# External Flow Volume creates 2 solid bodies, blocking single region simulation setup

**URL:** <https://www.simscale.com/forum/t/external-flow-volume-creates-2-solid-bodies-blocking-single-region-simulation-setup/101016>\
**Category:** Project Support\
**Created:** [May 4, 2026, 6:53am UTC](https://www.simscale.com/forum/t/external-flow-volume-creates-2-solid-bodies-blocking-single-region-simulation-setup/101016 "2026-05-04T06:53:38Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![RomeoL](https://www.simscale.com/forum/letter_avatar/romeol/32/5_5575768a8748004e209b776fc1b2916d.png) [@RomeoL](https://www.simscale.com/forum/u/RomeoL)\
**Post date:** [May 4, 2026, 6:53am UTC](https://www.simscale.com/forum/t/external-flow-volume-creates-2-solid-bodies-blocking-single-region-simulation-setup/101016/1 "2026-05-04T06:53:39Z")

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Hi SimScale team, I’m trying to set up an external aerodynamics simulation for a TT cyclist (STL). After uploading and running the External Flow Volume tool with default settings (no excluded parts, no seed face), the resulting geometry shows:

- Number of solid bodies: 2

- Volumes: Flow region + cyclist\_only

This creates a deadlock in Materials setup:

- Air assigned only to Flow region → “Wall 4 has body without material”

- Air assigned to both → “Multiple bodies with material assignments detected”

- Adding Darcy-Forchheimer to cyclist\_only → “not interfering with material body”

- Excluding cyclist\_only in External Flow Volume tool → “all bodies excluded”

What’s the correct way to configure this for an Incompressible single-region simulation? Should the cyclist\_only body be merged into Flow region somehow? Here’s the project link: **HJC With waterbottle**

My goal is to create a simple drag coefficient comparison for TT cyclist with/without bottle on their back

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**Author:** ![BryanArsenio](https://www.simscale.com/forum/user_avatar/www.simscale.com/bryanarsenio/32/71634_2.png) [@BryanArsenio](https://www.simscale.com/forum/u/BryanArsenio)\
**Post date:** [May 5, 2026, 11:16am UTC](https://www.simscale.com/forum/t/external-flow-volume-creates-2-solid-bodies-blocking-single-region-simulation-setup/101016/3 "2026-05-05T11:16:36Z")

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Hi @RomeoL

The error appears because there are 2 solids within the simulation geometry (shown in picture: blue and grey), while you did the right thing, create an external flow volume, you are missing one final step which is to delete the initial volume (in this case the volume of the cyclist), you can do this by using the Delete function in the CAD tool bar. The resulting flow region should only be the blue (fluid) volume (see second image).

Additionally, I would highly recommend you to take a look at [this article about creating flow region](https://www.simscale.com/knowledge-base/flow-volume-extraction/).

Thanks!  
Bryan

 ![Screenshot 2026-05-05 at 13.09.54](https://www.simscale.com/forum/uploads/default/original/3X/1/e/1e435e61887eb5c253533d922493d18e507bf6f2.jpeg)

 ![Screenshot 2026-05-05 at 13.15.50](https://www.simscale.com/forum/uploads/default/original/3X/0/d/0d1ed6437dcff065d6bec2eee8f6cad71a7f01a4.jpeg)