Flow Characteristics in the Wake Region of a Finite-Length Vegetation Patch in a Partly Vegetated Channel

The vegetation density λ affects turbulent flow type in the submerged vegetated river. This laboratory study investigates different types of vegetated turbulent flow, especially the flow at 0.04 < λ < 0.1 and λ = 1.44 by setting the experimental λ within a large range. Vertical distributions of turbulent statistics (velocity, shear stress and skewness coefficients), turbulence kinetic generation rate and turbulence spectra in different λ conditions have been presented and compared. Results indicate that for flow at 0.04 < λ < 0.1, the profiles of turbulent statistics manifest characteristics that are similar to those of both the bed-shear flow and the free-shear flow, and the turbulence spectral curves are characterized with some slight humps within the low-frequency range. For λ = 1.44, the turbulent statistics above the vegetation top demonstrate the characteristics of boundary-shear flow. The spectral curves fluctuate intensely within the low-frequency range, and the spectra of low-frequency eddies above vegetation top are significantly larger than the values below. The change of turbulent flow type induced by an increase of λ would increase the maximum value of turbulence kinetic generation rate G S and change the point where G S is vertically maximum upwards to the vegetation top.Vegetation density λ can be defined as the frontal vegetation area per unit bed area. Nepf [5] proposed that the bed-shear flow and free-shear flow occur at 0 < λ < 0.04 and 0.1 < λ < λ NB , respectively. Here, λ NB is the critical density which indicates the change from the free-shear flow to the secondary boundary-shear flow. The value of λ NB has not been determined yet, but according to the comparison and summary of many investigations, λ NB might lie in the range between 0.512 and 1.55 [6].Previous studies have investigated the bed-shear flow (0 < λ< 0.04) and the free-shear flow (0.1 < λ< λ NB ), especially the latter one in the vegetated channel. Flow at 0.04 < λ< 0.1 and λ> λ NB (the secondary boundary-shear flow may appear) have been rarely analyzed. In the present investigation, flow characteristics have been measured and analyzed as λvaries within a large range. Based on vertical distributions of turbulent statistics (velocity, shear stress, skewness coefficients), turbulence kinetic generation rate and turbulence spectra, different flow types have been verified. Furthermore, flow characteristics and velocity distribution laws at 0.04 < λ< 0.1 and λ> λ NB are discussed specifically. Experimental Setup and MeasurementLaboratory experiments were conducted in a 12-m-long, 60-cm-wide (B = 60 cm), 60-cm-deep recirculating flume (Figure 1). An ultrasonic flow meter (Krohne, Germany) with an accuracy of 0.001 L/s was used to monitor flow discharge Q, which was maintained constant by a variable frequency pump system. The flow from the water tank passed through a flow-straightener upstream of the vegetation section to smooth the inflow. A tailgate was mounted at the outlet of the flume to control the flow depth H in the experiment...

Section: Introductionmentioning

confidence: 99%

Effects of Submerged Vegetation Density on Turbulent Flow Characteristics in an Open Channel

Zhao

Yan

Yuan

et al. 2019

“…Here, x' refers to the distance from the patch edge and NV and NVlog correspond to the normalized velocity (U/U dm ) distribution and normalized logarithmic velocity (U log /U dm ) distribution, respectively, at non-vegetated flow conditions. You can find detailed information about the calculation of U log from Yılmazer et al [17]. A combination of a channel bed and suspended patch causes maximum velocity at the gap, with a flow similar to a wall jet beneath the vegetation patch.…”

Section: Resultsmentioning

confidence: 99%

“…Fishing activities and marine navigation can be at risk because of these types of floating vegetation patches [8]. Bed friction has a substantial effect on the velocity and turbulence below and within floating vegetation patches [6,13], however, it has a small role on the flow structure inside submerged or emergent canopies [14][15][16][17][18][19][20][21]. Despite the increasing importance of suspended canopies, there is still a limited number of studies on them.…”

