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Geometrical characterization of stall cells on rectangular wings
Wind Energy, Pages: n/a - n/a
Swansea University Author: Marinos Manolesos
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The onset of stall cells (SCs) is experimentally investigated on a flattop loaded 18% thick airfoil optimized for use on wind turbine blades, exhibiting trailing edge separation. SCs are dynamic coherent vortical structures that appear on wings under separated flow conditions. Although SCs have been...
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The onset of stall cells (SCs) is experimentally investigated on a flattop loaded 18% thick airfoil optimized for use on wind turbine blades, exhibiting trailing edge separation. SCs are dynamic coherent vortical structures that appear on wings under separated flow conditions. Although SCs have been known for long, neither are their characteristics completely documented nor their generating mechanisms fully understood. The present investigation aims at providing additional information on the geometric characteristics in terms of width, length and occupied area. The relevant data are presented as functions of Reynolds (Re) number, angle of attack and aspect ratio (AR) of the model. In the tests reported, the dynamic character of SCs is suppressed by imposing a localized flow disturbance. For the specific airfoil and for the Re and AR range tested, it is found that: the angle of attack at which SCs are initially formed decreases linearly with Re number and independently of the AR; unlike two‐dimensional separation, their chordwise length increases with Re; the SC area relative to the wing planform area (defined as the relative SC area) grows asymptotically with angle of attack and Re number reaching an upper bound, which is independent of the AR; at intermediate angles of attack, the SC relative area is higher for the lower AR wing; for a fixed increment in Re number, the growth of the SC relative area is independent of the initial Re number; at lower angles of attack, the actual SC area is independent of the wing span.
This paper is the first of the outcomes from the GZF032 / 2009–2010 Onassis PhD scholarship grant. It sets the ground for Stall Cell research and based on its findings a number of publications followed (including https://doi.org/10.1063/1.4869726, https://doi.org/10.1016/j.jweia.2015.03.020, https://doi.org/10.1002/we.1609). The main finding, which was not known at the time, is that it is possible to stabilize the inherently unsteady Stall Cells, which are 3D coherent structures of separated flow. The work is relevant to different types of separated flows from Unmanned Air Vehicles to Wind Turbine blades, as Stall Cell instability is linked to flutter and aeroelastic loads.