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 R18: CFD IX |
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Chair: Mehdi B. Nik, University of Pittsburgh Room: 306/307 |
Tuesday, November 26, 2013 1:05PM - 1:18PM |
R18.00001: Nonlinear optimisation of scalar mixing in plane Poiseuille flow with finite diffusivity C.P. Caulfield, Dimitry Foures, P.J. Schmid We consider the nonlinear optimisation of the mixing of a passive scalar, initially arranged in two layers, in 2D plane Poiseuille flow at finite Reynolds number and P\'eclet number, $Re \sim Pe \sim O(10^3)$. We use a nonlinear-adjoint-looping approach to minimise the variance of the scalar concentration $\theta$ at various target times $T$, subject either to a finite kinetic energy initial disturbance, or wall velocity perturbation. We show that both optimal initial perturbations and optimal wall excitation strategies which minimise the variance of $\theta$ are distinct from the equivalent perturbations which maximise the time-averaged energy gain of disturbance at $t=T$, and that these ``gain'' perturbations can often be poor at scalar mixing. We also identify perturbations and excitation strategies which minimise the distribution of $\theta$ at the target time relative to a particular Sobolev norm of negative index, a ``mix-norm'' as used in flows with no diffusion to measure ``mixing'' in the sense of ergodic theory (G. Mathew, I. Mezic, \& L. Petzold 2005 {\it Physica D}, {\bf 211}, 23-46). We show the close connection between these mix-norm perturbations and the optimal variance perturbations, all of which initially increase gradients to ensure good mixing at later times. [Preview Abstract] |
Tuesday, November 26, 2013 1:18PM - 1:31PM |
R18.00002: Simulation-based optimization using finite-time approximations of the infinite-time-average statistics Pooriya Beyhaghi, Thomas Bewley Simulated-based optimization problems are difficult, due both to the nonconvexity of the cost function and to the extreme cost of accurate function evaluations, especially if the cost function is derived from a finite-time-average approximation of the infinite-time-average statistics. In this work, we have developed an algorithm that controls both the location and the accuracy of each cost function evaluation. At each step of the algorithm, a Delaunay triangulation is created based on the existing evaluation points. In each simplex so created, the algorithm optimizes a cost function based on a polyharmonic spline regression. At each optimization step, an appropriately-modeled error function is combined with the regression, weighted with a tuning parameter governing the trade-off between local refinement and global exploration. In this way, the location of the new candidate point for the global minimum is found; then, based on the value of the minimum available cost function evaluations and the uncertainty associated with it, an efficient finite-time approximation at this point is calculated. The global convergence of this algorithm will be shown, and its efficiency will be tested on representatives test functions. [Preview Abstract] |
Tuesday, November 26, 2013 1:31PM - 1:44PM |
R18.00003: Determining wave resistance of a ship using a dissipative potential flow model Mirjam F\"urth, Mingyi Tan, Zhi-Min Chen Potential flow modelling is a common method to predict the wave resistance of ships. In its conventional form the flow is assumed to be free from damping due to the inviscid assumption of potential flow. However, it is evident by just looking at waves that they decay with time and distance. It is a reasonable assumption that, by including more of the actual physical aspect in mathematical model, the quality of the prediction will improve. As Havelock wrote almost 80 years ago ``It seems fairly certain that one of the main causes of differences between theoretical and experimental result is the neglect of fluid friction in the calculation of ship waves.'' In this study, the problem is modelled using Kelvin sources with a translating speed. Rayleigh damping is introduced in the model to emulate viscous damping. To calculate the source influences, a dissipative 3D Green function is derived. For initial validation of the Green function, thin ship theory is used to determine the wave pattern behind a Wigley hull and a modified form of the Eggers et al. transverse cut technique is used to calculate the wave resistance. To evaluate the method for fuller and more realistic hull shapes a panel method which calculates the resistance via the pressure on the ship hull is used. [Preview Abstract] |
