Rough Airfoils Research Articles

Numerical Analysis of Leading-Edge Roughness Effects on the Aerodynamic Performance of a Thick Wind Turbine Airfoil

The aerodynamic performance of wind turbine airfoils is crucial for the efficiency and reliability of wind energy systems, with leading-edge roughness significantly impacting blade performance. This study conducts numerical simulations on the DU 00-W-401 airfoil to investigate the effects of leading-edge roughness. Results reveal that the rough airfoil exhibits a distinctive “N”-shaped lift coefficient curve. The formation mechanism of this nonlinear lift curve is primarily attributed to the development of the trailing-edge separation vortex and variations in the adverse pressure gradient from the maximum thickness position to the trailing-edge confluence. The impact of different roughness heights is further investigated. It is discovered that when the roughness height is higher than 0.3 mm, the boundary layer can be considered fully turbulent, and the lift curve shows the “N” shape stably. When the roughness height is between 0.07 mm and 0.1 mm, a transitional state can be observed, with several saltation points in the lift curve. The main characteristics of different flow regimes based on different lift curve segments are summarized. This research enhances the understanding of the effects of leading-edge roughness on the aerodynamic performance of a thick wind turbine airfoil, and the simulation method for considering the effect of leading-edge roughness is practical to be applied on large-scale wind turbine blade to estimate the aerodynamic performance under rough leading-edge conditions, thereby supporting advancements in wind turbine technology and promoting the broader adoption of renewable energy.

Ice accretion simulation incorporating roughness effects and separation correction through a transition model

In the simulations of ice accretion, accurately modeling the impact of surface roughness and calculating the separated flow are crucial aspects. A modified laminar–turbulent transition model that incorporates both separation and roughness-induced transition correction is employed for icing simulations. An iced airfoil was selected to validate the modified model's predictive capability for large separation, and a rough airfoil was utilized to assess the modified model's capability in simulating roughness effects. Icing simulations under different conditions are conducted for an airfoil. The results indicate that roughness induces an earlier laminar–turbulent transition, thereby influencing the heat transfer coefficients. The effects of separation and roughness correction on rime icing simulation are minimal, but they are significant for glaze icing simulation, resulting in more accurate predictions of maximum ice thickness and ice horn orientation. The simulated glaze ice causes more pronounced separation and more substantial reductions in lift compared to simulated rime ice. The modified model can accurately predict the separation bubble and reduce the prediction error of lift over 60%.

Aerodynamic and Flow Characteristics of a Rough Airfoil: A Numerical Study

A computational study explores the aerodynamic and flow properties of rough-surfaced NACA-0018 airfoils using turbulent K-omega SST methodology and the control volume method. Ansys Fluent 2022 R2 was used to do the simulations, and Reynolds numbers between 5000 and 10000 were used. The analysis of how the rough surface affects the performance of the airfoil was the main goal of the study. The skin friction coefficient, drag force, lift force, pressure contours, and stream functions were among the many variables assessed. To have a thorough understanding of the flow behavior, these parameters were studied at various Reynolds numbers. The findings revealed a clear pattern: along the wall of the airfoil, the average drag force, lift force, and skin friction coefficient all increased linearly as the Reynolds number rose from 5000 to 10000. This discovery sheds important light on how the rough surface affects the overall aerodynamic performance of the airfoil. This study advances knowledge of the aerodynamic behavior of rough airfoils by illuminating the flow characteristics and their relationship to surface roughness. Significant results impact aeronautics, wind turbine design, and other airfoil applications.

Aerodynamic Exploration on Rough Airfoil Based on Overlapping Feathers of a Swift-Wing Structure

To investigate the flow mechanism of feather-like rough airfoils based on swift wings, computational simulations were employed to explore their overall aerodynamic characteristics in comparison to equivalent smooth airfoils. The study focused on angles of attack ranging from 0° to 20° at low Reynolds numbers. The results reveal that the rough airfoil exhibits higher lift and lower drag compared to the smooth airfoil at moderate angles of attack ranging from 6° to 10°, resulting in significantly improved aerodynamic efficiency. Notably, at an angle of attack of 8°, the aerodynamic efficiency is increased by 19%. However, at angles of attack smaller than 6°, the increase in drag outweighs the increase in lift, leading to lower aerodynamic efficiency for the rough airfoil. Conversely, when the angle of attack exceeds 16°, both airfoils experience separated flow-dominated flow fields, resulting in comparable effective aerodynamic shapes and similar aerodynamic efficiencies. Furthermore, the study found that increasing the Reynolds number results in greater pressure differences in the flow field, leading to higher aerodynamic efficiency. These preliminary conclusions are valuable for elucidating the flight mechanisms of bird-feather-like wings and can inform the design or morphing design of bio-inspired micro aerial vehicles in the near future.

