Channel flow simulation using CONVERGE CFD

CHANNEL FLOW SIMULATION USING CONVERGE CFD

AIM :

• To setup the channel flow case setup and analyze the steady-state flow simulation by Converge CFD.

OBJECTIVES :

• To setup the channel flow case in Converge CFD.
• To simulate the steady-state flow in the channel case by CYGWIN.
• To simulate the channel flow case physically using PARAVIEW.
• To create and analyze the plots of the simulation results by Converge CFD.

THEORY:

• To setup a channel flow case in Converge CFD:
• Open Converge CFD and click New Project.
• In Geometry Box, click Create, then click Shape ,click box and enter the dimensions of channel case to be created and then click Create.
• To create a boundary surfaces, make sure cursor picks the Triangle
• Select Boundary, in the Flag tab, click + to add the boundaries.
• Add the boundaries by clicking create multiple boundaries to create all the boundaries at a once and name them like Inlet, Outlet, Top and bottom walls, front 2D and back 2D by selecting the respective triangles and click
• Then click Normal Toggle option and click any one of the triangle which shows the normal that is opposite to the direction of fluid flow and click Apply.
• Click Options and under the tab of Visibility, click Geometry Bounding box to create Boundary line across the geometry.
• To ensure that the geometry is clean, click Diagnosis tab and click Find.
• All the parameters that got checked shows the green colour tick mark.

• In order to begin the case setup, click case setup and click begin case setup and click Time Based, since it is not a crank angle based like lC engine or Pumps but a steady state channel flow setup.
• By clicking Materials, uncheck the Reaction Mechanism since there is no chemical reaction takes place and select Air to be predefined mixtures.
• Since there is no role of turbulence here, click Physical Models and uncheck the Turbulence modelling.
• Type the values for the respective parameters and click ok for the Gas Simulation and Global Transport
• In Simulation Parameters, under Run Parameters in Solver tab fix the solver to be Steady state solver, the simulation mode to be Full Hydrodynamic, since no need to create mesh by No Hydrodynamic and it is also not a complex setup and it didn’t take much time for running simulations.
• In Miscellaneous tab, check the steady state monitor and uncheck the Use Shared memory because of we are running a steady state simulation flow here.
• In Boundary conditions, name the respective Boundary type like Inlet as Inflow , Outlet as Outflow, Top and bottom walls to be wall of No-slip, Front and back as Two-D
• In inlet, in Pressure boundary conditions, select the Specified value to let the inlet pressure be 100001 Pa and select the predefined mixture as Air which contains N2 as 77 and O2 as 0.23 and select the temperature to be specified value of 300 K and Outflow as 100000 Pa.
• Since we checked the Steady state monitor, in the steady state monitor, click Add config. And click Add to add the variables that needs to be monitored in the simulation.
• Then select the target type as Boundary and select the target as
• The Minimum no. of cycles to be executed for steady state solver is 5000 until which the simulation won’t stop despite the convergence is reached.
• Monitor Start Delay which is from which cycle the outputs needs to be monitored. Since it is steady state flow, we can keep it as 0.
• The Monitor Units is Cycles and the file that all the output results will be saved is selected as out.
• In Simulation time parameters, under General tab, the End time is 15000 cyc.
• The initial and minimum time-step is 1e-9 and click ok and click ok on Solver parameters with default values.
• In Regions and Initialization, click Add as Region 0 and set pressure as 100000 Pa since the flow shouldn’t get reversed and click Air as its Species.

• Then in Grid control, the mesh size that want to be created is typed in metres.
• Then in Post variable Selection, click on Species/Passive, check the Mass fraction and add the N2 and O2.
• For Output Files, Time interval for writing 3D output files is 100 cycles which is the time interval between the one and another output file that get dumped in output files successively and the maximum no. of restart files that saved is 3 which is a Factor Of Safety and click ok.
• Now, when we click on Validate all options for validating , we got error as foll. Picture:

• To resolve this, go to Regions and Initialization and rename the region 0 to volumetric region user defined and then go to Boundary to assign this region to all the boundaries we’ve created since the error is not assigning the region for those.

