Chuichi Arakawa

Wind Turbine Blade Tip Flow and Noise Prediction by Large-eddy Simulation

The purpose of this research is to investigate the physical mechanisms associated with broadband tip vortex noise caused by rotating wind turbines. The flow and acoustic field around a wind turbine blade is simulated using compressible large-eddy simulation and direct noise simulation, with emphasis on the blade tip region. The far field aerodynamic noise is modeled using acoustic analogy. Aerodynamic performance and acoustic emissions are predicted for the actual tip shape and an ogee type tip shape. For the ogee type tip shape the sound pressure level decreases by 5 dB for frequencies above 4 kHz.

A turbulence model for the pressure–strain correlation term accounting for compressibility effects

Abstract Compressibility effects on turbulence are examined with special focus on the intercomponent energy transfer via the pressure–strain term in compressible turbulence. Experimental studies often show that the growth rate of compressible mixing layers is reduced with increasing Mach number. The recent analysis of the direct numerical simulation data bases for compressible turbulence shows that this reduction is due to the suppression of the pressure–strain correlation in the compressible mixing layer. In this paper, the order of magnitude analysis in compressible turbulence is performed to derive a turbulence model for the pressure–strain term in which this compressibility effect is included. The derived model is used to simulate compressible mixing layers, showing that the model predicts the reduced growth rate observed in experimental studies.

Bubble dynamics in the thermal shock problem of the liquid metal target

The thermal shock stress in a mercury target vessel was analyzed. The target receives the incident proton beam at an energy of 1 MW with a pulse duration of 1 μs. A negative pressure of 61 MPa was generated following the dispersion of the compression field at 52 MPa which was generated by the proton beam injection. It is expected that cavitation may be caused by the negative pressure. In order to evaluate the cavitation behavior and the following material damage mechanism, a simulation study was carried out using the equation of motion based on bubble dynamics for a single bubble, and fundamental parameter analysis was carried out. It is found that a bubble has a volume expansion of more than 1000 times with a change of the pressure at the window of the target vessel. Consequently wave propagation will be affected. Theoretical consideration was given to the wave motion of propagation in a bubbly liquid. The equation of state in a bubbly liquid can be approximated by polynomials. The diameter of a bubble and the bubble volume fraction inherent in mercury can be estimated if the critical pressure, the sound velocity, and resonance frequency are measured by static and dynamic experiments.

Influence of Blade Profiles on Flow around Wells Turbine

The Wells turbine rotor consists of several symmetric airfoil blades arranged around a central hub, and the stagger angle is 90 degrees. These characteristics simplify the total construction of OWC type wave energy converters. Although the Wells turbine is simple, the turbine produces a complicated flow field due to the peculiar arrangement of blades, which can rotate in the same direction irrespective of the oscillating airflow. In order to understand these flows, flow visualization is carried out with an oil-film method in the water tunnel. This research aims to analyze the mechanism of the 3-D flows around the turbine with the flow visualization. The flow visualization explained the influence of attack angle, the difference between fan-shaped and rectangular wings, and the sweep angle.

WIND TURBINE FLOW AND NOISE PREDICTION BY LARGE EDDY SIMULATION

The purpose of this research is to investigate the physical mechanisms associated with broadband tip vortex noise caused by rotating wind turbines. The flow and acoustic field around a wind turbine blade is simulated using compressible Large-Eddy simulation and direct noise simulation, with emphasis on the blade tip region. The far field aerodynamic noise is modeled using acoustic analogy. Aerodynamic performance and acoustic emissions are predicted for the actual tip shape and an ogee type tip shape. For the ogee type tip shape the sound pressure level decreases by 5 dB for frequencies above 4 kHz. Initial results regarding winglet simulations are shown.

