Takashi Karasudani

Evolution of a vortex street in the far wake of a cylinder

The evolution of a primary vortex street shed from a circular cylinder in the far wake is experimentally examined for 70 < R < 154 (R is the Reynolds number). According to the vorticity fields obtained using digital image processing for visualized flow fields, the primary vortex street breaks down into a nearly parallel shear flow of Gaussian profile at a certain downstream distance, before a secondary vortex street of larger scale appears further downstream. The process leading to the nearly parallel flow can be explained as the evolution of the vortex regions of an inviscid fluid if we invoke the observation that the distance between the two rows in the primary vortex street increases with the downstream distance, although the viscous effect probably contributes to this increase. Numerical computations with the discrete vortex method also support this explanation. The wavelengths and speeds of the primary and secondary vortex street are also measured.

Stability of Flow between Eccentric Rotating Cylinders

The linear stability of two-dimensional steady flows between two long, eccentric, rotating circular cylinders is studied numerically under the condition that the inner cylinder rotates uniformly while the outer one is at rest. By using the pseudospectral method it is found that the critical Reynolds number increases with the eccentricity e . The critical axial wave number is found to remain nearly constant for small e and to increase with larger e . The eigenfunctions are distributed in the region from the position of the maximum gap to 180° downstream of that position. The Taylor-vortexlike three-dimensional steady flows are computed for several supercritical Reynolds numbers. The torques acting on the cylinders and the position of maximum vortex activity are calculated.

Improvement in Solar Chimney Power Generation by Using a Diffuser Tower

The solar chimney prototype, operated in Spain from 1982 to 1989, verified the concept of the solar chimney. The power generation mechanism in this system is to turn the wind turbine placed inside a high rise cylindrical hollow tower by an induced thermal updraft. As long as the thermal updraft is induced inside the tower by the solar radiation, this system can produce electricity. The disadvantage of this system is the low power generation efficiency compared to other solar energy power generation systems. To overcome this disadvantage, we improved the mechanism in order to augment the velocity of the air which flows into the wind turbine. By applying a diffuser tower instead of a cylindrical one, the efficiency of the systems power generation is increased. The mechanism that we investigated was the effect of the diffuser on the solar chimney structure. The inner diameter of the tower expands as the height increases so that the static pressure recovery effect of the diffuser causes a low static pressure region to form at the bottom of the tower. This effect induces greater airflow within the tower. The laboratory experiment, as does the computational fluid dynamics (CFD) analysis of the laboratory sized model, shows that the proposed diffuser type tower induces a velocity approximately 1.38‐1.44 times greater than the conventional cylindrical type. The wind power generation output is proportional to the cube of the incoming wind velocity into the wind turbine; therefore, approximately 2.6‐3.0 times greater power output can be expected from using the diffuser type tower. [DOI: 10.1115/1.4029377]

Numerical Studies of Flow around a Wind Turbine Equipped with a Flanged-Diffuser Shroud using an Actuator-Disk Model

Unsteady 3-D direct numerical simulations based on the finite-difference method (FDM) were performed for flow fields around a wind turbine equipped with a flanged-diffuser shroud. Generally, it is difficult to numerically simulate the flow around rotating bodies such as the blades of wind turbines because of the unsteady flow generated by moving bodies with complex geometry. Therefore, we have devised an actuator-disk model for a wind turbine for simulating the drag and rotational forces exerted on the fluid by the wind turbine. By incorporating the body forces derived from the actuator-disk model into the external terms in the Navier-Stokes equations, the unsteady flow around a wind turbine can be simulated. The results of numerical simulations were compared with the results from wind tunnel experiments and showed good agreement for the velocity and pressure fields.

