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Volumetric velocimetry study in a transitional wall jet flow with passive flow control via flaps
Birds are remarkably good flyers and show very special adaptations in their wings for stall delay. The pop-up of some cover feathers during starting and landing gave the idea for the present study to investigate the influence on a wall jet when inserting an array with flaps made of elastomer foil. In a wall jet with Re = 420 a flat plate and two different flap arrays (with a foil thickness of 100 and 200 µm) are measured by a time resolved 3D scanning PIV with 20 laser sheets. 2-dimensional analyses show the forming rolers between the jet flow and the surrounding fluid with a fundamental frequency of 13-14 Hz and the characteristically vortex pairing. By inserting the flap array the jet wallnormal spreading gets intensified and the vortex interaction process results in cooperative formation of larger vortices. The 3-dimensional analyses verify these results and show high 3-dimensional vortical structures which are growing when passing over a flap array. In case of the inserted flap array the vortex pairing process was delayed and accumulation of spanwise vorticity was forced to happen over the first rows of flaps, thus forming the larger structures. Already the used flap array configurations showed a significant impact influence on the jet evolution and the non-linear instabilities. Further investigations will analyze the influence of more parameters as the flap geometry or the distance to the jet flow and nozzle outlet
The interaction of helical tip and root vortices in a wind turbine wake
Analysis of the helical vortices measured behind a model wind turbine in a water channel are reported. Phase-locked measurements using planar particle image ve- locimetry are taken behind a Glauert rotor to investigate the evolution and breakdown of the helical vortex structures. Existing linear stability theory predicts helical vortex filaments to be susceptible to three unstable modes. The current work presents tip and root vortex evolution in the wake for varying tip speed ratio and shows a breaking of the helical symmetry and merging of the vortices due to mutual inductance between the vortical filaments. The merging of the vortices is shown to be steady with rotor phase, however, small-scale non-periodic meander of the vortex positions is also ob- served. The generation of the helical wake is demonstrated to be closely coupled with the blade aerodynamics, strongly influencing the vortex properties which are shown to agree with theoretical predictions of the circulation shed into the wake by the blades. The mutual inductance of the helices is shown to occur at the same non-dimensional wake distance
Direct numerical simulation of the oscillatory flow around a sphere resting on a rough bottom
The oscillatory flow around a spherical object lying on a rough bottom is
investigated by means of direct numerical simulations of continuity and
Navier-Stokes equations. The rough bottom is simulated by a layer/multiple
layers of spherical particles, the size of which is much smaller that the size
of the object. The period and amplitude of the velocity oscillations of the
free stream are chosen to mimic the flow at the bottom of sea waves and the
size of the small spherical particles falls in the range of coarse sand/very
fine gravel. Even though the computational costs allow only the simulation of
moderate values of the Reynolds number characterizing the bottom boundary
layer, the results show that the coherent vortex structures, shed by the
spherical object, can break-up and generate turbulence, if the Reynolds number
of the object is sufficiently large. The knowledge of the velocity field allows
the dynamics of the large scale coherent vortices shed by the object to be
determined and turbulence characteristics to be evaluated. Moreover, the forces
and torques acting on both the large spherical object and the small particles,
simulating sediment grains, can be determined and analysed, thus laying the
groundwork for the investigation of sediment dynamics and scour developments.Comment: 35 pages, 21 figure
Algebraic disturbances and their consequences in rotating channel flow transition
It is now established that subcritical mechanisms play a crucial role in the
transition to turbulence of non-rotating plane shear flows. The role of these
mechanisms in rotating channel flow is examined here in the linear and
nonlinear stages. Distinct patterns of behaviour are found: the transient
growth leading to nonlinearity at low rotation rates , a highly chaotic
intermediate regime, a localised weak chaos at higher , and complete
stabilization of transient disturbances at very high . At very low ,
the transient growth amplitudes are close to those for non-rotating flow, but
Coriolis forces already assert themselves by producing distinct asymmetry about
the channel centreline. Nonlinear processes are then triggered, in a
streak-breakdown mode of transition. The high regimes do not show these
signatures, here the leading eigenmode emerges as dominant in the early stages.
Elongated structures plastered close to one wall are seen at higher rotation
rates. Rotation is shown to reduce non-normality in the linear operator, in an
indirect manifestation of Taylor--Proudman effects. Although the critical
Reynolds for exponential growth of instabilities is known to vary a lot with
rotation rate, we show that the energy critical Reynolds number is insensitive
to rotation rate. It is hoped that these findings will motivate experimental
verification, and examination of other rotating flows in this light
Mean flow stability analysis of oscillating jet experiments
Linear stability analysis is applied to the mean flow of an oscillating round
jet with the aim to investigate the robustness and accuracy of mean flow
stability wave models. The jet's axisymmetric mode is excited at the nozzle lip
through a sinusoidal modulation of the flow rate at amplitudes ranging from 0.1
% to 100 %. The instantaneous flow field is measured via particle image
velocimetry and decomposed into a mean and periodic part utilizing proper
orthogonal decomposition. Local linear stability analysis is applied to the
measured mean flow adopting a weakly nonparallel flow approach. The resulting
global perturbation field is carefully compared to the measurements in terms of
spatial growth rate, phase velocity, and phase and amplitude distribution. It
is shown that the stability wave model accurately predicts the excited flow
oscillations during their entire growth phase and during a large part of their
decay phase. The stability wave model applies over a wide range of forcing
amplitudes, showing no pronounced sensitivity to the strength of nonlinear
saturation. The upstream displacement of the neutral point and the successive
reduction of gain with increasing forcing amplitude is very well captured by
the stability wave model. At very strong forcing (>40%), the flow becomes
essentially stable to the axisymmetric mode. For these extreme cases, the
prediction deteriorates from the measurements due to an interaction of the
forced wave with the geometric confinement of the nozzle. Moreover, the model
fails far downstream in a region where energy is transferred from the
oscillation back to the mean flow. This study supports previously conducted
mean flow stability analysis of self-excited flow oscillations in the cylinder
wake and in the vortex breakdown bubble and extends the methodology to
externally forced convectively unstable flows.Comment: submitted to the Journal of Fluid Mechanic
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