Freestream Flow Research Articles

Prediction model for multidirectional vortex-induced vibrations of catenary riser in convex/concave and perpendicular flows

This study presents an advanced numerical prediction model based on nonlinear wake oscillators for the investigation of multidirectional vortex-induced vibration (VIV) responses of a long catenary riser depending on the structural curved configuration versus the incoming flow direction. By considering a uniform free-stream flow aligned with the initial curvature plane of the catenary riser in a convex or concave shape, the normal flow velocity component is nonlinearly sheared owing to the spanwise variation of inclination angles. This is different from the perpendicular flow case in which the normal flow velocity is spatially constant. To capture the influence of flow direction on the three-dimensionally coupled cross-flow and in-line VIV, distributed wake oscillators are introduced and applied to simulate the fluctuating hydrodynamic lift and drag forces that account for the important effects of cylinder inclination, variable vortex excitation frequencies, global–local​ displacement relationships, and relative flow-cylinder velocities. The amplified mean drag force and the axial force depending on the tangential flow velocity are also considered. These empirical hydrodynamic effects are nonlinearly coupled with the equations of riser three-dimensional motion, and the overall governing equations are numerically solved to assess the multimode, multi-frequency and multi-degree-of-freedom responses. Validation of the model is carried out for VIV of flexible straight and curved cylinders based on experimental results in the literature. Parametric investigations are performed by varying the incoming flow velocities in perpendicular, convex and concave configurations for a long catenary riser with a low mass-damping ratio. In-plane (horizontal and vertical) and out-of-plane VIV features are presented and discussed in terms of space–time varying amplitudes, drag-amplified shape reconfigurations, oscillation frequencies, lock-in occurrences, dual resonances, orbital motion trajectories, fluid–structure energy transfer, and multimodal contributions. Overall prediction results highlight the influence of the cylinder geometric configuration versus the incoming flow orientation on multidirectional VIV characteristics that should be recognised and incorporated into practical analysis tools.

Experimental investigation of dynamic stall flow control using a microsecond-pulsed plasma actuator

To alleviate the performance deterioration caused by dynamic stall of a wind turbine airfoil, the flow control by a microsecond-pulsed dielectric barrier discharge (MP-DBD) actuator on the dynamic stall of a periodically pitching NACA0012 airfoil was investigated experimentally. Unsteady pressure measurements with high temporal accuracy were employed in this study, and the unsteady characteristics of the boundary layer were investigated by wavelet packet analysis and the moving root mean square method based on the acquired pressure. The experimental Mach number was 0.2, and the chord-based Reynolds number was 870 000. The dimensionless actuation frequencies F + were chosen to be 0.5, 1, 2, and 3, respectively. For the light dynamic regime, the MP-DBD plasma actuator plays the role of suppressing flow separation from the trial edge and accelerating the flow reattachment due to the high-momentum freestream flow being entrained into the boundary layer. Meanwhile, actuation effects were promoted with the increasing dimensionless actuation frequency F +. The control effects of the deep dynamic stall were to delay the onset and reduce the strength of the dynamic stall vortex due to the accumulating vorticity near the leading edge being removed by the induced coherent vortex structures. The laminar fluctuation and Kelvin–Helmholtz (K–H) instabilities of transition and relaminarization were also mitigated by the MP-DBD actuation, and the alleviated K-H rolls led to the delay of the transition onset and earlier laminar reattachment, which improved the hysteresis effect of the dynamic stall. For the controlled cases of F + = 2, and F + = 3, the laminar fluctuation was replaced by relatively low frequency band disturbances corresponding to the harmonic responses of the MP-DBD actuation frequency.

Mach 18 flow velocimetry with 100-kHz KTV and PLEET in AEDC Tunnel 9.

