Tip Velocity Research Articles

Novel scale-up strategy based on three-dimensional shear space for animal cell culture

Animal cell cultivations are widely used for producing antibodies, vaccines, and recombinant protein drugs. However, the extreme sensitivity of animal cells to hydrodynamic stress often hinders its scale-up in large-scale stirred tank bioreactors. This study introduced a new scale-up strategy based on three-dimensional (3D) shear space for large-scale animal cell culture. First, the shear environments of bioreactors ranging from lab-scale (7.5 and 42 L) to industrial-scale (30, 90, 350, and 1000 L) were quantitatively analyzed through computational fluid dynamics (CFD) methods and successfully validated by particle image velocimetry (PIV) experiments. Moreover, the quantitative relationships between shear parameters (including shear rates in the impeller and tank zone, overall average shear rate, and maximum shear rate) and impeller tip velocity were established. In addition, a correlation analysis between shear-related parameters and viable cell densities in Spodoptera frugiperda Sf9 cultivations indicated that shear rates in the impeller and tank zone, and the overall average shear rate were the three key shear parameters required for scale-up. Further, an optimized 3D operation space for shear rate was established according to the three key shear parameters obtained under preferable operation conditions in lab-scale bioreactors. Based on the results, agitation rates in large-scale bioreactors were determined using the proposed correlation. Ultimately, we achieved successful scale-up of Spodoptera frugiperda Sf9 in industrial bioreactors with volumes up to 1000 L using this strategy. Thus, this study introduces a highly efficient and economical scale-up strategy for shear-sensitive cells.

Thermo-solutal growth of a dendritic crystal in the form of an elliptical paraboloid with forced convection

The steady-state analytical solutions describing the thermo-solutal growth of a dendritic crystal in the form of an elliptical paraboloid with allowance for a forced convective flow are found. The system of transcendental equations consisting of the undercooling balance and the stability criterion that describes a stable mode of growth of sections of elliptical dendrites is formulated and analyzed. It is shown that the temperature distributions in the melt, the growth and flow Peclét numbers as well as the dendrite tip diameter and tip velocity in the case of elliptical crystals essentially differ from their analogs in the case of symmetric shapes of dendrites in the form of paraboloids of revolution.

Effects of equations of state and constitutive models on simulating copper shaped charge jets in ALEGRA

We investigate the effects of copper equations of state and constitutive models on simulations of shaped charge jets using the Sandia National Laboratories’ multiphysics hydrocode, ALEGRA. Specifically, we compare (1) the 3320, 3325, 3331, and 3337 SESAME copper equations of state (EOS), and (2) Johnson–Cook, Zerilli–Armstrong, Preston–Tonks–Wallace, Steinberg–Guinan–Lund, and Mechanical Threshold Stress constitutive models. Several of these cases are also compared against photon Doppler velocimetry measurements of the tip velocity. Lagrangian tracer particles are used to follow a part of the jet's evolution in state space for the various models. We monitored the jet topology and found that the various EOS produce similar results, although the 3325 model generates a cavity near the jet tip whose size decreases with increasing mesh resolution. Constitutive models generated more noticeable differences. We found the SGL and PTW models produced higher temperatures while the MTS and JC models returned very similar results at lower temperatures. The SGL model was the only strength model that reported a liquid region along the axis of the jet. For all material models, we found similar results for the velocity history of the jet tip as measured against experiment using photon Doppler velocimetry.

Effect of in-cylinder environment on spray characteristics of diesel and biodiesel

ABSTRACT The objective of this paper is to investigate the spray characteristics of diesel and biodiesel in different in-cylinder environments, including spray tip penetration, spray cone angle, projected spray area, and spray tip velocity. The in-cylinder environment was set to two different ambient pressures and five temperature gradients. The results showed that both ambient pressure and temperature had a significant effect on the spray characteristics of diesel and biodiesel. Higher ambient temperatures under non-evaporating conditions increased the spray tip penetration and projected spray area of the fuel. Biodiesel in the same in-cylinder environment exhibited different spray characteristics due to different physical properties compared to diesel. In addition, the initial breakup mechanism of the spray was analyzed using dimensionless numbers.

