# Department Term paper In this paper we will discuss

Department of Aerospace Engineering

Fundamentals of Aerodynamic Noise

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Instructor:

Dr. Yusuf Özyörük

Student:

MAHDI YAZDANPANAH

ID: 2204253

Term paper

In this paper we will discuss three papers related to acoustics and noise propagation in ducts. The papers that we have worked on them are as below:

·       Azimuthal modes measurement in an intake duct for a turbofan engine.

·       Analysis of Sources and Methods for Reducing Noise by Minimizing Vibrations of Engineering Technological Processes.

·       Helicopter Noise: State-of-the-Art.

First, we will go through the first paper.

This paper describes the acoustic mode measurements in duct of turbofan engine done with 100-microphone array placed around the intake. The array was tested before in the laboratory condition in the non-reflective room in PNRPU. The authors of this paper installed the array of microphones on an engine and they measured azimuthal structure field in intake duct while the engine was at stationary condition. The dominant modes were observed for different engine runs but the amplitude changed from each test to another and the reason of this is that no turbulence control is used while the test was performing. Therefore, engine noise reduction can be done by using the acoustic liners.

One of the major noise source of modern airplanes is the fan of aircraft engine. In order to reduce the noise, acoustic liners are used in intake of engines. Azimuthal modes in ducts of engines can be measured experimentally with a stationary microphone array installed around the duct of engine. In this work, direct measurements of azimuthal modes of turbofan engine noise are done in intake of engine to compare with azimuthal structure which is determined via noise measurement in the far field.

System of measurement

The array of 100 microphones were used along the circle of the radius 1.783 as shown in Fig.1. The microphones were placed in optimum place where dynamic range of the array for azimuthal mode is maximum. It is shown in Fig.1 the leakage spectrum F.

The method of cross-correlations is used for obtaining azimuthal modes and it is formulated as follows.

3. Results

The engine intake was placed on the floor and other parts were covered by blocks of non-reflected wedges as shown in Fig.2 and as it is clear from the Fig.2 the microphone array installed in intake of engine.

I the right side of the Fig.2 It is shown an example measurement results and here the synthesized mode m=+8 is determined.

After the validating the measurement system in laboratory, the microphones were installed on turbofan engine for static test. The engine axis was 5 m above the ground and the weather conditions was good and the maximum air speed was 5 m/s with no turbulence control during the test.

They tested the engine with three different engine runs with different thrusts. At each situation the acoustic measurement tested twice and the length of each measurement was 40 s and the range of frequency was 20 Hz – 128000 Hz.

As an illustration, the measured modal structure of sound field in intake of engine is shown Fig.3 where blade passing frequency of fan for two situations for each run is sketched. It is obvious from the Fig.3 that the amplitude of azimuthal modes are same but the amplitudes are different and the reason of this difference can be due to change in inflow condition while no turbulence control systems were used.

In this paper the authors tried to do same work for a microphone array in 200 degree angular range around the intake instead of 360 degree. The results from this setup were compared with previous setup and it is shown in Fig.4. According to Fig.4 it can be seen that the setup 2 which is limited array tend to overestimate the amplitude because of smaller dynamic range. In both cases the dominant mode is -21. The optimization of microphone positions in setup 1 will increase the dynamic range of array of microphones and makes better comparison of amplitudes. The Fig.4 is shown in the next page.

Conclusion

In this work, the limited angle range of an array of microphones is introduced. The developed system of measurement can be applied for determining the azimuthal structure of turbofan engines and increasing performance of engine noise reduction with liners.

Now we go through the second paper, ‘Analysis of Sources and Methods for Reducing Noise by Minimizing Vibrations of Engineering Technological Processes’.

Introduction

One of the factors which is the result of all of production process is noise. Vibration is an important noise source in mechanical systems in industries. In the industries, noise control system is conducted by minimizing the noise generated from vibration. In this regard, the noise generation at source of noise is controlled.

Acoustics noise is a source of environment pollution which has bad effects on human life. Industrial Noise like other forms of pollution has negative effects on human and makes some problems for hearing system. Sound more than specific sound pressure level causes hypertension and heart problem.

Formulation of the problem

A source of mechanical noise can be the interaction of elements of production equipment due to it’s shape or surface error. So in order to reduce noise in industries, noise reduction can be done by minimizing the noise at the source of noise and improving the equipment of production system. We should note that there are various types of noise absorption in industries such as sound insulation and liners and passive methods.

One of the important noise sources in industries is vibration. The actual problem in modern industries is the making correlation between noise and vibration for controlling noise by control systems. Having gears in this compositions in system of production is important to reduce the vibrations.

Vibration Sources and noise

The process of transferring mechanical energy from electrical motor causes energy loss in engineering production. The sources of technological equipment vibration can be classified in three sources as below:

·       Accidental sources of vibration

Steady sources can be because of imbalance of system elements of production equipment as well as impact processes. Unsteady state sources can be different processes in production equipment are used. Accidental sources can be like friction between elements.

