Faculty Research Publications (Civil and Construction Engineering)
http://hdl.handle.net/1957/28193
2014-12-26T09:24:41ZA dimensional analysis for determining optimal discharge and penstock diameter in impulse and reaction water turbines
http://hdl.handle.net/1957/54753
A dimensional analysis for determining optimal discharge and penstock diameter in impulse and reaction water turbines
Leon, Arturo S.; Zhu, Ling
This paper presents a dimensional analysis for determining optimal flow discharge and optimal penstock diameter when designing impulse and reaction turbines for hydropower systems. The aim of this analysis is to provide general insights for minimizing water consumption when producing hydropower. This analysis is based on the geometric and hydraulic characteristics of the penstock, the total hydraulic head and the desired power production. As part of this analysis, various dimensionless relationships between power production, flow discharge and head losses were derived. These relationships were used to withdraw general insights on determining optimal flow discharge and optimal penstock diameter. For instance, it was found that for minimizing water consumption, the ratio of head loss to gross head should not exceed about 15%. Two examples of application are presented to illustrate the procedure for determining optimal flow discharge and optimal penstock diameter for impulse and reaction turbines.
This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/renewable-energy/
2014-11-01T00:00:00ZLaboratory experiments on counter-propagating collisions of solitary waves. Part 2. Flow field
http://hdl.handle.net/1957/53081
Laboratory experiments on counter-propagating collisions of solitary waves. Part 2. Flow field
Chen, Yongshuai; Zhang, Eugene; Yeh, Harry
In the companion paper (Chen & Yeh, J. Fluid Mech., vol. 749, 2014, pp. 577–596),
collisions of counter-propagating solitary waves were studied experimentally by
analysing the measured water-surface variations. Here we study the flow fields
associated with the collisions. With the resolved velocity data obtained in the
laboratory, the flow fields are analysed in terms of acceleration, vorticity, and
velocity-gradient tensors in addition to the velocity field. The data show that flow
acceleration becomes maximum slightly before and after the collision peak, not in
accord with the linear theory which predicts the maximum acceleration at the collision
peak. Visualized velocity-gradient-tensor fields show that fluid parcels are stretched
vertically prior to reaching the state of maximum wave amplitude. After the collision
peak, fluid parcels are stretched in the horizontal direction. The boundary-layer
evolution based on the vorticity generation and diffusion processes are discussed. It is
shown that flow separation occurs at the bed during the collision. The collision creates
small dispersive trailing waves. The formation of the trailing waves is captured by
observing the transition behaviour of the velocity-gradient-tensor field: the direction
of stretching of fluid parcels alternates during the generation of the trailing waves.
This is the publisher’s final pdf. The published article is copyrighted by Cambridge University Press and can be found at: http://journals.cambridge.org/action/displayJournal?jid=FLM.
2014-08-19T00:00:00ZLaboratory experiments on counter-propagating collisions of solitary waves. Part 1. Wave interactions
http://hdl.handle.net/1957/50862
Laboratory experiments on counter-propagating collisions of solitary waves. Part 1. Wave interactions
Chen, Yongshuai; Yeh, Harry
Collisions of counter-propagating solitary waves are investigated experimentally.
Precision measurements of water-surface profiles are made with the use of the laser
induced fluorescence (LIF) technique. During the collision, the maximum wave
amplitude exceeds that calculated by the superposition of the incident solitary waves,
and agrees well with both the asymptotic prediction of Su & Mirie (J. Fluid Mech.,
vol. 98, 1980, pp. 509–525) and the numerical simulation of Craig et al. (Phys.
Fluids, vol. 18, 2006, 057106). The collision causes attenuation in wave amplitude:
the larger the wave, the greater the relative reduction in amplitude. The collision also
leaves imprints on the interacting waves with phase shifts and small dispersive trailing
waves. Maxworthy’s (J. Fluid Mech., vol. 76, 1976, pp. 177–185) experimental results
show that the phase shift is independent of incident wave amplitude. On the contrary,
our laboratory results exhibit the dependence of wave amplitude that is in support
of Su & Mirie’s theory. Though the dispersive trailing waves are very small and
transient, the measured amplitude and wavelength are in good agreement with Su
& Mirie’s theory. Furthermore, we investigate the symmetric head-on collision of
the highest waves possible in our laboratory. Our laboratory results show that the
runup and rundown of the collision are not simple reversible processes. The rundown
motion causes penetration of the water surface below the still-water level. This
penetration causes the post-collision waveform to be asymmetric, with each departing
wave tilting slightly backward with respect to the direction of its propagation; the
penetration is also the origin of the secondary dispersive trailing wavetrain. The
present work extends the studies of head-on collisions to oblique collisions. The
theory of Su & Mirie, which was developed only for head-on collisions, predicts well
in oblique collision cases, which suggests that the obliqueness of the collision may
not be important for this ‘weak’ interaction process.
This is the publisher’s final pdf. The published article is copyrighted by Cambridge University Press and can be found at: http://journals.cambridge.org/action/displayJournal?jid=FLM.
2014-05-19T00:00:00ZMicrowave backscattering from surf zone waves
http://hdl.handle.net/1957/50855
Microwave backscattering from surf zone waves
Catalán, Patricio A.; Haller, Merrick C.; Plant, William J.
The microwave backscatter properties of surf zone waves are analyzed using field observations.
By utilizing a preexisting, independent, water surface discrimination technique, the microwave returns were
extracted along individual waveforms and the data from shoaling (steepening) waves, surf zone breaking
waves, and remnant foam were separated and quantified. In addition, a wave tracking analysis technique
allows the returns to be examined on a wave-by-wave basis as individual waves progress through the shoaling
zone and break on a nearshore sand bar. Normalized radar cross sections (NRCS), polarization ratios,
Doppler spectra, and scatterer velocities were collected using a dual-polarized, X-band radar operating at
lower grazing angles than previously reported (1°–3.5°). The results indicate that the maximum NRCS levels
are from the active breaking portions of the wave and were consistently about -20 dB, regardless of radar
polarization, azimuth angle, wave height, or wind speed. In addition, breaking waves induce broadening of
the Doppler spectra and mean scatterer velocities that correlate well with the carrier wave celerity. Analysis
of the polarization ratios suggest that the active breaking portions of the wave are depolarized but that
higher polarization ratios (>0 dB) are found on the leading edges shoreward of the active breaking portions
of the waves, which indicates a clear distinction between two scattering regimes. These results are consistent
with scattering from a very rough surface that is being mechanically generated by the breaking process,
showing a good agreement with the expected grazing angle dependency of a Lambertian scatterer.
This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://onlinelibrary.wiley.com/journal/10.1002/%28ISSN%292169-9291.
2014-05-28T00:00:00Z