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The effect of periodic perturbation on the reattachment promotion has been experimentally investigated. A turbulent separated flow over a backward-facing step is chosen for a test case where periodic perturbation is applied from its step edge. When perturbed by the most effective frequency on the reattachment promotion, the Reynolds stress is remarkably increased in the reattachment region. This increase in Reynolds stress is realized by indirect effect of applied perturbation. The details of this effect is discussed in this paper., 資料番号: AA0032827013, レポート番号: NAL SP-54

航空宇宙技術研究所 16-18 Jun. 1999 東京 日本, National Aerospace Laboratory 16-18 Jun. 1999 Tokyo Japan, 計算機の急速な進歩とシミュレーション技術の不断の進歩により、CFD(数値流体力学)は製品設計、流れの予測の現場で広く普及している。一方、現在の工学CFDが抱える最も重要な課題は実用的な乱流モデルの開発であるが、この点ではまだ現在提案されている乱流モデルは複雑すぎ、実際にそれを用いることが難しい。乱流モデルを用いた計算には、その手法の根幹に関わる欠陥がないわけではないが、将来的には乱流モデルを用いた計算が工学CFDの現場で主流を占める可能性は大きいであろう。「乱流モデル応用研究会」は、実際に工学CFDにたずさわる若手研究者からなり、(1)各種実用問題に対する乱流の数値計算精度向上のための系統立った手法の確立と実行、および(2)既存乱流モデルの新しい利用法を確立すること、を目的として掲げている。乱流モデル自体の構築は研究会の主眼ではなく、モデルの純粋な利用者が無理なく参加できるようになっている。現在のメンバは24人で、メンバの数は徐々に増え続けている。会合は2ヶ月に1度で、幹事が中心となって設定した流れを研究会のメンバは各自のコードと乱流モデルを用いて計算し、結果を検討する。研究会のホームページでも結果を公開している。研究会の課題として、Karlssonら(1993)による2次元壁面噴流の実験を取り上げた。本稿では、低レイノルズ数型k-εモデル(Myong-Kasagiモデル)を用いて計算した計算結果の一例を報告した。平均速度や自己保存性などについて実験結果と計算結果を比較した。, By rapid progress in computer and steady development of simulation techniques, CFD (Computational Fluid Dynamics) is widely and practically used for product designs and flow predictions. Although one of the most important subjects in present engineering CFD is development of practical turbulence models, turbulence models having been proposed are too complicated for actual applications. Though computations using turbulence models have some problems, there is a possibility that computations using turbulence models are in the main stream of engineering CFD fields in future. The research group for the application of turbulence models consists of young researchers who actually work in the field of engineering CFD. The aims of the group are: (1) establishment and conduction of a systematic method which improves computation accuracy of the turbulent flows in various application problems; and (2) establishment of new applications of the existing turbulence models. The principal aim of the group is not for the construction of a turbulence model, so pure users of models can take part in the group without difficulty. Now there are 24 members in the group, and the members are increasing. A seminar is held once every two months, and the members compute a flow problem given by the host using their own code and model, and have discussions. The results are published on the Web site of the group. As a subject of the group, the two-dimensional wall jet experiment by Karlsson et al. (1993) is given. In this report, one example of the calculated results using low Reynolds number k-epsilon model (Myong-Kasagi model) is presented. The computation results are compared to the result of experiment on the average velocity, self-similarity, and so on., 資料番号: AA0001961044, レポート番号: NAL SP-44

航空宇宙技術研究所 7-9 Oct. 1998 (23th). 24-26 Mar. 1999 (24th) 東京 日本, National Aerospace Laboratory 7-9 Oct. 1998 (23th). 24-26 Mar. 1999 (24th) Tokyo Japan, マイクロコンピュータ用の磁気ディスクドライブの簡略化されたモデルとして、同時回転ディスクの間における流れの体積力不安定性を調べた。数値シミュレーションとともに、熱線による測定も行った。ディスク振動を測定して、弾性振動と流れの不安定性のフィードバック機構がディスクフラッタの発生に果たす役割を明らかにした。, As a simplified model of a magnetic disk drive for a micro-computer, a body force instability of flows between shrouded corotating disks has been investigated. The hot-wire measurements as well as the numerical simulation have been conducted. The measurement of disk vibration revealed the role of the feedback loop of elastic vibration and flow instability for establishing disk flutter., 資料番号: AA0001960005, レポート番号: NAL SP-43

The vortex method is applied to the calculation of a decaying homogeneous isotropic turbulence of Reλ=25,50 and the results are compared with a spectral method calculation. Vortex method calculations were accelerated by the use of a fast multipole method for periodic boundary conditions. The core spreading method and particle strength exchange were selected as the viscous diffusion scheme. The effect of spatial resolution was examined along with Reynolds number dependence and the effect of spatial adaption of elements.

