Smooth Airfoil Research Articles

Experimental Study of Owl-Like Airfoil Aerodynamics at Low Reynolds Numbers

This study experimentally investigates aerodynamic characteristics and flow fields of a smooth owl-like airfoil without serrations and velvet structures. This biologically inspired airfoil design is intended to serve as the main-wing for low-Reynolds-number aircrafts such as micro air vehicles. Reynolds number dependency on aerodynamics is also evaluated at low Reynolds numbers. The results of the study show that the owl-like airfoil has high lift performance with a nonlinear lift increase due to the presence of a separation bubble on the suction side. A distinctive flow feature of the owl airfoil is a separation bubble on the pressure side at low angles of attack. The separation bubble switches location from the pressure side to the suction side as the angle of attack increases and is continuously present on the surface within a wide range of angles of attack. The Reynolds number dependency on the lift curves is insignificant, although differences in the drag curves are especially pronounced at high angles of attack. Eventually, we obtain the geometric feature of the owl-like airfoil to increase aerodynamic performance at low Reynolds numbers.

Nanosecond laser ablation of the trapezoidal structures for turbomachinery applications

Correctly designed nature inspired micrometer size grooves i.e. riblets can be used to reduce drag. There exist several methods to manufacture these structures but all of them have their own problems and disadvantages, wherefore they are not perfectly suitable for all real industrial applications. Aim of this paper is to show how a short nanosecond pulse laser ablation can be applied in a micrometer size trapezoidal shape structures i.e. riblets manufacturing for a turbomachinery applications. It is shown that although the quality of the riblets, fabricated with this method, is not as good as with the best possible manufacturing methods, it is still enhancing the properties. Added to this, the processing speed is fast enough and acquisition costs of the system are economic so that method is suitable for the real industrial application. The wind tunnel studies indicate that the fabricated riblets on an airfoil reduce wall shear stress compared to a smooth airfoil without structuring.

Computational study of a pitching bio-inspired corrugated airfoil

The phenomenon of insect flight has been of scientific interest for many years and is more recently inspiring modern engineering devices such as Micro Aerial Vehicles (MAVs). Insect flight is characterized by unsteady fluid dynamics at low Reynolds numbers. The importance of viscous effects to the successful flapping flight of insects has been identified and with the current state of computing and Computational Fluid Dynamics (CFD) these effects can now be studied in detail. The present work attempts to simplify this complex phenomenon by considering symmetric oscillating rotational motion of a wing (pitching). What is of interest in this study is how the shape of a corrugated idealized insect wing affects the performance and flow characteristics around the pitching wing. Two dimensional CFD on an oscillating wing has been performed and reported. Measurements were taken to ensure the accuracy of the computational solution and the results validated against experimental PIV results. A range of frequencies and rotational amplitudes have been investigated. Lift and drag coefficients have been analyzed for all cases to quantify the effects of unsteady flow features on the performance of the oscillating wing. It was found that the wing shape used in this study resulted in the viscous features formed on the top of the wing exhibiting high sensitivity to the oscillating conditions and these influenced the performance of the wing. The flow features formed on the bottom of the wing remained similar throughout the cases tested. In the pitching regime this wing profile did not perform as well as published results for smooth airfoils in terms of thrust and propulsion efficiency. However this may be due to reduced frequency effects becoming important at our high pitching amplitude which need to be investigated further. There may be other oscillatory regimes that more accurately represent flapping flight in which the corrugated foil outperforms a smooth counterpart but these are yet to be investigated. Further research in this area may help answer the question as to how evolutionarily significant other benefits of a corrugated wing, such as being light and strong, are compared to its aerodynamic properties, the present results seem to favor the former.

Unsteady numerical investigation of two different corrugated airfoils

An unsteady, two-dimensional numerical study was conducted to investigate the aerodynamic and flow characteristics of two bio-inspired corrugated airfoils at Re = 14,000 and compared with those of a smooth NACA0010 airfoil. Mean aerodynamic results reveal that the corrugated airfoils have better lift performance compared to the NACA0010 airfoil but incur slightly higher drag penalty. Mean flow streamlines indicate that this favourable performance is due to the ability of the corrugated airfoils in mitigating large-scale flow separations and stall. Unsteady flow field results show persistent formations of small recirculating vortices that remain within the corrugations at 10° angle-of-attack or less for one of the corrugated airfoil and below 15° for the other. In contrast, the flow behaviour can be highly turbulent with regular pairings of large-scale flow separation vortices along the upper surface at higher angles-of-attack. This not only disrupts the small recirculating vortices and causes them to detach from the corrugated surfaces, but it gets increasingly dominant at higher angles-of-attack resulting in regular lift and drag oscillations. At the end of each cycle, there is a sudden ejection of flow perpendicular to the airfoil surface and these disruptions manifest themselves as “kinks” in the instantaneous lift and drag of the corrugated airfoils. Therefore instead of regular fluctuations, the lift and drag curves have additional undulations. Despite that, the corrugations are able to produce larger pressure differentials between the upper and lower surfaces than the smooth airfoil. The current study demonstrates the intricate relationships between different sharp surface corrugations and favourable aerodynamic performance. In particular, results from this paper supports earlier investigations that corrugated airfoils may be used to good effects even at low Reynolds numbers, where flow separations are more likely.

