Transactions of the Korean Society of Mechanical Engineers B, 2012
Fluctuating wall pressures were measured using an array of 16 piezoelectric transducers beneath a... more Fluctuating wall pressures were measured using an array of 16 piezoelectric transducers beneath a turbulent boundary layer. The coating used in this experiment was an open-cell, urethane-type foam with a porosity of approximately 50 ppi. The ultimate objective of the coating is to provide a mechanical filter to reduce the wall pressure fluctuations. The boundary layer on the flat plate was measured by using a hot wire probe, and the CPM method was used to determine the skin friction coefficient. The wall pressure autospectra and streamwise wavenumber-frequency spectra were compared to assess the attenuation of the wall pressure field by the coating. The coating is shown to attenuate the convective wall pressure energy. However, the relatively rough surface of the coating in this investigation resulted in a higher mean wall shear stress, thicker boundary layer, and higher low-frequency wall pressure spectral levels compared to a smooth wall.
The onset of cavitation causes head and efficiency of a main pump to be reduced significantly and... more The onset of cavitation causes head and efficiency of a main pump to be reduced significantly and generates vibration and noise. In order to avoid these phenomena, the inlet of the pump is fitted with a special rotor called an inducer, which can operate satisfactorily with extensive cavitation. The motivation of this study is to find out cavitation modes from the inducer inlet pressure signals and event characteristics from outlet ones at various operating conditions. The cavitation modes are analyzed by using a cross-spectral density of fluctuating pressures at the inducer inlet. The time-frequency characteristics of wall pressures downstream of the inducer are presented in terms of event frequency, its duration time, and number of events by using the Choi-Williams distribution.
The turbulent flow approaching an upright wall-mounted half cylinder has been numerically investi... more The turbulent flow approaching an upright wall-mounted half cylinder has been numerically investigated by using Large-Eddy Simulation. The formation and the downstream transport of horse-shoe vortex are numerically simulated and compared with experimental data. The interactions between horse-shoe vortex formed by reorientation of incoming vorticity and the Strouhal vortices in the near wake of the half cylinder are also examined in this study. The governing equations are formulated in terms of a structured, curvilinear grid system to accommodate the truncated deductive model proposed by for the subgrid-scale turbulent motions. The present results are produced at a Reynolds number of 8.0×10 3 , based on the cylinder diameter. The far-field sound is also computed using Lighthill's acoustic analogy. This LES results are examined with our experimental data in terms of both the global and local characteristics such as mean drag and base pressure coefficients, vortex shedding frequencies, wall pressure and farfield noise spectra, which are measured in an acoustic windtunnel facility.
Turbo-fans for a FFU unit should be aerodynamically designed to provide the FFU system with the g... more Turbo-fans for a FFU unit should be aerodynamically designed to provide the FFU system with the given flow-rate at the lowest rotational-speed by considering the interaction effect with the FFU casing. In this study, slim and highly efficient turbo-fans are designed to satisfy the given performance at the specific speed by using the hybrid-stacking method of an inducer and a 2D-bladed turbo fan. The mean-line analysis, cascade theory, and CFD technique are all together applied to control the passage areas on the meridional plane from the inlet to the exit of the blade. Furthermore, the torque control algorithm is adopted to improve the performances within the constraints by the motor rpm-torque characteristics, and the resulting measured performances of mock-up fans are discussed. † 인하대학교 기계공학과 대학원 * (주)신성이엔지 ** 책임저자, 회원, 인하대학교 기계공학부
The Journal of the Acoustical Society of America, 1991
The quadrupole-like directivity patterns of jet noise, created by jet impingement on a large fiat... more The quadrupole-like directivity patterns of jet noise, created by jet impingement on a large fiat plate were experimentally obtained by the authors previously [J. Acoust. Soc. Am. Suppl. 1 88, S8 (1990)]. Theoretical considerations have been examined to explain physically this directivity phenomena, and a model coherent structure called the “orientated vortex motion” is proposed. The model hypothesizes that a train of partially organized, large-scale vortices exists inside the boundary layer flow near the plate. By including their images in the rigid plate an oscillation is formed, resulting in a quadropole acoustical source, which explains the experiments quite well. This is analogous to the low Reynolds number free jet case that predicts a possible, similar directivity pattern for the noise from the free jet. A large eddy simulation (LES) computational technique for the simulation of turbulent flows seems to be a feasible approach to investigate the proposed organized motion. A si...
