scholarly journals Air Diffusion and Velocity Characteristics of Self-Aerated Developing Region in Flat Chute Flows

Water ◽  
2021 ◽  
Vol 13 (6) ◽  
pp. 840
Author(s):  
Liaochao Song ◽  
Jun Deng ◽  
Wangru Wei
Keyword(s):  
Free Surface ◽  
Flow Direction ◽  
The Self ◽  
Air Concentration ◽  
Mean Velocity ◽  
Velocity Fluctuations ◽  
Air Bubble ◽  
Chute Flow ◽  
Developing Region ◽  
Channel Bottom

Self-aerated flows in flat chutes are encountered downstream of the bottom outlet, in spillways with a small slope and in storm waterways. In the present study, the development of self-aeration in flat chute flow is described and new experiments are performed in a long flat chute with a pressure outlet for different flow discharge rates. The distribution of air concentration, time mean velocity and velocity fluctuation in flow direction in the self-aerated developing region—where air bubbles do not diffuse next to the channel bottom—were measured and analyzed. The region of self-aeration from free surface was about 27.16% to 51.85% of the water depth in the present study. The analysis results revealed that the maximum distance of air bubble diffusion to the channel bottom increased with the development of self-aeration along the flow direction. This indicates that for flat chute flow, the process of air bubble diffusion from free surface to channel bottom was relatively long. Cross-section velocities increased along the flow direction in the self-aerated developing region, and this trend was much more remarkable in the area near water free surface. The velocity fluctuations in flow direction in cross-sections flattened and increased with the development of self-aerated flow. Higher velocity fluctuations in flow direction corresponded to the presence of much stronger turbulence, which enhanced air bubble diffusion from the water free surface to channel bottom along the flow direction.

scholarly journals Influence of boundary conditions on the hydrodynamic forces of an oscillating sphere

2018 ◽  
Vol 180 ◽  
pp. 02067 ◽  
Author(s):  
Domenica Mirauda ◽  
Marco Negri ◽  
Luca Martinelli ◽  
Stefano Malavasi
Keyword(s):  
Free Surface ◽  
Flow Direction ◽  
Experimental Tests ◽  
Free Surface Flow ◽  
Transverse Flow ◽  
Hydrodynamic Forces ◽  
Surface Flow ◽  
Low Mass ◽  
Low Mass Ratio ◽  
Channel Bottom

The design of submerged structures in sea currents presents certain problems that are not only connected to the shape of the obstacle but also to the number of acting forces as well as the correct modelling of the structures dynamic response. Currently, the common approach is that of integrated numerical modelling, which considers the contribution of both current and structure. The reliability of such an approach is better verified with experimental tests performed on models of simple geometry. On the basis of these considerations, the present work analyses the hydrodynamic forces acting on a sphere, which is characterised by a low mass ratio and damping. The sphere is immersed in a free surface flow and can oscillate along the streamwise and transverse flow direction. It is located at three different positions inside the current: close to the channel bottom, near the free surface and in the middle, and equally distant from both the bottom and free surface. The obtained results for different boundaries and flow kinematic conditions show a relevant influence of the free surface on the hydrodynamic forces along both the streamwise and transverse flow directions.

Interaction of a turbulent round jet with the free surface

1994 ◽  
Vol 261 ◽  
pp. 305-332 ◽  
Author(s):  
C. K. Madnia ◽  
L. P. Bernal
Keyword(s):  
Free Surface ◽  
Large Scale ◽  
Flow Direction ◽  
Surface Current ◽  
Mean Velocity ◽  
Flow Measurements ◽  
Vortical Structures ◽  
Round Jet ◽  
Curvature Measurements ◽  
Hot Film

The interaction of a turbulent round jet with the free surface was investigated experimentally. Flow visualization, free-surface curvature measurements and hot-film velocity measurements were used to study this flow. It is shown that surface waves are generated by the large-scale vortical structures in the jet flow as they interact with the free surface. These waves propagate at an angle with respect to the flow direction which increases as the Froude number is increased. Propagation of the waves in the flow direction is suppressed by the surface current produced by the jet. Farther downstream the surface motions are caused by the large-scale vortical structures. Characteristic dark circular features are observed in shadowgraph images associated with concentrated vorticity normal to the free surface. The normal vorticity is believed to be the result of vortex line reconnection processes in the turbulent flow. Measurements of the mean velocity and turbulence intensity are reported. Owing to the confinement by the free surface, the decay rate of the maximum mean velocity is reduced by a factor of √2 compared to an unconfined jet.