Numerical investigations of six-fold dendritic icing process in subcooled water subject to natural and forced convective environments

Six-fold symmetric ice dendrite growth in subcooled water is investigated based on a GPU-accelerated hybrid lattice Boltzmann (LB) method and the cellular automation (CA) method. A reduction factor is proposed to reduce the discretization induced anisotropy in the hybrid method. The accuracy and feasibility of this hybrid method is validated by comparing simulated tip velocity of ice dendrite with existing experimental data under microgravity conditions, as well as comparing with the LM-K theory under small degrees of subcooling. Effects of natural convection and forced convection environments on dendrite morphology and tip velocity are further studied based on this improved method. It is shown that natural convection promotes ice dendrite to grow upward, and the arm growth velocity can be enhanced or weakened by natural convection, depending on the relative angle between intrinsic growth orientation and gravity direction. Under large natural convection effects, a growth competition phenomenon between different arms on a single ice dendrite is observed which had not been predicted by previous theoretical models. On the other hand, forced convection is shown to have strong enhancing effects on upstream arms and inhibit the growth of downstream arms. The arm growth is always enhanced with increasing inlet Re to various degrees for different relative angles between intrinsic growth orientation and gravity direction.

Experimental study on a Mach cone and trailing Rayleigh waves in a stress wave chasing running crack problem

This paper focuses on a blast-induced Mach cone followed by trailing Rayleigh waves during blast stress waves chasing a running crack in an epoxy plate, using the photoelasticity method and high-speed photography. A Mach cone and trailing Rayleigh waves were visualized and interpreted based on isochromatic fringes, agreeing well with theoretical wave paths. The Mach cone was the coalescence of shear wavelets generated by P waves on crack surfaces, which was verified by the half-cone angle, equal to arcsin (CS/CP). Effects of the Mach cone and trailing Rayleigh waves on the running crack were evaluated by measuring crack velocity and dynamic stress intensity factors (SIFs) of the crack tip. It was found that the Mach cone decreased both crack velocity and SIFs, while trailing Rayleigh waves had the opposite effect. Finally, because of similarities, the findings in our experiments were discussed with those in the supershear rupture. In particular, effects of the Mach cone and trailing Rayleigh waves on the running crack were consistent with particle motion near the fault in the supershear rupture.

Experimental investigation of the stochastic nature of end-gas autoignition with detonation development in confined combustion chamber

In the present work, end-gas autoignition formation, and the effects of oxygen concentration on the flame/shock waves propagation and pressure oscillation, are investigated in a self-designed constant-volume chamber equipped with a perforated plate. A hydrogen–oxygen–nitrogen mixture with adjustable oxygen to nitrogen ratio is chosen as the test fuel. In an oxygen-enriched condition, the probability of an end-gas autoignition occurrence increases significantly. End-gas autoignition with detonation development is further investigated, with a special emphasis on the stochasticity of the detonation development. In a low-oxygen condition, detonation occurs randomly owing to its stochastic physical behavior. However, when the oxygen concentration increases to 28%, the stochastic factors have a lower impact, and the detonation occurrence is certain. Nevertheless, the pressure and pressure oscillation in the autoignition exhibit random behaviors and are unrelated to the oxygen concentration. The variation tendency of the flame tip velocity remains constant under different oxygen concentrations. However, an increase in the oxygen concentration improves the flame tip velocity, thereby inducing stronger shock waves and promoting autoignition. Based on the start time of the autoignition, two types of autoignition modes were identified: Mode 1 and Mode 2. In Mode 2, the unburnt mixture experiences merely one compression by the shock wave before autoignition and only occurs under high oxygen concentrations of 27–28%. Under equal oxygen concentrations, the pressure and pressure oscillation of Mode 2 are higher than those of Mode 1 owing to the larger amount of unburnt mixture. Finally, the exhaust gas was introduced into the initial unburnt mixture to investigate the effect of an inert gas on the combustion.

A New Efficient Quantitative Multi-component Phase Field: Lattice Boltzmann Model for Simulating Ti6Al4V Solidified Dendrite Under Forced Flow

Ti6Al4V is a widely used, multi-component alloy in additive manufacturing, during which the fluid flow in the molten pool significantly affects the solidified dendrites. To predict and further control the microstructure, modeling and simulating the microstructure evolution play a critical role. In this study, a newly developed, efficient, quantitative multi-component phase-field (PF) model is coupled with a lattice Boltzmann (LB) model to simulate Ti6Al4V solidified dendrite evolution under fluid flow. The accuracy and convergence behavior of the model is validated by the Gibbs–Thomson relation at the dendrite tip. Single and multiple two-dimensional (2D) equiaxed dendrite evolution cases under forced flow were simulated. Results show that the dendrite pattern is influenced remarkably by the fluid flow. Underlying mechanisms of the asymmetrical evolution are revealed by discussing the interaction among the flow, composition distribution and dendrite morphology, quantitatively. The dendrite kinetics are also derived, which ascertains the relationship between tip velocity and undercooling and inlet velocity and is the foundation for larger-scale simulation. We believe that the coupled quantitative multi-component PF–LB framework employed in this study helps in investigating the solidified dendrite morphology evolution in a deep and quantitate manner.