Krypton Tagging Velocimetry (KTV) and Picosecond Laser Electronic Excitation Tagging (PLEET) velocimetry at a 100-kHz rate were demonstrated in Mach 18 flow conditions at the Arnold Engineering Development Center (AEDC) Tunnel 9 employing a burst-mode laser system and a custom optical parametric oscillator (OPO). The measured freestream flow velocities from both KTV and PLEET agreed well with the theoretical calculation. The increase in repetition rate provides better capability to perform time-resolved velocimetry measurements in hypersonic flow environments.

Rarefied gas effects on hypersonic flow over a transpiration-cooled flat plate

This paper presents the effect of blowing (transpiration flow) on hypersonic flow over a flat plate at different flow regimes. The investigation involves the study of the interaction between the free stream flow of argon gas at Mach 5 and transpiring gas introduced at the fluid–solid interface. The freestream Knudsen number considered for the present analysis are 0.002, 0.01, 0.05, and 0.25, extending from continuum to rarefied through transitional flow conditions. Flow simulations are performed using the open source particle-based direct simulation Monte Carlo (DSMC) solver called Stochastic PArallel Rarefied-gas Time-accurate Analyzer (SPARTA). In the DSMC framework, the transpiring gases are introduced as jets with specified velocity, number density, and temperature uniformly throughout the surface in the direction normal to the surface. The variation in flow field properties, such as density, temperature, and velocity with and without transpiration, is studied. The influence of rarefaction on surface heat flux distribution is studied at different flow Knudsen numbers. Furthermore, the effect of introducing the transpiring gas at different densities into the flow field is investigated and its impact on the surface heat flux is discussed. It is interesting to note that in certain cases, the heat flux actually increases locally as a result of the interaction between transpiring gas and freestream flow.

The influence of buoyancy upon pollution trapping and dispersal in the wake of a backward-facing step

The importance of buoyancy relative to free-stream flow is described using an adapted Froude number $Fr' = U/f_0^{1/3}$ , where $U$ is the flow speed and $f_0$ is the exhaust buoyancy flux per unit length. We varied $Fr'$ by changing the free-stream flow rate, the exhaust flow rate and the buoyancy of the exhaust. We have experimentally identified two flow regimes, depending on the value of $Fr'$ . For high $Fr'$ (low buoyancy), dispersion is driven by inertial forces in the wake and the amount of a pollutant in the wake is independent of $Fr'$ . For moderate $Fr'$ , a wall plume develops up the back of the step, directly feeding the pollutant into the shear layer, but without altering the shape of the wake. This wall plume reduces the amount of pollutant trapped behind the step. We developed an analytic model to describe the quantity of pollutant trapped behind the step. The model predicts the transition from buoyancy being negligible to being the dominant transport mechanism within the wake. We have hypothesised and observed some evidence of a third regime at low $Fr'$ , when the buoyancy is sufficient to distort the macrostructure of the shear layer and wake.

How swimming style and schooling affect the hydrodynamics of two accelerating wavy hydrofoils

Fish schools can make frequent accelerations that are almost simultaneous. While schooling at constant speed is well studied, far less is known concerning accelerating fish school across various body caudal fin swimming styles. The present study investigates the effects of swimming styles and schooling on two accelerating wavy hydrofoils in a free stream flow at wavelengths λ=0.5−8, Strouhal number St=0.2−0.7, front–back distance D=0,0.25,0.5,0.75, phase difference ϕ/π=0,0.5,1,1.5, and lateral gap distance G=0.25,0.3,0.35 with fixed Reynolds number Re=5000 and maximum amplitude Amax=0.1. In total, 591 cases were simulated using open-source software IBAMR based on immersed boundary method. Low and high wavelengths correspond to advantageous propulsive efficiency and thrust, respectively. The highest group propulsive efficiency is obtained at low wavelength λ<1.2. At a side-by-side arrangement, the thrust upon the two foils can be equivalent across various wavelengths, indicating a synchronised acceleration. At staggered arrangement, the follower can take significant advantage of the leader in locomotion performance by tuning phase difference, especially at high wavelengths and close distances. Front–back distance is a key factor affecting the follower’s propulsive efficiency for short-wavelength swimmers, but not for long-wavelength ones. Various combinations of wavelength and relative distance can lead to distinct flow structures, indicating a tunable stealth capacity of the accelerating fish schools.

