microsoft word - project report

8/22/2019 Microsoft Word - Project Report

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CHAPTER1

INTRODUCTION

A Nozzle is a device designed to control the direction or

characteristics of a fluid flow (especially to increase velocity) as it

exits (or enters) an enclosed chamber or pipe via an orifice. A nozzle is

often a pipe or tube of varying cross sectional area and it can be used to

direct or modify the flow of a fluid (liquid or gas). Nozzles are

frequently used to control the rate of flow, speed, direction, mass,

shape, and/or the pressure of the stream that emerges from the

Frequently the goal is to increase the kinetic energy of the flowing

medium at the expense of its pressure and internal energy.

Nozzles can be described as convergent (narrowing down from a

wide diameter to a smaller diameter in the direction of the flow) or

divergent (expanding from a smaller diameter to a larger one). A de

Laval nozzle has a convergent section followed by a divergent section

and is often called a nozzle. Convergent nozzles accelerate subsonic

fluids. If the nozzle pressure ratio is high enough the flow will reach

sonic velocity at the narrowest point (i.e. the nozzle throat). In this

situation, the nozzle is said to be choked.

Increasing the nozzle pressure ratio further will not increase the

throat Mach number beyond unity. Downstream (i.e. external to the

nozzle) the flow is free to expand to supersonic velocities. Note that

the Mach 1 can be a very high speed for a hot gas; since the speed of

sound varies as the square root of absolute temperature. Thus the speed

reached at a nozzle throat can be far higher than the speed of sound at


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sea level. This fact is used extensively in rocketry where hypersonic

flows are required, and where propellant mixtures are deliberately

chosen to further increase the sonic speed.

Divergent nozzles slow fluids, if the flow is subsonic, but

accelerate sonic or supersonic fluids. Convergent can therefore

accelerate fluids that have choked in the convergent section to

supersonic speeds. This CD process is more efficient than allowing a

convergent nozzle to expand supersonically externally. The shape of

the divergent section also ensures that the direction of the escaping

gases is directly backwards, as any sideways component would not

contribute to thrust. Jet exhaust produces a net thrust from the energy

obtained from combusting fuel which is added to the inducted air. This

hot air is passed through a high speed nozzle, a propelling nozzle

which enormously increases its kinetic energy.

For a given mass flow, greater thrust is obtained with a higher

exhaust velocity, but the best energy efficiency is obtained when the

exhaust speed is well matched with the airspeed. However, no jet

aircraft can maintain velocity while exceeding its exhaust jet speed,

due to momentum considerations. Supersonic jet engines, like those

employed in fighters and SST aircraft (e.g. Concorde), need high

exhaust speeds. Therefore supersonic aircraft very typically use a CD

nozzle despite weight and cost penalties. Subsonic jet engines employ

relatively low, subsonic, exhaust velocities. They thus employ simple

convergent nozzles. In addition, bypass nozzles are employed giving

even lower speeds.


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Rocket motors use convergent-divergent nozzles with very large

area ratios so as to maximise thrust and exhaust velocity and thus

extremely high nozzle pressure ratios are employed. Mass flow is at a

premium since all the propulsive mass is carried with vehicle, and very

high exhaust speeds are desirable.

Most modern passenger and military aircraft are powered by gas

turbine engines, which are also called jet engines. There are several

different types of gas turbine engines, but all turbine engines have

some parts in common. All gas turbine engines have a nozzle to

produce thrust, to conduct the exhaust gases back to the free stream,

and to set the mass flow rate through the engine. The nozzle sits

downstream of the power turbine.

A nozzle is a relatively simple device, just a specially shaped

tube through which hot gases flow. However, the mathematics which

describes the operation of the nozzle takes some careful thought. As

shown above, nozzles come in a variety of shapes and sizes depending

on the mission of the aircraft. Simple turbojets, and turboprops, often

have a fixed geometry convergent nozzle. Turbofan engines often

employ a co-annular nozzle as shown at the top left. The core flow

exits the centre nozzle while the fan flow exits the annular nozzle.

Mixing of the two flows provides some thrust enhancement and these

nozzles also tend to be quieter than convergent nozzles. Afterburning

turbojets and turbofans require a variable geometry convergent-

divergent - CD nozzle. In this nozzle, the flow first converges down to

the minimum area or throat, and then is expanded through the

divergent section to the exit at the right. The variable geometry causes

these nozzles to be heavier than a fixed geometry nozzle, but variable


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geometry provides efficient engine operation over a wider airflow

range than a simple fixed nozzle.

Rocket engines also use nozzles to accelerate hot exhaust toproduce thrust. Rocket engines usually have a fixed geometry CD

nozzle with a much larger divergent section than is required for a gas

turbine. All of the nozzles we have discussed thus far are round tubes.

Recently, however, engineers have been experimenting with nozzles

with rectangular exits. This allows the exhaust flow to be easily

deflected, or vectored. Changing the direction of the thrust with the

nozzle makes the aircraft much more manoeuvrable.

