arXiv subjects

Michael K. Rivera

Publications and source records attributed to Michael K. Rivera.

4 recordsLinked to original sources

arXiv2009

Mixing in stratified gravity currents: Prandtl mixing length

Shear-induced vertical mixing in a stratified flow is a key ingredient of thermohaline circulation. We experimentally determine the vertical flux of momentum and density of a forced gravity current using high-resolution velocity and density measurements. A constant eddy viscosity model provides a poor description of the physics of mixing, but a Prandtl mixing length model relating momentum and density fluxes to mean velocity and density gradients works well. For $ \approx 0.08$ and $Re_λ\approx 100$, the mixing lengths are fairly constant, about the same magnitude, comparable to the turbulent shear length.

physics.flu-dyn↗

arXiv2007

Eulerian and Lagrangian velocity statistics in weakly forced two-dimensional turbulence

We present statistics of velocity fluctuations in both the Lagrangian and Eulerian frame for weakly driven two-dimensional turbulence. We find that simultaneous inverse energy and enstrophy ranges present in the Lagrangian and Eulerian Fourier spectra are not directly echoed in real-space moments of velocity difference. The spectral ranges, however, do line up very well with ratios of the real-space moments {\em local} exponents, indicating that though the real-space moments are not scaling ``nicely'', the relative behavior of the velocity difference probability distribution functions is changing over very short ranges of length scales. Utilizing this technique we show that the ratios of the local exponents for Eulerian moments in weak two-dimensional turbulence behave in agreement with Kolmogorov predictions over the spectrally identified ranges. The Lagrangian local exponent ratios, however, behave in a different manner compared to their Eulerian counterparts, and deviate significantly from what would be expected from Kolmogorov predictions.

cond-mat.soft↗

arXiv2005

Lagrangian statistics and coherent structures in two-dimensional turbulence

Measurements of Lagrangian single-point and multiple-point statistics in a quasi-two-dimensional stratifed layer system are reported. The system consists of a layer of salt water over an immiscible layer of Fluorinert and is forced electromagnetically so that mean-squared vorticity is injected at a well-defined spatial scale. Simultaneous cascades develop in which enstrophy flows predominately to small scales whereas energy cascades, on average, to larger scales. Lagrangian velocity correlations and one- and two-point displacements are measured for random initial conditions and for initial positions within topological centers and saddles. The behavior of these quantities can be understood in terms of the trapping characteristics of long-lived centers, the slow motion near strong saddles, and the rapid fluctuations outside of either centers or saddles.

cond-mat.soft↗

arXiv2001

The Inverse Energy Cascade of Two-Dimensional Turbulence

This thesis presents an experimental study of the inverse energy cascade as it occurs in an electromagnetically forced soap film. It focuses on characterizing important features of the inverse cascade such as it's range, how energy is distributed over the range and how energy flows through the range. The thesis also probes the assumption of scale invariance that is associated with the existence of an inverse cascade. These investigations demonstrate that the extent of the inverse cascade range and the behavior of the energy distribution are in agreement with dimensional predictions. The energy flow in the inverse cascade range is shown to be well described by exact mathematical predictions obtained from the Navier-Stokes equation. At no time does the energy flow in the inverse cascade range produced by the e-m cell behave inertially or in a scale invariant manner. Evidence that the cascade could become scale invariant should an inertial range develop is presented, as are the requirements that a system must satisfy to create such an inertial range.

physics.flu-dyn↗