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C.P. van Dercreek
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10 records found
1
Conference paper
(2023)
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H.F. Mourão Bento, Colin VanDercreek, F. Avallone, D. Ragni, P. Sijtsma, M. Snellen
Aeroacoustic tests in closed wind tunnels are affected by reflections in the tunnel circuit and background noise. Reflections can be mitigated by lining the tunnel circuit. The present study investigates if lining exclusively the most accessible segment of a closed wind tunnel circuit, in particular the test section, is an approach which improves acoustic measurements. Literature shows that a wind tunnel lining material should have high acoustic absorption, low inertial resistivity and low surface roughness. Therefore, the test section of TU Delft's closed Low Turbulence Tunnel is lined with melamine foam wall liners. A total of 4 test section configurations were tested: baseline; test section with lining on the floor and ceiling; test section with lined side--panels; and test section lined at all surfaces (floor, ceiling and side--panels). An omnidirectional speaker is used for evaluating the wind tunnel's acoustic performance. A geometric modelling algorithm, based on the mirror-source method, is used to predict the effect of lining on primary reflections in the test section. In addition, reflections in the test section and in the tunnel circuit are characterized experimentally. The results show that the closed loop of the tunnel circuit is responsible for a long reverberation time in the test section. However, reflections inside the test section itself are the dominant source of acoustic interference at the microphone array location. The low fidelity geometric modelling algorithm is shown to be a valuable approach for an initial estimation of the acoustic benefit of lining, for both flow--off and --on conditions. Lining of the test section walls significantly reduces reflections from the reference source, as well as the aerodynamic background noise that reaches the array.
...
Aeroacoustic tests in closed wind tunnels are affected by reflections in the tunnel circuit and background noise. Reflections can be mitigated by lining the tunnel circuit. The present study investigates if lining exclusively the most accessible segment of a closed wind tunnel circuit, in particular the test section, is an approach which improves acoustic measurements. Literature shows that a wind tunnel lining material should have high acoustic absorption, low inertial resistivity and low surface roughness. Therefore, the test section of TU Delft's closed Low Turbulence Tunnel is lined with melamine foam wall liners. A total of 4 test section configurations were tested: baseline; test section with lining on the floor and ceiling; test section with lined side--panels; and test section lined at all surfaces (floor, ceiling and side--panels). An omnidirectional speaker is used for evaluating the wind tunnel's acoustic performance. A geometric modelling algorithm, based on the mirror-source method, is used to predict the effect of lining on primary reflections in the test section. In addition, reflections in the test section and in the tunnel circuit are characterized experimentally. The results show that the closed loop of the tunnel circuit is responsible for a long reverberation time in the test section. However, reflections inside the test section itself are the dominant source of acoustic interference at the microphone array location. The low fidelity geometric modelling algorithm is shown to be a valuable approach for an initial estimation of the acoustic benefit of lining, for both flow--off and --on conditions. Lining of the test section walls significantly reduces reflections from the reference source, as well as the aerodynamic background noise that reaches the array.
Aerodynamic noise produced by aircraft, wind turbines, and other objects subjected to airflow contribute to environmental noise pollution, which adversely affects human and animal health. Consequently, governments impose restrictions on aircraft and wind turbine noise levels. These restrictions can have an economic impact by limiting aircraft traffic and reducing wind turbine energy production. Accordingly, improving the design of aerodynamic surfaces to reduce their noise levels benefits health while enabling improved operational efficiency. Therefore, aeroacoustic research focuses on identifying and understanding the physical mechanisms behind aerodynamic noise to improve noise mitigation technologies. This research relies on acoustic wind tunnel measurements to validate simulations, theories, and design improvements.