Wake-Induced Vibration of a flexible plate behind a cylinder: Effect of structural parameters

The present Fluid flexible Structure Interaction (FfSI) study is on free-stream flow across a circular cylinder, with a detached flexible-plate that undergoes Wake-Induced Vibration. Effect of non-dimensional bending stiffness Kb0.01–0.521, and plate-fluid density ratio ρ∗10–500 is investigated numerically, for structural properties as well as cylinder–plate proximity induced coupled flow physics and vibrational characteristics of the plate. The study is performed at a Poisson’s ratio νs=0.4, Reynolds number Re=100, non-dimensional cylinder–plate gap G∗=1.5, and a constant size of the plate (non-dimensional length L∗=1 and thickness t∗=0.05). For most of the present simulated cases, the plate undergoes symmetric first-mode vibrations, for which three FfSI regimes are proposed. Regime-I corresponds to anti-phase ϕ≈180° periodic variation between the displacement Y of the plate-tip and lift coefficient CL,p, while Regime-III corresponds to in-phase ϕ≈0° variation, and Regime-II corresponds to a transition in the phase difference from anti-phase to in-phase with increasing Kb. For the transitional FfSI Regime-II, obtained at low and intermediate values of both ρ∗ and Kb, a larger plate-amplitude A∗ along with an oscillation frequency ratio f∗ close to lock-in (f∗=1) is found. Whereas Regime-I, obtained at intermediate ρ∗ for low Kb and high ρ∗ for all Kb, corresponds to low and intermediate amplitude A∗ and high or super-harmonic f∗; and Regime-III, obtained at low and intermediate ρ∗ for high Kb, corresponds to low A∗ and low or sub-harmonic f∗. Type of interaction between the cylinder-vortex and plate-vortex, in the near wake, is constructive for the Regime-II and destructive for the other regimes. The fluid-dynamic force-based damping is presented as positive in the Regime-I and negative in the other regimes. The qualitative vortex interaction-dynamics and the flow-induced damping show a strong correlation with quantitative vibrational parameters, indicating strongly coupled multi-physics characteristics of the present FfSI system.

Blade-Tip Vortex Noise Mitigation Traded-Off against Aerodynamic Design for Propellers of Future Electric Aircraft

We study noise generation at the blade tips of propellers designed for future electric aircraft propulsion and, furthermore, analyze the interrelationship between noise mitigation and aerodynamics improvement in terms of propeller geometric designs. Classical propellers with three or six blades and a conceptual propeller with three joined dual-blades are compared to understand the effects of blade tip vortices on the noise generation and aerodynamics. The dual blade of the conceptual propeller is constructed by joining the tips of two sub-blades. These propellers are designed to operate under the same freestream flow conditions and similar electric power consumption. The Improved Delayed Detached Eddy Simulation (IDDES) is adopted for the flow simulation to identify high-resolution time-dependent noise sources around the blade tips. The acoustic computations use a time-domain method based on the convective Ffowcs Williams–Hawkings (FW-H) equation. The thrust of the 3-blade conceptual propeller is 4% larger than the 3-blade classical propeller and 8% more than the 6-blade one, given that they have similar efficiencies. Blade tip vortices are found emitting broadband noise. Since the classical and conceptual 3-blade propellers have different geometries, especially at the blade tips, they introduce deviations in the vortex development. However, the differences are small regarding the broadband noise generation. As compared to the 6-blade classical propeller, both 3-blade propellers produce much larger noise. The reason is that the increased number of blades leads to the reduced strength of tip vortices. The findings indicate that the noise mitigation through the modification of the blade design and number can be traded-off by the changed aerodynamic performance.

