Development and testing of an acoustic measurement system for

the IST Aeroacoustic Wind Tunnel

Francisco Andre dos Inocentes Martins de Magalhaes Pereirafrancisco.pereira@tecnico.ulisboa.pt

Instituto Superior Tecnico, Universidade de Lisboa, Portugal

November 2017

Abstract

Aviation authorities have begun to impose stricter noise regulations and penalties to airlines toaddress the effects of noise pollution on communities. This is a problem that is gradually becomingmore evident in cities such as Lisbon where the airports is located near the city centre since the peaknoise from an aircraft is during its landing and take-off phase. Historically, the industry investment hasprioritized the reduction of noise originated by the engines but the vast development in the propulsionarea has made the noise produced by the airframe to be more significant. The goal of this work is toproduce the tools needed to acquire, process and visualize data captured by numerous sensors placedinside the Aeroacoustic Wind Tunnel located in the Aerospace Engineering Laboratory of IST, and tocarry out the characterization of the facility. This dissertation covers a throughout explanation of thedevelopment of the program using the LabVIEW software, the equipment used, established proceduresand the results of air flow and acoustic tests including the characterization of the flow and backgroundnoise levels. Flow uniformity tests revealed that flow profiles are generally consistent across all freestreamvelocities but there is a maximum 5.6% undershoot in freestream velocity near the centre of the jet whichmay pose a problem for future experiments. Background noise levels analysis showed the narrowbandbackground SPL spectra in the anechoic chamber for various velocities in different conditions as well asthe A-weighted sound pressure level measured during the studied scenarios.Keywords: Aeroacoustic wind tunnel, LabVIEW, anechoic chamber, background noise, aeroacousticperformance.

1. IntroductionHuman beings have only been flying for just over acentury but have already filled the skies with thou-sands of aircraft. The fact that air traffic is growingrapidly means that there is a huge increase in noisepollution in residential areas closer to airports, aproblem that is gradually becoming more evident inLisbon, for example. Aviation authorities have be-gun to impose stricter noise regulations to addressthe effects of noise pollution on communities.

The peak noise from an aircraft is during its land-ing and take off phase. Historically, the industryinvestment has prioritized the reduction of noiseoriginated by the engines and the ultra-high bypassengines and other low-noise propulsive technologieshave greatly reduced the overall noise levels in thelast few decades. Such development in the propul-sion area has made the noise produced by the air-frame to be more significant over the time. Whilethe engines originate most of the noise during thetake off, at approach conditions the engines havea low enough work rate that the airframe inducednoise reaches the same order of magnitude.Airframe

noise is dominated by the contributions from thehigh lift devices (leading edge slats, flap side-edge,trailing edge, etc.) and deployed landing gear sys-tems. In general, the noise of larger aircraft is dom-inated by the landing gear while in medium aircraftit is mainly generated by slats and flaps.

The sources of airframe noise are seen as thebarrier to overcome and this has motivated an in-creasing number of people in academia and indus-try to focus on aeroacoustic research. High qual-ity aeroacoustic wind tunnels play a very impor-tant role in facilitating interdisciplinary researchbetween airframe aerodynamics and aeroacoustics,and will continue to do so in the future.

The main purpose of this thesis is to characterizethe Aeroacoustic Wind Tunnel of the Aerospace En-gineering Laboratory of IST and produce the toolsand procedures necessary for potential future stud-ies using this facility. The first goal is to developa program using LabVIEW that allows the acquisi-tion and visualization of data captured by sensorsplaced inside the anechoic chamber. The secondmain objective is to run various tests and handle

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all the data acquired in a way that the final re-sults of the experiments can be compared betweeneach other so one can evaluate the air flow andthe acoustic performance of the aeroacoustic windtunnel. Before this work, a characterization of theacoustic quality of this wind tunnel has never beenperformed.

2. BackgroundAeroacoustics is a branch of acoustics that dealswith the production of noise as a result of aerody-namic forces interacting with surfaces or turbulentfluid motion. Noise generation can also be related toperiodically varying flows, such as the phenomenonreferred to as the Aeolian Tones. In fact, this phe-nomenon is the main topic of aeroacoustics, steadytones that result from trailing vortices with oscil-latory behaviour created when air passes over orthrough an obstacle [1].

