Wake Volume Research Articles

Penetration of a spherical vortex into turbulence

The penetration of a spherical vortex into turbulence is studied theoretically and experimentally. The characteristics of the vortex are first analysed from an integral perspective that reconciles the far-field dipolar flow with the near-field source flow. The influence of entrainment on the vortex drag force is elucidated, extending the Maxworthy (J. Fluid Mech., vol. 81, 1977, pp. 465–495) model to account for turbulent entrainment into the vortex movement and vortex penetration into an evolving turbulent field. The physics are explored numerically using a spherical vortex (initial radius $R_0$ , speed $U_{v0}$ ), characterised by a Reynolds number $Re_0(=2R_0U_{v0}/\nu$ , where $\nu$ is the kinematic viscosity) of 2000, moving into decaying homogeneous turbulence (root-mean-square $u_0$ , integral scale $L$ ) with turbulent intensity $I_t=u_0/U_{v0}$ . When the turbulence is absent ( $I_t=0$ ), a wake volume flux leads to a reduction of vortex impulse that causes the vortex to slow down. In the presence of turbulence ( $I_t> 0$ ), the loss of vortical material is enhanced and the vortex speed decreases until it is comparable to the local turbulent intensity and quickly fragments, penetrating a distance that scales as $I_t^{-1}$ . In the experimental study, a vortex ( $Re_0\sim 1490\unicode{x2013}5660$ ) propagating into a statistically steady, spatially varying turbulent field ( $I_{ve}=0.02$ to 0.98). The penetration distance is observed to scale with the inverse of the turbulent intensity. Incorporating the spatially and temporally varying turbulent fields into the integral model gives a good agreement with the predicted trend of the vortex penetration distance with turbulent intensity and insight into its dependence on the structure of the turbulence.

Correlation of the Structural Characteristics of an Artificial Oyster Reef with Its Wake Region

Oyster reefs are currently at risk of severe decline due to dangerous human interference and its aftermath; hence, artificial oyster reefs (AORs) have been utilized for their restoration. AORs with high vertical reliefs interact with the surrounding flow, constitute a reverse flow, and create a wake region in which concentrated nutrients and food organisms exist. However, the correlations of the structural characteristics of an AOR with its wake regions have not been studied. Thus, we established 96 AOR models, carried out flow analyses, and obtained their wake volumes, considering shell orientation, composition, penetration depth, and growth stage. We found that the growth stage is the most critical parameter for establishing a normalized wake volume. This implies that the number of oyster shells (N) is the most critical factor in securing a normalized wake volume, in which their correlation was linear and significant (R2=0.89). We also found that the correlations of the normalized wake volume with blocking and surface complexity indices were linearly significant, respectively. Additionally, wake volume efficiency increased with the number of oyster shells; specifically, the criterion for wake volume efficiency of EI (efficiency index) ≥ 2.0 was satisfied when N≥50 per 900 cm2.

Rugosity and blocking indices of artificial reefs and their correlations with wake volume

Like aquatic habitats in nature, artificial reefs (ARs) have been investigated to determine whether their complexity affects species diversity. Moreover, studies have been carried out to investigate wake regions of ARs, which provide a shelter, feeding ground, rest area, and temporary stopover for marine species; however, few have examined the correlation between the complexity of the AR and characteristics of the wake region. In this study, we first modelled three ideal scenarios and six real ARs, all designed with different degrees of rugosity and/or water-blocking effects. Second, we obtained the wake volumes of the target structures using the element-based finite volume method and investigated their correlations with the rugosity index (RI), blocking index (BI), and rugosity-blocking index (RBI). Our results showed that for securing a large wake region, it is necessary to implement not only a large upper surface with a certain degree of rugosity (e.g. 2.0 for a tunnel- or arch-type AR) but also a large front surface area with sufficient water-blocking effects (e.g. 0.5 for a cube- or box-type AR). Overall, the RI, BI, and RBI were useful for distinguishing the characteristics of the upper and front surfaces, and accordingly to estimate wake volumes.

