Physics of Fluids

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Table of Contents for Physics of Fluids. List of articles from both the latest and ahead of print issues.
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Numerical aero-thermal study of high-pressure turbine nozzle guide vane: Effects of inflow conditions

Fri, 03/27/2020 - 11:19
Physics of Fluids, Volume 32, Issue 3, March 2020.
Accurate predictability of high-pressure turbine nozzle guide vane aero-thermal performance is highly desired in the development campaign due to the exposure of the component to a frequent and high heat load. In this paper, the representative vane profile in modern aero-engines is numerically studied. Aerodynamics and aero-thermal validations of the blade profile have been performed in comparison with the available experimental data. It has been shown that a satisfactory agreement could be achieved with the use of the transitional turbulence model shear stress transport γ–θ due to its superiority in capturing the laminar–turbulent transition. Sensitivity studies on the increase in the inlet turbulence intensity, inlet endwall boundary layer thickness, and inlet total temperature profile have been performed to understand the impact of inflow conditions’ uncertainty on the aero-thermal predictability. Increasing the inlet turbulence intensity increases the pressure surface heat transfer coefficient and induces an earlier transition onset on the suction surface. Due to the rapid decay of turbulence intensity in the numerical model, the use of an artificially high inlet turbulence intensity has been shown to be effective in the prediction improvement. On the other hand, the change in the inlet boundary layer thickness influences the formation and strength of the secondary flow, namely, horseshoe vortex and passage vortex. These secondary flow phenomena affect the local blade surface heat transfer coefficient in the near-endwall region although the most significant rise in heat transfer is found on the endwall. The temperature distortion amplitude of a hot streak and its relative clocking position with the vane significantly affect the heat flux distribution. In contrast, the heat transfer coefficient is less sensitive to the change in hot streak conditions. However, it has been shown that increasing the temperature distortion amplitude could induce a larger difference among different clocking configurations. In addition, decreasing the difference between the fluid and wall temperature would delay the transition onset and stabilize the boundary layer. Further analysis of the unsteady effects has been carried out by comparing the steady and time-averaged flow solutions. It has been observed that the discrepancy between these solutions is attributed to the flow field nonlinearity. Thus, a significant discrepancy can be found in the laminar–turbulent transition as well as in the trailing edge region. However, since the contribution of these regions on the total area-averaged heat transfer is small, their influence on the total vane heat transfer is limited.

Vortex-induced vibrations of a confined circular cylinder for efficient flow power extraction

Fri, 03/27/2020 - 11:19
Physics of Fluids, Volume 32, Issue 3, March 2020.
The effects of confinement on vortex-induced vibration (VIV) of a circular cylinder (diameter D) and its flow power extraction capability are presented. A two-dimensional numerical study is performed on VIV of a circular cylinder inside a parallel plate channel of height H at Reynolds numbers 100 and 150. The blockage ratio (b = D/H) is varied from 1/4 to 1/2. The cylinder is elastically mounted with a spring such that it can only vibrate transversely to the flow. It has a fixed non-dimensional mass (m*) of 10. The energy extraction process is modeled as a damper (damping ratio ζ) attached to the cylinder. With increasing blockage, the vibration amplitude of the cylinder decreases, the lock-in happens at a larger non-dimensional natural frequency of the cylinder, and the initial branch of the vibration response of the cylinder shrinks. There is a minimum value of the channel height and Reynolds number for the existence of the initial branch. The extracted power is found to increase rapidly with the blockage. For the maximum blockage (b = 1/2), the flow power extracted by the cylinder is an order of magnitude larger as compared to what it would extract in an open domain with free stream velocity equal to the channel mean velocity. The optimal mass-damping (α = m*ζ) for extracting the maximum power is found to lie between 0.2 and 0.3. An expression is derived to predict the maximum extracted power from the undamped response of a confined/unconfined cylinder.

