Research has shown when a rectangular cylinder is located near a flat plate, the flat plate skin friction coefficient changes as a function of the rectangular aspect ratio, gap height between the rectangular and flat plate, distance of rectangular from the flat plate leading edge, and speed of free stream. However, there is no comprehensive experimental study on the comparison of the results of the flat plate skin friction coefficient for all the interactions between effective variables in the presence and absence of the obstacle. On the other hand, testing all possible combinations of effective variables will not be reasonable. In this paper, maximum and minimum ratios of the flat plate skin friction coefficients with and without the rectangular cylinder were determined using robust Taguchi design. Design of experiments method was applied for decreasing the number of experiments without losing the required information in the first step. Then, experimentation was done in a wind tunnel, the maximum speed of which was 13 m/s. Finally, the flat plate skin friction coefficient was optimized using Taguchi method and Minitab software. Results showed that presence of the rectangular cylinder near the flat plate decreased the average skin friction coefficient of the flat plate for all the possible combinations of the effective variables. Additionally, maximum value of the flat plate skin friction reduction was about 40%.
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