Solid Airfoil Research Articles

Numerical Study on Flow and Noise Characteristics of an NACA0018 Airfoil with a Porous Trailing Edge

An airfoil with a porous trailing edge has a low noise emission; thus, using a porous medium is a good technique for further reduction of wind turbine noise. In this paper, to reduce airfoil trailing edge noise while minimizing the negative influence of a porous medium on aerodynamic performance, a new filling method is proposed such that a porous medium is only used in the suction side half of the trailing edge, which is more sensitive to the noise generation. The large eddy simulation (LES) technique for flow and the Ffowcs Williams and Hawkings (FW-H) method for acoustics are used. At a Reynolds number of 2.63 × 105 and various angles of attack, an NACA0018 airfoil profile with a porous trailing edge covering 20% of the chord is studied under two porous configurations, namely a fully porous and a suction-side porous trailing edge type. The results show that the flow direction, velocity magnitude, and their distributions along the boundary layer of the two porous airfoils are significantly modified due to the presence of the porous medium. The fluctuation of the pressure coefficient and the increase in the boundary layer thickness are significant at low angles of attack. As compared to the solid airfoil counterpart, the noise radiation from the newly proposed suction-side porous airfoil achieves a noise reduction of 4.3 dB at an angle of attack α = 0°, and a noise reduction of 4.07 dB at an angle of attack α = 2°.

Performance Investigation of Airfoils for Axial Flow Fans in Low Solidity Cascades Operating at High Inlet Flow Angles

Abstract Axial flow fans are usually characterized by blade sections with low solidity and high inlet flow angles. Two main approaches are followed in the preliminary design phase, to take into account blade interaction effects: the use of available airfoil cascades data (or related correlations) and that of isolated airfoil data together with interference coefficients. The working conditions of low solidity airfoil cascades with highly tangential inflow are not widely studied in the literature, leaving the designer the possibility to rely on limited cascade data or often on the use of isolated airfoil data, with the assumption of negligible interference effects. A systematic investigation of the above working conditions for airfoil cascades is performed with Reynolds-averaged Navier–Stokes simulations. Three different airfoils are used to evaluate the influence of the design lift coefficient and maximum blade thickness. The results provide a better insight into the aerodynamic behavior of airfoil sections in such operating conditions, showing that interactions cannot be neglected. The use of metamodeling coupled with computational fluid dynamics (CFD) simulations is presented as a suitable tool for treating interference effects within the fan design process. The main findings of the present work can be used as a support for design choices as well as for developing design strategies both for the fan blades and for the airfoil sections.

Effect of the J-Shaped Wind Turbine Airfoil Opening Ratio and Thickness on the Performance of Symmetrical Airfoils

Abstract In this work, the effect of the inner opening ratio on the J-shaped airfoils aerodynamic performance was studied and documented for symmetrical airfoils. Three different airfoil thicknesses were investigated: small (NACA0008), medium (NACA0015), and large (NACA0024). For each airfoil thickness, effects of three inner opening ratios were analyzed: one-third, one-half, and two-thirds. The performance of each opening ratio was compared with the performance of the solid airfoil “zero opening ratio” for different angles of attack between 5 deg and 20 deg. All designs were simulated using the computational fluid dynamics (CFD) technology against experimental results for solid NACA4412 airfoil in the University of Wisconsin-Milwaukee (UWM) wind tunnel facility and other published experimental data. It was found that large eddy simulation yields accurate solutions with a smaller number of mesh cells compared to the k–ω turbulence model but with much longer computational time. The lift-to-drag ratio for all studied airfoils has a maximum value for solid airfoils compared to those equipped with openings. For airfoils equipped with 00.00% opening ratio “solid,” NACA0015 airfoil has the maximum lift-to-drag ratio. Furthermore, it was found that NACA0008 equipped with a 33.33% opening ratio has the best performance of all studied J-shaped airfoils.

A calculation method for modeling the flow characteristics of the wind turbine airfoil with leading-edge protuberances

An effective flow separation control method is urgently needed to improve the aerodynamic performance of the wind turbine blades. The leading-edge protuberances (LEPs) are the method that can inhibit flow separation and improve the aerodynamic performance of wind turbine airfoil. In order to achieve the optimal design of the LEPs on the wind turbine airfoil, it is necessary to develop a theoretical model of the effect of the LEPs on the flow characteristics. This paper proposes a novel modeling calculation method as achieved by substituting the momentum source into the N–S equation, and establishes the relationship between the geometrical parameters of the LEPs and the flow characteristics. The results show that the aerodynamic and flow characteristics of the modeled and solid airfoil with LEPs maintain good consistency under different stall conditions. Moreover, the high-strength streamwise vortices induced by LEPs increase the area of low pressure in the airfoil suction side, which effectively suppresses the flow separation and improves the aerodynamic performance of the airfoil. The modeling method provides the basis for the optimal design of the wind turbine airfoil with LEPs. The process of model construction explains the flow control mechanism of the LEPs from a new perspective.

