Eddy-resolving simulations of the flow around a vertical tail plane
Enhancing the ability to predict airflow around the Vertical Tail Plane (VTP) of an aircraft is vital in the aviation industry. The size of the VTP is driven by a particular flight condition - loss of an engine during take-off and low speed climb. Nowadays, Computational Fluid Dynamics (CFD) is the main tool used by engineers to assess VTP flows. However, due to uncertainties in the prediction of VTP effectiveness, aircraft designers keep to a conservative approach, which risks oversizing of the tail plane, adding more drag. Uncertainties emerge from difficulties in predicting the massive separation that occurs on the swept tail when it is approached by a flow at high incidence. Furthermore, the deployment of the control surface (the rudder) over the tail plane and the skewed flow along the span increase the CFD challenges. Improved predictive capabilities of the flow around VTPs would enable a more optimal design approach with potential drag saving. The correct prediction of flow separation is the essence of this study. Currently, the industry uses steady Reynolds-Averaged Navier-Stokes (RANS) simulations to analyse VTPs flow. In order to assess RANS performance, the study of airflow detaching from a backward rounded ramp is performed and the results are compared to Large-Eddy Simulations (LES). The analysis shows that, even though RANS may predict the onset of flow separation correctly, they completely miss the location of flow reattachment over the ramp, and this affects the whole flow solution. Moreover, the flow features a strong anisotropy at the onset of separation, difficult to be captured by RANS. The analysis shows that RANS cannot predict production of turbulent kinetic energy in the detached flow region correctly, discouraging flow mixing, and delaying flow reattachment. A hybrid RANS/LES carried out on the same test case shows the benefits of using eddy-resolving simulations for detached flows. The prediction of the locations of the separation and reattachment points differs by only 1% from the highly-resolved simulation. The VTP investigation carried out in this thesis uses a wind tunnel model tested at Airbus. The study starts with steady RANS approaches for different turbulence models. RANS simulations produce acceptable results for the flow at low incidence levels. On the contrary, at high incidence, when flow separation occurs, RANS methods fail. The second step of the research consists of using unsteady RANS (URANS) simulations for VTP flows at high sideslip angles. The introduction of time-accuracy brings important benefits. Nevertheless, the results still show some inaccuracies (around 20% error). Finally, restarting from the flow solutions obtained by URANS simulations, higher fidelity hybrid RANS/LES techniques in the form of Delayed Detached-Eddy Simulations (DDES) are used to assess the characteristics of the separated flow around the tail plane. Results show a remarkable improvement of the flow solution. The pressure distribution matches experimental results favourably, and this translates into an improved prediction of the aerodynamic loads over the VTP. This leads towards a new strategy for the assessment of the flow over aircraft VTPs, amounting to an important contribution to the design of future aircraft.
Eddy-resolving simulations, Aircraft tails, Flow separation, RANS, DDES, Vertical tail plane, CFD
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