Impact of turbulence level on intermittent-like events in the wake of a model wind turbine

Intermittent-like events in the wake of a model wind turbine are explored experimentally in a wind-tunnel environment under various turbulence levels. Hotwire anemometry is used to obtain single-point statistics at high temporal resolution at various spanwise and streamwise locations in the wake of a turbine exposed to freestream flow with high and low turbulence levels. High-order statistics and basic metrics are employed to quantify extreme events departing from the quasi-Gaussian distribution of velocity increments. Results indicate that inflow and rotor-generated turbulence and wake shear layer produce a distinct effect on scale-dependent intermittent patterns in the wake. In general, rare events decrease with an increase in the time scale of the velocity increments and approach to scale independence at sufficiently large scales roughly twice the rotor size. A distinct peak at scales of roughly one-tenth to one rotor diameter characterizes the asymmetry factor along the rotor axis. The shape parameter evidences shared features, exhibiting variability in the transverse and downwind directions. Non-Gaussian properties at scales on the order of the rotor diameter suggest that intermittent motions may impact wind farm power and loads.

Fluid properties of an unsteady MHD free stream flow over a stretching sheet in presence of radiation

AbstractVariable fluid properties with thermal radiation in an unsteady magnetohydrodynamics free stream incompressible flow over a stretching sheet has been considered. The thermal diffusivity and viscosity of the fluid varies linearly with temperature. The governing partial differential equations are moulded to ordinary differential equations using time‐dependent similarity variables and the stream function. RKF technique with shooting method has been implement to find the solution numerically. In the current analysis the impact of unsteadiness, magnetic field, radiative parameter, variable fluid viscosity and thermal diffusivity parameter on heat and flow behavior with the free stream parameter have been studied. Transition point observed in the velocity profiles with an change in unsteadiness parameter and the effect of magnetic field is reduced in the presence of free stram velocity. The velocity and the temperature gradient are computed on the surface and their outcomes with different parameters have been analyzed in the results shown graphically and in tabular form.

Numerical investigation of MHD convective free stream nanofluid flow influenced by radiation and chemical reaction over stretching cylinder

AbstractChemically reacting magnetohydrodynamic radiative flow of convective free stream nanofluid through a stretching cylinder using Buongiorno's model is discussed. The behavior of Brownian motion and thermophoresis is also appropriate. By adopting the similarity transformation, the partial differential equation is diminished into a first‐order ordinary differential equation (ODE). Since transformed equations are highly nonlinear these ODEs are solved by using mathematical simulation. The shooting procedure has been adopted to resolve converted equations along the attendant Runge–Kutta–Fehlberg technique. The reason behind the present work is to research the effects of different parameters of fluid, namely, magnetic parameter, free stream velocity, Brownian motion, thermophoresis, chemical reaction, heat radiation, Lewis number on nanoparticle concentration, temperature, and velocity distribution. The impact of significantly participating parameters on velocity, concentration, and temperature distribution is distinguished with appropriate physical significance. The convergence of solutions for temperature, velocity, and concentration profiles is studied carefully. The measured challenges of nanofluids are scale‐up capacity, increase in nanofluid viscosity, nanoparticle dispersion, and nanofluid cost. It is observed that nanoparticle temperature rises for more value of Brownian motion parameter while it declines for higher Lewis number. The current study in the cylindrical region is related to novel free stream flow in the presence of chemical reactions along with convective conditions which find applications in electronic systems like microprocessors and in a wide variety of industries and in the field of biotechnology. The current research helps control the transport phenomena, helping production companies to find the quality of the desired product.