Closed test section wind tunnels are widely used for aerodynamic testing but are less suitable for acoustic measurements because microphones must be installed in the wall. This location subjects the microphones to pressure fluctuations from the turbulent boundary layer (TBL), which contaminates acoustic measurements and reduces the signal-to-noise ratio (SNR). The impact of the TBL can be mitigated by recessing microphones within cavities and covering them with an acoustically transparent material. Modifying existing wind tunnel walls by installing cavity--mounted microphones is a straightforward and cost-effective improvement that enables combined aerodynamic and acoustic measurement campaigns.
The cavity geometry, i.e., depth, aperture size, wall angle, and presence of a covering determines the amount of TBL attenuation and consequently the improvement to SNR. While several studies have shown empirically that these parameters have an effect, few studies focus on identifying the physical mechanisms that explain the relationship between geometry and the reduction in TBL pressure fluctuations at the microphone. Thus, this thesis aims to identify these physical mechanisms through experiments and different modeling approaches to better explain the relationship between cavity geometry, the amount of TBL attenuation, and the subsequent impact on the measured acoustic signal.
Experimental data were collected to develop an empirical model to quantify how varying cavity geometry affects the measured pressure spectra. Moreover, experiments were also performed to validate simulation results and to quantify the SNR improvement when applying a beamforming algorithm to microphone array data. The modeling and simulation efforts focus on explaining the trends and phenomena identified in the experimental data. Initially, a physical model was developed that assumed acoustic propagation into an axisymmetric cavity with a constant cross-section. This model decomposes a pressure field, resulting from a TBL, into circular duct modes and was used to evaluate the relationship between cavity geometry and the propagation of these acoustic modes into the cavity. This model was followed up with a finite element method (FEM) simulation to study the influence of different cavity geometric parameters and wall materials on the acoustic response of the cavity when subjected to an acoustic wave.
The FEM simulation showed that the cavity's acoustic response is determined by the presence of standing waves in the form of acoustic depth modes. This simulation showed that cavities with angled walls have depth modes with lower amplitude waves and thus distort the acoustic signal less. Furthermore, it is shown that the acoustic responses of cavities formed out of sound-absorbing foam are driven by the shape of the foam holder and not the cavity shapes within the foam. Thus, the holder can be optimized to minimize the acoustic response, while the cavity itself can be optimized to reduce the influence of the TBL. Building upon these simulations, a Lattice Boltzmann based computational fluid dynamics (CFD) method was used to simulate the pressure and flow fields within three uncovered cavities and covered cavities resulting from the presence of a turbulent boundary layer.
The CFD simulations confirmed a significant finding of the physical model, that the amount of TBL attenuation increases as the cavity aperture size increases relative to the TBL streamwise coherence length. This is due to the resulting modal decomposition of the pressure field above larger cavities having more energy distributed across higher-order modes than for smaller cavities. These higher-order modes decay exponentially into the cavity, resulting in increased attenuation of the TBL. Smaller cavities have most of their energy in their first mode, which does not decay with increasing cavity depth. Furthermore, these simulations showed that the pressure field within covered cavities is primarily acoustic and can be decomposed into acoustic circular duct modes. Since the propagation of TBL pressure fluctuations into covered cavities is primarily acoustic, the shape of future cavities can be efficiently optimized using FEM simulations.
Finally, beamforming used with cavities improved the acoustic measurement SNR. Analysis shows that the improvements due to beamforming are independent of those attributed to the cavity geometry. Thus, combining the two approaches improves the SNR of acoustic measurements in closed test section wind tunnels.
...
Aerodynamic noise produced by aircraft, wind turbines, and other objects subjected to airflow contribute to environmental noise pollution, which adversely affects human and animal health. Consequently, governments impose restrictions on aircraft and wind turbine noise levels. These restrictions can have an economic impact by limiting aircraft traffic and reducing wind turbine energy production. Accordingly, improving the design of aerodynamic surfaces to reduce their noise levels benefits health while enabling improved operational efficiency. Therefore, aeroacoustic research focuses on identifying and understanding the physical mechanisms behind aerodynamic noise to improve noise mitigation technologies. This research relies on acoustic wind tunnel measurements to validate simulations, theories, and design improvements.
