The transition to turbulence in the incompressible flow around a NACA0012 wing at high incidence is studied by DNS in the Reynolds number range 800-10 000. Two main routes are identified for the two-dimensional transition mechanisms: that to aperiodicity beyond the von Kármán mode via a period-doubling scenario and the development of a shear-layer instability, forced by the fundamental oscillation of the separation point downstream of the leading edge. The evolution of the global parameters as well as the variation law of the shear-layer instability wavelength are quantified. The history of the three-dimensional transition mechanisms from a nominally two-dimensional flow structure is identified beyond the first bifurcation, as well as the preferred spanwise wavelengths.
The time-history of the development of the three-dimensional transition features in
a nominally two-dimensional flow configuration is established for Reynolds number
220 in a cylinder wake. The identification of the successive stages that evolve very fast
during experiments is possible by means of direct numerical simulation. The physical
processes related to the creation of streamwise and vertical vorticity components and
their impact on the spanwise waviness of the main von Kármán vortex filaments
are analysed by means of the Craik–Leibovich shearing instability mechanism and
a comparative discussion is given with respect to the elliptic stability theory. This
study proves the existence of a further stage in the three-dimensional transition,
which substantially modifies the regular spanwise undulation. This is a systematic
and repetitive development of natural vortex dislocations in the near wake. The
definition of this kind of structure is provided, as well as its properties related to a
drastic reduction of the fundamental frequency and to the selection of a lower path
in the Strouhal–Reynolds number relation. The induced amplitude modulation of the
flow properties along the span is also evaluated. Quantification of these properties is
carried out by using wavelet analysis and autoregressive modelling of the time series.
The reasons for the development of natural vortex dislocations are analysed and
related to specific modulations of the spanwise structure of the longitudinal velocity
upstream separation. From this part of the study an optimum shape for the spanwise
distribution of this component can be specified, able to trigger the vortex dislocations
in wake flows and therefore useful to apply in the context of stability theory analyses
and in further DNS studies.
The present study analyses the successive transition steps in the flow around a high-lift wing configuration, as the Reynolds number increases in the low and moderate range (800-10,000), by the Navier-Stokes approach. The flow system is mainly governed by two kinds of organised modes appearing successively as the Reynolds number increases: the von Kàrmàn and the shear layer mode. A period-doubling scenario characterises the first 2D stages of the von Kàrmàn mode up to Reynolds number 2000, where the shear-layer mode becomes predominant. The successive stages of the 3D transition are also analysed in detail. In a second step, the effect of wall suction has been studied both in 2D and 3D flows around the NACA0012 airfoil at 20˚of incidence and a Reynolds number of 800. This study has the objective to optimise the aerodynamic coefficients and to attenuate the mentioned 3D transition effects in the near wake. The receptivity of the flow to the suction is clearly shown and the suction position on the wall has been optimised according to the improvement of the aerodynamics coefficients.
Since a few years, a new wind measurement instrument has been competing with standard cup anemometers: the LiDAR. The performances of this instrument over complex terrain are still a matter of debate and this is mainly due to the flow homogeneity assumption made by the instrument. In this work, the error caused by this hypothesis was evaluated with the help of OpenFOAM 1.7, MeteoDyn WT 4.0 and WAsP Engineering for a LiDAR deployed on a complex site covered with dense forest. The assessment of the CFD model firstly revealed the significant impact of both the location and nature of the inlet boundary condition. Despite the presence of terrain complexity within a radius of 340 m around the remote sensor, an averaged error of less than 3% was observed, suggesting that the LiDAR is only affected by topographic variations in the immediate vicinity of the scanned volume.
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