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
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