Radial side fan for the Ultimaker 3 series and the S Series.
The fans at the side of the print head spin during printing. This ensures that the layer of filament is properly cooled down before the next layer is put on. Most materials require this extra cooling to retain optimal print quality.
Please see attached link for recommended steps to replace the fan.
https://ultimaker.com/en/resources/49922-replace-the-radial-fans