Closed test section wind tunnels are widely used for aerodynamic testing but are less suitable for acoustic measurements because microphones must be installed in the wall. This location subjects the microphones to pressure fluctuations from the turbulent boundary layer (TBL), which contaminates acoustic measurements and reduces the signal-to-noise ratio (SNR). The impact of the TBL can be mitigated by recessing microphones within cavities and covering them with an acoustically transparent material. Modifying existing wind tunnel walls by installing cavity--mounted microphones is a straightforward and cost-effective improvement that enables combined aerodynamic and acoustic measurement campaigns.
The cavity geometry, i.e., depth, aperture size, wall angle, and presence of a covering determines the amount of TBL attenuation and consequently the improvement to SNR. While several studies have shown empirically that these parameters have an effect, few studies focus on identifying the physical mechanisms that explain the relationship between geometry and the reduction in TBL pressure fluctuations at the microphone. Thus, this thesis aims to identify these physical mechanisms through experiments and different modeling approaches to better explain the relationship between cavity geometry, the amount of TBL attenuation, and the subsequent impact on the measured acoustic signal.
Experimental data were collected to develop an empirical model to quantify how varying cavity geometry affects the measured pressure spectra. Moreover, experiments were also performed to validate simulation results and to quantify the SNR improvement when applying a beamforming algorithm to microphone array data. The modeling and simulation efforts focus on explaining the trends and phenomena identified in the experimental data. Initially, a physical model was developed that assumed acoustic propagation into an axisymmetric cavity with a constant cross-section. This model decomposes a pressure field, resulting from a TBL, into circular duct modes and was used to evaluate the relationship between cavity geometry and the propagation of these acoustic modes into the cavity. This model was followed up with a finite element method (FEM) simulation to study the influence of different cavity geometric parameters and wall materials on the acoustic response of the cavity when subjected to an acoustic wave.
The FEM simulation showed that the cavity's acoustic response is determined by the presence of standing waves in the form of acoustic depth modes. This simulation showed that cavities with angled walls have depth modes with lower amplitude waves and thus distort the acoustic signal less. Furthermore, it is shown that the acoustic responses of cavities formed out of sound-absorbing foam are driven by the shape of the foam holder and not the cavity shapes within the foam. Thus, the holder can be optimized to minimize the acoustic response, while the cavity itself can be optimized to reduce the influence of the TBL. Building upon these simulations, a Lattice Boltzmann based computational fluid dynamics (CFD) method was used to simulate the pressure and flow fields within three uncovered cavities and covered cavities resulting from the presence of a turbulent boundary layer.
The CFD simulations confirmed a significant finding of the physical model, that the amount of TBL attenuation increases as the cavity aperture size increases relative to the TBL streamwise coherence length. This is due to the resulting modal decomposition of the pressure field above larger cavities having more energy distributed across higher-order modes than for smaller cavities. These higher-order modes decay exponentially into the cavity, resulting in increased attenuation of the TBL. Smaller cavities have most of their energy in their first mode, which does not decay with increasing cavity depth. Furthermore, these simulations showed that the pressure field within covered cavities is primarily acoustic and can be decomposed into acoustic circular duct modes. Since the propagation of TBL pressure fluctuations into covered cavities is primarily acoustic, the shape of future cavities can be efficiently optimized using FEM simulations.
Finally, beamforming used with cavities improved the acoustic measurement SNR. Analysis shows that the improvements due to beamforming are independent of those attributed to the cavity geometry. Thus, combining the two approaches improves the SNR of acoustic measurements in closed test section wind tunnels.
