Dynamics of a long tubular cantilever conveying fluid downwards,which then flows upwards around the cantilever as a confined annular flow

A theoretical model is developed for the dynamics of a hanging tubular cantilever conveying fluid downwards; the fluid, after exiting from the free end, is pushed upwards in the outer annular region contained by the cantilever and a rigid cylindrical channel. This configuration thus resembles that of a drill-string with a floating fluid-powered drill-bit. The linear equation of motion is solved by means of a hybrid Galerkin–Fourier method, as well as by a conventional Galerkin method. Calculations are conducted for a very slender system with parameters appropriate for a drill-string, for different degrees of confinement of the outer annular channel; and also for another, bench-top-size experiment. For wide annuli, the dynamics is dominated by the internal flow and, for low flow velocities, the flow increases the damping associated with the presence of the annular fluid. For narrow annuli, however, the annular flow is dominant, tending to destabilize the system, giving rise to flutter at remarkably low flow velocities. The mechanisms underlying the dynamics are also considered, in terms of energy transfer from the fluid to the cantilever and vice versa, as are possible applications of this work.     

Motion of a slender body in a fluid under a floating plate

This paper studies the three-dimensional unsteady problem of the hydroelastic behavior of a floating infinite plate under the impact of waves generated by horizontal rectilinear motion of a slender solid in a fluid of infinite depth. An analytic solution of the problem is found based on the known solutions for the unsteady motion of a point source of mass in a fluid of infinite depth under a floating plate. Asymptotic formulas are obtained which model the motion of a solid slender body in a fluid by replacing the body with a source-sink system. These formulas are used to numerically analyze the effect of plate thickness, depth of the body, its dimensions and the velocity of rectilinear motion on the amplitude of deflection of the floating plate. The motion of a submarine under a nonbreakable plate was modeled experimentally. Theoretical and experimental data are in good agreement.     

VIBRATION AND STABILITY OF VERTICAL UPWARD-FLUID-CONVEYING PIPE IMMERSED IN RIGID CYLINDRICAL CHANNEL

A theoretical model is developed for the vibration and stability of a vertical pipe subjected concurrently to two dependent axial flows. The external fluid, after exiting the outer annular region between the pipe and a rigid cylindrical channel, is conveyed upwards inside the pipe. This configuration thus resembles of a pipe that aspirating fluid. The equation of planar mo- tion is solved by means of the differential quadrature method （DQM）. Calculations are conducted for a slender drill-string-like and a bench-top-size system, for different confinement conditions of the outer annular channel. It is shown that the vibrations of these two systems are closely related to the degree of confinement of the outer annular channel. For a drill-string-like system with narrow annuli, buckling instability may occur in the second and third modes. For a bench-top-size system, however, both buckling and flutter may occur in the lowest three modes. The form of instability depends on the annuli size.     

Fluidelastic instability study in a rotated triangular tube array subject to two-phase cross-flow. Part I: Fluid force measurements and time delay extraction

Fluidelastic instability is a key issue in steam generator tube bundles subjected to cross-flow. The extension to two-phase flow of the existing theoretical models, developed and tested mostly for single phase flow, is investigated in this paper. The time delay is one of the key parameter for modeling fluidelastic instability, especially the damping controlled mechanism. The direct measurement of the time delay between the tube motion and the fluid force faces certain difficulties in two-phase flow since the high turbulence due to the interaction of the two components of the flow may increase the randomness of the measured force. To overcome this difficulty, an innovative method for extracting the time delay inherent to the quasi-steady model for fluidelastic instability is proposed in this study.Firstly, experimental measurements of unsteady and quasi-static fluid forces (in the lift direction) acting on a tube subjected to air–water two-phase flow were conducted. The unsteady fluid forces were measured by exciting the tube using a linear motor. These forces were measured for a wide range of void fractions, flow velocities and excitation frequencies. The experimental results showed that the unsteady fluid forces could be represented as single valued function of the reduced flow velocity. It was also found that for a given frequency, the unsteady fluid force phase was weakly dependent on the void fraction for the range of flow velocities considered.The time delay was determined by equating the unsteady fluid forces with the quasi-steady forces. The results given by this innovative method of measuring the time delay in two-phase flow were consistent with theoretical expectations. The time delay could be expressed as a linear function of the convection time and the time delay parameter was determined for void fractions ranging from 60% to 90%.     