Energy dissipation analysis based on velocity gradient tensor decomposition

Fri, 03/27/2020 - 11:19
Physics of Fluids, Volume 32, Issue 3, March 2020.
A velocity gradient tensor decomposition method based on a normal frame is introduced in this paper. The velocity gradient tensor is decomposed into a compression–stretching tensor, pure rotation tensor, and pure shear tensor. The analysis shows that both the strain rate tensor and vorticity tensor in Helmholtz velocity decomposition contain shear tensor components, and the total pure shear tensor is the combination of shear components in the two tensors. Based on this decomposition and the physical meaning of each tensor term, the energy dissipation of the channel flow with or without a pressure gradient and a turbine passage flow are analyzed. The results show that the energy dissipation is caused by shear deformation and expansion and contraction deformation of the motion fluid, and pure rotation does not cause energy dissipation. In particular, the pure shear is the primary factor of energy dissipation. Shear accounts for 99.9% of energy dissipation in the fully developed turbulence of zero-pressure gradient channel flow, 99% of the energy dissipation in the separated boundary layer flow is caused by the pure shear, and 81% of the energy dissipation in the turbine stage flow is caused by pure shear.

Investigation on the flow-induced noise propagation mechanism of centrifugal pump based on flow and sound fields synergy concept

Fri, 03/27/2020 - 04:15
Physics of Fluids, Volume 32, Issue 3, March 2020.
This paper explores the flow-induced noise propagation mechanism of centrifugal pump from the view of flow and sound field synergy concept. First, the unsteady synergetic relationship between flow and sound fields is deduced, and the synergy angle is defined to describe the synergy degree. It is shown that the domain-averaged synergy angle (θave) changes little with flow time, which implies that the synergy degree is basically unchanged with flow time. With increasing rotational speed or flow rate, the time-averaged θave (θtave) in the impeller and the volute moves far away from 90° gradually, i.e., the synergy degree increases. Meanwhile, the noise outside the pump increases, and the variation of both the noise outside the pump and θtave tends to be gradual. The results manifested that the flow-induced noise propagation mechanism of the centrifugal pump can be well described by the change in synergy degree and the increase in synergy degree can cause the noise tending to propagate outside. In addition, the impact of the blade outlet angle on the noise propagation characteristics is investigated. Considering the synergy degree in the impeller and the volute comprehensively, the deviation of θtave from 90° decreases from 6.48° to 4.74° as the angle increases from 15° to 35°, i.e., θtave tends to approach 90°, and the synergy degree decreases gradually, indicating that increasing the blade outlet angle can weaken the tendency of noise propagating outside by decreasing the synergy degree. These conclusions can guide noise control research and engineering design.

Transition effects on flow characteristics around a static two-dimensional airfoil

Wed, 03/25/2020 - 11:55
Physics of Fluids, Volume 32, Issue 3, March 2020.
Flows past a static NACA0015 airfoil are numerically investigated via Reynolds-averaged Navier–Stokes simulations at the Reynolds number 1.95 × 106, the Mach number 0.291, and the angle of attack (AoA) from 0° to 18°. Specifically, a one-equation local correlation-based transition model (γ model) coupled with Menter’s k–ω shear stress transport (SST) model (SST–γ model) is employed to approximate the unclosed Reynolds quantities in the governing equations. Distributions of mean velocity and Reynolds stresses as well as typical integral quantities, such as the drag coefficient, lift coefficient, and moment coefficient, are calculated and compared with previously reported experimental data and present numerical data based on Menter’s original k–ω SST model. It turns out that the SST–γ model enables the capture of a laminar separation bubble (LSB) near the leading edge of the airfoil and shows significant advantages over the traditional “fully turbulent” models for the prediction of static stall. As the AoA varies from 0° to 18°, the flow regime is affected by different processes, i.e., flow transition, flow separation, and interaction between the LSB and the trailing-edge separation bubble, which, respectively, correspond to the linear-lift stage, light-stall stage, and deep-stall stage.

Heat transfer enhancement and reduction in low-Rayleigh number natural convection flow with polymer additives

Wed, 03/25/2020 - 11:55
Physics of Fluids, Volume 32, Issue 3, March 2020.
The effects of viscoelasticity, here caused by polymer additives, on Rayleigh Bénard convection flows are investigated via direct numerical simulations at a marginally turbulent Rayleigh number. Simulations with a range of polymer length and relaxation time scales show heat transfer enhancement (HTE) and reduction (HTR). The selection of HTE and HTR depends strongly on the maximum extensional viscosity of the solution, whereas the magnitude of heat transfer modification is a function of both the maximum extensional viscosity and relaxation time of the polymer solution. The underlying physics of HTE and HTR are explored, and a mechanism of the interaction between convection cells and polymers is proposed. The findings are extrapolated to high Ra to shed some new light onto experimental observations of HTR.