The first publication of Sir James Lighthill [2]in 1952 originated the modern discipline of aeroa-coustics, when attempts to understand and predictthe noise generation associated with jet engine werebeginning to be done in a thorough way. There isno scientific theory of the generation of noise byaerodynamic flows that is completely established,but most aeroacoustic analysis relies upon the of-ten called ”Lighthill’s analogy”, which gave a for-mal definition of an acoustic field in a flow. AfterLighthill, other acoustic analogies have been formu-lated by Curle, Powell and Ffowcs Willams. Theyrelate the wave like propagating pressure/densityfluctuations to the hydrodynamic characteristics ofthe flow, by rearranging the Navier-Stokes equa-tions [3].

2.1. Properties of SoundIn physics, sound is defined as a vibration that typ-ically propagates as an audible wave of pressure, bymeans of a transmission medium such as air, wa-ter or other materials. That vibration propagatesaway from the sound source at a certain speed, thusforming a sound wave. In general, the behaviour ofsound waves is affected by the density and pressureof the medium, as well as the temperature, by themotion of the medium itself and by the viscosity ofthe medium. These factors can increase or decreasethe speed of sound and also the rate at which thesound is attenuated. Depending on the mediumcharacteristics, sound can also be refracted, eitherdispersed or focused, thus increasing or decreasingsound levels.

Sound has two fundamental elements to it: pres-sure and time. Any sound can be represented as amixture of its component sinusoidal waves of differ-ent frequencies. There is a group of generic prop-erties that can be used to easily describe a soundwave: frequency, wavelength, amplitude, speed of

sound and directionSound travels most slowly in gases, it travels

faster in liquids and even faster in solids. For anideal gas:

K = γ × p, (1)

c =√γ ×R× T =

√γ × p

ρ=

√K

ρ, (2)

Where γ is the ratio of specific heat of an ideal gas,R is the gas constant, T is temperature in Kelvin, pis pressure, ρ is the density, K is the bulk modulusand c is the speed of sound.

Speed of sound in an ideal gas is dependent nearlyonly on its temperature. The speed of sound (c) inair at 20◦C and acoustic impedance (z):

• ρ20 = 1.204 kg/m3, z20 = 413 N · s/m3 andc20 = 343 m/s

Human beings can only perceive sounds that havefrequencies from about 20 Hz to 20000 Hz and thepeak sensivity is around 3500 Hz. There are sixdifferent qualities of sound perception. They are:pitch, duration, loudness, timbre, sonic texture andspatial location.

Noise can be described as unwanted sound that isseen as unpleasant, loud or disruptive to hearing, anundesirable component that obscures a wanted sig-nal. Environmental noise caused by vehiclessuch asaircraft expose millions of people to noise pollutionthat creates not only annoyance, but is also associ-ated with several negative health outcomes such ashearing loss, high blood pressure and sleep distur-bances, depending on duration and level of exposure[4].

Sound Pressure Level is calculated in decibels(dB). The logarithmic scale is used because of thelarge range of rms values perceived by the humanear. The reference value was chosen conventionallyto correspond to the threshold of human hearing(pref = 2 · 10−5Pa).

SPL = 20 × log10

(prms

pref

), (3)

In acoustics, weighting is a process that involves em-phasizing the contribution of a certain set of data,giving it more weight in the analysis. When mea-suring loudness, estimates of sound annoyance typ-ically rely on weighting filters. The most used typeof weighting is the A-weighting curve, it is a cor-rection to the SPL spectra which results in units ofdB(A) sound pressure level. It emphasises frequen-cies around 3000 Hz to 6000 Hz where the humanear is presumed to be most sensitive, while attenu-ating very high and very low frequencies to whichthe ear is less sensitive. The A-weighting curve has

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been widely adopted for environmental noise mea-surement, and is standard in most sound level me-ters. It is used to measure environmental noise suchas roadway noise, rail noise and aircraft noise.

