Wind tunnel acoustic testing of wind generated noise on building facade elements

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2 Wind tunnel acoustic testing of wind generated noise on building facade elements Ivan BUBLIĆ 1 ; Ivan Tudor 2 ; Juraj Francetić 3 1 Brodarski institut d.o.o., Croatia 2 Brodarski institut d.o.o., Croatia 3 Brodarski institut d.o.o., Croatia ABSTRACT The aim of acoustic test conducted was to investigate possible occurrence of wind generated noise on building facade elements. The intention was to determine under which conditions there is occurrence of narrowband noise with noticeable tonal character, and to record and quantify any other acoustic phenomena that may occur. The object of the test was a building in construction in the city of Zagreb. The tests were conducted in the wind tunnel of the Faculty of Civil Engineering in Zagreb. The aerodynamic parameters were measured and analyzed parallel to the acoustic measurements. The results of acoustic testing in the wind tunnel show that there is wind generated noise present that is dependent on the wind speed. Also, the measurement showed the occurrence of tonal components in the spectrum at certain wind incidence angles. The tonal components became more pronounced and very audible at higher wind speeds. It can be concluded that the mounting of the certain type of facade elements on a building can result in significant wind generated noise levels, both broadband and narrowband. On the basis of the test results and available meteorological data it can be assumed that the noise on the facade elements could occur monthly. Keywords: Wind, Noise, Measurement 1. INTRODUCTION On the building in question, mounting of fixed vertical perforated sunscreens along the full height of the building for protection against excessive sunlight and mounting of perforated facade elements for the car park protection was envisioned. Based on the assumption that in the conditions of intensive wind activity the perforations would generate noise, the measurement of noise generated by the planned facade elements in a wind-tunnel was proposed. The aim of the measurements was to investigate acoustic phenomena due to the sunscreens exposure to wind. The intention was to determine whether and under which conditions there is occurrence of narrowband noise (which primarily refers to noise with noticeable tonal character), characterize the broadband noise (wind noise) resulting due to wind flow around and through a sunscreen, and record and quantify any other acoustic phenomena that may occur as a result of air made to move past the object. The tests were conducted in the wind-tunnel of the Faculty of Civil Engineering in Zagreb. The aerodynamic parameters were measured and analyzed by the employees of the Faculty of Civil Engineering, while the acoustic parameters were measured and analyzed by the employees of Brodarski institut d.o.o. 1 ivan.bublic@hrbi.hr 2 ivan.tudor@hrbi.hr 3 juraj.francetic@hrbi.hr 1

3 2. WIND TUNNEL ACOUSTIC TESTING DESCRIPTION 2.1 Problem description On the building in question installation of sunscreens is planned. The planned facade elements are made of aluminum sheet and are perforated. Perforations allow the air flow. The wind that will flow through and around a perforated sunscreen may cause the occurrence of broadband and narrowband noise of tonal character that occurs when the wind passes over the perforations and due to the excitation of facade element cavities acoustical resonance. These phenomena are the result of a complex interaction of air flow and geometry of the structural elements around which there is wind flow. Figure 1 Sample of ground floor facade element, and sunscreen element samples mounted on the building The resulting noise can be heard several hundred meters from the site of emission and can cause noise pollution. For this reason, the presented research was carried in order to avoid costly remedial measures. The noise caused by the action of wind on tall buildings can cause discomfort for people both inside the building and in its surroundings. Broadband noise caused by turbulent air flow can cause a very unpleasant humming and whistling. Tonal noise is most commonly associated with acoustic resonance of individual cavities or structural of flexible components, caused by vortex splitting mechanism. This resonance creates the equivalent effect as a musical instrument such as the flute or the organ that generate tones depending on the speed and direction of air flow. 2.2 The reference wind speeds Zagreb has a moderate continental climate and there are no frequent storm and gale force winds. The main wind directions are north and northeast, and spring (especially April) is the windiest time of the year. Figure 3 shows that from the NNE the moderately strong wind up to 5 Beaufort (approx 11 m/s) occurs. The measured data are related to wind speeds at a height of 10 meters above the ground and since the object has a height of about 30 meters, the development of boundary layer, i.e. increase of speed with distance from the ground, should be considered. 2