Mechanism of reattachment promotion of periodically forced backward-facing step flow is investigated in view of vortical structures. Based on it, a scaling law for the reattachment promotion is examined and appropriate dimensionless variable for the expression of the most effective frequency is proposed. Using this variable, the most effective frequency of back-step flow is expressed almost universally., 資料番号: AA0045947001, レポート番号: NAL SP-56

27-28 Sept. 1999 (25th). 27-28 Mar. 2000 (26th), マイクロバーストに関する実験室実験の一環として、垂直壁に衝突する渦輪の瞬間速度場を検討した。渦輪により誘起された瞬間速度場を相互相関法を用いたPIV(粒子画像速度測定)を適用して測定した。衝突時の速度ベクトル場の時間発展を図示した。, As a part of the laboratory experiment on a microburst, the instantaneous velocity fields of a vortex ring impinging onto a normal wall have been investigated. The instantaneous velocity fields induced by a vortex ring were measured by employing the PIV (Particle Image Velocimetry) in which the cross correlation method was used. The time evolution of velocity vector fields during the impingement was shown., 資料番号: AA0028636011, レポート番号: NAL SP-47

27-28 Sept. 1999 (25th). 27-28 Mar. 2000 (26th), 周期的摂動乱流はく離流における大規模渦構造を実験的に検討した。再付着促進に対し最も効果的な周波数を持つ摂動はせん断層中への誘導渦数が最も多い。一方、最効果的周波数よりも低いまたは高い周波数を持つ摂動は渦の誘導数が少ない。本論文では、せん断層内の誘導渦構造と運動量輸送の強化との間の関係を詳細に論じた。, The large-scale vortical structure in the periodically perturbed turbulent separated flow is experimentally investigated. The perturbation with the most effective frequency on the reattachment promotion induces the highest number of vortices into the shear layer. On the other hand, the perturbation with the lower or the higher frequency than the most effective frequency introduces fewer vortices. The details of the relationship between the induced vortical structures in the shear layer and the enhancement of the momentum transport are discussed in this paper., 資料番号: AA0028636020, レポート番号: NAL SP-47

航空宇宙技術研究所 27-29 Sept.2000 東京 日本, National Aerospace Laboratory 27-29 Sept.2000 Tokyo Japan, 周期的摂動を受ける剥離乱流中における大規模渦構造の流れ方向発展を実験的に検討した。摂動をバックステップの端から付加する試験事例に対してバックステップ流を選択した。位相平均流れ場はステップ不安定性のため渦を連続的に放出した。再付着の下流では、この渦運動は消滅し、新たな支配的振動数が出現した。この現象の機構を論じた。, The streamwise evolution of large-scale vortical structures in the periodically perturbed turbulent separated flow was experimentally investigated. Backward-facing step flow was chosen for the test case, where the perturbation was applied from its step edge. The phase averaged flow field showed successive shedding of the vortices due to the step instability. Downstream of the reattachment, this vortical motion disappeared and a new dominant frequency emerged. The mechanism of these phenomena is discussed in this paper., 資料番号: AA0028637020, レポート番号: NAL SP-48T

航空宇宙技術研究所 7-9 Oct. 1998 (23th). 24-26 Mar. 1999 (24th) 東京 日本, National Aerospace Laboratory 7-9 Oct. 1998 (23th). 24-26 Mar. 1999 (24th) Tokyo Japan, スパン方向に系が回転している場合の遷移境界層での熱伝達を研究した。試験壁は電気的に加熱し、一定熱流束条件になるようにした。表面温度は感温液晶を用いて測定し、裏面での熱損失を理論的に補償することにより、局所的な熱伝達係数を加熱器に対する入力電力から求めた。熱伝達は、コリオリ力が壁に向って、垂直に作用する場合は高圧面で促進され、負圧面ではまったく変化しなかった。これはコリオリ不安定性により生じた縦渦によるものである。, Heat transfer in a transitional boundary layer subjected to the spanwise system rotation has been investigated. The test wall has been electrically heated to give the constant heat flux condition. The surface temperature has been obtained by employing temperature-sensitive liquid crystal and the local heat transfer coefficient has been evaluated from the input power to the heater, by theoretically compensating the heat loss to the backside. The heat transfer is augmented on the pressure surface, on which the Coriolis force acts normal and toward the surface, while it does not change at all on the suction surface. This is due to the streamwise vortices generated by the Coriolis instability., 資料番号: AA0001960033, レポート番号: NAL SP-43