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Formation and instability of flow between two corotating disks in an enclosure is investigated numerically, in which the origin of polygonal flow pattern is explored utilizing numerical simulation and linear stability analyses. The outer cylindrical boundary of the flow field is assumed to be stationary, whereas the inner cylinder rotates together with the two disks. Polygonal flow patterns are known to appear due to instability of the axisymmetric flow with respect to the axis of rotation, which is a unique solution at small Reynolds numbers. The origin of the flow structure of so called shift-and-reflect flow pattern is clarified from the solution of the linear stability analyses and occurs due to Hopf bifurcation. In addition, we found that the most unstable mode of disturbance has an unexpected wavenumber as large as 20 to 30 for very narrow disk spacing., 資料番号: AA0063608007, レポート番号: JAXA-SP-07-009

The instability and transition of flow past two circular cylinders arranged perpendicularly to the stream are investigated numerically. It is a steady symmetric flow that is realized at small Reynolds numbers, but the flow becomes unstable above a critical Reynolds numbers. It has been believed that the resultant flow due to the instability is always oscillatory. However, we found a steady asymmetric flow as well as the oscillatory flow arising from the instability in a region of the gap width between the two cylinders. We evaluated the critical Reynolds number and clarified the region of the gap width in which the steady asymmetric flow is realized., 資料番号: AA0063156019, レポート番号: JAXA-SP-06-013

In this work, we present a methodology to analyze the stability of the steady-state propagation of a liquid plug in a capillary tube lined with a uniform film of the same liquid. To this end, the steady-state solutions are computed and the corresponding film thickness is obtained for different plug lengths and fixed capillary and Reynolds values. The evolution of this variable as a function of the plug length for each capillary and Reynolds number is the key to plug stability determination. Fil: Campana, Diego Martin. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Ubal, Sebastian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Giavedoni, Maria Delia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Saita, Fernando Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina

Possibility to stabilize the oscillatory thermocapillary convection is demonstrated using a proportional feedback control. The feedback control is realized by locally modifying the surface temperature by using the local temperature measured at different locations fed back through a simple control law. The control method was applied in simplified geometries such as the annular configuration and the half-zone for high Prandtl number liquids by means of experiments, numerical simulations, and formulation of a simple model equation system. Successful suppression of the oscillation was obtained especially in the weakly nonlinear regime where the control completely suppresses the oscillations. With a right choice of actuators, even with the local control, it was shown that it is possible to modify the linear and weakly-nonlinear properties of the three-dimensional flow system with linear and weakly nonlinear control., 資料番号: AA0049208015, レポート番号: JAXA-SP-05-012

Pattern formation in flow between two corotating disks in an enclosure is investigated numerically and experimentally. The outer cylindrical boundary of the flow field is assumed to be fixed, whereas the inner cylinder rotates together with the two disks. The flow is not only symmetric with respect to the inter-disk midplane but also axisymmetric around the axis of rotation at small Reynolds numbers. The axisymmetry of the flow field is broken due to instability at high Reynolds numbers. Such an instability occurs for small gap ratios, the ratio of the gap between two disks to the radius of the annulus, and yields a polygonal flow pattern in a plane normal to the rotation axis. We identified two kinds of three dimensional unsteady flow by numerical simulations, one of which is asymmetric with respect to the inter-disk midplane and the other has a shift-and-reflect symmetry with the midplane, and compared them with those obtained by experiment. We found that the critical Reynolds number at which the axisymmetric makes a transition to unsteady 3D flow is smaller than those evaluated in previous papers, although the flow patterns are consistent with those reported in them., 資料番号: AA0049208024, レポート番号: JAXA-SP-05-012

