scholarly journals Incipient bedforms in a bidirectional wind regime

2019 ◽  
Vol 862 ◽  
pp. 490-516 ◽  
Author(s):  
Cyril Gadal ◽  
Clément Narteau ◽  
Sylvain Courrech du Pont ◽  
Olivier Rozier ◽  
Philippe Claudin
Keyword(s):  
Three Dimensional ◽  
Wind Regime ◽  
Flow Strength ◽  
Longitudinal Patterns ◽  
Geometric Effect ◽  
Strong Winds ◽  
Bed Slope ◽  
Wind Regimes ◽  
Bidirectional Flow ◽  
Flat Sand

Most terrestrial sand seas form at ‘horse’ latitudes, where the wind direction exhibits seasonal variation. Here, we extend the two-dimensional linear stability analysis of a flat sand bed associated with a unidirectional wind to the three-dimensional case in order to account for multidirectional wind regimes. Focusing on the simplest case of bidirectional flow regimes, we show that the transition from transverse to oblique or longitudinal patterns is controlled by the transport ratio and the divergence angle between the two flows. Our predictions agree with previous results for dune orientation, and also provide a wider range of possible alignments depending on flow strength, especially when the two winds are perpendicular, at which the transition occurs. This analysis also predicts the selected pattern wavelength, which either decreases close to the transition angle for strong winds, due to a geometric effect, or increases at low winds, when the bed slope affects the transport. This theoretical analysis is complemented by analogous subaqueous experiments, where bedforms are submitted to alternate water flows. For transverse bedforms, the experimental data validate the model at strong flows, providing evidence for the predicted geometric effect, but also for the increase of the wavelength close to the transport threshold. For longitudinal bedforms, a discrepancy is observed, which we interpret as the sign of enhanced nonlinearities induced by the development of slip faces when the flow alternately blows on both sides of the dune.

scholarly journals Microphysical evidence of the transition between predominant convective/stratiform rainfall associated with the intraseasonal oscillation in the Southwest Amazon

2005 ◽  
Vol 35 (2) ◽  
pp. 175-184 ◽  
Author(s):  
Rachel Ifanger Albrecht ◽  
Maria Assunção Faus da Silva Dias
Keyword(s):  
Intraseasonal Oscillation ◽  
Convective Precipitation ◽  
Wind Regime ◽  
Tropical Regions ◽  
Stratiform Precipitation ◽  
Type Size ◽  
Global Atmospheric Circulation ◽  
Microphysical Processes ◽  
Wind Regimes ◽  
Precipitation Formation

The distinction between convective and stratiform precipitation profiles around various precipitating systems existent in tropical regions is very important to the global atmospheric circulation, which is extremely sensitive to vertical latent heat distribution. In South America, the convective activity responds to the Intraseasonal Oscillation (IOS). This paper analyzes a disdrometer and a radar profiler data, installed in the Ji-Paraná airport, RO, Brazil, for the field experiment WETAMC/LBA & TRMM/LBA, during January and February of 1999. The microphysical analysis of wind regimes associated with IOS showed a large difference in type, size and microphysical processes of hydrometeor growth in each wind regime: easterly regimes had more turbulence and consequently convective precipitation formation, and westerly regimes had a more stratiform precipitation formation.

scholarly journals Retrieval of High-Resolution Wind Fields over the Southern Korean Peninsula Using the Doppler Weather Radar Network

2009 ◽  
Vol 24 (1) ◽  
pp. 87-103 ◽  
Author(s):  
S-G. Park ◽  
Dong-Kyou Lee
Keyword(s):  
High Resolution ◽  
Korean Peninsula ◽  
General Structure ◽  
Three Dimensional ◽  
Small Scale ◽  
Wind Fields ◽  
Entire Area ◽  
Radar Network ◽  
Strong Winds ◽  
Rain Events

