Density currents or density wedges: boundary-layer influence and control methods

1987 ◽  
Vol 177 ◽  
pp. 187-206 ◽  
Author(s):  
Gerhard H. Jirka ◽  
Masamitsu Arita
Keyword(s):  
Boundary Layer ◽  
State Density ◽  
Density Current ◽  
Local Form ◽  
Density Currents ◽  
Viscous Effects ◽  
Long Distance ◽  
Ambient Flow ◽  
Wake Zone ◽  
And Control

Density currents and density wedges are two observed manifestations of interactions between an ambient flow and a horizontal buoyant intrusion. In a density current the buoyant pressure force is primarily balanced by the local form drag of the current head which has a blunt shape and abrupt depth change. In a density wedge a distributed interfacial drag is the primary balancing force, leading to a stretched-out shape and long-distance intrusions. A perturbation analysis of the approach flow to the inclined front of a density current shows that slight momentum changes caused by viscous effects in the ambient flow determine which of these two flow types is established. In a uniform ambient channel flow, any momentum deficit relative to the inviscid case will lead to a local flattening of the front and ultimate breakdown into a density wedge. On the other hand, a momentum surplus will support a steady-state density current. Several exploratory experiments on control of the ambient boundary layer through local non-uniformities were performed with the objective of achieving stable density-current forms with limited intrusion lengths. These methods include a small step, a barrier and suction and are applied for intrusions at either the bottom or surface of an ambient water flow. In all cases, good agreement is found with the force balances predicted by Benjamin's (1968) theory and its extension by Britter & Simpson (1978) which accounts for entrainment in the wake zone of the head.

Mobile Integrated Profiler System (MIPS) Observations of Low-Level Convergent Boundaries during IHOP

2006 ◽  
Vol 134 (1) ◽  
pp. 92-112 ◽  
Author(s):  
Haldun Karan ◽  
Kevin Knupp
Keyword(s):  
Boundary Layer ◽  
Cold Front ◽  
The Other ◽  
Density Current ◽  
Wind Profiler ◽  
Density Currents ◽  
General Increase ◽  
Convective Boundary ◽  
Convective Initiation ◽  
Slow Moving

Abstract Characteristics of convergent boundary zones (CBZs) sampled by the Mobile Integrated Profiling System (MIPS) during the 2002 International H2O Project (IHOP_2002) are presented. The MIPS sensors (915-MHz wind profiler, 12-channel microwave profiling radiometer, ceilometer, and surface instrumentation) provide very fine temporal kinematic and thermodynamic profiles of the atmospheric boundary layer and CBZ properties, including enhanced 915-MHz backscatter within the CBZ updraft (equivalent to the radar fine line), a general increase in integrated water vapor within the updrafts of the CBZ, an increase in the convective boundary layer (CBL) depth, and changes in ceilometer backscatter that are typically coincident with arrival of cooler, moister air (the case for density current CBZ). Three contrasting CBZs are analyzed. Convective initiation was associated with a slow-moving dryline as it passed over the MIPS on 19 June. Updrafts up to 6 m s−1 were measured, and the CBL attained its greatest depth within the CBZ. The CBZ in the other two cases were quite similar to density currents. The retrograding dryline of 18 June produced an enhancement in preexisting convection within 30 km of the MIPS. On 24 May, a shallow cold front, about 800 m deep, was sampled.

Direct Numerical Simulations of Planar and Cylindrical Density Currents

2005 ◽  
Vol 73 (6) ◽  
pp. 923-930 ◽  
Author(s):  
Mariano I. Cantero ◽  
S. Balachandar ◽  
Marcelo H. García ◽  
James P. Ferry
Keyword(s):  
Stokes Equations ◽  
Three Dimensional ◽  
Boussinesq Approximation ◽  
The Body ◽  
Density Current ◽  
Density Currents ◽  
Mean Flow ◽  
Viscous Effects ◽  
Spreading Velocity ◽  
Slip Boundary

