Froude Number Fr Research Articles

Quantifying the transition of impact mechanisms of geophysical flows against flexible barrier

Flexible barriers have been increasingly used worldwide for mitigating natural hazards involving various geophysical flows. The analysis and design of flexible barriers need to distinguish two major impact mechanisms, runup and pile-up, which remain poorly understood due to complicated interactions between the impacting flow and the deformable barrier. In this study, a unified computational approach based on coupled computational fluid dynamics and discrete element method (CFD-DEM) is employed to examine the impact mechanisms of a wide spectrum of geophysical flows against a deformable and permeable flexible barrier. We consider geophysical flows including rock avalanche, debris avalanche, debris flow, debris flood and mud flow, and a flexible barrier consisting of a barrier net, cables and brakes. The signatures of runup and pile-up mechanisms are thoroughly analyzed, in terms of flow features and barrier responses. The effects of Froude numbers (Fr = 0.46–7.40), solid volume concentrations (0.1–1), and fluid rheologies (Newtonian and non-Newtonian fluids) on the impact mechanism transitions are systematically examined. Two nondimensional indices, the static-peak load ratio of the barrier and the momentum reduction ratio of the flow, are proposed for identifying the transition of impact mechanism from pile-up to runup. The transition is found to occur with either an increase in Fr or solid volume concentration of the flow. Increased viscosity of the fluid in a solid-fluid mixture may result in a transition occurring at a higher Fr. This study helps gain insights into the two impact mechanisms and their transitions and may provide a useful reference for the future design of flexible barriers in mitigating hazardous geophysical flows.

Stratified flow past a sphere at moderate Reynolds numbers

A numerical study of stably stratified flows past spheres at Reynolds numbers Re=200 and Re=300 is reported. In these flow regimes, a neutrally stratified laminar flow induces distinctly different near-wake features. However, the flow behaviour changes significantly as the stratification increases and suppresses the scale of vertical displacements of fluid parcels. Computations for a range of Froude numbers Fr∈[0.1,∞] show that as Froude number decreases, the flow patterns for both Reynolds numbers become similar. The representative simulations of the lee-wave instability at Fr=0.625 and the two-dimensional vortex shedding at Fr=0.25 regimes are illustrated for flows past single and tandem spheres, thereby providing further insight into the dynamics of stratified flows past bluff bodies. In particular, the reported study examines the relative influence of viscosity and stratification on the dividing streamline elevation, wake structure and flow separation. The solutions of the Navier-Stokes equations in the incompressible Boussinesq limit are obtained on unstructured meshes suitable for simulations involving multiple bodies. Computations are accomplished using the finite volume, non-oscillatory forward-in-time (NFT) Multidimensional Positive Definite Transport Algorithm (MPDATA) based solver. The impact and validity of the numerical approximations, especially for the cases exhibiting strong stratification, are also discussed. Qualitative and quantitative comparisons with available laboratory experiments and prior numerical studies confirm the validity of the numerical approach.

Two-Analytical Comparison on Force Measurements and The Wave Pattern

This work presents a comparison of two analytical methods on the components of resistance, transverse and divergent waves, and the profile and contour of the waves. Michell’s thin linearization theory is applied to estimate the wave resistance and analysis of the method compared to the solution of the wave resistance coefficient based on the Rankine source method. The hull of series 60 is utilized as the initial hull, and varying the shape of the bow to the mid-ship with the hull offset variable. Computation of the wave resistance coefficient at low Froude number (Fr<0.3) shows the results of the wave resistance from the thin ship theory are higher, but it gives a graphical agreement with the Rankine source method. The wave profile provides a very similar profile line for both of them. However, the thin ship theory does not establish a significant difference in the wave profile from the initial hull of the improved hull variation. Comparison of contour displays is presented in different versions of the wave contour. Still, it affords a good contour match on the initial hull and the results of hull variations from both methods. At high Froude number (Fr>0.7), the initial hull and variations in the hull have minimal effects with small differences in wave resistance, profile and contour.

