Hossein Afzalimehr

Bio: Hossein Afzalimehr is an academic researcher from Iran University of Science and Technology. The author has contributed to research in topics: Turbulence & Shear stress. The author has an hindex of 15, co-authored 83 publications receiving 868 citations. Previous affiliations of Hossein Afzalimehr include Laval University & Isfahan University of Technology.

Accelerating shear velocity in gravel-bed channels

Abstract: The behaviour of the shear velocity along a gravel-bed channel is investigated experimentally in the presence of a negative pressure gradient (accelerating flow). Different methods of estimation of the shear velocity, derived from vertical profiles of the mean longitudinal point velocity, are examined and a new method is proposed. Results show that the proposed method of estimation is comparable to the St Venant and Clauser's methods. At a specific cross section, for constant bottom slope and relative roughness, shear velocity increases with discharge.

Determination of bed shear stress in gravel-bed rivers using boundary-layer parameters

Abstract: The behaviour of velocity profiles and shear velocity for non-uniform flow in gravel-bed rivers is studied, with the objectives: (a) to test a new method of shear velocity estimation in gravel-bed rivers that is based on boundary layer parameters, and to compare it with the log law and parabolic law; (b) to consider the influence of flow non-uniformity on the outer layer region of velocity profiles; and (c) to investigate the effect of aspect ratio on velocity profiles. For the primary study river, mid-channel velocity profiles were analysed with relative submergence ranging from 9.7 to 33.3 in channel sections with aspect ratios ranging between 16.2 and 50. Velocity profiles deviated from the log law in the outer region due to flow non-uniformity or pressure gradient effects, and the vertical extent of the inner region was variable. Estimates of shear velocity using the boundary layer parameters (δ* and θ) compared well with estimates from the log law. In a second study river, boundary-layer par...

Boundary Layer Theory

Abstract: The boundary layer equations for plane, incompressible, and steady flow are $$\matrix{ {u{{\partial u} \over {\partial x}} + v{{\partial u} \over {\partial y}} = - {1 \over \varrho }{{\partial p} \over {\partial x}} + v{{{\partial ^2}u} \over {\partial {y^2}}},} \cr {0 = {{\partial p} \over {\partial y}},} \cr {{{\partial u} \over {\partial x}} + {{\partial v} \over {\partial y}} = 0.} \cr }$$

Closure of "Bedload and Size Distribution in Paved Gravel-Bed Streams"

Abstract: Field data are used to study the size distribution of bedload in paved gravel-bed streams. Similarity analysis yields the results that all grain size ranges are of approximately equal transportability when the critical condition for breaking the pavement is exceeded. This result is only approximately correct due to deviations from similarity. However, it is adequate to justify development of a method for calculating total bedload, which requires only the subpavement median grain size rather than the size distribution. A method for calculating bedload size distribution that accounts for deviation from similarity is also developed.

Hydrodynamics of vegetated channels

Abstract: This paper highlights some recent trends in vegetation hydrodynamics, focusing on conditions within channels and spanning spatial scales from individual blades, to canopies or vegetation patches, to the channel reach. At the blade scale, the boundary layer formed on the plant surface plays a role in controlling nutrient uptake. Flow resistance and light availability are also influenced by the reconfiguration of flexible blades. At the canopy scale, there are two flow regimes. For sparse canopies, the flow resembles a rough boundary layer. For dense canopies, the flow resembles a mixing layer. At the reach scale, flow resistance is more closely connected to the patch-scale vegetation distribution, described by the blockage factor, than to the geometry of individual plants. The impact of vegetation distribution on sediment movement is discussed, with attention being paid to methods for estimating bed stress within regions of vegetation. The key research challenges of the hydrodynamics of vegetated channels ...

Turbulence in open-channel flows

Abstract: Part I presents the statistical theory of turbulence, and Part 2 the coherent structures in open-channel flows and boundary layers. The book is intended for advanced students and researchers in hydraulic research, fluid mechanics, environmental sciences and related disciplines. References Index.

The influence of roughness structure on flow resistance on steep slopes

Abstract: There is no standard flow resistance equation for the determination of mean flow velocity in mountain streams. The reason lies in the morphology of mountain streams, i.e. steep slopes, large roughness elements, bed forms and water depths of the same order of magnitude as the bed material size. Logarithmic, Froude and power-law approaches to determine flow resistance are discussed with respect to the roughness parameter which is usually a characteristic grain size. As a result of the irregular nature of gravel-bed profiles it is shown that the structure of these stream beds cannot be described sufficiently by a characteristic percentile of the grain size distribution. Statistical properties of a series of bed profiles are investigated in order to quantify the effect of roughness on flow resistance. The standard deviation of the bed elevations is introduced as characteristic roughness length and its applicability is verified by the analysis of experimental data. Based on this roughness parameter, an approac...