Coaxial Cylinders Research Articles

Weakly nonlinear stability analysis of non-isothermal parallel flow in a vertical porous annulus

The study examines the stability of parallel airflow within a tall vertical coaxial circular annulus filled with a highly permeable porous medium under stable stratification. The linear stability theory predicts that the least unstable mode can be either axisymmetric or non-axisymmetric, depending on the values of the governing parameters. The curvature parameter, defined as the gap between coaxial circular cylinders, contributes to the stabilization of the flow. Depending on various controlling parameter values, the stably stratified non-isothermal annular parallel flow (SNAPF) can manifest three instability modes: thermal-shear, thermal-buoyant, and mixed or interactive. The instability type can change abruptly with Reynolds number, curvature parameter, and media permeability. A finite-amplitude instability analysis is examined using weakly nonlinear stability theory. The analysis demonstrates both subcritical and supercritical bifurcations near the instability boundary. The range of Reynolds numbers for subcritical bifurcation expands when switching from axisymmetric to non-axisymmetric disturbance. Bifurcation transitions between supercritical and subcritical occur due to changes in the least stable azimuthal mode. The study also investigates the equilibrium/threshold amplitude within the supercritical/subcritical instability regime, and it verifies the existence of subcritical/supercritical bifurcation through nonlinear kinetic energy balance. Furthermore, the finite-amplitude analysis anticipates that the lower critical Rayleigh number increases as the Reynolds number, curvature parameter, or media permeability decreases. An increase in the Reynolds number shifts the point of inflection in the velocity profile toward the inner cylinder, which subsequently leads to a reduction in the size of the core vortex region. In the subcritical instability regime, the radius of the primary vortex cell decreased significantly as the Rayleigh number increased. When the curvature is altered, a structural transition occurs in the isotherms from multicellular to unicellular.

Air‐Entraining Effect on Rheological Properties of Cement‐Based Mixtures

The rheological properties and stability of cement‐based materials determine the technological properties of concrete mixtures and significantly influence concrete properties. Polycarboxylate ether (PCE) and other plasticizing admixtures are widely used in construction practice to plasticize concrete mixtures. Experimental research on the rheological characteristics of cement mortar modified with PCE and air‐entraining admixtures (AIRs) was carried out with a rotation rheometer with coaxial cylinders and water bleeding tests. Herschel–Buckley or modified Bingham models were used to approximate experimental flow curves and assess rheological parameters such as yield stress, plastic viscosity, and shear thinning/thickening index. The rheological test was carried out at 5 and 90 min after mixing the cement mortar. The rheological test established that the modification of cement mortar by PCE in a dosage of 0.2% (in dry material) leads to a decrease in plastic viscosity of 25.0% with a reduction in water demand of 18% for the same workability. Modification of cement mortar by AIR in 0.3% dosage leads to a decrease in yield stress by 37.7% and a decrease in plastic viscosity by 35.4% compared to the control specimen without PCE. The combined modification by PCE and AIR admixtures shows a synergistic effect. It leads to a significant decrease in yield stress of cement mortar by 92.2% and a decrease in plastic viscosity to about 0 Pa·s. In addition, there is a significant coefficient of water bleeding reduction up to 0%, when additional water reduction was carried out in the mixture while maintaining the same workability. The cement mortar, with or without PCE and AIR, exhibited shear‐thinning characteristics for up to 90 min after mixing. The solid, liquid, and gas (air) phases’ influence yield stress and plastic viscosity in the analysis mixtures are presented in this paper. It has been established that a modified exponential Mooney dependency can be used to predict the yield stress and plastic viscosity of the modified cement mortar (both plasticized and air‐entrained).

