# SOLIDWORKS Flow Tips and Tricks

Running SOLIDWORKS Flow, even if you are SOLIDWORKS Simulation users, will be a bit daunting. Tutorials, training and watching Internet videos can help, but the Flow interface is different and there are many more parameters to consider when analyzing fluid flow and heat transfer.

But don’t worry. Even if you find your knowledge of SOLIDWORKS Simulation is of little use with computational fluid dynamics (aka CFD, aka SOLIDWORKS Flow), Flow is not a mysterious and magical software. It may be a relatively small and specialized application but there is information available on its use—like this article where Flow users have assembled their tips and tricks over years of using Flow.

Before diving into SOLIDWORKS Flow’s capabilities, it is important to understand its limitations. It is a multi-physics simulation software based on the Navier-Stokes equations. There is a technical reference in PDF format in the help files that goes through all of the underlying mathematics, if you are curious.

### A Few Caveats

We should not confuse multi-physics with all-physics. SOLIDWORKS Flow does not solve electromagnetic problems (i.e., it does not deal with Maxwell’s equations). It is limited to fluid flow and heat transfer. It also does not handle phase change. There are no calculations for heat of fusion or heat of vaporization. Interestingly, it can manage cavitation. But, you can infer a phase change from the results.

For example, if the results of a simulation show water temperatures above the boiling point while at standard atmospheric pressure, you can infer that the liquid has turned to steam. It is up to the user running the simulation to understand such phenomena have occurred.

SOLIDWORKS Flow can run simulations at different pressures, including negative gauge pressure (vacuum). Perhaps not an absolute, inter-galactic outer-space or surface-of-the-moon vacuum, but a “medium” vacuum (0.1 Pa or larger) is manageable. This roughly equates to a mean free path of 10 cm for the gas molecules. There are exceptions and geometry does play a role, so be warned—and do read the help file and seek further advice in particular cases.

Figure 1. The General Settings window.

Figure 2. The Calculation Control options window.

Figure 3. The Engineering Database (SOLIDWORKS Flow specific).

Figure 4. The Flow feature tree.

## Set it Up

SOLIDWORKS Flow can be broken down into three different phases: setup, run and post-processing.

Setup is generally regarded as the most important phase and you will do well to remember this: garbage in, garbage out. Setup can also be broken down into three areas: Settings, Input Data and Goals.

The Settings portion of the Setup phase includes the General Settings window (a flavor of the setup wizard, shown in Figure 1), the Calculation Control Options window (Figure 2), the mesh settings and the Engineering Database (Figure 3).

The Input Data area includes things such as boundary conditions, fans and heat sources. It is basically the feature tree of the SOLIDWORKS Flow tab (Figure 4).

Goals are always mentioned in training, but the “why” is normally omitted. Goals are specific parameter-driven calculations that the application stores for further use. They can be created and used as part of the input data, and they are also used extensively during post-processing. An example of goals for input data is creating a temperature surface goal on the tip of a thermocouple. A surface heat source (input data) can be set up as a function of the temperature surface goal during a transient analysis (very important that it be transient, as a steady state will never finish solving), thereby simulating a thermostat in the study.

Working backwards, input data is where things such as fluid sub-domain, fans, boundary conditions and heat sources are created. A volumetric heat source can have a constant temperature; that is to say, a given volume (solid body) can always remain the same temperature and serve as an input parameter into the system being studied. A surface heat source will not offer this option.

## Don’t Mix It Up

Multiple fluid sub-domains are fine as long as the fluids do not come in contact with each other (like water traveling through a pipe and the pipe is in air). The sub-domain must be completely isolated in the model. There is a workaround that allows for multiple miscible fluids in the same domain: use a mass fraction or volume fraction—for example, X% propane and Y% methane in the same pipe. Just be sure to include all the fluids in the setup (General Settings window).

Since SOLIDWORKS 2018, it is also possible to have immiscible fluids (such as oil and water, or water and air) interacting. This is done with the “Free Surface” feature (also in General Settings).

## Join the Fan Club

You can create a custom fan from SOLIDWORKS Flow’s fan feature and, using Microsoft Excel, you can enter points on the fan-curve.  This is very useful when fan manufacturers provide fan properties and data sheets.

## Resist Contact

Be sure to insert a contact resistance if two solids of dissimilar materials are in contact with each other and heat transfer is important. This is normally reserved for high order, more accurate solutions after a design survives its preliminary simulation.

