 We have been talking about various kinds of axial flow compressors and their designs in the last few lectures. We have done various aerodynamic issues related to the axial flow compressors and then of course, we talked about how those aerodynamic theories and understandings can be used to create and design axial flow compressors, blades, rotors and stators. Now, all of it together also has another issue or a serious issue, which we will talk about in the today's lecture. This is regarding the noise that is created by the axial flow compressors and the fans used in turbo fans, which need to be seriously regulated as per present regulations or traffic regulations all over the world. Now, in the early days of jet engines, when the jet engines started powering aircraft, the main source of noise was the jet noise. The jet noise that came out of the rear of the engine and that jet noise was the most noise making device or element in the jet engine. Over a period of time as the engines became more and more powerful, there is another component that became bigger issue in terms of noise making device. Noise related to compressors and fans in most of the turbofan engines that are used in civil aircraft engines have become such a big problem that the regulatory authorities all over the world have put stringent restrictions on the noise created by various jet engine elements. So, now we have jet engines that noise coming out from the rear of the engine as the jet noise, it also has noise coming out from the front of the engine, which is created by the fans and compressors. And we shall see in today's lecture that one of the reasons these noises have become more and more annoying and need to be regulated is because the aircraft compressors and fans have actually become more powerful. And as we have discussed in the last few lectures, they have actually gone supersonic or transonic. So, when they go supersonic and transonic to create more compressor loading and more compression ratio in a short smaller size compressors, they actually end up creating more noise. They are more noisy elements or more noisy machines. Remember they are aerodynamic machines. So, the noise that comes out of them are essentially aerodynamic noise. And this aerodynamic noise has become such an annoying source of problem that many regulating authorities have put stringent regulation on the noise emanating from these compressors and fans. Many airports in the world have very stringent noise regulations. And if you do not conform to those noise regulations, you cannot operate from those airports. So, the noise regulation has become such a stringent thing, it has now started impacting how the compressors and fans should be designed. And it is one of the reasons. In fact, one of the main reasons why commercial aircraft compressors and fans, fan being the first component in the compressor, have not gone beyond the transonic or let us say middle transonic Mach numbers, where the tip relative Mach number is of the order of 1.5 or 1.6. The military engines have gone beyond that. So, the technology for making high Mach number fans and compressors do exist, but they are not being used in commercial aircraft engines. And one of the main reasons is noise, because moment you have high inlet relative Mach numbers, the noise coming out of these engines immediately start going up. And we shall see that the noise is very strongly related to the velocity or the Mach number with which the flow is indeed passing through the compressors. So, some of these issues have impacted how the blades should be designed, how the blades need to be finally configured. And we shall see that unless you conform to the noise regulation, your compressor is not going to be accepted commercially and your engine is not going to be certificated for operation worldwide. So, the issues are extremely important and we thought we should devote at least one lecture on this particular issue in which we will discuss noise related problems related to actual flow compressors and of course, fans. So, let us take a look at what are the fundamental issues that are involved here that is creating such a lot of annoyance to so many people all over the world. We shall stick ourselves to discussing noise problems in actual flow compressor. We will not go into the science of acoustics or science of sound making or sound measurement. We will just mention them in the passing, so that you become aware of the science that is involved in this particular problem related to aircraft engines. Because the science of acoustics is huge, it is a science by itself and it involves so many things and it does involve a lot of high level mathematics, all that is really speaking beyond the scope of this lecture series. So, we will stick to certain fundamental issues and I will try to bring to you certain fundamental definition that are used in quantifying the problem that we are facing that is the noise and then I will try to mention to you and show to you how this noise is actually measured and how it is actually related to the actual flow compressor operation and then finally, we will end up by discussing what people are doing to contain this noise. So, noise containment by design to begin with and then later on by certain containment methodology is a major technological issue with related to actual flow compressors and aircraft engines specifically. The land based gas turbine may not have such a big issue because most of those compressors are most likely to be subsonic compressors which do not make such a lot of noise and not certainly not as much as the aircraft engines. So, in land based gas turbine that noise is not that