Dead Water Zones Research Articles

Numerical Simulation of Water Tank Circulation and Water Age Under Different Working Conditions

AbstractWith increasing real estate and apartment vacancy rates, the utilization rate of the water in water tanks is decreasing; this leads to an increase in the water age and deteriorates the water quality in water tanks. The general hydrostatic numerical model cannot simulate the flow field and water age of a water tank correctly because the horizontal scale of the water tank is equivalent to the vertical scale, and the vertical hydrostatic approximation assumption is no longer applicable. In this paper, the water age module is added to a three‐dimensional numerical model based on estuarine and coastal oceans, and a nonhydrostatic numerical model is developed to simulate and analyze the three‐dimensional flow field and the temporal and spatial variations in the water age of the water tank under different working conditions. The total water consumption and the effective volume of the water tank are the main factors affecting the water age, followed by the water level change range. The water age can be reduced to a lesser extent by setting the position of the inlet and outlet at a diagonal angle and adopting a larger inflow velocity. The distributions of the circulation and water age are mainly determined by the inlet in the influent process. There is no obvious dead water zone in the tank due to the strong disturbance, advection and diffusion in the secondary water supply process. The water age distribution in the water tank is mainly affected by advection, while diffusion plays a secondary role.

Analysis of Flow Field Characteristics and Pressure Pulsation in Horizontal Axis Double-Runner Francis Turbine

Horizontal axis double-runner Francis turbines have great advantages in the development of small hydropower plants, but the arrangement of double runners aggravates the complexity of the water flow between runners, and the mutual influence of the two runners cannot be ignored. In order to explore the relationship between the performance and the internal flow field and investigate the pressure pulsation characteristics of the double-runner Francis turbine, the steady and unsteady numerical analysis of the full flow channel of a prototype turbine was carried out based on the Realizable k-epsilon model and the polyhedral mesh method. The results show that the relationship between the average efficiency of the two runners and the flow difference between the runners is negatively correlated. As the flow rate difference between the runners on both sides increases, the average efficiency of the runners decreases. The draft tube flow of a horizontal-axis turbine has a profound effect on the flow field characteristics in the runner. When the working conditions change, the turning and converging timing of the mainstream at the outlet of the two runners will change. The movement of the mainstream promotes the change in location of the dead water zone. The existence of the vortex zone makes the pressure distribution at the outlet of the runner uneven, which is an important reason for the asymmetry of the flow in the runner. The analysis of pressure pulsation and its frequency spectrum shows that when the working conditions change, the low-frequency, strong pressure pulsation area on the surface of the guide vane will regularly migrate between the two runners, while the high-frequency pressure pulsation that occurs in the bladeless zone will dissipate in the runner. The doubling of the blade frequency on the pressure surface and back surface of the blades gradually attenuates with the increase of frequency. The pressure pulsation attenuation on the surface of the high-position blade conforms to the linear law, and the attenuation of the pressure pulsation on the surface of the low-position blade conforms to the exponential law. The research in this paper provides a certain reference value for revealing the flow field mechanism and pressure pulsation characteristics of the double-runner Francis turbine.

APICAL VAPOUR LOCK EFFECT -PHENOMENON RELATED TO ENDODONTIC IRRIGATION - A REVIEW.

Complete debridement of the root canal system is a primary requirement for the successful root canal treatment 1. Thus for optimum disinfection , a thorough cleaning and shaping is mandatory ; so the current concept in Endodontics is ‘ Files shape, irrigants clean’2. Even with the use of rotary instrumentation, the nickel-titanium instruments currently available only act on the central body of the canal, leaving canal fins, isthmi, and cul-de-sacs untouched after completion of the preparation. These areas might harbor tissue debris, microbes, and their by-products, which might prevent close adaptation of the obturation material and result in persistent periradicular inflammation 3,4.So the irrigants must be brought into direct contact with the entire canal wall surfaces for effective action, particularly for the apical portions of small root canals. Recently, concern has been expressed about the possible presence of gas bubbles in the apical part of the root canal that could block irrigant penetration and may affect effective debridement.This phenomenon is termed as‘apical vapor lock’5 (AVL). It is also called as Dead Water Zone.

