Flow Simulation Basic Concepts
I had been a finite element user for many years before entering the world of flow simulation/computational fluid dynamics (CFD). Initially, I did not see many practical industrial applications for CFD and stuck with custom code, rules of thumb and spreadsheets to get the job done. However, as I more recently evaluated CFD software and its integration graphical interfaces, I came to realize its power not only in flow, but also in thermal analysis, which was my main area of interest at the time. So I write this article using the approach and models that introduced me to CFD and the eventual selection of SOLIDWORKS Flow Simulation as my CFD package.
Setting Up the Model
We will start with a simple model of water flow in a pipe. This is a model I used when I was evaluating various CFD packages. It is a 100-ft horizontal length of 4-in-diameter pipe. It’s nothing exciting but a result I knew I could verify from having done these calculations manually and from other technical references (first rule of simulation, trust but verify!).
Figure 1. Solid model of pipe geometry.
Figure 1 shows a simple solid model of a hollow cylinder to represent our pipe. The next step is to bring this model to the flow environment. The easiest way of doing this is to use the Flow Simulation Wizard. With the SOLIDWORKS Flow Simulation add-in activated, the display should appear similar to Figure 2.
Figure 2. Flow Simulation menu ribbon.
Selecting the wizard option from the menu brings up the dialog box shown in Figure 3.
Figure 3. Flow Simulation Wizard dialog box.
Clicking “Next,” the dialog box in Figure 4 appears for unit system selection. Conveniently, we can mix and match units. This is particularly useful when performing a flow/thermal simulation using U.S. units. We will continue with the default IPS units.
Figure 4. Unit System Wizard dialog box.
The next dialog is the analysis type, as shown in Figure 5. The type of analysis can usually be determined intuitively. Internal flow is bound by a solid at the flow outer boundary. Our current model is internal and the fluid is bound by the pipe walls. An external flow example would be airflow over an airplane wing.
Figure 5. Selecting the analysis type.
We then add the fluid we are simulating to the project. Selecting water in Figure 6 adds it to the project fluids section as the default fluid.
Figure 6. Defining the project fluids.
After completing the wizard, we are greeted with our first error, as seen in Figure 7.
Figure 7. Error message due to nonwatertight model.
Yes, the pipe must actually have its ends “sealed” and be watertight to be able to simulate flow through it. All flow simulation must happen over some contained volume—the “fluid volume.” The software does not know where to end the problem if we don’t cap the ends, so we select “Yes” in the dialog box. We are then asked to select the open ends that we need to close, and lids are created as shown in Figure 8. The purpose of the lids and closing the model will be clearer when we discuss boundary conditions.
Figure 8. Adding lids to the pipe ends.
Prior to placing boundary conditions, we basically have a water bottle. The model requires boundary conditions to define the inlets and outlets. However, prior to defining them, we will perform a model check using the Tools → Flow Simulation →Tools → Check Geometry command. This checks that the model has valid geometry to proceed with the analysis. We can also enable a Show Fluid option, which gives us the graphic shown in Figure 9.
Figure 9. The fluid volume.
Referring to Figure 9, we make the following definitions:
- This is the boundary referred to as the computational domain. It represents the mathematical boundary of the flow problem. For internal flow, it closely, if not identically, corresponds to the fluid volume. At a minimum, it must envelop the fluid volume.
- This is the fluid region/volume that the software recognized. It is important to note that this is the only volume in the model that the analysis is concerned with. The solid bodies (pipe wall and lids) are there as a convenient way of defining boundary conditions. They do not participate in the flow simulation.
- This is the lid that we added to close the ends of the pipe. It does not participate in the analysis but serves as a reference to define boundary conditions, as we will see next.
Applying Boundary Conditions
Boundary conditions are where we define inlets and outlets for the flow. For our problem, we know the flow rate and are interested in flow distribution and overall pressure drop. Figure 10 shows the explorer pane for the Flow Simulation environment.
Figure 10. Flow Simulation explorer pane.
Right-clicking on the Boundary Conditions item brings up the dialog box in Figure 11.
Figure 11. Boundary Condition dialog box.
The face that we select for the boundary is the interior of the lid we added to seal the model. We are not able to select the fluid directly. The software assigns those lid surface conditions to the fluid in contact with that face (see Figure 9). If the lid face is not coincident with the fluid, there will be an error applying the boundary condition.
We now have an inlet flow (aka volumetric flow rate) defined as 40 ft3/min. We need to define an outlet by putting a pressure boundary condition at the other end of the pipe, again at the internal lid face. As shown in Figure 12, the outlet pressure is defined as ambient (101 kPa or 14.7 psi).
