Advancements in the physical simulation of near-surface extreme wind phenomena using hybrid active-passive flow control in a large boundary layer wind tunnel

Abstract

This paper presents advancements in the reduced-scale physical simulation of near-surface synoptic and non-synoptic extreme wind phenomena in a long-fetch boundary layer wind tunnel (BLWT). This research was carried out at the University of Florida's BLWT, which is one of the National Science Foundation's (NSF) Natural Hazards Engineering Research Infrastructure (NHERI) Experimental Facilities (EF). Flow fields in the UF BLWT are created by three computer-controlled flow-conditioning stages: vaneaxial fans (VAF), automated roughness elements collectively called the Terraformer (TF), and a multi-fan array called the Flow Field Modulator (FFM). The VAF and TF together can generate traditional BLWT log law atmospheric surface layer (ASL) wind velocity profiles. The FFM provides additional mean flow shaping, active turbulence modulation, and the ability to create nonstationary flow fields toward simulating non-synoptic events. A novel governing convergence algorithm (GCA) was developed to provide seamless closed-loop control of the three flow-conditioning stages toward achieving target profiles in the downwind test section. Three case studies covering a broad range of target flow conditions are demonstrated. The first case is a stationary non-monotonic mean target profile, illustrating the mean flow shaping capabilities of the GCA. The second case is a nonstationary non-monotonic transitioning flow, illustrating how the FFM can be used to transition between two GCA-converged targets. The third case is a stationary ASL flow profile where the GCA is used to converge longitudinal mean, turbulence intensity, and integral length scale profiles simultaneously. The results demonstrate that a multi-stage flow control approach with closed-loop convergence can achieve user-specified complex and transient flow conditions. The resultant flow capabilities expand the traditional limits of flow simulation, bringing the breadth of behaviors in real wind events and our controlled study of them closer together.