On the well posedness of a mathematical model for a singular nonlinear fractional pseudo-hyperbolic system with nonlocal boundary conditions and frictional damping terms

<abstract><p>This paper is devoted to the study of the well-posedness of a singular nonlinear fractional pseudo-hyperbolic system with frictional damping terms. The fractional derivative is described in Caputo sense. The equations are supplemented by classical and nonlocal boundary conditions. Upon some a priori estimates and density arguments, we establish the existence and uniqueness of the strongly generalized solution for the associated linear fractional system in some Sobolev fractional spaces. On the basis of the obtained results for the linear fractional system, we apply an iterative process in order to establish the well-posedness of the nonlinear fractional system. This mathematical model of pseudo-hyperbolic systems arises mainly in the theory of longitudinal and lateral vibrations of elastic bars (beams), and in some special case it is propounded in unsteady helical flows between two infinite coaxial circular cylinders for some specific boundary conditions.</p></abstract>

Coupled vibration of a radially polarized piezoelectric cylinder with a co-axial elastic cylinder of varying height.

The cylindrical piezoelectric transducer has the advantages of large radiation area, high electromechanical coupled coefficient, and omni-direction radiation along the radius. In this paper, a piezoelectric transducer consisting of a radially polarized piezoelectric cylinder and an outer metal cylinder of varying height is presented. The metal cylinder of varying height is approximated as the radial superposition of multiple uniform height metal cylinders, and the equivalent impedance of the transducer's coupled vibration is obtained by using the impedance matrix method, and then the resonance frequency, anti-resonance frequency, effective electromechanical coupled coefficient, and displacement amplification coefficient are obtained. In this paper, the relationship between the vibration characteristics of the cylindrical piezoelectric transducer and its geometric dimensions is studied. An experimental sample of the transducer is fabricated and assembled, and its electrical impedance curve is measured. The measured results are in good agreement with the simulation results and the theoretical calculation results. The displacement distribution of the radiation surface of the transducer at resonance frequency is measured, which verifies that the two coupled vibration modes of the transducer can be effectively excited.

Dynamics of Ag–$${\text{TiO}}_{2} /{\text{H}}_{2} {\text{O}}$$ between two coaxial cylinders: a computational approach

Dynamics of Ag–$${\text{TiO}}_{2} /{\text{H}}_{2} {\text{O}}$$ between two coaxial cylinders: a computational approach

The distributed order models to characterize the flow and heat transfer of viscoelastic fluid between coaxial cylinders

The distributed order fractional derivatives can describe complex dynamic systems. In this paper, considering the periodic pressure gradient and magnetic field, the time distributed order fractional governing equations are established to simulate the two-dimensional flow and heat transfer of viscoelastic fluid between coaxial cylinders. Numerical solutions are obtained by the L1 approximation for the Caputo derivative (L1-scheme) and the finite difference method, and the effectiveness of numerical method is verified by a numerical example. Results demonstrate that the time distributed fractional Maxwell model can promote the flow while the distributed Cattaneo model can weaken heat transfer than the fractional Maxwell and Cattaneo model, and different weight coefficients have different effects on the fluid. The effect of physical parameters, such as the relaxation time of velocity and temperature λ 1, λ 2, the magnetic parameter M, the amplitude P 0 and frequency w of pressure gradient, and the Prandtl number Pr on velocity and temperature are discussed and analysed in detail.

Influence of Reynolds number and rotational number on the features of a transitional flow in short Taylor-Couette cavity

The paper reports on the DNS results of the flow in co- and counter-rotating coaxial cylinders of aspect ratios Γ = H/(R2 − R1) between 3.8 and 4.05, and radius ratio η = R1/R2 = 0.5, with the end-walls rotating with the angular velocity of the inner cylinder Ω1. The computations are performed for a wide range of rotational number RΩ = (1 − η)(Re1 + Re2)/(ηRe2 − Re1), from − 1.069 to 0.0, which includes both the linearly unstable flows and the Rayleigh stable flows. The considered Reynolds numbers of the inner cylinder Re1 = Ω1R1(R2 − R1)/ν are up to 3000 (Re2 = Ω2R2(R2 − R1)/ν). The obtained flow structures appearing at various stages of the laminar-turbulent transition and the radial profiles of statistical parameters are discussed in the light of the data published by other authors. The critical bifurcation lines are determined as functions of the inner and outer cylinder Reynolds numbers. Many interesting phenomena have been found.

