The volume flow rate f(t) from the CFD simulation during the fifth cutter motion cycle was used as an input to drive the transient structural simulation as follows. The accumulated volume flow of material into the cutter as a function of time v(t) was obtained by integrating f(t) and was converted into displacement d(t) applied to the end of the fiber bundle using the following equation:
\begin{eqnarray*}d\left( t \right) = \frac{{v\left( t \right)}}{{\left[ {\frac{{{A_p}}}{2}} \right]\rho }}\end{eqnarray*}
where
Ap is the cutter port area (the factor 2 appears because the geometry only represents half of the model) and
ρ is the ratio of the surface area of the circular cross-section of an equivalent aspirated cylinder divided by the port area. Port areas were 9.68 × 10
−8 m
2 for 27+-gauge and 1.29 × 10
−7/m
2 for 23-gauge. Observation of slow-motion videos of a cutter engaged in cutting porcine vitreous in vitro showed that approximately 1 mm of vitreous fiber is drawn into the cutter in one cycle for 27+-gauge. Thus we assign the length of the equivalent aspirated cylinder to be 1 mm. Because the amount of volume aspirated in one cycle (at a cut rate of 2500 cpm) for the 27+- and 23-gauge vitrectors is 2.48 × 10
−4 and 4.76 × 10
−4 mLs, respectively, the area of a cylinder representing the volume cut/aspirated in one cycle is 0.248 and 0.476 mm
2, respectively. Thus the values of
ρ used were 2.56 and 3.69 for the 27+- and 23-gauge vitrectors, respectively. The same values of
ρ for a particular gauge were used for all values of cut rate and duty cycle.
The simulation calculated the reaction force at the other end of the bundle that was attached to the force sensor. The geometric domain was meshed with ∼200,000 elements, and a few studies were conducted with varying mesh resolution to verify that simulation results would not change upon changing mesh resolution. FEA simulations typically took 40 minutes to complete engaging 16 cores in a distributed memory simulation. ANSYS software (version 17.1; ANSYS, Inc. Canonsburg, PA, USA) was used to perform simulations using a Lenovo P-900 workstation equipped with dual Intel E5-2687W v3 processors (3.10 GHz, 10 cores each) and 128 GB RAM.