CFD|NFX湍流模型-2方程K-ε

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k-ε模型

K 是湍流脉动动能(J),ε (epsilon)是湍流脉动动能的耗散率(%)

k-ε模型直接的好处是对于平板和圆柱射流的发散比率的更精确的预测。而且它对于旋转流动、强逆压梯度的边界层流动、流动分离和二次流有很好的表现

K 越大表明湍流脉动长度和时间尺度越大,ε 越大意味着湍流脉动长度和时间尺度越小，它们是两个量制约着湍流脉动。

The k-ε model is the most basic and strong 2-equation turbulence model widely applied in association with law of the wall. The model additionally solves the transport equation with respect to two turbulence variables k and ε. Because it is unable to effectively analyze turbulence flow near the wall due to suppressed flow turbulence fluctuation and dominant viscosity effect, it applies law of the wall together in many cases. It considers the transport equation (4.3.13) with respect to k and the transport equation with respect to ε as follows. It uses

：constants

Here,

and

are empirical constants acquired form simple experiment. Midas NFX-CFD uses

=1.45 and

=1.92, and calculates the turbulent viscosity as follows

Suggestions on the k-ε model are as follows

*The k-ε model accurately calculates turbulence in the low pressure gradient of the free shear layer. It provides accurate result if the pressure gradient is 0 or low in the wall bounded flow.

*The k-ε model is less accurate in the flow with the adverse pressure gradient.

*If fluid collides on the wall, turbulent kinetic energy is predicted excessively at the reattachment point,so it is less accurate at the edge or the bluff body boundary layer.

*Its result is less reliable in highly swirling flow due to complex shear effect. If the swirling flow exhibits recirculation, its value is underestimated.

*If the flow curvature radius is big when buoyance effect is high, its result accuracy on mixed phenomena drops.

*If surface flow separation phenomena occur by adverse pressure gradient, its result accuracy drops,and the separation phenomena can be calculated smaller than actual flow. The k-ω SST turbulence model can complement it.

*It is less accurate for flow recovery at the reattachment point. Avoide Application of wall functions.

*It inaccurately predicts the remote diffusive velocity from the jet flow phenomena.

*It is impossible to predict the secondary flow turbulence in the straight pipe other than the circular pipe.

*It analyzes the laminar zone and the flow for the transition zone less accurately. The k-ε two-layer model can complement it.

CFD|NFX湍流模型-Spalart-Allmaras

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来源：midas机械事业部