Improvement of aerodynamic characteristics of a thick airfoil with a vortex cell in sub- and transonic flow

The modified SST model (2005) is verified using Rodi– Leschziner–Isaev's approach and the multiblock computational technologies are validated in the VP2/3 code on different-structure overlapping grids by comparing the numerical predictions with the experimental data on transonic flow around an NACA0012 airfoil at an angle of attack of 4о for М=0.7 and Re=4×106. It is proved that the aerodynamic characteristics of a thick (20% of the chord) MQ airfoil mounted at an angle of attack of 2о for Re=107 and over the Mach number range 0.3–0.55 are significantly improved because an almost circular small-size (0.12) vortex cell with a defined volumetric flow rate coefficient of 0.007 during slot suction has been located on the upper airfoil section and an intense trapped vortex has been formed in it. A detailed analysis of buffeting within the self-oscillatory regime of flow around the MQ airfoil with a vortex cell has demonstrated the periodic changes in local and integral characteristics; the lift and the aerodynamic efficiency remain quite high, but inferior to the similar characteristics at М=0.55. It is found that the vortex cell at М=0.7 is inactive, and the aerodynamic characteristics of the MQ airfoil with a vortex cell are close to those of a smooth airfoil without a cell.

Comparing the effect of three transition models on the CFD predictions of a NACA0012 airfoil aerodynamics

The ability to accurately predict transition is important for flows around airfoils, especially at low Reynolds numbers. Transition from laminar to turbulent flow strongly influences the flow separation and the skin friction, thus affecting the airfoil aerodynamic characteristics. In the present work, three models for predicting flow transition were implemented in a 2D curvilinear, aerodynamic Navier–Stokes CFD code. These are Michel's empirical model, the eN model, and a newly proposed transition model (K–V model). The last two models are based on linear stability analysis employing the Orr-Sommerfeld equation to determine the amplification of spatial flow perturbations. The effect of the transition models on the airfoil aerodynamic characteristics at different Reynolds number and incidence angle are studied numerically. Both these parameters, when increased, promote the growth of flow perturbations. The test case is a 2D incompressible, low turbulence air flow around a smooth NACA0012 airfoil. The results of the new K–V transition model are in better agreement with existing experimental data.

Tonal noise production from a wall-mounted finite airfoil

This study is concerned with the flow-induced noise of a smooth wall-mounted finite airfoil with flat ended tip and natural boundary layer transition. Far-field noise measurements have been taken at a single observer location and with a microphone array in the Virginia Tech Stability Wind Tunnel for a wall-mounted finite airfoil with aspect ratios of L/C=1–3, at a range of Reynolds numbers (ReC=7.9×105–1.6×106, based on chord) and geometric angles of attack (α=0–6°). At these Reynolds numbers, the wall-mounted finite airfoil produces a broadband noise contribution with a number of discrete equispaced tones at non-zero angles of attack. Spectral data are also presented for the noise produced due to three-dimensional vortex flow near the airfoil tip and wall junction to show the contributions of these flow features to airfoil noise generation. Tonal noise production is linked to the presence of a transitional flow state to the trailing edge and an accompanying region of mildly separated flow on the pressure surface. The separated flow region and tonal noise source location shift along the airfoil trailing edge towards the free-end region with increasing geometric angle of attack due to the influence of the tip flow field over the airfoil span. Tonal envelopes defining the operating conditions for tonal noise production from a wall-mounted finite airfoil are derived and show that the domain of tonal noise production differs significantly from that of a two-dimensional airfoil. Tonal noise production shifts to lower Reynolds numbers and higher geometric angles of attack as airfoil aspect ratio is reduced.

