固気混相乱流における固体粒子のラグランジュ運動に対する乱流SGS成分の影響を検討するために、媒質と分散相の相互作用を考慮したダイナミックランダムウォークSGSモデルを提案した。提案したモデルのメリットはGermanoのダイナミック手順によって乱流SGS成分の正規分布および局所等方性を表わすことができることです。また、提案したSGSモデルを用いてLES解析を行い、同じレイノルズ数のDNS結果と比較し、提案したモデルの効果を検証した。さらに、異なった粒子の計算結果から、乱流のSGS成分は粒子の緩和時間の大きさによって、各々粒子運動の各種統計量へ選択的に複雑な影響を与えることが観察され、粒子の運動解析に対して、たとえ粒子が高くても、LES数値解析における解像されない乱流SGS成分が無視できないことが分かった。

抄録全体を表示

固気混相乱流における固体粒子の密度分布を明らかにするために、固体粒子を混入した垂直下向きチャネル乱流のLES解析を行った.著者らの提案したダイナミックRandom Walk SGSモデルを用いて, 5種類の固体粒子のと空間分布の相関, 乱流のSGS成分の粒子密度への影響, および粒子の衝突による粒子群の形成などについて調べた.その結果, 流れ場のレイノルズ数の変化に関らず, 乱流のコルモゴルフ時間スケールに基づく固体粒子のが1のオーダーである際に, 固体粒子がもっとも極端な濃度むらを形成し, 壁近傍の乱流低速ストリークに固体粒子のクラスター構造ができたことを確認した.また, 乱流渦のSGS成分から固体粒子分布に与える影響は選択的に粒子緩和時間に依存することも示した.

抄録全体を表示

The trajectories of small particles in a bifurcating laminar flow are studied by numerically solving the equations governing the viscous fluid and the particle. The following assumptions are introduced: The flow is incompressible, two-dimensional and steady; the shape of particle is spherical; the interaction between particles is negligible; the effect of gravity is ignored and the interaction between the particle and the fluid is governed by Stokes' drag law. The motion of the particle is governed by the Stokes number, the Reynolds number, the ratio of flow rates and that of the widths of the branches. After a very long calculation, the separating lines, by which the particles in the upstream channel are separated into particles leaving for the straight and for the side branch, are found numerically. The location of the separating line is found to depend strongly on the ratio of flow rates in the two branches, but the effect of the Reynolds number on the separating line is comparatively small. Collisions between the particles and the channel almost arise on the wall in the side branch. The distribution of collision frequency along the wall of the side branch is calculated and found to take the maximum value at a Stokes number.

抄録全体を表示

The motion of particles in a two-dimensional impinging jet is studied numerically. The fluid is assumed to be ideal. The motion of particles is governed by three dimensionless parameters: the Stokes number, the aspect ratio (the ratio of the distance between the nozzle and the wall to the half width of the nozzle) and the initial position of the particles. The critical conditions at which the particles first collide with the wall are obtained in the form of the function of these three parameters. The collision characteristics, namely, the location of collision, the collision frequency, the collision velocity and the force exerted on the wall due to collision, etc., are obtained. When the aspect ratio is larger than about 5, the collision characteristics are identified with those for the case of infinite aspect ratio. When the aspect ratio is smaller than about 1.3, no particle collides with the wall in regions near the stagnation point, and the locations of collision are focused at a point apart from the stagnation point. In the case where the distance between the nozzle and the wall is small, whether the particle collides with the wall or not is determined by the behavior of particles on the streamline, which do not pass through the stagnation point.

抄録全体を表示

背景の流体とは異なる質量をもつ微小粒子（慣性粒子）の２次元および３次元乱流中でのクラスタリングを直接数値計算を用いて調べた。その結果、慣性粒子のクラスタリングは多重尺度をもつ現象であり、（従来まで受け入れられてきた描像とは異なって）渦度場やストレイン場のような小スケールの性質では説明できないことが示された。我々の計算によれば、粒子のクラスタリングは加速度のよどみ点によって説明される。これは加速度場が粒子とともに大きなスケールの渦によって移流されるという事実に起因している。

抄録全体を表示

Direct numerical simulations are performed in plane Poiseuille flows with point-particles for varying density ratio of particle to fluid, or Stokes number. The purpose of our study is to understand subcritical transition in particle-laden flows. We investigated the effect of particle-fluid interactions on the robustness of intermittent localized turbulence, so-called turbulent stripe, in two-way coupling simulations. The stripe was found to disappear due to heavier particles with a density ratio over 4000, while it sustained with light particles, or small Stokes number. The particle-fluid interactions act favorably near the walls, but adversely in the channel central region with respect to the streamwise direction for all cases. The interaction force significantly is exerted in the case of heavier particle, or large Stokes number. In this paper, we also showed the instantaneous momentum transfer of the streamwise Reynolds normal stress. The production term gradually decreases with the increasing Stokes number. Furthermore, we found that the energy exchange due to particle-fluid interactions is from particle to fluid near the wall and is reversed in the channel central region. For larger-Stokes-number particles, the transports occur actively at each height during laminarization process.

