Leading Edge Contamination Research Articles

Hydrodynamic optimization of high-performance blade sections for stall regulated hydrokinetic turbines using Differential Evolution Algorithm

Hydrokinetic turbines are electromechanical devices, which convert the kinetic energy of freely flowing water into electricity without accumulating water. Turbine blades are generated by optimally combining one or more types of blade sections. The performance of blade sections is quite important directly affecting the power coefficient of the rotor. Since hydrokinetic technology is relatively a new branch of renewable energy, blade sections optimized for wind turbines or aviation applications had hitherto been used. However, hydrodynamics of water should be better considered during design processes. The main scope of this study is to optimize blade sections specifically for stall regulated hydrokinetic turbines considering high hydrodynamic forces, cavitation, leading-edge contamination, and ideal stall behavior. Differential Evolution Algorithm (DEA) was employed for the optimization process. Five different primary hydrofoil families were optimized and they have been scaled for various regions along the blade. Lift, drag, transition, and pressure coefficient performances of optimized sections have been analyzed and discussed with mostly used NACA, RISØ, and NREL sections. The optimized hydrofoils are found to deliver quite successful performance for hydrokinetic turbines based on design objectives, constraints, and comparing to the most preferred sections.

Critical Protuberance Height for Transition of Leading Edge Boundary Layers

Leading edge contamination on aircraft wings can occur through the accumulation of insect residues, ice, or dirt or through existing surface defects. Such protuberances can increase skin friction drag by causing an earlier transition of the boundary layer from laminar to turbulent flow. To determine the likelihood of this effect, a simple and approximate model was developed for determining the critical height for boundary-layer transition on the airfoil leading edge. The model assumes an outer potential flow based on an effective leading edge radius coupled with an inner flow based on semi-empirical laminar boundary profiles. The effective leading edge radius was based on leading edge pressure distribution as related to flow over a cylinder and tends to the geometric leading edge radius for the limits of no camber and zero angle of attack. The model was compared with available experimental transition results, which showed that the predictions are reasonable but approximate. The potential susceptibility of various airfoils to transition was next considered using sample insect residues heights. The results indicate that laminar flow airfoils for large commercial aircraft would be significantly susceptible to transition due to insect contamination, whereas high-altitude low-speed UAVs would generally not have such susceptibility.

Calculation of the power output loss based on thermographic measurement of the leading edge condition

Due to operation under harsh conditions, rotor blades suffer from soiling and erosion of the leading edge, which causes a premature laminar-turbulent transition of the boundary layer flow. A quantification of both the extent of the contamination and the impact on wind turbine performance is difficult and calculations are based on estimates. A new method for assessing the power output loss due to leading edge contamination is presented. The method is based on thermographic flow measurements along the rotor blade and the automated determination of the laminar-turbulent transition location. A comparison with the expected natural transition of the clean rotor blade position enables the extent of the leading edge contamination to be quantified. Measurements on a multi-MW rotor blade indicate a contamination level of up to 90.4 % in the given highly contaminated example. This information is then used in a blade element method (BEM) model to calculate the annual energy production (AEP) for both a clean and a contaminated case. Results indicate that for this particular case, the measured contamination level leads to a decrease in annual energy production of 4.7 % to 2.7 % for the investigated average wind speeds of 6 m/s to 9 m/s.

Airfoil optimization for wind turbine application

AbstractAn airfoil optimization method for wind turbine applications that controls the loss in performance due to leading edge contamination is developed and tested. The method uses the class‐shape‐transformation technique to parametrize the airfoil geometry and uses an adjusted version of the panel code XFOIL to calculate the aerodynamic performance. To find optimal airfoil shapes, the derivative‐free Covariance Matrix Adaptation Evolution Strategy is used in combination with an adaptive penalty function. The method is tested for the design of airfoils for the outer part of a megawatt‐class wind turbine rotor blade, and the results are compared with airfoils from Delft University. It is found that the method is able to automatically create airfoils with equal or improved performance compared with the Delft designs. For the tested application, the adjustments performed to the XFOIL code improve the maximum lift, post stall, and the overall drag predictions.