Electroosmosis of a viscoelastic fluid over non-uniformly charged surfaces: Effect of fluid relaxation and retardation time

Tue, 03/24/2020 - 05:41
Physics of Fluids, Volume 32, Issue 3, March 2020.
We investigate the electroosmotic flow of a quasi-linear viscoelastic fluid over a surface having charge modulation in narrow confinements. We obtain analytical solutions using a combination of regular and matched asymptotic expansions in order to describe the viscoelastic flow field and apparent slip velocity besides pinpointing variations of the flow rate and ionic currents due to the surface charge modulation. We demonstrate excellent agreement between the asymptotic analytical solution for the flow field and the full numerical solution in the limiting condition of a thin electrical double layer and weakly viscoelastic fluid. For a wide range of flow governing parameters, we analyze the flow velocity, vortex dynamics, flow rates, and streaming current. We demonstrate that the magnitude of the observed electroosmotic slip velocity is more sensitive to the thickness of the electrical double layer rather than the viscoelasticity of the fluid. We have observed that the contribution of fluid elasticity is prominent in breaking the axial symmetry in the electroosmotic flow with the presence of periodic charge distributions, which is in contrast to the symmetric electroosmotic flow field of a Newtonian fluid over the same charge modulated walls. The results hold the key toward understanding the flow of biological fluids in microfluidic flows by leveraging electrokinetic transport over charge modulated surfaces. We believe that the results of net throughput, streaming current, and vortex dynamics will aid our understanding of the complex fluid behavior and microfluidic mixers.

Large eddy simulation of the separated flow transition on the suction surface of a high subsonic compressor airfoil

Tue, 03/24/2020 - 04:57
Physics of Fluids, Volume 32, Issue 3, March 2020.
A large eddy simulation (LES) was conducted to investigate the separated flow transition on the suction surface of a high subsonic compressor airfoil at two Reynolds number (Re) conditions (1.5 × 105 and 0.8 × 105). The detailed vortex evolution in the separated shear layer was revealed. The instability amplification in the transition process and the associated loss mechanism were clarified. At Re = 1.5 × 105, the two-dimensional spanwise vortices shed periodically and were further distorted with the interaction of the streamwise evolving vortices, and then, small vortices were generated in the streamwise pairing of the neighboring spanwise vortices. Finally, three-dimensional hairpin vortices broke down into small-scale turbulent structures near the reattachment, along with the “ejection-sweeping” process near the wall. When the Reynolds number decreased to 0.8 × 105, the initial vortex shedding was not periodic, but the subsequent vortex evolution process was very similar to the case of Re = 1.5 × 105. The results have demonstrated the importance of the Tollmien–Schlichting (T–S) mechanism for the initial growth of disturbances in the attached boundary layer, but the transition process that occurred in the separated shear layer was dominated by the inviscid Kelvin–Helmholtz (K–H) instability. Moreover, a secondary instability observed in the vortex pairing process was supposed to have a great impact on the onset of transition. With the decrease in Re, the shear layer instability declined to a lower level, leading to a delayed transition. In addition, the deformation works associated with the Reynolds shear stress was found to be mainly responsible for the loss generation in the transitional flow. Compared with the traditional Reynolds average Navier–Stokes method, the LES was more accurate in predicting the profile loss at a low Re.

Effect of temperature on gelation and cross-linking of gelatin methacryloyl for biomedical applications

Tue, 03/24/2020 - 02:45
Physics of Fluids, Volume 32, Issue 3, March 2020.
Hydrogels with or without chemical cross-linking have been studied and used for biomedical applications, such as tissue repair, surgical sealants, and three dimensional biofabrication. These materials often undergo a physical sol–gel or gel–sol transition between room and body temperatures and can also be chemically cross-linked at these temperatures to give dimensional stability. However, few studies have clearly shown the effect of heating/cooling rates on such transitions. Moreover, only a little is known about the effect of cross-linking temperature or the state on the modulus after cross-linking. We have established rheological methods to study these effects, an approach to determine transition temperatures, and a method to prevent sample drying during measurements. All the rheological measurements were performed minimizing the normal stress build-up to compensate for the shrinking and expansion due to temperature and phase changes. We chemically modified gelatin to give gelatin methacryloyl and determined the degree of methacryloylation by proton nuclear magnetic resonance. Using the gelatin methacryloyl as an example, we have found that the gel state or lower temperature can give more rigid gelatin-based polymers by cross-linking under visible light than the sol state or higher temperature. These methods and results can guide researchers to perform appropriate studies on material design and map applications, such as the optimal operating temperature of hydrogels for biomedical applications. We have also found that gelation temperatures strongly depend on the cooling rate, while solation temperatures are independent of the heating rate.