Wave forces on a submerged horizontal plate - Part I: Theory and modelling

This paper is concerned with the propagation of nonlinear gravity waves over a thin horizontal plate submerged in water of shallow depth. An unsteady solution of the problem is obtained by use of the theory of directed fluid-sheets for the two-dimensional motion of an incompressible and inviscid fluid. Particular attention is paid to the calculation of the nonlinear wave-induced vertical and horizontal forces and overturning moment by solving the Level I Green–Naghdi equations. The theoretical formulation of the problem is given in this paper (Part I), while the results due to solitary and cnoidal waves, and comparisons with the available experimental data are given in a companion paper under the same title (Part II).     

Instantaneous Squeeze-Film Force Between a Heat Exchanger Tube and a Support Plate for Arbitrary Tube Motion

The instantaneous squeeze-film force between a heat exchanger tube and a support plate is studied. Based on a two-dimensional rectangular plate model, a short-sleeve squeeze-film model for arbitrary tube motion is developed. The instantaneous squeeze-film force is expressed in normal and tangential directions. The normal squeeze-film force consists of four nonlinear terms, the viscous, unsteady inertia, convective inertia and centripetal inertia terms. Three nonlinear terms, the viscous, unsteady inertia and Coriolis inertia terms, make up the tangential squeeze-film force. An experimental apparatus was developed in order to evaluate the theoretical models against measurements of a finite length squeeze film. A modified model based on the experimental data is obtained where the viscous terms for both directions are multiplied by the instantaneous Reynolds number. All the inertia terms are multiplied by constant coefficients. The modified model is in good agreement with most experimental cases for unsymmetrical linear motion, approximate circular motion and elliptical motion. The form of the modified model is suitable for predicting instantaneous squeeze-film forces in the simulation of heat exchanger tube vibration. Further work using different sized components and fluid properties is required in order to finalize coefficient values.     

Periodic solution of turbulent oscillating channel flows

The time-dependent turbulent Navier–Stokes equations are solved numerically by a finite element method with an algebraic eddy viscosity model (Baldwin–Lomax formulation) for oscillating turbulent channel flows. The method of averaging is used to analyse the resulting periodic motion of the fluid. Numerical results are obtained for various Strouhal numbers and relative amplitudes. A comparison is made between the numerical and published experimental results. It appears that for low relative amplitudes in a certain range of frequencies the agreement is satisfactory.     

An experimental and theoretical study of particle deposition due to thermophoresis and turbulence in an annular flow

The paper describes an experimental and theoretical study of the deposition of small particles from a turbulent annular-flow with cross-stream temperature variation, focusing on the effects of thermophoresis. Various expressions for the thermophoretic force on a spherical particle are critically discussed. The well-known composite formula of Talbot et al. (1980) does not include the ‘second mechanism of thermophoresis’ and it is concluded that the more recent theoretical approach of Beresnev and Chernyak (1995) is probably more reliable. New experimental measurements of particle deposition from a turbulent flow with cross-stream temperature gradients are then presented. The measurement technique is similar to the method of Liu and Agarwal (1974) but in the test section the aerosol flows vertically downwards in an annular gap between two concentric pipes. By heating the outer pipe and cooling the inner it is possible to establish a substantial, near-constant temperature difference between the two walls and hence a thermophoretic force which varies only with radius. Numerical calculations provide a comparison of theory with experiment. The theory is based on the turbulent deposition models of Young and Leeming (1997) and Slater et al. (2003) modified to include thermophoresis and the annular geometry. The theory of Beresnev and Chernyak gives good agreement with the experimental measurements.     

Finite element computations for unsteady fluid and elastic membrane interaction problems

The interaction between the hydrodynamic forces of a flow field and the elastic forces of adjacent deformable boundaries is described by elastohydrodynamics, a coupled fluid–elastic membrane problem. Direct numerical solution of the unsteady, highly non-linear equations requires that the dynamic evolution of both the flow field and the domain shape be determined as part of the solution, since neither is known a priori. This paper describes a numerical algorithm based on the deformable spatial domain space–time (DSD/ST) finite element method for the unsteady motion of an incompressible, viscous fluid with elastic membrane interaction. The unsteady Navier–Stoke and elastic membrane equations are solved separately using an iterative procedure by the GMRES technique with an incomplete lower-upper (ILU) decomposition at every time instant. One-dimensional, two-dimensional and deformable domain model problems are used to demonstrate the capabilities and accuracy of the present algorithm. Both steady state and transient problems are studied. © 1997 John Wiley & Sons, Ltd.     