Reduced Equations of Motion for Generalized Fluxes and Forces in the Continued-Fraction Expansion

With the aid of Moris projection operator method, it is shown that the mechanical equations of motion for flux variables can be transformed rigorously into the reduced form da, (t) / dt=iliJr a1 (t)- J: ds [,P1 (t-s) + ¢1 (t-s)] · a1 (s) + g1 (t) + jj(t), where a, (t) is a column matrix of generalized fluxes standing for the j·th order time derivative of the column matrix of state variables a, (t), <Po (t) =0, g0 (t) =0 and liJ1 is a frequency matrix. jj(t) is Moris j·th order random force and ¢1 (t) is the corresponding memory matrix. A new fluctuating force g1 (t) and the corresponding memory matrix cf;1 (t) represent the effects of lower· order fluxes. The temporal behaviour of g 1 (t) is quite different from that of jj(t), and the Laplace transform of cf;1 (t) is represented by an inverted continued fraction of finite order whose poles are all purely imaginary. The Laplace transform of time· correlation matrices of a1 (t) and a. (0) are given in terms of these memory·spectrum matrices cf;1 [z] and ¢1 [z].

Temperature Dependence of EPR Frequencies in Pure-and Pseudo-One Dimensional Heisenberg Magnets

The temperature dependence of EPR frequencies is studied theoretically for pure and pseudo-one dimensional Heisenberg magnets by means of Moris theory of generalized Brownian motions. In a system with uniaxial symmetry, simple expressions for resonance frequencies are obtained. It is seen that the resonance frequencies are shifted from the Zeeman frequency when the magnetic susceptibility is anisotropic. Fishers classical spin model is used to calculate the spin correlation functions. The anisotropy of the magnetic susceptibility is found to be enhanced by an effect of the short range order. The pseudo-one dimensional systems are treated with the help of the mean field approximation to interchain interactions. The result is reduced to that of a pure-one dimensional system, when the interchain interactions are absent. The effects of interchain interactions are negligible for antiferromagnets even near the Neel point, whereas those for ferromagnets are divergent near the Curie point. Nagata and Tazukes e...

On temperature dependence of EPR frequencies in Heisenberg linear chain magnets

Abstract The temperature dependence of paramagnetic resonance frequencies is studied theoretically for Heisenberg linear chain magnets. In the classical approximation two types of expressions for the resonance frequencies are obtained from Moris theory of generalized Brownian motions.

Non-Axis-Symmetric Taylor Vortex Flow in Eccentric Rotating Cylinders

An experimental study on the flow of an eccentric Couette-Taylor system is presented. In the system, the position of a fixed outer cylinder can be adjusted in relation to a rotating inner cylinder. The following results were obtained: when the displacement parameter was 0 0.3 the flow pattern of a no-axis-symmetric Taylor vortex flow was different from that of a concentric system. For e >0.6, there was no steady vortex flow. A pre-vortex flow changed drastically at about e =0.3, and a confined eddy occured for e >0.3.

Breakdown and Rearrangement of Vortex Streets in a Far Wake

Breakdown and rearrangement of regular vortex streets in the wake of a circular cylinder are examined experimentally. The evolution of vorticity field is obtained using digital image processing for visualized flow fields. For 100< R <140 ( R is the Reynolds number based on the velocity U 0 and diameter of the cylinder) a primary vortex street evolves to a parallel shear flow of Gaussian profile due to anisotropic extension of the vortex regions. Subsequently a secondary vortex street of larger scale appears. Its wavelength is within the instability region in the linear stability theory for the above velocity profile. The wavelength is, however, considerably smaller than that of the most unstable mode. Finally it is shown that the vortex regions in the primary and secondary vortex streets move at the speeds 0.15 U 0 ∼0.19 U 0 and 0.05 U 0 ∼0.10 U 0 , respectively, relative to the fluid at infinity.

On Temperature Dependence of EPR Frequencies in Pseudo-One-Dimensional Heisenberg Magnets

The temperature dependence of paramagnetic resonance frequencies is theoretically studied for classical pseudo-one-dimensional Heisenberg ferro- and antiferromagnets.