big issue as it is in a craft engines. So, we will stick to certain issues which are prevalent and really annoying issue with relation to aircraft engines. So, first let us get into a little bit just a little bit at just on the superficially the science of acoustics. We are looking at noise problems in actual flow compressors and fans. Now, compressor or fan noise as I mentioned is a different kind of noise than jet engine. So, jet engine creates a heavy noise that is due to the sharing action of the jet coming out from behind the nozzle of the engine. Whereas, the noise created by the compressors of fan is mainly due to the cutting of the blades rotating blades through the air at higher at high speed. So, as I mentioned higher the speed of this cutting higher is the noise that is created and this noise escalates as the speed relative to the rotating blade increase. Now, as we have seen this relative speed or relative velocity as we used to call it or relative Mach number may increase due to either increase of the rotating speed or due to higher incoming actual velocity coming through the intake of the engine. So, either of them could increase the relative velocity or relative air speed relative to the rotating blades. Then of course, the modern air engines have gone transonic certain of the air engines which are used in military applications have gone clearly supersonic and in those cases the noise is far more than that of the old subsonic compressors and fans. So, the problem is many fold today and as I see as we see it is directly related to the blade speed cutting through the air and as we shall see it is related to the fifth order or sixth order of the blade speed. So, the result is slight increase in blade speed could increase the noise tremendously hugely and that is something which the designers compressors and fan designers and aero engine designers are trying to contain by design and through containment methods. Let us get into certain fundamental issues related to the science of acoustics. Now, fundamentally sound is due to fluctuation of pressure levels and the travel of this fluctuation. So, if they travel in waves sound always travel in waves like light and these waves are due to the fluctuations of pressures. If we do not have any pressure fluctuation we are not going to have any sound. So, a silent zone means that there is no pressure fluctuation going on anywhere in that zone sound is typically measured in decibels. This has been derived over a period of years and it is measured in number of scales predominantly in A scale sometimes in B scale. There are many other scale which I will mention and finally, F scale which is called the flat scale and I will try to mention very briefly what these scales are just a little while later. The sound pressure level as I mentioned sound is actually a pressure fluctuation. So, sound pressure level is defined as SPL and that is 20 log 10 p by p reference and as I mentioned it is measured in dB or decibels. Now, dB measurement is in number of scales we will come to those scales just a little while later. Now, in this equation p is the RMS root mean square pressure fluctuation and p reference is the reference RMS root mean square of the pressure fluctuation and this reference is normally 2 into 10 to the power 5 newtons per meter square which is the normal units for pressure. So, there is a reference that has been sort of universally accepted at the moment and with respect to that reference what is the pressure that is operating at a particular instant for creation of sound. Acoustic power is defined as the power watt level of E W L in some literature in some books and this is given as 20 into log 10 W by W ref and that is also in dB and W is the acoustic power in watts and W ref is the reference acoustic power in that is normally given as 10 to the power minus 12 watts. So, you see the amount of power that is used by the noise is actually again coming out of the energy that is being transacted within the compressors or turbines. So, a measure of the power does tell us how much of power is literally getting frittered away in creation of noise rather than being used for purposeful use in aircraft engine. So, acoustic power gives us some idea how the power is going away in the form of noise energy. Now, these are the fundamental definitions of sound pressure level and acoustic power or sound power. Now, fundamentals of acoustics tells us that the pressure changes of fluctuations travel in waves with certain frequency and amplitude. Now, all waves travel with certain frequency and amplitude and normally higher the frequency lower is the amplitude and normally these are associated with the so called high pitched noise which are indeed also sometimes simply referred to as high frequency noise and these are normally producing or taking away low power. So, these are the low power noise making frequencies. On the other hand the high amplitude noise often has lower frequencies and these are the bass kind of sound and they often carry high power. So, this kind of sound is often associated with high amplitude. Now, when we are looking at a noise it is entirely possible and quite often it is so that it carries all spectrum of noise frequencies not just high frequency or low frequency high amplitude or low amplitude it carries noise of all many frequencies and many amplitudes. Now, this is often really the case as a result of which when you measure noise it becomes a problem because you are measuring all kinds of frequencies that are coming out of a particular noise source not just one particular frequency or amplitude and hence you need to actually measure all of them. So, measuring