Wind Tunnel Testing of a Subsonic Airfoil Employing Hybrid Laminar Flow Control

Surface suction provides an efficient way to delay boundary layer transitions. In order to verify the suction effects and determine the mechanism of suction control in transonic swept wing boundary layers, wind tunnel transition measurements in a hybrid laminar flow control (HLFC) wind tunnel model uses an infrared thermography technique in the Aircraft Research Association (ARA) 2.74 m × 2.44 m low turbulence level transonic wind tunnel. Based on the experimental data of stationary crossflow dominant transitions without and with surface suction in transonic swept wing boundary layers, in this paper, the effects on the receptivity and linear and nonlinear evolution of stationary crossflow vortices have been analyzed with the consideration of curvature. Theoretical analysis agreed with the experimental observations in regard to the transition delay caused by boundary layer suction near the leading-edge region.

Maintaining the laminar flow on surfaces through active control is a significantly promising technique for reducing fuel burn and alleviating environmental concerns in commercial aviation. However, there is a lack of systematic parameter studies for the hybrid laminar flow control (HLFC) together with natural laminar flow (NLF). To address this need, we optimize the infinite swept wings with different sweep angles and at various conditions, including different Mach numbers, Reynolds numbers, and lift coefficients. The Reynolds-averaged Navier-Stokes (RANS) solver coupled with the linear stability theory is applied for the laminar-turbulent transition prediction, and the traditional optimization method based on evolutionary algorithms is applied for laminar flow wing optimization. The optimization results found that HLFC is required when the NLF fails at a larger sweep angle (35°) and Reynolds number ( 20 × 10 6 ). The lower pressure peak with boundary-layer suction is found to delay the transition of the regional aviation condition. Besides, the pressure distribution of HLFC is similar to NLF results at the lower Reynolds number ( 10 × 10 6 ) or sweep angle (25°), i.e., a gentle negative pressure gradient near the leading edge and a small favorable pressure gradient behind it. Clarifying the characteristics of laminar flow wings will advance the application of the laminar flow technique within its field.

Resource-efficient flying is a central goal of current and future developments in aviation. Laminar technology is one of the most promising ways to contribute for the achievements to reach the goals of Flightpath 2050. The hybrid laminar flow control (HLFC), based on the combination of active flow control by boundary layer suction and natural laminar flow downstream, is a favourable way to laminarize leading edges with large sweep angle. While the resistance-reducing effective of HLFC technology has already been proven with wind tunnel and flight tests, solutions for the cost-effective production are still subject of current developments. The particular difficulty lies in the combination of industrially producible designs while maintaining the aerodynamic performance. For this purpose, DLR is developing a leading-edge design, based on a cost-effective, micro-perforated outer skin with intrinsic pressure drop distribution. Furthermore the leading edge will be fully demountable for maintenance and cleaning reasons. The resulting combination of low manufacturing and operational costs should leverage the breakthrough of HLFC technology for industrial application. In the following paper the aerodynamic and structural design of the leading edge as well as the manufacturing technology of the outer skin are described. In addition the structural concept for fin application is presented. The presented work and achieved results were developed in the CleanSky2 project NACOR (New Aircraft Configurations and Related Issues, Grant Agreement No. CS2-AIR-GAM-2014-2015-01), funded by the European Commission and the Federal Ministry for Economic Affairs and Energy.

The German research Cluster of Excellence SE2A (Sustainable and Energy Efficient Aviation) is investigating different technologies to be implemented in the following decades, to achieve more efficient air transportation. This paper studies the Hybrid Laminar Flow Control (HLFC) using boundary layer suction for drag reduction, combined with other technologies for load and structural weight reduction and a novel full-electric propulsion system. A multidisciplinary design optimisation framework is presented, enabling physics-based analysis and optimisation of a fully electric aircraft wing equipped with HLFC technologies and load alleviation, and new structures and materials. The main focus is on simulation and optimisation of the boundary layer suction and its influence on wing design and optimisation. A quasi three-dimensional aerodynamic analysis is used for drag estimation of the wing. The tool executes the aerofoil analysis using XFOILSUC, which provides accurate drag estimation through boundary layer suction. The optimisation is based on a genetic algorithm for maximum take-off weight (MTOW) minimisation. The optimisation results show that the active flow control applied on the optimised geometry results in more than 45% reduction in aircraft drag coefficient, compared to the same geometry without HLFC technology. The power absorbed for the HLFC suction system implies a battery mass variation lower than 2%, considering the designed range as top-level requirement (TLR).