Extensive research has been made in order to un-derstand how to mitigate flow induced noise. Flowinduced noise generation in presence of solid bodiescan be classified into three dominant mechanisms:vortex shedding noise, turbulence and structure in-teraction, trailing edge noise.

2.2. Wind TunnelWind tunnel testing is accurately reproducing windflow around full-scale or model-scale objects in atunnel-like construction under controlled conditionsto study their performance for aerodynamic designand research purposes, within a test environmentthat is also suitable for measuring the acoustics gen-erated by the interaction between the airflow andthe models. Despite CFD getting ever more power-ful and increasingly applied, it has not come closeto replacing the wind tunnel and it will never be,irrespective of the availability of computer power[5].

Almost every wind tunnel is a unique instrument.There is no mass production of wind tunnels, theyare almost always custom-made. They can be opencircuit or closed circuit, with an open test section orclosed test section, with each configuration havingits pros and cons. Although it is possible to conductaeroacoustic tests inside a typical hard-walled windtunnel, it is far from ideal. An anechoic open-jetwind tunnel is the most suitable to analyse noise in-tensity and directivity to predict full-scale far-fieldnoise emission levels. The purpose of having an ane-choic chamber surrounding the open-jet test sectionis to provide a non-reverberant, quiet, still-air en-vironment for noise measurement. The theoreticalfrequency cut-off limit above which the chamber isconsidered anechoic is determined via the equation[6]:

fc =c

4 × Lw, (4)

Where c is the speed of sound in the chamber, andLw the height of the wedges.

The aeroacoustic wind tunnel of the AerospaceEngineering Laboratory of IST is a closed circuit,U-shaped wind tunnel with an open test section.The anechoic chamber is lined with foam wedgeswith tip-to-base depth of 0.285 m and has a the-oretical anechoic cut-off frequency of 80 Hz. Thechamber inner dimensions as measured from wedge-tip to wedge-tip are approximately 4.3 m × 3.2 m× 2.7 m (L × W × H). It is equipped with a 200kW and 1500 rpm electric motor that powers a 7blade fan. Airflow maximum velocity can reach upto 50 m/s.

3. Experimental TechniquesAcoustic diagnostics are usually done using micro-phones, they give us a good idea on where noisesources are located and how they sound. However,there are techniques that were developed in order tobetter understand the mechanisms of the generationof sound. A big investment has been made in ad-vanced measuring methods for flow and acoustics,being two of those methods the PIV-based predic-tion of radiated sound and Acoustic Beamforming.

The Particle Image Velocimetry (PIV) methodmakes it possible to observe the source of the sound.This method measures movement of seeding parti-cles between two laser light pulses, applies cross-correlation to interrogation areas and the velocityvector map of whole target area is obtained. Theseeding particles used to accurately follow the fluidmotion do not alter its properties or flow charac-teristics. Commonly, the PIV method is used tomeasure near-field data that serves as an input invarious acoustic analogies that are then used to pre-dict far-field acoustic pressure fluctuations.

Figure 1: Typical PIV set up [7].

Microphones convert acoustic energy into electri-cal energy. Acoustic beamforming uses arrays ofmicrophones to investigate the sources of sound indifficult surroundings, and it has become a standardmethod. Microphones can be classified according tothe type of their transducer, the most common typeis the condenser microphone.

Figure 2: Condenser microphone.

When sound waves hit the diaphragm, it movesback and forth relative to the solid backplateand the distance between the two capacitor plateschanges. As a result, the capacitance changes ac-cording to the sound waves. That movement leadsto the conversion of sound into an electrical signal.The variation in voltage that is measured can be

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converted in terms of pressure fluctuations by cali-bration.

Beamforming microphone arrays consist of tensto hundreds of microphones. It is based on an aver-aging of sound signals from different receivers. Forthe system to be accurate each microphone mustbe calibrated to account for varying magnitude andphase frequency responses.

Figure 3: Principle of array beamforming [8].