them is actually a very tricky job. So, the noise meters or the sound meters are essentially rather complex machines or complex measuring machines many of them are indeed available in the market, but they have been created after a lot of you know scientific development to capture all these frequencies and then show them up in a certain standard manner in a certain standardized manner in db's and I will come to the scales in a few minutes now. So, capturing the noise in a measuring instrument is another kind of sound another kind of science. So, one we have the science of acoustics which defines various kinds of noise or various kinds of sounds and tones and so on and so forth which includes music which is you know nice to our ear, but we call something noise which is annoying to us. So, music is very pleasant thing noise is extremely unpleasant and we are dealing with noise which are indeed very very unpleasant and we have to ensure that those things are eliminated from or reduced as much as possible in operation of civil aircraft engines. So, we are dealing with the science of acoustics the measurement of sound then becomes a little more complex issue. The overall noise is often measured as overall sound pressure level or simply called OSPL which is RMS pressure level of the entire noise signal composed of all kinds of frequencies and amplitudes. So, this is kind of an overall average noise and is often simply measured in terms of db as the overall noise. So, any machine that you have the simplest possible noise meter actually gives this value where all the noise that is picked up by the measuring sensor is converted into an RMS pressure level and hence it just gives the overall noise in decibels. The more detailed understanding of the overall noise signal is then broken into what is known as spectral components using certain mathematical formats and this is of course, done through Fourier transform which is typically used in many wave measurements various kinds of measurement of various waves and sound being waveform it is also captured in Fourier transform. Now, Fourier transform produces spectral density for which the standard bandwidth is 1 hertz. So, the frequencies of noises are indeed expressed in terms of hertz and it is normally measured from 0 or 1 or in most measurement machines from something like 10 to 20 kilo hertz that is 20,000 hertz. So, that is the frequency range over which normally the measurements can be made it is possible that the noise generated could be higher than 20 kilo hertz the frequency of noise or lower than let us say 10 kilo hertz it is it is quite possible, but the point essentially is that those kind of noise are normally of not of interest to us. Let us see what are the noises that are measured in various scales A scale B scale C scale D scale and F scale. Now, before we talk about the scales the human perception of sound is restricted between 10 or more likely 100 hertz to about 12 kilo hertz. So, anything less than about 100 hertz some very perceptive people might hear less than that and anything above 12 kilo hertz that is 12,000 hertz is normally not heard by a human people human beings and so anything beyond 12 hertz which are measurable sound in normal measuring instruments is often called ultrasonic sound. Now, this ultrasonic sound can indeed be heard by some other animals for example, like dogs or dolphins or even whales. So, there are some other animals who seem to or owls who seem to be able to hear ultrasonic sound, but human beings do not hear those kind of sounds. So, as far as human beings are concerned the sound of interest is normally between something like 10 hertz to about 12 kilo hertz it depends on it varies from person to person some people actually may have even lower perception range than what is shown here. So, what the measurement units normally pick up that range to conform to the human hearing purpose. Now, the human hearing perception capability is captured in the A scale. So, A scale that has been created correspond to human hearing and lower than that and higher than 12 kilo hertz are artificially factored down or toned down in the A scale and as a result the noise from 400 hertz to about 10 kilo hertz are shown dominantly. That means they are shown on one is to one basis, whereas the noise is below that hertz and above 10 kilo hertz are often factored down by in the A scale. So, that is what is called A scale corresponding to human perception B C D scales are created for various industrial usages where some of the scales like D are actually factoring in the noise over a period of time something like may be 8 hours or 10 hours or 12 hours conforming to industrial noise regulatory requirements. So, many of these scales are conform to those regulatory requirements used in the industries and F scale typically conforms to flat scale where all the noise actually that is coming out is picked up on one is to one basis. So, the entire noise from 10 to 20 kilo hertz is picked up in the F scale. So, those are the scales that are typically used in noise measurement typically the noise is nowadays measured in what is known as noise level which was the acoustic science of acoustics, but it is measured now in what is known as perceived noise level or P N L and it is measured in terms of perceived noise noise decibel or P N D B. Over a period of time people have tried to figure out what is most annoying kind of noise certain kind of noise that is most annoying it actually can vary from person to person it can vary from one time of the day to another time of the day. Sometimes it carries certain particular tone of noise which is extremely annoying and