Environmental Engineering Reference
In-Depth Information
Figure 5-57. Examples of dynamic stall vortex topologies, illustrating the type of dia-
gram developed specifically for tracking the evolution of vortices with time. Yaw angles
are 20, 30, 40, and 50 deg (top planform to bottom) for U ¥ = 13 m/s. [Schreck et al. 2001
and 2005]
for wind turbine applications, and are summarized briefly below. Because all such models
are semi-empirical in nature, these should be considered to be reconstructions rather than
predictions of turbine aerodynamic response.
An early dynamic stall model was developed by Johnson [1980] based upon experi-
mental aerodynamic data. This model assumed that a dynamic stall vortex generates a large
lift increase, having a brief rise time to maximum lift and an equally brief decay time back
to static lift levels. Required inputs are few and easily acquired, consisting of static stall
angle of attack and lift for the blade airfoil section, blade chord length, rate of angle of at-
tack change, and local inflow speed. However, depending on the application, limitations in
specifying inputs may impose corresponding limitations in model generality.
The Boeing-Vertol model traces its origins to work by Harris et al. [1970]. In this meth-
od, unsteady inviscid stall angle is incremented by a factor that includes the a parameter
termed the reduced frequency and an empirical constant that is intended to approximate the
time lag in flow field development physically imposed by viscosity. Explicit inputs consist
of static lift curve characteristics, and the empirical constant accounts for effects of airfoil
thickness, Mach number, and Reynolds number.
The ONERA dynamic stall model [Tran and Petot 1981] superimposes two distinct com-
ponents to obtain dynamic stall aerodynamic loading. The first component corresponds to
Search WWH ::




Custom Search