## 1 Introduction  
The construction of a reservoir dam change the natural environment and the river course, and the water temperature in the reservoir may exhibit an obvious vertical stratification phenomenon [1]. The intake elevation of traditional hydropower stations is low because of the engineering reasons, especially in reservoir with large water level fluctuations. As a result, the discharged water temperature in spring and summer is lower than that in natural conditions, while that in winter is higher [2]. Low-temperature outflow water in spring and summer may have an important impact on downstream farmland irrigation, fish breeding, and water quality [3; 4; 5; 6] and can cause a series of ecological and environmental problems [7]. Regulation of the discharged water temperature and mitigation of the impact of low-temperature outflow water on the downstream ecology have become the focus of current environmental protection efforts [8]. Several selective withdrawal structures [9] have been adopted in thermally stratified reservoirs to regulate low-temperature outflow water, including stop log gates [10], temperature-control curtains(TCCs) [11], and side-type orifice intakes [12]. Traditional selective withdrawal structures, such as stop log gates and side-type orifice intakes, need to be built before reservoir operation and will cause a certain loss of power generation. TCCs can be constructed in reservoir water storage and anhydrous environments, so this structure can be used in reservoirs that have been put into operation but do not have selective withdrawal structures. Compared with a traditional selective withdrawal project, a TCC project has the advantages of low cost and simple construction and can even be added to operating reservoirs without impacting power generation. Top TCCs can effectively improve water quality and prevent algae blooms in thermally-stratified reservoirs [13]. [PERSON] [14] proposed using vertical curtains to control algae blooms in downstream areas of the Terauchi Dam Reservoir in Japan. [PERSON] [15] monitored the vertical curtain in front of a water intake and indicated that the vertical curtain could reduce harmful organic matter content and improve water quality in a drinking water intake zone during active algal growth and bloom periods. [PERSON] et al. [16] concluded that a vertically moveable curtain weir could effectively prevent eutrophic substances from flowing downstream and alleviate algae blooms in the Daecheong Reservoir. As a low-cost and easy-to-construct selective withdrawal structure, the TCC has been increasingly applied to change the outflow water temperature. A TCC is composed of a gravity anchorage system on the bottom, a main cable, a water-proof curtain wall and a floating bridge system on the top [17; 18]. [PERSON] [19] showed that top curtains could effectively stop surface high-temperature water and allow the survival and breeding of cold-water fish downstream through prototype observations of the Lewiston Reservoir and Whiskeytown Reservoir in the United States. [PERSON] et al. [20] studied the evolution of the flow field near a TCC through a two-layer flow model test in fresh water and saline water. [PERSON] et al. [21] adopted a three-dimensional model to simulate the effects of a TCC on the discharged water temperature of the McNary Dam and the temperature distribution in front of the dam. Through a three-dimensional hydrodynamic simulation model, [PERSON] et al. [22] analyzed the effect of a bottom TCC on low-temperature outflow water and the main influencing factors. [PERSON] et al. [23] analyzed the pressure on a TCC as a hydrostatic pressure difference and a hydrodynamic pressure difference and obtained the effect of thermal stratification on the hydrodynamic characteristics. Currently, no engineering guidelines for designing an effective TCC system are available due to the lack of understanding of the complex interactions between the flexible curtain structure and water flow.