The collapse of a heavy fluid column in a lighter environment is studied by direct numerical simulation of the Navier-Stokes equations using the Boussinesq approximation for small density difference. Such phenomenon occurs in many engineering and environmental problems resulting in a density current spreading over a no-slip boundary. In this work, density currents corresponding to two Grashof (Gr) numbers are investigated (105 and 1.5×106) for two very different geometrical configurations, namely, planar and cylindrical, with the goal of identifying differences and similarities in the flow structure and dynamics. The numerical model is capable of reproducing most of the two- and three-dimensional flow structures previously observed in the laboratory and in the field. Soon after the release of the heavier fluid into the quiescent environment, a density current forms exhibiting a well-defined head with a hanging nose followed by a shallower body and tail. In the case of large Gr, the flow evolves in a three-dimensional fashion featuring a pattern of lobes and clefts in the intruding front and substantial three-dimensionality in the trailing body. For the case of the lower Gr, the flow is completely two dimensional. The dynamics of the current is visualized and explained in terms of the mean flow for different phases of spreading. The initial phase, known as slumping phase, is characterized by a nearly constant spreading velocity and strong vortex shedding from the front of the current. Our numerical results show that this spreading velocity is influenced by Gr as well as the geometrical configuration. The slumping phase is followed by a decelerating phase in which the vortices move into the body of the current, pair, stretch and decay as viscous effects become important. The simulated dynamics of the flow during this phase is in very good agreement with previously reported experiments.

scholarly journals Observations of Shoaling Density Current Regime Changes in Internal Wave Interactions

2020 ◽  
Vol 50 (6) ◽  
pp. 1733-1751
Author(s):  
Aviv Solodoch ◽  
Jeroen M. Molemaker ◽  
Kaushik Srinivasan ◽  
Maristella Berta ◽  
Louis Marie ◽  
...  
Keyword(s):  
Propagation Speed ◽  
Finite Amplitude ◽  
State Density ◽  
Critical Regime ◽  
Density Current ◽  
Coastal Current ◽  
Density Currents ◽  
Ambient Conditions ◽  
Plume Front ◽  
Small Width

AbstractWe present in situ and remote observations of a Mississippi plume front in the Louisiana Bight. The plume propagated freely across the bight, rather than as a coastal current. The observed cross-front circulation pattern is typical of density currents, as are the small width (≈100 m) of the plume front and the presence of surface frontal convergence. A comparison of observations with stratified density current theory is conducted. Additionally, subcritical to supercritical transitions of frontal propagation speed relative to internal gravity wave (IGW) speed are demonstrated to occur. That is in part due to IGW speed reduction with decrease in seabed depth during the frontal propagation toward the shore. Theoretical steady-state density current propagation speed is in good agreement with the observations in the critical and supercritical regimes but not in the inherently unsteady subcritical regime. The latter may be due to interaction of IGW with the front, an effect previously demonstrated only in laboratory and numerical experiments. In the critical regime, finite-amplitude IGWs form and remain locked to the front. A critical to supercritical transition eventually occurs as the ambient conditions change during frontal propagation, after which IGWs are not supported at the front. The subcritical (critical) to critical (supercritical) transition is related to Froude number ahead (under) the front, consistently with theory. Finally, we find that the front-locked IGW (critical) regime is itself dependent on significant nonlinear speed enhancement of the IGW by their growth to finite amplitude at the front.

scholarly journals Cold smoke: smoke-induced density currents cause unexpected smoke transport near large wildfires

2015 ◽  
Vol 15 (13) ◽  
pp. 17945-17966
Author(s):  
N. P. Lareau ◽  
C. B. Clements
Keyword(s):  
Boundary Layer ◽  
Propagation Speed ◽  
Current Speed ◽  
Solar Heating ◽  
Density Current ◽  
Doppler Lidar ◽  
Density Currents ◽  
Smoke Transport ◽  
Smoke Layer

Abstract. First observations of smoke-induced density currents originating from large wildfires are presented. Using a novel mobile Doppler LiDAR and additional in situ measurements we document a deep (~ 2 km) smoke-filled density current that propagates more than 25 km at speeds up to 4.5 m s−1 near a large forest fire in northern California. Based on these observations we show that the dynamics governing the spread of the smoke layer result from differential solar heating between the smoke-filled and smoke-free portions of the atmospheric boundary layer. A calculation of the theoretical density current speed agrees well with the observed propagation speed. Additional LiDAR and photographic documentation of other smoke-filled density currents demonstrate that these previously unknown phenomena are relatively common near large wildfires and can cause severe and unexpected smoke inundation of populated areas.

scholarly journals Cold Smoke: smoke-induced density currents cause unexpected smoke transport near large wildfires

2015 ◽  
Vol 15 (20) ◽  
pp. 11513-11520 ◽  
Author(s):  
N. P. Lareau ◽  
C. B. Clements
Keyword(s):  
Boundary Layer ◽  
Propagation Speed ◽  
Current Speed ◽  
Solar Heating ◽  
Density Current ◽  
Doppler Lidar ◽  
Density Currents ◽  
Smoke Transport ◽  
Smoke Layer

