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  • 1.
    Dargahi, Bijan
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Hydraulic Engineering.
    Flow characteristics of bottom outlets with moving gates2010In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 48, no 4, p. 476-482Article in journal (Refereed)
    Abstract [en]

    This study investigates the discharge characteristics of a bottom outlet with a moving gate by Flow3D. Experimental results for a scale model outlet of the Aswan Dam, Egypt, were used. Two different flow features were found. Pressurized flow established if the flume was filled and then the gate was slowly opened. However, a free surface flow occurred if the gate was fully opened and the entire flume was slowly flooded with water. The numerical simulations successfully captured the two flow patterns as well as the discharges and water surface profiles. The discharges were predicted with sufficient accuracy using the first-order momentum advection scheme. In comparison with the k-epsilon turbulence model, the Re-Normalization Group model yields the best agreement with the experiments. The model performed with similar accuracy for both model and prototype cases.

  • 2.
    Dargahi, Bijan
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering (moved 20130630).
    Mitigation of sedimentation problems in the lower reach of the River Klaralven2008In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 46, no 2, p. 224-236Article in journal (Refereed)
    Abstract [en]

    The study focuses on the sedimentation problems found in the lower reach of the River Klaralven, where the river bifurcates into a west and an east channels at the city of Karlstad. During a period of thirty years, the sediment transport capacity of the west-channel has gradually diminished. This has caused a growing concern about the risk of flooding in the city. The study has aimed to find relevant methods for both increasing the hydraulic capacity of the west river channel, and mitigation of sedimentation problems. The main approach was numerical flow and sediment transport modelling. A two-dimensional depth-averaged model was used to study the influence of various river training measures. The model was found effective to address the sedimentation problem and the reduced hydraulic capacity in the river. To mitigate these problems, two effective engineering methods were controlling the water levels by partial gates and the use of groynes or vanes. The results suggested that a carefully calibrated and verified 2-D depth-averaged model can be used in supporting river restoration works and flood alleviation schemes.

  • 3.
    Dargahi, Bijan
    KTH, Superseded Departments, Land and Water Resources Engineering.
    Scour development downstream of a spillway2003In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 41, no 4, p. 417-426Article in journal (Refereed)
    Abstract [en]

    The scouring process downstream of spillways is an important research topic of value in engineering practice. The objectives of the present experimental study were to examine the similarity development of scour profiles, the controlling scour mechanism and predictions of the scour geometry. No experimental evidence was found in support of the similarity assumption for the scouring process. Scouring downstream of a spillway is controlled by large secondary flows and a vortex system induced by a hydraulic jump. The main dimensionless parameters controlling the scouring process are the relative operating head, the relative sediment size, and the relative roughness of the scour protective plate. The maximum scour depth and the rate of sediment transport can be estimated by using simple, power-type equations that relate the scour geometry to the controlling scour parameters.

  • 4.
    Ferdos, Farzad
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering.
    Dargahi, Bijan
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    A study of turbulent flow in large-scale porous media at high Reynolds numbers. Part I: numerical validation2016In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 54, no 6, p. 663-677Article in journal (Refereed)
    Abstract [en]

    Analyses of turbulent flows through the downstream slopes of embankment dams are important for dam safety assessments, especially considering high-risk scenarios such as a sudden release of water due to internal erosion. Flow prediction is difficult in such situations due to coarseness of construction materials and high Reynolds numbers. The present study addresses this issue through comprehensive numerical modelling. The novelty of the proposed approach lies in a combination of large-scale experiments and three-dimensional numerical simulations, leading to a fully calibrated and validated model that is applicable to flows through cobble-sized materials (100–160 mm in diameter) at high Reynolds numbers (>104). Comparing the results of the standard turbulence models to data from the large-sale experiments, the renormalization group theory-based model yielded the smallest relative errors based on the hydraulic gradients. Considering the flow field, the turbulent shear stress increased by a factor of 17, and the time-averaged vorticities intensified by factors of 2, 6 and 10 for vorticities in the x-, y- and z-directions, respectively, due to the presence of cobbles.