Figure 12. Defining the outlet conditions.
With these boundary conditions, the problem statement can be written as follows: “Calculate the pressure required to move 40 ft³/min of water through a 4-in-diameter pipe, 100 ft long, with an open discharge.”
Finally, we add goals to the model. Goals give the solver guidance on our solution objective. In this case, we add the inlet pressure as a goal. This is done by right-clicking “Goals” in Figure 10 and inserting a surface goal on the inlet lid face. This is the same face we used to define the inlet flow.
The fluid volume is meshed into a grid for the simulation to proceed. This is analogous to the mesh in finite element analysis. The default automatic mesh settings work very well for most flow simulation problems. However, even in the default automatic mode, there are refinement options available to the user. Figure 13 shows the dialog for setting mesh refinement from a scale of 1 to 7. The mesh preview is immediately updated in the model.
Figure 13. Mesh/grid settings.
With the problem properly defined and bound, we are ready to run the simulation. The Tools → Flow Simulation → Solve → Run command brings up the dialog in Figure 14.
Figure 14. Run dialog box.
We will choose “New calculation” and select “Run.” If there was a previous partial analysis performed, there is an option to continue the calculation so as to not have to rerun prior iterations.
The analysis progress and convergence can be monitored by selecting a goals plot. For our analysis, we are solving for the inlet pressure and we graph the convergence over time/iterations. This is useful to troubleshoot convergence issues. Figure 15 shows the value of the inlet pressure over the final iterations of the simulation.
Figure 15. Convergence plot.
There are many options for querying and viewing the results. Figure 16 shows the various graphics and result quantities that can be evaluated.
Figure 16. Results options.
Two of the most common are Flow Trajectories and Surface Parameters. For our problem, we are interested in a surface parameter, namely the pressure required at the inlet face/surface of the pipe to flow 40 ft³/min of water 100 feet. Right-clicking on the Surface Parameters option brings up the dialog box in Figure 17. We select the face of the lid to represent the inlet surface of the fluid and select “Show” to bring up the results in Figure 18.
Figure 17. Getting inlet pressure result.
Figure 18. Listing of calculated inlet pressure.
The results show that the average inlet pressure is 136,102 Pa. The outlet was set at ambient (101,325 Pa), giving a pressure drop of 35 kPa (5 psi).
The flow trajectories for our simple pipe model are essentially straight lines. Figure 19 better illustrates a flow trajectory plot. It’s taken from a sample model that combines flow and thermal effects.
Figure 19. Sample flow trajectory plot.
The flow model can be easily converted to a thermal model. We will modify the previous model to simulate a water heater by setting the pipe temperature to 500 °F and determining the water outlet temperature. Going to the General Settings dialog box and selecting “Wall conditions” brings up the dialog shown in Figure 20. We set the pipe wall to 500 °F (533 K).
Figure 20. Setting pipe wall temperature.
The steps to obtain a solution are the same as in the previous flow simulation.
The result we are interested in is the water discharge temperature to determine the amount of heat the 500 °F pipe transmits to the water. We right-click on the surface parameters item in Figure 16, which brings up the dialog box in Figure 21.
Figure 21. Water outlet temperature.
The outlet lid face is selected as the reference surface over which the water temperature will be evaluated.
Figure 22. Heat Transfer Coefficient parameter.
Figure 21 shows the results, indicating an average water discharge temperature of 316 K (109 °F). Other heat transfer–related parameters, such as the heat transfer coefficient (HTC), can also be determined. Figure 22 shows the results from selecting “Heat Transfer Coefficient” and the interior face of the pipe wall.
This shows that the average HTC acting at the pipe/water interface is 19,252 W/m²-K (3,390 Btu/hr/ft²-°F).
SOLIDWORKS Flow Simulation 2016 has capabilities for solving various flow and thermal problems. It uses the SOLIDWORKS modeling engine to define the physical geometry, and then the Flow Simulation environment defines boundary conditions and examines simulation results. In this article, we set up and solved a flow problem from a solid model through analysis and post-processing. The model was then converted to include thermal effects through a simple boundary condition change on the pipe wall. We also evaluated results pertinent to both flow and thermal simulations.
About the Author
Attilio Colangelo has more than 25 years of experience in engineering and project management in the chemical, process, ceramic and advanced-materials industries. His specialties include CAE, with an emphasis on FEA, high-temperature and heavy industrial design. His software skills include SOLIDWORKS Simulation, NASTRAN, Caesar II, ANSYS and iOS programming.