Oscillatory Flow Regimes Research Articles

Pattern selection and heat transfer in the Rayleigh–Bénard convection near the vicinity of the convection onset with viscoelastic fluids

The effect of viscoelasticity on the flow and heat transport in the Rayleigh–Bénard convection (RBC), a frequently encountered phenomenon in nature and industry, in a rectangular enclosure with horizontal periodic boundary is investigated via direct numerical simulation. The working fluid is described by a finitely extensible nonlinear elastic-Peterlin constitutive model almost all important features of viscoelastic fluid flow. Numerical simulations are conducted at a low concentration β=0.9, where β=μs/μ0, μs is the solvent viscosity, and μ0=μs+μp is the sum of μs and the polymer viscosity μp. A parametric analysis is performed to understand the influence of the Weissenberg number Wi, the viscosity ratio β, and the extension length L on the oscillating mode of the viscoelastic RBC. The results indicate that both Wi and β weakly inhibit the convection onset and the transition from steady to oscillatory convection. The amplitude and frequency of the oscillations in the oscillatory flow regime are both suppressed. However, the strongly elastic nonlinearity makes the flow transition irregular and even brings about the relaminarization or lead to the convection cells traveling in the horizontal direction. The increasing extension length L induces multiple pairs of roll flow patterns at a specific setting of (Ra, Wi). Heat transport is reduced (up to 8.5%) by elasticity but still obeys the power law with Ra if the flow pattern has one pair of rolls. However, heat transfer enhancement occurs if multiple pairs of rolls are induced.

Temperature evolution, flame behavior and extinction inside compartment with a ceiling vent under the ambient wind

This article presents an experimental and CFD simulation investigation and analysis on temperature evolution and flame behavior inside compartment with a ceiling vent under ambient wind. Experiments were conducted employing a reduced-scale model containing a cubic fire compartment with a ceiling vent under the external wind generated by a wind tunnel. The temperature of windward and leeward inside compartment as well as the flame behavior were recorded for various vent dimensions, heat release rates and wind speeds for a total of 720 test conditions. Results show that there are two types of fire behavior regimes inside the compartment for relatively small- (Bernoulli flow regime) or large vents (oscillatory exchange flow regime). With a relatively small vent, the flame is located at the center of compartment and the temperatures of windward and leeward are almost the same with or without the ambient wind. Then the flame turns to extinct due to lack of oxygen with increasing of fuel supply. However, with a relatively large vent, there is a flame transition from windward to leeward accompanied by the transition of temperature distribution with increasing of fuel supply when subject to ambient wind. Such extinction and transition mechanisms are interpreted by the aid of CFD simulation of global equivalence ratio and flow/oxygen field inside compartment. The critical heat release rate for the occurrence of flame transition decreases with raising wind speed while increases with vent size. Their complex dependence is found to be well represented in terms of a non-dimensional heat release rate as a function of the wind Froude number, employing the vent area-equivalent characteristic diameter as length scale. These new findings facilitate the understanding of the compartment fire evolution with a ceiling vent subject to ambient wind.

An electrochemical oscillatory flow reactor with pillar array electrodes improving mass transfer in electrosynthesis

Synthetic organic electrochemistry, due to its advantages in terms of environmental impact and energy demand, has in the past years increasingly attracted researchers and companies. Consequently, a variety of (commercially available) electrochemical reactors have been developed. However, the focus remained on conducting fundamental electrochemical research rather than on scalability and productivity. In this work, we present a novel electrochemical reactor concept by applying an oscillatory flow regime in a reactor fitted with conductive pillar field electrodes which serve both as electroactive interface and turbulence promotor. The use of this electrochemical oscillatory flow reactor (eOFR) enabled the control and improvement of mass transfer, irrespective of residence time. This resulted in current densities rise up to 4-fold versus conventional steady state flow regimes and as such, this novel reactor bridges the gap between fundamental research and industrial applications.