  • 5.
    Ferdos, Farzad
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering.
    Dargahi, Bijan
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    A study of turbulent flow in large-scale porous media at high Reynolds numbers. Part II: flow physics2016In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 54, no 6, p. 678-691Article in journal (Refereed)
    Abstract [en]

    The evaluation of hydraulic performance of coarse porous media at high Reynolds numbers is of significant importance for dam safety assessment. To address this task, the present study uses a numerical approach based on a rigorous theoretical framework. The novelty of the study lies in the application of a fully calibrated and validated numerical three-dimensional model and a set of modified equations for momentum transfer and the associated coefficients for flow in porous media. A Lagrangian particle tracking model was used to estimate the lengths of the flow channels that developed in the porous media. Gamma distributions were fitted to the normalized channel lengths, and the scale and shape parameters of the gamma distribution were found to be Reynolds number dependent. These shape parameters can be estimated from the suggested polynomial equations. The proposed normalized length parameter can be used to evaluate permeability, energy dissipation, induced forces, and diffusion. It was found that shear forces exerted on the coarse particles depend on the inertial forces of the flow and can be estimated using the proposed equation for the developed turbulent flows in porous media.

  • 6. Kung, C. S.
    et al.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Energy interpretation of hydraulic transients in pipelines1993In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 31, no 6Article in journal (Refereed)
    Abstract [en]

    The energy concept sheds light upon the hydraulic system that comprises the throttled surge tank, the tunnel and the penstock. The mathematical manipulation of the governing differential equations of water hammer yields an energy (power) equation in point form. By integration with respect to pipe distance, this relation is extended first to an individual pipe, then to the surge-tank system with branching connection. The system energy (power) equation states the relationship between the total (internal plus kinetic) energy, the convective energy, the work done on the system and the energy dissipation. A case study identifies the conversion between the internal and kinetic energy, and quantifies the behaviour and magnitudes of related energy components of the system.

  • 7.
    Liu, Ting
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Hydraulic Engineering.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering, Hydraulic Engineering.
    Experimental studies of air pocket movement in a pressurized spillway conduit2013In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 51, no 3, p. 265-272Article in journal (Refereed)
    Abstract [en]

    Undesired air entrainment in a bottom outlet conduit causes pressure transients, leading to conduit vibrations, blowbacks and discharge pulsations and thus endangers operational safety. In this study, the propagation velocity of a solitary air pocket and the characteristics of its critical velocity were examined in experiments conducted using a 240-mm-diameter pipe. Air pocket movement depends on the pipe diameter, slope, roughness and air pocket size. The critical pipe Froude number for initiating downstream movement of an air pocket is smaller in a larger pipe, most likely due to the scale effect and/or to a smaller reduction in the effective cross-sectional area. The critical velocity in rough pipes was found to be independent of the air pocket size. A minimum Froude number was suggested for a rough pipe instead of a critical pipe Froude number because the air removal process was found to involve successive air losses from the air pocket caused by turbulence.

  • 8. Lock, P.A.
    et al.
    Jing, X.D.
    Zimmerman, Robert
    University of London.
    Comparison of methods for upscaling permeability from the pore scale to the core scale2004In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 42, no SI, p. 3-8Article in journal (Refereed)
    Abstract [en]

    Several methods are compared for estimating the core-scale permeability of a rock whose pore space is idealized as consisting of a cubic network of pore tubes having a distribution of pore-scale hydraulic conductances. The conductance distribution of the pores is estimated from image analysis of scanning electron micrographs of rock sections. An explicit solution of the network equations is used as a benchmark to establish the "exact" macroscopic permeability. The other three upscaling methods used are Kirkpatrick's isotropic effective medium approximation, Bernasconi's anisotropic effective medium approximation, and the generalized perturbation ansatz (GPA) proposed by Gelhar and Axness. The analysis is carried out on a suite of petroleum reservoir sandstones from the North Sea, with measured core-scale permeabilities ranging from 20 to 500 mD. The log-variances of the pore-scale hydraulic conductance distribution were in the range of 2-3. The predictions of both the Kirkpatrick equation and the GPA are in each case within 10% of those computed by explicit network calculation, and all permeability predictions are generally within a factor of two of the core-scale values measured in the laboratory.

  • 9.
    Martinet, Philippe
    KTH, Superseded Departments, Land and Water Resources Engineering.
    Formation of arches and clusters of fine material in geotechnical filters2000In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 38, no 1, p. 7-14Article in journal (Refereed)
    Abstract [en]

    Two-dimensional mesoscale experiments and a theoretical analysis show that the formation of clusters in a geotechnical filter is associated with fine particles that form arches between the grains of the matrix. The analysis of the stability of such arches indicates two critical parameters. The characteristic size of a cluster is obtained as a function of the uniformity of the matrix. If the matrix is uniformly spaced, the clusters occupy large domains, whereas if the matrix is non-uniform the clusters form in small pores.

