Bottom Velocity Research Articles

Environmental factors controlling particulate mass fluxes on the Mallorca continental slope (Western Mediterranean Sea)

Settled material recorded by two near bottom sediment traps deployed from November 2009 to February 2011 at northern (Sóller) and southern (Cabrera) slopes of Mallorca Island (Western Mediterranean) is studied with the aim of discerning their possible origin. The total settled particulate mass fluxes (TMF) at Sóller station were found to be, on average, 2.8 times greater than at Cabrera location during the deployment period, although both time series had a similar temporal evolution. It is suggested that wind episodes affecting the entire area were the common forcing, causing a primary production enhancement and being responsible of the similar temporal behavior. The greater sediment amounts collected in Sóller are explained on the basis of two physical mechanisms: 1) a number of successive eddies generated by instabilities of the Balearic Current that are regularly observed on satellite images, some of which have been reported to reach the seabed, thus increasing near bottom velocities and causing sediment resuspension. And 2) bottom trapped waves that are evidenced from a wavelet analysis in Sóller which could affect the TFM by enhancing sediment resuspension or advecting material from the surrounding areas.

Numerical calculation for bed variation in compound-meandering channel using depth integrated model without assumption of shallow water flow

In a compound meandering channel, patterns of flow structures and bed variations change with increasing water depth owing to complex momentum exchange between high-velocity flow in a main channel and low-velocity flows in flood plains. We have developed a new quasi-three-dimensional model without the shallow water assumption, i.e., hydrostatic pressure distribution; our method is known as the general bottom velocity computation (BVC) method. In this method, a set of depth-integrated equations, including depth-integrated momentum and vorticity equations, are prepared for evaluating bottom velocity and vertical velocity distributions. The objective of this study is to develop a bed variation calculation method for both single and compound meandering channels by using the BVC method coupled with a sediment transport model. This paper shows that the BVC method can reproduce the pattern change of bed variation in a compound meandering channel flow with increasing relative depth. The variation in sediment transport rate due to overbank flow is explained by experimental and computational results.

Interactions between waves, sediment, and turbulence on a shallow estuarine mudflat

Measurements were collected on a shallow estuarine mudflat in northern San Francisco Bay to examine the physical processes controlling waves, turbulence, sediment resuspension, and their interactions. Tides alone forced weak to moderate currents of 10–30 cm s−1 in depths of 0–3 m, and maintained a background suspension of 30–50 mg L−1 of fine sediment. In the presence of wind waves, bottom orbital velocities spanned 20–30 cm s−1, suspended-sediment concentrations (SSC) at 15 and 30 cm above the bed (cmab) increased by 1–2 orders of magnitude, and vertical gradients in SSC were strong enough to produce turbulence-limiting stratification, with gradient Richardson numbers exceeding 0.25. Simultaneously, turbulent stresses (decomposed from wave motions) increased by an order of magnitude. The apparent contradiction of energetic turbulence in the presence of strong stratification was reconciled by considering the turbulent kinetic energy (TKE) budget: in general, dissipation and buoyancy flux were balanced by local shear production, and each of these terms increased during wave events. The classic wave-current boundary layer model represented the observations qualitatively, but not quantitatively since the velocity profile could not be approximated as logarithmic. Rather, the mean shear was elevated by the Stokes drift return flow and wind-generated surface stress, which diffused sediment upward and limited stratification. Our findings highlight a pathway for waves to supply energy to both the production and destruction of turbulence, and demonstrate that in such shallow depths, TKE and SSC can be elevated over more of the water column than predicted by traditional models.

