Edmond Yat-Man Lo

INCOMPRESSIBLE SPH METHOD FOR SIMULATING NEWTONIAN AND NON-NEWTONIAN FLOWS WITH A FREE SURFACE

Abstract An incompressible smoothed particle hydrodynamics (SPH) method is presented to simulate Newtonian and non-Newtonian flows with free surfaces. The basic equations solved are the incompressible mass conservation and Navier–Stokes equations. The method uses prediction–correction fractional steps with the temporal velocity field integrated forward in time without enforcing incompressibility in the prediction step. The resulting deviation of particle density is then implicitly projected onto a divergence-free space to satisfy incompressibility through a pressure Poisson equation derived from an approximate pressure projection. Various SPH formulations are employed in the discretization of the relevant gradient, divergence and Laplacian terms. Free surfaces are identified by the particles whose density is below a set point. Wall boundaries are represented by particles whose positions are fixed. The SPH formulation is also extended to non-Newtonian flows and demonstrated using the Cross rheological model. The incompressible SPH method is tested by typical 2-D dam-break problems in which both water and fluid mud are considered. The computations are in good agreement with available experimental data. The different flow features between Newtonian and non-Newtonian flows after the dam-break are discussed.

Simulation of near-shore solitary wave mechanics by an incompressible SPH method

Abstract An incompressible smoothed particle hydrodynamics (SPH) method together with a large eddy simulation (LES) approach is used to simulate the near-shore solitary wave mechanics. The incompressible Navier–Stokes equations in Lagrangian form are solved using a two-step fractional method. This method first integrates the velocity field in time without enforcing incompressibility. The resulting deviation in particle density is projected onto a divergence-free space to satisfy incompressibility through a pressure Poisson equation. SPH formulations are employed for discretization of relevant gradient and divergence operators. The spatial filtering of the LES approach produces sub-particle scale stresses, which are treated by the Smagorinsky model. The cases of a solitary wave against a vertical wall and running up a plane slope are treated. The wave profiles are in good agreement with reported experimental data or analytical solutions. It is found that the assumption of hydrostatic pressure holds almost everywhere except during the last stages of wave breaking. The dynamic viscosity is also found to be a maximum near the breaking front.

Nutrients and suspended solids in dry weather and storm flows from a tropical catchment with various proportions of rural and urban land use

The results of an investigation characterizing the nutrients and suspended solids contained in stormwater from Kranji Catchment in Singapore are reported in this paper. Stormwater samples were collected from 4 locations and analyzed for the following eleven analytes: TOC, DOC, TN, TDN, NH(4)(+), NO(2)(-)+NO(3)(-) (NO(x)), TP, TDP, OP, SiO(2) and TSS. Stormwater was sampled from catchments with various proportions of rural and urban land use, including forested areas, grassed areas, agricultural and residential and commercial areas. The event mean concentrations (EMCs) of nutrients and TSS from sampling stations which have agricultural land use activities upstream were found to be higher. Comparison of site EMCs (SMCs) with published data showed that the SMCs of the nutrients and TSS are generally higher than SMCs reported for forested areas but lower than published SMCs for urban areas. Positive correlations (p<5%) were found between loading and peak flow at locations most impacted by ubanisation or agricultural activities. Correlation between loading and rainfall variables was less distinct. EMC was found to correlate less with rainfall and flow variables compared to pollutant loading. Unlike loading, no consistent pattern exists linking EMC to any particular storm or flow variable in any of the catchments. Lastly, positive correlations were obtained between the particulate forms of nitrogen and phosphorus and TSS.

Characteristics of Monochromatic Waves Breaking over Fringing Reefs

ABSTRACT Yao, Y.; Huang, Z.H.; Monismith, S.G., and Lo, E.Y.M., 2013. Characteristics of monochromatic waves breaking over fringing reefs. A fringing reef is a reef that is directly attached to a shore. Since fringing reefs resemble plane beaches in some aspects, it is important to understand the similarities and discrepancies between the wave breaking over fringing reefs and the wave breaking over plane beaches. With an idealized fringing reef (a plane sloping fore reef and a submerged horizontal reef flat), a series of laboratory experiments were conducted in a wave flume to understand how the reef-flat submergence and the fore-reef slope may affect the characteristics of wave breaking over fringing reefs. The results show that the relative reef-flat submergence (the ratio of the reef-flat submergence to the wave height) is an important factor to characterize most wave-breaking features (the breaker type and location, the surf-zone width, and the incipient breaker depth index). The influence of the fore-reef slope appears to be insignificant in our experimental conditions. The findings in this study can be used to calibrate or improve some existing analytical or numerical models developed for cross-shore wave transformation, wave-induced setup and wave-driven flow over fringing reefs.

Surface-penetrating flexible membrane wave barriers of finite draft

The performance of surface-penetrating flexible membrane wave barriers of finite draft is studied. The interactions of a single membrane and dual membrane systems are examined with various system parameters. The analytical solutions are derived using eigenfunction expansions assuming linear wave theory and small membrane response. Wave transmission is determined for various combinations of membrane draft, protrusion above the water surface, membrane tension and membrane spacing. The numerical solutions are compared with data obtained from experiments conducted. Comparisons of transmission coefficient and energy loss show good agreement with suitably chosen parameters of viscous losses.

