The Francis-99 hydrofoil is simulated using a quasi two-way Fluid-Structure Interaction procedure. The structural domain is reduced by the use of modal decomposition, and solved for inside the commercial fluid solver ANSYS CFX. Both the first order Backward Euler and second order Crank-Nicolson time discretization scheme is used in the structural equations, with significantly different results. Several coupled fluid-structure phenomena is observed that would be unobtainable in a normal one-way approach. The most interesting is an “added stiffness” effect, where the eigenfrequency of the foil increases when the flow velocity is increased. This trend corresponds well with available experimental results. The same phenomenon is observed in the hydrodynamic damping on the foil. Self-induced vibration due to vortex shedding is also simulated with good results. The implemented two-way approach allows the different forcing terms to be tracked individually, due to the discretization of the second order structural system. This provides insight into the underlying physics behind the different FSI phenomena seen, and helps us explain why the damping and eigenfrequency characteristics change as the flow velocity passes the lock-in region.
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.
This paper contributes to the field of radial compressor design by proposing an adaptive, automated workflow incorporating the analysis of the compressor performance for a multitude of operation points by means of the respective operating maps. Most state-of-the-art approaches do not consider that the operating map limits are not conserved while changing geometric parameters which constraints these analyses to a rather small design space. In contrast, the presented methodology considers the varying operating map limits in regards to the corresponding mass flow and with that expands the possible input parameter range. The presented workflow integrates different software solutions, starting with the automated generation of the compressor geometry based on a parametric CAD model. For each geometry a mesh is generated that is used for all subsequent CFD simulations which finally result in the operating map. For every speed line, the choke point is identified by an adaptive CFD computation (based on the principle of similarity). By using the calculated choke mass flow, supplementary CFD simulations obtain additional operating points on the current speed line by a stepwise reduction of the mass flow. However, the identification of the surge line is not within the scope of the presented approach. Therefore, the range covered by the map is determined by the mass flow at the maximum efficiency and the mass flow at the choke line. The developed framework is applied to optimize the operating map of a radial compressor. A successful optimization shows that the optimized design has an enlarged choke mass flow for lower compressor speed while the pressure ratio and polytropic efficiency are comparable. At the same time, this design has a comparable choke mass flow and efficiency for higher compressor speed, but an improved maximal pressure ratio. The obtained results from the optimization show that the methodology is applicable to a wide parameter range. By adaptively calculating the operating map limits, the approach is not restricted to a small design space.
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