2 results
Characteristics of drag-reduced turbulent boundary layers with pulsed-direct-current plasma actuation
- Alan H. Duong, Thomas C. Corke, Flint O. Thomas
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- Journal:
- Journal of Fluid Mechanics / Volume 915 / 25 May 2021
- Published online by Cambridge University Press:
- 29 March 2021, A113
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Experiments were performed using an active flow control approach that has shown the ability to significantly reduce the viscous drag in turbulent boundary layers. The purpose of this work was to document the changes in the turbulence characteristics of the boundary layer with the drag reduction. The flow control involved generating a steady spanwise velocity component of the order of $u_{\tau }$, within the sublayer using an array of pulsed-DC plasma actuators. The intent was to reduce the wall-normal vorticity component, $\omega _y$, that is associated with the mean flow distortion caused by quasi-steady streamwise vorticity associated with the wall streak structure first observed by Kline et al. (J. Fluid Mech., vol. 30, 1967, pp. 741–773). The significance of the $\omega _y$ comes from Schoppa & Hussain (J. Fluid Mech., vol. 453, 2002, pp. 57–108), who proposed an autonomous mechanism for self-sustained wall turbulence generation of which the sublayer wall-normal vorticity component is a critical parameter. The results document the characteristics of a turbulent boundary layer in which the viscous drag was reduced by 68 %. This involved measurements of the $u$ and $v$ velocity components in a three-dimensional region within the boundary layer using a pair of dual (X) hot-wire probes. Under the reduced drag, these documented a decrease in $u$ and $v$ turbulence intensity levels through most of the boundary layer. When scaled by $u_{\tau }$, the impact on the $v$ fluctuations was larger than that on the $u$ fluctuations. Analysis based on [$uv$] quadrant splitting documented a decrease in duration, and an increase in the time between ‘ejections’ (Q2) and ‘sweep’ (Q4) events that substantially lowered the near-wall turbulence production in the drag-reduced boundary layers. Conditional averages used to reconstruct the two- and three-dimensional coherent motions including $\lambda _2$ vortical structures, indicate a suppression of coherent features in the wall layer. These results are consistent with an underlying mechanism for drag reduction that comes from a suppression of the turbulence producing events in the wall layer associated with the wall streak structure.
Behavioural and electrophysiological responses of the female malaria mosquito Anopheles gambiae (Diptera: Culicidae) to Limburger cheese volatiles
- Bart G.J. Knols, Joop J.A. van Loon, Alan Cork, Rosemary D. Robinson, Wim Adam, Jocelijn Meijerink, Ruurd De Jong, Willem Takken
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- Journal:
- Bulletin of Entomological Research / Volume 87 / Issue 2 / April 1997
- Published online by Cambridge University Press:
- 10 July 2009, pp. 151-159
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Limburger cheese, previously shown to attract female Anopheles gambiae Giles, was solvent extracted and chemically fractionated into acid and non-acid fractions. The extracts and aliquots of headspace odour of the cheese were analysed by gas chromatography and electron impact mass spectrometry. Nineteen saturated and unsaturated aliphatic fatty acids, ranging in carbon chain length from C2 to C18, were detected. The most abundant acids (>1 mg/g of cheese) identified in the acid extract were ethanoic, propanoic, butanoic, hexadecanoic and 9-octadecenoic acid. The same compounds were identified in analyses of headspace samples but only trace quantities of the less volatile acids (C10 to C16) were present, whilst C18 acids were absent. Behavioural responses of female A. gambiae towards a range of dilutions of the acid extract (in diethyl ether) were recorded in a windtunnel bioassay. The undiluted extract was found to be repellent, but became highly attractive (P «0.001) at lower doses, and was still significantly attractive (P<0.001) when diluted 106times. A synthetic mixture of 12 of the more abundant aliphatic acids identified in the acid extract was found to be significantly attractive (P<0.001) when diluted 108 times. Electroantennographic (EAG) studies showed significant and reproducible responses to (saturated) Limburger cheese headspace. At doses higher than 0.1%, the synthetic mixture of 12 acids elicited significantly higher EAG amplitudes than the solvent control (paraffin oil). EAG responses were recorded from mosquitoes stimulated with C5 to C8 acids, that were characterized by significant dose-dependencies. Weaker, though significant EAG responses were obtained with the less volatile acids (C9 to C14). Only hexadecanoic acid did not elicit a detectable response. The electrophysiological and behavioural responses obtained with fatty acids isolated from Limburger cheese suggests that together they could act as a kairomone for female A. gambiae. The implications of this are discussed together with the occurrence and bacterial production of these compounds on human skin.