Thickness Of Mixed Layer Research Articles

Vortex structure simulation for supersonic mixing layers using nonlinear PSE method

The method of nonlinear parabolized stability equations (PSE) is applied in the simulation of vortex structures in compressible mixing layer. The spatially-evolving unstable waves, which dominate the vortex structure, are investigated through spatial marching method. The instantaneous flow field is obtained by adding the harmonic waves to basic flow. The results show that T-S waves do not keep growing exponentially as the linear evolution, the energy transfer to high order harmonic modes, and that finally all harmonic modes get saturated due to nonlinear interaction. The mean flow distortion induced by the nonlinear interaction between the harmonic modes and their conjugate harmonic ones, makes great change of the average flow and increases the thickness of mixing layer. PSE methods can well capture the two- and three-dimensional large scale nonlinear vortex structures in mixing layers such as vortex roll-up, vortex pairing, and Λ vortex.

Comparative Study of the West African Continental, Coastal, and Marine Atmospheric Profiles during the Summer of 2006

We used sounding data of the Multidisciplinary Analysis of the African Monsoon experience in summer 2006 at continental and coastal sites of West Africa, respectively, to analyze the vertical profiles of relative humidity, temperature, dew point, and speed and wind direction for the JJAS rainy period. The vertical gradient method is applied to the profiles of some thermodynamic parameters estimated from sounding data to do a comparative study of the structure and thermal properties, moisture, and static stability of the atmospheric boundary layer of inland, coastal, and marine sites to show consistent differences related to geographic factors. In vertical profiles of relative humidity, the intensity is higher in Dakar than in Niamey particularly in the core of the season. There are dry intrusions in the low levels at the beginning and end of the season in Dakar, which do not exist in Niamey. The mixing layer on the continent during the day can reach a height greater than 1100 m, and the inversion layer height can exceed 1700 m. Therefore, the maximum thickness of the boundary layer is observed on the continent during the day, while at night the marine boundary layer is the thickest. The diurnal evolution shows that the mixing layer thickness decreases during the night over the continent but increases at the coast and at sea. In the night at the continental site there is a division of the mixing layer with a consistent residual mixing layer. Continental boundary layer is more unstable during the day, while at night it is the marine boundary layer that is more unstable than the coastal and inland ones.

NUMERICAL ANALYSIS OF DEVELOPING TURBULENT FLOW IN A CLOSED COMPOUND CHANNEL

The study of turbulence characteristics in compound channels is still focus of attention. A lot of experimental results have been produced. Main results have revealed a mixing layer formation between main subchannel and the gap region, implying the flow might be ruled by local scales. The outcomes have pointed to the instabilities of mixing layer are responsible for large structures formation between main channel and narrow gap. Furthermore, the periodical behavior of these structures seems to be ruled by mean mixing layer characteristics, as velocity difference, velocity of convection and mixing layer thickness. By using ANSYS-CFX-12, with unsteady Reynolds Average Navier-Stokes and as turbulence model Spalart-Allmaras (SA), a compound channel was studied. Numerical results predicted velocity profile with high vorticity peaks and flow instabilities starting at L/Dh = 15.

Lidar remote sensing of the mixed layer height over a tropical urban Indian station

Mixed layer is an important parameter which controls meteorological conditions in the lower atmosphere. Transport and diffusion of pollutants in the lower atmosphere is highly dependant on the structure of the planetary boundary layer, one important feature of which is the height of the well-mixed layer. In the present study, continuous wave, bistatic argon ion lidar-derived scattered signal strength from different heights in the lower troposphere over Pune (18˚ 32′ N, 73˚ 51′ E, 559 m above mean sea level), India during the period April 2007–January 2008 has been recorded remotely and by employing simple statistical tools, the mixed layer height (MLH) and transition layer thickness (TLT) have been estimated. The results show that sufficient mixing of atmospheric constituents such as aerosols exists in the boundary layer in the post-sunset hours during the summer season, enabling estimation of MLH and TLT. On the other hand, during winter months as mixing ceases/weakens by late evening hours, the mixed layer depth is either low or not easily discernible. In view of the importance of mixed layer depth information for various atmospheric applications, the remote sensing tool used and the simple methodology followed here seem promising.

