Layer Flow Research Articles

In Situ Observations of the Diurnal Variation in the Boundary Layer of Mature Hurricanes

AbstractRecent studies have suggested that the structure of tropical cyclones (TCs), especially the upper‐level clouds as indicated by satellite infrared brightness temperatures and precipitation, fluctuates with the diurnal cycle. The diurnal cycle of the low‐level structure, including the boundary layer, has not yet been investigated with observations. This study analyzes data from 2242 GPS dropsondes collected in mature hurricanes to investigate the diurnal variation of the mean boundary layer structure. A composite analysis is conducted to compare the kinematic and thermodynamic structure during nighttime (0–6 local time) vs in the afternoon (12–18 local time). The composites show that much stronger inflow occurs during nighttime and the moist entropy is also larger than that in the daytime. Grouping the dropsonde data into 6‐h time windows relative to the local time shows a clear diurnal signal of boundary layer inflow. The amplitude of the diurnal signal is largest at a radius of 250–500 km.

Stability and back flow of boundary layers for wind-driven oceanic current

Stability and back flow of boundary layers for wind-driven oceanic current

Thermocapillary migration of droplets and bubbles in a viscous liquid (review)

Investigation of the process of accumulation of gas bubbles in the aria of a heat source is, from a physical point of view, quite interesting problem that leads to important conclusions for practical applications. The peculiarity of the process under consideration is that the surface tension of the bubble changes in an alternating temperature field, which, in turn, leads to the appearance of a flow in the boundary layer of the liquid. In the world scientific literature, the discovery and description of the effect of gas bubble migration in the direction of the temperature gradient is usually associated with the experimental work of Yang, Goldstein and Block (1959). Without diminishing its significance, we note that the effect was first predicted in the theoretical work of Fedosov (1956) as a result of solving the problem of the onset of a microflow of a liquid near plane and spherical interphase boundaries in the presence of a temperature gradient. In both works, a significant factor in explaining the described phenomenon was the dependence of surface tension on temperature. After some time, after which it was realized the need to take into account the migration of not only bubbles, but also droplets, in inhomogeneous temperature fields in space technologies, biomedical and other applications, there was a significant number of publications on this subject, and this phenomenon was called thermocapillary migration. This review is devoted to the analysis of the main, in the opinion of the authors of the article, results of experimental, theoretical and applied research to establish the mechanism of migration bubbles and drops in temperature gradient fields. In most works, it is assumed that there is no dependence of the physical properties of a liquid, except for surface tension, on temperature. There are only a few studies where the influence of the temperature dependence of the viscosity coefficient was considered, which gives a new impetus to the continuation of research and the development of the theory of the effect, taking into account the thermorheological properties of working media.

Exact solutions for n-layer concentric flow of PTT fluids through a cylindrical pipe

Flows of multiple layers of fluids are encountered in many industrial and manufacturing processes. This paper investigates the concentric n-layer flow for Phan–Thien–Tanner (PTT) fluids through a cylindrical pipe. Finitely many immiscible non-Newtonian fluids are considered to be flowing concentrically in a tube. The flow is modelled using the exponential PTT fluid model and exact solutions for velocity fields and volume flow rates are computed. It has been shown that the corresponding results for linear PTT fluid model as well as Newtonian fluids can be deduced from the obtained expressions, and that they match with the present literature. It has also been observed that for such layered flow, the non-Newtonian parameters significantly affect the flow of fluids in adjacent layers. The effects of involved parameters on the velocity profiles are also shown graphically. We show that a unique velocity maximum exists along the axis of the pipe. Moreover, it is observed with the help of an example that layer thickness can be adjusted to obtain maximal flow rate with a given pressure gradient.