Effects of Friction at Movable Supports on Elasto-Plastic Seismic Responses of Continuous Girder Bridges

The effect of friction on movable supports on the elasto-plastic seismic responses of continuous girder bridges was analyzed on the basis of three aspects including stiffness contribution, friction energy dissipation of the movable supports, and kinetic interaction between the girders and movable piers. Finite element models were established, which considered the friction non-linearity of the movable supports and the material nonlinearity of the reinforced concrete piers. In addition, nonlinear time history analysis was performed to analyze the effects of friction at the movable supports on the pier bottom curvature, girder displacement, velocity, and acceleration. The results indicated that the frictional effect at movable supports is not always favorable. In some cases, the friction at these points must be considered. For the nonlinear time history analysis, the effect of natural vibration characteristics of the bridge and the response spectrum of the ground should also be considered. For some complicated continuous bridges, particularly those with the heights and stiffness of the fixed and movable piers, the friction at movable supports should be considered in seismic analysis.

Basin‐scale circulation and heat fluxes in ice‐covered lakes

A field and theoretical study of the mean circulation patterns, gravity currents, and heat fluxes in a mid‐latitude, ice‐covered lake is presented. Thermistor chains captured the lake‐wide pressure gradients and thermal regime, acoustic instruments measured bottom and interior velocities, and a sediment temperature probe measured sediment temperatures and heat fluxes. Typical Rossby numbers showed that the mean circulation was geostrophic, in the form of either an anti‐cyclonic or a cyclonic basin‐scale gyre. Each gyre reached a steady state after ∼ 10–15 d, circulating at a maximum velocity of ∼ 1 mm s−1. The gyres persisted throughout most of the winter and generally decayed with time. The cyclonic gyre was generated by the lateral temperature gradients created by sediment heat release. It was not possible to determine the driver of the anti‐cyclonic gyre. Because most ice‐covered lakes are controlled by the Earth's rotation, anti‐cyclonic and cyclonic circulation is postulated as being the most common mean interior flow pattern. The horizontal heat fluxes were of the same order as the vertical heat fluxes, which is consistent with the findings of previous research. The sediment heat flux decayed with time, stabilizing after ∼ 60 d. A scaling analysis of down‐sloping advection with dynamic, seasonal‐scale temperatures showed that a warmer lake and an increase in the length‐to‐depth geometric aspect ratio (both within a given lake) and an increase in lake size all increased advection time scales, and therefore circulation time scales, near the boundaries.

Steep Shelf Stabilization of the Coastal Bransfield Current: Linear Stability Analysis

Abstract In situ measurements obtained during the 2010 COUPLING cruise were analyzed in order to fully characterize the velocity structure of the coastal Bransfield Current. An idealized two-layer shallow-water model was used to investigate the various instability processes of the realistic current along the coastal shelf. Particularly studied is how the topographic parameter To (ratio between the shelf slope and the isopycnal slope of the surface current) impacts the growth and the wavelength of the unstable perturbations. For small bottom slopes, when the evolution of the coastal current is controlled by the baroclinic instability, the increase of the topographic parameter To yields a selection of smaller unstable wavelengths. The growth rates increase with small values of To. For larger values of To (To ≳ 10, which is relevant for the coastal Bransfield Current), the baroclinic instability is strongly dampened and the horizontal shear instability becomes the dominant one. In this steep shelf regime, the unstable growth rate and the wavelength selection of the baroclinic coastal current remains almost constant and weakly affected by the amplitude of the bottom velocity or the exact value of the shelf slope. Hence, the linear stability analysis of an idealized Bransfield Current predicts a typical growth time of 7.7 days and an alongshore scale of 47 km all along the South Shetland Island shelf. The fact that these large growth times are identical to the typical transit time of water parcels along the shelf may explain why the current does not exhibit any unstable meanders.

