Unsteady Discharge Research Articles

Experimental investigation of loop rating curve on a small 3D printed laboratory channel

The study of flood dynamics is generally complicated because it requires a lot of expensive sensors that might be inoperative when submerged during a flood event. Physical models can be helpful in order to get extra information on flood dynamics, but they are expensive in material, space occupation, and human investment (intervention, maintenance, assistance). The objective of this work is a feasibility study on the possibility of using a small 3D printed model to make hydraulic studies. For this we focus on the reproduction of a complex phenomenon which is the loop rating curve (LRC). In natural rivers or large channels, rating curve is a relation between stage and discharge. Under unsteady flow condition, rating curve may present a hysteresis behavior reflected by a more or less pronounced loop. LRC was historically observed on natural rivers or at large laboratory channels. This study is the first investigation of LRC on a small 3D printed laboratory channel. We measure stage and discharge at the channel outlet and we obtain rating curves as a response to an unsteady discharge hydrograph imposed at the channel inlet. We tested 11 scenarios corresponding to different flow conditions where we varied the following parameters: section control (freefall or weir), channel slope (flat = 0.1% or sharp = 10%), time step of the discharge hydrograph (5 s or 10 s). Our experimental results with a small 3D printed channel are in accordance with experimental observations on large channels or natural rivers, and our experimental rating curves are very well simulated by Jones formula that is classically used on large channels or rivers. These promising results allow the use of 3D printing technique to make small-scale hydraulic models. A further study of the rating curve, but also of other hydraulic characteristics can be considered on more complex geometries and more varied scenarios while paying attention to the scaling (which was not the case in this study). As real scenarios might be considered, this might be of interest from both a pedagogical and an application point of view with a reduced human, material and space occupation cost.

Morphodynamic equilibrium of alternate bar dynamics under repeated hydrographs

Discharge unsteadiness plays a key role in the dynamics of alternate bars, however, how migrating alternate bars determine their shape under unsteady discharge conditions, especially in the long-term, is still unclear. In this study, we numerically investigate long-term behaviors of migrating alternate bars subject to repeated hydrographs. For this, the same discharge variation but different hydrograph cycles were tested. The results show that even under unsteady flow conditions, specific combinations of hydrograph cycles and discharge variations result in an equilibrium state of migrating alternate bars (i.e., non-time dependent wavelength and migration period). This may be a unique feature of the alternate bar morphodynamics, since dynamic equilibrium state arising from steady discharge counterparts is time-dependent feature. Meanwhile, it is observed that when the discharge changes abruptly, the response of alternate bar morphodynamics changes along the time. Within a limited period, the response of alternate bars is not strong enough to destroy the original wavelength, suggesting that the hydrograph cycle that achieves this equilibrium state must be moderately long such that the timescale of the hydrograph is similar but slightly smaller than the bar growth.

Temporal dusty plasma afterglow: A review

In complex plasmas, dust particles are charged through their interactions with the electrons and ions of the surrounding plasma. In low-temperature laboratory plasmas, dust particles most commonly acquire a negative charge. In particular, in a laboratory glow-discharge plasma, the typical charge for a micrometer-size grain generally attains a few thousands of electronic charges. Under stable discharge conditions, this large negative charge is relatively well-characterized. However, for unsteady discharge conditions, the charge can differ and even fluctuate. In particular, when the power source of the discharge is turned off, the charged species of the plasma diffuse away and recombine into neutral species: this is a temporal afterglow. When dust particles are present inside a temporal plasma afterglow, the diffusion of charged species and the plasma decay dynamics are affected. Moreover, the dust particle charges also evolve during the afterglow period. In the late afterglow, dust particles are known to keep residual charges. The value of these residual charges strongly depends on the ambipolar-to-free diffusion transition. In addition, the presence of a constant electric field, causing ions to drift through the neutral gas, has a strong influence on the final dust particle residual charges, eventually leading to large positive residual charges. In this review article, the dynamics of temporal complex plasma afterglow are discussed. Experimental and theoretical results are presented. The basics of temporal afterglow modeling are also given.

