###### Abstract  
Floods control equipment in large dams is one of the most important requirements in hydraulic structures. Howell-Bunger valves and butterfly valves are two of these types of flow controls that are commonly used in bottom outlet dams. The optimal longitudinal distance (\(L\)) between the two Howell-Bunger and butterfly valves is such that the turbulence of the outlet flow from the butterfly valve should be dissipated before entering the outlet valve. Subsequently, the flow passing through the butterfly valves must have a fully developed flow state before reaching the Howell-Bunger valve. Therefore, the purpose of this study was to evaluate the optimal longitudinal distance between the Howell-Bunger and butterfly valves. For this purpose, different longitudinal distances were investigated using the Flow-3D numerical model. The ideal longitudinal distance obtained from the numerical model in the physical model was considered and tested. Based on the numerical study, the parameters of flow patterns, velocity profiles and vectors, turbulence kinetic energy, and formation of flow vorticity were investigated as criteria to determine the appropriate longitudinal distance. In addition, the most appropriate distance between the butterfly valve and the Howell-Bunger valve was determined, and the physical model was evaluated based on the optimal distance extracted from the numerical simulation. A comparison of the results from the numerical and the laboratory models showed that the minimum distance required in Howell-Bunger valves and butterfly valves should be equal to four times the diameter of the pipe (\(L=4D\)) so as not to adversely affect the performance of the bottom outlet system. turbulence kinetic energy; dam; bottom outlet; flood hazards; flow patterns Article  
[PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], K.  
control in dam construction. The outlet flow from these valves collides with a fixed cone at a 90-degree angle mounted at the end of the valve they result in a 45-degree deflection. In the absence of a terminal hood, the outlet flow can be distributed as a fountain, divergent and hollow over a large area. In cases of limited outside space of the valve, the use of a fixed outer cylinder or hood is common, and the conical flow becomes axial due to the impact on the external hood. The Howell-Bunger valve is simple, inexpensive, and relatively safe from cavitation; therefore, it is used in dams and structures with many heads up to about 200 m. In Howell-Bunger valves water spreads like a fountain, so the energy of water particles is reduced due to high dispersion in space, resulting in significant energy loss [4; 5]. Figure 1 shows the components of a Howell-Bunger valve along with the outlet flow conditions of this type of valve in the presence or absence of a hood. Under the conditions of using the hood in the flow path, the amount of spraying is reduced and, as a result, the flow rate coefficient is reduced. These structures are designed at a discharge coefficient of about 0.85 (about 0.78 with a hood) in 200-2500 mm sizes. Large valves operate for heights equivalent to heads of about 140 m while small valves operate for heights up to 300 m [6]. Butterfly valves are another type of valve widely utilized for flow regulation and control. Butterfly valves consist of three main parts: body, disc, and shaft. The advantageous characteristics of butterfly valves include a simple installation, low costs of installment, operation, and maintenance, as well as high flow capacity compared to other types of  
Figure 1: The Sectional view of typical (**a**) Howell–Bunger with hood; (**b**) Howell–Bunger without hood; (**c**) Howell–Bunger with hood of Salt River and Lower Bear Reservoirs, CA, USA; and (**d**) Howell–Bunger without hood of Jozini Dam, South Africa. valves [7; 8].