A rational method for determining intermittency in the transitional boundary layer

A new rational procedure is proposed for determining the intermittency in the streamwise direction. One of the key parameters for the intermittency determination is the selection of a threshold value, which often involves a certain level of subjectivity. Here, a reliable way of choosing the threshold value in a more objective manner is proposed. The proposed approach involves a single threshold value, equal to the magnitude of the maximum laminar perturbation in the transitional flow. The results obtained are validated with the widely used dual-slope method. In this paper, the measurements are carried out on an experimental arrangement, involving the interaction of an upstream aerofoil wake with a downstream flat plate boundary layer. As a by-product of the study, a scaling parameter has been identified which captures the length of the transition zone as the proximity of the aerofoil in the wall-normal direction is varied

Convergence of calculated transition loci during computational analysis of transonic aerofoils and infinite swept wings

The effect of intermittent re-calculation of transition position is analysed using a low-order VII platform coupled to an Orr-Sommerfeld-type stability analysis method. Transonic aerofoil and infinite-swept wing flows were studied. Where the transition mechanisms are weakly dependent on the gross flow field characteristics, as few as ten transition updates may be required over a 1000- iteration CFD solution, if correctly timed

Skin friction drag reduction over staggered three dimensional cavities

The effect of three-dimensional staggered circular cavities on a zero-pressure gradient incompressible turbulent boundary layer was studied. Two key parameters were varied, being the ratio of the diameter, d, to the depth, h, of the cavity, d/h and the Reynolds number based on the diameter of the cavity, Rd. Velocity profile measurements showed that for the cases of d/h>1 an increase in skin friction drag was experienced with respect to a smooth surface, but for d/h≤1 the drag increment was almost negligible and in some cases it was lower than that of a smooth surface by up to 10%. Measurements along the spanwise plane showed the presence of organised transverse velocity components which bear some resemblance with the flow over riblets. The skin friction drag appears to be a strong function of Rd, where for Rd>5500 a drag increment is experienced which could potentially be due to shear layer breakdown and more production of turbulence

Predicting the mission performance of a retrofit Hybrid Laminar Flow Control system

A technique is presented for assessing the cruise performance of an aircraft employing a hybrid laminar flow control (HLFC) system. Two-dimensional HLFC designs based on N-factor control of chamber pressures are extrapolated to a complete aircraft, leading to a full analysis of the potential drag reduction as well as the system power and weight penalties. These can remove over 30% of the aerodynamic drag benefit delivered by laminar flow. Simple trapeziumshaped suction distributions reduce the benefit still further, but the use of non-local stability methods would suggest a reduction of nearly 25% in suction requirements, increasing the net drag benefit by 10%. Modifications to the wing geometry indicate that changes favourable to laminar flow nevertheless introduce unacceptably large wave drag penalties. The most promising direction for future research appears to be extending the extent of the suction control system. Extrapolating the predicted HLFC system performance to the entire wing upper surface, horizontal tail plane and fin would suggest a potential 6½ - 7% reduction of total aircraft drag for the A310 at maximum L/D

New aerodynamic approaches to suction system design

A new approach to the aerodynamic design of Hybrid Laminar Flow Control suction systems is presented. The definition of suction chamber layout and pressures has been closely coupled with the boundary layer and stability analysis methodology to provide a numerical tool to help in the design of a suction system. The new approach also provides a direct link between the cost functions of suction system mass and power with the aerodynamic drag benefit, yielding a more streamlined design procedure. Practical constraints appear at an early stage in the process rather than late in the day after much effort has been expended. To demonstrate the power of the technique, the advantages and penalties associated with two different chamber layouts are discussed. Further research is required into the control of crossflow instability and the oversuction phenomenon before the method can be fully exploited

