Performance - Wind Energy: Renewable Energy and the Environment - page 170

stall flags are pressure driven. The stall flag responds to separated flow with an optical signal, which
exceeds the tuft signals by a factor of 1,000 [33]. Smoke shows the stream flow for airfoils in wind
tunnels and the generation of tip vortices from ends of blades and their propagation downstream
[34]. Smoke released from tethered smoke generators was used to observe the evolution of tip vor-
tices from the MOD-2 [35]. The vortex became unstable when it passed through the wake of the
turbine tower.
Blades on downwind turbines pass through the wake of the tower, so there is a change in
attack angle and flow across the blade, which also generates noise. Flow visualization was used
to study the flows [36] over the blades of an Enertech 21 (6.4 m diameter, 5 kW), with and without
tip brakes; a Carter 25 (10 m diameter, 25 kW); and an Enertech 44 (13.4 m diameter, 50 kW). All
three units were downwind, constant-rpm wind turbines. A video camera and a 35 mm camera
were mounted on a boom attached to the root of the blade. Tuffs and oil flow revealed the nature
and many of the details of the flows, such as laminar separation bubbles, turbulent reattachment,
and complete separation over part or almost all of the blade. Full or partial reattachment due to
tower shadow was observed on each unit (Figure 8.25). Spanwise, flow was observed near the
leading edge of the Enertech 21, and almost the whole blade was in stall at high wind speeds.
The tip brakes on the Enertech units are important in retaining attached flow near the tip. The
oil streak pattern after 4 minutes in winds from 7 to 15 m/s on the Enertech 44 blade shows
that below 0.5 blade length, the flow is completely separated. However, the flows on the highly
twisted and tapered Carter 25 blade are attached in medium winds. The flows show a turbulent
type edge separation, which begins at about half the radius and progresses forward. Pressure-
sensitive liquid crystals were tried, but field results were not good, as the lighting has to be just
right to observe the color changes.
It should be noted that vortices, alternating on each side, will be shed by cylinders in wind flow,
which can induce vibration in the cylinder. On the VAWT 34 m test bed, a spiral staircase on the
torque tube eliminated these vortices.
FIGURE 8.25 One blade of the Enertech 21 over one revolution. Shaded areas show representative pattern of
attached flow. Note the strong reattachment due to tower shadow.