Wake Entrainment Research Articles

Aerodynamic and production comparison of wind farms with downwind versus conventional upwind turbines

Ever-increasing turbine scales and their associated logistical challenges have reignited questions about the performance of downwind rotor configurations. A particular potential benefit of downwind rotor configurations is the farm-scale power increase that may be conferred by tilt-driven downward wake entrainment and associated wake recovery. In this work, a comprehensive aerodynamic analysis is carried out to understand the mechanisms for wake entrainment and recovery across a spectrum of velocity and inflow alignment conditions on a small, structured farm in order to understand the impact of downwind rotors on farm production. The results show that the benefits demonstrated previously in the literature for downwind-rotor farms in aligned flows are fragile, and, outside of strong farm/flow alignment conditions, power production benefits for small farms with downwind rotor configurations are significantly if not completely mitigated by misalignment effects. The work indicates that farm-scale benefits for downwind rotors must be realized either from large-scale entrainment benefits, with more exotic farm arrangements that can take advantage of the aerodynamic effects, or from beneficial fatigue impacts from entrainment of less turbulent outer boundary layer flows.

Experimental and numerical investigation on the bubble breakup and coalescence characteristics in the highly efficient static mixers

CFD-PBM and experiment methods were applied to study the bubble dispersion characteristics in the static mixers with three twisted blades (TKSM). The gas-liquid velocity ratio was selected as drag correction factors. The simulation data was compared with experimental values at different superficial gas velocity and the results revealed that the modified model could effectively enhance the calculation accuracy of gas holdup. Furthermore, bubble size distribution (BSD) was well predicted by Luo-Prince model and the mean relative error (MRE) of axial d32 between the prediction of Luo-Prince model and experimental value was 2.86 %. Meanwhile, the influences of bubble collision mechanisms and coalescence correctors on the numerical prediction were quantitatively analyzed. The effects of turbulent fluctuation, viscous shear, wake entrainment and buoyancy driven were considered for the prediction of bubble collision frequency in TKSM. The prediction precision of cumulative probability distribution (CPD) was promoted by 17.26 % when the correctors obtained from Energy-Minimization Multi-Scale (EMMS) method were used. In addition, empirical parameter CDas was evaluated and the results showed that the Das coalescence model could accurately describe the BSD when CDas was 0.01–0.1. The backmixing performance of the gas phase in the TKSM exceeds that of the liquid phase, which is distinct from that in vortex unit.

An Analysis of the Impact of Vertical Wind Shear on Convection Initiation Using Large-Eddy Simulations: Importance of Wake Entrainment

Abstract The initiation of thunderstorms in environments characterized by strong wind shear presents a forecast challenge because of the complexities of the interactions between growing cumulus clouds and wind shear. Thunderstorms that develop in such environments are often capable of producing high-impact hazards, highlighting the importance of convection initiation in sheared environments. Although recent research has greatly improved understanding of the structure and evolution of rising thermals in unsheared environments, there remains uncertainty in how wind shear influences the convection initiation process. Two large-eddy simulations (75-m horizontal grid spacing) were performed to study this problem. Convection initiation attempts are forced in the simulations through prescribed surface heat fluxes (the initial boundary layers are statistically horizontally homogeneous and quasi–steady state but contain turbulent eddies as a result of random initial temperature perturbations). The only difference between the two simulations is the presence or absence of wind shear above 2 km. Important differences in the entrainment patterns are present between sheared and unsheared growing cumulus clouds. As found in previous research, the overturning circulation associated with rising thermals drives dynamic entrainment in the unsheared clouds. However, in sheared clouds, wake entrainment resulting from the tilting of environmental vorticity is an important dynamic entrainment pathway. This result has implications for both the structure of sheared growing cumulus clouds and for convection initiation in sheared environments. Significance Statement Forecasts of thunderstorm hazards such as tornadoes, hail, and strong winds, require the accurate prediction of when and where thunderstorms form. Unfortunately, predicting thunderstorm formation is not easy, as there are a lot of different factors to consider. One such factor is environmental vertical wind shear, which describes how winds change speed and direction with height. The purpose of this study is to better understand how wind shear impacts developing clouds. Our results demonstrate a specific mechanism, called “wake entrainment,” through which wind shear can weaken developing clouds and potentially prevent them from becoming strong thunderstorms entirely. Understanding this mechanism may be useful for thunderstorm prediction in environments characterized by wind shear.

