In modern fluff pad machines, fine fibers are transported by the turbulent airflow in one or more ducts and are deposited on a rotating wire screen. The basis weight homogeneity of the fiber lay-down is subject to a) even distribution of the fibers in the incoming air, b) airflow uniformity in the screen vicinity and c) the strength of recirculating airflow. Curvature of the air ducts may generate substantial secondary flows. Therefore, fibers may be preferentially concentrated within the air stream at the end of the curved duct. Air turbulence may form recirculating, stagnant, or even oscillating flows. As the fibers are deposited, they increase the local hydraulic resistance to the airflow. All these phenomena may cause a non-uniform flow, leading to a non-uniform basis weight of the fiber. Additionally, the spinning screen itself can create air currents that can blow off already-deposited fibers.

CFD can be used to create a realistic computer model of the flow in a fluff pad machine. The figure below shows how the density of fibers affects the pad uniformity. Such a model offers an attractive alternative to expensive and time-consuming prototype testing. CFD helps engineers optimize the fiber-density and machine design to produce pads of the highest quality.

Pressure contours on two consecutive "limbs" on the fluff pad machine.

Transient fiber deposition and basis weight evolution during pad forming