Oscillatory Wave Effect on a Fin Motion: UDF

Oscillatory Wave and its Effect on Fin Motion, ANSYS Fluent CFD Training

Description

This project simulates the rotational motion of a fin within a two-phase flow field, driven by an oscillatory wave generated through ANSYS Fluent.

The 2D geometry was designed in Design Modeler, divided into three main regions: structured, unstructured, and stationary. Meshing was carried out in ANSYS Meshing, totaling 120,049 elements. An unstructured mesh was applied specifically in the region surrounding the fin, since this area undergoes deformation through the dynamic mesh process and requires high flexibility to accommodate remeshing, while the remaining regions retain a structured mesh.

The model is inherently unsteady, since it simulates the fin's rotational motion under a time-dependent oscillating fluid wave. Gravitational effects were included at 9.81 m/s² along the y-axis, given their influence on the torque acting on the fin.

Methodology

The two-phase flow was modeled using the VOF model, with air as the primary phase and water as the secondary phase, with no interaction or mass transfer between them. The motion of a rigid wall and its attached boundaries generates an oscillatory wave within the domain, which applies compressive force and shear stress to the fin mounted on the domain floor — causing the fin to rotate about its vertical axis as a rigid body.

Since the problem requires boundary displacement, a dynamic mesh technique was used to capture the fluid flow, with a UDF defining the reciprocating motion of the scaffold wall responsible for generating the waveform. The simulation ran for 100 seconds with a time step of 0.001 s.

Dynamic mesh smoothing was applied using a spring constant of 0.7, 500 iterations, and a convergence tolerance of 0.001, combined with the remeshing method using local cell sizing; spring-based smoothing alone was not used. The fin is constrained to a single degree of freedom (1-DOF), rotating about the z-axis around its pivot point, exhibiting reciprocating motion driven by wave impact. The moment of inertia applied to the fin was set to 0.1147 kg·m², equivalent to that of a rotating rod about its endpoint (I = 1/3·mL²).

Conclusion

The results include 2D contours of pressure, velocity, and the volume fraction of air and water, along with pathlines captured at t = 2s. By enabling the write motion history option within the 6-DOF definition settings, the fin's x-y position and angular orientation were recorded over time as a dataset, producing a graph of the fin's angular displacement across the full 22.5-second simulation window.