Rheological characterization of human follicular fluid under shear and extensional stress conditions

The rheology of human follicular fluid has been empirically evinced to be related to the reproductive health status of individuals, which supports its use as an indicator for improving the success rates of in vitro fertilization. However, there is a dearth of studies investigating the viscoelastic properties of human follicular fluid. Moreover, a comprehensive elucidation of the rheological properties of complex fluids necessitates the assessment of data regarding both shear and extensional viscosities. Nonetheless, to the best of our knowledge, the extant literature does not include reports on the behavior of follicular fluid under extensional conditions. Consequently, this study aimed to analyze the shear and extensional viscosities of human follicular fluid. Primarily, the impact of oocytes on the rheology of follicular fluid was evaluated by measuring the shear viscosity of this fluid using a high-resolution coaxial cylinder viscometer. The shear viscosity of follicular fluid exhibited marked differences depending on the presence or absence of oocytes. Subsequently, a measurement system that enables the handling of minute quantities of body fluid was developed to determine the extensional viscosity of follicular fluid, which contains albumin. A comparison of the acquired follicular fluid data with that of the protein solution containing albumin demonstrated that the follicular fluid alone displayed extensional behavior, whereas the protein solution did not. Therefore, it can be inferred that the protein solution is not its sole determinant, as other constituents of the fluid, such as peptides and cumulus cells, may determine its rheological properties. This observation was not attained through the conventional technique consisting in shear viscosity measurements.

Experimental analysis of a graphene oxide-enhanced paraffin PCM

The research here presented aims at characterizing the thermal behaviour of a PCM-based latent heat storage enhanced with graphene oxide. The heat storage tank is composed of two PVC coaxial cylinders and a smooth stainless-steel helical heat exchanger installed inside the smaller one. The heat exchanger is immersed in PCM, a commercial paraffin with a melting temperature of around 28°C. The melting and solidification of the PCM were forced through hot or cold water flowing in the heat exchanger. The purpose is to explore the melting and solidification process by monitoring the temperature in the PCM with multiple thermocouples placed at different heights and different radial distances from the centre of the heat exchanger. Initially, the system was studied with pure paraffin. Then, two different mass percentages (i.e., 1.5% and 3%) of graphene oxide were added, whose high thermal conductivity had to counterbalance the paraffin low thermal conductivity and thus enhance the overall performance of the system. These three configurations were then compared to a reference scenario, consisting of the inner cylinder filled with pure water. The use of graphene oxide has increased the heat flux by up to 24% during heating and up to 31% during cooling compared to the pure PCM.

MHD natural convection in an annular space between two coaxial cylinders partially filled with metal base porous layer saturated by Cu–water nanofluid and subjected to a heat flux

MHD natural convection in an annular space between two coaxial cylinders partially filled with metal base porous layer saturated by Cu–water nanofluid and subjected to a heat flux

Scattering and enhancement of electromagnetic waves energy by coaxial plasma cylinders

The scattering of plane electromagnetic waves by three configurations of coaxial plasma cylinders is investigated in perpendicular incidence.‏ The results show that the electromagnetic energy enhancement and redistribution of energy are significantly affected by plasma frequency, electron neutral collision frequency, incident wave frequency, and the configuration of coaxial plasma cylinders. With increasing the collision frequency at fixed plasma frequency and incident wave frequency, the maximum relative energy density of the focal spot increases, and with a further increase in the collision frequency, the maximum relative energy density of the focal spot reaches some maximum value and begins to decrease. Furthermore, for fixed values of collision and plasma frequencies, at incident frequencies where the focal length is large, the maximum relative energy density is small. One main result obtained in this research indicates that the energy enhancement at the focal spot for TM polarization scattering is more than for TE mode. The results of this paper may be used for applications related to the redistribution and enhancement of electromagnetic energy.