인공어초 후류역을 증가시키는 두 가지 관점

Wake region of an artificial reefs (AR) is a significant space to recruit marine species around the AR because the region gathers nutrients and prey (e.g., phytoplankton). Previous studies have focused on relatively small, simple ARs for the estimation of wake regions. Thanks to progressive artificial reef industry, nowadays relatively large, complex ARs are being installed on the seabed; hence, it is necessary to investigate how to secure wake region of those ARs. In this study, the concept of wake volume was used to estimate the wake regions of five relatively complex artificial reefs (ARs). The element-based finite volume method was used to estimate wake volume, and its fluctuation due to the size of the ARs and the inflow velocity was calculated. From the analysis results, it was found that the thickness and area of the structural members of the ARs are important to secure wake region. However, wake region was not created if the AR is composed of only a thin steel frame. This indicates that the function of the structural members of the ARs that blocks the flow is important for the creation of the wake region. In the case of AR in which a wake region occurs, the AR size (height) has a significant positive correlation with the magnitude of the wake region, and if the inflow flow velocity is 1.0 m·sSUP-1/SUP or more, a sufficient eddy currents occur and accordingly no variation in wake volume is observed. Thus, to increase wake region, it is shown that not only the size of AR, but also the thickness and area of the structural members play an important role.

Wake Region Estimates of Artificial Reefs in Vietnam: Effects of Tropical Seawater Temperatures and Seasonal Water Flow Variation

From the perspective of saving energy of marine species and creating feeding areas, the wake volume of an artificial reef (AR) should be considered as a parameter in any wake region estimation. Wake regions of AR modules (reef ball, cylinder reef, and cube reef) and sets were numerically estimated considering tropical seawater temperatures and water flow variation in Vietnamese coastal waters. In addition, we considered an efficiency index (i.e., total wake volume per reef volume) and wake volume diagram (i.e., wake volume dependency on water flow direction) to characterize wake volumes. From the results, first, it was found that the effect of temperature on the wake volumes was minor in comparison with the effect of flow direction. It was also found that the optimum installation angles were 30° (reef ball and its set), 30° (cylinder reef and its set), and 0° (cube and its set) along the major flow direction. Second, it was found that the cylinder reef and its set were attractive because they generated the maximum wake volumes, regardless of seawater temperature. Thus, the module and set showed better average efficiency indices (9.28 for module and 6.81 for set) than the other cases. We found that the wake volume was dominant in the efficiency index and, accordingly, wake volume diagrams are sufficient to indicate the dependence on flow direction.

Prediction of Primary Physical Measures for Cost-Effective Management of Artificial Seaweed Reefs

AbstractSeaweed plays a central role in supporting good habitats and spawning grounds for a number of fisheries resources. Artificial seaweed reefs (ASRs) should provide firm, stable substrates for seaweed. Hence, it is desirable to propose a physical measure of seaweed spore settlement, which can be characterized by the surface area of an ASR and its wake region characteristics. In this study, a so-called seaweed spore settlement contributor was proposed, 34 ASRs were characterized by their primary physical measures (contributors and wake volumes) and secondary measure (efficiency indices), and linear regression equations were obtained to predict the primary measures. It is found that the average values of the primary measures are 61.96 m2 (contributor) and 4.79 m3 (wake volume), and reef structures provoking the water flow blocking mechanism are critical to these parameters. The results also demonstrated that the effect of the seabed slope on the primary measures should be considered for reefs having a narrow top and wide bottom. We found that it is desirable to predict the primary physical measures for preinstallation and cost-effective management of ASRs.

Transient stratification force on particles crossing a density interface

We perform a series of experiments to measure Lagrangian trajectories of settling and rising particles as they traverse a density interface of thickness h using an index-matched water-salt-ethanol solution. The experiments confirm the substantial deceleration that particles experience as a result of the additional force exerted on the particle due to the sudden change in density. This stratification force is calculated from the measurement data for all particle trajectories. In absence of suitable parameterisations in the literature, a simple phenomenological model is developed which relies on parameterisations of the effective wake volume and recovery time scale. The model accurately predicts the particle trajectories obtained in our experiments and those of Srdić-Mitrović et al. (1999). Furthermore, the model demonstrates that the problem depends on four key parameters, namely the entrance Reynolds number Re1, entrance Froude number Fr, particle to fluid density ratio ρp/ρf, and relative interface thickness h/a.