Fruit fly impact on an aerodynamic surface: Types of outcomes and residue components

Leading edge contamination on aircraft wings and wind turbine blades can occur through the accumulation of insect residues, which can then increase drag by causing an earlier transition of the boundary layer from laminar to turbulence. To investigate the mechanisms of residue adherence and composition, this study focuses on the impact dynamics of insects striking an airfoil with a circular leading edge placed in a wind tunnel at a wind speed of about 100 mph. To mimic in-flight conditions, live flightless fruit flies (Drosophila melanogaster) were injected into the wind tunnel sufficiently upstream such that they reached the test section air-flow speed prior to impact on the airfoil. The aerodynamic surface had a leading edge radius of 3.8 cm and was tested with coatings of various surface wettability, ranging from hydrophilic to superhydrophobic. Based on previous studies and present high-speed imaging and surface imaging, five types of insect outcomes were defined (roughly in order of increasing impact speed): 1) insect bounce without rupture and no residue, 2) insect burst with release of internal fluid with adherence of full exoskeleton, 3) insect burst with release of internal fluid with little or no exoskeleton adherence, 4) disintegration of insect yielding residue of many pieces of exoskeleton, and 5) insect burst with neither fluid nor exoskeleton residue. The fifth outcome generally occurred for a superhydrophobic surface developed in this study. When insect residue did occur on a surface, it included up to three distinct components (in order of decreasing residue heights): exoskeleton pieces, yellow hemolymph from the insect abdomen and thorax, and red fluid from the insect head. Surfaces with lower surface wettability generally yielded reduced insect hemolymph residue area per insect release. In particular, a superhydrophobic coating yielded the lowest residue area and no exoskeleton residue, while the aluminum surface resulted in the highest residue area and several exoskeletons.

Characterization of a highly efficient chevron-shaped anti-contamination device

This paper is devoted to the characterization of an optimized chevron-shaped anti-contamination device (ACD). This device can prevent efficiently the propagation of turbulence from the fuselage along the attachment line (hypothetical streamline that spreads the flow going to suction side and the one going to pressure side) of swept wings and enables the development of a new laminar boundary layer downstream. More specifically, the aim is to prevent boundary-layer transition along the attachment line by a contamination process. This process is characterized by the typical Reynolds number $$\overline{R}$$ and the associated Poll’s criterion. Thus, ACD efficiency will be expressed in terms of $$\overline{R}$$ values. Some experiments performed on a new numerically optimized ACD have shown its ability to prevent leading-edge contamination up to $$\overline{R}$$ values close to the natural transition process of the laminar boundary layer along the attachment line. The corresponding stability analysis of the laminar boundary layer is made using the Gortler–Hammerlin stability approach. The study is completed with the different transition processes that can occur downstream the attachment line, around the airfoil, especially with crossflow analysis.

Constrained multi-objective aerofoil design using a multi-level optimisation strategy

AbstractA novel approach for the multi-objective design optimisation of aerofoil profiles is presented. The proposed method aims to exploit the relative strengths of global and local optimisation algorithms, whilst using surrogate models to limit the number of computationally expensive CFD simulations required. The local search stage utilises a re-parameterisation scheme that increases the flexibility of the geometry description by iteratively increasing the number of design variables, enabling superior designs to be generated with minimal user intervention. Capability of the algorithm is demonstrated via the conceptual design of aerofoil sections for use on a lightweight laminar flow business jet. The design case is formulated to account for take-off performance while reducing sensitivity to leading edge contamination. The algorithm successfully manipulates boundary layer transition location to provide a potential set of aerofoils that represent the trade-offs between drag at cruise and climb conditions in the presence of a challenging constraint set. Variations in the underlying flow physics between Pareto-optimal aerofoils are examined to aid understanding of the mechanisms that drive the trade-offs in objective functions.

Influence of surface characteristics on insect residue adhesion to aircraft leading edge surfaces

Leading edge contamination caused by insects is problematic for modern aircraft utilizing laminar flow aerofoils. The residue of crushed insect bodies adhering to aircraft leading edge surfaces can cause transition of the boundary layer, from laminar to turbulent, resulting in a significant increase in drag and therefore causing an increase in fuel consumption. Consequently, current research is focused on the evaluation of novel low surface energy coatings that will reduce or prevent insect adhesion. Insect residue adhesion tests were conducted on a range of surfaces, from superhydrophobic to hydrophilic. Surface free energy of the investigated substrates was obtained from measured dynamic contact angle values and surface roughness was measured using profilometry. Live insect testing with Drosophila melanogaster and Drosophila hydei was conducted using an insect delivery device inserted into a medium-speed wind tunnel. Tests were conducted at speeds ranging from 90 to 100m/s (speeds representative of those on take-off and landing of a commercial passenger aircraft). Topography of insect residues was characterized using scanning electron microscopy and confocal laser scanning microscopy. Results obtained indicate that coatings with high surface roughness values and low wettability exhibit good anti-contamination properties.

