Multi-element Airfoil Research Articles

On the Origin of Görtler Vortices in Flow over a Multi-Element Airfoil

The flow characteristics of a 30P30N three-element high-lift airfoil at low Reynolds numbers O104 are examined through three-dimensional simulations using a high-order spectral element method. This study primarily investigates the flow structures of the slat cove and Görtler vortices formed on the upper surface of the main airfoil. Spanwise instability grows exponentially in the slat cove with a constant wavelength, corresponding to that of the subsequently formed Görtler vortices. Görtler number calculations show that curvature-induced centrifugal instability at the slat cusp leads to the subsequent formation of Görtler vortices. Proper orthogonal decomposition (POD) is used to analyze the development of flow structures in the slat cove in different time ranges. At early time, the flow in the slat cove is dominated by shear layers that evolve into spanwise perturbations. These perturbations further evolve into distinct bell-shaped structures close to the slat cusp and are advected to the upper surface of the main airfoil, leading to the formation of Görtler vortices.

Comparative study on predicting turbulent kinetic energy budget using high-order upwind scheme and non-dissipative central scheme

The accurate computation of different turbulent statistics poses different requirements on numerical methods. In this paper, we investigate the capabilities of two representative numerical schemes in predicting mean velocities, Reynolds stress and budget of turbulent kinetic energy (TKE) in low Mach number flows. With concerns on numerical order of accuracy, dissipation and dispersion properties, a high-order upwind scheme with relatively good dispersion and dissipation and a second-order non-dissipative central scheme with perfect dissipation but poor dispersion are adopted for this comparative study. By carrying out a series of numerical simulations including Taylor-Green vortex, turbulent channel flow at Reτ=180\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Re_{\ au }=180$$\\end{document} and turbulent flow over periodic hill at Reb=10595\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Re_{b}=10595$$\\end{document}, it can be obtained that although the high-order upwind scheme lacks perfection of dissipation in high wave number range, it still demonstrates superior predictive capability compared with the second-order non-dissipative central scheme, especially with relatively coarse grids. Finally, by taking the high-order upwind scheme as a suitable selection for turbulence simulation, the turbulent flow over a 30P30N multi-element airfoil is investigated as an application study. After briefly comparing the simulated profiles and spectrum with reference experimental results as validation, the budget of TKE is analyzed to locate the dominant flow structures and regions. It is found that the production and dissipation terms behave in a “monopole” pattern in the locations with strong shears and wakes. Whereas the advection and diffusion terms show an “inward” pattern and an “outward” pattern, which indicate the spatial transport of TKE between the center of the shear layer and nearby locations.

Investigation of Aerodynamic Performance of Multi-element Airfoil in Free Surface Effect

Abstract This study examines the aerodynamic performance of the 30P30N multi-element airfoil flying on a free surface, a numerical computational model is established, the governing equations are addressed through the application of the finite volume method, while the volume of fluid method is utilized for the simulation of two-phase flow. Different mesh strategies and turbulence models are compared, with the numerical methods validated against experimental data. The aerodynamic properties of the 30P30N multi-element airfoil on the free surface at different wavelengths and wave heights are given by numerical computations, which show that the ground effect of the 30P30N multi-element airfoil differ from those on a clean airfoil, the aerodynamic coefficients of the 30P30N multi-element airfoil decrease as flight altitude lowers, with the pitching and drag moment of the slat is opposite to that of the main airfoil and the flap. Additionally, when the 30P30N multi-element airfoil flies over a regular wave surface, the aerodynamic coefficients of each component exhibit periodic behavior.

Wall-Modeled Large-Eddy Simulation Method for Unstructured-Grid Navier–Stokes Solvers

