Scott Wunsch

• Princeton University, BSE; University of Chicago, PhD
• Professor at Johns Hopkins University

Internal waves propagating in variable stratification exhibit nonlinear generation of harmonics. Here, synthetic schlieren is used to study a laboratory internal wave beam propagating from a uniformly stratified layer into a region of increased stratification (pycnocline) below a mixed layer. The results demonstrate the formation of harmonic (doubl...

The interaction of an internal wave beam (IWB) with an idealized oceanic pycnocline is examined using two-dimensional fully nonlinear direct numerical simulations based on a spectral multidomain penalty method in the vertical direction. The phenomenon of focus is the nonlinear generation of harmonics. A total of 24 simulations have been performed,...

Internal waves incident on a sheared ocean pycnocline are studied using analytic and numerical methods. Linear analysis of the unstable modes of a sheared ocean pycnocline is used to demonstrate interactions between internal waves and shear instabilities. A new analytic solution for an asymmetric shear layer over a stratified layer is presented, il...

Internal wave beams generated by oceanic tidal flows propagate upward and interact with the increasing stratification found at the pycnocline. The nonlinear generation of harmonic modes by internal wave beams incident on a pycnocline has recently been demonstrated by laboratory experiments and numerical simulations. In these previous studies, the h...

Recent work has explored linear internal wave transmission and reflection across a density staircase. Here, weakly nonlinear theory is used to extend previous linear internal wave solutions to include nonlinear effects at density interfaces within the staircase. Near-resonant forcing of freely propagating modes of the staircase by weakly nonlinear...

Weakly nonlinear theory is used to explore the dynamics of a single-mode internal tide in variable stratification with rotation. Nonlinear self-interaction in variable stratification generates a perturbation which is forced with double the original frequency and wavenumber. The dynamics of the perturbation is analogous to a forced harmonic oscillat...

Weakly nonlinear theory is used to explore the effect of vertical shear on surface gravity waves in three dimensions. An idealized piecewise-linear shear profile motivated by wind-driven profiles and ambient currents in the ocean is used. It is shown that shear may mediate weakly nonlinear resonant triad interactions between gravity and vorticity w...

Laboratory experiments have been performed to investigate the interaction of internal waves with a pycnocline. An oscillating cylinder generated internal wave beams, which were observed using the synthetic schlieren technique. Internal waves incident on the pycnocline layer excited higher-frequency modes. In the absence of shear, a discrete spectru...

Numerical simulations using the one-dimensional-turbulence (ODT) model are compared to water-tank measurements emulating convection and entrainment in stratiform clouds driven by cloud-top cooling. Measured dependences of the entrainment rate on Richardson number were numerically reproduced for water trials in which the initial stratification is du...

Numerical simulations using the one-dimensional turbulence (ODT) model are compared to water-tank measurements emulating convection and entrainment in stratiform clouds driven by cloud-top cooling. Measured dependences of the entrainment rate on Richardson number are reproduced. An additional parameter variation suggests more complicated dependence...

The phenomenon of nonlinear harmonic generation upon the encounter of an internal wave beam with a sharp oceanic pycnocline underlying a well-mixed layer is examined using 2-D direct numerical simulation. Use of a spectral multidomain penalty scheme enables the detailed resolution of phenomena within the pycnocline. Values of the ratio, r, of pycno...

Comparison of Entrainment Rates from a Tank Experiment with Results Using the One-Dimensional-Turbulence Model

The turbulent convection that takes place in a Chandrasekhar-mass white dwarf during the final few minutes before it explodes determines where and how frequently it ignites. Numerical simulations have shown that the properties of the subsequent Type Ia supernova are sensitive to these ignition conditions. A heuristic model of the turbulent convecti...

The parameterization of the stably stratified atmospheric boundary layer is a difficult issue, having a significant impact on medium-range weather forecasts and climate integrations. To pursue this further, a moderately stratified Arctic case is simulated by nineteen single-column turbulence schemes. Statistics from a large-eddy simulation intercom...

One-dimensional turbulence (ODT) is a single-column simulation in which vertical motions are represented by an unsteady advective process, rather than their customary representation by a diffusive process. No space or time averaging of mesh-resolved motions is invoked. Molecular-transport scales can be resolved in ODT simulations of laboratory-scal...

Controlled seeding of perturbations is employed to study the evolution of wire array z‐pinch implosion instabilities which strongly impact x‐ray production when the 3D plasma stagnates on axis. Wires modulated in radius exhibit locally enhanced magnetic field and imploding bubble formation at discontinuities in wire radius due to the perturbed curr...

The parameterization of the stably stratified atmospheric boundary layer is a difficult issue, having a significant impact on medium-range weather forecasts and climate integrations. To pursue this further, a moderately stratified Arctic case is simulated by nineteen single-column turbulence schemes. Statistics from a large-eddy simulation intercom...

Three-dimensional perturbations have been seeded in wire-array z pinches by etching 15 microm diameter aluminum wires to introduce 20% modulations in radius with a controlled axial wavelength. These perturbations seed additional three-dimensional imploding structures that are studied experimentally and with magnetohydrodynamics calculations, highli...

A stochastic one-dimensional model for thermal convection is formulated and applied to high-Rayleigh-number convection. Comparisons with experimental data for heat transfer in Rayleigh–Bénard cells are used to estimate two model parameters. Reasonable agreement with experimental results is obtained over a wide range of physical parameter values (si...

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 D(b)proportional to amplitude h(b)) o...

The one-dimensional turbulence (ODT) was used to simulate the GABLS stable boundary layer intercomparison case. The main goal is to obtain a cost-effective tool for simulation of vertical transport and mixing for ABL conditions that can be adequately treated within a single-column framework. The ODT modeling concept is introduced in an atmospheric...