Influence of particle residence time distribution on the biomass pyrolysis in a rotary kiln

Rotary kilns are widely used in industry to perform several different operations. When numerically modeling such systems, a common simplification is to neglect the residence time distribution of particles. The present study numerically investigates the influence of particle residence time distribution on the pyrolysis of biomass in a co-current heated rotary kiln. First, the model (balance equations, reaction scheme, flow model, and heat transfer mechanisms) is developed. The applied particle flow model (Seaman's model) is then validated by comparing the calculated mean residence time τ and bed height to the results of a discrete element method (DEM) simulation of an example case. The mean residence time (τ = 552 s) and the bed height (H(z=0) = 52 mm) of the Seaman's model was found to be in good agreement with the results of the DEM simulation (τ = 542 s, H(z=0) = 50 mm).A detailed discussion of the residence time distribution's influence on the simulated conversion for the example case is then presented. We performed 54 simulations with various Péclet numbers between 1 and 100 and showed that the product yield significantly changes for Péclet numbers below 10, that is, neglecting residence time distribution (RTD) may not be a true assumption in all operating conditions.Finally, performing a comprehensive parameter study, the RTD's influence under various operating conditions and kiln designs is investigated. The varied parameters are the kiln diameter (D = 0.1–1 m) the ratio of particle to kiln diameter (d/D = 5 × 10−3–40 × 10−3), the ratio of kiln length to kiln diameter (L/D = 1–10), the kiln's inclination angle (β = 0.1–8∘), the Froude number (Fr = 10−3–10−2), the rotational Reynolds number (Re = 102–16 × 103), and the Péclet number (Pe = 5–100). Using more than 13000 simulations, we show that in one third of the investigated cases the change of product yield is higher than 2% when changing from plug flow behavior to high axial dispersion (Pe = 5). For cases where the influence of the RTD on the final product yield is small, conditions along the kiln might still change significantly. The biggest change of the product yield from plug flow behavior to high axial dispersion was found to be 5.5 wt% for the following conditions: D = 1 m, d/D =5 × 10−3, L/D = 1, β = 0.1, Fr = 10−2 and Reω = 16,000.

Experimental assessment of the effective friction at the base of granular chute flows on a smooth incline.

We report on direct measurements of the basal force components for granular material flowing down a smooth incline. We investigate granular flows for a large range of inclination angles from θ=13.4^{∘} to 83.6° and various gate openings of the chute. We find that the effective basal friction coefficient μ_{B}, obtained from the ratio of the longitudinal force to the normal one, exhibits a systematic increase with increasing slope angle and a significant weakening with increasing particle holdup H (the depth-integrated particle volume fraction). At low angles, the basal friction is slightly less than or equal to tanθ.The deviation from tanθcan be interpreted as a contribution from the sidewall to the overall friction. At larger angles, the basal friction μ_{B} saturates at an asymptotic value that is dependent on the gate opening of the chute. Importantly, our data confirm the outcomes of recent discrete numerical simulations. First, for steady and fully developed flows as well as for moderately accelerated ones, the variation of the basal friction can be captured through a unique dimensionless number, the Froude number Fr, defined as Fr=U[over ¯]/(gHcosθ)^{1/2}, where U[over ¯] is the mean flow velocity. Second, the mean velocity scales with the particle holdup H with a power exponent close to 1/4, contrasting with the Bagnold scaling (U[over ¯]∼H^{3/2}).

Flow patterns and free-surface dynamics in hydraulic jump on pebbled rough bed

Some basic characteristics of a classic hydraulic jump flow over a pebbled rough bed, as well as on a smooth bed as a reference, are presented in this experimental study. For the experiments, an inflow Froude number Fr1from 1.54 to 4.94 and inflow Reynolds number Re1from 42 000 to 230 000 were considered. Visual observations and measurements suggested some differences between the formation of a hydraulic jump on rough and smooth bed configurations, including different air entrainment processes, larger vortical structures in the roller length and stronger backward flow in the upper layer. Furthermore, the jump roller and aerated flow lengths were shorter on a pebbled rough bed than on a smooth bed, while the dimensionless advection velocity of large vortices was the same for both bed types. The instantaneous jump toe perimeter showed the largest variation at the largest Fr1and was generally larger on rough bed than on smooth bed. Larger oscillations of the free-surface profile were observed on smooth bed, highlighting that roughness resulted in smaller free-surface oscillations, suggesting the higher rate of energy dissipation.