Interfacial area concentration correlation for bubbly flows in small-diameter pipes under normal and reduced gravity conditions

The accurate modeling of interfacial area concentration (IAC) is essential to improve the predictive performance of two-fluid-model-based simulations for state-of-the-art two-phase thermal systems under normal and reduced gravity conditions, such as two-phase heat sinks and space fission power systems. Although numerous empirical and semi-theoretical IAC correlations have been proposed to predict the IAC variations for different two-phase systems, the IAC correlation for small-diameter (hydraulic diameter from 1mm to 10mm) flow channels under normal gravity conditions is still underdeveloped and no IAC correlation has been developed for reduced gravity conditions. The present study collected 456 experimental IAC data measured in bubbly flows in small-diameter pipes under normal and reduced gravity conditions from the open literature and evaluated the predictive performance of 8 representative IAC correlations. The evaluation results illustrated that none of the existing IAC correlations could maintain acceptable predictive ability under both normal and reduced gravity conditions. Therefore, a new IAC correlation applicable to normal and reduced gravity conditions for bubble flows in small-diameter pipes was proposed semi-theoretically by (1) considering the IAC variation caused by the bubble breakup due to turbulent impact and the bubble coalescence mainly from wake entrainment and (2) employing the concept of effective body acceleration to extend the applicability of newly-developed IAC correlation to reduced gravity conditions. The performance of newly-developed IAC correlation has been evaluated against the collected bubbly IAC data taken in small-diameter pipes and the obtained mean relative errors are 0.134, 0.158 and 0.145 for 260 normal gravity data, 196 reduced gravity data and all collected data, respectively.

The research on interfacial area transport equation in rectangular channels: Experimental research and models development of sink and source terms

Interfacial area concentration is a crucial constitutive parameter to close the two-fluid model, the accuracy of whose prediction is vital, so the interfacial area transport equation (IATE) has been proposed. According to the analysis of experimental results involving bubble interaction mechanisms in the rectangular channel from the bubbly-to-slug flow, new sink and source terms in a one-group IATE are put forward in this study. The new source term aroused by random collision is considered with relative velocity inducing collision and the effect of gap size on coalescence efficiency. Besides, the number density of small and large bubbles is considered respectively to model the source term of wake entrainment. Combined with the source terms of break up brought by turbulent impact and expansion caused by the pressure gradient, a new one-group IATE applicable to rectangular channels are completed. The coefficients in the newly-derived model are determined by experimental results of 4 × 66 mm and 8 × 66 mm vertical rectangular channels under co-current upward air-water two-phase flow, and the average relative error is 11.9%. Furthermore, the new model is also testified by the experimental result in the 6 × 66 mm rectangular channel independently, and the average relative error is 14.4%. Generally, the new model is feasible to predict the change of the interfacial area concentration in different rectangular channels.

Flow around porous square cylinders with a periodic and scalable structure

This study experimentally explores the effect of permeability as an isolated control parameter on the downstream wake structure and the aerodynamic properties of the porous square cylinder. An addictive manufacturing technique was utilized to fabricate the porous cylinder, comprised of a simple cubic lattice structure. This unique manufacturing technique, coupled with a periodic and scalable lattice structure, allows us to decouple the permeability from the porosity. Based on permeability measurements in this study, Darcy number (Da) of the porous cylinder varies in a range of 2.44×10−5−4.07×10−4, while its porosity (Φ) is fixed at 0.7. Interesting features in the downstream wake are observed due to the mutual interaction between longitudinal and lateral bleeding flows. This mutual interplay prevents wake entrainment, relocating a recirculation bubble downstream and making its size smaller with increasing Da. When Da>2.0×10−4, in particular, a steady wake is observed in the near-wake despite the low degree of porosity. Finally, we proposed a new approach to defining the length scale of the flow adjustment that is suitable for the porous square cylinder and comparable to the porous cylinder with a circular cross-section.

