Virtual Plume Research Articles

The elements and richness of particle diffusion during sediment transport at small timescales

AbstractThe ideas of advection and diffusion of sediment particles are probabilistic constructs that emerge when the Master equation, a precise, probabilistic description of particle conservation, is approximated as a Fokker–Planck equation. The diffusive term approximates nonlocal transport. It ‘looks’ upstream and downstream for variations in particle activity and velocities, whose effects modify the advective term. High‐resolution measurements of bedload particle motions indicate that the mean squared displacement of tracer particles, when treated as a virtual plume, primarily reflects a nonlinear increase in the variance in hop distances with increasing travel time, manifest as apparent anomalous diffusion. In contrast, an ensemble calculation of the mean squared displacement involving paired coordinate positions independent of starting time indicates a transition from correlated random walks to normal (Fickian) diffusion. This normal behavior also is reflected in the particle velocity autocorrelation function. Spatial variations in particle entrainment produce a flux from sites of high entrainment toward sites of low entrainment. In the case of rain splash transport, this leads to topographic roughening, where differential rain splash beneath the canopy of a desert shrub contributes to the growth of a soil mound beneath the shrub. With uniform entrainment, rain splash transport, often described as a diffusive process, actually represents an advective particle flux that is proportional to the land surface slope. Particle diffusion during both bedload and rain splash transport involves motions that mostly are patchy, intermittent and rarefied. The probabilistic framework of the Master equation reveals that continuous formulations of the flux and its divergence (the Exner equation) represent statistically expected behavior, analogous to Reynolds‐averaged conditions. Key topics meriting clarification include the mechanical basis of particle diffusion, effects of rarefied conditions involving patchy, intermittent motions, and effects of rest times on diffusion of tracer particles and particle‐borne substances. Copyright © 2016 John Wiley & Sons, Ltd.

Parsing anomalous versus normal diffusive behavior of bedload sediment particles

AbstractBedload sediment transport is the basic physical ingredient of river evolution. Formulae exist for estimating transport rates, but the diffusive contribution to the sediment flux, and the associated spreading rate of tracer particles, are not clearly understood. The start‐and‐stop motions of sediment particles transported as bedload on a streambed mimic aspects of the Einstein–Smoluchowski description of the random‐walk motions of Brownian particles. Using this touchstone description, recent work suggests the presence of anomalous diffusion, where the particle spreading rate differs from the linear dependence with time of Brownian behavior. We demonstrate that conventional measures of particle spreading reveal different attributes of bedload particle behavior depending on details of the calculation. When we view particle motions over start‐and‐stop timescales obtained from high‐speed (250 Hz) imaging of coarse‐sand particles, high‐resolution measurements reveal ballistic‐like behavior at the shortest (10−2 s) timescale, followed by apparent anomalous behavior due to correlated random walks in transition to normal diffusion (>10−1 s) – similar to Brownian particle behavior but involving distinctly different physics. However, when treated as a ‘virtual plume’ over this timescale range, particles exhibit inhomogeneous diffusive behavior because both the mean and the variance of particle travel distances increase nonlinearly with increasing travel times, a behavior that is unrelated to anomalous diffusion or to Brownian‐like behavior. Our results indicate that care is needed in suggesting anomalous behavior when appealing to conventional measures of diffusion formulated for ideal particle systems. Copyright © 2016 John Wiley & Sons, Ltd.

Evolving a Neural Olfactorimotor System in Virtual and Real Olfactory Environments

To provide a platform to enable the study of simulated olfactory circuitry in context, we have integrated a simulated neural olfactorimotor system with a virtual world which simulates both computational fluid dynamics as well as a robotic agent capable of exploring the simulated plumes. A number of the elements which we developed for this purpose have not, to our knowledge, been previously assembled into an integrated system, including: control of a simulated agent by a neural olfactorimotor system; continuous interaction between the simulated robot and the virtual plume; the inclusion of multiple distinct odorant plumes and background odor; the systematic use of artificial evolution driven by olfactorimotor performance (e.g., time to locate a plume source) to specify parameter values; the incorporation of the realities of an imperfect physical robot using a hybrid model where a physical robot encounters a simulated plume. We close by describing ongoing work toward engineering a high dimensional, reversible, low power electronic olfactory sensor which will allow olfactorimotor neural circuitry evolved in the virtual world to control an autonomous olfactory robot in the physical world. The platform described here is intended to better test theories of olfactory circuit function, as well as provide robust odor source localization in realistic environments.

