粗糙床面风廓线统一对数区的空气动力学粗糙度及其空间变异特征

The Aerodynamic Roughness Length over Rough Surfaces Derived from Whole Wind Velocity Profiles with the Log Law and Its Spatial Variations

为了进一步理解粗糙床面阻力效应,减小空气动力学粗糙度测试中的不确定性,依据风沙风洞测试的3类粗糙元(细高粗糙元、孔隙粗糙元和粗矮粗糙元)覆盖的39个粗糙床面在不同自由风速下的风廓线数据,提出了风廓线统一对数区的概念并得出以下结论:粗糙床面风廓线统一对数区范围约在0.1~0.3h至边界层顶部,空气动力学粗糙度是变应力层内床面对气流阻力效应的垂向平均;在统一对数区内拟合的空气动力学粗糙度的垂向变异分为先增后减型(概率为71%)、减小型(20%)和增加型(9%)等类型,而采用统一对数区的空气动力学粗糙度可以避免垂向变异带来的不确定性;统一对数区的无量纲空气动力学粗糙度随粗糙元密度以幂函数形式增加的特征,进一步表明该指标能更好地表征粗糙床面对气流阻力效应;尾涡流风廓线统一对数区的空气动力学粗糙度约为街流区1~5倍,表明街流区风廓线统一对数区的空气动力学粗糙度是模拟跃移起动更合适的参数。

In terms of wind profiles over 39 surfaces covered with slender,porous and stocky roughness elements respectively at different density that were observed in a blown-sand wind tunnel under wind velocity around 4-20 m·s^-1,aerodynamic roughness length is redefined as value estimated from whole wind velocity profile following the log law rather than that from inertial sub-layer so as to understand further drag effect on airflow and to reduce uncertainty of estimation about aerodynamic roughness.These whole wind profiles with the log law（here called as WWPL）extend from0.1-0.3 h to the top of boundary layer except profiles over stocky roughness elements in wake flows which extend upward from top of the roughness elements,indicating extension of WWPL in street flows being stable in contrast with extension of inertial sub-layer.The trends of aerodynamic roughness lengths to observed heights are viewed as increase-decrease（probability 70%）,decrease（21%）and increase types（9%）at the range around 0.01-1 mm.The trends can be explained as variations of wind velocity gradients with height due to dissipation of airflows＇ momentum at the bottom and restoration above top of roughness elements.As a result,adoption of roughness length from WWPL resulting from vertical average of wind velocity gradient,is necessary for expressing drag effect of roughness surfaces on airflow.Increase of aerodynamic roughness length from WWPL with roughness elements＇ density as a power function shows further the index being better indicator of the resistance effect compared to the traditional roughness length.On average,aerodynamic roughness length from WWPL in wake flow being about 1-5 times higher than that from WWPL in street flow,indicates aerodynamic roughness from WWPL in street flow is a better parameter for predication saltation threshold.