Exploratory studies of modeling approaches for hydrogen triple flames

A review of triple flame modeling is first presented, which demonstrates the need for additional work in this area. Building on previous methods described in the literature, a hybrid model that uses a weighted average of one-dimensional premixed and diffusion flamelet reaction rates has been proposed and evaluated for a hydrogen triple flame. Results indicated that some type of progress variable is needed for application of the diffusion flamelet contribution. Weighting the premixed flamelet reaction rate contribution at 100%, it is shown that peak temperatures between the model and a case employing detailed chemistry vary 7.5%, while heat release rate, flame speed, and mass fraction contours agree well. A second model, based on a library of reaction rates built from numerical studies which directly resolve the propagating triple flame has also been tested. Computational time for the baseline case is shown to be reduced by a factor of 3 ½ in comparison to use of detailed chemistry. The role of scalar dissipation rate as a necessary independent variable to the library has also been investigated using simulations with variable mixing layer thicknesses. Overall, it is found that large changes in local mixture fraction gradient cause rather small changes in propagation speed and total heat release rate of the hydrogen triple flame. This implies that such a model may be useful for CFD simulations that do not employ spatial resolution capable of resolving the triple flame itself.

Reaction, transformation and delamination of samarium zirconate thermal barrier coatings

The rare earth zirconates have attracted interest for thermal barrier coatings (TBCs) because they have very low intrinsic thermal conductivities, are stable above 1200°C and are more resistant to sintering than yttria-stabilized zirconia (YSZ). Samarium zirconate (SZO) has the lowest thermal conductivity of the rare earth zirconates and its pyrochore structure is stable to 2200°C but little is known about its response to thermal cycling. Here, columnar morphology SZO coatings have been deposited on bond coated superalloy substrates using a directed vapor deposition method that facilitated the incorporation of pore volume fractions of 25 to 45%. The as-deposited coatings had a fluorite structure which transformed to the pyrochlore phase upon thermal cycling between 100 and 1100°C. This cycling eventually led to delamination of the coatings, with failure occurring at the interface between the TGO and a “mixed zone” that formed between the thermally grown alumina oxide (TGO) and the SZO. While the delamination lifetime increased with coating porosity (reduction in coating modulus), it was significantly less than that of similar YSZ coatings applied to the same substrates. The reduced life resulted from a reaction between the rare earth zirconate and the alumina-rich bond coat TGO, leading to the formation of a mixed zone consisting of SZO and SmAlO3. Thermal strain energy calculations show that the delamination driving force increases with TGO and mixed layer thicknesses and with coating modulus. The placement of a 10μm thick YSZ layer between the TGO and SZO layers eliminated the mixed zone and restored the thermal cyclic life to that of YSZ structures.

Global primary production of lakes: 19th Baldi Memorial Lecture

Global primary production of lakes is controlled by factors that are mainly deterministic (incident irradiance, temperature, ice cover) or mainly stochastic (nonalgal light attenuation, mixing depth, nutrient supply) with respect to latitude. The combined effect of these factors on the global lake population was estimated by Monte Carlo simulation based on lakes sampled randomly from the global lake size and latitudinal abundance density functions. Incident irradiance and temperature have strong and approximately equal effects globally on potential production. Ice cover has an additional effect that is approximately half as great as temperature or irradiance at latitudes where ice cover occurs. Together these 3 variables explain about half of the total constraint on global primary production. Among the stochastic factors, mixed layer thickness has a substantially stronger effect than nonalgal light attenuation. When nutrients and relative light availability in the water column (mixing depth, transparency) are considered together, nutrient limitation is the dominant control on algal biomass and production for about 80% of lake area. Estimated global net primary production per unit area for lakes averages 160 gC/m2/y (gross, 200 gC/m2/y; algal respiration 40 gC/m2/y) under background nutrient conditions, but under current conditions it is 260 gC/m2/y (gross, 360 gC/m2/y; algal respiration 100 gC/m2/y) because of eutrophication. Global totals of net primary production for current conditions are 1.3 PgC/y gross, 1.0 PgC/y net, 0.3 PgC/y algal respiration.