4. Equipment and Approach

In data acquisition projects, in particular whendeveloping a program in LabVIEW, knowing theequipment is essential to develop a simple and or-ganized programme that meets all the requirements.What DAQ device is available, how many sensorswill be used, cables needed, and most of all, whatdata is of interest and what measurements shouldthe software display and save.

The main pieces of equipment used in this workconsists on a computer with needed software in-stalled, the DAQ device NI PCIe-6353 (with 16 dif-ferential or 32 single ended channels, ADC resolu-tion of 16 bits and a multichannel maximum ag-gregate sample rate of 1.00 MS/s), six Bruel & Kjrmicrophones type 4958, a pitot tube sensor (usedto measure the speed of the flow inside the aeroa-coustic wind tunnel), two four channel ICP Sen-sor Signal Conditioners model 482C15 from PCBPiezotronics, one NI CB-68LP connector block andSHC68-C68-D4 NI custom cable connector, coaxialcables, a Sound Level Meter, multimeter and DCpower supply, a speaker, an oscilloscope and othertools like adhesive tape, scissors, screwdrivers andmeasuring tape.

There will be two very different stages to thisproject: the first stage being the development ofthe software and testing of the equipment beforemoving on to the wind tunnel facility, and the sec-ond stage being the experimental activities, whenexperimental proceedings are designed so that allthe tests are made following the same rules andultimately proceed to the characterization of theaeroacoustic wind tunnel.

5. Development of the Data Acquisition Pro-gram

LabVIEW has been widely adopted throughout in-dustry, academia, and research labs as the stan-dard for data acquisition and instrument controlsoftware. A LabVIEW program consists of oneor more virtual instruments (VIs) that can be di-vided into three parts: Front Panel (the interac-tive user interface of a VI), Block Diagram (VI’ssource code, it includes terminals, subVIs, func-tions, constants, structures, and wires that transferdata among other block diagram objects) and Icon(VI’s pictorial representation).

5.1. First VI

The data that will be transferred from the sensorsto the DAQ device and to the software is voltageconverted by each sensor. The first VI was made toshow a graph of the measured voltage and frequencyvalue of a known signal using the oscilloscope. Asignal of 0,6 V and 1000 Hz frequency were chosen,and when running the program the expected valueswere displayed. This proved that the DAQ devicewas working as intended.

5.2. Frequency Analysis and Sound Level Meter VIs

The microphones transmit voltage data and thatvoltage must be transformed into units that can beused in subVIs that are used to process and dis-play sound measurements. The engineering unitswere set as Pascal, sensor sensitivity was set to11,2 mV/Pa and the dB reference [Pa] was set to20 × 10−6. The signal conditioners will be operat-ing with voltage gain set to x100 in order to amplifythe signals captured by the microphones for betteranalysis translating into pregain settings of 40 dB.The SVFA FFT Spectrum VI was used to processthe signal acquired and display the dB vs Hz graph.It computes the Fast Fourier Transform spectrumof the scaled signal, then returns FFT results as themagnitude and phase of the FFT spectrum.

An indicator of Leq value was also added usingthe SVT Sound Level VI. Leq is the sound levelin dB, equivalent to the total sound energy over agiven period of time, and can be described mathe-matically by:

Leq = 10 log10

(1

t2 − t1×∫ t2

t1

(pApref

)2

dt

), (5)

Where the start time for measurement is repre-sented by t1 and t2 is the end time for measurement,pA is the acquired instantaneous sound pressure ofthe sound signal and pref is the reference soundpressure level of 20 µPa.

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5.3. Third-octave Analysis and Saving Data toMeasurement Files

A VI called SVT Third-octave Analysis was used,it performs a 1/3 octave analysis on scaled signalin accordance with the IEC 1260:1995 and ANSIS1.11-2004 standards. One-third octave analysisis widely accepted in many industries, such as theaeronautical industry. Several Write To Measure-ment File Express VIs were coded to save the ac-quired and processed data to different measure-ments files in the LVM file format that can be usedusing other programs such as MatLab and Excel.

