Display omitted
•Detailed air-water flow measurements were performed in a highly turbulent free-surface flow.•Three velocimetry approaches were applied, including intrusive and non-intrusive.•OF data ...were obtained from both side and top views.•Dual-tip phase-detection probe data were used to validate optical flow (OF) data.•The results highlighted both advantages and limitations of the three complementary metrologies.
Self-aerated free-surface flow studies have a more recent history compared to classical fluid dynamics. Traditional velocimetry techniques are adversely affected by the presence of gas–liquid interfaces. In the present study, detailed air-water flow measurements were performed in a highly turbulent free-surface flow, and three velocimetry approaches were applied: (a) centreline dual-tip phase-detection needle probe measurements at all step edges downstream of the inception location of free-surface aeration; (b) Optical Flow (OF) data based upon ultra-high-speed video movies through the left sidewall; and (c) Optical Flow (OF) data based upon ultra-high-speed video movies overlooking the self-aerated flow, in a direction normal to the pseudo-bottom formed by the staircase profile. The study was conducted in a steep channel and three stepped invert geometries were tested. The results highlighted both advantages and limitations of the three complementary metrologies in free-surface flows with strong turbulence. The dual-tip phase-detection probe delivered reliable interfacial velocity data, but in the form of point measurements. The sideview OF technique provided a great level of details of the cavity recirculation and shear zone between mainstream and cavity, but the data were unreliable for void fractions>0.30 and only limited to the sidewall region. The top view OF technique characterised the surface velocity field across the entire chute width, highlighting the occurrence of three-dimensional air-water surface patterns. One observed limitation of the OF was the requirements of a high frame rate (i.e. 10,000 fps or more) and high-quality light source. Overall, these measurement techniques provided complementary results for a better understanding of the physical behaviour of highly turbulent multiphase flows on stepped channels.
Fluid-structure interaction (FSI) of gas-liquid two-phase flow in the horizontal pipe is investigated numerically in the present study. The volume of fluid model and standard
k
-
ε
turbulence model ...are integrated to simulate the typical gas-liquid two-phase flow patterns. First, validation of the numerical model is conducted and the typical flow patterns are consistent with the Baker chart. Then, the FSI framework is established to investigate the dynamic responses of the interaction between the horizontal pipe and gas-liquid two-phase flow. The results show that the dynamic response under stratified flow condition is relatively flat and the maximum pipe deformation and equivalent stress are 1.8 mm and 7.5 MPa respectively. Meanwhile, the dynamic responses induced by slug flow, wave flow and annular flow show obvious periodic fluctuations. Furthermore, the dynamic response characteristics under slug flow condition are maximum; the maximum pipe deformation and equivalent stress can reach 4 mm and 17.5 MPa, respectively. The principal direction of total deformation is different under various flow patterns. Therefore, the periodic equivalent stress will form the cyclic impact on the pipe wall and affect the fatigue life of the horizontal pipe. The present study may serve as a reference for FSI simulation under gas-liquid two-phase transport conditions.
•Non-stationary vibration was identified on ducts conveying two-phase flows.•Elongated bubble velocity determined by using different methods on the vibration signal.•Different experimental procedures ...to identify and validate the technique.•Extensive test-matrix to evaluate the vibration signal over different flow conditions.
The multiphase flow occurs in various major industrial fields and nature. Furthermore, due to the single and multiphase flow characteristics, they generate vibration on the structure conveying or subjected to the flow. This paper investigates in an experimental apparatus, the possibility to use the flow-induced vibration from a vertical liquid-gas two-phase flow conveyed by a pipe to obtain the elongated bubble velocity. The vibration signal analysis in time domain showed that the structural excitation due to the elongated bubble rising in the stagnant liquid was significant to be distinguished from other excitations. The time-frequency analysis of slug and churn flow showed a significant amplitude variation in a specific frequency band. Finally, it was possible to obtain the elongated bubble velocity with reasonable accuracy by cross-correlating the envelope of a vibration signal filtered in a particular frequency band of two accelerometers. Thus, this paper presents a non-invasive and simple to mount technique to estimate the elongated bubble velocity in stagnant and moving liquid conditions.