This paper reports on the implementation and assessment of a wall-modeled large-eddy simulation (WMLES) methodology in an unstructured-grid, node-centered flow solver, FUN3D, that is developed and supported at the NASA Langley Research Center. Finite-volume (FV) and finite-element (FE) discretization schemes considered in the study provide formal second-order spatial accuracy. Large-eddy simulations (LES) resolve large-scale turbulent-flow features, and small-scale effects are modeled using the Vreman subgrid-scale model. At solid-wall boundaries, a shear-stress model is employed to provide a proper boundary-flux closure. The nonlinear equations are integrated in time using either an optimized backward difference formula or an implicit multistage Runge–Kutta temporal scheme. The implicit equations at each time step are solved by strong nonlinear iteration schemes. WMLES demonstrations are shown for two high-lift configurations, namely, the McDonnell Douglas 30P30N multi-element airfoil and a NASA High-Lift Common Research Model. Results show that the WMLES approaches implemented in the FV and FE discretization methods produce consistent solutions and are capable of capturing key aerodynamic characteristics and flow structures for high-lift configurations at a wide range of angles of attack, including maximum-lift conditions. In the 30P30N example, correct trends in the variations of integrated aerodynamic forces and moments, surface pressure distributions, and boundary-layer profiles are captured as the Reynolds number is increased.

An improved immersed boundary method for turbulent flow simulations on Cartesian grids: extension of a global geometric approach for thin boundary layers and strong flow incidence

This paper presents further improvements to the immersed boundary method introduced in [1]. The proposed developments take place during the pre- and post-processing stages of the simulation workflow and aim to further increase the accuracy of the approach for steady-state simulations of high Reynolds number turbulent flows around aerodynamic geometries facing strong incidence. To this end, the location of the forcing points is further optimized prior to simulation, with an adaptive and local modeling height that accounts for the evolution of the turbulent boundary layer thickness, especially at the leading edge. In addition, the direct extrapolation of the pressure solution at the wall is replaced by first- and second-order reconstructions using the normal pressure gradients interpolated at a new set of image points. This second approach is used only in post-processing, after the simulation, and prevents the degradation of the wall pressure in the presence of strong curvatures or thin boundary layers. These developments have been validated by simulating subsonic turbulent flows around the NACA0012 profile, 2D multi-element airfoil (2DMEA), and HL-CRM half-plane at significant angles of attack. Smooth and accurate skin pressure and friction coefficients are observed, in excellent agreement with body fitted wall-resolved solutions, even for coarser Cartesian meshes. Better drag and lift coefficients calculated by direct near-field integrations are obtained, along with accurate predictions of the near-wall flow physics throughout the turbulent boundary layer.

Impact of leading edge slat on stall performance of multi-element airfoils

Impact of leading edge slat on stall performance of multi-element airfoils

Multi-Element Airfoil in Jet Flows: Identifying Dominant Factors and Interactions

Propeller–wing–flap systems are subject to complex aerodynamic interactions between each part of the system. Although the propeller–wing interaction in cruise conditions is well defined, the high-lift condition is relatively unexplored. Effective analysis of the complex aerodynamic relationship between propeller, wing, and flap is being impeded by a lack of understanding of the underlying mechanisms. In this paper, we therefore investigate the effects of a 2D jet impinging on a multisection airfoil. We quantify which factors that define a jet–wing–flap configuration dominate lift, drag, and moment responses. We further investigate interactions between these factors and discuss how they affect the flow. We find that the jet velocity ratio is by far the dominant factor in lift, drag, and moment responses, but it does not have strong interactions with other factors. The sensitivities of the multi-element airfoil do not change significantly when impinged upon by a jet, except when critical Mach numbers are exceeded. This strongly affects the aerodynamic response and dominant sensitivities. We furthermore conclude that the immersion of the flap is a key aspect when it comes to augmenting the lift by increasing the dynamic pressure in the flowfield. The conclusions from this paper can provide key insights for propeller–wing–flap flows.

Revisiting the wake-triggered secondary vortices over a circular-cylinder/flat-plate configuration

The wake-vortex/boundary-layer interaction is significant for both fundamental and practical aspects of fluid mechanics. The wake vortex triggered the well-known single-secondary vortex over a canonical circular-cylinder/flat-plate configuration. However, recent investigations reported wake-triggered double-secondary vortices over a multi-element airfoil or a simplified circular-cylinder/airfoil configuration. Motivated by this contrast, this paper revisits this topic with a circular-cylinder/flat-plate configuration and finally observes the double-secondary vortices in the boundary layer. The disturbances related to the shedding frequency of the wake vortex and its second harmonic are found to penetrate the boundary layer and be further amplified to form the double-secondary vortices.

