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D. Ragni - One of the best experts on this subject based on the ideXlab platform.
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Measurements of near-wall pressure fluctuations for trailing-edge serrations and slits
Experiments in Fluids, 2018Co-Authors: D. Ragni, F. Avallone, W. C. P. Velden, D. CasalinoAbstract:Pressure fluctuations on the suction side of a NACA 0018 with trailing-edge add-ons are obtained from integration of time-resolved stereoscopic and tomographic particle image velocimetry data and compared to the ones computed from Lattice–Boltzmann simulations. The airfoil is retrofitted with solid and slitted serrated trailing edges and measured at 0° and at 12° Angles of attack. At 0° Angle of attack, the boundary-layer thickness and the intensity of the pressure fluctuations are found to decrease along the edge of the serration from its root to its tip. The spectra of the pressure fluctuations additionally show a change of decay in frequency along the serration edge. This last finding has important repercussions for noise-prediction models, which usually assume the turbulence and the slope of the pressure spectra to be “frozen” in the streamwise direction. Results from this study also indicate that the pressure-fluctuation modification along the serrations scales with the local boundary-layer parameters, which can be obtained from experimental and numerical data. In particular, the pressure spectra collapse into a single profile when the local boundary-layer thickness and skin-friction coefficient is employed, instead of the parameters of the incoming flow. The analysis is further extended to flow fields at Positive Angle of attack, where serrations are known to exhibit lower performance in noise reduction. At incidence Angle, the scaling with the local parameters shows that the spatial distribution of boundary-layer thickness and pressure fluctuations is uniform along the serration. This evidence might indicate a Positive correlation between the noise-reduction performance of serrations and the spatial change of pressure spectra (and local boundary-layer thickness) along their edge. Graphical abstract
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Measurements of near-wall pressure fluctuations for trailing-edge serrations and slits
Experiments in Fluids, 2018Co-Authors: D. Ragni, W.c.p. Van Der Velden, F. Avallone, D. CasalinoAbstract:Abstract: Pressure fluctuations on the suction side of a NACA 0018 with trailing-edge add-ons are obtained from integration of time-resolved stereoscopic and tomographic particle image velocimetry data and compared to the ones computed from Lattice–Boltzmann simulations. The airfoil is retrofitted with solid and slitted serrated trailing edges and measured at 0° and at 12° Angles of attack. At 0° Angle of attack, the boundary-layer thickness and the intensity of the pressure fluctuations are found to decrease along the edge of the serration from its root to its tip. The spectra of the pressure fluctuations additionally show a change of decay in frequency along the serration edge. This last finding has important repercussions for noise-prediction models, which usually assume the turbulence and the slope of the pressure spectra to be “frozen” in the streamwise direction. Results from this study also indicate that the pressure-fluctuation modification along the serrations scales with the local boundary-layer parameters, which can be obtained from experimental and numerical data. In particular, the pressure spectra collapse into a single profile when the local boundary-layer thickness and skin-friction coefficient is employed, instead of the parameters of the incoming flow. The analysis is further extended to flow fields at Positive Angle of attack, where serrations are known to exhibit lower performance in noise reduction. At incidence Angle, the scaling with the local parameters shows that the spatial distribution of boundary-layer thickness and pressure fluctuations is uniform along the serration. This evidence might indicate a Positive correlation between the noise-reduction performance of serrations and the spatial change of pressure spectra (and local boundary-layer thickness) along their edge. Graphical abstract: [Figure not available: see fulltext.].
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Three-dimensional flow field over a trailing-edge serration and implications on broadband noise
Physics of Fluids, 2016Co-Authors: F. Avallone, Stefan Pröbsting, D. RagniAbstract:The three-dimensional flow field over the suction side of a NACA 0018 airfoil with trailing-edge serrations was studied by means of time-resolved tomographic particle image velocimetry. Mean flow results show that the boundary layer thickness decreases along the streamwise direction with a corresponding reduction of the size of the turbulent structures developing over the suction side of the serrations. At a Positive Angle of attack, streamwise-oriented and counter-rotating vortices aligned with the edge of the serrations are found to be the main features of the mean flow field. Their formation is attributed to the pressure imbalance between the two sides of the airfoil and the mixing layer at the edge. They locally modify the effective Angle seen by the turbulent flow approaching the serrated edge. This effect may contribute to the serration underperformance in terms of noise reduction reported in literature. The spatial distribution of the spectra of the source term of the Poisson equation, which relate...