With the goal of providing the first example of application of a recently proposed method, thus demonstrating its ability to give results in principle, global stability of a version of the rotating Couette flow is examined. The flow depends on the Reynolds number and a parameter characterizing the magnitude of the Coriolis force. By converting the original Navier–Stokes equations to a finite-dimensional uncertain dynamical system using a partial Galerkin expansion, high-degree polynomial Lyapunov functionals were found by sum-of-squares of polynomials optimization. It is demonstrated that the proposed method allows obtaining the exact global stability limit for this flow in a range of values of the parameter characterizing the Coriolis force. Outside this range a lower bound for the global stability limit was obtained, which is still better than the energy stability limit. In the course of the study, several results meaningful in the context of the method used were also obtained. Overall, the results obtained demonstrate the applicability of the recently proposed approach to global stability of the fluid flows. To the best of our knowledge, it is the first case in which global stability of a fluid flow has been proved by a generic method for the value of a Reynolds number greater than that which could be achieved with the energy stability approach.

The first part of this paper reviews the application of the sum-of-squares-of-polynomials technique to the problem of global stability of fluid flows. It describes the known approaches and the latest results, in particular, obtaining for a version of the rotating Couette flow a better stability range than the range given by the classic energy stability method. The second part of this paper describes new results and ideas, including a new method of obtaining bounds for time-averaged flow parameters illustrated with a model problem and a method of obtaining approximate bounds that are insensitive to unstable steady states and periodic orbits. It is proposed to use the bound on the energy dissipation rate as the cost functional in the design of flow control aimed at reducing turbulent drag., Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 372 (2020), ISSN:1364-503X, ISSN:1471-2962

A more detailed look is given on the scenario of three-dimensional (3-D) instabilities in the magnetohydrodynamic cylinder flow when the oncoming flow and the magnetic field are parallel. The results presented here are in the frame of linear stability analysis in the range 100

Linear stability of the Stuart vortices in the presence of an axial flow is studied. The local stability equations derived by Lifschitz & Hameiri (Phys. Fluids A, vol. 3 (11), 1991, pp. 2644–2651) are rewritten for a three-component (3C) two-dimensional (2D) base flow represented by a 2D streamfunction and an axial velocity that is a function of the streamfunction. We show that the local perturbations that describe an eigenmode of the flow should have wavevectors that are periodic upon their evolution around helical flow trajectories that are themselves periodic once projected on a plane perpendicular to the axial direction. Integrating the amplitude equations around periodic trajectories for wavevectors that are also periodic, it is found that the elliptic and hyperbolic instabilities, which are present without the axial velocity, disappear beyond a threshold value for the axial velocity strength. Furthermore, a threshold axial velocity strength, above which a new centrifugal instability branch is present, is identified. A heuristic criterion, which reduces to the Leibovich & Stewartson criterion in the limit of an axisymmetric vortex, for centrifugal instability in a non-axisymmetric vortex with an axial flow is then proposed. The new criterion, upon comparison with the numerical solutions of the local stability equations, is shown to describe the onset of centrifugal instability (and the corresponding growth rate) very accurately.

International audience; The role of interphase triggered from interdiffusion process at neighboring layers on controlling the interfacial flow instability of multilayer coextrusion have been highlighted in this study using a compatible bilayer system. The polymers used are based on poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF). The interdiffusion kinetics and the rheological and geometrical properties of the generated interphase have been modelized in real experimental conditions of the coextrusion process. Polymer chain orientation in coextrusion process was demonstrated to decelerate the interdiffusion coefficient. Furthermore, the interfacial shear stress was able to promote mixing and homogenizing process at the vicinity of the interface, which favors the development of the interphase. The convective mixing was evidenced by performing a pre-shear mode on PMMA/PVDF multilayer structures. The rheological and morphological properties of the interphase are related to a lot of parameters like contact time, processing temperature, interfacial shear stress and compatibility of the polymers, etc. Some key classical decisive parameters concerning the interfacial instability phenomena such as viscosity ratio, thickness ratio and elasticity ratio, etc. were highlighted during the coextrusion process. These key factors which are significant for the interfacial stability of coextrusion of incompatible multilayered polymers seem not that important for the studied compatible systems. The coextrusion of PMMA/PVDF compatible bilayers appears to be more stable. This would be attributable to the presence of the interphase generated from interdiffusion and favored from convective mixing. The interfacial flow instability of coextrusion can be reduced (or even eliminated) despite of the very high viscosity ratio and elasticity ratio of PMMA versus PVDF, especially at low temperatures. Overall, apart from the classical mechanical parameters, we have demonstrated that the creation of diffuse interphase that favors the homogenization should be taken into consideration as an important factor to remove the interfacial instability properties.