Resolution assessment of Reynolds stress model-based DES methods for high-lift device flow over 30P30N configuration

Flows over high-lift devices (HLD) contain abundant complex physical characteristics such as flow separation, free shear layer transition, and turbulent vortex evolution, which cause challenges to the numerical prediction of unsteady flow characteristics over the HLD. The recently developed detached-eddy simulation (DES) based on the Reynolds-stress model (RSM) is expected to resolve the complex flow characteristics with high fidelity due to the RSM model holding fewer modeling assumptions than classical eddy-viscosity models (EVM). To assess the flow resolution performance of the RSM-based DES method and its dynamic version on the HLD, they are adopted in the numerical investigation of flow over a 30P30N multielement airfoil, a canonical HLD configuration with sufficient experiment measurements. The widely used EVM-based DES method, the shear-stress transport (SST)-based DES method, is also applied for the numerical investigation as the benchmark DES method for the flow resolution performance assessment of RSM-based DES methods. The results are compared with the available experimental data and the benchmark DES method, including average quantities, fluctuation quantities, and instantaneous flow field. It is found that RSM-based DES methods can overcome the prediction delay matter of free-shear layer instability appearing in the SST-based DES method. The dynamic RSM-based DES method can predict the fluctuation flow characteristics in the slat cove of 30P30N configuration more exactly than the other two methods and resolve finer vortex structures in the flow field.

Vortex-induced sound prediction of slat noise from time-resolved particle image velocimetry data

A data-driven method for predicting the vortex-induced sound from time-resolved velocimetry data is presented and applied to the sound generated by flow passing through the slat in a multi-element high-lift airfoil. The time-dependent velocity fields in the slat-cove region of the 30P30N multi-element airfoil are obtained from time-resolved particle image velocimetry measurements, and a low-order reconstruction is achieved by using the rank-one vbnm modes from spectral proper orthogonal decomposition. The pressure force and associated dipole sound are then computed via the application of the force and acoustic partitioning methods (Seo et al. in Phys Fluids 34(5):053607, 2002) which involve volume integrals of the product of the second invariant of the velocity gradient tensor and geometry-dependent influence fields. The method enables estimation of the dipole sound generated by local flow structures, and the results are shown to be consistent with theory of vortex sound. Comparison with the measured sound data suggests that while the shear layer modes are responsible for the tonal noise, the interactions between the shear layer modes and other parts of the wing also generate a substantial level of flow noise.

Effect of leading-edge parameters of multi-element airfoil on aerodynamic characteristic

Studying the effect of leading-edge parameters of 30P30N on aerodynamic characteristic was carried out. Effect of changing the gap and overlap of the slat of the airfoil were mainly investigated. Numerical simulation was applied to obtain aerodynamic characteristics, results show that: the lift coefficient in the linear segment basically does not change with the gap, the drag coefficient decreases with change of the gap. The overlap and gap of slat have obvious effect on the lift-to-drag ratio. The smaller the gap between slat and fixed wing, the greater the lift-to-drag ratio is achieved. With the increase of overlap between slat and fixed wing, lift to drag ratio increases.

Computation of plane potential flow around multi-element airfoils using the Schottky–Klein prime function

Potential flow is computed around multi-element airfoils in two dimensions using numerical conformal mapping combined with the calculus of the Schottky–Klein prime function. The corners at the trailing edges of the airfoils are successively removed by Kármán–Trefftz maps. The map from the domain exterior to disks to the domain exterior to the smooth images of the airfoils is computed using an extension of a Fourier series method for the disk due to Fornberg. The velocity potential for flow in the circle domain, with circulation calculated to satisfy the Kutta condition in the airfoil domain, is computed using the Schottky–Klein prime function. The computations are compared to examples from the aerospace engineering literature.

Optimisation of a Multi-Element Airfoil for a Fixed-Wing Airborne Wind Energy System

Airborne wind energy systems benefit from high-lift airfoils to increase power output. This paper proposes an optimisation approach for a multi-element airfoil of a fixed-wing system operated in pumping cycles to drive a drum-generator module on the ground. The approach accounts for the different design objectives of the tethered kite’s alternating production and return phases. The airfoil shape is first optimised for the production phase and then adapted for the requirements of the return phase by modifying the flap setting. The optimisation uses the multi-objective genetic algorithm NSGA-II in combination with the fast aerodynamic solver MSES. Once the optimal shape is determined, the aerodynamic performance is verified through CFD RANS simulations with OpenFOAM. The resulting airfoil achieves satisfactory performance for the production and return phases of the pumping cycles, and the CFD verification shows a fairly good agreement in terms of the lift coefficient. However, MSES significantly underpredicts the airfoil drag.

