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Localized heavy rainfall which resulting from isolated cumulonimbus often brings severe human and economic damages. Nakakita et al. developed a prediction system for torrential rainfall using vertical vorticity in a first echo using X-band Polarimetric Doppler radar (X-band MP radar) and Katayama et al. implemented this system. Besides, research on vertical vortex tube structure has been conducted. Nakakita et al. showed that with the development of the cumulonimbus cloud, a vertical vorticity structure with positive and negative pairs is generated. In this study, we analyzed the cumulous cloud with updraft, vertical vorticity, and hydrometeor. As a result, we succeeded in observing the behavior of the vertical vortex tubes, updraft, and hydrometeor.

In this research, potential temperature gradient based vorticity which is the expression of baroclinic is obtained to analyze the energy structure of the vorticity field in developing clouds. First, applying the variational method enables us to obtain a diagnostic equation in which the equation of motion, conservation law of mass, and entropy are considered as constraints. Second, Fourier analysis was performed on the vorticity field in the cross-section of the convective core in the isolated cumulonimbus simulation. The temporal change of the spectrum of vorticity field indicates that the rotational intensity of potential temperature gradient based vorticity increases as the degree of baroclinicity increases. Third, we conducted the data assimilation experiment assuming the use of vertical vorticity estimated by doppler radar observation. The change in the potential temperature and vertical wind through the error covariance matrix generates coherent convection in the computations.

Suddenly-generated and rapidly-developed isolated cumulonimbus often cause flash flood and inundation disasters. For these disasters prediction, it is significant to analyze flow structure in a cumulonimbus cloud before raindrop generation. To investigate the initiation process, we conducted observation in Kobe City, on August 2018 using a Ka-band Doppler radar, X-band radars and some sensors. Developing cumulonimbus clouds and non-precipitating cumulus were observed by Ka-band radar, and the former has high Radar reflectivity (Zh) in the upper part. This implies that strong updraft lifted up the cloud particles and particle size grew up in the developing cell. In addition, we evaluated the effect of spatial resolution and direction on the radar observation by analyzing the vortex structure simulated by a Large Eddy Simulation model. The simulation result show that the vorticity can be observed irrespective of the direction if a cell has a circulation in a spatial scale larger than observation.

Localized heavy rainfall which resulting from isolated cumulonimbus often brings severe human and economic damages. Nakakita et al.(2015) developed a prediction system for torrential rainfall using vertical vorticity in a first echo using by X-band Polarimetric Doppler radar. Besides, research on vortex tube structure in guerrilla heavy rainfall has been conducted. Nakakita et al. (2016) showed that with the development of the cumulonimbus cloud, a vertical vorticity structure with positive and negative pairs. Nakakita et al(2017) showed that there is also vertical vorticity structure with positive and negative pairs is generated in early developing stage. In this study, we analyzed the cumulous cloud with higher spational temporal resolution by using Phased Array Radar (PAR). As a result, we succeeded in observing the detail behavior of the vortex tube which cannot be captured by X-band MP radar.

In Japan, the frequency of Guerrilla-heavy rainfall is increasing, so it is quite important to predict heavy rainfall even within a few minutes. Nakakita et al. (2016) discovered the large vertical vorticity inside the baby cell aloft is useful to predict the Guerrilla-heavy rainfall, and also discovered the baby cells have vertical vortex tubes. By these results, we considered that the horizontal vortex tubes were caused by vertical shear of horizontal wind and these were tilted into vertical by an updraft. This hypothesis is popular in large scaled clouds, but it is not unclear in small scaled clouds. This paper shows two findings. One is that the strongest updraft exists between positive and negative vorticity using high ZDR column and multi Doppler technique. The other is that Guerrilla-heavy rainfall is caused by not so strong vertical shear as liner convective system. These unlock the developing process of small scaled cumulonimbus cloud.

