550 research outputs found
High-resolution simulations of the flow around an impulsively started cylinder using vortex methods
The development of a two-dimensional viscous incompressible flow generated from a circular cylinder impulsively started into rectilinear motion is studied computationally. An adaptative numerical scheme, based on vortex methods, is used to integrate the vorticity/velocity formulation of the Navier–Stokes equations for a wide range of Reynolds numbers (Re = 40 to 9500). A novel technique is implemented to resolve diffusion effects and enforce the no-slip boundary condition. The Biot–Savart law is employed to compute the velocities, thus eliminating the need for imposing the far-field boundary conditions. An efficient fast summation algorithm was implemented that allows a large number of computational elements, thus producing unprecedented high-resolution simulations. Results are compared to those from other theoretical, experimental and computational works and the relation between the unsteady vorticity field and the forces experienced by the body is discussed
The finite element method in low speed aerodynamics
The finite element procedure is shown to be of significant impact in design of the 'computational wind tunnel' for low speed aerodynamics. The uniformity of the mathematical differential equation description, for viscous and/or inviscid, multi-dimensional subsonic flows about practical aerodynamic system configurations, is utilized to establish the general form of the finite element algorithm. Numerical results for inviscid flow analysis, as well as viscous boundary layer, parabolic, and full Navier Stokes flow descriptions verify the capabilities and overall versatility of the fundamental algorithm for aerodynamics. The proven mathematical basis, coupled with the distinct user-orientation features of the computer program embodiment, indicate near-term evolution of a highly useful analytical design tool to support computational configuration studies in low speed aerodynamics
Turbulence and secondary motions in square duct flow
We study turbulent flows in pressure-driven ducts with square cross-section
through direct numerical simulation in a wide enough range of Reynolds number
to reach flow conditions which are representative of fully developed
turbulence. Numerical simulations are carried out over extremely long
integration times to get adequate convergence of the flow statistics, and
specifically high-fidelity representation of the secondary motions which arise.
The intensity of the latter is found to be in the order of 1-2% of the bulk
velocity, and unaffected by Reynolds number variations. The smallness of the
mean convection terms in the streamwise vorticity equation points to a simple
characterization of the secondary flows, which in the asymptotic high-Re regime
are found to be approximated with good accuracy by eigenfunctions of the
Laplace operator. Despite their effect of redistributing the wall shear stress
along the duct perimeter, we find that secondary motions do not have large
influence on the mean velocity field, which can be characterized with good
accuracy as that resulting from the concurrent effect of four independent flat
walls, each controlling a quarter of the flow domain. As a consequence, we find
that parametrizations based on the hydraulic diameter concept, and
modifications thereof, are successful in predicting the duct friction
coefficient
The role of non-uniqueness in the development of vortex breakdown in tubes
Numerical solutions of viscous, swirling flows through circular pipes of constant radius and circular pipes with throats have been obtained. Solutions were computed for several values of vortex circulation, Reynolds number and throat/inlet area ratio, under the assumptions of steady flow, rotational symmetry and frictionless flow at the pipe wall. When the Reynolds number is sufficiently large, vortex breakdown occurs abruptly with increased circulation as a result of the existence of non-unique solutions. Solution paths for Reynolds numbers exceeding approximately 1000 are characterized by an ensemble of three inviscid flow types: columnar (for pipes of constant radius), soliton and wavetrain. Flows that are quasi-cylindrical and which do not exhibit vortex breakdown exist below a critical circulation, dependent on the Reynolds number and the throat/inlet area ratio. Wavetrain solutions are observed over a small range of circulation below the critical circulation, while above the critical value, wave solutions with large regions of reversed flow are found that are primarily solitary in nature. The quasi-cylindrical (QC) equations first fail near the critical value, in support of Hall's theory of vortex breakdown (1967). However, the QC equations are not found to be effective in predicting the spatial position of the breakdown structure
Coherent structures and secondary motions in open duct flow
This thesis investigates the turbulent secondary motions observed in the straight open and closed duct flow with a rectangular cross-section. The turbulent flow was simulated by means of direct numerical simulations, which resolve all the relevant spatio-temporal scales. Reynolds number and geometrical aspect ratio dependences of the mean secondary flow were investigated with a spacial emphasis upon coherent structure analysis
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