Kolmogorov-Kraichnan theory power law of EðkÞ /
k5=3 at wave numbers k 1500 m1. At higher wave
numbers, k 1500 m1, spectra are somewhat steeper
than expected from the direct enstrophy cascade fit of
k3, probably due to higher damping at large wave numbers.
If we define the turbulence forcing wave number from
the position of the kink on the turbulence spectrum, e.g.,
kf 1550 m1 at 60 Hz [Fig. 3(b)], it appears that the
turbulence forcing wave number is roughly twice that of
the surface elevation wave number [Fig. 2(c)]. The energy
injection scale for the horizontal particle transport is thus
related to the oscillon size, which is about half of a period
of the Faraday wave.
The kinetic energy of the horizontal flow can be estimated
as E0 ¼ Rkf
klow EðkÞdk. Here, klow is the lowest wave
number determined by the field of view. In Fig. 3(c), E0 is
shown as a function of the vertical acceleration a. By
changing the vertical acceleration from 0.7 to 2.4 g, we
can vary the kinetic energy in the flow by over 2 orders of
magnitude.
Similar measurements were performed at several frequencies
of the vertical vibrations: 30, 45, and 60 Hz
[Fig. 4(a)]. A k5=3 power law is consistently observed in
the energy spectra at lower wave numbers. A distinct kink
is present in all spectra, and it is associated with the forcing
wave number. The turbulence forcing wave number kf
decreases with the decrease in frequency, in accordance
with the capillary-gravity wave dispersion relation. A
k5=3 range in the horizontal velocity spectra is consistent
with the Kolmogorov-Kraichnan theory.
An important feature of 2D turbulence is the presence of
the inverse energy cascade, or the transfer of energy from
smaller to larger scales in the inertial range rf