Browsing by Author "Tsai, WT"

Now showing 1 - 8 of 8

A numerical study on the evolution and structure of a stress-driven free-surface turbulent shear flow
(2005-12-25)
Tsai, WT
;
Chen, SM
;
Moeng, CH
;
土木工程學系
;
Department of Civil Engineering
Turbulent shear flow beneath a flat free surface driven by a surface stress is simulated IF numerically to gain a better understanding of the hydrodynamic processes governing the scalar transfer across the air-water interface. The simulation is posed to mimic the subsequent development of a wind-driven shear layer as in a previous experiment except that the initiation of the surface waves is inhibited, thus focusing on the boundary effect of the stress-imposed surface on the underlying turbulent flow and vice versa. Despite the idealizations inherent in conducting the simulation, the computed flow exhibits the major surface features, qualitatively similar to those that appear in the laboratory and field experiments. Two distinct surface signatures, namely elongated high-speed streaks and localized low-speed spots, are observed in the simulated flow. Including temperature as a passive tracer and describing an upward heat flux at the surface, we obtain high-speed streaks that are colder and low-speed spots that are warmer than the surrounding regions. The high-speed streaks, arranged with somewhat equal cross-spacing of centimetres scale, are formed by an array of streamwise jets within the viscous sublayer immediately next to the surface. Beneath the streaks, counter-rotating streamwise vortex pairs are observed among other prevailing elongated vortices. However, they are significantly shorter in length and more irregular than their corresponding high-speed streaks at the surface. Accompanying the more organized high-speed streaks, localized regions of low streamwise velocity emerge randomly on the surface. These low-speed spots are attributed to strong upwelling flows which disrupt the viscous sublayer and also bring up the submerged fluids of low streamwise velocity. The occasional interruptions of the streamwise high-speed Jets by the upwelling flows account for bifurcation or dislocation of the surface streaks. Statistics of the turbulence are presented and their implications for the formation of the flow structures are discussed.
An assessment of the effect of sea surface surfactant on global atmosphere-ocean CO2 flux
(2003-04-26)
Tsai, WT
;
Liu, KK
;
土木工程學系
;
Department of Civil Engineering
[1] We assess the possible impact of the distribution of naturally occurring surfactants on the direct integration of the global atmosphere-ocean CO2 flux across the ocean surface. The global atmosphere-ocean CO2 flux is calculated using the monthly mean DeltapCO(2) climatology compiled by Takahashi et al. [1997] as well as satellite wind speed and sea-surface temperature data. In the absence of any global map of surfactant coverage and as it is known that phytoplankton exudates and degradation products are the major sources of marine surfactants, ocean primary productivity, which can be derived from the satellite-based estimate of chlorophyll concentration, is used as an indicator of the presence of surfactants as proposed by Asher [1997]. From the calculated results it is found that suppression of the upward and downward CO2 fluxes by marine surfactants exhibits an asymmetric effect: The average percent reduction of absorption flux by surfactants is about twice that of outgassing, which results in an overall decrease in the net global CO2 uptake by the oceans. For almost half of the year (between January and May) the presence of surfactants does not affect CO2 outgassing from global oceans. In contrast, throughout the entire year the presence of surfactants suppresses CO2 absorption by the oceans. The major reduction in absorption fluxes occurs in the northern Pacific and Atlantic (10degreesN to 70degreesN) in all seasons and in the Southern Ocean (south of 40degreesS) in austral spring and summer. However, the most significant decrease in outgassing fluxes occurs in the equatorial and southern Pacific Ocean (40degreesS to 10degreesN), particularly in the eastern equatorial and subtropical waters off the southern American coast, in the period of austral spring and summer. Annual net CO2 flux is reduced by approximately 20% under the surfactant coverage condition that the primary productivity is above a threshold value of 25 g-C m(-2) mom(-1) and by about 50% with a threshold of 15 g-C m(-2) mom(-1).
Catalytic decomposition of ozone in the presence of water vapor
(1998)
Tsai, WT
;
Chang, CY
;
Jung, FH
;
Chiu, CY
;
Huang, WH
The catalytic decomposition of ozone in the presence of water vapor at the relative humidity (RH) of about 90 % was investigated by using a tubular fixed-bed reactor. The performance tests and reaction kinetic studies were conducted for catalyst, i.e., Pt/gamma-Al2O3 (symbolized as Dash-220). The experiments were carried out at various constant reaction temperatures (298 similar to 323 K) and space velocities (24,000 similar to 44,000 hr(-1)). About 75 % decomposition efficiencies of ozone with inlet concentration of about 0.17 mol/m(3) was achieved at catalyst contact time of 015 sec (space velocity of 24,000 hr(-1)), reaction temperature of 323 K and RH of about 90 % for the Dash-220 catalyst. However, about 97 % decomposition efficiencies of ozone without the presence of water vapor at the same reaction conditions. The results indicated that the Dash-220 catalyst with the presence of water vapor gave lower performance of ozone decomposition than without the presence of water vapor can be achieved at the same reaction conditions. For the reaction kinetics, it was found that the pseudo-first-order chemical reaction rate equation was appropriate for the catalytic decomposition of ozone in the presence of water vapor investigated in this study. The apparent activation energies for the decomposition reaction of ozone using the Dash-220 catalyst in the presence of water vapor at RH of 90 % is 19.32 kJ mol(-1). The corresponding pre-exponential factor is 0.0041 mol/(s g-catalyst (mol/m(3))).
