The 12th International Conference on Hydrodynamics
18 – 23 september 2016, Egmond aan Zee, The Netherlands
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PHASE LEAD OF THE BED SHEAR STRESS TO THE FREE STREAM VELOCITY OF ASYMMETRIC WAVES


Go-down ichd2016 Tracking Number 157

Presentation:
Session: Linear and-linear waves III
Room: Room 1
Session start: 10:30 Wed 21 Sep 2016

Jifu Zhou   zhoujf@imech.ac.cn
Affifliation:

Shuhui Yang   ysh@bwsti.com
Affifliation:


Topics: - Linear and non-linear waves and current

Abstract:

The phase lead of the bed shear stress to the free stream velocity of water waves is of significant importance for sediment transport in coastal areas (Nielsen and David, 2003; Dohmen-Janssen et. al., 2002). In case of linear or sinusoidal water waves, the phase lead is exactly 45° in laminar flow regime, and approximately 10° or so in fully turbulent flow regime (Jensen, et. al., 1989). However, water waves in coastal areas are generally nonlinear or asymmetric, exhibiting asymmetric velocity profiles with different amplitudes of crest and trough, or/and different acceleration and deceleration periods in half a wave cycle (Tanaka, 1998). The phase lead of the bed shear stress to the free stream velocity of these nonlinear or asymmetric water waves is not understood. To reveal the phase lead, we have proposed to use an infinite immersed horizontal plate oscillating non-harmonically in its own plane in a quiescent water to simulate asymmetric wave boundary layers, and have established a large eddy simulation model to manifest the flow characteristics of asymmetric wave boundary layers. We have verified the model by analytical velocity profiles and bed shear stresses of laminar flow under cnoidal waves, experimental results of intermittent turbulent flow under asymmetric waves, and experimental results of fully turbulent flow under sinusoidal waves. We further investigate the behavior of the bed shear stress under cnoidal waves and forth-leaning waves. Particular attention is drawn to the phase lead of the bed shear stress to the free stream velocity at high Reynolds number never studied. The results show that the behavior of the bed shear stress is very much different from that of linear waves. The peak phase lead (the phase lead of the maximum shear stress over the maximum value of the free-stream velocity) and the trough phase lead (the phase lead of the minimum shear stress over the minimum value of the free-stream velocity) are different. We have revealed the dependences of the peak and trough phase leads on the asymmetric degree and the velocity-leaning index.