Theoretical and experimental investigation of the performance of back-pass solar air heaters

This paper presents a study dealing with the experimental and theoretical analysis of a flat plate solar air heater. The air collectors were tested experimentally for the tilt angle of 35$^{\circ}$ and 2 m $s^{-1}$ air velocity, and ambient temperature, inlet temperature, outlet temperature, absorber plate temperature, bottom plate temperature, solar radiation, air velocity, and airflow velocity from the duct were measured. The outlet air temperature and energy and exergy efficiencies of the collector were calculated theoretically. The results showed that the collector has the maximum mean outlet temperature for the airflow velocity of 1 m $s^{-1}$, duct height of 0.001 m, triple glass cover, and length of 3 m; the maximum mean energy efficiency for the airflow velocity of 4 m $s^{-1}$, duct height of 0.04 m, triple glass cover, and length of 1 m; and the maximum mean exergy efficiency for the airflow velocity of 1 m $s^{-1}$, duct height of 0.005 m, triple glass cover, and length of 3 m. This study demonstrated the superiority of exergy analysis over energy analysis before the decision regarding design parameters.

Variability of Sand Mobility Surrounding Cylinder Object Freely Resting on the Seabed under the Action of Typhoon

In sandy sediments, scour and fill is the key process contributed to mine burial. The scour processes surrounding the cylinder mines freely resting on the sandy seabed under the 12-hr combined action of tidal currents and wind-generated waves, especially over typhoon events are numerically simulated using the DRAMBUIE model. The East China Sea is a good case study due to the dominant impact of summer typhoon events on sediment transport and scour. The numerical results show that the scour depth generally increases with time under the combined current and wave stresses exerted on the seabed, while the depth of the scour pit depends on infill once the currents subside. There is a positive relationship between the scour depth and the bottom orbital velocity after experiencing 12-hr wave action including storm waves, while the relation is not linear. The experimental results also display an elevated trend for scour depth with the increase of orbital velocity. The numerical results reveal a surprising phenomenon: the mobility of sand altering with the increasing bed shear stress larger than the certain threshold, which is also manifested as the curves of scour depth with the different grain size might cross each other. For laboratory experiments, the variability of sand mobility does not occur, likely because typhoon storm waves cannot be reproduced in the flume. More numerical tests indicate that the intersection will be triggered by the division of critical Shields parameter. The preliminary analysis suggests that the phenomenon never documented is likely generated from the error of empirical formulae.

A Hydropower Station Energy Dissipation Shape Optimization Test

In this study, hydraulic model testing is used to optimize the flood discharge and energy dissipation body type of some power plant. Compared from the flow regime, bottom velocity, dynamical pressure and so on, it is found that the fluctuating pressure and bottom velocity at the exit side wall stilling pool bottom, are impoved by surface outlet contraction and intermediate outlet contraction, the largest bottom flow rate of surface outlet is about 13.6m/s, the maximum fluctuating pressure of stilling basin is approximately 6.7×9.81kPa. The problem of large bottom flow rate and fluctuating pressure are solved by this testing result, the result provided references for design.

Experimental investigation of the impacts of coherent flow structures upon turbulence properties in regions of crescentic scour

ABSTRACTThis study examines the spatial distributions of third‐order moments of velocity fluctuations, the turbulent kinetic energy (TKE) fluxes, and the conditional statistics of Reynolds shear stress across the equilibrium crescentic scour structures generated upstream of short horizontal static cylinders. Detailed velocity data were collected using three‐dimensional (3D) micro‐acoustic Doppler velocimeter (ADV) across and within the equilibrium scour marks. The analysis reveals that the positive and negative values of third‐order moments associated with the level bed surface and the scour holes are directly related to coherent structures. The components of TKE flux are discussed for the near‐bed region of the level bed surface and scour holes in relation to sweep–ejection events. A cumulant‐discard method is applied to the Gram‐Charlier probability distribution of two variables to describe the statistical properties of the term u′w′. The conditional statistics of the Reynolds shear stress show a good agreement with the experimental data. The distribution of the joint probability density function in the near‐bed region changes cyclically along the scour hole depending on the bottom fluid velocity, which implies a change from upward to downward flux of momentum and vice versa. Both the ejection and sweep events at near‐bed points on the level surface are more important than within the scour region; and in contrast, both events are stronger for the scour marks than the level bed surface at the outer layer. Sweeps dominate over ejections for the scour hole induced by smaller diameter and ejections dominate for larger diameter. Copyright © 2013 John Wiley & Sons, Ltd.