抄録全体を表示

We propose that the Stokes-number dependence of inertial-particle clustering in turbulence is explained in terms of the structures extracted from the coarse-grained fluid acceleration, and investigate the spatial correlation between the structures and the clusters of particles. To verify this proposal, we conduct direct numerical simulations of the inertial particles subjected to the linear Stokes drag and suspended in homogeneous isotropic turbulence.

抄録全体を表示

The motion of a small particle in a viscous flow which is driven toward a plane wall by a steady uniform injection at a given distance from the wall is investigated. The particle is assumed to start with the same velocity as the injection velocity of the fluid and to be driven by the interaction force due to Stokes' drag law. The governing equations of the fluid and the particle are found to be simultaneously solvable in the form of similar solutions. That is, they can be reduced to the ordinary differential system. The following results are obtained: the collision between the particle and the wall depends only on the fluid velocity in the direction perpendicular to the wall; the critical Stokes number, above which a particle can collide with the wall, decreases as the Reynolds number increases; the collision can occur more easily in the case of a three-dimensional flow compared with the case of a two-dimensional flow; the critical Stokes number is asymptotically close to that in the case of an ideal fluid with increasing the Reynolds number when the value of the Reynolds number is several thousands, but the critical Stokes number in the case of viscous flow is rather different from that in the case of an ideal fluid, that is, the effect of viscosity is important; the collision in the case of this flow can occur more easily compared with the case of the stagnation-point flow; in the case where the particle collides with the wall, the collision must always occur on the wall at a finite distance from the origin, the frequency of collision increases as the Stokes number increases, and the frequency of collision increases as the Reynolds number increases.

抄録全体を表示

In order to find the optimum atomizing condition for combustion efficiency, a combustion evaluation concept is introduced on the basis of the atomizing characteristics, and the dimensionless parameter Stk (Stokes Number) is used to analyze the trajectories of CWM spray droplets. Based on the these concepts, the Combustion Index CI is defined. The larger the CI values, the better the combustion. It is also found that the atomizing condition at a W_a/W_l of 0.26 is optimum for combustion efficiency. When W_a/W_l>0.26 blow-off occurs due to the increased injecting speed. On the other hand, the burn-out times become much longer because of worse atomization (in the condition of W_a/W_l<0.26). This is a very useful prediction method to evaluate the performance of an atomizer and the combustibility of CWM with various properties. The prediction results above are verified in a small-scale test furnace.

抄録全体を表示

　近年、固気混相流の分野において乱流変調問題や乱流中の粒子運動に対して乱流数値解析法を適用するシミュレーションが盛んになされるようになってきた。特に連続相の解析には非定常、高レイノルズ数の乱流場に対して有効なラージ・エディ・シミュレーション（LES）を、分散相の解析には個々粒子の動きを決定するラグランジュ 追跡法を用いる手法が脚光を浴びている。本報では、著者らの提案したダイナミックRandom Walk SGS モデルを用いて、さらに粒子間の衝突を考慮する決定論的DNS手法を加えてLES解析を行い、粒子と粒子空間分布の相関、乱流のSGS成分の固体粒子密度への影響、および粒子の衝突による粒子群の形成などについて計算結果の可視化により調べた結果を述べる。

抄録全体を表示

We propose that the Stokes-number dependence of inertial-particle clustering in turbulence is explained in terms of the structures extracted from the coarse-grained fluid acceleration. To verify this proposal, we conduct direct numerical simulations of small heavy particles subjected to the linear Stokes drag in homogeneous isotropic turbulence. We investigate the surfaces, in three-dimensional space, satisfying the condition for the fluid acceleration which was introduced by Goto and Vassilicos [“Sweep-stick mechanism of heavy particle clustering in fluid turbulence”, Physical Review Letters, Vol. 100 (2008), 054503] and we apply the condition to the coarse-grained fluid acceleration. Our DNS results show that the surfaces thus extracted from the fluid acceleration coarse-grained at different lengths well correlate with the clusters of particles at different Stokes numbers.

