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1.
Passive and active tracers are used to examine particle mixing and reactive front dynamics in an open shallow flow of water past a circular cylinder. A quadtree grid based Godunov-type shallow water equation solver predicts the unsteady flow hydrodynamics of the wake behind the cylinder. The resulting periodic flow field consisting of a von Kármán vortex street is decomposed and stored over one oscillatory period using Singular Value Decomposition (SVD). Particles are advected according to the reconstructed flow field from the SVD modes, with continuous spatial velocity information obtained via bilinear interpolation. Passive particle dynamics driven by different SVD flow modes is investigated, and it is found that the flow field recovered from the mean flow and the first pair of time varying modes is adequate to represent the complicated dynamical properties induced by the original flow field. Active autocatalytic reaction, A + B → 2B, is incorporated into the particle advection model, assuming surface reaction. Active particles are found to trace out an expanded version of the unstable manifold of the chaotic saddle in the wake, in qualitative agreement with published analytical results. The numerical model is applicable to mixing and transport processes in more complicated shallow environmental flows.  相似文献   

2.
A series of Synthetic Aperture Radar (SAR) images acquired during the summer of 2006 revealed island wakes in the lee of the Izu Islands, south of Japan. The wakes were formed of not only meandering disturbances but also a series of eddies with diameters on the order of those of the islands. The Kuroshio flowed near these islands in the summer of 2006, indicating that the island wakes were induced by a Kuroshio-island interaction. Satellite sea-surface temperature (SST) and Chlorophyll-a (Chl-a) images observed during the summer of 2006 revealed low-SST and high Chl-a wakes, some of which included low-SST eddy trains. It is thus inferred that the phenomenon entailed upwelling and mixing processes and biological productivity known as the “island-mass effect.” High-spatial-resolution SST derived by combining a LANDSAT infrared channel and AVHRR SST clearly revealed a well-defined SST front associated with the island wakes and a 1 km-scale low-SST wake pattern. A numerical simulation was performed to investigate the formation mechanism. The simulation qualitatively reproduced the cold-eddy pattern, with eddy-driven mixing developing a mixed layer down to 200 m, causing low-SST island wakes. The shedding frequency and spacing of the model-produced eddies were roughly close to those of the Kármán vortex theory, suggesting that Kármán-type cold-eddy trains are commonly formed behind the islands when the Kuroshio strong flow impinges on them.  相似文献   

3.
The effectiveness of a sensor configuration for feedback flow control on the wake of a circular cylinder is investigated in both direct numerical simulation as well as in a water tunnel experiment. The research program is aimed at suppressing the von Kármán vortex street in the wake of a cylinder at a Reynolds number of 100. The design of sensor number and placement was based on data from a laminar two-dimensional simulation of the Navier-Stokes equations for the unforced condition. A low-dimensional proper orthogonal decomposition (POD) was applied to the vorticity calculated from the flow field and sensor placement was based on the intensity of the resulting spatial eigenfunctions. The numerically generated data was comprised of 70 snapshots taken over three cycles from the steady state regime. A linear stochastic estimator (LSE) was employed to map the velocity data to the temporal coefficients of the reduced order model. The capability of the sensor configuration to provide accurate estimates of the four low-dimensional states was validated experimentally in a water tunnel at a Reynolds number of 108. For the experimental wake, a sample of 200 particle image velocimetry (PIV) measurements was used. Results show that for experimental data, the root mean square estimation error of the estimates of the first two modes was within 6% of the desired values and for the next two modes was within 20% of the desired values. This level of error is acceptable for a moderately robust controller.  相似文献   

4.
The identification of successive stages in the transition of unsteady viscous transonic flow around an aerofoil is carried out by solving the time-dependent Navier-Stokes equations for a compressible fluid in two-dimensional approach. The numerical simulation is carried out at the Mach number range (0.2-0.98). At a fixed Reynolds number (Re=10,000), it is found that this flow undergoes the following four transition steps: It remains steady up to the Mach number values (0.2-0.35) and afterwards it develops spontaneously, without any imposed artificial perturbation, an inherent unsteadiness corresponding to a near-wake von Kármán instability, in the Mach number range (0.35-0.9). It is found that there exists a critical Mach number between the values (0.90-0.95) for which the flow returns to a steady-state. Furthermore, the flow is found to be governed by two instability processes in the Mach number range (0.75-0.8), where, apart from the von Kármán mode (mode I), a lower frequency mode II appears, due to the formation of weakly supersonic alternating zones in the region upstream of the aerofoil, related to the buffeting phenomenon. A triple role played by the increasing compressibility effects to trigger the instability processes, to maintain and to inhibit them in the transonic flow regime is therefore analysed in detail.  相似文献   

5.
In this paper post-buckling analysis of carbon fibre reinforced plastic cylindrical shells under axial compression is considered. Reissner-Mindlin-Von Kármán type shell facet model is used in the computations. The effect of geometric imperfection shape and amplitude on nonlinear analysis results is discussed. Numerical-experimental correlation is performed using the results of experimental buckling tests found in the literature. Results show that bringing the diamond shape geometric imperfection in the model significantly improves the correlation and gives good accuracy in simulating cylindrical shell post-buckling behaviour.  相似文献   

