Computational Simulation of Dynamic Stall on the NLR 7301 Airfoil

2012

The aim of the computational study is to present different approach for numerical modeling of airfoil dynamic stall as the airfoil is pitched at a constant rate from zero incidences to a high angle of attack. An application of the Detached-Eddy Simulation model on a NACA 0012 airfoil is presented. The DES model is a method for predicting turbulence in CFD computations, which combines a Reynolds Averaged Navier-Stokes (RANS) method in the boundary layer with a Large Eddy Simulation (LES) in the free shear flow. (DES) turbulence model gives a good accuracy of the flow field because its solves an additional equation for turbulent Reynolds number in a shear stress transport version (SST), which solves a first equation for the turbulent energy K and a second equation for the specific turbulent dissipation rate w. The approach using DES turbulence model is effective because it gives better visualization of flow field, the unsteady separation flow and vortex shedding. Consequently the sugg...

Aerospace Science and Technology, 2005

For the present investigations of dynamic stall a supercritical airfoil was chosen. This new airfoil designed by DLR will be used in dynamic stall control research activities (project ADASYS) planned for the near future: the leading edge portion of the airfoil will be drooped down dynamically to improve dynamic stall characteristics on the retreating side during blade motion. The optimised transonic properties of the airfoil, i.e., reduction of shock strength over a Mach number range will improve in addition the performance of the advancing rotor blade. Dynamic stall experiments on the rigid supercritical airfoil have first been carried out in the DNW-TWG transonic wind tunnel with a 1 m × 1 m cross section of the test section and adaptive top and bottom-walls. This tunnel has the advantage to cover the speed range of both retreating and advancing blade. Emphasis has been placed on unsteady pressure measurements along the adaptive walls simultaneously with the unsteady pressure measurements on the pitching model. In addition to the experiments corresponding numerical simulations with a RANS-code have been carried out and their results are compared with the experimental data. Of main concern are the influence of laminarturbulent boundary-layer transition as well as wind-tunnel-wall interference effects on the unsteady results.

Unsteady Aerodynamics, Aeroacoustics, and Aeroelasticity of Turbomachines and Propellers, 1993

A fully factorized two-dimensional Navier-Stokes flow solver has been developed and applied to the problem of predicting subsonic airfoil flutter in the light stall regime. The inviscid fluxes are evaluated with a central difference ADI scheme and fourth and second order numerical dissipation is used to obtain oscillation-free solutions. The performance of algebraic and one-equation turbulence models in predicting separated flow is explored for computing high Reynolds number steady flow and unsteady flows over an oscillating NACA 0012 airfoil. Comparisons of the computed results with available experimental data indicate that even though the lift response is fairly well predicted, the computation of the pitching moment hysteresis loops is very sensitive to tuibulence modeling. Results computed with several current models are in good agreement whenever the steady stall angle is exceeded only slightly. However, they fail to capture the vortex shedding process leading to the onset of stall flutter.

36th AIAA Aerospace Sciences Meeting and Exhibit, 1998

Compressible dynamic stall flow over an NACA 0012 airfoil undergoing large amplitude pitching oscillations (α= 10 o +10 o sin ωt) at a freestream Mach number of 0.3 and a Reynolds number of 1.1 x 10 6 has been computed by incorporating the transition physics that are an integral part of the compressible dynamic stall process. The computed results have been compared with interferogram-generated pressure distributions for a range of angles to validate the model. The resulting model has been applied in a preliminary study of the flow over a dynamically deforming leading edge (DDLE) airfoil. The DDLE airfoil is being used in a separate study as a novel concept for controlling dynamic stall for a variety of flow conditions. The fundamental fluid physical aspects of the flow are discussed along with the observed influence of transition modeling on the proper representation of the airfoil flow. Nomenclature C D drag coefficient C L lift coefficient C M pitching moment coefficient C f skin friction coefficient C p pressure coefficient C pmin peak suction pressure coefficient c airfoil chord f frequency of oscillation, Hz k reduced frequency = π f c /U ∞ M freestream Mach number U e velocity at edge of boundary layer U ∞ freestream velocity Re Reynolds number based on chord x,y chordwise and vertical distance α angle of attack α o mean angle of attack α a amplitude of oscillation ω circular frequency, radians/sec

Results in Engineering, 2023

Over the last few decades, researchers have been focusing on determining the critical attack angle at which dynamic stall occurs. This angle is usually determined by solving the Navier-Stokes equations, which include viscosity, pressure, gravity, and acceleration terms. However, Navier-Stokes equations are quite complex to solve and consequently difficult to simulate, thus the simulation is not accurate enough. Therefore, this article predicts the critical attack angle for the first time using Euler equations devoid of viscous terms. One of the key advantages of Euler equations is their ability to capture the vortices and predict stall dynamics. The Euler equations are thus integrated and the resulting equations are discretized using the finite volume method. A first-order upwind-based method is used to calculate the convective fluxes at the cell boundaries in the finite volume approach. A NACA 0012 airfoil is chosen for this study at various attack angles with a Mach number of 0.3. Based on the justification of Crocco's theorem, the Euler equations successfully act as Navier-Stokes equations. The vortex patterns are found to behave independently of the artificial dissipation. All the vortices are successfully predicted using the inviscid governing equations. The numerical results obtained are validated by other published experimental and numerical data.

