Week 3: Flow over a backward facing step

As Converge-studio, Cygwin, and Paraview are installed in the system, we begin with importing the geometry, and for this we downloaded the model file and imported it by dragging the file into Converge-studio. After this, go for the normal checks, which must be pointed in the direction of the flow. Then, a diagnosis is done to check for any kind of geometrical error. After verifying it, the case setup is set as per the given inputs and boundary conditions. In this case, all the basic parameters are selected with specific values, and all these values are setup together as input files for the model. At last, export the input files into the folder.

The simulation that follows can be converging by using the Cygwin interface, which is required for giving the command to the software. In Cygwin, first we go to the directory where the input files are present and then command them for simulation. This simulation can be done automatically at the back end, and the output files are generated in the same folder as the results of the simulation.

Then those output files have to be exported in Paraview to see the results in the different interfaces and also in their plots in the Converge Studio. For importing the output files in Paraview, first we need to convert the data into a ".vtm file," which is done in the "Post-Processing 3-D" option in Converge-Studio. Then all the 3D and 2-D analyses of the results are analysed separately. The rest of the plots can be seen in Converge-studio itself, specifically under the "line-plotting option."

By doing so, I was able to obtain all of the required results for this particular challenge and take screenshots of them. The CFD analysis of this problem is done three times for three different mesh sizes, and the results are attached in a word file. Kindly find it below.

Detailed explanations about the plots:

1. The cell count plot shows that as the mesh size decreases, the number of total cell counts increases, indicating that decreasing the mesh size increases the number of cells. We can also understand that, by the same token, in the fixed volume, if the mesh size decreases, that means the per-mesh volume also reduces. So to equalise the total volume of meshes with the total volume of the geometry, more cells are required, which increases the cell count.
2. The mass flow rate plots appear to be very unstable at first touch, but as the flow cycled on, convergence took effect. After achieving the convergence shown in the graph, we observe that the mass of the flow that enters the geometry from the inlet per unit of time is equal to the mass of the flow that exits from the outlet. But due to the sign conventions, the input and output flows are represented, respectively, as positive and negative values for comparison.
3. The velocity plots also look unstable in the starting phase, but as the flow loops after a few circulations, the flow achieves its convergence, as in the case of flow rate. After achieving the convergence shown in the graph, we observe that there is a huge difference between the initial and outlet velocities. More than that, the velocity at the input is much higher than the velocity at the outputs. But due to the sign conventions, the input and output flows are respectively represented as positive and negative values for differentiation. As the flow goes on initially, the velocity is very high and constant until the surface area for the flow doesn’t increase steeply. So in the corner part of back flow, the velocity is minimal because of the swirl. Overall, output exit has a lower velocity than input due to a comparatively large area. Apart from this, as the mash size decreases, the local variation in the velocity flow becomes more apparent.
4. The pressure graphs look unstable in the starting phase, but as the flow goes on, after some time, the flow achieves its convergence, as in the case of flow rate. It initially rises, but over time, the overall graph falls and eventually converges.The initial unstable pressure plot is very similar to the velocity plot. The pressure at the inlet and outlet remains the same throughout the time, but at different magnitudes because of the different initial conditions that are given to both ends in the case setup for making fluid flow inside the channel. But there is a wide range of change observed from the inlet to the outlet in the simulated results, in the direction of the flow, in the Paraview file. As the mash size decreases, the local variation in the velocity flow becomes finer and clearer. The variation in the pressure came because of the variation in the surface areas between the inlet and outlet; the minimum of the area is observed near the centre because of the divergence that came in the model due to a sudden increase in the flow area.

Thankyou

Animation link: https://youtu.be/_plrtGDWdvk

Anubhav