Approach flow conditions of intake structures should be in compliance with state of the art acceptance criteria for all operating conditions, to provide the required flow rates of cooling or circulating water properly. Specially for high specific speed vertical pumps the direct inflow should be vortex free, with low prerotation and symmetric velocity distribution. Flow separation in front of open and covered intake structures can lead to free surface vortices. Depending on the strength, vortices can emerge a coherent air core, starting from the surface and entering the inlet nozzle directly. High mechanical load of the pump and decreasing hydraulic performance are an immediate consequence. Energy content, stability and position of a free surface vortex are determinated by intake system geometry and operating conditions. By installing flow guiding devices, the generation of vortex formations can be prevented. Optimization steps should be accomplished with respect to installation costs and complexity on-site. Therefore the effectiveness of the improvements has to be verified. Physical model investigations are common practice and state of the art to evaluate, to optimize and to document flow conditions inside intake structures. Nowadays, computational analysis is more and more adopted in this work field. A combination of both leads to well-defined and reproducible results within a wide application range. By this means prototype intake structures can be investigated and, if necessary, optimized for their best approach flow conditions. In the report the optimization of insufficient approach flow by computational analysis and physical model tests is presented. Therefore various intake structures for cooling water systems — all having approach flow not in compliance with the acceptance criteria of common standards — were physically modeled and investigated. Simultaneously initial and optimized layouts were reviewed by numerical calculations of different kinds. Calculated results are compared with model test and prototype data for different cases and operating points. Focusing on the occurrence of free surface vortices, methods of reproducing free surface vortices with numerical approaches will be presented and evaluated.

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