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Sensitivity analysis, modeling of axial air drag

Purpose

The purpose of the sensitivity analysis is to determine how the numerical solution is affected by changes in the constants for calculation of axial air drag.

Background

The fly line will be subject to shear stresses in the axial or lengthwise direction as it travels through the air. What is referred to as boundary layers form in the vicinity of the fly line. The properties of the boundary layer determine the axial air drag. A fly line will, in general, move at an angle in relation to air. The angle will influence the boundary layer increasing the shear stresses and the drag compared to a boundary layer developing along a line at zero angle. Here, the air drag is calculated as function of the Reynolds number based on experimental data. The increase in shear stress due to the angle is accounted for approximately. Hence, there is some uncertainty in the calculation of axial air drag.

Method

The sensitivity of the solution to changes in axial air drag is analyzed as follows:

· The 50ft oh cast reference is used as reference, the solution for this cast is denoted axial drag 100% (constants as for all calculated casts except the two below).

· A solution, denoted axial drag 150%, is calculated with a 50% increased axial air drag as the only change in relation to the reference cast.

· A solution, denoted axial drag 67%, is calculated with a 33% decreased axial air drag as the only change in relation to the reference cast.

The three solutions are compared in the animation below (click on image to start animation).

Conclusions

· Changing the axial air drag +50% and -33%, in relation to the reference, has a strong impact on the simulated casts.

· It may be concluded that the calculation of axial air drag remains an uncertain aspect of the simulation model.