Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session H5: CFD V 
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Chair: Tomasz Drozda, NASA Room: 327 
Monday, November 25, 2013 10:30AM  10:43AM 
H5.00001: Simulation of Reacting Flow with a Discontinuous Spectral Element Method Zia Ghiasi, Farzad Mashayek, Jonathan Komperda While using high order methods is desirable in order to accurately capture the small scale mixing effects in reacting flows, the challenge is to develop and implement such methods for complex geometries. In this work, a highorder Discontinuous Spectral Element Method (DSEM) code, which solves for the NavierStokes equations, has been modified by adding the appropriate components to solve for scalar transport equations in order to simulate the chemical reaction. Dealing with discontinuous solution at element interfaces is a challenge that is met by patching the fluxes at mortars thus making them continuous on interfaces. The patching is performed using the LaxFredrichs numerical flux for scalars, whereas a generalized Riemann solver is used for the NavierStokes equations. Direct numerical simulation is conducted in a temporally developing mixing layer to validate the method for a single step reaction ($F + rO \rightarrow [1+r]P$). Next, the method is implemented to simulate a subsonic reacting flow in a slanted cavity combustor with gaseous fuel injectors to demonstrate the capability of the method to handle complex geometries. The results will be used for physical understanding of mixing and reaction in this type of combustors. [Preview Abstract] 
Monday, November 25, 2013 10:43AM  10:56AM 
H5.00002: A numerical investigation of the influence of aspect ratio in three dimensional separated flows Nikolaos Malamataris The influence of aspect ratio in three dimensional separated flows is investigated numerically by solving the full three dimensional Navier Stokes equations for Newtonian fluids using standard Galerkin finite elements. As a prototype flow, the backward facing step is chosen with an expansion ratio of 1:2. The Reynolds number is of the order of 1000 where steady state, laminar flow conditions prevail. The computational domain is designed as an actual laboratory experiment with lateral walls and aspect ratios from 1:10 up to 1:40. The results focus on the spanwise variation of the length and the strength of both eddies for this flow that appear along the bottom and top wall. Depending on what attributes of the flow are taken for comparison, aspect ratios of 1:20 up to 1:40 are considered adequate for calling two dimensional flow conditions along the plane of symmetry. The results are contrary to the common wisdom in this field where the aspect ratio of 1:10 is still considered satisfactory two dimensional flow conditions. This is the first computational study for separated flows that raises the issue of two dimensionality along the plane of symmetry and computes the eddy along the top wall for aspect ratios less than 1:35. [Preview Abstract] 
Monday, November 25, 2013 10:56AM  11:09AM 
H5.00003: Ablation patterns driven by simple flows Ryan Crocker, Daniel Hagan, Michael Allard, Yves Dubief, Christopher White The erosion (here through thermal ablation) of a surface driven by a turbulent, or at least nonlinear, flow may offer an interesting variety of erosion patterns. The present work is interested in the interactions between the flow coherent structures and the topology and erosion rates of such structures. The investigation involves different flows including natural convection flow and flows parallel and perpendicular to the ablated surface. The simulation algorithm is based on momentum and thermal immersed boundary techniques in a finite volume direct numerical simulation flow solver. The interface is tracked by a level set method and the ablation velocity is governed by the Stefan condition. The analysis focuses on the nonequilibrium nature of the flow and the possible prediction of erosion rates. [Preview Abstract] 
Monday, November 25, 2013 11:09AM  11:22AM 
H5.00004: Rich 3tori dynamics in smallaspectratio highly counterrotating TaylorCouette flow  reversal of spiraling vortices Sebastian Altmeyer, Bj\"orn Hof, Francisco Marques, Juan M. Lopez We present numerical simulations concerning the reversal of spiraling vortices in short highly counterrotating cylinders. Increasing the differential cylinder rotation results in global flowinversion which develops various different and complex flow dynamics of several quasiperiodic solutions that differ in their number of vortex cells in the bulk. The dynamics change from being dominated of the inner cylinder boundary layer to be dominated by the outer cylinder boundary layer. Solutions exist on either two or three tori invariant manifolds whereby they appear as symmetric or asymmetric states. We find for either moderate and high inner cylinder rotation speed the quasiperiodic flow to consist of only two vortex cells but differ in its spiraling direction. These both flows live on 2tori but differ in number of symmetries. While for the quasiperiodic flow at lower rotation speed a pair of symmetrically related 2tori exists the quasiperiodic flow at higher rotation speeds is symmetric living on a single 2torus. In addition these both flows differ due to their dominant azimuthal m modes. The 2tori states are separated by an further quasiperiodic flow living on pair of symmetrically related 3tori and offer periodical competition between a two and three vortex cell states. [Preview Abstract] 