Because the nozzle conducts the hot exhaust back to the free

stream, there can be serious interactions between the engine exhaust

flow and the airflow around the aircraft. On fighter aircraft, in

particular, large drag penalties can occur near the nozzle exits. As with

the inlet design, the external nozzle configuration is often designed by

the airframes and subjected to wind tunnel testing to determine the

performance effects on the airframe. The internal nozzle is usually the

responsibility of the engine manufacturer.

1.1 INTRODUCTION TO JET

Jet is a free shear flow driven by momentum introduced at the

nozzle exit of, usually, a nozzle or am orifice which exhibits a

characteristic that, the ratio of width to axial distance is a constant.

The jet may also define as a continuous fluid flow issuing from an

orifice into a medium of lower speed fluid. As the jet fluid travels

further away from its origin, it slows down due to mixing with slower

speed ambient fluid. This is due to boundary layer at the nozzle exit


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which develops roll up structure, or ring vortices, which grow in size

when they move downstream, due to the entrainment of ambient fluid

into jet stream. Thus, mass flow at any cross-section of the jet

progressively increases along the downstream direction. Hence, to

converse momentum the centreline velocity decreases with

downstream distance. The resulting centreline velocity decay as

proportional to gradient across the shear layer and is a strong function

of distance downstream of the exit. The vast quanta of knowledge

presently available and continuous research currently being carried out

stand testimony to the importance associated with the jet flows.

1.1.1 CLASSIFICATION OF JETS

Basically jets can be classified into two categories namely;

incompressible and compressible jets Fig.1.1. The jets with Mach

number less than 0.3, up to which the compressibility effects are

negligible are called incompressible jets. Compressible jets can be

again subdivided into subsonic, sonic and supersonic jets. Jets with

Mach number 1.0 are called sonic jets, which can be correctly

expanded or under expanded. Supersonic jets are the jets with Mach

number more than one. These can be further classified into over

expanded, correctly expanded and under expanded jets.

1.1.2REGIMES OF JETS

A Schematic diagram of a typical subsonic jet and the different

flow zones are shown in Fig. 1.2 the flow regimes in the subsonic jets

are classified as follows:


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Fig 1.1 Classification of jets.

a. Potential core region: This the region consists of a coreconstant axial velocity close to the jet exit velocity surrounded

by a rapidly growing and strongly sheared annulus of mixing

layer or shear layer with intense turbulence. Potential core region

extends about 5 times the nozzle exit diameter (D) downstream

from the nozzle exit. This is because, the mixing initiated at the

jet boundaries has not yet permeated into the entire flow field,

thus leaving a region that is characterized by a constant axial

velocity.

b. Transition region: This is the region where the centerlinevelocity begins to decay. This characteristic decay zone extends

from about 5D to 10D downstream over which the turbulence

JETS

INCOMPRESSIBLE

0


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changes from its annular to a somewhat pseudo-cylindrical

distribution. As a result, the velocity difference between the

ambient fluid and the high speed core of the jet decrease and

attenuates the shear that supports the vortical rings in the jet and

thus the velocity profiles become smoother with jet propagation.

c. Fully developed region: Beyond the transition region the jetbecomes similar in appearance to a flow of fluid from a source of

infinitely small thickness In reality the jet velocity becomes

insignificant after about 30D

Fig.1.2 Schematic of different zones in a subsonic jet.

1.2 NEED OF JET CONTROL

There are numerous system, especially in the aerospace, where

the ability to enhance the mixing characteristics of a jet will greatly

improve their performance. For example, by increasing the rate of

mixing between air and fuel, the efficiency of a combustion cycle can

be improved. Other examples of technological application requiring

control of mixing in compressible flows include thrust augmenting


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ejectors, thrust vector control, metal deposition, and gas dynamic

lasers.

1.3 TYPES OF CONTROLS

Control may be defined as the ability to modify the flow

characteristics of jets. Jet controls can be broadly classified into active

and passive controls. Both active and passive controls mainly aim at

modifying the flow and noise characteristics.

1.3.1 ACTIVE CONTROL

In active control, an auxiliary power source (like micro jets) is

used to control the jet characteristics. Many active jet control methods

use energized actuators to dynamically manipulate flow phenomena.

Pulsed jets, piezoelectric actuators, micro jets and oscillating jets are

among the most effective controls for active mixing enhancement.

1.3.2 PASSIVE CONTROL

In passive control the controlling energy is drawn directly from

the flow to be controlled. Passive controls are mostly desired because

no external power source is required. Passive control methods use

geometrical modifications which alter the flow structure. Some of the

commonly used passive control methods are shown below.


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Fig.1.3 Schematic of different type of passive controls.

