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Summary:

Ready to dive into fluid simulation with nTop? Learn how to perform a fluid analysis.

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Applies to:

  • nTop 5.23 or later, as this is when nTop Fluids was introduced.

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Procedure:

Before we start, please ensure you have generated the Fluid Geometry.
  • Fluid Geometry: Have an implicit body that defines your internal fluid domain.
    • For example, this could be a manifold’s negative space, the heat exchanger’s internal channels, or any other part through which fluid will flow.
PipesManifoldsCold plates
Pipe geometry that can be used for an nTop Fluids Flow AnalysisManifold geometry that can be used for an nTop Fluids Flow AnalysisCold plate geometry that can be used for an nTop Fluids Flow Analysis
1. Review the Fluid Geometry. For this example, we are starting with CAD Geometry, but you can also create your fluid geometry using Modeling blocks in nTop. CAD geometry that has been imported into nTop 2. Add a Flow Analysis block The Flow Analysis block that is used to run fluid simulations in nTop 2. We need to add a Simulation Model (Virtual Model overload) as the Model input of the Flow Analysis block. You’ll use its “Virtual Model” overload for flow analysis. A Virtual Model is designed for simulations that don’t require traditional body-fitted meshing in nTop, like LBM-based CFD. It acts as a container for your simulation setup. An example setup of a Simulation Model used to run a Flow Analysis
  • Add a Fluid Domain : This block defines the region where the fluid flow will be simulated. You’ll input your implicit body representing the fluid volume here.
  • Add a Fluid Attribute : This block assigns material properties to your Fluid Domain. You’ll specify the type of fluid using the Isotopic Fluid Property block (e.g., Water, Air) and its relevant characteristics (Density, Kinetic Viscosity). Water and Air blocks are directly available for use.
Note: These blocks will differ from traditional Structures setup in nTop as they are tailored for methods like LBM, streamlining the setup for complex implicit geometries.
3. Once we have added the Domain, we need to move the input of the Flow Analysis block’s Boundary Conditions (How to use a CAD Face in a boundary condition). For our example, we will use two Velocity Inlets and one Pressure Outlet.
Note: To run Flow Analysis , you need at least one pressure and one velocity boundary condition, and outlets need to have the same pressure value.
Every remaining boundary not defined as a Velocity or Pressure boundary is set to a no-slip boundary condition. A diagram showing how boundary conditions are applied to pipe geometry for a Flow Analysis
First Inlet (Velocity)
  1. Drag in a Velocity block
  2. Switch to the CAD overload and then select the Inlet Face 1
  3. Enter the Velocity Vector value (0,10,0)
Second Inlet (Velocity)
  1. Drag in a Velocity block
  2. Switch to the CAD overload and then select the Inlet Face 2
  3. Enter the Velocity Vector value (10,0,0)
Outlet (Pressure)
  1. Drag in a Pressure block
  2. Switch to the CAD overload and then select the Outlet Face
  3. Enter the Magnitude of 0 Pa.
An example setup of a Flow Analysis including boundary conditions and the Simulation Model 4. The last input of the Flow Analysis block is Cell Size. This is the size of the cells used to create a Cartesian grid across your domain. For this example, we can use a value of 5mm. Tips on Cell Size:
  • The flow analysis uses a uniform Cartesian mesh, defined by a single parameter: Cell Size.
  • For quantitative results, use at least seven cells to resolve the smallest flow diameter, gap, or hole; for turbulent boundary layers, 20+ cells are recommended.
  • At least three cells should resolve the smallest flow features for qualitative results. Smallest Flow Feature
A diagram showing the smallest acceptable cell size for a fluid when running a Flow Analysis
  • Solid structures should be resolved with at least two cells.
A diagram showing the smallest acceptable cell size for a solid body when running a Flow Analysi 5. While the Flow Analysis is running, you can see the progress updated in the Bottom Panel (Ctrl +3) The log panel in nTop displaying each calculation step of the Flow Analysis process 7. Once the Flow Analysis is completed, we can review the results. Isolate the Flow Analysis block to visualize the results. Visualized results of an example Flow Analysis through a pipe When you toggle the Pressure option on the HUD, you can switch between different available results such as Velocity and Pressure. The default Display Mode is Domain, but you can switch it to visualize Streamlines in the Display tab of the Right Panel. Flow analysis results example with blue number bubbles to indicate how to change the display settings

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Visualized results of an example flow analysis with the display set to Streamlines

Streamline Settings: Seed Count: This input allows you to control the number of seed points. Increase the number for a denser, streamlined plot. Specify Seed: When you check this input, you can place a Sphere with Center [mm] and Radius [mm] to specify the region where you want to select the seed points. Flow Analysis Results on Boundary We have a Flow Analysis Results on Boundary , which allows you to calculate the average value of the Flow Analysis results, such as Velocity and Pressure, on the selected Boundary. Like other analysis blocks in nTop, Flow Analysis block will have access to all the underlying fields from which you can calculate values. An example of how to use the Flow Analysis Results on Boundary block using the Flow Analysis results That’s it: you have completed your flow analysis.

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Keywords:

analysis fluid flow cfd document fluids