5.4. Weighting Filter and Other Considerations

The SVT Weighting Filter VI applies the selectedweighting to the scaled signal and operates on time-continuous data. This VI complies with ANSIS1.4-1983, ANSI S1.42-2001, IEC 61672-1:2002, andJIS C 1509-1:2005 standards. When the the A-weighting setting is turned on the SVT Sound LevelVI indicator displays the LAeq value. LAeq mea-surements are extensively used in environmentalnoise standards as well as many other regulationsand documents.

A Trigger Loop was created to be run before themain While Loop, once the trigger conditions aremet, an internal trigger signal is sent to initiate theacquisition. The Elapsed Time Express VI was usedto indicate and control the amount of time that theprogram is set to run. Front panel controls and in-dicators were added to make it easier to customizeeach execution of the program. Finally, graphs thatdisplay the combined data of all the microphones to-gether were implemented for a better understandingand evaluation of the results. SVT STFT vs TimeVI was also added to perform a short-time Fouriertransform on the scaled signal and return the col-ormap at the time specified.

6. Experimental Set-up

The aeroacoustic wind tunnel is located in theAerospace Laboratory, while part of the work doneuntil this phase was done in a small room outsidethe laboratory. It was crucial to transport all theequipment to the Aerospace Laboratory and pre-pare it to be used. A couple of desks were putright next to the entrance of the anechoic chamber.The computer, signal conditioners, block connectorand cables were all mounted on and around thesedesks. The cables that link the microphones to thesignal conditioners are long (10 meters), because ofthat they can be installed inside the anechoic cham-ber and easily connected to the signal conditionersoutside that in turn are connected via coaxial ca-bles to the block connector. The connections weremade following the instructions on the NI PCIe-6353, ICP Sensor Signal Conditioners and NI CB-68LP user guides. Inside the anechoic chamber, the

microphones were mounted on a support bar thatexists for this purpose. Carbon sticks were usedto secure the microphones in the desired positions.Each sensor uses 3 channels on the NI PCIe-6353pinout, connector 0 (AI 0-15).

After mounting the equipment and having all mi-crophones connected, the main VI was launched andrun to try out the data acquisition system with arandom sound source to have an idea of how ev-erything would behave. This was the first time allthe sensors installed were working simultaneouslyand the system was acquiring, processing, display-ing and saving data at the same time. Unfortu-nately, the program was too big for the computer tohandle as delays were clearly noticed when the Runbutton was clicked and the time set for the programto run was not being respected. It was concludedthat the best way to solve this problem was to di-vide the main VI into two separate programs. Onewould consist on the VI that runs each microphoneindividually, and would be used when testing themicrophones individually. The second one wouldbe used in the main tests running all the micro-phones at the same time and displaying the com-bined processed data. During working hours thereare numerous sources of sound that are unavoid-able and may influence the results of the measure-ments inside the anechoic chamber. Machines likeventilation and equipment used in the surroundingfacilities, produce sounds that were registered bythe microphones during preliminary tests, as wellas the fact that the laboratory is constantly beingused by other students working on other projects.In order to try to make the interior of the anechoicchamber more sound proof, additional foam wedgeswere used to cover some key areas of the anechoicchamber.

6.1. Microphones: individual testsMicrophones were placed in a central position in thesupport bar, 1.45 meters high. A portable speaker,capable of producing sounds with frequencies rang-ing from 20 Hz to 20000 Hz, was placed 3 metersaway at the same height and facing in the micro-phone’s direction.

Figure 4: Frequency range test.

This first test consisted on evaluating the fre-

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quency range of the speaker and the microphones.A sound file that produces a sinusoidal wave goingthrough the entire human audio spectrum, startingat 20 Hz and ending at 20 kHz, was played for 90seconds. Note that the frequency increases expo-nentially during the test.

A test consisting on analysing pink noise was alsomade. Pink noise is often used as a reference tone tocheck frequency responses and becomes particularlyuseful when coupled with a 1/3 octave spectrumanalyser.