1.4. CHEVRONS

Chevrons are saw tooth-like patterns at the trailing edge of jet engine

nozzles that help reduce noise from the ensuing jet. It has been known

from past experimental studies with laboratory-scale jets that small

protrusions at the nozzle lip, called tabs, would suppress screech

tones. In the 1980s and 1990s the tabs were explored extensively for

mixing enhancement in jets. These studies advanced the understanding

of the flow mechanisms and suggested that the technique might have a

potential for reduction of turbulent mixing noise that is the

dominant component of jet noise for most aircraft. Driven by stringent

noise regulations, such a potential first received serious attention

on an application level in the mid 1990s. Engine companies expressed

interest and some proposed their own concepts for tests. In 1996-97,

concepts from General Electric Aircraft Engines (GEAE), Pratt &


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Whitney (P&W) and others were combined into a test program under

NASAs Advanced Subsonic Technology (AST) Program. Various

tab/chevron configurations were evaluated for noise reduction with

models of separate flow nozzles in free-jet tests and encouraging results

were obtained. However, scepticism lingered and there was reluctance to

embrace the technology primarily out of concerns about thrust penalty.

In 1998 the impact on thrust was evaluated and found to be less than

0.25%. This was the turning point in the development of the technology

when industry started to invest heavily with product development

programs. The effort under AST culminated in flight tests in 2001 on

NASAs Lear jet 25 and Honeywells Falcon 20 test aircraft proving the

noise reduction.

Today, chevrons are implemented on various engines,

However, as stated, the evolution of the technology can be traced back

to decades of fundamental studies with tabs and similar devices at

universities, NASA as well as in industry. The concerted NASA /

industry studies in the 1990s eventually led to designs that produced

significant noise reduction while keeping the thrust loss within acceptable

limits. The objective of this paper is to provide an account of this

evolution, starting with a summary of the earlier fundamental studies.

1.4.1. EARLIER STUDIES ON THE EFFECT OF TABS

It has been known for a long time that tabs, small protrusions placed near

the nozzle exit, suppress screech noise. Screech is a phenomenon typical of

small, clean, laboratory jets that, under imperfectly expanded supersonic

condition, involve a feedback loop to produce a sharp tone. In laboratory

experiments the curious suppression effect is readily demonstrated by

inserting a small obstacle, such as the tip of a pencil, near the nozzle exit.


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One of the earliest studies of noise suppression by such devices is that of

Westley & Lilley . A picture of the teeth patterns used in their

experiment, in the then newly established program of jet noise research at

Cranfield, UK.

The authors observed large reduction of supersonic jet noise by these

devices apparently in part due to suppression of screech. Later experiments

usually deployed a single tab or two tabs that were sufficient to suppress

screech. Suppression of screech was desired in order to allow a clearer

study of other components of jet noise.

With regards to the effect of tabs on the jet flow field, The authors of this

work noted that the insertion of small rectangular tabs into the jet flow on

the nozzle perimeter had a profound effect; the apparent potential core

length was reduced to about two diameters followed by a rapid decay of

the centerline mean velocity.

With previous studies when compared with the rectangular protrusion at

the nozzle exit, it was soon recognized that a triangular tab with same

base width worked just as well. Moreover, when the apex of the triangular

tab was tilted downstream it appeared to work even better.

Thats why here we have chosen the triangular protrusion in addition to

that we have added 8 tabs for better reduction of noise.


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CHAPTER 2

LITERATURE SURVEY

2.1EFFECT OF CHEVRON COUNT AND PENETRATION

Experimental investigations (1) have been carried out on chevron

nozzles to assess the importance of chevron parameters such as the

number of chevrons (chevron count) and chevron penetration. Acoustic

measurements such as overall sound pressure level, spectra, directivity,

acoustic power, and broadband shock .Noise has been made over a

range of nozzle pressure ratio from sub-critical to under expansion

levels. Shadowgraph images of the shock-cell structure of jets from

various chevron nozzles have also been captured for different nozzle

pressure ratios. The results indicate that a higher chevron count with a

lower Level of penetration yields the maximum noise suppression for

low and medium nozzle pressure ratios. Of all the geometries studied,

chevron nozzle with eight lobes and 0degree penetration angle gives

the maximum noise reduction. Chevron nozzles are found to be free

from screech unlike regular nozzles. Acoustic Power index has been

calculated to quantitatively evaluate the performance of the various

chevron nozzles. Chevron count is the pertinent parameter for noise


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reduction at low nozzle pressure ratios, whereas at high nozzle pressure

ratios, chevron penetration is crucial. The results illustrate that by

careful selection of chevron parameters substantial noise reduction can

be achieved.

2.2 NUMERICAL PREDICTIONS OF NOISE IN

NOZZLESWITH AND WITHOUT CHEVRONS

Numerical simulations (2) of round, compressible, turbulent jets

using the Shear Stress Transport (SST kx)model have been carried

out. The three-dimensional calculations have been done on a

tetrahedral mesh with 0.9 million cells. Two jets, one cold and hot,

have been simulated. The Mach number for both the Cases is 0.75.