Indonesian Throughflow variability during the last 140 ka: the Timor Sea outflow

Abstract The transfer of surface and intermediate water from the Pacific to Indian Ocean through the Indonesian passages (Indonesian Throughflow: ITF) strongly influences the heat and freshwater budgets of tropical water masses, in turn affecting global climate. Here, we use combined δ 18 O and Mg/Ca analyses of surface and thermocline planktonic foraminifers to estimate variations in sea surface temperature, salinity and mixed layer thickness over the last 140 ka. Comparison of water mass properties reveals a steeper thermocline temperature gradient in the Timor Strait than in the eastern Indian Ocean during glacials, implying a decrease in ITF cool thermocline outflow. A major freshening and cooling of thermocline waters occurred at c . 9.5 ka, when sea level rose above a critical threshold, allowing establishment of a shallow marine connection from the South China Sea to the Java Sea. Comparison of benthic δ 13 C profiles ( c . 1800 to 3000 m water depth) suggests vigorous mixing of Indian Ocean and ITF outflow intermediate waters during interglacials. In contrast, deep and intermediate water masses became more stratified during glacials. Lower δ 13 C values at c . 3000 m water depth reflect a decrease in deepwater ventilation, probably related to slowdown of the global thermohaline circulation during glacials.

Surface and near-surface modifications of ultralow dielectric constant materials exposed to plasmas under sidewall-like conditions

The authors describe the temporal evolution of the surface and near-surface regions of a porous SiCOH ultralow k (ULK) dielectric during exposure under sidewall-like exposure conditions to various plasma processing environments. The authors studied the exposure of the ULK material to Ar plasma, C4F8/Ar-based etching plasma, and O2 or CO2 ashing plasmas, as well as various sequences of these processes. Real-time monitoring of the ULK surfaces during plasma processing was performed by in situ ellipsometry employing a novel gap structure. Additionally, changes in ULK surface properties were characterized by x-ray photoelectron spectroscopy and selective dilute hydrofluoric acid wet etching in combination with ex situ ellipsometry measurements. Pristine ULK material exposed to O2 plasma without ion bombardment shows the formation of a near-surface porous layer. For exposure of the ULK to CO2 plasma operated at comparable plasma operation conditions, the modification depth for a given exposure time is reduced relative to O2, but otherwise an identical ellipsometric trajectory is followed. This is indicative of a similar ULK damage mechanism for the two discharges, although at different rates. Energetic (∼400 eV) ion bombardment on the surface of ULK with line-of-sight Ar plasma exposure introduced a ∼12 nm thick SiO2-like densified layer on the ULK surface meanwhile sputtering off the ULK material. The sidewall-like modifications of ULK due to metastable Ar, if present, were too subtle to be measurable in this article. For ULK exposed under sidewall-like geometry to C4F8/Ar-based etching plasma, fluorocarbon quickly permeated into the subsurface region and showed saturation at a mixed layer thickness of about 14 nm. For additional exposure to O2 or CO2 discharges, a strong decrease of the CO2 plasma induced ULK surface modifications with increasing fluorocarbon (FC) film thickness was found, indicative of surface protection by FC surface deposition along with pore-sealing by the FC material. Attempts to increase the protective nature of the FC film by additional plasma processing, e.g., by exposure to Ar or He plasma after FC plasma etching, did not reduce CO2 plasma induced ULK surface modifications further.

An experimental comparison of non-premixed bio-butanol flames with the corresponding flames of ethanol and methane

Butanol non-premixed flames were studied in a counter-flow burner configuration in view of the emergence of methods of production of this fuel from biological agricultural sources. Major combustion species were measured using line-Raman imaging and K-type thermocouples were used in order to perform temperature scans across the flame. Also, extinction strain rates were measured as a function of overall stoichiometry. Of particular importance was the comparison with flames of methane (which is not oxygenated) as well as ethanol which is a currently widely employed bio-fuel. It was shown that butanol flames could sustain higher strain rates at extinction than ethanol flames but significantly smaller than methane flames. The extinction strain rate of methane–butanol mixtures were shown to be smaller than weighted averages of the extinction strain rates of the flames of the two pure components. No leakage of C–H containing species through the flame was measured for either of the flames. For the strongly diluted flames under consideration, it was shown that temperature followed a very closely linear relation with nitrogen concentration. For the same nitrogen concentration, butanol exhibited smaller temperature at the same overall stoichiometry and heat release, because of the higher average molecular weight of the fuel stream. Consistently with theoretical predictions, a decrease in mixing layer thickness was measured with increasing strain.