This test was carried out because pink noise isused for acoustic applications, such as loudspeakertests, and since pink noise has a flat spectrum whenviewed on a third-octave spectrum analyser it wasused to see if the developed VI would give the ex-pected results.

Figure 5: Pink noise test result, front panel.

For the remaining tests pure tones of 500 Hz, 1000Hz, 2500 Hz and 5000 Hz were played one at a time,for the duration of 1 minute. The same tests weremade positioning the speaker at a distance of 2 me-ters and 1 meter from the microphones. The follow-ing figures present the results captured during the1000 Hz pure tone tests.

Figure 6: Intensity chart for the pure tone of 1000Hz.

Figure 7: Narrowband SPL spectra of the 1000 Hzpure tone test.

Figure 8: SPL levels in dB acquired during the testsfor each of the distances using the same pure tonesound.

Microphones are usually calibrated by means of aprofessional microphone calibrator. The right wayof doing it is using a calibrator that emits a 1000 Hzreference tone at 94 or 114 dB SPL, in order to de-termine the sensitivity of the microphone and pre-amplifier pairing. However, there is no unit like thatavailable in the school and because of that there wasthe need of using a different approach here. Afterstudying the numerous results obtained during theindividual tests of the microphones, it was decidedto pick up the 1000 Hz tests results at 1 meter dis-tance between the speaker and the microphones andgo from there. The mean value of the sound pres-sure levels measured was calculated and correctingpregain values were calculated in order to match allthe microphones so they would read the same valuein the exact same condition. The microphones werenot calibrated having the results of a Sound LevelMeter in mind but to approximate the behaviour ofthe microphones to each other.

6.2. Positioning of the sensors

The microphones were placed in a support bar in-stalled in the far side of the anechoic chamber rela-tive to the test section, but due to its dimension itwas only possible to distribute 5 microphones in aspan of 60 degrees. The 6th microphone was placedin the wall of the anechoic chamber, opposite to theentrance, oriented 45 to tunnel centreline.

Figure 9: View of free-field microphone locationsfor tunnel noise measurements. Figure not to scale.

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After getting all the sensors in the planned posi-tions preliminary tests with the wind tunnel weremade to ascertain that everything was correctlymounted. This opportunity was taken to test themicrophones by measuring pure tones of 1000 Hzand 5000 Hz in two different tests, played by thespeaker used in previous tests. This speaker pro-duces a relatively clean sound when listening to iton axis, but when one gets 20 to 30 degrees off axisthere is a slight loss in sound clarity and intensity.This test was intended to verify this fact.

Sensor SPL (dB) SPL (dB)1000 Hz test 5000 Hz test

Mic 0 60.11 66.30Mic 1 59.10 63.44Mic 2 59.08 63.61Mic 3 58.57 62.83Mic 4 58.38 62.95Mic 5 57.50 61.25

Table 1: Pure tone tests with microphones placedin different positions.

7. Facility Characterization

Tests were performed with a slightly convergentnozzle mounted at the entrance of the chamber/testregion, and were repeated after removing the nozzleusing the same procedures.

7.1. Flow Speed

The flow velocity was measured using a pitot tubesituated at the centre of the test section. This pitottube was connected to a differential pressure trans-ducer, that in turn was connected to a multimeterpowered by a DC Power Supply. The signal was alsoacquired by the NI PCIe-6353 via a CB-68LP blockconnector. The values presented by the multimeterwere compared to the values acquired by the Lab-VIEW program to be sure that the results were sim-ilar. The freestream velocity was calibrated againstthe frequency output of the tunnel motor controlpanel up to 12 Hz which means that the flow speedcharacterization was made using speeds between 0rpm and 360 rpm.

Figure 10: Flow velocity versus output frequencyfor fan blades.

7.2. Flow uniformityThe spacial uniformity of the test section flow wascharacterized using a flow velocity measurementsystem that consisted on mounting the pitot tubeon a base that would be easy to move horizontallyand vertically. Data was acquired using a total of35 different points in the horizontal axis and 20 dif-ferent points in the vertical axis, at 8 different ve-locities which correspond to the electrical motor op-erational settings of 5 Hz to 12 Hz, for 30 seconds ata sampling rate of 10000 Hz, at each measurementlocation.

