Oscillatory Wave and its Effect on Fin Motion CFD Simulation

Project Description (Oscillatory Wave and its Effect on Fin Motion)

The present problem simulates the rotational motion of a fin in a two-phase flow field under the influence of the generated oscillatory flow wave. The two-phase flow used in the 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 the fin to rotate as a rigid object around its vertical axis. 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, the UDF (user-defined function) is used to define the reciprocating motion of the scaffold wall that causes the waveform within the domain. The simulation was performed in a 100s time and the time step is 0.001s.

It should be noted that the industrial use of these rotational fins in the water flow is to generate hydraulic forces in the hydraulic levers and thereby generate electricity in special hydraulic generators. The following figure shows a schematic of the angular shift of a ballet sample due to wave motion.

Geometry & Mesh

The 2-D geometry of the present model is designed by Design Modeler software. The geometry of the model is divided into three main areas: structure, unstructured and stationary. In the structure section, a rigid wall with the name wall-dynamic is used as the main factor in the flow waveform, while the upper and lower walls are used as wall-bottom-deforming and outlet-deforming, respectively. In the unstructured section, the entire unstructured region is defined as the deformation acceptor zone and the wall-flap-dynamic walls as a rigid, reciprocating rotational object. The figure below shows an outline of the model’s geometry.

The meshing of the present model was carried out using ANSYS Meshing software. The element number is equal to 120049. In the special region created around the blade, the unstructured mesh is used because it is subject to a dynamic mesh process and is deformed, and therefore, this area must be capable of high flexibility against mesh change (remeshing), while in other areas of the model, the mesh is structured. The following figure shows an overview of the mesh.

Assumption

To simulate the present model, we consider several assumptions:

The solver is Pressure-Based. Simulation only deals with fluid behavior; in other words, we don’t perform heat transfer simulation. 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. We consider the effect of gravity on the flowing fluid to be 9.81 m.s-2 along the y-axis of the present model because the gravitational force on the applied torque will affect the fin completely.

CFD Simulation

Here is a summary of the steps to define and solve the problem:

Models (Oscillatory Wave and its Effect on Fin Motion)
k-epsilon	Viscous model
standard	k-epsilon model
scalable wall function	near-wall treatment
VOF			Multiphase model
air and water	phases
implicit	formulation
sharp	interface modeling
Dynamic mesh
smoothing and re-meshing			mesh methods
rigid body and deforming			type of dynamic mesh zone
six DOF			options
Boundary conditions (Oscillatory Wave and its Effect on Fin Motion)
pressure-outlet			The outlet for the upper outlet
0 Pa	gauge pressure
wall			Walls type for all walls
Solution Methods (Oscillatory Wave and its Effect on Fin Motion)
PISO			Pressure-velocity coupling
PRESTO	pressure		Spatial discretization
second-order upwind	momentum
second-order upwind	turbulent kinetic energy
second-order upwind	turbulent dissipation rate
compressive	volume fraction
Initialization (Oscillatory Wave and its Effect on Fin Motion)
Standard, patch	Initialization method
0 m.s-1	velocity (x,y)
0 pascal	gauge pressure
1	Phase-2 volume fraction
0 (in the defined region)	Phase-1 volume fraction

Dynamic Mesh

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 is equal to 500 and the convergence tolerance criterion of 0.001. We don’t use the layering method for the dynamic mesh. We apply 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).

In the present model, the geometrical shape, outlet-deforming wall, unstructured area, and wall-bottom-deforming wall have a dynamic mesh behavior of deforming type, meaning that these boundaries and the areas are shifting. On the other hand, the wall-dynamic wall and the wall-flap-dynamic wall have a dynamic mesh behavior of rigid body type, meaning that these walls and boundaries are subject to mesh movement. They do not change shape and maintain their rigidity.

Results

After the solution process, we obtained two-dimensional contours related to pressure, velocity, the volume fraction of liquid and air and pathlines in 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 set of data in a text file and the graph of the data shows the angular changes of the fin position over time in 22.5 seconds.

There is a mesh file in this product. By the way, the Training File presents how to solve the problem and extract all desired results.