How to stir turbulence
In strong turbulence there is a large separation between the large-scale vortices where we stir, and the smallest ones that are just powerful enough to survive the smoothing action of viscosity. In the end, the smallest vortices, turbulence becomes homogeneous and isotropic. However, our windtunnel may not be large enough to see this happen. The question is if there are smart ways to stir turbulence, such that we can tune the "flavour" of the turbulent flow.
Ideal (homogeneous and isotropic) turbulence in a windtunnel can be made by passing the flow though a grid, but then the turbulence is not very strong. In our experiments we use an active grid which creates much stronger turbulence: it effectively increases the size of the windtunnel by a factor four.
An active grid is a grid of axies with attached vanes that can be rotated by servo motors. Our grid is unique in that we can prescribe the instantaneous angle of each axis. Thus we can stir turbulence perfectly periodically, or we can stir randomly by prescribing axies angles that change randomly in time.
There is a very large number of ways to move the grid for stirring turbulence. The simplest manner is to do it periodically, by cycling through a fixed number of patterns. It appears that the tapestry below produces the strongest and most homogeneous turbulence at a very particular frequency. No surprise is that this frequency is the internal frequency of turbulence: the turnover rate of its largest vortices.