Wake volume of injected bubbles in fluidized beds: A magnetic resonance imaging velocimetry study

Rapid magnetic resonance imaging (MRI) was used to determine the volume of wakes of bubbles injected into incipiently fluidized beds. MRI was used to generate 2D maps of particle velocity surrounding bubbles with a temporal resolution of 18 ms. The bubble rise velocity, ub, was determined from the change in bubble position in successive maps, and pixels of particles which had an upward velocity greater than or equal to 0.8 ub were considered to be in the “wake” if they were below the bubble and in the “crown” if they were above the bubble. Results show that (i) the wake volume is much larger than the crown volume and (ii) the ratio of the wake volume to bubble volume increases as the bubble rises in the bed but decreases with increasing bubble volume. The influence of two bubbles interacting on bubble wake volume is also investigated. Comparison with geometric theory for wake volume exposes flaws in this theory for fully developed bubbles, bubbles near the distributor and interacting bubbles.

Efficiency and unit propagation indices to characterize wake volumes of marine forest artificial reefs established by flatly distributed placement models

Marine forest artificial reefs (MFARs) have been used in relatively shallow seawaters to provide light intensity and firm, stable substrates for seaweeds. Their design should be oriented to enlarge surface areas with relatively small reef sizes and to spread spores with a productive reef placement. Regarding such productive placement, it is necessary to investigate how seaweed spores spread out from a point. Consequently, the water flows around MFARs are important in predicting the spreading pattern and obviously estimating the corresponding wake region is essential. In this study, we present the wake volumes of seven MFARs and associated flow characteristics, such as efficiency and unit propagation indices, and finally recommended better placement models, optimizing the interaction between ARs and the corresponding wake volumes. From the demonstrations, it was found that the indices were sensitive to facilitated AR modules, adapted AR size, and mixed use of different AR modules. Additionally, it was demonstrated that the intervals between neighboring ARs should be the major wake length of each flatly distributed placement model. It was shown that the analysis procedure demonstrated and the efficiency and unit propagation indices proposed here are useful for the efficient design and management of MFARs.

Efficient placement models of labyrinth-type artificial concrete reefs according to wake volume estimation to support natural submerged aquatic vegetation

The present study presents efficient placement models of labyrinth-type artificial concrete reefs (LT-ACRs) to support natural submerged aquatic vegetation beds or enhance macrophyte beds [Undaria pinnatifida (Harv.) Suringar, Laminaria sp., Ecklonia stolonifera Okamura, and Sargassum fulvellum (Turner) C. Agardh] in the coastal waters of South Korea. To reach an efficient solution, wake regions were obtained for 12 LT-ACR placement models using the element-based finite volume method and wake volume concept. The optimal flatly distributed placement models were identified in intervals between neighboring reefs (i.e., longitudinal width: 1 m ≤ W1 ≤ 3 m; transverse width: W2 = 5 m), which resulted in wake volumes of 43.8, 38.5, and 39.5 m3. The optimal models are regarded as more efficient than the maximum wake volume case because their geometries sustain the intervals between neighboring reefs. Overall, labyrinth-type artificial concrete reefs are efficient because they have relatively wide surfaces and small drag coefficients, and their optimal placement models satisfy Korean practices, which dictate that the intervals should not exceed 5 m. However, the hydrodynamic and economic pros and cons of intervals >5 m were not assessed in the present study.

후류역 효율성 증대를 위한 해중림초 배치 모델

This study presents placement models for marine forest artificial reefs (MFARs) to increase their wake region efficiency. For the purpose, first, ten line placement models are considered to investigate the effect of height difference between neighboring reefs on wake volumes. Second, seven single type placement models and eight mixed placement models are constructed. Third, wake volume of each placement model is calculated using the so-called element-based finite volume method. Finally, efficiency and unit propagation indices are estimated using the corresponding wake volumes and geometric characteristics of ARs. From the analysis results, it is shown a strong positive linear correlation between the efficiency indices and the height differences of the neighboring ARs. In addition, it is found that the mixed placement model with R1s (half-ball type reefs) improved both indices in comparison with the results of the single type placement models. Thus, a mixed placement model can be an alternative to improve the efficiency of wake region.

