Flow structure over square bars at intermediate submergence : Large Eddy Simulation study of bar spacing effect

Peer reviewedPublisher PD

Impact of environmental turbulence on the performance and loadings of a tidal stream turbine

A large-eddy simulation (LES) of a laboratory-scale horizontal axis tidal stream turbine operating over an irregular bathymetry in the form of dunes is performed. The Reynolds number based on the approach velocity and the chord length of the turbine blades is approximately 60,000. The simulated turbine is a 1:30 scale model of a full-scale prototype and both turbines operate at very similar tip-speed ratio of λ ≈ 3. The simulations provide quantitative evidence of the effect of seabed-induced turbulence on the instantaneous performance and structural loadings of the turbine revealing how large-scale, energetic turbulence structures affect turbine performance and bending moments of the rotor blades. The data analysis shows that wake recovery is notably enhanced in comparison to the same turbine operating above a flat-bed and this is due to the higher turbulence levels generated by the dune. The results demonstrate the need for studying in detail the flow and turbulence characteristics at potential tidal turbine deployment sites and to incorporate observed large-scale velocity and pressure fluctuations into the structural design of the turbines

Influence of bubble size, diffuser width and flow rate on the integral behaviour of bubble plumes

A large-eddy simulation based Eulerian-Lagrangian model is employed to quantify the impact of bubble size, diffuser diameter, and gas flow rate on integral properties of bubble plumes, such as the plume's width, centerline velocity, and mass flux. Calculated quantities are compared with experimental data and integral model predictions. Furthermore, the LES data were used to assess the behavior of the entrainment coefficient, the momentum amplification factor, and the bubble-to-momentum spread ratio. It is found that bubble plumes with constant bubble size and smaller diameter behave in accordance with integral plume models. Plumes comprising larger and non-uniform bubble sizes appear to deviate from past observations and model predictions. In multi-diameter bubble plumes, a bubble self-organisation takes place, i.e., small bubbles cluster in the center of the plume whilst large bubbles are found at the periphery of the plume. Multi-diameter bubble plumes also feature a greater entrainment rate than single-size bubble plumes, as well as a higher spread ratio and lower turbulent momentum rate. Once the plume is fully established, the size of the diffuser does not appear to affect integral properties of bubble plumes. However, plume development is affected by the diffuser width, as larger release areas lead to a delayed asymptotic behavior of the plume and consequently to a lower entrainment and higher spread ratio. Finally, the effect of the gas flow rate on the integral plume is studied and is deemed very relevant with regards to most integral plume properties and coefficients. This effect is already fairly well described by integral plume models

Hydrodynamic loadings on a horizontal axis tidal turbine prototype

Until recently tidal stream turbine design has been carried out mainly by experimental prototype testing aiming at maximum turbine efficiency. The harsh and highly turbulent environments in which tidal stream turbines operate in poses a design challenge mainly with regards to survivability of the turbine owing to the fact that tidal turbines are exposed to significant intermittent hydrodynamic loads. Credible numerical models can be used as a complement to experiments during the design process of tidal stream turbines. They can provide insights into the hydrodynamics, predict tidal turbine performance and clarify their fluid–structure interaction as well as quantify the hydrodynamic loadings on the rotor. The latter can lead to design enhancements aiming at increased robustness and survivability of the turbine. Physical experiments and complementary large-eddy simulations of flow around a horizontal axis tidal turbine rotor are presented. The goal is to provide details of the hydrodynamics around the rotor, the performance of the turbine and acting hydrodynamic forces on the rotor blades. The simulation results are first compared with the experimental and good agreement between measured and simulated coefficients of power are obtained. Acting bending and torsional moment coefficients on the blade-hub junction are computed for idealised flow conditions. Finally, realistic environmental turbulence is added to the inflow and its impact on the turbine’s performance, hydrodynamics and rotor loadings is quantifie

Effect of blade cambering on dynamic stall in view of designing vertical axis turbines