Journal article
(2022)
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Hugo F. Mourão Bento, Colin P. Vandercreek, Francesco Avallone, Daniele Ragni, Mirjam Snellen
Microphone measurements in a closed test section wind tunnel are affected by turbulent boundary layer (TBL) pressure fluctuations. These fluctuations are mitigated by placing the microphones at the bottom of cavities, usually covered with a thin, acoustically transparent material. Prior experiments showed that the cavity geometry affects the propagation of TBL pressure fluctuations toward the bottom. However, the relationship between the cavity geometry and the flowfield within the cavity is not well understood. Therefore, a very large-eddy simulation was performed using the lattice Boltzmann method. A cylindrical, a countersunk and a conical cavity are simulated with and without a fine wire-cloth cover, which is modeled as a porous medium governed by Darcy's law. Adding a countersink to an uncovered cylindrical cavity is found to mitigate the transport of turbulent structures across the bottom by shifting the recirculation pattern away from the cavity bottom. Covering the cavities nearly eliminates this source of hydrodynamic pressure fluctuations. The eddies within the boundary layer, which convect over the cover, generate a primarily acoustic pressure field inside the cavities and thus suggesting that the pressure fluctuations within covered cavities can be modeled acoustically. As the cavity diameter increases compared to the eddies' integral length scale, the amount of energy in the cut-off modes increases with respect to the cut-on modes. Since cut-off modes decay as they propagate into the cavity, more attenuation is seen. The results are in agreement with experimental evidence.
...
Microphone measurements in a closed test section wind tunnel are affected by turbulent boundary layer (TBL) pressure fluctuations. These fluctuations are mitigated by placing the microphones at the bottom of cavities, usually covered with a thin, acoustically transparent material. Prior experiments showed that the cavity geometry affects the propagation of TBL pressure fluctuations toward the bottom. However, the relationship between the cavity geometry and the flowfield within the cavity is not well understood. Therefore, a very large-eddy simulation was performed using the lattice Boltzmann method. A cylindrical, a countersunk and a conical cavity are simulated with and without a fine wire-cloth cover, which is modeled as a porous medium governed by Darcy's law. Adding a countersink to an uncovered cylindrical cavity is found to mitigate the transport of turbulent structures across the bottom by shifting the recirculation pattern away from the cavity bottom. Covering the cavities nearly eliminates this source of hydrodynamic pressure fluctuations. The eddies within the boundary layer, which convect over the cover, generate a primarily acoustic pressure field inside the cavities and thus suggesting that the pressure fluctuations within covered cavities can be modeled acoustically. As the cavity diameter increases compared to the eddies' integral length scale, the amount of energy in the cut-off modes increases with respect to the cut-on modes. Since cut-off modes decay as they propagate into the cavity, more attenuation is seen. The results are in agreement with experimental evidence.
Cavities placed along wind tunnel walls can attenuate the turbulent boundary layer (TBL) fluctuations as they propagate into the cavity. Placing microphones within the cavities can thus improve the signal-to-noise ratio of acoustic data. However, standing waves form within these cavities distorting the acoustic measurements. This work uses a finite element (FE) solver to evaluate how cavity geometry (depth, diameter, and wall angle) and wall material (hard-walled and melamine foam) affect the amplitude and eigenfrequency of standing waves when excited by an incident acoustic plane wave. Good agreement between predicted and measured acoustic transfer functions is shown. Compared to cylindrical cavities, countersunk and conical cavities improve the overall response, i.e., reducing the quality factor quantifying the resonance and damping characteristics. Stainless steel coverings also reduce the quality factor. A finding is that the shape of the external foam holder rather than the cavity shape drives the standing wave characteristics for the melamine foam cavities. The optimization problem of minimizing the acoustic response while also attenuating the TBL is thus decoupled by using the melamine foam. Consequently, these considerations can be addressed independently by optimizing the outer cavity shape for acoustics and the melamine foam insert for TBL attenuation.
...