Breaking bore roller characteristics: Turbulence statistics using optical techniques

Surface wave breaking induces strong turbulence in the two-phase flow region. Detailed turbulence statistics were experimentally obtained using non-intrusive optical techniques in a breaking bore roller, at relatively large scale, with a bore Froude number Fr1 = 2.15 and Reynolds number Re = 2.3 × 105. These novel velocity data were ensemble-averaged based upon an instantaneous dataset of 24,320 images. In terms of the velocity field, the breaking bore roller was classified into three regions: the impinging jet, developing shear layer and flow reversal region. The vertical profiles of the longitudinal velocity data exhibited some self-similarity. The Reynolds stress data showed an anisotropic turbulent flow immediately downstream of the roller toe, and tended towards isotropy away from the roller toe. The vorticity data suggested that the breaking at the roller toe was responsible for the generation of vortices. The turbulent structures in the shear layer presented significantly smaller length and time scales with higher dissipation rate than other regions. A discussion between present turbulence statistical data and bubble dynamics from literature was developed. The comparison between present and past studies suggested a similarity in two-phase physical processes in the breaking roller region between the tidal bore, hydraulic jump, swash zone bore and breaking wave.

Rotational Mass Shedding from Asteroid (6478) Gault

Abstract The ∼4 km diameter main belt asteroid 6478 Gault has ejected dust intermittently since at least 2013. The character of the emission, including its episodic nature and the low speed of the ejected particles (V ∼ 0.15 m s−1), is most consistent with mass loss from a body rotating near rotational breakup. Owing to dust contamination of the nucleus signal, this conclusion had not yet been confirmed. To test this idea, we have obtained new images of Gault in 2020 August in the absence of dust. Our photometry shows a lightcurve with a very small amplitude (maximum ∼0.05 mag) and a periodicity of 2.55 ± 0.10 hr. The new observations are consistent with a model in which Gault is rotating near breakup, with centrifugal forces responsible for its episodic mass loss. Approximated as a strengthless (fluid) spherical body, the implied density is ρ = 1700 kg m−3. We use the Froude number Fr, defined here as the ratio between centrifugal force and gravitational force, as a way to investigate mass-loss regimes in fast-spinning asteroids and find that mass shedding starts at Fr ∼ 0.5.

Scaling effects on the free surface backward facing step flow

A set of large eddy simulations for the free surface backward facing step (FSBFS) are carried out to study wave formation behind the step. The volume-of-fluid ghost fluid method is employed to capture the free surface. Previous studies have indicated that the wave physics depend on the step draught-based Froude number (Fr). For small Fr, the rear face of the step (transom) becomes wet, while for large Fr, the wave separates smoothly from the transom. Close to a critical Fr separating wet and dry transoms, both conditions may occur. Here, we study wet, critical, and dry conditions based on the Fr classification with three different inflow boundary layer profiles (ReL=1,2,3×106). For Fr = 1.75 (wet conditions), we observe a weak dependence on the ReL. A proper orthogonal decomposition of the velocity field at Fr = 1.75 shows a coherent vortex street forming beneath the free surface. At Fr = 2.66 (critical conditions), we observe that an increase in the ReL results in a decrease in the wavelength and pronounced gas entrainment due to wave breaking. For Fr = 3.17 (dry conditions), we also observe shorter wavelength at increased ReL. Further, in the dry conditions, a breaking wave is noticed to occur at higher ReL, while breaking waves are not observed for the smallest studied ReL. Based on the results, we conclude that the wave shape for FSBFS cannot be characterized by the Froude number alone.