Heat Transfer Correlations for Smooth and Rough Airfoils

Low-fidelity methods such as the Blade Element Momentum Theory frequently provide rotor aerodynamic performances. However, these methods must be coupled to databases or correlations to compute heat transfer. The literature lacks correlations to compute the average heat transfer around airfoil. The present study develops correlations for an average heat transfer over smooth and rough airfoil. The correlation coefficients were obtained from a CFD database using RANS equations and the Spalart–Allmaras turbulent model. This work studies the NACA 0009, NACA 0012, and NACA 0015 with and without the leading roughness representative of a small ice accretion. The numerical results are validated against lift and drag coefficients from the literature. The heat transfer at the stagnation point compares well with the experimental results. The database indicates a negligible dependency on airfoil thickness. The work presents two correlations from the database analysis: one for the smooth airfoils and one for the rough airfoils. For the zero lift coefficient, the average Nusselt number is maximum. This increases with Re0.636 for the smooth surface and with Re0.85 for the rough surface. As the lift increases, the average Nusselt is reduced by values proportional to the square of the lift coefficient for the smooth surface, while it is reduced by values proportional to Re and the square of the lift coefficient for the rough surface.

Performance analysis of wind turbines with leading-edge erosion and erosion-safe mode operation

For offshore wind turbines, Leading-Edge Erosion (LEE) due to rain is posing a serious risk to structural integrity and can lead to a performance loss of the order of a few percent of the Annual Energy Production (AEP). A proposed mitigation strategy is the so-called Erosion-Safe Mode (ESM). In this work, the AEP losses caused by LEE or by operating in the ESM are compared for two reference turbines, i.e. the IEA 15MW and the NREL 5MW turbines. For both turbines, the performance is evaluated in uniform and sheared inflow conditions. The effects of erosion are modeled by creating clean and rough airfoil polars in XFOIL. It is assumed that erosion occurs once a critical blade element section speed is exceeded. Power curves for LEE and ESM are calculated by using the free-wake vortex method CACTUS. Results show that LEE negatively affects the power production below rated capacity, while operating in ESM predominantly sheds performance at rated power of the turbine. This study, therefore, shows that a break-even point for the ESM exists. The AEP loss due to erosion can be successfully mitigated with the ESM at sites with low mean wind speed, however, at sites with higher mean wind speed, operation with erosion leads to a lower AEP loss. The break-even point shows little sensitivity to the blade design and to mean shear variations, but strongly depends on the frequency ESM needs to be applied. The latter is driven by the predicted amount of damaging rain events. In conclusion, erosion-optimal operation is strongly governed by the site characteristics and much less by turbine design, and the viability of an ESM strategy can be significantly expanded by a better understanding of blade damage mechanisms and improved forecasting of the related weather events.

INFLUENCE OF SURFACE ROUGHNESS ON THE DYNAMIC STALL OF A ROTARY WING SECTION IN SUBSONIC FLOW

Wear involves the losses of some material from the surface and leads to the surface roughness. In the present work, the influence of surface roughness on the dynamic stall of a rotary wing section in subsonic flow has been investigated with different degree of surface roughness. The airfoil surface roughness was estimated using arithmetic mean roughness value (ISO-4287/1). For this purpose an indicial response method for unsteady, two dimensional flow is used to calculating the unsteady lift and pitching moment of NACA 0012 airfoil undergoing a pitching oscillation in the deep dynamic stall regime. Using the ANSYS-5.8 software as a source for the preliminary static data that required by the indicial method. The investigation is done through a number of test cases with different mean angles of attack, amplitude and reduced frequencies for airfoil oscillating around its quarter chord axis. The results include the time dependent behaviour and hysteresis loops of the lift and pitching moment for the airfoil with different degree of surface roughness. For rough airfoil an aerodynamic characteristic showed a reduction in the lift coefficient, separation point moved forward to the leading edge, the boundary layer reattachment take place at smaller angle of attack, the stall angle decrease and the pitching moment coefficient decrease as compared with smoother airfoil. So the periodic test must be done and re-coating the rotary wing if the surface roughness increased.

