Statistical analysis of turbulent super‐streamwise vortices based on observations of streaky structures near the free surface in the smooth open channel flow

Abstract

Long streamwise-elongated high-and low-speed streaks are repeatedly observed near the free surface in open channel flows in natural rivers and lab experiments. Super-streamwise vortex model has been proposed to explain this widespread phenomenon for quite some time. However, statistical evidence of the existence of the super-streamwise vortices as one type of coherent structures is still insufficient. Correlation and proper orthogonal decomposition (POD) analysis based on PIV experimental data in the streamwise-spanwise plane near the free surface in a smooth open channel flow are employed to investigate this topic. Correlation analysis revealed that the streaky structures appear frequently near the free surface and their occurrence probability at any spanwise position is equal. The spanwise velocity fluctuation usually flows from low-speed streaks toward high-speed streaks. The average spanwise width and spacing between neighboring low (or high) speed streaks are approximately h and 2h respectively. POD analysis reveals that there are streaks with different spanwise width in the instantaneous flow fields. Typical streamwise rotational movement can be sketched out directly based on the results from statistical analyses. Pointby-point analysis indicates that this pattern is consistent everywhere in the measurement window and is without any inhomogeneity in the spanwise direction, which reveals the essential difference between coherent structures and secondary flow cells. The pattern found by statistical analysis is consistent with the notion that the super-streamwise vortices exist universally as one type of coherent structure in open channel flows. Key Points:Statistical results directly sketch out streamwise rotational movement without any hypothesis Super-streamwise vortices are one type of coherent structure rather than secondary flow cells Transportation of super-streamwise vortices results in the streaky structures (2016), Statistical analysis of turbulent super-streamwise vortices based on observations of streaky structures near the free surface in the smooth open channel flow, Water Resour.

Wavelet Coherency Structure in Open Channel Flow

“…It indicates that the 10h order motions consist of two different types of smaller structures, hairpin packets, and Q4 events. This agrees with the phenomena reported by Zhong et al [12], Adrian and Marusic [22], Zhong et al [12] and Zhong et al [13] suggested a super-streamwise vortex model for the 10h order motions in open channel flows. The super-streamwise vortices rotate around the x axis.…”

“…This two-energetic-mode feature of the pre-multiplied spectrum is similar to the results from other wall-bounded flows [15][16][17]. By the coherent structure classification in open channel flow [12,13], the h and 10h order motions can be classified as large-and very-large-scale structures.…”

“…The window size in the final iterative step is 16 × 16 pixels with a 50% overlap. A detailed description of the experiment system can be found in [12,13]. Figure 1a presents the wavenumber spectrum E uu for streamwise velocity fluctuation at point (x/h = 0, y/h = 0.5) from both Fourier and wavelet analysis.…”

Modeling the Effects of Turbulence on Hyporheic Exchange and Local‐to‐Global Nutrient Processing in Streams

“…Stream turbulence can influence vertical exchange in at least two ways, by controlling (1) the rate nutrients are delivered from the bulk stream to the streambed (or vice versa) and (2) mixing and transport of nutrients within the streambed (Figure a). In both cases, the vertical transport of mass (and momentum) is facilitated by eddies in the stream that are spatially coherent; that is, the positive or negative velocity fluctuations are spatially correlated (Grant & Marusic, ; Sinha et al, ; Vollmer et al, ; Zhong et al, ). At the sediment‐water interface, coherent turbulence manifests as sweep and ejection events, in which water parcels in the bulk stream with nutrient concentration C B (units of moles per cubic meter) sweep down to the interface, while water parcels near the interface with interfacial nutrient concentration C S (units of moles per cubic meter) are ejected into the bulk stream.…”

Modeling Suspended Sediment Transport Under Influence of Turbulence Ejection and Sweep Events

“…On the other hand, del Álamo et al () analyzed the direct numerical simulation data and reported “wall‐detached/attached vortex clusters,” which have different properties as opposed to the hairpin vortex. Zhong et al () observed the existence of the coherent structure near the free surface.…”