LARGE-EDDY SIMULATION OF TIP VORTEX FLOW AT HIGH REYNOLDS NUMBER

Simulations using up to 300 million grid points are performed on the Earth Simulator in Japan, the fastest supercomputer in the world. The present work is the first attempt to simulate a finite blade with a tip in an incident flow and its associated tip vortex and acoustic field at a Reynolds number in the order of a million using direct compressible LES (Large-Eddy Simulation). For validation purposes, the flows around a NACA0012 blade section as well as finite blade at lower Reynolds number are simulated. It is a wall resolved simulation using the Smagorinsky eddy viscosity model. Angles of attack between 5 and 11 degrees are studied at Reynolds numbers ranging from 4.06×10 5 to 2.87×10 6 . The simulation results showed that the tip vortex plays a major role in aerodynamic noise generation. The present large-scale simulation can provide information about the physical phenomena causing tip vortex flow and tip noise at high Reynolds numbers. This information can be used in various engineering applications such as aerospace and wind energy and could help to design blade tips for reduced noise emission.

Three Dimensional Navier-Stokes Flow-Field Computations through Horizontal Axis Wind Turbine Blade

This paper presents the three-dimensional (3D) flow analysis around a horizontal axis wind turbine using pseudo-compressibility method combined with overset grid method. The computational results of 3D Navier-Stokes solution agree with two-dimensional (2D) ones of corresponding relative angle of attack at the middle span. However, it is found that the angle of attack at near the tip region is predicted to be smaller than that derived with 2D simulation which neglects the effect of tip vortex. The data obtained with the proposed algorithm of Computational Fluid Dynamics (CFD) are compared with the Blade Element and Momentum theory (BEM) data for a given angle of attack at each span section. Both sets of data are in good agreement with each other at the middle span position. Both BEM and CFD are able to analyze the down wash effect induced by a tip vortex and other variance of bound vortex in the span wise direction. However, the BEM underestimates the angle of attack at the hub and tip side where 3D flow effects are strong. In other words, the BEM estimates larger induced velocity than CFD.

Flow on Blades of Wells Turbine for Wave Power Generation

Wells turbine has the cascade whose stagger angle is 90°, namely the blades are perpendicular to the axial velocity. Good performance is required from 0° to 90° angle of attack because the turbine is operated in the oscillating airflow produced with wave energy. Furthermore, very interesting and complex flows are experimentally observed by the oil film method for large angles of attack where the performance is strongly influenced, especially, the self-starting. This paper tries to analyze the mechanism of these three-dimensional flows around the turbine with the flow visualization and the numerical analysis, focusing on the off-design condition.

TURBULENT FLOW SIMULATION OF FRANCIS WATER RUNNER WITH PSEUDO-COMPRESSIBILITY

The three-dimensional Navier-Stokes code with the pseudo-compressibility, the implicit formulation of finite difference, and the turbulence model has been developed for the Francis water runner. Two turbulence models are used; one is the so called Baldwin-Lomax zero-equation model and the other is the k — ∈ two-equation model. The viscous flow in the rotating field can be simulated well with these turbulence models, and their results agree with the experimental data and the Euler code in the design flow condition. Because these codes employing the wall function near the wall for the k — ∈ model do not require large CPU time, they can be used on the small computer like the so-called engineering work station.

Field Operation and Track Tests of 1-kW Small Wind Turbine Under High Wind Conditions

A 1-kW small horizontal-axis wind turbine “Airdolphin,” capable of high wind operation up to 50 m/s without pitch control system, is now under global round robin tests. The present paper reports a series of technical approach including design/analysis, track tests, and field tests conducted in support to the design process. One windy site “Erimo” and one offshore site “Fukushima” were chosen. For example, at “Erimo,” a record of one-day generation was 8.831 kWh on November 13, 2006 (day-averaged wind speed; 11.8 m/s) with 36.8% of capacity factor. An operation data under an attack of typhoon with 50 m/s maximum wind speed demonstrated the technical concepts of high wind operation and safety. A new term “capatureability” as an indicator of WT performance was proposed.