## 2 Material and Methods
### Particle Image Velocimetry (PIV) Measurements  
The flow velocity was measured using a two-dimensional PIV system (TSI, USA), which is a whole-flow-field and non-intrusive laser optical measurement technique for research and diagnostics into flow. It provides instantaneous velocity vector measurements in a cross-section of the flow. The PIV system mainly includes tracer particles, a laser generator, image acquisition, synchronizer and image-processing software, see Figure 2 for detail. PIV systems measure velocity by determining particle displacement over a precisely selected time using a double-pulsed laser technique. A laser light sheet illuminates a plane in the flow, and the positions of particles (naturally present or added to the flow to have a sufficient number of tracers) in that plane are recorded using a digital camera. A short time (micro or milliseconds) later, a second pulse illuminates the same plane, creating a second set of particle images. From these sets of images, PIV analysis algorithms obtain the particle displacements for the imaged region to give the velocity information at thousands of locations--quickly, easily, reliably. Flow properties such as the vorticity and strain rates are obtained. Mean velocity, turbulence intensity, and higher order statistics are also obtained.  
In recent years, given the marked improved accuracy and visualization of PIV measurements, PIV has become a popular tool for examining complex and chaotic structure-induced flow patterns. [PERSON] et al. [57] studied a swirling flow combined with a vibrating wall in conical pipes by 2D PIV. [PERSON] et al. [58] used PIV, observing the turbulence performance of combining the honeycomb with the screens in a small open circulating flume. Under two filling rates and three baffle positions, [PERSON] and [PERSON] [59] applied the PIV technique to monitor the cone-accumulating flow for a whirlpool tank in rotational motion. Based on previous work, the improvement of flow conditions in a whirlpool with a  
Figure 3: Different groin configuration scenarios. (**a**) Top view; (**b**) side view.  
modified bottom is further proposed by [PERSON] et al. [60] using PIV measurements. In the research mentioned above, PIV is utilized to study the complicated flow structures by rigid structures with a range of hydraulic properties. As shown with these few examples, PIV is a great and universal tool with which to investigate turbulent flow.  
The choice material and size of tracer particles are key components of PIV measurements. In this experiment, polyvinyl chloride (PVC-6500) micro-powders with a diameter of 50 \(\upmu\)m, which have excellent flow-tracking and stable chemical properties, were used as the tracer particle. The high-resolution 2048 \(\times\) 2048-pixel CCD (Charge Coupled Device) camera was used to continuously capture 300 instantaneous particle images at the frequency of 4.83 Hz, which is the optimum value of the synchronization between camera and laser by a digital delay pulse generator. The test section was illuminated by a double pulsed Nd: YAG laser with a pulse intensity of 25 mJ focused on a sheet of 2-m width. The model of the laser pulse synchronizer is 610036. The flow field measurement was performed with a horizontal region situated at 0.05 m above the flume bottom. The time-averaged flow fields over a 0.05-s interval were processed using the Insight 4G software PIV system. The PIV images were taken over an area of 380 mm (length) \(\times\) 370 mm (width), which resulted in 3969 vectors (63 \(\times\) 63) over the measured area. The measurement uncertainty was less than 0.005 pixels based on the particle diameter and particle density.