An alternative permeable topology design space for trailing-edge noise attenuation

This study focuses upon a new permeable topology design concept as an alternative to porous metal foams, for turbulent boundary layer trailing-edge (TBL-TE) noise attenuation. The present permeable topology has unconventional characteristics with respect to the metal foams: a combination of low flow resistivity r and high form drag coefficient C. The unconventional characteristics are realized by a Kevlar-covered 3D-printed perforated structure. An experimental study featuring a NACA 0018 airfoil model with a Kevlar-covered 3D-printed TE insert at chord-based Reynolds numbers up to [Formula: see text] is carried out. The airfoil with this TE insert gives a broadband TBL-TE noise reduction up to approximately 5 dB, compared to a solid TE. This reduction varies only slightly with airfoil loading (lower than 1 dB variation), in contrast to the porous metal foams (up to 3 dB variation). When comparing the variation of noise attenuation given by all the permeable materials considered, the variation is found to decrease with the increasing C. This is because C specifies the permeable material's ability to withstand the increasing pressure difference, which causes cross flow that might interfere with the noise attenuation mechanism. Additionally, the drag coefficients as well as the roughness noise of the airfoil equipped with the present TE insert are also significantly lower than those of the metal-foam TE, and are mostly negligible compared to the fully solid airfoil. Based on the findings, design guidelines for permeable TE are proposed: the permeable material shall have a combination of a low flow resistivity and a high form drag coefficient as well as a negligible surface roughness.

Numerical investigation of porous materials for trailing edge noise reduction

In the framework of the German Collaborative Research Center CRC 880: Fundamentals of High Lift for Future Civil Aircraft porous materials as a means towards the reduction of airfoil trailing edge noise are investigated. At DLR, both experimental and numerical approaches are pursued to understand the physics behind the noise reduction. The present paper focuses on the numerical investigations, for which the experimental data serves as an evaluation basis. From the analysis of homogeneous materials, first steps are made towards the design of aeroacoustically tailored materials. It is assumed that materials with locally varying permeability may be suitable to achieve maximum noise reduction, as they provide a smooth transition from the solid airfoil to the free flow in the wake. The simulation results support this understanding, however it is revealed that high local gradients in the material properties themselves may act as acoustic sources.

Geometry-Grid Generation for Three-Dimensional Multidisciplinary Simulations in Multistage Turbomachinery

A procedure is presented for automated and fast geometry-grid generation of three-dimensional, multistage, axial turbomachinery for multidisciplinary simulations involving fluid–thermal, fluid–structural, or fluid–thermal–structural interaction. The procedure rapidly generates geometry for solid airfoils and endwalls, thermal barrier coatings, heat transfer pedestals, cooling plenums, cooling flow control tubes, and interblade row endwall leakage slots as well as the computational grids for these components automatically based on prescribed input parameters. The computational grids that are generated in general consist of multiblock, point-matched structured grids. For the cooling flow control tubes, heat transfer pedestals, and interblade row leakage slots, the computational grids are embedded, multiblock, overset grids to allow for arbitrary placement to allow optimization without the need for regridding the entire domain. The techniques used to construct the geometry and various computational grids are described. Demonstration of the procedure is provided for a one-and-a-half stage film-cooled turbine and a single-stage compressor. The geometry parameterization, as well as the automated geometry and grid topology construction, makes this a unique and rapid approach for multidisciplinary grid generation of multistage turbomachinery configurations during design optimization.

DGFEM for the analysis of airfoil vibrations induced by compressible flow

AbstractThe subject of the paper is the numerical simulation of the interaction of two‐dimensional compressible viscous flow and a vibrating airfoil. A solid airfoil with two degrees of freedom performs rotation around an elastic axis and oscillations in the vertical direction. The numerical simulation consists of the solution of the Navier‐Stokes system by the discontinuous Galerkin method coupled with a system of nonlinear ordinary differential equations describing the airfoil motion. The time‐dependent computational domain and a moving grid are taken into account by the arbitrary Lagrangian‐Eulerian (ALE) formulation of the Navier‐Stokes equations. The developed method is robust with respect to the magnitude of the Mach number and Reynolds number. Its applicability is demonstrated by numerical experiments.

