Automated large-scale and terrain-induced turbulence modulation of atmospheric surface layer flows in a large wind tunnel

This study leverages a multi-fan flow control instrument and a mechanized roughness element grid to simulate large- and small-scale turbulent features of atmospheric flows in a large boundary layer wind tunnel (BLWT). The flow control instrument termed the flow field modulator (FFM), is a computer-controlled 3 m × 6 m (2D) fan array located at the University of Florida (UF) Natural Hazard Engineering Research Infrastructure (NHERI) Experimental Facility. The system comprises 319 modular hexagonal aluminum cells, each equipped with shrouded three-blade corotating propellers. The FFM enables the active generation of large-scale turbulent structures by replicating user-specified velocity time signals to inject low-frequency fluctuations into BLWT flows. In the present work, the FFM operated in conjunction with a mechanized roughness element grid, called the Terraformer, located downstream of the FFM array. The Terraformer aided in the production of near-wall turbulent mixing through precise adjustment of roughness element heights. A series of BLWT velocity profile measurements were carried out at the UF BLWT test section for a range of turbulence intensity and integral length scale regimes. Input commands to the FFM and Terraformer were iteratively updated via a governing convergence algorithm (GCA) to achieve user-specified mean and turbulent flow statistics. Results demonstrate the capabilities of the FFM for significantly increasing the longitudinal integral length scales compared to conventional BLWT approaches (i.e., no active large-scale turbulence generation). The study also highlights the efficacy of the GCA scheme for attaining prescribed target mean and turbulent flow conditions at the measurement location. Graphical abstract