Research by choice of excipients ingredients of the gel for the therapy of radiation lesions of the skin based on rheological studies

The aim of this work is to conduct a research analysis of changes in the structural and mechanical properties of the gel from the addition of active substances and surfactants.
 Materials and methods. In the presented work, model samples of the gel with active substances were studied: freshly obtained stonecrop juice, sea buckthorn oil, rosehip oil, St. John's wort oil and quercetin. As substances for the manufacture of bases hydrocolloids of cellulose derivatives were used: sodium carboxymethylcellulose (NaCMC) and hydroxyethylcellulose (HEC) with a surfactant Lanette SX.
 A study of the rheological properties of the samples was performed using a rheoviscometer Rheolab QC (Anton Paar, Austria) using a system of coaxial cylinders C-CC27 / SS. The Rheolab QC rheometer is equipped with RheoPlus software, which allows you to set the necessary experimental conditions (shear rate gradient range, number of measuring points and measurement time of one point).
 Results. The rheological properties of model samples of NaCMC and HEC gels, depending on the used concentration, were studied. With increasing concentrations of both NaCMC and HEC thixotropy of dispersed systems decreases. The change of structural and mechanical parameters of hydrogels from the addition of active ingredients and surfactant is analyzed, and established that adding them into the hydrogels increases the structural and mechanical parameters of model samples.
 A study of gels with a constant concentration of sodium carboxymethylcellulose and hydroxyethylcellulose and different content of surfactant Lanette SX established that as the surfactant concentration increases, the area of the hysteresis loop and the resistance of the dispersed system to the applied mechanical fracture increase.
 Conclusions. According to the results of the obtained structural and mechanical parameters and rheograms of the studied model samples of gels, it is rational to use in further studies of sodium carboxymethyl-cellulose (1.5 %) and hydroxyethylcellulose (1.25 %) stabilized Lannete SX in concentrations of 4-5 % and 3-5 % respectively

Development and characterization of fission chamber neutron detectors in Isfahan miniature neutron source reactor

In this research, two fission chamber neutron detectors have been designed and constructed, and their performance has been evaluated in the radiation field of the Isfahan miniature reactor neutron beam line. Each detector consists of two coaxial cylinders called anode and cathode made of stainless steel. The outer layer of the anode is covered with a very thin layer of uranium-235 oxide as fissile material, and a mixture of argon and nitrogen gas is used as the filling gas. The thickness of the deposited layer on the anode is different, so that on one of them, named FC400A, the thickness of the uranium oxide layer was more than 5 μm, and on the other one, named FC400B, it was less than 1.5 μm. A spectrum collection software has also been designed and developed in LabVIEW. The developed detectors are irradiated in the neutron field resulting from the neutron radiography beamline of the miniature neutron source reactor in the range of neutron flux of about 2E+02 to 2E+05 n.cm−2.s−1. In order to determine the sensitivity of the detection response, the results of these two detectors are also compared with a commercial fission chamber neutron detector. The results showed that the working voltage range of these detectors is in the range of 400–600 V and both of these detectors have a suitable linear response in the mentioned neutron flux range. Also, the sensitivity of FC400A and FC400B detectors is 0.1 and 1 counts/1E+04 neutrons, respectively.

Research on the development of composition and technology of an emulgel with a probiotic component for the treatment of infectious and inflammatory diseases

The aim was conducted research to develop the composition and technology of a new soft preparation under the conventional name "Probioskin" for topical use with a probiotic component in the form of an emulgel.
 Materials and methods of work. The homogeneity was determined by visual inspection of the experimental samples, using a biological microscope XSP-128 ULAB. The rheological (structural and mechanical) properties of the samples were studied using a Rheolab QC rheoviscosimeter (Anton Paar, Austria) with a C-CC27/SS coaxial cylinder system. The Rheolab QC rheometer is equipped with RheoPlus software. Microbiological studies and biotesting on a biological model of infusoria were performed under aseptic conditions of a laminar flow box (biological safety cabinet AC2-4E1 "Esco", Indonesia) of the Department of Biotechnology of the National University of Pharmacy. The identification of lactobacilli and the number of live cells as critical indicators of a preparation containing live microorganisms were performed at each stage of the study. Bacterioscopic control was carried out by examining smears prepared from samples of the dissolved drug and stained by Gram stain. Bacteriological control was performed by inoculation on thick MRS-4 medium and liquid MRS-1 medium. The number of live lactobacilli (when lactobacilli were co-cultured with the drug components at the stages of development and in the finished product) was determined by surface sowing on Petri dishes with thick MRS-4 medium; after 48 h of incubation at (37±1)°C, colonies grown on the surface of the medium were counted and the number of live lactobacilli expressed in CFU was calculated in 1 ml of the drug.
 Results and discussion. Based on the conducted research, the composition of a soft preparation for topical use containing dexpanthenol, lactic acid, lyophilized lactobacillus biomass, propylene glycol, peach oil, polysorbate-80, aristoflex AVC, tocopherol, and purified water was developed. The production process of the developed Probioskin product is carried out according to a standardized scheme and consists 8 stages: preparation of raw materials, preparation of oil concentrate of the probiotic component, preparation of aqueous concentrate of active substances, preparation of gel base, preparation of emulsion, dosing into tubes, packaging of tubes into packs, packaging of packs into group containers.
 Conclusions. On the basis of a complex research, the composition and rational technology of a soft preparation for topical use under the conventional name "Probioskin" in the form of an emulsion was developed.
 Key words: pharmacological research, soft drug, emulgel, probiotics, dermatological diseases

