Direct numerical simulations of free convection beneath an air–water interface at low Rayleigh numbers

2003 ◽  
Vol 15 (10) ◽  
pp. 3181 ◽  
Author(s):  
R. I. Leighton ◽  
G. B. Smith ◽  
R. A. Handler
Keyword(s):  
Free Convection ◽  
Numerical Simulations ◽  
Direct Numerical Simulations ◽  
Water Interface ◽  
Air Water Interface ◽  
Rayleigh Numbers

Pressure fluctuations and interfacial robustness in turbulent flows over superhydrophobic surfaces

2015 ◽  
Vol 783 ◽  
pp. 448-473 ◽  
Author(s):  
J. Seo ◽  
R. García-Mayoral ◽  
A. Mani
Keyword(s):  
Drag Reduction ◽  
Numerical Simulations ◽  
Superhydrophobic Surface ◽  
Stagnation Pressure ◽  
Direct Numerical Simulations ◽  
Superhydrophobic Surfaces ◽  
Water Interface ◽  
System Parameters ◽  
Wide Range ◽  
Air Water Interface

Superhydrophobic surfaces can entrap gas pockets within their grooves when submerged in water. Such a mixed-phase boundary is shown to result in an effective slip velocity on the surface, and has promising potential for drag reduction and energy-saving in hydrodynamic applications. The target flow regime, in most such applications, is a turbulent flow. Previous analyses of this problem involved direct numerical simulations of turbulence with the superhydrophobic surface modelled as a flat boundary, but with a heterogeneous mix of slip and no-slip boundary conditions corresponding to the surface texture. Analysis of the kinematic data from these simulations has helped to establish the magnitude of drag reduction for various texture topologies. The present work is the first investigation that, alongside a kinematic investigation, addresses the robustness of superhydrophobic surfaces by studying the load fields obtain from data from direct numerical simulations (DNS). The key questions at the focus of this work are: does a superhydrophobic surface induce a different pressure field compared to a flat surface? If so, how does this difference scale with system parameters, and when does it become significant that it can deform the air–water interface and potentially rapture the entrapped gas pockets? To this end, we have performed DNS of turbulent channel flows subject to superhydrophobic surfaces over a wide range of texture sizes spanning values from $L^{+}=6$ to $L^{+}=155$ when expressed in terms of viscous units. The pressure statistics at the wall are decomposed into two contributions: one coherent, caused by the stagnation of slipping flow hitting solid posts, and one time-dependent, caused by overlying turbulence. The results show that the larger texture size intensifies the contribution of stagnation pressure, while the contribution from turbulence is essentially insensitive to $L^{+}$. The two-dimensional stagnation pressure distribution at the wall and the pressure statistics in the wall-normal direction are found to be self-similar for different $L^{+}$. The scaling of the induced pressure and the consequent deformations of the air–water interface are analysed. Based on our results, an upper bound on the texture wavelength is quantified that limits the range of robust operation of superhydrophobic surfaces when exposed to high-speed flows. Our results indicate that when the system parameters are expressed in terms of viscous units, the main parameters controlling the problem are $L^{+}$ and a Weber number based on inner dimensions; We obtain good collapse when all our results are expressed in wall units, independently of the Reynolds number.

Blast Wave Mitigation from the Straight Tube by Using Water Part II - Numerical Simulation

2018 ◽  
Vol 910 ◽  
pp. 78-83 ◽  
Author(s):  
Yuta Sugiyama ◽  
Tomotaka Homae ◽  
Kunihiko Wakabayashi ◽  
Tomoharu Matsumura ◽  
Yoshio Nakayama
Keyword(s):  
Experimental Data ◽  
Numerical Simulation ◽  
Energy Transfer ◽  
Numerical Simulations ◽  
Blast Wave ◽  
Blast Waves ◽  
Straight Tube ◽  
Water Interface ◽  
Effect Of Water ◽  
Air Water Interface

This paper investigates explosions in a straight square tube in order to understand the mitigation effect of water on blast waves that emerge outside. Numerical simulations are used to assess the effect of water that is put inside the tube. The water reduces the peak overpressure outside, which agrees well with the experimental data. The increases in the kinetic and internal energies of the water are estimated, and the internal energy transfer at the air/water interface is shown to be an important factor in mitigating the blast wave in the present numerical method.