Physical influence on biological production along the western shelf of Madagascar

In September 2009, the R.V. Dr. Fridtjof Nansen surveyed the western coast of Madagascar. Environmental parameters of temperature, salinity, fluorescence and oxygen were profiled with a CTD probe and continuously underway at 5m utilising a thermosalinograph equipped with a fluorescence sensor. A ship mounted Acoustic Doppler Current Profiler (ADCP) provided current profiles down to 250m, while estimates of biomass were obtained from acoustics and trawling was used for species identification. In addition, visual whale observations were conducted. The survey revealed three areas that were identified as upwelling regions, namely the Southern Coast (26°S), offshore from Cap St. André (16°S) and near Nosy Be Island (13°S). In these upwelling regions, acoustic estimates, trawling and whale observations indicated high biological productivity. The total acoustic estimate for the whole western coast was as low as 62000t, typical for tropical waters. In addition to the upwelling areas, high biological productivity was also found outside river mouths. Ship born wind measurements, as well as re-analysed wind fields, indicated that the southern coast upwelling cell was wind-driven and had a larger extent than reported earlier. The wind conditions were not favourable for upwelling in the two northernmost upwelling cells. Here the ADCP showed high bottom velocities (>1ms−1) oriented northeast. These currents were probably forced by the migrating eddies in the area as indicated by the remotely sensed Sea Level Anomaly (SLA). Such currents induce bottom friction layer transport oriented towards the coast, thus driving upwelling, although not necessarily penetrating all the way to the surface layer as was the case near Cap St. André.

주변지형을 포함한 3차원 하상단면 표출에 관한 연구

최근 기후변화 및 대규모 국가사업으로 인한 하도준설, 보 운영 등의 하천환경 변화는 유역 내 수리 수문현상에 많은 영향을 끼칠 것으로 예상되며, 이로 인하여 하도의 물리적 변화에 따른 하상 및 지형 변동 예측, 하도와 시설물의 운영 및 유지관리 등에 대한 연구가 필요할 것으로 예상된다. 이를 위해 본 연구에서는 첨단 기술을 이용하여 2차원 위치와 수심에 대한 정밀한 측정을 수행하고 이를 바탕으로 3차원 하상단면을 표출하는 연구를 수행하였다. 정밀 측정을 위한 장비로는 자유롭게 이동하며 측정할 수 있는 하천유량 계측보트인 R2V2 (River Robot for Velocity and Volume)를 이용하였다. R2V2는 ADCP센서와 RTK-GPS 모듈을 장착하여 실시간으로 하천 전단면에 대한 유속, 유량, 하상 등에 대한 자료를 측정할 수 있고, RTK-GPS를 이용하여 정밀한 위치측정이 가능한 장비이다. 향후 본 연구결과를 바탕으로 3차원 하상단면을 표출한다면, 하천 횡단 또는 하천 내 구조물 계획 수립시 주변지형 뿐 아니라 하천 지형도 고려하는 계획 수립이 이루어질 수 있을 것으로 판단된다. Recently the environmental change in river such as channel dredging and weir operation caused by climate change and large scale national project should affect hydraulic and hydrological phenomena. Therefore, study for topographical change and operation/ maintenance of facilities in river is necessary. In this study, the method of three dimensional (3D) expression of river bed was proposed using detailed measurement in section with the advanced technique. The remote controlled boat called River Robot for Velocity and Volume (R2V2) was used to measure the detailed measurement. A remote controlled boat R2V2 equipped with ADCP and RTK GPS were integrated to measure bottom geometry, velocity profile, and river discharge simultaneously. Therefore, if river bed is expressed in 3D based on this study, it is possible to plan considering surrounding and river topography when planning of river transverse facilities or structures in river.

