TY - JOUR
T1 - Laboratory observation of boundary layer flow under spilling breakers in surf zone using particle image velocimetry
AU - Huang, Zhi Cheng
AU - Hwung, Hwung Hweng
AU - Hsiao, Shih Chun
AU - Chang, Kuang An
N1 - Funding Information:
The authors would like to thank the National Science Council of Taiwan for their financial supports (grant numbers: NSC 96-2221-E-006-064 , NSC 96-2628-E-006-249-MY3 ). They would also like to thank the Tainan Hydraulics Laboratory for providing the facilities, and to thank Mr. C. H. Pao for conducting the experiments.
PY - 2010/3
Y1 - 2010/3
N2 - A boundary layer flow under spilling breakers in a laboratory surf zone with a smooth bottom is investigated using a high resolution particle image velocimetry (PIV) technique. By cross-correlating the images, oscillatory velocity profiles within a viscous boundary layer of O(1) mm in thickness are resolved over ten points. Using PIV measurements taken for an earlier study and the present study, flow properties in the wave bottom boundary layer (WBBL) over the laboratory surf zone are obtained, including the mean velocities, turbulence intensity, Reynolds stresses, and intermittency of coherent events. The data are then used to estimate the boundary layer thickness, phase variation, and bottom shear stress. It is found that while the time averaged mass transport inside the WBBL is onshore in the outer surf zone, it changes to offshore in the inner surf zone. The zero Eulerian mass transport occurs at h/hb ≈ 0.92 in the outer surf zone. The maximum overshoot of the streamwise velocity and boundary layer thickness are not constant across the surf zone. The bottom shear stress is mainly contributed by the viscous stress through mean velocity gradient while the Reynolds stress is small and negligible. The turbulence level is higher in the inner surf zone than that in the outer surf zone, although only a slight increase of turbulent intensity is observed inside the WBBL from the outer surf zone to the inner surf zone. The variation of phase inside and outside the WBBL was examined through the spatial velocity distribution. It is found the phase lead is not constant and its value is significantly smaller than previous thought. By analyzing instantaneous velocity and vorticity fields, a remarkable number of intermittent turbulent eddies are observed to penetrate into the WBBL in the inner surf zone. The size of the observed large eddies is about 0.11 to 0.16 times the local water depth. Its energy spectra follow the - 5/3 slope in the inertial subrange and decay exponentially in the dissipation subrange.
AB - A boundary layer flow under spilling breakers in a laboratory surf zone with a smooth bottom is investigated using a high resolution particle image velocimetry (PIV) technique. By cross-correlating the images, oscillatory velocity profiles within a viscous boundary layer of O(1) mm in thickness are resolved over ten points. Using PIV measurements taken for an earlier study and the present study, flow properties in the wave bottom boundary layer (WBBL) over the laboratory surf zone are obtained, including the mean velocities, turbulence intensity, Reynolds stresses, and intermittency of coherent events. The data are then used to estimate the boundary layer thickness, phase variation, and bottom shear stress. It is found that while the time averaged mass transport inside the WBBL is onshore in the outer surf zone, it changes to offshore in the inner surf zone. The zero Eulerian mass transport occurs at h/hb ≈ 0.92 in the outer surf zone. The maximum overshoot of the streamwise velocity and boundary layer thickness are not constant across the surf zone. The bottom shear stress is mainly contributed by the viscous stress through mean velocity gradient while the Reynolds stress is small and negligible. The turbulence level is higher in the inner surf zone than that in the outer surf zone, although only a slight increase of turbulent intensity is observed inside the WBBL from the outer surf zone to the inner surf zone. The variation of phase inside and outside the WBBL was examined through the spatial velocity distribution. It is found the phase lead is not constant and its value is significantly smaller than previous thought. By analyzing instantaneous velocity and vorticity fields, a remarkable number of intermittent turbulent eddies are observed to penetrate into the WBBL in the inner surf zone. The size of the observed large eddies is about 0.11 to 0.16 times the local water depth. Its energy spectra follow the - 5/3 slope in the inertial subrange and decay exponentially in the dissipation subrange.
UR - http://www.scopus.com/inward/record.url?scp=74449088182&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=74449088182&partnerID=8YFLogxK
U2 - 10.1016/j.coastaleng.2009.11.004
DO - 10.1016/j.coastaleng.2009.11.004
M3 - Article
AN - SCOPUS:74449088182
SN - 0378-3839
VL - 57
SP - 343
EP - 357
JO - Coastal Engineering
JF - Coastal Engineering
IS - 3
ER -