Malcolm J. Andrews

A comparative study of the turbulent Rayleigh–Taylor instability using high-resolution three-dimensional numerical simulations: The Alpha-Group collaboration

The turbulent Rayleigh–Taylor instability is investigated in the limit of strong mode-coupling using a variety of high-resolution, multimode, three dimensional numerical simulations (NS). The perturbations are initialized with only short wavelength modes so that the self-similar evolution (i.e., bubble diameter Db∝amplitude hb) occurs solely by the nonlinear coupling (merger) of saturated modes. After an initial transient, it is found that hb∼αbAgt2, where A=Atwood number, g=acceleration, and t=time. The NS yield Db∼hb/3 in agreement with experiment but the simulation value αb∼0.025±0.003 is smaller than the experimental value αb∼0.057±0.008. By analyzing the dominant bubbles, it is found that the small value of αb can be attributed to a density dilution due to fine-scale mixing in our NS without interface reconstruction (IR) or an equivalent entrainment in our NS with IR. This may be characteristic of the mode coupling limit studied here and the associated αb may represent a lower bound that is insensiti...

Experimental investigation of Rayleigh-Taylor mixing at small Atwood numbers

The self-similar evolution to turbulence of a multi-mode Rayleigh–Taylor mix at small density differences (

Rayleigh–Taylor and shear driven mixing with an unstable thermal stratification

A_{t} \sim 7.5 \times 10^{ - 4}

A numerical study of the influence of initial perturbations on the turbulent Rayleigh-Taylor instability

), is investigated through particle image velocimetry (PIV), and high-resolution thermocouple measurements. The density difference has been achieved through a temperature difference in the fluid. Cold fluid enters above the hot in a closed channel to form an unstable interface. This buoyancy-driven mixing experiment allows for long data collection times, short transients, and is statistically steady. First-, second-, and third-order statistics with spectra of velocity and temperature fields are presented. Analysis of the measurements has shed light on the structure of mixing as it develops to a self-similar regime in this flow. The onset of self-similarity is marked by the development of a self-preserving form of the temperature spectra, and the collapse of velocity profiles expressed in self-similar units. Vertical velocity fluctuations dominate horizontal velocity fluctuations in this experiment, with a ratio approaching 2:1 in the self-similar regime. This anisotropy extends to the Taylor microscales that undergo differential straining in the direction of gravity. Up to two decades of velocity spectra development, and four decades of temperature spectra, have been captured from the experiment. The velocity spectra consist of an inertial range comprised of anisotropic vertical and horizontal velocity fluctuations, and a more isotropic dissipative range. Buoyancy forcing occurs across the spectrum of velocity and temperature scales, but was not found to affect the structure of the spectra, resulting in a

Prediction of turbulent heat transfer in rotating smooth square ducts

-5/3

Three-Dimensional Modeling of a Helixchanger® Heat Exchanger Using CFD

slope, similar to other canonical turbulent flows. A scaling argument is presented to explain this observation. The net kinetic energy dissipation, as the flow evolves from an initial state to a final self-similar state was measured to be 49% of the accompanying loss in potential energy, and is in close agreement with values obtained from three-dimensional numerical simulations.

Spectral measurements of Rayleigh–Taylor mixing at small Atwood number

A new water channel experiment has been used to study turbulent mixing driven by buoyancy, and by combined buoyancy and shear. Density differences were produced by thermal stratification. The experiment was statistically steady, and a space–time transformation in the streamwise direction permitted a continuous study of the mixing evolution. Dye and digitized photographs were used to study the mixing process. An ensemble average of images gave the average mixing layer growth rate and the distribution of light and heavy fluid in the mixing layer. The structure of the early growth of buoyancy dominated mixing and of combined shear and buoyancy mixing is presented. The mixing transition from combined shear and buoyancy mixing to buoyancy dominated mixing occurred at Richardson numbers from −5 to −11. It was found that buoyancy dominated a self‐similar mixing stage for the range of flows (ΔU=0 to 2 cm/s) and density differences (Δρ=0.38 to 2.4 kg/m3). Transition to self‐similar mixing occurred at a Reynolds nu...

Detailed measurements of a statistically steady Rayleigh-Taylor mixing layer from small to high Atwood numbers

The effect of initial conditions on the growth rate of turbulent Rayleigh–Taylor (RT) mixing has been studied using carefully formulated numerical simulations. A monotone integrated large-eddy simulation (MILES) using a finite-volume technique was employed to solve the three-dimensional incompressible Euler equations with numerical dissipation. The initial conditions were chosen to test the dependence of the RT growth coefficient (

Statistically steady measurements of Rayleigh-Taylor mixing in a gas channel

\alpha_{b})

On shock driven jetting of liquid from non-sinusoidal surfaces into a vacuum

and the self-similar parameter (