Cavities placed along wind tunnel walls can attenuate the turbulent boundary layer (TBL) fluctuations as they propagate into the cavity. Placing microphones within the cavities can thus improve the signal-to-noise ratio of acoustic data. However, standing waves form within these cavities distorting the acoustic measurements. This work uses a finite element (FE) solver to evaluate how cavity geometry (depth, diameter, and wall angle) and wall material (hard-walled and melamine foam) affect the amplitude and eigenfrequency of standing waves when excited by an incident acoustic plane wave. Good agreement between predicted and measured acoustic transfer functions is shown. Compared to cylindrical cavities, countersunk and conical cavities improve the overall response, i.e., reducing the quality factor quantifying the resonance and damping characteristics. Stainless steel coverings also reduce the quality factor. A finding is that the shape of the external foam holder rather than the cavity shape drives the standing wave characteristics for the melamine foam cavities. The optimization problem of minimizing the acoustic response while also attenuating the TBL is thus decoupled by using the melamine foam. Consequently, these considerations can be addressed independently by optimizing the outer cavity shape for acoustics and the melamine foam insert for TBL attenuation.
Aeroacoustic measurements with microphones in closed-section wind tunnels are typically hampered by the pressure fluctuations of the turbulent boundary layer (TBL) on the tunnel's walls, ceiling, or floor, where phased microphone arrays are usually mounted. This paper evaluates the performance of several advanced acoustic imaging methods that aim at overcoming this limitation for localizing and quantifying sound sources in closed-section wind-tunnel measurements. The acoustic data employed was obtained using a phased microphone array installed in two different configurations: flush-mounted on the wind-tunnel wall and recessed within cavities behind an acoustically transparent covering. The acoustic imaging methods considered are conventional frequency domain beamforming (CFDBF, as a baseline), functional (projection) beamforming (FUNBF), orthogonal beamforming (OB), CLEAN-SC, and the deconvolution approach for the mapping of acoustic sources (DAMAS). Two sound sources are analyzed: (1) a single speaker emitting broadband sound as a reference signal, and (2) a flat plate inside of the flow as a distributed aeroacoustic source. In general, it is observed that the array with cavities provides considerably better results as it benefits from a higher signal-to-noise ratio. In addition, removing the main diagonal from the cross-spectral matrix also helps obtaining clearer acoustic source maps and more accurate quantitative sound spectra estimations. Overall, DAMAS and OB are the best performing methods for the case with a single speaker. CFDBF and FUNBF are the most suitable methods for the case with the distributed sound source of the trailing edge of the flat plate, whereas the other techniques fail to properly identify it.
...
Aeroacoustic measurements with microphones in closed-section wind tunnels are typically hampered by the pressure fluctuations of the turbulent boundary layer (TBL) on the tunnel's walls, ceiling, or floor, where phased microphone arrays are usually mounted. This paper evaluates the performance of several advanced acoustic imaging methods that aim at overcoming this limitation for localizing and quantifying sound sources in closed-section wind-tunnel measurements. The acoustic data employed was obtained using a phased microphone array installed in two different configurations: flush-mounted on the wind-tunnel wall and recessed within cavities behind an acoustically transparent covering. The acoustic imaging methods considered are conventional frequency domain beamforming (CFDBF, as a baseline), functional (projection) beamforming (FUNBF), orthogonal beamforming (OB), CLEAN-SC, and the deconvolution approach for the mapping of acoustic sources (DAMAS). Two sound sources are analyzed: (1) a single speaker emitting broadband sound as a reference signal, and (2) a flat plate inside of the flow as a distributed aeroacoustic source. In general, it is observed that the array with cavities provides considerably better results as it benefits from a higher signal-to-noise ratio. In addition, removing the main diagonal from the cross-spectral matrix also helps obtaining clearer acoustic source maps and more accurate quantitative sound spectra estimations. Overall, DAMAS and OB are the best performing methods for the case with a single speaker. CFDBF and FUNBF are the most suitable methods for the case with the distributed sound source of the trailing edge of the flat plate, whereas the other techniques fail to properly identify it.