6.
This work describes the development and validation of a parallel high-order compact finite difference Navier–Stokes solver for application to large-eddy simulation (LES) and direct numerical simulation. The implicit solver can employ up to sixth-order spatial formulations and tenth-order filtering. The parallelization of the solver is founded on the overset grid technique. LES were then performed for turbulent channel flow with Reynolds numbers ranging from Re τ=180 to 590, and flow past a circular cylinder with a transitional wake at Re D =3900. The channel flow solutions were obtained using both an implicit LES (ILES) approach and a dynamic sub-grid scale model. The ILES method obtained virtually identical solutions at half the computational cost. The original vector and new parallel solvers produce indistinguishable mean flow solutions for the circular cylinder. Repeating the cylinder simulation on a much finer mesh resulted in significantly better agreement with experimental data in the near wake than the coarse grid solution and other previous numerical studies.  相似文献   

7.
The pulsating cross-flow over a single circular cylinder at the subcritical Reynolds number ReD = 2580 is studied with the large eddy simulation (LES) technique using the standard Smagorinsky model as well as a dynamic model in which the test filtered quantities are evaluated through a truncated Taylor series expansion. The filtered equations are discretised using the finite volume method in an unstructured, collocated grid arrangement with a second-order accurate method, in both space and time. The predictions are compared against very detailed experiments for mean velocities and Reynolds stresses that were performed in a duct of cross-section 72 mm × 72 mm using the PIV technique. The effects of mesh refinement close to the cylinder as well as of subgrid scale model are also examined. The numerical predictions are in very good agreement with the measurements in terms of mean as well as turbulence quantities. The instantaneous flow patterns of the flow field are examined and the effect of the external flow pulsation on the wake characteristics such as vortex formation length, vortex strength, Strouhal number as well as the lift and drag coefficients is quantified. The vortex formation length is decreased while the mean drag, as well as the rms values of the drag and lift coefficients increase significantly under pulsating flow conditions. The performance of the LES technique is analysed in the light of the wake characteristics.  相似文献   

8.
Large-eddy simulations (LES) of the turbulent flow in a swirl tube with a tangential inlet have been performed. The geometry, and flow conditions were chosen according to an experimental study by [Escudier MP, Bornstein J, Zehnder N. Observations and LDA measurements of confined turbulent vortex flow. J Fluid Mech 1980;98:49-63]. Lattice-Boltzmann discretization was used to numerically solve the Navier-Stokes equations in the incompressible limit. Effects of spatial resolution and choices in subgrid-scale modeling were explicitly investigated with the experimental data set as the testing ground. Experimentally observed flow features, such as vortex breakdown and laminarization of the vortex core were well represented by the LES. The simulations confirmed the experimental observations that the average velocity profiles in the entire vortex tube are extremely sensitivity to the exit pipe diameter. For the narrowest exit pipe considered in the simulations, very high average velocity gradients are encountered. In this situation, the LES shows the most pronounced effects of spatial resolution and subgrid-scale modeling.  相似文献   

9.
This paper studies a two-dimensional incompressible viscous flow past a rotating cylinder with cross flow oscillation using a finite element method based on the characteristic based split (CBS) algorithm to solve governing equations including full Navier–Stokes and continuity equations. Dynamic unstructured triangular grid is used employing lineal and torsional spring analogy which is coupled with the solver by an Arbitrary Lagrangian–Eulerian (ALE) formulation. After verifying the accuracy of the numerical code, simulations are conducted for the flow past a rotating cylinder with cross flow oscillation at moderate Reynolds numbers of 50, 100, and 200 considering different non-dimensional rotational speeds based on the free-stream velocity in the range 0–2.5, and various oscillating amplitudes and frequencies. Effects of the oscillation and rotation of the cylinder on the vortex shedding both in lock-on and non-lock-on regions, the mean drag and lift coefficients, and the Strouhal number are investigated in detail. It is found that similar to the fixed cylinder beyond a critical non-dimensional rotational speed the vortex shedding is highly suppressed. In addition, by increasing the rotational speed of the cylinder, the lift coefficient increases while decreasing the drag coefficient. However, in the vortex lock-on region both the lift and the drag coefficients increase significantly.  相似文献   

10.
A numerical method for the solution of the Navier-Stokes equations is developed using an integro-differential formulation of the equations. The method permits the actual computation to be confined to the viscous region of the flow and offers a drastic reduction in the number of data points required in the numerical procedure. The integro-differential formulation is presented along with discussion of the kinetic and kinematic aspects of the problem and the interplay between the two aspects. Results for several parallel flow problems and for the flow past a circular cyliner are presented. For the circular cylinder, it is shown that the introductions of a splitter plate behind the cylinder suppresses vortex shedding.  相似文献   

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