Experimental and Computational Fluid Dynamics Studies on Straight and U-Bend Double Tube Heat Exchangers with Active and Passive Enhancement Methods

In this work, the authors wanted to demonstrate the possibility to increase the heat transfer efficiency by using simple wire coil inserts to create turbulent flow in the boundary layer as well as air blowing into the annulus of the pipe. Experimental investigations were carried out for four heat exchanger constructions, i.e., plain double tube, turbulized double tube, plain U-bend double tube, U-tube with turbulator, plain double tube and U-bend double tube working with two-phase flow conditions (air-water mixture). For these geometries, the values of pressure drop and heat flux were determined. The comparison of heat transfer efficiency was performed based on NTU-ε method. The computational fluid dynamics (CFD) technique was also employed to investigate the influence of coiled wire inserts and air bubbly injection at heat transfer, pressure drops and overall efficiency in double tube heat-exchangers. For this purpose, six configurations of heat exchangers were meshed and simulated at various flow conditions by using the CFD package Autodesk CFD 2016 (Students & Educators version).

ТЕОРЕТИЧЕСКОЕ МОДЕЛИРОВАНИЕ БЕЗНАПОРНЫХ ТЕЧЕНИЙ ЖИДКОСТИ С РАСТЕНИЯМИ, ПЛАВАЮЩИМИ НА СВОБОДНОЙ ПОВЕРХНОСТИ

A mathematical model of a fluid flow in the rectangular channel or in the illuminator with plants floating on the free surface is proposed, based on the use of the Navier-Stokes equations for the plane problem of a slow stationary flow of a viscous fluid in two areas: a free pressureless flow of fluid and a fluid flow in a porous layer formed with the roots of hyacinths floating on the surface of the liquid. The mathematical model of the non-pressure flow of the fluid with plants floating on the free surface is made and give the determination of the water flow speed depending on the parameters of the surface layer, where the plants root systems are concentrated. It is shown that when using water hyacinth for water purification in a settling pond of the enrichment waste storage, the parameters of the layer floating on the free surface of plants do not affect the fluid flow under it. Moreover, the speed in the layer of floating plants is directly proportional to the porosity of this layer and in a complex way depends on the resistance coefficient of the layer. The formulas obtained are the basis for experiments to determine the dependence of the resistance coefficient of the layer with hyacinths on the parameters of the layer and the geometric characteristics of the plants used. The mathematical models proposed in this work for the first time make it possible to determine the fluid velocity through a layer of hyacinths floating on the free surface, depending on the compactness of their upsetting, which makes it possible to choose the parameters of a layer with hyacinths necessary for clarifying technical water from particles of the given hydraulic size, taking into account the geometric dimensions of the settling pond or cleaning channel of rectangular cross section.

Vortex Spinup Process in the Extratropical Transition of Hurricane Sandy (2012)

Abstract This study utilizes the quasi-Lagrangian azimuthal momentum equation (i.e., budget calculation) and 1.667-km-resolution numerical simulation data to study the intensity and structural changes in Hurricane Sandy’s extratropical transition. The results indicate that after the onset of extratropical transition, Sandy maintains an eyewall-like convection and warm core in the core region and has a frontal structure in the outer region. In the outer region, baroclinicity-driven frontal convection induces extensive planetary boundary layer (PBL) inflow, causing an inward advection of absolute angular momentum (AAM) per unit radius, which generates outer local wind maxima and expands Sandy’s outer wind field through a spinup process. Moreover, because the outer tangential wind velocity accelerates in a frontal convection, local wind maxima associated with fronts can expand to the outer sides of frontal regions. Frontal convection increases AAM in the outer region, providing the precondition for reintensification; however, the front itself cannot cause Sandy’s reintensification. The eyewall-like convection in the core region still plays an important role in Sandy’s reintensification. When the baroclinic zone, where a strong horizontal temperature gradient exists, approaches the core region, the eyewall-like convection is enhanced because the warm, moist air of the core region is lifted by the cold, dry air associated with the approaching baroclinic zone. Consequently, owing to the enhancement of eyewall-like convection, the PBL inflow, which extends from the outer region to the core region, develops. This inflow increases the inward transportation of the outer frontal region’s high-AAM air, thus leading to spinning up the core region’s wind and reintensification.

