Roy P. Koomullil

Iced airfoil simulation using generalized grids

A new strategy for simulating the flow around iced airfoils is presented in this paper. Two different approaches are used to generate quality grids over the iced airfoil. Both grids can be categorized as generalized grids since multiple element types are employed in each. In the first approach, a structured grid is generated near the body using a marching scheme and the rest of the domain is filled with an unstructured grid. In the second approach, the marching strategy is coupled with a node deletion/insertion algorithm to generate a quad-dominant grid. An edge based data structure is used to store the grid information to handle polygons with an arbitrary number of sides. A finite-volume, cell-centered scheme is used to solve the integral form of the full Navier-Stokes equations. The numerical fluxes crossing the cellfaces are calculated using Roes approximate Riemann solver. The turbulent viscosity is estimated using Spalart-Allmaras one equation turbulence model. The results of computations are presented, together with comparison to NPARC results.

Numerical Simulation of Ablation for Reentry Vehicles

A computer program is developed for numerical simulation of ablation problems, taking into account in-depth pyrolysis; surface recession; nonequilibrium chemistry in the flow of pyrolysis gases through a variable porosity char; and thermal non-equilibrium between the char and the pyrolysis gases. The program provides a general and easy to use interface for coupling with a reacting flow solver and a radiation solver. The program is verified using available analytical solutions for heat conduction in non-receding and receding solids; the program will then be validated using available experimental data.

Computational Fluid Dynamic Analysis of the Posterior Airway Space After Maxillomandibular Advancement for Obstructive Sleep Apnea Syndrome

PURPOSE This study evaluated the soft tissue change of the upper airway after maxillomandibular advancement (MMA) using computational fluid dynamics. MATERIALS AND METHODS Eight patients with obstructive sleep apnea syndrome who required MMA were recruited into this study. All participants underwent pre- and postoperative computed tomography and then MMA by a single oral and maxillofacial surgeon. Upper airway computed tomographic datasets for these 8 patients were created with high-fidelity 3-dimensional numerical models for computational fluid dynamics. The 3-dimensional models were simulated and analyzed to study how changes in airway anatomy affect the pressure effort required for normal breathing. Airway dimensions, skeletal changes, apnea-hypopnea index, and pressure effort of pre- and postoperative 3-dimensional models were compared and correlations were interpreted. RESULTS After MMA, laminar and turbulent air flows were significantly decreased at every level of the airway. The cross-sectional areas at the soft palate and tongue base were significantly increased. CONCLUSIONS This study showed that MMA increased airway dimensions by increasing the distance from the occipital base to the pogonion. An increase of this distance showed a significant correlation with an improvement in the apnea-hypopnea index and a decreased pressure effort of the upper airway. Decreasing the pressure effort will decrease the breathing workload. This improves the condition of obstructive sleep apnea syndrome.

Computational Tools for Re-entry Aerothermodynamics: Part II. Surface Ablation

An extension of the recently developed numerical model for surface ab lation is reported here. In order to capture the chemical processes happening insid e the char and the pyrolyzing zone, equilibrium chemistry and finite-rate chemi stry modules have been incorporated into the numerical model. The developed numerical model is then validated using standard benchmark test cases available from the lit erature. A detailed discussion about the results is given, followed by a brief note on th e ongoing future work.

Original articles: Patient-specific geometry modeling and mesh generation for simulating Obstructive Sleep Apnea Syndrome cases by Maxillomandibular Advancement

The objective of this paper is the reconstruction of upper airway geometric models as hybrid meshes from clinically used Computed Tomography (CT) data sets in order to understand the dynamics and behaviors of the pre- and postoperative upper airway systems of Obstructive Sleep Apnea Syndrome (OSAS) patients by viscous Computational Fluid Dynamics (CFD) simulations. The selection criteria for OSAS cases studied are discussed because two reasonable pre- and postoperative upper airway models for CFD simulations may not be created for every case without a special protocol for CT scanning. The geometry extraction and manipulation methods are presented with technical barriers that must be overcome so that they can be used along with computational simulation software as a daily clinical evaluation tool. Eight cases are presented in this paper, and each case consists of pre- and postoperative configurations. The results of computational simulations of two cases are included in this paper as demonstration.

