Alain Kassab

A BEM-based Inverse Algorithm To RetrieveMulti-dimensional Heat Transfer CoefficientsFrom Transient Temperature Measurements

We present a BEM-based inverse algorithm to retrieve multi-dimensional heat transfer coefficients (h) from surface temperature measurements. The time history of temperatures at convective BEM nodes is measured (or simulated). At each time level, a regularized least-squares functional is minimized to retrieve current fluxes and simultaneously smooth out temperature measurement errors. The regularization term weighs the square of deviations of BEM-computed fluxes from a mean flux to allow adjustment of temperatures and fluxes to obtain a best fit through noisy input data. Results for retrieval of h over a square and a backward facing step show the 1-D approach fails to reproduce ft, while our approach reproduces h accurately while being robust to input errors.

A Localized Radial-Basis-Function Meshless Method Approach to Axisymmetric Thermo-Elasticity

Current methods for solving thermoelasticity problems involve using finite element analysis, boundary element analysis, or other meshed-type methods to determine the displacements under an imposed temperature/stress field. This paper will detail a new approach using localized meshless methods based on multi-quadric radial basis function interpolation to solve these types of coupled thermoelasticity problems. Here, a point distribution is used along with a localized collocation method to solve the Navier equation for the components of the displacement vector. The specific application considered in this paper is that of axisymmetric thermo-elasticity. With rapidly increasing availability and performance of computer workstations and clusters, the major time requirement for solving a thermoelasticity model is no longer the computation time, but rather the problem setup. Defining the required mesh for a complex geometry can be extremely complicated and time consuming, and new methods are desired that can reduce this time. The proposed meshless method features the complete elimination of a mesh, be it structured or unstructured, and the associated complexities involved in its generation and control. The reduction of initial model setup time makes the meshless approach an ideal method of solving coupled thermoelasticity problems. Several examples with exact solutions are used to verify this method for various geometries and boundary condition combinations.

BEM Model of Ablation Characteristics of a Thrust Vector Control Vane

A dual reciprocity boundary element method is implemented to predict ablation in model TVC vanes. A moving front algorithm is described. Experimental data are available from tests performed on scaled vanes. Numerical results for recession of quarter-scale and half-scale vanes compare well with experimental data. Future work includes accounting for temperature variation of the thermophysical properties and full coupling of the flowfield and conduction solutions.

Computational Fluid Dynamics Simulation of United Launch Alliance Delta IV Hydrogen Plume Mitigation Strategies

During the launch sequence of the United Launch Alliance Delta IV launch vehicle, large amounts of pure hydrogen are introduced into the launch table and ignited by RadialOutward-Firing-Igniters (ROFIs). This ignition results in a significant flame, or plume, that rises upwards out of the launch table due to buoyancy. The presence of the plume causes increased and unwanted heat loads on the surface of the vehicle. A proposed solution to this problem is to add a series of fans and structures to the existing launch table configuration that are designed to inject ambient air in the immediate vicinity of the launch vehicle’s nozzles to suppress the plume rise. In addition to the air injection, secondary blockages and fan systems can be added around the launch table openings to further suppress the hydrogen plume. The proposed air injection solution is validated by computational fluid dynamics simulations that capture the combustion and compressible flow observed during the Delta IV launch sequence. A solution to the hydrogen plume problem will have direct influence on the efficiency of the launch vehicle: lower heat loads result in thinner vehicle insulation and thus allow for a larger payload mass. Current results show that air injection around the launch vehicle nozzles and air suppression around the launch table openings significantly reduces the size of the plume around the launch vehicle prior to liftoff.

Time-Accurate Simulation of Shock Propagation and Reflection in an Axi-Symmetric Shock Tube

An axi-symmetric shock-tube model has been developed to simulate shock propagation and reflection in both inviscid and viscous nonreactive flows. Simulations were performed for the full shock-tube geometry of the high-pressure shock tube facility at Texas A&M University. The CFD code FLUENT was employed to simulate the shock propagation and reflection processes in the shock tube, and the flow properties behind the reflected shock wave were obtained by solving the axi-symmetric, unsteady Euler and Navier-Stokes equations. Computations were carried out based on the finite volume approach and the AUSM+ flux differencing scheme. Adaptive mesh refinement (AMR) algorithm was applied to the time-dependent flow fields to accurately capture and resolve the shock and contact discontinuities as well as the very fine scales associated with the viscous effects. The bifurcation phenomenon resulting from the interaction of the reflected shock wave and the boundary layer has been accurately simulated. Conjugate heat transfer modeling was made possible in conjunction with the viscous model which enhanced the credibility of the results. The robustness of the numerical model and the accuracy of the simulations were assessed through validations with the analytical ideal theory and experimental measurements. The model is shown to be capable of accurately simulating the shock and expansion wave propagations and reflections as well as the flow non-uniformities behind the reflected shock wave associated with the viscous non-ideal effects.

Boundary Integral Formulation For Non-linear Heat Conduction In Anisotropic Heterogeneous Media

The recently developed BEM formulation of the authors for heat conduction in media with spatially dependent thermophysical properties is herein implemented to approximate the solution of problems in which the temperature dependency of the material properties results into a non-linear governing equation. The thermal conductivity of the medium is also considered to be anisotropic. An iterative approach is used to lag the thermal conductivity as a spatially varying function Numerical examples of the present application are provided in regular and irregular geometries. BEM solutions are found to be in agreement with analytical solutions. Convergence of the iterative scheme is very fast satisfying convergence criteria in a small number of updates.

An efficient inverse BEM algorithm to characterize space dependent thermal conductivity with an evolutionary-genetic approach

A Genetic Algorithm approach is used as an optimization tool combined with the generalized BEM formulation for heat conduction in heterogeneous media to estimate space varying thermal conductivities. A least-squares functional measuring the difference between over-imposed observed temperatures and current temperatures generated by the BEM under current thermal conductivity is minimized using a Genetic Algorithm.