CFD analysis of a turbine, interpreting torque values

[Jeviah]

Homework Statement

Use CFD to determine the characteristics of a turbine.

Relevant Equations

N/A

Hello, I am using CFD to analyse a Kaplan turbine and am unsure on how to interpret the torque values as they come as x, y and z components. My understanding is that the z component in this case will determine the rotation direction depending on whether it is positive or negative however I am unsure how to interpret the x and y components, my thinking is that I would use vector addition to find the resulting magnitude of torque with the z component determining whether the part is rotating clockwise or anticlockwise? or would a have to do something different?

Current torque values:
195.3 (x component)
-60.2 (y component)
-6.9 (z component)

 

[BvU]

There is something wrong here: from symmetry I would say ##\tau_x = \tau_y = 0## for an even number of blades. The ##\tau_z## should all be in one and the same direction and add up constructively.
So to see ##|\tau_z|## as the smallest value is unexpected !

The sign is negative, I assume because your flow is in the ##+z## direction ?

 

[Jeviah]

There is something wrong here: from symmetry I would say ##\tau_x = \tau_y = 0## for an even number of blades. The ##\tau_z## should all be in one and the same direction and add up constructively.
So to see ##|\tau_z|## as the smallest value is unexpected !

The sign is negative, I assume because your flow is in the ##+z## direction ?

Correct my low is in the z+ direction. It seems I have got incorrect information somewhere when asking about torque...
So to clarify the torque value which determines the rotation and therefore power etc is the z component? So on any orientation you would take the torque value from the rotating axis for calculations?

Just for a better understanding as well would I be correct in saying the the X and Y components are essentially trying to tilt the whole turbine?

 

[BvU]

Correct my low is in the z+ direction. It seems I have got incorrect information somewhere when asking about torque...
So to clarify the torque value which determines the rotation and therefore power etc is the z component? yes
So on any orientation you would take the torque value from the rotating axis for calculations?

Just for a better understanding as well would I be correct in saying the the X and Y components are essentially trying to tilt the whole turbine? yes

Torque is around an axis and points along that axis of rotation.

 

[Jeviah]

Torque is around an axis and points along that axis of rotation.

Thank you very much for the quick reply.
One final question about how CFD works, am I correct in saying that when using surface goals, the surface goal will only record the torque applied to said surface by the fluid, opposed to including bodies attached to it?
For example a surface goal on an axle will only record the torque generated directly by the fluid, it won't include how the torque from the blades will actually alter it

 

[BvU]

Sorry, I have no idea -- you'll have to ask the program's customer support.