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research-article

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

[+] Author and Article Information
David Holst

Chair of Fluid Dynamics, Hermann-Föttinger-Institut, Technische Universität Berlin, Müller-Breslau-Str. 8, 10623 Berlin, Germany
david.holst@tu-berlin.de

Benjamin Church

Chair of Fluid Dynamics, Hermann-Föttinger-Institut, Technische Universität Berlin, Müller-Breslau-Str. 8, 10623 Berlin, Germany
church.benni@gmail.com

Felix Wegner

Chair of Fluid Dynamics, Hermann-Föttinger-Institut, Technische Universität Berlin, Müller-Breslau-Str. 8, 10623 Berlin, Germany
felix.wegner5@googlemail.com

Georgios Pechlivanoglou

Chair of Fluid Dynamics, Hermann-Föttinger-Institut, Technische Universität Berlin, Müller-Breslau-Str. 8, 10623 Berlin, Germany
george@pechlivanoglou.com

Christian Navid Nayeri

Chair of Fluid Dynamics, Hermann-Föttinger-Institut, Technische Universität Berlin, Müller-Breslau-Str. 8, 10623 Berlin, Germany
christian.nayeri@tu-berlin.de

Christian Oliver Paschereit

Chair of Fluid Dynamics, Hermann-Föttinger-Institut, Technische Universität Berlin, Müller-Breslau-Str. 8, 10623 Berlin, Germany
oliver.paschereit@tu-berlin.de

1Corresponding author.

ASME doi:10.1115/1.4041146 History: Received July 03, 2018; Revised July 20, 2018

Abstract

The wind industry needs reliable and accurate airfoil polars to properly predict wind turbine performance, especially during the initial design phase. Medium- and low-fidelity simulations directly depend on the accuracy of the airfoil data and even more so if e.g. dynamic effects are modeled. This becomes crucial if the blades of a turbine operate under stalled conditions for a significant part of the turbine's lifetime. In addition, the design process of vertical axis wind turbines (VAWTs) needs data across the full range of angles of attack between 0 and 180 deg. Lift, drag and surface pressure distributions of a NACA 0021 airfoil equipped with surface pressure taps were investigated based on time-resolved pressure measurements. The present study discusses full range static polars and several dynamic sinusoidal pitching configurations covering two Reynolds numbers Re = 140k and 180 k, and different incidence ranges: near stall, post stall and deep stall. Various bi-stable flow phenomena are discussed based on high frequency measurements revealing large lift-fluctuations in the post and deep stall regime that exceed the maximum lift of the static polars and are not captured by averaged measurements. Detailed surface pressure distributions are discussed to provide further insight into the flow conditions and pressure development during dynamic motion. The experimental data provided within the present paper is dedicated to the scientific community for calibration and reference purposes, which in the future may lead to higher accuracy in performance predictions during the design process of wind turbines.

Copyright (c) 2018 by ASME
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