Experimental Analysis of a NACA 0021 Airfoil Under Dynamic Angle of Attack Variation and Low Reynolds Numbers

Holst, David; Church, Benjamin; Wegner, Felix; Pechlivanoglou, George; Nayeri, Christian Navid; Paschereit, Christian Oliver

doi:10.1115/gt2018-76514

Cited by 4 publications

References 18 publications

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Experimental Analysis of a NACA 0021 Airfoil Section Through 180-Degree Angle of Attack at Low Reynolds Numbers for Use in Wind Turbine Analysis

Holst

Church

Pechlivanoglou

et al. 2017

Volume 9: Oil and Gas Applications; Supercritical CO2 Power Cycles; Wind Energy

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Wind turbine industry has a special need for accurate post stall airfoil data. While literature often covers incidence ranges [−10deg,+25deg] smaller machines experience a range of up to 90 deg for horizontal axis and up to 360 deg for vertical axis wind turbines (VAWTs). The post stall data of airfoils is crucial to improve the prediction of the start-up behavior as well as the performance at low tip speed ratios. The present paper analyzes and discusses the performance of the symmetrical NACA 0021 airfoil at three Reynolds numbers (Re = 100k, 140k, and 180k) through 180 deg incidence. The typical problem of blockage within a wind tunnel was avoided using an open test section. The experiments were conducted in terms of surface pressure distribution over the airfoil for a tripped and a baseline configuration. The pressure was used to gain lift, pressure drag, moment data. Further investigations with positive and negative pitching revealed a second hysteresis loop in the deep post stall region resulting in a difference of 0.2 in moment coefficient and 0.5 in lift.

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Experimental Analysis of a NACA 0021 Airfoil Section Through 180-Degree Angle of Attack at Low Reynolds Numbers for Use in Wind Turbine Analysis

Holst

Church

Pechlivanoglou

et al. 2017

Volume 9: Oil and Gas Applications; Supercritical CO2 Power Cycles; Wind Energy

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Combined Numerical and Experimental Study on the Use of Gurney Flaps for the Performance Enhancement of NACA0021 Airfoil in Static and Dynamic Conditions

Balduzzi

Holst

Melani

et al. 2020

Volume 12: Wind Energy

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The use of power augmentation devices in wind energy applications has been receiving increasing interest from both the scientific and the industrial community. In particular, Gurney Flaps (GFs) showed a great potential thanks to the passive functioning, the simple construction and the possibility to add them as a retrofit to existing rotors. The possibility of having a high quality set of airfoil data for a wide range of both angle of attack (AoA) and Reynolds number is pivotal in the design phase of newly developed machines. Such data are usually available in the technical literature for smooth airfoils, while there is a lack of generalized results in case of airfoils with GFs. Within this context, the authors have performed an extended investigation on the lift increase capabilities of GFs for the well-known NACA 0021 airfoil, which has been used in several wind energy applications up to now. The present paper shows the results of a combined experimental and numerical analysis considering different geometrical configurations of the flaps under both static and dynamic conditions. Experimental data were first obtained for the AoA range of 180 degrees at a low value of Reynolds number (i.e. Re = 180 k) to analyze the impact of three different geometrical configurations of the GF on the aerodynamic behavior for the full range of incidence angles. The geometrical configurations were defined by varying the length of the flap (i.e. 1.4% and 2.5% of the chord length) and its inclination angle with respect to the blade chord (i.e. 90 degrees and 45 degrees). The experimental investigation involved also dynamic sinusoidal pitching movements at multiple reduced frequencies to evaluate the stall hysteresis cycle. Subsequently, an unsteady CFD numerical model was first calibrated against wind tunnel data at low values of the Reynolds number. Then, the virtual model was exploited to extend the investigation to a wider range of Reynolds number for dynamic AoA rates of change typical for example of vertical-axis wind turbines, i.e. characterized by higher reduced frequencies with a non-sinusoidal motion law.

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Cartographing dynamic stall with machine learning

Lennie

Steenbuck

Noack

et al. 2019

Preprint

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Abstract. Airfoil stall is bad for wind turbines. Once stall has set in, lift collapses, drag increases and then both of these forces will fluctuate strongly. The result is higher fatigue loads and lower energy yield. In dynamic stall, separation first develops from the trailing edge up the leading edge, eventually the shear layer rolls up and then a coherent vortex forms and then sheds downstream with it’s low pressure core causing a lift spike and moment dump. When 50+ experimental cycles of lift or pressure values are averaged, this process appears clear and coherent in flow visualizations. Unfortunately, stall is not one clean process, but a broad collection of processes. This means that the analysis of separated flows should be able to detect outliers and analysis cycle to cycle variations. Modern data science/machine learning can be used to treat separated flows. In this study, a clustering method based on dynamic time warping is used to find different shedding behaviors. This method captures that secondary and tertiary vorticity vary strongly and in static stall with surging flow; the flow can occasionally reattach. A convolutional neural network was used to extract dynamic stall vorticity convection speeds and phases from pressure data. Finally, bootstrapping was used to provide best practices regarding the number of experimental repetitions required to ensure experimental convergence.

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Experimental Analysis of a NACA 0021 Airfoil Under Dynamic Angle of Attack Variation and Low Reynolds Numbers

Cited by 4 publications

References 18 publications

Experimental Analysis of a NACA 0021 Airfoil Section Through 180-Degree Angle of Attack at Low Reynolds Numbers for Use in Wind Turbine Analysis

Experimental Analysis of a NACA 0021 Airfoil Section Through 180-Degree Angle of Attack at Low Reynolds Numbers for Use in Wind Turbine Analysis

Combined Numerical and Experimental Study on the Use of Gurney Flaps for the Performance Enhancement of NACA0021 Airfoil in Static and Dynamic Conditions

Cartographing dynamic stall with machine learning

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