개수로 흐름에서 사다리꼴 돌출줄눈의 벽면조도 효과

Wall-roughness effects of trapezoidal ribs on the flow of open channel

Abstract

산지하천 만곡부의 홍수피해를 줄이고자 사다리꼴 단면의 돌출줄눈을 하천옹벽에 설치하였다. 본 연구에서는 사다리꼴 형상에 의한 흐름저항 효과를 파악하고자 개수로 측벽에 사다리꼴 돌출줄눈을 설치하여 수리실험을 수행하였다. 벽면조도가 k형에 해당하는 ${\lambda}_{nv}$가 6, 9, 12인 경우에 대해 유량에 따른 흐름특성을 파악하였다. 흐름저항은 돌줄줄눈의 설치간격이 멀어짐에 따라 전반적으로 증가하였다. 고유량 조건에서 최대마찰계수는 ${\lambda}_{nv}$가 9일 때 발생하였다. 사다리꼴 돌출줄눈은 정사각형 돌출줄눈과 비교해 흐름저항은 상대적으로 작았지만, 사다리꼴의 형상저항은 전체 흐름저항의 평균 62%를 차지했다. 벽면조도 증가에 따른 흐름저항 효과를 극대화하기 위한 사다리꼴 돌출줄눈의 최적 설치간격은 돌출줄눈 높이의 9~12배 범위임을 확인하였다.

The trapezoidal ribs had been installed in the retaining wall in order to reduce to flood damage in the impingement of mountain rivers. In this study, experiments in open channel with the trapezoidal ribs on sidewall were conducted to evaluate the effect of flow resistance by the trapezoidal shape. The hydraulic flow characteristics according to the flow rates were surveyed where the wall roughness is k-type that dimensionless spacings, ${\lambda}_{nv}$, are 6, 9, and 12. The flow-resistance factors such as roughness and friction coefficients increased generally with increase of the spacing of ribs. In high flow rate the friction coefficient showed the maximum value when ${\lambda}_{nv}$ is 9. Though the trapezoidal ribs has the relatively smaller flow resistance compared to the square ribs, their form drag accounted for mean 62% of the total flow resistance. It was confirmed that the optimal spacing of trapezoidal ribs to maximize the effect of flow resistance as the wall roughness increases are 9 to 12 times of the height of trapezoidal ribs.

Fig. 1. Application of vertical ribs to reduce flow velocity in out bend bank of mountain river (Hwangji River in Teabaek, Gangwon)

Fig. 2. Mean streamlines for simulated flow according to types of rib roughness (Cui et al., 2003)

Fig. 3. Layout of open channel for hydraulic experiment of retaining wall with vertical trapezoidal ribs

Fig. 4. Longitudinal variation of water depth according to spacing of ribs in discharges of 103 l/s (left) and 120 l/s (right)

Fig. 5. Variations of manning’s roughness coefficients (left) and Darcy-Weisbach's friction factors (right) according to flow discharge (Small, Medium, and Large) and spacing of square ribs (SR) and trapezoidal ribs (TR)

Fig. 6. Relationships between F_r and f/f₀ for square ribs (SR) and trapezoidal ribs (TR)

Fig. 7. Distributions of x-direction velocity (cm/s) of longitudinal direction on open channel installed vertical trapezoidal ribs in case of λ _nv= 0, 6, 9 and 12 of Q = 120 l/s; vertical axis is S_n/S₀ and horizontal axis is y/B

Fig. 8. Distributions of x-direction velocity (cm/s) of cross section at cavity between vertical trapezoidal ribs in case of λ _nv = 9 and 12 of Q = 120 l/s; vertical axis is z/H and horizontal axis is y/B

Fig. 9. Relationship between u/U_* and y/B in open channel with vertical trapezoidal ribs installed on a retaining wall in case of λ _nv = 0, 6, 9 and 12 of Q = 120 l/s

Table 1. Experimental values for flow characteristics and resistance factors

Table 2. Ratios of form drag to flow resistance of square ribs and trapezoidal ribs