SIMULATION OF DUST DEVILS

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SIMULATION OF DUST DEVILS

• Zhaolin GU, PhD, Professor
• Xian Jiaotong University
• October , 2006, CCFD Forum , Tokyo University

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Background

• Atmospheric dust has important impacts on global
  and regional climates.
• Some specific convective wind systems in the
  convective boundary layer (CBL), such as, dust
  storms, dust devils etc., can carry dust into the
  atmosphere.

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Dust transportation in Northwestern arid area of
China
Dust storm Meso-scale meteorological process
Dust devil Micro-scale meteorological process
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About dust storms Some pictures
The vision of Lanzhou Train Station at 1600,
April 10, 2004
The vision of Dunhuang Grottoes at 1600, April
10, 2004
The vision of Xian City Wall, April 9, 2006
The destroyed window by the dust storm, April 9,
2006
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About dust storms Meteorological aspects

• A Mesoscale meteorological process, composed of
  strong convection cells
• Height of dust front 300-400 m
• Length of dust front around 100 km
• second dust front 10-20 km
• Horizontal velocity more than 20
  ms-1
• Vertical velocity more than
  15 ms-1
• Temperature increment (DT ) 48 K
• Pressure depression (Dp) 2.03.5 hPa
• ?T and ?p are departures from ambient values of
  temperature and pressure.

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About dust devils Meteorological aspects

• A special case of convective vortex occurring in
  the atmosphere boundary layer.
• The most common, small-scale dust transmitting
  system.
• Maybe the primary atmospheric dust-loading
  mechanism in non-storm seasons
• Mixing grains in dust devils becoming
  tribo-electrified.

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Typical Observed Dust Devil Physical
Characteristics

• Diameter Tens to 141 m
• Height 300660 m
• Horizontal velocity 5-20 ms-1
• Vertical velocity 315 ms-1
• Rotation sense Random
• Temperature increment (?T ) 28 K
• Pressure depression (?p) 2.54.5 hPa
• ?T and ?p are departures from ambient values of
  temperature and pressure.

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Inducement of dust devilsBackground vorticity
The Benards Convection in atmospheric boundary
layer
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Inducement of dust devilsBackground vorticity
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Inducement of dust devils Surface roughness
At h5.2m and h9.4m (V7 and V31 in the figure),
the value of tangential velocities at r50m is
different, where is far from the dust devil
center and maybe the boundary of the dust devil.
(P. C. Sinclair, 1973)
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Inducement of dust devils Buoyancy

• Heat radiation from the sun causing the rise of
  surface temperature
• The temperature difference between the ground
  surface and the near-surface air parcels is over
  20-30K at sunny mid-day in deserts (Li J. F. ,
  Desert Climate, Meteorological Press, Beijing,
  2002)
• The temperature difference is related to the heat
  flux on the surface.

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Tool and methods for dust devil study

• Field observation and test
• Laboratory experiment, e.g. dry ice simulator in
  Arizona University
• Numerical simulation
• gas-solid two-phase flow

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Numerical simulation

• C. B.Leovy, Nature, 424 ,2003
• Promise to be an important tool for interpreting
  laboratory and field observations of dust devils
  
• (C. B.Leovy, Nature, 424 ,2003)
• Getting insights into the dynamics of
  boundary-layer vortices
• Two scale methods convective boundary layer
  (CBL) scale simulation, and dust devil-scale
  simulation

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Dust devil-scale simulation method LES-
Lagrangian discrete phase model (LDPM) model

• LES for the turbulent flow
• LDPM for the grain movementone way coupling
• Lifting of dust not actually appearing to be of
  major dynamical importance for the development of
  these vertical vortices ( P. C. Sinclair, J.
  Appl. Meteorol. 8,1969)

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LES Equations
Continuity equation
Momentum equation
Energy equation
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Dynamic subgrid scheme
Least-squares approach, Lilly (1992)
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Dust devil scale LES modelboundary conditions
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Involution of dust devils

• a) Weak vortex phase b)
  Single cell phase
• c) Transition phase of single cell to double
  cell d) Double cell phase .

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Fine flow structure in the weak vortex phase
The updraft vectors and contours of updraft
velocity
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Fine flow structure in the single cell phase
The updraft vectors and contours of updraft
velocity
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Fine structure in transition phase of single cell
to double cell
The updraft vectors and contours of updraft
velocity
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Fine flow structure in the double cell phase
The updraft vectors and contours of updraft
velocity
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Daughter vortices in the double cell phase
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Grain tracks in the mature phase flow
100mm
200mm
300mm
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Dust lifting patterns in a dust devil
1-track of fine dust grains 2-track of medium
grains 3-track of large grains 4-small vortices
induced by the interaction of different
sized grains 5-general pattern of
interactions.
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An illustration of the electric dust
devilFarrell et al. J. Geophys. Res., 109,2004
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Observed near-surface patterns of terrestrial
dust devils
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Modeled dust devils and typical parameters
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Simulated near-surface patterns of dust devils

• Near-surface shapes of dust devils simplified by
  the periphery of the rotating velocity contour of
  their cores.

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Some problems in the numerical simulation

• Grain size distribution
• The influence of electrostatic field on the
  movement the positive-charged or negative-charged
  particles
• The collision of particles resulting in charge
  neutralization and/or production, and then the
  particle movement

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Particle population balance modelan example
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Particle population balance model

• A method connecting the microcosmic behaviors
  with the macroscopic token of dispersed phase
• Description for different microcosmic behaviors
  in various process, such as crystal, growth,
  dissolution, breakage, aggregation, erosion,
  sinter and so on
• Dealing with some process formation or
  transformation of rain, snow and hail, aerosol,
  sand storm, dust devils

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Particle population balance equation (PPBE)

• n (L, us, t, x) particle number density
  function
• us particle velocity
• L particle properties
• X dimension
• T time
• S(L, us, x, t ) source tem, relating to the
  dispersed phase behaviors
• F forces acting on the dispersed
  phase.

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Particle population balance equation (PPBE)
Source term with no reaction
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Solution of the PPBE

• Classification method (CM)(N1)-fluid model, one
  fluid corresponding to the gas phase and N fluids
  to the different size dispersed phase
• Quadrature method of moment (QMOM) (McGraw,
  Aerosol. Sci. Technol., 27, 1997)
• Direct Quadrature method of moment (DQMOM)
  (Marchisio et al., Chem. Eng. Sci., 58, 2003)
• Monte Carlo Methods

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Prospect of CFD coupling with PPBE-1

• Extending the physical understanding of the
  dispersed phase behaviors and process
• QMOM and DQMOM, based on the fundamental
  statistical concepts on the microscopic level,
  are the promise to solve the PPBE
• Reformulation of the coalescence and breakage,
  consistent with QMOM and DQMOM
• Improving the formulation of the turbulence
  effects, interfacial transfer fluxes
• Improving the formation of the boundary
  conditions (inlet and outlet conditions, wall
  boundary)

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Prospect of dust simulation

• The evaluation of dust flux at a level of dust
  devil on different surface
• The possibility evaluation of occurring of dust
  devils in the region
• The evaluation of regional dust flux from dust
  devils

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Thanks for your attention!