A study of the vibration of a horizontal U-bend subjected to an internal upwards flowing air-water mixture

U-bends are a common geometry in heat exchangers. In this paper, a U-bend in the vertical plane connected to horizontal straight pipes is considered. An initially stratified water/air flow moves upwards against gravity. The aim of this research is to investigate the internal flow profile and resulting force when the U-bend is subjected to a stratified air-water flow at the inlet. This is done numerically, i.e. by solving the unsteady Reynolds-averaged Navier-Stokes equations. For low mass flow rates, large gas bubbles are naturally formed at the entrance of the bend. The transient force on the tube allows to determine precisely the time instants of bubble initiation and thus to quantify the bubble frequency. Firstly, the tube is assumed to be rigid and the dependence of force oscillation on the inlet conditions is investigated. Secondly, the influence of the viscosity, wall wetting and the mass flow rate is analyzed. Finally, a fluid-structure interaction calculation is performed in order to quantify the vibration characteristics of the tube

Numerical and analytical investigation of subcycling in the flow problem of a strongly-coupled partitioned fluid-structure interaction simulation

Fluid-structure interaction (FSI) simulations can be used to quantify the frequency, damping constant and amplitude of the vibration of equipment such as piping and heat exchangers. Typically, the time step is the same in the flow and structural equations, but this causes long computational times when the time step is restricted due to stability requirements of only one solver. In that case, a more efficient approach is to use so-called subcycling with a different time step in the flow and structural solver. In this paper, only subcycling with a smaller time step in the flow solver compared to the structural solver is analyzed. The research presented here is split into two parts: an analytical study and a numerical computation of the one-dimensional flow in an elastic cylindrical tube. Firstly, a monolithic analytical FSI calculation is analyzed with a Fourier stability analysis. This allows to verify the stability of the solution by considering the eigenvalues of the problem as a function of the perturbation wavenumber. The conclusions drawn from the analytical study are subsequently verified in a partitioned numerical FSI simulation, coupling the flow solver Fluent with the structural solver Abaqus. The implicit coupling is achieved using an interface quasi-Newton method with an approximation of the inverse of the Jacobian (IQN-ILS), implemented in the in-house code Tango. The research shows that a stable solution is attained for significant subcycling in the flow problem: the results indicate that the solution remains temporally stable even if the time step in the flow solver is only one tenth of the structural time step. However, some (temporally stable) oscillations in the resulting pressure profile on the pipe wall arise when the time discretization schemes applied in the flow and structural solvers are different.These oscillations do not persist when the same time discretization scheme is applied

Evaluation of a new implicit coupling algorithm for the partitioned fluid-structure interaction simulation of bileaflet mechanical heart valves

The movement of the leaflets of Bileaflet Mechanical Heart Valves (BMHVs) strongly interacts with the surrounding fluid motion and therefore it needs to be modeled through a Fluid-Structure Interaction (FSI) scheme with implicit coupling. Therefore, when using partitioned solvers, a subiteration loop within each time step is needed. The stability of such a scheme depends on the value of the under-relaxation factor. For the simulation of a BMHV, several methods can be used to find such an appropriate under-relaxation factor, like fixed under-relaxation or the dynamically changing Aitken Δ2 under-relaxation. Also, a stable scheme can be achieved with a newly developed algorithm which uses the Jacobian with the derivatives of the pressure and viscous moments acting on the leaflets with resprect to the angular accelerations of the leaflets. In this paper, this new algorithm is presented and compared to existing coupling schemes. It is shown through numerical experiments that our newly developed algorithm outperforms these existing coupling schemes

Parametric numerical analysis of fire-induced pressure variations in a well-confined and mechanically ventilated compartment

The investigation of a fire in a well-confined and mechanically ventilated compartment is of primary importance for the nuclear industry. In normal operating conditions, a ventilation network system is set-up to ensure confinement via an appropriate pressure cascade. In the event of a fire, the subsequent pressure build-up alters the confinement level significantly and therefore changes the level of safety of the installation. The fire-induced pressure variations depend mainly on the: (1) HRR (Heat Release Rate) history of the fire, (2) heat losses to the walls, (3) leakage area, and (4) operating conditions of the fans. A numerical parametric analysis on the latter three parameters, using the Fire Dynamics Simulator (FDS 5.5.3), have shown that a change in the initial ventilation parameters (i.e. operating conditions of the fans and/or leaks), which can be sometimes difficult to determine, may lead to substantial changes in the pressure profiles. However, only a change in the thermal boundary conditions (i.e. presence or no of insulation) produces significant changes in the gas temperature