Circular Wake Research Articles

Comparison of hydrokinetic energy harvesting performance of a fluttering hydrofoil against other Flow-Induced Vibration (FIV) mechanisms

Despite the valuable previous studies on energy harvesting from flutter instability, no decisive ranking still exists between fluttering generators and harvesters based on other Flow-Induced Vibration (FIV) instabilities. In the current empirical study, the hydroelastic response and the energy performance of a self-sustained NACA 0015 hydrofoil are determined and compared to those of oscillators working based on Vortex-Induced Vibration (VIV), galloping, and Wake-Induced Vibration (WIV). Moreover, the effects of an unsteady circular wake on the performance of the hydrofoil are analyzed. With the scale and the aspect ratio considered in the current study, the upstream wake slightly mitigates the FIV response, power, and efficiency of the hydrofoil. Results of the current study show that the three top energy harvesting performances belong to oscillators with instability and combined instability-resonance responses. The high-power production capability of the galloping plus the high efficiency of the VIV makes WIV of the circular oscillator the optimum option for energy harvesting. Specific galloping oscillators like triangular cross-sections can even surpass WIV of the circular oscillator. The average performance of the fluttering hydrofoil makes it the third top instability above all resonance oscillators.

An entrainment‐based model for annular wakes, with applications to airborne wind energy

AbstractSeveral novel wind energy systems produce wakes with annular cross‐sections, which are qualitatively different from the wakes with circular cross‐sections commonly generated by conventional horizontal‐axis wind turbines and by compact obstacles. Since wind farms use arrays of hundreds of turbines, good analytical wake models are essential for efficient wind farm planning. Several models already exist for circular wakes; however, none have yet been proposed for annular wakes, making it impossible to estimate their array performance. We use the entrainment hypothesis to develop a reduced‐order model for the shape and flow velocity of an annular wake from a generic annular obstacle. Our model consists of a set of three ordinary differential equations, which we solve numerically. In addition, by assuming that the annular wake does not drift radially, we further reduce the problem to a model comprising only two differential equations, which we solve analytically. Both of our models are in good agreement with previously published large eddy simulation results.

Investigation on aerodynamics and active flow control of a vertical axis wind turbine with flapped airfoil

A 2D unsteady numerical simulation with dynamic and sliding meshing techniques was conducted to solve the flow around a threeblade Vertical axis wind turbine (VAWT). The circular wakes, strip-like wakes and the shedding vortex structures interact with each other result in an extremely unstable performance. An airfoil with a trailing edge flap, based on the NACA0012 airfoil, has been designed for VAWT to improve flow field around the turbine. Strategy of flap control is applied to regulate the flap angle. The results show that the flapped airfoil has an positive effect on damping trailing edge wake separation, deferring dynamic stall and reducing the oscillating amplitude. The circular wake vortices change into strip vortices during the pitch-up interval of the airfoils. Examination of the flow details around the rotating airfoil indicates that flap control improves the dynamic stall by diminishing the trend of flow separation. Airfoil stall separation has been suppressed since the range of nominal angle of attack is narrowed down by an oscillating flap. Vortices with large intensity over rotational region are reduced by 90 %. The lift coefficient hysteresis loop of flapped airfoil acts as an O type, which represents a more stable unsteady performance. With flap control, the peak of power coefficient has increased by 10 % relative to the full blade VAWT. Obviously, the proposed flapped airfoil design combined with the active flow control significantly has shown the potential to eliminate dynamic stall and improve the aerodynamic performance and operation stability of VAWT.

