Friction drag reduction by a perforated plate
Date
2025
Authors
Hoang, Van Thuan
Editors
Advisors
Arjomandi, Maziar
Cazzolato, Benjamin
Jafari, Azadeh
Cazzolato, Benjamin
Jafari, Azadeh
Journal Title
Journal ISSN
Volume Title
Type:
Thesis
Citation
Statement of Responsibility
Conference Name
Abstract
Turbulence control in wall-bounded flows has gained considerable attention from many researchers. It has the potential to reduce energy consumption in various engineering applications, such as airplanes and cars by reducing skin friction drag. As a result, carbon dioxide (CO2) emissions from these applications are reduced. The generation of friction drag is directly linked to turbulent coherent structures. Coherent structures, like streamwise vortices and streaks, contribute to turbulent momentum transfer and shear stresses. Streamwise vortices, particularly those exhibiting both ejection and sweep events, dominate these coherent structures in the near-wall region of turbulent boundary layers, causing burst events which are the major contributors to turbulence generation and skin friction drag in turbulent boundary layers. This project aims to develop an understanding of the mechanisms of drag reduction in turbulent boundary layers due to a perforated plate with a backing cavity. As a passive device, it does not require external energy input and is simpler to install than an active device. The turbulent boundary layer over a perforated plate with a backing cavity has been simulated in a wind tunnel. The flow field was measured by hot-wire anemometry, which was then analysed by a variable-interval time-averaging (VITA) technique, Fourier transformation, and Fourier decomposition. The modification of the near-wall region, the interaction between small and large scales, the turbulence energy spectrum, and the turbulence generation of the turbulent boundary layers were estimated to evaluate if a perforated plate with a backing cavity can decrease the skin friction drag of a fully formed boundary layer. To develop an understanding of the effect of the perforated plate with a backing cavity on the turbulent characteristics of a turbulent boundary layer, the modification of nearwall turbulence was first investigated using hot-wire measurements. The perforated plate was tested in a low Reynolds number wind tunnel at the University of Adelaide in two flow conditions at momentum-thickness-based Reynolds numbers of 1165 and 2294, which correspond to the viscous-scaled cavity volumes of 2.6×106 and 11.5×106. Their experiments found a reduction of up to 9% in burst intensity and a decrease of approximately 33.5% in burst frequency within the near-wall region at an viscous-scaled wall-normal location below 30, indicating that the perforated plate weakened burst events in this region. Additionally, the perforated plate transferred turbulence energy away from the wall. Consequently, there was a reduction of up to approximately 9% in turbulence intensity near the wall, contributing to a reduction of 8.7% in local skin friction drag. After determining the effect of the perforated plate on the near-wall region of turbulent boundary layers, the impact of the perforated plate on the interaction between large-scale motions primarily located in the log region and small-scale motions in the near-wall region of the boundary layer was investigated. The perforated plate was tested at momentumthickness- based Reynolds numbers from 1165 to 3002, using hot-wire measurements. As a result, the viscous-scaled cavity volume increased from 2.6×106 to 27.8×106. The results showed that the perforated plate strengthened the large-scale amplitude modulation of small scales. An increase in the viscous-scaled cavity volume increases the large-scale amplitude modulation of the small scales. Consequently, the location where the amplitude modulation is zero moves further away from the wall. The increase in the amplitude modulation is associated with reducing skin friction drag in the turbulent boundary layers over the perforated plate. The interaction between the backing cavity and the turbulent boundary layer generates unsteady wall-normal velocities through the perforated plate. It is hypothesised that the reduction in turbulence energy and skin friction in the turbulent boundary layers is contributed by the wall-normal velocities. To develop an understanding of the effect of wall-normal velocities through a perforated plate on turbulent boundary layers, a synthetic jet actuator was used to manipulate the turbulent boundary layers. The effect of jet frequency and amplitude on turbulent boundary layers is investigated. Different jet characteristics including an viscous-scaled forcing frequency from 0.025 to 0.063 at two inner-scale amplitudes of 2.23 and 4.47 were considered to manipulate a turbulent boundary layer at a momentum-thickness-based Reynolds number of 1050. The results demonstrate a reduction of up to 5% in burst intensities and a decrease of approximately 20% in burst durations within the near-wall region at viscous-scaled wall-normal locations below 12, indicating that synthetic jets lifted turbulent energy and weakened burst events in this region. Consequently, there was a reduction