Studies on Directional Characteristics of Supersonic Jet Noise

B Vinay Kumar 06AE1016

What is Sound ?• Pressure disturbance in a medium produces

sound which may be composed of different frequencies and travels at a speed of 330m/s at 20˚C

• The average person lower threshold of hearing correspond to 20µPa.

• Frequency range that can be heard by humans

Audible range ultra sonicinfrasonic

20 Hz 20,000 Hz ∞

• Sound pressure less than 20µPa cannot be perceived by human ear no matter what the frequency is.

• If the frequency of sound is less than 20 Hz then that particular sound cannot be heard no matter what the sound pressure is

• Audible sound pressure range from 20 µPa to 100 Pa. The ratio of lower and upper limits of audibility is very high. So, we need to measure the sound in a different scale

Measurement of sound (decibel)

• It is more practical to express acoustic parameters as a logarithm of the ratio of the measured value to a reference value. This logarithm of the ratio is called a decibel(dB).

• Reference value(Pref) is taken as the weakest sound pressure that can be heard by human i.e. 20 µPa

• Sound pressure level (SPL) = 20log (P/Pref)• SPL at Pref is 0 dB

Noise• It is an unwanted sound or undesired sound• Whether sound is perceived as unwanted or not depends on

subjective factors such as the sensitivity of the listener and the situation, age, health and personal attitudes towards the sound source, frequency of exposure and many such psychological and social factors

• Usually 80 dB is the level at which sound becomes physically painful

• Humans, animals, plants and even inert objects like buildings and bridges have been victims of the increasing noise pollution

• Low frequency noise is less damaging to the structure of the ear than the high frequency noise

Causes of noise• The sound caused by the exhaust system of autos, trucks, buses and

motorcycles is the chief reason for noise. • Aircraft noise, the rapid growth of air travel in the recent years

contributes to the increase in noise pollution• horns and whistles and switching and shunting operation in rail yards• Plumbing, boilers, generators, air conditioners and fans create a lot of

noise in the buildings• Household equipments, such as vacuum cleaners, mixers and some

kitchen appliances are noisemakers of the house• Machinery, motors and compressors used in the industries create a

lot of noise which adds to the already detrimental state of noise pollution………..and many more

Effects of noise

• The first and foremost effect of noise is a decrease in the efficiency in working.

• Sometimes, being surrounded by too much of noise, people can be victims of certain diseases like blood pressure, mental illness, etc.

• Deafness, temporary or permanent, is one of the most prevalent effects of noise pollution.

• Noise pollution indirectly affects the vegetation• Animals are susceptible to noise pollution as well. It damages

the nervous system of the animals.• Noise indirectly weakens the edifice of buildings, bridges and

monuments. It creates waves, which can be very dangerous and harmful and put the building in danger condition.

Sources of noise from a typical aircraft

Engine related noise•Fan/turbine•Jet•Combustion

Nose Gear noise

Flap edge noiseMain Gear noiseWing tip noise

Slat noise

Why study jet noise

Fan Turbine Airframe Jet Total0

20

40

60

80

100

120

93.784.3 86.6

102.2 103.7

Noise(dB)

B-7X7 Noise components Jet Noise is the most dominant source of Noise

Our main objective

• Experimental study of different components of supersonic jet noise and its directivity spectrum for various Nozzle pressure ratios

Step by step procedure

• Setting up the anechoic chamber, nozzle and the reservoir

• Calibration of microphones• Arranging the mic’s in the arc as required• Setting the pressure ration inside the Lab View• Running the tunnel until all data is collected• Data Analysis

Measuring Jet noise

• Sound intensity can be measured by using a high quality unidirectional microphone with less reflections from the microphone head i.e. size of the microphone must not be more than the wavelength of the sound which we are going to measure.

• As intensity of sound is different in different directions we need to have a array of mic’s spread in all directions or simply a rotating mic around the sound source.

• As sound has properties like reflection, refraction, diffraction we cannot measure exact SPL in a given place.

• We need to have a “free field” condition like free from reflections etc.. This can be achieved by designing the room with special structures and special materials, namely an anechoic(free from echo's) chamber

Experimental setup

Setting the pressure ratios inside the Lab View

Microphone• ¼ ‘’ free field microphone• Features Sensitivity 4mV/Pa

Frequency 4Hz-100KHzDynamic Range 28-164 dBTemperature -40 to +150 ° CPolarization 200 V external

Free field response of the microphone without protection grid

Anechoic Chamber requirements• The anechoic chamber must be large enough to allow acoustics as well as the

geometric far fields of the jet• The anechoic chamber must be large enough to allow the measurements in

the rear and the forward arc measurements• The room must be anechoic at all frequencies of interest in the noise

spectrum of the model scale noise • The wedges must have flame retardant characteristics ,if the facility is used

for heated jets.• Easy removal of wedges• There must be adequate means of exhausting air from the room.• The jet must see the anechoic termination.• Noise entering the jet exhaust collector from outside should be insignificant.• For heated jets cooling arrangement is required

Anechoic Chamber inside NAL

• The function of the anechoic chamber is to eliminate or minimize the reflected or scattered sound energy from a source. To mimic far field conditions.

• Height of the wedge is 300mm it decides the minimum cut off frequency which is 330 Hz in this case.

• A catcher is placed diametrically opposite to the nozzle to collect the jet exhaust

• Microphones are arranged in horizontal plane radial directed in the far field conditions.

• At a particular angle we measure the sound internsity with time

Data AnalysisMicrophone Amplifier Computer Analysis

Analysis of Data in time domain

1. In time domain it is impossible to determine the dominant frequencies. Hence we transform the time domain data in to frequency domain using Fast Fourier Transform(FFT).

