Papers by Ravishankar Subramanya
Users often can not easily express their queries. For example, in a multimedia/image by content s... more Users often can not easily express their queries. For example, in a multimedia/image by content setting, the user might want photographs with sunsets; in current systems, like QBIC, the user has to give a sample query, and to specify the relative importance of color, shape and texture. Even worse, the user might want correlations between attributes, like, for example, in a traditional, medical record database, a medical researcher might want to find "mildly overweight patients", where the implied query would be "weight/height ~ 4 lb/inch". Our goal is to provide a user-friendly, but theoretically solid method, to handle such queries. We allow the user to give several examples, and, optionally, their 'goodness' scores, and we propose a novel method to "guess" which attributes are important, which correlations are important, and with what weight. Our contributions are twofold: (a) we formalize the problem as a minimization problem and show how to ...
modeling of wave breaking induced by fixed or moving boundaries
Recent advances in the BEM modelling of nonlinear water waves
International Journal of Turbo and Jet Engines, 2001
The numerical simulation of unsteady flow in multi-stage turbomachinery is computationally expens... more The numerical simulation of unsteady flow in multi-stage turbomachinery is computationally expensive. A parallel code using message-passing interface libraries, which was developed to reduce the turnaround time and the cost of computation, is presented in this paper. The paper describes the details of the parallel algorithm, including the parallelization rationale, domain decomposition and processor allocation, and communication patterns. The numerical algorithm was used to simulate the unsteady flow in a six-row test turbine. The numerical results present the salient features of the turbine flow, such as the temporal and spatial vari-ation of velocity, pressure, temperature and blade force. To illustrate the advantages of parallel processing, the turnaround time of the turbine flow simulation was compared on several computers where the code was run in parallel and sequential mode. Nomenclature C p -Pressure coefficient F -Force f -Frequency p -Pressure r -Radius T -Temperature γ -Ratio of specific heats of a gas µ -Viscosity ρ -Density τ -Skin friction ω -Angular velocity Subscripts F -Half-amplitude Fourier transform hub -Hub location nd -Non-dimensional tip -Tip location -∞ -Upstream infinity Superscripts * -Total (or stagnation)
2001 Eighteenth IEEE Symposium on Mass Storage Systems and Technologies, 2001
On August 3rd, 2000, the National Science Foundation announced the award of $45 million to the Pi... more On August 3rd, 2000, the National Science Foundation announced the award of $45 million to the Pittsburgh Supercomputing Center to provide "terascale" computing capability for U.S. researchers in all science and engineering disciplines. This Terascale Computing System (TCS) will be built using commodity components. The computational engines will be Compaq Alpha CPU's in a four processors per node configuration. The final system will have over 682 quad processor nodes, for a total of more than 2700 Alpha CPUs. Each node will have 4 gigabytes of memory, for a total system memory of over 2.5 terabytes. All the nodes will be interconnected by a high speed, very low latency Quadrics switch fabric, constructed in a full "fat-tree" topology.
Min-dreader: Query databases through multiple examples
Parallel Computation of Rotor-Stator Interaction
Unsteady Aerodynamics and Aeroelasticity of Turbomachines, 1998
Numerical simulation of rotor-stator interaction is crucial for turboma-chinery improvement since... more Numerical simulation of rotor-stator interaction is crucial for turboma-chinery improvement since it allows the optimization of blade path design. In addition, the numerical simulation of rotor-stator interaction makes it possible to predict the location and variation of hot spots on blades, which is extremely useful in the turbine thermal design. However, simulation of rotor-stator interaction is notorious for being computationally expensive. To reduce the turnaround time and cost/MFLOP, a parallel code based on the message-passing interface libraries was developed. This code runs on symmetric multi-processors (Silicon Graphics Challenge) and massively parallel processors (Cray T3E). The parallel code was used to compute the rotor-stator interaction in two turbine configurations. A super-linear speedup of 15.6 was obtained on a 12 CPU Silicon Graphics Challenge for a geometry distributed over ten processors.
