This paper presents a numerical simulation and experiment on the effect of the variable pitch angle on the performance of a small vertical-axis wind turbine (VAWT) with straight blades. The power coefficient of the VAWT was measured in an open-circuit wind tunnel. By conducting two-dimensional unsteady computational fluid dynamics simulations using the RNG k-ε, Realizable k-ε, and SST k-ω models, the power and torque of the VAWT and the flow around the straight blades were also analyzed. The numerical simulation of the power performance results were validated using wind tunnel experimental data. The results of both the numerical simulations and experiments showed that a VAWT with variable-pitch blades has better performance than a VAWT with fixed-pitch blades. The numerical simulation of the performance using the RNG k-ε turbulence model had good qualitative agreement with the experimental results. The numerical simulation was able to capture the flow separation on a blade, and it was shown that a variable-pitch blade can suppress the flow separation on its blades at a tip speed ratio lower than that of fixed-pitch blades.
We present a numerical method based on hierarchical Cartesian grids for the simulation of multiphysics problems, here in particular for the conjugate heat transfer between a fluid and a solid. The data structures developed for this simulation method allow to account for moving objects and are especially well suited for massive parallelization. The major problem of a suitable domain decomposition for a coupled fluid and heat conducting domain is solved by discretizing both domains on a joint hierarchical Cartesian mesh, where the individual cells can be marked according to the underlying physics, i.e. to be a fluid cell, a heat conducting cell or both. Individual solvers for the Navier-Stokes equations and the heat conduction equation are implemented which only operate on the Cartesian cells belonging to the fluid or solid subset of the joint hierarchical mesh. The solution strategy is validated against the analytical solution of the convective heat transfer between a heat-conducting solid flat plate and a laminar incompressible boundary layer. The applicability of the method for moving objects is then demonstrated by solving a conjugate heat transfer problem for a heated and moving cylinder in a laminar flow.
A butterfly wind turbine (BWT) is a kind of vertical axis wind turbine (VAWT) with closed-loop blades. These blades form a double-blade structure, which is expected to improve self-starting properties and reduce energy costs because of their simple construction. Two models of micro BWTs (diameter: 0.4 m; height: 0.3 m) were built and subjected to wind tunnel testing. One of the models had a symmetrical blade section and the other had a cambered blade section with a mean line that followed a curved path in a flow curvilinear relative to the blade. Experimental results showed that the cambered blade rotor was superior to the symmetrical blade rotor in terms of torque and power coefficients at higher tip speed ratios (TSR). However, at low TSRs, the performance of the symmetrical blade rotor tended to be higher than that of the cambered blade rotor. To investigate the effects of blade section on the performance and flow field of the double-blade rotor, two-dimensional computational fluid dynamics (2D-CFD) analysis was carried out for two double-blade rotors with symmetrical and cambered blades. Although 2D-CFD analysis is not suitable for the quantitative performance analysis of the three-dimensional BWT, the CFD results showed the same tendency of the torque and power performance as the experimental results. If the outer blades alone are considered, the cambered blades generate larger torque (or power) than the symmetrical blades at all TSR values, in the case of a large chord-to-radius ratio as with the present rotors. On the other hand, the inner symmetrical blades generate more torque (or power) than the inner cambered blades at TSRs less than 1.5. A TSR of 0.75, at which the symmetrical blade rotor showed the highest torque coefficient, was intensively analyzed in terms of the aerodynamic forces and torques calculated by the 2D-CFD. Under this condition, the inner blade of the symmetrical blade rotor generated positive torque at a wider range of azimuth angles than the cambered blade rotor.
Direct numerical simulation of a turbulent mixing layer with a transversely oscillated inflow is performed. The inlet flow is generated by two driver parts of turbulent boundary layers. The Reynolds number based on the freestream velocity on the low speed side, UL, the 99% boundary layer thickness of the inflow, δ, and the kinematic viscosity, v, is set to be Re = 3000. In order to compare the results with the experimental study of Naka et al. [Naka, Tsuboi, Kametani, Fukagata, and Obi, J. Fluid Sci. Technol., Vol. 5, pp. 156-168 (2010)], the angular frequency of the oscillation was set to be Ωc = 0:83 and 3.85 (referred to as Case A and Case B, respectively). From the three-dimensional visualization, large-scale spanwise vortical structures are clearly observed in the controlled cases. The momentum thickness and the vorticity thickness indicate that the mixing is enhanced in Case A, while it is temporarily suspended in Case B. In both cases, the Reynolds normal stresses are increased in the region right downstream of the forcing point due to the periodic forcing. Furthermore, in Case B, the Reynolds shear stress (RSS), -u´v´, is suppressed in the region downstream of the forcing point. The spatial development of the turbulent energy thickness, δk, and the Reynolds shear stress thickness, δrss, show that the Reynolds shear stress in Case B is decreased by the control despite the increase of the turbulent kinetic energy. From the spectral analysis, large-scale spanwise structures are found to be caused by the periodic forcing, while the spectra of the spanwise velocity fluctuations are nearly unchanged. Co-spectra of the Reynolds stresses show that the present forcings generally enhance the long wavelength component. In Case B, however, the long wavelength component of the Reynolds shear stress is not increased in the downstream region.