FFT Theory

Frequency domain analysis

• The dominant frequencies can now be determined

Principal components of the supersonic Jet noise

• Turbulent jet flows contain both fine and large-scale turbulence structures.

• For subsonic jets the large turbulence structures are ineffective noise generetors. The dominant noise is produced by the fine scale turbulence.

Ideally Expanded Jets

• For supersonic ideally expanded jets the large turbulence structures propagate downstream at a supersonic mach number relative to the ambient sound speed

• They produce intense Mach Wave radiations • The Mach Wave radiations easily

predominates over noise from the fine scale turbulence

Imperfectly Expanded Jets

• For imperfectly expanded jets, a shock shell structure is formed in the jet plume.

• There are two shock associated noise components– One has discrete frequencies, which are

commonly referred as Screech tones.– The other component is broadband shock

associated noise.

Characteristics of Turbulent Mixing Noise

• In the upstream direction, the noise intensity is low and is nearly uniform.

• This is the background noise that is believed to be generated by the fine-scale turbulence of the jet flow.

• Dominant part of Turbulent mixing noise is radiated into an angular sector of about 45 to 60 degrees measured from the jet flow direction.

• This noise is believed to be generated directly by the large turbulence structures of the jet flow.

Noise Generation by the large turbulence structures

• An approximate picture of the physical problem is to regard the instability wave as a wavy wall.

• The wavy wall has the same wavelength and wave speed as the instability wave.

Characteristics of the Broad Band shock associated noise

• The very distinct peak to the right of the screech tone is the broadband shock associated noise. Clearly, this is the dominant noise component in the upstream direction.

• The frequency corresponding to the peak of the spectrum changes with the direction of radiation.

• The frequency corresponding to the peak of the spectrum is increasing with the inlet angle.

• The half-width of the spectral peak increases with inlet angle.

Noise generation mechanism of BBSN• The interaction between the downstream

propagating large turbulence structures/ instability waves and the shock cells will give rise to disturbance.

• Using simple one dimensional model the disturbance represents a traveling wave with wave number(k- ki).

• If ki is slightly larger than k, then the phase speed of interaction wave is negative and supersonic.

Characteristics of Screech tones

• It is found that fundamental screech tone radiates primarily in the upstream direction.

• They are generated due to an acoustic feedback phenomenon.

• Near the nozzle lip acoustic disturbances impinging on this area excite the intrinsic instability wave of the jet flow.

• As the instability wave propagates downstream it extracts energy from the mean flow and grows rapidly in amplitude.

• The instability wave having acquired a large enough amplitude interacts with the quasi-periodic shock cells in the jet plume.

• The unsteady interaction generates acoustic radiation. For the same reason as in the case of broadband shock-associated noise, the acoustic radiation is primarily in the upstream direction.

• The feedback acoustic waves propagate upstream outside the jet.

• Upon reaching the nozzle lip region, they excite the shear layer of the

jet, which leads to the generation of new instability waves.

• In this way, the feedback loop is closed.

ResultsNPR Jet Characteristics Distance of Mic’s Diameter of Nozzle

Case 1 3.6 Ideally Expanded 50.6 diameters 1.5 in

Case 2 2.5 Over Expanded “ “

Case 3 3.0 Over Expanded “ “

Case 4 4.0 Under Expanded “ “

Case 1 [NPR=3.6 ideally expanded]

Case 1 (contd…)

There are no screech tones or broadband shock noise as it is an ideally expanded case Case 1 (contd…)

OASPL variation

The OASPL at higher angles is higher as these angles are dominated by mach waves generated by large scale turbulence structures.At other angles the OASPL is flat as it is due to small scale turbulence only.

Case 2 [NPR=2.5 over expanded jet]

Case 2(contd…)

The peaks correspond to screech tones. The frequency of the first harmonic is 5716 Hz.Screech frequency calculated using TAM’s formula is 5309 Hz. The hump on left of the screech is due to large scale turbulence noise. The hump on the right side is due to Broadband shock associated noise (BBSN).

As angle Turbulence noise BBSN Case 2(contd…)

OASPL variation

Case 3[NPR=3 over expanded jet]

Case 3(contd…)

Screech tones upto third harmonics are visible.The frequency of the first harmonic observed from the graphs is 3957 Hz. The frequency calculated using TAM’s formula is 4084 Hz.

Case 3(contd…)

OASPL variation

OASPL at all angles is more than NPR=2.5.But it is less than the ideally expanded case of NPR=3.66

Case 4[NPR=4 under expanded jet]

Case 4(contd…)

There are no screech tones observed as the degree of under expansion is small.

Case 4(contd…)

OASPL variation

The OASPL at all angles is observed to be higher than all previous cases thus leading to the inference that OASPL increases with increase in NPR.

References

• Selecting a Microphone: On The Differences Between Pressure-Field and Free-Field Mic’s

• Tam, C K W, “Jet Noise: Since 1952”, Theoretical. Computational. Fluid Dynamics (1998) 10: 393–405

• Tam, C K W, “On The Two Sources of Jet Noise” , • Syed R. Ahmed, “Introduction to Aero acoustics of Aircraft” • J. Panda, “An experimental investigation of screech noise generation”,

J. Fluid Mech. (1999), vol. 378, pp. 71{96 • Christopher K. W. Tam, “Supersonic Jet Noise”, Annu. Rev. Fluid Mech.

1995.27:17-43 • Hao Shen and Christopher K.W. Tam “The Effects of Jet Temperature

and Nozzle Lip Thickness on Screech Tones”

Thank you