**1. Introduction**

Flow-based optimization of products and devices is an immature field compared to corresponding topology optimization based on solid mechanics. However, it is an essential part of component development with both internal and/or external flow.

Flow-based optimization can be achieved by e.g., coupling of computational fluid dynamics (CFD) and optimization software; both open-source and commercial options exist. The motivation for flow-based optimization can be to improve performance, reduce size/cost, extract additional information or a combination of these objectives. The outcome of the optimization process may be geometries which are more suitable for additive manufacturing (AM) instead of traditional subtractive manufacturing.

This MDPI Fluids Special Issue (SI) is a two-fold effort to:



**Table 1.** Overview of Special Issue research contributions: Applications and Quantities of Interest.
