Mete Koken

An investigation of the dynamics of coherent structures in a turbulent channel flow with a vertical sidewall obstruction

The physics of the flow around a vertical-wall obstruction attached to one of the sidewalls of a straight channel is numerically investigated using detached eddy simulation (DES) at a high channel Reynolds number, Re=5×105 (case HR). In particular, the study investigates the role played by the large-scale coherent structures in the sediment entrainment processes at the bed for conditions close to the initiation of scour (flat bed) in a loose-bed channel. Scale effects are investigated by comparing the results of the present DES simulation with results from a large eddy simulation performed at a much lower Reynolds number, Re=18 000 (case LR). Similar to laboratory flume studies of flow and scour around in-stream obstructions, the incoming flow in the simulations was fully turbulent and contained unsteady velocity fluctuations. The main necklace vortex of the horseshoe vortex (HV) system forming near the upstream base of the flow obstruction was subject to bimodal large-scale oscillations. The intensity of...

Coherent structures around isolated spur dikes at various approach flow angles

The turbulent flow structures around an isolated spur dike located at three different approach flow angles are investigated. Detached eddy simulation is used at a channel Reynolds number of 45,000. Experimental velocity measurements were also conducted using acoustic Doppler velocimetry for validating the simulations. It is found that the size and orientation of the horseshoe vortex system change considerably with the approach flow angle. The main necklace vortex is largest in size and most coherent for 90°. Within this orientation it has larger amplitude bimodal oscillations compared to the 60° and 120° cases. A new scour mechanism is defined for the 60° case which is expected to generate more scour upstream of the spur dike. The possible scour area around the spur dike is found to be the maximum in the 90° case which is roughly by 15% larger than that of the 120° case.

Effect of spur dike length on the horseshoe vortex system and the bed shear stress distribution

ABSTRACT Turbulent flow structures forming around isolated spur dikes in a horizontal channel are investigated in this study. In the analysis detached eddy simulation is used under fully turbulent incoming flow conditions at a channel Reynolds number of 45,000. Changes in the structure of the horseshoe vortex system, bed shear stress and pressure standard deviation on the bed are investigated for three different spur dike lengths. In all of the cases the main horseshoe vortex undergoes bimodal oscillations, which leads to an amplification in the turbulence quantities such as turbulent kinetic energy and pressure fluctuations along its axis. The main horseshoe vortex disappears over a much shorter distance in the flow direction for the short spur dike than those of medium and long spurs. Large bed shear stress values and pressure standard deviation values observed around the tip of the spur dike, beneath the upstream part of the main horseshoe vortex and beneath the separated shear layers increase with the increasing length of the spur dike. Different from the long and medium spur dike, in the short spur dike case, it is shown that the secondary horseshoe vortex is as coherent as the main horseshoe vortex and it contains bimodal oscillations together with the main horseshoe vortex.

INVESTIGATION OF FLOW AROUND A BRIDGE ABUTMENT IN A FLAT BED CHANNEL USING LARGE EDDY SIMULATION

Large Eddy Simulation (LES) is used to numerically investigate the horseshoe vortex (HV) system around a vertical bridge abutment located on a flat bed in a straight channel with vertical lateral walls corresponding to conditions at the initiation of the scour process. The simulation is performed with upstream fully turbulent flow including the turbulent fluctuations obtained from a precalculated LES simulation. The dynamics of the instantaneous coherent structures associated with the HV system around the base of the abutment and the spectral content of the flow in this region are analyzed. Due to the flat bed conditions, for which the HV system is not stabilized by the presence of the scour hole, very large random variations in the structure, position, size and overall intensity of the turbulent HV system are observed. In particular, the HV system appears to oscillate between a zero-flow mode in which the main HV is situated closer to the abutment and the near-bed jet flow beneath it is weak and separates early, and a back-flow mode in which the near-bed jet flow is stronger and separates at a larger distance from the abutment, and the main HV eddy is larger. It is observed that the legs of the horseshoe vortices can interact, at times, with the eddies shed inside the detached shear layer (DSL). The distribution of the bed shear stress shows that the largest values are present in the strong acceleration region near the tip of the abutment, but high bed shear stress values are observed beneath the HV system. It is found that the bed shear stress fluctuations around the local mean values can be very high, especially in the region beneath the separated shear layer. The pressure fluctuations and resolved kinetic energy levels are found to be very high inside the HV region compared to the surrounding flow. These high values are produced primarily by the low-frequency chaotic switching of the HV system between the zero-flow mode and the back-flow mode.

Coherent structures at different contraction ratios caused by two spill-through abutments

ABSTRACT Detached eddy simulation (DES) at a channel Reynolds number of 45,000 is used to investigate the coherent structures forming around spill-through abutments located at opposite sides of a rectangular channel which contract the flow section. Changes in the coherent structures are investigated for two contraction ratios at two different scour stages. It is observed that at the small contraction ratio the coherent structures forming around the abutments are similar to the ones observed for isolated abutments. However, at the larger contraction ratio there are important changes in these structures. At the beginning of the scour process two counter rotating contraction vortices form close to the bed at the centre of the channel, oriented in the streamwise direction. At this initial scour stage, the upstream part of the main horseshoe vortex is incoherent. On the other hand, at a later stage of scour with the development of the scour hole contraction vortices disappear and the main horseshoe vortex regains its original shape.