PERFORMANCE OF A FLEXIBLE MEMBRANE WAVE BARRIER OF A FINITE VERTICAL EXTENT

This study investigates the interaction of water waves with a vertical flexible membrane of a finite extent which is less than the water depth. The cases considered include that of a membrane descending from the water surface or ascending from the seabed, and that of an immersed membrane with gaps at both the top and the bottom. Linear wave theory and small membrane response are assumed and used. A numerical solution is developed based on eigenfunction expansions for the wave velocity potential and membrane motion. The computed results agree well with the reported results of a rigid barrier and of a flexible membrane spanning the entire depth. It is found that cancellation can occur from wave diffraction across the gap and that arising from the membrane motion resulting in a small transmitted wave. The membrane extent, location in the water column and tension are significant factors governing the wave interaction. The effect of these parameters, as well as wave frequency and wave incidence direction, on the transmitted wave is discussed.

An Experimental Study of Wave-Induced Set-Up Over a Horizontal Reef With an Idealized Ridge

Wave action has been the primary focus of near-shore hydrodynamic studies over decades. The wave-induced set-up due to wave breaking is one of the important factors to consider in determining both water level and mass transport above the reef-top, which has ecological as well as engineering significance. Previous investigations into reef hydrodynamics (including laboratory experiments, field investigations, theoretical analysis and numerical simulations) focused mainly on wave-induced set-up on reefs with a horizontal reef-top (Gourlay, 1996. J. Coastal Eng. 27:161–193). It has been observed that a ridge (reef crest) may be present at the reef edge, but so far we are not aware of any published studies on the effects of this type of ridge on the wave-induced set-up over the reef-top. To understand the role of the ridge in wave-breaking mechanism and wave-induced set-up over the reef, a series of experiments were carried out in a wave flume of 36m long and 0.55m wide, with idealized reef-ridge models being installed at the reef edge to simulate fringing reefs with rectangle ridges. The surface elevations at four locations over the reef were measured with Ultralab sensors (General Acoustics), revealing the variation of wave-induced set-up along the reef. Experimental results are reported for two water depths and eighteen regular wave conditions. Also discussed are the effects of the ridge width on the wave-induced set-up. The focus of this paper will be given to the comparison of wave-induced set-ups with and without the rectangular ridges. Preliminary analysis shows that the ridge controls the water level above the reef-top in a way similar to that a broad crest weir controls the water level in open channel flows. Furthermore, the presence of the ridge is found to alter significantly the wave transformation process near the reef edge, especially the strength of the reflected waves and the locations of breakers. Experiments also show that the wave-induced set-up over reef-top with a ridge is generally much larger than that without a ridge. Finally, an attempt is made to introduce a new dimensionless parameter in order to take the ridge configuration into consideration and achieve a better agreement between experiments and predictions when ridges are present.© 2009 ASME

A preliminary laboratory study of motion of floating debris generated by solitary waves running up a beach

Destructive tsunamis can destroy coastal structures and move huge amounts of tsunami debris. Our current understanding of motion of tsunami debris in tsunami flows is limited. In this paper, we present a preliminary laboratory study of motion of model debris under the action of solitary waves running up a beach. The difference between the waterline of maximum inundation and the final position of debris was examined under various conditions. Effects of solitary wave height, water depth, and the distance of debris source to the shoreline on the maximum inundation, the debris limit, and the final position of debris were examined. In general, the final positions of the debris are different from the waterline at maximum inundation and there is a low possibility that a large amount of debris can be carried by retreating water offshore into the sea.

Seasonal and Interannual Variability of Wet and Dry Spells over Two Urban Regions in the Western Maritime Continent

AbstractDaily rainfall data from two urban regions in Southeast Asia are analyzed to study seasonal and interannual variability of wet and dry spells. The analysis is carried out using 35 years of data from Singapore and 23 years of data from Jakarta. The frequency distribution of wet (dry) spells and their relative contribution to the total number of wet (dry) days and to the total rainfall are studied using 15 statistical indicators. At the annual scale, Singapore has a greater number of wet spells and a larger mean wet spell length compared to Jakarta. However, both cities have equal probability of extreme wet spells. Seasonal-scale analysis shows that Singapore is drier (wetter) than Jakarta during boreal winter (summer). The probability of extreme wet spells is lower (higher) for Singapore than Jakarta during boreal winter (summer). The results show a stronger contrast between Singapore and Jakarta during boreal summer. The study also examined the time series of Singapore wet and dry spell indicators...

Assessing urban growth dynamics of major Southeast Asian cities using night-time light data

ABSTRACT This study analysed urban growth patterns for 15 Southeast Asian cities using remotely sensed night-time light data from 1992 to 2012. We extracted three urban categories (countryside, peri-urban, and core-urban) for each city using objectively derived thresholds from the brightness gradient (BG) approach. The peri-urban and core-urban combined categories were generally found to increase over time for all cities whereas countryside urban category decreased implying strong spatial and temporal trends in urbanization. These trends were also found to be sensitive to geographic characteristics of cities. The study showed that the BG approach can be successfully applied to extract and study growth dynamics of different urban categories for Southeast Asian cities having range of demographic and socioeconomic conditions. The BG derived urban categories compared favourably with Landsat derived impervious areas, where the former was found to envelope the high percentage impervious region derived from the latter. The BG-derived urban areas are lastly compared against the population data to explore linkages with population growth.