Journal of the Brazilian Society of Mechanical Sciences and Engineering, 2019

A new semiempirical dynamic stall model is presented to predict the aerodynamic coefficients of an airfoil in unsteady conditions. The model is mainly based on two modifications on the airfoil angle of attack which introduce an equivalent angle of attack to implement in the proposed aerodynamic coefficients. The first modification includes unsteady wake effects of the airfoil in the proposed model which are not considered in most of the semiempirical models. Hence, an effective angle of attack is introduced based on an unsteady aerodynamic theory using the Wagner function for pitching and plunging oscillations of the airfoil. The second modification takes into account time delay effects and dynamic effects due to the flow separation on the airfoil. These effects are included in the model by developing a semiempirical relationship between static and dynamic angle of attack published in the literature. Finally, the modifications form the equivalent angle of attack to implement in a set of equations based on Kirchhoff flow theory. The proposed approach dynamically approximates trailing edge separation point on the airfoil by a completely new way and contributes its effects in the aerodynamic coefficients. Moreover, apparent mass effects and the airfoil leading edge vortex contributions are sufficiently included in the model. The presented method effectively predicts unsteady lift and pitching moment coefficients of the airfoil undergoing stall conditions. Consequently, the proposed model is verified against experimental data under various test cases where obtained numerical results are in good agreement with the test data. Furthermore, the performance of the proposed model is compared with various dynamic stall models including Leishman-Beddoes model, ONERA model and Boeing-Vertol model. The comparison shows that the proposed model minimizes the number of the semiempirical parameters determined from an experiment in dynamic stall modeling compared with Leishman-Beddoes model and ONERA model. It also demonstrates that the presented model performs as well as other well-known models using the various approaches. Dynamic stall • Wake effects • Effective angle of attack • Dynamic angle of attack • Leishman-Beddoes

46th AIAA Aerospace Sciences Meeting and Exhibit, 2008

In this research, viscous, unsteady and turbulent fl uid fl ow is simulated numerically around a pitching NACA0012 airfoil in the dynamic stall area. The Navier-Stokes equations are discretized based on the fi nite volume method and are solved by the PIMPLE algorithm in the open source software, namely OpenFOAM. The SST kω model is used as the turbulence model for Low Reynolds Number fl ows in the order of 10 5. A homogenous dynamic mesh is used to reduce cell skewness of mesh to prevent non-physical oscillations in aerodynamic forces unlike previous studies. In this paper, the effects of Reynolds number, reduced frequency, oscillation amplitude and airfoil thickness on aerodynamic force coeffi cients and dynamic stall delay are investigated. These parameters have a signifi cant impact on the maximum lift, drag, the ratio of aerodynamic forces and the location of dynamic stall. The most important parameters that affect the maximum lift to drag coeffi cient ratio and cause dynamic stall delaying are airfoil thickness and reduced frequency, respectively.

2013

In this paper, the influence of turbulence modeling, Reynolds number and stall condition on the fluid numerical solution over NACA0012 airfoil is studied by using the OpenFOAM package. Large-eddy simulation (LES) turbulence model is used to simulate flow at a high angle of attack in near-stall or stall condition. Some vortexes are formed and caused to appear a fluctuation behavior of pressure distribution on the upper surface of the airfoil. Three RANS turbulence models, i.e., K- SST, K-e and RNG K-e are used at low angles of attack than stall condition and lift/drag coefficients of them are compared. Among different turbulence models, the best result for lift coefficient is obtained once K-w SST turbulence model is used and the best drag coefficient is obtained by RNG K-e turbulence model. K SST turbulence model predicted the beginning of stall condition accurately whereas two other models were not able to predict it. The effects of Reynolds number on the lift coefficient have be...

Unsteady, turbulent flow simulations are carried out for a NACA 0012 airfoil. The objective is to assess the performance of various turbulence models for application to fluid-structure interaction and airfoil optimization problems. Unsteady simulations are carried out at an angle of attack of 20 degrees and a chord Reynolds number of 10 5 . The Spalart Allmaras and Speziale k−τ models are used in unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations. Filtered Navier Stokes simulations in conjunction with an adaptive k − τ model proposed by Magagnato are also presented. This adaptive model can be used at all grid or temporal resolutions and tends assymptotically to DNS or RANs in the limit of fine grid resolution or steady state simulations. This model produces physically realistic flow patterns, rich vortex dynamics and non-periodic lift and drag signals. The k − τ model also yields unsteady flow patterns, but these remain purely periodic, whereas only steady state solutions are obtained with the Spalart Allmaras model.

The recent development of large wind turbines poses new challenges with regard to understanding the mechanisms surrounding unsteady flow-structure interaction. The larger and more flexible blades imply risks from an aeroelastic point of view and urge the need to properly understand and model these phenomena. Due to limited experimental data available in this field, Computational Fluid dynamics (CFD) techniques provide an invaluable alternative to identify and model aerodynamic and aeroelastic phenomena around the wind blades. The study is part of the coupling between aerodynamic and elastic models of the commercial code-CFX with ANSYS, respectively. In this paper we are modeling the dynamic stall that describes the complex events that result in the delay of stall on airfoils undergoing unsteady motion. This study is realized on the S809 airfoil for which experimental data are available in literature, specifically for the study of periodic unsteady motion. The study will use the k-ω SST intermittency model. The k-ω SST model is based on a blending of the Wilcox k-ω model and the k-ε model and combines the advantages of the two models but it also accounts for the transport of the turbulent shear stress and so proposed good predictions especially for separated flows. A transition model is added to the k-ω SST model that allows the user to choose the intermittency factor in order to describe the transition. The domain discretization was carried using an unstructured mesh. The simulations are made on the S809 airfoil with a Reynolds number of one million and an angle of attack of 8° ±5.5°, 14° ±5.5 and 20° ±5.5° with a reduced frequency of k=0.026.