## 1 Introduction
For small water flow rates (see Table 1), the Archimedes screw can operate at high efficiency. In very-low-head or ultra-low head conditions, the Archimedes screw turbine (\(AST\)) has proven to be a suitable solution for this particular scenario, demonstrating satisfactory performance [17]. [PERSON] et al. [17] presented a study comparing several micro hydropower turbines installed at sites characterised by low head and low flow. The results showed that \(AST\)s remain very efficient at sites with a head of less than 5 (m), even as the available head approaches zero [17]. Another study also showed that \(AST\)s work best at sites with less than 10 (m) head and 10 (m\({}^{3}\)/s) of the water flow rate [18]. Therefore, its high efficiency (80-90%) is the main advantage [19]. Environmental authorities have acknowledged that this technology is fish-friendly, meaning it is environmentally much better than other turbines [20]. However, there are also disadvantages, such as its rather limited regulation capabilities and the high gear ratio. The introduction of frequency converters in this technology allows for rotation speed control, which prevents these disadvantages. An Archimedes screw is inserted around a cylinder of a certain diameter, which works like an axis. The helical planes (blades) are placed in this cylinder to form the screw. The cylinder is extended by a shaft of a smaller diameter to support it on bearings. The \begin{table}
\begin{tabular}{c c c} \hline \hline
**Hydraulic Turbine** & **Head (m)** & **Flow Rate (m\({}^{3}\)/s)** \\ \hline Pelton & 60–2000 & 0.02–50 \\ [PERSON] & 8–800 & 0.2–1000 \\ Cross-flow & 3–200 & 0.04–12 \\ [PERSON] & 2–70 & 0.5–1000 \\ Archimedes screw & 1–9 & 0.25–10 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Operating range of hydro turbines. gearbox and the electric generator are placed on the upper end of the shaft. Direct coupling to an electric generator is not possible due to the low speed of an Archimedean screw. Therefore, a speed increaser must be used to connect an Archimedes screw to an electric generator. A gearbox is normally used for this purpose. Figure 1 shows a diagram of a typical Archimedean screw turbine system. The parameters of an Archimedean screw can be divided into external and internal [21]. The external parameters are [21] the radius of the screw's outer cylinder (\(R_{o}\)), the total length of the screw (\(L\)), and the angle of the screw turbine (\(\theta\)). The internal parameters are [21] the radius of the screw's inner cylinder (\(R_{i}\)), the pitch (or period) of one blade (\(p\)), and the number of blades (\(N\)). These parameters are related to each other and are heavily dependent on the available installation conditions for the Archimedean screw, such as the length, inclination, and riverwater flow. The Archimedean screw has been the subject of numerous studies. Most researchers focus on analysing the influence of its parameters on the power and performance of an Archimedes screw turbine. Some of these are presented below:  
1. [label=()]
2. Studies related to the parameters affecting the performance of an \(AST\). [PERSON] et al. [22] studied the influence of the tilt angle and the number of blades of an Archimedes screw turbine on the power output and performance using computational fluid dynamics simulation and laboratory tests. The results showed good agreement between the model and experiment, with relative errors in hydraulic efficiency of less than 2% in optimal cases. [PERSON] et al. [23] evaluated the performance of \(AST\)s at different rotational speeds, flow rates, and tilt angles using computational fluid dynamics. [PERSON] et al. [24] experimentally analysed the influence of variations in tilt angle, water flow, and rotational speed on the performance of an \(AST\). [PERSON]. [23] investigated the effect of several parameters (the number of blades, the tilt angle, the pitch angle, the screw's outer diameter, the rotation speed, the head, and the flow rate) on the efficiency of \(AST\)s, using computational fluid dynamics.