ
๐๐ด
๐ด
1
1 โ๐๐2
๐๐ด > 0
Diffuser
๐๐ > 0
๐๐ < 0
Nozzle
๐๐ < 0
๐๐ > 0
Subsonic Flow ๐๐ < 1 Supersonic Flow ๐๐ > 1
๐๐ด < 0
๐๐ด < 0
๐๐ด > 0
4-27
Simple Area Change Flows (contโd)
โข Sonic flow is a very special case. It can occur
โข when the cross sectional area goes through a minimum, i.e. ๐๐ด = 0
โข or at the exit of a subsonic nozzle or a supersonic diffuser
๐๐ < 1
๐๐ > 1
Sonic flow may occur at the throat.
Sonic flow may occur at these exits.
4-28
Simple Area Change Flows (contโd)
Exercise : The nozzle shown on the right is called a converging diverging nozzle (de Laval nozzle). Show that it is the only way to
โข isentropically accelerate a fluid from subsonic to supersonic speed.
โข isentropically decelerate a fluid from supersonic to subsonic speed.
Exercise : When subsonic flow is accelerated in a nozzle, supersonic flow can never be achieved. At most Mach number can be unity at the exit.
What happens if we add another converging part to the exit of such a nozzle?
de Laval nozzle
๐๐ < 1 ๐๐๐๐ฅ๐๐ก = 1
๐๐ < 1
๐๐๐๐ฅ๐๐ก =?
4-29
Critical State
โข Critical state is the special state where Mach number is unity.
โข It is a useful reference state, similar to stagnation state. It is useful even if there is no actual critical state in a flow.
โข It is shown with an asterisk, like ๐โ, ๐โ, etc.
โข Ratios derived in Slide 4-23 can be written using the critical state
๐๐๐โ
= 1 +๐ โ 1
2
๐/(๐โ1)
๐๐๐โ
= 1 +๐ โ 1
2
๐๐๐โ
= 1 +๐ โ 1
2
1/(๐โ1)
๐๐๐= 1 +
๐ โ 1
2๐๐2
๐/(๐โ1)
๐๐๐= 1 +
๐ โ 1
2๐๐2
๐๐๐= 1 +
๐ โ 1
2๐๐2
1/(๐โ1)
๐๐ = 1
4-30
Critical State (contโd)
Exercise : Similar to the ratios given in the previous slide, following area ratio is also a function of Mach number and specific heat ratio only. Derive it.
๐ด
๐ดโ=
1
๐๐
1 +๐ โ 12
๐๐2
๐ + 12
๐+12(๐โ1)
3.0
0.5
0 0.5 1 1.5 2 2.5 3
Adapted from Fox, McDonald and Pritchardโs textbook ๐ = 1.4
๐๐
2.5
2.0
1.5
1.0
0
๐ด
๐ดโ
4-31
Isentropic Flow Table
โข It provides the following ratios at different Mach numbers.
๐
๐0
๐
๐0
๐
๐0
๐ด
๐ดโ
Akselโs Fluid Mechanics textbook
4-32
Simple Area Change Flows (contโd)
Exercise : Derive an expression for the mass flow rate term that appears in the last column of the table given in the previous slide.
Exercise : A converging duct is fed with air from a large reservoir where the temperature and pressure are 350 K and 200 kPa. At the exit of the duct, cross-sectional area is 0.002 ๐2 and Mach number is 0.5. Assuming isentropic flow
a) Determine the pressure, temperature and velocity at the exit.
b) Find the mass flow rate.
Exercise : Air is flowing isentropically in a diverging duct. At the inlet of the duct, pressure, temperature and velocity are 40 kPa, 220 K and 500 m/s, respectively. Inlet and exit areas are 0.002 ๐2 and 0.003 ๐2.
a) Determine the Mach number, pressure and temperature at the exit.
b) Find the mass flow rate.
4-33
Simple Area Change Flows (contโd)
Exercise : Air flows isentropically in a channel. At an upstream section 1, Mach number is 0.3, area is 0.001 ๐2, pressure is 650 kPa and temperature is 62 โ. At a downstream section 2, Mach number is 0.8.
a) Sketch the channel shape.
b) Evaluate properties at section 2.
c) Plot the process between sections 1 and 2 on a ๐ โ ๐ diagram.
4-34
Shock Waves
โข Waves are disturbances (property changes) moving in a fluid.
โข Sound wave is a weak wave, i.e. property changes across it are infinitesimally small.
โข โ๐ across a sound wave is in the order of 10โ9 โ 10โ3 ๐๐ก๐.
โข Shock wave is a strong wave, i.e. property changes across it are finite.
