Vortex-Induced Vibrations of a Cylinder at Subcritical Reynolds Number

The present work focusses on the numerical study of Vortex-Induced Vibrations (VIV) of an elastically mounted cylinder in a cross flow at moderate Reynolds numbers. Low mass-damping experimental studies show that the dynamic behavior of the cylinder exhibits a three-branch response model, depending on the range of the reduced velocity. However, few numerical simulations deal with accurate computations of the VIV amplitudes at the lock-in upper branch of the bifurcation diagram. In this work, the dynamic response of the cylinder is investigated by means of three-dimensional Large Eddy Simulation (LES). An Arbitrary Lagrangian Eulerian framework is employed to account for fluid solid interface boundary motion and grid deformation. Numerous numerical simulations are performed at a Reynolds number of 3900 for both no damping and low-mass damping ratio and various reduced velocities. A detailed physical analysis is conducted to show how the present methodology is able to capture the different VIV responses.

Analysis Techniques for Modeling the Pressure Wave Effects of Guillotine Ruptures of Steam Lines

The rupture of a pipe containing gas or steam at high pressure will cause a shock wave. In order to assess the potential damage that such a shock wave may cause to the surrounding structures, systems and components, it is necessary to determine the amplitude and propagation properties of the shock. A CFD model has been developed for the purpose of predicting shock propagation transients resulting from a sudden pipe rupture in terms of the fluid properties, pipe geometry, and surroundings. A simplified shock propagation model also is included, which offers verification of the CFD model results.

Microscale Flow Pumping Inspired by Rhythmic Tracheal Compressions in Insects

Inspired by the physiological network of insects, which have dimensions on the order of micrometers to millimeters, we study the airflow within a single model insect tracheal tube. The tube undergoes localized rhythmic wall contractions. A theoretical analysis is given to model the airflow within the tracheal tube. Since flow motions at the microscale are dominated mainly by viscous effects, and the tube has radius, R, that is much smaller than its length, L, (i.e. δ = R/L ≪ 1), lubrication theory for axisymmetric, viscous, incompressible flows at low Reynolds number (Re ∼ δ) is used to model the problem mathematically. Expressions for the velocity field, pressure gradient, wall shear stress and net flow produced by the driving tube wall contractions are derived. The effect of the contraction amplitudes, time lag, and spacing between two sequences of contractions on the time-averaged net flow over a single cycle of wall motions is investigated. The study presents a new, insect-inspired mechanism for valveless pumping that can guide efforts to fabricate novel microfluidic devices that mimic these physiological systems. A x-ray image that shows the tracheal network of the respiratory system of an insect (Carabid beetle) and the associated locations of these rhythmic contractions are shown in figure (1) to promote this study.

LOCA Simulation: Analysis of Rarefaction Waves Propagating Through Geometric Singularities

The propagation of a transient wave through an orifice is investigated for applications to Loss Of Coolant Accident in nuclear plants. An analytical model is proposed for the response of an orifice plate and implemented in the EUROPLEXUS fast transient dynamics software. It includes an acoustic inertial effect in addition to a quasi-steady dissipation term. The model is experimentally validated on a test rig consisting in a single pipe filled with pressurized water. The test rig is designed to generate a rapid depressurization of the pipe, by means of a bursting disk. The proposed model gives results which compare favourably with experimental data.

Vibration Excitation Forces in a Normal Triangular Tube Bundle Subjected to Two-Phase Cross Flow

This paper presents test results of vibration forces in a normal triangular tube bundle subjected to air-water cross-flow. The dynamic lift and drag forces were measured with strain gage instrumented cylinders. The array has a pitch-to-diameter ratio of 1.5, and the tube diameter is 38 mm. A wide range of void fraction and fluid velocities were tested. The experiments revealed significant forces in both the drag and lift directions. Constant frequency and quasi-periodic fluid forces were found in addition to random excitation. These forces were analyzed and characterized to understand their origins. The forces were found to be dependent on the position of the cylinder within the bundle. The results are compared with those obtained with flexible cylinders in the same tube bundle and to those for a rotated triangular tube bundle. These comparisons reveal the influence of quasi-periodic forces on tube motions.

Influence of Fluid Viscosity and Structural Specifications on Stability in the Vicinity of Critical Velocity of an Elastic Beam Subjected to Confined Axial Flow

Evaluation methodologies for the flow-induced vibration of an elastic beam subjected to an axial flow confined in a narrow passage are reported. In this paper, by using the analytical method already proposed by the author, a parameter study is performed on the dynamic stability of an elastic beam subjected to axial flow confined a narrow passage. The effects of support conditions of structures, structural damping, and fluid characteristics on the dynamic stability are clarified. Especially, paying attention on the vicinity of critical velocity, the effects of stabilization or destabilization on flutter and divergence are investigated by changing fluid viscosity and structural damping.