Figure 11: Flow profile for each velocity tested, hor-izontal axis. Nozzle installed.

Figure 12: Flow profile for each velocity tested, hor-izontal axis. Nozzle not installed.

The graphs displayed in Figures 12 and 13 showthe results of the flow uniformity tests carried outin the horizontal axis, with and without the noz-zle installed, respectively. The profiles at eachstreamwise location are generally consistent acrossall freestream velocities. Though, with the nozzleinstalled there is a difference between the minimumand maximum flow speeds measured in a profilethat corresponds to a maximum of 5,6% undershootin freestream velocity near the centre of the jet. Af-ter removing the nozzle, the difference between theminimum and maximum flow speeds measured ina profile corresponds to a 2.7927% undershoot infreestream velocity near the centre of the jet. Theseresults show that without the nozzle installed theair flow is slightly more uniform, what points tothe possibility that the nozzle influences the flowsignificantly mainly in its extremities. These devi-ations may have been more apparent because the

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wind tunnel was operated at low speeds which maynot be the most efficiently way to use it. At higherspeeds the flow may become more uniform in thenozzle exit. The measurements were also made closeto the nozzle exit instead of being made in the mid-dle of the test section in the anechoic chamber, asone moves away from the nozzle exit the flow maybecome slightly more uniform so further testing isneeded. These deviations from the mean freestreamvelocity present in the different profiles tested maypose a problem for future experiments, and furthertesting in future projects is required.

7.3. Background Noise LevelsThe first test was carried out in the quietest envi-ronment possible to achieve in the facility duringthe day. Data was acquired at a sampling rate of50 kHz for 60 seconds each test.

Figure 13: Narrowband background SPL spectra inanechoic chamber in silent environment.

Some peaks in under 100 Hz frequencies canbe seen and may originate from artificial sourcesof low-frequency noise such as ventilation or air-conditioning units and working equipment fromnearby laboratories. In terms of evaluating the per-formance of the anechoic chamber in what concernsto its sound proof capabilities, the SPL values mea-sured in a silent environment were around the 39dBA mark although it should ideally be closer to25-30 dBA. Considering the big size of the diffuserand nozzle exit, 39 dBA may be considered satis-factory since the test was carried out during busyhours but there is still room to improve the sound-proofing of the anechoic chamber.

Background noise measurements were then car-ried out over the range of flow velocities from 7.58m/s to 19.83 m/s with the nozzle installed.

There are some noticeable peaks in the spectraat flow velocities of 7.58 m/s to 16.77 m/s, lowfrequency mechanical vibrations could be reducedthrough motor and fan blade balancing and bear-ing greasing. Shear layer impingement on the col-lector may produce low frequency broadband noise.Environmental sources are unpredictable and couldcontribute both broadband and tonal noise to thespectra, the main method of attenuating this typeof noise is through acoustic baffles. There are alsopeaks around 2000 Hz and 4000 Hz frequencies

Figure 14: Background SPL spectra by microphone0 in all different velocities. Nozzle installed.

characteristic of the motor. The background noisein the anechoic chamber mainly comes from the fanand motor, open-jet shear layer impingement onthe collector, flow turbulence, and noise transmittedinto the open-circuit tunnel. As the flow speed wasincreased, the data acquired showed more uniformresults. The peak at 2000 Hz and 4000 Hz were notobserved after the flow speed increase from 16.77m/s to 18.30 m/s.

Figure 15: Comparison of the results obtained formicrophones 0, 3 and 4, in two different velocities.

There are considerable differences between the re-sults from each microphone due to the angle thattheir diaphragm makes with the jet centreline.

Figure 16: Comparison of A-weighted SPL betweenthree microphones and Sound Level Meter.