Assessment of Surgical Effects on Patients with Obstructive Sleep Apnea Syndrome Using Computational Fluid Dynamics Simulations.

Obstructive sleep apnea syndrome is one of the most common sleep disorders. To treat patients with this health problem, it is important to detect the severity of this syndrome and occlusion sites in each patient. The goal of this study is to test the hypothesis that the cure of obstructive sleep apnea syndrome by maxillomandibular advancement surgery can be predicted by analyzing the effect of anatomical airway changes on the pressure effort required for normal breathing using a high-fidelity, 3-D numerical model. The employed numerical model consists of: 1) 3-D upper airway geometry construction from patient-specific computed tomographic scans using an image segmentation technique, 2) mixed-element mesh generation of the numerically constructed airway geometry for discretizing the domain of interest, and 3) computational fluid dynamics simulations for predicting the flow field within the airway and the degree of severity of breathing obstruction. In the present study, both laminar and turbulent flow simulations were performed to predict the flow field in the upper airway of the selected patients before and after maxillomandibular advancement surgery. Patients of different body mass indices were also studied to assess their effects. The numerical results were analyzed to evaluate the pressure gradient along the upper airway. The magnitude of the pressure gradient is regarded as the pressure effort required for breathing, and the extent of reduction of the pressure effort is taken to measure the success of the surgery. The description of the employed numerical model, numerical results from simulations of various patients, and suggestion for future work are detailed in this paper.

Evaluation of in-stent stenosis by magnetic resonance phase-velocity mapping in nickel-titanium stents.

To evaluate different grades of in‐stent stenosis in a nickel‐titanium stent with MRI.

Moving-body simulations using overset framework with rigid body dynamics

The simulation of flow past bodies in relative motion is a challenging task due to the presence of complex flow features, moving grids, and rigid body movements under the action of external forces and moments. A generalized grid-based overset framework is presented for the simulation of this class of problems. The equations that govern the fluid flows are cast in an integral form and are solved using a cell-centered finite volume upwind scheme. The rigid body dynamics equations are formulated using quaternion and are solved using fourth-order Runge-Kutta (RK) time integration. The overset framework and the six degree of freedom (6-DOF) rigid body dynamics simulators are developed in a library form for easy incorporation into existing flow solvers. The details of the flow solver, the 6-DOF library, and the overset framework are presented in this paper along with the validation results of the developed system.

Development of a Parallel Hybrid Method for the DSMC and NS Solver

A parallel 3-D hybrid DSMC-NS method using the unstructured grid topology has been proposed and developed such that near continuum flows or a region consisting of continuum, slip, and free-molecular flows can be simulated efficiently. A continuum breakdown parameter proposed by Wang and Boyd is used in the present work for determining the choice of solver in spatial domain. A hypersonic flow over a 2-D wedge and other test cases are employed to evaluate and validate the present hybrid method. Preliminary results show the simulation data of the present hybrid method is in good agreement with those of the pure DSMC method. In general, computational time using the hybrid method is lower than that using the pure DSMC method. In addition, the region with strong thermal non-equilibrium cannot be precisely identified by the continuum break down parameter proposed by Wang and Boyd. Future study is needed to develop an appropriate method to identify the thermal non-equilibrium region.

Solution of Radiative Boundary Design Problems Using a Combined Optimization Technique

In this work, a novel combined strategy has been developed and verified for radiative boundary design problems. It is highly efficient and simple to implement and appears promising for obtaining appropriate results in practical applications. In the proposed approach, the determination of the unknown profile of heat flux through active constraints includes utilizing a new search optimization technique that is merged with the maximum entropy method (MEM). It is shown that use of the merged MEM algorithm minimizes both the occurrence of negative values for physically non-negative components, and the oscillatory profile of heat flux. A generalized computational grid based on finite-volume scheme is devised to solve the radiative transfer equation and sensitivity equations. Numerical simulations are conducted to evaluate the performance and accuracy of the present approach with regard to the classical methods involved in its derivation.