Application of a Multi Level CFD-Technique for the Design Optimisation of Hydraulic Machinery Bladings

To achieve maximum hydraulic efficiency at a wide range of operation, efficient numerical tools are inevitable for the shape optimisation of hydraulic machinery, e.g. rotodynamic pumps and water turbines. A suitable geometry representation by means of B-Spline techniques for the three-dimensionally (3D) curved bladings is necessary as well as a powerful CFD technique in order to evaluate and analyze the hydraulic properties of the design. Both of these modules are integrated in one program system so as to allow the design engineer to achieve maximum performance. There is a natural contradiction between the numerical effort to describe a complex geometry and to compute the flow field as exactly as necessary for an accurate evaluation, and the industrial need to lower the costs and the duration of development of a new design. The 3D curved bladings of rotodynamic pumps and water turbines are characterized by a big number of parameters. Thus, the shape optimisation of this type of blades is still a problem to be solved by an efficient approach. To simplify this complexity, the blade profiles are defined by a reduced set of B-Spline describers on a number of stream surfaces in the conformal mapping representation. This provides an efficient and quick method for modifying the blade shape. To reduce the numerical effort in an iterative optimisation process, a new idea of using the Multi Level CFD-Technique (MLCFD) is described more detailed. The CFD tools used in this process, can be chosen by different criterions such as accuracy on the one hand and computation time on the other hand: There are quasi-3D and full-3D Euler codes as well as quasi-3D and full-3D Navier-Stokes codes for the flow computation and analysis. Starting from an initial design which may come from an artificial neural network having been trained from an existing database, a rough estimation of the quality of a geometry modification can be performed by a quick quasi-3D Euler computation, whereas the final evaluation of the optimum design should be carried out by an accurate 3D Navier-Stokes code. The MLCFD-technique is applied to optimize the blade shape of a centrifugal pump impeller with a specific speed nq = 39 l/min. The numerical results show that significant improvements of the local and integral flow quantities as well as a remarkable reduction of the numerical effort may be reached by using this new approach.Copyright © 2004 by ASME