ME 6139: High Speed Aerodynamicsteacher.buet.ac.bd/toufiquehasan/ME 6139 (October 2017)-L-01.pdf ·...

1

ME 6139: High Speed Aerodynamics 1

ME 6139: High Speed Aerodynamics

Dr. A.B.M. Toufique HasanProfessor

Department of Mechanical Engineering, BUET

Lecture-0104 November 2017

teacher.buet.ac.bd/toufiquehasan/[email protected]

M.Sc. Eng., ME, BUET (Oct. 2017)

M.Sc. Eng., ME, BUET (Oct. 2017) ME 6139: High Speed Aerodynamics 2

Aerodynamics is the study of dynamics of gaseous fluids (air/gas), especially the

atmospheric interaction with moving objects.

This field of engineering deals with the aerodynamic forces namely lift and drag

and moments and the heat transfer rates acting on a vehicle in flight.

These mechanical parameters greatly depends on the pattern of flow around the

vehicle. And the resultant flow pattern depends on

• the geometry of the vehicle (i.e. shape of the airfoil: NACA, SC, RAE, ONERA,

BGK, NLR, DLR, OAT… …)

• its orientation with respect to undisturbed free-stream (angle of attack-AOA),

• speed (Mach No.) and

• altitude (density) at which the vehicle is moving.

2

Very recent….

M.Sc. Eng., ME, BUET (Oct. 2017) ME 6139: High Speed Aerodynamics 3

Geosynchronous Satellite Launch Vehicle GSLV-F09, carrying the 2230-kg GSAT-9South Asia satellite, lifted off from the Satish Dhawan Space Centre in Sriharikota,Andhra Pradesh, on Friday 05 May 2017 at 4.57 pm.

This communication satellite can be used for a variety of purposes like broadcast services,tele-medicine and tele-education, banking networks, and direct-to-home television.

Indian Space Research Organisation (ISRO) Satellite GSAT-9 blasts into orbit on 05/05/2017

*Indian Express (India)*Daily Star (Bangladesh)

High speed flows & Compressibility


Compressibility of a fluid is basically a measure of the change of density that will beproduced in the fluid by a specific change in pressure.

In a fluid flow there are usually changes in pressure associated, for example, withchange in velocity in the flow. These pressure changes will in general induce densitychanges, which will have an influence on the flow.

If these density changes are important, the temperature changes are becomingalso important.

The study of flows in which the changes in density and temperature are importantis basically what is known as Aerothermodynamics, compressible fluid flow or gasdynamics.

It usually only being in gas flows that compressibility effects are important.


High speed flows deals with the fluid velocity which is atleast comparable to thespeed of sound. In this massive velocity, the change of fluid density becomes verysignificant and must be accounted for the prediction of other fluid dynamic andthermodynamic property changes. Compressibility plays a great role.

3

Compressibility cont…


A change in the pressure applied to a certain amount of a substance (solid, liquidor gas) always produces some change in its volume.

The proportionate change in volume of a particular material during thecompression is directly related to the change in the pressure.

The compressibility of fluid is defined by

dp

dv

v

1

Here dp represents a small increase in pressure applied to the material and dv the corresponding smallincrease in the original volume v. Since a rise in pressure always causes a decrease in volume, dv is alwaysnegative, and the minus sign is included in the equation to give a positive value of τ

TT p

v

v

1

In case of isothermal process i.e. if the temperature of the fluid element is heldconstant, the isothermal compressibility is defined by

For water, τT = 5x10-10 m2/N at 1 atmair, τT = 10-5 m2/N at 1 atm (more than four orders of magnitude

higher than water)



Mach Number

a

VM

whereV is the flow velocity and a is the local speed of sound in the fluid.

In fluid mechanics, the effect of compressibility in the flow field can be assessedby a number called the “Mach number”. This dimensionless number is defined as

Why speed of sound ? ? ?

This is the speed at which “signal” (disturbance) can travel through the medium. Incase when an object moves through a fluid, it generates disturbances (infinitesimalpressure waves, which are sound waves) that emanate from the object in all director.

When the speed of object becomes comparable or higher than the speed of sound,then the propagation and interaction of disturbance (signal) become complicated anddifferent compared to low speed cases.


4

Flow Classification


In aerodynamics, the following flow classes are classified roughly depending onthe Mach number-

M < 0.3 : Incompressible flow (density effects are negligible)

0.3< M < 0.8 : Subsonic flow, where density effects are important butno appearance of shock waves

0.8< M < 1.2 : Transonic flow, where shock waves first appear, dividing thesubsonic and supersonic flows.