Laplace transform solution of the time-dependent annular Couette flow with dynamic wall slip

The annular Couette flow has several industrial applications, particularly for the characterization of the fluid flow and deformation behavior of fluids. The inclusion of the dynamic wall slip into the flow boundary conditions seems to be necessary for an efficient continuum description of motion of nanofluidics as it reflects the importance of fluid–structure interface related phenomena. Dynamic wall slip introduces a dissipative boundary condition and thus increases the difficulties of finding solutions to related problems. In the present work we investigate the behavior of fluid flow between two infinitely long coaxial circular cylinders, when the inner cylinder is axially moving due to sudden constant velocity, while the outer cylinder is held stationary. The boundary condition on the outer cylinder is that of dynamic wall slip, in addition to the usual Navier slip. The medium considered here is a Newtonian viscous fluid. The solution of the governing equations, initial and boundary conditions for this flow is obtained using the Laplace transform technique and inversion by Laguerre polynomials. This method may be useful, when applied in conjunction with perturbation methods, to solve nonlinear Couette flow problems involving temperature changes. Numerical results are presented and discussed.

Experimental Study on Dynamic Barite Sag and Effects of Inclination and Pipe Rotation

Summary Barite sag causes pressure fluctuations in the wellbore, which is undesirable. These problems usually occur with oil-based muds (OBMs; invert emulsion muds) and are associated with fluid properties and operation parameters. Drilling issues related to this undesirable phenomenon include wellbore instability, lost circulation, and stuck pipes. As barite sagging is a complex phenomenon, the mechanisms that cause and aggravate it still need to be fully understood to mitigate these problems. This study examines barite sagging in the wellbore with inner pipe rotation to understand the process and develop prevention strategies. Thus, a flow loop study with OBM is conducted in a concentric annular test section with varying inner pipe rotation and inclination angles. The tests were performed at an elevated temperature (49°C) to simulate borehole conditions. By measuring the pressure profile in a mud sample trapped in the test section, barite sagging was evaluated. Using the data, we calculated the density difference between the top and bottom sections of the column. The novelty of the work lies in continuous monitoring of the density profile of the mud column, which is sheared between two coaxial cylinders to simulate drillstring rotation in the wellbore, and utilizing the data for evaluating barite sag. The results show the evolution of the pressure profile with time, indicating the sagging of barite particles at the bottom of the test section. Due to barite sagging, the density of the top portion of the mud column decreased over time, while the density of the bottom part increased. The lateral sedimentation of barite particles toward the annulus outer wall enhances barite sag in inclined configurations. The sedimentation creates two suspension layers with different densities, leading to secondary flow, which enhances sagging. Hence, the primary factor driving barite sagging is inclination. An increase in inclination angle from 0° to 50° resulted in a significant (more than twofold) increase in mud density difference. Also, the rotation of the pipe delayed sagging during the early phases of the testing process (less than 20 minutes). However, it did not have much effect as the sagging progressed, resulting in approximately the same density difference in both cases (i.e., with and without rotation).