Nowadays, the frequency of tragic disasters caused by localized torrential rainfall is increasing. This tragic disaster was brought by suddenly generated and rapidly developed isolated cumulonimbus cloud. For earlier risk detection of torrential rain fall, some researchers use Ka-band cloud radar and they revealed it can detect cells 20minutes before X-band radar can. In this study we analyze the cumulonimbus cloud in earlier stage in different aspects and it was found the relationship between Ka-band radar first echo height and cloud particle development by comparison Lifting Condensation Level with first echo height. Secondly, we found the growing speed was the highest in time of X-band radar first echo appeared. Finally, we reveals Ka-band radar can detect the vorticity in earlier stage than it had observed by X-band radar. These result shows usefulness of Ka-band radar for applying Early detection system and risk judge system.

In Japan, the frequency of Guerrilla-heavy rainfall is increasing, so it is quite important to predict the risk of heavy rainfall even within a few minutes. Nakakita et al. discovered the large vertical vorticity inside the baby cell aloft is useful to predict the Guerrilla-heavy rainfall, and also discovered the baby cells have vortex tubes which are similar to ones inside the initial cumulonimbus clouds of supercells. From this results, we considered that the vortex tubes inside the baby cells support fluid dynamics. This paper shows that the strongest updraft exists between positive vorticity and negative vorticity inside one sample cell using high ZDR column.

Today, frequency of Guerilla-heavy rainfall are increased in Japan. Nakakita et al. (2014) developed the risk prediction system with the vertical vorticity information in a convective cell at its first stage (hereafter, baby-cell). For more accurate system, we tried to understand how a cell develops to heavy rainfall by analyzing the structure of vortex tubes inside baby-cells. Vortex tubes are observed inside all sample cells with X-MP Doppler radar in Kansai area. For more detailed analysis, we are challenging to analyze vortex tubes with phased array radar in Osaka University.

近年のゲリラ豪雨災害事例を例に挙げ，1分1秒でも早くゲリラ豪雨を探知・予測する超短時間降雨予測を実現するために，中北ら(2011)によるゲリラ豪雨の早期探知に関する研究を踏まえ，探知したゲリラ豪雨のタマゴが発達するか否かの「危険性予知」を本研究の目的と設定した．国土交通省XバンドMPレーダで観測されるドップラー風速から二つのスケールの渦度，メソγ渦およびミクロ渦に関して，豪雨との関連性を調べた．その結果，メソγ渦が形成されている時は地上では豪雨となることがわかった．またミクロ渦に関しては，発達する全事例の半数以上の事例においてタマゴ時点での存在が確認できた．発達しない事例においてもミクロ渦の存在が確認されたことから，タマゴ時点では発達するかどうかの判断は難しいものの，タマゴの5分後の渦度分布をみることでタマゴの危険性予知に資する可能性が十分に明らかになった．, Localized torrential rainfall caused heavy rainfall disasters in Summer and Baiu season of Japan called as "Guerrilla-heavy-rainfall" in Japanese media. The rainfall was produced by an isolated cumulonimbus that grew rapidly. To prevent these disasters, Japanese government recently installed networks of X-band polarimeric Doppler radars over major urban areas in Japan. X-band radar is suitable to realize earlier and more detailed detection of the baby cell, because X-band radar has higher sensitivity and spatial resolution. This research investigates the growth of the baby rain cell to become large cumulative clouds by focusing on its vorticity using Doppler velocity. In this research, we define two vortexes with different size, and evaluate a relationship qualitatively between rainfall and the existence of vortex.