Large eddy simulation of turbulent flow structure in a rectangular embayment zone with different population densities of vegetation.

Dead-water zones (DWZs) in natural open channels, formed by embayment or consecutive groins, can provide favorable growth conditions for aquatic organisms. Although flow hydrodynamics in side-cavity zones have been well studied, the impact of vegetation on recirculating flow is rarely considered. This study adopts large eddy simulation (LES) to examine the flow field in a rectangular embayment zone with different population densities of vegetation. The numerical model is validated by mean streamwise velocity data collected near mid-depth in the physical experiment. Vegetation rearranges the circulation structure in the DWZ and weakens the velocity and turbulent kinetic energy. This negative effect increases with increasing population density. With the development of the shedding vortex induced in the front edge of the channel-embayment interface, the large-scale coherent structure forms in the mixing layer and is hardly affected by the variation of population density. As the population density increases, the mean retention time first decreases and then increases as a result of the combined action of three factors, namely, the large-scale coherent structure, the plant-induced Karman vortex street, and the blocking effect of dense vegetation.

Case Study on Water Quality Improvement in Xihu Lake through Diversion and Water Distribution

Eutrophication in lakes and reservoirs is a serious environmental problem that has damaged ecosystem health worldwide. Water diversion is one of the most popular methods for improving the water quality in shallow lakes, as it dilutes pollutants in and diverts them out of the lake. However, simple diversion without rational water distribution cannot significantly enhance water exchange in the entire lake because dead water zones always exist. This paper illustrates a case study on water quality improvement in Xihu Lake by diversion and water distribution. Based on theoretical calculation, the diversion water discharge was determined and rationally distributed into four different locations. According to the field observations after the implementation of the diversion and water distribution project, the average velocity over the dead water zones increased approximately 50 times over that of prior to the project. The average water exchange period reduced from 68 days to 22.5 days. The average turbidity was 8.8% and 12.4% lower than before after two and four months of diversion, respectively. The maximum turbidity reduced from the original 27.5 NTU (Nephelometric Turbidity Unit) to 20.1 NTU after two months of diversion, then to 16.1 NTU after four months of diversion. It shows that this diversion and rational water distribution eliminates most of the dead water zones and achieves a favorable flow field, thus reducing the turbidity and increasing water transparency, which is conducive to the improvement of water quality.

B-6 簡易シミュレーションによる連結完全混合槽型水蓄熱槽の死水域検知手法に関する研究

B-6 簡易シミュレーションによる連結完全混合槽型水蓄熱槽の死水域検知手法に関する研究

Bulk velocity measurements by video analysis of dye tracer in a macro-rough channel

Steep mountain rivers have hydraulic and morphodynamic characteristics that hinder velocity measurements. The high spatial variability of hydraulic parameters, such as water depth (WD), river width and flow velocity, makes the choice of a representative cross-section to measure the velocity in detail challenging. Additionally, sediment transport and rapidly changing bed morphology exclude the utilization of standard and often intrusive velocity measurement techniques. The limited technical choices are further reduced in the presence of macro-roughness elements, such as large, relatively immobile boulders. Tracer tracking techniques are among the few reliable methods that can be used under these conditions to evaluate the mean flow velocity. However, most tracer tracking techniques calculate bulk flow velocities between two or more fixed cross-sections. In the presence of intense sediment transport resulting in an important temporal variability of the bed morphology, dead water zones may appear in the few selected measurement sections. Thus a technique based on the analysis of an entire channel reach is needed in this study. A dye tracer measurement technique in which a single camcorder visualizes a long flume reach is described and developed. This allows us to overcome the problem of the presence of dead water zones. To validate this video analysis technique, velocity measurements were carried out on a laboratory flume simulating a torrent, with a relatively gentle slope of 1.97% and without sediment transport, using several commonly used velocity measurement instruments. In the absence of boulders, salt injections, WD and ultrasonic velocity profiler measurements were carried out, along with dye injection technique. When boulders were present, dye tracer technique was validated only by comparison with salt tracer. Several video analysis techniques used to infer velocities were developed and compared, showing that dye tracking is a valid technique for bulk velocity measurements. RGB Euclidean distance was identified as being the best measure of the average flow velocity.