Journal article
(2021)
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Colin van Dercreek, Roberto Merino-Martínez, Pieter Sijtsma, Mirjam Snellen
Aeroacoustic measurements performed by flush-mounted microphone arrays on the walls of closed-section wind tunnels are contaminated by the hydrodynamic pressure fluctuations of the wall's boundary layer. This study evaluates three different microphone cavity geometries for mitigating this issue. Their improvement to the signal-to-noise ratio (SNR) and the accuracy of their acoustic imaging results are compared to a flush-mounted microphone array. The four geometries include: (1) an array of flush-mounted microphones as the baseline, (2) a cylindrical hard-plastic cavity with a countersink, (3) a conical cavity made of melamine acoustic absorbing foam, and (4) a conical cavity with star-shaped protrusions, also made of melamine. The three arrays with cavities were covered with a steel-wire cloth to reduce the boundary layer fluctuations at the microphone while the baseline array was uncovered. Two sound sources were tested in an aeroacoustic wind tunnel for assessing the performance of the different cavities: a speaker placed outside the flow and a distributed sound source generated by a flat plate inside of the flow. When using conventional frequency domain beamforming, both cavities made of melamine offer up to a 30 dB increase in SNR with respect to the flush-mounted case, followed by the hard-walled cavity with up to a 20 dB increase. This is a 20 dB improvement when compared to the single microphone cases. The melamine cavities also provide cleaner acoustic source maps and accurate spectral estimations for a wider frequency range. The effect of cavity placement and geometry on the coherence, which affects the beamforming analysis of the acoustic signal was negligible for all cases. Distributed sound source measurements using the three arrays agreed with predictions using the Brooks, Pope, and Marcolini (BPM) model, showing that the cavities could detect vortex shedding that was undetectable by the flush array.
...
Aeroacoustic measurements performed by flush-mounted microphone arrays on the walls of closed-section wind tunnels are contaminated by the hydrodynamic pressure fluctuations of the wall's boundary layer. This study evaluates three different microphone cavity geometries for mitigating this issue. Their improvement to the signal-to-noise ratio (SNR) and the accuracy of their acoustic imaging results are compared to a flush-mounted microphone array. The four geometries include: (1) an array of flush-mounted microphones as the baseline, (2) a cylindrical hard-plastic cavity with a countersink, (3) a conical cavity made of melamine acoustic absorbing foam, and (4) a conical cavity with star-shaped protrusions, also made of melamine. The three arrays with cavities were covered with a steel-wire cloth to reduce the boundary layer fluctuations at the microphone while the baseline array was uncovered. Two sound sources were tested in an aeroacoustic wind tunnel for assessing the performance of the different cavities: a speaker placed outside the flow and a distributed sound source generated by a flat plate inside of the flow. When using conventional frequency domain beamforming, both cavities made of melamine offer up to a 30 dB increase in SNR with respect to the flush-mounted case, followed by the hard-walled cavity with up to a 20 dB increase. This is a 20 dB improvement when compared to the single microphone cases. The melamine cavities also provide cleaner acoustic source maps and accurate spectral estimations for a wider frequency range. The effect of cavity placement and geometry on the coherence, which affects the beamforming analysis of the acoustic signal was negligible for all cases. Distributed sound source measurements using the three arrays agreed with predictions using the Brooks, Pope, and Marcolini (BPM) model, showing that the cavities could detect vortex shedding that was undetectable by the flush array.