Laboratory equipment for practice learning in the framework of educational course “Molecular Physics and Thermodynamics”

This article presents laboratory equipment for practice learning of physics specialties students in the framework of educational course “Molecular Physics and Thermodynamics”. The presented laboratory equipment allows measuring the temperature dependence of the thermal conductivity coefficient of molecular liquids under atmospheric pressure and saturated vapors pressure in the temperature range from 150K to 400K. A general scheme of the developed experimental setup for measuring the thermal conductivity of liquids by the steady-state method under isobaric condition has been described. The created laboratory setup uses general design solutions of the coaxial cylinder method, preserving its features and measurement precision in general. The measurements are carried out on coaxial-geometry cell under atmospheric pressure. A modified heat potentiometer method with one thermometer was used, which allowed us to minimize the error in the estimation of the thermal conductivity coefficient due to the uncontrollable heat flows and different calibration of thermometers. The total systematic error in the measurement of the molecular liquids thermal conductivity does not exceed 3%. The procedures for direct measurement and calculation of the thermal conductivity coefficient of molecular liquids using the developed laboratory equipment are described.

Designing a Cylindrical Capacitive Based Sensor for Monitoring the Table Salt’s Moisture Content during Iodization

Coaxial cylinder capacitive moisture sensor for determination of moisture content of table salts has been fabricated. The CMS method for determination of moisture content of table salts overcomes the effect of evaporation of iodine in table salt which results from heating in the oven drying method. The CMS overcomes the polarization effect result from the resistive methods of moisture content determination and it is non expensive to construct and it has high accuracy since measurements involves programming system in interpretation of the results. In this research, moisture content sensor based on capacitive principal created using two coaxial cylinders with height 5.207 cm, and different diameters of 4.303 cm and 1.198 cm embedded inside one another. The input signal of the CMS system was provided by the AC signal from the sensor capacitor, the capacitance of the sensor capacitor is directly proportional to the moisture content of the salt sample placed between the two coaxial cylinders of the sensor capacitor. The moisture content was obtained from the model m=6.6145×10-4c+4.54044 for calibration of the CMS by using the OVD as a reference method for determination of moisture content in the table salts. The model was then validated by constructing the comparison plot of OVD and CMS moisture contents. The comparison graph of OVD and CMS MC against test had conceding error bars between OVD and CMS MC. The coefficient of determination of 0.94 was obtained and the correlation plot was a linear relation expressed as y=0.971x+0.773. The model was then validated by solving for moisture content from measured values of capacitance.

On the limitations of the complex wave velocity for the heterogeneous swirling flows

Abstract The hydrodynamic instability of variable density swirling flows under gravity between two infinite coaxial cylinders is investigated for axisymmetric disturbances. It is shown that the complex wave velocity of any arbitrary unstable axisymmetric mode must lie within the semi-elliptical region whose minor axis depends on the density stratification parameter J(r). The stabilizing effect of density stratification is shown by reducing the semi-circular instability region of [G. K. Batchelor and A. E. Gill, “Analysis of the stability of axisymmetric jets,” J. Fluid Mech., vol. 14, p. 529, 1962]. Furthermore, we have obtained two parabolic instability regions which intersect and reduce the semi-elliptical instability region for density-stratified flows. These parabolic instability regions are uniformly valid for both variable density and density homogeneous flows also.

SIMULATION OF HEAT EXCHANGE PROCESSES IN THE “THERMAL CORE” FILLED WITH HEAT ACCUMULATION MATERIAL

The presented article proposes an analytical method for modeling heat exchange processes inside the “thermal core”, which is a coaxial cylinder filled with heat storage material with a phase change material. The equation for heating the cooled phase change material inside the “thermal core” is derived, taking into account the radial transfer of heat in any i-th zone and in the first and last ring zones, separately. The dependence of the heat capacity of the phase change material on the temperature was studied, taking into account the change in the heat capacity according to the cosine law. Also, a comparison of the analytical data of the mathematical modeling with the results obtained experimentally in previous studies is presented and obtained a high degree of similarity, which indicates the reliability of the proposed analytical methodology. Recommendations are provided for the practical implementation of the research results, namely the use of a coaxial pipe “thermal core” filled with a pre-selected material with a phase change material in the construction of storage batteries used in the thermal power industry. As a result, capacitive batteries with a “thermal core” filled with ceresin are used in mobile thermal batteries MTA-0,5MW. Bibl. 14, Fig. 4, Tab. 2.