A Wind Tunnel Rig to Study the External Fan Cowl Separation Experienced by Compact Nacelles in Windmilling Scenarios

Abstract

Ultra high-bypass ratio aircraft engines adopt slim fan cowl profiles which are sensitive to separation on their external surfaces under diversion windmilling conditions, when engine shutdown occurs during cruise. A two-dimensional wind tunnel rig is developed to investigate the separation mechanisms experienced by aero-engine nacelles in such scenarios to establish the detailed aerodynamics. The tunnel flow field with entry Mach number 0.65 broadly replicates the expected diversion windmilling flow behaviour for a full-scale nacelle, featuring a supersonic region which terminates in a normal shock wave on the external fan cowl surface. For the three conditions investigated, which relate to different engine mass-flow rates, the response of the boundary layer in this region progresses from remaining fully attached through to well-established separation. However, the rig Reynolds number based on nacelle lip thickness is about 1.2 × 10 6 , roughly one-third of full scale, resulting in a transitional rather than turbulent shock-boundary-layer interaction. Nevertheless, simple simulations correctly predict the wind tunnel flow field, except for the precise boundary-layer response to normal shock waves, and so experimental data from the rig can be used to validate relevant computational methods.

Characteristics of Shock-Induced Boundary-Layer Separation on Nacelles Under Windmilling Diversion Conditions

“…Thus, the slightly different shock location and pre-shock Mach number for CFD compared to the experiments may be associated to different characteristics of the boundary layer. The temperature distribution on the nacelle cowl obtained in the rig through infrared thermography showed that the interaction between the shock and the boundary layer was transitional [9]. The estimated recovery factor ahead of the shock was approximately 0.8, which is close to a theoretical value of 0.84 for a laminar boundary layer over a flat plate with zero pressure gradient [34] based on a Prandtl number for air of 0.71.…”

“…A recent experimental campaign using a quasi-2D rig representative of the aerodynamics of an aero-engine nacelle under windmilling diversion conditions (Fig. 1, [8,9]) provides a dataset to determine the characteristics of the boundary layer on the nacelle cowl and the interaction with the shock wave. A similar arrangement of the rig was previously used to investigate Shock-wave Boundary Layer Interaction (SBLI) on a configuration representative of an intake at high-incidence [10,11].…”

Infrared Thermography Techniques for Boundary Layer State Visualisation

“…Transient tunnels, such as the Cambridge supersonic tunnel (Sabnis et al, 2023), the Oxford turbine research facility (Hilditch et al, 1994) and the von Karman Institute compression tube facility (Paniagua et al, 2013), are common in high-speed testing, so it is important to understand thermal effects in them when using IR. In this experiment thermal transients are investigated by pre-heating the flow and allowing the test piece to thermally soak.…”

“…The application in this example is an engine nacelle being studied in a transient blow-down transonic wind tunnel at the University of Cambridge. Details of the study, rig and experimental procedure are given by Sabnis et al (2023). In off-design conditions the stagnation point on the nacelle moves inside the intake, resulting in strong flow acceleration around the lip.…”