CFD investigation on the maximum lift coefficient degradation of rough airfoils

<abstract> Ice accretion can reduce the performance of aircraft's wings, which results in higher fuel consumption and risk of accidents. Experiments proved that even in its very earlier stages (increased roughness), icing could cause a reduction of 25% in the maximum lift, and an increase of 90% in drag of an aspect ratio 6 wing. In this work, we propose a correlation to predict the degradation of the maximum lift coefficient caused by roughness effects on flows over two airfoils, a NACA 0012 and a model 5–6. In addition, a second correlation is proposed to find the minimum Reynolds number that are useful for higher Reynolds number applications when roughness is considered. The SA roughness extension is implemented into an open-source code called SU2. The verification and validation of the implementation is performed in two steps. First, the behavior of the flow over a smooth NACA 0012 is investigated to confirm whether the implementation has no influence on the original model when roughness is not activated. Then, roughness is activated, and estimations of lift coefficients and velocity profiles inside the boundary layer are evaluated and compared to numerical and experimental results. Finally, investigations on the maximum lift coefficient reduction caused by different equivalent sand grain roughness heights and Reynolds numbers are performed. Our results demonstrated that, for the equivalent sand grain roughness heights investigated, the variation of sufficiently small heights has no significant influence on the maximum lift coefficient degradation. Moreover, when roughness is continuously increased, a saturation point seems to be approached, in which the variation of the maximum lift coefficient degradation is reduced. We noticed that although the reduction of the maximum lift coefficient caused by different equivalent sand-grain roughness heights and Reynolds number present similar behavior, they fall into different curve formats. </abstract>

Surface Roughness Impact on Low-Pressure Turbine Performance Due to Operational Deterioration

The overall efficiency and operational behavior of aircraft engines are influenced by the surface finish of the airfoils. During operation, the surface roughness significantly increases due to erosion and deposition processes. The aim of this study is to analyze the influence of roughness on the aerodynamics of the low-pressure turbine (LPT) of a midsized high bypass turbofan. In order to gain a better insight into the operational roughness structures, a sample of new, used, cleaned, and reworked turbine blades and vanes are measured using the confocal laser scanning microscopy technique. The measurement results show local inhomogeneities. The roughness distributions measured are then converted into their equivalent sand grain roughness ks,eq to permit an evaluation of the impact on aerodynamic losses. The numerical study is performed using the computational fluid dynamics (CFD)-solver turbomachinery research aerodynamics computational environment (TRACE) which was validated before with the existing data from rig experiments. It is observed that the influence of the surface roughness on the turbine efficiency is significant at take-off but negligible at cruise. A detailed analysis on the aerodynamics at take-off shows that very rough airfoils lead to higher profile and secondary loss. Due to the higher disturbances present in flows circulating over rough walls, the transition occurs earlier, and the momentum thickness increases in the turbulent boundary layer. The service-induced roughness structures cause an efficiency drop in the LPT of ηT=−0.16% compared to new parts. A gas path analysis showed that this results in an increased fuel flow of Δm˙f=+0.06% and an exhaust gas temperature (EGT) rise of ΔEGT=+1.2K for fixed engine pressure ratio which is equivalent to roughly 4% of the typical EGT margin of a fully refurbished engine. This result stresses the importance of roughness-induced loss in LPTs.

Numerical study of the aerodynamic performance of blunt trailing-edge airfoil considering the sensitive roughness height

For rough wind turbine airfoil and its blunt trailing-edge modification, the aerodynamic performance has been numerically investigated to facilitate a greater understanding of the effects of the blunt trailing-edge modification on the aerodynamic performance enhancement of airfoil with sensitive roughness height. The S834 airfoil from National Renewable Energy Laboratory is used for the simulation. The lift and drag coefficients of S834 airfoil with smooth or rough surface are calculated by the k-ω SST turbulence model, and are compared with wind tunnel test results. The aerodynamic performance of airfoils with different roughness heights is studied to obtain the sensitive roughness heights of suction and pressure surfaces. The mathematical expression of the blunt trailing-edge airfoil profile is established using the coordinate's rotation combined with the zoom coefficient of coordinate. Then, the S834 airfoil with sensitive roughness height is modified to be symmetrical blunt trailing-edge modification, and the lift and drag coefficients and the lift-drag ratio are also calculated and analyzed. Results indicate that the sensitive roughness height of suction surface is 0.5 mm, and the pressure surface is insensitive to the roughness height. Through the blunt trailing-edge modification, the lift coefficient and the maximum lift-drag ratio obviously increase for rough airfoil, and the sensitivity of airfoil to roughness height is reduced. The research provides significant guidance for designing the wind turbine airfoil under conditions of rough blade.

Lift sensitivity analysis for a Whitcomb airfoil with aileron deflections

Transonic flow past a two–element Whitcomb airfoil is studied in the range of free–stream Mach numbers between 0.81 and 0.86. Numerical solutions of the Reynolds–averaged Navier–Stokes (RANS) equations are obtained with a finite–volume solver on unstructured meshes using several turbulence models. Both smooth and rough airfoil surfaces are considered. The study reveals free–stream conditions that admit extreme sensitivity of aerodynamic coefficients to small changes of the aileron deflection. In addition, computations demonstrate free–stream conditions in which aileron deflections produce no effect on the lift.