Vertically layered flow structure at confluence of a reservoir and tributary carrying high sediment loads

Enhanced understanding of flow structure at a river confluence is essential for predictions of sediment transport and morphological evolution. To date, however, the confluent flow structure of a reservoir and tributary carrying high sediment loads has remained poorly understood, and may be vertically layered sharply, featuring subaqueous sediment-laden flow, i.e., turbidity currents underneath subaerial clear water. Here a recently established 2D double layer-averaged model, able to resolve the formation, propagation, and recession of turbidity currents, is used to investigate a series of idealized laboratory-scale cases and a prototype case study of the Guxian Reservoir on the Yellow River, China. Four primary patterns of the stable, vertically layered flow structure at a reservoir-tributary confluence are identified: 1) single layers of sediment-laden inflow in both the main channel and tributary, sustained by sufficient vertical mixing; 2) a double layer in the main channel and a single layer of sediment-laden inflow in the tributary, when the sediment-laden flow in the tributary suffices to block intrusion of flow in the main channel; 3) a single layer of sediment-laden inflow in the main channel and a double layer in the tributary, induced by the intrusion of sediment-laden flow from the main channel into clear-water flow with small discharge in the tributary; and 4) double layers in both the main channel and tributary, which may be further divided into three subpatterns, as turbidity current exists in both the main channel and tributary, or in the main channel (tributary) intruding into the tributary (main channel). In response to unsteady discharge and sediment inputs from upstream, the vertically layered flow structure evolves in time, and may fall into one of the patterns identified above. Although bed deformation in the long term may modify the confluent flow, the vertically layered flow pattern remains so far as the present cases are concerned. The findings have implications for sediment transport and morphological evolution at a reservoir–tributary confluence, for which further studies are suggested to inform the optimization of reservoir operation schemes to mitigate capacity loss caused by sedimentation.

Analysis of the effect of unsteady reservoir flow and sand excavation on the channel conditions of the Yangtze River (Yibin-Chongqing)

This paper systematically describes the current research methods for analyzing unsteady flows in an open channel. In particular, it focuses on elaborating a discrete method to solve the one-dimensional Saint–Venant equations for unsteady flow in an open channel, compares the advantages and disadvantages of different discrete methods, and proposes a new implicit iterative mathematical model applicable to the calculation of the multiwavelength unsteady flow in the Yangtze River from Yibin to Chongqing. This paper also simulates three typical unsteady flow processes and analyzes the effect of sand excavation on the water level in the upper reaches of the Yangtze River from Yibin to Chongqing considering flood, normal-water, and low-water conditions. The spreading characteristics of the daily adjusted unsteady flow were obtained in the Shuifu-Chongqing river reach. Under the conditions of the daily minimum unsteady discharge flow rate, the designed water level of the reach from Yibin to Chongqing increases by 0.19 m compared to the natural water level. With the increase in concentrated sand excavation in the upper reaches of the Yangtze River, a large number of shingle beaches have disappeared, and the riverbed has been drastically lowered. Affected by the excessive and unordered sand excavation in the river channel, the water level of the upper reaches of the Yangtze River from Chongqing to Yibin has decreased by 0.5–2.0 m.

Dominating factors influencing rapid meander shift and levee breaches caused by a record-breaking flood in the Otofuke River, Japan

The Otofuke River is a tributary of the Tokachi River, Hokkaido Japan, consisting of a channelized reach with a steep slope and a relatively large designed width. In August 2016, the river witnessed four major flooding events with a record-breaking discharge within a time span of 15 days. The Otofuke River originally had a straight channelized reach. However, because of a series of flooding events, the river promptly followed a sinusoidal path, damaging the riverbank embankments. The straight channelized reach of Otofuke River faced active erosion of both banks, leading to the formation of a meandering channel planform. This also led to levee breaches at the seven locations. In order to understand the mechanism of such a quick shift in river channel planform, we collected various field survey data and conducted computational analysis. We investigated the underlying processes by capturing the complex interactions between the rapid channel migration, unsteady discharge, and sediment transport. Field surveys including aerial photographs and images showed rapid formation of a meandering planform in the Otofuke River as a result of the typhoon on 31 August 2016. According to the maximum water level traces, bank erosion developed after the peak discharge. Numerical investigation revealed that alternate sandbars continuously developed and propagated downstream until the moment of peak discharge, and a significant accumulation of sediment subsequently started on the bars at the mid-channel during the flood falling stage. Such morphological processes led to strong flow deflections transversely toward the embankment that resulted in lateral channel shifting and levee breaches at a location where vegetation along the embankments had already disappeared.

