W H I T E P A P E R
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Video 4 below shows an example of fiber breakage using the shear stress criterion.
Video 4. Fiber breakage using the shear stress breakage criterion.
What if it is a fluid-driven problem?
In many cases, in order to predict the particle trajectory, it is important to consider the fluid
forces acting on particles . For many applications, the fluid drag is the driving force for the
fibers in the system.
For fibers, Rocky uses the Marheineke & Wegener drag model [2] that was developed for long
slender fibers immersed in turbulent dilute flows. This model decomposes the force into normal
and tangential components and is uniformly valid for all Reynolds number regimes. By taking
into account the fiber orientation and alignment with the incident flow, the the drag coefficient
estimate is more accurate.
It is also important to note that the drag is computed per element. This is important for fibers
with large aspect ratios, as the fiber can span different fluid cells, and therefore cross regions
with different velocities. Using this approach, the drag force for each segment is calculated
based on the fluid velocity at the centroid of that segment.
The videos below show two cases in which this effect can be clearly seen. In Video 5, the fiber
is injected on a parabolic flow. If the drag were computed based on the centroid of the fiber,
the fiber would move to the right keeping the same shape. But given the drag is computed per
segment and as the velocity is higher for the center region, this fiber bends and flows to the
right, forming a C shape.
