# Enhancing Flow Field Performance of a Small Circulating Water Channel Based on Porous Grid Plate
## 2 Structure and Working Principle  
The small circulating water channel designed in this work adopts the type of a vertical form, and the power section is under the water channel. The hydrodynamic characteristics of the underwater device in the axial direction (\(x\) direction) and lateral direction (\(y\) direction) can be tested by a flow rate meter or by using PIV testing technology. Figure 1 shows the structure and fundamental dimensions of the circulating water channel. The power section mainly relies on the water pump to pump the fluid with a rated flow. The suction section transmits the pumped fluid to the rectification section. In the rectification section, the honeycomb and screen are used to suppress the axial and lateral turbulence. The contraction section uniformly accelerates the fluid from the rectification section to the test section, depending on the specific boundary curve. The flow characteristics of the test section are stable and uniform, which are suitable for the simulation of the actual underwater working environment of the tested device. The effluent section leads the fluid from the test section to the power section to complete the whole circulation process. The wall of the test section is made of acrylic plate with high transmittance, so the fluid field characteristics of the \(x\)-\(y\) section and \(x\)-\(z\) section can be measured by PIV technology. The PIV system mainly includes a tracer particle, laser generator, and an image acquisition and image-processing method. The tracer particles are uniformly distributed in the measured fluid domain, the laser is emitted at the observed position, and particle images at different times are collected by a CCD (charge coupled device) camera and processed by the image-processing method. The characteristics of the measured fluid domain can be obtained after the image processing. The particle motion data is mainly obtained by the CCD camera; the size of the tracer particle should be larger than the single pixel of the camera. The effective measurement range depends on the accuracy and location of the CCD camera. It can be seen that a better test performance can be obtained by using a camera with high resolution, which also increases the cost as well. Besides, as the main raw test data of the PIV system is the particle position, when the PIV system is applied to the mixture fluid with a high concentration of the dispersed phase, the test performance has substantial limitations. More details about establishing the PIV visualization technology can be found in the works of [PERSON] et al. [23; 24]. The boundary of the contraction section, which is located between the rectification section and the test section, is a smooth curve. The cross-sectional area of the contraction section decreases along the flow direction according to certain rules, so as to increase the flow speed uniformly. In this work, the fifth power curve is selected as the contraction curve, as shown in Figure 2. When using this curve, fluid separation is not easy to be produced at the entrance, and the energy loss is less than selecting other curves [10]. The curve equation can be described as:  
\[\frac{R-R_{2}}{R_{1}-R_{2}}=1-10(\frac{x}{L})^{3}+15(\frac{x}{L})^{4}-6(\frac{ x}{L})^{5} \tag{1}\]  
Figure 1: Structure diagram of the designed circulating water channel. where \(R_{1}\) is the radius at the inlet of the contraction section, \(R_{2}\) is the radius at the outlet, \(L\) is the length of the contraction section, and \(R\) is the radius at a location where the distance from the inlet is \(x\). Considering the overall occupancy space and cost of the circulating water channel, the length of the contraction section should not be too long [25]. The contraction ratio of the contraction section is described as \(C=(R_{1}/R_{2})^{2}\), and the turbulence intensity is inversely proportional to the contraction ratio. In order to reduce the turbulence intensity, the contraction ratio needs to be larger [26; 27].