How to Easily Connect 1D Pipes to 3D Flow Domains

Niklas Rom November 27, 2014
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In earlier versions of COMSOL Multiphysics, it was cumbersome to connect 1D pipes to 3D flow domains. However, did you know that a new feature in COMSOL Multiphysics version 5.0 now allows you to easily accomplish this? Let me demonstrate how this feature works.

The Pipe Flow Module

The Pipe Flow Module has been around for quite some time now. My impression is that this add-on to COMSOL Multiphysics is well received by its users. Many of them view it as a natural extension of the CFD Module and the Heat Transfer Module.

The whole idea with the Pipe Flow Module is that fluid flowing in extended piping and channels largely follows the laws for fully developed flow. As such, you don’t need to resolve the complete flow field with volume elements. You can simply make the assumption that the flow is fully developed.

A diagram depicting pipe flow equations.
The pipe flow equations (right) only solve for an average velocity and the pipe is only represented by a line in the geometry (middle). This approach saves computational time and cuts memory requirements.

Connecting Pipe Ends to Volume Flow Models

In the image below, pipes are embedded in a solid mold. The heat transfer between the cooling fluid inside the pipes and the mold is studied. In this scenario, however, many users want to connect a pipe section to a 3D fluid flow domain.

An image of pipes embedded in a solid mold.
Pipes embedded in a solid mold. Image taken from the Cooling of an Injection Mold model entry.

This is a common occurrence when you have long stretches of slender pipes that suddenly open up to a larger fluid volume that cannot be approximated by a pipe equation.

Fluid volume attached to a pipe segment.
The fluid volume is attached to pipe segment, a 3D curve in the geometry.

The Pipe Connection Feature

In earlier versions of the COMSOL software, this coupling was possible to do. The old process, however, was rather involved, as it required users to set up coupling operators and do a lot of bookkeeping to get it right. In COMSOL Multiphysics version 5.0, we decided to make this easier by providing the Pipe Connection, a feature in the Pipe Flow interface.

This feature couples a 1D pipe segment (modeled with the Pipe Flow interface) with a 3D single-phase flow body. After adding the Pipe Connection feature to both the Single-Phase Flow interface and the Pipe Flow interface, you can choose an option from the list to couple the two physics interfaces. The Pipe Flow interface adds the Pipe Connection to an end point of a pipe and the Single-Phase Flow interface adds the Pipe Connection to a 2D surface that comprises an orifice in a 3D body.

The Pipe Connection feature is used to connect 1D pipes to 3D flow domains in COMSOL Multiphysics.
The Pipe Connection feature as seen in the COMSOL Multiphysics UI.

A model showing pipe flow sections.
A detailed schematic from the Convective Flow in a Heat Exchanger Plate model. The wavy segments are pipe flow sections and the block-shaped body is a 3D flow domain.

Model Download


  1. Jesus Lucio February 25, 2018   7:51 am

    Very useful blog, nice presentation, thanks a lot. The ‘Pipe Connection’ feature allows a connection for the fluid part of the model. But what about the ‘Nonisothermal Pipe Flow Interface’? Does it also easily allow a connection for the heat (temperature) part?

  2. Niklas Rom February 26, 2018   7:06 am

    Hi, Jesus the temperature connection is not built in to the Pipe Connection feature yet, but you can couple the temperature manually like in the example below:
    Kind regards
    Estimated size: 11.7 MB
    This link expires March 5, 2018. Please make sure to download before that date.

    Included files:
    - Pipe_CFD_k-eps_test_53a_T.mph

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