Computational Fluid Dynamics modelling of hydrodynamic characteristics of oscillatory flow past a square cylinder using the level set method

This paper presents a numerical study of the three-dimensional hydrodynamic characteristics of oscillatory flow past a square cylinder including the effect of the variation of blockage ratio and the Keulegan–Carpenter (KC) number on those characteristics. The oscillatory flow is simulated using an open-source Computational Fluid Dynamics (CFD) modelling tool REEF3D. The CFD model solves the Reynolds-Averaged Navier–Stokes (RANS) equations in all the three dimensions. The location of the free-surface is represented using the level-set method (LSM), which calculates the complex motion of the free-surface in a very realistic manner. For the implementation of the waves, the CFD code is used as a numerical wave tank. First, the model has been extensively validated against experimental measurements present in the literature. Thereafter, further analysis is carried out using the CFD code to study the effect of blockage ratio on the hydrodynamic characteristics (drag coefficient, inertia coefficient, lift coefficient and coefficient of pressure) for different oscillatory flow regimes. To this effect KC numbers ranging between 3 and 13 (implying different flow regimes) have been selected for blockage ratios of 0.05, 0.0625, 0.1, 0.2, 0.3, 0.5 (for wall effect). The results indicate that the drag coefficient and the lift coefficient increases with the increase of blockage ratios for a particular KC number. Further, both these coefficients increases with the increase of KC number at a fixed blockage ratio. The coefficient of inertia, however, increases up to a blockage ratio of 0.2 and decreases thereafter for a particular KC number. The results also show that for a particular blockage ratio the coefficient of inertia decreases on escalating KC number and the coefficient of pressure increases for large blockage ratios at a fixed KC number on the upstream of the cylinder and reduces on the downstream side of the cylinder.

A Novel Computational Model of the Dynamic Response of the Evaporating Liquid-Vapor Interface in a Capillary Channel

A computational model is developed to investigate the dynamic response of an evaporating meniscus during the capillary flow between vertical parallel plates. A previously developed arbitrary Lagrangian-Eulerian (ALE) model was extended to directly track the formation and evolution of the evaporating meniscus during spontaneous liquid penetration within a capillary channel. The two-dimensional time-dependent conservation equations for mass, momentum, and energy were solved in a finite-volume framework implemented on a moving and deforming grid. The sharp interface tracking method developed here enables direct access to the flow variables and transport fluxes at the meniscus with no need for averaging techniques. The model was validated by comparing the predicted dynamic response of the capillary height subject to interfacial evaporation against theoretical results. The effects of wall spacing and liquid superheat on the capillary flow and the evaporation rate were studied. It was found that thermal diffusion adjacent to the meniscus has a critical effect on the evaporation rate, and neglecting it leads to significant overprediction of the evaporation rate. Results show that, in general, inclusion of evaporation causes a reduction of the liquid column height compared to the non-evaporating case. It was also observed that the equilibrium capillary height is inversely proportional to the liquid superheat. Analyses of the transient regime show that evaporation tends to dampen the oscillatory flow regime compared to the non-evaporating meniscus case.

A numerical lift force analysis on the inertial migration of a deformable droplet in steady and oscillatory microchannel flows.

Inertial migration of deformable particles has become appealing in recent years due to its numerous applications in microfluidics and biomedicine. The physics underlying the motion of these particles is contingent upon the presence of lift forces in microchannels. Therefore, in this work, we present a lift force analysis for such migration of a deformable droplet in steady and oscillatory flow regimes and identify the effects of varying Capillary number and oscillation frequency on its dynamics. We then propose an expression that mimics the lift force behavior in oscillatory flows accurately. Finally, we introduce a procedure to derive and predict a simple expression for the steady and averaged oscillatory lift for any given combination of Capillary number and oscillation frequency within a continuous range.

Study of the wave period effect on the time-averaged suspended sediment concentration distribution under the oscillatory sheet flow condition

ABSTRACT Existing sheet flow experimental data show the measured time-averaged suspended sediment concentration (TSSC) may increase, decrease, or even remain constant if the wave period decreases. With respect to the available sheet flow experimental data, it is confirmed that the TSSC of relatively fine sand undergoes three stages, i.e. first decreases, then increases, and finally decreases again in the case that the wave period decreases from 12 s to 2 s, whereas for the relatively coarse sand, its vertical profile remains almost unchanged. The criterion for identifying the relatively fine or coarse sand was proposed in terms of the nondimensional ratio between the settling velocity and the root-mean-square flow velocity. If this ratio is less than 0.04–0.043, it is deemed as relatively fine sand. Otherwise, it is classified as relatively coarse sand. Applying the classical gradient diffusion model, sediment diffusivity is confirmed to be insensitive to the wave period. Keeping other experimental conditions uniform, TSSCs with different wave periods are proportional to each other and only affected by the reference concentration. Subsequently, a simple quantitative expression considering the wave period effect on the TSSC under the oscillatory sheet flow regime was proposed and verified with the experimental data.

