Programmable Synthesis of Unsteady Turbulent and Shear Flows Using a Modular Fan-Array Wind Wall

Programmable fan-array wind tunnels have emerged as a powerful approach for generating controlled flow environments; however, the ability to systematically prescribe turbulence characteristics relevant to urban boundary-layer (UBL) flows remains limited. In this work, a modular fan-array wind tunnel, hereafter referred to as the wind wall, is developed as a research platform for controlled synthesis of unsteady flow fields. The wind wall spans a cross-section of 1.44 m x 1.44 m driven by 288 independently addressable fans, with a modular architecture that allows expansion and introduction of orthogonal walls to allow three-dimensional flow generation. A forcing framework is introduced in which a prescribed mean duty cycle signal is superimposed with a stochastic fluctuation component, enabling independent control of mean velocity and turbulence intensity. Using this approach, uniform and shear flow profiles are generated for mean velocities up to 20 m/s, corresponding to Reynolds numbers up to 1.5e6, with turbulence intensity exceeding 42% in uniform flow. The temporal response of the system is characterized using square- and triangle-wave inputs, demonstrating repeatable unsteady flow generation governed by fan dynamics. Results demonstrate that the wind wall enables systematic control of mean flow shear, turbulence intensity, and unsteadiness over a broad range exceeding conventional active-grid approaches, providing a pathway toward controlled reconstruction of UBL-relevant flows for studies in urban aerodynamics, environmental flows, and data-driven modeling.