The research works of the present author are reviewed. The beginning of turbulence research is about compressible fluid and then extended to transition and drag reduction by making use of higher order accurate difference method and LES (Large Eddy Simulation). The vorticity-stream function method, which is used in theoretical research, as well as vorticity-vector potential method, is extended in order to study flows with engineering importance which are associated with multi-element airfoil, journal bearing, lobe pump, natural thermal convection, viscous micropump and micropump on silicon wafer., 資料番号: AA0063742001, レポート番号: JAXA-SP-07-016

The effect of small perturbations on steady nonlinear transonic small disturbance flow-fields, in the context of two-dimensional flows governed by the general-frequency transonic small disturbance equation with nonreflecting far-field boundary conditions, is investigated. This paper presents a time-linearized time-domain solution method that includes effects due to the shock-generated entropy and vorticity and shock wave motions. The solution procedure correctly accounts for the small-amplitude shock wave motion due to small unsteady changes in the aerofoil boundary conditions, and correctly models a flow-field with embedded strong shock waves. Steady and first harmonic pressure distributions for the NACA 0003 aerofoil with a harmonically oscillating flap, and NACA 0012 aerofoil undergoing a sinusoidal pitching oscillation, are predicted and compared with the Euler results., 資料番号: AA0063824001, レポート番号: JAXA-SP-07-008E

Development of vorticity family and the role of its family members to the flow phenomena and structures are discussed., 資料番号: AA0063156017, レポート番号: JAXA-SP-06-013

In this paper, the gamma-Re(sub theta) transition model proposed by Menter et al. is investigated. Correlations not described in the original paper are reproduced based on other literature and validated by several test cases: T3A- (Tu = 1 percent), T3A (3 percent), T3B (6 percent) for flat-plate boundary layers with bypass transition, and Schubauer & Klebanof (0.2 percent) for flatplate boundary layer with natural transition. T3LC test case is used for separated flow transition which is important for RANS simulation applied to separated flow problems. Transition position on flatplate for each free-stream turbulent intensity has good agreement with experiment, although transition lengths estimated for low free-stream turbulent intensity are too short. For T3LC test case, the length of laminar separation bubble is correctly predicted by using the present transition model., 資料番号: AA0063156010, レポート番号: JAXA-SP-06-013

Two examples of flows dominated by vortical structures are discussed: In the first interaction and decay of vortex structures in in-cylinder flows of automotive engines are described. Numerical studies revealed clearly identifiable vortex rings, generated during the intake stroke. The influence of compressibility on the vortex formation was studied by using Mach-Zehnder interferometry in a specially designed test stand of a towed one-cylinder engine, and with numerical solutions of the Navier-Stokes equations. In an industrial application a stable vortex ring could be generated by direct high-pressure liquid fuel injection, issuing a conical flow of evaporating droplets and stratifying the evaporated fuel to an ignitable air-fuel mixture. This concept led to a substantial reduction of fuel consumption. In the second example the state of the art of investigations aimed at finding means to destabilize tip vortices of aircraft wakes is briefly reviewed. Enforcing a more rapid decay compared to the natural one and reducing the downwash of large aircraft during take-off and landing are presently pursued in several studies. Discussed are the influence of the spanwise position of the engines, of winglets, of preset and oscillating ailerons on the vorticity distribution in the tip vortices and on their destabilization. The results are preliminary in nature. Further investigations in more detail are needed to substantiate previous findings., 資料番号: AA0063154001, レポート番号: JAXA-SP-06-010

In order to accurately solve the unsteady fluid motion by large eddy simulation, we have to use a grid-resolution as uniform as possible for the whole flow field. It is often the case that the grid-resolution is deteriorated far from the wall when the structured grid system such as an O-type is employed. There have been many researches on the Adaptive Mesh Refinement (AMR) since 1980s. Note, however, that we should take advantage of the knowledge acquired in the structured grids in the AMR code. Then, we employ the Blocked Adaptive Mesh Refinement (BAMR) to remain the grid-resolution to be preferably unchanged in the computational domain. In this study, we develop an algorithm to generate the blocks with the finer mesh automatically, and apply the BAMR code to the viscous flow analyses around a two-dimensional cylinder, the NACA0012 airfoil configuration, and a three-dimensional cylinder. These test cases show feasible resolutions with the limited CPU resources., 資料番号: AA0063154028, レポート番号: JAXA-SP-06-010

