Ralph W. Noack

CFD Analysis of Gear Windage Losses: Validation and Parametric Aerodynamic Studies

A computational fluid dynamics (CFD) method has been applied to gear configurations with and without shrouding. The goals of this work have been to validate the numerical and modeling approaches used for these applications and to develop physical understanding of the aerodynamics of gear windage loss. Several spur gear geometries are considered, for which experimental data are available. Various canonical shrouding configurations and free spinning (no shroud) cases are studied. Comparisons are made with experimental data from open literature, and data recently obtained in the NASA Glenn Research Center Gear Windage Test Facility, Cleveland, OH. The results show good agreement with the experiment. The parametric shroud configuration studies carried out in the Glenn experiments and the CFD analyses elucidate the physical mechanisms of windage losses as well as mitigation strategies due to shrouding and newly proposed tooth contour modifications.

A Finite-Volume Approach to Modeling Ice Accretion

In this work we present a novel, generalized, multiscale physics, unstructured finite-volume, CFD approach for simulating ice accretion on aircraft. A multi-physics solver that evaluates the (1) air flow, (2) droplet trajectories, (3) surface-liquid flow, (4) solidification, and (5) computes the deformed ice shape, is presented. Initial results show promise in the developed methods and solvers, that are expected to later be extended for future rotorcraft ice-accretion analysis. Initial validation cases are presented for the various components of the solver, and compare reasonable well with LEWICE and experiments for simple geometries. This initial capability displays a capability that could be extended, in future efforts, with more detailed models and provide ice shapes of similar quality as the current methodologies, while providing a capability that extends to more complex configurations such as rotorcraft.

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.

A Non-Cut Cell Immersed Boundary Method for Use in Icing Simulations

This paper describes a computational fluid dynamic method used for modelling changes in aircraft geometry due to icing. While an aircraft undergoes icing, the accumulated ice results in a geometric alteration of the aerodynamic surfaces. In computational simulations for icing, it is necessary that the corresponding geometric change is taken into consideration. The method used, herein, for the representation of the geometric change due to icing is a non-cut cell Immersed Boundary Method (IBM). Computational cells that are in a body fitted grid of a clean aerodynamic geometry that are inside a predicted ice formation are identified. An IBM is then used to change these cells from being active computational cells to having properties of viscous solid bodies. This method has been implemented in the NASA developed node centered, finite volume computational fluid dynamics code, FUN3D. The presented capability is tested for two-dimensional airfoils including a clean airfoil, an iced airfoil, and an airfoil in harmonic pitching motion about its quarter chord. For these simulations velocity contours, pressure distributions, coefficients of lift, coefficients of drag, and coefficients of pitching moment about the airfoils quarter chord are computed and used for comparison against experimental results, a higher order panel method code with viscous effects, XFOIL, and the results from FUN3Ds original solution process. The results of the IBM simulations show that the accuracy of the IBM compares satisfactorily with the experimental results, XFOIL results, and the results from FUN3Ds original solution process.

A Numerical Investigation of Droplet/Particle Impingement on a Dynamic Airfoil

In this paper we investigate phenomena associated with particle impingement on dynamic airfoils. Using an Eulerian based solver, a droplet velocity field is computed throughout the computational domain. This solver is used to scrutinize particular concepts, with the intent to save computation time, listed as follows: (i) The applicability of different time-averaging techniques to calculate an average local collection efficiency for dynamic simulations. (ii) Comparison between static and dynamic bodies undergoing particle surface interactions. (iii) The use of a numerically viscous solution to the Euler equations instead of a fully viscous solution to predict proper particle trajectories on dynamic airfoils. Guidance is provided herein, for the applicability of solution methods to reduce computational costs.

A coupled overset vorticity transport and compressible Euler solver for vortex-dominated flows

An efficient solver for the incompressible vorticity transport equations on adaptive Cartesian grids is coupled to an unstructured spectral volume solver for the conservative form of the compressible Euler equations using overset grid assembly and interpolation provided by SUGGAR and DiRTlib, respectively. Vortical flow structures originate at solid surfaces in near-body regions employing the Euler solver and are transported into the wake region that employs an Eulerian Vorticity Transport (EVT) solver. The excessive numerical dissipation common to most grid-based Navier-Stokes solvers is avoided in the wake region by solving the fluid dynamic equations in vorticity conservation form. In addition, the adaptive Cartesian mesh utilized in the wake region allows for efficient transport and preservation of vortical structures by means of localized adaptive mesh refinement and coarsening. Two different approaches for evaluating the EVT velocity field, one involving a fast summation technique based on the Cartesian Treecode method, and the other utilizing a multigrid Poisson approach, are presented and compared. The implementation of both the solution algorithm and overset coupling methodology is described in detail, and results for several different test cases are presented which demonstrate the effectiveness of the hybrid EVT-Euler simulation capability for accurately transporting vortical structures between a near-body compressible Euler solver and an off-body EVT solver.

A Library Based Overset Capability Development for Density- and Pressure- Based Flow Solvers

Computational simulation of flows involving bodies in relative motion is a challenging problem, which requires the capability of relocating the grid associated with moving bodies as well as the formulation of grid speed terms in the flux evaluation of the governing equations. An overset grid framework enables the computational fluid dynamics (CFD) flow solver to model this class of problems. However, implementing this technology into an existing CFD code can be very tedious and time consuming due to the need for laborious book-keeping as well as proper data interpolation between different layers of grids. This paper describes an overset grid approach to alleviate these problems and enhance the capability of flow solvers to handle problems involving bodies in relative motion. A library approach has been developed and adopted to handle the bookkeeping of the overlapping grids among different layers and the transfer of information across them. To have a wide applicability, this library approach should be able to support structured, unstructured, and generalized grid topologies. This approach makes it easier to incorporate the overset capability into legacy CFD solvers. In the present study, a set of developed overset grid libraries (SUGGAR and DiRTLib) was implemented into a density-based generalized grid flow solver and a pressure-based structured grid flow solver as demonstration. This implementation of the overset capability has been validated with static and moving grids and is presented in the paper.