Fluent post-processing: getting velocity component in arbitrary direction

Question:

How to get velocity components in arbitrary direction?

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Solution:

The essence of this question is computing the projection of velocity vector onto arbitrary vector.

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1 Projection

According to the definition of projection, velocity's projection on vector n, named vn, can be calculated:

where:

• vector vn is parallel to n
• absolute value of scalar vn is the length of vector vn
• negative value of vn means vn has opposite direction respect with n

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In Fluent, vn can be calculate by expression:

In expression, ne_x、ne_y、ne_z are coordinates of ne defined by named expressions, with the unit of meter. The unit of expression is m/s.

Also, coordinates of vn can be calculated by expression:

2 Rejection

Rejection is the vector’s projection onto a plane. It is apparent that vp is orthogonal with vn.

For any vector v, vp can be calculated:

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Similarly, coordinates of vp can be calculated by expression:

Notes

The dot product of vector is a scalar, the formula is:

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