Approximate Boundary Layer Methods for a 3D Panel Code for Helicopter Simulations

Kurzfassung

The implementation of approximate boundary layer analysis methods is a key step in the process of modernizing and extending DLR's UPM code, an unsteady panel and free wake code for helicopter simulations. The identification of flow separation and the estimation of friction forces were identified as important features required to better supplement the industrial helicopter development process and addressed in the project CHARME. Two approaches for approximate boundary layer analysis within UPM were selected and implemented in order to provide the desired functionality. The first approach is based on stripwise analysis using integral boundary layer methods for laminar and turbulent flow in combination with empirical transition criteria. The current implementation takes the inviscid velocity distribution as input and allows the prediction of skin friction, transition, laminar and turbulent separation points. However, it is only intended for the analysis of lifting bodies. For arbitrary unstructured non-lifting bodies, a simplified approach based on local flow properties and flat plate analogy was selected. It was extended by a simple flow separation criterion. The newly implemented methods were validated and compared for several test cases including airfoils, wings, a rotor in hover and two fuselage geometries. The results were used to deduce best practices and default settings. When considering these, both approaches showed to be robust and able to deliver useful results, within the limits of the underlying models. The frictional forces and the transition and separation points calculated by the stripwise analysis method for lifting surfaces are in good accordance with most experiments and reference results. Additionally, the results were found to be conservative. The simplified analysis for non-lifting bodies also proved to be robust and tends to provide conservative estimates, but its accuracy lags behind the one of the stripwise analysis method. The need for identification of flow separation regions and estimation of friction forces using mid-fidelity tools in the industrial helicopter development process was addressed in the project CHARME. Therefore, approximate boundary layer analysis methods were evaluated and implemented in UPM, DLR's unsteady panel and free-wake code for helicopter simulations. Two approaches were selected in order to provide the desired functionality. The first approach is based on stripwise analysis using integral boundary layer methods for laminar and turbulent flow in combination with empirical transition criteria. The current implementation takes the inviscid velocity distribution as input and allows the prediction of skin friction, transition, laminar and turbulent separation points. However, it is only intended for the analysis of lifting bodies. For arbitrary non-lifting bodies, a simplified approach based on local flow properties and flat plate analogy was selected. It was extended by a simple flow separation criterion. The newly implemented methods were validated and compared for several test cases including airfoils, wings, a rotor in hover and two fuselage geometries. Both approaches showed to be robust and able to deliver good results, within the limits of the underlying models. The frictional forces and the transition and separation points calculated by the stripwise analysis method are in good accordance with most experiments and reference results. Additionally, the results were found to be conservative. The simplified analysis for non-lifting bodies also proved to be robust and tends to provide conservative estimates, but its accuracy lags behind the one of the stripwise analysis method.