Overall sound pressure levels (SPL) at far-field observer locations have

been calculated using fowcs WilliamsHawkins equation. The

numerical predictions have been compared with experimental results

available in the literature. Axial and radial variation of the mean axial

velocity and overall SPL levels are compared. The potential core

length is predicted well, but the predicted centreline velocity decay is

faster than the measured value. The URANS calculations are not able

to predict the absolute values for the overall SPL, but predict the trends

reasonably well. The calculations predict the trends and absolute values

of the variations of the spectral amplitude well for the aft receivers, but

not for the forward receivers. Effect of chevrons on the noise from the

jet is also investigated for cold and hot jets. In each case, two chevron

taper angles, namely, 0degree and 5degree are considered. The latter

nozzle produces the most significant modification to the baseline

spectra and is less effective at high frequencies in abating the noise.

The present calculations predict a reduction in the overall SPL for the


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chevron nozzle with 0degree taper angle and a slight increase for

chevron nozzle with 5degree taper angle, for both cold and hot jets.

2.3 LARGE-EDDY SIMULATION OF CHEVRON JET FLOWWITH NOISE PREDICTIONS

Hybrid large-eddy(3) type simulations for chevron nozzle jet

flows are performed at Mach 0.9 and Re = 1.03*10^6. Without using

any sub grid scale model (SGS), the numerical approach applied in the

Present study is essentially implicit large-eddy simulation (ILES).

However, a Reynolds-averaged NavierStokes (RANS) solution is

patched into the near wall region. This makes the overall solution

strategy Hybrid RANSILES. The disparate turbulence length scales,

implied by these different modelling Approaches are matched using a

HamiltonJacobi equation. The complex geometry features of the

chevron Nozzles are fully meshed. With numerical fidelity in mind,

high quality, hexahedral multi-block Meshes of 12.5*10^6 cells are

used. Despite the modest meshes, the novel RANSILES approach

shows Encouraging performance. Computed mean and second-order

fluctuating quantities of the turbulent near field compare favourably

with measurements. The radiated far-field sound is predicted using the

Ffowcs Williams and Hawkins (FWH) surface integral method.

Encouraging agreement of the predicted far field sound directivity and

spectra with measurements is obtained.

Key purpose of this study is to highlight the penetration effects

of chevrons. Hence, the most severe 18.2bend SMC006 nozzle is

chosen as our primary focus. For comparative purposes the 5SMC001

is also fully studied. To explore the altered chevron shear layer mixing


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by the chevrons, the near nozzle instantaneous flow field is examined.

Only the results for the much more strongly penetrating SMC006 are

presented here gives computational snapshots of density Gradient

magnitude or Numerical Schlieren on both cut planes In spite of

some density variations within it, the potential core is clearly visible,

its length being slightly shorter than 5D. Strong density Variations are

located in the shear layer next to the potential Core. The outer edge of

the shear layer can also be easily identified.

The numerical simulation confirms the experiment with respect

to the influence of chevron penetration. With a bend angle of 18.2,the

potential core is shorter, downstream Reynolds stresses lower, but the

sound source distribution wider in the side line direction. The

characteristics of far-field sound for the 18.2 bend are therefore to the

bend angle of 5. Namely, the more server bend has louder Side line but

quieter downstream noise. It also increases the high-frequency sound

intensity but decreases the low-frequency. This consistency of the

current numerical study with measurements encouraging and

suggesting a reliable numerical frame work has been developed.

2.4 AN EXPERIMENTAL INVESTIGATION OF FLOW WITH

A SINGLE DELTA TAB

A single inverted(4) delta tab attached to the trailing edge of a

splitter plate in a two-stream mixing layer has been examined

experimentally using a three-component laser-Doppler anemometer.

Detailed mean flow and turbulence measurements were obtained at a

velocity ratio of 2:1 between the two co-flowing streams. The results


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showed that, when the tab was tilted to the high-speed side, stream

wise vortices generated and the subsequent mixing were stronger and

more intense than when tilting it to the low-speed side. The strength of

stream wise vorticity appeared to have a direct correlation with the

level of turbulence generated in the cross-stream directions. Attempts

were also made to quantify the effect of each (streamwise vorticity)

production term in the streamwise vorticity transport equation.

The effects of a single inverted delta tab in a two-stream mixing

flow situation have been investigated using a three component laser-

Doppler anemometer. The distortion in the streamwise mean velocity

flow fields and the generation of the streamwise vorticity that resulted

from the introduction of tab.

The tab has been established via measurements of three mean velocity

components and six Reynolds stress components in the present

investigation. The results have clearly shown that when the tab is tilted

to the high-speed side, it enhances mixing between the two streams

better than when tilted on the side of the low-speed flow. When the tab

is placed on the high-speed side, both sources for stream wise vorticity

generation are operative. When the tab is placed on the low-speed side,

only source 2 is operative. This observation suggests that the

production of turbulence in the lateral directions is strongly associated

with stream wise vorticity and that the imbalance of production and

dissipation leads to significant diffusion in the stations further

downstream.