Hydrodynamic characteristics and related mass-transfer properties in open-channel flows with rectangular embayment zone

Consecutive groynes and embayments form dead water zones, where sedimentation and high concentrations of pollutants are often observed. It is thus very important to understand the mass and momentum exchange between the main channel and side cavities in rivers and hydraulic engineering structures. The spanwise gradient of the streamwise velocity near the junction produces small-scale turbulent vortices because of shear instability. Furthermore, large-scale horizontal circulation is also generated in the cavity zone. These coherent turbulent structures play a significant role in mass and sediment transfer at the boundary between the mainstream and embayment. However, the relation between turbulence and mass transfer is poorly understood. In this study, we performed particle image velocity and laser-induced fluorescence experiments using a laboratory flume, laser light sheets and a high-speed CMOS camera. We examined the exchange properties of a dye as a function of bed configuration and sedimentation effect. Both primary and secondary gyres were observed in the flat bed and downward-sloping bed, whereas the primary gyre was prevalent in the upward-sloping bed. Moreover, the horizontal circulation strongly affected the mass-transfer properties between the mainstream and side cavity.

連続水制を伴う開水路乱流における自由水面のガス輸送に関する基礎的研究

In this study, gas transfer velocity through free surface is investigated in open channel flow with a series of spur dikes. Spur dikes formed closed dead-water zones between the neighboring members. The slow-speed circulation observed in such cavity zones is quite different from high-speed mainstream. Therefore, the cavity zones and the mainstream are assumed to have different reaeration coefficient. The reaeration coefficients are evaluated by means of laboratory measurements. The measurement theory is also specially developed together with consideration of gas transfer through the junction between main-channel / side-cavity. It is found out from present results that reaeration coefficient depends strongly on the aspect ratio (width/length) of dead water zones under same hydraulic condition.

連続水制群周辺の水理乱流特性と物質交換に関する基礎的研究

It is well known that groynes navigate mainstream effectively to protect a river basin. An associated dead water zone can be a valuable space for various aquatic eco-system. In contrast, such a closed water environment reduces water quality rapidly, and it is therefore of vital importance to consider not only hydrodynamic properties but also mass transfer mechanism beneath the mainstream and the dead water zone. We conducted turbulence and dye concentration measurements in a laboratory flume by using PIV and LIF techniques, respectively, and based on the present results, a phenomenological flow model was developed which describes comparison of large-scale gyre structure and mass transfer coefficient among the different rank number.

EFFECTS OF SLOPE-CHANGED BED IN A GROYNE FIELD ON ITS FLOW STRUCTURES AND MASS-EXCHANGEING

PIV and LIF techniques were conducted to reveal relationship between sedimentation and mass exchange property in a groyne field. It was found from the PIV measurements that different bed-form conditions, that is to say, with and without bed slope, yielded corresponding gyre formations. The other experiment, the LIF results allowed us reasonably to evaluate transfer velocity of dye through boundary between mainstream and dead water zone, They suggest that mass exchange properties have significant relations with horizontal gyres, thereby, with the conditions of bed slope. The results normalized by turbulence intensity and length scale of gyre were in good agreement with a large-eddy model.

Dynamics analysis of bladder-urethra system based on CFD

A mathematical model for a bladder-urethra system can provide basic analysis for the disabled urethra closure of stress urinary incontinence (SUI) patients in a clinic. Based on computational fluid dynamics (CFD), we developed a three-dimensional urodynamic bladder-urethra system, which includes bladder, bladder neck, prostate, and urethra. The realistic recirculation process of the urinary bladder during the physiologic voiding process in conjunction with a flow simulation through the female urinary bladder and urethra is presented. The computational results show that a dead-water zone and the zone of secondary flow occur, independent of the shape of the prostatic urethra. For the pathological prostata, the extreme constriction of the prostatic urethra results in an additional wide-stretched dead-water zone. The simulation results does not only improve urinary incontinence surgery for clinicians, but can also provide a basis for theoretical analysis.