some of these are actually can be captured in the spectrum analysis of the noise meter and then a correction can be added to account for this extremely annoying tone that is present in the noise and then this corrected noise is referred to as effective perceived noise and is measured as effective perceived noise decibel. So, E P N D B is what nowadays noise is being measured especially with related to aircraft engine noise. The pressure fluctuations that we are talking about which is a source of noise can be expressed in terms of certain parameters that are indeed used in the noise capturing or noise measurement where P is equal to cos of alpha divided by 4 pi r into f by a into capital A. Now, capital A here is the amplitude of the oscillating force in which case there is a pressure fluctuation f is a frequency of the oscillation it is being measured at a distance r from the point of observation or the source of the noise is at a distance r from the point of measurement. So, where you measure is extremely important noise indeed does increase as you go towards the source of the noise and it of course, you measure less and less as you go away from the source of the noise. So, the distance r is extremely important in the noise capturing of measurement business and A of course, is the speed of sound and alpha is the angle at which over which the noise is being captured typically you would be measuring at number of angular locations or at an angular distance and alpha is the measure of that. Now, this allows for an approximate estimate of the pressure fluctuations and may provide an approximate estimate of the power associated with it. Now, as I was trying to mention little while earlier the power that goes with the noise is proportional to 6 power of the blade speed. So, a slight increase or decrease of the blade speed would strongly impact the noise and the power that is associated with the noise. So, we have to keep an eye on that blade speed or the relative blade speed very stringently if you want to keep a check on the noise that is emanating from the rotating blades. As I was mentioning the spectra of the noise can be captured or shown in different ways. One way is the noise meters are designed to measure either in one third octave spectra which is shown here at the top which misses some of the details, but it captures one third octave spectra. You can see here the band that is being captured here is from something like 50 hertz to all the way up to 10000 hertz which is the normal range of human hearing, but it is captured in one third octave spectra or it can be captured in full spectra as it is shown below here and entire detail again from low hertz something like 1050 hertz to 10000 hertz has been captured and you can see here all the details have been many of the details which were missing in the one third octave spectra can be captured in the full spectra noise. The instrument that can capture full spectra indeed is as can be well expected is a costlier instrument whereas one third octave spectra is little cheaper instrument and both the instruments of course can give the overall noise which we mentioned a little while earlier. That is the overall RMS of all these frequencies averaged very quickly and gives in one single value of db whereas as you can see here at various frequencies the db levels or spl levels are different and as a result at various frequencies the db levels are different. So, some frequencies the db is lower at some frequencies the db is very high and those are the annoying noises which need to be somehow cut out and the designers and the engine designers the containment people have to work out how to cut out those noises which are coming from particular frequencies. Now, what happens is with relation typically to aircraft blades engine blades compressor blades certain noise peaks are associated with the blade passing frequency. Now, what happens in a blade passing frequency is that one particular blade while rotating passing a particular point in rotation is referred to as a blade passing frequency is very easy to calculate that particular blade passing frequency and that blade passing frequency or blade cutting frequency a peak is observed here. Now, that is an obvious and most easily identifiable tone that is captured by the noise measuring instrument and in a old fashioned subsonic compressor that blade passing frequency was the most identifiable noise source or tone which is easily at least 10 db above the rest of the noises as it used to be. In the recent compressors which have gone supersonic and transonic we can see multiple pure tones that means there are many of them mainly due to the shocks that are coming out of the blades and those shocks create noise and blade passing frequency tone or that particular frequency noise is kind of mixed up or buried in all the multiple tones coming out of the supersonic compressors or fans. So, the modern engines and the 10 db you know gap or safety margin that we had is gone because there are so many other frequencies at which high noise is indeed happening some of them are indeed higher than even the blade passing frequency. So, the noise coming out from axial flow compressors or fans the modern ones are indeed of multiple tones and they are of course, more difficult to contain. Now, let us look at what is happening in case when you have a rotor and stator there is interaction of aerodynamic interaction between the rotor and the stator and this interaction also creates noise. There is a cutting of the flow that is coming out of the rotor from the trailing edge of the rotor and then of course, you have blade 1, blade 2, blade 3, blade 4 and