J0504-5-1 プロペラファンの離散周波数騒音と環状後流の破れ([J0504-5])流体関連の騒音と振動(5))

In order to clarify the relation between the discrete frequency noise of the propeller fan and the circular flow formed in the wake, the characteristics of the propeller fans were analyzed by the measurement of the flow and the CFD. The absolute velocity in the main flow domain of the meridional plane in the wake of the propeller fan became 30 m/s while the velocity in the inclined plane was decreased to 25 m/s. The velocity fluctuations on the vertical plane in the wake were weakened at the diagonal line by the velocity deformation formed to the circumferential direction around the impeller. Then the circular wake formed in the propeller fan was broken by the velocity deformation. It is considered that the discrete frequency noise of the propeller fan in the second harmonics became large since the fourteen dynamic rotors and the pseudo four stators made by the broken circular wake resonated.

Numerical simulations of a cylinder wake under a strong axial magnetic field

We study the flow of a liquid metal in a square duct past a circular cylinder in a strong externally imposed magnetic field. In these conditions, the flow is quasi-two-dimensional, which allows us to model it using a two-dimensional (2D) model. We perform a parametric study by varying the two control parameters Re and Ha (Ha2 is the ratio of Lorentz to viscous forces) in the ranges [0…6000] and [0…2160], respectively. The flow is found to exhibit a sequence of four regimes. The first three regimes are similar to those of the non-magnetohydrodynamic (non-MHD) 2D circular wake, with transitions controlled by the friction parameter Re∕Ha. The fourth one is characterized by vortices raising from boundary layer separations at the duct side walls, which strongly disturbs the Kármán vortex street. This provides the first explanation for the breakup of the 2D Kármán vortex street first observed experimentally by Frank, Barleon, and Müller [Phys. Fluids 13, 2287 (2001)]. We also show that, for high values of Ha (Ha⩾1120), the transition to the fourth regime occurs for Re∝0.56Ha, and that it is accompanied by a sudden drop in the Strouhal number. In the first three regimes, we show that the drag coefficient and the length of the steady recirculation regions located behind the cylinder are controlled by the parameter Re∕Ha4∕5. Also, the free shear layer that separates the recirculation region from the free stream is similar to a free MHD parallel layer, with a thickness of the order of Ha−1∕2 that is quite different to that of the non-MHD case, and therefore strongly influences the dynamics of this region. We also present one case at Re=3×104 and Ha=1120, where this layer undergoes an instability of the Kelvin–Helmholtz-type.

Models for the counter-gradient-transport phenomena

The counter gradient transport phenomena on momentum, energy and passive scalar in turbulent flows were studied by use of the single response function for TSDIA. As a result, models that can describe qualitatively the phenomena are obtained. Then the results are simplified by use of the inertial range theory, and the results for lower degrees agree with results of predecessor. Finally the counter gradient-transport phenomena in channel flow and circular wake flow are analyzed.

Full-potential circular wake solution of a twisted rotor blade in hover

A solution for transonic flow past a twisted rotor blade in hover is obtained using a modified version of the full-potential code ROT22 and a circular wake. The flow is also evaluated for a fixed-wing-type straight wake. The solutions for the straight wake and circular wake, and the circular wake and a two-dimensional wake are compared. The data reveal that the circular wake and the general two-dimensional wake solutions have similar characteristics.

An experimental and theoretical study of internal waves generated by the collapse of a two-dimensional mixed region in a density gradient

Previously reported experiments with a self-propelled body submerged in a fluid with a stable vertical density gradient have demonstrated that the turbulently mixed wake first expands more or less uniformly and then collapses vertically while continuing to expand horizontally (Schooley & Stewart 1963). It was also shown that the vertical collapse of the wake generates internal waves. Essentially two-dimensional experiments have also been used to explore some of the build-up and decay characteristics of vertical wake collapse induced by a sub-merged burst of turbulent mixing (Wu 1969; Schooley 1968). The present paper reports new experimental measurements and a linear theoretical analysis of the internal wave field created in stratified water by a burst of submerged turbulent mixing. The forcing function has been obtained in integral form for an initial-value model of wake collapse in terms of a general Brunt-Väisälä frequency profile, using normal mode theory. Numerical results have been determined for the specialized case of a completely mixed circular wake in a constant Brunt-Väisälä profile. These results are compared to the experimental measurements.