4 According to meteorological data for dimensioning of objects with respect to wind influence, the city of Zagreb belongs to P-1 area that includes the western interior from Požega valley to the western border of Croatia, for which the reference wind speed of 22 m/s has been adopted. On the basis of the presented meteorological parameters it was decided that the sunscreens would be tested up to a wind speed of 20 m/s, and facade elements located at the ground level up to a speed of 10 m/s. Figure 2 Wind rose form the meteorological station Zagreb-Maksimir 2.3 Materials and methods For the purpose of test conducting, four facade elements were provided (one shorter and two taller sunscreen elements and one facade panel). They were tested in life-size (scale 1:1). The shorter sunscreen had a height of 50 cm and was mounted in the working section of the wind tunnel where it is possible to generate wind speeds up to 50 m/s. The sunscreen was "closed" to the top and bottom ends as not to generate additional noise and was mounted on a support so that it could be pivoted. Figure 3 Sunscreen mounted in the wind tunnel working section Above the sunscreen a sound level meter was mounted. The system for sound pressure measurements included an integrating sound level meter (Type 1, B&K 2250), a microphone (B&K 4189), a sound calibrator (B&K, 4231), an integrating sound level meter (Type 1, B& K2250L) and a microphone (B&K 4950). In order to reduce wind noise generated on the microphone, noise cone B&K UA0386 as described in (1) was used in the wind tunnel. For the measurement of air velocity a KIMO Pitot tube anemometer and a KIMO hot wire anemometer were used. Tests started with mounting a segment of perforated sunscreen having a height of 50 cm in the small working section of the wind tunnel. The sound pressure level was measured for different wind 3

5 incidence angles and wind speeds. The perforated sunscreen and the supporting structure were turned relative to wind incidence angle so that the measurements were conducted under conditions when the sunscreen horizontal axis was parallel with the wind direction up to the conditions in which the horizontal axis was at an angle of 130⁰ in relation to the wind direction. In this way a span of 130⁰ with increments of 5 ⁰ was covered. For various wind incidence angles the measurements of wind speed, sound pressure level at the perforated sunscreen, and sound pressure level of environmenta l noise were measured. Figure 4 Sound level meters used for measurements and microphone with the aerodynamic cap Noise was simultaneously measured with two microphones, one of which was located within the working section of the wind tunnel, while the other one was located outside the wind tunnel. The microphone has a cap with favorable aero dynamical shape in order to avoid the noise that the wind could generate flowing around it. During the tests continuous broadband A-weighted and narrowband (1/3 octave) Z-weighted noise samples lasting 100 ms were recorded. Also, direct audio recording was done and FFT analysis was subsequently made. Figure 5 Testing of perforated facade element 4

6 Figure 6 Testing of mutual influence of two sunscreens In the second series of tests, in the larger section of the wind tunnel a structure with two perforated sunscreens at a mutual distance as planned by the project was mounted in order to study their mutual influence. These tests were conducted in a large working section where it is possible to obtain speeds of up to 11 m/s, which corresponds to a wind speed of 5 Beaufort. The third series of tests were carried out on flat perforated elements (facade elements) that will be mounted in the ground floor intended for the car park. These elements were tested in the larger working section of the wind tunnel at different incidence angles. During the tests all elements were rigidly connected to the supporting structure in order to avoid the occurrence of noise caused by vibration of the whole structure. Vibration of the entire facade structure could not be simulated in the wind tunnel due to technical conditions. 3. WIND TUNNEL ACOUSTIC TESTING RESULTS 3.1 Single segmet The basic tests were conducted for a segment having a height of 50 cm. The results of measurements of the average wind speed of 5 m/s are shown in Figure 7. The blue line indicates the sound pressure measured close to the sunscreen, the red line shows the values recorded by the microphone outside the wind tunnel, while the purple line indicates the sound pressure in the wind tunnel in the conditions without the mounted sunscreen. The purple line represents the level of the ambient noise resulting from electric motor operation and air flow through the wind tunnel. The difference between the purple line and the blue line indicates the level of the noise generated by the sunscreen, and in the considered conditions it amounts to an average of 6.5 db. The greatest value of noise increase at a wind speed of 5 m/s was measured at incidence angles of about 60⁰. Increasing the wind speed to 10 m/s, the noise increases as well (Figure 8), both within the tunnel (blue and purple lines) and outside it (red line). In these conditions, the level of the noise is lowest when the wind acts perpendicularly to the facade, i.e. in the direction of the sunscreen axis. The average noise increase in these conditions is 12 db. With a further increase in wind speed to the average value of 15 m/s, the level of the noise increases too (Figure 9), and the average increase of the noise generated by the sunscreen is 13 db. In addition to the noise intensity measurements, the FFT analysis (Figure 10) was also performed. The appearance of tonal components within the spectrum can be noticed at certain incidence angles. At a wind speed of 15 m/s the appearance of tonal components can be observed within the signal spectrum, however, they are not audible. With a further increase in the wind speed, the noise level further increases, thus at a speed of 20 m/s the noise level due to mounting of the sunscreen increases (Figure 11) by an average of 13.5 db. In addition to the noise increase, at the wind speeds of 20 m/s the tonal components get more pronounced and very audible (Figure 12). 5