Secondary instability of stationary Görtler vortices originating from first/second Mack mode

Mon, 03/23/2020 - 10:04
Physics of Fluids, Volume 32, Issue 3, March 2020.
This work investigates the origination of the secondary instability in Görtler vortices using the linear stability theory, BiGlobal analysis, three-dimensional linear parabolized stability equations (3DLPSEs), and direct numerical simulation (DNS). The flow over a concave wall suffering from the Görtler instability and first/second Mack mode instability is selected. Furthermore, this work simulates the evolution of infinitesimal Mack mode disturbance in a flow perturbed by finite-amplitude Görtler vortices by using DNS and 3DLPSE methods. The 3DLPSE approach accurately predicts the process of Mack mode disturbance evolving into the secondary instability of Görtler vortices, and a perfect agreement with results by DNS is obtained. The results indicate that the secondary instability of stationary Görtler vortices can originate from the first/second Mack mode. The evolutions of first/second Mack mode with different spanwise wavenumbers are performed based on 3DLPSE and compared against the BiGlobal method. The results show that the shape functions and growth rates of disturbances always tend to the results of dominant modes obtained by the BiGlobal method. Because the dominant mode might shift from one to another, the overall evolution cannot be predicted only by the BiGlobal method based on a fixed mode. According to our computations, it is deduced that the Mack modes with the same frequency and symmetric characteristics would finally develop into the secondary instability with similar shapes.

Pattern method for higher harmonics of first normal stress difference from molecular orientation in oscillatory shear flow

Fri, 03/20/2020 - 01:36
Physics of Fluids, Volume 32, Issue 3, March 2020.
This study examines the simplest relevant molecular model of a polymeric liquid in large-amplitude oscillatory shear (LAOS) flow: rigid dumbbells suspended in a Newtonian solvent. For such suspensions, the viscoelastic response of the polymeric liquid depends exclusively on the dynamics of dumbbell orientation. Previously, the explicit analytical expressions of the zeroth, second, and fourth harmonics of the alternating first normal stress difference response in LAOS have been derived. In this paper, we correct and extend these expressions by seeking an understanding of the next higher harmonic. Specifically, this paper continues a series of studies that shed light on molecular theory as a useful approach in investigating the response of polymeric liquids to oscillatory shear. Following the general method of Bird and Armstrong [“Time-dependent flows of dilute solutions of rodlike macromolecules,” J. Chem. Phys. 56, 3680 (1972)], we derive the expression of the first normal stress coefficient up to and including the sixth harmonic. Our analysis relies on the extension of the orientation distribution function to the sixth power of the shear rate. Our expression is the only one to have been derived from a molecular theory for a sixth harmonic and thus provides the first glimpse of the molecular origins of a first normal stress difference higher than the fourth.

Universal scaling parameter for a counter jet drag reduction technique in supersonic flows

Fri, 03/20/2020 - 01:36
Physics of Fluids, Volume 32, Issue 3, March 2020.
The reduction in aerodynamic drag by injecting a gaseous jet from the nose of a blunt body into a supersonic stream is investigated numerically. The penetration of the jet into the supersonic flow modifies the shock structure around the body and creates a low pressure recirculation zone, thereby decreasing the wave drag significantly. Combining various theoretical estimates of different flow features and numerical simulations, we identify a universal parameter, called the jet to freestream momentum ratio (RmA), which uniquely governs the drag on the blunt body. The momentum ratio fundamentally decides the penetration of the jet as well as the extent of low pressure envelope around the body. In addition, various influencing parameters reported in the literature are reviewed for different steady jet flow conditions. Furthermore, their limitations in regulating the flowfield are explained by correlating the facts with the jet to freestream momentum ratio. We perform the simulations for various combinations of physical and flow parameters of the jet and the freestream to show a universal dependence of drag on the momentum ratio.