Direct numerical simulation of a turbulent shear flow beneath a wind-driven water surface
(2003)
Tsai, WT
;
土木工程學系
;
Department of Civil Engineering
Direct numerical simulation of a turbulent shear flow beneath a wind-driven water surface is performed to gain a better understanding of the hydrodynamic processes governing the vertical exchanges of heat and gases across the air-water interface. Coherent flow structures are identified which serve as the renewal eddies that pump the submerged fluids toward the water surface and bring down the upper fluids, and therefore enhance the scalar exchange between the air and the water body. The computed vertical distributions of the mean velocity, temperature and gas concentration compare well the parameterized profiles derived based on the surface renewal model.
Evidence and quantification of the correlation between radar backscatter and ocean colour supported by simultaneously acquired in situ sea truth
(2002-05-15)
Lin, II
;
Wen, LS
;
Liu, KK
;
Tsai, WT
;
Liu, AK
[1] Near simultaneously (within 2 hours) acquired ERS-2 Synthetic Aperture Radar (SAR) and Sea-viewing Wide Field-of-view Scanner (SeaWiFS) data over an upwelling region shows remarkable similarity in feature's location, scale, and boundary. Under uniform wind and sea states, reduction of Normalised Radar Cross Section (NRCS) from SAR is found highly correlated with the increase of SeaWiFS Chlorophyll-a (Chl-a) concentration. Typically, 1 mg/m3 of chlorophyll-a is correspondent with a 5 dB reduction of NRCS. This relationship is supported by simultaneously collected in situ sea truth measurements. This work provides a direct evidence and exceptional quantification of the relationship between radar backscatter and ocean colour that has long been speculated.
Molecular sublayers beneath the air-sea interface relative to momentum, heat and gas transports
(2003-09-27)
Tsai, WT
;
Chen, SM
;
Lin, MY
;
Hung, LP
;
土木工程學系
Numerical simulations of the wind-driven aqueous turbulent flow and the underlying heat and dissolved gas transports are conducted with sufficiently fine grid resolution to resolve the molecular sublayers immediately beneath the air-water interface. The simulated mean distributions of velocity, temperature and gas concentration all exhibit exponential profiles across the sublayers in accordance with the theoretical postulation of Liu and Businger [1975] which they derived on the basis of the conceptual surface renewal model. The numerical results identify two major coherent renewal processes within the flow: intermittent upwellings induced by uprising horseshoe-like eddies in the well-mixed region, and elongated, high-speed, cool streaks within the sublayer reflecting the cool-skin thermal structure.
On the formation of streaks on wind-driven water surfaces
(2001-10-15)
Tsai, WT
;
土木工程學系
;
Department of Civil Engineering
Evidence derived from numerical experiments is presented to show that, contrary to the prevalent notion, the generation of elongated wind-induced streaks on water surfaces with spacing of centimeters is not necessarily attributed to interactions between surface waves and the shear current. Although the water surface is artificially designated as a flat boundary in the numerical simulations here, high-speed streaks appear on the surface arising from an underlying shear current. Floating Lagrangian tracers cluster along the streaks and, in accordance with field and laboratory observations, undergo dislocation and amalgamation.
Transport of exogenous surfactants on a thin viscous film within an axisymmetric airway
(2004-02-26)
Tsai, WT
;
Liu, LY
;
土木工程學系
;
Department of Civil Engineering
The transport of exogenous surfactant in an axisymmetric pulmonary airway lining is analyzed using the model based on lubrication theory to understand the parameters involved in surfactant replacement therapy for neonatal respiratory distress syndrome, including the ratio of film thickness-to-airway radius epsilon, and the initial exogenous and endogenous surfactant distributions. Numerical simulation of the model indicates that the film disturbance induced by Marangoni flow in low generation airways (small epsilon, such as trachea) would be more significant than that in high generation airways (large epsilon, such as bronchioles) for the same amount of instilled surfactant. The non-dimensional transport properties of the exogenous surfactant, however, are virtually identical in various airway generations considered. Increasing the initial local exogenous concentration enhances the surfactant transport and also intensifies the film disturbance. The presence of endogenous surfactant tends to moderate the shock-like film disturbance and also prolongs the extent of surfactant monolayer front. This, however, does not mean that the spread of exogenous surfactant is enhanced with the presence of endogenous surfactant. The extended monolayer is attributed to the compression of endogenous surfactant concentration in regions ahead of the advancing exogenous surfactant front. (C) 2003 Elsevier B.V. All rights reserved.