Impacts of native vegetation on the hydraulic properties of the concentrated flows in bank gullies

To quantify the impacts of native vegetation on the spatial and temporal variations in hydraulic properties of bank gully concentrated flows, a series of in situ flume experiments in the bank gully were performed at the Yuanmou Gully Erosion and Collapse Experimental Station in the dry-hot valley region of the Jinsha River, Southwest China. This experiment involved upstream catchment areas with one- and two-year native grass (Heteropogon contortus) and bare land drained to bare gully headcuts, i.e., Gullies 1, 2 and 3. In Gully 4, Heteropogon contortus and Agave sisalana were planted in the upstream catchment area and gully bed, respectively. Among these experiments, the sediment concentration in runoff in Gully 3 was the highest and that in Gully 2 was the lowest, clearly indicating that the sediment concentration in runoff obviously decreased and the deposition of sediment obviously increased as the vegetation cover increased. The concentrated flows were turbulent in response to the flow discharge. The concentrated flows in the gully zones with native grass and bare land were sub- and supercritical, respectively. The flow rate and shear stress in Gully 3 upstream catchment area were highest among the four upstream catchment areas, while the flow rate and shear stress in the gully bed of Gully 4 were lowest among the four gully beds, indicating that native grass notably decreased the bank gully flow rate and shear stress. The Darcy-Weisbach friction factor (resistance f) and flow energy consumption in the gully bed of Gully 4 were notably higher than those in the other three gully beds, clearly indicating that native grass increased the bank gully surface resistance and flow energy consumption. The Reynolds number (Re), flow rate, shear stress, resistance f, and flow energy consumption in the gully beds and upstream areas increased over time, while the sediment concentration in runoff and Froude number (Fr) decreased. Overall, increasing vegetation cover in upstream catchment areas and downstream gully beds of the bank gully is essential for gully erosion mitigation.

KAJIAN NUMERIK FENOMENA UNDULAR TIDAL BORES DALAM MEMPENGARUHI PROSES EROSI PADA DAERAH ALIRAN SUNGAI (Numerical study of undular tidal bores phenomenon in influencing erosion processes of watersheds)

Riverbank erosion is one indication of watershed damage. One of the causes is the phenomenon of tidal bores waves that occur in a river channel.The strength of tidal bores wave's can be measured based on its shear force parameter and dissipation energy. Wave shear force and dissipation energy are the parameters that play a role in a riverbank erosion process. Both of them are characterized by the Froude number (Fr) which is a function of the upstream river flow velocity (V0), the tidal bores flow velocity from the estuary (Vb), the river depth (h1), and the gravity acceleration (g). A numerical study of the phenomenon of undular tidal bores has been carried out in this article. Five undular bores simulations have been built using the open-source Computational Fluid Dynamics (CFD) software, OpenFOAM. This study aims to analyze the effect of the Froude number variations (Fr) on the magnitude of the wave shear coefficient (ϵ) and dissipation energy ( ) on undular bores cases. Five simulations of undular bores have been generated based on five Froude's numbers, Fr = 1.0, 1.1, 1.2, 1.3, and 1.4. The validation has been performed by comparing the experimental and numerical results from the scientific literature. The analysis results show that the increase in Fr has a significant effect on the increase in the ϵ and .These results indicate that the Froude number variations have influenced the wave shear coefficient and dissipation energy on the undular bores cases. Increasing the Fr values have triggered an increase in the value of ϵ linearly and exponentially. Thus, the erosion that occurs on the riverbank in the undular tidal bores phenomenon could be determined based on Froude's number.

Simulations of stably stratified flow past two spheres at Re = 300

Flows past two spheres immersed in a horizontally moving, linearly stratified fluid are investigated at a moderate Reynolds number of 300. Characterization of flow patterns considers representative geometrical configurations defined by varying both the distance between the spheres and their relative orientation to the free stream direction. Simulations are performed on unstructured meshes chosen to accurately resolve the dynamics of fluids in regions close to the spheres for Froude numbers Fr∈[0.25,∞]. Results illustrate the evolution of boundary layers, separation, and the wakes interaction under the influence of a gravity induced buoyancy force. Computations utilize a limited area, nonhydrostatic model employing non-oscillatory forward-in-time integration based on the multidimensional positive definite advection transport algorithm. The model solves the Navier–Stokes equations in the incompressible Boussinnesq limit, suitable for describing a range of mesoscale atmospheric flows. Results demonstrate that stratification progressively dominates the flow patterns as the Froude number decreases and that the interactions between the two spheres' wakes bear a resemblance to atmospheric flows past hills.

