于 代表薄层涡流旋的S=0.16，能观察到均方根值在垂直变化的类似趋势 。对于S=0.25，均方根的值随着高度增大渐渐增加。这可以归因于上升龙卷风旋涡两个层结构的存在。对于S>0.4，切向应力与最大切向速度随高度的变化是非常相似(Fig. 5b所示)。所有常压的最高值出现在面层的极限和观察[12]到的速度梯度最高值粘性区域的核心处。常应力值的比较表明，湍流能量的主要部分是由径向拉力产生。
Fig.9 r=0.18r0时径向速度的均方根
Fig.10 r=0.18r0时切向速度的均方根
Fig. 11展示了在两个相同的径向位置雷诺剪应力的变化。与大涡旋相比当S<0.4剪应力表现出了独特的模式。对S=0.08和0.15，雷诺剪应力几乎等于零，这表明有个类似Snow[18]提出的核心层。对于S=0.25，在外域里剪切应力几乎为零(Fig. 11 b所示)，而在核心，他们随着高度类似的切向应力的变化逐渐增加(Fig. 11 a所示)。
对于S=0.4雷诺应力呈现出一个准对称跨核行为，而S=0.68时，好像没有对称性。可以断定在S= 0.68时涡旋没有任何对称并展现出一个类似Church等[5]人提出的如Fig. 4d所示涡旋核心是湍流的形式。对于0.4和0.68两种情况，雷诺应力表现出与在射流中的相似行为，这是发生在射流与周围流体之间夹带一个相对较高的剪切力在面层接口处产生的区域。在核心出的最大值是外层区域的两倍。对于外层区域，表面剪切应力增加并最终在上升接口应力中占主导地位。随着涡流比的增大，壁的摩擦效果也增大。对于S= 0.4和0.68，能观察到剪应力的值在核心与外部区域的一定高度后变成负值。这种标志的变化的一个合理的答案可能是由于高涡旋的双层结构导致轴向分量的波动部分向下方向的出现。发生这一边的位置更接近相对外部区域粘性核占主要地位的核心区域内的地面。根据切向和径向波动速度
(u’v’)雷诺剪应力也能得到计算。它们显示出与涡流比有关的其他湍流特性方面类似的行为，为简洁起见，这里就不介绍了。用于描述在给定流中湍流的特点的重要参数是在湍流动能方程中的湍能生成项(
等人，能得出一个结论：涡流比在流的动态中起到主导作用，在涡流与表面相互作用的上方存在一个关键的涡流比，造成一个与低涡流相比更高的量级的能量和动量区域。这也能得到靠近地面时湍流场而不是均匀流场导致了与龙卷风事件有关的损害的结论。
Fig. 11 r=0.18r0 和r=0.375r0时雷诺应力
Fig. 12 r=0.18r0 和r=0.375r0时湍流能量
4. 结束语
对于龙卷风涡般涡流比旋流的灵敏度是使用一个小型的龙卷风旋涡模拟器实验室研究的。研究采用粒子图像测速（PIV）技术测定均值和湍流涡旋的特征。 发现径向和切向的平均速度分量随着漩涡增加而增加。最大速度的位置是接近表面并随着漩涡增加向表面下降。涡核随着涡旋变化对于 S<0.4宽展为小层核，对于对于 S>0.4变成更湍流核。 湍流的特征表明正应力和剪应力随着漩涡的增加而增加，明显接近表面。该径常应力高于轴向，切向和剪应力，也就是说湍流能量主要由径向拉力产生。最湍急的生产位置是接近龙卷风旋涡的核心区域内的地面。最重要的是，在S=0.4时对应龙卷风旋涡触地的最大湍动能产生。这导致湍流能量而不是平均速度与龙卷风在地面产生强烈破坏的可能性。
参考文献
[1] Baker, G.L., 1981. Boundary layers in a laminar vortex ?ows. PhD Thesis, 143 pp.Purdue University, West Lafayette, IN, USA.
[2] Baker, G.L., Church, C.R., 1979. Measurements of core radii and peak velocities in modeled atmospheric vortices. J. Atmos. Sci. 36, 2413–2424.
[3] Bluestein, H.B., Weiss, C.C., Pazmany, A.L., 2004. The vertical structure of a tornado near Happy, Texas on 5 May 2002: high-resolution mobile W-band Doppler radar observations. Mon. Weather Rev. 132 (10), 2325–2337.
[4] Church, C.R., Snow, J.T., 1993. Laboratory models of tornadoes. Geophys. Monogr. 79,284–295.