The Effect of Surface Roughness on Efficiency of Low Pressure Turbines

The effect of surface roughness on the efficiency of low pressure turbines (LPTs) was experimentally investigated in a multistage turbine high-speed rig. The rig consisted of three stages of a state-of-the-art LPT. The stages were characterized by a very high wall-slope angle, reverse cut-off design, very high lift, and very high aspect ratio airfoils. Two sets of airfoils (both stators and rotors) were tested. The first set was made of airfoils with a roughness size of 0.7 μm Ra (25–35 × 10−5 ks/Cm), which was representative of LPT polished airfoils. The surface finish for the second set of airfoils was 1.8 μm Ra for blades and 2.5 μm Ra for stators (approximately 90 × 10−5 in terms of ks/Cm for both stators and blades). The resulting roughness of this set was representative of “as-cast” airfoils of low pressure turbines. The airfoil geometries, velocity triangles, leading and trailing edge locations, and flowpath were maintained between both sets. They were tested with the same instrumentation and at the same operating conditions with the intention of determining the isolated impact of the surface roughness on the overall efficiency. The turbine characteristics: sensitivity to speed, specific work, Reynolds number, and purge flows, were obtained for both sets. The comparison of the results suggests that the efficiency and capacity of both types of airfoils exhibit the same behavior. No significant differences in the results can be distinguished for the range of operating conditions in this study. The results agree with previous studies of distributed roughness in turbines: the use of as-cast rough airfoils in some low pressure turbines at high altitude does not introduce additional pressure losses.

Degradation of Film Cooling Performance on a Turbine Vane Suction Side due to Surface Roughness

After an extended period of operation, the surfaces of turbine airfoils become extremely rough due to deposition, spallation, and erosion. The rough airfoil surfaces will cause film cooling performance degradation due to effects on adiabatic effectiveness and heat transfer coefficients. In this study, the individual and combined effects of roughness upstream and downstream of a row of film cooling holes on the suction side of a turbine vane have been determined. Adiabatic effectiveness and heat transfer coefficients were measured for a range of mainstream turbulence levels and with and without showerhead blowing. Using these parameters, the ultimate film cooling performance was quantified in terms of net heat flux reduction. The dominant effect of roughness was a doubling of the heat transfer coefficients. Maximum adiabatic effectiveness levels were also decreased significantly. Relative to a film cooled smooth surface, a film cooled rough surface was found to increase the heat flux to the surface by 30%–70%.

Reynolds Number and Roughness Effects on Thick Airfoils for Wind Turbines

The effects of increased Reynolds number (Re) on clean and rough airfoils were investigated by wind tunnel experiments and by numerical simulation with an improved Navier-Stokes solver. By cooling the cryogenic wind tunnel in Cologne (KKK) of the German-Dutch Wind-Tunnel (DNW) down to 100 K, values of Re up to 10 × 106 were reached, based on a model chord of 500 mm. Whereas the clean surface configuration shows no drastic loss in maximum lift, the lift-to-drag ratio decreases from 95 to approximately 85, mainly due to an increase of drag. In contrast to that, rough surface configurations, including zig-zag tape for transition fixing and carborundum based distributed roughness around the nose, show increased performance for increased Re. In parallel, numerical simulations were carried out with a boundary layer stability code coupled to a Navier-Stokes solver. The trends in behaviour of the boundary-layer based properties were predicted well, in contrast to the separation pattern responsible for maximum lift.

Determination of rough-surface skin friction coefficients from wake profile measurements

A technique for determining the skin friction coefficients from wake profile measurements is described, and is applied to symmetric turbine airfoils with rough surfaces, which operate in a compressible, high-speed flow environment. The procedure involves the measurement of profiles of streamwise momentum in the wakes which form downstream of different airfoils with different levels of surface roughness. Different physical phenomena which affect such wake profiles are discussed and related to different effects, such as surface roughness, form drag, flow separation zones, and laminar-to-turbulent transition. With the same inlet experimental condition for each case, overall skin friction coefficients for the rough airfoils are determined. Resulting values increase considerably as the magnitude of equivalent sandgrain roughness size increases.

Calculation of Flow Over Iced Airfoils

Progress toward the development of a method for predicting the flowfield of an iced airfoil is described and shown to offer the prospect of a priori calculations of the effects of ice accretion and roughness on airfoil performance. The approach is based on interaction of inviscid flow solutions obtained by a panel method and improved upon by a finite-difference boundary-layer method which, operating in an inverse mode, incorporates viscous effects including those associated with separated flows. Results are presented for smooth, rough and iced airfoils as a function of angle of attack. Those for smooth and rough airfoils confirm the accuracy of the method and its applicability to surfaces with roughness similar to that associated with insect deposition and some forms of ice. Two procedures have been developed to deal with large ice accretion and their performance is examined and shown to be appropriate to the engineering requirements.