Modeling an increase in the lift and aerodynamic efficiency of a thick Göttingen airfoil with optimum arrangement

The Reynolds equations closed using the Menter shear-stress-transfer model modified with allowance for the curvature of flow line have been numerically solved jointly with the energy equation. The obtained solution has been used to calculate subsonic flow (at M = 0.05 and 5° angle of attack) past a thick (24% chord) Gottingen airfoil with variable arrangement of a small-sized (about 10% chord) circular vortex cell with fixed distributed suction Cq = 0.007 from the surface of a central body. It is established that the optimum arrangement of the vortex cell provides a twofold decrease in the bow drag coefficient Cx, a threefold increase in the lift coefficient Cy, and an about fivefold increase in the aerodynamic efficiency at Re = 105 in comparison to the smooth airfoil.

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.

Local Acoustic Forcing of a Wing at Low Reynolds Numbers

At transitional Reynolds numbers , many smooth airfoils experience laminar flow separation and possible turbulent reattachment, where the occurrence of either state is strongly influenced by small changes in the surrounding environment. The Eppler 387 airfoil is one of many airfoils that can have multiple lift and drag states at a single wing incidence angle. Prestall hysteresis and abrupt switching between stable states occur due to sudden flow reattachment and the appearance of a separation bubble close to the leading edge. Here, we demonstrate control of the flow dynamics by localized acoustic excitation through small speakers embedded beneath the suction surface. The flow can be controlled not only through variations in acoustic power and frequency, but also through spatial variations in forcing location. Implications for control and stabilization of small aircraft are considered.

Passive separation control by acoustic resonance

At transitional Reynolds numbers, the laminar boundary layer separation and possible reattachment on a smooth airfoil, or wing section, are notoriously sensitive to small variations in geometry or in the fluid environment. We report here on the results of a pilot study that adds to this list of sensitivities. The presence of small holes in the suction surface of an Eppler 387 wing has a transformative effect upon the aerodynamics, by changing the mean chordwise separation line location. These changes are not simply a consequence of the presence of the small cavities, which by themselves have no effect. Acoustic resonance in the backing cavities generates tones that interact with intrinsic flow instabilities. Possible consequences for pas- sive flow control strategies are discussed together with potential problems in measurements through pressure taps in such flow regimes.

An investigation on the effects of irregular airfoils on the aerodynamic performance of small axial flow fans

In this paper, the effects of airfoils on the aerodynamic performance of small axial flow fans were investigated to reduce airflow turbulence on the blade surface and to improve the aerodynamic performance of small axial flow fans. Irregular airfoils where several convex grooves are bound in the blade pressure surface of the fans have two kinds. The wave-shaped edge is bound to the blade trailing edge of the fans and is designed from the smooth airfoil of the fans. The filtered N-S equations with the finite volume method and the standard k-ɛ turbulence model were adopted to carry out the steady simulation calculation. The large eddy simulation and the FH-W noise models were adopted to carry out the unsteady numerical calculation and aerodynamic noise prediction. The results of simulation calculation are in good agreement with the tests, which proves that the numerical calculation method is feasible. The spectrum characteristics of aerodynamic noise of the smooth airfoil and the two kinds of irregular airfoils were analyzed. Although the fans of the three airfoils are regarded as noise sources, the vortex distribution features in the unsteady flow field are also described. Noise reduction mechanisms of the irregular designs of the airfoils were also discussed. The results of this research may provide proof of the parameter optimization and the structural design of small axial flow fans with low noise.

Stall Onset on Airfoils at Moderately High Reynolds Number Flows

The inception of leading-edge stall on two-dimensional smooth thin airfoils at moderately high Reynolds number flows [in the range O(104) to O(106)] is investigated by an asymptotic approach and numerical simulations. The asymptotic theory is based on the work of Rusak (1994) and demonstrates that a subsonic flow about a thin airfoil can be described in terms of an outer region, around most of the airfoil chord, and an inner region, around the nose, that asymptotically match each other. The flow in the outer region is dominated by the classical thin airfoil theory. Scaled (magnified) coordinates and a modified (smaller) Reynolds number are used to correctly account for the nonlinear behavior and extreme velocity changes in the inner region, where both the near stagnation and high suction areas occur. It results in a model (simplified) problem of a uniform flow past a semi-infinite parabola with a far-field circulation governed by a parameter à that is related to the airfoil’s angle of attack, nose radius of curvature, and camber and to the flow Mach number. The model parabola problem consists of a compressible and viscous flow described by the steady Navier-Stokes equations. This problem is solved numerically for various values of à using a Reynolds-averaged Navier-Stokes flow solver, and utilizing the Spalart-Allmaras viscous turbulent model to account for near-wall turbulence. The value Ãs where a large separation zone first appears in the nose flow concurrent with a sudden increase in the minimum pressure coefficient is determined. The change of Ãs with the modified Reynolds number is determined. These values indicate the stall onset on the airfoil at various flow conditions. The predictions according to this approach show good agreement with results from both numerical simulations and available experimental data of the stall of thin airfoils. This simplified approach provides a criterion to determine the stall angle of airfoils with a parabolic nose and the effect of airfoil’s thickness ratio, nose radius of curvature, camber and flaps, and flow compressibility on the onset of stall. This approach also presents an analysis method that can be used to predict the stall of airfoils with alternative nose geometry.

