Conceptual Design of Boundary Layer Suction System for Energy Efficient Laminar Flow Control

Hybrid Laminar Flow Control (HLFC) is one of the technologies that shows significant drag reduction potential for the aircraft. Even though the concept of HLFC was proven in the 1960s, a reliable, economically feasible, and energy-efficient system is still not commercially used. One of the main reasons that restricts the use of HLFC in commercial aviation is the complex system design, which can lead to additional power and mass requirements. As a solution to this problem, the thesis proposes a numerical tool for the systematic design of a suction system for laminar flow control application. The proposed design tool, named Actual Suction and Power Calculation Tool (ASPeCT), uses a modular architecture and incorporates global and local level dependencies through a multi-level input module. Three sub-routines are developed within ASPeCT to design the suction system layout and estimate the suction power and weight requirement of the system. The present study hypothesizes that by using the proposed suction system design tool, the actual benefits in terms of suction power and system weight could be derived during the conceptual design phase, leading to potential higher actual benefits and easier suction system integration to the aircraft wing. As a first step towards testing the hypothesis, a novel suction system concept is proposed. The new suction system consists of a microperforated outer sheet supported on an inner structure and a plenum chamber driven by the compressor. The inner structure is composed of straight stringers with throttle holes provided on it. These throttle holes provide additional control on the static pressure below the perforated sheet that drives the boundary layer suction. The diameter of the throttle holes can be numerically estimated to match any desired target suction velocity distribution based on the pressure loss characteristics of the suction system components. A new flowbench test stand is designed to determine these pressure loss characteristics. Eight micro-perforated sheet samples manufactured using Stereolithography (SLA), Etching, Micro-laser drilling, and Selective Laser Melting (SLM) are measured in the current study. The measurement data shows a significant deviation from analytical models based on straight cylindrical holes, especially for smaller perforation diameters in the range of 60-120 µm. Two different modelling approaches are then considered to determine the effect of perforation shape and flow behaviour. A single-hole modelling approach is used to study the influence of the perforation shape and size. A continuum modelling approach based on the Bohning-Doerffer (B-D) model is used to study the effect of flow behaviour. The single-hole modelling approach shows that the conicity of the perforation affects the pressure loss characteristics, especially at low Reynolds numbers based on hole diameter and velocity through the single hole. A continuum modelling approach shows that in the Darcy regime (Re < 1), the B-D models fail to predict the pressure loss accurately. However, in the inertial regime (Re > 1), the experimental data agrees well with the B-D model. Wind tunnel experiments are then carried out to estimate the pressure loss through the micro-perforated sheet when suction takes place from the boundary layer. The measurements reveal that the shear effect of the boundary layer on the wall imparts an additional pressure loss for the micro-perforated sheet. The original B-D model fails to predict the additional pressure loss accurately. Therefore, new coefficients are determined based on the curve fit of the experimental data. The present study formulates a reduced-order model based on the Priest model as well as the B-D model to model the total pressure loss through the micro-perforated sheet accurately. A similar modelling approach is also followed to model the pressure loss through the throttle holes of the inner structure. An inner structure with throttle hole variation similar to that expected in the HLFC system is measured to validate the reduced-order models. Two case studies are presented using the developed toolchain ASPeCT to showcase the functionality. The first case study deals with the design of the suction system for the eXtended Hybrid Laminar Flow Control (xHLFC) wing for subsonic flight conditions. The impact of the pressure characteristics of the micro-perforated sheet and inner structure on the total power requirement of the suction system is estimated. The case study reveals that the micro-perforated sheet accounts for less than 5% of the total suction power. The contribution from the inner structure is approximately 15% of the total suction power. The remaining significant amount of the suction power is used to re-energize the sucked air back to freestream conditions. The second case study discusses the suction system design for swept wings at transonic flight conditions. The suction system design is performed on an aerodynamically optimized backward-swept wing of the SE2A mid-range aircraft for a cruise Mach number of 0.71. Four suction panels in the spanwise direction of the wing are designed using ASPeCT. ASPeCT predicts a maximum fuel reduction of 7% for the aircraft with the use of the HLFC system, even accounting for the additional weight and power off-take from the engine. Finally, the suction power of different airfoils from the final Pareto front is compared to that of the minimum total drag airfoil selected by the aerodynamic wing optimizer. The selection of a different airfoil based on the analysis leads to a reduction of 1 kW in suction power for an approximately similar total drag coefficient. The lower suction power corresponds to significant savings in compressor weight, leading to an increase in actual benefit and energy efficiency of the aircraft.