Forced and Self-Excited Instabilities From Lean Premixed, Liquid-Fuelled Aeroengine Injectors at High Pressures and Temperatures

2013 ◽  
Author(s):  
Simone Hochgreb ◽  
David Dennis ◽  
Isil Ayranci ◽  
William Bainbridge ◽  
Stewart Cant
Keyword(s):  
Heat Release ◽  
Direct Injection ◽  
Excitation Frequency ◽  
The Self ◽  
Normal Operation ◽  
Mean Velocity ◽  
Velocity Fluctuations ◽  
Lean Direct Injection ◽  
Flame Response ◽  
The Mean

Measurements of unsteady pressure and chemiluminescence during flow forced operation of aeroengine lean direct injection fuel spray nozzles were made, with a goal to determine the response of the flame, subject to a range of air fuel ratios, fuel flow splits between pilot and mains injectors, and cooling flows. A rotating shutter installed at the downstream choked nozzle provided the excitation for forcing the mass flow rate between 100 to 600 Hz, at normalized intensities of 0.1 to 0.7 relative to the mean velocity at the injector. The experiments were performed at inlet conditions of 800 K and 5.7 bar. Self-excitation created by the coupling between the flame and the combustor cavity was observed, in the form of a broad peak around 275 Hz. Numerical studies indicate that the peak is associated with an entropy spot (a region of non-uniform temperature) travelling from the flame to the choked nozzle, followed by the ensuing expansion wave towards the injector and amplification of the excitation. Investigation of previous studies suggests that similar phenomena may have been present in other studies at high pressure. The main impact of the self-excitation is the significant amplification of the velocity fluctuations from 0.1 of the mean velocity away from the self-excitation frequency to around 0.7 at the peak. The flame response, represented by the ratio of the fractional fluctuations in OH* chemiluminescence to the fractional velocity fluctuations at the injector, can be determined under conditions where the self-excitation heat release contributes only a small portion of the forced heat release, based on the measured background. The flame response shows a significant dependence on both air fuel ratio and fuel splits, with a decreasing gain towards higher frequency. The results show that it is possible to generate high amplitude fluctuations on the flow using this method, and demonstrate the role of entropy spots during normal operation in lean direct injection systems. Finally, the results suggest that there is an interaction between the forcing frequency and the self-excitation, which may behave in a non-linear manner, and which deserves further investigation.

scholarly journals Experiments on Turbulence Intensity and Bubble Frequency in Self-Aerated Open Channel Flows

Water ◽  
2018 ◽  
Vol 10 (9) ◽  
pp. 1201 ◽  
Author(s):  
Ruidi Bai ◽  
Faxing Zhang ◽  
Shanjun Liu ◽  
Wei Wang
Keyword(s):  
Turbulence Intensity ◽  
Open Channel ◽  
Water Velocity ◽  
The Self ◽  
Air Concentration ◽  
Bubble Frequency ◽  
Air Bubble ◽  
Comprehensive Properties ◽  
Self Similar ◽  
Velocity Turbulence

Although spillways have been investigated experimentally by various researchers, only a few studies have been conducted on the comprehensive properties of a self-aerated air-water flow. In this study, new experimental data were recorded and discussed for the distribution of the air concentration, air-water velocity, turbulence intensity and bubble frequency in the completely developed regions for spillways. It was observed that both the turbulence intensity and bubble frequency increased from the bottom and subsequently decreased near the free surface. The positions of maximum air bubble frequency and turbulence intensity gradually approached air concentration to 0.50 in the self-aerated developed region. Self-similar relationships between the turbulence intensity and bubble frequency were proposed.

Study on Evaluation of Sound Speed in Duct With Tube Banks

2010 ◽  
Author(s):  
Kunihiko Ishihara
Keyword(s):  
Heat Exchanger ◽  
Resonance Frequency ◽  
Vortex Shedding ◽  
Sound Speed ◽  
Flow Direction ◽  
Fem Analysis ◽  
The Self ◽  
Resonance Phenomenon ◽  
Acoustic Characteristics ◽  
Tube Banks

As tube banks are set in a duct in a boiler and a heat exchanger, the resonance phenomenon or the self sustained tone are generated due to the interference between vortex shedding and the acoustic characteristics of the duct. It is necessary to know the resonance frequency of the duct, namely sound speed, for avoiding any trouble that may arise. In general, it is said that the sound speed decreases in the duct with tube banks and an evaluation formula is given. However, this formula is often used for the perpendicular direction of the flow. We wanted to know whether this formula would be able to be used for the flow direction and for various arrays of patterns or not. In this paper, the applicability of this expression is discussed by using FEM analysis and experiments.

scholarly journals On the structure of the self-sustaining cycle in separating and reattaching flows

2018 ◽  
Vol 857 ◽  
pp. 907-936 ◽  
Author(s):  
A. Cimarelli ◽  
A. Leonforte ◽  
D. Angeli
Keyword(s):  
Shear Layer ◽  
Rectangular Plate ◽  
Large Scale ◽  
Reverse Flow ◽  
Leading Edge ◽  
Streamwise Velocity ◽  
The Self ◽  
Small Scale ◽  
Spectral Evolution ◽  
Mean Velocity

