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[ACG42-P04] Structural characteristics of airflow and blown sand flux over fine-grained gravel surfaces in wind tunnel
Keywords:wind erosion, vertical profiles, blown sand flux, piezoelectric blown sand meter, wind tunnel developing boundary layer
To reduce damage caused by wind-blown sand, it is necessary to clarify the mechanism of soil erosion under various surface conditions. In the present study, we investigated the influence of gravel coverage on aerodynamic characteristics of airflow and blown sand flux in a compact wind tunnel developing boundary layer equipped with a turbulence generator and a piezoelectric blown sand meter.
The vertical profile of wind-blown sand over a flat sand surface showed an exponential distribution at all wind speeds, whereas the profile over fine-grained gravel surfaces of 20% or greater coverage showed a non-monotonic vertical distribution. At 20% to 30% gravel coverages, a peak of wind-blown sand flux developed between 6 and 10 cm above the ground at all wind speeds because of less energy loss due to grain-bed collisions at that level. We found that the wind-blown sand flux at 4 cm height accounted for about 20% of the total flux regardless of wind speed and gravel coverage. This finding can simplify future estimations of total near-surface wind-blown sand flux based on field observations because such measurements can be taken at just one height.
The effect of fine-grained gravel coverage on drag coefficient, wind-speed gradient, and turbulent transfer coefficient was investigated to understand the structure characteristic of blown sand flux. The drag coefficient of the gravel surface reached the maximum value at 15% coverage and then tended to stabilize at gravel coverage 20% and greater. At a height of 4 cm, near-surface airflow on gravel surfaces can be divided clearly into upper and lower sublayers, defined as the inertial and roughness sublayers, respectively. The coefficient of variation of wind speed over gravel surfaces in the roughness sublayer was 8.6 times as much as that in the inertial sublayer, indicating a greater effect of gravel coverage on wind-speed fluctuations in the lower layer. At the height of 4 cm, changes in gravel coverage had the same degree of effect on wind-speed fluctuations under the observed wind speeds. In addition, an energy-exchange region, where sand particles can absorb more energy from the surrounding airflow, was found between the roughness and inertial sublayers (6-10 cm), enhancing the erosional state of wind-blown sand. This finding can be applied to evaluate the aerodynamic stability of the gravel surface and provide a theoretical basis for elucidation of the vertical distributions of wind-blown sand flux.
The vertical profile of wind-blown sand over a flat sand surface showed an exponential distribution at all wind speeds, whereas the profile over fine-grained gravel surfaces of 20% or greater coverage showed a non-monotonic vertical distribution. At 20% to 30% gravel coverages, a peak of wind-blown sand flux developed between 6 and 10 cm above the ground at all wind speeds because of less energy loss due to grain-bed collisions at that level. We found that the wind-blown sand flux at 4 cm height accounted for about 20% of the total flux regardless of wind speed and gravel coverage. This finding can simplify future estimations of total near-surface wind-blown sand flux based on field observations because such measurements can be taken at just one height.
The effect of fine-grained gravel coverage on drag coefficient, wind-speed gradient, and turbulent transfer coefficient was investigated to understand the structure characteristic of blown sand flux. The drag coefficient of the gravel surface reached the maximum value at 15% coverage and then tended to stabilize at gravel coverage 20% and greater. At a height of 4 cm, near-surface airflow on gravel surfaces can be divided clearly into upper and lower sublayers, defined as the inertial and roughness sublayers, respectively. The coefficient of variation of wind speed over gravel surfaces in the roughness sublayer was 8.6 times as much as that in the inertial sublayer, indicating a greater effect of gravel coverage on wind-speed fluctuations in the lower layer. At the height of 4 cm, changes in gravel coverage had the same degree of effect on wind-speed fluctuations under the observed wind speeds. In addition, an energy-exchange region, where sand particles can absorb more energy from the surrounding airflow, was found between the roughness and inertial sublayers (6-10 cm), enhancing the erosional state of wind-blown sand. This finding can be applied to evaluate the aerodynamic stability of the gravel surface and provide a theoretical basis for elucidation of the vertical distributions of wind-blown sand flux.