抄録全体を表示

The removal of snowdrifts resulting from snowstorms is often physically intensive work in Antarctica. Therefore, it is important to consider snowdrift around buildings in construction planning. In this study, we predicted snowdrift using a small experimental facility and existing numerical-analysis software that can be easily implemented. First, the snow cover around a cube model resulting from a snowstorm was calculated through numerical analysis. And the validity of numerical analysis was examined. It was shown clearly that the solid-gas two phase flow numerical analysis could respond to the wide range value of the number of Stokes which is a parameter with which snow particles follow a motion of an air current enough. To consider the effect of the form of a proposed Antarctic building on snowdrift, the snowdrift around buildings of an Antarctic base was then predicted by numerical analysis. As a result, it is thus expected that work involved in removing snowdrifts can be reduced through the appropriate design of curved-roof Antarctic buildings employing numerical analysis.

抄録全体を表示

粒子径20–300µmの粒子を対象として，気相における粒子と壁の衝突挙動を実験的および理論的に検討した．顕微高速度カメラによって得られた画像を用いて，衝突前後の速度におよぼす粒子径の依存性を定量的に解析した．平板近傍における流体抵抗の増加（すなわち，潤滑効果）を考慮した衝突速度の数値計算を行い，実験値と比較した結果，両者は良好に一致することがわかった．微粒子では，潤滑効果を含めた流体抵抗が粒子慣性と比べて大きくなることに起因して速度低下が生じることを明らかにした．さらに，衝突速度と粒子密度を変化させた実験によって，反発係数はの減少とともに小さくなることを示した．

抄録全体を表示

A combustion evaluation concept is introduced on the basis of the spray droplet size distributions in order to determine the optimum atomizing condition for combustion efficiency. The Combustion Index CI to predict the combustibilities is then defined using the dimensionless parameter Stk (Stokes number) which analyzes the trajectories of spray droplets. This is a very useful prediction method to evaluate the combustibilities of CWMs. However the application to various liquids appears to be invalid, since the droplet size distributions of typical fuel oils are different from those of CWM. The major objective of this study is to apply this prediction method to"more homogeneous"liquid. The atomizing tests for fuel oil are carried out varying the air/fuel ratio and liquid flow rate. Based on the results, the CI developed for CWM is then modified to predict the optimum atomizing condition for fuel oil. The predicted results are verified by heavy oil combustion tests.

抄録全体を表示

飛来塩分量・腐食センサを含む構造物表面での海塩粒子の沈着量分布推定のため，センサを模した典型的な物体である鉛直平板周りの粒子接近風に対してURANSによる数値シミュレーションを実施した．平板への衝突率について，風向の影響は小さいが，粒子径とともに急増した．小さな粒子の場合，前部でのはく離により円柱に対するものより大きくなる．衝突効率の予測に，接近風速と構造物寸法に基づく

が有効である．

抄録全体を表示

A descending rate of falling particles through a quiescent or a turbulent airflow, in which the turbulent level is kept constant by rotating grids as a turbulent generator, has been investigated considering particle volume fraction and Stokes number as experimental conditions. In the experiment, turbulent flow properties and downward flow induced by particles are measured by means of LDV. Particles velocity is measured by means of 2 D-PTV with laser sheet and CMOS camera. As a consequence, in the quiescent flow a descending rate is reduced for the particles with diameter 0.5mm and volume fraction of 10^<-4>, and further reduction is expected by increasing volume fraction. While, the effect of volume fraction range is expected to be different depending on the diameter. On the other hand, in the turbulent flow (St〜10^3), the descending rate is increased but its mechanism is not explained by existing studies. In the case of solid-air two phase flow, the motion of particles are governed by drag and gravity so that a discrepancy of descending rate between present experiment and prediction by existing equation indicates a change of drag under present experimental conditions.

抄録全体を表示

In order to study the effect of turbulence sub-grid scale (SGS) component on particle motion, a new model named Dynamic Random Walk (DRW) SGS Coupling Model based on Eulerian-Lagrangian approach was presented. The advantage of the new model is that the Gaussian statistical distribution and local isotropic properties of turbulence SGS fluctuation magnitude could be parameterized by Germano's (1991) dynamic procedure. Using the present model, Large Eddy Simulation was performed for downward channel flow at Reynolds numbers of 180 that was identical to the DNS done by Rouson & Eaton in 1997. Through comparing of statistical properties of particles diffusion with DNS, the capability and limitation of presented DRW model was verified. Moreover, it was found that turbulence SGS component was strongly associated with particles motion because preferred particles were affected by preferred length scale of the eddy structure around, and fluid SGS component is indispensable in calculating particles motion with LES even though the particle Stokes number is high.

抄録全体を表示