There are different noises in production system like aerodynamic noise from engines. The sources in industries are from different noise processes and we can mention some of them as below:

·       Interaction between the elements of systems because of presence of imbalance or surface errors;

·       Fluctuation of elements in electromechanical systems which leads to electromagnetic noise;

·       Interaction of air flows

The scientists have done much work to understand the effects of noise on human beings and they developed standards that can describe acceptable noise in environment. Then these standards made the regulations which can be adapted by different communities for industries.

We must check and compare the measurement of noise from a standard regulation otherwise the measurement is meaningless. If the measurement of noise can not satisfy the standard regulations, then there must be some actions done to reduce the noise level. An example of this problem, we can look at the design of aircraft engines these days. The engines must have a maximum noise generation level to get some certificates.

For controlling the noise, we can do it in three different ways as below:

·       We can control the noise at the source of noise;

·       We can control the noise between the source and microphone;

·       We can control the noise at a microphone.

Among the ways above, the controlling the noise at the source is the simplest and cheapest way. First we should identify the noise source and it’s properties, then analyze vibration parameters. The most important source in mechanical systems is vibration and thus establishing the relation between noise and vibration can help us to reduce the noise.

Results

For experimental part we have electric motor (I) which is source of noise and vibration and we have base of equipment (II) and a converter (III) and a device for transmitting electric signals to a vibration measuring device (IV). The Fig.1 we can see the schematic of the system.

In experimental part we have two different cycles of measurements. The first cycle of measurement is fixed loads in position 1 and second cycle in position 2 in the figure above. Sound pressure level (SPL) is the parameter characterizing the noise and vibration acceleration level is the parameter characterizing vibration.

By analyzing the experimental research results they could stablish a relation between the parameters characterizing industrial noise and vibration. This correlation between noise and vibration is done over entire frequency range and at low frequencies the amplitudes of oscillations is greater in comparison with high frequencies. An important result from the experimental research is that the authors could determine that in position 1, we have highest value of SPL due to  the highest value of electric motor rotor imbalance and at the position 2, we have the lowest value of sound pressure level due to lowest value of electric motor imbalance. The experimental results is shown in Fig.2 where we can see the correlation between parameters corresponding to industrial noise and vibration.

Modeling the correlation of noise from vibration is in the form of equations as below:

Where X is the vibration acceleration level and Y is the sound pressure level and unit of both of them is dB and A, B are linear regression function. The linearized correlation from Fig.2 and equation above for frequency of 63 Hz is as below:

These equations introduce the noise control algorithms by vibration control system. In Fig.3 we can see the results of linearized noise correlation function from vibration.

The process of transferring mechanical energy from electrical motor causes energy loss in engineering production. Reduction of energy losses is essential for analyzing the active control methods at source of noise or vibration. In order to minimize the vibrations, the angular magnetostrictive device can be used in mechanical system elements for the automated control system on production equipment.

Sound pressure level sensor should be used in an automated noise control system in order to generate a control signal and the schematic of this configuration can be seen in Fig.4. Here the sensor receives information about actual value of sound pressure created by the control device. After that the information is given to the device from the sensor to compare the values. Now this device compares these values with standard values and after that, the device compares the values with magnetostriction device and finally the mag.dev. orders a control action on the control object. This algorithm is used to minimize the noise at the source.

Conclusion

One of the factors which is the result of all of production process is noise. Vibration is an important noise source in mechanical systems in industries. In the industries, noise control system is conducted by minimizing the noise generated from vibration. In this regard, the noise generation at source of noise is controlled.

Some conclusions that we can have are as below:

·       One of the important sources of noise in industries is vibration of mechanical system elements.

·       Experimental results  can help to perform the correlation between industrial noise and vibration and analysis of this correlation helps us to make a relation between these two parameters, therefore the results from the functions with help of mathematical analysis linearized the correlation of noise by vibration of elements.

·       By minimizing the vibrations of processes in industries we can reduce the noise

·       By using the control algorithm at the source of vibration, it is possible to minimize the noise at source with help of noise control automated system.

Third paper: Helicopter Noise: State-of-the-Art

Introduction

There are different of noise sources related with helicopters, and their relative importance depends on the helicopter design and the criteria that is considered. In this paper helicopter external noise and specially the noise due to helicopter main and tail rotors is reviewed. The important issues regarding annoyance and audibility are discussed. Sources of rotor noise include steady, periodic, and random loads on the rotor blades and volume displacement and nonlinear aerodynamic effects at high blade Mach numbers. Main rotor or tail rotor of helicopter can be the dominant noise source at different positions of observer.

Regarding the engine noise, all components of engine makes noise. In this paper just noise from main and tail rotor of helicopter is discussed.

Annoyance and Audibility

The frequency weighting characteristic of the weighted sound level is shown in Fig. 1. It is obvious that lower frequencies of noise are less annoying compare with higher frequencies. Sound generated by an aircraft propagates through the air and there are some absorbtions due to viscosity.

