Determining fluid drag through ground vegetation for a forest-fire simulation model

Fluid drag data for flows through ground vegetation is needed by those studying the atmospheric boundary layer near the earth in wind turbine design, by those performing drainage calculations for fields awash from a great river, by CFD modelers of the lowest levels of a forest fire, and by agriculturalists investigating the lodging (wind overturning) of crops. All of these researchers lament that there is little plant-drag data available. This work presents an on-going, initial study into a plant-drag model for a forest-fire simulation CFD program; but hopefully will be useful to the other professionals mentioned.

The model develops a friction factor as a function of Reynolds number for various percentage coverage of plants. Percentage coverage is a variable used by ecologists and is easily determined from aerial surveys.

Artificial plant arrangements are mounted in a clear acrylic frame, backlit from underneath, and, using a mirror, are photographed from above from sixty feet away. From the photographic data, fractal dimension and lacunarity can be determined as a function of percentage coverage. Then the arrangement is placed in a wind tunnel and drag forces on the plant-frame system measured for upstream velocities ranging up to thirty-five miles an hour. Plots of friction factor as a function of Reynolds number are generated and then collapsed into single curves using the photographically generated variables.

Vorticity and flow through the plant canopy causing plant oscillation are not considered.