RoughnessHeightfluent壁面粗糙度设置

1、Flue nt 关于壁面设置中的Rough ness Height 与 Rough ness Con sta nt1页Setti ng the Rough ness ParametersThe roughness parameters are in the Momentum tab of theWall Dialog Box(seeFigure 6.34: The Wall Dialog Boxfor Mara ngoni Stress), which is opened from theBoun dary Con diti ons Task Page(as described inSetti

2、ng CellZone and Boundary Conditions).To model the wall roughness effects, you must specify two roughness parameters: theRoughnessGKsHeight , and the Roughness Constant ,. The default roughness height () is zero, whichcorresponds to smooth walls. For the roughness to take effect, you must specify a n

3、on-zero value forFor a uniform sand-grain roughness, the height of the sand-grain can simply be taken for. For anon-uniform sand-grain, however, the mean diameter (-, ) would be a more meaningful roughness,氐height. For other types of roughness, an“egrriVraulghne”s Isgdit could be used forThe above a

4、pproaches are only relevant if the height is considered constant per surface. However, if the roughnessconstant or roughness height is not constant, then you can specify a profile (seeProfiles).Similarly, user-defined functions may be used to define a wall roughness height that is not constant. For

5、details on theformat of user-defined functions, refer to theUDF Manual .qChoosing a proper roughness constant (一) is dictated mainly by the type of the given roughness. Themodels, it reproduces Nikuradses resistance data for pipes roughened with-pahked, uniformsand-grain roughness. You may need to a

6、djust the roughness constant when the roughness you want toKsCsdefault roughness constant (CS=Q.5)was determined so that, when used withturbulence2页model departs much from uniform sand-grain. For instance, there is some experimental evidence that, forC5=0.5-1.0non-uniform sand-grains, ribs, and wire

7、-mesh roughness, a higher value () is moreqappropriate. Unfortunately, a clear guideline for choosingfor arbitrary types of roughness is notavailable.Note: The rough wall formulation using the shift introduced inEquation 6 -93eliminates all restrictionswith respect to mesh resolution near the wall a

8、nd can therefore be run on arbitrary fine meshes.两个参数的含义是roughness height 粗糙度厚度 Ks,也就是通常说的粗糙度，如果 Ks=0，则认为是光滑的。 如要考虑粗糙度，对于均匀砂粒状表面，可简单任务Ks 为砂粒高度；对不均匀表面，可用砂粒平均直径。空气动力学概念粗糙度即粗糙高度或粗糙参数，是地表的一种空气动力学参数。可表示地表(包括陆面、植被和水面)的粗糙程度，具有长度的量纲。在数值上被定义 为贴近地面平均风速为零处的高度，但在物理上这一高度并不真正存在。近地层对数分布风速廓线满足此边界条件。粗糙度一般与气流无关，而只决定于

9、地表粗糙单元的几何形状、大小和排列等。对于水面和具有弹性的植被，粗糙度还与风速有关。作为十分粗略的近似，蒙蒂思(J. L. Monteith )总结了经验关系：粗糙度与粗糙单元平均高度h (如植被平均高度)的比值为 0.13。但实际上比值是表面特征参数的复杂函数，如莱托(H. Lettau)提出的关系为=0.5Lh/，式中 L 为粗糙单元迎风面上的平均截距，D 为粗糙单元的平均间距。这类公式都带有局限性和经验性。 粗糙度可利用中性大气条件下实测的风速廓线推算。典型的粗糙度高度如下：拖拉管 0.0015 mm钢 0.046 mm水泥 0.3-3 mmroughness constant 粗糙度常

10、数 Cs: 般为 0.5,对于均匀砂粒表面，不用调整此值；对非 均匀砂粒表面，如带有筋板或网眼的表面，一般Cs 为 0.51。来自论坛我做的清水离心泵 CFD 模拟计算，请教一下大家：Wall Boundaries 对话框中，有两个 Roughness 参数，分另 U 是 Roughness Height 禾口 Roughness Constant,这两个参数需要设置吗，该如何设置？我模拟的扬程和效率跟试验值总有差距，怀疑是泵壁铸造精度差，过于粗糙，因此想在 CFD计算中反映这种粗糙度。谢谢！ 下来仔细看了一下 help 文件，了解了这两个参数的含义。经过几次运算和对比，发现flue nt 输

11、出结果对这两个参数十分敏感。应该取什么数值，还要进一步研究。欢迎大家探讨啊！两个参数的含义是：roughness height 粗糙度厚度 Ks,也就是通常说的粗糙度,如果 Ks=0，则认 为是3页光滑的。roughness constant 粗糙度常数 Cs:一般为 0.5，对于均匀砂粒表面，不用调整此值；对非均 匀砂粒表面，如带有筋板或网眼的表面，一般Cs 为 0.51。我曾经对考虑壁面粗糙度和不考虑的情况分别进行过计算,采用相同的结构和网格 .发现对扬程和效率的影响很细微 ,可以忽略 .关于模拟值与试验值差别较大的问题.一般计算出来的扬程要高出试验值不多,而效率值则高不少.这是由于在模拟时