F. Avallone - One of the best experts on this subject based on the ideXlab platform.
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Measurements of near-wall pressure fluctuations for trailing-edge serrations and slits
Experiments in Fluids, 2018Co-Authors: D. Ragni, F. Avallone, W. C. P. Velden, D. CasalinoAbstract:Pressure fluctuations on the suction side of a NACA 0018 with trailing-edge add-ons are obtained from integration of time-resolved stereoscopic and tomographic particle image velocimetry data and compared to the ones computed from Lattice–Boltzmann simulations. The airfoil is retrofitted with solid and slitted serrated trailing edges and measured at 0° and at 12° Angles of attack. At 0° Angle of attack, the boundary-layer thickness and the intensity of the pressure fluctuations are found to decrease along the edge of the serration from its root to its tip. The spectra of the pressure fluctuations additionally show a change of decay in frequency along the serration edge. This last finding has important repercussions for noise-prediction models, which usually assume the turbulence and the slope of the pressure spectra to be “frozen” in the streamwise direction. Results from this study also indicate that the pressure-fluctuation modification along the serrations scales with the local boundary-layer parameters, which can be obtained from experimental and numerical data. In particular, the pressure spectra collapse into a single profile when the local boundary-layer thickness and skin-friction coefficient is employed, instead of the parameters of the incoming flow. The analysis is further extended to flow fields at Positive Angle of attack, where serrations are known to exhibit lower performance in noise reduction. At incidence Angle, the scaling with the local parameters shows that the spatial distribution of boundary-layer thickness and pressure fluctuations is uniform along the serration. This evidence might indicate a Positive correlation between the noise-reduction performance of serrations and the spatial change of pressure spectra (and local boundary-layer thickness) along their edge. Graphical abstract
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Measurements of near-wall pressure fluctuations for trailing-edge serrations and slits
Experiments in Fluids, 2018Co-Authors: D. Ragni, W.c.p. Van Der Velden, F. Avallone, D. CasalinoAbstract:Abstract: Pressure fluctuations on the suction side of a NACA 0018 with trailing-edge add-ons are obtained from integration of time-resolved stereoscopic and tomographic particle image velocimetry data and compared to the ones computed from Lattice–Boltzmann simulations. The airfoil is retrofitted with solid and slitted serrated trailing edges and measured at 0° and at 12° Angles of attack. At 0° Angle of attack, the boundary-layer thickness and the intensity of the pressure fluctuations are found to decrease along the edge of the serration from its root to its tip. The spectra of the pressure fluctuations additionally show a change of decay in frequency along the serration edge. This last finding has important repercussions for noise-prediction models, which usually assume the turbulence and the slope of the pressure spectra to be “frozen” in the streamwise direction. Results from this study also indicate that the pressure-fluctuation modification along the serrations scales with the local boundary-layer parameters, which can be obtained from experimental and numerical data. In particular, the pressure spectra collapse into a single profile when the local boundary-layer thickness and skin-friction coefficient is employed, instead of the parameters of the incoming flow. The analysis is further extended to flow fields at Positive Angle of attack, where serrations are known to exhibit lower performance in noise reduction. At incidence Angle, the scaling with the local parameters shows that the spatial distribution of boundary-layer thickness and pressure fluctuations is uniform along the serration. This evidence might indicate a Positive correlation between the noise-reduction performance of serrations and the spatial change of pressure spectra (and local boundary-layer thickness) along their edge. Graphical abstract: [Figure not available: see fulltext.].
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Three-dimensional flow field over a trailing-edge serration and implications on broadband noise
Physics of Fluids, 2016Co-Authors: F. Avallone, Stefan Pröbsting, D. RagniAbstract:The three-dimensional flow field over the suction side of a NACA 0018 airfoil with trailing-edge serrations was studied by means of time-resolved tomographic particle image velocimetry. Mean flow results show that the boundary layer thickness decreases along the streamwise direction with a corresponding reduction of the size of the turbulent structures developing over the suction side of the serrations. At a Positive Angle of attack, streamwise-oriented and counter-rotating vortices aligned with the edge of the serrations are found to be the main features of the mean flow field. Their formation is attributed to the pressure imbalance between the two sides of the airfoil and the mixing layer at the edge. They locally modify the effective Angle seen by the turbulent flow approaching the serrated edge. This effect may contribute to the serration underperformance in terms of noise reduction reported in literature. The spatial distribution of the spectra of the source term of the Poisson equation, which relate...
Ulf Lindefelt - One of the best experts on this subject based on the ideXlab platform.