Abstract The performance of a radar network for retrieving high-resolution wind fields over South Korea is examined. The network consists of a total of 18 operational radars. All of the radars possess the Doppler capability and carry out plan position indicator (PPI) volume scans comprising 6–15 elevation steps at every 6 or 10 min. An examination of the coverage of the radar network reveals that the radar network allows the retrieval of three-dimensional high-resolution wind fields over the entire area of the southern Korean Peninsula as well as nearby oceans above a height of approximately 3 km. After the quality control procedures of the radar measurements, the high-resolution wind fields (a few kilometers) are extracted using multiple-Doppler wind synthesis in the Custom Editing and Display of Reduced Information in Cartesian Space (CEDRIC) package developed by NCAR. The radar-retrieved winds are evaluated using the following two rain events: 1) Typhoon Ewiniar in 2006, which resulted in strong winds and heavy rainfall over the entire southern Korean Peninsula, and 2) a well-developed hook echo with a relatively small-scale diameter of about 30 km. The wind fields retrieved from the radar network exhibit counterclockwise rotation around the typhoon center and a general structure around a hook echo such as a cyclonically rotating updraft (i.e., mesocyclone). Comparisons with the wind measurements from four UHF wind profilers for the typhoon case reveal that the u- and υ-wind components retrieved from the radar network deviate by standard deviations of 3.6 and 4.5 m s−1 over ranges from −30 to 20 m s−1 and from 0 to 40 m s−1, respectively. Therefore, it is concluded that the operational radar network has the potential to provide three-dimensional high-resolution wind fields within the mesoscale precipitation systems over almost the entire area of the southern Korean Peninsula.

scholarly journals Directional solidification of a binary alloy into a cellular convective flow: localized morphologies

1999 ◽  
Vol 395 ◽  
pp. 253-270 ◽  
Author(s):  
Y.-J. CHEN ◽  
S. H. DAVIS
Keyword(s):  
Directional Solidification ◽  
Binary Alloy ◽  
Temporal Dynamics ◽  
Three Dimensional ◽  
Multiple Scale ◽  
Solute Diffusion ◽  
Two Dimensional ◽  
Morphological Instability ◽  
Flow Strength ◽  
Weakly Nonlinear

A steady, two-dimensional cellular convection modifies the morphological instability of a binary alloy that undergoes directional solidification. When the convection wavelength is far longer than that of the morphological cells, the behaviour of the moving front is described by a slow, spatial–temporal dynamics obtained through a multiple-scale analysis. The resulting system has a parametric-excitation structure in space, with complex parameters characterizing the interactions between flow, solute diffusion, and rejection. The convection in general stabilizes two-dimensional disturbances, but destabilizes three-dimensional disturbances. When the flow is weak, the morphological instability is incommensurate with the flow wavelength, but as the flow gets stronger, the instability becomes quantized and forced to fit into the flow box. At large flow strength the instability is localized, confined in narrow envelopes. In this case the solutions are discrete eigenstates in an unbounded space. Their stability boundaries and asymptotics are obtained by a WKB analysis. The weakly nonlinear interaction is delivered through the Lyapunov–Schmidt method.

Numerical Anaysis of a Thermopneumatic Micropump

2010 ◽  
Author(s):  
S. Shahsavari ◽  
M. B. Shafii ◽  
M. H. Saidi
Keyword(s):  
Fluid Flow ◽  
Numerical Study ◽  
Three Dimensional ◽  
Working Fluid ◽  
Structure Interaction ◽  
Positive Displacement ◽  
Dimensional Simulation ◽  
Center Deflection ◽  
Bidirectional Flow ◽  
Secondary Fluid

Thermopneumatic micropump is one type of positive displacement micropump, which has many applications due to its relatively large stroke volume, low working voltage, and simple fabrication in microscale. In this paper, a numerical study of heat transfer and fluid flow in a valveless thermopneumatically driven micropump is presented. For rectifying the bidirectional flow, a nozzle and a diffuser are used as the inlet and outlet channels of the chamber. Since the fluid flow is induced by the motion of a diaphragm, the numerical simulation includes fluid structure interaction, which requires applying a dynamic mesh. The domain of solution is divided into two sections; the actuator unit, which contains the secondary fluid, and the main chamber through which the working fluid is passing. The temperature distribution, the pressure variations, and the center deflection of the diaphragm are obtained. In order to validate the model, the numerical results are compared with some experimental data, which shows fair consistency. According to the results of the three dimensional simulation, the rectification efficiency for the nozzle and diffuser channels depends on the frequency.