then you have the stator 1, stator 2, stator 3, stator 4. Now, what happens is there is a interaction of this trailing edge which is coming out with the stationary blades with relation to the stationary blade remember the rotor is moving component. So, the wigs that are coming out are moving wigs and hence there is a relative velocity associated with the stator and the moving wigs. Now, so those interactions create another kind of noise and those noises can be two kinds depending on the dynamics of the situation. It can be propagating noise as is shown on top here. So, it can propagate in waves like this and it can keep on propagating or it can be somewhat decaying interaction field where the noise kind of every particular noise source from B 1, S 1, B 2, S 2 kind of sort of dies off. They do not propagate along as is shown on top here and each of those interactions die off on their own. So, it could be a propagating interaction field or it could be decaying interaction field. So, interaction between the rotor and the stator. So, not only the rotor creates noise the interaction between the rotor and the stator also create noise and some of the interactions are indeed in the modern compressors transonic or supersonic. Now, let us see what happens when you have the blades set apart. We know that between the rotor and stator there is a spacing that is decided by the aerodynamics of the design and designers actually do a lot of analysis aerodynamic analysis before they fix certain rotor stator axial gap or axial spacing. Now, as we can see here the interaction between the two create a annoying noise and it has impact on the noise. This spacing between the rotor and the stator has an impact on the noise. As we can see here the lower the spacing that is more close they are higher is the noise. It can be as high as 140 dB which is extremely annoying and painful to human perception or human ear. If you increase the spacing or the distance between rotor stator to from half chord to 2 chord the noise comes down and if it is increased as much as 4 chords noise comes down even more. The trouble is if you increase the distance between the rotor and the stator it is known that the noise would go down, but it would also reduce the compressor stage efficiency. That means the rotor and the stator would not then work as one single aerodynamic unit of a compressor and as a result the efficiency of the compression would actually indeed go down. Now, when the efficiency goes down it impacts on the overall engine efficiency overall engine fuel efficiency, but even more importantly if you separate the rotor and the stator remember the overall compressor length is now going up. So, rotor stator rotor stator if you keep on increasing the distance between the rotor and the stator the overall length of the compressor is going up overall length of the engine is going up. Now, this is unacceptable this is unacceptable to the engine designer to the compressor designer nobody wants a long engine anymore. So, what is good for noise is not likely to be accepted as an engineering solution to the engine designer. So, there has to be a certain amount of compromise between the noise that is happening and the engineering solution of a compact powerful engine which will power a flying aircraft. So, this is a compromise that has to be done a priori at the time of design before the engine is actually made. So, this is where most of the modern aero engine designers the compressor designers the fan designers are spending a lot of time lot of analysis in trying to keep the basic source of noise down under operating conditions various operating conditions of the aircraft engine. If we look at what happens to the noise we can see that certain under certain operating conditions the flow that is going into the engine is indeed different. For example, under static flow condition when the aircraft is at the top of let us say a take off condition take off run this flow is going into the engine from all sides. So, the noise that is coming out will also be impeded from all sides or it will be carried with this air on all sides. So, the inflow characteristics during the static test flow or during take off is quite different from when the aircraft is flying when the air is going into the engine in a more uniform manner. So, the manner by which the air is ingested into the engine also would in a way decide how the noise comes out of the engine and is propagated. So, as we can see here the noise coming out it is in waves. So, it will propagate in waves like this whereas, in situation like this it will propagate in waves in all directions. So, in a take off condition the noise very quickly spreads in all directions whereas, in flying condition it normally spreads straight in the front of the compressor or fan of an aero engine. So, it depends on what the operation is of the engine as to how the noise actually propagates from the source of noise creation. Now, depending on that the noise that actually can be measured shows that the blade passing frequency if it is a stationary engine the noise measured is often higher under take off conditions is normally of a much higher order. Whereas, in flight condition the noise are of a much lower order it depends on the fan relative tick mark number. Typically, during the aircraft take off it uses maximum thrust and as a result of which the noise created indeed is somewhere over here which is indeed quite high could be very high. Whereas, during a cruise at some altitude the fan relative tick mark number is normally somewhere down here and as a result of which the noise as we can see is substantially lower than the take off or