Effects of conjunction channel on water exchange in riverside embayment

A riverside embayment is expected to provide a good habitat for aquatic life. It is often connected with primary river flow through a conjunction channel. It is important to maintain water quality and to prevent siltation in such a dead water zone. In this study, we investigated the effects of the length, location and orientation angle of a conjunction channel on flow structures and water exchange rate. The experiments were conducted in a laboratory flume and velocity vectors were measured by PIV method in horizontal planes. The flow patterns in embayment zones were extremely changed by the angle of the conjunction channel. When the length of conjunction channel is long relatively to the width, an isolated vortex was generated in a conjunction zone. This vortex fluctuates and penetrates intermittently into embayment zone. The water exchange rate was investigated by a dye-injection method. The water exchange rate is generally proportional to the velocity scale in the embayment area but it was affected by vortex structures in a conjunction channel.

Detection Method of Dead Water Zone for Water Thermal Energy Storage Tank of Multi-connected Complete Mixing Type

Detection Method of Dead Water Zone for Water Thermal Energy Storage Tank of Multi-connected Complete Mixing Type

Experiments on Mass Exchange between Groin Fields and Main Stream in Rivers

Laboratory experiments are presented that model the exchange processes between a river and dead-water zones created by groin fields. The measurement methods include particle image velocimetry (PIV) at the water surface to capture the dynamics of the flow field as well as planar concentration analysis (PCA) to determine the depth integrated mass fluxes of dissolved tracer. Idealized groins with variable length-to-width ratios and inclination angles relative to the flow direction are investigated. Two-dimensional large coherent structures (2DLCS) are found in the mixing layer between the groin field and the river, and these flow structures control the majority of mixing with the main stream. The PIV data allow quantification of the turbulence intensity distribution in these structures throughout the groin field and mixing region. The PCA data provide measurements of the exchange rate between the groin field and the main stream. Both methods give comparable measures of the entrainment coefficient for the exchange flow. The entrainment coefficient is found to depend on the groin field hydraulic radius (a measure of the average size of 2DLCS in the mixing layer) and the water depth (a parameter controlling the stability of the shallow water flow). These data provide a link between the exchange parameter in groin field dead zones and the river properties that can be used to efficiently develop river alarm models to predict contaminant transport without requiring a large amount of field dye studies for calibration.

EFFECTS OF CONJUNCTION-CHANNEL CONFIGURATION ON WATER EXCHANGE IN RIVERSIDE EMBAYMENT

Riverside embayment is often connected with primary river flow through a conjunction channel. It is important to maintain water quality in such a dead water zone during ordinary water-stage. In this study, we concentrated on the effects of length and orientation angle of the conjunction channel on flow structures and water exchange rate. The flow patterns in embayment zones were extremely changed by the orientation angle of the conjunction channel. When the length of conjunction channel is long relatively to the width, an isolated vortex was generated in a conjunction zone. This vortex fluctuates and penetrates intermittently into embayment zone. This indirect inflow generates consequential vortices but the magnitude of these vortices are considerably smaller than those caused by direct inflow. The water exchange rate was generally proportional to the velocity scale but it was affected by the vortex structures.

