Page 165 - Wind Energy Handbook

STALL DELAY 139

3.12 Stall Delay

A phenomenon first noticed on propellers by Himmelskamp (1945) is that of lift
coefficients being attained at the inboard section of a rotating blade which are
significantly in excess of the maximum value possible in two-dimensional static
tests. In other words the angle of attack at which stall occurs is greater for a rotating
blade than for the same blade tested statically. The power output of a rotor is
measurably increased by the stall-delay phenomenon and, if included, improves
the comparison of theoretical prediction with measured output. It is noticed that
the effect is greater near the blade root and decreases with radius.
The reason for the stall delay has been the cause of much discussion but a
convincing physical process has not yet been established. What is agreed is that, for
whatever reason, the adverse pressure gradient experienced by the flow passing
over the downwind surface of the blade is reduced by the blade’s rotation. The
adverse pressure gradient slows down the flow as it approaches the trailing edge of
the blade after the velocity peak reached close the the leading edge. In the boundary
layer viscosity also slows down the flow and the combination of the two effects, if
sufficiently large, can bring the boundary layer flow to a standstill (relative to the
blade surface) or even cause a reversal of flow direction. When flow reversal takes
place the flow separates from the blade surface and stall occurs giving rise to loss of
lift and a dramatic increase in pressure drag.
Aerodynamic analyses (Wood (1991) and Snel et al. (1993)) of rotating blades
using computational fluid dynamic techniques, which include the effects of viscos-
ity, do show a decrease in the adverse pressure gradient but it is not obvious from
these numerical calculations as to what exactly is occurring physically.
It is also agreed that the parameter that influences stall delay predominantly is
the local blade solidity c(r)=r. The evidence which does exist shows that for
attached flow conditions, below what would otherwise be the static (non-rotating)
stall angle of attack, there is little difference between two-dimensional flow condi-
tions and rotating conditions. When stall does occur, however, the air in the
separated region, which is moving very slowly with respect to the blade surface, is
rotating with the blade and so is subject to centrifugal force causing it to flow
radially outwards. Prior to stalling taking place, centrifugal forces on the fluid in
the boundary layer, again causing radial flow, may reduce the displacement
thickness and so increase the resistance to separation.
Blade surface pressures have been measured by Ronsten (1991) on a blade while
static and while rotating. Figure 3.74 shows the comparison of surface pressure
coefficients for similar angles of attack in the static and rotating conditions (tip
speed ratio of 4.32) for three span-wise locations. At the 30% span location the
estimated angle of attack at 30:418 is well above the static stall level which is
demonstrated by the static pressure coefficient distribution. The rotating pressure
coefficient distribution at 30% span shows a high leading edge suction pressure
peak with a uniform pressure recovery slope over the rear section of the upper
surface of the chord. The gradual slope of the pressure recovery indicates a reduced
adverse pressure gradient with the effect on the boundary layer that it is less likely
to separate. The level of the leading edge suction peak, however, is very much less