Effects of Upstream Wakes on the Boundary Layer Over a Low-Pressure Turbine Blade

Abstract In the present work, the evolution of the boundary layer over a low-pressure turbine blade is studied using direct numerical simulations, with the aim of investigating the unsteady flow field induced by the rotor-stator interaction. The freestream flow is characterized by the high level of freestream turbulence and periodically impinging wakes. As in the experiments, the wakes are shed by moving bars modeling the rotor blades and placed upstream of the turbine blades. To include the presence of the wake without employing an ad-hoc model, we simulate both the moving bars and the stationary blades in their respective frames of reference and the coupling of the two domains is done through appropriate boundary conditions. The presence of the wake mainly affects the development of the boundary layer on the suction side of the blade. In particular, the flow separation in the rear part of the blade is suppressed. Moreover, the presence of the wake introduces alternating regions in the streamwise direction of high- and low-velocity fluctuations inside the boundary layer. These fluctuations are responsible for significant variations of the shear stress. The analysis of the velocity fields allows the characterization of the streaky structures forced in the boundary layer by turbulence carried by upstream wakes. The breakdown events are observed once positive streamwise velocity fluctuations reach the end of the blade. Both the fluctuations induced by the migration of the wake in the blade passage and the presence of the streaks contribute to high values of the disturbance velocity inside the boundary layer with respect to a steady inflow case. The amplification of the boundary layer disturbances associated with different spanwise wavenumbers has been computed. It was found that the migration of the wake in the blade passage stands for the most part of the perturbations with zero spanwise wavenumber. The non-zero wavenumbers are found to be amplified in the rear part of the blade at the boundary between the low- and high-speed regions associated with the wakes.

Effect of Base Fluids on the Initiation of Vortex Shedding for Nanofluid Flow Over a Circular Object

Abstract An inherently steady separated flow of a nanofluid around a two-dimensional circular object can be transformed into an unsteady periodic flow by controlling the nanoparticle concentration. This has been established in the contemporary literature 2022 by Garai et al. (2022, “Triggering Vortex Shedding for the Free Stream Flow of Nanofluids Around Bluff Objects,” ASME J. Fluids Eng., 144(3), p. 034502.) using Cu–H2O and Ag–H2O nanofluids, where the base fluids remain the same (H2O) and the nanoparticles are different (Cu, Ag). However, it may be anticipated that changing the base fluid may cause certain alterations in the flow dynamics. Two different base fluids, e.g., propylene glycol (C3H8O2) and ethylene glycol (C2H6O2) are selected in which copper (Cu) nanoparticles are mixed to produce Cu–C3H8O2 and Cu–C2H6O2 nanofluids. The nanofluids flow over a circular object with Reynolds number in the range 15≤Re≤40. The Cu nanoparticle concentration is gradually increased and it is observed that the vortex shedding can be initiated with a lesser concentration of Cu in case of C3H8O2 based nanofluid compared to C2H6O2 and also H2O based nanofluids. Hence, the critical solid volume fractions (φcr) for Cu–C2H6O2 based nanofluid are more compared to Cu–C3H8O2 and Cu–H2O nanofluids. Further, φcr for all the nanofluids are found a decreasing function of Re in its chosen range. The critical solid fraction is estimated from an extended Stuart–Landau model and the phenomena are demonstrated qualitatively through the streamlines and vorticity contours and quantitatively through the lift signal analysis and phase diagrams. Finally, a regime diagram is constructed to demarcate the steady and unsteady zones of operation.

Effects of free-stream turbulence on motion-induced aerodynamic forces of a long-span flexible flat roof using LES simulations

This study aims to investigate the effects of free-stream turbulence on motion-induced aerodynamic forces of a long-span flexible flat roof. Large-eddy simulations (LES) of both rigid and forced vibrating roofs under different free-stream turbulence conditions are performed to obtain the aerodynamic forces and flow field data simultaneously. Firstly, the LES simulations of the rigid and vibrating roofs are validated against the reference experiments. And then, characteristics of the distributed pressures on vibrating roofs under free-stream flow turbulence conditions with different turbulence intensities are elucidated. Moreover, the motion-induced forces acting on the vibrating roof under different turbulence conditions are identified and compared, in which the power spectra of wind forces and aerodynamic damping and stiffness are examined. Furthermore, the underlying flow mechanism of the turbulence effects on the motion-induced forces is investigated based on POD analysis of the flow around the vibrating roof. It is found that dual effects exist between the influences of free-stream turbulence and roof motion on the aerodynamic forces, the motion-induced aerodynamic forces on the roof would be greatly decreased with the increase of free-stream turbulence, and the motion-induced effects become less dominant under high free-stream turbulence. The outcomes of this study would facilitate our understanding of the role played by the motion-induced aerodynamic forces and improve the wind-resistant design of long-span roof structures for engineering practices.