The separating and reattaching flows and the wake of a finite rectangular plate are studied by means of direct numerical simulation data. The large amount of information provided by the numerical approach is exploited here to address the multi-scale features of the flow and to assess the self-sustaining mechanisms that form the basis of the main unsteadinesses of the flows. We first analyse the statistically dominant flow structures by means of three-dimensional spatial correlation functions. The developed flow is found to be statistically dominated by quasi-streamwise vortices and streamwise velocity streaks as a result of flow motions induced by hairpin-like structures. On the other hand, the reverse flow within the separated region is found to be characterized by spanwise vortices. We then study the spectral properties of the flow. Given the strongly inhomogeneous nature of the flow, the spectral analysis has been conducted along two selected streamtraces of the mean velocity field. This approach allows us to study the spectral evolution of the flow along its paths. Two well-separated characteristic scales are identified in the near-wall reverse flow and in the leading-edge shear layer. The first is recognized to represent trains of small-scale structures triggering the leading-edge shear layer, whereas the second is found to be related to a very large-scale phenomenon that embraces the entire flow field. A picture of the self-sustaining mechanisms of the flow is then derived. It is shown that very-large-scale fluctuations of the pressure field alternate between promoting and suppressing the reverse flow within the separation region. Driven by these large-scale dynamics, packages of small-scale motions trigger the leading-edge shear layers, which in turn created them, alternating in the top and bottom sides of the rectangular plate with a relatively long period of inversion, thus closing the self-sustaining cycle.

Flow over a stepped chute with and without macro-roughness elements

2004 ◽  
Vol 31 (5) ◽  
pp. 880-891 ◽  
Author(s):  
Mehmet Ali Kökpinar
Keyword(s):  
Water Flow ◽  
High Speed ◽  
Length Distribution ◽  
Air Entrainment ◽  
Air Concentration ◽  
Bubble Frequency ◽  
Two Phase ◽  
Air Bubble ◽  
Roughness Elements ◽  
Stepped Chute

High-speed two-phase flows over a 30° stepped flume were experimentally investigated using macro-roughness elements. The roughness elements included combinations of steps and horizontal strips. Local values of air concentration, air bubble frequency, and mean chord lengths were measured by a fiber-optical instrumentation system in the air–water flow region. The range of unit discharge of water was varied from 0.06 to 0.20 m2/s. Three step configurations were studied: (i) without macro-roughness elements, (ii) with macro-roughness elements on each step, and (iii) with macro-roughness elements on each second step (AMR configuration). The results were compared in terms of onset flow conditions and internal air–water flow parameters such as local air concentration, mean air bubble chord length distribution, and air bubble frequency in the skimming flow regime. It was observed that the AMR configuration produced the maximum free-surface aeration among the other configurations. This alternative step geometry has potential for less cavitation damage than conventional step geometry because of the greater air entrainment.Key words: stepped chute, air-entrainment, air-water flow properties, macro-roughness elements, skimming flow.

Turbulence measurements in axisymmetric jets of air and helium. Part 2. Helium jet

1993 ◽  
Vol 246 ◽  
pp. 225-247 ◽  
Author(s):  
N. R. Panchapakesan ◽  
J. L. Lumley
Keyword(s):  
Density Ratio ◽  
Effective Diameter ◽  
Mean Velocity ◽  
Velocity Fluctuations ◽  
Air Jet ◽  
Plume Region ◽  
Turbulent Schmidt Number ◽  
Radial Profiles ◽  
The Mean ◽  
Mean Concentration

A turbulent round jet of helium was studied experimentally using a composite probe consisting of an interference probe of the Way–Libby type and an × -probe. Simultaneous measurements of two velocity components and helium mass fraction concentration were made in the x/d range 50–120. These measurements are compared with measurements in an air jet of the same momentum flux reported in Part 1. The jet discharge Froude number was 14000 and the measurement range was in the intermediate region between the non-buoyant jet region and the plume region. The measurements are consistent with earlier studies on helium jets. The mass flux of helium across the jet is within ±10% of the nozzle input. The mean velocity field along the axis of the jet is consistent with the scaling expressed by the effective diameter but the mean concentration decay constant exhibits a density-ratio dependence. The radial profiles of mean velocity and mean concentration agree with earlier measurements, with the half-widths indicating a turbulent Schmidt number of 0.7. Significantly higher intensities of axial velocity fluctuations are observed in comparison with the air jet, while the intensities of radial and azimuthal velocity fluctuations are virtually identical with the air jet when scaled with the half-widths. Approximate budgets for the turbulent kinetic energy, scalar variance and scalar fluxes are presented. The ratio of mechanical to scalar timescales is found to be close to 1.5 across most of the jet. Current models for triple moments involving scalar fluctuations are compared with measurements. As was observed with the velocity triple moments in Part 1, the performance of the Full model that includes all terms except advection was found to be very good in the fully turbulent region of the jet.

scholarly journals Freshwater Mussel Bed Habitat in an Alluvial Sand-Bed-Material-Dominated Large River: A Core Flow Sediment Refugium?