Optimum supply and utilization of pure oxygen along with nitrogen on the cathode side for thermal stability of a proton exchange membrane fuel cell

ABSTRACT As the economical operation of Fuel Cells is essential, in this work, a simulation study is carried out on a 3D geometry of the cathode side of a single-layered Proton Exchange Membrane Fuel Cell (PEMFC) since the cathode reaction is the limiting step. The oxygen and nitrogen mass fraction in the inlet feed on the cathode side are varied as well as the total feed flow rate consisting of traces of water vapor. The worst-case scenario of no-cooling that is with an insulated bipolar plate is assumed. This is to test the maximum thermal effect on the membrane. The flow, heat, and mass transfer multiphysics phenomena in the gas diffusion layer and catalyst scaffold layer are simulated using a simplified geometry and an own program in COMSOL Multiphysics software. The electrochemical reaction of oxygen reacting with hydrogen is considered instantaneous reaction at the catalyst surface. A performance factor comprising of power density generated, the maximum temperature and oxygen utilization is defined. It is optimized for the purpose of determining the optimum oxygen mass fraction in feed and net feed flow rate that gives long life thermally and low operating cost of a PEMFC.

Impacts of Assimilating High-Resolution Atmospheric Motion Vectors Derived from Himawari-8 on Tropical Cyclone Forecast in HWRF

AbstractThe impact of the assimilation of high spatial and temporal resolution atmospheric motion vectors (AMVs) on tropical cyclone (TC) forecasts has been investigated. The high-resolution AMVs are derived from the full disk scan of the new generation geostationary satellite Himawari-8. Forecast experiments for three TCs in 2016 in a western North Pacific basin are performed using the National Centers for Environmental Prediction (NCEP) operational Hurricane Weather Research and Forecasting Model (HWRF). Two different ensemble–variational hybrid data assimilation configurations (using background error covariance created by global ensemble forecast and HWRF ensemble forecast), based on the Gridpoint Statistical Interpolation (GSI), are used for the sensitivity experiments. The results show that the inclusion of high-resolution Himawari-8 AMVs (H8AMV) can benefit the track forecast skill, especially for long-range lead times. The diagnosis of optimal steering flow indicates that the improved track forecast seems to be attributed to the improvement of initial steering flow surrounding the TC. However, the assimilation of H8AMV increases the negative intensity bias and error, especially for short-range forecast lead times. The investigation of the structural change from the assimilation of H8AMV revealed that the following two factors are likely related to this degradation: 1) an increase of inertial stability outside the radius of maximum wind (RMW), which weakens the boundary layer inflow; and 2) a drying around and outside the RMW. Assimilating H8AMV using background error covariance created from HWRF ensemble forecast contributes to a significant reduction in negative intensity bias and error, and there is a significant benefit to TC size forecast.

Modeling of heat and fluid flow in granular layers using high-order compact schemes and volume penalization method

The article presents a novel computational algorithm for modeling of heat and mass transfer processes around neighboring solid objects and inside granular layers. It is based on a volume penalization approach formulated in the framework of an immersed boundary technique, which allows using Cartesian computational meshes for modeling of solid objects with virtually arbitrarily complex shapes and in any form of contact (point-to-point, point-to-surface, etc.). The spatial discretization is performed using a high-order compact discretization/interpolation method on the meshes in which the locations of velocity, density, and temperature are partially staggered with respect to the locations of pressure nodes. Compared to the conventional collocated node arrangement, the present approach has a stabilizing effect, which is very desirable in case of a sudden change of geometry inside the computational grids, especially when high-order discretization is applied. The formulated algorithm is verified based on the following test cases: (i) three hot solid spherical objects immersed in a stream of cold flow; (ii) two granular beds with hundreds of hot solid objects placed in a flowing cold fluid. The results are compared to the solutions obtained using ANSYS-Fluent software and experimental data. In all the cases, the agreement of the results is very good.