1.2 < M < 3.0 : Supersonic flow, where shock waves are present but thereare no subsonic regions.

M > 3.0 : Hypersonic flow, where shock waves and other flow changesare especially strong.(Surface Chemistry, Plasma dynamics)


Applications of High speed flows


The most obvious applications of compressible flow theory are in the design of highspeed aircraft. These includes:

• Commercial civil aircraft• Military fighters• Ramjet vehicle• Scramjet vehicle• Rockets etc.

However, the knowledge of compressible fluid flow theory is required in the design andoperation of many devices commonly encountered in engineering practice. Amongthese applications are:

• Gas turbines: The flow in the blades and nozzles is compressible.• Steam turbines: here the flow in the nozzles and blades must be treated ascompressible.• Reciprocating engines: the flow of the gases through the valves and in theintake and the exhaust systems must be treated as compressible.• Combustion chambers: the study of combustion, in many cases requires aknowledge of compressible fluid flow.


5

Flight Envelope


Source: T. A. Ward, Aerospace Propulsion Systems, John Wiley & Sons (Asia), Pte Ltd., Singapore (2010)

Engines can operate only over a certain range of altitudes and velocities (Mach numbers)which correspond to differing atmospheric pressure, temperature and densities (allchanges with altitude). This range is known as the engine’s flight envelope.


Atmosphere


Reynolds number and Mach number significantly vary with altitude at the same flow velocity.


6

Shock Waves


Shock waves are obvious in internal or external aerothermodynamics.

The extremely thin region in which the transition from the initial supersonic velocity

(M>1), relatively low-pressure state to the state that involves a relatively low velocity

(M<1) and high pressure is termed as a normal shock wave.

The thickness is usually only a few mean free paths.

Supersonic initial flow (M>1) is mandatory for the generation of a shock wave.


Physical Examples


shadowgraph of supersonic flowaround space crew modules: Mach2.2 flow around an Apollo-like capsuleat 25° angle-of-attack

the essential ingredients of these flowsincluding: presence of multiple shockwaves, separated zones, and wakes, andlarge scale structures.*

shocks

*Source: T. B. Gatski & J P Bonnet, Compressibility, Turbulence and High Speed Flow, Elsevier, The Netherlands (2013)


7


Physical Examples contd…

Flow, M = 2.3


Shock wave/boundary layer interaction (SWBLI)



M>1

Shock waves in Compression ramp flow


8



Flow




Shock wave structure in propulsion C-D nozzle

Symmetric shock waves

Asymmetric shock waves

Flow

Flow

Flow

λ-shock wave


9

17

A generic missile body with Mach 5 embedsmany flow structures occurring simultaneously;oblique shock wave at the tip of the body,expansion fan at the shoulder, a dead airregion at the compression corner due toshockwave- boundary layer interaction(SWBLI).

Schlieren photograph of high speed (transonic)flow over an airfoil.The nearly vertical shock wave is followed byboundary layer separation that adverselyaffects lift, drag, and other flight parameters.


M.Sc. Eng., ME, BUET (Oct. 2017) ME 6139: High Speed Aerodynamics


Shock diamond

Test of a rocket nozzle F-16 Fighter plane



10



Interaction of shock waves in compressor cascade



High speed flow (Compressible flow) in fan (a) and compressor blades (b)

(a) (b)



11


Course OutlineIntroduction to low speed flow and high speed flows; Review of thermodynamics of compressible flow; Integral form of conservation equations;Isentropic flow relations; Normal shock waves; One‐dimensional flow with heat addition and friction; Oblique shock waves; Shock reflections and interactions; Prandtl-Meyer expansion waves; Airfoils and wings in transonic and supersonic flows; Supercritical airfoils;Linearized theory; Method of characteristics for supersonic flow; Design of propulsion nozzles; Characteristics of supersonic jets; Thrust vectoring; Measurement in high speed flows.

Reference TextsModern compressible flow- J D AndersonIntroduction to compressible fluid flow- P H Oosthuizen and W E CarscallenFundamentals of gas dynamics- R D Zucker and O Biblarz


ME 6139: High Speed Aerodynamicsteacher.buet.ac.bd/toufiquehasan/ME 6139 (October 2017)-L-01.pdf ·...

Documents

Transcript of ME 6139: High Speed Aerodynamicsteacher.buet.ac.bd/toufiquehasan/ME 6139 (October 2017)-L-01.pdf ·...