7 Figure 7 Sound pressure levels for wind speed of 5 m/s Figure 8 Sound pressure levels for wind speed of 10 m/s Figure 9 Sound pressure levels for wind speed of 15 m/s 6

8 Figure 10 FFT analysis for the first testing phase at speed of 15 m/s and incidence angle of 105 Figure 11 Sound pressure levels for wind speed of 20 m/s Figure 12 FFT analysis for the first testing phase at speed of 20 m/s and incidence angle of 100 7

9 3.2 Interaction of two segments After single segment testing, the measurement of noise generated by two segments spaced at a distance of 110 cm was done in order to assess their possible interaction. The measurement results are presented in Figure 13 and they show that there is no negative interaction of the sunscreens on noise generation. Figure 13 Sound pressure level for wind speed of 10 m/s interaction of two sunscreens Based on the experience and measurement results it can be said that the interaction between two sunscreens is favorable, because at certain wind incidence angles the wind speed decreases at the slipstream. 3.3 Facade element After testing the sunscreens, a ground floor facade element was also tested. The measurement results are presented in Figure 14. Blue line represents noise levels with ground floor facade element present in the wind tunnel work section. Purple line represents noise levels in the empty wind tunnel work section. Figure 14 Sound pressure level for wind speed of 10 m/s ground floor facade element 4. CONCLUSIONS The measurement of noise generated by facade elements on was performed. The measurements were carried out in a wind tunnel where one sunscreen segment having a height of 50 cm was tested at wind speeds up to 20 m/s, and two connected elements having a height of 100 cm were tested at wind 8

10 View publication stats INTER-NOISE 2016 speeds up to 10 m/s. The elements were tested in the conditions when the wind acts perpendicularly to the facade, which is an angle of 0⁰ in relation to the sunscreen axis, up to the angle of 135⁰ between the sunscreen axis and the wind direction. The results of measurements in the wind tunnel show that the tested facade elements increase the sound pressure level at a wind speed of 20 m/s for a maximum of db, and that there is no significant increase in noise due to the interaction of two sunscreens (up to a speed of 10 m/s). Tests showed some correlation between noise levels and wind incidence angle, and a clear correlation between noise levels and wind speed. Based on the measurements it can be concluded that close to the sunscreen there will be present significant noise levels caused by the wind having a speed of 20 m/s. The generated noise is broadband and increases with the increase of wind speed. The measurements showed the occurrence of tonal components in the spectrum at certain wind incidence angles. The tonal component appeared in the signal spectrum at a wind speed of 15 m/s, however, they were not audible. At a wind speed of 20 m/s tonal components became more pronounced and very audible. It can be concluded that the mounting of the sunscreens on the facade will at certain wind speeds generate a significant level of noise. On the basis of the available meteorological data it can be assumed that the noise on the sunscreens (at higher floors) could occur with a one-month frequency. The interaction between two sunscreens is favorable, because at certain angles it reduces the wind speed at the slipstream sunscreen. Facade elements at the ground floor are not exposed to high wind speeds due to the development of the boundary layer and will not cause an increase in the sound pressure. REFERENCES 1. Master Catalogue, Bruel&Kjær electronic instruments, Nærum, Denmark, 1974, p 134 9