Investigation into contributions of static and dynamic loads to compressive residual stress fields caused by ultrasonic surface rolling

A compressive residual stress field caused by ultrasonic surface rolling process is produced due to association of both the static and dynamic loads. The present work is an analytical approach to finding contributions of static and dynamic loads to this field. This study considered various rolling parameters and material properties based on a rectangular Hertz contact area. In the ultrasonic-rolling simulation, a quasi-static model and a dynamic model were developed based on the effects of strain hardening and strain rate hardening, respectively. Moreover, in order to validate the proposed analytical approach, experimental verification was carried out on the Al6061-T6 aluminum alloy. By investigating rolling parameters such as rolling depth, radius of tool tip, linear velocity of tool tip, friction coefficient, amplitude and frequency, relationships between external loads and those parameters were established and contributions of static and dynamic loads to this field were also found out. Finally, an approach to effective enlargement of compressive residual stress field was obtained based on the finding results.

Phase-field–lattice Boltzmann simulation of dendrite growth under natural convection in multicomponent superalloy solidification

The thermosolutal convection can alter segregation pattern, change dendrite morphology and even cause freckles formation in alloy solidification. In this work, the multiphase-field model was coupled with lattice Boltzmann method to simulate the dendrite growth under melt convection in superalloy solidification. In the isothermal solidification simulations, zero and normal gravitational accelerations were applied to investigate the effects of gravity on the dendrite morphology and the magnitude of melt flow. The solute distribution of each alloy component along with the dendrite tip velocity during solidification was obtained, and the natural convection has been confirmed to affect the microsegregation pattern and the dendrite growth velocity. In the directional solidification simulations, two typical temperature gradients were applied, and the dendrite morphology and fluid velocity in the mushy zone during solidification were analyzed. It is found that the freckles will form when the average fluid velocity in the mushy zone exceeds the withdraw velocity.

Residual stress prediction in ultrasonic vibration–assisted milling

In the current study, an analytical predictive model on residual stress after ultrasonic vibration–assisted milling is proposed in an effort to provide an accurate and reliable reference. Three types of tool-workpiece separation criteria are checked based on the tool center instantaneous position and velocity. Type I criterion examines the instantaneous velocity of tool tip under combined effects of feed movement and vibration. Type II criterion examines the position of tool center. Type III criterion describes the smaller chip size due to the overlaps between current and previous tool paths as a result of vibration. If none of these criterions is satisfied, the mechanical and thermal stresses are nonzero. The residual stress is then predicted through the calculation of stress distribution in loading process, incremental stress change considering kinematic hardening in plasticity, and the elastic stress release during relaxation process. The proposed predictive residual stress model in ultrasonic vibration–assisted milling is validated through comparison with experimental measurements on AISI 316L alloy. The proposed predictive model is able to match the measured residual stress with high accuracy of 6.4% average error and 23.6% maximum error among all cases. In addition, a sensitivity analysis is conducted. Higher axial depth of milling results in less compressive residual stress. Moreover, both higher ultrasonic vibration amplitude and higher spindle rotation frequency result in more compressive residual stress for AISI 316L alloy.

Experimental study on effects of ethanol-diesel fuel blended on spray characteristics under ultra-high injection pressure up to 350 MPa

In the present study, the atomization characteristics of ethanol-diesel blends fuel spray and shock waves were investigated by using experimental methods under ultra-high injection pressure in the range 200Mpa–350 MPa. A high-speed camera and Schlieren technology were used to visualize the fuel spray morphology and shock waves. The spray tip penetration, velocity and shock wave angle were determined via experimental approaches. Furthermore, the special spray shape under ultra-high injection pressure, such as liquid tail, expanded body and bifurcated structures, was analyzed. To validate the classical spray empirical models, predicted spray penetration results based on two typical models (Dent's and Hiroyasu's model) were compared with experimental data. The results indicated that fuel injection pressure and ethanol volume fraction presented significant effects on the spray evolution. With increasing injection pressure, the spray tip penetration became faster and the induced attached shock waves reveal clearer. The fuel spray jets achieved the larger penetration velocity and more induced shock waves under higher injection pressure or lower ethanol volume fraction.

