| Popis: |
Adequate modelling of aerodynamic autorotation in the stall region is critical for a more realistic loss-of-control in-flight (LOC-I) pilot training [1]. Wind tunnel rotary-balance testing of scaled aircraft models with measurements of aerodynamic forces and moments are commonly used to collect data for static conditions to investigate aircraft departures in stall region and spin regimes at high angles of attack [2, 3]. These data for steady conditions with constant angle of attack ?, sideslip ?, and constant conical rotation rate ? are helpful but not sufficient [4]. Rotary balance oscillatory coning tests, in which the axis of rotation is misaligned with the tunnel flow on some angle ?, creates a periodic variation in angle of attack and sideslip with amplitude ?. The balance reading time histories have periodic variation of aerodynamic characteristics with periodic time defined by conical rotation rate ? so that the mean values of aerodynamic characteristics are representing the rotary-balance data, while the amplitudes of unsteady periodical components inform about unsteady aerodynamic derivatives which are required for evaluation of dynamic stability in stall lateral departures and unsteady spins. The rotary-balance tests are carried out at different angles of attack ?, sideslip ? and rotation with angular velocity ? which coincides with the flow velocity V in the wind tunnel at ?=0 as shown in Fig. 1. In such pure conical motion, the angle of attack and sideslip remain constant, which provides kinematic conditions as in standard static tests at given ?,?, but with a steady conical rotation ?. When the axis of rotation is misaligned with the tunnel flow on some angle ? the unsteady aerodynamic derivatives in pitch and yaw can be extracted using the Fourier approximation of periodical variation of aerodynamic coefficients. Stall aerodynamics largely depends on the Reynolds number, but when tested in a wind tunnel, the values of the Reynolds number that can be achieved are usually much lower than in real flight conditions. The CFD methods to predict stall aerodynamics based on CFD methods, as shown in [1], can be effectively used for extrapolating results to higher Reynolds numbers, as well as for eliminating interference effects produced by a support system in wind tunnel. In this paper, we use open-source CFD software OpenFOAM to develop and validate the methodology of predicting stall aerodynamics in rotary-balance and oscillatory coning testing conditions [6]. An inverse quaternion transformation is applied to present projections of aerodynamic forces and moments in the body-fixed axes, which allows comparison of simulated results with experimental data and their direct use in aerodynamic modelling. The dual time stepping method is implemented in OpenFOAM to speed up the simulation. The OpenFOAM simulation results for the NASA Common Research Model (CRM) rolling moment coefficient C_l (?) at Re=10^6 have been validated via comparison with the NLR ENFLOW CFD code results [1,5] (see Fig. 2). This comparison confirms the reliability of the implemented rotary-balance procedure in OpenFOAM with reasonably high accuracy. Additionally, the oscillatory conning data with ?=3^o and Fourier approximation of the unsteady aerodynamic derivatives in pitch and yaw will be also presented. The effect of the top sting supporting a scaled model in wind tunnel rotary-balance tests allowed to evaluate the level of interference and make important corrections to improve the fidelity of the aerodynamic model. We also present CFD simulation results showing the transformation of the aerodynamic autorotation zone with the increase of the Reynolds number typical for real flight conditions. |
