流体中的表面摩擦系数(skin friction coefficient, C f C_f Cf)可以定义为:
C f ≡ τ w 1 2 ρ U ∞ 2 C_f \equiv \frac{\tau_w}{\frac{1}{2} \, \rho \, U_\infty^2} Cf≡21ρU∞2τw
其中, τ w \tau_w τw表示局部壁剪切应力(wall shear stress), ρ \rho ρ是流体密度。 U ∞ U_\infty U∞是自由流速度(free-stream velocity),通常在边界层外或入口处取。
关于一个平板湍流边界层的几种局部表面摩擦近似公式如下:
C f = 0.0576 R e x − 1 / 5 f o r 5 ⋅ 1 0 5 < R e x < 1 0 7 C_f = 0.0576 Re_x^{-1/5} \quad {for} \quad 5 \cdot 10^5 < Re_x < 10^7 Cf=0.0576Rex−1/5for5⋅105<Rex<107
C f = 0.0592 R e x − 1 / 5 f o r 5 ⋅ 1 0 5 < R e x < 1 0 7 C_f = 0.0592 \, Re_x^{-1/5} \quad {for} \quad 5 \cdot 10^5 < Re_x < 10^7 Cf=0.0592Rex−1/5for5⋅105<Rex<107
C f = [ 2 l o g 10 ( R e x ) − 0.65 ] − 2.3 f o r R e x < 1 0 9 C_f = [ 2 \, log_{10}(Re_x) - 0.65 ] ^{-2.3} \quad {for} \quad Re_x < 10^9 Cf=[2log10(Rex)−0.65]−2.3forRex<109
C f = 0.370 [ l o g 10 ( R e x ) ] − 2.584 C_f = 0.370 \, [ log_{10}(Re_x) ]^{-2.584} Cf=0.370[log10(Rex)]−2.584
(文献[1]式(38)):
1.0 / C f 1 / 2 = 1.7 + 4.15 l o g 10 ( R e x C f ) 1.0/C_f^{1/2} = 1.7 + 4.15 \, log_{10} (Re_x \, C_f) 1.0/Cf1/2=1.7+4.15log10(RexCf)
下面是从文献[2],p19页中获取的表面摩擦公式,可以适当参考:
C f = 0.074 R e x − 1 / 5 C_f = 0.074 \, Re_x^{-1/5} Cf=0.074Rex−1/5
C f = 0.34 R e x − 1 / 3 + 0.0012 C_f = 0.34 \, Re_x^{-1/3} + 0.0012 Cf=0.34Rex−1/3+0.0012
C f = 0.455 [ l o g 10 ( R e x ) ] − 2.58 C_f = 0.455 \, [ log_{10}(Re_x)]^{-2.58} Cf=0.455[log10(Rex)]−2.58
C f = 0.0586 [ l o g 10 ( R e x C f ) ] − 2 C_f = 0.0586 \, [ log_{10}(Re_x \, C_f )]^{-2} Cf=0.0586[log10(RexCf)]−2
C f = 0.427 [ l o g 10 ( R e x ) − 0.407 ] − 2.64 C_f = 0.427 \, [ log_{10}(Re_x) - 0.407]^{-2.64} Cf=0.427[log10(Rex)−0.407]−2.64
C f = 0.055 R e x − 0.182 C_f = 0.055 \, Re_x^{-0.182} Cf=0.055Rex−0.182
C f = 0.0648 [ l o g 10 ( R e x C f 0.5 ) − 0.9526 ] − 2 C_f = 0.0648 \, [log_{10}(Re_x \, C_f^{0.5})-0.9526]^{-2} Cf=0.0648[log10(RexCf0.5)−0.9526]−2
C f = 0.0816 [ l o g 10 ( R e x ) − 1.703 ] − 2 C_f = 0.0816 \, [log_{10}(Re_x) - 1.703]^{-2} Cf=0.0816[log10(Rex)−1.703]−2
C f = 0.067 [ l o g 10 ( R e x ) − 2 ] − 2 C_f = 0.067 \, [log_{10}(Re_x) - 2 ] ^{-2} Cf=0.067[log10(Rex)−2]−2
C f = 0.52 [ l o g 10 ( R e x ) ] − 2.685 C_f = 0.52 \, [log_{10}(Re_x)] ^{-2.685} Cf=0.52[log10(Rex)]−2.685
C f = 0.075 [ l o g 10 ( R e x ) − 2 ] − 2 C_f = 0.075 \, [log_{10}(Re_x) - 2 ] ^{-2} Cf=0.075[log10(Rex)−2]−2
C f = 0.0113 [ l o g 10 ( R e x ) − 3.7 ] − 1.15 C_f = 0.0113 \, [log_{10}(Re_x) - 3.7 ] ^{-1.15} Cf=0.0113[log10(Rex)−3.7]−1.15
C f = 0.0776 [ l o g 10 ( R e x ) − 1.88 ] − 2 + 60 R e x − 1 C_f = 0.0776 \, [log_{10}(Re_x) - 1.88 ] ^{-2} + 60 \, Re_x^{-1} Cf=0.0776[log10(Rex)−1.88]−2+60Rex−1
C f = [ 4.06 l o g 10 ( R e x C f ) − 0.729 ] − 2 C_f = [4.06 \, log_{10}(Re_x \, C_f) - 0.729]^{-2} Cf=[4.06log10(RexCf)−0.729]−2
[1] von Karman, Theodore (1934), “Turbulence and Skin Friction”, J. of the Aeronautical Sciences, Vol. 1, No 1, 1934, pp. 1-20.
[2] Lazauskas, Leo Victor (2005), “Hydrodynamics of Advanced High-Speed Sealift Vessels”, Master Thesis, University of Adelaide, Australia (download).
[3] Schlichting, Hermann (1979), Boundary Layer Theory, ISBN 0-07-055334-3, 7th Edition.