โข Shock waves are very thin, in the order of 10โ7 ๐.
โข Fluid particles decelerate with tens of millions of ๐โs through a shock wave.
โข They can be stationary or moving.
โข They can be normal (perpendicular to the flow direction) or oblique (inclined to the flow direction).
โข In ME 306 weโll consider normal shock waves for 1D flows inside channels.
4-35
Oblique shock wave ahead of a bullet moving at a supersonic speed
Normal shock wave in a supersonic nozzle. Flow is from left to right. Extra waves are due to surface roughness
Whiteโs Fluid Mechanics textbook
Shock Waves (contโd)
4-36
Formation of a Strong Wave
โข Strong wave are formed by the accumulation of weak compression waves.
โข Compression waves are the ones across which pressure increase and velocity decrease in the flow direction.
โข Sound wave is an example of weak compression waves.
โข Consider a piston pushed with a finite velocity ๐ in a cylinder filled with still gas.
โข We can decompose pistonโs motion into a series of infinitesimally small disturbances.
โข Weak compression waves will emerge from the piston, one after the other.
โข The first two of such waves are sketched below.
๐ ๐1 ๐ ๐
๐ = 0
First wave front
๐2
Second wave front
๐ + ๐๐ ๐ + ๐๐ ๐๐
4-37
Formation of a Strong Wave (contโd)
โข First wave will cause an increase in temperature behind it.
โข Second wave will move faster and eventually catch the first one.
๐2 > ๐1
โข A third one, which is not shown, will move even faster and catch the first two waves.
โข At the end all the waves will accumulate into a strong wave of finite strength.
โข Weak expansion waves thatโll be generated by pulling the piston to the left will not form such a strong wave.
๐ ๐ ๐
๐ = 0
Strong wave
๐ + โ๐ ๐ + โ๐ โ๐
4-38
Property Changes Across a Shock Wave
โข Consider a stationary normal shock wave in a
duct of variable cross sectional area.
โข Upstream and downstream states are denoted
by ๐ฅ and ๐ฆ.
๐ฅ ๐ฆ
โข Due to very sudden and finite property changes, the process across the wave is
considered to be non-isentropic.
โข Therefore there are two different stagnation states, state 0๐ฅ for the flow before the
shock and state 0๐ฆ for the flow after the shock.
๐0๐ฅ โ ๐0๐ฆ
โข However, considering the flow to be adiabatic, stagnation temperatures of these
states are identical.
๐0๐ฅ = ๐0๐ฆ = ๐0 and โ0๐ฅ = โ0๐ฆ = โ0
โข Stagnation state concept can also be used for non-isentropic flows, but there will be multiple such states.
โข โ0 , ๐0 and ๐0 will be unique but not other stagnation properties such as ๐0 or ๐0.
4-39
Stagnation State of a Non-isentropic Flow
State 1 โ1, ๐1, ๐1, ๐1, etc.
Isentropic deceleration
Isentropic deceleration
State 2 โ2, ๐2, ๐2, ๐2, etc.
Stagnation state 2 โ0, ๐0 = 0, ๐02, ๐02, ๐0, etc.
Stagnation state 1 โ0, ๐0 = 0, ๐01, ๐01, ๐0, etc.
Non-isentropic flow
โข Adiabatic stagnation is reached if the deceleration from a state is not isentropic, but only adiabatic.
4-40
Adiabatic Stagnation State
โข During adiabatic deceleration entropy increases.
โข But the achieved adiabatic stagnation state will have the same stagnation enthalpy โ0 and therefore same stagnation temperature ๐0 as isentropic stagnation state.