<div class="htmlview paragraph">This paper describes a program of wind tunnel tests of the hybrid laminar flow control (HLFC) concept at near full scale Reynolds number. The tests were performed in the low speed 5′ X 8′ Boeing Research Wind Tunnel on a large (20 Foot Chord) section of an infinite swept wing having a sweep angle of 30°degrees. Boundary layer suction was provided over the first 20 percent chord on the wing upper surface through an electron beam perforated titanium suction surface. The extent of laminar run beyond the suction region on the wing surface was measured for various external pressure distributions and suction distributions. Two transition detection techniques were employed: an off-the-surface-pitot and a relocatable hot film. Depending upon the external pressure distribution the laminar run extended as far back as 45 percent chord. This corresponds to a transition Reynolds number of approximately 11 x 10<sup>6</sup>. Significant spanwise non-uniformity of the transition location was encountered at “off-design” model incidences. This was caused by spanwise non-uniformity of suction induced by spanwise external pressure gradients. For given chordwise pressure distribution and Reynolds number conditions, the maximum chordwise extent of laminar run was found to be insensitive to the suction level over a wide range.</div> <div class="htmlview paragraph">Evidence of “aerodynamic roughness” induced transition was found under strong suction applied through multiple rows of discrete holes.</div>

A series of test campaigns at DLR’s large flow meter (LFM) has been conducted to investigate the discharge characteristics of cylindrical throttle orifices, drilled in fiber-reinforced plastics, with respect to their diameter and manufacturing imperfections. In hybrid laminar flow control (HLFC) applications following the simplified so-called ALTTA concept, those throttle holes interfacing a common plenum to multiple chambers play an important role in regulating the local amount of suction through the porous wing surface. The work performed aims at the further development and industrialized application of HLFC in an aircraft environment. This question is tackled by the industry-oriented drilling of throttle holes into coupon-sized flat samples. Drilling parameters allowing a stable and reproducible process were established through optical inspection. The major experimental findings are discussed in this article alongside some general manufacturing recommendations for throttle holes in HLFC applications.

Aerodynamic drag reduction through a hybrid laminar flow control and variable camber coupled wing

The represented paper is a brief overview of some activities in laminar flow control area being performed in Central Aerohydrodynamic Institute named after prof. N.E. Zhukovsky (TsAGI) in Russia. These efforts are focused on the improvement of the existing laminar flow control methods and on the developm ent the new ones. The investigations have demonstrated effectiveness of aircraft surface laminarization application with the aim of friction drag r eduction. The opportunity of considerable delaying of laminar -turbulent transition due to special wing profil e geometry (natural laminar flow – NLF) and using boundary layer su ction and surface cooling (forced laminar flow control – FLFC) has been verified at sub - and supersonic speeds through various wind tunnels testing in TsAGI and during flying labor atory exp eriments in Flight Research Institute (LII). The investigations on using of hybrid laminar flow control (HLFC) system for friction drag reduction were carried out. The new techniques of laminar flow control were developed, in particular the method of local heating of the wing leading edge, boundary layer laminarization by means of receptivity control, electrohydrodynamic control of laminar -turbulent transition.

The energy efficiency of emerging aircraft designs plays a key role, not only in reducing environmental impact, but also in reducing operating costs in the anticipated rise in fuel prices. The European Clean Sky 2 project HLFC-Win is investigating the feasibility of hybrid laminar flow control (HLFC) technology integrated into the outer wing leading edge for a long-haul aircraft. HLFC technology reduces aerodynamic friction drag by means of suction of the boundary layer through a micro-perforated skin to achieve laminarity and thereby improving aircraft performance. However, integrating such a system is not without its drawbacks, as the integration has an impact on the geometry, mass, aerodynamics and engine offtakes that need to be considered. Therefore, the aim of this current work is to assess the HLFC system based on a fair, objective and transparent comparison between the HLFC aircraft and an aircraft of the same technology level without HLFC. The assessment of the HLFC system is twofold, firstly estimating the mission-based performance at the overall aircraft level and secondly performing a lifecycle simulation with three scenarios to determine realistic fuel and cost savings. The mission-based performance assessment indicates a block fuel reduction of over 3 % for the design mission which averages 1.6 to 2.5 % considering a realistic route scenario and expected degradation. The economic assessment suggests a dependency on the scenario chosen, ranging from a 0.7 % increase in total cost (in an unfavourable scenario) to almost a 1 % reduction in total cost (in a favourable scenario), equivalent to $15 million saved per HLFC aircraft over its lifetime. These results support the commercial viability of HLFC technology, which offers significant aerodynamic and fuel efficiency improvements and operating cost savings to the aviation industry. Importantly, no critical barriers were identified for the integration of HLFC technology, further underscoring its potential to improve aircraft performance.