Experiment and analysis of spray characteristics of biodiesel blending with di-n-butyl ether in a direct injection combustion chamber

In this study, spray characteristics of biodiesel blending with di-n-butyl ether (DBE15, DBE30) were investigated by experiment and analysis and compared with diesel. Spray characteristics parameters including spray tip penetration, spray cone angle, maximum spray width and spray tip velocity were used to compare under different injection pressures and ambient pressures. The spray images were analyzed according to light intensity level, while the gas entrainment mass and equivalent ratio were analyzed by empirical model. The results showed that the spray tip penetration of biodiesel added with DBE was reduced and the spray cone angle and maximum spray width were increased. The range of light intensity level of biodiesel was the narrowest, while the light intensity level of biodiesel-DBE blends was similar to that of diesel. Empirical models showed that biodiesel shows poor gas-fuel mixing. DBE30 had a larger gas entrainment mass and a lower equivalence ratio, similar to diesel. It was proved that the biodiesel added with DBE enhanced the gas-fuel mixing.

Study on Interaction Mechanism between the Shaped Charge Jet and Thick Moving Target

AbstractThe traditional anti‐aircraft and anti‐missile warhead mainly hit the target by fragments or MEFPs. However, these killing elements are only effective for thin‐shell ammunition (10–30 mm), but not for thick‐shell ammunition (>100 mm). This paper proposes an attacking method towards thick moving target by shaped charge jet. Through the analysis of the whole penetrating process between the shaped charge jet and the thick moving target, the paper divides the process into two stages: undisturbed jet penetration and disturbed jet penetration. Based on virtual origin theory, this paper carried out detailed theoretical analysis for each stage and established theory algorithms of changes of cavity when shaped charge jet penetrates the thick moving target, penetration depth of undistributed jet, the lateral velocity of jet and penetration depth contributed by distributed jet. Since the current experimental method cannot accurately analyze the dynamic penetration process of each jet element, in order to verify the theoretical model, a diameter of 66 mm shaped charge was designed, and through the LS‐DYNA software, the jet penetrates steel target with the transverse velocity of 0∼600 m/s was calculated. The study also carried out detailed analysis of the jet tip and tail velocity, the shape of the penetration hole, the lateral velocity of the disturbed jet and the total depth of penetration. At the same time, we combine with the experimental data of E. Marmor et al.[22], to compare with the theoretical results. The calculations are in good agreement with the E. Marmor's experimental datas. The results indicate that shaped charge warhead is an effective method to tackle high speed thick moving target. The theoretical model established in this paper can effectively guide the structural design of the warhead.

Method of evaluation for the non-stationary period of primary dendritic crystallization

Theoretical predictions by many models of solidification kinetics rest on assumption of the steady-state regime of crystal growth with constant velocity. Such approximation holds if the time for solidification of a region under study is much longer than the non-stationarity time for solidification velocity. Even though a lot of experimental evidences which support the existence of long quasi-stationary periods of crystal growth or solidifying layers with constant velocity exists, direct quantitative estimations of such regimes are given usually for planar fronts or essential simplifications which, for instance, do not take into account the finiteness of solidifying bulk. The present work suggests a method to quantify non-stationary periods of crystal growth in comparison with solidification time of finite bulks. In essence we discuss and quantify limits of applicability of steady state crystal growth theories. With this aim, an acceleration- and velocity-dependent interfacial condition [A. Salhoumi, P.K. Galenko, Physica A 447 (2016) 161] is used for the analysis of various regimes of dendrite growth as particular case of crystalline solidification. The time dependence of the dendrite tip velocity is obtained for the growth from a pure (chemically one component) undercooled liquid. The developed theoretical model shows a drastic reducing the non-stationarity time with the increase of undercooling. The present model can be advanced to the arbitrary undercooling and further compared with data of experimental measurements on crystal growth kinetics.

Nonlinear Rayleigh–Taylor instability with horizontal magnetic field

In this research, the height, curvature and velocity of the bubble tip in Rayleigh–Taylor instability at arbitrary Atwood number with horizontal magnetic field are investigated. To support the earlier simulation and experimental results, the vorticity generation inside the bubble is introduced. It is found that, in early nonlinear stage, the temporal evolution of the bubble tip parameters depends essentially on the strength and initial perturbation of the magnetic field, although the asymptotic nature coincides with the nonmagnetic case. The model proposed here agrees with the previous linear, nonlinear and simulation observations.