Numerical Study of Flow Control on Simplified High-Lift Configurations

Enhancement in the aerodynamic performance of wings and airfoils is very notable when Active Flow Control (AFC) is applied to Short Take-off and Landing aircraft (STOL). The present numerical study shows the application of steady, pulsed and synthetic tangential jets applied to the plain flap shoulder of a modified NASA Trapezoidal Wing. Pulsed jets are modeled by sinusoidal and square waveforms while synthetic jets are modeled only by pure sine waveform. The freestream airflow conditions are Mach number equal to 0.2 and Reynolds number equal to 4.3 million based on the mean aerodynamic chord. The presented simulations are two-dimensional and based on RANS for steady jet cases and URANS for pulsed and synthetic cases, compiled with the open-source suite SU2 and adapted for time varying boundary conditions. Numerical results for modified configurations based on the same baseline wing profile considering different leading edges, jet slot height, flap position, blowing mass flow, type and frequency of the jets are presented. Curves of pressure coefficient distribution revealed a substantial influence upstream of the AFC, around the slat and main element. The jet slot height analysis showed that the lift gain is also influenced by the slot size due to the change of the local flow velocity considering the same blowing momentum coefficient. Regarding the jet frequency, no significant differences on the lift coefficients were found between the reduced frequencies F+ equal to 1 and 2. Results of aerodynamic loads showed an improved lift coefficient in relation to the baseline airfoil when pulsed and steady jets are employed. Pulsed jets under square waveform were effective even at high deflected flap condition at 50°, with a significant gain in the lift coefficient of 36%, in relation to the uncontrolled case, combined with a drag reduction of 20%, and a decrease in mass flow up to 49% in relation to the steady jet for the same lift gain. Although sine and square waveform results presented similar improvements for lift, the drag is around 15% higher for the former. When compared with the steady jet case, the mass flow reduction is 36% for the sinewave. Synthetic jets with zero-net-mass-flux proved superior to the baseline conventional multi-element airfoil only with deployed flap at 50°, where a modest lift improvement of 5% was observed.

Slat cove dynamics of multi-element airfoil at low Reynolds number

Slat cove dynamics of multi-element airfoil at low Reynolds number

Validation of SST-SCM correlation-based transition model by the simulation of wall-bounded flows

A modified correlation-based transition model coupled with SST turbulence model(k−ω−γ−R˜eθt) is proposed in this work. For better verification of intermittency, the constraint definition strictly based on local variables is cited to ensure physical solution. In order to obtain more details of laminar and transition boundary layer for separation-induced transition, a local correction method based on streamline curvature modification (SCM) has been developed to extend model ability in predicting adverse-pressure-gradient flows. The mean strain rate involved with mean rotation rate and shear rate is modified according to wall-bounded condition with rotation and non-rotation features to preserve the anisotropic characteristics encountered in large range flows. Furthermore, separation-induced intermittency is corrected by considering the effect of average strain rate to prevent outliers in local intermittency whenever the laminar boundary layer separates. Considering that results of test cases simulation with different empirical correlations vary huge from each other, all cases are matched with its suitable empirical correlations to ensure the stability and accuracy calculated by proposed model. Transitional flat-plate cases are calculated to validate the basic ability for near-wall flow simulation.Massive separated flow over two-dimensional circular cylinder has been studied to verify the predicted accuracy for non-equilibrium flows. Flow over multi-element airfoil 30P30N and the simulation of vortex system over three-dimensional Low Pressure Turbine(LPT) cascade T106A are given to examine the predictions of Reynolds shear stress profiles and pressure distribution with flow disturbance; reasonable results show a good match between experimental data and calculation by proposed model.