The losses at off-design points from a compressor cascade occur due to the deviation from a desig... more The losses at off-design points from a compressor cascade occur due to the deviation from a design incidence angle at the inlet of the cascade. The self-noise from the blade cascade at off-design points comes from a separated boundary layer and vortex sheddings. If the incidence angle to the cascade increases, stalling in blades may occur and the noise level increases significantly. This study applied Large-Eddy Simulations (LES) using deductive and deductive dynamic SGS models to low Mach-number, turbulent flow with each incidence angle to the cascade ranging from -40°to +20°, and compared numerical predictions with measured data. It was observed that the oscillating separation bubbles attached to the suction surface do not modify wake flows dynamically for cases of negative incidence angles. However, an incidence angle greater than 8°caused a separated vortex near the leading edge to be shed downstream and created stalling. The computed performance parameters such as drag coefficient and total pressure loss coefficient showed good agreement with experimental results. Noise from the cascade of the compressor is summarized as sound generated by a structure interacting with unsteady, turbulent flows. The hybrid method using acoustic analogy was observed to closely predict the measured overall sound powers and directivity patterns at design and off-design points of blade cascade.
Curved displays have been actively developed beyond the limits of the conventional flat displays.... more Curved displays have been actively developed beyond the limits of the conventional flat displays. The curved displays are known to be advantageous for immersion, preferences, and viewing convenience. As such, it is necessary to conduct ergonomic studies on the curvature of the curved display. In this study, experiments were conducted to determine the optimal curvature of the curved display for six kinds of contents. The results of the experiments showed that the optimal curvature is 560.9 mm at the standard viewing distance of 600 mm, which can be considered an evidence that there is an intriguing correlation between the viewing distance and the display curvature. It is expected that the results of this study would improve the existing display products and would provide ergonomic guidelines for the users.
This paper presents a systematic procedure for three-dimensional noise analysis of an axial-flow ... more This paper presents a systematic procedure for three-dimensional noise analysis of an axial-flow fan by using computational aero-acoustics based on Ffowcs Williams-Hawkings equation. Flow-fields of a basic fan model are simulated by solving three-dimensional, unsteady, Reynolds-averaged Navier-Stokes equations using the commercial code ANSYS CFX 11.0. Starting with steady flow results, unsteady flow analysis is performed to extract the fluctuating pressures in the time domain at specified local points on the blade surface of the axial flow fan. The perturbed density wave by rotating blades reaches at the observer position, which is simulated by an in-house noise prediction software based on Ffowcs Williams-Hawkings equation. The detailed far-field noise signatures from the axial-flow fan are analyzed in terms of source types, field characteristics, and interpolation schemes.
An experimental investigation of the near-field pressure fluctuations on a hemisphere-nose of an ... more An experimental investigation of the near-field pressure fluctuations on a hemisphere-nose of an axisymmetric body was conducted in three regions-laminar separation, turbulent shear layer, and developing turbulent boundary layer-at a Reynolds number(Re D) of 2.43×10 5 in an anechoic wind tunnel facility. A significant increase of energy level, especially at low frequencies, was noticed downstream of the laminar separation on the nose, and the positive events were observed in the turbulent shear layer region.
The Journal of the Acoustical Society of America, 1993
The breakdown process of the initial large-scale structures into substructures near the end of th... more The breakdown process of the initial large-scale structures into substructures near the end of the potential core is examined and the production of aerosound as a result is discussed. The quadrupole-like directivity pattern of jet noise is numerically obtained by applying the large-eddy simulation (LES) technique for the fluid flow. The Lighthill analogy is used to obtain the acoustic far field. Most of the sound comes from a region near the end of the potential core. A discussion to explain this quasiaxisymmetric directivity pattern is given. The directivity maximum is obtained at about 45° from the axis of the jet exhaust. This will be explained in terms of turbulent velocities. The convective effect on sources is introduced by ray tracing to calculate retarded times. The result is to obtain more radiation downstream, as is observed. The relation of this correction to the Lighthill theory will be discussed.