• And click on validate all again and there will be no errors and the validation has been successfully completed.
• Since a case setup of three different meshes have been asked , change the value of base grid and save it separately.
• Now these input files has to be export by:

File-->Export-->Export input files.

Now, the input files of all 3 cases have been exported and these input files will be ran by CYGWIN in order to get output results.

The simulations for channel flow case setups of each mesh size have been done by CYGWIN and the output files have been saved.

Analyzing the plots in Converge CFD:

• For mesh size 2e-4 m :

1. velocity plot:

• Here, from this graph, the velocity of the channel flow case of mesh size 1e-4 is shown. At the inlet, the velocity is seems to be decreasing gradually and very low due to the increase in pressure and at the outlet, the velocity is seems to be increasing gradually and becomes high due to the decrease in pressure.
• At inlet, velocity is decreased and reached to -1m/s and outlet velocity is increased to 1m/s.
• It occurs since the Pressure is inversely proportional to the velocity. (P=k.1/v)

2.Pressure plot:

• Here, in the Pressure plot, at inlet, the value of Total pressure and Static pressure (100001 Pa) is same at initially because the pressure of the fluid that isn’t moving (static pressure) is equal to the total pressure (static pressure + dynamic pressure), which also means dynamic pressure is zero.
• As the cycles going on, the static pressure is getting decreased due to the increase in dynamic pressure.
• After 2000 cycles, the static pressure is decreasing fast to 100000.9 Pa as the fluid starts to move fast.
• At the outlet, finally the static pressure reaches to 100000 Pa and the total pressure value is increasing with respect to cycles due to the increase in dynamic pressure.

1. Mass flow rate plot:

• Mass flow rate is defined as the mass of a substance which passes per unit of time.
• Here, the mass flow rate at inlet is gradually decreasing due to the high pressure and low velocity and after 6000 cycles, the mass flow rate is decreasing so gradually than the initial condition because of slight decrease in pressure and slight increase in velocity.
• Finally, at inlet the mass flow rate reaches to -0.01kg/s and at outlet, reaches to 0.01kg/s.

1. Total cell counts:

• From the graph, the total cell count of the channel case is shown.
• It shows that we’ve used 4 processors from rank 0, rank 1, rank 2, rank3.
• The total cells are about 25000.

• For mesh size 1.5e-4m :

1. Velocity plot:

• Here also, the velocity at the inlet is keeps on decreasing gradually up to -0.8 due to high pressure.
• The Velocity at the outlet is increasing gradually nearly to 1 because of low pressure as we discussed in previous case.
• Here the simulation has been converged at nearer to 24000 cycles.

2. Pressure plots:

• In the pressure plot graph, at inlet, initially the static pressure and total pressure is same value of 100001 Pa until nearly 1500 cycles.
• After that the static pressure is begin to drop gradually due to increase in dynamic pressure up to 5000 cycles and reaches 100000.95 Pa.
• Then, it starts to begin little bit faster than the initial case due to respective increase in dynamic pressure and finally reaches to 100000.6 Pa.
• Similarly, at the outlet, total pressure is getting increase as the static pressure is getting decrease respectively, finally reaches 100000.5 Pa.

3. Mass flow rate plot:

• Here, in the mass flow rate, the mass flow is decreasing gradually up to nearly 5200 cycles and after that it decreases little bit faster to it’s convergence of 24000 cycles due to high pressure to low pressure and finally, at inlet reaches nearly to -0.01kg/s and at oulet to 0.01kg/s.

4. Total cell counts:

• The total cells that takes in the channel case are about nearly 45000 cells.

• For mesh size 1e-4m:

1. Velocity plots:

• Here also, the Velocity decreases to -1 at inlet due to high pressure and increases at oulet to 1 due to low pressure.
• The simulation has been converged at nearer to 16500 cycles.