of up to approximately 12.5% in turbulence intensity near the wall, contributing to diminished shear stresses and local skin-friction drag. Furthermore, the synthetic jets generated a displacement of the inner peak of turbulent energy away from the wall, indicating that the synthetic jets shifted turbulent energy away from the wall. As the jet frequency or amplitude increased, the modification of the boundary layer became more pronounced. These findings show that the wall-normal velocities through the perforated plate can be used to manipulate the turbulent energy in turbulent boundary layers, which supports the hypothesis about the effects of the the wall-normal velocities on the reduction in turbulence energy and skin friction in the turbulent boundary layers. The interaction between the wall-normal velocities and the pressure at the wall can lead to suppression of the near-wall cycles as well as friction drag. This interaction is represented by wall impdeance of the perforated plate. To understand the impact of the wall impdeance on the near-wall turbulence, different wall impedances generated by a perforated plate with a backing cavity were used to control turbulent boundary layers. Acoustic impedance was varied by the use of a perforated plate with a backing cavity, providing normalised specific acoustic reactance from 5.5 to 69.6 at the angle of the acoustic impedance of approximately 0.25×(2π). The correlation between the wall impedance and the modification of the burst events, turbulence intensity, and turbulence energy of the turbulent boundary layers with the wall impedance also was discussed. The findings indicate that the acoustic impedance of the perforated plate with a backing cavity has a correlation with near-wall turbulence. When the normalised specific acoustic reactance increases, the turbulence intensity in the near-wall region reduces. A maximum reduction of about 7.8% in sweep intensity and 7% in ejection intensity within the near-wall region was observed at a normalised specific acoustic reactance of 69.6. These are associated with a reduction of about 7% in the turbulence intensity in the near-wall region and about 8% in the estimated local friction drag. This indicates that the perforated suppressed the near-wall cycles and consequently reduced the near-wall turbulence. An increase in the normalised specific acoustic impedance affects the relationship between the wall-normal velocities and the pressure at the wall in a way that the wall-normal velocities lift the turbulence kinematic energy further from the wall. These wall-normal velocities also suppress the near-wall cycles by weakening sweep and ejection events in the near-wall region. As a result, friction drag, shear stress in the near-wall area, and turbulence intensity are all decreased. This research provides an improved understanding of the interaction between a perforated plate and turbulent boundary layers for skin drag reduction. The findings show a correlation between friction drag reduction and the acoustic impedance of the perforated plate. When the normalised specific acoustic reactance of the perforated plate increases, the friction drag reduction to become more pronounced. This evidences that the wall-normal velocities interact with the pressure at the wall in a way that suppresses the near-wall cycles and reduces friction drag. The findings provide recommendations for designing perforated plates with a backing cavity with to generate preferable acoustic impedance to achieve friction drag. Furthermore, the presented research shows the potential of perforated plates to reduce turbulence intensity and skin friction drag in turbulent boundary layers. A reduction of 9% in turbulence intensity near the wall and 8.7% in local skin friction drag were found in the turbulent boundary layers over the perforated plate at the momentum-thickness-based Reynolds number of 2294. The findings highlight that the perforated plate weakens burst events in the near-wall region, which consequently reduces near-wall turbulence, shear stresses near the wall, and friction drag. The perforated plate also increases turbulence energy in the logarithmic region. As a result, the amplitude modulation of the near-wall region small scale by the large scales increases, which contributes to friction drag reduction. These effects of the perforated plate on turbulent boundary layers could result from the wall-normal velocities exchange of the flow inside and outside the backing cavity. A larger backing cavity would dampen kinetic energy more effectively. As a result, an increase in the backing cavity volume is associated with a decrease in the local skin friction drag in the turbulent boundary layers over the perforated plate. The results of this research show the potential of perforated plates for turbulent boundary layer control, which suggests a need for further investigation into the flow inside the backing cavity in the future.
School/Discipline
School of Electrical and Mechanical Engineering
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Electrical and Mechanical Engineering, 2025
Provenance
This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at: http://www.adelaide.edu.au/legals