A double ITCZ phenomenology of wind errors in the equatorial Atlantic in seasonal forecasts with ECMWF models

Abstract. Modern coupled general circulation models produce systematic biases in the tropical Atlantic that hamper the reliability of long-range predictions. This study focuses on a common springtime westerly wind bias in the equatorial Atlantic in seasonal hindcasts from two coupled models – ECMWF System 4 and EC-Earth v2.3 – and in hindcasts also based on System 4, but with prescribed sea-surface temperatures. The development of the equatorial westerly bias in early April is marked by a rapid transition from a wintertime easterly, cold tongue bias to a springtime westerly bias regime that displays a marked double intertropical convergence zone (ITCZ). The transition is a seasonal feature of the model climatology (independent of initialisation date) and is associated with a seasonal increase in rainfall where a second branch of the ITCZ is produced south of the Equator. Excess off-equatorial convergence redirects the trade winds away from the Equator. Based on arguments of temporal coincidence, the results of our analysis contrast with those from previous work, and alleged causes hereto identified as the likely cause of the equatorial westerly bias in other models must be discarded. Quite in general, we find no evidence of remote influences on the development of the springtime equatorial bias in the Atlantic in the IFS-based models. Limited evidence however is presented that supports the hypothesis of an incorrect representation of the meridional equatorward flow in the marine boundary layer of the southern Atlantic as a contributing factor. Erroneous dynamical constraints on the flow upstream of the Equator may generate convergence and associated rainfall south of the Equator. This directs attention to the representation of the properties of the subtropical boundary layer as a potential source for the double ITCZ bias.

Temperature Factor Effect on the Disturbance Propagation in Hypersonic Flow past a Yawed Flat Plate

The flow in the three-dimensional laminar boundary layer on a yawed flat plate of finite length is studied in the regime of strong viscous-inviscid interaction with a hypersonic flow. In the vicinity of the leading edge the flow functions are expanded in series under the assumption that the base pressure dependent on the transverse coordinate is given at the trailing edge of the plate. It is established that the expansions obtained include an indefinite function and its derivative with respect to the transverse coordinate. The corresponding boundary value problems are formulated and numerically solved, the eigenvalues are found, and it is shown that the exponent in the third term of the expansion differs from that in the second term only by one. The plate surface temperature effect on the flow parameters and the initiation of three-dimensional disturbances is investigated.

Latitudinal dependence of the dry air effect on tropical cyclone development

The impacts of dry air on tropical cyclone (TC) development at different latitudes with no mean flows are investigated with idealized simulations. It is found that the effective radius of the dry air is sensitive to its vertical distribution and the background earth rotation. The effect of low-level dry-air layer in inhibiting TC development decreases with increasing latitude. At lower latitudes, the greater boundary layer gradient wind imbalance results in a strong low-level inflow, and the dry air can easily penetrate into the TC inner-core region. The intruding dry air inhibits the inner-core deep convection and leads to marked asymmetric convective structure, which significantly suppresses TC development. In contrast, at higher latitudes, the dry air gets moistened before reaching the TC inner-core region due to a weaker radial inflow but can suppress the development of the outer spiral rainbands. The suppressed outer spiral rainbands lead to a weaker barrier effect to the boundary layer inflow and help TC development. Furthermore, the lower the altitude of dry-air layer resides, the greater the impact on TC intensification. The low-level pathway associated with the boundary layer inflow plays an important role on how dry-air layer acts on a TC without considering the mean flow effects. Through examining the climatological distribution of the moisture field, we expect that the intrusion of dry air can be more frequent in the North Atlantic area and therefore has more effects on TC development than in the western North Pacific.

