Yu Fukunishi

Identification of multi-scale coherent eddy structures in a homogeneous isotropic turbulence

The coherent vortices of various scales are identified from a turbulent flow field obtained by a Direct Numerical Simulation (DNS). In this paper, the vorticity field of the database is filtered before the vortex-identifying method is applied, which enables the identification of large and middle scale coherent vortices from the turbulent flow field. The lattice Boltzmann method with a forcing scheme is used to obtain a statistically stationary three dimensional homogeneous and isotropic turbulence in a periodic box with a resolution of 128³. The vorticity field is filtered using sharp cut-off filters of different cut-off-wavenumbers to obtain different scales of coherent eddy structures. The Q criterion is applied to visualise the coherent vortices and the conditional sampling technique is used to investigate the energy spectrum of the coherent and incoherent parts, separately. By applying the filtering process prior to identification, flow fields of different scale structures are extracted and the energy spectrum of each is investigated.

Formation of intermittent region by coherent motions in the turbulent boundary layer: Coherent motions and intermittent region

Coherent motions in the turbulent boundary layer and their relation to the large-scale structures in the intermittent region are investigated experimentally. Using conditional sampling technique, we have shown that a coherent motion is a pair of counter-rotating fluid motions. These coherent motions of various sizes take place in the turbulent boundary layer. Most of them are short and are found only near the wall, but some are tall enough to reach the outer edge of the boundary layer. The turbulent bulges of the intermittent region are formed by these tall coherent motions.

Flap -edge Flowfield and Noise in Civil Transport Aircraft Model

The results of Wind Tunnel Testing (WTT) using a fully configured aircraft provided by Aviation Program Group / Japan Aerospace Exploration Agency (APG/JAXA) and designed for High-Lift Device (HLD) research are discussed in detail to clarify the very complex flowfield in the flap-tip region. To understand the basic flow features, Fast Fourier Transform (FFT) data as well as the coherence of unsteady pressure fluctuations at the flap tip are discussed. Steady flow data obtained from Computational Fluid Dynamics (CFD) capturing the complicated flap-tip flowfield in detail using an unstructured grid are used to understand the locations of the vortical structures and streamlines. This combination enabled clarifying the mechanism of noise generation in the flap-tip region.

G0701 Promotion of Destabilization of a Boundary Layer Using Interaction Between Streaky Structures and Jet

1 緒言 境界層の乱流遷移は,2 次元的な T-S 波の線形成長から始 まるものと,この過程を経ないバイパス遷移と呼ばれるものに 大別される.一様流乱れが強い場合に見られるバイパス遷移で は,ストリークと呼ばれるスパン方向に交互に並ぶ高速と低速 の筋状の構造が現れる.ストリーク構造自身の不安定性により 速度変動が成長してやがてストリークが崩壊すると,境界層中 に局所的な乱流領域である乱流斑点が生じる. 乱流斑点を 人為的に励起するには,一様流の 2倍程度の速度に達する強い 噴流を壁面から吹いたり ,境界層中でスパークを起こす必要 がある. 自然遷移では,このような強い刺激を必要とせずに 乱流斑点が次々に現れることを考えると,適切な環境のもとで あればもっと弱い刺激で乱流斑点を作り出すことができるはず である.本研究の目的は,乱流斑点の発生環境やその生成過程 を探ることでバイパス遷移への理解を深めることにある.本稿 では,3次元数値シミュレーションにより平板境界層中にスト リーク構造を励起し,低速ストリークに対して噴流を噴射する ことで境界層の不安定化を促進することを試みた.

609 Numerical Study of Active Control of Oblique Waves in a transitional boundary layer

The aim of this paper is to actively control oblique waves in a transitional boundary layer using piezo-ceramic actuators. Each actuator attached on the flat plate surface is thin enough to keep the boundary layer in the laminar state. The actuators introduce velocity fluctuation into the flow field by moving their edge for about several tens of t -meters. The results show that these actuators are capable of generating two-dimensional T-S wave, a pair of oblique waves crossing each other and a one-sided oblique wave. The capability of the actuator to generate a pair of oblique waves upstream and then weakening it downstream was successfully demonstrated using two rows of actuators.