Unsteady flow generation in a wind tunnel using an active grid

Unsteady flows are pervasive in nature and engineering applications, such as unmanned aerial vehicles and wind turbines. Simulating these flows in an experimental setting is important to enable a better understanding of the effect of unsteady flow conditions in many applications and prevent detrimental changes in performance or dangerous conditions. This study explores the capabilities of an active grid to produce unsteady flows in a recirculating wind tunnel. An important advantage of using an active grid for this purpose is that relatively high levels of turbulence intensity can be superimposed on the resulting unsteady flow field if desired without the need for additional turbulence production devices. The active grid was tested with different excitation modes that allow control over the unsteady flow frequency, amplitude, waveform and turbulence intensity. Hot-wire measurements demonstrate that the grid can generate flows typical of those used in previous experiments of unsteady aerodynamics. Mean grid blockage, freestream speed and the tunnel size are all shown to be the critical factors influencing the resulting flow. An analytical model of the wind tunnel and active grid system is presented that can predict the resulting unsteady flow characteristics. The model shows good agreement with experimental results. Finally, it is shown that the active grid produces turbulence with an intensity independent of the Reynolds number, in a similar manner to a passive grid, when operated to generate unsteady flow conditions at frequencies lower than the characteristic frequency of the turbulent large scales of the flow. When the unsteady flow frequency is comparable to that of the large scales, the turbulence intensity increases and shows a dependence on the flow Reynolds number.