Physical Bases of Rotor Noise Generation

We consider the Lightill’s acoustic theory. Beginning from continuity equation and momentum conservation, but having mass source and dipoles in the fluid, Lightill could show that these equations can be put in the form of wave equation which on the left hand side we have these equations and on the right hand side we have mass source and dipoles and the formulation is as follows:

Lightil’s simplified this equation by considering that the right hand side of equation we have known source terms. So in this way the inhomogeneous equation can be solved for radiated sound. Here they assumed that rotor blade is comprised of three different sources;

1.     Moving sources and sinks to model the displacement of the rotor;

2.     Moving forces to model the forces between the fluid and the rotor;

3.     Moving Tij distribution which is related to nonlinear flow effects.

For understanding better of Lightill’s stress we can say that this includes the turbulence effect, compressible flow and shock-wave near the tip of the blade and viscosity.

Lightill’s equation can be written as:

We can write both equation (1) and (2) in the inhomogeneous wave equation form and thus we can write:

And we can write it’s solution as below:

or by using the retarded time we can rewrite it as:

Typical Helicopter Noise Time Histories and Spectra

A helicopter noise spectrum is made of two types of sound:

1.     Periodic part

2.     Non-periodic part

Fig.3 Spectrum of acoustic signal of UH-1H: 80 knots, 400-ft/min descent

Fig.4 Measured acoustic spectrum from a hovering helicopter and
calculated spectra based on atmospheric turbulence induced random

In this section we will discuss about noise generated by forces.

When the main rotor fundamental frequency is on the order of 15 Hz, and only the higher harmonics are important for annoyance and audibility.

For some helicopters, tail rotor rotational noise is more important than main rotor noise in some specific parts of the spectrum. This is usually from 100 to 500 Hz, a range which is very important to audibility and annoyance. Tail rotors produce a large number of rotational harmonics and combination tones with the main rotor, as their inflow is generally quite non-uniform because of ingestion of the periodically distorted main rotor wake and the effect of the nearby tail boom or pylon the on the flow. However, by reducing tail rotor tip speed and changing the position of the tail rotor with respect to the main rotor wake can be useful in reducing the radiation.

When the rotor blade is passing from the vortex that exists in the air, we have blade-vortex interaction. Because of complexity of trailing tip vortex geometry, the prediction of the noise is hard. Even for a single rotor we have many blade-vortex interactions which depend on the speed and rate of decreasing the altitude. We can indicate when the blade-vortex interaction occurs.

D. Stall and Shock Effects in Blade-Vortex Interactions

Unsteadiness and shock-wave in tip of the blade give loadings that are different from those found from classical acoustics. The best noise control technique is to eliminate the close passage of a blade.

E. Radiation Due to Vortex Streets and Related Phenomena

If we have fluctuating forces on a body, it generates sound. Although blades have streamlined shapes, similar vortex shedding occur on the blades in some specific Re number. This source only occurs when the B.L of one side of the blade is laminar. Prototype of helicopter rotor has turbulent boundary layer and thus this source is unimportant.

Other blade loadings can be because of the interaction of rotor blade and the turbulence which the blade has produced by it’s own motion. This interaction is still poorly understood experimentally and theoretically. Turbulent B.L noise is not as important as incident turbulence noise. Blade tip shapes also affect trailing tip vortices.

G. Noise Due to Turbulent Inflow

Fluctuating loading related with ambient inflow turbulence is an important source of random part of rotor noise. We know that turbulent upwash fluctuations causes the unsteady load fluctuations which can radiate the sound. The interactions with larger eddies generates lower frequencies and vice versa.

Some people call this noise as thickness noise and the displacement of air volume because of the motion of the rotor blade causes the thickness noise.

Sound Due to Ttj Terms

Lightill’s stress tensor or quadrupole source has different mechanisms in itself. Fowck-Williams and Hawkings showed that the mean flow turbulence interaction is important here.

Noise Reduction Techniques

Now we arrived to important part of this paper where we can see some techniques for reducing the noise of helicopter rotor blade. Noise reduction techniques are related to prediction of noise. In helicopter noise we have different sources and we must know which of these sources is dominant source and after that we should know how they are designed and about operating conditions to be able to reduce the noise. One of the ways for reduction of noise is a reduction in rotor tip speed. In this way we reduce the rotational noise because of slower source motion and it reduces random noise by reducing loadings due to velocity fluctuations and finally reduces high Mach number effect by reducing the advancing side of blade M numbers. But here we have some limitations for reducing the tip speed because of adverse effect on helicopter performance and we should find optimum tip speed.

Other noise reduction techniques are reduced disk loading, changes the number of the blades, changing the area, changing the twist and shape. Again some of these parameters can have negative effects on performance of the helicopter and the design point. As an illustration for effect of number of blades and chord on sound spectrum, we can take a look on Fig.5.

Fig.5 Comparison of turbulent inflow noise spectra envelopes for changes in number of blades (B) and blade chord (c)

We can see that an increase number of blades, increases turbulent inflow noise however, it reduces and raises the frequencies of rotational noise.

Conclusions