Tuesday, November 26, 2013 1:44PM - 1:57PM |
R18.00004: Thermal Fluctuations in Smooth Dissipative Particle Dynamics simulation of mesoscopic thermal systems Nikolaos Gatsonis, Jun Yang The SDPD-DV is implemented in our work for arbitrary 3D wall bounded geometries. The particle position and momentum equations are integrated with a velocity-Verlet algorithm and the entropy equation is integrated with a Runge-Kutta algorithm. Simulations of nitrogen gas are performed to evaluate the effects of timestep and particle scale on temperature, self-diffusion coefficient and shear viscosity. The hydrodynamic fluctuations in temperature, density, pressure and velocity from the SDPD-DV simulations are evaluated and compared with theoretical predictions. Steady planar thermal Couette flows are simulated and compared with analytical solutions. Simulations cover the hydrodynamic and mesocopic regime and show thermal fluctuations and their dependence on particle size. [Preview Abstract] |
Tuesday, November 26, 2013 1:57PM - 2:10PM |
R18.00005: Boundary conditions for coupling molecular dynamics simulations to continuum simulations Liv Herdman, Yves Dubief Coupling continuum simulations to molecular dynamics simulations require implementing boundary conditions that constrain the atomic motions to match the physical properties of the large-scale simulations. The traditional wall and periodic boundary conditions used in molecular dynamics present difficulties for simulating non-equilibrium and spatially evolving flows. We are working toward creating an evolving boundary condition to match temperature and momentum conditions in atomistic simulations that are driven by coupled continuum simulations. We have developed an inlet boundary condition that utilizes a periodic buffer cell to drive the variable thermodynamic and flow conditions. In this work we demonstrate the new inlet boundary condition with simulations of a Lenard-Jones fluid in a channel and compare the effects of different outlet boundary conditions [Preview Abstract] |
Tuesday, November 26, 2013 2:10PM - 2:23PM |
R18.00006: Patient Specific Multiscale Simulations of Blood Flow in Coronary Artery Bypass Surgery Abhay Bangalore Ramachandra, Sethuraman Sankaran, Andrew M. Kahn, Alison L. Marsden Coronary artery bypass surgery is performed to revascularize blocked coronary arteries in roughly 400,000 patients per year in the US.While arterial grafts offer superior patency, vein grafts are used in more than 70{\%} of procedures, as most patients require multiple grafts. Vein graft failure (approx. 50{\%} within 10 years) remains a major clinical issue. Mounting evidence suggests that hemodynamics plays a key role as a mechano-biological stimulus contributing to graft failure. However, quantifying relevant hemodynamic quantities (e.g. wall shear stress) invivo is not possible directly using clinical imaging techniques. We numerically compute graft hemodynamics in a cohort of 3-D patient specific models using a stabilized finite element method. The 3D flow domain is coupled to a 0D lumped parameter circulatory model. Boundary conditions are tuned to match patient specific blood pressures, stroke volumes {\&} heart rates. Results reproduce clinically observed coronary flow waveforms. We quantify differences in multiple hemodynamic quantities between arterial {\&}venous grafts {\&} discuss possible correlations between graft hemodynamics {\&} clinically observed graft failure.Such correlations will provide further insight into mechanisms of graft failure and may lead to improved clinical outcomes. [Preview Abstract] |
Tuesday, November 26, 2013 2:23PM - 2:36PM |
R18.00007: Coupling surface and subsurface flows with curved interfaces Pu Song, Ivan Yotov A mortar multiscale method is developed for the coupled Stokes andDarcy flows with the Beavers--Joseph--Saffman interface condition in irregular domains. Conforming Stokes elements and multipoint flux mixed finite elements in Darcy are used to discretize the subdomains on the fine scale. A coarse scale mortar finite element space is used to approximate interface stresses and pressures and impose weakly continuity of velocities and fluxes. Matching conditions on curved interfaces are imposed by mapping the physical grids to reference grids with flat interfaces. [Preview Abstract] |
Tuesday, November 26, 2013 2:36PM - 2:49PM |
