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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 layers determine the axial air drag. A fly line will, in general, move at an angle in the 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 a 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 ref. cast is used as reference (drag constants as for all calculated casts except the one below).

· A solution, denoted “50ft ax drag flyline +50%”, is calculated with a 50% increased axial air drag as the only change in relation to the reference cast.

Results

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

Conclusions

· Increasing the axial air drag by 50%, in relation to the reference, has a significant impact on the simulated cast.

· It may be concluded that the calculation of axial air drag introduces a degree of uncertainty.