FSI Analysis of Airflow around an Airfoil Simulation, ANSYS Fluent Training
The present problem simulates the airflow around an airfoil using the Fluid Solid Interaction (FSI) method in ANSYS Fluent software.
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FSI Project Description
The present problem simulates the airflow around an airfoil using the Fluid Solid Interaction (FSI) method in ANSYS Fluent software. In this simulation, a circular computational domain of airflow is designed; So that an airfoil is located in this area. Due to the fact that this airfoil is moving in the air with a considerable speed, the air flow collides with its body and exerts force on it. As a result, we can say that a two-way confrontation occurs between the fluid and the solid. Therefore, we use the FSI method in the ANSYS Workbench software environment.
When using the FSI method, due to the change in the structure of the fluid flow mesh around the geometric model, it is necessary to define a Dynamic Mesh; Because the dynamic mesh technique allows changing the mesh structure of the model in a time-dependent manner. In the determination of dynamic mesh methods, smoothing and remeshing methods have been used. According to the smoothing method, the number of nodes does not change and only adjusts the mesh of an area by moving or deforming the borders.
We also use Remeshing, when the displacement of the borders is large relative to the size of the local cells to regenerate the destructive cells of the critical size limit. To define two-way fluid-solid interaction, we should use system coupling in ANSYS Workbench software. To do this, you must first define the model in each of the Fluent and Transient Structural software and then couple the process of solving them with this system coupling, and also considering that the geometry is the same in both fluid and solid design modes. We also should establish a coupling between the geometry of each of these two software.
Now it is necessary to define the walls or boundaries of the model that are affected by the FSI in the environment of Fluent and Transient Structural software. To define the desired boundary in Fluent software, we should use dynamic mesh; Because the meshing around solid boundaries changes over time due to the deformation of solids, and as a result, the mesh around these boundaries must be considered moving. To do this, the boundary of the airfoil body must be defined in the dynamic mesh section in system coupling. This means that the instantaneous change in the structure of the mesh is due to coupling with solid analysis.
To define the desired boundary in transient structural software, the same surface related to the peripheral body of the airfoil should be defined as fluid solid interface, ie the boundary between fluid and solid. In this model, a circular hole inside the airfoil is distinguished to define the boundary of the inner surface around this circle as fixed support; That is, this boundary or surface is constantly affected by the interaction with the fluid flow and does not change position or deform.
However, the two sides of the airfoil are displaced due to the collision of the fluid flow with the airfoil body, and therefore, these boundaries must be defined as displacement. Finally, to make a connection or coupling between fluid and solid and to define their effect on each other, data transfer must be defined; In this way, the results of these two solutions in the two mentioned software are transferred to each other. Therefore, we define two data transfer in the system coupling section; Thus, we must define this data transfer for a specific area or boundary from a source to a target.
To do this, we should define a data transfer from the model wall in fluent software to the same model wall in transient structural software as Force. This means that the flow of fluid around the airfoil wall strikes the wall and exerts a force on it. Also, we should define a data transfer from the model wall in transient structural software to the same model wall in fluent software as displacement. This means that the wall changes the flow of fluid around it. It should be noted that in the present modeling, a circular computational domain is considered to define the air flow, the circumference of which is defined as the inlet boundary of the air flow.
Therefore, the velocity inlet boundary condition is used at this border; So that the air flow velocity and the directions of air flow are variable as a function of time. Being variable in the direction of airflow means that the angle of attack of the airfoil varies with time. Therefore, the UDF was used to determine the variable velocity and to determine the variable attack angle. Due to the main nature of the model based on the use of dynamic mesh, we should apply a transient solver for the simulation process.
In the present model, the simulation process is performed in 0.32 seconds with a time step size of 0.0005 seconds. Since the simulation process is performed in both fluid and solid software, we define a same time period for both software.
Geometry & Mesh
We design the present model in three dimensions using Design Modeler software. The model includes a circular computational domain with a diameter of 4.8 m and an airfoil inside this area. This computational area has only the inlet air flow and the lateral faces of this area have a symmetry condition.
We carry out the meshing of the model using ANSYS Meshing software, and the mesh type is unstructured. The element number is 56220. The following figure shows the mesh.
FSI CFD Simulation
We consider several assumptions to simulate the present model:
- We perform a density-based solver, since the air velocity is very high.
- The simulation is unsteady.
- We ignore the gravity effect on the fluid.
The following table represents a summary of the defining steps of the problem and its solution:
|mesh methods||smoothing & remeshing|
|x-/y-component of flow direction||udf|
|Wall of Airfoil||Wall|
|wall motion||stationary wall|
|heat flux||0 W.m-2|
|flow||first order upwind|
|turbulent kinetic energy||first order upwind|
|specific dissipation rate||first order upwind|
|gauge pressure||0 Pascal|
After the solution process, we obtain the results in both Fluent and Transient Structural software. In transient structural software, we represent deformation, strain and stress contours on the outer surface of the airfoil. These contours correspond to the final second (0.32 s) of the simulation process. In fluent software, we show two-dimensional contours related to speed, pressure and temperature on the symmetrical surface of the computational area around the mentioned airfoil. These contours also correspond to the final second (0.32 s) of the simulation process.
Also, we present the diagram of changes in drag coefficient and lift coefficient applied on airfoil over time in 0.32 s.
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.