Editing Stall (fluid dynamics) (section)

===Dynamic stall===
{{anchor|Stall delay}}
Dynamic stall is a non-linear unsteady aerodynamic effect that occurs when airfoils rapidly change the angle of attack. The rapid change can cause a strong [[vortex]] to be shed from the leading edge of the aerofoil, and travel backwards above the wing.<ref>{{cite journal |last1=Buchner |first1=A. J. |last2=Soria |first2=J. |title=Measurements of the flow due to a rapidly pitching plate using time resolved high resolution PIV |journal=Aerospace Science and Technology |volume=44 |pages=4–17 |year=2015 |doi=10.1016/j.ast.2014.04.007|bibcode=2015AeST...44....4B }}</ref><ref>{{cite journal |last1=Khalifa |first1=Nabil M. |last2=Rezaei |first2=Amir S. |last3=Taha |first3=Haithem E. |date=2021 |title=Comparing the performance of different turbulence models in predicting dynamic stall |journal=AIAA Scitech 2021 Forum |pages=1651 |doi=10.2514/6.2021-1651|isbn=978-1-62410-609-5 |s2cid=234321807 }}</ref> The vortex, containing high-velocity airflows, briefly increases the lift produced by the wing. As soon as it passes behind the trailing edge, however, the lift reduces dramatically, and the wing is in normal stall.<ref name="Filippone">{{cite web|url=http://aerodyn.org/Dstall/dstall.html |title=Dynamic Stall, Unsteady Aerodynamics |access-date=25 March 2016 |url-status=unfit |archive-url=https://web.archive.org/web/20071229110350/http://aerodyn.org/Dstall/dstall.html |archive-date=29 December 2007 }}</ref>

Dynamic stall is an effect most associated with helicopters and flapping wings, though also occurs in wind turbines,<ref>{{cite journal |doi=10.1017/jfm.2018.112 |title=Dynamic stall in vertical axis wind turbines: Scaling and topological considerations |journal=Journal of Fluid Mechanics |volume=841 |pages=746–66 |year=2018 |last1=Buchner |first1=A-J. |last2=Soria |first2=J. |last3=Honnery |first3=D. |last4=Smits |first4=A.J. |bibcode=2018JFM...841..746B |s2cid=126033643 |doi-access=free }}</ref> and due to gusting airflow. During forward flight, some regions of a helicopter blade may incur flow that reverses (compared to the direction of blade movement), and thus includes rapidly changing angles of attack. Oscillating (flapping) wings, such as those of insects like the [[bumblebee]]—may rely almost entirely on dynamic stall for lift production, provided the oscillations are fast compared to the speed of flight, and the angle of the wing changes rapidly compared to airflow direction.<ref name="Filippone" />

Stall delay can occur on [[airfoils]] subject to a high angle of attack and a three-dimensional flow. When the angle of attack on an airfoil is increasing rapidly, the flow will remain substantially attached to the airfoil to a significantly higher angle of attack than can be achieved in steady-state conditions. As a result, the stall is delayed momentarily and a lift coefficient significantly higher than the steady-state maximum is achieved. The effect was first noticed on [[Propeller (aircraft)|propellers]].<ref>{{cite book
|last=Burton
|first=Tony
|author2=David Sharpe |author3=Nick Jenkins |author4=Ervin Bossanyi
 |title=Wind Energy Handbook
|url=https://books.google.com/books?id=4UYm893y-34C&pg=PA139
|year= 2001
|publisher= John Wiley and Sons
|isbn= 978-0-471-48997-9
|page=139
}}
</ref>