These authors used measured values of air velocity in surface boundary layers
related to the airflow velocity field and turbulent energy for parametrization of
initial and boundary conditions for the simulations of sand–dust particles
entrainment.
The numerical simulations assumed particle densities of 1500, 1700, and 2650
kgm
−3 for dust, sand and dust mixtures, and sand, respectively, with a stationary
horizontal velocity of 13 ms
−1 . Particles were injected into the flow randomly over
the entire ground surface, with the injection angle and particle vertical velocity
being considered as Gaussian randomly distributed. The trajectories of particles
were computed over 5000 m with three mean diameters of 5, 20, and 40 lm,
considering suspension as the only transportation mechanism without rebound or
ejection of new particles after returning to the ground surface.
The first simulation of sand–dust entrainment in a purely horizontal flow with
three suspensions with 50 hypothetical particles each regarding dust, sand, and sand
and dust mixtures, showed that under the boundary turbulent velocity field assumed
very few particles could reach a 150 m height. This height was about 100 times
greater than the thickness of the estimated initial sand/dust 1.5 m two-phase
boundary layer of fluid flow. The calculations estimated that about 34, 48, and 88%
remained in the 1.5 m layer. The greater height that dust or mixtures of dust and
sand could reach was about 250 m. For sand, the estimated maximum height
achieved was 180 m.
The second simulation concerned entrainment of particles injected in a descending airflow over the ground. A vertical component was introduced in the wind
velocity profile, and it was shown that in three suspensions with 50 particles each,
about 92, 98, and 72%, of particles of dust, sand, and sand and dust mixtures
remained in the 1.5 m double phase layer. The larger height achieved by dust was
20 m, with only one sand particle reaching 50 m and the remaining lift by no more
than 1 m. For dust and sand mixture, only one particle reached 50 m height and the
remaining reached 30 m at maximum.
The third type of simulation concerned wind gust, as the only element of the
airflow velocity field with a downward velocity of 1 ms
−1 . A major change in the
boundary and initial conditions was imposed, with new variables concerning vertical and horizontal amplitudes of wind gust changes with height above the ground
surface. The results of these simulations with three hypothetical particle suspensions showed that, with the coherent structure of wind gust, particles could indeed
reach the top of the atmospheric boundary layer.
As expected, lower quantities of heavier particles should reach the top height by
comparison with lighter particles. Sand/dust particles with a diameter ranging
between 5 and 40 lm can overcome the descending air motion, penetrate the
middle and upper levels of the atmospheric boundary layer, and then propagate
further and diffuse into the troposphere where ascending air motion prevails. This is
a typical scenario for soil erosion, sand/dust emission, and entrainment, e.g., in
eastern Asia during spring (Cheng et al. 2012).
6.5 Mass Transfer
223
related to the airflow velocity field and turbulent energy for parametrization of
initial and boundary conditions for the simulations of sand–dust particles
entrainment.
The numerical simulations assumed particle densities of 1500, 1700, and 2650
kgm
−3 for dust, sand and dust mixtures, and sand, respectively, with a stationary
horizontal velocity of 13 ms
−1 . Particles were injected into the flow randomly over
the entire ground surface, with the injection angle and particle vertical velocity
being considered as Gaussian randomly distributed. The trajectories of particles
were computed over 5000 m with three mean diameters of 5, 20, and 40 lm,
considering suspension as the only transportation mechanism without rebound or
ejection of new particles after returning to the ground surface.
The first simulation of sand–dust entrainment in a purely horizontal flow with
three suspensions with 50 hypothetical particles each regarding dust, sand, and sand
and dust mixtures, showed that under the boundary turbulent velocity field assumed
very few particles could reach a 150 m height. This height was about 100 times
greater than the thickness of the estimated initial sand/dust 1.5 m two-phase
boundary layer of fluid flow. The calculations estimated that about 34, 48, and 88%
remained in the 1.5 m layer. The greater height that dust or mixtures of dust and
sand could reach was about 250 m. For sand, the estimated maximum height
achieved was 180 m.
The second simulation concerned entrainment of particles injected in a descending airflow over the ground. A vertical component was introduced in the wind
velocity profile, and it was shown that in three suspensions with 50 particles each,
about 92, 98, and 72%, of particles of dust, sand, and sand and dust mixtures
remained in the 1.5 m double phase layer. The larger height achieved by dust was
20 m, with only one sand particle reaching 50 m and the remaining lift by no more
than 1 m. For dust and sand mixture, only one particle reached 50 m height and the
remaining reached 30 m at maximum.
The third type of simulation concerned wind gust, as the only element of the
airflow velocity field with a downward velocity of 1 ms
−1 . A major change in the
boundary and initial conditions was imposed, with new variables concerning vertical and horizontal amplitudes of wind gust changes with height above the ground
surface. The results of these simulations with three hypothetical particle suspensions showed that, with the coherent structure of wind gust, particles could indeed
reach the top of the atmospheric boundary layer.
As expected, lower quantities of heavier particles should reach the top height by
comparison with lighter particles. Sand/dust particles with a diameter ranging
between 5 and 40 lm can overcome the descending air motion, penetrate the
middle and upper levels of the atmospheric boundary layer, and then propagate
further and diffuse into the troposphere where ascending air motion prevails. This is
a typical scenario for soil erosion, sand/dust emission, and entrainment, e.g., in
eastern Asia during spring (Cheng et al. 2012).
6.5 Mass Transfer
223