We perform direct numerical simulation of air and water turbulence in a Couette flow. The air-water interface is kept flat, and the coupling between the two fluids is through continuity of velocity and shear stress at the interface. True air-to-water ratios of density and viscosity are used in the simulation to illustrate features of air-water coupled boundary layers. Our analysis of statistics of the velocity, vorticity, and turbulent kinetic energy budget confirms known features, notably the similarity of the airside interface boundary layer to wall boundary layer. Our study obtains new insights on the characteristics of the waterside motions. Compared to the airside, waterside turbulence structures are more persistent and larger in scale, which dominate the interface signatures. The interface boundary layer on the waterside possesses unique features that are intermediate between but qualitatively different from wall boundary layer and free-slip surface layer. On the waterside, as the interface is approached, enstrophy and viscous dissipation first decrease together, with higher turbulence mixing and production than the airside, and then increase sharply in response to airside stress fluctuations. The waterside turbulence is characterized by interface-connected, hairpin, and quasistreamwise vortices that are closely related to each other in their evolutions. Conditional average using a variable-interval space averaging approach illustrates the strong air-water coupling in splat, hairpin vortex, and quasistreamwise vortex events. It is found that waterside velocity gradients are amplified significantly across the interface, with the presence of horizontal jets at splat and quasistreamwise vortex regions. We also study the transport of passive scalars near the air-water coupled boundary layers. As expected, vertical advection associated with coupling air-water coherent structures greatly enhances interfacial transfer of scalars. By considering scalars of different diffusivity and solubility values, we investigate the partition of interfacial transfer between the waterside and airside. We illustrate statistical characteristics of gaslike and heatlike scalars and perform detailed analysis of the scalar variance budget for these to reveal unique features that are substantially different from wall boundary layer or free-slip surface layer.

We perform direct numerical simulation to study the transport of gas and heat as passive scalars in free-surface turbulence. Our analysis focuses on the surface age of surface fluid particles, i.e. the time elapsed since the last surface renewal they experienced. Using Lagrangian tracing of fluid particles combined with heat diffusion analysis, we are able to directly quantify surface age to illustrate scalar characteristics at different stages of interfacial transfer. Results show that at the early stage of surface renewal, vertical advection associated with upwellings greatly enhances surface gas flux; random surface renewal model does not apply at this stage when most of the interfacial gas transfer occurs. After a fluid particle leaves the upwelling region, it may enter a nearby downwelling region immediately, where the gas flux is sharply reduced but the variation in surface temperature is small; alternatively, the fluid particle may travel along the surface for some time before it is absorbed by a downwelling, where the surface temperature has changed significantly due to long duration of diffusion and the gas flux is also reduced. To gain further insight into the relationships between surface velocity and scalar quantities, we perform a statistical analysis of upwellings using clustering and nonlinear regression. With this analysis, we are able to provide qualitative and quantitative descriptions of the skewed probability density functions associated with the surface divergence, temperature and gas flux that support our physics-based investigation of surface renewal and surface age.