Isentropic deceleration
โ0
โ
โ
๐
๐0
๐
Any state
(Isentropic ) stagnation state ๐0,๐๐๐๐๐๐๐ก๐๐
Adiabatic stagnation state Adiabatic
deceleration
4-41
Property Changes Across a Shock Wave (contโd)
โข Governing equations for the 1D flow inside the control volume enclosing the shock wave are
โข Continuity : ๐ = ๐๐ฅ๐๐ฅ๐ด = ๐๐ฆ๐๐ฆ๐ด where ๐ด = ๐ด๐ฅ = ๐ด๐ฆ
โข Momentum : ๐๐ฅ โ ๐๐ฆ ๐ด = ๐ ๐๐ฆ โ ๐๐ฅ
โข Energy : โ0 = โ๐ฅ +๐๐ฅ2
2 = โ๐ฆ +
๐๐ฆ2
2
โข Second Law : ๐ ๐ฆ > ๐ ๐ฅ
๐ฅ ๐ฆ
4-42
Property Changes Across a Shock Wave (contโd)
โข For the flow of an ideal gas with constant specific heats, these equations can be simplified as follows
โข Donwstream Mach number :
โข Temperature change :
โข Pressure change :
โข Density change :
โข Velocity change :
๐๐๐ฆ =๐ โ 1 ๐๐๐ฅ
2 + 2
2๐๐๐๐ฅ โ (๐ โ 1)
๐๐ฆ
๐๐ฅ=
1 +๐ โ 12 ๐๐๐ฅ
2 2๐๐ โ 1
๐๐๐ฅ2 โ 1
๐ + 1 2
2(๐ โ 1)๐๐๐ฅ
๐๐ฆ
๐๐ฅ=
2๐
๐ + 1๐๐๐ฅ
2 โ๐ โ 1
๐ + 1
๐๐ฆ
๐๐ฅ=
(๐ + 1)๐๐๐ฅ2
2 + (๐ โ 1)๐๐๐ฅ2
๐๐ฆ
๐๐ฅ=๐๐ฅ๐๐ฆ
4-43
Property Changes Across a Shock Wave (contโd)
โข Stagnation pressure change :
โข Critical area change :
โข Entropy change :
โข According to the last equation for all known values of ๐ entropy increase occurs only if ๐๐๐ฅ > 1.
โข Therefore a shock wave can occur only if the incoming flow is supersonic.
Exercise : Show that flow after the shock should be subsonic, i.e. ๐๐๐ฆ < 1
๐0๐ฆ
๐0๐ฅ=
๐ + 12
๐๐๐ฅ2
1 +๐ โ 12 ๐๐๐ฅ
2
๐๐โ1
2๐
๐ + 1๐๐๐ฅ
2 โ๐ โ 1
๐ + 1
๐ด๐ฆโ
๐ด๐ฅโ =
๐0๐ฅ๐0๐ฆ
๐ ๐ฆ โ ๐ ๐ฅ
๐ = โ๐๐
๐0๐ฆ
๐0๐ฅ
4-44
Property Changes Across a Shock Wave (contโd)
โข All these relations are given as functions of ๐๐๐ฅ and ๐ only.
โข Usually graphical or tabulated forms of them are used.
Akselโs Fluid Mechanics textbook
4-45
Property Changes Across a Shock Wave (contโd)
6
1 1.5 2 2.5 3 3.5 4
Adapted from Whiteโs Fluid Mechanics book
๐๐๐ฅ
5
4
3
2
1
0
๐๐ฆ
๐๐ฅ
๐๐ฆ
๐๐ฅ
๐ด๐ฆโ
๐ด๐ฅโ =
๐0๐ฅ๐0๐ฆ
๐๐ฅ๐๐ฆ
=๐๐ฆ
๐๐ฅ
๐0๐ฆ
๐0๐ฅ
๐๐๐ฆ
โข Across a normal shock wave
๐๐, ๐, ๐0 decreases
๐, ๐, ๐, ๐ดโ, ๐ increases
๐0 remains the same
โข Kinetic energy of the fluid after the shock
wave is smaller than the one that would
be obtained by a reversible compression
between the same pressure limits.
โข Lost kinetic energy is the reason of
temperature increase across the shock
wave.
4-46
Normal Shock Wave (contโd)
Exercise : Air traveling at a Mach number of 1.8 undergoes a normal shock wave.
Stagnation properties before the shock are known as ๐0๐ฅ = 150 ๐๐๐, ๐0๐ฅ = 350 ๐พ.
Determine ๐๐ฆ , ๐๐ฆ , ๐๐๐ฆ , ๐๐ฆ , ๐0๐ฆ , ๐0๐ฆ , ๐ ๐ฆ โ ๐ ๐ฅ
Exercise : Supersonic air flow inside a diverging duct is slowed down by a normal
shock wave. Mach number at the inlet and exit of the duct are 2.0 and 0.3. Ratio of
the exit to inlet cross sectional areas is 2. Pressure at the inlet of the duct is 40 kPa.
Determine the pressure after the shock wave and at the exit of the duct.
4-47
Operation of a Converging Nozzle
โข Consider a converging nozzle.
โข Gas is provided by a large reservoir with stagnation properties, ๐0 and ๐0.
โข Back pressure ๐๐ is adjusted using a vacuum pump and a valve to obtain different flow
conditions inside the nozzle.
โข Weโll differentiate between exit pressure ๐๐ and back pressure ๐๐. They are often
equal, but not always.
๐0
๐0 ๐๐
๐๐
4-48
Operation of a Converging Nozzle (contโd)
โข First set ๐๐ = ๐0. There will be no flow.