Application of computational fluid dynamics to predict hydrodynamic downpull on high head gates

Purpose High head gates are commonly used in hydropower plants for flow regulation and emergence closure. Hydrodynamic downpull can be a critical parameter in design of the lifting mechanism. The purpose of this paper is to show that a simplified two-dimensional (2D) computational fluid dynamics solution can be used in the prediction of the downpull force on the gate lip by comparison of computed results to experimentally measured data. Design/methodology/approach In this study, ANSYS FLUENT CFD software was used to obtain 2D numerical solution for the flow field around a generic gate model located in a power intake structure which was previously used in an experimental study. Description of the flow domain, computational grid resolution, requirements on setting appropriate boundary conditions and methodology in describing downpull coefficient are discussed. Total number of 245 simulations for variable gate lip geometry and gate openings were run. The downpull coefficient evaluated from the computed pressure field as function of gate opening and lip angle are compared with the experimental results. Findings The computed downpull coefficient agrees well with the previous experimental results, except one gate with small lip angle where a separation bubble forms along the lip, which is responsible from this deviation. It is observed that three-dimensional (3D) effects are confined to the large gate openings where downpull is minimum or even reversed. Research limitations/implications In large gate openings, three dimensionality of the flow around gate slots plays an important role and departure from 2D solutions become more pronounced. In that case, one might need to perform a 3D solution instead. Practical implications This paper presents a very fast and accurate way to predict downpull force on high head gates in the absence of experimental data. Originality/value An extensive amount of simulations are run within the scope of this study. It is shown that knowing its limitations, 2D numerical models can be used to calculate downpull for a wide range of gate openings without the need of expensive experimental models.

Formation of air-entraining vortices at horizontal intakes without approach flow induced circulation

The aim of this experimental study is to investigate the effects of hydraulic parameters on the formation of air-entraining vortices at horizontal intake structures without approach flow induced circulation. Six intake pipes of different diameters were tested in the study. The intake pipe to be tested was horizontally mounted to the front side of a large reservoir and then for a wide range of discharges experiments were conducted and critical submergences were detected with adjustable approach channel sidewalls. Empirical equations were derived for the dimensionless critical submergence as a function of the relevant dimensionless parameters. Available data is also checked for the possible scale effect. Then, these obtained equations were compared with the similar ones in the literature which showed a quite good agreement.

Time Evolution of the Horseshoe Vortex System Forming around a Bridge Abutment

Scour forming around an isolated bridge abutment with curved end is investigated in this study. Clear water scour experiments are conducted for an abutment in a 25 m long sediment flume at a Reynolds number of 45000. 3D scour patterns forming around the abutments are obtained using an array of acoustic transducers along a grid refined up to 1–2 cm spacing at different stages of the scour. Detached Eddy Simulation (DES) is performed at the same channel Reynolds number for flat bed case (initiation of the scour) and for two deformed bed cases (intermediate stages of scour). Incoming flow in the simulations were fully turbulent containing unsteady velocity fluctuations. Variations in the structure and intensity of the horseshoe vortex system are investigated. At the initial stage of the scour the main and secondary horseshoe vortices undergo aperiodic bimodal oscillations. Those oscillations cause the horseshoe vortices to induce large bed shear stress values beneath them. As the scour hole starts forming secondary necklace vortex, HV2, gets closer to the primary necklace vortex, HV1, and merges with it at a location close to the abutment tip.

AN INVESTIGATION OF COHERENT STRUCTURES IN THE WAKE OF A BRIDGE ABUTMENT AT EQUILIBRIUM BED SCOUR CONDITIONS

The database from a Large Eddy Simulation (LES) calculation is used to investigate the dominant coherent structures in the wake of a vertical bridge abutment in a straight channel with deformed bed corresponding to equilibrium scour bathymetry. The bathymetry is obtained from an experiment conducted at the same Reynolds number. The incoming flow is fully turbulent in both experiment and simulation. Dye visualizations are used to validate the numerical predictions. It is observed the directions at which the eddies are shed in the separated shear layer change as the bed is approached. In average, the separated shear layer curved toward the lateral wall as the bed is approached. The eddies shed into the separated shear layer can entrain sediment as they are convected. This may explain one of the mechanisms responsible for the development of the scour hole in the region behind the abutment. It is also observed the formation of large scale bed forms strongly increases the complexity and three-dimensionality of the flow in the region situated between the detached shear layer and the lateral wall downstream of the abutment. In particular, a streamwise elongated submerged mount of sediment (hill) is observed to form downstream of the abutment. As a result, two streamwise-oriented vortices are identified in between the submerged hill and the lateral wall. It is found the movement of the fluid inside these vortices may be either in the flow direction or in the reverse flow direction depending on the streamwise position along their axes. The influence of these vortices on the sediment transport within the recirculation region past the abutment in the later stages of the scouring process is discussed.