Conference paper
(2020)
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Colin van Dercreek, Roberto Merino Martinez, Mirjam Snellen, Daniele Ragni, Dick Simons
This paper analyses the influence of microphone cavity geometry on beamforming measurements with a turbulent boundary layer present on the microphone array. A 16- microphone array was tested in the anechoic open-jet wind tunnel of the Delft University of Technology. The array was placed on a flat plate mounted flush with the exit nozzle of the wind tunnel. Microphones were installed in three different cavity geometries along with a flush mounted microphone array which was used as a baseline for comparison. The geometries include a chamfered-cylindrical hard-plastic cavity, a chamfered-cylindrical cavity made of melamine foam, and a chamfered-cylindrical cavity with star-shaped protrusions, also made of melamine. The recessed cavities were covered with a 0.026 mm gauge steelwire cloth. A speaker emitting white noise outside of the flow was employed as a sound source. Different flow velocities at the same sound power level for the speaker (and hence different signal-to-noise ratios) were studied. The results obtained with the three cavity geometries are compared with the flush-mounted case in terms of the quality of the acoustic source maps (location and strength of the sound source). The signal-to-noise ratio of the beamforming measurements increased by as much as 30 dB by placing microphones within a conical cavity with melamine foam walls when compared to the flush mounted case.
...
This paper analyses the influence of microphone cavity geometry on beamforming measurements with a turbulent boundary layer present on the microphone array. A 16- microphone array was tested in the anechoic open-jet wind tunnel of the Delft University of Technology. The array was placed on a flat plate mounted flush with the exit nozzle of the wind tunnel. Microphones were installed in three different cavity geometries along with a flush mounted microphone array which was used as a baseline for comparison. The geometries include a chamfered-cylindrical hard-plastic cavity, a chamfered-cylindrical cavity made of melamine foam, and a chamfered-cylindrical cavity with star-shaped protrusions, also made of melamine. The recessed cavities were covered with a 0.026 mm gauge steelwire cloth. A speaker emitting white noise outside of the flow was employed as a sound source. Different flow velocities at the same sound power level for the speaker (and hence different signal-to-noise ratios) were studied. The results obtained with the three cavity geometries are compared with the flush-mounted case in terms of the quality of the acoustic source maps (location and strength of the sound source). The signal-to-noise ratio of the beamforming measurements increased by as much as 30 dB by placing microphones within a conical cavity with melamine foam walls when compared to the flush mounted case.
This study investigates how embedding microphones in different cavity geometries along the wall of a wind tunnel reduces the measured turbulent boundary layer pressure fluctuations. The effect of these cavities on the measured signal-to-noise ratio of an acoustic source with flow present was also quantified. Twelve cavity geometries defined by their depths, diameters, chamfer, opening percentage, and mesh covering were tested. The cavity geometries were selected using a design of experiments methodology. The application of design of experiments enabled a statistically sound and efficient test campaign. This was done by applying a D-optimal selection criterion to all potential cavity geometries in order to select 12 cavities to allow for the individual effect of the geometric parameters such as depth and diameter to be quantified with statistical confidence. The resulting wind tunnel test data were fit to a generalized additive model. This approach quantified the relative effect of these parameters on the turbulent boundary layer pressure spectral energy and signal-to-noise ratio while accounting for non-linear frequency dependence. This experimental investigation quantified how much increasing depth reduces the turbulent boundary layer spectral energy and increases signal-to-noise ratio. It also showed that a mesh covering reduces the boundary layer noise by 8 dB. It was also quantified how much reducing the cavity area from the opening of the cavity to the base of the microphone reduces the measured boundary layer spectral energy. Additionally, the model quantified the interactions between the mesh and cavity area as well as the change in area.
...
This study investigates how embedding microphones in different cavity geometries along the wall of a wind tunnel reduces the measured turbulent boundary layer pressure fluctuations. The effect of these cavities on the measured signal-to-noise ratio of an acoustic source with flow present was also quantified. Twelve cavity geometries defined by their depths, diameters, chamfer, opening percentage, and mesh covering were tested. The cavity geometries were selected using a design of experiments methodology. The application of design of experiments enabled a statistically sound and efficient test campaign. This was done by applying a D-optimal selection criterion to all potential cavity geometries in order to select 12 cavities to allow for the individual effect of the geometric parameters such as depth and diameter to be quantified with statistical confidence. The resulting wind tunnel test data were fit to a generalized additive model. This approach quantified the relative effect of these parameters on the turbulent boundary layer pressure spectral energy and signal-to-noise ratio while accounting for non-linear frequency dependence. This experimental investigation quantified how much increasing depth reduces the turbulent boundary layer spectral energy and increases signal-to-noise ratio. It also showed that a mesh covering reduces the boundary layer noise by 8 dB. It was also quantified how much reducing the cavity area from the opening of the cavity to the base of the microphone reduces the measured boundary layer spectral energy. Additionally, the model quantified the interactions between the mesh and cavity area as well as the change in area.
