Safer UAV landings

Dr. Ulf Specht, IABG mbH, Germany

Performing landing operations of a helicopter or an UAV (unmanned air vehicle) on a helideck of a battle ship is a critical situation for both the helicopter pilot and the crew of the ship. A detailed understanding of the flow structures and the magnitude of the turbulent fluctuations is therefore necessary. Taking into consideration the very detailed superstructure of the complete ship, STAR-CD can be used, as it is in this case, to predict turbulent fields.

Background

Reconnaissance is one of the main tasks of the German Navy. An UAV will ensure identification of objects beyond the horizon of the ship even under insufficient optical conditions. Since the landing operation should be a completely automated task, the software of the control system has to take into account anything that might affect the smooth landing of the UAV, such as the environmental conditions. Therefore it is necessary to predict the flow field around the ship, by considering different wind speeds and wind directions relative to the ship.

CFD simulations

Starting from the water surface level, a narrow box was discretized using ICEM/Tetra containing the complete ship. This included the complicated structure with a refined region near the helideck. The mesh was completed by adding blocks of hexahedral cells in front of the bow, behind the stern, portside, starboard, and using STAR-CD’s ‘arbitrary couples’ methodology above the tetrahedral cells. The final mesh consisted of approximately 2 million cells.
The flow was assumed to be steady, incompressible and turbulent. Turbulence is modeled by the standard high Reynolds k- model.

Results

A general view of the flow field containing isosurfaces of the turbulent kinetic energy is shown in Fig.1. There is a remarkable production of the turbulent kinetic energy due to the superstructure. Since most of the turbulent kinetic energy has been dissipated before reaching the helideck these turbulent fields hardly affect the situation there.Fig. 1: Streamlines and isosurfaces of the turbulent kinetic energy (general view)

A large vortex generated by the main flow at the end of the hangar (similar to a backward facing step) however, has a significant impact on the landing procedure (Fig. 2). This vortex interacts with the main flow generating a shear layer and producing a region of high turbulent kinetic energy.

Fig. 2: Streamlines and isosurfaces of the turbulent kinetic energy (detailed view of the helideck)

Simulating many different flow directions, these computations can provide a “Best Practice Guide” on how to maneuver the ship prior to a landing operation. Furthermore the data sets of the velocities and the turbulent kinetic energy can be prepared as an input for a real time UAV approach and touch down simulation.

Conclusions

The detailed information of complex flow patterns obtained by STAR-CD simulations improves significantly the understanding of how the turbulent fields are generated. Furthermore the complete flow field, which is impossible to achieve in experiments, can be used as an input for an UAV simulation environment.For more information contact Specht@iabg.de