2.5 CHARACTERISTICS OF SONIC JETS WITH TABS


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The result of an experimental investigation(5) on the effect of

vortex generation in the form of a mechanical tab placed at the nozzle

exit on the evolution of jet and its decay are reported in this paper. Jets

from a sonic nozzle with and without tabs operated at nozzle pressure

ratio from 2 to 7 were studied. Tabs with two combinations of length -

to-width ratio were investigated by keeping the blockage area constant.

The tabs offered a blockage of 10.18% of the nozzle exit area. The

centreline pitot pressure decay shows that for the tabbed jet a

maximum core reduction of about 75% can be achieved at a nozzle

pressure ratio (NPR) 7 compared to an uncontrolled jet. So that the

tabs drastically weaken the shock structure in the jet core and disperse

the supersonic zone of the flow making them occupying a greater zone

of the flow field compare to the plane nozzle. This causes the waves to

become weaker and the jet to spread faster. The tabs are found to shed

counter-rotating vortices all along the edges, resulting in enhanced

mixing. Isobaric contours reveal that the streamwise vortices cause an

inward indentation of the entrained mass into the jet core and an

outward ejection of core flow. To understand the distortion introduced

by tabs on the jet cross section and its growth leading to bifurcation of

the jet, a surface coating visualization method was developed and

employed.

Vortex generators in the form of tabs have been found to be quite

effective in influencing jet evolution and mixing. The tabs are found to

be effective in bringing down the centreline pitot pressure oscillations

for all nozzle pressure ratios. Shadow graph pictures reveal that the

tabs diffuse the shocks, resulting in weaker shocks in the core region.

Further, it is found that the tabs disperse the supersonic zone of the


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flow field compared to the plane nozzle. The tab is found to generate a

pair of counter-rotating vortices, which result in enhanced mixing.

Isobaric contours reveal that generated stream wise vortices cause an

inward indentation of entrained mass into jet core and an outward

ejection of core flow. Surface visualization explains the distortion

introduced by the tabs on the jet cross section. The distortion produced

by the two tabs grows which downstream distance and result

essentially in bifurcation of the jet. The tab length is found to be more

effective in mixing enhancement then the width for the same blockage

area and the limit for tab length are the nozzle radius and not the

boundary layer thickness.

2.6 EFFECT OF TAB ON MIXING CHARACTERISTICS OF

SUBSONIC AND SONIC JETS

The effect of tabs placed (6) at the exit of a circular nozzle of

10mm diameter on the near flow field of the jet was investigated

experimentally for subsonic mach numbers. The tab used was a hollow

semi circular tube of diameter 1.5mm and length 2mm. The near jet

flow field was studied for three configurations of the tab, namely, the

concave surface facing the flow exiting nozzle (arc tab facing -in) and

convex surface facing the flow (arc tab facing -out) for the blockage

ratio of 7.64 %.the center line mach number decay shows that for the

jet with arc tab facing in, a maximum reduction in core length of

about 80% was achieved at all subsonic and sonic correctly expanded

conditions of jet. Arc tab facing-out and rectangular tab configuration

reduces the core length to about 50%. The decay of arc tab controlled

jet was compared with that obtained for a plain rectangular tab of same


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blockage and a plain circular nozzle. The jet was found to decay at a

faster rate in the case of arc- tab facing in configuration as compared to

the facing out and rectangular tab configurations. Mach number

profiles show that the arc tab facing in distorts the jet efficiently by

spreading the jet wider in the plane normal to the tab and the effect of

spread is more pronounced in the jet with arc tab facing in as compared

to the arc tab facing out. The effect of tab orientation and shape seem

to have a profound influence on the development of the jet in the near

field.

Among the three configurations arc tab facing in was found to be

more efficient in reducing the potential core length and distorting three

jet structure compared to the other two configurations in all subsonic

and sonic correctly expanded mach numbers .arc tab facing in is

showing consistently better performance for all the mach numbers

compared to other two configurations. It may be due to the fact that the

arc tab facing in creates stronger pressure gradient shedding stronger

stream wise vortices causing reduction in potential core and enhanced

mixing.


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CHAPTER 3

COMPUTATIONAL WORK

3.1 MODELLING

Designing three dimensional nozzle with different configurations

at exit we used CATIA modelling software which is flexible one anduser friendly.

3.1.1 THREE DIMENSIONALMODELLING

For this project, the model of nozzle has been created by using

modelling software CATIA. Initially the sketch has been created

with the given dimension, and at the exit the domain for the

visualization of the potential core region has been designed with the

respective dimensions.

NOZZLE SPECIFICATIONS:

Specifications Dimensions

Length 30mm

Inlet diameter 30mm

Exit diameter 16mm

Chevron length 5.44mm

Wedge 3.08mm

Table 3.1 Nozzle specifications


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Fig 3.1 Two dimensional free jet nozzle


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Fig 3.2Three dimensional free jet nozzle

Fig 3.3 Drafted views of free jet nozzle


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Fig 3.4 Chevron nozzle with 8 count

Fig 3.5 Drafted views of chevron nozzle

Fig 3.3 is the three dimensional diagram of nozzle with chevron. Todraw the nozzle with chevron follows the same as nozzle. After

developing the free jet nozzle then draw chevron which seems like

saw tooth. To draw chevron first choose the appropriate plane and

go to sketcher. Then draw the chevron with correct dimensions for our

specifications. After finishing sketch exit the sketcher. Then select the

chevron and choose the pocket option to pocket the chevron. After

pocketing the chevron choose the circular pattern in the transformation

features. Thus the chevrons are created in the exit of nozzle with our

dimensions and the chevron count is 8.