Journal of Mechanical Science and Technology, 2011
The coupling between the emitted energy and the acoustic energy in a thermal system, e.g. combust... more The coupling between the emitted energy and the acoustic energy in a thermal system, e.g. combustor, may result in system instability accompanying severe vibration. In this study, the limit curve for thermo-acoustic instability is numerically obtained by using the CFD-CAA combined method. The response function for pulsating release of heat in the Rijke tube and the distribution of mean density in the tube are computed accurately by the CFD analysis and used as inputs to CAA predictions. To verify the accuracy of the prediction method, the thermo-acoustics inside the horizontal, one meter long Rijke tube of square cross-section, which has the heating part at the location of one quarter length, is simulated. The predicted instability curve agrees well with the experimental one except the low flowrate region of Re Do < 1.
Journal of Mechanical Science and Technology, 2012
Urban wind turbines are recommended for installation on a building roof-top to capture more wind ... more Urban wind turbines are recommended for installation on a building roof-top to capture more wind energy. It is critical to decide an exact location for the wind turbine installation on the roof-top area. In this paper a joint probability density function of wind speed and direction is proposed as a statistical model for wind distribution, and a CFD approach is taken to predict the wind acceleration effect by the geometry of the building. Singly Truncated Normal Weibull (TNW) PDF mixture (TNW-PDF) is used as the wind speed distribution function, and von Mises mixture distribution is utilized for the wind direction distribution. These two functions are jointly tested to accurately describe the wind database. The method proposed by Johnson and Wehrly is used to obtain angular-linear distributions for this purpose. The inevitable acceleration and secondary effects of building's roof-top upon the wind distribution is assessed quantitatively by means of the CFD technique. The CFD simulation provides time-averaged wind speeds of various directions with their calculated probabilities at a specific location over the roof top. Thereby the micrositing method is suggested for determination of optimal location for the installation of small wind turbine in the urban roof-top environment.
Journal of Mechanical Science and Technology, 2005
A database obtained by direct numerical simulation of turbulent channel flow is used to compute t... more A database obtained by direct numerical simulation of turbulent channel flow is used to compute the frequency/wave number spectra of the wall shear-stress fluctuations. Auto- and cross-correlations of the wall shear-stress and pressure fluctuations are obtained in space and time domains. The convection velocities as a function of frequency, wave number, and spatial and temporal separations are calculated. The overall convection velocities of wall shear-stress fluctuations for the most energetic structures are obtained and the validity of Taylor’s hypothesis is confirmed.
Transactions of the Korean Society of Mechanical Engineers B, 2012
Fluctuating wall pressures were measured using an array of 16 piezoelectric transducers beneath a... more Fluctuating wall pressures were measured using an array of 16 piezoelectric transducers beneath a turbulent boundary layer. The coating used in this experiment was an open-cell, urethane-type foam with a porosity of approximately 50 ppi. The ultimate objective of the coating is to provide a mechanical filter to reduce the wall pressure fluctuations. The boundary layer on the flat plate was measured by using a hot wire probe, and the CPM method was used to determine the skin friction coefficient. The wall pressure autospectra and streamwise wavenumber-frequency spectra were compared to assess the attenuation of the wall pressure field by the coating. The coating is shown to attenuate the convective wall pressure energy. However, the relatively rough surface of the coating in this investigation resulted in a higher mean wall shear stress, thicker boundary layer, and higher low-frequency wall pressure spectral levels compared to a smooth wall.
The onset of cavitation causes head and efficiency of a main pump to be reduced significantly and... more The onset of cavitation causes head and efficiency of a main pump to be reduced significantly and generates vibration and noise. In order to avoid these phenomena, the inlet of the pump is fitted with a special rotor called an inducer, which can operate satisfactorily with extensive cavitation. The motivation of this study is to find out cavitation modes from the inducer inlet pressure signals and event characteristics from outlet ones at various operating conditions. The cavitation modes are analyzed by using a cross-spectral density of fluctuating pressures at the inducer inlet. The time-frequency characteristics of wall pressures downstream of the inducer are presented in terms of event frequency, its duration time, and number of events by using the Choi-Williams distribution.
The turbulent flow approaching an upright wall-mounted half cylinder has been numerically investi... more The turbulent flow approaching an upright wall-mounted half cylinder has been numerically investigated by using Large-Eddy Simulation. The formation and the downstream transport of horse-shoe vortex are numerically simulated and compared with experimental data. The interactions between horse-shoe vortex formed by reorientation of incoming vorticity and the Strouhal vortices in the near wake of the half cylinder are also examined in this study. The governing equations are formulated in terms of a structured, curvilinear grid system to accommodate the truncated deductive model proposed by for the subgrid-scale turbulent motions. The present results are produced at a Reynolds number of 8.0×10 3 , based on the cylinder diameter. The far-field sound is also computed using Lighthill's acoustic analogy. This LES results are examined with our experimental data in terms of both the global and local characteristics such as mean drag and base pressure coefficients, vortex shedding frequencies, wall pressure and farfield noise spectra, which are measured in an acoustic windtunnel facility.