Oscillatory thermocapillary convection in deformed half zone liquid bridges of low Prandtl number fluids

A series of three-dimensional (3D) transient simulations are carried out to study the onset and features of oscillatory thermocapillary convection in deformed half zone liquid bridges of low Prandtl number fluids (Pr = 0.01) with unit aspect ratio (As). The critical conditions for different volume ratios on ground and under microgravity are determined. It turns out that the threshold of the second bifurcation is more sensitive to volume ratio than the first bifurcation. The thresholds in deformed liquid bridges are usually higher than those in cylindrical liquid bridges. The critical azimuthal wavenumber increases from 2 to 3 with volume ratio. Gravity affects the flow through three factors. Compared with the hydrostatic deformation and heating orientation, buoyancy is marginal to the flow transition. It only affects the threshold and the deviation caused by buoyancy is within 2%. The effects of gravity on the critical conditions are dependent on volume ratio. Except for the threshold, gravity may also affect the critical azimuthal wavenumber and the critical oscillation mode. 3D oscillatory flow regimes with two different oscillation modes are present. A new type of “Rocking” mode is discovered in the present study.

Study of Compact Heat Exchangers Operating in Self-Sustained Oscillatory Flows

Abstract Computer codes were developed to study the performance of compact heat exchangers (CHEs) operating in self-sustained oscillatory flow (SSOF) regimes. The methods were based on a control volume-based finite element (CVFEM) method for geometric discretization and the explicit first stage, single diagonal coefficient, diagonally implicit, Runge–Kutta (ESDIRK) method for temporal discretization. The developed codes were validated for both steady and unsteady cases. A study of nine geometrically related domains of flat tubes in staggered configurations was performed. Grid independence was established subject to double cyclic conditions—periodically fully developed flow and heat transfer in the stream-wise direction and cyclic or repeating flow and heat transfer in the cross-stream direction. The maximum Reynolds number was established at approximately 2000 for the cases studied to avoid the turbulent flow regime. Parameters of interest like Nusselt number, friction factor, and pumping power were calculated for steady and SSOF regimes. An approach was proposed to determine critical Reynolds number (Recrit) for the SSOFs such that for Reynolds number below Recrit the flow remains steady, and above Recrit, the flow exhibits the characteristics of SSOFs before finally transitioning to fully turbulent conditions. The results indicated a sensitivity of performance parameters to transverse spacing but not to longitudinal spacing.

Hydrodynamic stress maps on the surface of a flexible fin-like foil.

We determine the time dependence of pressure and shear stress distributions on the surface of a pitching and deforming hydrofoil from measurements of the three dimensional flow field. Period-averaged stress maps are obtained both in the presence and absence of steady flow around the foil. The velocity vector field is determined via volumetric three-component particle tracking velocimetry and subsequently inserted into the Navier-Stokes equation to calculate the total hydrodynamic stress tensor. In addition, we also present a careful error analysis of such measurements, showing that local evaluations of stress distributions are possible. The consistency of the force time-dependence is verified using a control volume analysis. The flapping foil used in the experiments is designed to allow comparison with a small trapezoidal fish fin, in terms of the scaling laws that govern the oscillatory flow regime. As a complementary approach, unsteady Euler-Bernoulli beam theory is employed to derive instantaneous transversal force distributions on the flexible hydrofoil from its deflection and the results are compared to the spatial distributions of hydrodynamic stresses obtained from the fluid velocity field.

Thermocapillary effects during the melting of phase-change materials in microgravity: steady and oscillatory flow regimes

Abstract

Pressure and Heat Flux Oscillations Induced by Gas-Dynamic Interaction between a High Inertia Particle and a Shock Layer

Oscillatory flow and heat transfer regimes arising from the gas-dynamic interaction between a high-inertial particle and a shock layer in a supersonic flow around bodies are under numerical research. Detailed space-time flow patterns were obtained using high-resolution adaptive grids and parallelization of computations using the graphic processing units.

Comparative Analysis of Calculated and Experimental Data on an Oscillating Flow Induced by the Gasdynamic Interaction of a Particle with a Shock Layer

A supersonic flow around a cylinder with a flat end is studied for the case in which a single particle is launched from the end surface towards the incident flow. Numerical modeling is carried out with allowance for the gas-dynamic interaction of the particle with the shock layer. High-resolution, adaptive, rectangular grids are used to obtain detailed spatial and temporal flow patterns. The calculation results are compared with experimental data. It is shown that a single, large particle moving along the axis of symmetry against the incident flow and crossing the bow shock wave causes a significant restructuring of the flow and the formation of complex shock-wave and vortex structures. An important role in the formation of the flow structure is played by the formation of a toroidal vortex, which leads to an “inviscid” separation of the incident flow from the axis of symmetry. A distinctive feature of the flow around a cylinder with a flat end is the oscillatory flow regime caused by alternating stages of growth and decay of a toroidal vortex. The patterns of the oscillating flow and the oscillation frequency obtained via numerical simulation are in good agreement with the experimental data.

