Simulation of Wind-Induced Vortex Flow and the Effect on a Helicopter Structural Failure

摘要

Wind flow over topography is often converted to vortex flow due to the interaction of specific features of the terrain such as two mountain peaks. An example of this is found in the area of Straumsfjorden in Northern Norway at certain wind directions. During an emergency flight along that coastline a rescue helicopter of type Eurocopter SA 365N Dolphin 2 experienced a sudden pitch-down change of attitude, which was subsequently corrected by the pilot and the automatic flight control system. After a safe landing the crew observed that both sides of the horizontal stabilizer were missing. Since there were strong winds at the time of the incident, the Norwegian Aircraft Accident Investigation Board ordered a numerical wind analysis of the mountainous area around the pitch-down location. This paper reports on that study and sets forth a hypothesis of dynamic stall together with torsional loading as the cause of the mishap. The physical domain under study is an area of 15 by 20 km and a height of 2 km. This domain was transformed into a numerical space based on a digital representation of the topography. Hence, about 400,000 computational cells were generated in which the non-viscous Euler equations for rotational flow were solved. The numerical method applied is based on the time-stepping finite volume technique and incorporates cell-centered fluxes for spatial discretization. The numerical simulation of steady-state wind conditions revealed strong vortex flow patterns in two areas of the flight path, one coinciding with the location of the sudden nose-down movement of the helicopter and the recovery of the stabilizer in the fjord. This wind flow structure will continue to introduce strong transient flow conditions on flight vehicles and their horizontal stabilizers, including the two vertical fins. This highly 3D wind-induced flow could be the cause for the structural failure of the stabilizer, however, no aerodynamic proof could be put forward. (12 figures, 4 refs7).