## 2 Experimental Setup  
The experiment was carried out in a medium-sized, low-speed circulating water channel at the Fluid Mechanics Laboratory of Hebei University of Technology. The channel, constructed with polymethyl methacrylate (PMMA) side walls and bottom, consists of a straightening section, a contraction section, a test section, a power section, and a return section. Flow uniformity is ensured by a honeycomb and a damping net in the straightening section, while the contraction section minimizes flow separation. The test section measures 600 mm in height, 500 mm in width, and 2500 mm in length. A PMMA flat plate (2200 mm \(\times\) 500 mm \(\times\) 15 mm) with an 8:1 elliptical leading edge is mounted inside, and a 5 mm diameter trip wire positioned 100 mm downstream from the leading edge promotes a fully developed turbulent boundary layer. A 300 mm \(\times\) 300 mm recess located 1400 mm from the plate's leading edge allows for interchangeable test surfaces with different wall morphologies. Water flow is driven by an axial-flow pump powered by a three-phase AC motor, with speed controlled via a frequency converter. A schematic of the experimental setup is shown in Figure 1.  
The riblet walls are arranged in a convergent and divergent pattern, with a sawtooth shape. The actual width \(a\) of the riblet walls is 240 mm, the top angle \(\alpha\) of the ribs is 60\({}^{\circ}\), the spacing \(s\) between the ridges is set to 1.2 mm, and the depth \(h\) of the ribs is 1.039 mm. The riblet plates were fabricated from acrylic sheets using CNC machining to ensure precision. To investigate the effect of different yaw angles on the riblet turbulent boundary layer, the yaw angle \(\beta\) was set to 0\({}^{\circ}\), \(\pm\)30\({}^{\circ}\). Figure 2 is a schematic diagram of the riblet wall plate. Figure 1: Schematic diagram of experimental apparatus: (**a**) medium-sized low-speed circulating water tank physical diagram; (**b**) physical image of the experimental smooth plate; (**c**) medium-sized low-speed circulating water tank physical schematic diagram. To ensure the existence of a single variable and to maintain the dimensionless size of the riblets within the optimal drag reduction range [23], adjustments were made to the motor during the experiment to achieve an incoming free stream velocity of \(U_{\infty}\) = 0.21 m/s. The experimental environment was kept at 20 \({}^{\circ}\)C, with the density of the experimental water being 996.7 kg/m\({}^{3}\) and the kinematic viscosity of water equaling 1.006 mm\({}^{2}\)/s. The experimental setup and layout are shown in Figure 3. In the experiment, the flat plate was placed vertically, and by adjusting the trailing edge plate, the static pressure gradient along the flow direction on the plate surface was adjusted to be close to zero. Photographs were taken of the \(x\)-\(y\) and \(x\)-\(z\) planes under each wall surface. The working surface was 260 mm away from the side wall of the test section, and after measurement, the influence of the water channel side wall boundary layer on the flow in the measurement plane could be neglected. The \(x\)-\(y\) shooting plane is located at the center of the symmetrical plane in the model, that is, directly above the convergence and divergence junction of the convergent and divergent riblet wall surfaces. The \(x\)-\(z\) shooting plane is parallel to the smooth flat plate. The \(x\)/\(y\)/\(z\) directions in the figure represent the streamwise direction, normal direction, and spanwise direction, respectively, with the corresponding velocities represented by \(u\), \(v\), and \(w\). For the \(x\)-\(y\) shooting plane, the origin of the smooth flat plate's streamwise coordinate is located 150 mm from the front edge of the recess, and the normal coordinate is at the smooth wall surface. The streamwise coordinate position of the riblet wall surface origin remains unchanged, and the normal coordinate is the coordinate after the zero point is offset.