Vertical shear alters chemical front speed in thin-layer flows

The mixing of a reactive scalar by a fluid flow can have a significant impact on reaction dynamics and the growth of reacted regions. However, experimental studies of the fluid mechanics of reactive mixing present significant challenges and puzzling results. The observed speed at which reacted regions expand can be separated into a contribution from the underlying flow and a contribution from reaction–diffusion dynamics, which we call the chemical front speed. In prior work (Nevins & Kelley,Chaos, vol. 28 (4), 2018, 043122), we were surprised to observe that the chemical front speed increased where the underlying flow in a thin layer was faster. In this paper, we show that the increase is physical and is caused by smearing of reaction fronts by vertical shear. We show that the increase occurs not only in thin-layer flows with a free surface, but also in Hele-Shaw systems. We draw these conclusions from a series of simulations in which reaction fronts are located according to depth-averaged concentration, as is common in laboratory experiments. Where the front profile is deformed by shear, the apparent front speed changes as well. We compare the simulations to new experimental results and find close quantitative agreement. We also show that changes to the apparent front speed are reduced approximately 80 % by adding a lubrication layer.

Distinctive Features of Flow in the Hypersonic Boundary Layer in the Vicinity of the Plane of Symmetry of a Planar Wing Having a Bend of the Leading Edge

The flow in the three-dimensional laminar boundary layer in the vicinity of the plane of symmetry of a semi-infinite planar wing having a bend of the leading edge is investigated in the case of strong interaction with the external hypersonic flow. The flow functions are asymptotically expanded in power series in the angular coordinate in the vicinity of the plane of symmetry. The corresponding boundary value problem for the principal terms of the expansion is formulated and solved. The possibility of the existence of several solutions in the vicinity of the plane of symmetry is shown. The sweep angle effect on the distinctive features of the flow is considered.

Effect of Unidirectional Vertical Wind Shear on Tropical Cyclone Intensity Change—Lower‐Layer Shear Versus Upper‐Layer Shear

AbstractIn this study, a quadruply nested, nonhydrostatic tropical cyclone (TC) model is used to investigate how the structure and intensity of a mature TC respond differently to imposed lower‐layer and upper‐layer unidirectional environmental vertical wind shears (VWSs). Results show that TC intensity in both cases decrease shortly after the VWS is imposed but with quite different subsequent evolutions. The TC weakens much more rapidly for a relatively long period in the upper‐layer shear than in the lower‐layer shear, which is found to be related to the stronger storm‐relative asymmetric flow in the middle‐upper troposphere and the larger vertical vortex tilt in the former than in the latter. The stronger storm‐relative flow in the former imposes a greater ventilation of the warm core in the middle‐upper troposphere, leading to a more significant weakening of the storm. The storm in the lower‐layer shear only weakens initially after the VWS is imposed but then experiences a quasi periodic intensity oscillation with a period of about 24 hr. This quasi periodic behavior is found to be closely related to the boundary layer thermodynamic “discharge/recharge” mechanism associated with the activity of shear‐induced outer spiral rainbands. There is no significant intensity oscillation for the storm embedded in the upper‐layer shear, even though outer spiral rainbands develop quasi periodically also. The boundary layer inflow is very weak in that case and the low equivalent potential temperature air induced by downdrafts in outer spiral rainbands therefore cannot penetrate into the inner core but remains in the outer region.

The Use of Wall Functions for Simulating the Turbulent Thermal Boundary Layer

An important problem in the numerical simulation of turbulent heat exchange in fluids is accurate prediction of hydrodynamic characteristics of the flow in the boundary layer, which requires a fine grid near rigid surfaces. In applications, it is not always possible to have a fine grid and the use of a coarser grid results in significant loss of accuracy. A well-known approach to improving the accuracy of the numerical simulation of the boundary layer is the use of universal wall functions for computing the friction and thermal flux. In this paper, we consider the known wall functions for computing the thermal flux. The accuracy of these functions in problems of turbulent nonisothermal flow of fluid is studied. These are the flow in a plane channel, Couette flow, and flow along a heated plate. Each of these problems is solved on grids with various near wall resolutions. The results of solving these problems provide a basis for estimating the accuracy of the wall functions used for solving them. It is shown that the wall functions considered in this study yield nonmonotonic convergence of the results as the grid is refined.