โข Gradually decrease ๐๐. Following pressure distributions will be observed.
๐
๐ฅ
1 : No flow (๐๐ = ๐0) ๐0
๐โ
2 : ๐โ < ๐๐ < ๐0
3 : Critical (๐๐ = ๐โ)
Subcritical regime
4 : ๐๐ < ๐โ Supercritical regime
4-49
Choked Flow
โข Flow inside the converging nozzle always remain subsonic.
โข For the subcritical regime as we decrease ๐๐ mass flow rate increases.
โข State shown with * is the critical state. When ๐๐ is lowered to the critical value , exit
Mach number reaches to 1 and flow is said to be choked.
โข If ๐๐ is lowered further, flow remains choked. Pressure and Mach number at the exit
does not change. Mass flow rate through the nozzle does not change.
โข For ๐๐ < ๐โ, gas exits the nozzle as a supercritical jet with ๐๐ > ๐๐. It undergoes
through a number of alternating expansion waves and shocks and its cross sectional
area periodically becomes thinner and thicker.
4-50
Operation of a Converging Nozzle (contโd)
โข Case 1 is the no flow case.
โข From case 1 to case 3 ๐๐ drops and ๐ increases.
โข Case 3 is the critical case with minimum possible ๐๐ and maximum possible ๐ .
๐๐
2
1
3 4
๐โ ๐0
๐
๐ ๐๐๐ฅ
Variation of ๐ with ๐๐
2
1
3 4
๐0
๐๐ ๐โ ๐0
๐๐
Variation of ๐๐ with ๐๐
4-51
Operation of a Conv-Div Nozzle
โข Similar to the case of converging nozzle, we again first set ๐๐ = ๐0 and than gradually
decrease ๐๐.
Throat
๐
๐ฅ
1 : No flow (๐๐ = ๐0) ๐0
๐โ 5y : Shock at the exit
Same ๐
2 : Subsonic Flow 3 : Choked flow (๐๐๐กโ๐๐๐๐ก = 1)
4 : Flow with shock
6 : Overexpansion
8 : Underexpansion
7 : Design condition
4-52
Operation of a Conv-Div Nozzle
โข Flow inside the converging section is always subsonic.
โข At the throat the flow can be subsonic or sonic.
โข If ๐๐ = 1 at the throat than the flow is called choked. This corresponds to the
maximum flow rate that can pass through the nozzle.
โข Under choked conditions the flow in the diverging part can be subsonic (case 3) or
supersonic (cases 6, 7 ,8).
โข Depending on ๐๐ there may be a shock wave in the diverging part. Location of the
shock wave is determined by ๐๐.
โข Design condition corresponds to the choked flow with supersonic exit without a shock.
โข Overexpansion : ๐๐ < ๐๐. Exiting jet finds itself in a higher pressure medium and
contracts. Underexpansion : ๐๐ > ๐๐. Exiting jet finds itself in a lower pressure
medium and expands. For details and pictures visit http://aerorocket.com/Nozzle/Nozzle.html
and http://www.aerospaceweb.org/question/propulsion/q0224.shtml
4-53
Operation of a Conv-Div Nozzle (contโd)
5y
1 3
8
๐0
๐๐ ๐0
๐๐
5x 7 6
4 2
Variation of ๐๐ with ๐๐
๐๐๐๐ ๐๐๐
๐
๐ฅ
1 ๐0
๐โ 5y
2 3 4
6
8 7
Throat
2
1
3 4
5y
6
7
8
๐๐ ๐0
๐
๐ ๐๐๐ฅ
Variation of ๐ with ๐๐
4-54
Operation of a Conv-Div Nozzle (contโd)
Exercise : Air is supplied to a C-D nozzle from a large reservoir where stagnation
pressure and temperature are known. Determine
a) the Mach number, pressure and temperature at the exit
b) the mass flow rate
๐0 = 318 ๐พ
๐0 = 327 ๐๐๐
๐ด๐ = 0.0038 ๐2
๐๐ = 30 ๐๐๐
๐ด๐ก = 0.0022 ๐2
4-55
Operation of a Conv-Div Nozzle (contโd)
Exercise : Air flows in a Conv-Div nozzle with an exit to throat area ratio of 2.1.
Properties at a section in the converging part are as shown below.
a) Determine the ranges of back pressure for subsonic, non-isentropic (with shock),
overexpansion and underexpansion flow regimes.
b) If there is a shock wave where the area is twice of the throat area, determine the
back pressure.
๐1 = 128 ๐๐๐
๐1 = 294 ๐พ
๐1 = 135 ๐/๐
1