2,536 research outputs found
Environmental Restoration Disposal Facility Lessons Learned
The purpose of lessons learned is to identify insight gained during a project – successes or failures – that can be applied on future projects. Lessons learned can contribute to the overall success of a project by building on approaches that have worked well and avoiding previous mistakes. Below are examples of lessons learned during ERDF’s ARRA-funded expansion project
AN INVESTIGATION INTO THE EFFECTS OF A SIMULATED EFFUSION IN HEALTHY SUBJECTS ON KNEE KINEMATICS AND LOWER LIMB MUSCLE ACTIVITY DURING A SINGLE LEG DROP LANDING
Arthrogenic muscle inhibition (AMI) is defined as an ongoing reflex inhibition of the musculature surrounding a joint following distension or damage to the structures of that joint [Hopkins and Ingersoll, 2000]. AMI following joint injury may affect movement and muscle recruitment which may impair rehabilitation and delay the return to activity. Knee angular displacement and velocity as well as lower limb EMG were measured in the period 250 milliseconds pre initial contact to 250 milliseconds post initial contact during a single leg drop jump in 8 healthy subjects before and after a simulated knee joint effusion of 60 millilitres. Repeated measures ANOVA and post hoc testing revealed no statistically significant differences in pre and post effusion in knee kinematic or lower limb EMG measures undertaken. A simulated knee effusion did not result in significant alterations to knee joint mechanics or lower limb muscle activation patterns during a single leg drop landing. The mechanism by which an effusion affects motor control during functional and dynamic weight bearing tasks warrants further investigation
Quantifying mixing and available potential energy in vertically periodic simulations of stratified flows
Turbulent mixing exerts a significant influence on many physical processes in
the ocean. In a stably stratified Boussinesq fluid, this irreversible mixing
describes the conversion of available potential energy (APE) to background
potential energy (BPE). In some settings the APE framework is difficult to
apply and approximate measures are used to estimate irreversible mixing. For
example, numerical simulations of stratified turbulence often use triply
periodic domains to increase computational efficiency. In this setup however,
BPE is not uniquely defined and the method of Winters et al. (1995, J. Fluid
Mech., 289) cannot be directly applied to calculate the APE. We propose a new
technique to calculate APE in periodic domains with a mean stratification. By
defining a control volume bounded by surfaces of constant buoyancy, we can
construct an appropriate background buoyancy profile and
accurately quantify diapycnal mixing in such systems. This technique also
permits the accurate calculation of a finite amplitude local APE density in
periodic domains. The evolution of APE is analysed in various turbulent
stratified flow simulations. We show that the mean dissipation rate of buoyancy
variance provides a good approximation to the mean diapycnal mixing
rate, even in flows with significant variations in local stratification. When
quantifying measures of mixing efficiency in transient flows, we find
significant variation depending on whether laminar diffusion of a mean flow is
included in the kinetic energy dissipation rate. We discuss how best to
interpret these results in the context of quantifying diapycnal diffusivity in
real oceanographic flows.Comment: 28 pages, 10 figures, accepted to J. Fluid Mec
Shear-induced breaking of internal gravity waves
Motivated by observations of turbulence in the strongly stratified ocean
thermocline, we use direct numerical simulations to investigate the interaction
of a sinusoidal shear flow and a large-amplitude internal gravity wave. Despite
strong nonlinearities in the flow and a lack of scale separation, we find that
linear ray tracing theory is qualitatively useful in describing the early
development of the flow as the wave is refracted by the shear. Consistent with
the linear theory, the energy of the wave accumulates in regions of negative
mean shear where we observe evidence of convective and shear instabilities.
Streamwise-aligned convective rolls emerge the fastest, but their contribution
to irreversible mixing is dwarfed by shear-driven billow structures that
develop later. Although the wave strongly distorts the buoyancy field on which
these billows develop, the mixing efficiency of the subsequent turbulence is
similar to that arising from Kelvin-Helmholtz instability in a stratified shear
layer. We run simulations at Reynolds numbers of 5000 and 8000, and vary the
initial amplitude of the internal gravity wave. For high values of initial wave
amplitude, the results are qualitatively independent of . Smaller initial
wave amplitudes delay the onset of the instabilities, and allow for significant
laminar diffusion of the internal wave, leading to reduced turbulent activity.
We discuss the complex interaction between the mean flow, internal gravity wave
and turbulence, and its implications for internal wave-driven mixing in the
ocean.Comment: 27 pages, 12 figures, accepted to J. Fluid. Mec
Two-color holography concept (T-CHI)
The Material Processing in the Space Program of NASA-MSFC was active in developing numerous optical techniques for the characterization of fluids in the vicinity of various materials during crystallization and/or solidification. Two-color holographic interferometry demonstrates that temperature and concentration separation in transparent (T-CHI) model systems is possible. The experiments were performed for particular (succinonitrile) systems. Several solutions are possible in Microgravity Sciences and Applications (MSA) experiments on future Shuttle missions. The theory of the T-CHI concept is evaluated. Although particular cases are used for explanations, the concepts developed will be universal. A breadboard system design is also presented for ultimate fabrication and testing of theoretical findings. New developments in holography involving optical fibers and diode lasers are also incorporated
A method to construct refracting profiles
We propose an original method for determining suitable refracting profiles
between two media to solve two related problems: to produce a given wave front
from a single point source after refraction at the refracting profile, and to
focus a given wave front in a fixed point. These profiles are obtained as
envelopes of specific families of Cartesian ovals. We study the singularities
of these profiles and give a method to construct them from the data of the
associated caustic.Comment: 12 pages, 5 figure
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