Influence of solid–fluid interaction on impact dynamics against rigid barrier: CFD–DEM modelling

Both the solid and fluid phases in a debris flow vitally influence the impact dynamics against a rigid barrier. However, previous numerical and analytical models commonly adopt an equivalent fluid approach, which does not explicitly consider solid–fluid interaction when impacting a barrier. This study investigates the role of solid–fluid interaction on the impact mechanism and load exerted on a rigid barrier. A coupled computational fluid dynamics and discrete-element method (CFD–DEM) is used to study the effect of solid–fluid interaction on impact by varying the solid fraction from 0 to 0·5. The mesoscopic insight from CFD–DEM modelling reveals that the particle–fluid interaction plays two roles in increasing the kinematic energy of particles: (a) imposing driving force to the particles; (b) reducing inter-particle contact forces and therefore energy dissipation from shearing among grains by applying buoyancy to the particles. Consequently, the run-up height and impact pressure of the water–particle mixture flows at the barrier are higher than those of dry granular flows with the same Froude number (Fr). While impacting the rigid barrier, most of the energy of water–particle mixtures (exhibiting a run-up mechanism) is dissipated as the fluid phase segregates from the mixture and rolls back towards subsequent flow. This differs from the conventional impact mechanism observed for dry granular flows, where the energy is mainly dissipated by way of shearing between layers of sand piling up behind a barrier.

Turbulence Characteristics in Submerged Hydraulic Jump Below Silled Sluice Gates besides Studying alos The Scale Effects in Free Efflux Below Gate of Variable Openings.(Dept.C)

In the present research, an experimental set up was arranged to investigate the scale effects in the free efflux below the sluice gate of variable openings and also to study the turbulence structure in the generated submerged hydraulic jump below the silled gate. The objective of the present study research was is to analyze the collected experimental data on the effect of constructing a sill under a sluice gate on the free and submerged flow bellow it. Also, this paper presents the results of a laser Doppler Anemometry (LDA) study of submerged hydraulic jumps in a horizontal rectangular channel with the different submergence factor S and different inlet Froude number Fr1. Measurements include surface profiles , mean velocity components of ū andV– ;turbulence intensities u and v' and turbulence shear stress, u vꞌ Major flow characteristics of submerged hydraulic jumps were discussed and analyzed , The flow in the fully developed region is found to have some degree of similarity . It was also found that a submerged jump was three dimensional in nature. The maximum vertical velocity in the recirculating zone for all submerged jumps is about 7% of U1. Also, the results show that the maximum streamwise velocity near the center plane was smaller than that near the side wall . The turbulence shear stress near the center is about 46% higher than that near the side wall. After the jump the flow will recover into a two dimensional flow. The coefficient of discharge becames smaller at constant opening ratio Y1/h1. The results showed that there was a close relationship between the scale effects in the contraction coefficients and in the discharge coefficients. The energy dissipation in effluxes was also related to the scale effect in the discharge coefficients closely. The scale effects in the contraction coefficient and in the coefficients of energy loss which are the main factors composing the discharge coefficients showed the same trend as in the discharge coefficient.

Performance evaluation of venturi aeration system

The venturi aeration is an effective practice to increase the dissolved oxygen accessibility in the water bodies. This study aims to optimize the various geometrical parameters of the venturi aeration system. A non-dimensional technique was applied to find the optimum performance of various geometric parameters i.e. throat lengths (tl), number of air holes (N), and converging and diverging angles (α and β). These experiments have been carried out using 1124 L capacity of tank having dimensions of 105 cm long, 105 cm wide and 102 cm deep. The experiments were conducted at a constant flow velocity of water (1.096 m/s) with varying throat length (tl = 20–100 mm keeping 20 mm as interval between two consecutive length), number of air holes (N = 1–17 at an equal hole to hole distance of 5 mm between them), and converging and diverging angles (α and β = 10°, 15°, 20° and 25°). Multiple non-linear regression equations were also developed from the linear relation with the dependent variable (Non-dimensional form of standard aeration efficiency, NDSAE) and independent variables (tl and N). With the geometrically optimized venturi aerator the optimum performance was found for tl =100 mm, N = 17, and α and β = 15°. The maximum value of standard oxygen transfer rate (SOTR) and standard aeration efficiency (SAE) obtained was 0.0216 kgO2/h and 0.611 kgO2/kWh respectively. From the non-dimensional study, it was found that the NDSAE is the function Reynolds number (Re) and Froude number (Fr). The simulation equations were developed on the basis of Re and Fr for NDSAE, and subjected to 7.378 × 10−6 <Re < 3.689 × 10-5 and 0.163 <Fr < 0.817, respectively.

