![Hier klicken, um den Treffer aus der Auswahl zu entfernen](images/unchecked.gif) |
Titel |
Sub-harmonic frequency lock-in and vortex shedding synchronization due to regular and irregular surface waves. |
VerfasserIn |
Hans Gunnoo, Nizar Abcha, Anne-Claire Bennis, Daniel Levacher |
Konferenz |
EGU General Assembly 2017
|
Medientyp |
Artikel
|
Sprache |
en
|
Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 19 (2017) |
Datensatznummer |
250151152
|
Publikation (Nr.) |
EGU/EGU2017-15707.pdf |
|
|
|
Zusammenfassung |
The influence of both regular and irregular surface waves on a turbulent Von Karman vortex
street has been investigated experimentally with the help of the phase modulation of the
signals. The experiments have been performed in a hydrodynamic flume, where the Von
Karman Street created behind a vertical cylinder is created by a steady current. The
surface waves propagating upstream are excited by a computer controlled wave
maker. It is found that the regular surface wave can give rise to different regimes
of vortex shedding in the Von Karman Street: Sub-harmonic frequency lock-in
arise if the frequency of the wave is approximately two times that of the vortex
shedding frequency. We also observed another regime called harmonic frequency
lock-in when the shedding frequency coincides with the frequency of the surface
wave.
Phase synchronization can also be obtained with irregular surface waves. In
this case, the peak frequency of the applied surface wave spectrum is twice that
of the vortex shedding frequency. The relationship between the vortex shedding
frequency and the amplitude of the surface waves has been studied. It was found that an
increase in the amplitude of the applied waves causes the shedding frequency to
decrease and tends to half of the frequency of the applied wave. For a more detailed
study of this effect, Hilbert transformation is applied. Using this transformation, the
continuous phases of the surface waves fw and that of the Von Karman vortex street fk
are calculated. The relation between the phases combination F= 2fk- fwand the
surface wave amplitude is studied. It was found that the time-evolution of F has a
step-like shape. The time interval of F having a slope around 0, corresponds to
synchronization-like effect. This explains the changes obtained in the spectrum of the
turbulent vortex shedding with an increase in the amplitude of the irregular surface
waves.
We then decided to compare the evolution of the scouring and the sandy patterns created
in the wake of the cylinder for three different regimes: current only, sub-harmonic frequency
lock-in and harmonic frequency lock-in. For each regime the time evolution of the
bed profiles have been observed in a region of 1mx0.35m around the cylinder. It
was found that sub-harmonic frequency lock-in regime leads to the intensification
of vortices and the formation of a more regular sand ripple structure behind the
cylinder compared to that created by current only. Furthermore, in the harmonic
frequency lock-in regime, the sand ripples generated are more important than in
the first and second regimes. We can observe that the increase in sand ripples is
due to the intensification of vortices in the wake of the cylinder caused by surface
waves.
Keywords: Surface waves, von Karman Street, Frequency lock-in, Synchronization,
Scouring |
|
|
|
|
|