This paper introduces a computational technique to compensate for the added numerical diffusion that is generated when uniform Cartesian coordinates are used to describe the flow around bluff bodies. Because of the staircase-like representation of the surface object, it was found that the added surface 'roughness' causes larger than expected separation region for some test cases (flow around a sphere, and flow around a cylindrical body). In order to control the velocity profile in the boundary layer, a new parameter blvr (boundary layer velocity ratio) is define, and it is used to set the negative value of the viscosity along the surface. Extensive visualizations of flow past bluff bodies are performed using the present technique. Numerical solutions of the governing Navier-Stokes equations are carried out in a uniform Cartesian coordinates using a multi-directional finite difference scheme with a third-order upwinding. No explicit turbulence model is incorporated into the model, and the dependence of the solution on the blvr parameter is investigated., 資料番号: AA0063154006, レポート番号: JAXA-SP-06-010

An advanced Large Eddy Simulation (LES) technique with high accuracy and resolution is proposed. This method solves compressible 3-D Navier-Stokes equations with the Approximated Deconvolution Method approach as LES approach and has several sophisticated techniques such as highly tuned compact schemes, the generalized characteristic interface conditions, and the generalized Navier-Stokes boundary conditions. Several applications to flows around 2-D NACA0012 aerofoil are presented and discussed. Numerical results show that this method can solve very complicated subsonic flow including the laminar flow separation at the leading edge region, the transition in the bubble and the reattachment. It also has capability to capture the natural laminar to turbulent transition phenomena in the order of Reynolds number of 10(exp 6). In order to validate the present LES technique in more detail, the results are compared with existing wind-tunnel experiments and linear stability analyses with the e(sup N) method and discussed., 資料番号: AA0063154022, レポート番号: JAXA-SP-06-010

Rocket Based Combined-Cycle (RBCC) engines are currently being explored as advanced propulsion for space transportation. JAXA has been conducting RBCC engine research by using various experimental facilities. In order to clarify the experimental results and contribute to the improvement of designing, the analysis of the RBCC engine in an ejector-jet mode was carried out using the CFD (Computerized Fluid Dynamics) code developed in-house for unstructured grids. CFD replicated the basic flow structures in regard with barrel shocks and pseudo-shock waves. Two cases of Mach numbers (i.e., M = 1, 1.1) entering to the combustor were simulated and remarkable differences were found on the subsonic areas in the flowfields. The effect of Mach number was found to be one of the important parameters for better match to the experimental data. Vorticity and mixing were also investigated. In addition to the transverse vorticity, the streamwise vorticity were generated in the air flow, and they stirred and affected the nitrogen/air mixing., 資料番号: AA0049500027, レポート番号: JAXA-SP-06-006

This study presents numerical simulation results of combustion dynamics in a lean premixed combustor. The combustor is a swirl-stabilized gas turbine model combustor installed at Aerospace Research Center (ARC), Japan Aerospace Exploration Agency (JAXA). The methane/air lean premixed gas is used at the preheated temperature of 700 K, pressure 1 atm and equivalence ratio 0.5. Without control, large pressure oscillations are observed in the combustor mostly with longitudinal acoustic modes. The basic mode is quarter-wave mode. The pressure waves induce periodical vortex shedding from the combustor dump plane, thus changing, the flame shape and location. This leads to temporal and spatial fluctuations in heat release rate, and the Rayleigh index, the correlation between pressure and heat release fluctuations, indicates that these fluctuations are coupled to sustain combustion oscillations. Secondary fuel injection control is introduced to charge the coupling between these factors. This numerical simulation succeeds in reproducing reduced combustion oscillations with constant injection of 3 percent fuel through the swirler hub injection ports. The change in flame shape is reproduced and the reduced amplitude can be attributed to the change in heat release distribution as the Rayleigh index implies. Thus, it is expected that the amplitude may be reduced further when injection phase control is additionally introduced with a feedback control system, and this is our future research subject., 資料番号: AA0049212011, レポート番号: JAXA-SP-05-017