Validity of Uniform Flow Hypothesis in One-Dimensional Morphodynamic Models

Local-uniform-flow (LUF) hypothesis is a simplification of the governing equations describing river morphodynamics, which is needed to determine the evolution of the bed profile and bed-material composition in the case of large time and space scales. This paper presents a rigorous analysis of the full one-dimensional river hydrodynamic and morphodynamic mathematical model compared to its LUF approximation. The analysis establishes two criteria to assess the validity of the LUF hypothesis: (1) a criterion for rivers in equilibrium and (2) a criterion for evolving rivers (i.e., in nonequilibrium). The first criterion is based on the concept of the morphological box. Variations of the river bed longer than the box length are adequately reproduced by the LUF hypothesis, whereas only spatially averaged values are resolved within the box. The second criterion is based on the concept of an evolution window. Temporal variations represented by wave periods larger than the evolution window can be adequately reproduced by the LUF hypothesis, whereas variations with shorter periods are averaged within this window. The minimum size of morphological box and evolution window that limit the error introduced by the LUF hypothesis increases when the Froude number decreases. Further, the minimum size of the evolution window increases for decreasing sediment concentration and increasing mixing layer thickness (i.e., for larger bed forms). The LUF hypothesis is therefore best applied to small mountain rivers for which both the minimum size of the morphological box and the evolution window is relatively small, so that spatial and temporal variations can be resolved in more detail. Applications using the LUF hypothesis for large watersheds (including the lowland portion of the fluvial network) are possible, but are limited to simulations over larger spatial and temporal intervals.

Climate variability in the southern Indian Ocean as revealed by self-organizing maps

Using a non-linear statistical analysis called “self-organizing maps”, the interannual sea surface temperature (SST) variations in the southern Indian Ocean are investigated. The SST anomalies during austral summer from 1951 to 2006 are classified into nine types with differences in the position of positive and negative SST anomaly poles. To investigate the evolution of these SST anomaly poles, heat budget analysis of mixed-layer using outputs from an ocean general circulation model is conducted. The warming of the mixed-layer by the climatological shortwave radiation is enhanced (suppressed) as a result of negative (positive) mixed-layer thickness anomaly over the positive (negative) SST anomaly pole. This contribution from shortwave radiation is most dominant in the growth of SST anomalies. In contrast to the results reported so far, the contribution from latent heat flux anomaly is not so important. The discrepancy in the analysis is explained by the modulation in the contribution from the climatological heat flux by the interannual mixed-layer depth anomaly that was neglected in the past studies.

Offshore Detachment Process of the Low-Salinity Water around Changjiang Bank in the East China Sea

Abstract A patchlike structure of low-salinity water detached from the Chanjiang “Diluted Water” (CDW) is frequently observed in the East China Sea (ECS). In this study, the offshore detachment process of CDW into the ECS is examined using a three-dimensional numerical model. The model results show that low-salinity water is detached from the CDW plume by the intense tide-induced vertical mixing during the spring tide period when the tidal current becomes stronger. During the spring tide, thickness of the bottom mixed layer in the sloping bottom around Changjiang Bank reaches the mean water depth, implying that the stratification is completely destroyed in the entire water column. As a result, the offshore detachment of CDW occurs in the sloping side of the bank where the tidal energy dissipation is strong enough to overcome the buoyancy effect during this period. On the other hand, the surface stratification is retrieved during the neap tide period, because the tidal current becomes substantially weaker than that in the spring tide. Wind forcing over the ECS as well as tidal mixing is a critical factor for the detachment process because the surface wind primarily induces a northeastward CDW transport across the shelf region where tide-induced vertical mixing is strong. Moreover, the wind-enhanced cross-isobath transport of CDW causes a larger offshore low-salinity patch, indicating that the freshwater volume of the low-salinity patch closely depends on the wind magnitude.

