Semnan University Press Journal of Heat and Mass Transfer Research 2345-508X 11 1 2024 06 01 Physical Overview of the Instability in Laminar Wall-Bounded Flows of Newtonian Fluids at Subcritical Reynolds Numbers مرور فیزیکی ناپایداری در جریان های آرام محدود به دیوار سیالات نیوتنی در اعداد رینولدز زیر بحرانی 15 32 8491 10.22075/jhmtr.2024.31381.1466 EN Hamed Mirzaee Faculty of Mechanical Engineering, Semnan University, Semnan, Iran 0000-0002-5292-1130 Goodarz Ahmadi Department of Mechanical and Aerospace Engineering, Clarkson University, Potsdam, NY, USA 0000-0001-5277-7960 Roohollah Rafee Faculty of Mechanical Engineering, Semnan University, Semnan, Iran 0000-0002-4289-3420 Farhad Talebi Faculty of Mechanical Engineering, Semnan University, Semnan, Iran Journal Article 2023 07 29 This paper reviews the latest findings on instability and subcritical transition to turbulence in wall-bounded flows (i.e., pipe Poiseuille flow, plane channel flow, and plane Couette flow). The main focus was on the early stage of transitional flow and the appearance of coherent structures. The scaling of threshold disturbance amplitude for the onset of natural transition was discussed. Generally, the scaling proved to be in the form of A_c = O(Re^g) for Newtonian fluids where Re is the Reynolds number, g ≤ -1, and A_c is the critical perturbation amplitude. It was noted that exploration of perturbations like vortices, streaks, and traveling waves together with their amplitudes could clarify the instability and transition process. Hence, this paper focused on physical behavior and realizations of the transitional flow. Finally, a summary of consequential implications and some open issues for future works were presented and discussed. https://jhmtr.semnan.ac.ir/article_8491_ae4503ec3da32f5e9033604744ec45ae.pdf