Flow-induced variations in odour boundary formation

Odour tracking is vital for many biological organisms, underpinning behaviours such as foraging and navigation. However, due to the lack of systematic investigation, relevant applications remain underdeveloped. This study simulates the dispersion of odours from a single source, discussing the concept of an ‘odour boundary’. Utilising Schmidt numbers $Sc$ ranging from 0.96 to 3.08 and Reynolds numbers $Re$ from 195 to 47,665, we explored the structure and variability of the odour boundary at different concentrations. Results demonstrate that the odour boundary width remains largely consistent within the $Sc$ range, with minor variations at lower $Re$. As $Re$ increases, the boundary narrows, although low $Re$ values limit odour spread due to incomplete lateral flow development. Additionally, the potential core length inversely affects the odour boundary width; a more extended core facilitates downstream airstream propagation while restricting lateral dispersion. When $Re$ exceeds 10,000, the boundary width greatly decreases and stabilises. High-concentration regions within the lateral boundaries contract with higher wind speeds and at specific high $Re$, the odour distribution adopts an M-shaped pattern rather than a bell-shaped curve, suggesting that the highest concentration may not align with the centreline. This supports the notion of an ‘odour barrier’ and could elucidate biological tracking mechanisms, such as those in silk moths. Insights from this study may inform robotic navigation systems, enabling more efficient odour source localisation in turbulent environments.