R18.00008: An efficient pressure-correction method for incompressible multifluid flows M. Dodd, A. Ferrante We present a new pressure-correction (PC) method for solving incompressible multifluid flows with large density ratios. The novelty of the method is that the variable coefficient Poisson equation that arises in solving the variable-density Navier-Stokes equations has been reduced to a constant coefficient equation, which can then be solved directly using a fast Poisson solver. The new method is coupled to our mass-conserving volume-of-fluid (VoF) method to capture the interface between the moving fluids. First, we verified the new PC/VoF solver using the capillary wave test-case up to density and viscosity ratios of 10,000. Then, we validated the new flow solver by simulating the motion of a falling water droplet in air by comparing the droplet terminal velocity with the experimental value (Beard, 1976) for $95.6 \leq \mathrm{Re} \leq 473$, $0.06 \leq \mathrm{We} \leq 0.61$, and $0.05 \leq \mathrm{Bo} \leq 0.26$. We also verified the solver for a rising air bubble in water. The algorithm is shown to be second-order accurate, and stable for density and viscosity ratios up to 10,000. Also, we show that our fast Poisson solver is more than ten times faster than the Hypre multigrid solver up to a $1024^3$ grid and 1024 cores. [Preview Abstract] |
Tuesday, November 26, 2013 2:49PM - 3:02PM |
R18.00009: A Controls-CFD Approach for Estimation of Concentration from a Moving Aerial Source: Advantages of a Finite Volume-TVD implementation with Guidance-Based Grid Adaptation Tatiana Egorova, Nikolaos A. Gatsonis, Michael A. Demetriou In this work the process of gas release into the atmosphere by a moving aerial source is simulated and estimated using a sensing aerial vehicle (SAV). The process is modeled with atmospheric advection diffusion equation, which is solved by the finite volume method (FVM). Advective fluxes are constrained using total variation diminishing (TVD) approach. The estimator provides on-line estimates of concentration field and proximity of the source. The guidance of the SAV is dictated by the performance of the estimator. To further improve the estimation algorithm from the computational prospective, the grid is adapted dynamically through local refinement and coarsening. The adaptation algorithm uses the current sensor position as a center of refinement, with the areas further away from the SAV being covered by a coarse grid. This leads to the time varying state matrix of the estimator and the variation depends on the SAV motion. Advantages of the adaptive FVM-TVD implementation are illustrated on the examples of estimator performance for different source trajectories. [Preview Abstract] |
Tuesday, November 26, 2013 3:02PM - 3:15PM |
R18.00010: Interface capturing using a compressive advection method and a compositional modelling approach: Applications Dimitrios Pavlidis, Zhihua Xie, James Percival, Jefferson Gomes, Christopher Pain, Omar Matar Progress on a consistent approach for interface-capturing in which each component represents a different phase/fluid is described. The aim is to develop a general multiphase modelling approach based on fully-unstructured meshes that can exploit the latest mesh adaptivity methods, and in which each fluid phase may have a number of components. The method is based on the P1DG-P2 finite element pair, in which the velocity has a linear discontinuous variation and the pressure has a quadratic continuous variation. The method is compared against experimental results for a collapsing water column test case and a convergence study is performed. The method is then used to simulate horizontal slug flow. [Preview Abstract] |
Tuesday, November 26, 2013 3:15PM - 3:28PM |
R18.00011: Diffused interface ghost fluid method for incompressible multiphase, phase change simulations Moon Soo Lee, Amir Riaz Sharp interface methods for simulating multiphase flow often suffer from unstable pressure and velocity fluctuations for problems involving mass transfer. An improved sharp interface method is developed for multiphase flow with phase change using both sharp and diffused interfacial properties. The approach is based on defining continuous, phase averaged velocity and density fields within a diffused interfacial region while using the sharp treatment for the implementation of the jumps in the pressure and the temperature gradient. The method implements interface advection with diffused and stable velocity field but can represent accurate movement of the sharp interface. Two-dimensional film boiling problems are solved on a horizontal surface to demonstrate the performance of the new approach. [Preview Abstract] |
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