scholarly journals Mixed convection in turbulent channels with unstable stratification

2017 ◽  
Vol 821 ◽  
pp. 482-516 ◽  
Author(s):  
Sergio Pirozzoli ◽  
Matteo Bernardini ◽  
Roberto Verzicco ◽  
Paolo Orlandi
Keyword(s):  
Free Convection ◽  
Numerical Simulations ◽  
Turbulent Flows ◽  
Momentum Flux ◽  
Direct Numerical Simulations ◽  
Mean Values ◽  
Developed Turbulence ◽  
Wide Range ◽  
Channel Thickness ◽  
The Individual

We study turbulent flows in pressure-driven planar channels with imposed unstable thermal stratification, using direct numerical simulations in a wide range of Reynolds and Rayleigh numbers and reaching flow conditions which are representative of fully developed turbulence. The combined effect of forced and free convection produces a peculiar pattern of quasi-streamwise rollers occupying the full channel thickness, with aspect ratio considerably higher than unity; it has been observed that they have an important redistributing effect on temperature and momentum, providing for a substantial fraction of the heat and momentum flux at bulk Richardson numbers larger than$0.01$. The mean values and the variances of the flow variables do not appear to follow Prandtl’s scaling in the free-convection regime, except for the temperature and vertical velocity fluctuations, which are more directly affected by wall-attached turbulent plumes. We find that the Monin–Obukhov theory nevertheless yields a useful representation of the main flow features. In particular, the widely used Businger–Dyer flux-profile relationships are found to provide a convenient way of approximately accounting for the bulk effects of friction and buoyancy, although the individual profiles may have wide scatter from the alleged trends. Significant deviations are found in direct numerical simulations with respect to the commonly used parametrization of the momentum flux in the light-wind regime, which may have important practical impact in wall models of atmospheric dynamics. Finally, for modelling purposes, we devise a set of empirical predictive formulae for the heat flux and friction coefficients, which are within approximately$10\,\%$standard deviation from the numerical results in a wide range of flow parameters.

Sampling and analysis of the air-water interface with SEM and EDS of microlayers

1989 ◽  
Vol 47 ◽  
pp. 1004-1005
Author(s):  
Randall W. Smith ◽  
John Dash
Keyword(s):  
Heavy Metals ◽  
Surface Microlayer ◽  
Surface Films ◽  
Water Interface ◽  
Physical Processes ◽  
Infrared Spectroscopic Analysis ◽  
Eds Analysis ◽  
Air Water Interface ◽  
Significant Area ◽  
Bulk Chemical

The structure of the air-water interface forms a boundary layer that involves biological ,chemical geological and physical processes in its formation. Freshwater and sea surface microlayers form at the air-water interface and include a diverse assemblage of organic matter, detritus, microorganisms, plankton and heavy metals. The sampling of microlayers and the examination of components is presently a significant area of study because of the input of anthropogenic materials and their accumulation at the air-water interface. The neustonic organisms present in this environment may be sensitive to the toxic components of these inputs. Hardy reports that over 20 different methods have been developed for sampling of microlayers, primarily for bulk chemical analysis. We report here the examination of microlayer films for the documentation of structure and composition.Baier and Gucinski reported the use of Langmuir-Blogett films obtained on germanium prisms for infrared spectroscopic analysis (IR-ATR) of components. The sampling of microlayers has been done by collecting fi1ms on glass plates and teflon drums, We found that microlayers could be collected on 11 mm glass cover slips by pulling a Langmuir-Blogett film from a surface microlayer. Comparative collections were made on methylcel1ulose filter pads. The films could be air-dried or preserved in Lugol's Iodine Several slicks or surface films were sampled in September, 1987 in Chesapeake Bay, Maryland and in August, 1988 in Sequim Bay, Washington, For glass coverslips the films were air-dried, mounted on SEM pegs, ringed with colloidal silver, and sputter coated with Au-Pd, The Langmuir-Blogett film technique maintained the structure of the microlayer intact for examination, SEM observation and EDS analysis were then used to determine organisms and relative concentrations of heavy metals, using a Link AN 10000 EDS system with an ISI SS40 SEM unit. Typical heavy microlayer films are shown in Figure 3.

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