1,721,049 research outputs found
Optimal perturbations and minimal defects
No full textTwo possible initial paths of transition to turbulence in simple
shear flows are examined. The first is the – by now classical –
transient (or algebraic) growth scenario which may have an important role
in the by-pass transition of those flows for which traditional
eigen-analysis predicts asymptotic stability. Transient growth is
optimally excited by certain initial disturbances now known as
"optimal perturbations"; they can be found through a classical variational
analysis initiated by Farrell (1988). The second path starts with the
exponential amplification, in nominally subcritical conditions, of
modal disturbances developing over a base flow mildly distorted with
respect to its idealized counterpart. The base flow distortion of
given norm that excites the largest growth of the instability wave is
called the “minimal defect", and its study was initiated by Bottaro et al. (2003). Both paths provide feasible initial conditions
for the transition process and it is likely that in most practical
situations algebraic and exponential growth mechanisms are concurrently at
play in provoking transition to turbulence in shear flows
Apparent slip and drag reduction for the flow over superhydrophobic and lubricant-impregnated surfaces
No full textThe motion of a liquid above textured surfaces, impregnated with either a gas or a lubricant oil, is considered by homogenizing the microscopic near-wall flow to obtain protrusion heights (or Navier slip lengths), which can then be used in (macroscopic) direct numerical simulations of turbulence. The microstructure of the wall consists in longitudinally aligned ridges; this renders the small-scale Stokes problem amenable to decoupling into two problems, solved by the boundary element method with full account of the deformation of the liquid-lubricant interface, finally yielding two different protrusion heights, a longitudinal and a transverse one, respectively h|| and h. Such heights are then employed in direct numerical simulations of the turbulent flow in a channel bounded by superhydrophobic or lubricant-impregnated walls, demonstrating the reduction in skin-friction drag which can be achieved with the rise in protrusion height difference, Δh=h||-h. The results are in good agreement with an analytical approximation, provided Δh+ (Δh scaled in wall units) remains below 2 and the protrusion heights small in magnitude
Three dimensional mixed convection in an inclined pipe
No full textMixed convection in a cylindrical channel inclined at different angles has been studied computationally. A three-dimensional approach is essential for retaining the features of the flow, which include separation at the wall, fluid moving along spiral paths, and asymmetry. Such characteristics have considerable influence on heat transfer and pressure drop
Flow Separation Control on a NACA0012 Airfoil via a Porous, Compliant Coating
No full textThe purpose of this paper is to study the effect of a porous, compliant, anisotropic coating on the fluid-dynamical performance of a NACA0012 airfoil, under conditions of boundary-layer separation. This fluid-structure interaction problem is studied computationally in two dimensions by performing a detailed parametric study of the coating, with respect to its physical dimensions, characteristics and placement on the airfoil. Aerodynamic performances are quantified in terms of the non-dimensional mean drag and lift coefficients. The configuration described here is a separated flow at Re = 1100 and alpha = 70 degrees. Coating parameters are found which decrease the amplitude of the drag oscillations by about 11% and increase the mean lift by about 9%
Goertler vortices: are they amenable to local eigenvalue analysis?
No full textEUROPEAN JOURNAL OF MECHANICS B/FLUID
Laminar flow in a channel bounded by porous/rough walls: Revisiting Beavers-Joseph-Saffman
No full textFlow through porous metamaterials formed by TPMS-based unit cells: Effects of advection
No full textThe design of metamaterials based on triply periodic minimal surfaces (TPMS) is currently a very active field of research. An upscaling approach is used here to study the flow in TPMS-based porous media, with focus on the effects of advection. The effective medium permeability, function of the Reynolds number Re of the flow through the pores, is numerically evaluated for varying porosity & theta;, for six types of TPMS-based structures, namely Gyroid, I-WP, Schwarz P, Split P, Fischer-Koch S, and Neovius. Inertial effects are found to be significant; for instance, the permeability is reduced by 15 - 50% (according to the surface type) as Re increases from 0 to 50000, when & theta; = 0.98.& COPY; 2023 Elsevier Masson SAS. All rights reserved
Refraction of instability waves in non-parallel boundary layers and their adjoint-based receptivity analysis
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