Experimental Study of Surface Roughness Effects on a Turbine Airfoil in a Linear Cascade— Part I: External Heat Transfer

Abstract The present experimental study is part of a comprehensive heat transfer analysis on a highly loaded low pressure turbine blade and endwall with varying surface roughness. Whereas a former paper (Lorenz et al., 2009, “An Experimental Study of Airfoil and Endwall Heat Transfer in a Linear Turbine Blade Cascade—Secondary Flow and Surface Roughness Effects,” International Symposium on Heat Transfer in Gas Turbine Systems, Aug. 9–14, Antalya, Turkey) focused on full span heat transfer of a smooth airfoil and surface roughness effects on the endwall, in this work further measurements at the airfoil midspan with different deterministic surface roughness are considered. Part I investigates the external heat transfer enhancement due to rough surfaces, whereas part II focuses on surface roughness effects on aerodynamic losses. A set of different arrays of deterministic roughness is investigated in these experiments, varying the height and eccentricity of the roughness elements, showing the combined influence of roughness height and anisotropy of the rough surfaces on laminar to turbulent transition and the turbulent boundary layer as well as boundary layer separation on the pressure and suction side. It is shown that, besides the known effect of roughness height, eccentricity of roughness plays a major role in the onset of transition and the turbulent heat transfer. The experiments are conducted at several freestream turbulence levels (Tu1=1.4–10.1%) and different Reynolds numbers.

On the Development of Turbomachine Blade Aerodynamic Design System

A turbomachine blade aerodynamic design process is proposed to design turbomachine blades. The design system, including a global optimization of through flow for whole machine and a local optimization of the airfoil design and airfoil section, stacks up. The airfoil generator code employs Bezier polynomial curves to produce smooth airfoil shapes. A meanline program combined with an optimizer was used to perform the global optimization. In the airfoil section design, for fast calculations of the airfoil pressure distributions a discrete vortex method is developed. A novel Navier-Stokes (N-S) solver is developed for further examination of the airfoil performance for final section design. The N-S code is used to obtain the blade-to-blade quasi-three-dimensional, turbulent, and viscous flow characteristics. The time-dependent N-S equations are discretized and integrated in a coupled manner based on a finite-volume formulation, as well as a flux-difference splitting. The flux-difference splitting method enables us to compute with a rapid convergence. The present N-S code can handle computations of both subsonic and supersonic flows and can be connected to an external optimizer code with the airfoil generator. After optimized airfoil was designed, a three-dimensional code was used for final tuning of the design.

Control of Flow Separation Using Self-Supplying Air-Jet Vortex Generators

Wind-tunnel experimental investigations were conducted to investigate the use of conventional outsupplying and proposed self-supplying air-jet vortex generators to delay flow separation over an NACA 0012 airfoil. The conventional air-jet vortex generators were supplied with the air from an external compressor, and proposed self-supplying generators got the air from the overpressure region situated in the nose part of the airfoil lower surface. The lift, drag, and pitching moment for the smooth airfoil equipped with air-jet vortex generators were determined based on the pressure distribution measurements. The experimental tests were carried out in both the low-speed wind tunnel (M=0.05-0.1) with the airfoil model of 0.5-m chord length and the high-speed wind tunnel (M = 0.1-0.85) with the airfoil model of 0.18-m chord length. In the high-speed wind tunnel, the influence of using only the self-supplying air-jet vortex generators was tested. It was found that using both types of air vortex generators can delay the flow separation on the airfoil, which leads to the increase of both the lift coefficient and the critical angle of attack.

