# An Experimentally Validated CFD Code to Design Coanda Effect Screen Structures
## 1 Introduction  
While turbines and generators of hydroelectric power plants are already highly developed [1], water capture in general, or the \"rakes\" for separating the bedload and purifying the useful water, represents the greatest weakness, especially in mountainous regions [2]. Whereas simple technologies for water catchment (e.g., horizontal rakes or filters) are sufficient for calm watercourses with constant flow rates and little bedload material, which also only have small to medium sizes, the requirements in alpine environments are much more complex [3, 4, 5]. There, the available water quantity and the quantity and size structure of the bedload are highly variable and cover an enormous range [6]. In addition, the effects of local extreme weather events such as heavy precipitation, hail, or storms are felt very quickly. Therefore, the filtering or purification of the water plays a central role in the collection of service water in the alpine region in order not to damage the turbine wheel [7] of the hydroelectric power plants. The Coanda effect is named after the Romanian scientist [PERSON] [8]. When a liquid or gas flows along a solid surface, it tends to stick to the surface. Due to fluid flow viscosity, a velocity boundary layer is formed at the wall. Based on the Bernoulli equation [9], a higher velocity results in a smaller static pressure. A pressure gradient is responsible for the fact that the flow is redirected. This effect is, for example, used to separate solids from a fluid flow in order to use the flow within hydroelectric power plants. A device for that is the so-called Coanda effect screen (CES) structure, see Figure 1. Water with solids flows to the weir edge, over theacceleration plate, and to the screen, where the solids are separated. The screen consists of struts with a usually triangular profile that are transverse to the direction of the flow. Each strut is mounted at a small angle with respect to the flow. The separation effect is based on the distance between the struts and the screen continuously overflowing [10]. The CES structure is self-cleaning and provides reliable operation with minimum maintenance [11]. It has been successfully applied several times in hydroelectric power plants and has been constantly optimized [12; 13; 14]. Furthermore, experiments showed that CES performed well in cold climates, in spite of some periods of reduced intake capacity [15]. In [16], a theoretically based computational model was developed to predict the hydraulic performance. Model predictions were compared based on results from nearly 100 clean water laboratory tests with several different prototype-size screen structures. This model allows designers to estimate the screen capacity and economical screen structures for a variety of objectives and in situ conditions. The combination of experimental investigations and computational fluid dynamics (CFD) can further support the design process and evaluate the flow capacity of the CES [17]. [PERSON] and [PERSON] [18] investigated fluid flow conditions based on a physical and numerical approach. The open-source numerical modeling tool Delft3D-FLOW, Deltares (NL), was used to observe the flow velocity vector field, the water depth, and the Froude number in a diversion structure with Coanda effect screens. The main weakness is the very coarse grid (mesh) used for the spatial discretization in the numerical analysis, without consideration of the fluid flow boundary layer with a refined mesh towards the walls. Furthermore, the flow through the Coanda effect screen was not investigated. In [19], 2D CFD simulations of the Coanda effect screen were quantitatively compared with experiments. Compared to [18], a much better grid  
Figure 1: Experimental setup of the CES. For simplicity, the borders are not shown, see Figure 2. At the inlet, the flow rates for the three different experiments (7 m\({}^{3}\)h\({}^{-1}\), 8 m\({}^{3}\)h\({}^{-1}\), and 9 m\({}^{3}\)h\({}^{-1}\)) are set and controlled with a PI controller.