Heat Transfer for the Film-Cooled Vane of a 1-1/2 Stage High-Pressure Transonic Turbine—Part I: Experimental Configuration and Data Review With Inlet Temperature Profile Effects

This paper investigates the vane airfoil and inner endwall heat transfer for a full-scale turbine stage operating at design corrected conditions under the influence of different vane inlet temperature profiles and vane cooling flow rates. The turbine stage is a modern 3D design consisting of a cooled high-pressure vane, an un-cooled high-pressure rotor, and a low-pressure vane. Inlet temperature profiles (uniform, radial, and hot streaks) are created by a passive heat exchanger and can be made circumferentially uniform to within ±5% of the bulk average inlet temperature when desired. The high-pressure vane has full cooling coverage on both the airfoil surface and the inner and outer endwalls. Two circuits supply coolant to the vane, and a third circuit supplies coolant to the rotor purge cavity. All of the cooling circuits are independently controlled. Measurements are performed using double-sided heat-flux gauges located at four spans of the vane airfoil surface and throughout the inner endwall region. Analysis of the heat transfer measured for the uncooled downstream blade row has been reported previously. Part I of this paper describes the operating conditions and data reduction techniques utilized in this analysis, including a novel application of a traditional statistical method to assign confidence limits to measurements in the absence of repeat runs. The impact of Stanton number definition is discussed while analyzing inlet temperature profile shape effects. Comparison of the present data (Build 2) to the data obtained for an uncooled vane (Build 1) clearly illustrates the impact of the cooling flow and its relative effects on both the endwall and airfoils. Measurements obtained for the cooled hardware without cooling applied agree well with the solid airfoil for the airfoil pressure surface but not for the suction surface. Differences on the suction surface are due to flow being ingested on the pressure surface and reinjected on the suction surface when coolant is not supplied for Build 2. Part II of the paper continues this discussion by describing the influence of overall cooling level variation and the influence of the vane trailing edge cooling on the vane heat transfer measurements.

J101034 離散渦法を用いた二自由度真の流体関連振動と空力音の解析

This work is concerned with the stability and sound generation analysis of a vibrating airfoil which receives energy by the aerodynamic force. A solid airfoil with two degrees of freedom, which can rotate about the elastic axis and oscillate in the vertical direction, is considered. The stability analysis considers two different models for the external force term in the equations of motion. In the first case a simple lift coefficient is considered, and the flutter critical flow speed is found via eigenvalue analysis. In the second case the external force term is estimated by a vortex method as a more detailed estimate, and the flutter critical flow speed is found via the Newmark (3 method. Finally, sound generated by the fluttering airfoil is analysed.

Numerical analysis of flow-induced nonlinear vibrations of an airfoil with three degrees of freedom

The subject of the paper is the numerical simulation of the interaction of two-dimensional incompressible viscous flow and a vibrating airfoil with large amplitudes. A solid airfoil with three degrees of freedom performs rotation around an elastic axis, oscillations in the vertical direction and rotation of a flap. The numerical simulation consists of the finite element solution of the Navier–Stokes equations coupled with a system of nonlinear ordinary differential equations describing the airfoil motion. The time-dependent computational domain and a moving grid are treated by the arbitrary Lagrangian–Eulerian (ALE) formulation of the Navier–Stokes equations. High Reynolds numbers require the application of a suitable stabilization of the finite element discretization. With the aid of numerical experiments we analyze the convergence of the method, if the computational mesh is refined and the time step is successively decreased. Also the effect of the loose (weak) and strong coupling of the flow and structure problems is tested. The applicability of the method is demonstrated by the comparison with NASTRAN computations and the solution of the coupled fluid–structure problem with increasing far-field velocity up to and behind the flutter instability. The developed method can be used in theoretical prediction of dynamic behaviour of aeroelastic systems especially when the system stability has been lost.

Numerical simulation of flow induced airfoil vibrations with large amplitudes

The subject of this paper is the numerical simulation of the interaction of two-dimensional incompressible viscous flow and a vibrating airfoil . A solid airfoil with two degrees of freedom , which can rotate around the elastic axis and oscillate in the vertical direction, is considered. The numerical simulation consists of the finite element solution of the Navier–Stokes equations, coupled with the system of ordinary differential equations describing the airfoil motion. The high Reynolds numbers considered 10 5 – 10 6 require the application of a suitable stabilization of the finite element discretization . The method presented in this paper is based on the laminar model and the turbulence modelling is not applied here. The time-dependent computational domain and a moving grid are taken into account with the aid of the arbitrary Lagrangian–Eulerian (ALE) formulation of the Navier–Stokes equations. Special attention is paid to the time discretization and the solution of the nonlinear discrete problem on each time level is performed. As a result, a sufficiently accurate and robust method is developed, which is applied to the case of flow-induced airfoil vibrations with large amplitudes after the loss of aeroelastic stability. The computational results are compared with known aerodynamical data and with results of aeroelastic calculations obtained by NASTRAN code for a linear approximation.