Simulating sediment transport processes in San Pablo Bay using coupled hydrodynamic, wave, and sediment transport models

Abstract San Pablo Bay is a tidal sub-embayment of the San Francisco Estuary which is dominated by broad shallow shoals bisected by a deep channel. Under the conceptual model of sediment transport in San Pablo Bay proposed by Krone (1979) , sediment typically enters San Pablo Bay during large winter and spring flows and is redistributed during summer conditions through wind wave resuspension and transport by tidal currents. Numerical simulations of sediment resuspension due to wind waves and the subsequent transport of this sediment by tidal currents show that sediment concentrations and fluxes throughout the channel–shoal system result from a complex temporal and spatial interaction of the waves and tides. The three-dimensional UnTRIM San Francisco Bay-Delta Model was coupled with the Simulating WAves Nearshore (SWAN) wave model and the SediMorph morphological model, to develop a three-dimensional (3D) hydrodynamic, wind wave, and sediment transport model of San Francisco Bay and the Sacramento–San Joaquin Delta. The coupled model was validated using water level, velocity, wind waves and suspended sediment data collected in San Pablo Bay, and then used to quantify the spatial and temporal variability of sediment fluxes on the extensive shoals in San Pablo Bay under a range of tidal and wind conditions. The model validation shows this modeling system can accurately predict hydrodynamics, waves, and suspended sediment concentration in San Pablo Bay. The predicted bottom orbital velocities are elevated across the shoals during large wave events regardless of tidal stage, but near low tide orbital velocities are increased even under low to moderate waves. Sediment fluxes between the shoals and the deeper channel are highest during spring tides, and are elevated for up to a week following wave events, even though the greatest influence of the wave event occurs abruptly.

Contourites in the Gulf of Cadiz: a cautionary note on potentially ambiguous indicators of bottom current velocity

Facies associations in cores collected in the deep part of the Gulf of Cadiz, which is under the influence of the lower branch of the Mediterranean Outflow Water, are investigated in terms of the classical contourite model using grain-size analyses and thin sections of indurated sediment. Cores include both low-energy (contourite drift) and high-energy (channel) environments. The thin sections and grain-size distributions show that clayey fine silts and sandy coarse silts are the most common facies associations in the studied contourite sequences, while coarse-grained, gravelly contourites are less common. Grain-size distributions are unimodal in the fine-grained and bi- or trimodal in the coarser-grained contourites. This change in grain-size composition is related both to the partial removal of the fine-grained fraction and to the replenishment of the coarser-grained one. In addition, most of the contacts between individual facies are sharp rather than transitional. This suggests that the contourite sequence is only in part related to changes in bottom current velocity and flow competency, but may also be related to the supply of a coarser terrigeneous particle stock, provided by either increased erosion of indurated mud along the flanks of confined contourite channels (mud clasts), or by increased sediment supply by rivers (quartz grains) and downslope mass transport on the continental shelf and upper slope. The classical contourite facies association may therefore not be solely controlled by current velocity, but may be the product of a variety of depositional histories. The classical contourite depositional sequence should therefore be interpreted with greater care and in the light of the regional sedimentological background. In addition, the wisdom of exclusively using mean or modal particle size for the interpretation of depositional contourite processes is questioned. Instead, it is proposed that the vertical evolution of grain-size populations in the facies successions forming contourite sequences be assessed.