The same noise measurement tests were made af-ter removing the nozzle since the absence of the noz-zle may influence the data captured by the micro-phones. It should be noted that in the test withoutthe nozzle no peaks around the frequency of 2000 Hzwere seen, this is one characteristic of the electricmotor and some times when turning on the motor itwould produce a slightly different sound each time,

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sometimes originating those peaks around the 2000Hz that fade away when the velocity is increased.The biggest differences can be seen when analysingthe results of microphone 4 that are a lot differentfrom the results obtained with the nozzle installed.

Figure 17: Comparison between the results ob-tained for microphones 0, 3 and 4.

There are considerable differences between the re-sults from each microphone due to the angle thattheir diaphragm makes with the jet centreline. Inthis case, microphone 3, that is oriented 60o to tun-nel centreline facing upstream, clearly shows a sub-stantial deviation from its counterparts.

Figure 18: Comparison of A-weighted SPL betweenthree microphones and Sound Level Meter.

By observing the results, one should also high-light the fact that there is a significant fluctuationaround the 15.5 m/s mark that overlaps with theintense 4000 Hz sound that could be heard duringthe test and that suddenly stops. During the previ-ous tests with the nozzle installed the same soundcould be heard but it faded away gradually untilthe wind tunnel velocity setting reached the 12 Hz.

Considering that the flow velocities tested are rel-atively low, in the future higher speeds should beused in order to obtain more diverse results.

Figure 19: Third-octave SPL in dB, as measured bymicrophone 0.

Figure 20: Third-octave SPL in dB, as measured bymicrophone 3.

Figures 19 and 20 present third-octave analysisusing the data captured with microphones 0 and 3respectively, with and without the convergent noz-zle installed, U = 19.83 m/s.

7.4. Noise measurements with a Wind Turbine inthe Test Section

There was also the opportunity to carry out thesame noise measurement tests with a vertical axiswind turbine placed in the center of the test section.This object has a main component, the vertical axiswind turbine that is painted red, and secondary andsmaller white turbine. Both of them will producenoise, as well as the remaining of the body struc-ture. This experiment was made with the nozzleinstalled.

Figure 21: SPL spectra, comparison of the resultsobtained with wind tunnel turned off, with emptychamber and with wind turbine. U = 7.58 m/s.

Figure 22: SPL spectra, comparison of the resultsobtained with wind tunnel turned off, with emptychamber and with wind turbine. U = 18.30 m/s.

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In the first case there are not many differencesbetween the results obtained with and without thewind turbine placed in the test section, mainly dueto the low speed of the flow. However, in the sec-ond case, due to a higher air flow velocity of 18.30m/s, the wind turbine rotated faster and originatedhigher sound pressure levels across the frequencyspectrum.

Figure 23: Comparison of A-weighted SPL mea-sured in tests with empty anechoic chamber andwith the wind turbine.

Figure 24: Third-octave analysis. Results obtainedby microphone 0 in silent environment, and withand without the wind turbine placed in the testsection, at speed of 7.58 m/s.

Figure 25: Third-octave analysis. Results obtainedby microphone 0 in silent environment, and withand without the wind turbine placed in the testsection, at speed of 18.30 m/s.

8. ConclusionsThe tunnel characterization was performed, whichaddressed the flow velocity, flow uniformity uni-formity, background noise measurements with andwithout the convergent nozzle installed and a testwith a vertical axis wind turbine was also made.

It is possible to observe, by looking through theresults presented in this thesis, the influence of thenoise that comes from the fan and motor, flow tur-bulence, open-jet shear layer impingement and noise

transmitted by nearby machinery. There are char-acteristic frequency peaks produced by the electricmotor and fan that show up in the acoustic tests.

The flow uniformity tests exhibit unwanted devi-ations that despite being small still are disappoint-ing. Testing at higher velocities is essential in thefuture to confirm if the undershoot in freestreamvelocity near the center of the jet remains.

Unfortunately, the highest flow speed measuredand used during the tests was about 20 m/s, only40% of the theoretical maximum airflow velocity ofup to 50 m/s, this happened due to limitations ofthe equipment, time availability and security rea-sons.

The work performed in this thesis is good startingpoint for future projects that can go further in thecharacterization and setting up of the wind tunnelto be used in diverse aeroacoustic acoustic research.

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