Mixed layer depth variability in the tropical boundary of the California Current, 1997–2007

The variability of the mixed layer depth (MLD) is examined over a decade (1997–2007) for the tropical boundary of the California Current (24–32°N), using conductivity‐temperature‐depth observations collected by quarterly survey cruises. Results indicate that salinity gradients control MLD rather than temperature gradients. The mean state of the upper ocean indicates that contours of constant MLD are parallel to the coast, with mixed layer thickness decreasing toward the coastal zone. The deepest (∼70 m) thickness is reached in January and the shallowest (∼15 m) occurs in July. The warmer conditions (summer) are reproduced for a simple thermal energy equation. The rest of the seasons are reproduced for a one‐dimensional momentum balance for the upper ocean, which includes Ekman dynamics and stratification. This comparison indicates that the variability of MLD is mainly due to wind‐driven phenomena except during the heating period. In particular, seasonal and interannual variability of the MLD are correlated with offshore Ekman transport. An abrupt MLD change occurs between January 1998 and January 2000 associated with the strong El Niño‐La Niña cycle shift that occurred in this period.

A scaling law of internal run-up duration

Wave tank experiments with long internal waves of elevation, of different initial length l, moving in a two-fluid system, interacting with a weak slope of 0.045 rad, show an onshore flow of the dense water, at the undisturbed pycnocline-slope intersection, of duration $11.3\sqrt{l/g'}$ (g′ reduced gravity). This period corresponds to that of a strong bottom current event measured in the stratified ocean at the Ormen Lange gas field, at 850 m depth, lasting for 24 hrs, corresponding to $11.2\sqrt{l/g'}$ , using the width l = 300 km of the Norwegian Atlantic Current (NAC) at the site as length scale, suggesting a lateral sloshing motion of the NAC causing the event. The onshore velocity of the dense fluid has a maximal velocity of $0.4\sqrt{g'h_2}$ in laboratory and 0.5 ms $^{-1}=0.3\sqrt{g'h_2}$ in the field (h 2 mixed upper layer thickness). Run-up of the dense fluid, beyond the undisturbed pycnocline-slope intersection, has initially a front velocity of $0.35\sqrt{g'h_2}$ , corresponding to the velocity of the head of a density current on a flat bottom. Due to disintegration, an initially depressed pycnocline results in comparatively smaller run-up and velocity. While moving past the turning point, a dispersive wave train is formed in the back part of the depression wave, developing by breaking into a sequence of up to eight boluses moving by the undisturbed pycnocline-slope intersection.

Numerical study of hydrogen triple flame response to mixture stratification, ambient temperature, pressure, and water vapor concentration

Mixing layer simulations of spark-ignited H 2/air that detail the influence of mixing layer thickness, domain pressure and air temperature, and water vapor concentration have been carried out. The stabilization speed of the propagating flame front is shown to increase with increasing mixing layer thickness as a result of streamline divergence ahead of the triple flame structure. The pressure changes considered are small (1, 2 bar), and within this range the pressure has only a modest effect on propagation speeds, while air layer temperature effects (298 K, 750 K) are pronounced. Higher adiabatic flame temperatures produced by elevated air temperature have been shown to lead to increased formation of nitrogen oxides. This tendency can be counteracted by increasing inert concentration. Therefore, the influence of water vapor concentration on emissions has been considered in studies where air layer species concentrations are replaced with up to 50% water by mole. Resulting changes in flame structure, heat release rates, flame speed, and emission concentrations – estimated through use of the standard Zeldovich NO mechanism, are given. Data trends for all parameter effects have been related to expected qualitative predictions for direct-injection hydrogen internal combustion engine behavior.

