*2.1. Wind Channel*

The open-circuit wind channel used in the experiment is shown in Figure 1. The length, width, and height 2*h\** of the working section are 4.5, 1.0, and 0.01 m, respectively. The flow is driven by three centrifugal fans with 1.5 kW induction motors, and the midplane velocity in the working section is controlled by a frequency converter to vary between 0.4 and 28 m·s<sup>−</sup>1. In order to isolate the vibration noise generated by the centrifugal fans, a soft connection is attached just in front of the expansion section. A perforated screen and 5 stainless-steel screens are mounted near the honeycomb layer to

stabilize the flow and decrease the turbulence intensity. Two contractions with the contraction ratios of 4:1 and 9:1 are used to further reduce the turbulence intensity to a level less than 0.2%.

**Figure 1.** Sketch of the wind channel. (**a**) The components of the wind channel: (1) centrifugal fan, (2) soft connection, (3) fine damping screen, (4) expansion section, (5) perforated screen, (6) honeycomb, (7) screen, (8) first contraction, (9) second contraction, (10) first working section (steel), and (11) second working section (tempered glass). Unit of length, mm; (**b**) 3D drawing of the wind channel.

The channel walls of the first working section are polished to achieve a surface roughness less than 15 μm and are supported by steel frames, to avoid deflection. According to the finite element analyses, the maximum deflection of the whole test section is less than 3.7 μm. The second part is a transparent test section with a length of 0.5 m, granting optical access to the Particle Image Velocimetry (PIV) setup. Two 10-mm-thick side walls are sandwiched between the top and the bottom walls, and the error of channel height in the working section is less than 0.01 mm. In all experiments, the ambient temperature variation is less than 2 degrees centigrade. For non-dimensionalization, the half channel height *h\** and the time averaged velocity at the midplane *U*∗ *<sup>c</sup>* are chosen as the characteristic length and velocity, respectively, and the dimensionless parameters have no superscript. For laminar flows, *Uc* = 1.5*Ub*. The origin of the coordinates lies at the entrance center of the working section, and the dimensionless *x*, *y*, and *z* represent the streamwise, the wall-normal, and the spanwise directions, respectively.