  • 10.
    Yang, James
    et al.
    Vattenfall R&D.
    Cederström, Malte
    Vattenfall.
    Modification of spillways for higher discharge Capacity2007In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 45, no 5Article in journal (Refereed)
    Abstract [en]

    In Sweden, many dams are to be refurbished in light of the new dam-safety guidelines that lead to higher design floods. To modify the spillway of a dam is often one of the alternatives proposed to increase its discharge capacity. Spillway rebuilding proposals for Ajaure, Bergeforsen, Midskog and Stenkullafors have been examined in hydraulic models. This paper deals with modification of the bottom outlet at Ajaure and proposed crest lowering at the other three overflow spillways. The hydraulic characteristics after the modifications are summarized and compared with the existing situation. The results are shown mainly in terms of water stage - discharge relationship and discharge coefficient, reasons for discrepancy are discussed and explained for each case.

  • 11.
    Yang, James
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Kung, C. S,
    Stability of air-cushion surge tanks with throttling1992In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 30, no 6Article in journal (Refereed)
    Abstract [en]

    The throttling orifice is introduced into air-cushion surge chambers to improve stability. The presence of the orifice adds another nonlinear term to the dynamic system, and its effect becomes predominant for oscillations with large displacements. By means of linearization, the type of singularities in the phase plane and their stability criteria in case of small oscillations are identified. The nonlinear analysis by direct numerical integration indicates that the system manifests itself as Hopf bifurcation with the surge-chamber size as its controlling factor. The bifurcation point corresponds to Svee's stability criterion. Before bifurcation, an unstable limit cycle may occur around the equilibrium state of practical interest, and it defines the domain of asymptotic stability. After bifurcation, two limit cycles may occur. Because of the existence of the stable one, the chamber size can be smaller than that specified by the Svee criterion. The orifice has stabilizing effect on surge motions.

  • 12.
    Yang, James
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Kung, C. S.
    Cederwall, Klas
    Large-amplitude oscillations in closed surge chamber1992In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 30, no 3Article in journal (Refereed)
    Abstract [en]

    The governing equations for surge oscillations in a closed surge chamber yield a second-order nonlinear differential equation for constant power. The surge stability is investigated by the direct method of Liapunov, which introduces the Liapunov function from the energy consideration. The nonlinear terms arising from large surges are included. The resulting stability criterion is demonstrated on a phase plane. For the equilibrium point of practical interest, the stability diagram specifies domains of asymptotic stability as a function of the safety factor of the surge-chamber. The diagram indicates that large-amplitude surges necessitate larger chamber size than that laid down by Svee for damping and the surge stability deteriorates with downward oscillations. By postulating sinusoidal surge motion, the critical chamber area is equal to the product of the critical area in case of small oscillations (Svee's formula) and a factor greater than 1.0. The factor is a function of surge amplitude and air cushion parameters for given turbine head. Stability criteria for open surge tanks can be obtained as special cases of those for closed ones.

  • 13.
    Yang, James
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Liu, Ting
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Bottacin Busolin, Andrea
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Lin, Chang
    National Chung Tsing University, Taiwan.
    Effects of intake-entrance profiles on free-surface vortices2014In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 52, no 4, p. 523-531Article in journal (Refereed)
    Abstract [en]

    Intake free-surface vortices can cause efficiency losses, flow fluctuations and even structural damages. Experiments were performed to examine the effect of entrance shapes on the critical submergence. Seven entrance shapes were devised and tested, including a square-edged, a bell-mouthed, three symmetrical conical and two conical profiles with eccentricity. The focus of the study was on a range of Froude numbers from 0.25 to 0.65. The square-edged shape appeared to show the highest local head-loss compared to other shapes. Steady counter-clockwise vortices characterize all the intake profiles except in a narrow water tank. The experiments show both discrepancy and similarity between the intake profiles. The critical submergence of the bell-mouthed intake is lower when compared to the square-edged shape. For the other profiles, it is proportional to the Froude number. A closer sidewall may lead to larger critical submergence in the case of weak circulations. The results demonstrate that the intake-entrance profile has an important effect on the critical submergence.

  • 14. Ye, F.M.
    et al.
    Yang, James
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Hydraulic Engineering.
    Wang, S. R.
    Turbine governing and surge-tank stability1992In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 30, no 1Article in journal (Refereed)
    Abstract [en]

    The stability of water-surface oscillations is investigated for the case of a tailrace surge chamber shared by two or more generating units in a hydroelectric power plant. Two kinds of turbine governing are applied for the condition of small-amplitude surge perturbations. Critical stability conditions for the surge chamber are derived by linearizing the ordinary differential equations describing the lumped system. As compared with the result achieved by previous researchers, the common surge chamber requires smaller horizontal crosssection for surge stability. A combination of the proposed manners of governing can be utilized in certain power plants in order to achieve an economical design of the surge chambers.

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