Performance trade-off studies for a retrofit Hybrid Laminar Flow Control system

Recent work in the UK, studying the possible retrofit of a hybrid laminar flow control (HLFC) system to a medium-sized aircraft, is reviewed. The key feature of the work was the use of robust boundary layer tools to design HLFC systems based on direct control of Nfactors using a discrete suction chamber technique. The improved HLFC designs were applied to a representative aircraft configuration, leading to a significant reduction in predicted suction mass flow rates, and therefore in pump power requirements and suction system weight. The use of PSE methods to assess suction requirements was further found to reduce predicted suction rates by nearly 25%, increasing the net drag benefit by 10%. Modifications to the wing geometry that were advantageous for laminar flow usually introduced unacceptably large wave drag penalties: the most promising direction for future research therefore appears to be increasing the chord-wise extent of the suction control system. Nevertheless, extrapolating the predicted retrofit HLFC system performance to the entire wing upper surface, tailplane and fin would suggest a potential 6½ - 7% reduction of total aircraft drag for the representative aircraft at cruise

Progress in linear stability methods for design applications

Despite recent developments in the understanding of boundary layer receptivity and non-linear stability, linear stability methods remain the state-of-the-art in industry for aerodynamic design and analysis. A conceptual model is presented to explain why the eN approach is used and the circumstances under which it might be expected to work. The paper reviews the latest results and conclusions from a series of recent collaborative projects, supported by the European Commission, which have contributed significantly to the confidence and ease with which linear stability methods can now be used for design. Recent experimental work has allowed local, linear stability N-factor correlations to be derived, for the first time in Europe, for HLF systems. A range of N-factor integration strategies have been evaluated during the analysis of these experiments. The use of non-local theory has demonstrated a significant effect on crossflow N-factors which warrants further, systematic correlation of these N-factors against experiment. The authors feel that the use of advanced non-linear transition prediction techniques can be used to provide guidance in the avoidance of pathological situations in the design of commercial HLF systems, but that linear stability theory is today's best tool for design purposes. Database methods derived from linear theory can considerably accelerate the design process provided that they are validated appropriately against stability computations

Systems driven HLFC design

The paper presents an aerodynamic study carried out in parallel with the EU AFLoNext project to assess the issues involved in combining hybrid laminar flow control (HLFC) technology for drag reduction with wing ice protection systems (WIPS). The paper describes the selection of appropriate test cases in the literature which are representative of wings designed for HLFC system and the progression from an initial baseline HLFC chamber layout to layouts driven by practical constraints such as WIPS requirements and aircraft structure. The resulting HLFC system is a compromise between all concerned systems. Conclusions are drawn about design driven not purely by performance but by the ability to physically implement the system on a commercial aircraft

Transition modelling for viscous flow prediction.

The paper describes a method for the calculation of viscous flows that includes the prediction of the transition onset location. It is an engineering approach aimed at providing a useful and necessary tool for air vehicle design and assessment. Some background that led to the adoption of the present approach is first given and the method is then described. Particular problems associated with the approach are identified and solutions are provided. Results from the initial validation of the method for both two- and three-dimensional flows are presented. The results illustrate the effects on flow-field predictions of modelling transition and the need to incorporate transition modelling in CFD methods is clearly demonstrated. The results also illustrate that the improved modelling can lead to a better understanding of the complex flows associated with some recent air vehicle designs, and the sensitivity of those flows to transition

Lean Six-Sigma in Aviation Safety: An implementation guide for measuring aviation system’s safety performance

The paper introduces a conceptual framework that could improve the safety performance measurement process and ultimately the aviation system safety performance. The framework provides an implementation guide on how organisations could design and develop a proactive, measurement tool for assessing and measuring the Acceptable Level of Safety Performance (ALoSP) at sigma (σ) level, a statistical measurement unit. In fact, the methodology adapts and combines quality management tools, a leading indicators programme and Lean-Six Sigma methodology to formally measure and continuously improve a stable and in-control safety management process by reducing safety defects and variability from core organisational processes and objectives. The implementation guide was empirically tested and validated with data collected and analysed within a period of nine months by the safety department of a complex aviation organisation operating a large transport aircraft fleet