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首页> 外文期刊>Earth Surface Processes and Landforms: The journal of the British Geomorphological Research Group >Simulation and measurement of surface shear stress over isolated and closely spaced transverse dunes in a wind tunnel
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Simulation and measurement of surface shear stress over isolated and closely spaced transverse dunes in a wind tunnel

机译：风洞中孤立且间隔较近的横向沙丘表面剪切应力的模拟和测量

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Topographic interactions generate multidirectional and unsteady airflow that limits the application of velocity profile approaches for estimating sediment transport over dunes. Results are presented from a series of wind tunnel simulations using Irwin-type surface-mounted pressure sensors to measure shear stress variability directly at the surface over both isolated and closely spaced sharp-crested model dunes. Findings complement existing theories on secondary airflow effects on stoss transport dynamics and provide new information on the influence of lee-side airflow patterns on dune morphodynamics. For all speeds investigated, turbulent unsteadiness at the dune toe indicates a greater, more variable surface shear, despite a significant drop in time-averaged measurements of streamwise shear stress at this location. This effect is believed sufficient to inhibit sediment deposition at the toe and may be responsible for documented intermittency in sand transport in the toe region. On the stoss slope, streamline compression and flow acceleration cause an increase in flow steadiness and shear stress to a maximum at the crest that is double that at the toe of the isolated dune and 60-70 per cent greater than at flow reattachment on the lower stoss of closely spaced dunes. Streamwise flow accelerations, rather than turbulence, have greater influence on stress generation on the stoss and this effect increases with stoss slope distance and with incident wind speed. Reversed flow within the separation cell generates significant surface shear (30-40 per cent of maximum values) for both spacings. This supports field studies that suggest reversed flow is competent enough to return sediment to the dune directly or in a deflected direction. High variability in shear at reattachment indicates impact of a turbulent shear layer that, despite low values of time-averaged streamwise stress in this region, would inhibit sediment accumulation. Downwind of reattachment, shear stress and flow steadiness increase within 6h (h = dune height) of reattachment and approach upwind values by 25 h. A distance of at least 30 h is suggested for full boundary layer recovery, which is comparable to fluvial estimates. The Irwin sensor used in this study provides a reliable means to measure skin friction force responsible for sand transport and its robust, simple, and cost-effective design shows promise for validating these findings in natural dune settings.
机译：地形相互作用产生多方向且不稳定的气流，从而限制了速度剖面方法在估算沙丘上的泥沙输送量方面的应用。通过使用Irwin型表面安装压力传感器对风洞进行的一系列模拟得出了结果，这些传感器直接在孤立且间隔很近的尖顶模型沙丘上直接在表面测量切应力变化。这些发现补充了有关二次气流对储层运输动力学的影响的现有理论，并提供了关于风侧气流模式对沙丘形态动力学影响的新信息。对于所研究的所有速度，沙丘脚趾处的湍流不稳定性都表明更大的，更大的表面剪切力，尽管在该位置的沿河方向的剪切应力的时间平均测量值显着下降。据信这种作用足以抑制沉淀物在脚趾上的沉积，并且可能导致在脚趾区域中砂运输的间歇性。在高应力斜坡上，流线压缩和流动加速导致波峰的流动稳定性和剪切应力增加到最大值，是孤立沙丘脚趾的两倍，比下部沙丘重新流动时高60-70％紧密间隔的沙丘。沿河的水流加速而不是湍流，对地应力产生的影响更大，并且这种影响随着地应力的倾斜距离和入射风速的增加而增加。分离单元内的反向流动会在两个间距上产生明显的表面剪切力（最大值的30％至40％）。这为野外研究提供了支持，这些研究表明逆流具有足够的能力将沉积物直接或以偏转的方向返回沙丘。重新附着时剪切的高变化性表明湍流剪切层的影响，尽管该区域的时间平均流向应力值较低，但仍会抑制沉积物的堆积。重新附着的顺风，切应力和流动稳定度在重新附着的6h（h =沙丘高度）内增加，并接近逆风值25 h。对于完全边界层恢复，建议至少30 h的距离，这与河流估算值相当。这项研究中使用的Irwin传感器提供了一种可靠的方法来测量引起沙粒运输的皮肤摩擦力，其坚固，简单且经济高效的设计显示了在自然沙丘环境中验证这些发现的希望。

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