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temperature dependence of avalanche breakdown for epitaxial diodes in 4h silicon carbide
Applied Physics Letters, 1998Co-Authors: Andrey O Konstantinov, N Nordell, Q Wahab, Ulf LindefeltAbstract:The temperature dependence of avalanche breakdown is investigated for uniform and microplasma-related breakdown in epitaxial 4H SiC p-n junctions. P-n mesa diodes fabricated with Positive Angle beveling and oxide passivation can withstand temperatures of up to 300–400 °C in the breakdown regime. Uniform avalanche breakdown in 4H silicon carbide appears to have a Positive temperature coefficient, in contrast to the 6H polytype, where the temperature coefficient is negative. The influence of deep levels on avalanche breakdown in epitaxial diodes is of minor importance for uniform breakdown, but appears to be significant for breakdown through microplasmas. A negative temperature coefficient for the avalanche breakdown voltage can be observed even for 4H SiC if the breakdown is dominated by microplasmas.
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ionization rates and critical fields in 4h silicon carbide
Applied Physics Letters, 1997Co-Authors: Andrey O Konstantinov, N Nordell, Q Wahab, Ulf LindefeltAbstract:Epitaxial p-n diodes in 4H SiC are fabricated showing a good uniformity of avalanche multiplication and breakdown. Peripheral breakdown is overcome using the Positive Angle beveling technique. Photomultiplication measurements were performed to determine electron and hole ionization rates. For the electric field parallel to the c-axis impact ionization is strongly dominated by holes. A hole to electron ionization coefficient ratio of up to 50 is observed. It is attributed to the discontinuity of the conduction band of 4H SiC for the direction along the c axis. Theoretical values of critical fields and breakdown voltages in 4H SiC are calculated using the ionization rates obtained.
Andrey O Konstantinov - One of the best experts on this subject based on the ideXlab platform.
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temperature dependence of avalanche breakdown for epitaxial diodes in 4h silicon carbide
Applied Physics Letters, 1998Co-Authors: Andrey O Konstantinov, N Nordell, Q Wahab, Ulf LindefeltAbstract:The temperature dependence of avalanche breakdown is investigated for uniform and microplasma-related breakdown in epitaxial 4H SiC p-n junctions. P-n mesa diodes fabricated with Positive Angle beveling and oxide passivation can withstand temperatures of up to 300–400 °C in the breakdown regime. Uniform avalanche breakdown in 4H silicon carbide appears to have a Positive temperature coefficient, in contrast to the 6H polytype, where the temperature coefficient is negative. The influence of deep levels on avalanche breakdown in epitaxial diodes is of minor importance for uniform breakdown, but appears to be significant for breakdown through microplasmas. A negative temperature coefficient for the avalanche breakdown voltage can be observed even for 4H SiC if the breakdown is dominated by microplasmas.
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ionization rates and critical fields in 4h silicon carbide
Applied Physics Letters, 1997Co-Authors: Andrey O Konstantinov, N Nordell, Q Wahab, Ulf LindefeltAbstract:Epitaxial p-n diodes in 4H SiC are fabricated showing a good uniformity of avalanche multiplication and breakdown. Peripheral breakdown is overcome using the Positive Angle beveling technique. Photomultiplication measurements were performed to determine electron and hole ionization rates. For the electric field parallel to the c-axis impact ionization is strongly dominated by holes. A hole to electron ionization coefficient ratio of up to 50 is observed. It is attributed to the discontinuity of the conduction band of 4H SiC for the direction along the c axis. Theoretical values of critical fields and breakdown voltages in 4H SiC are calculated using the ionization rates obtained.
S. S. Yoon - One of the best experts on this subject based on the ideXlab platform.
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Permeability and magnetoimpedance in Fe77.5Si7.5B15 wire under torsional stress
Journal of Applied Physics, 2000Co-Authors: G. H. Ryu, C. G. Kim, I. H. Nahm, S. S. YoonAbstract:The complex permeability spectra and the magnetoimpedance (MI) are measured as a function of torsional stress and torsion Angle in Fe77.5Si7.5B15 amorphous wire. The permeability spectra show irreversible and reversible relaxation with relaxation frequencies of 40 and 300 kHz, respectively. The static permeability resulted from the irreversible magnetization decreases with torsion Angle for a negative direction, but increases with a Positive Angle. The MI profile shows one symmetric peak for the Positive torsion Angle, which is same MI behavior as in the case of zero torsion. However, there is a hysteresis in the MI profile for negative torsion, that is, there are two asymmetric peaks for decreasing field and one peak for increasing field.