Experimental assessment of the active flow control over the flight deck of a frigate
Creators: name:  E. DERHILLE 
name:   L. KEIRSBULCK 
name:   Q. GALLAS 
name:   L. BORDIER 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: FLOW CONTROL
AEROSPACE

description: Helicopters landing operations on ship are challenging for the pilots due to the aerodynamic interaction of the helicopter wake with the flow perturbations caused by the superstructures of the ship [1]. The flow on the helideck is highly turbulent and pilots must go through strong airwake for landing operations. The addition of helidecks movements, rotor wake, visibility and pilot experience to this unsteady flow conditions is often characterized by an increased workload for the pilots [2]. In addition to these constraints during the landing procedure, there is the difficulty of establishing - and even more so of predetermining - the operational limits. To date, only the Ship Helicopter Operating Limits (SHOL) allows to answer this question. It defines the boundaries of safe operation for a particular ship-helicopter combination in terms of maximum allowable wind speed and direction relative to the ship. The development of SHOL requires to perform numerous trials at-sea that are extremely expensive, time-consuming and associated with high risk. To alleviate these constraints, researches have shown promising results with active flow control applied on the helideck [3]. This paper presents an experimental framework to identify the effect of active flow control on the helideck with and without an hovering helicopter rotor. To this aim, two main steps are proposed: identification of flow characteristics according to the control strategies implemented followed by the strong coupling between ship airwake with and without control and the rotor wake while helicopter is in an hover position above the flight deck. To study the flow topology, static pressure and Stereo PIV (SPIV) are used in the test campaign presented in this paper. The experimental setup consists of a simplified but representative frigate geometry and a 4-bladed rotor model. The ship model is a 1:50 scaled-model. The active flow control is realized by continuous blowing through eight slots of 2 mm width located along the sides of the hangar. The blowing jets exit tangentially to the geometry side toward the flight deck platform or with an angle of 30 deg toward the center flight deck platform. The incoming mass flow comes from underneath the wind tunnel and separates into four branches that independently feed four pair of slots. This power supply allows independent adjustment by pair of the eight slots. The experiments were conducted in the L2 low-speed wind tunnel at ONERA Lille. The test section of the wind tunnel is closed and rectangular (6 m width by 2.4 m height by 13 m long). A turntable setup of 5.96 m in diameter allows the azimuthal adjustment. In these experiments, the drift angle varies from -40 to 40 deg by 10 degrees steps. The upstream velocity U0 is measured by pitot probes located at the test section inlet and is set to 15 m/s, equivalent to about 30 kn. Here three velocity ratios (Uj/U0 equal to 2, 3 and 4) are used in the wind tunnel experiments, where Uj is the mean jet velocity at the slot exit. The helideck platform is equipped with 88 static pressure taps, 32 additional static pressure taps are on the superstructure and the hangar door. A specific setup is used for the SPIV measurements. The laser source and the mirror are directly integrated into the model and an optical glass replaces a part of the hangar door. Two configurations are used, one parallel to the platform with the plane at mid-height of the hangar and a perpendicular one with the plane oriented along the deck centreline. The two cameras used for the PIV measurements are located either on the wind tunnel roof or on the floor for, respectively, the parallel or perpendicular configuration. By such embedded setup, the recording of the flowfield at different drift angles is made easier. First results show a reduction of the recirculation area with continuous steady blowing around the hangar, observed at different drift angles. The jets direction towards the center of the deck looks to be more efficient in terms of flows modification and requires a lower velocity ratio.