Tip-vortex flow characteristics investigation of a novel bird-like morphing discrete wing structure

Wed, 03/18/2020 - 02:08
Physics of Fluids, Volume 32, Issue 3, March 2020.
A bird-like morphing discrete wing, inspired by primary feathers of birds’ wings, was designed to control the wing-tip vortex strength. The influence of both the morphing process and discrete (non-continuous) surface feature for the bird-like wing structure on the tip-vortex flow characteristics was investigated in detail at Re = 87 000. The results reveal that the morphing process of the bird-like discrete wing structure can achieve the effective control of the core vortex strength by changing the flow structures around the tip-vortex core center(s). The induced drag yielded by the bird-like morphing wing structure is tightly related to its vorticity distribution in the near-wake region. Moreover, compared with the fully extended fixed-wing model with a continuous surface structure, the bird-like discrete wing model with the fully extended morphing state can suppress the core vortex strength by destroying the tip-vortex merging process. Meanwhile, the core vortex strength of the fully extended discrete wing model decays more sharply with the increase in x/c. The maximum proportions of the induced drag relative to the total drag for both the discrete and continuous wing models with the fully extended shape are 14.33% and 19.97%, respectively. However, the fully folding process of the bird-like wing structure significantly weakens the induced-drag reduction effect of the discrete surface structure. The maximum proportions of the induced drag relative to the total drag for both the discrete and continuous wing models with the fully folded shape are 17.59% and 18.41%, respectively.

Numerical simulations of inert and reactive highly underexpanded jets

Wed, 03/18/2020 - 02:08
Physics of Fluids, Volume ISPF2020, Issue 1, March 2020.
In this study, the high-resolution numerical simulations of the two-dimensional (2D) multi-component inert and reactive highly underexpanded jets are conducted to quantify the influences of the injected gas mixture properties on the flow structure. First, the gas mixture with the specified species mass fractions is imposed to exhaust into the quiescent air with a Mach number of 1.0, of which the specific heat ratios (γe) range from 1.3 to 1.6. Our results indicate that the larger γe yields a relatively shorter and thinner jet core under the same inlet pressure ratio due to the sound speed increasing. Next, we focus on the chemical reaction effects on the jets with a premixed hydrogen–air mixture injection. The results reveal that the shock-induced combustion develops into a detonation, inducing numerous vortices behind the combustion wave, while the combustion in the mixing layer cannot be preserved due to the instability of the supersonic shearing. During the detonation process, the increasing pressure accompanied by the heat release forces the Riemann wave to move upstream compared with the inert one. The violent detonation periodically propagates between the two jet triple points. The detonation collision leads to the intersection of their slip lines, which causes distinct vortex formation. In addition, the main frequencies, corresponding to the Riemann wave movement, the oscillation of the shock-induced ignition positions, the periodical propagation of the detonation, and the collision of the detonation triple points, are explored to explain the unsteady process of the reactive highly underexpanded jet.

Numerical simulations of inert and reactive highly underexpanded jets

Wed, 03/18/2020 - 02:08
Physics of Fluids, Volume 32, Issue 3, March 2020.
In this study, the high-resolution numerical simulations of the two-dimensional (2D) multi-component inert and reactive highly underexpanded jets are conducted to quantify the influences of the injected gas mixture properties on the flow structure. First, the gas mixture with the specified species mass fractions is imposed to exhaust into the quiescent air with a Mach number of 1.0, of which the specific heat ratios (γe) range from 1.3 to 1.6. Our results indicate that the larger γe yields a relatively shorter and thinner jet core under the same inlet pressure ratio due to the sound speed increasing. Next, we focus on the chemical reaction effects on the jets with a premixed hydrogen–air mixture injection. The results reveal that the shock-induced combustion develops into a detonation, inducing numerous vortices behind the combustion wave, while the combustion in the mixing layer cannot be preserved due to the instability of the supersonic shearing. During the detonation process, the increasing pressure accompanied by the heat release forces the Riemann wave to move upstream compared with the inert one. The violent detonation periodically propagates between the two jet triple points. The detonation collision leads to the intersection of their slip lines, which causes distinct vortex formation. In addition, the main frequencies, corresponding to the Riemann wave movement, the oscillation of the shock-induced ignition positions, the periodical propagation of the detonation, and the collision of the detonation triple points, are explored to explain the unsteady process of the reactive highly underexpanded jet.