Three dimensional vortex structures in the wake region behind two dimensional cylinder in the laminar flow are analyzed. In the wake-transition regime of Reynolds number between about 190 and 260, experiments show discontinuous changes of Strouhal number and the existence of two modes; Mode A and Mode B whose span wise characteristic wave length are about 4 D (D: diameter of cylinder) and D respectively. The compressible Navier-Stokes equations are solved with the finite difference method. 4th order penta-diagonal compact scheme with 4th order Runge-Kutta time integration scheme are used for high accuracy and resolution. NSCB which is generalized into the body fitted grid system is applied to the boundary condition. Numerical results show good agreements with experimental data of Strouhal number for both Mode A and B in the wake-transition regime. Span wise characteristic wave lengths for both modes are captured clearly. These results show the validity of effectiveness of highly accurate and resolution methods used here for the analysis of complex flow structures., 資料番号: AA0049212024, レポート番号: JAXA-SP-05-017

In this study, a flow around a blunt-tipped wing is solved using Reynolds Averaged Navier-Stokes (RANS) equations, and Large Eddy Simulation (LES). The main objective is to investigate the dependencies of mean flow velocity and angle-of-attack. A rectangular wing with NACA0012 airfoil section is chosen as a model. The computed results are compared with that of wind tunnel experiments, which were also performed at JAXA for the purpose of CFD validation. The present unsteady computation using LES indicates that angle-of-attack has a great influence on the vortical structure around the wingtip. As the angle-of-attack increases, the merging location of two vortices, one at the side edge and another on the upper surface, moves to the upstream side. Consequently, the large pressure fluctuating region appears on the upper surface., 資料番号: AA0049212008, レポート番号: JAXA-SP-05-017

Helicopters are used in various fields such as EMS (Emergency Medical Service), fire fighting, disaster relief, news report, and so on because of the capabilities of hovering and VTOL. However, noise, cost, and VFR (Vision Flight Rules) problems prevent helicopters from being widely used as a means of inter-city transportation in densely populated area. Prediction of the flow field of helicopter rotors in forward night is regarded as one of the most challenging problems in current computational fluid dynamics. Addition of the fuselage underneath the rotor further amplifies the complexity, requiring the solution of unsteady three-dimensional flows involving multiple bodies in relative motion. The periodic airloads, which is principally due to the rotating main rotor blades, impact the undesirable noise and vibration on the fuselage in the way of a mutual aerodynamic interference. Therefore, elaborate aerodynamic analyses of the rotor-fuselage interaction are needed to understand the physics and as an aid to the designers. In the preset study, three dimensional parallel Euler flow solvers have been developed for the simulation of unsteady rotor-fuselage interaction aerodynamics on both structured and unstructured grids using the ROBIN (ROtor Body INteraction) configuration. Validations with experimental data are conducted by simulating the flows around the ROBIN fuselage. The merit/demerit of each grid system is discussed., 資料番号: AA0049212004, レポート番号: JAXA-SP-05-017

Based on the idea that turbulence is the flow phenomena caused by the movement of vortex, the structures of turbulent boundary layer are discussed. The circulation is the most important factor for characterizing the flow structures. The large scale motions such as coherent structure of the outer layer and the burst structure of the inner layer can be explained by the interactions between vortices., 資料番号: AA0047776002, レポート番号: JAXA-SP-04-002

Reflection and refraction of linear disturbance waves by an oblique shock wave is studied by a linear analysis. Several different cases are considered, when the incident plane wave is a fast acoustic, slow acoustic, entropy, or vorticity wave. Results show that 1) a critical angle of the wave incidence exists, beyond which the regular wave solution of the linear problem can not be realized, 2) strongest sound waves are generated behind the shock wave, if the incident wave strikes the shock wave at the critical angle, and 3) at large Mach numbers of the upstream flow, the sound amplification factor asymptotically increases as the Mach number squared, while at the critical incidence, it is increased as the Mach number cubed., 資料番号: AA0047427005, レポート番号: JAXA-SP-03-002

