Flow through a backward facing step
The case setup :
The model was designed and imported to the converge studio and the bondary flagging was done.
The simulation done was a time-based, steady-state simulation. Normal vectors were alligned in such a
way that the flow was made to visualize inside the backward facing step body. An inlet pressure of 110.325kPa
and an outlet pressure of 100.325kPa was applied. The simulation was made for diffrent mesh sizes. The simulation
began at 0 cyc and was made to end at 15000 cyc. Converging pressure and velocity contours were observed for each
simulation run.
Surface with edges :
1) base mesh of 2e-3
ie,,, around 136 elements or cells were created in x-direction and around 10 elements in y-direction and around
13 cells in z-direction to visualize the behaviour of flow through each of them.
2) base mesh of 1.5e-3
The mesh refinment here in this case is more fine and the number of elements or cell created are also more.
Around 181 elements were created in the x-direction and 13 cells in y-direction and 16cells in z-direction to
visualize the behaviour of flow through each of them.
3) base mesh of 1e-3
The mesh done here is a finer one and there are much more number of elements created in the geometry. There are around
272 cells in x-direction and 20 cells in y-direction and 25 cells in z-direction. The result obtained for this mesh will be a much
more accurate and convergent than previous meshes.
Pressure and velocity contours
1) base mesh of 2e-3
PRESSURE
Initially at the inlet the pressure is 110.325kPa and exit pressure is 100.325kPa and due to this pressure diffrence there will be a
flow that takes place from inlet to the exit if the backward facing step. As the fluid passes from inlet, initially the pressure will be high and
starts to increase with increase in area. As the flow continues, the flow tend to expand as it encounters an increase in area region, for any flow
to expand the flow requires energy hence the flow consumes it\'s pressure energy and pressure starts decreasing as the flow exits through the
geometry
VELOCITY
Here the flow can be considered as Rayleigh flow ie,,, non-frictionless flow, where the effect of friction is neglected.
Initially when the fluid flows the velcoity tends to increase in first half portion of the geometry and remains almost constant
till it encounters any changes in area, once there is a increase change in area of the geometry the flow tends to expands and as
the flow expands kinetic energy per unit mass of it decreases thereby decreasing the velocty of the flow in the second half of the
geometry. And as the fluid reaches the exit point it\'s velocity is still reduced.
RECIRCULATION
A glyph plot is shown in the above figure, once the flow encounters an increase in area region, the flow starts expanding and this causes an
adverse pressure gradient in the flow and is responsible for the recirculation in the regions as shown above. Recirculstion in the above region
causes both the decrease in pressure and velocity as it\'s a waste phenomenon which consumes pressure and kinetic energy of the flow.
2) base mesh of 1.5e-3
VELOCITY
The mesh refinement is much more finer and there are more number of elements in the gemoetry to get a better and accurate results.
Even here the flow can be considered as Rayleigh flow ie,,, non-frictionless flow, where the effect of friction is neglected.
Initially when the fluid flows the velcoity tends to increase in first half portion of the geometry and remains almost constant
till it encounters any changes in area, once there is a increase change in area of the geometry the flow tends to expands and as
the flow expands kinetic energy per unit mass of it decreases thereby decreasing the velocty of the flow in the second half of the
geometry. And as the fluid reaches the exit point it\'s velocity is still reduced.
3) base mesh of 1e-3
PRESSURE
A better and much more finer mesh is done and the behaviour of pressure is observed throughout the geometry to get a better results.
Initially at the inlet the pressure is 110.325kPa and exit pressure is 100.325kPa and due to this pressure diffrence there will be a
flow that takes place from inlet to the exit if the backward facing step. As the fluid passes from inlet, initially the pressure will be high and
starts to increase with increase in area. As the flow continues, the flow tend to expand as it encounters an increase in area region, for any flow
to expand the flow requires energy hence the flow consumes it\'s pressure energy and pressure starts decreasing as the flow exits through the
geometry
VELOCITY
The mesh refinement is best and there are more number of elements in the gemoetry to get a better and accurate results.
Even here the flow can be considered as Rayleigh flow ie,,, non-frictionless flow, where the effect of friction is neglected.
Initially when the fluid flows the velcoity tends to increase in first half portion of the geometry and remains almost constant
till it encounters any changes in area, once there is a increase change in area of the geometry the flow tends to expands and as
the flow expands kinetic energy per unit mass of it decreases thereby decreasing the velocty of the flow in the second half of the
geometry. And as the fluid reaches the exit point it\'s velocity is still reduced.
RECIRCULATION
A better mesh quality yields in a better ananlysis for the region of recirculation.
A glyph plot is shown in the above figure, once the flow encounters an increase in area region, the flow starts expanding and this causes an
adverse pressure gradient in the flow and is responsible for the recirculation in the regions as shown above. Recirculstion in the above region
causes both the decrease in pressure and velocity as it\'s a waste phenomenon which consumes pressure and kinetic energy of the flow.
A clear view of recirculation can be visualized in the above plot.
PLOTS OF VELOCITY, PRESSURE, MASS-FLOW RATE AND TOTAL CELL COUNT
for base mesh of 2e-3
mass-flow rate (kg/s) v/s cycles
mass-flow rate through inlet boundary
As mass flow rate involves the components like density, pressure, area of the geometry and the velocity of the flow taking place through it.
Mass flow rate is the amount of fluid in \'kg\' passing through a given area in unit interval of time(s). Initially when the flow starts the mass-flow rate decreases
and reaches a steady state value when the solution reaches convergence.
mass-flow rate through outlet boundary
As mass flow rate involves the components like density, pressure, area of the geometry and the velocity of the flow taking place through it.