Section: Introductionmentioning

confidence: 99%

Near-Wake Flow Structure of a Suspended Cylindrical Canopy Patch

Ozan

Yilmazer

2019

Self Cite

Urban stormwater is an important environmental problem, especially for metropolitans worldwide. The most important issue behind this problem is the need to find green infrastructure solutions, which provide water treatment and retention. Floating treatment wetlands, which are porous patches that continue down from the free-surface with a gap between the patch and bed, are innovative instruments for nutrient management in lakes, ponds, and slow-flowing waters. Suspended cylindrical vegetation patches in open channels affect the flow dramatically, which causes a deviation from the logarithmic law. This study considered the velocity measurements along the flow depth, at the axis of the patch, and at the near-wake region of the canopy, for different submerged ratios with different patch porosities. The results of this experimental study provide a comprehensive picture of the effects of different submergence ratios and different porosities on the flow field at the near-wake region of the suspended vegetation patch. The flow field was described with velocity and turbulence distributions along the axis of the patch, both upstream and downstream of the vegetation patch. Mainly, it was found that suspended porous canopy patches with a certain range of densities (SVF20 and SVF36 corresponded to a high density of patches in this study) have considerable impacts on the flow structure, and to a lesser extent, individual patch elements also have a crucial role.

“…In previous studies [30], the mean flow velocity stabilized when the measurement duration was greater than 45 s. Thus, the test time of the flow velocity measurement time was set to 60 s with a 6000 sampling number at 100 Hz, and the sampling volume was 7 × 10 −2 m 3 . The distance from the probe to the measurement point was 2.4 × 10 −6 m. To ensure the quality of the measurement data, the signal-to-noise ratio of the measurements was maintained a 15 dB [31]. The average velocity at each point was calculated from the observations of 3000 samples.…”

Section: Locations Of Flow Measurementsmentioning

confidence: 99%

Experimental Study of Overland Flow through Rigid Emergent Vegetation with Different Densities and Location Arrangements

Wang

Zhang

Yang

et al. 2018

The effect of vegetation density on overland flow dynamics has been extensively studied, yet fewer investigations have focused on vegetation arrangements with different densities and position features. Flume experiments were conducted to investigate the hydrodynamics of flow through rigid emergent vegetation arranged in combinations with three densities (Dense, Middle, and Sparse) and three positions (summit, backslope, and footslope). This study focused on how spatial variations regulated hydrodynamic parameters from two dimensions: direction along the slope and water depth. The total hydrodynamic parameters of bare slopes were significantly different from those of vegetated slopes. The relationship between Re and f illustrated that Re was not a unique predictor of hydraulic roughness on vegetated slopes. In the slope direction, all hydrodynamic parameters on vegetated slopes exhibited fluctuating downward/upward trends due to the clocking effect before the vegetated area and the rapid conveyance effect in the vegetated area, whereas constant values were observed on bare slopes. The performance of hydrodynamics parameters suggested that the dense rearward arrangement (SMD) was the optimal vegetation pattern to regulate flow conditions. Specifically, the vertical profiles of the velocity and turbulence features of the SMD arrangement at different sections demonstrated the significant role of vegetation density in identifying the velocity layers along the water depth. The maximum velocity and Reynolds Stress Number (RSN) indicated the position where local scour was most likely to occur, which would improve our basic understanding of the mechanisms underlying hydraulic and soil erosion processes.Thus, determining the optimum conditions for flow within vegetated slopes is vital for water and soil conservation and control.To understand the mechanisms underlying the hydraulic and soil erosion processes on vegetated hillslopes, hydrodynamic characteristics, which were mainly quantified by hydraulic parameters, e.g., water depth, mean velocity, flow pattern and resistance, kinetic energy and momentum distributions were considered as fundamental variables [6,7]. The hydrodynamic effects of emergent vegetation have been widely investigated in controlled laboratory experiments. In previous studies, well-defined rigid cylinder rods were assumed to represent an appropriate model for groups and trees or reeds and to generate accurate representations of the geometric characteristics [8]; thus, this method has been utilized as vegetation stems in numerous laboratory studies [7]. Generally, vegetation promotes the water depth [9], slows down the flow [10,11], enhances flow resistance [12], raises energy and momentum coefficients [13], attenuates the turbulence intensity [14], and promotes diffusion and deposition processes [15]. Dunkerley et al. [16] pointed out that plants could stabilize the flow state as laminar flow, whereas Hamilton et al. [17] stated that mixed flow states occurred on vegetated slopes. The definition of t...