The aerodynamic sound generated from a protuberance has been simulated in this study. The protuberance, which is placed on a flat plate, has three kinds of shape at the leading edge. The propagation of sound waves is examined by solving the three-dimensional N-S equations to observe small pressure fluctuations both in the near and far fields. Numerical results show that vortices created from near the leading edge may make pressure waves, leading to acoustic waves. Moreover, FFT analysis shows that the frequency profile partly agrees with experimental data., 資料番号: AA0047427004, レポート番号: JAXA-SP-03-002

航空宇宙技術研究所 24-26 Jun. 1998 東京 日本, National Aerospace Laboratory 24-26 Jun. 1998 Tokyo Japan, 翼先端漏れ渦の崩壊を、中程度の翼負荷を持つ低速の軸流圧縮機翼について調べた。渦崩壊の動翼空力に及ぼす影響をナビエ・ストークススキームによる流れのシミュレーション、および流れの可視化手法を崩壊の同定に用いて検討した。シミュレーションの結果によると、漏れ渦の崩壊は最高圧上昇作動条件よりも低い流量において、回転翼の内部で発生することが示される。崩壊は、よどみ点の存在およびそれに続く気泡様の循環流領域によって特徴づけられる。崩壊の開始に伴って、翼先端漏れ渦に顕著な変化が現れる。すなわち、渦のおおきな拡張および、渦の内部に集中している流れ方向の渦度の消失である。拡張は、先端エッジの上流にまで達するきわめて大きなブロッケージ効果を持つ。集中していた渦度の消滅は、回転翼下流における渦の巻き上がりの消失とケーシングへの圧力の消失をもたらす。翼下流の漏れの流れ場は、崩壊領域の気泡様構造の収縮によってもたらされる外向きの軸流によって支配されている。漏れ渦の崩壊は失速点近傍における翼の作動特性に主要な役割を果たすことが分かる。流量が最高圧上昇点作業条件から減少すると、崩壊域は流れ方向、スパン方向およびピッチ方向に急激に成長する。崩壊の成長はブロッケージをもたらし、急激な損失の増大を招く。これにつれて、崩壊域と翼負圧面の相互作用により、負圧面境界層の3次元的な分離が起こり、失速する。この3次元的な失速は、2次元的な翼の失速と本質的に異なっている。, The breakdown of tip leakage vortex has been investigated on a low speed axial compressor rotor with moderate blade loading. Effects of the breakdown on the rotor aerodynamics are elucidated by Navier-Stokes flow simulations and visualization techniques for identifying the breakdown. The simulations show that the leakage vortex breakdown occurs inside the rotor at a lower flow rate than the peak pressure rise operating condition. The breakdown is characterized by the existence of the stagnation point followed by a bubble like recirculation region. The onset of breakdown causes significant changes in the nature of the tip leakage vortex: large expansion of the vortex and disappearance of the streamwise vorticity concentrated in the vortex. The expansion has an extremely large blockage effect extending to the upstream of the leading edge. The disappearance of the concentrated vorticity results in no rolling up of the vortex downstream of the rotor and the disappearance of the pressure through on the casing. The leakage flow field downstream of the rotor is dominated by the outward radial flow resulting from the contraction of the bubble-like structure of the breakdown region. It is found that the leakage vortex breakdown plays a major role in characteristic of rotor performance at near stall conditions. As the flow rate is decreased from the peak pressure rise operating condition, the breakdown region grows rapidly in the streamwise, spanwise and pitchwise directions. The growth of the breakdown causes the blockage and the loss to increase drastically. Then, the interaction of the breakdown region with the blade suction surface gives rise to the three dimensional separation of the suction surface boundary layer, thus leading to a stall. This three dimensional stall is substantially different from a two dimensional blade stall., 資料番号: AA0001958026, レポート番号: NAL SP-41