Diversity ◽  
2020 ◽  
Vol 12 (5) ◽  
pp. 174
Author(s):  
Alan D. Christian ◽  
Andrew J. Peck ◽  
Ryan Allen ◽  
Raven Lawson ◽  
Waylon Edwards ◽  
...  
Keyword(s):  
Flow Direction ◽  
Freshwater Mussel ◽  
Physical Habitat ◽  
Stream Power ◽  
Mussel Bed ◽  
Mean Velocity ◽  
Core Flow ◽  
White River ◽  
Bed Material ◽  
Mussel Habitat

Habitat degradation, organismal needs, and other effects influencing freshwater mussel declines have been subject to intense focus by conservationists for the last thirty plus years. While researchers have studied the physical habitat requirements and needs of mussels in small- to medium-sized rivers with variable levels of success, less research has been conducted on mussel habitat in larger non-wadeable rivers, especially at the reach scale, where core flow environmental conditions provide and maintain habitat for freshwater mussels. We designed a quasi-experimental observational field study to examine seven hydrologic energy and material variables laterally and longitudinally at Current and Extirpated mussel bed habitat reaches in lower White River, Arkansas, a large non-wadeable, sand-bed-material-dominated river. As expected, lateral and longitudinal hydrologic variable differences were identified within a reach. Mean velocity, bed velocity, the Froude number, and stream power were all significantly lower at Current mussel bed habitat stations within a sampling reach. Energy regime differences in shear stress and, marginally, stream power were higher at Extirpated mussel bed habitat reaches. Several factors emerged as important to mussel habitat in the White River. First, bed velocity warrants further exploration in terms of both flow strength and flow direction. Second, bedload appears to be the primary contributor to mussel habitat but requires additional exploration within the context of core and secondary flow pathway interactions. The combined empirical evidence from our study supports the flow refugium concept identified for mussel habitats in smaller systems but expands the concept to large non-wadeable streams and includes reach-scale refuge from sediment transport conditions.

scholarly journals Bidirectional cyclical flows increase energetic costs of station holding for a labriform swimming fish, Cymatogaster aggregata

2020 ◽  
Vol 8 (1) ◽  
Author(s):  
Sarah M Luongo ◽  
Andreas Ruth ◽  
Connor R Gervais ◽  
Keith E Korsmeyer ◽  
Jacob L Johansen ◽  
...  
Keyword(s):  
Steady Flow ◽  
Flow Direction ◽  
Energy Budgets ◽  
Constant Flow ◽  
Energetic Costs ◽  
Mean Velocity ◽  
Unidirectional Flow ◽  
In The Wild ◽  
Additional Costs ◽  
Temperate Fish

Abstract Wave-induced surge conditions are found in shallow marine ecosystems worldwide; yet, few studies have quantified how cyclical surges may affect free swimming animals. Here, we used a recently adapted respirometry technique to compare the energetic costs of a temperate fish species (Cymatogaster aggregata) swimming against a steady flow versus cyclical unidirectional and bidirectional surges in which unsteady swimming (such as accelerating, decelerating and turning) occurs. Using oxygen uptake (ṀO2) as an estimate of energetic costs, our results reveal that fish swimming in an unsteady (i.e. cyclical) unidirectional flow showed no clear increase in costs when compared to a steady flow of the same average speed, suggesting that costs and savings from cyclical acceleration and coasting are near equal. Conversely, swimming in a bidirectional cyclical flow incurred significantly higher energetic costs relative to a steady, constant flow, likely due to the added cost of turning around to face the changing flow direction. On average, we observed a 50% increase in ṀO2 of fish station holding within the bidirectional flow (227.8 mg O2 kg−1 h−1) compared to a steady, constant flow (136.1 mg O2 kg−1 h−1) of the same mean velocity. Given wave-driven surge zones are prime fish habitats in the wild, we suggest the additional costs fish incur by station holding in a bidirectional cyclical flow must be offset by favourable conditions for foraging and reproduction. With current and future increases in abiotic stressors associated with climate change, we highlight the importance of incorporating additional costs associated with swimming in cyclical water flow in the construction of energy budgets for species living in dynamic, coastal habitats.

Sign in / Sign up

Export Citation Format

Share Document