102,593 research outputs found
Optimal and robust controllers for periodic and multirate systems
No full textIncludes bibliographical references (p. [7]).Caption title. Series from publisher's list.Supported by the Army Research Office. DAAL-86-K-0171 Supported by the the National Science Foundation. 8810178-ECS Supported by the Air Force Office of Scientific Research, Elgin Air Force Base, Grant F087635-87-0031 Supported by a gift from the Boeing Corporation and the NASA Ames and Langley Research Centers. NASA/NAG-2-297Munther A. Dahleh, Petros G. Voulgaris, Lena S. Valavani
Linear features on side-scan sonar images: an algorithm for the correction of angular distortion
No full textThe distortion ellipse graphical method is used regularly by scientists for the correction of compressional effects caused in side-scan sonar images by variable ship speeds. An algorithm is derived for this correction for the calculation of the true angle of linear objects (e.g. sandwaves) in relation to the ship's track:where R is the ratio of the ship's speed to the optimal ship speed (i.e. for no distortion) and ? is the false angle between the ship's track and the linear object measured directly from the image (sonograph) (0° < ? < 90°). The algorithm is applicable for any combination of chart paper speed and width. The results obtained using this approach are considered to be more accurate than conventional graphical methods of analysis of analogue output. Interpretation of imagery for sedimentological investigations should be enhanced by the adoption of this procedure.<br/
Sediment resuspension on beaches: response to breaking waves
No full textField data of time-averaged suspended sediment concentration (ssc) are presented from three different UK beaches and a variety of wave conditions. The data are collated into a single analysis to examine the mechanisms associated with sand resuspension in the surf zone. Emphasis is placed upon the relative contribution of bottom and surface generated turbulence in controlling suspended sediment concentration distribution. Inside the surf zone, vortices induced by breaking waves and bores are the main mechanism for sediment resuspension; this process dominates over bottom boundary layer processes. Time-averaged reference concentration, defined as the ssc at 3.5 cm above the bed, does not correlate with bottom-induced shear stress or predicted ripple steepness. Both reference concentration and vertical distribution of sediment are controlled by the breaking wave (plunging, spilling) characteristics. Within the inner surf zone, hydraulic jumps (associated with strong offshore flows) are responsible for sediment resuspension. Vertical distribution of sediment is parameterised using a convective type time-averaged suspended sediment concentration profile. Reference concentration is highly correlated with wave breaking type, as defined by the local breaker parameter. The mixing parameter depends also on the wave breaking type and may be expressed as a fraction of the local water depth
H [infinity] and H2 optimal controllers for periodic and multi-rate systems
No full textCaption title. On t.p. "[infinity]" is represented by the infinity sign in superscript.Includes bibliographical references (p. 18-19).Research supported by the AFOSR-ELGIN AFB. F08635-87-K-0031 Research supported by the NASA Ames and Langley Research Centers. NASA/NAG-2-297 Supported by a gift from the Boeing Corp. Second author supported by the Army Research Office. DAAL-86-K-0171 Second author supported by the NSF. 8810178-ECSPetros G. Voulgaris, Munther A. Dahleh, Lena S. Valavani
VELSSCPHASE: A code for examining the phase relationships between suspended fine sediment and tidal flows using the Bass et al. (2002) method.
No full textVELSSCPHASE.m is a MATLAB script of the code of the simplified model of Bass et al (2002). It can be used diagnostically to examine the phase relationships between suspended fine sediment and tidal flows. The theory and assumptions involved are described in Bass et al.(2002). It is provided as is without any warranties and/or support. The code has been tested against the figures in Bass et al (2002) and it was found to be performing accurately. Sometimes a more refined time step is required for better accuracy at the expense of run time.
Please read Bass et al. 2002 before attempting to use.
Bass, S.J., J.N. Aldridge, I.N. McCave, and C.E. Vincent, 2002. Phase relationship between fine sediment suspension and tidal currents in coastal seas. J. Geophys. Res. 107(C10), 3146 doi: 10.1029/2001JC01269.
