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

Description

The present problem simulates the rotational motion of a fin in a two-phase flow field under the influence of the generated Oscillatory Wave flow using ANSYS Fluent software. We perform this CFD project and investigate it by CFD analysis.

We design the 2-D geometry of the present model by Design Modeler software. The model’s geometry is divided into three main areas: structured, unstructured, and stationery.

We carry out the meshing of the present model using ANSYS Meshing software. The element number is equal to 120049. The unstructured mesh is used in the special region created around the blade because it is deformed and subject to a dynamic mesh process.

Therefore, this area must be capable of high flexibility against mesh change (remeshing) while the mesh is structured in other areas of the model.

The present model is unsteady because the model is concerned with simulating the rotational motion of a fin under the influence of a fluid oscillating wave that completely depends on time.

Also, the effect of gravity on the flowing fluid has been taken into account and equals 9.81 m/s2 along the y-axis of the present model because the gravitational force will affect the applied torque exerted on the fin completely.

Oscillatory Wave Methodology

The two-phase flow in this problem is defined by the VOF model and consists of two phases: air as the primary phase and water as the secondary phase, with no interaction or mass transfer between the two phases.

The mixture of air and water flow within the main domain, and the movement of the rigid wall and the walls attached to that moving boundary create a wave of oscillation in the water flow.

This oscillatory wave produced by the flow applies compressive force and shear stress on the fin attached to the floor of the domain. As a result, the water forces acting on the fin cause it to rotate around its vertical axis as a rigid object.

Due to the nature of the problem requiring displacement at the model boundaries, a dynamic mesh technique was used to define the fluid flow.

Also, a UDF (user-defined function) defines the reciprocating motion of the scaffold wall that causes the waveform within the domain. The simulation was performed for 100 seconds, and the time step size was 0.001s.

In the present model, we use the dynamic mesh smoothing method, with a constant coefficient of spring equal to 0.7, the number of iterations equal to 500, and the convergence tolerance criterion of 0.001. We apply the remeshing method and local cell type. The current model does not use the spring mode.

The present model only has one degree of freedom (1-DOF) to rotate around the z-axis and the point of the fin’s rotation, which appears in a reciprocal motion around the center of its rotation due to the collision fluid wave.

In the present model, the moment of inertia applied to the fin is 0.1147 kg.m2, which is equivalent to the moment of inertia applied to the rotary rod around the endpoint of the rod (I = 1 / 3mL2).

Oscillatory Wave Conclusion

After the solution process, we obtained two-dimensional contours related to pressure, velocity, the volume fraction of liquid and air, and pathlines at t=2s.

Also, by enabling the write motion history option in the six DOF definition section, the fin location in the x and y coordinates and its angular placement at different times are stored as a data set in a text file. The data graph shows the angular changes of the fin position over time in 22.5 seconds.