Boundary-layer separation control on a thin airfoil using local suction

High-speed incompressible flow past a thin airfoil in a uniform stream is considered. When the angle of attack for a solid airfoil exceeds a certain critical value, the boundary layer in the leading-edge region separates in a process known to lead to dynamic stall. Here suction near the leading edge is studied as a means of controlling separation and thereby inhibiting dynamic stall. First, steady boundary-layer solutions are obtained to determine the nature of suction distributions required to suppress separation on an airfoil at an angle of attack beyond the critical value (for a solid wall). Unsteady boundary-layer solutions are then obtained, using a combination of Eulerian and Lagrangian techniques, for an airfoil at an angle of attack exceeding the critical value; the effects of various parameters associated with the finite-length suction slot, its location and the suction strength are considered. Major modifications of the Lagrangian numerical method are required to account for suction at the wall. It is determined that substantial delays in separation can be achieved even when the suction is weak, provided that the suction is initiated at an early stage.

Computational analysis of buffet alleviation in viscous transonic flow over a porous airfoil

RANSONIC flow past an airfoil at a high freestream Mach number is traditionally defined as consisting of large supersonic regions embedded within a subsonic flowfield. The pressure fluctuations initiated by the shock/boundary-layer interaction may excite intense levels of buffeting. 1 Therefore, to extend the buffet boundaries for a particular airfoil, a technique is required which is capable of controlling either the boundary layer, or alternatively, the shock strength. Boundary-layer control may be performed by employing either blowing or suction within the interaction region. Blowing has a twofold effect, namely, an increase in the boundary-layer thickness coupled with a reduction in the skin friction, whereas suction induces a contrasting decrease in boundary-layer thickness and increase in skin friction. Passive control of the shock/boundary-layer interaction is a technique which provides a combination of blowing and suction forward and aft of the shock, respectively, without the need for a pump. A passive control device (Fig. 1) consists of a porous region and plenum which are positioned at the interaction region. The static pressure rise across the shock wave results in the flow traveling through the plenum to the region of lower pressure forward of the shock position. The increased communication across the shock stimulates the development of a thicker boundary layer forward of the shock. This process effectively changes the airfoil's geometry in a manner which produces a weaker lambda shock system. Moreover, passive control's inherent damping characteristics have been shown experimentally25 to reduce the pressure fluctuations initiated by the shock/boundary-layer interaction and, therefore, delay buffet onset. In the present study, time-accurate flow over an 18%-thick porous circular-arc airfoil is computed in an attempt to ascertain whether passive control will damp, or even eliminate, the flow periodicity detected in the corresponding solid airfoil (CSA) computation,6 thus extending the airfoil's buffet boundary. The following sections briefly describe both the numerical procedure and the results. Numerical Model An explicit Navier-Stokes finite volume scheme MGENS has been developed which is capable accurately predicting viscous flow over an arbitrary airfoil using an orthogonal boundary conforming hyperbolic grid MGHYPR. A detailed description of both the

Closure to “Discussions of ‘Forcing Function Effects on Unsteady Aerodynamic Gust Response: Part 2—Low Solidity Airfoil Row Response’” (1993, ASME J. Turbomach., 115, pp. 759–761)

Closure to “Discussions of ‘Forcing Function Effects on Unsteady Aerodynamic Gust Response: Part 2—Low Solidity Airfoil Row Response’” (1993, ASME J. Turbomach., 115, pp. 759–761)

Forcing Function Effects on Unsteady Aerodynamic Gust Response: Part 2—Low Solidity Airfoil Row Response

The fundamental gust modeling assumption is investigated by means of a series of experiments performed in the Purdue Annular Cascade Research Facility. The unsteady periodic flow field is generated by rotating rows of perforated plates and airfoil cascades, with the resulting unsteady periodic chordwise pressure response of a downstream low-solidity stator row determined by miniature pressure transducers embedded within selected airfoils. When the forcing function exhibited the characteristics of a linear-theory vortical gust, as was the case for the perforated-plate wake generators, the resulting response on the downstream stator airfoils was in excellent agreement with the linear-theory models. In contrast, when the forcing function did not exhibit linear-theory vortical gust characteristics, i.e., for the airfoil wake generators, the resulting unsteady aerodynamic responses of the downstream stators were much more complex and correlated poorly with the linear-theory gust predictions. Thus, this investigation has quantitatively shown that the forcing function generator significantly affects the resulting gust response, with the complexity of the response characteristics increasing from the perforated-plate to the airfoil-cascade forcing functions.

Calculation of the hydrodynamic reactions on a solid airfoil oscillating in a motionless liquid

Calculation of the hydrodynamic reactions on a solid airfoil oscillating in a motionless liquid