Turbo-fans for a FFU unit should be aerodynamically designed to provide the FFU system with the g... more Turbo-fans for a FFU unit should be aerodynamically designed to provide the FFU system with the given flow-rate at the lowest rotational-speed by considering the interaction effect with the FFU casing. In this study, slim and highly efficient turbo-fans are designed to satisfy the given performance at the specific speed by using the hybrid-stacking method of an inducer and a 2D-bladed turbo fan. The mean-line analysis, cascade theory, and CFD technique are all together applied to control the passage areas on the meridional plane from the inlet to the exit of the blade. Furthermore, the torque control algorithm is adopted to improve the performances within the constraints by the motor rpm-torque characteristics, and the resulting measured performances of mock-up fans are discussed. † 인하대학교 기계공학과 대학원 * (주)신성이엔지 ** 책임저자, 회원, 인하대학교 기계공학부
The Journal of the Acoustical Society of America, 1991
The quadrupole-like directivity patterns of jet noise, created by jet impingement on a large fiat... more The quadrupole-like directivity patterns of jet noise, created by jet impingement on a large fiat plate were experimentally obtained by the authors previously [J. Acoust. Soc. Am. Suppl. 1 88, S8 (1990)]. Theoretical considerations have been examined to explain physically this directivity phenomena, and a model coherent structure called the “orientated vortex motion” is proposed. The model hypothesizes that a train of partially organized, large-scale vortices exists inside the boundary layer flow near the plate. By including their images in the rigid plate an oscillation is formed, resulting in a quadropole acoustical source, which explains the experiments quite well. This is analogous to the low Reynolds number free jet case that predicts a possible, similar directivity pattern for the noise from the free jet. A large eddy simulation (LES) computational technique for the simulation of turbulent flows seems to be a feasible approach to investigate the proposed organized motion. A si...
The losses at off-design points from a compressor cascade occur due to the deviation from a desig... more The losses at off-design points from a compressor cascade occur due to the deviation from a design incidence angle at the inlet of the cascade. The self-noise from the blade cascade at off-design points comes from a separated boundary layer and vortex sheddings. If the incidence angle to the cascade increases, stalling in blades may occur and the noise level increases significantly. This study applied Large-Eddy Simulations (LES) using deductive and deductive dynamic SGS models to low Mach-number, turbulent flow with each incidence angle to the cascade ranging from -40°to +20°, and compared numerical predictions with measured data. It was observed that the oscillating separation bubbles attached to the suction surface do not modify wake flows dynamically for cases of negative incidence angles. However, an incidence angle greater than 8°caused a separated vortex near the leading edge to be shed downstream and created stalling. The computed performance parameters such as drag coefficient and total pressure loss coefficient showed good agreement with experimental results. Noise from the cascade of the compressor is summarized as sound generated by a structure interacting with unsteady, turbulent flows. The hybrid method using acoustic analogy was observed to closely predict the measured overall sound powers and directivity patterns at design and off-design points of blade cascade.
Curved displays have been actively developed beyond the limits of the conventional flat displays.... more Curved displays have been actively developed beyond the limits of the conventional flat displays. The curved displays are known to be advantageous for immersion, preferences, and viewing convenience. As such, it is necessary to conduct ergonomic studies on the curvature of the curved display. In this study, experiments were conducted to determine the optimal curvature of the curved display for six kinds of contents. The results of the experiments showed that the optimal curvature is 560.9 mm at the standard viewing distance of 600 mm, which can be considered an evidence that there is an intriguing correlation between the viewing distance and the display curvature. It is expected that the results of this study would improve the existing display products and would provide ergonomic guidelines for the users.
