بررسي تأثيرالمان زبري نصب شده در وجه بالادست پايه ي پل در جريان‌هاي پايين رونده با استفاده از آناليز طيف قدرت

Turbulence structures and sediment transport of different countermeasure roughness of the squire bridge pier

استفاده از زبري در بدنه‌ي بالادست و پايين‌دست پايه‌ي پل‌ها، تأثير مستقيمي در كاهش آبشستگي خواهد داشت. از اين رو، در جهت تعيين شرايط بهينه، 108 آزمايش به ازاء شرايط مختلف زبري نصب شده براي بررسي تأثير در آبشستگي اطراف پايه‌ي پل، طرح‌ريزي شده است. مقايسه‌ها بين مدل‌هاي زبري پيوسته و ناپيوسته نشان داد كه با اجراي ناپيوسته‌ي زبرهاي مستطيلي و مثلثي، ميزان بيشينه‌ي عمق آبشستگي به صورت محسوسي كاهش پيدا كرده است. درنهايت، با استفاده از آناليز طيف قدرت و به‌كارگيري داده‌هاي سرعت‌سنجي، به بررسي ميزان قدرت مؤلفه‌هاي سرعت در نقطه‌ي بيشينه‌ي آبشستگي پرداخته شده است. نتايج نشان داد كه بيشترين مقدار شدت آشفتگي، در زمان بيشترين مقدار طيف قدرت رخ داده است. بررسي داده‌هاي سرعت نشان داد كه سرعت در جهت قائم در حالت پايه‌ي پل بدون زبري، بيشترين مشاركت را در افزايش شدت آشفتگي در جلوي پايه‌ي پل ايفا كرده است. همچنين ناپيوسته كردن زبري، باعث تغيير در ساختار گردابه‌هاي پايين‌رونده به سمت جلو شده است كه درنهايت تغيير مشاركت سرعت‌ها در بيشينه‌ي مقدار آشفتگي را به دنبال داشته است.

Laboratory experiments were carried out to investigate the hydraulics, turbulence structure, and sediment removal capacity of bridge pier with different adjusted roughness. Four experimental models of the roughness were employed to measure the effect of the geometry of the roughness on the scour hole formation and depth. The effects of roughness geometry on the turbulence structure and local erosion through the bridges were studied for one sand sediment size. The three-dimensional velocity profiles and turbulence characteristics of flow downstream and upstream of the bridge pier were measured by Acoustic Doppler velocimetry (ADV) technique due to different adjusted roughnesses. After filtration process by WinADV software, the contour plots of the turbulence kinetic energy were depicted due to triangular roughness. The maximum longitudinal eroded section, which is commonly located by bridge pier corners, was measured through the experiments. The results of the scour hole measurements indicate that due to increasing the length of the roughness, the scour hole depth values decreased; however, increasing the vertical distance has the same effect on the scour hole formation. Furthermore, different geometries of the employed roughness illustrate that the triangular roughness has less scour hole depth than other experimental models with the same hydraulic condition. The detailed information of the magnitude, distribution, and probability of turbulence structures was extracted from time-series data using power spectra. The turbulence data were compared with the topography of upstream erosion. Analysis of the power spectrum density function indicated that the discontinuous adjusted roughness remarkably reduced the downward energy level of vortexes at the upstream of bridge pier, which is supposed that the gap opening through the roughness can increase downward jets impact. It appears that this condition reduces the turbulence kinetic energy near the bed elevation at the upstream face of squire bridge pier.