Reducing Harbor Siltation. I: Methodology

This paper describes a decade of research on siltation-reducing measures for harbor basins and the methodology and measuring techniques developed. A classification of harbor systems is made on the basis of siltation mechanisms, i.e. horizontal entrainment by mixing layers, tidal filling, and density currents. This classification distinguishes between harbors in various environments, i.e. stagnant water systems such as shallow lakes, where wind-induced currents prevail, riverine systems, tidal systems, either homogeneously fresh or saline, and estuarine systems with fresh/salt water induced density gradients. Case studies and detailed design studies yielded large reductions in siltation rates of many tens of percents for all harbor basins located in flowing water systems, using a variety of siltation-reducing techniques. These reductions were attained by reducing the exchange flow rates between the harbor and ambient water, and by reducing the sediment concentration of the water entering the basin. In the case of stagnant water systems with very fine, mainly organic matter, a reduction in exchange flow rate cannot be successful, and a reduction in siltation rates can only be achieved through the prevention of dead water zones in the harbor basins. The methodology developed is cost efficient and therefore applicable for large commercial ports as well as small marinas.

Velocity and turbulence distribution around lotic macrophytes

Flow velocity and turbulence patterns were measured in and around a common lotic macrophyte, Ranunculus penicillatus subspecies pseudofluitans (stream water-crowfoot), using a two-dimensional electromagnetic current meter (EMCM). Due to the high shooting density of this species, there was a sharp velocity gradient at the plant boundary, with velocities dropping to a constantly low value after no more than 5 cm into the plant, thus forcing most of the flow over and around the macrophyte. There was a dead-water zone immediately downstream of the plant, beyond which the current moved in from the sides to allow flow under the trailing shoots. High turbulence intensities were recorded for both downstream and cross-stream velocity components at the lateral margins and downstream of the plant. Meanwhile, pulses of water upstream of the plant produced turbulence in the downstream component, but not in the cross-stream component.

On the behavioural response ofRanaandBufotadpoles to echinostomatoid cercariae: implications to synergistic factors influencing trematode infections in anurans

We used high-speed videography of staged encounters between tadpoles of either Bufo americanus Holbrook, 1836 or Rana sylvatica LeConte, 1825 and Echinostoma Rudolphi, 1809 cercariae to understand why echinostomatoid trematodes, such as species from the genera Echinostoma and Ribeiroia Travassos, 1939 (implicated in anuran limb deformities), attack specific anatomical regions of tadpoles. Bufo and Rana tadpoles can shed cercariae on their skin from some parts of their body more easily than others. In particular, cercariae that enter the "dead-water zone" at the junction of a tadpole's body and tail appear particularly difficult for tadpoles to brush off. Cercariae that reach this recess can easily enter the inguinal region of tadpoles (as do Ribeiroia spp.) or ascend the tadpole's cloaca (as do Echinostoma spp.). When tadpoles sense cercariae contacting their skin they make explosive movements to shed those parasites. Factors that reduce tadpoles' activity, such as predator threat or certain pesticides, may increase a tadpole's susceptibility to echinostomatoid infection. Because Bufo tadpoles are unpalatable to many predators, they can afford to make more conspicuous evasive maneuvers than Rana tadpoles, and do so in the laboratory. Bufo tadpoles in the field also have a lower rate and different anatomical distribution pattern of Ribeiroia infection than Rana tadpoles. Factors that reduce tadpole activity in the field may act synergistically to increase parasite loads and subsequent deformities in anurans.

Large scale PIV-measurements at the surface of shallow water flows

To measure the flow dynamics at the surface of shallow water flows over a large measuring field, a simple and reliable method has been developed using the advantages of Particle Image Velocimetry (PIV). Besides the determination of mean flow conditions and turbulent flow characteristics, this method makes it possible to track two-dimensional large coherent structures, which are the dominating flow phenomena in many shallow flow applications. As basic equipment, a commercial PIV software system has been used. The measurements are carried out at the water surface, which means that no laser light sheet is needed. Depending on the time scales of the flow and camera characteristics, it is even possible to work with a constant light source. A particle dispenser to provide a homogeneous distribution of particles on the water surface is also presented. Because floating particles have a strong tendency of sticking together, different types of particles and special coatings have been tested to reduce this problem. A laboratory application of this method is presented to analyze the effects of shallow dead-water zones on exchange processes in rivers where large coherent two-dimensional flow structures in the mixing layer dominate the flow characteristics.