Research on bubbly to slug flow regime transition of air-water co-current upward flow in vertical rectangular channels with different gap sizes

To investigate the transitional criterion of bubbly to slug flow influenced by gap size in rectangular channels, an experimental investigation of air-water two-phase co-current upward flow has been conducted in adiabatic vertical channels under atmospheric conditions. Experiments were carried out in three channels with different cross-sections of 4 × 66 mm, 6 × 66 mm, 8 × 66 mm, respectively. The superficial velocities are 0≤jg≤20m/s and 0.25≤jf≤3m/s for gas and liquid phase. The range of liquid superficial Reynolds numbers is 1900≤Ref≤42000 which have covered purely laminar flow to turbulent flow. Flow regimes were defined according to photos captured by a high-speed camera and characteristics of void fraction measured by flat electrodes, and a random forest algorithm was used to identify the flow regimes at the transitional region. Results show that with the increase of the gap size, the transition from bubbly to slug flow is delayed, and the regime transitional void fraction changes from 0.17 to 0.25. The mechanisms of random collision, wake entrainment, and turbulence breakup were analyzed according to the experimental phenomena. A new criterion of the bubbly to slug flow regime transition applied in rectangular channels was proposed considering the impact of the gap size.

Radial distribution of solid holdup and its fluctuation characteristics in a three-phase scrubbing-cooling chamber

The local solid holdup in the three-phase scrubbing-cooling chamber was measured using optical fiber probes under different solid-phase properties, liquid-phase properties, and superficial gas velocities. The root-mean-square (RMS) fluctuations of the solid fraction (v′¯) were used to describe the local solid volume fraction fluctuations quantitatively. The results showed that the bubble wake is the dominant factor affecting the radial solid holdup distribution uniformity. The bubble wake entrainment in the central area and the fluid reflux in the near-wall area resulted in the radial distribution of solids holdup with “low center and high sidewalls.” In addition, the increase of solid volume fraction and the enhanced entrainment ability of bubble wake both encourage the growth of local v′¯ and lead to a more uneven radial distribution of v′¯. A regional model of v′¯ is established, and its predicted values agree well with the experimental values.

Interfacial Area Transport Equation for Bubble Coalescence and Breakup: Developments and Comparisons.

Bubble coalescence and breakup play important roles in physical-chemical processes and bubbles are treated in two groups in the interfacial area transport equation (IATE). This paper presents a review of IATE for bubble coalescence and breakup to model five bubble interaction mechanisms: bubble coalescence due to random collision, bubble coalescence due to wake entrainment, bubble breakup due to turbulent impact, bubble breakup due to shearing-off, and bubble breakup due to surface instability. In bubble coalescence, bubble size, velocity and collision frequency are dominant. In bubble breakup, the influence of viscous shear, shearing-off, and surface instability are neglected, and their corresponding theory and modelling are rare in the literature. Furthermore, combining turbulent kinetic energy and inertial force together is the best choice for the bubble breakup criterion. The reviewed one-group constitutive models include the one developed by Wu et al., Ishii and Kim, Hibiki and Ishii, Yao and Morel, and Nguyen et al. To extend the IATE prediction capability beyond bubbly flow, two-group IATE is needed and its performance is strongly dependent on the channel size and geometry. Therefore, constitutive models for two-group IATE in a three-type channel (i.e., narrow confined channel, round pipe and relatively larger pipe) are summarized. Although great progress in extending the IATE beyond churn-turbulent flow to churn-annual flow was made, there are still some issues in their modelling and experiments due to the highly distorted interface measurement. Regarded as the challenges to be addressed in the further study, some limitations of IATE general applicability and the directions for future development are highlighted.