Drag reduction experiments using boundary layer heating

The application of Hybrid Laminar Flow Control (HLFC) on transport aircraft is one of many promising fuel reduction technologies. Even though research and development around this topic is ongoing for some decades, HLFC has not been commercially deployed yet. One major factor is the insecurity of operators regarding the overall efficiency, especially due to the additional maintenance of the system and its components or the laminar effectivity under realistic operational conditions. In this study, we provide a quantification of the impact of several critical elements on the economic benefit of aircraft with HLFC throughout the lifecycle. These are additional line and base maintenance, degreadation due to cloud encounter and insect contamination as well as an increased aircraft price. The influence of each element is thoroughly analyzed and discussed. Results show that the overall economic benefit is significantly less sensitive towards variations of additional maintenance than towards different insect contamination and cloud encounter degradation scenarios. The highest sensitivity was found when varying the aircraft price of the HLFC aircraft. A final combined impact analysis revealed that under the realistic assumptions made in this study for all critical elements, the HLFC aircraft is still economically superior to its conventional turbulent counterpart.

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.

An investigation of surfactant and enzyme formulations for the alleviation of insect contamination on Hybrid Laminar Flow Control (HLFC) surfaces

Experimental investigation was carried out to study the effect of suction positions and suction flow rates on the aerodynamic performance of a compressor cascade with a large camber angle. The ink-trace flow visualization was conducted and the flow fields of the cascade were also measured. Three types of boundary layer suction configurations are compared, i.e. the suction surface suction, the endwall suction and the compound suction. Experimental results show that the large amount of suction flow rate gains more losses reduction than the small amount for a certain proper suction configuration, but the speed of loss decline slows down as the suction flow rate goes on increasing. Boundary layer suction on the suction surface obviously enhances the ability of the boundary layer around the midspan to withstand the negative pressure gradient of the flow passage. The range of the suction surface corner is also decreased. If the suction slot locates in the corner separation region where severe separation has happened, the flow separation will be terminated at the slot and redevelop downstream. And boundary layer suction on the endwall mainly influences the endwall corner region in remarkably delaying, lessening and reorganizing the corner separation. While the whole flow field is remarkably improved at both midspan and the corner region in the compound suction schemes. At higher suction flow rates, the aerodynamic performance of the compressor cascade is better than that with boundary layer suction simply on the suction surface or on the endwall. When the suction flow rate is 1.5% of the inlet mass flow, the compound suction scheme achieves a maximum loss reduction of 17% compared with the cascade without boundary layer suction.

A quasi-three-dimensional aerodynamic solver is developed for the aerodynamic analysis of wings in a transonic regime that is able to capture the effect of BLS in hybrid laminar flow control (HLFC) application or transition to turbulent flow for natural laminar flow (NLF). The tool provides accurate results, but without the high computational cost of high-fidelity tools. The solver combines the use of an Euler flow solver characterized by an integral boundary-layer method and linear stability analysis using a approximation for transition prediction. In particular, a conical transformation is adopted, including the determination of the shock-wave position. The solver is implemented in a multidisciplinary design optimization (MDO) framework, including wing weight estimation and aircraft performance analysis. The framework consists of different modules: aerodynamics, structure, suction system analysis, and performance evaluation. Using a genetic algorithm and considering HLFC technology, wing MDO has been performed to find the optimum wing planform and airfoil shape. A backward-swept wing (BSW) aircraft, developed inside the Cluster of Excellence–Sustainable and Energy Efficient Aviation (A) is studied. Novel technologies such as active flow control, limited maximum load factors due to load alleviation, and novel materials allow a fuel weight reduction of 6%.