This paper presents a systematic procedure for three-dimensional noise analysis of an axial-flow ... more This paper presents a systematic procedure for three-dimensional noise analysis of an axial-flow fan by using computational aero-acoustics based on Ffowcs Williams-Hawkings equation. Flow-fields of a basic fan model are simulated by solving three-dimensional, unsteady, Reynolds-averaged Navier-Stokes equations using the commercial code ANSYS CFX 11.0. Starting with steady flow results, unsteady flow analysis is performed to extract the fluctuating pressures in the time domain at specified local points on the blade surface of the axial flow fan. The perturbed density wave by rotating blades reaches at the observer position, which is simulated by an in-house noise prediction software based on Ffowcs Williams-Hawkings equation. The detailed far-field noise signatures from the axial-flow fan are analyzed in terms of source types, field characteristics, and interpolation schemes.
An experimental investigation of the near-field pressure fluctuations on a hemisphere-nose of an ... more An experimental investigation of the near-field pressure fluctuations on a hemisphere-nose of an axisymmetric body was conducted in three regions-laminar separation, turbulent shear layer, and developing turbulent boundary layer-at a Reynolds number(Re D) of 2.43×10 5 in an anechoic wind tunnel facility. A significant increase of energy level, especially at low frequencies, was noticed downstream of the laminar separation on the nose, and the positive events were observed in the turbulent shear layer region.
The Journal of the Acoustical Society of America, 1993
The breakdown process of the initial large-scale structures into substructures near the end of th... more The breakdown process of the initial large-scale structures into substructures near the end of the potential core is examined and the production of aerosound as a result is discussed. The quadrupole-like directivity pattern of jet noise is numerically obtained by applying the large-eddy simulation (LES) technique for the fluid flow. The Lighthill analogy is used to obtain the acoustic far field. Most of the sound comes from a region near the end of the potential core. A discussion to explain this quasiaxisymmetric directivity pattern is given. The directivity maximum is obtained at about 45° from the axis of the jet exhaust. This will be explained in terms of turbulent velocities. The convective effect on sources is introduced by ray tracing to calculate retarded times. The result is to obtain more radiation downstream, as is observed. The relation of this correction to the Lighthill theory will be discussed.
Journal of Mechanical Science and Technology, 2011
The coupling between the emitted energy and the acoustic energy in a thermal system, e.g. combust... more The coupling between the emitted energy and the acoustic energy in a thermal system, e.g. combustor, may result in system instability accompanying severe vibration. In this study, the limit curve for thermo-acoustic instability is numerically obtained by using the CFD-CAA combined method. The response function for pulsating release of heat in the Rijke tube and the distribution of mean density in the tube are computed accurately by the CFD analysis and used as inputs to CAA predictions. To verify the accuracy of the prediction method, the thermo-acoustics inside the horizontal, one meter long Rijke tube of square cross-section, which has the heating part at the location of one quarter length, is simulated. The predicted instability curve agrees well with the experimental one except the low flowrate region of Re Do < 1.
Journal of Mechanical Science and Technology, 2012
Urban wind turbines are recommended for installation on a building roof-top to capture more wind ... more Urban wind turbines are recommended for installation on a building roof-top to capture more wind energy. It is critical to decide an exact location for the wind turbine installation on the roof-top area. In this paper a joint probability density function of wind speed and direction is proposed as a statistical model for wind distribution, and a CFD approach is taken to predict the wind acceleration effect by the geometry of the building. Singly Truncated Normal Weibull (TNW) PDF mixture (TNW-PDF) is used as the wind speed distribution function, and von Mises mixture distribution is utilized for the wind direction distribution. These two functions are jointly tested to accurately describe the wind database. The method proposed by Johnson and Wehrly is used to obtain angular-linear distributions for this purpose. The inevitable acceleration and secondary effects of building's roof-top upon the wind distribution is assessed quantitatively by means of the CFD technique. The CFD simulation provides time-averaged wind speeds of various directions with their calculated probabilities at a specific location over the roof top. Thereby the micrositing method is suggested for determination of optimal location for the installation of small wind turbine in the urban roof-top environment.
Journal of Mechanical Science and Technology, 2005
A database obtained by direct numerical simulation of turbulent channel flow is used to compute t... more A database obtained by direct numerical simulation of turbulent channel flow is used to compute the frequency/wave number spectra of the wall shear-stress fluctuations. Auto- and cross-correlations of the wall shear-stress and pressure fluctuations are obtained in space and time domains. The convection velocities as a function of frequency, wave number, and spatial and temporal separations are calculated. The overall convection velocities of wall shear-stress fluctuations for the most energetic structures are obtained and the validity of Taylor’s hypothesis is confirmed.
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