DEVELOPMENT OF A NUMERICAL MODEL FOR THE STUDY OF AN OSCILLATING WATER COLUMN DEVICE CONSIDERING AN IMPULSE TURBINE

The present work brings a numerical study of an energy conversion device which takes energy from the waves through an oscillating water column (OWC), considering an impulse turbine with rotation in the chimney region through the implementation of a movable mesh model. More precisely, a turbulent, transient and incompressible air flow is numerically simulated in a two-dimensional domain, which mimics an OWC device chamber. The objectives are the verification of the numerical model with movable mesh of the impulse turbine in the free domain from the comparison with the literature and, later, the study of the impulse turbine inserted in the geometry of the OWC device. In order to perform the numerical simulation on the generated domains, the Finite Volume Method (FVM) is used to solve the mass and momentum conservation equations. For the closure of the turbulence, the URANS (Unsteady Reynolds Averaged Navier-Stokes) model k-ω SST is used. To verify the numerical model employed, drag coefficients, lift, torque and power are obtained and compared with studies in the literature. The simulations are performed considering a flow with a Reynolds number of ReD = 867,000, air as the working fluid and a tip speed ratio of λ = 2. For the verification case, coefficients similar to those previously predicted in the literature were obtained. For the case where the OWC device was inserted it was possible to observe an intensification of the field of velocities in the turbine region, which led to an augmentation in the magnitude of all coefficients investigated (drag, lift, torque and power). For the case studied with the tip velocity ratio λ = 2, results indicated that power coefficient was augmented, indicating that the insertion of the turbine in a closed enclosure can benefit the energy conversion in an OWC device.

Experimental study on spray and atomization characteristics under subcritical, transcritical and supercritical conditions of marine diesel engine

In this research, detailed characteristics of diesel spray and atomization based on marine engine scale have been studied under subcritical, transcritical and supercritical conditions. A large constant volume chamber with an inner diameter of 300 mm and a 300 μm single-nozzle injector are used to mimic the in-cylinder thermodynamic condition of marine diesel engine. Sub/trans/supercritical conditions are composed of nine cases by means of different diesel phase transition regions. Optical diagnostic measurements are conducted through backlight illumination technology and schlieren imaging technology in an inert atmosphere. The images are processed with multi-threshold technique to obtain quantitative spray parameters, such as jet and liquid penetrations, averaged lengths, liquid steady time, spray cone angles and R-parameter. Moreover, some new finding about the structure of spray under supercritical condition is also discussed. The results show that the spray liquid periphery is quite smooth and stable with shorter penetration under supercritical condition, meanwhile, the jet of spray behaves as thinner periphery near the injector nozzle region and shows lower density at downstream region. Higher ambient pressure produces a negative effect on the increase of the tip velocity and development of spray. With the transition from transcritical condition to supercritical condition, the liquid length decreases by 32 mm and there exists an obvious decrease of 35 mm from jet length to liquid length. A trade-off relationship between the jet cone angle and jet penetration is also found. Generally, this work is helpful to understand the detailed process of diesel atomization under transcritical and supercritical conditions.

Phase field study of spacing evolution during wire and laser additive manufacturing under transient conditions

Understanding the dynamic evolution of primary dendritic spacing in the laser melt pool is significant from a technological viewpoint because primary spacing is one of the foremost parameters that control the final mechanical properties of additive manufactured products. In this work, a multi-scale computational framework that couples FEM and a developed quantitative phase field method is employed to simulate the evolution of microstructure and primary spacing of a nickel-based superalloy during wire and laser additive manufacturing (WLAM) solidification. Transient conditions in the laser melt pool are considered in which both temperature gradient G and solidification speed VP are made time-dependent. Through the use of this model, the dendritic morphology, tip velocity and spacing evolution during the solidification are investigated to provide the relationship between the laser processing parameters and the final spacing. Moreover, we attempted to clarify the intrinsic mechanism of spacing adjustment under different laser processing parameters from a novel perspective. This work provides meaningful understanding of spacing evolution in nickel-based superalloy and demonstrates the potential of controlling the complex microstructure morphologies and final primary spacing during wire and laser additive manufacturing process.

A comparative investigation of premixed flame propagation behavior of syngas-air mixtures in closed and half-open ducts

A comparative investigation has been developed in conjunction with high-speed cameras and piezoelectric gauge to gain deep insight into the propagation behavior of premixed syngas-air flames. Experiments were carried out in two constant rectangular ducts, one closed duct (C-D) and one half-open duct (HO-D). One important finding is that the flame behavior in the C-D is different than that in the HO-D. The repeatable distorted tulip flame forms in the C-D while an unrepeatable distorted tulip shape forms in the HO-D. The repeated pressure-flame interactions result in a repetitive process of the tulip distortion. The downstream opening end of the duct has a significant influence on the flame propagation characteristics after the flame surface reaches the lateral walls. The opening end results in a higher maximum flame tip velocity, a lower overpressure and a longer plane formation time in the HO-D. Meanwhile, the position of the plane flame formation decreases as hydrogen fraction increases in the C-D while the opposite is true in the HO-D. Furthermore, the pressure build-up in both ducts are examined, and the maximum flame tip velocity is of importance for distorted tulip flame formation.