Verification of a Pressure-Based Compressible Flow Solver

This paper presents Solution Verification exercises with the pressure-based compressible flow solver ReFRESCO for five test cases available in the NASA Turbulence Modeling Resource: the two-dimensional flows over a flat plate, a bump-in-channel, a DSMA661 airfoil and a multi-element airfoil and the three-dimensional flow of a bump-in-channel. Simulations are performed with the Favre-averaged continuity and Navier-Stokes equations using the Spalart & Allmaras turbulence model. ReFRESCO results are compared with reference data from density-based compressible flow solvers (CFL3D and FUN3D). two aspects of the implementation of the turbulence model are addressed: the calculation of the distance to the wall and the discretization scheme used in the convective terms of the turbulence model transport equation. Results of this study show perfect consistency with the reference data for the test cases that are not affected by the determination of the distance to the wall.

“Turbulent/non-turbulent interface” in a low-Reynolds-number transitional boundary layer over a multi-element airfoil

Akin to the turbulent/non-turbulent interface, which envelops a fully developed turbulence, the iso-vorticity interface that separates the irrotational fluids from the free stream is investigated in a transitional boundary layer over a multi-element airfoil. Geometrically, the interface is steady and flat at the initial transitional stage. With the progress of transition, the interface fluctuates more and more violently. Statistically, the evolution of interface properties, including the fluctuation of interface height σI, the fractal dimension Df, and the thickness of interface δI, are tracked along the streamwise direction. It is revealed that all the three quantities grow with transition. Both σI and δI become invariant at the latter half part of the airfoil where the transition is completed. Counter-intuitively, the growth of Df stops at a more upstream location, indicating the fractal property of turbulence is already existing during transition. Instantaneously, the above-mentioned quantities are found to be related to the periodical vortices inside the boundary layer. The vortex will elevate/sink the above interface by the induced ejection/sweep flow and increases Df at slightly downstream of the vortex. Furthermore, it is highlighted that the variations of interface properties will greatly influence the entrainment by two opposite effects: the length of interface LI and the vorticity gradient across the interface; and it is found that the entrainment flux is mainly controlled by LI. This suggests that the recently reported dominance of LI on entrainment for wall-bounded turbulence could be a universal phenomenon in boundary layers, despite the flow states and wall configurations.

Investigating the Performance of a Novel Multi-Element Airfoil Concept Using Numerical Analysis

Separation control is one of the trending topics recently for Vertical Axis Wind Turbine (VAWT) applications, flow separation that occur on the suction side of an airfoil is a major limitation for VAWTs starting and power coefficient; passive control of flow separation has been attracting wind energy research community recently. The principle objective of this paper is to investigate the effect of the multi-element airfoil on the aerodynamic characteristics at wide range of angles of attack at a Reynolds number of 140,000. The multi element airfoil modification has been implemented by replacing the baseline airfoil with two airfoil members within the same geometrical profile of the original baseline airfoil, cambered and symmetrical airfoil members were investigated in this work. The cambered airfoil sections were made of the NACA 4412 airfoil while the symmetrical airfoil sections consisted of the NACA 0015 airfoil. The baseline airfoil is a profile of NACA 0021. The numerical investigation was executed using computational fluid dynamics (CFD) by the usage of ANSYS FLUENT and the turbulence model used was the k-ε Standard. The results of the present research show that the optimized multi-element airfoil configurations have better performance than the baseline airfoil at high angles of attack. A significant delay of separation was observed using the multi element airfoil, resulted in an increase of 200% in the glide ratio using cambered airfoil members and of 100% using symmetric airfoil members. These results are expected to have a significant effect on the performance of low wind speed wind turbines. In addition, a potential benefit for vertical axis wind turbines, whereas the airfoil generates lift at wider range of angles of attack.

Multi-element airfoil configuration for HAWT: A novel slot design for improved aerodynamic performance

Wind energy has a vast potential to provide low-carbon energy in the future. However, to improve the wind turbine blade efficiency, longer blades are being manufactured, which causes expensive production, complicated logistics and structural instability. For good structural integrity, the root airfoils are kept thicker, which brings unsteady aerodynamic characteristics to the inboard region of the blade. Therefore, multi-element airfoil configuration is introduced. In the present study, DU-99-W-405 airfoil is chosen, and a 2D CFD analysis is performed using ANSYS Fluent. Novel slot design shows the increment in lift coefficient with the angle of attack, which helped delay the stall.