Special Joint LCDS and PDE Seminar
Penn State Universiy, University Park, PA 16802 | |
An Unified Energetic Variational Approach | |
Abstract: In this talk, I will discuss several micro-macro coupling models for viscoelastic fluids. The focus will be on the transport and the induced elastic stress.
Center for Fluid Mechanics
And The Fluids, Thermal And Chemical Processes Group
OF
The Division of Engineering
Seminar Series
Department of Physics, Lehigh University, Bethlehem, PA | |
Brown Analysis Seminar
Scientific Computing Seminar
Abstract: The superlinear O(N log N) time requirement is the bottleneck for the Fast Fourier Transform (FFT) to process huge amount of data for sparse signals. We analyze a sublinear RAlSFA (Randomized Algorithm for Sparse Fourier Analysis) that finds a near-optimal B-term Sparse Representation R for a given discrete signal S of length N, in time and space poly(B,log(N)) (instead of O(N logN)). A straightforward implementation of the RAlSFA, as presented in the theoretical paper by Gilbert et al, turns out to be very slow in practice. Our main result is a greatly improved and practical RAlSFA (more than a factor of 4000 times faster than original algorithm!). It beats the FFTW for reasonably large N for sparse signals. We also extend the algorithm to higher dimensional cases. The crossover point lies at N~70000 in one dimension, and at N~ 900 for data on a N*N grid in two dimensions for small B signals. We also find this algorithm is very robust to the noise.
PDE Seminar
in Gradient Reaction-diffusion Systems | |
Brown University
Joint Materials/Solid Mechanics Seminar Series
Physics Department and Courant and Courant Institute, New York University | |
Abstract: The locomotion of most fish and birds is realized by flapping wings or fins transverse to the direction of travel. Here, we study experimentally the dynamics of a wing that is "flapped" up and down but is free to move in the horizontal direction. We show that flapping flight occurs abruptly at a critical flapping frequency as a symmetry-breaking bifurcation. We then investigate the separate effects of flapping frequency, wing thickness and flexibility. We further seek the optimal parameters of the flapping locomotion. In particular, we study at what flapping amplitude the forward flight speed is highest (minimum Strouhal number) and at what amplitude the flapping wing has the lowest threshold to forward flight. Overall, we emphasize the robustness of the thrust-generating mechanisms determining the forward flight speed of a flapping wing, as observed in our experiments.
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