The water environment of swimming fish in nature is always complex which includes various vortices and fluctuations. In order to study the interaction between the fish and its surrounding complex flow, the physical model with a D-section cylinder placed at the front of a flapping foil is employed. The D-section cylinder is used to produce vortices to contact with the foil as well as the vortices shed from the foil. According to the experimental work of Gopalkrishnan et al., there are three interaction modes between vortices shed from the cylinder and the flapping foil, which are expanding wake, destructive interaction and constructive interaction. Here in this article, three of those typical cases are picked up to reproduce the vortices interaction modes with the modified immersed boundary methods and their hydrodynamic performances are studied further. Results show that, for expanding wake mode and destructive interaction mode, the incoming vortices contact with the foil strongly, inducing relative low pressure domains at the leading-edge of the foil and enlarging the thrust of foils. For constructive mode, the foil slalom between the shed vortices from the D-section cylinder do not contact with them obviously and the foil＇s thrust is only enlarged a little.
In this article,we employ a fully-resolved numerical simulation method（the fictitious domain method）to investigate the effects of large neutrally-buoyant particles on the turbulent flow in a pipe at low Reynolds number and non-dilute regimes.The tube Reynolds number is fixed to be 4 900,the particle-pipe diameter ratio is 0.1,and the particle volume fraction ranges from 0.33%to 10%.Our results indicate that the presence of large particles decreases the maximum root-of-mean-square（rms）of the streamwise velocity fluctuation near the wall by weakening the intensity of large-scale streamwise vortices,although in the region very close to the wall the particles increase the rms of streamwise velocity fluctuation.On the other hand,the particles induce small-scale vortices in the near-wall region,resulting in the enhancement of the rms of radial and circumferential velocity fluctuations there.
In this paper, we combine the direct-forcing fictitious domain （DF/FD） method and the sharp interface method to resolve the problem of particle dielectrophoresis in two dimensions. The flow field and the motion of particles are solved with the DF/FD method, the electric field is solved with the sharp inter- face method, and the electrostatic force on the particles is computed using the Maxwell stress tensor method. The proposed method is validated via three problems： effective conductivity of particle compos- ite between two planar plates, cell trapping in a channel, and motion of particles due to both conventional and traveling wave dielectrophoretic forces.