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Fig 3.6 Chevron nozzle with wedge thickness 1mm

Fig 3.7 Drafted view


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Fig 3.8 Chevron nozzle with wedge thickness 2mm

Fig 3.9 Drafted view


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3.2PREPROCESSING

ICEM CFX is the pre-processor tool used to mesh the model in

order to get the accurate result.

3.2.2THREE DIMENSIONAL MESHING

Import the Three dimensional iges file or step file into the ICEM

CFX. Then do any clean up operations if necessary. Then create the

domain according to requirement which may be rectangular brick or

cylinder for our convenient. The zone height is to be 5D to 10D and

length to be 30D to 40D range or to be greater than that too. After

creating the zone we place the domain at the exit of the nozzle by using

copy option. We should unite the two volumes as a single volume

.Then the domain can be decomposed according to the requirement.

Then the mesh has been made with the size of 1 which can be reduced

according to our requirement for better results.

MESH TYPE

Combination of Quadra and Hexa mesh for nozzle and for

boundary domain tetra mesh type.

3.2.3 CHEVRON NOZZLE WITH AND WITHOUT WEDGES

In this follow the same as in above instead of uniting the

volumes here subtract the volumes by using the subtract option i.e.,

subtracting the nozzle from domain. After subtracting there will be one

volume only. Before subtracting place the nozzle inside the domain for

the convenience. Then mesh the faces using the face mesh operation by

tria elements under pave scheme. We can mesh the edges of the

volume and then do face and volume mesh if needed. In the edge mesh


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select each edge of the volume and to give number of nodes in each

edge based on the need. After finishing edge mesh then select the face

mesh. In the face mesh we have to select each face and mesh. In the

face mesh we may select the elements as QUAD or TRIA.After

finishing the face mesh then go for the zones operation. In the Zones

we have to select the INLET, OUTLET,WALL, AXIS, PRESSURE

INLET, PRESSURE OUTLET, etc,. After applying the zones we may

check the quality of the mesh by using quality operation. After

finishing all above steps have to save the mesh and to export the mesh.

3.2.4BOUNDARY CONDITIONS

The below diagram illustrates the boundary conditions of 3dimensional

models. In ICEM CFX select the pre-processor modules and feed the

input for the respective Mach number.

Fig 3.10boundary conditions


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Fig 3.11 Three Dimensional free jet Mesh

Fig 3.11 is the meshing model of base line nozzle. The meshing

process for this model follows the above procedure with little variation.

In which nozzle volume and zone volume are united to a single

volume. Then this volume is meshed by using volume mesh operation

using tetra and hex elements scheme and it contains 835710 elements.


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Fig 3.12Chevron nozzle mesh

Fig 3.12 is the meshing model of chevron nozzle. The meshing


In which nozzle volume and zone volume are subtracted to a single


using tetrahedral elements under and it contains 363094 elements.


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Fig 3.13 Chevron nozzle with wedge thickness1mm mesh

Fig 3.13 is the meshing model of base line nozzle. The meshing


In which nozzle volume and zone volume are subtracted to a single


using tetrahedral (boundary) & hex(nozzle) elements under scheme and

it contains 453731 elements.

Fig 3.14 Chevron nozzle with wedge thickness 2mm mesh

The above meshing model contains 456208 tetrahedral elements

and meshing under scheme. The procedure is same as for the previous

one.


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3.3SOLVER

ICEM CFX has a post processor or analyzing software used to

simulate the model for their application.

3.3.1STEPS TO SOLVE

After finishing the pre-processor and then have to import our

meshed model into the software by read the case file such as mesh file.

After reading the case file have to check the grid and zones. Then go to

define select the model and select the solver. Then click the viscous

model and select the appropriate viscous model for your problem. Here

we selected k-omega (used for turbulent analysis).

Then click materials in define option and change the material to

ideal gas and click ok. Then click operating condition and give the

operating condition value in Pascal or any other unit. Then click

Boundary conditions and give the pressure values for boundary

conditions such as pressure inlet, pressure outlet, pressure far field, etc,

Then go to solve select initialize and select initialize in which select the

inlet condition and click apply. Then go to monitors in solve and select

residual and click ok for print and plot.

After finishing all above steps click iterate in solve and give

value for number of iterations and start the iteration. After the solution

is converged and iteration will stop and we see the results. Go to the

display menu select the contours and select which one to display such

as velocity, pressure, etc, and click display and the contour will display

in different colours for variations of values. We can also see the XY

plot for velocity, pressure, etc, by selecting plot menu and select XY


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plane and select pressure ,velocity ,etc,. Thus the meshed model is

analyzed using CFX following the above procedure.

