A typical turbulent junction flow characteristics upstream of boulders mounted atop a rough, permeable bed and the effects of submergence

Authors: WYSSMANN, MICAH - University Of Missouri; CODER, JAMES - Pennsylvania State University; SCHWARTZ, JOHN - University Of Tennessee; Papanicolaou, Athanasios - Thanos

Submitted to: Experiments in Fluids
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/26/2023
Publication Date: N/A
Citation: N/A

Interpretive Summary: Understanding flow patterns around objects such as boulders in rivers and vegetation patches in landscapes can provide unique information of the level of turbulence and friction characteristics within the vicinity of these objects. The focus of this study is, using new technology that involves laser techniques and imaging analysis, to describe the patterns of turbulence around objects (here the application is boulders). For large size objects that typically protrude through the water surface turbulent patterns that match the shape of a horseshoe have been identified. Horseshoe patterns spiral down in front of the objects affecting pressure distribution and velocity with implications to soil erosion and deposition in front of the objects. The same phenomenon has been observed in overland flows in front of vegetation patch. In the latter case horsehoe patterns affect soil moisture levels as well as radiation levels. This research provides research ecosystem scientists, as well as water researchers, potential mechanisms for future studies aiming to quantify turbulence effects on soil attributes such as moisture and radiation for landscapes with vegetation and no vegetative cover.

Technical Abstract: While turbulent flow characteristics in the region upstream of wall-mounted obstacles (often called the junction region) are well described for many engineering systems, uncertainty remains about flow characteristics upstream of the large, immobile boulders that are commonly found in natural, steep gravel-bed streams. This uncertainty is largely due to the unique bed features (e.g., relatively large bed roughness elements, a permeable bed) and hydraulic characteristics (e.g., variable submergence) present at boulders, which may affect flow characteristics. This study reports results of laboratory volumetric particle image velocimetry (PIV) experiments performed upstream of model boulders for fully submerged (FS) and partially submerged (PS) conditions. Several atypical junction flow characteristics are documented that are common to both tested submergence conditions. The first and most striking result is that mean-flow separation is substantially delayed compared to previous junction flow studies. The data also suggest that there is notable mass flux into the permeable bed upstream of boulders via downward directed mean near-bed velocity and 3D streamlines terminating just above the roughness crests. Additionally, the mean horseshoe vortex system characteristics are atypical since no vortices are easily discerned via spiralling streamlines or concentrated vorticity peaks. Several effects of submergence on junction flow characteristics are also uniquely identified. Compared with the FS condition, the PS condition exhibits more rapid deceleration of the mean streamwise velocity, stronger downward velocities near the bed that extend over a wider transverse area, stronger increases of the near-bed turbulent kinetic energy, and oppositely signed streamwise-vertical Reynolds stresses near the water surface.