The Effect of Unsteady Water Discharge through Dams of Hydroelectric Power Plants on Hydrodynamic Regimes of the Upper Pools of Waterworks

The hydrological regimes of surface water bodies, as a rule, are unsteady. However, accounting for the non-stationarity substantially complicates the hydrodynamic calculations. Because of this, the scenario approach is traditionally used in the calculations. Characteristic scenarios are set with constant hydrological characteristics throughout the time covered in the calculations. This approach is fully justified if the characteristic time of the change in water flow rate is much longer than the calculation time. However, nowadays, tasks are becoming more and more urgent when accounting for flow variability during calculation period becomes crucial. First of all, such a problem arises when assessing the effect of non-stationary water discharge through hydroelectric power plant dams on the hydrodynamic regime of both the upper and lower pools of the reservoir. In the present paper, the effect of the intraday variability of the Kamskaya Hydroelectric Power Plant (Kamskaya HEPP) operation on the peculiarities of the hydrodynamic regimes of the near-dam part of the upper pool of the Kama reservoir is described. The importance of the problem is determined by the location of the main drinking water intake of Perm city and one of the largest thermal power plants (TPP) in Europe, Permskaya TPP, in this part of the reservoir. This TPP uses a direct-flow cooling system from the Kama reservoir, which is very sensitive to the peculiarities of the hydrodynamic regime of the reservoir. The computational experiments based on the combined hydrodynamic models in 2D/3D formulations have shown that the intraday oscillations of the discharge flow rate through the dam of the HEPP have a very significant effect on the hydrodynamic regime of the reservoir in the vicinity of the Permskaya TPP; therefore, these effects must be taken into account when minimizing the risks of thermal effluents entering the intake channel of the Permskaya TPP.

Unsteady Stage-Discharge Relationships for Sharp-Crested Weirs

Currently, discharges passing measurement devices and structures in unsteady flow conditions are determined based on relationships developed for steady flow condition due to lack of sufficient data. In this study, the operation of triangular and rectangular sharp-crested weirs in unsteady flow conditions was experimentally investigated. The compiled data indicated the presence of looped rating curves, but with transposition of the rising and falling limbs compared to that commonly observed in stream gauge ratings. The presence of such looped rating curves was justified, and the validity of the steady rating curve relationships, which are presently being applied to unsteady flow conditions, was questioned. Accordingly, relationships for evaluating unsteady discharge passing different rectangular and triangular sharp-crested weirs were presented. The proposed formulas considerably improve the discharge measurements accuracy in unsteady flow conditions and could be incorporated in hydrodynamic models.

Investigation of nanofluid conduction heat transfer within a triplex tube considering solidification

In current simulation, Galerkin approach was employed to achieve the best design of heat storage. Utilizing nanoparticles as well as mounting fins were promising ways to improve the efficiency of unit. The unsteady discharge process was analyzed in various time steps and outcomes were reported as contours, profiles and solid front. Acceleration in discharging means that more uniform process and better performance. Increasing the fin length is effective way to achieve greater discharging rate. Outcomes revealed that greater size of nanoparticles yielding higher performance when dp < 40 nm.

Long‐Term Cumulative Effects of Intra‐Annual Variability of Unsteady River Discharge on the Progradation of Delta Lobes: A Modeling Perspective

AbstractRivers, regardless of their scales and geographic locations, are characterized with natural and human‐induced variability in their discharges. While previous studies have established the effects of both interannual and intra‐annual variabilities of unsteady river discharge on delta morphological evolution, the long‐term cumulative effects of intra‐annual unsteadiness on the progradation of delta lobes has remained hitherto elusive. To address this issue, numerical experiments using simplified unsteady discharges were performed in Delft3D and compared with those assuming constant bank‐full discharges. A modified box model was further used to explore the effects of varying intra‐annual unsteadiness on the progradation of delta lobes at reduced computational cost. While the overall trends of the progradation and the ultimate area created were found to be similar between the unsteady discharge scenarios and their corresponding constant bank‐full discharge scenarios, the nuances of intermittent zig‐zag variation in natural delta lobe area were well reproduced by model simulations assuming unsteady river discharges. In addition, long‐term predictions suggested the potential existence of a tipping point in the area growth trajectory beyond which the delta lobe area declines during periods of low discharge. When confounding factors such as waves and variable sediment capture ratio were further taken into consideration, simulation results for unsteady river discharge scenarios exhibit significant deviations from constant bank‐full discharge scenarios. The implications of the modeling results for delta protection and restoration measures, such as the water‐sediment regulation scheme in the Yellow River and artificial channel diversions in the Mississippi River Delta, are also discussed.