Experimental Investigation of a Tandem Cylinder System With a Yawed Upstream Cylinder

The unsteady flow field produced by a tandem cylinder system with the upstream cylinder yawed to the mean flow direction is investigated for upstream cylinder yaw angles from α = 60° to α = 90°. Multi-point fluctuating surface pressure and hotwire measurements were conducted at various spanwise positions on both the upstream and downstream cylinders. The results indicate that yawing the front cylinder to the mean flow direction causes the pressure and velocity spectra on the upstream and downstream cylinders to become more broadband than for a regular tandem cylinder system, and reduces the magnitude of the peak associated with the vortex-shedding. However, span-wise correlation and coherence measurements indicate that the vortex-shedding is still present and was being obscured by the enhanced three-dimensionality that the upstream yawed cylinder caused and was still present and correlated from front to back, at least for the larger yaw angles investigated. When the cylinder was yawed to α = 60°, the pressure fluctuations became extremely broadband and exhibited shorter spanwise correlation.

Pressure Coefficient Distribution Over Tube Surfaces of Tube Bundle Subjected in Two-Phase Flow

Two-phase cross flow exists in many shell- and tube heat exchangers such as condensers, evaporators and nuclear steam generators. During the last two decades, research devoted to two-phase flow induced vibrations has increased, mainly driven by the nuclear industry. Flow-induced vibration excitation forces can cause excessive vibration which will result in long-term fretting-wear or fatigue. To avoid potential tube failures in heat exchangers, it is required for designer to have guidelines that incorporate flow-induced vibration excitation forces. The phenomenon of the vibration of tubes in two-phase flow is very complex and depends on factors which are nonexistent in single-phase flows. To understand the fluid dynamic forces acting on a structure subjected to two-phase flow, it is essential to get detailed information about the characteristics of two-phase flow. Pressure distributions generated by two-phase flow over tube surfaces yield more general information than the local velocity distribution. The pressure coefficient distribution obtained by experimental test has been evaluated.

New Trends in Accurate Simulations for the Verification of Safety Margins in the Nuclear Power Plant Industry: Application of Virtual Performance Solution™ for the Response of Immersed Structures Subjected to Earthquakes

Constraints on the safety of nuclear power plant components have increased recently along with the necessity to extend the lifespan of existing plants. For example, the acceleration levels to be sustained by the plant equipment during an earthquake have been increased many folds by the safety regulation agencies. Industrial and economic requirements plead for a verification of unknown safety margins, by accurate and physics based models taking into account all non-linear effects (for example contacts and fluid structure interaction). These effects are only approximately represented by standard linear analysis tools. Virtual Performance Solution (VPS), developed by ESI Group, includes (among other capabilities) a structural finite element software for non-linear, high velocity, dynamic simulations (PAM-CRASH), as well as a coupled, mesh free CFD module, FPM (Finite Point Method), developed in partnership with Fraunhofer ITWM. This solution accurately predicts fluid structure interactions, taking into account non-linear structural effects (contacts, friction, damping…) as well as complex fluid influences.

Numerical Simulations of Nonlinear Post-Critical Vibrations of an Airfoil After Flutter Instability

The contribution deals with the numerical simulation of the flutter of an airfoil with three degrees of freedom (3-DOF) for rotation around an elastic axis, oscillation in the vertical direction and rotation of a flap. The finite element (FE) solution of two-dimensional (2-D) incompressible Navier-Stokes equations is coupled with a system of nonlinear ordinary differential equations describing the airfoil vibrations with large amplitudes taking into account the nonlinear mass matrix. The time-dependent computational domain and a moving grid are treated by the Arbitrary Lagrangian-Eulerian (ALE) method and a suitable stabilization of the FE discretization is applied. The developed method was successfully tested by the classical flutter computation of the critical flutter velocity using NASTRAN program considering the linear model of vibrations and the double-lattice aerodynamic theory. The method was applied to the numerical simulations of the post flutter regime in time domain showing Limit Cycle Oscillations (LCO) due to nonlinearities of the flow model and vibrations with large amplitudes. Numerical experiments were performed for the airfoil NACA 0012 respecting the effect of the air space between the flap and the main airfoil.