Blade sections for wind turbine and tidal current turbine applications-current status and future challenges

The designers of horizontal axis wind turbines and tidal current turbines are increasingly focusing their attention on the design of blade sections appropriate for specific applications. In modern large wind turbines, the blade tip is designed using a thin airfoil for high lift : drag ratio, and the root region is designed using a thick version of the same airfoil for structural support. A high lift to drag ratio is a generally accepted requirement; however, although a reduction in the drag coefficient directly contributes to a higher aerodynamic efficiency, an increase in the lift coefficient does not have a significant contribution to the torque, as it is only a small component of lift that increases the tangential force while the larger component increases the thrust, necessitating an optimization. An airfoil with a curvature close to the leading edge that contributes more to the rotation will be a good choice; however, it is still a challenge to design such an airfoil. The design of special purpose airfoils started with LS and SERI airfoils, which are followed by many series of airfoils, including the new CAS airfoils. After nearly two decades of extensive research, a number of airfoils are available; however, majority of them are thick airfoils as the strength is still a major concern. Many of these still show deterioration in performance with leading edge contamination. Similarly, a change in the freestream turbulence level affects the performance of the blade. A number of active and passive flow control devices have been proposed and tested to improve the performance of blades/turbines. The structural requirements for tidal current turbines tend to lead to thicker sections, particularly near the root, which will cause a higher drag coefficient. A bigger challenge in the design of blades for these turbines is to avoid cavitation (which also leads to thicker sections) and still obtain an acceptably high lift coefficient. Another challenge for the designers is to design blades that give consistent output at varying flow conditions with a simple control system. The performance of a rotating blade may be significantly different from a non-rotating blade, which requires that the design process should continue till the blade is tested under different operating conditions. Copyright © 2012 John Wiley & Sons, Ltd.

Transition measurement and prediction on a generic high-lift swept wing

This article presents experimental and numerical studies on the effect of the Reynolds number on transition location for a simplified multi-element wing. The main phases of the test preparation and the main results of the data analysis carried out within the EUROLIFT EC funded programme are given in the first part. In the second part of the article, recent assessments of the ONERA elsA CFD software where transition is computed during the Navier-Stokes calculation are presented for both the effect of change in incidence at a given Reynolds number and the change in Reynolds number at constant angle of attack. The upper slat region is the main concern of this article, as this element was better equipped and analysed after the experiments. Flap transition is also considered in the computations. For themain wing, turbulent flow is imposed in the computations, as leading-edge contamination occurred in the experiments.

Subcritical instability on the attachment-line of an infinite swept wing

The leading-edge contamination (LEC) problem of an infinite swept wing is shown here as vortex-induced instability. The governing equation for receptivity is presented for LEC in terms of disturbance energy based on the Navier-Stokes equation. The unperturbed shear layer given by the swept Hiemenz boundary-layer solution is two-dimensional and an exact solution of incompressible the Navier-Stokes equation. Thus, the LEC problem is solved numerically by solving the full two-dimensional Navier-Stokes equation. The contamination at the attachment-line is shown by solving a receptivity to a convecting vortex moving outside the attachment-line boundary layer, which triggers subcritical spatio-temporal instability

Crossflow Induced Transition Prediction Using Coupled Navier-Stokes eN Method Computations

The flow around two infinite swept wings and an infinite swept cylinder model is computed using a Reynolds averaged Navier-Stokes method coupled to a boundary-layer and transition prediction method based on the e(N) approach. The configurations were wind-tunnel tested, including transition location measurements. In all test cases, transition is provoked purely by crossflow instabilities. The infinite swept wing experiments represent critical test cases because they contradict two assumptions made in the conventional e(N) approach, where the limiting cross-flow N factor is supposed to be independent of the surface curvature and the quality of the wing surface finish. It will be shown that the extended version of the e(N)- method can resolve this problem and produce acceptable results. The infinite swept cylinder model tested at high sweep angles between 53 and 71 deg is excellently suited to demonstrate that the e(N)-method is capable of describing the leading-edge contamination problem. For sweep angles between 53 and 58 deg, a relative constant limiting crossflow N factor exists, whereas for higher sweep angles the crossflow N factors at the measured transition location decrease gradually. Applying the leading-edge contamination criterion of Pfenninger, the e(N)-method predicts the sweep angle, where leading-edge contamination starts to influence the laminar boundary-layer flow. Furthermore, the sweep angle, which produces, according to an empirical correlation, fully turbulent leading-edge flows, is well predicted by the e(N)-method.

