Knowledge Based Airfoil Aerodynamic and Aeroacoustic Design

Abstract

A systematic investigation of the unsteady flows around a series of NACA airfoils is carried out. The main objective is to conduct manual design case studies on the connections between an airfoil shape characteristics and it s aerodynamic and aeroacoustic performance. The approach employs the unsteady CFD flow simulations in the near field of an airfoil and the FW-H integral method for the far field noise prediction. The work focuses on analyzing the aerodynamic and aeroacoustic performance of an airfoil and examining the sensitivities of the objective functions to various weightin g factors. The results include identifying the optimum symmetric and asymmetric airfoils among the airfoils and suggesting the possible optimum airfoil characteristics. The results ca n be used to guide the selections of the geometric parameters and constraints in a fully automated aerodynamic and aeroacoustic optimization. 3-5 in a constrained aeroacoustic optimization, an airfoil shape can be expressed as a function of some design parameters. Idea lly, the constrained aeroacous tic shape optimization could be performed by combining an optimizer with a CFD solver coupled with an automated grid generator and a sound propagation formula based on the Ffowcs Williams & Hawkings (FW-H) equation. A CFD simulation for the unsteady viscous flow in the immediate vicinity of an airfo il can be carried out in order to provide a reliable input to the FW-H equation for calculations of the far field noise. An optimizer would then evaluate hundreds or thousands of airfoil designs to achieve an optimum design with respect to both aerodynamic performance (through constraints) and aeroacoustic consideration. However this optimization process is cumbersome due to the fact that a high fidelity unsteady CFD simulation must also be performed hundreds or thousands of times while an optimum set of design parameters is being searched during an optimization. The airfoil parameterization methods that can accurately represent a variety of dramatically different airfoil shapes and can reflect subtle changes in local areas usually use a large number of design variables, so it would be very difficult to use such an approach due to an unrealistic demand on CPU time. Instead of choosing a parameterization method and conducting a constrained aeroacoustic optimization (that could well be unrealistic), in the current work, we directly study the effects of an airfoil shape characteristics on its combined aerodynamic and aeroacoustic performance characteristics. In the study, a two-dimensional unsteady flow field around a series of 4-digit NACA airfoils is consid ered. Firstly, the unsteady viscous flow fields around the airfoils (symmetric and asymmetric) are calculated using a high fidelity CFD solver. The aerodynamic performance of the airfoils can then be evaluated. Secondly, the near field unsteady CFD solutions are used to calculate the far field noise using the FW-H equation. This hybrid method allows us to effectively evaluate the aeroacoustic performance of the airfoils. As a result of the systematic evaluations, a link between our knowledge on the overall performance and a variety of characteristics of an airfoil shap e is established. In Sec. II, the details of the CFD code and the flow condition for the current study are described. The FW-H equation and the integral code verification are