An initial study to determine a friction-factor model for ground vegetation

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 wind overturning of crops. All of these researchers lament that there is little plant-drag data available. This work presents an initial study into a plant friction-factor 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 cover of plants. Artificial plant arrangements were mounted in a clear acrylic frame, backlit from underneath, and, using a mirror, are photographed from sixty feet away. From the photographic data, fractal dimension and an average gap diameter can be determined as a function of percentage coverage. Then the arrangement is placed in a wind tunnel and drag forces on the system measured for upstream velocities ranging up to thirty-eight miles an hour. Most arrangements had axes of greatest and least flow resistance. Averaging the friction-factor values computed for these extremes a friction-factor model is developed for three different artificial plants. These individual models are, in turn, combined into a single model using the fractal dimension data for the plants. Vorticity and flow through the plant canopy causing plant oscillation are not considered.