Journal of Waterway, Port, Coastal, and Ocean Engineering, 1994
Shoaling of solitary waves on both gentle (1:35) and steeper slopes (-<1:6.50) is analyzed up to ... more Shoaling of solitary waves on both gentle (1:35) and steeper slopes (-<1:6.50) is analyzed up to breaking using both a fully nonlinear wave model and high-accuracy laboratory experiments. For the mildest slope, close agreement is obtained between both approaches up to breaking, where waves become very asymmetric and breaking indices reach almost twice the value for the largest stable symmetric wave. Bottom friction does not seem to affect the results at all. Wave celerity decreases during shoaling and slightly increases before breaking. At breaking, the crest particle velocity is almost horizontal and reaches 90% of the crest celerity, which is two to three times larger than the bottom velocity. The nonlinear shallow water (NSW) equations and the Boussinesq approximation both fail to predict these results. Finally, shoaling rates for various wave heights and bottom slopes differ from the predictions of Green's or Boussinesq shoaling laws. On the mildest slope, shoaling rates roughly follow a "two-zone" model proposed earlier but on steeper slopes reflection becomes significant and wave heights change little during shoaling.
Journal of Fluid Mechanics, 2000
A Boussinesq-type model is derived which is accurate to O(kh)4 and which retains the full represe... more A Boussinesq-type model is derived which is accurate to O(kh)4 and which retains the full representation of the fluid kinematics in nonlinear surface boundary condition terms, by not assuming weak nonlinearity. The model is derived for a horizontal bottom, and is based explicitly on a fourth-order polynomial representation of the vertical dependence of the velocity potential. In order to achieve a (4,4) Padé representation of the dispersion relationship, a new dependent variable is defined as a weighted average of the velocity potential at two distinct water depths. The representation of internal kinematics is greatly improved over existing O(kh)2 approximations, especially in the intermediate to deep water range. The model equations are first examined for their ability to represent weakly nonlinear wave evolution in intermediate depth. Using a Stokes-like expansion in powers of wave amplitude over water depth, we examine the bound second harmonics in a random sea as well as nonline...
Computational Mechanics, 1996
In this paper, several numerical aspects of an existing model for fully nonlinear waves are impro... more In this paper, several numerical aspects of an existing model for fully nonlinear waves are improved and validated to study ware breaking due to shoaling over a gentle plane slope and wave breaking induced by a moving lateral boundary. The model is based on fully nonlinear potential flow theory and combines a higher-order Boundary Element Method (BEM) for solving Laplace's equation at a given time and Lagrangian Taylor expansions for the time updating of the free surface position and potential. An improved numerical treatment of the boundary conditions at the intersection between moving lateral boundaries and the free surface (corner) is implemented and tested in the model, and the free surface interpolation method is also improved to better model highly curved regions of the free surface that occur in breaking waves. Finally, a node regridding technique is introduced to improve the resolution of the solution dose to moving boundaries and in breaker jets. Examples are presented for solitary wave propagation, shoaling, and breaking over a 1:35 slope and for wave breaking induced by a moving vertical boundary. Using the new methods, both resolution and extent of computations are significantly improved compared to the earlier model, for similar computational efforts. In all cases computations can be carried out up to impact of the breaker jets on the free surface.
Engineering Analysis with Boundary Elements, 1994
The model by Grilli eta/., 5,s based on fully nonlinear potential flow equations, is used to stud... more The model by Grilli eta/., 5,s based on fully nonlinear potential flow equations, is used to study propagation of water waves over arbitrary bottom topography. The model combines a higher-order boundary element method for the solution of Laplace's equation at a given time, and Lagrangian Taylor expansions for the time updating of the free surface position and potential. In this paper, both the accuracy and the efficiency of computations are improved, for wave shoaling and breaking over gentle slopes, in domains with very sharp geometry and large aspect ratio, by using quasi-singular integration techniques based on modified Telles 17 and Lutz H methods. Appfcations are presented that demonstrate the accuracy and the efllciency of the new approaches.
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Papers by Ravishankar Subramanya