Sep 09 2025

A research group led by KARISAWA Hiroaki, a second-year doctoral student in the Graduate School of Science and Technology, Niigata University (selected for the Next-Generation Researchers' Challenging Research Program), and Research Professor YASUDA Hiroyasu of the Institute for Disaster and Reconstruction Science/Research Coordination Organization, has experimentally discovered a phenomenon where turbulence¹ is suppressed and an ordered structure (pseudo-laminar flow) forms in high-speed shallow water flows descending steep slopes. Conventionally, flow states have been classified as laminar flow2 or turbulent flow based on the Reynolds number3. This research challenges that understanding. Such ordered structures suggest new possibilities for fluid design, such as future control of river flows and engineered manipulation of friction losses4.
The research was published online in the international scientific journal Physics of Fluids on August 19, 2025 (EDT).

Key points

• We experimentally confirmed that in sheet flow over smooth steep slopes, accelerated flow suppresses turbulence growth, forming an organized velocity structure (pseudo-laminar flow).
• The formed pseudo-laminar flow persists over a long distance (more than 50 times the average water depth) even under high Reynolds number conditions that are fully turbulent, providing new insights beyond the dichotomous understanding of laminar versus turbulent flow.
• The measured data on pseudo-laminar flow stability and its formation conditions indicate new possibilities for fluid design in real flow fields, such as levee overtopping. This research is expected to lead to the design parameterization of friction losses and increased design flexibility for river and urban drainage systems.

[Term explanations]

1. Turbulence
Where vortices and disturbances of varying sizes occur irregularly within a flow, accompanied by complex spatial and temporal fluctuations. While velocity and pressure fluctuate violently, making prediction difficult, many actual flows, such as natural river flows and stormwater runoff, are in this turbulent state.

2. Laminar flow
Refers to a fluid flowing in orderly layers, characterized by flows like water or air sliding past each other with little mixing, flowing in parallel. Large vortices or turbulence are not observed within the flow, which exhibits properties where the velocity distribution and pressure changes are smooth and predictable.

3. Reynolds number
A representative dimensionless number used to classify the fluid motion, serving as a guideline for determining whether the flow is laminar or turbulent. Generally, a low Reynolds number indicates laminar flow, while a high Reynolds number indicates turbulent flow.

4. Friction loss
Refers to the energy loss that occurs when a fluid comes into contact with surfaces such as walls or the bottom. Caused by shear stress (a force that resists flow) within the fluid and results in a gradual loss of energy along the direction of flow. Generally, varies with flow velocity, water depth, and bottom surface roughness.

Publication Details

Journal: Physics of Fluids
Title:Acceleration-induced laminarization of sheet flow over smooth steep slopes
Authors:H. Karisawa and H. Yasuda
Doi: 10.1063/5.0279763