Attachment-Line Approach for Design of a Wing-Body Leading-Edge Fairing

A simplified procedure is presented for the design of a leading-edge fairing of a wing-body combination. The design aims at optimizing the design of the fairing, which is to serve the double purpose of eliminating separation near the wing-body junction, as well as minimizing leading-edge contamination of the laminar wing. The design optimization procedure is based on the analysis of the viscous-flow performance of a fairing of prescribed geometry. First, a panel method is used to determine the inviscid flow around the fairing. This is then followed by an integral-method calculation of the boundary-layer development on the attachment line along the body and the fairing. As an application, a fairing was designed for a straight NACA0015 wing mounted on a flat plate. Tests in the wind tunnel confirmed the effectiveness of the fairing

Computation of leading-edge contamination

The leading-edge contamination (LEC) problem of an infinite-swept wing is computed by solving the full two-dimensional Navier–Stokes equation using a very high spectral accuracy compact scheme. The initial condition is given by the steady attachment-line boundary layer forming over the leading-edge of an infinite swept wing or a yawed cylinder. The contamination is shown as due to either a convecting or a stationary vortex far outside the attachment-line boundary layer at the leading edge. This mechanism is proposed to account for the sub-critical instability onset observed for attachment-line boundary layer. (The criticality is for Reynolds number (based on momentum thickness) exceeding 100––an empirically suggested value based on earlier experiments.) The presented results show the problem to be essentially two-dimensional. Inflow and outflow of computational domain are chosen so that we consider cases of both sub- and super-critical instabilities. The receptivity mechanism of leading-edge contamination for both cases is investigated with respect to different parameters. Finally, we have also analyzed the computed solutions by proper orthogonal decomposition (POD) to highlight the low-dimensional nature and to provide a quantitative measure of leading-edge contamination events.

Transition measurement and analysis on a swept wing in high lift configuration

In the framework of the European Research Program EUROLIFT (European High Lift Program), the ONERA experimental contribution is a basic experiment to study the transition phenomenon on a swept model equipped with a slat and a flap in high lift configuration. In preparation for the tests, the 3D high lift configuration has been computed at ONERA with a Navier Stokes code to get pressure distributions on the different elements. Preliminary transition prediction work was performed by different EUROLIFT partners (FOI, Airbus-D and ONERA), taking into account leading edge contamination and using the ONERA database method. The tests have been performed in the F1 ONERA low speed pressurised wind tunnel at two sweep angles and different Reynolds numbers. The model well equipped in pressure taps was completed by devices to determine the transition location (infrared thermography and hot films). The tests results have been analysed and the transition determined for all the elements, tested in the different configurations. Moreover we tried to analyse the transition behaviour under the Reynolds number influence in order to illustrate the different transition mechanisms found on this high lift generic model.

Effect of wall suction on leading edge contamination

This paper is devoted to an experimental study of swept wing leading edge contamination by the turbulence emanating from the wing-wall junction. The main objective is to delay the contamination onset by applying surface suction along the attachment line. Two series of experiments are described; the first one was performed in a small wind tunnel at CERT ONERA, the second one was carried out in the F2 wind tunnel at Le Fauga Mauzac centre. Hot film measurements showed that leading edge contamination could be delayed up to very large Reynolds numbers. We also studied the behaviour of the relaminarized boundary layer downstream of the sucked region, along the span as well as in the chordwise direction.

20] Double mixing methods for kinetic studies of ligand binding in partially liganded intermediates of hemoglobin

This chapter focuses on the differences between the two methods of double mixing stopped-flow and single mixing, the type of reactions that can be studied by the double mixing method, experimental design, data treatment, and a brief discussion of the results obtained. Unless otherwise mentioned, all examples discussed in the chapter are for normal human hemoglobin (Hb). The double-mixing method , along with the micro peroxidase method for studying carbon mono oxide (CO) dissociation reactions and the high pressure liquid chromatography (HPLC) method for preparation of the valency hybrids, allows for the study of kinetics of CO association with, and dissociation from, partially liganded intermediates of Hb. All of these studies so far have been made by only two groups and, therefore, a certain degree of bias in perspective may have been unavoidable, particularly in formulating reaction schemes to fit the data. These studies have been made using a first-generation double mixing stopped-flow spectrophotometer with limited capabilities, such as a long dead time, use of large sample volumes for each kinetic shot, and some probability of leading-edge contamination at longer aging times. More information and precision can be obtained by using state-of-the-art double-mixing instruments.