Flow over a backward facing step

Simulating flow over a backward facing step using converge cfd

Here a backward facing step is created using converge cfd to analyse the flow variation at the step, and how does the plot of different parameter behave at that step, bellow are the steps disscused taken in forming the geometry and creating case setup and results of the analysis done.

GEOMETRY PREPARATION

• Here a geometry is prepared using cad tools in converge, having a step of 90deg
• The geometry is created by using the following steps

1. The vertices are created first
2. Then triangles are formed by joining the 3 vertices and likewise all the vertices are joined to form a prefect geometry formed out of triangles
3. Then the geometry is copied at an offset 0.250m
4. Then loft triangles are used using the open edges options, and a complete box type object is formed.

Here the values allong all the axes are kept the same as shown in the given stl file.

CASE SETUP

• Then the geometry is diagnosed to check if there are any errors.
• Boundaries are set for each face.
• Then in case set up, a time based approach is used
• Air is used as a standard material
• A steady state, hydrodynamic, compressible flow is used.
• Then the inflow , outflow, walls and 2D boundaries are set accordingly and a reaction mixture of air is used with a temperature value of 300K
• In this challenge fixed embeding is used for the top and bottom walls, it helps us refile particular area in the domain like in this a mesh size of 1 is selected for both the walls mentioned above.which will be seen in the mesh image
• The base MESH sizes are set up for 3 cases:

Case 1 = dx = dy = dz =2e-3m

Case 2 = dx = dy = dz =1.5e-3m

Case 3 = dx = dy = dz =1.0e-3m

• Then the simulation is run for all the 3 cases using CYGWIN and the results are analysed using paraview.

CASE 1: AT MESH SIZE OF 2e-3m

• The grid at this size looks like:

Here we can see that as the mesh size are quite big the simulation con

• Velocity in this case:
• Explanation of where and why the flow seperation takes place

Here we can see that there is a flow seperation taking place just after the step, and the velocity at the bottom part of the step(the area in blue) it drastically reduces and this area goes on increasing as the flow advances.

• Velocity plot at 2e-3m

Here the velocity is plotted by taking a vertical line few distance after the step, and it clearly shows the variation.

• Pressure plot at 2e-3

Here we can see that the pressure decreases at the step and continues to a few distance after the step and then gradually starts increasing.

• Mass flow rate vs cycles

The orange line is for mass flow rate at the inlet, and blue if for the outlet

• Total cell count vs cycles

Here the total no of cell count is approx 1850

CASE 2: AT MESH SIZE OF 1.5e-3m

• Grid size at 1.5e-3m

Here we see that the mesh size are more small as compared to the first case

• Velocity variation at 1.5e-3m

• Velocity plot

Here the velocity curve is more smooth than case 1 due to fine mesh size

• Pressure plot

• Mass flow rate vs cycles for case 2
• Total no of cells vs cycles

Here the no of cells increases to approximately 3000

CASE 3: AT MESH SIZE OF 1e-3m

• Grid size at 1e-3

Here we can see that the mesh are more fine than the above 2 cases

• Velocity variation

• Velocity plot

• Pressure plot

• Mass flow rate vs cycles

• Total no of cells vs cycles

Here the no of cells further increase to 5600

Video of animation for velocity at 2e-3m

Video of animation for velocity at 1.5e-3m

Video of animation for velocity at 1e-3m

CASES OF STEP AT AN ANGLE OF 30,45 AND 60 DEGREE

in all the above cases the step was only at 90deg and the variation in the graph of different parameters were dure to change in the mesh sizes and we can note that as the mesh size goes on decreasing the cell count goes on increasing and the plots become finer.

To take this a bit more further i have generated 3 more cases at an angle of 30, 45 and 60 degrees respectively, to see how the velocity approaches further after the step in all the 3 cases and hoe does the flow behave.(the mesh for all the 3 cases are same that is 2e-3m)

CASE 1 : AT AN ANGLE OF 30 DEG

VELOCITY PLOT

Here we see that the velocity plot gradually increases after the step

ANIMATION OF VELOCITY AT 30DEG STEP

here in the video we can see that the the velocity variation has a much smaller length for the lowest velocity and it increases at a much faster rate as compared to the above 3 cases

CASE 2 : AT AN ANGLE OF 45 DEG

VELOCITY PLOT

Here the velocity takes a much smaller time to go higher than at the 30deg step

ANIMATION OF VELOCITY AT 45DEG STEP

here in the video the blue portion is for a much smaller distance

CASE 3 : AT AN ANGLE OF 60 DEG

VELOCITY PLOT

Here the velocity increases at a much faster rate

ANIMATION OF VELOCITY AT 60DEG STEP

here the distance for the blue part is much smaller than the above 2 cases of 30,45 deg

conclusion

from the above 3 cases we can conclude that there is not much variation in the velocity plots and the step at an angle of 60 deg shows very less variation , so the pressure drop will be for a much lower level and if 2 straight ducts are to be joined with a step it would be better if the transition would be having a step of 60 deg.