The One-Dimensional Turbulence (ODT) model is a single-column simulation in which vertical tur- bulent transport is modeled by an unsteady, stochastic advective process, rather than the customary representation by a diffusive process. Unlike conventional single-column models, ODT resolves small- scale, unsteady motions and transport processes in th...

Summary form only given. Evidence for non-uniform mass ablation has been observed on various wire array experiments. Non-uniform wire ablation leads to non-uniform wire breakup, providing the means for wire array material to be left behind the implosion of the bulk array mass. Mass ablation variation along the length of the wire vary periodically,...

The GABLS arctic stable boundary layer intercomparison case is simulated using the One-Dimensional Turbulence (ODT) model. ODT is a single-column simulation in which vertical turbulent transport is represented by an unsteady, stochastic advective process, rather than the customary representation by a diffusive process. Unlike conventional single-co...

In most turbulent convection experiments, the flow is driven by an unstable temperature difference between two plates. However, in many natural flows, the convection is driven by the heating of the fluid in a region far from any hard walls. An example is convection in stars, in which thermonuclear combustion in the core releases the heat which driv...

Capsule radiation symmetry is a crucial issue in the design of the double z-pinch hohlraum approach to high-yield inertial confinement fusion [1]. Recent x-ray backlit capsule experiments in double z-pinch hohlraums on Sandia National Laboratories' Z facility have demonstrated the ability to predictably control time-integrated P2 asymmetry [2], and...

The double-z-pinch approach to ICF decouples wire array dynamics from capsule implosion physics and provides a robust way to control P2 capsule symmetry via secondary hohlraum length[1]. This source has produced even-mode symmetry of 3.0+/-1.4(case-to-capsule ratio=8.9)[2]. Symmetry control experiments are being done with 4.7-mm-diam capsules at ca...

Via x-ray backlit capsule shell distortion measurements, the time-averaged polar radiation uniformity in a 70 eV high-yield-scale double-z-pinch-driven hohlraum (cylindrical) on Sandia National Laboratories Z accelerator has recently been measured to the 3.0 +/- 1.4% level on the best shots [Bennett et al., Phys. Plasmas, in press]. This 3.0% symme...

The observable properties of a Type Ia supernova are sensitive to how the nuclear runaway ignites in a Chandrasekhar mass white dwarf - at a single point at its center, off-center, or at multiple points and times. We present a simple analytic model for the runaway based upon a combination of stellar mixing-length theory and recent advances in under...

Buoyancy reversal occurs when the mixing of two fluids, initially stably stratified, produces a mixture which is more dense than either pure fluid. The resulting instability generates turbulent mixing, and may play an important role in geophysical and astrophysical flows. In this work, a stochastic one-dimensional model is used to simulate these sy...

A novel near-wall LES closure model is developed based on a revised form of the one-dimensional turbulence (ODT) model of Kerstein and is tested by performing LES calculations of turbulent channel flow at Reynolds numbers based on friction velocity ranging from 395 to 10,000. In contrast to previous models, which invoke Reynolds averaging, near-wal...

Using two-dimensional direct numerical simulations, the statistics of temperature differences in stably stratified turbulence are studied. Comparison with passive scalar statistics in similar flows suggests that the stably stratified case is qualitatively similar to the passive case. Probability distribution functions of temperature differences bet...

One-dimensional turbulence is a stochastic simulation method representing the time evolution of the velocity profile along a notional line of sight through a turbulent flow. In this paper, the velocity is treated as a three-component vector, in contrast to previous formulations involving a single velocity component. This generalization allows t...

A stochastic one-dimensional turbulence (ODT) model for thermal convection is applied to Rayleigh-Bénard flow in order to investigate the physics underlying the various turbulent thermal convection regimes that have been identified in experimental studies. The model is an unsteady simulation representing flow evolution along a vertical line of sigh...

We study the statistics of the temperature fluctuations in stably stratified turbulence using two dimensional direct numerical simulations. Comparison with passive scalar statistics in similar flows suggest that the stably stratified case is qualitatively similar to the passive case. The stabilizing stratification is found to suppress large scale m...

Stably stratified turbulent flows are common in geophysics and astrophysics, and frequently exhibit layered structures in which large regions of nearly constant fluid density are separated by sharp density gradients. Experiments have demonstrated that, under suitable conditions, the stirring of a stably stratified fluid generates these layer struct...

Experiments have demonstrated that the stirring of a stably-stratified fluid sometimes generates a series of layers of nearly constant density separated by steep density gradients. In this paper, mixing length arguments based on the Kolmogorov picture of turbulence are used to suggest scalings for the layer sizes, gradients, and buoyancy fluxes in...

This paper presents a simple, one-dimensional model of a randomly advected passive scalar. The model exhibits anomalous inertial range scaling for the structure functions constructed from scalar differences. The model provides a simple computational test for recent ideas regarding closure and scaling for randomly advected passive scalars. Results s...

It has been established experimentally that a passive scalar at high Peclet number exhibits markedly non-gaussian statistics when advected by a high Reynolds number velocity field. This has generally been attributed to the non-gaussian statistics of the underlying velocity field. However, a recently proposed model (R. Kraichnan, Phys. Rev. Lett. 72...

This paper is a set of notes about work in progress. Since the work is directed toward scaling, it seems quite appropriate to report this in a context devoted to Ben Widom's scientific contributions.Model equations for the motion of a passive scalar in one dimension are set up and some scaling properties of their solutions are derived. All these mo...

This report describes research and development of the large eddy simulation (LES) turbulence modeling approach conducted as part of Sandia's laboratory directed research and development (LDRD) program. The emphasis of the work described here has been toward developing the capability to perform accurate and computationally affordable LES calculation...