3.3.2FORMULA TO FIND THE TOTAL PRESSURE

PO/P M2)/-1

P= 101325 Pascal

PO/P 0.42

)1.4/1.4-1

PO= 113134.6279 Pascal

3.3.3 BOUNDARY CONDTION PRESSURE VALUES

Mach number Total pressure

(pascal)

Gauge pressure

(pascal)

Operating

pressure

(pascal)

0.4 113134.6279 101325 0

0.6 129240.4201 101325 0

0.8 154453.7514 101325 0

Table 3.2 Boundary conditions pressure values


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CHAPTER 4

RESULT AND DISCUSSION

Potential core region in the subsonic flow is the region where thecenter line velocity is same as the velocity at the exit of the nozzle. The

Mach number along the center line was calculated from the total

pressure and static pressure. The static pressure across the jet is

assumed to be the same as the surrounding ambient pressure. The

assumption is perfectly valid for subsonic jets.

In this chapter we are discussed about the centreline mach

number decay for chevron, chevron with 1mm wedge and chevron with

2mm wedge. Here we plot dimensional and non-dimensional graphs

for all subsonic mach number for all configuration. The plots are

obtained from the post processor ICEM CFX. We discussed and

compared the results of center line Mach number decay for all subsonic

mach number to each configurations.

Streamwise vortices are generated when wedges and chevron are

introduced at the exit of the nozzle. They act as effective mixing

promoters and enhance the jet mixing. These faster mixing results in

rapid jet decay of chevron with wedge compared to a jet from a plane

nozzle. The chevron increase vortices in the exit nozzle along with


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wedges also induce vortices and increase the jet mixing and help to

reduce the potential core.

The contour plot obtained from the CFD is dimensional and

convert it in to non dimensional by excel. The graph is plotted between

X/Dj and M/Mj. The length of the potential core is divided by the exit

diameter and Mach number is divided by the jet Mach number. Both in

dimensional and non dimensional plot we can view the Mach number

decay clearly.

4.1 EFFECT OF CENTER LINE MACH NUMBER FOR FREE

JET

Fig 4.1Mach number plot for free jet M= 0.4


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Fig 4.2Centerline Mach number decay of free jet

The above graph shows the centerline Mach number decay for

the Mach number 0.4 and is plotted between X/Dj and M/Mj. Here the

potential core decays at 5.5D.



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Fig4.4Centerline Mach number decay of free jet

The above graph shows the centerline Mach number decay for


potential core decays at 5.81D. When comparing with 0.4 Mach

number of free jet the potential core length is high.



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Fig 4.6Centerline Mach number decay of free jet

The above graph shows the center line Mach number decay for


potential core decays at 6.12D. When comparing with 0.4 and 0.6

Mach number of free jet the potential core length is high.

4.2 CENTER LINE MACH NUMBER DECAY OF CHEVRON


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Fig 4.7Mach number plot for chevron M= 0.4

Fig 4.8Centerline Mach number decay of chevron

The above centerline Mach number decay graph is plotted

between M/Mj and X/Dj for the chevron nozzle for the Mach number

0.4. The potential core decay starts from 3.88D. When comparing with

0.4 Mach number of free jet the potential core length is 30 % reduced.


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4.3 CENTER LINE MACH NUMBER DECAY OF CHEVRON

WITH WEDGE

Fig 4.13Mach number plot for chevron with wedge thickness 1mm

M= 0.4

Fig 4.14Centerline Mach number decay of chevron with

wedge1mm


between M/Mj and X/Dj for the chevron nozzle with 1mm wedge


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thickness for the Mach number 0.4. The potential core decay starts

from 3.17D. When comparing with 0.4 Mach number of free jet and

chevron the potential core length is reduced to 42 % and 18%

respectively.

Fig 4.15Mach number plot for wedge thickness1mm M= 0.6


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Fig 4.16Centerline Mach number decay of chevron with wedge

1mm

The above graph is plotted between M/Mj and X/Dj for the

chevron nozzle with 1mm Wedge thickness for the Mach number 0.6.

The potential core decay starts from 3.05D. When comparing with 0.6

Mach number of free jet and chevron the potential core length is

reduced to 47 % and 22% respectively.

Fig 4.17Mach number plot for wedge thickness1mm M= 0.8


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1mm


between M/Mj and X/Dj for the chevron nozzle with 1mm Wedge



chevron the potential core length is reduced to 50% and 22%

respectively.

Fig 4.19Mach number plot for wedge thickness 2mm M= 0.4


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2mm



thicknessfor the Mach number 0.4. The potential core decay starts from

2.0D. When comparing with 0.4 Mach number of free jet and chevron

and Wedgewith 1mmthickness the potential core length is reduced to

63%, 48% and 40% respectively.