Experimental and theoretical studies on the pressure fluctuation of an internal gear pump with a high pressure

In this paper, the flow ripple equation is derived to analyze the effect of working condition on pressure pulsations of an internal gear pump. Results indicate that working pressure has a significant effect on pressure fluctuation of the internal gear pump, while the rotating speed has a complex influence on the pressure pulsation behavior. Then, pressure pulsations of the internal gear pump under different working conditions are discussed by experimental investigations. Results show that the internal gear pump taken for analysis has a low-pressure pulsation at a high working pressure and a relatively high rotational speed. Regarding the frequency spectrum of the pressure pulsation, the dominant frequency is Z* fn, i.e. the product of the tooth number of the driving gear (gear shaft) and its rotational frequency for many working conditions, caused by the inevitable unsteady discharge process of gear pumps. It transforms to the rotational frequency of the gear shaft ( fn) for a high rotational speed or a high operating pressure, but to the rotating frequency of the internal gear ring (2/3 fn) only for a high operating pressure. The occurrence of the two frequencies (2/3 fn and fn) may result from the deformation of the gear ring and the gear shaft under the unbalanced radial forces caused by a high working pressure. Moreover, the frequency spectrum of the inlet pressure pulsation presents some differences from that of the outlet pressure pulsation. This is because the inlet pressure may be influenced by cavity generated at the suction side of the pump.

Modeling Plasma via Electron Impact Ionization

Variable and potential plasma applications in aerospace engineering are exemplified by ion thrusters, flow control by plasma actuator, enhanced ignition and combustion stability. The operational environments span a range from the rarefied to continuum gasdynamic regimes; however, the ionization process in practical applications is mostly by electron impact. The fundamental ionization mechanisms by electron impact consist of electron secondary mission and the cascading process. In an alternating electric field, unsteady and random micro discharges or streamers are always presented; therefore the discharge physics imposes a formidable challenge for incisive understanding. Meanwhile, the ionized species constitute hundreds of metastable chemical species; under this circumstance the physics-based modeling for analyzing the inhomogeneous medium becomes necessary. A summary of the physics-based modeling for electron impact ionization from the Boltzmann distribution equation to the inelastic particle kinetics formulation is delineated.

Technical Note: Monitoring of unsteady open channel flows using the continuous slope-area method

Abstract. The advent of low-cost pressure transducers capable of directly measuring water surface elevation enables continuous measurements of dynamic water surface slopes. This opens up a new possibility of dynamically monitoring unsteady flows (i.e., hysteresis) during the course of flood wave propagation. Hysteresis in this context refers to a looped stage–discharge rating caused by unsteadiness of flows. Hysteresis is monitored in this study using a continuous slope area (CSA) method, which uses Manning's equation to calculate unsteady discharges based on continuously measured water surface slopes. In the rising stage, water surface slopes become steeper than a steady water surface slope, resulting in higher discharges than steady-based discharges, while the trends are reversed in the falling stage. The CSA method applied to Clear Creek near Oxford (Iowa, USA) estimates the maximum differences of peak discharges by 30–40 %, while it shows sound agreements for a low to medium range of discharges against USGS steady-based records. The primary cause of these differences is the use of a single channel bed slope in deriving Manning's roughness coefficients. The use of a single channel bed slope (conceptually equal to the water surface slopes at every stage in uniform flow conditions) causes substantial errors in estimating the channel roughness, specifically at high stages, because non-uniformities of natural channels result in varying (non-uniform) steady water surface slopes at each stage. While the CSA method is promising for dynamically tracking unsteady water surface slopes and flows in natural streams, more studies are still needed to increase the accuracy of the CSA method in future research.

Revealing pMDI Spray Initial Conditions: Flashing, Atomisation and the Effect of Ethanol.

Sprays from pressurised metered-dose inhalers are produced by a transient discharge of a multiphase mixture. Small length and short time scales have made the investigation of the governing processes difficult. Consequently, a deep understanding of the physical processes that govern atomisation and drug particle formation has been elusive. X-ray phase contrast imaging and quantitative radiography were used to reveal the internal flow structure and measure the time-variant nozzle exit mass density of 50µL metered sprays of HFA134a, with and without ethanol cosolvent. Internal flow patterns were imaged at a magnification of 194pixels/mm and 7759 frames per second with 150ps temporal resolution. Spray projected mass was measured with temporal resolution of 1ms and spatial resolution 6µm × 5µm. The flow upstream of the nozzle comprised large volumes of vapour at all times throughout the injection. The inclusion of ethanol prevented bubble coalescence, altering the internal flow structure and discharge. Radiography measurements confirmed that the nozzle exit area is dominantly occupied by vapour, with a peak liquid volume fraction of 13%. Vapour generation in pMDIs occurs upstream of the sump, and the dominant volume component in the nozzle exit orifice is vapour at all times in the injection. The flow in ethanol-containing pMDIs has a bubbly structure resulting in a comparatively stable discharge, whereas the binary structure of propellant-only flows results in unsteady discharge and the production of unrespirable liquid masses.

