# Wind Turbine, wind direction, air direction, drag direction?

1. Oct 14, 2015

### gamz95

In the aeroplanes, the drag is in the opposite way of the direction of the aeroplane. My questions for wind turbine:

1) Why "wind speed" and "the drag on the blade" is in the same direction? Shouldn't be the opposite?

2) Are freestream velocity and the wind speed same thing?

3) Are the drag and the air resistance same thing?

2. Oct 14, 2015

### Staff: Mentor

Generally, lift and drag are defined to be perpendicular components of the single resultant aerodynamic force. They are typically defined parallel and perpendicular to freesstream velocity.

3. Oct 14, 2015

### cjl

Think of the airplane's frame of reference (so the airplane is stationary, with a strong wind coming from ahead of it). This should help visualize the similarities.

4. Oct 14, 2015

### rcgldr

I'm wondering about which frame of reference to use here, and if the blades on a turbine could be considered as similar to a sail. If considered similar to a sail, then each blade sees an apparent wind (free stream?) relative to the blade, but the speed varies with the radius, so yet another issue to consider.

5. Oct 14, 2015

### CWatters

Notwithstanding #2...

Just about anything that moves through air will experience a force that is in the same direction as the wind. Drop a brick off a cliff.. the airspeed is vertically upwards relative to the brick. The drag force is also vertical upwards

6. Oct 14, 2015

### RandomGuy88

The direction of lift and drag are defined relevative the local flow and this changes along the blade. The local freestream direction and speed depend on the wing and the rotational velocity of the turbine. As you move along the blade the exact direction of these forces will change. On a turbine it is generally most useful to decompose the forces in a direction in that generates torque and a direction perpendicular to the plane of the rotor.

7. Oct 15, 2015

### gamz95

Yes, in this case makes sense. But, then we couldn't say that drag is opposite of the motion, right? Because we think aeroplane as fixed?
Aha! So, we are saying that wind speed and the drag in the same direction, right? When we run with wind, drag is with us? When we run towards the wind, drag is towards us? Is this true? In your example, the airspeed actually caused by the velocity of the brick; so the brick velocity downward, the airspeed should be upward. Where is wind in this case? Do we have three velocities as; airspeed, drag, and the wind?

8. Oct 15, 2015

### Staff: Mentor

Exactly. Note though that this isn't different from an airplane climbing or descending. It's still the relative wind that matters.

9. Oct 15, 2015

### A.T.

The wind here is relative the body you analyze, which for a windmill blade has a different direction and speed than the wind relative to the ground.

10. Oct 15, 2015

### rcgldr

The issue here is the frame of reference, which is a rotating frame of reference in the case of the blades on a turbine, complicating matters since the relative speed is zero at the center and maximum at the outer edges of the blades. Like any wing or sail, the blades divert a relative flow. As mentioned previously in this thread, the change in the relative flow due to diversion can be seperated into two components, the perpendicular change is related to lift, and the decrease in the component in the direction of flow is related to drag.

The relative airflow approaches a direction perpendicular to the horizontal axis of a windmill at the outer edges of the rotor blades (for a large turbine, close to 320 kph with perhaps less than 50 kph wind). Only part of the lift generates the torque that drives the windmill, while much of the drag reduces the torque, so a high lift to drag ratio is needed. Switching to a ground frame of reference, the rest of the lift and drag push downwind (relative to the ground) against the windmill. The aerodyanmic torque and force are part of a Newton third law pair, so from a ground perspective, the air exerts a torque on the windmill, and the windmill exerts an equal in magnitude but opposing torque on the air. The air exerts a downwind force on the windmill, and the windmill exerts an equal in magnitude but opposing upwind force on the air. The wind ends up being rotated and slowed down.

11. Oct 16, 2015

### gamz95

Thank you. I kind of understand now. It would be perfect if I can see these relationships drawn in a paper though:)