Aerodynamic Investigation of Air Inlets on Aircrafts with Application of Computational Fluid Dynamics

Abstract: Air inlets in some form are used on all commercial airliners today. The type of air inlet investigated in this report is a NACA inlet submerged into a surface. This surface is within this thesis a test section wall of a wind tunnel. The considered wind tunnel is TWG in Göttingen (Germany) that operates in transonic speeds. Submerged inlets have the main advantage of low aeroynamic drag from the inlet itself. The importance of reducing drag, and the attention given to this subject is increasing as fuel prices rise as well as public awareness of environmental impact by all of us. The outcome of this thesis contributes to the government-funded project ECOCENTS which deals with the design of innovative new aircraft cooling systems and the detailed flow analysis of these systems. This thesis was carried out at the company Airbus in Bremen, Germany. The main objective of this report was the evaluation of the ram pressure efficiency of four different ramp angles of a NACA inlet and the estimation of the drag caused by these geometries with the use of Computational Fluid Dynamics (CFD). The flow solver used was TAU, a Reynolds-Averaged Navier-Stokes (RANS) solver developed by the German Aerospace Center (DLR). The inlet consisted of one ramp section where the ramp angle was fixed at 7 degrees, and a second variable ramp section. The following different angles were investigated: 4, 7, 10 and 15 degrees. These configurations were evaluated at a velocity of Mach 0.8 and a Reynolds number of 10'10^6. The ramp angle of 7 degrees was evaluated at two additional velocities (Mach 0.73 and Mach 0.87) and at two additional Reynolds numbers (5'10^6 and 15'10^6) at Mach 0.8. The inlet efficiency outcome of this study was located between two other investigations. The results of this RANS computation predicted a higher total pressure at the inlet throat plane compared to a previous CFD investigation where a different RANS solver at the same geometry was used. In comparison to an estimation method mainly based on experimental data (ESDU method), the recent study showed a lower total pressure at the inlet throat plane. The aerodynamic drag that arised by the presence of the inlet system was calculated within this thesis to be higher than the outcome of the experimental data based (ESDU) method. The advantage of using a NACA type inlet was observed to be highly related to the ramp angle. Vortices are originated and develop along the edges of the intake ramp walls. These two vortices help to transport higher energy flow from the free stream into the inlet and therefore reduce the boundary layer thickness in the inlet region. For lower mass flows (0.10 - 0.20 kg/s) a ramp angle of 7 degrees was seen to be prefered in view of ram pressure efficiency. At a higher mass flow (0.25 kg/s) the 10 degrees ramp angle was prefered, followed by the 15 degrees ramp angle at the highest investigated mass flows (0.30 - 0.35 kg/s). In view of drag, the lowest ramp angle possible for a given mass flow was seen to be most advantagous. Future work on this subject will include simulation of an inlet in combination with a heat exchanger and a ram air outlet. This arrengement will be the same as in the investigation at the TWG test campaign and therefore comparable. The difference in outcome of the separate CFD analysis was discussed within this investigation but could not be completely cleared.