Numerical coupling of fluid and structure in cardiac flow and devices

Numerical simulations are a powerful tool in investigation of flow and structure dynamics in biological systems and in the design of biomedical devices. Time-dependent fluid-structure interaction (FSI) problems in biological systems are often characterized by a periodic nature and relatively low Reynolds number. In order to solve the dynamics of the fluid and structure of coupled systems, different approaches may be used. Several parameters such as geometrical complexity, degree of displacement, convergence to steady periodicity, and the system stability may determine the coupling method. In the talk, four numerical studies of biological and implanted systems will be presented, each with a different FSI approach. The first study is of flow through mechanical heart valves, using finite-volume (FV) fluid solver coupled with an external structural solver using a weak coupling scheme for large displacements. The second study is of flow inside a pulsatile ventricular assist device with FV fluid solver coupled with finite-element (FE) structure solver using a strong staggered coupling assuming small displacements. The third study is of flow through vulnerable plaque in the coronary arteries, with FE solvers for both the fluid and structure domains, using a fully-coupled iterative scheme assuming small displacements. The fourth simulation is of an impedance pump using a direct FE coupling method for large displacements. In addition to the methodology, the applicative design and hemodynamic aspects of the cases will be discussed, including washout properties and risk for thrombosis. The results obtained from the studies will be compared to experimental analyses

Large-Scale Mass Power Spectrum from Peculiar Velocities

This is a brief progress report on a long-term collaborative project to measure the power spectrum (PS) of mass density fluctuations from the Mark III and the SFI catalogs of peculiar velocities. The PS is estimated by applying maximum likelihood analysis, using generalized CDM models with and without COBE normalization. The application to both catalogs yields fairly similar results for the PS. The robust result is a relatively high PS, with P(k)\Omega^{1.2}=(4.5+/-2.0)X10^3 (Mpc/h)^3 at k=0.1 h/Mpc. An extrapolation to smaller scales using the different CDM models gives \sigma_8\Omega^{0.6}=0.85+/-0.2. The general constraint on the combination of cosmological parameters is of the sort \Omega \h_{50}^{\mu} n^{\nu}=0.75+/-0.25, where \mu=1.3 and \nu=3.7,2.0 for \Lambda CDM models with and without tensor fluctuations respectively. For open CDM, without tensor fluctuations, the powers are \mu=0.9 and \nu=1.4.Comment: 3 pages, 1 figure, uses mprocl.sty. To appear in Proceedings of the Eighth Marcel Grossmann Meetin

Fishing in the Stream: Similarity Search over Endless Data

Similarity search is the task of retrieving data items that are similar to a given query. In this paper, we introduce the time-sensitive notion of similarity search over endless data-streams (SSDS), which takes into account data quality and temporal characteristics in addition to similarity. SSDS is challenging as it needs to process unbounded data, while computation resources are bounded. We propose Stream-LSH, a randomized SSDS algorithm that bounds the index size by retaining items according to their freshness, quality, and dynamic popularity attributes. We analytically show that Stream-LSH increases the probability to find similar items compared to alternative approaches using the same space capacity. We further conduct an empirical study using real world stream datasets, which confirms our theoretical results

The Growth of Galaxy Stellar Mass Within Dark Matter Halos

We study the evolution of stellar mass in galaxies as a function of host halo mass, using the "MPA" and "Durham" semi-analytic models, implemented on the Millennium Run simulation. The results from both models are similar. We find that about 45% of the stellar mass in central galaxies in present-day halos less massive than ~10^{12} Msun/h is already in place at z~1. This fraction increases to ~65% for more massive halos. The peak of star formation efficiency shifts toward lower mass halos from z~1 to z~0. The stellar mass in low-mass halos grows mostly by star formation since z~1, while in high-mass halos most of the stellar mass is assembled by mergers. These trends are clear indications of "halo downsizing". We compare our findings to the results of the phenomenological method developed by Zheng, Coil & Zehavi (2007). The theoretical predictions are in qualitative agreement with these results, however there are large discrepancies. The most significant one concerns the amount of stars already in place in the progenitor galaxies at z~1, which is about a factor of two larger in both semi-analytic models. We also use the semi-analytic catalogs to test different assumptions made in that work, and illustrate the importance of smooth accretion of dark matter when estimating the mergers contribution. We demonstrate that methods studying galaxy evolution from the galaxy-halo connection are powerful in constraining theoretical models and can guide future efforts of modeling galaxy evolution. Conversely, semi-analytic models serve an important role in improving such methods.Comment: 13 pages, 8 figures, submitted to Ap

Distributed Sparse Signal Recovery For Sensor Networks

We propose a distributed algorithm for sparse signal recovery in sensor networks based on Iterative Hard Thresholding (IHT). Every agent has a set of measurements of a signal x, and the objective is for the agents to recover x from their collective measurements at a minimal communication cost and with low computational complexity. A naive distributed implementation of IHT would require global communication of every agent's full state in each iteration. We find that we can dramatically reduce this communication cost by leveraging solutions to the distributed top-K problem in the database literature. Evaluations show that our algorithm requires up to three orders of magnitude less total bandwidth than the best-known distributed basis pursuit method

Computational studies of resonance wave pumping in compliant tubes

The valveless impedance pump is a simple design that allows the producion or amplification of a flow without the requirement for valves or impellers. It is based on fluid-filled flexible tubing, connected to tubing of different impedances. Pumping is achieved by a periodic excitation at an off-centre position relative to the tube ends. This paper presents a comprehensive study of the fluid and structural dynamics in an impedance pump model using numerical simulations. An axisymmetric finite-element model of both the fluid and solid domains is used with direct coupling at the interface. By examining a wide range of parameters, the pump's resonance nature is described and the concept of resonance wave pumping is discussed. The main driving mechanism of the flow in the tube is the reflection of waves at the tube boundary and the wave dynamics in the passive tube. This concept is supported by three different analyses: (i) time-dependent pressure and flow wave dynamics along the tube, (ii) calculations of pressure–flow loop areas along the passive tube for a description of energy conversion, and (iii) an integral description of total work done by the pump on the fluid. It is shown that at some frequencies, the energy given to the system by the excitation is converted by the elastic tube to kinetic energy at the tube outlet, resulting in an efficient pumping mechanism and thus significantly higher flow rate. It is also shown that pumping can be achieved with any impedance mismatch at one boundary and that the outlet configuration does not necessarily need to be a tube