Acoustic Performance of Lobed Nozzles

SEMINAR REPORT SUBMITTED FOR PARTIAL FULFILMENT OF THE

REQUIREMENTS FOR THE DEGREE

B.Tech Aerospace + M.Tech Avionics (Dual Degree Program)

By

M Nishant

(A4717210012)

Under the guidance of

Prof. A.K.Verma

Amity Institute of Space Science and Technology

Amity University, NOIDA

Certificate

It is certified that the project titled ‘Acoustic Performance of Lobed

Nozzles’ which is submitted to Amity Institute of Space Science and Technology,

Amity University, NOIDA by M NISHANT in partial fulfilment of the requirement

for the award of the degree Bachelors in Aerospace engineering and Masters in

Avionics (DUAL DEGREE) has been carried out under my supervision.

Date:

Faculty Guide Name & Signature

Acknowledgement

Words would not be sufficient to express the gratitude that I feel towards

Prof A.K.Verma for giving this wonderful opportunity to help me study the

acoustic properties of different nozzle configurations

I also take the opportunity to thank Prof M.S.Prasad, Head, Amity Institute of

Space Science and Technology for his intellectual guidance and support.

I also owe a debt of gratitude towards Prof K. L. Anand for his guidance and

support at every instance in the seminar preparation.

INDEX

Acknowledgement

List of Figures

Introduction

Jet Noise

Noise Suppression

Lobed Nozzle

Experiment and results

Conclusions

References

List of figures

Fig 1: Lobed Nozzle Geometry

Fig 2: Geometry and mesh for 10 lobed nozzle

Fig 3: Graph showing Acoustic power level variation with distance

Introduction

Aircraft noise is noise pollution produced by any aircraft or its components,

during various phases of a flight. Its effect is felt mostly in settlements around an

airport during the take-off, climb, descent and landing of the aircraft.

A moving aircraft causes compression and rarefaction of air molecules resulting

in their motion. This motion propagates through air as pressure waves. These

pressure waves have a wide spectrum of frequency with some lying in the

audible range. Different types of noise have different noise signatures. The most

important noise generating sources are:

Aerodynamic noise

Engine noise

Aircraft System noise

Like every other form of noise, aircraft noise also has ill effects on human body

and wildlife. A statistical study conducted in Germany concluded that elevated

levels of noise (of the order greater than 65dB) can increase chance coronary

heart failure by 80% [1]. Apart from that, elevated noise levels can cause

psychological damage, high stress levels, hearing loss, sleep disturbances and

other harmful effect which lead to further health problems.

Due to this strong influence of aircraft noise, there is always a need to reduce the

acoustic level as far as possible. This is done by using quieter jet engines,

aerodynamic aircraft frame and other techniques. This paper talks about one of

these techniques which has been used in jet engines for some time now.

Jet Noise

Jet noise is a field which deals with the acoustic pressures formed by high speed

jets and the turbulent eddies created by the shearing flow. It is caused by the

violent turbulent mixing of the exhaust gases with the atmosphere and is

influenced by the shearing action caused by the relative speeds between the

exhaust jet and the atmosphere. The primary sources of jet noise for a high speed

air jet are ‘jet mixing noise’ and ‘shock associated noise’ in case of supersonic

flows alone. Also, acoustic sources inside the engine contribute to the jet noise.

These include combustion noise and sound produced by the rotating components

like compressors and turbines.

The jet mixing sound is created by the turbulent mixing of a jet with the ambient

air. The mixing initially occurs in an annular shear layer, which grows with the

length of the nozzle. Turbulence created near the exhaust exit causes a high

frequency noise (small eddies) and further downstream of the exhaust,

turbulence causes low frequency noise (large eddies).

In supersonic jets, certain cells are created in the exhaust through which the flow

continuously expands and contracts. This results in a ‘screeching’ noise which

can be controlled using suitable nozzle configurations. [2]

Noise Suppression

As mentioned, exhaust jets are major sources of jet noise. This can be suppressed

by inducing a rapid and short mixing region. This is achieved by increasing the

contact area between the exhaust gas stream and the atmosphere by using

suitable nozzle geometry. Effective control of turbulent structures also helps in

suppressing jet noise. This type of suppression reduces the low frequency noise

while increasing the high frequency noise.

