Marilena Giangi

Melting of a pure metal on a vertical wall : Numerical simulation

Melting of pure gallium in a bidimensional rectangular cavity with aspect ratio 1.4 is presented. The paper is focused on pattern formation in the fluid phase during the process of melting. The formation of a multiple cells structure has been found during the first stage of the transient, while a merging of the small recirculating cells into larger ones is observed during the following stage of the transient, generating a quite complex evolution of the flow pattern. To ensure the validity of the flow pattern evolution found, a deep meshsensitivity analysis has beenperformed and repeated during the different phases of flow evolution. Three different meshes have been tested, the finest of those, assuming as unity the shorter vertical dimension, with a Delta x = 1/640 and a Delta y = 1/320. To optimize computational resources requests, an optimal solution strategy has been adopted, using different meshes during the various phases of the transient, depending on the size of the melted zone. Because of the large request of CPU time only one test case is presented and compared with available results at Ra = 7 x 10 5 , Pr = 0.0216, and Ste = 4.6 x 10 - 2. Results show that only the use of a fine mesh allows the observation of the multicellular flow structure described.

A NUMERICAL STUDY OF SOLIDIFICATION IN THE PRESENCE OF A FREE SURFACE UNDER MICROGRAVITY CONDITIONS

A numerical study of the relative importance of Marangoni effects under microgravity conditions is presented. The mathematical formulation adopted is based on the enthalpy porosity method. One of the advantages of the fixed grid method is that a unique set of equations and boundary conditions is used for the whole domain, including both solid and liquid phases. The governing equations written in a vorticity-velocity formulation are discretized using a finite volume technique on a staggered grid. A fully implicit method has been adopted for the mass and momentum equations, while the temperature field is solved separately in order to evaluate the variation in the local liquid mass fraction. The resulting algebraic system of equations is solved using a preconditioned BI-CGStab method. Numerical results modelling the free surface, including the effects on it of Marangoni convection, are presented. The influence of the presence of argon in the gap above the free surface is investigated. During the numerical simulations presented in this paper 161 2 41 and 641 2 161 uniform meshes on the whole computational domain for values of Marangoni number ( Ma ) up to 16,120 and Rayleigh number ( Ra ) of 5 have been used.

Sensitivity Analysis for the Dynamic Aeroelasticity of a Launch Vehicle

launcherintermsofthe firstnonzeronaturalfrequenciesandmodesofvibrationiscarriedout.Moreover,areducedorder model for the unsteady transonic aerodynamics is obtained, performing several prescribed modal transient boundary conditions by laminar-based computational fluid dynamics. Thus, a modal input/output system identification for the aerodynamics, performed in the frequency domain, allows one to identify the linearized unsteady aerodynamic operator in the neighborhood of the specific transonic flight condition. Both the structural andaerodynamic modelsare finally employedin the aeroelastic coupledmodel given bythe generalized Lagrangian equations of motion. An eigenanalysis, in terms of aeroelastic-system poles and complex eigenvectors on the linearized model, is performed to check the local dynamic stability of the launch vehicle. Finally, the proposed approach also allows one to give an evaluation of the uncertainty in the obtained stability scenario in terms of perturbing flight parameters like angle of attack, Mach number, flight speed, and air density.

Numerical Simulation of Re-Entry Flow: Heat Flux Evaluation

A method for the study of the flow field around a re-entry vehicle is discussed. Flow field at re-entry is characterized by high energy with the presence of dissociative effects. To take into account such dissociative effects, a simplified model based on the use of a modified specific heat has been adopted and discussed. A numerical method has been validated by means of a comparison with available data of Riley and De Jarnette [1]. Numerical results are also presented for a YES-2 re-entry capsule in the correspondence of three of the most critical flight conditions.