76 research outputs found
Directed percolation in aerodynamics: resolving laminar separation bubble on airfoils
In nature, phase transitions prevail amongst inherently different systems,
while frequently showing a universal behavior at their critical point. As a
fundamental phenomenon of fluid mechanics, recent studies suggested
laminar-turbulent transition belonging to the universality class of directed
percolation. Beyond, no indication was yet found that directed percolation is
encountered in technical relevant fluid mechanics. Here, we present first
evidence that the onset of a laminar separation bubble on an airfoil can be
well characterized employing the directed percolation model on high fidelity
particle image velocimetry data. In an extensive analysis, we show that the
obtained critical exponents are robust against parameter fluctuations, namely
threshold of turbulence intensity that distinguishes between ambient flow and
laminar separation bubble. Our findings indicate a comprehensive significance
of percolation models in fluid mechanics beyond fundamental flow phenomena, in
particular, it enables the precise determination of the transition point of the
laminar separation bubble. This opens a broad variety of new fields of
application, ranging from experimental airfoil aerodynamics to computational
fluid dynamics.Comment: 8 pages, 11 figure
Description of laminar-turbulent transition of an airfoil boundary layer measured by differential image thermography using directed percolation theory
Transition from laminar to turbulent flow is still a challenging problem.
Recent studies indicate a good agreement when describing this phase transition
with the directed percolation theory. This study presents a new experimental
approach by means of differential image thermography (DIT) enabling to
investigate this transition on the suction side of a heated airfoil. The
results extend the applicability of the directed percolation theory to describe
the transition on curves surfaces.
The experimental effort allows for the first time an agreement between all
three universal exponents of the (1+1)D directed percolation for such airfoil
application. Furthermore, this study proves that the theory holds for a wide
range of flows, as shown by the various conditions tested. Such a large
parameter space was not covered in any examination so far. The findings
underline the significance of percolation models in fluid mechanics and show
that this theory can be used as a high precision tool for the problem of
transition to turbulence.Comment: 9 pages, 8 figure
Detailed analysis of the blade root flow of a horizontal axis wind turbine
The root flow of wind turbine blades is subjected to complex physical
mechanisms that influence significantly the rotor aerodynamic performance.
Spanwise flows, the Himmelskamp effect, and the formation of the root vortex
are examples of interrelated aerodynamic phenomena that take place in the
blade root region. In this study we address those phenomena by means of
particle image velocimetry (PIV) measurements and Reynolds-averaged
Navier–Stokes (RANS) simulations. The numerical results obtained in this
study are in very good agreement with the experiments and unveil the details
of the intricate root flow. The Himmelskamp effect is shown to delay the
stall onset and to enhance the lift force coefficient Cl even at moderate angles of attack. This improvement in the aerodynamic performance
occurs in spite of the negative influence of the mentioned effect on the
suction peak of the involved blade sections. The results also show that the
vortex emanating from the spanwise position of maximum chord length rotates
in the opposite direction to the root vortex, which affects the wake
evolution. Furthermore, the aerodynamic losses in the root region are
demonstrated to take place much more gradually than at the tip
On different cascade-speeds for longitudinal and transverse velocity increments
We address the problem of differences between longitudinal and transverse
velocity increments in isotropic small scale turbulence. The relationship of
these two quantities is analyzed experimentally by means of stochastic
Markovian processes leading to a phenomenological Fokker- Planck equation from
which a generalization of the Karman equation is derived. From these results, a
simple relationship between longitudinal and transverse structure functions is
found which explains the difference in the scaling properties of these two
structure functions.Comment: 4 pages, 5 figures, now with corrected postscrip
Mandelbrot set in coupled logistic maps and in an electronic experiment
We suggest an approach to constructing physical systems with dynamical
characteristics of the complex analytic iterative maps. The idea follows from a
simple notion that the complex quadratic map by a variable change may be
transformed into a set of two identical real one-dimensional quadratic maps
with a particular coupling. Hence, dynamical behavior of similar nature may
occur in coupled dissipative nonlinear systems, which relate to the Feigenbaum
universality class. To substantiate the feasibility of this concept, we
consider an electronic system, which exhibits dynamical phenomena intrinsic to
complex analytic maps. Experimental results are presented, providing the
Mandelbrot set in the parameter plane of this physical system.Comment: 9 pages, 3 figure
On the universality of small scale turbulence
The proposed universality of small scale turbulence is investigated for a set
of measurements in a cryogenic free jet with a variation of the Reynolds number
(Re) from 8500 to 10^6. The traditional analysis of the statistics of velocity
increments by means of structure functions or probability density functions is
replaced by a new method which is based on the theory of stochastic Markovian
processes. It gives access to a more complete characterization by means of
joint probabilities of finding velocity increments at several scales. Based on
this more precise method our results call in question the concept of
universality.Comment: 4 pages, 4 figure
Stochastic analysis of surface roughness
For the characterization of surface height profiles we present a new
stochastic approach which is based on the theory of Markov processes. With this
analysis we achieve a characterization of the complexity of the surface
roughness by means of a Fokker-Planck or Langevin equation, providing the
complete stochastic information of multiscale joint probabilities. The method
was applied to different road surface profiles which were measured with high
resolution. Evidence of Markov properties is shown. Estimations for the
parameters of the Fokker-Planck equation are based on pure, parameter free data
analysis
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