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2mm


between M/Mj and X/Dj for the chevron nozzle with 2mm Wedgethickness for the Mach number 0.6. The potential core decay starts


chevron and Wedge with 1mmthickness the potential core length is

reduced to 65%, 49% and 34% respectively.



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2mm





chevron and Wedge with 1mmthickness the potential core length is

reduced to 67%, 49% and 34% respectively.


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CHAPTER 5

CONSTRAINTS AND RECTIFICATION

To evaluate the Reduction in noise we need experimental setupbut we have preceded in such a way that if there is a reduction in

pressure, noise will reduce.

CONCLUSION

The Wedges and chevrons were used in the presents study,

namely chevron, chevron with 1mm wedge thickness, chevron with

2mm Wedge thickness were found to be effective in distorting the jet

structure. From the figure 5.1, fig 5.2, fig 5.3 among the three

configurations, chevron with 2mm Wedge was found to be more

efficient in reducing the potential core length and distorting the jet

structure compared to the other two configurations in all subsonic

mach numbers. From the above figures, Nozzle with the chevron with

2mm Wedge , the maximum core length reduction achieved was 67 %

of uncontrolled jet , followed by rapid decay of jet center line mach

number, where the chevron with 1 mm Wedge reduce the core length

to 50 % and chevron reduce the core length to 40 % at all subsonic

mach numbers. Chevron with 2 mm Wedge is showing consistently

better performance for all mach numbers compare to other two

configurations. It may due to that fact the chevron with 2mm Wedge

creates stronger pressure gradient shedding stronger streamwise


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vortices causing reduction in potential core and enhanced mixing.

Mach number profiles at different X/D locations shows the chevron

with 2mm Wedge is more effective spreading of the jet normal to the

wedge compare to other two configurations.

Chevron technology has provided a modest relief. Unfortunately,

a complete understanding of jet noise mechanisms is still not in our

grasp. The insight of fundamental experiments coupled with

application of CFD allowed the development of the subject

technology with tools slightly better than cut-and-try. Hope forfurther control and reduction of jet noise hinges on advancement of

our understanding of the relevant mechanisms.

Fig 5.1 Comparison of Centerline Mach number decay


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COMPARISON OF POTENTIAL CORE LENGTH


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MACH

NO

FREE JET CHEVRON WEDGE 1mm WEDGE 2mm

0.4 5.50D 3.88D 3.17D 2.00D

0.6 5.81D 3.93D 3.05D 2.00D

0.8 6.12D 3.93D 3.05D 2.00D

Table 5.1Comparison of potential core length

CHAPTER6REFERENCES

1. D. Casalino, F. Diozzi, R. Sannino, A. Paonessa, Aircraft noisereduction technologies: A bibliographic review, Journal of

aerospace science andtechnology.Aerospace Science andTechnologyVolume 12, Issue 1, January 2008, Pages 117

2. Hao Xia, Paul G. Tucker, Simon Eastwood, Large-eddysimulations of chevron jet flows with noise predictions,

International Journal of Heat and Fluid Flow. InternationalJournal of Heat and Fluid Flow,Volume 30,Issue6 December

2009, Pages 1067-1079


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3. M.L. Shur, P.R. Spalart, M.Kh. Strelets, A.K. Travin, Towardsthe prediction of noise from jet engines,: International

Journal of Heat and Fluid Flow, Volume 24, Number 4, August

2003 , pp. 551-561(11) Publisher: Elsevier

4. P.S. Tide, K. Srinivasan, Effect of chevron count and penetrationon the acoustic characteristics of chevron nozzles.2009.08.010.Applied Acoustics, Volume 71, Issue 3, March 2010, Pages 201-

220

5. P.S. Tide, V. Babu, Numerical predictions of noise due tosubsonic jets from nozzles with and without

chevrons.j.apacoust.2008.03.006. Applied Acoustics, Volume70, Issue 2, February 2009, Pages 321-332

6. S.C.M. Yu, P.K. Koh, L.P. Chua, An experimental investigationof two-stream mixing flow with a single delta tab. International

Journal of Heat and Fluid Flow. International Journal of Heat

and Fluid Flow, Volume 22,

Issue 1, February 2001, Pages 62-71

7. Shibu Clement, E.Rathakrishnan, Characteristics of sonic jetswith tabs.

July 2006, Volume 15, Issue 3-4, pp 219-227

8. S.Thanigaiarasu, S. Elangovan, E.Rathakrishnan, Effect of tabon mixing characteristics of subsonic and sonic jets International

Review of Aerospace Engineering;Feb2010, Vol. 3 Issue 1, p1

9. S. Lardeau, E. Collin, E. Lamballais, J.P. Bonnet, Analysis of ajet mixing layer interaction, International Journal of Heat and

Fluid Flow.International Conference on Engineering TurbulenceModelling and Measurements Volume 24, Issue 4, August 2003,

Pages 520528


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10. You-Hong Liu, Experimental and numerical investigation ofcircularly lobed nozzle with/without central plug, International

journal of heat and mass transfer 45(2002) 2577-2585. Publisher:

Elsevier

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