Time-course of behavioural changes induced by ethanol in zebrafish (Danio rerio)

The zebrafish has been proposed for the study of the effects of ethanol on the vertebrate brain. Behavioural tests have been successfully employed in the phenotypical characterization of these effects. However, the short scale (minute to minute) time course of ethanol induced changes of zebrafish behaviour has not been analyzed. The current study alleviates this need using a 2×3 chronic×acute ethanol exposure experimental design. We first expose zebrafish to ethanol chronically using a dose escalation procedure in which fish are kept in a final concentration of 0.5% vol/vol ethanol for 10 days while control fish receive identical dosing procedures but no ethanol. Subsequently, we expose zebrafish for 1h to an acute dose of ethanol (0.00, 0.50, or 1.00% vol/vol) and monitor their behaviour throughout this period. We quantify the mean and within-individual temporal variance of distance travelled, distance from bottom and angular velocity using video-tracking, and establish temporal trajectories of ethanol induced behavioural changes in zebrafish. For example, we find fish of the highest acute dose group previously not exposed to chronic ethanol to exhibit an inverted U shaped temporal trajectory in distance travelled (biphasic alcohol effect). We find this response to be blunted after chronic ethanol exposure (development of tolerance). We also describe an acute ethanol withdrawal induced increase in angular velocity. We conclude that temporal analysis of zebrafish behaviour is a sensitive method for the study of chronic and acute ethanol exposure induced functional changes in the vertebrate brain

Effects of substrate and water velocity on migration by early-life stages of kaluga, Huso dauricus (Georgi, 1775): an artificial stream study

Summary The effect of two environmental factors (substrate type and water velocity regime) was studied in the ontogenetic migration intensity of kaluga, Huso dauricus, a protected species in the Amur River. Early-life stages studied were: free embryos = days 0–8; larvae = days 9–49; and juveniles = days 50–66. Effect of treatments on fish migration intensity was observed in circular channels, allowing migration in an endless stream. Daily migration intensity of fish was characterized by counting the number of daily upstream or downstream fish passes past a specific site in the channel observed for 5 min every hour by vision or video camera. The hypothesis that substrate type (bare bottom or sand-small cobble) affects migration intensity was accepted, depending on the life stage. For example, the substrate type had no effect on migration intensity of days 0–6 free embryos. However, the intensity in days 7–8 free embryos and days 9–29 larvae was greater over rock substrate, while intensity in days 46–49 larvae and days 50–66 juveniles was greater when the bottom was bare. Thus, the effect of the substrate on migration varied in intensity according to the life stage, and within the larva life stage, by age. The velocity regime had a positive effect on migration intensity of free embryos, but a high velocity (mean, 29.9 cm∙s−1) delayed the resting period of day 8 free embryos and day 9–15 larvae. Free embryos and larvae in low vs fast velocities showed that they may have a drive to migrate a similar distance, and moreover, that a triggering velocity may be needed to initiate or to stop migration. The hypothesis was accepted that the rearing velocity affects migration intensity, e.g. larvae reared in still water and then placed in moving water had a compensatory migration intensity response. For population enhancement stocking of H. dauricus, the results indicate culture practices must insure that: (i) same-population individuals are mated to produce early-life stages with the correct behaviour, (ii) migrating larvae, not juveniles, should be stocked, and (iii) larvae should be released in a river reach with a bottom velocity ≥30 cm∙s−1 containing sand and small pebbles.

Microhabitat use of the diamond darter

AbstractThe only known extant population of the diamond darter (Crystallaria cincotta) exists in the lower 37 km of Elk River, WV, USA. Our understanding of diamond darter habitat use was previously limited, because few individuals have been observed during sampling with conventional gears. We quantified microhabitat use of diamond darters based on measurements of water depth, water velocity and per cent substrate composition. Using spotlights at night‐time, we sampled 16 sites within the lower 133 km of Elk River and observed a total of 82 diamond darters at 10 of 11 sampling sites within the lower 37 km. Glides, located immediately upstream of riffles, were the primary habitats sampled for diamond darters, which included relatively shallow depths (<1 m), moderate‐to‐low water velocities (often < 0.5 m·s−1) and a smooth water surface. Microhabitat use (mean ± SE) of diamond darters was estimated for depth (0.47 ± 0.02 m), average velocity (0.27 ± 0.01 m·s−1) and bottom velocity (0.15 ± 0.01 m·s−1). Substrate used (mean ± SE) by diamond darters was predominantly sand intermixed with lesser amounts of gravel and cobble: % sand (52.1 ± 1.6), % small gravel (12.2 ± 0.78), % large gravel (14.2 ± 0.83), % cobble (19.8 ± 0.96) and % boulder (1.6 ± 0.36). Based on our microhabitat use data, conservation and management efforts for this species should consider preserving glide habitats within Elk River. Spotlighting, a successful sampling method for diamond darters, should be considered for study designs of population estimation and long‐term monitoring.