This study investigated how embedding microphones in different cavity geometries reduce the measured turbulent boundary layer pressure fluctuations at the microphones. The cavity geometries were systematically varied using a design of experiments (DOE) methodology. This approach tested different cavity depths, diameters, chamfers, and opening sizes as well as the effect of a fine mesh covering. The resulting wind-tunnel test data was analyzed using a generalized additive statistical model (GAM). This approach quantified the relative effect of these parameters on the response variables of interest while accounting for non-linear frequency dependence. This experimental investigation showed that a mesh reduces the boundary layer noise by 8 dB. It was also shown that reducing the cavity area from the wall to the base of the microphone reduces the measured boundary layer spectral energy. Additionally, the model quantified the complex interactions between the mesh and area as well as the change in area.
...
This study investigated how embedding microphones in different cavity geometries reduce the measured turbulent boundary layer pressure fluctuations at the microphones. The cavity geometries were systematically varied using a design of experiments (DOE) methodology. This approach tested different cavity depths, diameters, chamfers, and opening sizes as well as the effect of a fine mesh covering. The resulting wind-tunnel test data was analyzed using a generalized additive statistical model (GAM). This approach quantified the relative effect of these parameters on the response variables of interest while accounting for non-linear frequency dependence. This experimental investigation showed that a mesh reduces the boundary layer noise by 8 dB. It was also shown that reducing the cavity area from the wall to the base of the microphone reduces the measured boundary layer spectral energy. Additionally, the model quantified the complex interactions between the mesh and area as well as the change in area.
Conference paper
(2019)
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Colin van Dercreek, Pieter Sijtsma, Mirjam Snellen, Daniele Ragni, Francesco Avallone, Dick Simons
A mathematical model is proposed that evaluates acoustic propagation of turbulent boundary layer waves on top of a cavity enclosing a microphone. The goal is to optimize these cavity geometries to improve the signal-to-noise ratio of acoustic measurements. This model predicts the attenuation of the turbulent boundary layer fluctuations propagating within the cavity for a given wind tunnel speed, porous surface resistance and cavity geometry. Duct acoustics predict that the spatial wave numbers of the turbulent boundary layer pressure fluctuations are shorter than the acoustic wavelength and are therefore evanescent in the cavity. These cavities are also covered with a porous material such as a metallic mesh or kevlar, to attenuate hydrodynamic fluctuations through the cavity. This model supports the investigation of 3D cylindrical cavities and incorporates both soft and hard cavity walls. Good agreement was found with experimental data.
...
A mathematical model is proposed that evaluates acoustic propagation of turbulent boundary layer waves on top of a cavity enclosing a microphone. The goal is to optimize these cavity geometries to improve the signal-to-noise ratio of acoustic measurements. This model predicts the attenuation of the turbulent boundary layer fluctuations propagating within the cavity for a given wind tunnel speed, porous surface resistance and cavity geometry. Duct acoustics predict that the spatial wave numbers of the turbulent boundary layer pressure fluctuations are shorter than the acoustic wavelength and are therefore evanescent in the cavity. These cavities are also covered with a porous material such as a metallic mesh or kevlar, to attenuate hydrodynamic fluctuations through the cavity. This model supports the investigation of 3D cylindrical cavities and incorporates both soft and hard cavity walls. Good agreement was found with experimental data.