Three-Dimensional Structures in Experimental Density Currents

2007 ◽  
Author(s):  
B. Firoozabadi ◽  
H. Afshin ◽  
E. Safaaee
Keyword(s):  
Salt Solution ◽  
Chemical Elements ◽  
Three Dimensional ◽  
Lateral Spreading ◽  
Current Data ◽  
Density Current ◽  
Density Currents ◽  
Ambient Water ◽  
Data Set ◽  
Bed Slope

Density currents are continuous currents which move down-slope due to the fact that their density is greater than that of ambient water. The density difference is caused by temperature differences, chemical elements, dissolved materials, or suspended sediment. Many researchers have studied the density current structures, their complexities and uncertainties. However, there is not a detailed 3-D turbulent density current data set perfectly. In this work, the structure of 3-dimensional salt solution density currents is investigated. A laboratory channel was used to study the flow resulting from the release of salt solution into freshwater over an inclined bed. The experiments were conducted with different bottom slopes, inlet concentrations and flow rates. In these tests, the instantaneous velocities are measured by an ADV apparatus (Acoustic Doppler Velocimeter). Results show that by increasing the bed-slope and inlet concentrations, the height of the current decreases. As the density current moves downward the channel or by increasing the discharge, the height of the density current increases. Finally, the effects of different variables such as the bed slope, concentration and flow rate of entering fluid on the velocity profile in different distances from the entrance is studied. The entrainment coefficient, lateral spreading and drag coefficient of the bed and shear layer between salt solution and ambient water is discussed.

Heat Transfer for a Turbine Blade With Nonaxisymmetric Endwall Contouring

2010 ◽  
Vol 133 (1) ◽  
Author(s):  
Stephen P. Lynch ◽  
Narayan Sundaram ◽  
Karen A. Thole ◽  
Atul Kohli ◽  
Christopher Lehane
Keyword(s):  
Heat Transfer ◽  
Turbine Blade ◽  
Three Dimensional ◽  
High Heat ◽  
Secondary Flows ◽  
Flow Strength ◽  
Pressure Losses ◽  
High Heat Transfer ◽  
Oil Flow ◽  
Transfer Benefit

Complex vortical secondary flows that are present near the endwall of an axial gas turbine blade are responsible for high heat transfer rates and high aerodynamic losses. The application of nonaxisymmetric, three-dimensional contouring to the endwall surface has been shown to reduce the strength of the vortical flows and decrease total pressure losses when compared with a flat endwall. The reduction in secondary flow strength with nonaxisymmetric contouring might also be expected to reduce endwall heat transfer. In this study, measurements of endwall heat transfer were taken for a low-pressure turbine blade geometry with both flat and three-dimensional contoured endwalls. Endwall oil flow visualization indicated a reduction in the passage vortex strength for the contoured endwall geometry. Heat transfer levels were reduced by 20% in regions of high heat transfer with the contoured endwall, as compared with the flat endwall. The heat transfer benefit of the endwall contour was not affected by changes in the cascade Reynolds number.

scholarly journals Winter Wind Regimes over Israel Using Self-Organizing Maps

2017 ◽  
Vol 56 (10) ◽  
pp. 2671-2691 ◽  
Author(s):  
Sigalit Berkovic
Keyword(s):  
Surface Wind ◽  
Pressure Gradients ◽  
Wind Regime ◽  
Self Organizing Maps ◽  
Diurnal Variability ◽  
Easterly Wind ◽  
Synoptic Classification ◽  
Wind Measurements ◽  
Wind Regimes ◽  
Self Organizing

