POST: Calculating value based on velocity at another height...

MarkHerskind · September 22, 2014, 07:29

I'm guessing this is probably a simple question, but I just can't seem to find where to get this....

I'm doing simulations of flow over terrain..
I need to visualize a surface with value at a certain height, which is based on values from two other heights...

Basically I want to plot the Wind Shear at for a wind turbine, which is based on the wind speed at upper and lower tip, but I need it displayed at hub height..

So I need something like
velocity u@(x,y,z+dz)

Thanks in advance..

JuPa · September 22, 2014, 11:49

Attach an annotated image of what you want. Your description isn't the best.

singer1812 · September 22, 2014, 16:43

Is User Surface >> Offset From Surface what you need?

You can create a user surface offset from a surface either via Surface Normal or Translational. The distance can be an expression (so it could be f(v) if you so desire).

MarkHerskind · September 23, 2014, 04:35

Hi again..

Sorry, if my question was unclear.. I'll try to explain..

When positioning a wind turbine, one of the values needed to evaluate is the wind shear exponent..

Wind shear (alpha) is calculated between two heights like this:
alpha=ln(u1/u2)/ln(h1/h2)..

(u=wind speed, h=height above ground level)

The wind shear can be calculated between any heights, but for wind turbine purposes, it's often done between upper tip/lower tip or lower tip/hub height..

I would like a surface, located at hub height, showing the windshear exponents... So in short: I need to visualize a plane, showing data combining data from two other planes/heights...

Did that make more sense?

singer1812 · September 23, 2014, 10:11

Still dont understand. Why don't you just use a Plane?

MarkHerskind · September 24, 2014, 04:20

As far as I can figure, when I'm using a plane (or a user surface, as it needs to at the same offset from the ground), I can only use the data on that plane.. The "velocity u" will give me the velocity at that any point, but only on the active plane... To be able to display the wind shear as I described in my previous post, I need to combine the velocity on the plane with another velocity not on the plane...

As an example. If we look at a single points on the display surface; lets say this surface is at 80m above the ground, which is the hub height of the turbine.. To draw the wind shear between lower tip and hub, I need the wind speed at hub (80m above ground) and lower tip (lets say 30m above ground)..
My usersurface will be located 80m offset from the ground-face, so the wind speed at hub is given.. But how do I get the wind speed at 30m from the ground?

singer1812 · September 24, 2014, 08:39

Ahh I see where you are going now. I am not sure you can do this easily within post. You might have to export the 2 surface data set to MATLAB (or some other tool) and do it within that.

Perhaps someone else has done something like this in post.

If you dont need a full surface contour, you could do points in post and use perl to roll through your points to get data out that way, but it is not as continous as what I think you want.

MarkHerskind · September 24, 2014, 09:42

My temporary solution is to export the data from two usersurfaces, sending it through matlab and just develop a plot.. That solution works..

However, what I really want, is to be able to make it as a surface in post and export .cvf files to have it in 3D to view through the Ansys Viewer...

I have considered the point-by-point option, but as far as I can figure it wont give me nearly the result I'm hoping for..

Maybe this isn't as simple as I thought.. :/

Thank you for trying..

September 22, 2014, 07:29	POST: Calculating value based on velocity at another height...	#1
MarkHerskind New Member Mark Herskind Join Date: Oct 2010 Posts: 12 Rep Power: 16	I'm guessing this is probably a simple question, but I just can't seem to find where to get this.... I'm doing simulations of flow over terrain.. I need to visualize a surface with value at a certain height, which is based on values from two other heights... Basically I want to plot the Wind Shear at for a wind turbine, which is based on the wind speed at upper and lower tip, but I need it displayed at hub height.. So I need something like velocity u@(x,y,z+dz) Thanks in advance..

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September 22, 2014, 11:49		#2
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 15	Attach an annotated image of what you want. Your description isn't the best.

September 22, 2014, 16:43		#3
singer1812 Senior Member Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 511 Rep Power: 21	Is User Surface >> Offset From Surface what you need? You can create a user surface offset from a surface either via Surface Normal or Translational. The distance can be an expression (so it could be f(v) if you so desire).

September 23, 2014, 04:35		#4
MarkHerskind New Member Mark Herskind Join Date: Oct 2010 Posts: 12 Rep Power: 16	Hi again.. Sorry, if my question was unclear.. I'll try to explain.. When positioning a wind turbine, one of the values needed to evaluate is the wind shear exponent.. Wind shear (alpha) is calculated between two heights like this: alpha=ln(u1/u2)/ln(h1/h2).. (u=wind speed, h=height above ground level) The wind shear can be calculated between any heights, but for wind turbine purposes, it's often done between upper tip/lower tip or lower tip/hub height.. I would like a surface, located at hub height, showing the windshear exponents... So in short: I need to visualize a plane, showing data combining data from two other planes/heights... Did that make more sense?

September 23, 2014, 10:11		#5
singer1812 Senior Member Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 511 Rep Power: 21	Still dont understand. Why don't you just use a Plane?

September 24, 2014, 04:20		#6
MarkHerskind New Member Mark Herskind Join Date: Oct 2010 Posts: 12 Rep Power: 16	As far as I can figure, when I'm using a plane (or a user surface, as it needs to at the same offset from the ground), I can only use the data on that plane.. The "velocity u" will give me the velocity at that any point, but only on the active plane... To be able to display the wind shear as I described in my previous post, I need to combine the velocity on the plane with another velocity not on the plane... As an example. If we look at a single points on the display surface; lets say this surface is at 80m above the ground, which is the hub height of the turbine.. To draw the wind shear between lower tip and hub, I need the wind speed at hub (80m above ground) and lower tip (lets say 30m above ground).. My usersurface will be located 80m offset from the ground-face, so the wind speed at hub is given.. But how do I get the wind speed at 30m from the ground?

September 24, 2014, 08:39		#7
singer1812 Senior Member Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 511 Rep Power: 21	Ahh I see where you are going now. I am not sure you can do this easily within post. You might have to export the 2 surface data set to MATLAB (or some other tool) and do it within that. Perhaps someone else has done something like this in post. If you dont need a full surface contour, you could do points in post and use perl to roll through your points to get data out that way, but it is not as continous as what I think you want.

September 24, 2014, 09:42		#8
MarkHerskind New Member Mark Herskind Join Date: Oct 2010 Posts: 12 Rep Power: 16	My temporary solution is to export the data from two usersurfaces, sending it through matlab and just develop a plot.. That solution works.. However, what I really want, is to be able to make it as a surface in post and export .cvf files to have it in 3D to view through the Ansys Viewer... I have considered the point-by-point option, but as far as I can figure it wont give me nearly the result I'm hoping for.. Maybe this isn't as simple as I thought.. :/ Thank you for trying..