Numerical study on the drag characteristics of rigid submerged vegetation patches

Aquatic plants play a crucial role in the hydrodynamic and material transport processes within the aquatic environments due to the additional flow resistance induced by vegetation stems. In this study, high-resolution numerical experiments were performed to investigate the drag characteristics of circular vegetation patches fully immersed in a turbulent open channel flow. The submerged vegetation patch was modeled as a rigid cylinder array with a diameter D composed of N cylinder elements with a diameter d. The effects of vegetation density Φ (0.023 ≤ Φ ≤ 1) and relative diameter d/D (d/D = 0.051 and 0.072) were tested. The simulation results show that Φ and d/D affect the flow resistance exerted by the vegetation patch by modifying the bleeding flow intensity. With the increase in Φ, the drag forces acting on the individual cylinder elements decrease, whereas the total drag forces of the patch increase. The oscillation strength of the drag force of individual cylinders depends on Φ and the fixed positions within the patch. The presence of the free end of submerged cylinder array leads to enhanced wake entrainment with the increase in Φ. The drag coefficient of the submerged patch is smaller than that of the emergent patch when the dimensionless frontal area aD > 3. However, the two patches exhibit comparable drag coefficients for smaller aD values.

Interfacial area transport for bubbly-to-slug transition flows in small diameter pipes

This study aims to experimentally investigate the two-group interfacial area transport in small diameter pipes. Experimental data focusing on the bubbly to slug transition regime, namely one-group to two-group transport region, are collected in a 12.7 mm vertical pipe under adiabatic, air-water conditions. The result shows the intergroup transfer in the small diameter pipe can be drastic, especially under low superficial liquid velocities. The cause of this phenomenon is mainly due to the large relative bubble size comparing to the pipe cross-sectional area. The wake entrainment effect could be enhanced by the small spherical bubbles that are acting like cap or slug bubbles in a medium-size pipe. Based on the experimental observation, a throughout analysis of the dependence of the drastic intergroup transfer is provided in this study. The models predicting the initiation of drastic intergroup transfer in small diameter pipes in terms of the bubble diameter and the void fraction are developed. These models are compared with the two-phase data among the different pipe sizes and the results show a good agreement. These newly developed models are applied to the IATE wake entrainment model by developing a transition function analogous to the sigmoid function. With the transition function, the revised IATE model is given the new ability on predicting the drastic intergroup transfer phenomenon.

A Simple Passive Device for the Drag Reduction of an Ahmed Body

In this paper, a simple passive device is proposed for drag reduction on the 35° Ahmed body. The device is a simple rectangular flap installed at the slant surface of the model to investigate the effect of slant volume, formed between the device and the slant surface, on the flow behaviour. The slant volume can be varied by changing the flap angle. This investigation is performed using the FLUENT software at a Reynolds number of 7.8 ×〖10〗^5 based on the height of the model. The SST k-omega model is used to solve the Navier-stokes equations. It is found that this passive device influences the separation bubbles created inside the slant volume and provides a maximum drag reduction of approximately 14% at the flap angle of 10°. Moreover, the device delays the main separation point, which changes the flow conditions at the back of the model. The drag reduction was found to mainly dependent on the suppression of the separation bubbles formed inside the slant volume, which leads to faster pressure recovery. The cause of this pressure recovery is found to be the reduction in recirculation length and width. Also, the addition of a flap reduces the turbulent kinetic energy, which lessened the wake entrainment in the recirculation region, leading to a drag reduction. Also, it hinders the formation of horseshoe vortex that provides a pressure recovery and influence the wake width. However, the investigation also reveals that this device does not reduce the induced drag due to longitudinal vortex from the side edges.

Developmental effects on sleep\u2013wake patterns in infants receiving a cow\u2019s milk-based infant formula with an added prebiotic blend: a Randomized Controlled Trial

