Fluid Dynamics for Virtual Molecules:An Engineering Guide to the Principles & Assumptions Underpinning Turbulence Modelling. Part 1: RANS

This paper provides an overview of the presentation materials prepared for the AIAA AVIATION 2025 Special Session “Meet the Turbulence Modellers,” organized under the auspices of the Integrated Collaborative Experimentation (ICE) Technical Activity Group 1 (TAG-1). The session, held on July 22, 2025, sought to create a common conceptual framework between turbulence measurers and turbulence modellers in support of advancing verification and validation (V&V) for computational fluid dynamics (CFD). The accompanying slides, titled Fluid Dynamics for Virtual Molecules: An Engineering Guide to the Principles and Assumptions Underpinning Turbulence Modelling (Part I: RANS), present an accessible narrative of the foundations and limitations of Reynolds-averaged Navier–Stokes (RANS) models. This paper summarizes those materials, providing an overview and commentary for each of the six major sections of the slide deck, with the goal of helping engineers and researchers outside the CFD community understand the conceptual basis and scope of modern turbulence models.