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The meteorological conditions markedly affect the energy efficiencies and cost/power rate of the wind turbines. The dusty environment is common in arid windy areas and airborne particles generally degrade wind turbine performance. The National Renewable Energy Laboratory (NREL) Phase VI wind turbine has been designed to be insusceptible to surface roughness, a consequence of facing dusty winds. This study numerically investigates the wake flow topology and analyses the performance of this turbine for either clean or dusty air. First, the numerical approach is validated against the available experimental data for clean air. Following this, the model is developed into a Lagrangian-Eulerian multiphase approach to comprehensively analyze the effects of the dusty air. The dependence of aerodynamic performance on the wind speed (U∞ = 5–25 m/s), particle diameter (dp = 0.025–0.9 mm), and angle of attack (AoA = 0°–44°) is investigated. The turbine performance deteriorates acutely for dp ≥ 0.1 mm and post-stall state. As such, the generated power is reduced by 4.3% and 13.3% on average for the air with the dp = 0.05 and 0.9 mm, respectively. The particles change the flow field profoundly, declining the pressure difference between the suction/pressure sides of the blade-airfoil, advancing the boundary layer separation, and strengthening the recirculation zones. The above changes account for a lower lift coefficient and a higher drag coefficient. [ABSTRACT FROM AUTHOR]

Numerical simulations are conducted to analyze flow characteristics around two tandem sharp-edged cylinders with cross sections of square ( b 1 * = 1) for the upstream cylinder and rectangle ( b 2 *) for the downstream cylinder (b * = b / a , where a and b are the sides of the cylinder). This study investigates the effects of Reynolds numbers (Re = 30–150), cross-sectional aspect ratios of the downstream cylinder ( b 2 * = 1 – 4) , and scaled gap-spacing between cylinders ( S * = 1–6) on the flow structure, onset of vortex shedding, hysteresis, and aerodynamic parameters (such as drag coefficient, Strouhal number, root mean square lift and drag coefficients, and so on). The results reveal that increasing b 2 * suppresses the vortex shedding of the upstream cylinder, depending on S * . The suppression is attributed to the interference effect and the adhesion of the shear layers on the downstream cylinder. Three distinct time-mean flow patterns are identified based on the separation and reattachment of shear layers. Flow pattern Ι exhibits parallel flow along the side faces of the upstream cylinder, while the separation bubbles associated with reattachment points are formed in flow pattern II on these faces. For pattern III, no reattachment point is observed, and the separation bubbles cover the upstream cylinder' side faces. Additionally, two instantaneous flow patterns of extended-body and co-shedding are apperceived within the ranges of examined Re and S * . The behaviors of time-mean and varying forces as well as the vortex shedding frequency are correlated with the flow structures. The onset of vortex shedding and hysteresis dependence is discussed comprehensively. The results show that the critical Reynolds numbers for the onset of vortex shedding decrease from 127.5 ± 2.5 to 49.5 ± 2.5 with S * increasing from 1 to 6 ( b 1 * = 1 and b 2 * = 4). The hysteresis limit is found within the range of 3.5 ≤ S * ≤ 4.5 for flow over two tandem cylinders ( b 1 * = 1 and b 2 * = 4) at Re = 150. [ABSTRACT FROM AUTHOR]

This paper presents the dependence of flow transitions and flow regimes on Reynolds number Re (≤ 105) and scaled cylinder center-to-center spacing S* (≤ 10) for two tandem cylinders. Both circular and square cross-sectional geometries are considered, and a comparison of results is made between the two geometries. To do so, data available from the literature are collected, and supplementary numerical simulations are done to complement the literature data so as to provide clear-cut flow maps in the Re–S* domain. We identified the borders of Re- and S*-dependent flows, including steady, unsteady, transition, slender/extended body, alternating reattachment, bistable, and co-shedding flows. The flow interference between two tandem cylinders with S* < S*cr is susceptible to stabilize the flow that requires a higher Re for the onset of vortex shedding than the isolated cylinder counterpart, where S*cr is the critical spacing for the onset of co-shedding flow. Besides, the flow interference generally postpones and advances the onset of secondary vortex shedding associated with the transition from two- to three-dimensional vortex shedding for S* < S*cr and S* > S*cr, respectively. Two flow maps are made for the two geometries, both yielding fundamental contributions toward a better understanding of the flow interference effect on various flow transitions. The flow maps determine the regions of a dearth of knowledge, which future studies may pay attention to. Finally, the drag inversion, hysteresis, and flow patterns in the proximity of the onset of vortex shedding for tandem square cylinders (1.5 ≤ S* ≤ 7) are discussed. [ABSTRACT FROM AUTHOR]

A direct numerical simulation is applied to investigate three-dimensional unsteady flow characteristics around a finite wall-mounted square cylinder with an aspect ratio of 7 at a Reynolds number (Re) of 40-250. Determination of Re for the onset of vortex shedding and Re influence on the wake structure and integral parameters are the major objectives of the current research. The results show that the vortex shedding inception occurs within the range of 75 < Re < 85. Re has a considerable effect on the mean wake topology and integral parameters. As such, the wake flow changes from a dipole to a quadrupole type, when the flow changes from steady to unsteady. A transition flow commences at Re = 150-200, where the wake instabilities are intensified with increasing Re, and the force signal oscillation alters from a sinusoidal to a chaotic type. Finally, the wake flow becomes turbulent at Re > 200. [ABSTRACT FROM AUTHOR]

In the present research work, we introduce a new method for estimating the slip length on superhydrophobic surfaces. Hence, a dynamic force is added to momentum equations and velocity boundary condition is rewritten in a new form. Laminar and turbulent channel flows are considered and two force functions are used with different profiles to investigate their effects on results. The turbulent channel flow is considered at Reτ=180 and the Large Eddy Simulation (LES) method has been applied to analyze this flow. All results indicate that this method can predict the streamwise slip length with a good accuracy, which is comparable with the Navier's method. So, using this numerical solution and also measuring pressure drop and mass flow rate in the channel, slip length can be calculated. Consequently, the errors and difficulties of slip length measurements in typical methods such as AFM and £gPIV would be eliminated. [ABSTRACT FROM AUTHOR]