Bottom ShearGenerated Turbulence in Stream or River Flows

The main source of turbulence in flowing streams comes from the shearing stress on the bottom. This turbulence generation is in balance with the local dissipation near the bottom. In higher regions, including near the water surface, the turbulence that diffuses up to the interface is in balance with dissipation.

Experiments with grid-generated turbulence in a tank provide a convenient analogy to the near-surface turbulence in open-channel flows. Although the mechanism of the turbulence generation at the bottom is different, the interaction between the interface and the decaying turbulence that diffuses up to the surface has similar characteristics. Grid-stirred tank systems generate near homogeneous turbulence with almost zero mean velocity. In comparison with open-channel flows, the absence of strong advection reduces the intricacy in performing measurements. Recent laboratory investigations in grid-stirred tanks using particle image velocimetry (PIV) and laser-induced fluorescence (LIF) techniques enabled detailed visualization and quantification of the velocity and concentration fields near the interface, respectively. Figure 2 shows an example of a sequence obtained with the PIV-LIF technique for an 8-mm-deep and 13-mm-wide vertical section below the water surface, with 0.75 s intervals between the shown images. The dark and light color scaling in the figure represent the regions with high and low oxygen concentration, respectively. In the experiments, the gas-transfer process was induced by lowering the dissolved oxygen concentration of the water in the test tank so that a concentration gradient is forced between the interface and the water in the bulk region. Thus, the darkest layer occurs at the water surface where the oxygen concentration reaches saturation. In the vicinity of the interface, a very thin dark layer indicating the boundary layer in which the oxygen concentration decreases rapidly from saturated to the bulk concentration can be observed. Below this boundary layer, the images are dominated with light color corresponding to the low oxygen concentration in the bulk region that is constantly being mixed by the turbulence. The visualization of the instantaneous concentration and velocity fields enables a good insight into the transport mechanism. The sequence shows the interplay between the turbulence and the boundary layer. Turbulent eddies impinging on the interface obviously squeeze the boundary layer which leads to a higher gas-transfer rate. Furthermore, the eddies are also responsible for the transport mechanism related to surface renewal events as can be observed in the sequence. Here, an eddy structure approaching the boundary peels off part of the concentration boundary layer and subsequently transports this oxygen-rich portion into the bulk region where the oxygen concentration is low.

The high spatial resolution of the LIF images in such experiments enables us to elucidate the details of the s

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Figure 2 A sequence of DO distribution and velocity fields during a reaeration process in an experiment in which turbulent flow is induced by a vertically oscillating grid. The water depth in the tank was 45 cm and the grid oscillated with 5 cm stroke and 4 Hz. The sequence (0.75s intervals between the shown images) visualizes a surface renewal event, in which an eddy structure approaching the boundary peels off part of the concentration boundary layer and transports this oxygen-rich portion downward into the bulk region.

Figure 3 Mean concentration profiles. Data from gas transfer experiments with grid-generated turbulence conducted by Herlina and Jirka GH (2007) Turbulent gas flux measurements near the air-water interface in a grid-stirred tank. In: Garbe C, Handler R, and Jahne B (eds.) Transport at the Air Sea Interface - Measurements, Models and Parameterizations. Berlin: Springer.

Figure 3 Mean concentration profiles. Data from gas transfer experiments with grid-generated turbulence conducted by Herlina and Jirka GH (2007) Turbulent gas flux measurements near the air-water interface in a grid-stirred tank. In: Garbe C, Handler R, and Jahne B (eds.) Transport at the Air Sea Interface - Measurements, Models and Parameterizations. Berlin: Springer.

saturation value at the interface to the concentration in the bulk within a very thin layer. This layer becomes thinner with increasing turbulent intensity. The boundary layer thickness (defined as equal to the depth where the normalized concentration has a value of 1/e) ranges between 800 and 250 mm for the ReT values between 260 and 780.

The concentration fluctuation c (root-mean-squared values) profiles for different grid conditions are shown in Figure 4. The fluctuations increase from smaller values near the interface to a maximum at about the boundary layer thickness. The normalized maximum peaks c'/(Cs — C) range between 0.15 and 0.2. Below this level, the c values decrease with further submergence.

concentration distribution near the water surface. Figure 3 depicts the time-averaged (mean) profiles for different turbulent intensity levels, represented by the turbulent Reynolds number (ReT = 2u L/v, where u is the turbulent velocity scale, L the integral length scale, and v the kinematic viscosity). The concentration is shown in a normalized form (c — C)/(Cs — C), where c is the local concentration and Cs and C the concentrations at the interface and bulk region, respectively. The concentration decreases from its

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