In this paper, computational fluid dynamics (CFD) simulations are employed to characterize the effects of bubble aerator layouts (i.e. spatial arrangement) on the hydrodynamics in activated sludge ...(AS) reactors. The first configuration considered is a channel reactor with aerators placed alongside one lateral wall, for which velocity measurements are available in literature. CFD results were in good agreement with experimental data, which proves that the model is sufficiently accurate and predictive. Accordingly, simulations and numerical residence time distribution tests were conducted for different aerator layouts to determine their effects on the reactor hydrodynamics. The results revealed that the flow characteristics are extremely sensitive to the aerators arrangement given the high gas flow rates used in AS processes. Among the layouts investigated, the one where diffusers are placed all over the reactor floor has led to the least dispersive flow, i.e. which characteristics best tend toward that of an ideal plug flow reactor. Indeed, this flow field presented the lowest average turbulent diffusion and the most uniform axial velocity and turbulence fields. Such a flow behaviour is expected to be highly beneficial for biological treatment since it reduces pollutant dilution by axial diffusion and limits raw wastewater channelling to the outlet.
The convective flow generated by a source located at a water surface containing an insoluble surfactant has been investigated experimentally. The application of various sources, which differ in the ...way in which they drive the interface into motion, and two surfactants with different rheological properties made it possible to generalize the results and to develop a unified approach to describing the problem. We show that the threshold occurrence of a two-zone flow structure with an axisymmetric flow nearby the source and stagnant zone at the periphery results from the competition between two counter-directed shear stresses caused by surfactant concentration inhomogeneity and source influence. We demonstrate that, regardless of the source type, the ratio of these stresses, known as the elasticity number, can be used to predict the formation of a two-zone flow and to define the position of the boundary between the zones. We examined in detail the conditions for formation of the multi-vortex flow within the stagnant zone. Although this phenomenon has been observed by many researchers, it has not yet met a consistent physical explanation. We hypothesized that the formation of vortices can be considered as the instability of the mechanical equilibrium of the surfactant layer caused by the shear on the side of the underlying bulk flow. Based on the mechanism suggested, we introduced a non-dimensional parameter, called the surface Rayleigh number, and estimated its critical value. Finally we analysed the conditions for the occurrence of this instability, called the shear-driven surfactant layer instability in some famous problems of interfacial hydrodynamics.
This article provides theoretical conditions for the use and meaning of a stability analysis around a mean flow. As such, it may be considered as an extension of the works by McKeon & Sharma (J. ...Fluid Mech., vol. 658, 2010, pp. 336–382) to non-parallel flows and by Turton et al. (Phys. Rev. E, vol. 91 (4), 2015, 043009) to broadband flows. Considering a Reynolds decomposition of the flow field, the spectral (or temporal Fourier) mode of the fluctuation field is found to be equal to the action on a turbulent forcing term by the resolvent operator arising from linearisation about the mean flow. The main result of the article states that if, at a particular frequency, the dominant singular value of the resolvent is much larger than all others and if the turbulent forcing at this frequency does not display any preferential direction toward one of the suboptimal forcings, then the spectral mode is directly proportional to the dominant optimal response mode of the resolvent at this frequency. Such conditions are generally met in the case of weakly non-parallel open flows exhibiting a convectively unstable mean flow. The spatial structure of the singular mode may in these cases be approximated by a local spatial stability analysis based on parabolised stability equations (PSE). We have also shown that the frequency spectrum of the flow field at any arbitrary location of the domain may be predicted from the frequency evolution of the dominant optimal response mode and the knowledge of the frequency spectrum at one or more points. Results are illustrated in the case of a high Reynolds number turbulent backward facing step flow.
•Stable Ag/water nanofluid has been prepared by a novel method of electrical explosion of wire.•Thermal and electrical efficiencies of the utilized PV/T have been evaluated applying the ...nanofluids.•Energy and exergy analysis have been carried out for the tested PV/T.•Total efficiency of the tested PV/T has been compared with the case of using water as the working fluid.
This paper aims to investigate the electrical, thermal and exergy efficiencies of a photovoltaic/ thermal (PV/T) system cooled by Ag/water nanofluid. The utilized nanofluid was prepared by a one-step method of electrical explosion of wire (EEW) and was tested for long-term stability and uniformity. The performance of the PV/T system was measured by considering a wide range of parameters to determine key performance indicators such as electrical and thermal energy efficiency as well as exergy efficiency of the system. The effects of mass flow (i.e. different flow regimes of laminar, transient, and turbulent) on efficiencies were studied. The results showed that using nanofluids for cooling of the PV/T system can enhance both the energy and exergy efficiencies of the system significantly. It was also found that this positive impact is more pronounced by increasing the concentration of the nanofluid and increasing the flow rate (i.e. moving towards a turbulent flow). By using 4 wt% nanofluid (with turbulent flow) the power output of the panel increased by ∼35% and ∼10% compared to when no cooling and water cooling were applied respectively; and the exergy efficiency was also determined to be 50% and 30% higher than when no cooling and water cooling were used, respectively.
