Fiber spinning process may involve high take-up velocity. It is not unusual that drag force originating from the air surrounding the free boundary of the fiber becomes significant with respect to the take-up force. When simulating fiber spinning process with a 2D axisymmetric model, it is possible to include the drag force into the calculation. From the point of view of modeling, the vector of drag force density is given as:

(15–21)

In the equation above, and are independent parameters, while and are the local Reynolds number and the density of the surrounding gas, and is the magnitude of the local velocity on the surface of the fiber. The quantity denotes a unit vector parallel to the local velocity. The Reynolds number is evaluated on the basis of the local fiber radius , the magnitude of the fiber velocity, and the kinematic viscosity of the surrounding gas. The density of the gas is evaluated from the law of perfect gases, that involves the mole mass of the gas, the local pressure , the universal thermodynamic molar gas constant and the absolute temperature. For the temperature, reference temperature and offset temperature can be defined as well.

From the point of view of the physics, the relationship indicates that the intensity of the drag force increases with the density of the surrounding gas and with its velocity as well. The quadratic dependence of the drag force is affected by the local Reynolds number.