Figure 5. Right Click on Solid Materials.

## Material Properties

Keep the material of the model current and material properties as complete as possible. If heat transfer (conduction) is involved, the density, specific heat and thermal conductivity of the material are required. If these properties are all up to date, then importing them into the simulation is as easy as right-clicking on “Solid Materials” and “Import Material from Model” (Figure 5). It will upload all unknown materials to the Engineering Database at the same time.

The SOLIDWORKS Flow Engineering Database is unique from the SOLIDWORKS material database. If “Solid Materials” does not appear, it is because “conduction” has not been enabled in the General Settings.

## Understand Gravity

Gravity can be a function of time when running a transient analysis (Figure 6). This allows the solution of objects turning upside down (or pouring out).

Figure 6. Gravity as a function of time.

## Making a Big Mesh of It

The Run phase is straightforward. There are a handful of options available to make sure the simulation is converging to a solution. You can monitor the goals that have been defined.

Check your mesh. You can get an accurate number of elements with a run without a solution. The solver will show the mesh count (once completed) and it will be easier to determine a rough solve time for future studies. For example, a 300,000 cell model took 52 minutes.

Look at the model with the mesh visible to see if the mesh should be refined anywhere.

## Batch Run and Go

The “Batch Run” option, in conjunction with the Clone Project feature (which in layman’s terms means “copy simulation”), allows up to two simulations to be run either concurrently or multiple simulations to be run sequentially without user interaction. You can set the number of cores to process with, so you can continue to do other work during the simulation.

If multiple simulations need to be run after changing just one parameter, use the “parametric study” option to set up multiple values for the same parameter/variable. The number of variables translates to the number of simulations solved. If it takes three hours to run one simulation and you have five different values, it will take 15 hours. It is often best to set up a batch run or parametric study at the end of the day.

The post-processing phase is the most gratifying—because that is where all the pretty pictures are. Every screenshot you grab from the post-processing phase should tell a story about that particular result.

## Red is Not Always to the Swift

It can be very confusing to see the same color in one screen shot represent different values in another screen shot. It is better to keep the colors consistent, with the gradient in every screenshot showing the same range of values. For example, screenshot A has a minimum temperature of 153°C in blue and a maximum temperature of 347°C in red. Screenshots B, C and D should have the same minimums and maximums.

## When in Doubt…

Trust your gut when you are doing the post-processing. When in doubt, throw it out. It is better to ask for an extension than to present bad data.

## Cut to the Plot

The Cut Plot feature has an extra field below the surface selection field for planes that may be hard to find the first time. Be careful when inspecting a cut across solid and fluid phases. Some properties are material-phase specific but will be indicated by the color gradient of fluid temperatures after the Cut Plot command.

Use Isosurfaces in the post-processing to analyze an entire 3D volume of any given parameter. For example, use Isosurfaces to show the temperature of the volume of air surrounding a PCB board that is above 80°C.

Use the flow trajectories feature in post-processing to create animations of the fluid flows. But keep in mind that the color of streamlines can represent temperature or pressure, not just velocity. It may be counterintuitive to see the little arrows moving fast and colored red and realize too late that you are seeing temperature , not velocity.

## Get to the Point

There is a feature called “Point Parameters” in the Results section of the feature tree. If you create a sketch with a point, use Point Parameters to analyze any available parameter (temperature, velocity, pressure, etc.) at that point. This will allow you to repeat multiple design changes using that point as a reference. The same can be done across a line in a sketch using the XY Plot feature.

The Goal Plots feature is fabulous for exporting goal plots across iterations (if steady state) or time (if transient) to Microsoft Excel for further data mining and analysis. This feature alone makes creating goals a higher necessity than explained during those introductory courses. It is important to note that the goals need to be established prior to running the simulation if you intend to use this feature.

## Under Pressure

Finally, the pressure field of a SOLIDWORKS Flow simulation can be exported to SOLIDWORKS Simulation for structural examination. However, this option must be enabled inside of SOLIDWORKS Flow. Do this in the Flow Simulation dropdown, Tools and then selecting “Export Results to Simulation.” This feature is in different locations depending on the SOLIDWORKS version being used.

In conclusion, SOLIDWORKS Flow is a virtual sandbox of possibilities for CFD simulation. With the introductory examples and lessons provided, the few resources available to apply the program practically, and a few common-sense rules (“garbage in, garbage out,” “when in doubt, throw it out”), your Flow simulations will be a success.