[5] Church, C.R., Snow, J.T., Baker, G.L., Agee, E.M., 1979. Characteristics of tornado-like vortices as a function of swirl ratio: a laboratory investigation. J. Atmos. Sci. 36,1755–1766. [6] Church, C.R., Snow, J.T., Agee, E.M., 1977. Tornado vortex simulation at Purdue University. Bull. Am. Meteorol. Soc. 58, 900–908.
[7] Davies-Jones, R.P., 1973. The dependence of core radius on swirl ratio in a tornado simulator. J. Atmos. Sci 30, 1427–1430.
[8] Haan, F.L., Sarkar, P.P., Gallus,W.A., 2007. Design, construction and performance of a large tornado simulator for wind engineering applications. Eng. Struct. J. Earthquake Wind Ocean Eng. 30, 1146–1159.
[9] Hangan, H., Kim, J.-D., 2008. Swirl ratio effects on tornado vortices in relation to Fujita scale. J. Wind Struct. 11, 291–302.
[10] Lee, W.C., Wurman, J., 2005. The diagnosed structure of the Mulhall tornado. J. Atmos. Sci.
62, 2373–2393.
[11] Lewellen, W.S., 1962. A solution of three dimensional vortex ?ows with strong circulation. J.
Fluid Mech. 14, 420–432.
[12] Lewellen, D.C., Lewellen, W.S., Xia., J., 2000. The in?uence of a local swirl ratio on tornado
intensi?cation near the surface. J. Atmos. Sci. 57, 527–544.
[13] Lund., D.E., Snow, J.T., 1993. Laser Doppler velocimetry measurements in tornado like
vortices. In: Church, C., Burgess, D., Doswell., C., Daxies-Jones., R. (Eds.), The Tornado: Its Structure, Dynamics, Prediction, and Hazards. Geophysical Monograph Series, vol. 79. American Geophysics Union Press, pp. 297–306.
[14] Mishar, A.R., James, D.J., Letchford, C.W., 2008. Physical simulation of a single-celled
tornado-like vortex, part A. Flow ?eld characterization. J. Wind Eng. Ind. Aerodyn. 96, 1243–1257.
[15] Rotunno, R., 1979. A study in tornado-like vortex dynamics. J. Atmos. Sci. 36,140–155.
[16] Sarkar, P., F. Haan, W. Gallus Jr.K. le, 2005. Velocity measurements in a laboratory tornado
simulator and their comparison with numerical and full scale data. In:Proceedings of the 37th Joint Meeting Panel on Wind and Seismic Effects,Tsukuba, Japan, May 2005.
[17] Snow, J.T., Church, C.R., Barnhart, B.J., 1979. An investigation of the surface pressure ?elds
beneath simulated tornado cyclones. J. Atmos. Sci. 37, 1013–1026.
[18] Snow, J.T., 1982. A review of recent advances in tornado vortex dynamics. Rev.Geophys.
20,953–964.
[19] Twisdale, L.A., Vickery, P.1., 1992. Research on thunderstorm wind design parameters. J.
Wind Eng. Ind. Aerodyn. 41 (44), 545–556.
[20] Wan, C.A., Chang, C.C., 1972. Measurement of the velocity ?eld in a simulated tornado-like
vortex using three-dimensional velocity probe. J. Atmos. Sci. 29,116–127.
[21] Wang Fl., James D., Letchford C., Peterson R., Snow J., 2001. Development of a prototype
tornado simulator for the assessment of ?uid structure interaction. In: Proceedings of the First Americas Conference on Wind Engineering,Clemson, SC.
[22] Ward, N.B., 1972. The exploration of certain features of tornado dynamics using a laboratory
model. J. Atmos. Sci 29, 1194–1204.
[23] Wilson, T., Rotunno, R., 1986. A numerical simulation of a laminar end-wall vortex and
boundary layer. Phys. Fluids 24, 3993–4005.
[24] Wind Hazard Coalition, 01/31/2006
[25] Wurman, J., Gill, S., 2000. Fine scale radar observations of the Dimmitt, Texas (2 June 1995),
tornado. AMS Monthly Weather Review 2000 128, 2135–2164.
[26] Wurman, J., Alexander, C.R., 2005. The 30 May 1998 Spencer, South Dakota, storm,part II.
Comparison of observed damage and radar-derived winds in the tornadoes. AMS Mon. Weather Rev. 2005 (133), 97–118.