The present study proposes “a flagmill”, which is a new power generator utilizing the flutter of a flexible sheet such as a flag. The sheet flutter causes the angular oscillation of the supporting pole. Since the supporting pole is connected to the power generator axis, the angular oscillation produces the electromotive power. The flutter and the power generation characteristics were investigated experimentally and analytically. The flutter occurs by decreasing the relative stiffness or increasing the mass ratio. The electromotive force cause the increase of the critical flutter speed. The higher flutter frequency and the larger amplitude of the angular oscillation of the sheet leading edge can generate the larger electromotive power.
A vortex generator (VG) is one of the most successful devices used for suppression of flow separation. A conventional VG is a passive control device, so that it may suffer from an inherent drag penalty. As an alternative, we use a dielectric-barrier-discharge plasma actuator (PA) as a VG. Since few studies have been made on such a vortex generator-type plasma actuator (VG-PA), many questions still remain open, e.g., the effectiveness and the performance of the VG-PA for suppression of vortex shedding from a bluff body. In the present study, we experimentally evaluate the performance of the VG-PA by applying it to a pantograph head model of high-speed trains. The Reynolds number based on the freestream velocity and the width of the pantograph head model is 7200. The spanwise velocity induced by the VG-PA is about 33% of the freestream velocity. The flow field is measured by using particle image velocimetry. The present results show that the vortex shedding from a pantograph head model can significantly be suppressed by the VG-PA, which amounts to about 60% reduction in the transverse velocity fluctuations. It is also shown that a burst discharge control significantly improves the control effect in the single-side configuration, in which the vortex shedding is not much suppressed in the continuous mode.
Control valves have been widely used in thermal and nuclear power plants to regulate the flow entering the turbine. The safety and stable operation of the turbine is threatened by the control valves' vibration and noise, which are directly related to the flow pattern. Many turbulence models have been used to show the mechanism in previous research, but few studies compared their abilities. In this study, four turbulence models, realizable k-ε, SST, DES based on SST and DES based on realizable k-ε, are selected to predict the flow pattern. A novel simulating method with a continuously changing pressure ratio is employed in the numerical simulation. Using a numerical method, the turbulence models are tested by two challenges in aerodynamic flow:(I)growth and separation of the boundary layer and (II)boundary layer reattachment. Compared to the traditional method with a constant pressure ratio, the new method offers more clear and credible results. The results are successfully verified by the sound mutation in the experiment. It is found that the DES based on the realizable k-ε model is the most powerful turbulence model for the two processes of boundary layer separation and reattachment.
We develop a pressure-based method using a mixture model for solving low Mach number compressible two-phase flows which contain surface tension, viscous stress, and heat conduction. We utilize the inviscid mixture model with isobaric-one velocity conditions imposed at the interface, and simply append viscous dissipation and heat conduction, where an isothermal condition is assumed. The continuum surface force model proposed by Brackbill et al. (1992) is used for surface tension effects. The pressure-based, segregated solution procedure is employed, the advection terms are explicitly integrated to suppress the numerical diffusion, and the other terms are implicitly solved by an iterative method. For the stability of the iterative calculation, the mixture sound speed included in the pressure equation is linearized by using the solution of the previous iteration, and the density is directly calculated from the equation of state. The proposed method is validated by conducting a shock tube problem, a one-dimensional oscillating bubble problem, and a two-dimensional static bubble problem governed by the Laplace law. In these problems, the numerical results are in good agreement with exact solutions. Finally, a two-dimensional collapsing bubble problem is presented to demonstrate the applicability of the proposed method.
The characteristics of a straight-bladed Darrieus turbine (Gyro-mill turbine) with elastic blades are experimentally studied. The same structured turbine with rigid blades is also studied for comparison. The turbine with elastic blads is shown to have the following characteristics. When the T.S.R. (tip speed ratio) is small, less than 1, it generates very large torque like a drag-type turbine, and it can start by itself. On the other hand, when the T.S.R. is large, it shows the ordinary characteristics of the lift-type turbine, but the T.S.R. is limited less than 2. The deformation of the turbine blades is observed by a high-speed video movie, and it is shown that the torque rises not because of the wind drag but of the lift due to the deformation when the T.S.R. is small.The starting torque is also increased. Operation under too high rotation speed, which is sometimes dangerous, is also avoided by the deformation due to the centrifugal force which becomes significant when the rotation speed is high.
Effects of pressure balance between multi-stages on part-load performance of centrifugal compressor for turbo chiller were analyzed by using CFD (Computational Fluid Dynamics). The analysis domains were two stages including the first and second variable inlet guide vanes (IGVs) arranged upstream of each compressor stage. The numerical simulation involved three-dimensional, steady, and compressible flow analysis with ANSYS-CFX®. The flash gas injected from the intermediate inlet of the economizer passage was taken into account. The part-load efficiency was measured while changing the opening ratios for the first and second variable IGVs. The predicted part-load efficiency corresponded to the measured one. We confirmed that the pressure balance between the first and second stages improved by adjusting the opening ratios of the first and second IGVs according to the flow ratio. The predicted flow pattern indicated that higher efficiency could be obtained by adjusting the opening ratio of the first and second variable IGVs to suppress excessive prewhirl from the first impeller and stall in the second impeller.
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