Kinematic problem of an ideal incompressible fluid flow around an arbitrarily moving airfoil

An unsteady kinematic problem for arbitrary two-dimensional motion of an airfoil in an ideal incompressible fluid with formation of one and two vortex wakes is solved. The problem is solved by the method of conformal mapping of the flow domain onto a circle exterior; solution singularities in the vicinity of a sharp edge are analyzed, and the initial asymptotics of the solution is taken into account. The calculated results are found to be in good agreement with available experimental data on visualization of the flow pattern. The necessity of correct modeling of the initial stage of vortex-wake formation is demonstrated. A regular flow pattern is found to form after three and more periods of oscillations. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 50, No. 2, pp. 120–128, March-April, 2009.     

Vibrations of a body in a bounded volume of viscous fluid

The problem of the vibrations of a body in a bounded volume of viscous fluid has been studied on a number of occasions [1–4]. The main attention has been devoted to determining the hydrodynamic characteristics of elements in the form of rods. Analytic solution of the problem is possible only in the simplest cases [2]. In the present paper, in which large Reynolds numbers are considered, the asymptotic method of Vishik and Lyusternik [5] and Chernous' ko [6] is used to consider the general problem of translational vibrations of an axisymmetric body in an axisymmetric volume of fluid. Equations of motion of the body and expressions for the coefficients due to the viscosity of the fluid are obtained. It is shown that in the first approximation these coefficients differ only by a constant factor and are completely determined if the solution to the problem for an ideal fluid is known. Examples are given of the determination of the “viscous” added mass and the damping coefficient for some bodies and cavities. In the case of an ideal fluid, general estimates are obtained for the added mass and also for the influence of nonlinearity. Ritz's method is used to solve the problem of longitudinal vibrations of an ellipsoid of revolution in a circular cylinder. The hydrodynamic coefficients have been determined numerically on a computer. The theoretical results agree well with the results of experimental investigations.     

Exact solution for the unsteady flow of a semi-infinite micropolar fluid

The unsteady motion of an incompressible micropolar fluid filling a half-space bounded by a horizontal infinite plate that started to move suddenly is considered. Laplace transform techniques are used. The solution in the Laplace transform domain is obtained by using a direct approach. The inverse Laplace transforms are obtained in an exact manner using the complex inversion formula of the transform together with contour integration techniques. The solution in the case of classical viscous fluids is recovered as a special case of this work when the micropolarity coecient is assumed to be zero. Numerical computations are carried out and represented graphically.     

Sequential approximation method for one-dimensional unsteady problems of gasdynamics

The considered sequential approximation method for one-dimensional unsteady problems makes it possible to reduce the solution to the integration of systems of ordinary differential equations. The gas motion is bounded, on the one hand, by a piston moving in accordance with an arbitrary law, and, on the other hand, by either a compression shock or a rarefaction wave propagating through the gas with given arbitrary parameters. A comparison is made of certain exact solutions with the resulting approximate solutions.     

Numerical and experimental studies on the hydrodynamic damping of a zero-thrust propeller

Fluid added mass and damping are significant parameters when predicting the dynamic response of a submerged structure. The hydrodynamic damping of underwater rotating machinery is numerically and experimentally investigated by a zero-thrust propeller in this paper. The lifting surface method(LSM) combined with forced vibration was introduced as the numerical method to compute the corresponding unsteady thrust, while the experimental method of measuring added damping was accomplished by a propeller undergoing rotation combined heave motion. Results of the theoretical method are in good agreement with the experimental results before cavitation occurs, as cavitation is regarded to weaken the unsteady response of the propeller partly. The calculation results also show that both the frequency ratio(vibration frequency divided by rotation frequency) and the blade angle have a significant influence on the hydrodynamic damping. Therefore, the effect of blade angle on hydrodynamic damping should be considered during the design phase.     

