Xu, Peng (2009) Turbulent flow control using spanwise travelling wave via Lorentz forcing. PhD thesis, University of Nottingham.
Lorentz-forcing spanwise travelling wave actuation in the turbulent boundary layer has been studied in a water channel at various experimental conditions (St = 139.2, 186 and 232; T+ = 17, 42 and 82). At the Reynolds number of Reτ = 388, a maximum skin friction drag reduction of 30% is achieved in some cases, while up to 22.8% of viscous drag increase is also observed.
The results of the turbulent boundary layer profiles show that the turbulence intensities for both the drag-reducing and the drag-increasing cases are reduced. The higher moments of turbulence statistics such as the skewness and the kurtosis increase near the wall when T+ = 42, St = 232 in the drag-reducing case. For the drag-increasing case (T+ = 17, St = 232), the skewness and the kurtosis are decreased when very close to the wall (y+ < 6), while they are increased for y+ > 6, similar to the drag-reducing case. The reduction in the turbulent intensities as well as the changes in VITA velocity profiles suggest that the drag changes are due to the modified near-wall activities by the Lorentz forcing.
Flow visualisation shows that the low-speed streaks are twisted into the spanwise directions in both the drag-reducing and the drag-increasing cases. For the drag-reducing case, the low-speed streaks are clustered together to form a wide low-speed region similar to what Du et al (2002) have found. This low-speed region seems to act as the ‘storage’ of low-speed fluid to help reduce the skin friction drag. To achieve the drag reduction, the spanwise displacement of low-speed streaks must be greater than 115 wall units in the present configuration, which compares well with the average spacing of low-speed streaks in the turbulent boundary layer.
When the drag increase occurs, only pseudo-local spanwise oscillation is observed without a formation of a wide low-speed region. The pseudo-local spanwise oscillation appears to produce converging and diverging motions around the forcing-activation area. The induced streamwise vorticity layers are believed to enhance the effect of the sweep motion, which results in the increasing skin-friction drag.
|Item Type:||Thesis (PhD)|
|Faculties/Schools:||UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering|
|Deposited By:||Dr. Peng Xu|
|Deposited On:||09 Dec 2009 11:28|
|Last Modified:||09 Dec 2009 11:28|
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