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

Peer reviewedPublisher PD

Alternatives to punishment: Counterterrorism strategies in Algeria

Rational choice theory has been one of the key theories used to explain the effectiveness of counterterrorism policies (Dugan, LaFree & Piquero, 2005; Enders & Sandler, 1993; 2003; Frey, 2004; LaFree, Dugan & Korte, 2009). These investigations have examined policies focused on increasing the costs of committing political violence, such as criminalization, increased police presence, and government strikes. However, few investigations have looked at policies that increase the benefits of not committing political violence such as negotiations and amnesties. In this study, I investigate the effectiveness of counterterrorism policies that seek to increase the benefits of not committing terrorism. I use Algeria as a case study and examine three counterterrorism policies between 1994 and 2004. One of the policies is a traditionally deterrent policy that increases the consequences of committing terrorism while the two other policies represent alternatively deterrent policies that increase the benefits of not committing terrorism. To analyze these policies, I use ARIMA modeling (N=120 months) and the Global Terrorism Database to determine whether each policy led to a significant change in overall attacks and the proportion of fatal attacks. While researchers have found mixed results when studying the effectives of traditional deterrence counterterrorist measures (Dugan, LaFree & Piquero; Enders & Sandler, 1993; Enders, Sandler & Cauley, 1990; LaFree, Dugan & Korte, 2009), I found that the Civil Concord Act, an amnesty program, as well as the Rome Platform, a negotiation policy, were related to a significant reduction in terrorism in Algeria

Flow Separation Dynamics in Three-Dimensional Asymmetric Diffusers

The mean and instantaneous flow separation of two different three-dimensional asymmetric diffusers is analysed using the data of large-eddy simulations. The geometry of both diffusers under investigation is based on the experimental configuration of Cherry et al. (Int J Heat Fluid Flow 29(3):803–811, 2008). The two diffusers feature similar area ratios of AR=4.8 A R = 4.8 and AR=4.5 A R = 4.5 while exhibiting differing asymmetric expansion ratios of AER=4.5 A E R = 4.5 or AER=2.0 A E R = 2.0 , respectively. The Reynolds number based on the averaged inlet velocity and height of the inlet duct is approximately Re=10,000 Re = 10,000 . The time-averaged flow in both diffusers in terms of streamwise velocity profiles or the size and location of the mean backflow region are validated using experimental data. In general good agreement of simulated results with the experimental data is found. Further quantification of the flow separation behaviour and unsteadiness using the backflow coefficient reveals the volume portion in which the instantaneous reversal flow evolves. This new approach investigates the cumulative fractional volume occupied by the instantaneous backflow throughout the simulation, a power density spectra analysis of their time series reveals the periodicity of the growth and reduction phases of the flow separation within the diffusers. The dominating turbulent events responsible for the formation of the energy-containing motions including ejection and sweep are examined using the quadrant analysis at various locations. Finally, isourfaces of the Q-criterion visualise the instantaneous flow and the origin and fate of coherent structures in both diffusers

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

A Biomechanical Investigation of Scaphoid and Lunate Kinematics During Wrist Motion