航空宇宙技術研究所 24-26 Jun. 1998 東京 日本, National Aerospace Laboratory 24-26 Jun. 1998 Tokyo Japan, 非有限非圧縮性流に対する4つの開境界条件をLeith型3次風上法(QUICKEST法)の枠組みの中で評価し、それぞれの有効性を、開境界条件による流れの違いと、短距離あるいは長距離にある開境界による流れの違いとの2つの手段によって比較検討した。開境界条件の評価には、流路中に後方に向かう階段がある流れ、太く短い形の物体がある流れ、および矩形柱の障害物がある流れの3種類を検討対象として用いた。開境界条件に関する4つの候補を検討し、均一な入口速度をSommerfeld放射条件における位相速度とすることを提案した。結論として、4つの開境界条件の中でこれが最も優れていることを示した。, Four open boundary conditions for incompressible unbounded flows are evaluated in the framework of the Leith type third order upwind scheme (QUICKEST scheme), and each effectiveness is compared by two means of difference of flows among open boundary conditions and between short and long open boundaries. Three test problems used for the open boundary condition evaluation are the backward facing step flow, the blunt based body flow and the rectangular cylinder obstacle flow in a channel. The investigated open boundary conditions are four. The author proposes to take the uniform inlet velocity as the phase speed in the Sommerfeld radiation condition. As the conclusion, this is shown to be the most excellent open boundary condition among the four open boundary conditions., 資料番号: AA0001958045, レポート番号: NAL SP-41

航空宇宙技術研究所 7-9 Jun. 1995 東京 日本, National Aerospace Laboratory 7-9 Jun. 1995 Tokyo Japan, 数値風洞(NWT)を用いて減衰等方性乱流の直接数値シミュレーション(DNS)を行った。完全に発達した乱流と受動スカラー場、温度が、最高512{3}の空間格子点数を有するDNSにより得られた。流れ解析の結果、乱流渦度の強い部分が渦管領域に集中するが、温度勾配はシート状空間領域を示すことが分かった。渦管領域に密接するほとんどの渦度ベクトルは、歪み速度テンソルの中間固有値に相当する固有ベクトルの方向に1列に並ぶ。これとは反対にシート状領域に密着する温度勾配ベクトルのほとんどは、歪み速度テンソルの負の固有値に相当する固有ベクトルの方向を向く。以上の結果、渦管領域ではBurgerの渦と類似の特性を有すること、一方、温度勾配のシート状領域の形成メカニズムは未解明であることを示した。, Direct Numerical Simulation (DNS) of decaying isotropic turbulence was made using the Numerical Wind Tunnel (NWT). A fully developed turbulent flow and a passive scalar field, temperature, were obtained by the DNS with up to 512(exp 3) mesh points in space. From the analysis of the flow, it is obtained that intense portions of the turbulent vorticity concentrate into worm-like regions, but those of the temperature gradient into sheet-like regions in space. Most vorticity vectors nearby the worm-like regions align into the direction of the eigen vectors corresponding to the intermediate eigenvalue of the strain rate tensor. In contrast with this, most temperature gradient vectors nearby the sheet-like regions align into the direction of the eigen vectors corresponding to the negative eigenvalue of the strain rate tensor. These results show that the worm-like regions have a similar property to Burger's Vortex. On the other hand, the mechanism forming the sheet-like regions of temperature gradient is not yet clear., 資料番号: AA0000294026, レポート番号: NAL SP-30