[1] We use direct numerical simulation to study characteristics of interfacial transfer of gas and heat in free-surface turbulence. Using Lagrangian tracing, we are able to directly quantify surface age of surface renewal for the first time. The physical meaning of surface renewal and various representations of surface age, including the use of heat as a proxy, are discussed. Results show that the Higbie penetration theory and the Danckwerts random surface renewal model are inappropriate for gas transfer. [2] Surface renewal is a critical process in turbulent interfacial transport that is important to many applications including sea surface temperature and atmosphere -ocean gas transfer. Scalar transport in the upper ocean is governed by the interplay of molecular diffusion at the sea surface and turbulent mixing underneath. When a surface renewal occurs, fluid is brought from the bulk towards the surface, the scalar is highly mixed, and the gradient of scalar concentration is increased at the surface to enhance interfacial diffusion.[3] An influential model of surface renewal is the penetration theory of Higbie [1935], which stated that the surface is intermittently exposed to turbulent upwelling flows. The turbulent mixing process was considered instant, with the scalar well mixed. After this surface renewal, molecular diffusion was assumed to dominate in the scalar boundary layer till the next surface renewal is generated by the turbulence below. Take gas as an example. Its concentration c is governed by the diffusion equation:2 ). Here D is the molecular diffusivity and z is the vertical coordinate. Subject to the boundary conditions at the surface c(z = 0, t) = c 0 and in the deep region c(z = À1, t) = c bulk , and the initial condition c(z, t = 0) = c bulk , the gas flux at the surface q g obtains asIn the above, the time elapsed since the surface renewal is called the surface age t.[4] Danckwerts [1951] elaborated the penetration theory in his random surface renewal model by assuming that the chance of a surface element being renewed by fresh fluid from the bulk flow is independent of its surface age. Therefore, the probability density function (pdf) of the surface age governed by dp(t)/dt = Às p(t), where p is the pdf and s is the fractional rate of surface elements being renewed (assumed to be constant by Danckwerts), has an exponential solution: p(t) = s exp(Àst). With this pdf, average surface age can be obtained as t = 1/s, and average gas flux at the surface is q g = (c 0 1951] did not account for effects of vertical turbulent advection after t = 0. Near the surface, the advection is in the form of upwelling and can be measured by surface divergency a = @u/@x + @v/@y = À@w/@z (so that w = Àaz), which was considered by Ledwell [1984] and Banerjee [1990]. The relative effects of diffusion and advection can be seen in the upwelling stagnation flow model of Chan and Scriven [1970], in which the advectiondiffusion equation @c/@t = az(@c/@z) + D(@ 2 c/@z 2 ) is solved to obtain the surface gas flux base...

It is widely assumed in clinical practice that drug treatment associated with hypotension can result in falls and syncope, but there is actually very little evidence to support this. Therefore the data in all patients whose cardiovascular medications were stopped at a falls/syncope clinic were analysed to see if their symptoms were altered and if renewal of these medications was necessary at subsequent visits.Of 338 consecutive referrals, cardiovascular medications had been stopped in 65 (19%). At follow up 78% reported improvement in their original presenting symptoms and renewal of medication was not necessary in 77% oV antianginals, 69% oV antihypertensives, and 36% oV antiarrhythmics. It was concluded that adjusting cardiovascular medications could help in the management of falls and syncope and may obviate the need for other treatment. These medications can be stopped in select patients if there is regular monitoring and this should reduce unwanted side eVects and costs of these drugs. (Postgrad Med J 2001;77:403-405) Keywords: withdrawal of medications; falls; syncope Hypotensive responses are associated with recurrent falls and syncope and, a high prevalence of cardiovascular diagnoses in elderly fallers with and without syncope has been reported. 1 2 Some observational studies suggest that cardiovascular medications are associated with falls and syncope, 3-5 while casecontrol data refute this concept. 6 The main objectives of this study were to examine if patients attending a falls/syncope clinic reported any change in symptoms after their cardiovascular medications were adjusted, and to see if it was necessary to renew these drugs at follow up visits. Methods and resultsAll attendances at the syncope clinic in a two year period were reviewed noting those in whom cardiovascular drugs were deliberately stopped. Referrals to the clinic included those with recurrent unexplained falls and presyncope as well as syncope. Certain cardiovascular medications were stopped altogether if there was evidence of unwanted side eVects including a cardiovascular diagnosis attributable to their use, if there was no indication, that is, digoxin in those in sinus rhythm, and if supine blood pressure was <120/80 mm Hg in those taking antihypertensives.Twenty four hour ambulatory blood pressure monitoring (ABPM) was carried out after a six week period in all patients whose antihypertensive medications were stopped and at intervals thereafter in certain cases. The initial follow up visits were scheduled within a two month period and repeat visits were arranged at 2-3 month intervals for those needing ongoing review.Indications for restarting the medications at follow up visits were angina for antianginals and symptoms attributable to arrhythmias for antiarrhythmics. Three or more systolic blood pressure readings >160 mm Hg, three or more diastolic blood pressure readings >90 mm Hg, and/or a mean daytime blood pressure value >134/81 mm Hg 7 recorded during the 24 hour ABPM were considered indications to resume antihyp...