Quantitative simulation of the particle size distribution of eroded sediment on grass slopes with intact plants and root slopes with the aboveground parts removed

AbstractTo quantitatively simulate the particle size distribution (PSD) of eroded sediment on grass and root slopes, simulated rainfall experiments were implemented on grass slopes with intact plants and root slopes with the aboveground parts removed. There were four coverage degrees (Cr) (0, 20, 40, and 60%), two rainfall intensities (R) (60 and 90 mm h–1) and two slope gradients (J) (15° and 25°). The content of sand‐sized particles was greatest in the eroded sediment from the grass slope, whereas the content of fine silt‐sized particles was greatest in the eroded sediment from the root slope. The PSD of eroded sediment was influenced by Cr, R, J, and hydrodynamic characteristics [flow velocity (u), flow depth (h), Reynolds number (Re), Froude number (Fr), resistance coefficient (f), runoff shear force (t), runoff power (w), unit runoff power (P)]. Through grey relational analysis, the relational degree (γ) of these impact factors to the PSD of eroded sediment was ranked γR &gt; γRe &gt; γJ &gt; γh &gt; γw &gt; γu &gt; γt &gt; γP &gt; γFr &gt; γCr &gt; γf on the grass slope and γR &gt; γRe &gt; γJ &gt; γh &gt; γt &gt; γw &gt; γu &gt; γFr &gt; γp &gt; γf &gt; γCr &gt; on the root slope. The radial basis function neural network performed well at simulating the PSD of eroded sediment on both the grass and root slopes. These results lay the foundation for the establishment of a physics‐ based soil erosion model and a chemical transport model.

Interface dynamics at the impact of a drop onto a deep pool of immiscible liquid

The present study investigates the impact of a droplet onto a deep pool of immiscible liquid for a wide range of drop impact Froude numbers (10 ≤ Fr ≤ 1075) to characterize the crater and jet dynamics. The radial expansion of the crater is followed by contraction of its sidewalls before the inception of vertical retraction at low drop impact Froude numbers (Fr ≤ 400), whereas the gravity-induced buoyant force of displaced liquid resulted in the simultaneous horizontal and vertical retraction of deeper cavities at high Froude numbers (Fr > 400). Four different jet regimes such as short jet, singular jet, slender jet, and compound thin thick jet are observed and their formation is associated with specific flow behaviors around the retracting crater. The short, singular, and slender jets are observed when retraction of the crater is capillary dominant. The compound thin thick jets are mainly attributed to gravity dominant deeper cavities.

Sediment transport and roughness coefficients generated by flexible vegetation patches in the emergent and submerged conditions in a semiarid alluvial open-channel