Oscillations of aerosols in the tubes near to resonances

Experimental investigations longitudinal oscillations of a small-dispersed aerosol were carried out for tubes with various geometry on the end in a shock-free modes near to natural resonances. Di-ethyl-hexyl-sebacate was used as the working fluid to generate aerosol. Number concentration of drops for all experiments decreases with time scale and with growth of the excitation frequency. In the closed tube this process includes the coagulation and deposition of aerosol droplets on the tube walls. In the open tube except the coagulation and deposited of droplets on the walls of tube processes as unsteady discharge of aerosol from the open end of the tube. The time dependence of the numerical concentration of aerosol droplets is determined, and the effect of frequency and amplitude of oscillation on the process of coagulation and deposition of droplets is studied.

Modelling of spatially and temporally-varying cavity pressures in air-permeable, double-layer roof systems

This paper discusses the development of an analytical model to simulate time-varying pressure distributions (Cpi) in the cavity of air-permeable layer, double-layer systems, given pressure data on the external surface (Cpe). The model was derived considering the pressure drops associated with the flow through the gaps between panels, which is like an orifice flow, and the cavity flow between parallel plates. Thus, the model uses two primary equations: the unsteady discharge equation and the equation for unsteady flow between two parallel plates. The model accounts for several geometric parameters including the gap (G) between the panels, the height (H) of the panel above the airtight layer and the length (L) of the panels, as well as the loss coefficients for the orifice and cavity flows. The proposed model is able to capture the fluctuations of Cpi and a good agreement is found between the numerical and experimental results for the mean, RMS and peak coefficients, to a great extent, when spanwise-averaged external pressure coefficients are used as input. For large gap-to-height ratios (i.e., G/H = 12.5), the model accuracy decreases.

Unsteady discharge calibration of a large V-notch weir

Thin-plate weirs are commonly used as measuring devices in flumes and channels, enabling an accurate discharge measurement with simple instruments. The calibration formulae of such devices rely upon some empirical coefficients and there is a need to obtain new accurate physical data to complement the existing evidence. In the present study, the discharge calibration of a large 90° V-notch thin plate weir was performed using an unsteady volume per time technique. The V-notch weir was initially closed by a fast-opening gate. The sudden opening induced an initial phase of the water motion dominated by the free-falling motion of a volume of fluid in the vicinity of the weir, followed by a gradually-varied phase, during which some seiche was observed in the tank. The relationship between water discharge and upstream water elevation was derived from the integral form of the continuity equation. The results yielded a dimensionless discharge coefficient Cd=0.58 close to previous experiments for 90° V-notch weirs. The findings showed that the unsteady discharge calibration of the V-notch weir yielded similar results to a more traditional calibration approach based upon steady flow experiments, allowing a rapid testing over a broad range of flow rates.

Simulation of heat dissipation with phase change material for cylindrical power battery

AbstractIn order to better understand the heat dissipation performance of power battery for electrical vehicles, a three-dimensional model based on phase change material (PCM) cooling for individual cylindrical battery cell has been developed. The model takes into account the effects of unsteady heat generation, internal conduction and external natural convection to investigate the temperature response and distribution. The simulation results showed that the PCM with thermal conductivity higher than 0·8 W m−1 K−1 can be used for keeping the maximum temperature below 50°C in a battery cell under a constant heat generation rate of 170 000 W m−3. The unsteady discharge simulated results demonstrated that the heat transfer coefficient was another important factor for heat transfer enhancement. The PCM based power battery thermal management system indicated effective cycle application because the PCM can store most of the thermal energy during discharge and release it to the outer air during charge or non-star...

Numerical errors in discharge measurements using the KDP method

The kinetic differential pressure (KDP) method for measuring unsteady discharge in pressurized fluid conduits is introduced. The method utilizes two pressure measurements to estimate the unsteady discharge at the sampling rate of the pressure sensors. The technique is based on the linear approximation of the one-dimensional governing equations for transient flow. The application of the method is affected by two sources of error: the numerical error due to linear approximation of the one-dimensional governing equation and the modelling error representing the inability of the one-dimensional model to replicate the real transient behaviour. This research provides a first step in the verification of the KDP method by investigating the numerical error caused by the linear approximation. The numerical error is quantified as a function of the measurement location, system and signal characteristics. The method exhibits minor numerical errors if the system resonance is avoided.

Analytical and Numerical Investigation of Transient Gas Blow down

An analytical and numerical solution for unsteady discharge of gas through blow down stack open to atmosphere based upon the volume model analysis is developed. The time dependent, one-dimensional mass conservation equation was used in the analysis with neglecting the effects of flow and friction along the main pipe. Also isothermal gas expansion is assumed inside the main pipe and isentropic flow through the stack. The results obtained were compared very well with available data and published model.