ground static noise. So, noise does depend on the fan relative tick mark number. We can see here it also depends on the compressor operating condition. As we can see here the when the compressor is operating under normal open throttle operating condition in flight the noise is normally somewhat on the lower side. The tip speed of course, impacts on the other hand if it is operating at peak pressure that means, when the pressure created is high the noise is higher this is expected because noise depends on the pressure that is the source of noise. When the compressor has gone into surge which is the worst situation from stall the noise is much higher and in fact an experienced compressor operator can make out that the compressor has indeed gone into stall or surge by hearing the noise coming out of it. So, noise actually is easily identifiable when the compressor is surged it depends on the tip speed. So, as we can see here the noise values would depend on the operating condition and the operating speed of the compressor or fan. So, these are the issues that are to be considered while indeed designing a particular aircraft compressor or fan and together they would have to be put together in final design. Now the noise levels which as I mentioned are of annoyance to people who are passengers and others who are around the aircraft at the time of operation the jet noise which typically creates noise of the order of you know 92 about 140 dB. Now, 140 dB is extremely painful the human hearing is normally good up to about 100, 110 anything more than that becomes painful 140 is extremely painful in the sense if you listen to if you have to listen to 140 dB for a few minutes it could impact your hearing capability for a long time and if you are subject to that kind of noise over a certain period of time your hearing human hearing may be impaired for life. So, the noise is very important issue and it can be extremely dangerous to human beings subjected to such very high noise. Now, fan or compressor noise of subsonic fans where of the order of 100 dB or thereabouts moment it has gone transonic supersonic the noises are now of the order of 100 or more and as we just saw certain tones the noises are of the order of 140 dB's. So, noise permitted by regulations is much lower coming out under certain conditions measured at a certain distance something like 50 meters away from the engine at a certain angle under certain operating conditions have to be between 80 and 95 dB all most for all kinds of aircraft engines and this are the noise regulations that have been put in place to safeguard the human beings that are present inside the aircraft or outside the aircraft in the airport which means that to conform to regulation the engine designer have to use noise containment or noise suppressing methods because normally some of the noises indeed actually much higher than that. So, let us take a quick look at what all things can be done the noise attenuation as it is called or acoustic treatment as it is called in engineering terminology is normally with the help of perforated metal sheets with certain perforation or porosity level that is prescribed by the designer and it surrounds the compressor or fan. So, the casing of the compressor or fan is made up of such made with such perforated metal sheet and around the metal sheet you have honeycomb structures which also absorb a lot of noise you remember noise carrying power or energy that has to be absorbed and that results in large amount of attenuation or absorption of noise depth of the honeycomb structure determines the frequency which is mostly attenuated or absorbed. So, honeycomb structure will have to be designed by specialist to absorb noise of certain kinds. So, that most of the high pitch noise is indeed absorbed porosity of the metal sheet also has to be you know determined in a manner which actually attenuate certain kind of noise frequencies. Now, this treatment of using metal porous metal sheet or perforated metal sheet and honeycomb works best for high frequency noise which I mentioned earlier actually carries low power. The high amplitude noise which carry high power are actually more difficult to attenuate and now we know why because they actually carry high power. So, most of the noise that is annoying to our ear are essentially high frequency noise and they need to be indeed attenuated or absorbed by certain mechanical means. So, that at least the minimum safety is guaranteed for the people who are around the aircraft. So, we have seen that a number of issues are involved here. There is a lot of aerodynamics, there is a lot of mathematics which we have not done the science of acoustics all that needs to be factored into the understanding of the noise and then the fan or compressor designer would have to design a compressors that conform to this noise and only then it goes on to the aircraft. So, the modern compressor designers would have to conform to noise regulations very stringently otherwise his compressor of fan is not likely to be accepted by the commercial engine designers. So, this brings us to the end of a discussion on various aspect of axial flow compressors and fans. We have discussed various things, but more predominantly we have discussed the aerodynamic issues that are related to the axial flow compressors and fan. Now, we will move on to the turbines and to begin with axial flow turbines. So, in the next class we will introduce the axial flow turbines, the fundamentals of axial flow turbines and we shall see that it is a different kind of machine and that is what we will do in the next class, axial flow turbines and introduction to axial flow turbines.