Mass flow rate is the amount of fluid in \'kg\' passing through a given area in unit interval of time(s). Initially when the flow starts the mass-flow rate decreases
in the sense at the exit the mass-flow rate is increasing and as the solution reaches the steady state value the mass-flow rate at the exit also comes
to a constant value at the exit boundary.
total cell count
The total cell count is a constant value throughout the simulation and it only depends on the base mesh size of what we give.
The main intention of the cell count is that. by increasing the number of elements or the cells the solution can be improved to a converged value
and the result obatined will be a accurate one. And also by increasing the total cell count the behaviour of the flow through each element or cell
can be observed and results can be inferred.
Pressure v/s cycles
A plot of pressure v/s cycles is shown. Initially at the inlet the pressure is high and as the flow starts from the inlet the pressure increases
as the first half is a constant area channel. Once the flow reaches the second half the geometry with increase in area, pressure increases to some
extent, because the flow starts expanding due to increasing in area the pressure drops gradually till the flow completely expands and reaches a steady
state value once there is no pressure energy left in the flow.
Velocity v/s cycles
velocity through inlet boundary
Same behaviour as in case of mass-flow rate can be observed here.
velocity through outlet boundary
Once the flow crosses the first half of the geometry with increase in area the flow starts to expand and there by the flow starts accelerating
which increases the velocity of the flow and when the solution reaches the steady state, the velocity becomes almost a constant value.
for base mesh of 1.5e-3
mass-flow rate (kg/s) v/s cycles
mass-flow rate through inlet boundary
As mass flow rate involves the components like density, pressure, area of the geometry and the velocity of the flow taking place through it.
Mass flow rate is the amount of fluid in \'kg\' passing through a given area in unit interval of time(s). Initially when the flow starts the mass-flow rate decreases
and reaches a steady state value when the solution reaches convergence.
mass-flow rate through outlet boundary
As mass flow rate involves the components like density, pressure, area of the geometry and the velocity of the flow taking place through it.
Mass flow rate is the amount of fluid in \'kg\' passing through a given area in unit interval of time(s). Initially when the flow starts the mass-flow rate decreases
in the sense at the exit the mass-flow rate is increasing and as the solution reaches the steady state value the mass-flow rate at the exit also comes
to a constant value at the exit boundary.
total cell count
The total cell count is a constant value throughout the simulation and it only depends on the base mesh size of what we give.
The main intention of the cell count is that. by increasing the number of elements or the cells the solution can be improved to a converged value
and the result obatined will be a accurate one. And also by increasing the total cell count the behaviour of the flow through each element or cell
can be observed and results can be inferred.
Pressure v/s cycles
Much finer bheaviour and results of pressure are observed here. A plot of pressure v/s cycles is shown. Initially at the inlet the pressure is high and as the flow starts
from the inlet the pressure increases as the first half is a constant area channel. Once the flow reaches the second half the geometry with increase in area, pressure increases to some
extent, because the flow starts expanding due to increasing in area the pressure drops gradually till the flow completely expands and reaches a steady
state value once there is no pressure energy left in the flow.
Velocity v/s cycles
velocity through inlet boundary
velocity through outlet boundary
Once the flow crosses the first half of the geometry with increase in area the flow starts to expand and there by the flow starts accelerating
which increases the velocity of the flow and when the solution reaches the steady state, the velocity becomes almost a constant value.
for base mesh of 1.0e-3
mass-flow rate (kg/s) v/s cycles
mass-flow rate through inlet boundary
A better plot is obtained here when compared to above two meshes. As mass flow rate involves the components like density, pressure, area of the geometry and
the velocity of the flow taking place through it. Mass flow rate is the amount of fluid in \'kg\' passing through a given area in unit interval of time(s).
Initially when the flow starts the mass-flow rate decreasesand reaches a steady state value when the solution reaches convergence.
mass-flow rate through outlet boundary
As mass flow rate involves the components like density, pressure, area of the geometry and the velocity of the flow taking place through it.
Mass flow rate is the amount of fluid in \'kg\' passing through a given area in unit interval of time(s). Initially when the flow starts the mass-flow rate decreases
in the sense at the exit the mass-flow rate is increasing and as the solution reaches the steady state value the mass-flow rate at the exit also comes
to a constant value at the exit boundary.
total cell count
With the increase in mesh size the number of elements are increased here. The total cell count is a constant value throughout the simulation and it
only depends on the base mesh size of what we give.The main intention of the cell count is that. by increasing the number of elements or the cells
the solution can be improved to a converged value and the result obatined will be a accurate one. And also by increasing the total cell count the
behaviour of the flow through each element or cell can be observed and results can be inferred.
Pressure v/s cycles
The mesh done here is finer one and the bheaviour and results of pressure are observed here are accurate. A plot of pressure v/s cycles is shown.
Initially at the inlet the pressure is high and as the flow starts from the inlet the pressure increases as the first half is a constant area channel.
Once the flow reaches the second half the geometry with increase in area, pressure increases to some extent, because the flow starts expanding
due to increasing in area the pressure drops gradually till the flow completely expands and reaches a steady state value once there is no pressure energy left in the flow.
Velocity v/s cycles
velocity through inlet boundary
velocity through outlet boundary
Once the flow crosses the first half of the geometry with increase in area the flow starts to expand and there by the flow starts accelerating
which increases the velocity of the flow and when the solution reaches the steady state, the velocity becomes almost a constant value.