Laminar boundary layer suction has significant potential for reducing aircraft drag, thereby diminishing its environmental impact. This study presents wind tunnel experiments conducted on a flat plate to examine the effectiveness of laminar boundary layer suction in delaying the transition and compares the measured data with the en method based on linear stability theory (LST). The experiments, performed over a range of freestream velocities from 15 to 50 m/s, comprised infrared thermography, pressure measurements, and hot-wire anemometry. The boundary layer suction is implemented through interchangeable suction boxes mounted on the flat plate, with two types of suction surfaces tested, featuring hole diameters of 120 and 60μm and a constant porosity of 0.9%. The study examines the influence of various parameters on transition, as the intensity of the suction coefficient, particularly at elevated values, as well as the impact of the micro-holes diameter, the chordwise distribution of the suction velocity and the freestream Reynolds number. A discrepancy between the experimentally measured transition location and the predictions from LST is observed. To identify the origin of this deviation, boundary layer measurements are taken on the porous surface while varying both the suction coefficient and its spatial distribution. A particular flow disturbance near the porous surface, amplified by the suction intensity, is identified, leading to increased velocity fluctuations in the near-wall measurement points. The difference depends on both the suction coefficient and the suction velocity distribution. For this reason, a configuration is investigated in which only the first and last of the four suction chambers are used to aspirate the boundary layer. It is observed that the flow disturbances are significantly reduced, and the boundary layer predictions align more closely with the experimental data.

Hybrid laminar flow control (HLFC) can be a possible solution for future sustainable energy-efficient aviation. The current study proposes a MATLAB-based numerical tool for the design of the suction system for an airfoil optimized for a subsonic short-range HLFC application. Considerable energy losses may occur when the air passes through the perforated metallic outer surface and the inner structure of the suction system. A semi-empirical approach is used to design a layout that provides a target suction velocity based on measured pressure losses through porous medium and substructures. Flowbench measurements were performed on 3D-printed internal core test samples to quantify the pressure losses that can be used to create a lower pressure below the porous sheet matching the target suction velocity. The actual suction realized on the airfoil using this substructure concept has a discrete nature that increases with the distance between two adjacent walls. Finally, the suction system’s power requirement is calculated. The power requirement for distributed suction accounts for the pressure loss characteristics of the porous material, the internal core structure, and throttling holes. However, the study does not include the ducting losses from the substructure to the compressor. Approximately 80% of the total suction power is utilized to eject the sucked air back to the freestream conditions for a system with a compressor and propulsive system efficiency equal to one. The study analyses the performance of the designed internal core layout to different flight conditions and addresses the suction power requirement variation with lift coefficient and flight altitude.

The Energy System Transition in Aviation research project of the Aeronautics Research Center Niedersachsen (NFL) searches for potentially game-changing technologies to reduce the carbon footprint of aviation by promoting and enabling new propulsion and drag reduction technologies. The greatest potential for aerodynamic drag reduction is seen in laminar flow control by boundary layer suction. While most of the research so far has been on partial laminarization by application of Natural Laminar Flow (NLF) and Hybrid Laminar Flow Control (HLFC) to wings, complete laminarization of wings, tails and fuselages promises much higher gains. The potential drag reduction and suction requirements, including the necessary compressor power, are calculated on component level using a flow solver with viscid/inviscid coupling and a 3D Reynolds-Averaged Navier-Stokes (RANS) solver. The effect on total aircraft drag is estimated for a state-of-the-art mid-range aircraft configuration using preliminary aircraft design methods, showing that total cruise drag can be halved compared to today's turbulent aircraft.