BackgroundFew studies have evaluated nutritive effects of prebiotics on infant behavior state, physiology, or metabolic status.MethodsIn this double-blind randomized study, infants (n = 161) received cow’s milk-based infant formula (Control) or similar formula with an added prebiotic blend (polydextrose and galactooligosaccharides [PDX/GOS]) from 14–35 to 112 days of age. Infant wake behavior (crying/fussing, awake/content) and 24-h sleep–wake actograms were analyzed (Baseline, Days 70 and 112). Salivary cortisol was immunoassayed (Days 70 and 112). In a subset, exploratory stool 16S ribosomal RNA-sequencing was analyzed (Baseline, Day 112).ResultsOne hundred and thirty-one infants completed the study. Average duration of crying/fussing episodes was similar at Baseline, significantly shorter for PDX/GOS vs. Control at Day 70, and the trajectory continued at Day 112. Latency to first and second nap was significantly longer for PDX/GOS vs. Control at Day 112. Cortisol awakening response was demonstrated at Days 70 and 112. Significant stool microbiome beta-diversity and individual taxa abundance differences were observed in the PDX/GOS group.ConclusionsResults indicate faster consolidation of daytime waking state in infants receiving prebiotics and support home-based actigraphy to assess early sleep–wake patterns. A prebiotic effect on wake organization is consistent with influence on the gut–brain axis and warrants further investigation.ImpactFew studies have evaluated nutritive effects of prebiotics on infant behavior state, cortisol awakening response, sleep–wake entrainment, and gut microbiome.Faster consolidation of daytime waking state was demonstrated in infants receiving a prebiotic blend in infant formula through ~4 months of age.Shorter episodes of crying were demonstrated at ~2 months of age (time point corresponding to age/developmental range associated with peak crying) in infants receiving formula with added prebiotics.Results support home-based actigraphy as a suitable method to assess early sleep–wake patterns.Prebiotic effect on wake organization is consistent with influence on the gut–brain axis and warrants further investigation.

Wakes of wall-bounded turbulent flows past patches of circular cylinders

The properties of the wake generated by a porous body fully immersed in a turbulent boundary layer are experimentally assessed. The body consists of an array of cylinders, with diameter , covering a circular patch of diameter . For fixed and , by increasing the number of cylinders, , within the patch, the wake properties are systematically tested under different levels of density ( covered planar area per total surface) and compared to the flow past a solid body of equivalent diameter and height ( ). Some insights on the complex flow developing in the wake are captured: varying in the range 2 %–24 % results in the flow meandering among the cylinders and bleeding from the top, the sides and the trailing edge of the patch. The interplay between trailing edge and top bleeding prevents wake entrainment, locking the wake longitudinal extent to 5–7 patch diameters, regardless of the density level. Due to the finite body vertical extent, a third shear layer develops from the top of the patch. The interaction between the top shear layer and the lateral ones leads to a mutual alteration, namely a nonlinear growth not captured by the classical mixing layer theory. Nevertheless, on the horizontal plane at the patch mid-height, the mean flow recovers, exhibiting a self-similar decay. Surprisingly, the recovery is well described by the classical planar wake theory and the characteristic scales, namely the maximum velocity deficit and the wake half-width, evolve linearly as proposed by Wygnanski et al. (J. Fluid Mech., vol. 168, 1986, pp. 31–71).

Effects of bubble coalescence and breakup on CO2 absorption performance in nanoabsorbents

The complexity and dynamics of bubble coalescence and breakup have fascinated scientists for decades. In this study, we performed experiments and simulations involving successively rising bubbles in a rectangular bubble column for carbon dioxide absorption in nanoabsorbents (methanol with various concentrations of alumina nanoparticles). The variations of the bubble behavior were captured by a high-speed camera in experiments and simultaneously visualized via simulations. Firstly, we distinguished the orifice region and the bulk liquid region. The bubble diameter was determined by only the gas flow rate and orifice diameter in the orifice region; however, it changed significantly in the bulk liquid region, experiencing approximately four stages. Wake entrainment and eddy capture dominated the bubble coalescence in the bulk liquid region; however, the bubble breakup was mainly caused by eddy collision and the coalescence of two bubbles with a small surface tension force. Both coalescence and breakup were more likely to occur in liquids with a higher concentration of nanoparticles. Additionally, we considered the mass transfer coefficient in the liquid phase and found that it can be enhanced by the coalescence and breakup, through the generation of polydisperse bubble swarms. A correlated parameter was deduced that can be substituted in the Hughmark equation to predict the mass transfer coefficient of the CO2–methanol–nanoparticle system.