Lobed Nozzles

Lobed forced mixers consist of a splitter plate with convoluted trailing edge for

efficient mixing of two co-flow streams with different velocity, temperature

and/or species. The mixing is achieved by the introduction of pairs of counter

rotating streamwise vortices that efficiently transport momentum across the

mixing layer. The streamwise vortices deform the normal vortices into pinch-off

structures and increase the stirring effect in the mixing flow. These result in the

creation of intense small-scale turbulence and mixing. It is capable of mixing the

core and bypass streams of turbofan engines to improve propulsive efficiency,

reduce specific fuel consumption. It has also been used to supress the infrared

radiation emission [3]. Due to its improved mixing performance, lobed nozzles

also find use inside the combustion chamber for enhanced mixing of fuel and air

[4].

Many researchers have investigated jet flows from nozzles with lobed exit

configurations. In all cases, faster spreading of the jet was observed. Further,

reduction in jet noise has been observed with use of lobed nozzles in some cited

work and industry as well.

Unfortunately, because of the complex geometry and large parameters, the

underlying mixing mechanism is still under research.

Fig 1: Lobed Nozzle Geometry

Experimental set up

Computational investigations are conducted for three different configurations of

nozzles. All of them have a convergent contour with a constant nozzle exit area of

1256m2. Of the three nozzles, one is a circular nozzle which acts as the baseline

case. The rest are of lobed configurations having six (6) and ten (10) lobes

respectively.

The geometry is made on CAD software like CATIA and grid is generated in

GAMBIT software using suitable size functions. The number of cells in each of the

case is kept in the range of ten (10) to fifteen (15) lakhs.

An acoustic problem is set up in ANSYS FLUENT. A k-ω SST model is used for

turbulence. The boundary conditions were set for an exit Mach number of 0.6

which typically depicts real life scenario.

Fig 2: Geometry and mesh for 10 lobed nozzle

Results

Fig 3: Graph showing Acoustic power level variation with distance

On the basis of the created flow animation, it is evidently found that the overall

acoustic performance of lobed nozzle is better than a conventional circular

nozzle.

There is a significant reduction in the total sound power level in the overall

computational domain. At a distance of x/D = 35 (x=800m), there is a noise

improvement of almost 20dB when comparing the circular and 10 lobe

configurations.

From this results we can conclude that by increasing the number of lobes in the

exit geometry of nozzle, the mixing process is improved thereby resulting in

improved noise suppression.

Conclusion

The study done shows that there is significant improvement in the jet acoustics

by increasing the number of lobes in the nozzle exit geometry. With higher

computational capabilities, the frequency spectrum of the acoustic noise can be

obtained and effectively compared.

As in the case of any improvement, there is always a compromise to be made.

Lobed nozzles suffer a setback in form of thrust reduction. Work done by Zaman

and Wang [5] clearly compares the improvement in acoustic performance to that

of the thrust produced for rectangular lobe configurations.

Also as mentioned earlier, the mixing mechanism of lobed geometries is under

research and hence the optimization process is purely by hit and trial method.

With an environmental approach in mind, the drawbacks of lobed nozzle

geometry can be compensated for with the highly effective noise suppression

mechanism which , with the help of other noise suppression techniques, can

make air travel more environment friendly.

References

Cited references

[1] Tödlicher Lärm - Spiegel, Nr. 51, 14 Dezember 2009, Page 45 (German)

[2] Howe, M.S. (1998). Acoustics of Fluid-Structure Interactions. Cambridge:

Cambridge University Press. pp. 153

[3] Power G. D., McClure M. D. and Vinh D. (l994) Advanced IR Suppresser

Design Using A Combined CFD/Test Approach.

AIAA94-32l5

[4] Smith L.L, Majamak A.J., Lam I.T. Delabroy O.,Karagozian A.R., Marble

F.E. and Smith, O. I., (l997) Mixing

Enhancement in a Lobed Injector. Phys. Fluids, Vol.9_No.3_PP667-678

[5] Zaman K.B.M.Q, Wang F.Y (2002) Noise, Turbulence and Thrust of Subsonic

free jets from Lobed nozzles, AIAA-2002-0569

Tetsuo SAGA, Hui HU, Toshio KOBAYASHI (1999) Mixing Process in the

Jet Flow of Lobed Nozzle

Purdue AAE Propulsion

Jet noise- Wikipedia