Intensively Stacked Placement Models of Artificial Reef Sets Characterized by Wake and Upwelling Regions

AbstractThe wake and upwelling regions of artificial reefs (ARs), and sets, groups, and complexes thereof, are critical factors in engineering AR design and estimating fishing grounds, which are often facilitated by ARs. This study proposes six AR set placement models, constructed based on previous field observations, to characterize their wake and upwelling regions quantitatively through an element-based finite-volume method. These AR sets are hexahedral (Hexa), rectangular (Rect), pyramidal (Pyra), trigonal (Tri), tetragonal (Tetra), and pentagonal (Pent) sets, and their usable volumes are all fixed at 800 m3 (the minimum requirement of South Korean AR provision) in the flow analysis models. The analyses showed that Rect and Hexa provide the largest wake volumes of 496 and 352 m3, respectively, while Rect and Tetra provide the largest upwelling volumes of 13,106 and 6,116 m3, respectively. Finally, the maximum reductions in the wake and upwelling regions due to uniform settlement of 20%, which is equivalent to a usable volume loss of 160 m3, were 67% and 35%, respectively. Thus, wake regions are more sensitive to usable volume loss than upwelling regions.

후류체적선도를 이용한 인공어초 후류역 평가

To evaluate the wake regions of six artificial reefs (ARs) frequently used in the marine forest creation project in Korea, we consider the effect of water flow directions on the wake regions and accordingly propose a wake region diagram, which is characterized by parameters such as wake volume fluctuations, averaged wake volume, fundamental symmetric angle, secure angle, and principal direction. To demonstrate the parameters, seven water flow directions (0 to <TEX>$90^{\circ}$</TEX>) were considered and consequently the variations in wake volumes were investigated by using the concept of wake volume, adopting element-based finite volume method, and utilizing numerical flow domain and boundary conditions. From the analysis results, it was shown that the wake region diagrams have a period of either 45 or <TEX>$90^{\circ}$</TEX> according to the geometrical symmetry of each artificial reef. Also, it was found that the secure angle ranges fluctuate depending on the shapes and sizes of the artificial reefs considered. Thus, it is demanded to consider those parameters during installation of artificial reefs for establishing a larger wake region and accordingly attracting more marine fauna and flora in the region.

​Efficiency, tranquillity and stability indices to evaluate performance in the artificial reef wake region

The wake region of an artificial reef (AR) is defined as the space consisting of the recirculating water flow immediately behind the AR. This study proposes three performance indices to evaluate the wake region and pinpoints how to determine the indices for ARs. First, we established dimensionless performance indices, such as the so-called wake region efficiency index, tranquillity index and stability index. Second, we considered these three indices with respect to two ARs (AR1 and AR2) and determined wake volumes using the element-based finite volume method. AR2 was found to have better efficiency and tranquillity indices than AR1 because of its size and complexity. The AR1 stability index was slightly better than that of AR2. Overall, AR2 (box type with a steel box inside) showed better wake region performance, mainly because of a higher efficiency index (9.55) compared with that of AR1 (2.00). The results show that performance indices can be used to evaluate efficiency, tranquillity and stability of wake regions in ARs.