The focus of this overview is to give a state-of-the-art knowledge on coextrusion and rheology at the interface of multilayer polymers. In this article, we summarize important experimental and fundamental aspects given in the literature in regard to structure–processing–properties relationships. In addition, the modeling of the interfacial phenomena and the role of the interphase on the flow stability in coextrusion are also discussed. Keywords: multilayer polymer; coextrusion; rheology; interphase; flow stability

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The hydrodynamic stability of plane Poiseuille flow of superfluid is studied using modal and non-modal analysis. Two modes of instability are predicted, in normal mode stability analysis of the normal fluid, one caused by viscosity similar to the classical mode and another due to mutual friction between superfluid and normal fluid. The mutual friction mode occurs at high wavenumbers, which are stable wavenumbers in classical plane Poiseuille flow. A high superfluid vortex line density alone is not enough to induce instability in normal fluid; a localization of vortex lines is shown to play a major role. The extent of vortex line concentration required to cause instability depends on the density itself. Non-modal instability analysis shows that oblique waves are stronger than streamwise waves, unlike the scenario in classical plane Poiseuille flow.

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International audience; The temporal stability of a Carreau fluid flowing down an inclined porous substrate is considered. A reduced model is derived under the assumption of small permeability which decouples the flow in the liquid layer from the filtration flow in the porous medium and incorporates the effect of the porous medium by means of an effective slip condition at the liquid-solid interface. The slip coefficient in the effective slip condition is a function of the structure, permeability of the porous medium and the rheology of the fluid saturating the porous medium. The effects of shear-thinning rheology and permeability of the substrate on the stability of the film flow system are investigated. This problem gives rise to a generalized eigenvalue formulation which is solved through two approaches. The problem is solved analytically for long waves in the limiting cases of weakly and strongly non-Newtonian behaviors (power-law limit). A numerical investigation is carried out in the general case. The results are shown to agree well for the weakly non-Newtonian limit. Further, the power-law model and the Carreau model agree on a wide range of shear-thinning parameter values for a thin film over a rigid substrate. However, when considering a porous medium, this trend is not observed. The Carreau model gives valid results for the entire range of shear-thinning parameter values for a film over a rigid/porous substrate. The novelty of the present investigation lies in the inclusion of both the effects of bottom permeability and shear-thinning rheology. Both permeability and shear-thinning rheology have a destabilizing effect on the film flow system. The numerical results indicate the correlation between the effects due to shear-thinning properties and permeability. An energy balance analysis performed on the perturbation fields shows that destabilization induced by both shear-thinning and permeability is linked to the viscous shear work rate on the free surface.

Rotating cavitation in inducers is known as one type of cavitation instability, in which an uneven cavity pattern propagates in the same direction as the rotor with a propagating speed ratio of 1.0-1.2. On the other hand, cavitation in cryogenic fluids has a thermodynamic effect because of the thermal imbalance around the cavity. To investigate the influence of the thermodynamic effect on rotating cavitation, we conducted experiments in which liquid nitrogen was set at different temperatures (74 K, 78 K and 83 K) with a focus on the cavity length. At higher cavitation numbers, super-synchronous rotating cavitation occurred at the critical cavity length of Lc/h approximately equal to 0.5 with a weak thermodynamic effect in terms of the fluctuation of cavity length. In contrast, synchronous rotating cavitation occurred at the critical cavity length of Lc/h approximately equal to 0.9-1.0 with a strong thermodynamic effect in terms of the unevenness of cavity length. Furthermore, we confirmed that the amplitude of the shaft vibration depended on the degree of the unevenness of the cavity length through the thermodynamic effect., 資料番号: AA0064094000, レポート番号: JAXA-RM-08-004