Authors: Douglas Cahl and George Voulgaris , School of the Earth, Ocean and Environment, University of South Carolina, Columbia, SC, USA
Published in Zenodo, - doi:10.5281/zenodo.3363922
If you use the code please cite Bass et al. (2002) and Cahl and Voulgaris (2019)
Citation:
Cahl, D. and G. Voulgaris, 2019. VELSSCPHASE: A code for examining the phase relationships between suspended fine sediment and tidal flows using the Bass et al (2002) method. Zenodo, doi:10.5281/zenodo.336392
The influence of clay on the threshold of movement of fine sandy beds
No full textThe erosion threshold (i.e. surface shear strength) of mixed sediment deposits has been tested under the action of unidirectional currents (mean critical speeds, 4 mm above the bed, ranging from 12.4 cm s?1 to 18.2 cm s?1) or simulated waves (mean critical periods ranging from 2.0 s to 6.7 s, for near-bed wave amplitudes of 0.28–0.57 m) using an oscillating tray in a rectangular recirculating flume. The sediment mixtures investigated consisted of angular fine-grained quartz sands (D50 = 152.5 and 215 ?m) and cohesive estuarine mud. The investigation has shown that the cohesive material increases generally the erosion threshold of sandy deposits. The rate of increase is smaller for mud contents < 30% (clay mineral content < 11–14%, by dry weight) and is attributed to an increase of the internal angle of friction. When the clay mineral content exceeds 11–14% by dry weight, then the sand particles are not in contact with each other. Consequently, the clay component controls the erodibility of the sediment mixture. Finally, a prolonged and intense stress history can increase considerably (by a factor of 1.3–1.7) the original erosion threshold value of these sediments
Optimal H_2 controllers for spatially invariant systems with delayed communication requirements
No full textWe consider the problem of optimal H_2 design of semi-decentralized controllers for a special class of spatially dis-
tributed systems. This class includes spatially invariant and distributed discrete-time systems with an inherent temporal
delay in the interaction of neighboring sites. We consider the problem of optimal design of distributed controllers that have
the same information passing delay structure as the plant. We show how for stable plants, the YJBK parameterization of
such stabilizing controllers yields a convex parameterization for this class. We then show how the optimal H_2 problem
can be solved
Optimal decentralized controllers for spatially invariant systems
No full textWe consider the problem of optimal H/sub 2/ design of semi-decentralized controllers for a special class of spatially distributed systems. This class includes spatially invariant and distributed discrete-time systems with an inherent temporal delay in the interaction of neighbouring sites. Such a structure arises naturally from spatio-temporal discretizations of many physical systems described by partial differential equations. We consider the problem of optimal design of distributed controllers that have the same information passing delay structure as the plant. We show how the YJBK parametrization of such stabilizing controllers yields a convex parametrization for this class. We then show how the optimal H/sub 2/ problem can be solved exactly
Optimal rejection of bounded persistent disturbances in periodic systems
No full textCover title.Includes bibliographical references (p. 15-16).Supported by the Army Research Office. DAAL03-86-K-0171 Supported by the NSF. 8810178-ECS Supported by the AFOSR-Elgin A.F.B. F08635-87-K-0031 Supported by the NASA Ames and Langley Research Centers. NASA/NAG-2-297 Supported by a gift from the Boeing Corporation.Munther A. Dahleh, Petros G. Voulgaris, Lena S. Valavani
Laboratory investigations into wave period effects on sand bed erodibility, under the combined action of waves and currents
No full textLaboratory experimental data on sediment threshold conditions for fine to coarse sands, under the combined action of waves and currents are presented. Higher flows are needed to satisfy threshold under short period waves than for longer period waves. The combined threshold velocities indicate that under the shorter (5 s) period waves, the oscillatory and steady flows are independent of each other, but interact to a greater extent as the wave period increases. The derived threshold flow conditions are converted into threshold shear stress values using existing linear and non-linear models. The performance of the models for various periods of oscillation is then examined in view of the assumption that a unique threshold value exists for a particular grain size. All models suggest that the critical stress of a particular grain is higher under shorter period waves, for the range of grain sizes presented. Using different models for waves, of different periods, appears to remove the effect of wave period upon the data. It is suggested that theories used by the existing models are insufficient to fully describe the interaction of the steady and oscillatory flows during the experiments
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