평면 분산된 인공어초 집합의 어초협곡 간격에 따른 후류체적 특성

평면 분산된 인공어초 집합의 어초협곡 간격에 따른 후류체적 특성

Bubble generated turbulence and direct numerical simulations

Gas–liquid two phase flows are widely encountered in industry. The design parameters include two phase pressure drop, mixing and axial mixing in both the phases, effective interfacial area, heat and mass transfer coefficients. Currently, there is a high degree of empiricism in the design process of such reactors owing to the complexity of coupled flow and reaction mechanism. Hence, we focus on synthesizing recent advances in computational and experimental techniques that will enable future designs of such reactors in a more rational manner by exploring a large design space with high-fidelity models (computational fluid dynamics) that are validated with high-fidelity measurements (hot film anemometry (HFA), Laser Doppler anemometry (LDA), particle image velocimetry (PIV), etc.) to provide a high degree of rigor. Understanding the spatial distributions of dispersed phases and their interaction during scale up are key challenges that were traditionally addressed through pilot scale experiments, but now can be addressed through advanced modelling.For practically complete knowledge of the fluid mechanical parameters, it is desirable to implement direct numerical simulations (DNS). However, the current computational power does not permit full DNS for real bubble columns. Therefore, we have been using simplified turbulence models (such as large eddy simulation, Reynolds stress, k–ε, etc.) which need the knowledge of turbulence parameters. For the estimation of these parameters, currently semi-empirical procedures are being used pending the knowledge of turbulence. Further, the formulation of governing equations in all the CFD models (except DNS), the knowledge of interface forces (drag, lift, virtual mass, Basset, etc.) is needed and for their estimations empirical correlations are being employed, again pending the knowledge of fluid mechanics under turbulent conditions in bubble columns.In gas–liquid dispersions, the gas is sparged in the form of bubbles. During the bubble rise, the mechanism of wake detachment creates turbulence which can be called as wake generated turbulence. In addition, energy gets transferred from the gas phase to liquid phase. The quantitative amounts are negligible when bubble motion is not hindered and the gas–liquid dispersion is homogenous. The amounts increase with an increase in the extent of hindrance. However, in the homogenous regime, even under extreme conditions, the extent of energy transfer in the bulk gas–liquid dispersions (volume other than wake volume) is fairly limited. On contrast, in the heterogeneous regime, the rates of energy transfer become sizeable. The energy received by the liquid (in both the regimes) also creates turbulent motion and termed as bulk generated turbulence. In turbulent flows a compendium of eddies (flow structures) of different length and time scales contribute towards improved/enhanced mixing, momentum transfer, heat transfer, and mass transfer (transport phenomena). Hence, a proper understanding of the dynamics of these turbulent flow structures, and their role in the transport phenomena, can bring substantial improvement in the scale-up and design procedures. The present paper brings out the current status of knowledge on bubble generated turbulence. All the published literature in experimental measurements and DNS simulations has been critically analysed and coherently presented.

The effect of a uniform through-surface flow on a cylinder and sphere

The effect of a uniform through-surface flow (velocity$U_{b}$) on a rigid and stationary cylinder and sphere (radius$a$) fixed in a free stream (velocity$U_{\infty }$) is analysed analytically and numerically. The flow is characterised by a dimensionless blow velocity${\it\Lambda}\,(=U_{b}/U_{\infty })$and Reynolds number$Re\,(=2aU_{\infty }/{\it\nu}$, where${\it\nu}$is the kinematic viscosity). High resolution numerical calculations are compared against theoretical predictions over the range$-3\leqslant {\it\Lambda}\leqslant 3$and$Re=1,10,100$for planar flow past a cylinder and axisymmetric flow past a sphere. For$-{\it\Lambda}\gg 1$, the flow is viscously dominated in a thin boundary layer of thickness${\it\nu}/|U_{b}|$adjacent to the rigid surface which develops in a time${\it\nu}/U_{b}^{2}$; the surface vorticity scales as$Re|{\it\Lambda}|U_{\infty }/a$for a cylinder and sphere. A boundary layer analysis is developed to analyse the unsteady viscous forces. Numerical results show that the surface pressure and vorticity distribution within the boundary layer agrees with a steady state analysis. The flow downstream of the body is irrotational so the wake volume flux,$Q_{w}$, is zero and the drag force is$F_{D}=-{\it\rho}U_{\infty }Q_{b}$, where${\it\rho}$is the density of the fluid and$Q_{b}$is the normal flux through the body surface. The drag coefficient is therefore$-2{\rm\pi}{\it\Lambda}$or$-8{\it\Lambda}$for a cylinder or sphere, respectively. A dissipation argument is applied to analyse the drag force; the rate of working of the drag force is balanced by viscous dissipation, flux of stagnation pressure and rate of work by viscous stresses due to sucking. At large$Re|{\it\Lambda}|$, the drag force is largely determined by viscous dissipation for a cylinder, with a weak contribution by the normal viscous stresses, while for a sphere, only$3/4$of the drag force is determined by viscous dissipation with the remaining$1/4$due to the flux of stagnation pressure through the sphere surface. When${\it\Lambda}\gg 1$, the boundary layer thickness initially grows linearly with time as vorticity is blown away from the rigid surface. The vorticity in the boundary layer is weakly dependent on viscous effects and scales as$U_{\infty }/a{\it\Lambda}$or$U_{\infty }/a{\it\Lambda}^{3/2}$for a cylinder and sphere, respectively. For large blow velocity, the vorticity is swept into two well-separated shear layers and the maximum vorticity decreases due to diffusion. The drag force is related to the vorticity distribution on the body surface and an approximate expression can be derived by considering the first term of a Fourier expansion in the surface vorticity. It is found that the drag coefficient$C_{D}$for a cylinder (corrected for flow boundedness) is weakly dependent on${\it\Lambda}$while for a sphere,$C_{D}$decreases with${\it\Lambda}$.