Unsteady behaviors of separated flow over a finite blunt plate at different inclination angles

Tue, 03/17/2020 - 02:47
Physics of Fluids, Volume 32, Issue 3, March 2020.
This study comprises an extensive analysis of unsteady behaviors of a separated flow over a finite blunt plate at three different inclination angles, θ = 0°, 3°, and 6°. It was found that these three distinctly different flow patterns resulted from increasing inclination angles: reattachment, intermittent reattachment, and non-reattachment. The separated flow fields and wall-pressure fluctuations were experimentally measured with planar particle image velocimetry (PIV) operating at 1 Hz and a microphone array sampling at 5 kHz, respectively. Flow patterns were discussed in terms of the time-averaged flow fields and distributions of the statistical quantities (i.e., the reverse-flow intermittency, or velocity fluctuation intensity). A slender separation bubble formed in the front area of the plate (0 < x/D < 4.6) in the system where θ = 0° and then it enlarged to the whole surface of the plate in the system where θ = 3°. In contrast, in the system where θ = 6°, the plate surface was entirely engulfed by a large recirculation zone extending to the near-wake region. In the wall-pressure fluctuation analysis, two characteristic frequencies, St = 0.036 and 0.107, could be readily identified in all three systems; these corresponded to the flapping of separation bubble and the shedding of large-scale vortical structures, respectively. In addition, in the system where θ = 0°, wall-pressure fluctuations of the Karman vortex were detected at St = 0.154 but were suppressed in the systems where θ = 3° and 6° due to the extensive interaction between the shedding of large-scale vortical structures and the unsteady wake. Subsequently, a field-programmable gate array taking full advantage of dynamic mode decomposition (DMD) on the wall-pressure fluctuations was constructed, and a real-time conditional signal corresponding to individual unsteady behavior was generated to fire the phase-locking PIV measurement. High-resolution spatiotemporal evolutions of dominant flow behaviors (i.e., enlargement-and-shrinkage motion of the separation bubble and shedding motion of the large-scale vortical structures) were determined. In the system where θ = 6°, a separation bubble enlarged and shrank dramatically together with the shedding of large-scale vortical structures, leading to a large recirculation zone over the blunt plate, distinct from the behavior in the systems where θ = 0° and 3°. Finally, a joint dominant mode analysis of flow structures and wall-pressure fluctuations was further evaluated, which delineated the complex unsteady processes in separated flow clearly and provided more information and references for other studies on wind engineering, fluid-induced structure vibrations, and acoustic emissions.

Linear global stability of liquid metal mixed convection in a horizontal bottom-heating duct under strong transverse magnetic field

Tue, 03/17/2020 - 02:16
Physics of Fluids, Volume 32, Issue 3, March 2020.
Two-dimensional steady-state solutions of liquid metal mixed convection in a horizontal bottom-heating duct under a strong magnetic field are first computed numerically by the Newton iteration method along with the spatial discretization of the Taylor–Hood finite element. Two branches of steady solutions with symmetrical rolls and a pair of asymmetrical solutions with a single roll are identified and can be regarded as the base flow for linear global stability analysis. The symmetrical steady solution for the first branch has a nearly uniform distribution for the temperature field in the transverse direction, while the second branch occurs at much larger Grashof numbers and the temperature field becomes nonuniform transversely. The linear stability analysis is performed for a fixed Reynolds number and Prandtl number with Re = 5000 and Pr = 0.0321. For the symmetrical rolls of the first branch, with an increase in the Grashof number, two-dimensional stationary instabilities first occur at small Hartmann numbers, while three-dimensional oscillatory instabilities first appear at moderate or large Hartmann numbers. From the further study of the two-dimensional instabilities, it is revealed that the asymmetrical solution is actually bifurcated supercritically from the symmetrical solution at a two-dimensional critical Grashof number. In addition, the critical curve of the Grashof number with respect to the Hartmann number for the three-dimensional oscillatory mode shows that there exists a minimum critical Grashof number, which occurs at a moderate Hartmann number. The critical curves of the one-roll asymmetrical solution are also exhibited and determined by two three-dimensional oscillatory unstable modes. It is revealed that there exists a minimum Hartmann number below which the asymmetrical steady-state can always remain stable for all Grashof numbers (5 × 105–107). The energy analyses at the oscillatory critical thresholds with different Hartmann numbers are performed to exhibit that buoyancy is the dominant destabilizing term, and the magnetic force is always the main stabilization term for both symmetrical and asymmetrical solutions. In addition, both streamwise and cross-sectional shears of the basic flow are important for the determination of the linear stability boundary of the asymmetrical solution.

