Flow over a backward facing step using CONVERGE CFD

Objective:

In this case flow of air over a backward facing step to be simulated for 2D case condition , using three base grid sizes to be meshed and simulated with presuure and velocity contours, even the pressure, velocity, massflow rate, total cell count plots be plotted for three different cases.

Mesh sizes:

These are the three mesh sizes for the diffrent simulation ,

case 1

dx=2e-3 dy=2e-3 dz=2e-3

case 2

dx=1.5e-3 dy=1.5e-3 dz=1.5e-3

case 3

dx=1e-3 dy-1e-3 dz=1e-3

Geometry:

A case stepup for a backward step flow was created with vetrices for the given dimenssions and then using the vertices created the triagngles after creating the traingles lofted with edges and completed the model.

Boundry conditions:

For all the saperated boundries boundry regions were created such as input , output, front, back, top and bottom walls.

Case steup:

• Fluid used - Air
• Solver - pressure based steady slover
• Run parameters:
• Start - 0 cycle
• End time - 15000 cycles
• Inital and minimum time step - 1e-6
• Maximum time step - 2
• Turbulence modeling - RNG
• Inlet Pressure - 110325 pa
• Outlet pressure - 101325 pa
• Inlet and outlet Temp - 300k
• Top and bottom wall - law of wall
• Top and bottom wall Temperature - 300k
• At front and back - 2D

Mesh/surface with edges using PARAVIEW:

case 1

case 2

case 3

velocity and pressure contours :

velocity :

The below shown is the velocity contours for all the three cases :

case 1

case 2

case 3

Pressure:

The below are pressure contours for the different base grid sizes,

Case 1

Case 2

Case 3

Velocity plots:

Case 1

In this case velocity of air at inlet reaches the steady state around 1700 cycles and velocity of air at outlet reaches the steady state around 2000 cycles

Case 2

In this case velocity of air at inlet reaches the steady state around 6000 cycles and velocity of air at outlet reaches the steady state around 4000 cycles

Case 3

In this case velocity of air at inlet reaches the steady state around 6000 cycles and velocity of air at outlet reaches the steady state around 4000 cycles

Pressure plots:

The below shown are the presuure plots for diffrent mesh sizes,

Case 1

In the above case the static pressure of the inlet attains steady state at 1800 cycles but the total pressure at outlet attains the steady state around 6000 cycles are more.

Case 2

In the above case the static pressure and inlet reaches the steady state after 6000 cycles and th total pressure at outlet reaches more than 7000 cycles .

Case 3

In the above case the static pressure at inlet reaches the steady state more than 6000 cycles and total pressure reaches the steady state around 4000 cycles as shown in the above plot.

Mass flow rate plot:

Below plots shows the difference in the mass of air as it passes through inlet and comes out of outlet through no slip walls.

case 1

In this case the inlet steady state occurs around 3500 cycles and in outlet around 2000 cycles .

case 2

In this case the inlet steady state occurs around 6000 cycles and in outlet around 4500 cycles .

case 3

In this case the inlet steady state occurs around 4000 cycles and in outlet around 4000 cycles .Both are under the steady state of 4000 cycles

Total cell count:

Case 1

Above shows the total cell count and the individual variation of cells of 4 processors used for processing the simulation.

Case 2

Above shows the total cell count and the individual variation of cells of 4 processors used for processing the simulation.

Case 3

Above shows the total cell count and the individual variation of cells of 4 processors used for processing the simulation.

conclusion:

• Separation happens due to higher static pressure which is caused by the creation of recirculation zone near the step.
• Due to excessive momentum loss near the step, a pressure gradient called as recirculation zone is created. As a result there might be more time consumed by the system to attain steady state for velocity.
• As the base grid size is decreased the total cell is increased.
• For finer mesh much more cycles or itterations needs to be run for the exact steadt state.