On the stability of a Blasius boundary layer subject to localised suction

In this study the origins of premature transition due to oversuction in boundary layers are studied. An infinite row of circular suction pipes that are mounted at right angles to a flat plate subject to a Blasius boundary layer is considered. The interaction between the flow originating from neighbouring holes is weak and for the parameters investigated, the pipe is always found to be unsteady regardless of the state of the flow in the boundary layer. A stability analysis reveals that the appearance of boundary layer transition can be associated with a linear instability in the form of two unstable eigenmodes inside the pipe that have weak tails, which extend into the boundary layer. Through an energy budget and a structural sensitivity analysis, the origin of this flow instability is traced to the structures developing inside the pipe near the pipe junction. Although the amplitudes of the modes in the boundary layer are orders of magnitude smaller than the corresponding amplitudes inside the pipe, a Koopman analysis of the data gathered from a nonlinear direct numerical simulation confirms that it is precisely these disturbances that are responsible for transition to turbulence in the boundary layer due to oversuction.

Detachment of particles from surfaces by thermocapillary flows induced by a moving laser beam

The thermocapillary flows produced by heating with the moving laser beam in a thin liquid layer on a light-absorbing solid surface have recently been recognized to be effective for the removal of particulate impurities from the surface. We performed the comparative analysis of adhesion and thermocapillary removal forces acting on particles attached to the solid surface. A simplified hydrodynamic model was used to evaluate the velocity field of the thermocapillary flow in the liquid layer covering particles. Hydrodynamic forces such as the drag force, the lifting force induced by the shear flow far away from the receding contact line and the maximal lifting force caused by the transversal velocity in the layer near the receding contact line were identified as the cleaning forces, which are determined by the temperature gradient along the surface. It was found out that the lifting force is not enough to overcome the adhesion force and detach particles from the solid surface. The drag and the maximal lifting forces were shown to be responsible for removal of particles by using the proposed method. A reasonable qualitative agreement between experimental results and the quantitative force analysis was observed.

Simulation of urban boundary and canopy layer flows in port areas induced by different marine boundary layer inflow conditions

Computational fluid dynamics (CFD) simulations and wind-tunnel (WT) tests can be considered as boundary-value problems, where the inlet boundary condition, which is usually obtained inferring inlet mean wind profiles from on-site measurements or other type of experimental data, represents the large-scale atmospheric forcing exerted at the outer limit of the urban model. It is not clear, however, to which extent the choice of different inflow wind speed profiles may affect WT and CFD results in the urban environment. In the present study, this aspect is investigated through the comparison of the wind flow fields simulated numerically and tested experimentally in an atmospheric boundary layer wind tunnel (ABLWT) within a district of Livorno city, Italy, called “Quartiere La Venezia”. Three different shapes of inflow profiles were tested using the CFD technique and the results were compared with each other: one is based on the approach-flow profiles measured upstream of the urban model in the WT test section (WT profile) and two are based on anemometric data corresponding to the approach-flow profile measured by means of a LiDAR wind profiler (LiDAR profile 1 and 2). The analysis showed that using different wind speed profiles does not affect significantly the results in the urban canopy layer (UCL), where correlations of 95% and 98% were found between the LiDAR profile 1 and 2 data and the WT profile data (at z = 0.02 m above the bottom), respectively. Conversely, the different inflow profiles strongly affected the results above the UCL. This means that the local-scale effects induced on the wind field in the UCL by the urban texture are dominated mainly by the larger-scale forcing, as within the canopy the flow remains topologically invariant despite the different inflow conditions.