This paper presents large-eddy simulations of symmetric and asymmetric (cambered) airfoils forced to undergo deep dynamic stall due to a prescribed pitching motion. Experimental data in terms of lift, drag, and moment coefficients are available for the symmetric NACA 0012 airfoil and these are used to validate the large-eddy simulations. Good agreement between computed and experimentally observed coefficients is found confirming the accuracy of the method. The influence of foil asymmetry on the aerodynamic coefficients is analysed by subjecting a NACA 4412 airfoil to the same flow and pitching motion conditions. Flow visualisations and analysis of aerodynamic forces allow an understanding and quantification of dynamic stall on both straight and cambered foils. The results confirm that cambered airfoils provide an increased lift-to-drag ratio and a decreased force hysteresis cycle in comparison to their symmetric counterpart. This may translate into increased performance and lower fatigue loads when using cambered airfoils in the design of vertical axis turbines operating at low tip-speed ratios

An immersed boundary-based large-eddy simulation approach to predict the performance of vertical axis tidal turbines

Vertical axis tidal turbines (VATTs) are perceived to be an attractive alternative to their horizontal axis counterparts in tidal streams due to their omni-directionality. The accurate prediction of VATTs demands a turbulence simulation approach that is able to predict accurately flow separation and vortex shed- ding and a numerical method that can cope with moving boundaries. Thus, in this study an immersed boundary-based large-eddy simulation (LES-IB) method is refined to allow accurate simulation of the blade vortex interaction of VATTs. The method is first introduced and validated for a VATT subjected to laminar flow. Comparisons with highly-accurate body-fitted numerical models results demonstrate the method’s ability of reproducing accurately the performance and fluid mechanics of the chosen VATT. Then, the simulation of a VATT under turbulent flow is performed and comparisons with data from exper- iments and results from RANS-based models demonstrate the accuracy of the method. The vortex-blade interaction is visualised for various tip speed ratios and together with velocity spectra detailed insights into the fluid mechanics of VATTs are provided

Effect of three-dimensional mixing conditions on water treatment reaction process

The performance of water disinfection facilities traditionally relies on Hydraulic Efficiency Indicators (HEIs), extracted from experimentally derived Residence Time Distribution (RTD) curves. This approach has often been undertaken numerically through computational fluid dynamics (CFD) models, which can be calibrated to predict accurately RTDs, enabling the assessment of disinfection facilities prior to the construction of disinfection tanks. However, a significant drawback of the conventional efficiency methodology prescribed for disinfection tanks is associated with the respective indicators, as they are predominantly linked to the internal flow characteristics developed in the reactor, rather than the disinfection chemistry which should be optimized. In this study three-dimensional numerical models were refined to simulate the processes of chlorine decay, pathogen inactivation and the by-product formation in disinfection contact tanks (CTs). The main objective of this study was to examine the effect of three-dimensional mixing on the reaction processes which were modelled through finite-rate kinetic models. Comparisons have been made between pathogen inactivation and disinfection by-product accumulation results produced by a RANS approach against the findings of a Segregated Flow Analysis (SFA) of conservative tracer transport. CFD Results confirm that three-dimensional mixing does have an effect on the reaction processes, which, however, is not apparent through the SFA approach

Large eddy simulation of free-surface flows

This paper introduces and discusses numerical methods for free-surface flow simulations and applies a large eddy simulation (LES) based free-surface-resolved CFD method to a couple of flows of hydraulic engineering interest. The advantages, disadvantages and limitations of the various methods are discussed. The review prioritises interface capturing methods over interface tracking methods, as these have shown themselves to be more generally applicable to viscous flows of practical engineering interest, particularly when complex and rapidly changing surface topologies are encountered. Then, a LES solver that employs the level set method to capture free-surface deformation in 3-D flows is presented, as are results from two example calculations that concern complex low submergence turbulent flows over idealised roughness elements and bluff bodies. The results show that the method is capable of predicting very complex flows that are characterised by strong interactions between the bulk flow and the free-surface, and permits the identification of turbulent events and structures that would be very difficult to measure experimentally