Velocity measurements in a hypersonic flow using acetone molecular tagging velocimetry.

In the present work, a non-intrusive diagnostic technique known as molecular tagging velocimetry was used to collect quantitative freestream velocity measurements in the Mach 7 Ludwieg Tube Wind Tunnel located at The University of Texas at San Antonio. This laser-based diagnostic technique used a single Nd:YAG 4th harmonic laserline to excite acetone molecules seeded in the flow field. From the resulting emitted light, mean and instantaneous velocity profiles in the hypersonic freestream flow and facility boundary layer were measured. Uncertainty in the velocity measurements for individual test runs is estimated at ≤ 8% while overall 1D freestream mean velocity measurements were recorded with ±2.4% (± 21 m/s) accuracy. The effect of acetone seeding on the speed of sound was also quantified.

Wind tunnel investigations of aircraft airfoil in cruise conditions

Presented work is focused on analysis of the flow over the 2D model of Airbus A320 airfoil wing in cruise phase of the flight. For this purpose the measurement channel with an airfoil model was designed and assembled in a transonic wind tunnel to obtain a similar flow pattern as in the reference two-dimensional freestream flow. Experimental investigations were conducted in the IMP PAN transonic wind tunnel with a relative narrow test section which is a novel approach in terms of these type of research. The test section was designed using CFD simulations based on 2D freestream flow for the tested wing profile and in the next step the research were continued experimentally in transonic wind tunnel with a measurement chamber width of 100 mm. This paper presents the results of reference investigations on the A320 wing profile which combines experimental tests and CFD calculations. The obtained results show that the approach presented in the paper is appropriate and the obtained flow features in the tunnel do not differ much from the freestream conditions.

The effect of increasing rarefaction on the formation of Edney shock interaction patterns: type-I to type-VI

A shock–shock interaction problem can arise in high-speed vehicles where an oblique shock from one part of the body impinges on a bow shock from a different part of the body. The nature of the interaction can change as the vehicle increases in altitude to a more rarefied environment. In this work, the outcomes of a numerical study investigating the formation of Edney shock patterns from type-I to type-VI as a result of shock–shock interactions at different rarefaction levels are presented. The computations are conducted with a direct simulation Monte Carlo solver for a free-stream flow at a Mach number of 10. In shock–shock interaction problems, both geometrical and rarefaction parameters determine what type of Edney pattern is formed. The region on the shock impinged surface that experiences enhanced thermo-mechanical loads increases when the free-stream flow becomes more rarefied, but the peak values decrease. It is known that these shock interactions can have unsteady behavior in the continuum regime; the current work shows that although increasing rarefaction tends to move the flow toward steady behavior, under some conditions the flow remains unsteady.Graphical abstract

Study of an air bubble curtain along a wall in water and radiated noise mitigation