In axial flow pump systems, not only are the flow rates typically large but the water leaving the guide vanes almost invariably possesses a residual rotation. Since such pumps often generate only low ...heads, the energy content of this residual rotation often has a notable influence on the hydraulic and economic performance of the outlet conduit. The hydraulic losses and flow fields associated with a specific siphon outlet conduit under different rotational speeds are herein studied both experimentally and using a 3D turbulent flow numerical simulation. In order to experimentally verify the simulation results, the hydraulic losses are measured and flow patterns are observed for a siphon outlet conduit with a series of different guide vanes. It is observed that as the rotational speed increases the hydraulic loss within this outlet conduit increases gradually until the rotation speed reaches 300
; as the rotational speed surpasses this value, the hydraulic losses increase sharply. Yet surprisingly, residual swirl can have benefits, with the general trend being that larger rotational speeds are associated with a smaller vortex zones and a better flow field in the outlet conduit, an observation confirmed both numerically and experimentally. This research has considerable significance, both theoretically and practically, for the design of outlet conduits and guide vanes in axial flow pump system.
Display omitted
•Two-phase flows were investigated experimentally in a radial centrifugal pump.•Hysteresis, head degradation, pump surging and flow instabilities were considered.•The two-phase flow ...regimes were recorded and identified using a high-speed camera.•Effects of the tip clearance gap and of adding an inducer were thoroughly studied.•Increasing the gap or installing the inducer improved transport of two-phase flows.
The transport of two-phase (air/water) was experimentally investigated in a semi-open pump impeller of radial type. The influence of increasing the tip clearance gap and installing an upstream inducer on the pump performance were thoroughly investigated and compared, helping to select the optimal operation based on the considered flow conditions. Three experimental procedures were used to set the desired flow conditions to check a possible hysteresis due to gas accumulation. The head degradation, pump surging and flow instabilities were investigated for all cases. The whole pump was made of transparent acrylic glass, providing high-quality flow visualization. The two-phase flow regimes were recorded and identified using a high-speed camera. For single-phase flow, the results show a considerable performance reduction for the increased gap, while the inducer has only insignificant effects. For gas volume fractions within 1%⩽ε⩽3%, the performance deterioration of the semi-open impeller with standard gap is very gradual. However, the performance is strongly reduced for ε⩾4% due to the onset of big gas accumulations in the impeller. The abrupt performance drop could be delayed to ε=7% for the increased gap. Similarly, installing the inducer resulted in more robust performance up to ε=5% and ε=7% for overload and part-load flow conditions, respectively. For ε⩾8%, the performance is very low in all cases due to the occurrence of gas-locking phenomenon. Comparing different experimental procedures, obvious performance hysteresis could be seen within 4%⩽ε⩽6% when the standard gap is used. However, installing the inducer could strongly reduce the performance hysteresis, while increasing the gap could completely eliminate it. Flow instabilities and pump surging occur mainly in overload conditions for 4%⩽ε⩽5%. Having the inducer installed could positively damp the instabilities and reduce the surging region. The obtained two-phase maps show the strongly increased resistance to gas accumulation for the larger gap and the moderately improved resistance when installing the inducer. All the obtained experimental results are used for checking and validating CFD numerical models in a companion study.
A simulation of stably stratified plane Poiseuille flow at a moderate Reynolds number ($\textit {Re}_\tau = 550$) and Richardson number ($\textit {Ri}_\tau = 480$) is presented. For the first time, ...the dynamics in the channel core are shown to be described as a series of internal waves that approximately obey a linear wave dispersion relationship. For a given streamwise wavenumber $k_x$ there are two internal wave solutions, a dominant low frequency mode and a weaker-amplitude high-frequency mode, respectively corresponding to ‘backward’ and ‘forward’ propagating internal waves relative to the mean flow. Analysis of linearised equations shows that the dominant low-frequency mode appears to arise due to a particularly sensitive response of the mean flow profiles to incoherent forcing. Instantaneous visualisations reveal that hairpin vortices dominate the outer region of the channel flow, neighbouring the buoyancy dominated channel core. These hairpins are fundamentally different from those observed in canonical unstratified boundary layer flows, as they arise via quasi-linear local processes far from the wall, governed by background shear. Outer region ejection events are common and can be induced by high amplitude waves. Ejected hairpins are transported into the channel core, in turn ‘ringing’ the prevailing strong buoyancy gradient and thus generating high-amplitude internal waves, high dissipation and wave breaking, induced by spanwise vortex stretching and baroclinic vorticity generation. Such spontaneous and sustained generation of quasi-linear internal waves by wall-bounded sheared turbulence may provide novel idealised solutions for, and insight into, large-scale turbulent mixing in a wide range of environmental and industrial flows.
PDF