Impact of personal factors and furniture arrangement on the thermal plume above a sitting occupant

The impact of thermal insulation and the design of clothing and chair, the blocking effect of a table and breathing on the thermal plume above a sitting thermal manikin was studied in a climate chamber. Air speed and temperature in the plume cross-section 0.7 m above the manikin head were measured. Results show that loose clothing increases the volume flux by 24%, changes the shape of the plume and thus should be carefully simulated. Tight clothing and chair design do not affect the plume volume flux and need not be considered in full-scale experiments. However, the convective part of the sensible heat loss increases with thermal insulation of a chair, and may be important in detailed CFD predictions. A wig on the manikin head appears to be important because a bald head decreases volume flux by 15%. Positioning the table tightly in front of the body affects air temperature and velocity distributions in the plume and increases volume flux by almost 50%. Exhaling through the mouth expands the plume, resulting in the integral characteristics being 40% greater and therefore should be considered in numerical simulations or experiments with thermal manikins, while exhaling through the nose can be disregarded. The volume flux of the thermal plume 0.7 m above the head of a sedentary occupant can be calculated with 25% uncertainty, using values for the convective heat loss equal to 30% of the sensible heat generated by the body and the distance of the virtual plume origin equal to −1.60 m.

Evaluation of Exploration and Monitoring Methods for Verification of Natural Attenuation Using the Virtual Aquifer Approach

Though natural attenuation (NA) is increasingly considered as a remediation technology, the methods for proper identification and quantification of NA are still under discussion. Here the "Virtual Aquifer" approach is used to demonstrate problems which may arise during measurement of concentrations in observation wells and for interpolation of locally measured concentrations in contaminated heterogeneous aquifers. The misinterpretation of measured concentrations complicates the identification and quantification of natural attenuation processes. The "Virtual Aquifer" approach accepts the plume simulated with a numerical model for a heterogeneous aquifer as "virtual reality". This virtual plume is investigated in the model with conventional methods like observations wells. The results of the investigation can be compared to the virtual "reality", evaluating the monitoring method. Locally determined concentrations are interpolated using various interpolation methods and different monitoring set-ups. The interpolation results are compared to the simulated plume to evaluate the quality of interpolation. This evaluation is not possible in nature, since concentrations in a heterogeneous aquifer are never known in detail.

Chemical plume tracking. 3. Ascorbic acid: a biologically relevant marker.

Dynamic characteristics of the amperometric sensing system for tracking of turbulent chemical plumes have been studied. The correlation analysis requires that such sensors respond rapidly and have long-term stability and minimal or low-flow sensitivity. Moreover, for practical purposes, such sensors must respond to an electrochemical marker that would be found in or at least be compatible with the diet of marine animals. Ascorbic acid is such a compound and can be electrochemically oxidized. Its long-term dynamic behavior on several types of electrodes has been studied. It has been found that a Pt electrode coated with polyaniline satisfies all the above requirements. However, this system has a peculiar dynamic behavior that affects the results of the correlation analysis, namely, at the higher frequencies. The behavior of such system in the virtual plume setup has been characterized and its usability for detection of fluctuating concentration of ascorbic acid in saline solution has been confirmed.

Chemical plume tracking. 2. Multiple-frequency modulation.

The purpose of this research is to estimate the effect of turbulent mixing on the information content in a chemically encoded signal, to investigate the effect of the presence of multiple encoded frequencies, and to evaluate the information contained in the higher harmonics of the coherence spectra. The virtual plume instrument is used to mimic the flow conditions and signal patterns in a real chemical plume. Two-frequency modulation experiments are performed using solenoid valves to introduce concentration plugs of a marker into the carrier flows at certain constant frequencies. In our experiments, the length of the delay elements and the dispersion were varied to mimic different characteristics of the turbulent plume. In addition, an artificial but uncorrelated white noise was added to the raw amperometric signals in order to simulate the "noisy" conditions existing in a real plume. Our experiments reveal that the introduced turbulence has only a marginal effect on the coherence spectra. Moreover, it is shown that when the second frequency is present in the plume, both fundamental frequencies can be unambiguously assigned. Higher harmonics in the coherence spectra have been found to depend on the distance from the source. These findings are important for understanding of the mechanism of chemotaxis and may also lead to the design of optimized search algorithms for chemical plume tracking robots.

Virtual Plume

A bench-top model of a chemical plume in a turbulent flow, a Virtual Plume, was built to simulate the behavior of a turbulent chemical plume. It is based on an eight-channel FIA/SIA system and an array of eight amperometric sensors. A plug of sample solution was delivered to the sensors after passing through delay and dispersion elements. Adjusting these two elements enabled us to simulate the time-concentration profile at any arbitrary point in the plume. It has been shown that an eight-channel system can simulate an arbitrary sensor array positioned in a turbulent plume. Preliminary correlation of the shape of the peaks obtained in the VP with those recorded using planar laser-induced fluorescence in a turbulent flow shows the value of this approach.