Carpal kinematics have been previously investigated, yet there remains no consensus regarding the relative contribution of each bone to total wrist motion. A more detailed understanding of carpal kinematics is essential in the effective diagnosis and treatment of injuries of the wrist, as many injuries manifest as an alteration in intercarpal kinematics. The scapholunate (SL) ligament is one of the most commonly injured intercarpal articulations resulting in a cascade of degenerative changes included cartilage wear and altered joint kinematics. The SL ligament is considered the primary stabilizer of the SL joint but is surrounded by a complex network of secondary ligamentous constraints, each contributing to the maintenance of normal SL kinematics. The ligamentous anatomy of the SL ligament and secondary stabilizers has been well established, although the functional and stabilizing role of each structure remains unclear. This work investigates the relative role and contribution of the scaphoid, lunate, and surrounding ligamentous restraints during planar wrist motions. An in vitro study examined the kinematics of the scaphoid, lunate, and capitate during planar motions of wrist flexion and extension. Scaphoid and lunate motion was found to correlate linearly with wrist motion throughout flexion and extension, with the scaphoid contributing at a greater degree throughout flexion-extension. Both the scaphoid and lunate were found to contribute more to wrist motion during flexion when compared to extension. A subsequent in vitro study examined the effect of the sequential sectioning of the SL ligament and two secondary stabilizers, the scaphotrapezium-trapezoid (STT) ligament and the radioscaphocapitate (RSC) ligament, on scaphoid and lunate kinematics during wrist flexion-extension and radial-ulnar deviation. The SL ligament was found to be the primary stabilizer of the SL joint, as sectioning caused the largest angular change in SL kinematics, and the STT and RSC ligaments are secondary stabilizers, as the additional sectioning induced further postural changes in SL kinematics. A more detailed understanding of role and stabilizing function of the SL ligament and secondary stabilizers may assist in the development of more effective treatment strategies following injury to the SL articulation

Large-eddy simulation of free-surface turbulent channel flow over square bars

The results of large-eddy simulations of free-surface turbulent channel flow over spanwise-aligned square bars are used to investigate the effects of bed roughness and water surface deformations on the root-mean-squared velocity fluctuations, dispersive shear stress, double-averaged Reynolds shear stress, wake kinetic energy and double-averaged turbulent kinetic energy. Two bar spacings, corresponding to transitional and k-type roughness, at similar Reynolds and Froude numbers are considered. The main peak of all statistical quantities occurs at the bar crest height. The effects of a standing wave at the water surface in flow over k-type roughness is marked by a local peak under the water surface for all statistical quantities considered here except wall-normal and spanwise velocity fluctuations. Quadrant analysis shows that sweeps and ejections are the strongest events contributing to both dispersive and double-averaged Reynolds shear stress but their contributions are different for the two bar spacings. Examining the budgets of dispersive and double-averaged Reynolds shear stress reveals that the dominant terms of these stresses are pressure–strain correlation and pressure transport and the contribution of wake production is similar for both of these stresses but with opposite sign. In addition to the main role of the bars in consuming or producing wake kinetic energy through production and transport and convection, the standing wave at the water surface in flow over k-type roughness induces large convection in the bulk flow too. The dominant terms in the double-averaged turbulent kinetic energy budget are similarly production, transport and convection. Large shear production renders large temporal fluctuations than spatial fluctuations of flow variables. The interaction of bars, bed and water surface is seen in the convection term in flow over k-type roughness

Reynolds and dispersive shear stress in free-surface turbulent channel flow over square bars

Reynolds and dispersive shear stresses in turbulent flow over spanwise-aligned square bars in an open channel flow are examined. Results of large-eddy simulation of flow over two different bar spacings corresponding to transitional and k -type (reattaching flow) roughness are analyzed. The Reynolds shear stress contribution to the momentum loss (or the friction factor, respectively) is greater than the dispersive shear stress contribution. By increasing the bars spacing, however, the contribution of the dispersive shear stress increases while the Reynolds shear stress contribution decreases, which is due to a standing wave at the water surface in the flow over k -type roughness which results in significant spatial variations in the time-averaged velocities. Strong sweep events take place and contribute to the friction coefficient. Investigating the dynamics of the flow reveals that there is momentum source below the crest of the bars and momentum sink above them, leading to acceleration or deceleration of flow, respectively. The contribution of dispersive shear stress is significant only in the deceleration of the flow near the crest of the bars and in the acceleration of the flow under the water surface. Quantification of the three components of total kinetic energy, i.e. mean, turbulent, and wake kinetic energy, reveals that the largest contribution is that of the mean flow in both geometries. By increasing the bar spacing, the contributions of turbulent and wake kinetic energy, which are localized at the bar height, increase, while the kinetic energy of the mean flow decreases

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

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 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