Computation of unsteady transonic flow over a fighter wing using a zonal Navier–Stokes/full‐potential method

An improved hybrid method for computing unsteady compressible viscous flows is presented. This method divides the computational domain into two zones. In the inner zone, the Navier–Stokes equations are solved using a diagonal form of an alternating‐direction implicit (ADI) approximate factorisation procedure. In the outer zone, the unsteady full‐potential equation (FPE) is solved. The two zones are tightly coupled so that steady and unsteady flows may be efficiently solved. Characteristic‐based viscous/inviscid interface boundary conditions are employed to avoid spurious reflections at that interface. The resulting CPU times are about 60% of the full Navier–Stokes CPU times for unsteady flows in non‐vector processing machines. Applications of the method are presented for a F‐5 wing in steady and unsteady transonic flows. Steady surface pressures are in very good agreement with experimental data and are essentially identical to the full Navier–Stokes predictions. Density contours show that shocks cross the viscous/inviscid interface smoothly, so that the accuracy of full Navier–Stokes equations can be retained with significant savings in computational time. Copyright © 1999 John Wiley & Sons, Ltd.     

Surrogate-based aeroelastic loads prediction in the presence of shock-induced separation

A surrogate-based modeling strategy is presented for robust and efficient prediction of unsteady aeroelastic loads in the presence of shock-induced separation. Enriched piston theory predictions are extended with a data-driven nonlinear autoregressive with exogenous inputs model to account for hysteresis from the interplay of a dynamically deforming surface with the separation bubble in a shock/boundary layer interaction. The approach is evaluated for prescribed surface motion and shock-induced panel flutter responses, with good agreement observed in each scenario relative to unsteady Reynolds-averaged Navier–Stokes simulations. For the latter, excellent agreement is observed in the prediction of the stability boundary and oscillation frequency. In contrast, the oscillation amplitude conservatively deviates from the Reynolds-averaged Navier–Stokes solution with increasing dynamic pressure. The online computational cost of the extended approach is orders of magnitude less than that required for predictions using an unsteady Reynolds-averaged Navier–Stokes model.     

Numerical investigation of a perturbed swirling annular two-phase jet

A swirling annular gas–liquid two-phase jet flow system has been investigated by solving the compressible, time-dependent, non-dimensional Navier–Stokes equations using highly accurate numerical methods. The mathematical formulation for the flow system is based on an Eulerian approach with mixed-fluid treatment while an adjusted volume of fluid method is utilised to account for the gas compressibility. Surface tension effects are captured by a continuum surface force model. Swirling motion is applied at the inlet while a small helical perturbation is also applied to initiate the instability. Three-dimensional spatial direct numerical simulation has been performed with parallelisation of the code based on domain decomposition. The results show that the flow is characterised by a geometrical recirculation zone adjacent to the nozzle exit and by a central recirculation zone further downstream. Swirl enhances the flow instability and vorticity and promotes liquid dispersion in the cross-streamwise directions. A dynamic precessing vortex core is developed demonstrating that the growth of such a vortex in annular configurations can be initiated even at low swirl numbers, in agreement with experimental findings. Analysis of the averaged results revealed the existence of a geometrical recirculation zone and a swirl induced central recirculation zone in the flow field.     

A numerical modelling of stator–rotor interaction in a turbine stage with oscillating blades

In real flows unsteady phenomena connected with the circumferential non-uniformity of the main flow and those caused by oscillations of blades are observed only jointly. An understanding of the physics of the mutual interaction between gas flow and oscillating blades and the development of predictive capabilities are essential for improved overall efficiency, durability and reliability. In the study presented, the algorithm proposed involves the coupled solution of 3D unsteady flow through a turbine stage and the dynamics problem for rotor-blade motion by the action of aerodynamic forces, without separating the outer and inner flow fluctuations. The partially integrated method involves the solution of the fluid and structural equations separately, but information is exchanged at each time step, so that solution from one domain is used as a boundary condition for the other domain. 3-D transonic gas flow through the stator and rotor blades in relative motion with periodicity on the whole annulus is described by the unsteady Euler conservation equations, which are integrated using the explicit monotonous finite volume difference scheme of Godunov–Kolgan. The structural analysis uses the modal approach and a 3-D finite element model of a blade. The blade motion is assumed to be constituted as a linear combination of the first natural modes of blade oscillations, with the modal coefficients depending on time. A calculation has been done for the last stage of the steam turbine, under design and off-design regimes. The numerical results for unsteady aerodynamic forces due to stator–rotor interaction are compared with results obtained while taking into account blade oscillations. The mutual influence of both outer flow non-uniformity and blade oscillations has been investigated. It is shown that the amplitude-frequency spectrum of blade oscillations contains the high-frequency harmonics, corresponding to the rotor moving past one stator blade pitch, and low-frequency harmonics caused by blade oscillations and flow non-uniformity downstream from the blade row; moreover, the spectrum involves the harmonics which are not multiples of the rotation frequency.