Stochastic interpretation of magnetotelluric data, comparison of methods

Global optimization and stochastic approaches to the interpretation of measured data have recently gained particular attraction as tools for directed search for and/or verification of characteristic structural details and quantitative parameters of the deep structure, which is a task often arising when interpreting geoelectrical induction data in seismoactive and volcanic areas. We present a comparison of three common global optimization and stochastic approaches to the solution of a magnetotelluric inverse problem for thick layer structures, specifically the controlled random search algorithm, the stochastic sampling by the Monte Carlo method with Markov chains and its newly suggested approximate, but largely accelerated, version, the neighbourhood algorithm. We test the algorithms on a notoriously difficult synthetic 5-layer structure with two conductors situated at different depths, as well as on the experimental COPROD1 data set standardly used to benchmark 1D magnetotelluric inversion codes. The controlled random search algorithm is a fast and reliable global minimization procedure if a relatively small number of parameters is involved and a search for a single target minimum is the main objective of the inversion. By repeated runs with different starting test model pools, a sufficiently exhaustive mapping of the parameter space can be accomplished. The Markov chain Monte Carlo gives the most complete information for the parameter estimation and their uncertainty assessment by providing samples from the posterior probability distribution of the model parameters conditioned on the experimental data. Though computationally intensive, this method shows good performance provided the model parameters are sufficiently decorrelated. For layered models with mixed resistivities and layer thicknesses, where strong correlations occur and even different model classes may conform to the target function, the method often converges poorly and even very long chains do not guarantee fair distributions of the model parameters according to their probability densities. The neighbourhood resampling procedure attempts to accelerate the Monte Carlo simulation by approximating the computationally expensive true target function by a simpler, piecewise constant interpolant on a Voronoi mesh constructed over a set of pre-generated models. The method performs relatively fast but seems to suggest systematically larger uncertainties for the model parameters. The results of the stochastic simulations are compared with the standard linearized solutions both for thick layer models and for smooth Occam solutions.

Characteristics of Annular Mixing Layer in High Subsonic Jet

Experimental studies were conducted on an annular mixing layer which evolved from freely expanding adiabatic circular jet produced by a conical nozzle. The mixing layer region shrouding the potential core was explored at four different jet-exit Mach numbers - 0.5, 0.6, 0.7 and 0.8. Pressure measurements were carried out by means of a miniature Pitot probe attached to an automated traverse. Mean velocity profiles were obtained from the radial distribution of pressures across the annular mixing layer thickness at various axial locations. The iso-velocity curves, plotted in longitudinal plane, exhibited that the velocity in the line of the nozzle lip was always about 60% of the nozzle exit velocity for the entire range of Mach numbers studied. The mixing layer thickness, assessed in terms of vorticity thickness, was found to grow linearly with the longitudinal distance downstream of the nozzle. The mixing layer growth rate was found to decline with increasing Mach number. However, the mixing layer was found ...

Coherent Vortex Simulation of weakly compressible turbulent mixing layers using adaptive multiresolution methods

An adaptive mulitresolution method based on a second-order finite volume discretization is presented for solving the three-dimensional compressible Navier-Stokes equations in Cartesian geometry. The explicit time discretization is of second-order and for flux evaluation a 2-4 Mac Cormack scheme is used. Coherent Vortex Simulations (CVS) are performed by decomposing the flow variables into coherent and incoherent contributions. The coherent part is computed deterministically on a locally refined grid using the adaptive multiresolution method while the influence of the incoherent part is neglected to model turbulent dissipation. The computational efficiency of this approach in terms of memory and CPU time compression is illustrated for turbulent mixing layers in the weakly compressible regime and for Reynolds numbers based on the mixing layer thickness between 50 and 200. Comparisons with direct numerical simulations allow to assess the precision and efficiency of CVS.

Experiments on the Flow Field and Acoustic Properties of a Mach number 0·75 Turbulent Air Jet at a Low Reynolds Number

In this paper we present the experimental results of a detailed investigation of the flow and acoustic properties of a turbulent jet with Mach number 0·75 and Reynolds number 3·5 103. We describe the methods and experimental procedures followed during the measurements, and subsequently present the flow field and acoustic field. The experiment presented here is designed to provide accurate and reliable data for validation of Direct Numerical Simulations of the same flow. Mean Mach number surveys provide detailed information on the centreline mean Mach number distribution, radial development of the mean Mach number and the evolution of the jet mixing layer thickness both downstream and in the early stages of jet development. Exit conditions are documented by measuring the mean Mach number profile immediately above the nozzle exit. The fluctuating flow field is characterised by means of a hot-wire, which produced radial profiles of axial turbulence at several stations along the jet axis and the development of flow fluctuations through the jet mixing layer. The axial growth rate of the jet instabilities are determined as function of Strouhal number, and the axial development of several spectral components is documented. The directivity of the overall sound pressure level and several spectral components were investigated. The spectral content of the acoustic far field is shown to be compatible with findings of hot-wire experiments in the mixing layer of the jet. In addition, the measured acoustic spectra agree with Tam’s large-scale similarity and fine-scale similarity spectra (Tam et al., AIAA Pap 96, 1996).