The effect of axial flow on bubbly drag reduction has been experimentally investigated in a vertical Couette-Taylor flow system. The water flow is combined from circumferential and axial flow. Flow condition is fully turbulence and Taylor vortices have appeared in the annulus gap. The shear stress modification in the simultaneous presence of air bubbles and axial flow in the system has been studied by measuring torque acting on the inner cylinder. The results show that axial flow improves the effect of bubbles on drag reduction by damping Taylor vortices and increasing upward velocity of bubbles. In this case, drag reduction of more than 25% has been achieved, which corresponds to lower tested ω Re and this amount is gradually decreased with increasing ω Re in each Re a and Q a . Increasing Q a causes drag reduction enhancement which could be due to the effect of bubbles on flow density reduction, flow fluctuations and Taylor vortices. Moreover, it is observed that skin friction is affected by axial flow solely and by increasing its volume rates, drag reduction reaches 11%. It is concluded that when bubbles and axial flow are simultaneously applied into the Couette-Taylor flow, the amount of achieved drag reduction is more than when they are separately applied.

A Large Eddy Simulation (LES) has been employed in order to study the flow field in a single-wind turbine and in two in-line wind turbines. The present study focuses on the flow around a horizontal axis wind turbine in a virtual wind tunnel. The anisotropic residual stress tensor is driven by the Smagorinsky model. The results are consistent with experimental data presented in literature. Streamwise velocity is increased and cross stream velocity is decreased as wake moves in downstream direction. A faster rate of wake recovery is seen for the two in-line setup. The results reveal that turbulence intensity is increased by increasing the downstream distance and two in-line turbines show greater intensity. Wind turbine performance can be affected by the turbulent structures. If this phenomenon occurs, information about turbulent structures would be useful in order to investigate the effect of turbulent structures on wind turbine performance. As a result, we aim to reveal the effect of turbulent structures, by using the ci  technique in this study, and to investigate the performance of wind turbines in different conditions. Furthermore, the effects of blade rotation direction are studied in this paper. It is concluded that wind turbine efficiency is increased by 4%.

In the present work, flow around a horizontal axis wind turbine is investigated in co-rotating and counter-rotating configurations. Large Eddy Simulation (LES) has been employed in order to study the effect of rotation direction of the downstream turbine in a two in-line setup. The results are in good agreement with presenting experimental data in literatures. The power efficiency of the downstream turbine has increased about 4% in the counterrotating configuration rather than co-rotating configuration with the same separation distance (3D). The power efficiency of the downstream turbine increased about 7% by decreasing the separation distance to 1D. The improved efficiency in the counter-rotating configuration is due to the flow rotation direction of the upstream turbine. The results revealed in both configurations that streamwise velocity is almost identical, while lateral velocities are changed greatly. Lateral velocities are decreased by moving in a downstream direction. Hence, the benefit of this configuration is obtained by lower separation distance. The authors suggested counter-rotating configuration for the terrain with a high density of installed wind turbines and believed that operating the wind turbines in counter-rotating configuration will not only improve the total efficiency of two in-line turbines, but will also considerably reduce the space they require. Since the oscillations of power coefficient are so high, the exact location of encountering the vortices on downstream blades should be determined to improve the power coefficient of two in line turbines. So, further studies are suggested to extract and track the vortices for future work to increase the efficiency of wind farms.

No abstract

The effect of small bubbles on reduction of energy dissipation has been numerically investigated in a vertical Couette-Taylor system. Flow is in the annular space between two concentric cylinders as the internal cylinder is rotating while the outer cylinder is stationary. Main fluid between cylinders is silicone, while air bubbles are constantly injected into the main flow at the bottom of cylinders' gap. The air bubbles rise through the flow when they are injected into the silicone flow. The flow is fully turbulence and Taylor vortices have appeared in the annulus gap. The rotational Reynolds number (Re x) varies from 700 to 3000. The fully two-phase turbulent flow has been studied using a discrete phase model and Euler-Lagrange approach. Air bubbles distribution or bubbles pattern through the main flow, which is acquired using numerical method, shows a good agreement to those acquired via experimental data in all Reynolds numbers. To investigate the changes of skin friction drag, the rate of energy dissipation in the system is calculated. The effect of injected air with constant flow rate on the total energy dissipation rate and the drag coefficient is also investigated. The results confirmed reduction of energy dissipation and about 25% of drag reduction when small bubbles were injected in the system. This reduction was the effect of the bubbles on the density of fluid and transformed momentum. Moreover, the acquired numerical results were in good agreement with those found in the previous experimental works, in which maximum Re x is up to 3000. Keywords Bubbly flow Á Taylor coquette system Á Energy loss List of symbols F D Drag force Fr Froude number G Gravity (m/s 2) I Turbulence intensity M Mach number Q Volume rate (m 3 /s) Re Reynolds number R Radius Ta Taylor number U The average velocity U Velocity(m/s) u 0 Velocity in Reynolds stress (m/s) m w The kinematic viscosity of the pure water Y Dissipation term We Weber number Subscripts B Bubble D Drag K Power gain factor i X direction j Y direction k Z direction T Temperature Greek symbols a Volume fraction (closure coefficient in the dissipation rate equation) b* Closure coefficient in the turbulent-kenetic energy equation d Radial gap