The injection of air bubbles into the liquid phase of a freestream flow has several impacts on the flow structure, which depends on the volume and size of the bubbles. This work experimentally investigates the characteristics of air bubble injection into freestream flow using three different injector models. The effects of the bubble curtain on the sound wave attention are studied. A wide range of air injection rates from 2 to 50 standard liters per minute is injected into the freestream water at Froude numbers (Fr) of 50.5, 70.7, and 90.9. The injector model is placed on the sidewall, which is where the bubble curtain is generated. High-speed cameras and an image processing technique are used to visualize and quantify the projected void fraction (PVF) of air bubbles. The sound measurement system consists of two hydrophones. The first hydrophone projects sound waves at discrete frequencies ranging from 17 to 50 kHz, and the second receives the transmitted sound waves. The bubble PVF is observed to increase with the air injection rate as the Froude number increases. There is no significant increment in the thickness of the bubbly curtain when the air injection rate or Froude number increases. The different air injector models produce various air bubble flow patterns, and model II provides the highest PVF. The bubble curtain greatly suppresses sound waves at all frequency ranges. However, the sound attenuation rate varies along the frequency range. The insertion loss increases depending on the PVF of the bubbles, freestream velocity, and frequency range. At a high Froude number of 90.9, the increased air injection rate does not affect the insertion loss. Model II has a relatively higher insertion loss rate at frequencies &amp;gt;26 kHz.

Gaseous Entrainment Dynamics in a Viscous Pool Due to Combined Influence of Asymmetric Rotational Field and Crossflow of Air

Abstract Efforts are made to perform simulations to describe the gaseous entrainment dynamics in a viscous liquid pool due to the combined influence of asymmetric converging rotational field and continuous freestream flow of air. A pair of counter-rotating and equal sized rollers is placed inside the pool along a horizontal line. Gerris is an open-source solver, which is employed to carry out the present computational study. Complex interfacial configurations are illustrated with the influence of relevant input parameters, such as rotation of rollers 1 and 2 (measured by Capillary number, Ca1=Rω1μl/σ and Ca2=Rω2μl/σ, where R=D/2 is roller radius), submersion depth (b*), the gap between the rollers (2a*), and strength of air stream flow (measured by Reynolds number, Reflow=ρgUD/μg). It has been observed that the depth of steady entrainment is reduced at Reflow≠0 compared to Reflow=0 because the hydrodynamic force acts as an opposing force to viscous pumping and rotating inertia. A complete understanding of disintegration of and subsequent accumulation of gaseous bubbles from the cusp tip is characterized in detail. In addition, the influence of viscous drag (specified by Morton number, Mo=gμl4(ρl−ρg)/(ρl2σ3)) and gravitational pull (estimated by Archimedes number, Ar=gD3ρl2/μ2) on the phase contours are also reported. Finally, an analytical formulation is proposed to analyze the structure of entrainment, and this model reports an excellent match with the numerical findings.

Behavior of Gas Entrainment Inside Viscous Pool Due to Combined Influence of Symmetric Rotational Field and Freestream Flow of Air

Abstract The present investigation elucidates the interfacial characterization caused by the simultaneous effect of a symmetric converging rotational field and continuous air stream flow above the free surface. The converging rotational field is developed by a couple of counter and equal rotating rollers fully immersed inside the viscous liquid medium and their centers are aligned along a horizontal line. Such phenomenon is abundantly encountered in various engineering devices, where the interactions and transfer of mass, momentum, and energy are quite important through gas–liquid interfaces. Behavior of entrainment profile is observed due to the influence of various relevant pertinent parameters, namely, rotational of speed (measured by Ca), submersion depth (b*), the gap between the rollers (2a*), and strength of air stream flow (measured by Reflow). An upper rounded structured interfacial configuration is obtained for all cases of Ca when the rollers are located very close to each other. The length of the entrainment of cusp decreases with the rise of Reflow for the same value of Ca. The value of Cac increases continuously with the increase of Reflow for a particular of 2a* and b*. Bubble ejection from filament tip and subsequent accumulation increases significantly with the rise of Ca for a particular case of Reflow. The cusp tip progressively traverses in upward direction with the continuous increase of gravitational pull for a particular value of Ca and Reflow. Entrainment length progressively grows with the continuous rise of viscous drag for a particular value of Ca and Reflow. Finally, an analytical formulation is proposed to analyze the structure of entrainment and this model reports an excellent match with the numerical findings.