Air Entraiment 模型

在一些自由面流中，由紊流或流動(dòng)條件改變（例如，沖擊射流）造成空氣卷入自由水面。摻氣模型在模擬水流通過(guò)溢洪道，bottle filling（不知道該如何翻譯，應(yīng)該跟鑄模有關(guān)）以及金屬鑄模的缺陷預(yù)測(cè)等方面從有著廣泛的用途。

In some free-surface flows, gas may be entrained at the free surface due to turbulence or the flow conditions (e.g., an impinging jet). This has a wide variety of applications, from modeling the flow down a spillway, to bottle filling, and defect prediction in metal casting.

在 FLOW3D 中，流體摻氣可通過(guò)兩相流模型或者使用了 Air entraiment 模型的單向流來(lái)模擬。Air entrainment 通過(guò) Model Setup > Physics 激活，通過(guò)平衡穩(wěn)定力（重力和表面張力）和擾動(dòng)力（紊動(dòng)力）來(lái)預(yù)估流體中的摻氣量（空泡區(qū)域）。Air entraiment 模型不僅包括了自由表面擾動(dòng)產(chǎn)生的摻氣，還包括了層流射入自由表面造成的摻氣。因此，此模型在層流或者紊流中均可使用。當(dāng)模擬紊流時(shí)，推薦使用 k-ε 雙方程，RNG 或者 k - ω 雙方程模型。可通過(guò) Physics > Viscosity and Turbulence 激活，詳細(xì)信息將在 Turbulence（紊流）章節(jié)中討論。

In FLOW-3D, the entrainment of gas can be modeled using the two-fluid model or, in one-fluid cases, by using the Air entrainment model. The Air entrainment model is activated on the Model Setup–>Physics tab and estimates the rate at which gas (represented by the void regions) is entrained into the flow using a balance of stabilizing forces (gravity and surface tension) and destabilizing forces (turbulence). Although the primary focus of the model is to compute the air entrainment resulting from turbulent disturbances at a free surface, the model also contains logic for handling entrainment resulting from laminar jets impinging on a free surface. Therefore, the model may be used for both laminar and turbulent cases. When modeling turbulent flows, the Two-equation , RNG or Two-equation turbulence models are recommended. These models can be activated in Physics →Viscosity and Turbulence and are discussed in more detail in Turbulence.

同樣可以設(shè)置流體的 Surface tension coefficient（表面張力系數(shù)），包含以表面張力作為平衡力來(lái)定義摻氣量的方式。這種情況下無(wú)需激活 Surface tension 模型。

When using the Air entrainment model, the Entrainment rate coefficient must be defined to be a positive value; the default value of 0.5 is a suitable starting point for most cases. The Surface tension coefficient for fluid can also be defined to include surface tension in the balance of forces defining the amount of entrained air. It does not activate the surface tension model.

基本模式下，摻氣模型因不會(huì)影響流動(dòng)，一般充當(dāng)主流模型影響下的一種被動(dòng)增加。當(dāng)摻氣量相對(duì)較小（< 10%）時(shí)，顯然是足夠的。如果不是這種情況，則流動(dòng)會(huì)受流體中摻氣量的影響。這些影響可通過(guò)選擇 Activat bulking，Activate buoyancy 選項(xiàng)以及定義相關(guān)參數(shù)來(lái)模擬。

In this basic mode, the model acts as a passive addition to the main flow model in that it does not affect the fluid flow. This is sufficient when the amount of entrained air is relatively small (<10%). If this is not the case, the flow may be affected by the air entrained in the fluid. These effects can be included by selecting the Activate bulking and Activate buoyancy options and defining the associated parameters. The Activate bulking option enables the Density evaluation model to account for the non-uniform fluid density resulting from the entrained air (see Flows with Density Variations). In this case, input the value for Density of phase #2 in the model panel.

Activate bulking 選項(xiàng)使得 Density evaluation 模型解釋了由摻氣造成非均勻流密度的改變。在這種情況下，需要輸入 Density of phase #2 的值。Activate buoyancy 選項(xiàng)使得 Drift-flux 模型引起了兩相之間的相互作用。選中 Activate bulking 情況下，由于密度的不同，氣泡在流體中移動(dòng)影響流動(dòng)。具體參見(jiàn) Drift Flux 模型。如果選中 Allow gas to escape at free surface（允許氣體躍離液面 ），默許了液體質(zhì)點(diǎn)躍離頁(yè)面，回落時(shí)帶入空氣，詳見(jiàn) Escape of Gas through a Free Surface。

The Activate buoyancy option enables the Drift-flux model to account for the interaction between the two phases. It requires that the Activate bulking option is already enabled. In this case, the air bubbles can move within fluid due to the difference in their densities and affect the fluid motion. See the full description of the model in Drift Flux. If the Allow gas to escape at free surface check box is selected, the entrained air can also escape back into the void, as described in Escape of Gas through a Free Surface.

Air entrainment 模型模擬存儲(chǔ)的值為 Vol.fraction of entrained air（摻氣量），可在后處理中導(dǎo)出。

The Air entrainment model evaluates and stores a quantity called Vol. fraction of entrained air, which is available for postprocessing in spatial and history plots. 
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