Response of the "Boundary Layer Wind Tunnel 1" to a 10% variation of the fan rotational speed

The aim of our research is the possibility to obtain a pulsating flow or a controlled unsteady flow in the experimental zone of the boundary layer wind tunnel as a result of varying the fan rotational speed. Fan rotational speed is controlled by a frequency converter. In this paper, a series of velocity measurements for the same frequency step (magnitude equal to 10% of the maximal frequency) with different initial frequencies and for different roughness values on the floor of the tunnel are presented. These measurements are a first step in characterizing the response time of the whole system to such a step function imposed on the frequency.