1,114 research outputs found
Theory and Application of Hot-wire Instruments in the Investigation of Turbulent Boundary Layers
Characteristics of turbulence in boundary layer with zero pressure gradient
The results of an experimental investigation of a turbulent boundary layer with zero pressure gradient are presented. Measurements with the hot-wire anemometer were made of turbulent energy and turbulent shear stress, probability density and flattening factor of u-fluctuation (fluctuation in x-direction), spectra of turbulent energy and shear stress, and turbulent dissipation. The importance of the region near the wall and the inadequacy of the concept of local isotropy are demonstrated. Attention is given to the energy balance and the intermittent character of the outer region of the boundary layer. Also several interesting features of the spectral distribution of the turbulent motions are discussed
Characteristics of turbulence in a boundary layer with zero pressure gradient
The results of an experimental investigation of a turbulent boundary layer with zero pressure gradient are presented. Measurements with the hot-wire anemometer were made of turbulent energy and turbulent shear stress, probability density and flattening factor of u-fluctuation, spectra of turbulent energy and shear stress, and turbulent dissipation. The importance of the region near the wall and the inadequacy of the concept of local isotropy are demonstrated. Attention is given to the energy balance and the intermittent character of the outer region of the boundary layer. Also several interesting features of the spectral distribution of the turbulent motions are discussed
Optimal streaks in a Falkner-Skan boundary layer
This paper deals with the optimal streaky perturbations (which maximize the
perturbed energy growth) in a wedge flow boundary layer. These three
dimensional perturbations are governed by a system of linearized boundary layer
equations around the Falkner-Skan base flow. Based on an asymptotic analysis of
this system near the free stream and the leading edge singularity, we show that
for acute wedge semi-angle, all solutions converge after a streamwise transient
to a single streamwise-growing solution of the linearized equations, whose
initial condition near the leading edge is given by an eigenvalue problem first
formulated in this context by Tumin (2001). Such a solution may be regarded as
a streamwise evolving most unstable streaky mode, in analogy with the usual
eigenmodes in strictly parallel flows, and shows an approximate
self-similarity, which was partially known and is completed in this paper. An
important consequence of this result is that the optimization procedure based
on the adjoint equations heretofore used to define optimal streaks is not
necessary. Instead, a simple low-dimensional optimization process is proposed
and used to obtain optimal streaks. Comparison with previous results by Tumin
and Ashpis (2003) shows an excellent agreement. The unstable streaky mode
exhibits transient growth if the wedge semi-angle is smaller than a critical
value that is slightly larger than , and decays otherwise. Thus the
cases of right and obtuse wedge semi-angles exhibit less practical interest,
but they show a qualitatively different behavior, which is briefly described to
complete the analysis
Investigation of Separation of the Turbulent Boundary Layer
An investigation was conducted on a turbulent boundary layer near a smooth surface with pressure gradients sufficient to cause flow separation. The reynolds number was high, but the speeds were entirely within the incompressible flow range. The investigation consisted of measurements of mean flow, three components of turbulence intensity, turbulent shearing stress, and correlations between two fluctuation components at a point and between the same component of different points. The results are given in the form of tables and graphs. The discussion deals first with separation and then with the more fundamental question of basic concepts of turbulent flow
Some Features of Artificially Thickened Fully Developed Turbulent Boundary Layers with Zero Pressure Gradient
Report gives an account of an investigation conducted to determine the feasibility of artificially thickening a turbulent boundary layer on a flat plate. A description is given of several methods used to thicken artificially the boundary layer. It is shown that it is possible to do substantial thickening and obtain a fully developed turbulent boundary layer, which is free from any distortions introduced by the thickening process, and, as such, is a suitable medium for fundamental research
Some features of artificially thickened fully developed turbulent boundary layers with zero pressure gradient
Anisotropic eddy-viscosity concept for strongly detached unsteady flows
The accurate prediction of the flow physics around bodies at high Reynolds number is a challenge in aerodynamics nowadays. In the context of turbulent flow modeling, recent advances like large eddy simulation (LES) and hybrid methods [detached eddy simulation (DES)] have considerably improved the physical relevance of the numerical simulation. However, the LES approach is still limited to the low-Reynolds-number range concerning wall flows. The unsteady Reynolds-averaged Navier–Stokes (URANS) approach remains a widespread and robust methodology for complex flow computation, especially in the near-wall region. Complex statistical models like second-order closure schemes [differential Reynolds stress modeling (DRSM)] improve the prediction of these properties and can provide an efficient simulationofturbulent stresses. Fromacomputational pointofview, the main drawbacks of such approaches are a higher cost, especially in unsteady 3-D flows and above all, numerical instabilities
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