a b s t r a c t Large Eddy Simulation/particle Probability Density Function (LES/PDF) approaches are now well developed, and can be applied to turbulent combustion problems involving complex flows with strong turbulence-chemistry interactions. However, these methods are computationally expensive, restricting their use to simple fuels with relatively small detailed chemical mechanisms. To mitigate the cost in both CPU time and storage requirements, an adaptive strategy tailored for particle PDF methods is presented here, which provides for each particle a specialized reduced representation and kinetic model adjusted to its changing composition. Rather than performing chemical reduction at runtime to determine the optimal set of equations to use for a given particle, an analysis of the composition space likely to be accessed during the combustion simulation is performed in a pre-processing stage using simple Partially Stirred Reactor (PaSR) computations. In the pre-processing stage, the composition space is partitioned into a user-specified number of regions, over which suitable reduced chemical representations and kinetic models are generated automatically using the Directed Relation Graph with Error Propagation (DRGEP) reduction technique. A computational particle in the combustion simulation then carries only the variables present in the reduced representation and evolves according to the reduced kinetic model corresponding to the composition space region the particle belongs to. This region is identified efficiently using a low-dimensional binary-tree search algorithm, thereby keeping the run-time overhead associated with the adaptive strategy to a minimum. The performance of the algorithm is characterized for propane/air combustion in a PaSR with pairwise mixing. The results show that the reduction errors are well controlled by the specified error tolerance, and that the adaptive framework provides significant gains in cost and storage compared to traditional non-adaptive reduction approaches.

Flow over a traveling wavy foil with a passively flapping flat plate has been investigated using a multiblock lattice Boltzmann equation and the immersed boundary method. The foil undergoes prescribed undulations in the lateral direction and the rigid flat plate has passive motion determined by the fluid structure interaction. This simplified model is used to study the effect of the fish caudal fin and its flexibility on the locomotion of swimming animals. The flexibility of the caudal fin is modeled by a torsion spring acting about the pivot at the conjuncture of the wavy foil and the flat plate. The study reveals that the passively oscillating flat plate contributes half of the propulsive force. The flexibility, represented by the nondimensional natural frequency F, plays a very important role in the movement and propulsive force generation of the whole body. When the plate is too flexible, the drag force is observed. As the flat plate becomes more rigid, the propulsive force that is generated when the undulation is confined to last part of the wavy foil becomes larger. The steady movement occurs at F=5. These results are consistent with the observations of some swimming animals in nature.

No abstract

No abstract