Abstracts - KTH Mechanics

130
Global drag fluctuations of disks having different sizes in a
turbulent jet : averaging effect of the turbulent scales
B. Thiria ∗ , J. -F. Beaudoin † and O. Cadot ∗ .
The fluctuating drag of a disk δF(t) with a surface S (the corresponding diameter
vary between 1cm and 5cm) facing a turbulent jet (of radial size 10cm, velocity
U = 10m/s, Reynolds number Re = 65000) is measured with a piezoelectric transducer.
A special care was taken in order to increase the frequency response of the
drag fluctuations that crucially depends on the transducer holding stand. The transfer
function is measured with a white noise electromagnetic excitation and shows the
measurements to be reliable for frequencies up to 1Khz. The fundamental question is:
how are the local turbulent fluctuations averaged in space and time to form the resulting
global drag fluctuation on the disk ? As far as we know there are not much studies
devoted to this while it is of great interest for industrial applications. In the present
study, the experimental parameter being the disk surface S and the drag an extensive
variable, we focus on the drag divided by S, sayδF(t)/S. The first important
result is that whatever the disk surface the standard deviation δF rms /S is constant.
However, the skewness of the fluctuations are significantly reduced as the disk surface
increases showing somehow a filtering effect. The filtering effect is confirmed by a
cut-off at lower frequencies for larger disk’s surface. The effect is also accompanied
by an increase of energy contained at low frequencies. This redistribution of energy
fluctuations is equivalent to the constancy of standard deviation.
∗Unité deMécanique, Ecole Nationale Supérieure de Techniques Avancées, Chemin de la Hunière,
91761 Palaiseau Cedex, France.
† Department of Research and Innovation, PSA Peugeot-Citroën, 2 route de Gisy, 78943 Vélizy-
Villacoublay, France.
Figure 1: (a): Density probability function (PDF) of the drag reduced by its standard
deviation. (b): power spectrum of drag fluctuations δF(t)/S. For both cases, the
arrows signalize the increase of the surface S.