Resistance to Flow over Two-Dimensional Strip Roughness

Wind tunnel and open channel experimental data on the resistance of two-dimensional strip roughnesses kept on a plane boundary have been analyzed. The analysis indicates that, at low Froude number, in the range of relative spacing from 2.5 to 40.0, the total (effective) resistance of the plane boundary can be taken as equal to the form resistance of the roughness elements. The resistance relation was put in the form of an empirical equation. Comparison of this equation with the conventional resistance equations for artificial roughnesses shows that the datum may be located at the level of the plane boundary for relative spacing less than 5.0 and should be located higher at smaller spacings, if the conventional equation is to be used. The study also reveals that large errors can result if the method proposed by Morris is used to predict the resistance in case of flow over strip roughnesses.