A computational study of the aerodynamic performance of a dragonfly wing section in gliding flight

A comprehensive computational fluid-dynamics-based study of a pleated wing section based on the wing of Aeshna cyanea has been performed at ultra-low Reynolds numbers corresponding to the gliding flight of these dragonflies. In addition to the pleated wing, simulations have also been carried out for its smoothed counterpart (called the ‘profiled’ airfoil) and a flat plate in order to better understand the aerodynamic performance of the pleated wing. The simulations employ a sharp interface Cartesian-grid-based immersed boundary method, and a detailed critical assessment of the computed results was performed giving a high measure of confidence in the fidelity of the current simulations. The simulations demonstrate that the pleated airfoil produces comparable and at times higher lift than the profiled airfoil, with a drag comparable to that of its profiled counterpart. The higher lift and moderate drag associated with the pleated airfoil lead to an aerodynamic performance that is at least equivalent to and sometimes better than the profiled airfoil. The primary cause for the reduction in the overall drag of the pleated airfoil is the negative shear drag produced by the recirculation zones which form within the pleats. The current numerical simulations therefore clearly demonstrate that the pleated wing is an ingenious design of nature, which at times surpasses the aerodynamic performance of a more conventional smooth airfoil as well as that of a flat plate. For this reason, the pleated airfoil is an excellent candidate for a fixed wing micro-aerial vehicle design.

Criteria for the Separation of Unsteady Ideal Fluid Flow past a Smooth Airfoil

Criteria for the separation of unsteady flow past a closed smooth airfoil are studied using the ideal fluid model and the Brillouin-Villat criterion. The necessary separation conditions are formulated.

Brazier Effect in Multibay Airfoil Sections

Although it has long been recognized that all long thin-walled hollow structures exhibit a nonlinear response to bending moments, the majority of analytical research has focused on circular cross sections. A method of predicting the nonlinear behavior of multibay airfoil sections is presented. Although the approach is simple, the algebraic complexity lends itself toward a solution using algebraic manipulation software. Comparison of the analytical models with finite element analysis shows good correlation and demonstrates the ability of the model in predicting the nonlinear bending response of smooth orthotropic two-bay airfoils. Nomenclature [a ]= shell in-plane compliance matrix C = overall curvature [D ]= shell bending stiffness matrix E =Y oung’s modulus G = shear modulus h = height I = second moment of area l = section Y dimension M = longitudinal bending moment m = cross-sectional bending moment p = cross-sectional reaction force S =w all length U = energy w = cross-section displacement z α = cos(β) β = horizontal wall angles to midplane e = strain κ = cross-sectional curvature λ = element length correction factor µ = I correction factor ν = Poisson’s ratio ψ = distributed Brazier crushing force I. Introduction

Control of flow around a NACA 0012 airfoil with a micro-riblet film

The flow structure of the wake behind a NACA 0012 airfoil covered with a V-shaped micro-riblet film (hereafter, MRF) has been investigated experimentally. The results were compared with the corresponding results from an identical airfoil covered with a smooth polydimethylsiloxane (PDMS) film of the same thickness. The drag force acting on each airfoil, as well as the spatial distributions of turbulence statistics in the near wake behind each airfoil, were measured for Reynolds numbers (calculated based on the chord length, C = 75 mm ) ranging from Re=1.03×10 4 to 5.14×10 4. At Re=1.54×10 4 ( U 0=3 m/s), the drag force on the MRF-covered airfoil was about 6.6% lower than that on the smooth airfoil. In contrast, at the higher Reynolds number of Re=4.62×10 4 ( U 0=9 m/s), application of the MRF increased the drag force by about 9.8%. To determine the spatial distributions of turbulence intensity, including the mean velocity, turbulence intensity and turbulent kinetic energy, 500 instantaneous velocity fields of the wake behind each airfoil were measured using a 2-frame PIV technique and ensemble averaged. For the case of drag reduction (Re=1.54×10 4), the near wake behind the MRF-covered airfoil had a shorter vortex formation region and higher vertical velocity component compared with that behind the smooth airfoil. At the downstream end of vortex formation region, the Reynolds shear stress and turbulent kinetic energy for the MRF-covered airfoil were similar or slightly larger than for the smooth airfoil. Smoke-wire flow visualization showed that the presence of the MRF on the airfoil surface caused the smoke filaments to become thinner and to be separated by a smaller lateral spacing, indicating suppression of spanwise movement. For the drag-increasing case (Re=4.62×10 4), the presence of MRF grooves on the airfoil seemed to increase the vertical velocity component and decrease the height of the large-scale streamwise vortices, which interacted actively. This active interaction increased the turbulent kinetic energy and Reynolds shear stress in the near-wake region, increasing the drag force acting on the airfoil.