چكيده به لاتين
Turbulence is not a completely random process; instead, there are some detectable spatial and temporal patterns which are known as coherent flow structures. These structures have considerable effects on the distribution of Reynolds stress in natural streams and play a key role in the sedimentation, mixing and distribution of nutrient materials and formation of aquatic flora and fauna. During the last three decades, investigation on the coherent flow structures and the related parameters has been a critical research subject. The aquatic plants not only are very sensitive to human activities and to climate change, but also they have substantial interaction with the flow structures. This research investigates the impact of vegetation patch changes on the flow structures using the field observation and the empirical experiments. A complete pattern of fade was simulated for a vegetation patch in a step by step procedure in which the dimensions of vegetation patches reduced in each step. An experimental flume was used to simulate a natural gravel-bed open channel, and synthetic elements (with close similarity to a real world vegetation patch) were used to simulate a submerged vegetation patch.
Acoustic Doppler Velocimeter (ADV) was used to capture instantaneous velocity vectors of the flow in various points. The outputs of ADV were used to calculate the values and distributions of Reynolds shear stress, Turbulent Kinetic Energy (TKE), coherent Reynolds shear stress, turbulence intensities and quadrant and octant occurrences of momentum transfer. These parameters were used to identify the flow structures downstream of the vegetation patch. According to the results, three flow layers were detected in the downstream region of the vegetation patch: wake layer, mixing layer and shear layer. For a relatively wide patch, the flow structures were almost 2D; however, it turns to a 3D pattern as the dimensions of the patch reduce. Highly Intermittent turbulent regime was observed in the downstream of wide patches accompanied by high frequency of outward interaction of quadrant occurrences (Q1). In the smaller patches, presence of von-Karman Vortex Street was observable. These vortexes were also detectable through 3D octant analysis in a certain frequency of particular classes of occurrences. In addition, in the presence of vegetation patches, local scouring was observed in very low Shields parameters. The extent of scouring was related to the turbulence intensity rather than the Shield parameter. However, the scouring reduces in smaller patches. In all cases, regardless of the length and width of the patches, turbulence intensity reaches its maximum value in a distance between 5Hv and 8 Hv (where Hv is the height of patch) and then reduces suddenly to a distance of 12Hv of the downstream face of patch. This phenomenon is also accompanied by formation of convex velocity profiles in the same distance. In addition, octant analysis showed that in the small patches (with blockage ratio lower than 0.5), von-Karman vortex street caused reduction of accuracy in Boundary Layer Characteristic Method (BLCM); a method which is traditionally used to determine shear velocity and Shield parameter.