航空宇宙技術研究所 Sep. 1994 (15th). Mar. 1995 (16th) 東京 日本, National Aerospace Laboratory Sep. 1994 (15th). Mar. 1995 (16th) Tokyo Japan, 水素気泡法による渦構造の理解への定量的情報を与える技法を提案した。この技法を用いて、円柱の後流近傍の渦領域での速度のみでなく、速度勾配テンソルも得た。この技法に基づいて得た値より、滞留点を通過する断面流線のパターンや、それの渦の中心近傍領域への拡大や渦度の等高線などによって、渦の全体構造を示した。それにより、剪断応力の局所値や渦領域での発散、さらには、全般的な渦の特性値、例えば循環、エンストロフィー、運動量、角運動量など、を決定した。それらの結果は、柱体の背後で形成される渦が、柱体の後流近くの下流で対流を起す時、シート状の渦から棒状の渦への渦構造の変化が起こることを示唆した。, A technique for quantitative information was proposed to understand the vortex structure by means of hydrogen bubble techniques. Using the technique, not only the velocity but the velocity gradient tensor was obtained in the vortex region in the near wake of a circular cylinder. Based on the values obtained in this technique, the patterns of sectional streamlines passing through stagnation points and extending to the region near the center of vortices and the contour of vorticity were illustrated to see the overall structure of the vortices. Thus, the local values of shear stress and divergence were determined in the vortex region as well as the global characteristic values of vortices, e. g. circulation, enstrophy, momentum, angular momentum, etc. These results suggest that the vortices change in structure from sheet-like vortices to rod-like vortices when the vortices formed behind the cylinder were convected downstream in the near wake of the cylinder., 資料番号: AA0000109017, レポート番号: NAL SP-28

航空宇宙数値シミュレーション技術シンポジウム 宇宙航空研究開発機構 20050623-20050624 東京 日本, Aerospace Numerical Simulation Symposium Japan Aerospace Exploration Agency 20050623-20050624 Tokyo Japan, 資料番号: AA0049212000, レポート番号: JAXA-SP-05-017

Extraction and traction of hierarchical vortical structures in a homogeneous isotropic turbulence are attempted. Fourier and Wavelet decompositions are applied to extract three scales, namely the large, intermediate and fine scaled structures from the flow field. Both filtering methods captured almost the same structures. An automatic tracking scheme that could handle vortex division is developed and presented., 資料番号: AA0063608004, レポート番号: JAXA-SP-07-009

The purpose of the present investigation is to reveal the effect of buoyancy on vertical movement of a laminar vortex ring whose density is smaller than ambient. Its downward motion may be obstructed by buoyancy and travel distance may be reduced. The numerical simulation based on the finite volume method is performed assuming axisymmetry and incompressibility. Vertical movement is significantly obstructed by small density difference even below 0.1 percent. The ring is decelerated, its diameter is reduced and the circulation decreases with higher rate than uniform density. Due to the baroclinic generation of negative vorticity, the outer part of a ring is gradually peeled away and finally stops to descend. The buoyancy effect decreases relatively with increasing initial circulation., 資料番号: AA0063156008, レポート番号: JAXA-SP-06-013

The structures of the scalar dissipation field are studied by performing direct numerical simulations of a turbulent channel flow with passive scalar transport. The Reynolds number is set to be 180, 395 and 640 based on the friction velocity and the channel half-width. The molecular Prandtl number is 0.71. Near the wall, the scalar dissipation field is associated primarily with anisotropic structures, which reflect the presence of quasi-streamwise vortices, whereas away from the wall, it is mainly sheet-like in form., 資料番号: AA0063154025, レポート番号: JAXA-SP-06-010

Means to explain the flow structure generated by thin piezo-ceramic actuators are discussed. In the previous experiments, it was shown that the oblique waves, observed in an initial stage of the transition, could be cancelled by generating intercepting-waves using thin piezo-ceramic actuators. The initial aim of this study is to explain the generation process of these intercepting-waves by modeling the flow structure using a large number of the vortex tubes. However, it is found that this approach is not useful in the near wall region, because in these areas the diffusion of vorticity pattern on the wall is dominant compared with the convection., 資料番号: AA0047776003, レポート番号: JAXA-SP-04-002