Young Coastal Scientists and Engineers Conference 2007, PlymouthPeer reviewedPostprin

O'Donoghue, Thomas

Ribberink, Jan S

Van Der A, Dominic Alexander

English

Young Coastal Scientists and Engineers Conference 2007, PlymouthPeer reviewe

Aberdeen University Research

Boundary layer flow induced by waves with acceleration skewnessDominic A. van der A1∗, Tom O’Donoghue1 and Jan S. Ribberink21University of Aberdeen, Department of Engineering, Aberdeen AB24 3UE2University of Twente, PO BOX 217, Enschede, The Netherlands∗d.a.vandera@abdn.ac.ukMotivation• Sand transport rates induced bysawtooth shaped waves are found todepend on acceleration skewness β(See Figure 1).•Practical quasi-steady transportmodels where Φ(t) ∝ θ(t)n predictzero net transport rates for thesewaves.•New research at Aberdeen withinthe SANTOSS project focusses onsheet flow transport processes forflows with acceleration skewness.•To understand better the funda-mental processes, the first stagefocusses on boundary layer flowover non-mobile fixed rough beds.0.6 0.8 1 1.2 1.4 1.60510152025303540umax (m/s)Φ (−)  β=0.6β=0.7β=0.8Figure 1: Sand transport rate for various degrees ofacceleration skewness after Watanabe & Sato (2004).Test facilityThe Aberdeen Oscillatory Flow Tunnel(AOFT) has an overall length of 16mwith a glass-sided, rectangular testsection 10 m long, 0.75m high by 0.3mwide (Figure 2).Figure 2: The Aberdeen Oscillatory Flow TunnelThe AOFT is capable of generatingflow amplitudes in the test section of1.5m with periods greater than 5 s.Sheet-flow conditions can be producedover a wide range of periods fortypical sand sizes.Fixed bed experimentsVelocities were measured using cross-correlation Particle Image Velocimetry(PIV) for flows of different period(5-7 s), amplitude (1-1.5m) andacceleration skewness (0.5-0.75). Thefixed bed consists of coarse sand witha median diameter of 0.44mm.Velocity measurements were acquiredfor 50 consecutive wave cycles toobtain consistent results (Figure 3, 4)Results−1.5 −1 −0.5 0 0.5 1 1.5010203040506070u (m/s)z (mm)Figure 3: Ensemble averaged velocity profiles for theentire flow cycle over a fixed course sand bed(D50 = 0.44mm) for a 7 s sawtooth flow (β = 0.75),measured with PIV at 12 Hz. The lowest measurementis at z = 1.6mm where z = 0 corresponds to the top ofthe roughness elements.0 0.25 0.5 0.75 1−101u∞ (m/s)t/T (−)0 0.5 10204060|u∞| = 0.5 m/s|u| (m/s)z (mm)0 0.5 10204060|u∞| = 0.8 m/s|u| (m/s)Figure 4: Velocity profiles for two selected phases withsimilar free stream velocities but different accelerations.Near bed velocities are higher when the flowacceleration is greater. These profiles imply highershear stresses for stronger flow accelerations.Future work1. Continue fixed bed experiments fordifferent bed roughness and variousflow conditions.2. Detailed measurements of velocitiesand concentrations formobile bedsheet flow with accn. skewness.3. Implementing acceleration effects inthe practical transport model that isbeing developed within theSANTOSS project.Acknowledgements: This work has been carried out within the UK/Dutch project SANTOSS funded by the UK’s EPSRC (GR/T28089/01) and the Dutch research organisation STW (TCB.6586).

Boundary layer flow induced by waves with acceleration skewness

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Boundary layer flow induced by waves with acceleration skewness

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