We examine effects of the divergence of the viscous terms on the numerical results of an incompressible flow by using an exact solution of the governing equation. When the Poisson equation is solved using fractional steps, the divergence in the velocity field may have nonzero magnitude. It should be noted that the divergence of the viscous terms can be larger than that of the velocity field. We use an exact solution, which is commonly used for benchmarking, to examine the effects of the divergence of the viscous terms. The divergence of the viscous terms affects the equilibrium relations found in the exact solution. In the present numerical results, the divergence of the viscous terms reduces the rate of decay of the kinetic energy and the pressure, and it also causes some harmonic waves in the flow field. We examine these results quantitatively by using approximations to the numerical results. Skewness and kurtosis factors of the physical quantities are also affected by the divergence of the viscous terms. These results show that the divergence of the viscous terms significantly affects the flow field.
Wet-steam flows under high-pressure conditions are numerically investigated using a newly derived condensation model that assumes a real gas and uses a real gas Equation of State (EOS). Under high-pressure conditions, the present condensation model calculates the critical radius of a water droplet that is larger than that of the conventional model. The results indicate that nucleation is initiated farther downstream in the nozzle and the nucleation rate is smaller. Wet-steam nozzle flows under high-pressure conditions are simulated and the results are compared with experimental results. The present model simulates the pressure increase due to the release of latent heat of condensation more accurately than the conventional method based on the ideal gas assumption. The results obtained are also in good agreement with the experimental results.
In this paper, we present a shallow water flow field estimation analysis, based on the Kalman filter FEM, for locating the optimal positions for tidal stream power generation systems. In our flow field analysis, we adopt the shallow water equation as the governing equation. The Galerkin and the selective lumping methods are employed as discretization techniques in space and time, respectively. The Kalman filter theory is applied to estimate the flow field. As a numerical example, we carry out a flow estimation analysis for Tokyo Bay. The high estimation accuracy of the flow field estimation based on the Kalman filter FEM is confirmed by comparison with conventional FEM. The electric power generation potential is also computed using the estimated flow field.
Thixotropy is a common behavior of soft matters widely used in various industries. Numerous models, both empirical and theoretical, have been proposed for different soft matters. In this paper the Maxwell viscoelastic model has been modified by decoupling the second order tensor type equation into vector type equations, without introducing new material parameters or assuming the dependence of the viscosity and/or elasticity on time and/or shear rate. The proposed Maxwell model is known as the vector type Maxwell model while the original one as the second order tensor type Maxwell model. The vector type Maxwell model was applied to both the stationary and the single step shear rate shear flow and the numeric results were compared with the experiments in the literature. It is found that the vector type Maxwell model predicted better both the shear and the normal stresses in the stationary shear flow than the second order tensor type Maxwell model, and the vector type Maxwell fluids may show thixotropic behavior in the single step shear rate flow as observed in experiments.
The operating conditions of Horizontal Axis Wind Turbine (HAWT) are extremely complex in natural surroundings. One of the wind conditions is called extreme wind direction change which has been specified in the International Electrotechnical Commission (IEC) 61400-1 standard. External conditions can generate extreme loads which may affect the power coefficient and the lifetime of HAWTs. This paper attempted to compile the evaluation of the aerodynamic forces acting on a small HAWT under extreme wind direction change condition in the wind tunnel experiments. An Avistar airfoil is used for the two-bladed and three-bladed wind turbines. This study is intended to clarify the load fluctuation when sudden wind direction change reacts to two-bladed and three-bladed wind turbines. A vane system is used to generate the wind direction change. A 6-component balance is used to measure the forces and the moments acting on the entire wind turbine in the three directions of x, y and z-axes. The results show that when the extreme wind direction change is generated, the yaw moment fluctuation amplitude of two-bladed wind turbine is about 39% larger than that of the three-bladed one. The strong dependence of the inflow timing of the wind direction change on the fluctuation amplitude can be seen only for the two-bladed wind turbine.