# Enhancing Flow Field Performance of a Small Circulating Water Channel Based on Porous Grid Plate
## 1 Introduction  
A circulating water channel is one of the important equipments used in hydrodynamic studies [1; 2]. The power system, such as the pump, is usually used to drive the water circulation in the water channel, and the tested device is generally positioned in the test section of the channel. Compared with other underwater test systems, the circulating water channel has the advantages of low cost, high efficiency, and quick response. Combined with PIV (particle image velocity) technology, it can effectively observe and record the flow characteristic around the tested equipment [3]. Therefore, a circulating water channel is widely used as an efficient hydrodynamic test system. A circulating water channel can be divided into a horizontal type and vertical type, according to the structural forms. Compared to a horizontal water channel, a vertical water channel can obtain higher efficiency and better flow quality. The actual working environment of the tested device can be simulated by controlling the flow characteristics in the test section. A circulating water channel system usually includes the power section, suction section, rectification section (including the honeycomb and screens), contraction section, test section, and effluent section. The function of the contraction section is to uniformly accelerate the fluid from the rectification section, to ensure that the fluid characteristics in the test section meet the requirements of stability and uniformity [4; 5; 6]. The structure of the contraction section is mainly determined by the contraction curve. In order to achieve the required flow characteristics of the test section, researchers have done many studies on the optimization of the contraction curve. The commonly used curves are the Witzozinsky curve, Batchelor-Shaw curve, cubic curve, the fifth power curve, etc. [7; 8; 9]. [PERSON] et al. analyzed the influences of several typical contraction curves on the field uniformity in the high-speed water channel [10]. The key components of the rectification section are the porous honeycomb and screen, whose structure size and location parameters have great influences on the flow characteristics of the test section [11]. In normal conditions, these parameters are selected based on experience [12; 13]. The honeycomb is mainly used to suppress the lateral turbulence, and the screen is mainly used to suppress the axial turbulence. In order to obtain the required turbulence characteristics of the test section, parameter optimization studies of the honeycomb and screen should be carried out. [PERSON] and [PERSON] [12] thought that a large length-diameter ratio of the honeycomb could effectively reduce the turbulence in the downstream fluid field, and the type selecting workflow of the honeycomb was given. [PERSON] et al. [14] gave the design methods of the honeycomb and screen based on experience to meet the requirements of controlling the downstream turbulence and noise characteristics. [PERSON] et al. studied the influences of the hole shape of the honeycomb and the position in the wind tunnel on the downstream turbulence characteristics through experiments [15]. [PERSON] et al. optimized the parameters of the screen by the experiments to obtain the required turbulence suppression performance [16; 17]. [PERSON] and [PERSON] thought that the turbulence of the downstream fluid field could be reduced by arranging screens at downstream of the honeycomb [18]. [PERSON] [19] compared the flow characteristics under the conditions of a single honeycomb and combining the honeycomb with the screens. The effectiveness of the honeycomb and screens in reducing the turbulent intensity of the fluid field has been proved by the existing research [20; 21; 22]. However, the systematic studies of the circulating water tunnel are insufficient.