Hole expansion from a bubble at a liquid surface

Tue, 03/17/2020 - 02:16
Physics of Fluids, Volume 32, Issue 3, March 2020.
For millimetre to micron sized bubbles, floating at the free surface of different low viscosity fluids with different surface tensions, and then collapsing, we study the ensuing expansion of the outer radius of the hole (ro) at the free surface, as well as its velocity of expansion (uo). Since the thin film cap of the bubble disintegrates before the hole in it reaches the static rim, the hole expansion at intermediate times occurs as if it initiates at the bubble’s static rim of radius Rr; the evolution of ro then results to be a strong function of gravity, since Rr depends strongly on the bubble radius R. A scaling analysis, which includes the increase in the tip radius due to mass accumulation and the resulting change in the retraction force, along with the gravity effects by considering the hole radius in excess of its initial static radius, re = ro − Rr, results in a novel scaling law [math], where [math] is the capillary time scale; this scaling law is shown to capture the evolution of the hole radii in the present study. The dimensionless velocities of hole expansion, namely, the Weber numbers of hole expansion, [math], scale as [math], independent of gravity effects, matching the observations. We also show that these Weber numbers, which reduce with time, begin with a constant initial Weber number of 64, while the viscous limit of the present phenomena occurs when the bubble Ohnesorge number [math].

Unified prediction of reshocked Richtmyer–Meshkov mixing with K-L model

Mon, 03/16/2020 - 05:39
Physics of Fluids, Volume 32, Issue 3, March 2020.
Hydrodynamic instabilities, including Rayleigh–Taylor, Richtmyer–Meshkov (RM), and Kelvin–Helmholtz, induced turbulent mixing broadly occur in both natural phenomena, such as supernova explosions, and high-energy-density applications, such as inertial confinement fusion. Reshocked RM mixing is the most fundamental physical process that is closely related to practical problems, as it involves three classical instabilities. In complex applications, the Reynolds-averaged Navier–Stokes model analysis continues to play a major role. However, there are very few turbulence models that facilitate unified predictions of the outcome of reshocked RM mixing experiments under different physical conditions. Thus, we aim to achieve this objective using the K-L model based on three considerations: deviatoric shear stress is considered when constructing Reynolds stress tensor; the model coefficients used are derived based on a new systematic procedure; the performance of different numerical schemes are studied to ensure high resolution but basically no numerical oscillation. Consequently, a unified prediction is obtained for the first time for a series of reshocked RM mixing experiments under incident shock Mach numbers Ma = 1.2–1.98, Atwood numbers At = ±0.67, and test section lengths 8 cm ≤ δ ≤ 110 cm. The results reveal the feasibility of demonstrating different reshocked RM processes using a single model, without adjusting the model coefficients, which sheds light on the further application of the present model to practical engineering, such as inertial confinement fusion.

Effect of geometric parameters on synthetic jet: A review

Fri, 03/13/2020 - 12:49
Physics of Fluids, Volume 32, Issue 3, March 2020.
A synthetic jet actuator is a fluidic device that produces a jet flow by the periodic ingestion of fluid into and expulsion of fluid out of a cavity across an orifice. Since such a mechanism transfers linear momentum to the fluid without introducing a net mass into the system over an actuation cycle, the synthesized jet is also termed a zero-net-mass-flux jet. Over the last two decades, synthetic jets have been the subject of intense research. It has been shown that the geometric parameters of a synthetic jet actuator can strongly influence the flow characteristics and performance of synthetic jets. The aim of this paper is to provide a comprehensive review of the influence of the geometric parameters of a synthetic jet actuator on the characteristics and performance of synthetic jets. These parameters include the height and diameter of the cavity and the orifice and the shape and edge configuration of the orifice.

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