In the semiarid environment, low vegetation cover and deforestation on the banks of rivers represent a strong impact on the flow resistance process, altering the interaction of the flow with the banks, one of the conditioning factors of the morphological balance of the river channel. In extreme hydrological events, turbulence and high speeds can cause a marked erosion process at the banks, increasing the sediment yield. This study aimed to monitor water and solid discharges through direct hydrosedimentometric measurements, as well as to estimate, using a simplified model based on force balance, the hydraulic roughness coefficients generated by flexible vegetation element of the Ipomoea pes-caprae species under emergent and submerged conditions. The total annual rainfall for 2019 was 748.5 mm year−1, above the historical mean (642.80 mm year−1). The mean depth (h = 0.24 m) and flow (Ql = 0.15 m3 s−1) rates were highly correlated as a power function (R2 = 0.8041). In general, without specifically considering the effect of vegetation, the flow regime was characterized as turbulent and subcritical flow (Re = 66,634.65 dim. and Fr = 0.16 dim.), respectively. The suspended sediment concentration (CSS), suspended sediment (QSS) and bedload (QBed) discharges presented mean values of 409.41 mg L−1, 6.23 t day−1 and 0.069 t day−1, respectively. The total sediment yield (Yt) was 1.90 t km−2 year−1. The vegetation effect was analyzed on the right (RB) and left (LB) banks, where the stems of I. pes-caprae were concentrated between 0.22 and 0.40 m and 0.66–1.20 m from the banks, respectively. On the RB, the shear (1.0 < Sr < 1.95), free flow (0.27 < Sr < 1.0) and the transition (Sr = 1) zones were identified. On the LB, just emergent condition (Sr < 1.0) was indetified. The following relationship were found between vegetation drag coefficient (C′D), stem Reynolds number (Red), plant density Reynolds number (Rev) and Froude number (Fr) on the RB: C′D (1.22–12.46 m−1); Red (567 < Red < 2224), Rev (1305 < Rev < 35,011) and Fr = 0.14; on the LB: C′D (2.10–1168.29 m−1); Red (57 < Red < 1427), Rev (2615 < Rev < 167,534) and Fr = 0.08. These results demonstrate that the aquatic vegetation directly influences the river dynamics; the density vegetation having a hydraulic impact on the flow transport capacity, that is, as the C′D increases, it increases the resistance of the vegetation to the flow and decreases the capacity of transport of sediments.

Airborne transmission of pathogen-laden expiratory droplets in open outdoor space

Virus-laden droplets dispersion may induce transmissions of respiratory infectious diseases. Existing research mainly focuses on indoor droplet dispersion, but the mechanism of its dispersion and exposure in outdoor environment is unclear. By conducting CFD simulations, this paper investigates the evaporation and transport of solid-liquid droplets in an open outdoor environment. Droplet initial sizes (dp = 10 μm, 50 μm, 100 μm), background relative humidity (RH = 35%, 95%), background wind speed (Uref = 3 m/s, 0.2 m/s) and social distances between two people (D = 0.5 m, 1 m, 1.5 m, 3 m, 5 m) are investigated.Results show that thermal body plume is destroyed when the background wind speed is 3 m/s (Froude number Fr ~ 10). The inhalation fraction (IF) of susceptible person decreases exponentially when the social distance (D) increases from 0.5 m to 5 m. The exponential decay rate of inhalation fraction (b) ranges between 0.93 and 1.06 (IF=IF0e-b(D-0.5)) determined by the droplet initial diameter and relative humidity. Under weak background wind (Uref = 0.2 m/s, Fr ~ 0.01), the upward thermal body plume significantly influences droplet dispersion, which is similar with that in indoor space. Droplets in the initial sizes of 10 μm and 50 μm disperse upwards while most of 100 μm droplets fall down to the ground due to larger gravity force. Interestingly, the deposition fraction on susceptible person is ten times higher at Uref = 3 m/s than that at Uref = 0.2 m/s. Thus, a high outdoor wind speed does not necessarily lead to a smaller exposure risk if the susceptible person locating at the downwind region of the infected person, and people in outdoors are suggested to not only keep distance of greater than 1.5 m from each other but also stand with considerable angles from the prevailing wind direction.

Design and analysis of Trimaran feeder ships as a connector for small islands in the Maluku

Maluku waters have limited seas, which are still categorized as calm seas because the wave height is about 1.0 meter. The existence of feeder ships for transportation services from and to areas that are still considered remote, outermost and border will behave more smoothly in operation. The selection of three-body vessel (Trimaran) has the advantage of having a large deck area so that it can function as a feeder between small islands effectively and efficiently. This paper focuses on analysing resistance and seakeeping of Trimaran vessel for limited seas using CFD techniques with variations in transverse distance S/L = 0.2, 0.3, 0.4, 0.5 and at a speed designated by the Froude number Fr = 0.21. The results of numerical analysis show that at S/L = 0.4 the vessel has very good characteristics in resistance and seakeeping. Therefore, results of the calculation can be adopted as preliminary data for the preliminary design.