Monday, November 25, 2013 11:22AM  11:35AM 
H5.00005: Numerical Study of Flow Structure of the Taconis Oscillations in an Axisymmetric Closed Tube Katsuya Ishii, Shyun Kitagawa, Shizuko Adachi Spontaneous thermoacoustic oscillations of a helium gas in a closed cylindrical tube are studied by solving the axisymmetric compressible NavierStokes equations. The wall temperature of the hot part near both ends (300K) and that of the cold central part (20K) are fixed. The computations are done for various values of the length ratio of the hot part to the cold part between 0.3 and 1.0. The oscillation states are divided into three groups according to the magnitude of the pressure amplitude, which are the fundamental mode and the second mode of a standing wave, and the oscillation with a shock wave. The states in each group have distinguished features of the vortical flow field. We analyze the effect of vortices on the structure of the temperature distribution and the flow of energy fluxes to gain a better understanding of the mechanism of the thermoacoustic oscillations. [Preview Abstract] 

H5.00006: ABSTRACT WITHDRAWN 
Monday, November 25, 2013 11:48AM  12:01PM 
H5.00007: CFD methodology of a model quadrotor Burak Sunan This paper presents an analysis of the aerodynamics characteristics of a quadrotor for both steady and unsteady flows. For steady flow cases, aerodynamics behaviour can be defined readily for any aerial vehicles in wind tunnels. However, unsteady flow conditions in wind tunnels make experimental aerodynamics characterizations difficult. This article describes determination of lift, drag and thrust forces on a model quadrotor by using CFD (Computational Fluid Dynamics) software ANSYS Fluent. A significant issue is to find a new CFD methodology for comparison with the experimental results. After getting sufficiently close agreement with some benchmarking experiments, the CFD methodology can be performed for more complicated geometries. In this paper, propeller performance database experiments from Ref. 1 will be used for validation of the CFD procedure. The results of the study reveals the dynamics characteristics of a quadrotor. This demonstrates feasibility of designing a quadrotor by CFD which saves time and cost compared to experiments. [Preview Abstract] 
Monday, November 25, 2013 12:01PM  12:14PM 
H5.00008: Using NavierStokes to Characterize ReEntry of Microscale Vehicles Sudharsan Thiruvenkadam, Harris Ben Atmospheric reentry vehicles experience different flow regimes during flight due to the change in atmospheric density. This change in density creates nonequilibrium regions on the order of one mean free path, called as Knudsen layer. In the design of atmospheric reentry vehicles, the flux variations near solid surface are of critical importance. The traditional CFD simulations which use Navier Stokes equations fail to predict the flow in Knudsen layer. These areas where the rarefaction effects begin to dominate can be quantified by the Knudsen breakdown parameter. The Direct Simulation Monte Carlo (DSMC) method, although accurate for all flow regimes, it is computationally expensive as the number of simulating molecules increases. We developed a method that models the Knudsen Layer by using Navier Stokes equations with MaxwellSmoluchowski slip boundary conditions and DSMC for low (Kn \textless\ 0.1) and high (Kn \textgreater\ 0.1) Knudsen numbers respectively. This study investigates the surface properties of a flat plate with Nitrogen gas flow from continuum to rarefied regimes. Computational fluid dynamics and DSMC results are obtained for different test conditions. The results demonstrate that the Knudsen layer can be predicted with DSMC and continuum approach for all flow regimes. [Preview Abstract] 
Monday, November 25, 2013 12:14PM  12:27PM 
H5.00009: von Neumann Stability Analysis of PressureBased Formulation of 1D and 2D Euler Equations Santosh Konangi, Urmila Ghia The stability properties of a pressurebased scheme for the Euler equations are investigated, as such schemes are widely employed in commercial computational fluid dynamics codes. The published literature often focusses on model equations, and does not consider the solution scheme used in the parent code. The present study conducts a von Neumann stability analysis for a pressurebased, segregated scheme, SIMPLE (SemiImplicit Method for PressureLinked Equations). The 1D and 2D Euler equations, closed by an ``artificial'' equation of state, are discretized using finite differences on a staggered grid, which permits equivalence to finitevolume discretization. As a first effort, firstorder accurate spatial and temporal schemes are analyzed, to determine error amplification matrices, identify stable and unstable regimes, and predict practical stability limits in terms of the maximum allowable CFL number as a function of Mach number. The predictions are verified using the Riemann problem at several Mach numbers, and very good agreement is obtained between the predicted and the ``numerically'' observed CFL values. Hence, the present results should prove useful in guiding the stability of a simulation using the parentcode and the scheme tested here. [Preview Abstract] 
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