We present results on parametrically forced capillary waves in a circular cylinder, obtained in the limit of large fluid depth, using two low-viscosity liquids whose surface tensions differ by an order of magnitude. The evolution of the wave patterns from the instability to the wave-breaking threshold is investigated in a forcing frequency range (f= ω/2π = 25–100 Hz) that is around the crossover frequency (ωot) from gravity to capillary waves (ωot/2≤ω/2≤4ωot). As expected, near the instability threshold the wave pattern depends on the container geometry, but as the forcing amplitude is increased the wave pattern becomes random, and the wall effects are insignificant. Near breaking, the distribution of random wavelengths can be fitted by a Gaussian. A new gravity–capillary scaling is introduced that is more appropriate, than the usual viscous scaling, for low-viscosity fluids and forcing frequencies 3Hz. In terms of these scales, a criterion is derived to predict the crossover from capillary- to gravity-dominated breaking. A wave-breaking model is developed that gives the relation between the container and the wave accelerations in agreement with experiments. The measured drop size distribution of the ejected drops above the breaking threshold is well approximated by a gamma distribution. The mean drop diameter is proportional to the wavelength determined from the dispersion relation; this wavelength is also close to the most likely wavelength of the random waves at drop ejection. The dimensionless drop ejection rate is shown to have a cubic power law dependence on the dimensionless excess acceleration ε′dan inertial–gravitational ligament formation model is consistent with such a power law.

本研究では、液糸からの液滴生成過程についての数値解析を実施し、これまでとは異なる理解を得るに至った結果を報告する。物性値を適正に取ることによってスケールアップされた微小重力実験では、端面でレーリー解析から予測されるよりも短い波長での分断が観察された。梅村らの提唱する理論では、端面からの伝播性表面張力波の役割が考慮され、これらの波の不安定化によって分断が引き起こされることが示されている。ここでは、端面と内部流れの影響をより詳しく見るために対応する数値計算を行う。その結果、端面からの表面張力波によって短波長の分断が引き起こされていることが示された。気体による液柱の伸張も分断を促進する方向に働き、レーリーのモードとは異なる分断を起こしている。また、一定時間経過後に液柱長さが長くなったときに、今度は長波長のレーリーモードが出現する。これは短波長の伝播速度の速い表面張力波が表面を遡り、ノズル出口で反射されて不安定化することによって出現するモードである。このように、液滴分断は2つのモードが関係していることが分かったが、いずれも端面からの表面張力波によって駆動されており、液滴分断に新しい理解を与える結果となった。, A novel understanding of droplet formation mechanism from a ligament is presented. In the scaled-up microgravity experiment, it is observed that droplets are always created at the ligament tip at a shorter wavelength than that of Rayleigh's analysis. In the new theoretical analysis by Umemura, the role of capillary waves from the tip is considered and it is shown that destabilization of these waves lead to pinch-off from the tip. In this report, a numerical study on this phenomenon is conducted to investigate the effects of ligament tip and internal liquid flow. It is shown that there is a short-wave self-sustained cycle of droplet breakup that is driven by the capillary force at the tip. Liquid jet elongation due to gas flow creates internal liquid flow and this modifies the characteristics of each region of the liquid column. This type of breakup mechanism is different from Rayleigh's analysis. After a certain time, however, the liquid column length no longer increases and converges to an average value. At the same time with this change, longer waves appear on the liquid surface upstream of the tip. The wavelength corresponds to Rayleigh's most unstable wave. It is observed that small-wavelength waves that have higher propagation speed have reached the nozzle exit and reflected. As the liquid column becomes longer, these perturbations have come to get enough time to grow to Rayleigh's wave. Thus, the liquid breakup phenomenon from the tip is not a single phenomenon, but a combination of two mechanisms. This study gives a new insight into liquid atomization mechanism., 資料番号: AA0063887000, レポート番号: JAXA-RR-07-041