Ir/Ni Photoredox Dual Catalysis with Heterogeneous Base Enabled by an Oscillatory Plug Flow Photoreactor

Continuous flow reactor technology has a proven track record in enabling photochemical transformations. However, transfer of a photochemical batch process to a flow protocol often remains elusive, especially when solid reagents or catalysts are employed. In this work, application of an oscillatory plug flow photoreactor enabled a heterogeneous MacMillan-type C(sp2)–C(sp3) cross-electrophile coupling. Combination of an oscillatory flow regime with static mixing elements imparts exquisite control over the mixing intensity and residence time distribution, pinpointing a mindset shift concerning slurry handling in continuous flow reactors. The C(sp2)–C(sp3) cross-electrophile coupling was successfully transferred from batch to flow, resulting in an intensified slurry process with significantly reduced reaction time and increased productivity (0.87 g/h).

Seagrass coastal protection services reduced by invasive species expansion and megaherbivore grazing

Abstract Seagrasses provide an important ecosystem service by creating a stable erosion‐resistant seabed that contributes to effective coastal protection. Variable morphologies and life‐history strategies, however, are likely to impact the sediment stabilization capacity of different seagrass species. We question how opportunistic invasive species and increasing grazing by megaherbivores may alter sediment stabilization services provided by established seagrass meadows, using the Caribbean as a case study. Utilizing two portable field‐flumes that simulate unidirectional and oscillatory flow regimes, we compared the sediment stabilization capacity of natural seagrass meadows in situ under current‐ and wave‐dominated regimes. Monospecific patches of a native (Thalassia testudinum) and an invasive (Halophila stipulacea) seagrass species were compared, along with the effect of three levels of megaherbivore grazing on T. testudinum: ungrazed, lightly grazed and intensively grazed. For both hydrodynamic regimes, the long‐leaved, dense meadows of the climax species, T. testudinum provided the highest stabilization. However, the loss of above‐ground biomass by intensive grazing reduced the capacity of the native seagrass to stabilize the surface sediment. Caribbean seagrass meadows are presently threatened by the rapid spread of the invasive opportunistic seagrass, H. stipulacea. The dense meadows of H. stipulacea were found to accumulate fine sediment, and thereby, appear to be effective in reducing bottom shear stress during calm periods. This fine sediment within the invasive meadows, however, is easily resuspended by hydrodynamic forces, and the low below‐ground biomass of H. stipulacea make it susceptible to uprooting during storm events, potentially leaving large regions vulnerable to erosion. Overall, this present study highlights that intensive megaherbivore grazing and opportunistic invasive species threaten the coastal protection services provided by mildly grazed native species. Synthesis. Seagrass meadows of dense, long‐leaved species stabilize the sediment surface and maintain the seabed integrity, thereby contributing to coastal protection. These services are threatened by intensive megaherbivore grazing, which reduces the stability of the surface sediment, and opportunistic invasive species, which are susceptible to uprooting in storms and thereby can leave the seabed vulnerable to erosion.

Oscillatory Flow Regimes Resulting from the Shock Layer–Particle Interaction

The paper is devoted to the numerical study of the shock layer–particle interaction in a supersonic flow past bodies. The shock wave associated with a moving particle and the flow in the particle wake is taken into consideration. Distinctive features of the numerical technique are the adaptive Cartesian sliding grids of high resolution, a ghost cells immersed boundary method for the realization of conditions on curvilinear boundaries, parallelization of computations on graphics processors. The results of computational experiment obtained allows to study the characteristic shock and vortex structures formed during the passage of a particle rebounding from the surface through the bow shock wave. The oscillations observed for a flow over the fla-tended cylinder are discussed.

