Darrieus Vertical Axis Water Turbine, Dynamic Mesh, ANSYS Fluent Training
The present problem simulates the water flow around a vertical-axis water turbine submerged in water using the dynamic mesh method in ANSYS Fluent software.
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Darrieus Turbine Project Description
The present problem simulates the water flow around a vertical-axis water turbine submerged in water using the dynamic mesh method in ANSYS Fluent software. The water turbine is from the category of vertical axis turbines and is of the darrieus type; So that the axis of the turbine is perpendicular to the direction of water flow. For this simulation, a vertical axis turbine with three blades is designed within a relatively large computational area in which the computational area has only water flow. In this simulation, it is assumed that the vertical turbine is rotating in the water flow, and therefore, this rotational motion of the turbine blade body affects the surrounding meshing.
So that it causes instantaneous and time-dependent change in meshing based on the type of displacement at adjacent boundaries of cells. Therefore, the dynamic mesh model is used to define the instantaneous change of meshing around the rotating turbine blades. To better define the dynamic mesh, a cylindrical area is separated from the entire computational area, so that this area includes the turbine blades. Now, to define the areas under the dynamic mesh, the walls of the turbine blade body, as well as the differentiated area around the turbine blades, are defined as Rigid Body.
This means that the turbine blade body and its surrounding area act as a rigid and integrated body and can be moved in a transient and rotational manner. This rigid body behavior means that the body itself does not change and only the meshing of the surrounding areas changes over time. Two methods can be used to define the motion with several degrees of freedom for turbine blades; You can either use a UDF to define this motion, or you can define it manually in fluent software. Since this current model is a turbine, it only has a rotational motion, and on the other hand, this rotational motion can only take place around an axis.
Therefore, to define the type of motion of a rigid body, a rotational motion with a degree of freedom (1-DOF) should be specified; Thus, the mass of the blades was considered equal to 1 kg and the moment of inertia of the blades was considered equal to 3.09 kg.m2. Also, according to the geometric structure of the present model, the axis of rotation is defined in the direction of the central axis of the vertical turbine, ie the Z-axis, and the center of rotation is defined as the center of the turbine, ie coordinates (0., 0,0). The boundary conditions are also defined in such a way that the incoming water flow enters the computational domain with an average flow velocity of 1 m.s-1 in the direction of the horizon (x-axis).
The water flow exits the computational domain with a pressure equal to atmospheric pressure. The upper and lateral surfaces of the computational area are also defined by the symmetry type boundary condition. Due to the main nature of the model based on the use of dynamic mesh, the simulation process should be defined transient (unsteady) that in the present model, the simulation process is performed in 50 seconds with a time step of 0.05 seconds.
Darrieus Turbine Geometry & Mesh
We design the present model in three dimensions using Design Modeler software. The model includes a computational domain and we design a water turbine within this domain. Darrieus type water turbine is a category of vertical axis turbines. This turbine has three blades with a height of 0.5 m. This computational area is defined so that the center of the turbine is 3 m far from the inlet section, 10 m from the outlet section, and 0.75 m from the top and bottom surfaces.
We carry out the meshing of the model using ANSYS Meshing software, and the mesh type is Hybrid. Thus, the meshing in the areas around the turbine body is unstructured and in other areas of the model, it is structured. The element number is 7422668. The following figure shows the mesh.
Darrieus Turbine CFD Simulation
We consider several assumptions to simulate the present model:
- We perform a pressure-base solver.
- Since we applied Dynamic Mesh model and the turbine rotates as a function of time, the solver is unsteady.
- The gravity effect on the fluid is equal to zero.
The following table represents a summary of the defining steps of the problem and its solution:
|near-wall treatment||standard wall function|
|velocity magnitude||1 m.s-1|
|gauge pressure||0 pascal|
|wall motion||stationary wall|
|wall motion||stationary wall|
|Top & Sides||Symmetry|
|momentum||second order upwind|
|turbulent kinetic energy||second order upwind|
|turbulent dissipation rate||second order upwind|
|gauge pessure||0 Pascal|
|y-velocity & z-velocity||0 m.s-1|
At the end of the solution process, we obtain two-dimensional contours relating to velocity, pressure, and kinetic turbulence energy, as well as path lines and velocity vectors on a plane passing through the center of the turbine.
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.