Bubble coalescence and breakup model evaluation and development for two-phase bubbly flows

The performance of thermal-hydraulics analysis codes heavily depends on the reliable models developed for interfacial transfer terms in the two-fluid model. This requires the accurate modeling of the interfacial area concentration (IAC). The interfacial area transport equation (IATE) developed from the population balance equation is one of the promising ways to be able to dynamically describe the IAC evolutions in the two-phase flows. A lot of efforts have been made to develop the IATE and its bubble coalescence and breakup models in the past two decades. The one-group IATE with its bubble coalescence and breakup models serves as an important and promising model to predict the IAC behaviors in the bubbly flows now. This paper made an extensive survey on the recent theoretical and experimental researches on the IATE and collected the available 8 sets of bubble coalescence and breakup models and the latest experimental databases measured with the up-to-date measuring techniques. The 8 sets of bubble coalescence and breakup models are originally developed to model the bubble evolution in bubbly flows and the small bubble evolution in the flows beyond bubbly flow. The collected data covers the bubbly flows taken the vertical pipes with the diameter ranging from 0.0508 m to 0.200 m and the height-to-diameter ratio ranging from 4 to 113. To confirm the reliability of the 8 sets of bubble coalescence and breakup models, this paper systematically compared and evaluated them with the collected experimental data in the 1D one-group IATE numerical calculations. Although the bubble coalescence and breakup models of Sun et al. (2004, Modeling of bubble coalescence and disintegration in confined upward two-phase flow. Nucl. Eng. Des. 230, 3–26) and Hibiki and Ishii (2000, One-group interfacial area transport of bubbly flows in vertical round tubes. Int. J. Heat Mass Transfer 43, 2711–2726) were found to show the best and second-best predicting performance for the collected latest experimental databases of bubbly flows with data-number-averaging mean absolute relative errors of 33.5% and 61.7% respectively, their relative errors are not satisfactory. Hibiki and Ishii (2000, One-group interfacial area transport of bubbly flows in vertical round tubes. Int. J. Heat Mass Transfer 43, 2711–2726) model only consisting of the contributions of the bubble random collision coalescence and turbulent impact breakup mechanisms can reasonably predict the IAC experimental data in the bubbly flows with high-liquid flow-rate flow conditions and significantly overpredict the IAC experimental data in the bubbly flows with low-liquid flow-rate flow conditions. To further improve the performance of the 1D one-group IATE, this paper has developed a bubble coalescence and breakup model by greatly improving the model of Hibiki and Ishii (2000, One-group interfacial area transport of bubbly flows in vertical round tubes. Int. J. Heat Mass Transfer 43, 2711–2726) with the addition of the contribution of the wake entrainment bubble coalescence mechanism which prevails in low-liquid flow-rate bubbly flows and optimized the adjustable coefficients of the 3 major constitutive models with the collected latest experimental databases of bubbly flows. The data-number-averaging mean absolute relative error of the newly-proposed bubble coalescence and breakup model is accordingly improved to be 17.1% in the predictions of the collected latest experimental databases of bubbly flows.

Evaluation of two-group interfacial area transport equation model for vertical small diameter pipes against high-resolution experimental data

Two-phase flow is ubiquitous in industrial, chemical and thermal plants alike. The current state-of-the-art system-code model for predicting fluid transport in two-phase flows is the two-fluid model. In the two-fluid transport model, the coupling of mass, momentum and energy transfer between phases is highly dependent on interfacial area transfer terms. Several research efforts in the past have been focused on the development of an interfacial area transport equation model (IATE) in order to eliminate the drawbacks of static regime flow maps currently used in best-estimate thermal-hydraulic system codes. The IATE attempts to model the dynamic evolution of the vapor/liquid interface by accounting for the different interaction mechanisms affecting gaseous phase transport.The further development and validation of IATE models has been hindered by the lack of adequate experimental data in regions beyond the bubbly flow regime. At the Helmoltz-Zentrum Dresden-Rossendorf (HZDR) experiments utilizing wire-mesh sensors have been performed over all flow regimes, establishing a database of high-resolution (in space and time) data. A 52.3mm diameter pipe with a 16 by 16 wire-mesh sensor operating at 2.5kHz is utilized in the air-water experimental database used in this work. There are a total of 37 tests (with varying superficial gas and liquid velocities, at approximately 0.25MPa). Analysis of IATE performance in the bubbly flow and slug flow regimes is presented.The performance of the Fu-Ishii two-group IATE model is evaluated. In all regions, the interfacial area concentration for small bubbles is predicted well. The model performs poorly in high void fraction regimes, in which large irregularly shaped bubbles are present. The interaction mechanisms that support and deter performance of the IATE model are highlighted. A sensitivity analysis indicates modification of the group-2 wake entrainment coefficient may extend the validity of the Fu-Ishii model. An optimization study is presented to further explore improving IATE performance. It is concluded that the availability of accurate data at high void fractions from the HZDR facility provides a path to improve IATE performance.