Abstract. The first observations of smoke-induced density currents originating from large wildfires are presented. Using a novel mobile Doppler lidar and additional in situ measurements, we document a deep (~ 2 km) smoke-filled density current that propagates more than 25 km at speeds up to 4.5 m s−1 near a large forest fire in northern California. Based on these observations we show that the dynamics governing the spread of the smoke layer result from differential solar heating between the smoke-filled and smoke-free portions of the atmospheric boundary layer. A calculation of the theoretical density current speed agrees well with the observed propagation speed. Additional lidar and photographic documentation of other smoke-filled density currents demonstrate that these previously unknown phenomena are relatively common near large wildfires and can cause severe and unexpected smoke inundation of populated areas.

RELATIONSHIPS BETWEEN COMMAND, MANAGEMENT, AND COMMUNICATIONS. TO CONTINUE IN DIFFERENT DIRECTIONS OR TO REMAIN IN SAME FIELD

2018 ◽  
Vol 28 (6) ◽  
pp. 1887-1891
Author(s):  
Todor Kalinov
Keyword(s):  
Data Exchange ◽  
Content Management ◽  
System Structure ◽  
Power Comparison ◽  
Long Distance ◽  
Military Alliances ◽  
Military Systems ◽  
Military Command ◽  
And Control ◽  
The Military

Management and Command253 are two different words and terms, but military structures use them as synonyms. Military commanders’ authorities are almost equal in meaning to civilian managers’ privileges and power. Comparison between military command and the civilian management system structure, organization, and way of work shows almost full identity and overlapping. The highest in scale and size military systems are national ministries of defense and multinational military alliances and coalitions. Military systems at this level combine military command structures with civilian political leadership and support elements. Therefore, they incorporate both military command and civilian management organizations without any complications, because their nature originated from same source and have similar framework and content. Management of organizations requires communication in order to plan, coordinate, lead, control, and conduct all routine or extraordinary activities. Immediate long-distance communications originated from telegraphy, which was firstly applied in 19th century. Later, long-distance communications included telephony, aerial transmitting, satellite, and last but not least internet data exchange. They allowed immediate exchange of letters, voice and images, bringing to new capabilities of the managers. Their sophisticated technical base brought to new area of the military command and civilian management structures. These area covered technical and operational parts of communications, and created engineer sub-field of science, that has become one of the most popular educations, worldwide. Communications were excluded from the military command and moved to separate field, named Computers and Communications. A historic overview and analysis of the command and management structures and requirements shows their relationships, common origin, and mission. They have significant differences: management and control are based on humanities, natural and social sciences, while communications are mainly based on engineering and technology. These differences do not create enough conditions for defragmentation of communications from the management structures. They exist together in symbiosis and management structures need communications in order to exist and multiply their effectiveness and efficiency. Future defragmentation between military command and communications will bring risks of worse coordination, need for more human resources, and worse end states. These risks are extremely negative for nations and should be avoided by wide appliance of the education and science among nowadays and future leaders, managers, and commanders.

Comparison of Inviscid Computations With Theory and Experiment in Vibrating Transonic Compressor Cascades

1990 ◽  
Author(s):  
G. A. Gerolymos ◽  
E. Blin ◽  
H. Quiniou
Keyword(s):  
Boundary Layer ◽  
Plate Theory ◽  
Strong Shock ◽  
Acoustic Resonance ◽  
Boundary Layer Separation ◽  
Computational Method ◽  
Viscous Effects ◽  
Transonic Compressor ◽  
Boundary Layer Interaction ◽  
Interaction Regions

The prediction of unsteady flow in vibrating transonic cascades is essential in assessing the aeroelastic stability of fans and compressors. In the present work an existing computational code, based on the numerical integration of the unsteady Euler equations, in blade-to-blade surface formulation, is validated by comparison with available theoretical and experimental results. Comparison with the flat plate theory of Verdon is, globally, satisfactory. Nevertheless, the computational results do not exhibit any particular behaviour at acoustic resonance. The use of a 1-D nonreflecting boundary condition does not significantly alter the results. Comparison of the computational method with experimental data from started and unstarted supersonic flows, with strong shock waves, reveals that, notwithstanding the globally satisfactory performance of the method, viscous effects are prominent at the shock wave/boundary layer interaction regions, where boundary layer separation introduces a pressure harmonic phase shift, which is not presicted by inviscid methods.

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