Cross-Shelf Dynamics in a Western Boundary Current Regime: Implications for Upwelling

Abstract The cross-shelf dynamics up- and downstream of the separation of the South Pacific Ocean’s Western Boundary Current (WBC) are studied using two years of high-resolution velocity and temperature measurements from mooring arrays. The shelf circulation is dominated by the East Australian Current (EAC) and its eddy field, with mean poleward depth-integrated magnitudes on the shelf break of 0.35 and 0.15 m s−1 up- and downstream of the separation point, respectively. The high cross-shelf variability is analyzed though a momentum budget, showing a dominant geostrophic balance at both locations. Among the secondary midshelf terms, the bottom stress influence is higher upstream of the separation point while the wind stress is dominant downstream. This study investigates the response of the velocity and temperature cross-shelf structure to both wind and EAC intrusions. Despite the deep water (up to 140 m), the response to a dominant along-shelf wind stress forcing is a classic two-layer Ekman structure. During weak winds, the shelf encroachment of the southward current drives an onshore Ekman flow in the bottom boundary layer. Both the bottom velocity and the resultant bottom cross-shelf temperature gradient are proportional to the magnitude of the encroaching current, with similar linear regressions up- and downstream of the WBC separation. The upwelled water is then subducted below the EAC upstream of the separation point. Such current-driven upwelling is shown to be the dominant driver of cold water uplift in the EAC-dominated region, with significant impacts expected on nutrient enrichment and thus on biological productivity.

Resolution analysis of the inverse problem for geoacoustic experiment design

This work explores effects of experiment geometry and array configuration on the resolving power of a continuous ocean bottom geoacoustic inverse problem in a shallow water environment. The uncertainty and resolution of this problem, in which ocean bottom P-wave velocities as a function of depth are estimated from noisy acoustic pressure waveforms received on vertical and horizontal line arrays in the water, can be investigated before the experiment is conducted, allowing one to improve or optimize the problem parameters to best configure an experiment during its planning phase. In this work, the resolution results of complete synthetic geoacoustic inversions at varying geometries and array configurations are compared with resolution results at various candidate seabottom profiles, initially using standard techniques of linearized inverse theory. Singular value decomposition is used to interpret the tie between geometry and regularization in the inverse problem, which directly affects the resolution. Then, additional comparisons and analysis address the nonlinearity of the problem, which causes a dependence of the resolution results on the bottom profile being solved for—which is unknown, and Monte Carlo analysis is used to show where the linearity approximation breaks down in the resolution results. [Work partially funded by ONR.]

A numerical simulation of metallic cylindrical sandwich shells subjected to air blast loading

The dynamic response of cylindrical sandwich shells with aluminum foam cores subjected to air blast loading was investigated numerically in this paper. According to KNR theory, the nonlinear compressibility of the air and finite shock conditions were taken into account in the finite element model. Numerical simulation results show that the compression strain, which plays a key role on energy absorption, increases approximately linearly with normalized impulse, and reduces with increasing relative density or the ratio of face-sheet thickness and core thickness. An increase of the impulse will delay the equalization of top and bottom face-sheet velocities of sandwich shell, but there is a maximum value in the studied bound. A limited study of weight optimization was carried out for sandwich shells with respect to the respective geometric parameters, including face-sheet thickness, core thickness and core relative density. These numerical results are of worth to theoretical prediction and engineering application of cellular metal sandwich structures.