AbstractThe aim of this study is to objectively define and automatically reconstruct surface wind regimes over Israel. Unlike other previous studies that subjectively examined case studies or applied the semiobjective synoptic classification (SOC), this study shows the ability of the method of self-organizing maps (SOM) to directly define well-known wind regimes at the synoptic hours (0000, 0600, 1200, and 1800 UTC) during the winter. This ability sets the groundwork for future automatic climatological analysis and applications. The investigation is performed by analyzing surface wind measurements from 53 Israel Meteorological Service stations. The relation between the synoptic variables and the wind regimes is revealed from the averages of ECMWF interim reanalysis (ERA-Interim) variables at each SOM wind regime. The inspection of wind regimes and their average pressure anomalies has shown that wind regimes relate to the gradient of the pressure anomalies rather than to the specific isobaric pattern. Two main wind regimes—strong western and strong eastern—are well known over this region. During daytime, SOM classification identifies these two regimes while SOC reveals only strong western regimes since SOC considers depth of the pressure gradients only in the case of low pressure centers. In accordance with previous studies, two main groups—winter low and high pressure centers and/or Red Sea troughs—are related to the strong westerly or easterly wind regimes with low diurnal variability and high daily persistence. Regimes under weak pressure gradients have higher diurnal variability, relatively lower steadiness, and weaker speed. Their daily persistence is not necessarily low.

Diurnal Cycle of Shallow and Deep Convection for a Tropical Land and an Ocean Environment and Its Relationship to Synoptic Wind Regimes

2006 ◽  
Vol 134 (10) ◽  
pp. 2688-2701 ◽  
Author(s):  
L. Gustavo Pereira ◽  
Steven A. Rutledge
Keyword(s):  
Diurnal Cycle ◽  
Large Scale ◽  
Field Experiments ◽  
Deep Convection ◽  
Shallow Convection ◽  
Wind Regime ◽  
Diurnal Variability ◽  
Synoptic Wind ◽  
Wind Regimes ◽  
Rain Rates

Abstract The characteristics of shallow and deep convection during the Tropical Rainfall Measuring Mission/Large-Scale Biosphere–Atmosphere Experiment in Amazonia (TRMM/LBA) and the Eastern Pacific Investigation of Climate Processes in the Coupled Ocean–Atmosphere System (EPIC) are evaluated in this study. Using high-quality radar data collected during these two tropical field experiments, the reflectivity profiles, rain rates, fraction of convective area, and fraction of rainfall volume in each region are examined. This study focuses on the diurnal cycle of shallow and deep convection for the identified wind regimes in both regions. The easterly phase in TRMM/LBA and the northerly wind regime in EPIC were associated with the strongest convection, indicated by larger rain rates, higher reflectivities, and deeper convective cores compared to the westerly phase in TRMM/LBA and the southerly regime in EPIC. The diurnal cycle results indicated that convection initiates in the morning and peaks in the afternoon during TRMM/LBA, whereas in the east Pacific the diurnal cycle of convection is very dependent on the wind regime. Deep convection in the northerly regime peaks around midnight, nearly 6 h before its southerly regime counterpart. Moreover, the northerly regime of EPIC was dominated by convective rainfall, whereas the southerly regime was dominated by stratiform rainfall. The diurnal variability was more pronounced during TRMM/LBA than in EPIC. Shallow convection was associated with 10% and 3% of precipitation during TRMM/LBA and EPIC, respectively.

scholarly journals Enhanced Wind-Driven Downwelling Flow in Warm Oceanic Eddy Features during the Intensification of Tropical Cyclone Isaac (2012): Observations and Theory

