Unsteady wind force on an elliptic cylinder subjected to a short-rise-time gust from steady flow

The properties of unsteady wind force on an elliptic cylinder under a rapid change from steady flow were investigated using a gust wind tunnel. Short-rise-time gusts from steady flow were generated by the control of blade rows which were installed for generating a pulsation flow. We measured the unsteady drag and lateral forces on an elliptic cylinder under such gusts with a target wind velocity of between 2.0 m/s and 7.0 m/s, with a ratio of initial wind velocity to target wind velocity of between 0.0 and 0.9 and a rise time of between 0.1 s and 1.4 s. As a result, we confirmed that the overshoot phenomenon of wind force occurred under a short-rise-time gust from steady flow as well as from calm. And it was found that the overshoot coefficient, defined by the ratio of maximum to a steady-state value of wind force, decreased with an increase in the ratio of the initial wind velocity to the target wind velocity. For each ratio of the initial wind velocity to the target wind velocity, the overshoot coefficient was determined by an expanded non-dimensional rise time composed of rise time, body size, and initial and target wind velocities.