第35回 20040930-20041001;第36回 20050328-20050329, 35th, 20040930-20041001;36th, 20050328-20050329, 資料番号: AA0049208000, レポート番号: JAXA-SP-05-012

We investigate numerically the transition to turbulence in a flat-plate boundary layer controlled by electromagnetic forces. The fluid considered is incompressible, Newtonian and low conductive. Similar to boundary layer suction, when applying a steady, wall-parallel, and streamwise orientated Lorentz force as suggested by Gailitis and Lielausis [1] in the early 1960s, the Blasius velocity profile is transformed to an exponential one gaining a critical Reynolds number which is increased by two orders of magnitude. Two and three dimensional direct numerical simulation (DNS) of both linear and nonlinear stages of the transition process were performed, as well as a linear stability analysis (LSA) of the intermediate velocity profiles. DNS and also LSA results confirm the expected increased stability of the controlled flow. Depending on Lorentz force strength transition to turbulence is delayed or even stopped by either damping primary instability, or, in the nonlinear case, by suppressing the emerge of Omega-vortices which usually preceeds the breakdown to turbulence. Surprisingly, both DNS and LSA results suggest interesting linear stability characteristics of the intermediate velocity profiles. In DNS (Re based on inflow displacement thickness is 360), to initiate transition, small amplitude disturbances are introduced near the inflow boundary, forming Tollmien-Schlichting waves (TSW) which grow and decay in uncontrolled case corresponging to linear stability theory. When applying a Lorentz force, all investigated TSW are damped. The decay rate is maximum in a region near the onset of control and decreases as the velocity profile evolves towards the exponential shape. This observation could suggest that in the intermediate region there are profiles more stable than the exponential one, although we are aware that from these decay rates one cannot conclude directly the stability of a velocity profile, notably its critical Reynolds number. However, our assumption is confirmed by LSA results where critical Reynolds numbers of intermediate profiles are found to be larger than for the exponential profile. References: 1. A. Gailitis, O. Lielausis , „On a possibility to reduce the hydrodynamic resistance of a plate in an electrolyte,“ Applied Magnetohydrodynamics, Reports of the Physics Institute Riga, v. 12, p. 143-146, 1961

28-29 Sep. 1995 (17th). 28-29 Mar. 1996 (18th), 本報告の目的は、脈動流の線形安定性解析に由来する実験的および理論的研究結果を示すことである。最初に取り組んだ解析では、流線曲率による遠心力により不安定性の生ずるTaylor-Couette形状での脈動流の発達を理論的および実験的に取り扱った。次に、前後に振動する長方形タンクにおいて発達する脈動流の安定性を扱った。コリオリ力が基底流に作用し不安定性の原因となる。理論解析結果をBoltonとMaurerの実験結果(1994年)と対照して示す。, The purpose of this communication is to present experimental and theoretical results stemming from the linear stability analysis of pulsed flows. The first analysis tackled theoretically and experimentally deals with a pulsed flow developing in a Taylor-Couette geometry where the instability is driven by the centrifugal forces due to the curvature of the streamlines. The second investigation deals with the stability of a pulsed flow developing in a rectangular tank oscillating back and forth. The Coriolis force drives the base flow and is at the origin of the instability. Theoretical results will be presented in the light of the experiments of Bolton & Maurer (1994)., 資料番号: AA0000867006, レポート番号: NAL SP-33