Sediment resuspension due to near-bed turbulent coherent structures in the nearshore

Laboratory and field experiments have suggested that near-bed sediment resuspension is an intermittent process influenced by turbulent coherent structures. This is in contrast to many sediment transport theories that relate sediment transport rates to mean near bed flow parameters. This study presents observations of turbulent bursting events obtained from two contrasting environments dominated by unidirectional and oscillatory flow regimes with mean currents higher than the critical value for sand transport. In agreement with previous laboratory and field experiments the data investigated in this study (both in unidirectional current and oscillatory flow environments) indicated that sediment resuspension was an event based system with sediment flux controlled by ejections and sweeps. Both the magnitude of the Reynolds stress and duration of the events contributed to sediment resuspension. Wavelet analysis confirmed that over time turbulence occurred in slowly evolving clusters that were closely followed by periods of strong resuspension near the bed, evolving from the primary leading scales at low frequencies, and decaying in time after the termination of the turbulent agitation. The results highlight the necessity of considering the instantaneous Reynolds shear stress effects in the existing sediment transport equations which are developed based on a time-averaged flow parameters and mean a critical velocity. Field observations presented in this study highlight the importance of considering turbulence events along with the concept of impulse as an essential parameter to be considered into future modelling of sediment movement under both unidirectional and oscillatory flows.

Simulation of the Friction Pressure Loss in the Bottomhole Zone of a Reservoir Hydraulic Fracture

Abstract—The mathematical model that describes changes in pressure in a hydraulic fracture after the stoppage of fracturing fluid injection is proposed. The model takes into account the friction loss in the bottomhole zone of a well as a result of deviation of the fracture trajectory and the presence of the perforation holes. In the case of monotonic and oscillatory flow regimes in the fracture the analytical solutions are obtained for the pressure in the hydraulic fracture after the stoppage of fracturing fluid injection together with the analytical expressions for the pressure drop in the bottomhole zone. The dependence of the solutions obtained on the problem parameters is investigated.

Swimming in unsteady water flows: is turning in a changing flow an energetically expensive endeavor for fish?

Unsteady, dynamic flow regimes commonly found in shallow marine ecosystems such as coral reefs pose an energetic challenge for mobile organisms that typically depend on station holding for fitness-related activities. The majority of experimental studies, however, have measured energetic costs of locomotion at steady speeds, with only a few studies measuring the effects of oscillatory flows. In this study, we used a bidirectional swimming respirometer to create six oscillatory water flow regimes consisting of three frequency and amplitude combinations for both unidirectional and bidirectional oscillatory flows. Using the goldring surgeonfish, Ctenochaetus strigosus, a pectoral-fin (labriform) swimmer, we quantified the net cost of swimming (swimming metabolic rate minus standard metabolic rate) associated with station-holding under these various conditions. We determined that the swimming costs of station-holding in the bidirectional flow regime increased by 2-fold compared with costs based on swimming over the same range velocities at steady speeds. Furthermore, as we found minimal differences in energetic costs associated with station-holding in the unidirectional, oscillating-flow compared with that predicted from steady swimming costs, we conclude that the added acceleration costs are minimal, while the act of turning is an energetically expensive endeavor for this reef fish species.

Oscillatory flow regimes around four cylinders in a diamond arrangement

Oscillatory flow around a cluster of four circular cylinders in a diamond arrangement is investigated using two-dimensional direct numerical simulation over Keulegan–Carpenter numbers (KC) ranging from 4 to 12 and Reynolds numbers (Re) from 40 to 230 at four gap-to-diameter ratios (G) of 0.5, 1, 2 and 4. Three types of flows, namely synchronous, quasi-periodic and desynchronized flows (along with 14 flow regimes) are mapped out in the (G,KC,Re)-parameter space. The observed flow characteristics around four cylinders in a diamond arrangement show a few unique features that are absent in the flow around four cylinders in a square arrangement reported by Tonget al. (J. Fluid Mech., vol. 769, 2015, pp. 298–336). These include (i) the dominance of flow around the cluster-scale structure at$G=0.5$and 1, (ii) a substantial reduction of regime D flows in the regime maps, (iii) new quasi-periodic (phase trapping)$\text{D}^{\prime }$(at$G=0.5$and 1) and period-doubling$\text{A}^{\prime }$flows (at$G=1$) and most noteworthily (iv) abnormal behaviours at ($G\leqslant 2$) (referred to as holes hereafter) such as the appearance of spatio-temporal synchronized flows in an area surrounded by a single type of synchronized flow in the regime map ($G=0.5$). The mode competition between the cluster-scale and cylinder-scale flows is identified as the key flow mechanism responsible for those unique flow features, with the support of evidence derived from quantitative analysis. Phase dynamics is introduced for the first time in bluff-body flows, to the best knowledge of the authors, to quantitatively interpret the flow response (e.g. quasi-periodic flow features) around the cluster. It is instrumental in revealing the nature of regime$\text{D}^{\prime }$flows where the cluster-scale flow features are largely synchronized with the forcing of incoming oscillatory flow (phase trapping) but are modulated by localized flow features.