ORCID Profile
0000-0003-1951-6106
Current Organisation
University of Adelaide
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Publisher: AIP Publishing
Date: 2019
DOI: 10.1063/1.5117532
Publisher: Elsevier BV
Date: 07-2022
Publisher: Cambridge University Press (CUP)
Date: 22-03-2023
DOI: 10.1017/JFM.2023.149
Abstract: The potential of frequency-tuned surfaces as a passive control strategy for reducing drag in wall-bounded turbulent flows is investigated using resolvent analysis. These surfaces are considered to have geometries with impedances that permit transpiration and/or slip at the wall in response to wall pressure and/or shear and are tuned to target the dynamically important structures of wall turbulence. It is shown that wall impedance can suppress the modes resembling the near-wall cycle and the very-large-scale motions and the Reynolds stress contribution of these modes. Suppression of the near-wall cycle requires a more reactive impedance. In addition to these dynamically important modes, the effect of wall impedance across the spectral space is analysed by considering varying mode speeds and wavelengths. It is shown that the materials designed for suppression of the near-wall modes lead to gain reduction over a wide range across the spectral space. Furthermore, a wall with only shear-driven impedance is found to suppress turbulent structures over a wider range in spectral space, leading to an overall turbulent drag reduction. Most importantly, the present analysis shows that the drag-reducing impedance is non-unique and the control performance is not sensitive to variations of the surface impedance within a favourable range. This implies that specific frequency bandwidths can be targeted with periodic material design.
Publisher: AIP Publishing
Date: 2022
DOI: 10.1063/5.0085677
Publisher: Elsevier BV
Date: 11-2019
Publisher: AIP Publishing
Date: 04-2020
DOI: 10.1063/5.0005594
Abstract: The mean and spectral characteristics of turbulence in the wake flow of a flat plate model resembling a heliostat in the atmospheric boundary layer flow are investigated in a wind tunnel experiment. Mean velocity and turbulence kinetic energy were characterized in the wake of a heliostat model at three elevation angles up to a distance of eight times the characteristic dimension of the heliostat panel. An increase in turbulence intensity and kinetic energy was found in the wake flow, reaching a peak at a distance equal to approximately twice the characteristic dimension of the heliostat panel. Furthermore, spectral and wavelet analysis of velocity fluctuations in the wake showed that the dominant mechanism in the immediate downstream of the plate was the breakdown of large inflow turbulence structures to smaller scales. In the end, the wake-induced turbulence patterns and wind loads in a heliostat field were discussed. It was found that compared to a heliostat at the front row, the heliostats positioned in high-density regions of a field were subjected to a higher turbulence intensity and, consequently, larger dynamic wind loading. The results show that it is necessary to consider the increased unsteady wind loads for the design of a heliostat in high-density regions of a field, where the gap between the rows is less than three-times the characteristic length of the heliostat panel.
Publisher: Springer Science and Business Media LLC
Date: 30-01-2021
Publisher: AIP Publishing
Date: 2020
DOI: 10.1063/5.0028676
Publisher: Elsevier BV
Date: 09-2020
Publisher: AIP Publishing
Date: 04-2022
DOI: 10.1063/5.0084505
Abstract: This study investigates the potential of finite-length porous surfaces with a subsurface chamber for the control of the turbulent boundary layer. The effect of the subsurface chamber on the boundary layer is investigated by hot-wire anemometry measurements of the boundary layer response to different chamber configurations. Three different chamber configurations were investigated: a common cavity that connected the array of surface perforations, a locally reacting chamber with in idual cavities underneath each perforation, and chambers that connected the perforations in streamwise or spanwise flow directions. It was found that a common backing cavity and in idual cavities reduced the peak turbulence intensity, whereas the test case with streamwise or spanwise channels increased the turbulence intensity and strengthened large-scale turbulent structures within the boundary layer. While both common and in idual cavities were effective in reducing turbulence, the in idual cavities created a larger reduction in the pre-multiplied spectrum with an average of 80% at large scales compared to between 40% and 60% reduction at large scales for common cavities with different volumes. Hence, a short porous surface with in idual cavities underneath each perforation was found to be the most effective turbulence-reducing configuration among the investigated cases.
Publisher: Elsevier BV
Date: 09-2021
Publisher: AIP Publishing
Date: 2020
DOI: 10.1063/5.0028678
Publisher: Elsevier BV
Date: 10-2019
Publisher: Elsevier BV
Date: 06-2019
Publisher: AIP Publishing
Date: 2020
DOI: 10.1063/5.0028609
No related grants have been discovered for Azadeh Jafari.