Bluff body drag manipulation using pulsed jets and Coanda effect

The impact of fluidic actuation on the wake and drag of a three-dimensional blunt body is investigated experimentally. Jets blowing tangentially to the main flow force the wake with variable frequency and amplitude. Depending on the forcing conditions, two flow regimes can be distinguished. First, in the case of broadband actuation with frequencies comprising the natural wake time scale, the convection of the jet structures enhances wake entrainment, shortens the length of the recirculating flow and increases drag. Secondly, at higher actuation frequencies, shear-layer deviation leads to fluidic boat tailing of the wake. It additionally lowers its turbulent kinetic energy thus reducing the entrainment of momentum towards the recirculating flow. The combination of both mechanisms produces a rise in the base pressure and reduces the drag of the model. Both actuation regimes are characterized by complementary velocity, pressure and drag measurements at several upstream conditions and control parameters. By adding curved surfaces to deviate the jets by the Coanda effect, periodic actuation is reinforced and drag reductions of approximately 20 % are achieved. The unsteady Coanda blowing not only intensifies the flow deviation and the base pressure recovery but also preserves the unsteady high-frequency forcing effect on the turbulent field. The present results encourage further development of fluidic control to improve the aerodynamics of road vehicles and provide a complementary insight into the relation between wake dynamics and drag.

Implementation of an improved bubble breakup model for TFM-PBM simulations of gas–liquid flows in bubble columns

An improved bubble coalescence and breakup model was implemented in the inhomogeneous TFM-PBM model for buoyancy-driven gas–liquid bubbly flows. The bubble coalescence was modeled by considering bubble collisions induced by turbulent fluctuations, buoyancy driven, wake entrainment, and viscous shear, and the liquid film drainage model was used for the description of the coalescence efficiency of collisions. The bubble breakup was analyzed in terms of bubble interactions with turbulent eddies which coupled the restriction of surface energy with the capillary pressure. A generic TFM-PBM model was developed and applied for the simulations of bubble columns operated in different flow regimes. The evolutions of the bubble size distributions were simulated and validated for literature data of a D=0.14m cylindrical bubble column at gas superficial velocities 0.03 and 0.45m/s, respectively. Furthermore, a well documented D=0.44m bubble column was simulated, and the predicted distributions of the gas holdup and liquid velocity were compared with the experimental measurements of Chen et al. (1998). The model showed the capacity of describing the gas–liquid fluid dynamics of bubble columns operated in both bubbly and churn-turbulent flow regimes.

Experimental study of non-uniform inlet conditions and three-dimensional effects of vertical air–water two-phase flow in a narrow rectangular duct

To study the three-dimensional interfacial structure development in vertical two-phase flow, air–water upflow experiments were performed in a rectangular duct. Various non-uniform two-phase profiles were created by injecting air from individually controlled spargers at the duct inlet into uniformly injected water flow. A four-sensor conductivity probe was used to measure local void fraction, interfacial area concentration, bubble velocity and Sauter mean diameter at three axial locations to record the development of two-phase parameters. Experimental results showed that the lateral development across the wider dimension of the duct was significant with a non-uniform inlet profile when compared to a uniform inlet profile. It is postulated that lift, wall and turbulent forces are the major contributors to the lateral distribution of the two-phase interfacial structures making this an useful experiment for benchmarking three-dimensional two-fluid models. In examining the interfacial area, the shearing-off of group 1 bubbles (defined as the smaller spherical and distorted bubbles) from the skirt region of group 2 bubbles (defined as the bigger cap and churn bubbles), the coalescence of group 2 bubbles due to wake entrainment, and random collision are the major source and sink mechanisms of interfacial area concentration.