1. Pressure plot:

• Here, at inlet, the total and static pressure is same (100000 Pa), at outlet, also same(100001 Pa) up to 1500 cycles.
• Then at inlet, static pressure is reducing due to high pressure until 16500 cycles and at oulet, total pressure gets increased up to 100000.6 Pa and static pressure is constant at 100000 Pa because of the same reason as it is in of mesh size 1e-4m.

1. Mass flow rate:

• Here, the mass flow rate is decreased up to -0.01kg/s at inlet and 0.01 kg/s is increased at outlet due to pressure difference.

4. Total cell counts

• The total cells for channel case of mesh size 1e-4m is about 100000 cells.

Mesh size for channel case setups:

• For mesh size 2e-4m :

• For mesh size 1.5e-4m :

• For mesh size 1e-4m:

• Simulation in PARAVIEW:
  • Velocity Magnitude:
    • Initial cycles:

• In each mesh size, it is seen that the velocity of the fluid at the initial cycles are very less.
• This occurs due to the high pressure.
• From Bernoulli’s principle, it is well known that the velocity of the fluid will get decrease, if the pressure of the fluid increases.
• So that due to the accumulation of masses at the inlet which causes high pressure resists the fluid to flow which tends to the low velocity at initial cycles.

• Half Cycles:

• At half cycles, from the animation each mesh size the Velocity seems to develop gradually.
• Because the fluid or air, will always tends to move from the higher pressure area to the lower pressure area.
• As at the inlet, there has been the high pressure of fluid at the initial cycles and after half of the cycles get completed, it is seen from the animation that the velocity of the fluid gets higher and forms a parabolic shape of flow of fluid.

• Final Cycles:

• At final cycles, after the pressure of the fluid drops in high rate, the velocity of the fluid gets increased
• But the velocity nearer to the Top and bottom walls are seems to be very less value since the Area is inversely proportional to the Velocity.
• From the scale, the value of maximum velocity attained is 2m/s.
• The value of velocity is more exactly in the mesh 1e-4m

• Pressure Magnitude:

• Initial Cycles:

• At the initial cycles of the simulation of each mesh sizes, the pressure of fluid at the inlet area is higher because the area is also inversely proportional to pressure.
• The value of the pressure obtained here is maximum that is 100001Pa.

• Half Cycles:

• Here, since the air or fluid tends to move from higher pressure area to lower pressure area, the pressure of the fluid gradually decreases at the inlet area in the half of the cycles.
• Also, at inlet, static pressure is decreasing and dynamic pressure is increasing.

• At Final Cycles:

• At final cycles, in each mesh size, eventually, the pressure at inlet area gets decreases to its maximum possible extent.
• And at outlet, the total pressure will be increasing.
• The value of pressure is seems to be less at the mesh 1e-4m at the inlet.

• Conclusion:
• From the three different cases of mesh size, it seems that as the value of mesh size gets smaller, the value of the variables will be more exactly in both plots and animations.
• Here, in this case, the velocity of the smaller mesh 1e-4m gives us the exact value or more parabolic form of result.
• Thus, the channel flow case of three different case of mesh size has been setup.
• And, the Plots of velocity, pressure, mass flow rate and total cell counts have been created and analysed through the converge plots.
• The physical mode of animation has also been done by PARAVIEW and the velocity and pressure contours are simulated and analysed.

Links for the PARAVIEW simulation animation:

1. For mesh size 2e-4m :

velocity magnitude: https://www.youtube.com/watch?v=HntjZWofnIw

pressure magnitude: https://www.youtube.com/watch?v=vANCLCezkXg

2. For mesh size 1.5e-4m :

velocity magnitude : https://www.youtube.com/watch?v=L6ui9YGpC_U

pressure magnitude: https://www.youtube.com/watch?v=sQhAcMFPfSo

3. For mesh size 1e-4m :

velocity magnitude : https://www.youtube.com/watch?v=0Q-WlKJ0eX8

pressure magnitude : https://www.youtube.com/watch?v=afUivevXP9A