The complex aerodynamic footprint of desert locusts revealed by large-volume tomographic particle image velocimetry.

Particle image velocimetry has been the preferred experimental technique with which to study the aerodynamics of animal flight for over a decade. In that time, hardware has become more accessible and the software has progressed from the acquisition of planes through the flow field to the reconstruction of small volumetric measurements. Until now, it has not been possible to capture large volumes that incorporate the full wavelength of the aerodynamic track left behind during a complete wingbeat cycle. Here, we use a unique apparatus to acquire the first instantaneous wake volume of a flying animal's entire wingbeat. We confirm the presence of wake deformation behind desert locusts and quantify the effect of that deformation on estimates of aerodynamic force and the efficiency of lift generation. We present previously undescribed vortex wake phenomena, including entrainment around the wing-tip vortices of a set of secondary vortices borne of Kelvin–Helmholtz instability in the shear layer behind the flapping wings.

Simulation of slug flow systems under laminar regime: Hydrodynamics with individual and a pair of consecutive Taylor bubbles

Slug flow is an extremely important gas–liquid flow pattern that can be frequently found in oil extraction processes causing instabilities and randomness in the hydrodynamic environment of an oil well. The dynamics of individual and a pair of Taylor bubbles rising in vertical columns of stagnant and co-current liquids was numerically studied using the volume of fluid (VOF) methodology implemented in the commercial code ANSYS FLUENT. The first step was to validate the application of the numerical code to slug flow with in-house experimental data about the rising of single Taylor bubbles obtained by photographic studies and PIV/PST measurements. Then, the application of this CFD tool was extended to a system with a pair of Taylor bubbles. The numerical results of the single Taylor bubble velocity, frontal bubble shape, liquid film thickness, wall shear stress in the stabilized film, axial velocity in the liquid film, bubble shape at the rear, wake volume and length are favorably compared with available experimental and theoretical values. Regarding systems with a pair of consecutive Taylor bubbles, which are the structural units of continuous slug flow, the hydrodynamics was simulated under stagnant and co-current liquid flow conditions, with physical properties that can be representative of some gas–oil systems. The behavior of the most important hydrodynamic features, as the bubbles approach process evolves, is addressed to infer the major effects caused by this kind of bubble–bubble interaction. The information gathered about the bubble–bubble interaction can be crucial to improve already developed bubble train flow simulators.

Flow patterns in the wake of a Taylor bubble rising through vertical columns of stagnant and flowing Newtonian liquids: An experimental study

The flow in the wake and near-wake regions of individual Taylor bubbles rising through stagnant and co-current vertical columns of Newtonian liquids was studied, employing simultaneously particle image velocimetry (PIV) and pulsed shadowgraphy techniques (PST). Experiments were made with water and aqueous glycerol solutions covering a wide range of viscosities ( 1 × 10 - 3 Pa s < μ < 1.5 Pa s ) , in an acrylic column of 32 mm ID. Different wake structures (laminar, transitional and turbulent) are identified, in both stagnant and co-current flow conditions. In stagnant liquids, the wake flow pattern is only dependent on the dimensionless group N f . The different types of wakes obtained are in accordance with the critical N f numbers proposed in previous works. For co-current flow conditions, the flow patterns in the wake depend on the Reynolds number based on the relative (to the bubble) average velocity of the upward liquid flow, the laminar-transitional and transitional-turbulent limits being for the first time experimentally determined. The wake flow patterns are quantified by means of instantaneous and average flow fields. Values for the wake length and wake volume are also presented and compare well with correlations found in literature. Study of the flow in the near-wake zone enabled determination of the distance needed to recover the undisturbed liquid velocity profile. The detailed study of the flow in the wake and near-wake regions is an important contribution to better understanding the interaction and coalescence mechanisms between Taylor bubbles. The data reported are relevant to the validation of numerical simulation codes in the vertical slug flow regime.