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What's the difference between /frame/mov and /skew/mov

Hi all,

1. What's the difference between /frame/mov and /skew/mov? From the user manual, I don't find the difference. In my opinion, this two keywords are defining a local system by selecting three nodes.

2. In keyword /impvel, it can select a skew or a frame. What's the difference? I cannot understand the comments in the manual about 6 and 7 figure below.

frame_mov.JPG.e0c76a378b623b91135c6db2d3ad0f47.JPG skew_mov.JPG.a0fcd84e81b564c2d7e312a62b635894.JPG impvel.JPG.2ae1d00211a76ef299dd56272f68fef4.JPG impvel_2.JPG.82437484399d37a8dc1244b3f2e14ed9.JPG

Anyone can explain the difference? Thank you

Roy

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Altair Forum User

Anyone can help to explain?

Altair Forum User

Hi Roy,

Apologies for the delay.

Skews and frames are used to define local directions. These directions can be used to apply:

• boundary conditions

• concentrated load

• fixed velocity

The /SKEW/MOV and /FRAME/MOV are used to creating moving skew or moving frame. For most applications /SKEW is used rather then /FRAME.

Basically, FRAME measures relative motion where as SKEW measures global motion and projects it to skew. Only few options use FRAME like imposed velocity.

I'm attaching a document on this which explains these concepts with example.

Unable to find an attachment - read this blog

Altair Forum User

Hi George P,

Thank you for the document. However, I think I am more confused. How to understand the statement in the document: In fact, the origin of the skew remains at the initial position during the motion even though a moving skew is defined. Does the direction of the axis change during motion?

Suppose I define a /skew/mov and a /frame/mov by using the same three nodes. Is there any difference by defining an /impvel using a skew or a frame?

Best Wishes

Roy

Altair Forum User

Hi Roy,

The definitions in the Help may slightly confuse.

Please check the video file attached along where a /FRAME/MOV is used.

Two similar components are provided with an imposed velocity. For the left side component a function for imposed velocity is created and a moving frame is defined for that component . For the right side component the imposed velocity function defined is 0, but the created reference frame for the other component is recalled here. The relative motion with respect to the reference frame is measured which makes the other component to rotate. But for /SKEW/MOV this will not work.

For most applications /SKEW is used rather then /FRAME. Only few options use FRAME like imposed velocity.

Hope this clears your doubt.

<?xml version="1.0" encoding="UTF-8"?>

Unable to find an attachment - read this blog

Altair Forum User

Hi George,

Could you please share the complete input file (_0000.rad and _0001.rad) about this simple example to me? It will help me to understand.

Thank you

Roy

Altair Forum User

Hi Roy,

The definitions in the Help may slightly confuse.

Please check the video file attached along where a /FRAME/MOV is used.

Two similar components are provided with an imposed velocity. For the left side component a function for imposed velocity is created and a moving frame is defined for that component . For the right side component the imposed velocity function defined is 0, but the created reference frame for the other component is recalled here. The relative motion with respect to the reference frame is measured which makes the other component to rotate. But for /SKEW/MOV this will not work.

For most applications /SKEW is used rather then /FRAME. Only few options use FRAME like imposed velocity.

Hope this clears your doubt.

<?xml version="1.0" encoding="UTF-8"?>

Hi George,

Could you please share the input file about this example to me? Thank you.

Roy

Unable to find an attachment - read this blog

Altair Forum User

Hi Roy,

Please find the attached model file.

Unable to find an attachment - read this blog

Altair Forum User

Hi Roy,

Please find the attached model file.

Hi George,

I have run your test model. The result is the same as you said. When using /frame/mov system, the right side component rotates with left. However, when using /skew/mov system, the right side component remain static.

I think the result of /frame/mov is easy to understand, but how to understand the result of /skew/mov (the right side component remain static)? As far as I know, the orientation of the skew is changed with time, why the velocity of the right side component is zero? How to understand?

In addition, what situation should the /skew/mov be used? In other words, the moving skew system is defined for what purpose? Any example?

Thank you.

Roy

Unable to find an attachment - read this blog

Altair Forum User

Hi Roy,

For /SKEW/MOV, at each time, the actual orientation of the skew is recalculated according to the actual position of these nodes. In fact, the origin of the skew remains at the initial position during the motion even though a moving skew is defined. In this case, a simple projection matrix is used to compute the kinematic quantities in the reference.

For most applications /SKEW is used rather then /FRAME.We rarely use /FRAME

Altair Forum User

Hi Roy,

For /SKEW/MOV, at each time, the actual orientation of the skew is recalculated according to the actual position of these nodes. In fact, the origin of the skew remains at the initial position during the motion even though a moving skew is defined. In this case, a simple projection matrix is used to compute the kinematic quantities in the reference.

For most applications /SKEW is used rather then /FRAME.We rarely use /FRAME

Hi George,

Thank you for your reply. I still have two major questions.

1. How to calculate the simple projection matrix? In other words, as I mentioned before, how to understand the right component remain static when using /skew/mov system?

2. You mentioned for most applications /skew is used rather then /frame. In my opinion, maybe /skew/fix is frequently used. Could you provide an example about using /skew/mov?

Thank you.

Roy

Altair Forum User

Hi Roy,

The simple projection matrix calculation is done in the background. The user has to select the right component when defining the system.

For follower forces in RADIOSS we use /SKEW/MOV, When the nodes move with the deformation the skew is updated accordingly.

Altair Forum User

Hi Roy,

The simple projection matrix calculation is done in the background. The user has to select the right component when defining the system.

For follower forces in RADIOSS we use /SKEW/MOV, When the nodes move with the deformation the skew is updated accordingly.

Hi George,

Thank you for the reply. It will be better if a simple example can be shown.

Best Wishes

Roy