2015 ◽  
Vol 45 (6) ◽  
pp. 1667-1689 ◽  
Author(s):  
Benjamin Jaimes ◽  
Lynn K. Shay
Keyword(s):  
Mixed Layer ◽  
Numerical Models ◽  
Three Dimensional ◽  
Ocean Warming ◽  
Sea Surface ◽  
Upper Ocean ◽  
Ekman Pumping ◽  
Flow Strength ◽  
Oceanic Mixed Layer ◽  
Oceanic Eddy

AbstractTropical cyclones (TCs) typically produce intense oceanic upwelling underneath the storm’s center and weaker and broader downwelling outside upwelled regions. However, several cases of predominantly downwelling responses over warm, anticyclonic mesoscale oceanic features were recently reported, where the ensuing upper-ocean warming prevented significant cooling of the sea surface, and TCs rapidly attained and maintained major status. Elucidating downwelling responses is critical to better understanding TC intensification over warm mesoscale oceanic features. Airborne ocean profilers deployed over the Gulf of Mexico’s eddy features during the intensification of tropical storm Isaac into a hurricane measured isothermal downwelling of up to 60 m over a 12-h interval (5 m h−1) or twice the upwelling strength underneath the storm’s center. This displacement occurred over a warm-core eddy that extended underneath Isaac’s left side, where the ensuing upper-ocean warming was ~8 kW m−2; sea surface temperatures >28°C prevailed during Isaac’s intensification. Rather than with just Ekman pumping WE, these observed upwelling–downwelling responses were consistent with a vertical velocity Ws = WE − Rogδ(Uh + UOML); Ws is the TC-driven pumping velocity, derived from the dominant vorticity balance that considers geostrophic flow strength (measured by the eddy Rossby number Rog = ζg/f), geostrophic vorticity ζg, Coriolis frequency f, aspect ratio δ = h/Rmax, oceanic mixed layer thickness h, storm’s radius of maximum winds Rmax, total surface stresses from storm motion Uh, and oceanic mixed layer Ekman drift UOML. These results underscore the need for initializing coupled numerical models with realistic ocean states to correctly resolve the three-dimensional upwelling–downwelling responses and improve TC intensity forecasting.

scholarly journals Dominant Regimes of the Ross Ice Shelf Surface Wind Field during Austral Autumn 2005

2007 ◽  
Vol 46 (11) ◽  
pp. 1933-1955 ◽  
Author(s):  
Mark W. Seefeldt ◽  
John J. Cassano ◽  
Thomas R. Parish
Keyword(s):  
Wind Field ◽  
Surface Wind ◽  
Wind Regime ◽  
Ice Shelf ◽  
Sea Level Pressure ◽  
Ross Ice Shelf ◽  
Pressure Fields ◽  
Wind Regimes ◽  
Surface Wind Field ◽  
Austral Autumn

Abstract An analysis of the surface wind field across the Ross Ice Shelf, Antarctica, is conducted for austral autumn 2005. The airflow is divided into dominant wind regimes identifying similar wind patterns and the associated typical atmospheric forcing. The results of previous research and a seasonal analysis of the recently expanded network of automatic weather stations in the Ross Ice Shelf region are used to define the dominant wind regimes. Events composing each wind regime are identified by matching wind speed and wind direction observations at several automatic weather station sites for durations of at least 10 h. The four different dominant wind regimes are barrier wind, strong katabatic, weak katabatic, and light wind. Each wind regime is studied through the use of wind rose plots and sea level pressure fields from the Antarctic Mesoscale Prediction System. The sea level pressure fields are used to characterize the forcing of the surface wind field by synoptic pressure gradients. The four dominant wind regimes result in classifying less than 50% of the total hours for austral autumn 2005. The results indicate that previous studies of the Ross Ice Shelf surface wind field, focusing on katabatic winds and barrier winds, represent less than one-half of the observed winds. This study provides a better understanding of the composition of the surface wind field in Antarctica and more insight into the characteristics of the Ross Ice Shelf airstream.

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