Turbulent swirling flows are important in many applications including gas turbines, furnaces and cyclone dust separators among others. Although the mean flow fields have been relatively well studied, a complete understanding of the flow field including its dynamics has not been achieved. The work contained in this dissertation attempts to shed further light on the behavior of turbulent swirling flows, especially focused on the dynamic behavior of a turbulent swirling flow encountering a sudden expansion. Experiments were performed in a new isothermal turbulent swirling flow test facility. Two geometrical nozzle configurations were studied. The \cb\ nozzle configuration exhibits a cylindrical \cb\ in the center of the nozzle. The free vortex nozzle configuration is obtained when the cylindrical \cb\ is removed. Detailed laser velocimeter measurements were performed to map out the flow field near the sudden expansion of the 2.9" (ID) nozzle leading to the 7.4" (ID) downstream section. In addition to presenting detailed flow profiles for both nozzle and downstream flow fields, representative frequency spectra of the flow dynamics are presented. Along with the flow time histories and histograms, the wide variety of dynamic behavior was thus described in great detail. The dynamics observed in the experiment can be classified into three main categories: coherent and large scale motion, intermittent motion and coherent periodic motion. Free vortex geometry flows, in the parameter space of the experiments (Swirl number = 0 - 0.21), exhibited mostly coherent and large scale motion. The spectra in these cases were broadband with very light concentration of spectral energy observed in some specific cases. Center--body geometry flows exhibited all three categories of flows as swirl strength was increased from zero. Flows with little or no swirl exhibited broad--band spectra similar to those for the free vortex geometry. Intermediate swirl levels resulted in a large amount of low frequency energy which, with the aid of the time histories, was identified as a large scale intermittence associated with radial movement of the annular jet as it enters the sudden expansion. Large swirl levels resulted in high magnitude coherent oscillations concentrated largely just downstream of the sudden expansion. Linear stability analysis was used to help in the interpretation of the observed dynamics. Although, as implemented here (using the parallel flow assumption), the analysis was not successful in quantitatively matching the experimentally observed dynamics, significant insight into the physical mechanisms of the observed dynamics was obtained from the analysis. Specifically, the coherent oscillations observed for larger swirl levels were able to be described in terms of the interaction between the inner and outer shear layers of the flow field. Ph. D.

Experiments on the linear instability of flow in a wavy-wall channel are carried out to examine the effects of 2D distributed surface roughness on the stability of wall-bounded shear flows. One of the channel walls is modified by introduction of the wavy wall model with the amplitude of 4 percent of the channel half height and the wave number of 1.02. The experiment is focused on the two-dimensional traveling wave instability. The results are compared with the theory proposed by Floryan. It is shown that the flow is destabilized by the wall corrugation at subcritical Reynolds numbers below 5,772, as predicted by the theory. For the present corrugation geometry, the critical Reynolds number is decreased down to about 4,000. The spatial growth rates, the disturbance wave numbers and the distribution of disturbance amplitude measured over such wavy wall also agree well with the theoretical results., 資料番号: AA0049208025, レポート番号: JAXA-SP-05-012

This paper investigates the flowability properties of Labisia pumila (L. pumila) powder. Material properties of the powder and their influence on flowability were discussed. L. pumila, a popular Malaysian herb with therapeutic and medicinal functions was compressed into tablets using a stainless steel cylindrical uniaxial die with 13mm diameter and compaction pressures of 7 to 25MPa. Two feed weights, 0.5 and 0.7g were used to form tablets. Compaction strength and density increased as compaction pressure increased resulting in tablets porosity reduction. Cohesion index, flow stability and caking strength were also determined to enhance the understanding of flowability properties of the L. pumila powder.

Flow velocity and sedimentation patterns were investigated experimentally and numerically in shallow rectangular reservoirs with different asymmetric locations of the inlet and outlet channels. Velocity fields were measured in the entire reservoir, both for clear water flow and with suspended sediments. Thickness of sediment deposits was mapped in the whole reservoir by means of a laser light method. In one of the studied geometric configurations, injection of suspended sediments led to a complete change in the observed flow field. Experimental results were compared with numerical simulations performed with the depth-averaged flow model WOLF 2D, using a depth-averaged k–ε turbulence model. The simulations lead to accurate predictions of the velocity profiles and the change in flow pattern as a result of sediment deposits was successfully reproduced.