To gain insight into the turbulent mixing processes at small scales, we study the time evolution of an initially regular passive scalar pattern in three-dimensional homegeneous turbulence. We generate turbulence (R_lambda <= 100) by... more
To gain insight into the turbulent mixing processes at small scales, we study the time evolution of an initially regular passive scalar pattern in three-dimensional homegeneous turbulence. We generate turbulence (R_lambda <= 100) by towing a grid (mesh size M=1.27cm, solidity sigma=0.44) down a 120 cm tall tank of cross-section 30cm× 30cm containing water. The initial pattern consists of a vertical line of highly concentrated KMnO4 solution with an approximate initial width w=0.5 mm, and is formed by dropping a small crystal of the dye in the tank after the grid has passed. Using back lighting and a high resolution digital camera, we resolve the integrated concentration field at scales ranging from 40mu m to 40 mm. We present data on the probability density functions (pdf) of both the concentration and concentration gradient fields. In some range, the pdf for the concentration falls off exponentially and the exponent decreases as the turbulence decays. The concentration gradient along the vertical direction, as well as the concentration gradient along the projected line of highest concentration have approximately exponential pdfs.
Research Interests:
We present results for the dynamics of shaken granular systems. We consider first purely vertical shaking, and focus chiefly on tall layers of material for which friction between walls and grains and gas flow are responsible for a... more
We present results for the dynamics of shaken granular systems. We consider first purely vertical shaking, and focus chiefly on tall layers of material for which friction between walls and grains and gas flow are responsible for a collection of interesting dynamical states. We then briefly consider pure horizontal shaking and finally mixed horizontal and vertical shaking.
Research Interests:
Abstract We report on novel experimental observations in binary mixtures of differently shaped, but same sized particles subjected to vertical vibrations. We find that above a threshold frequency, strong mixing is observed. Below the... more
Abstract We report on novel experimental observations in binary mixtures of differently shaped, but same sized particles subjected to vertical vibrations. We find that above a threshold frequency, strong mixing is observed. Below the threshold, several new segregated patterns occur. There are two necessary conditions for segregation: 1) The free surface inclines (heaping) and grains roll down the free surface. 2) The two species roll with significantly different speeds. Condition 1) is satisfied both below and above the threshold ...
Research Interests:
Research Interests:
Research Interests:
ABSTRACT
Research Interests:
Research Interests:
We consider layers of non-cohesive granular materials lying on a vertically vibrating surface. Above a threshold acceleration, Γ = Γc internal convective motion begins, and the free surface of the granular material becomes unstable to the... more
We consider layers of non-cohesive granular materials lying on a vertically vibrating surface. Above a threshold acceleration, Γ = Γc internal convective motion begins, and the free surface of the granular material becomes unstable to the formation of a heap. As Γ increases further, there are several higher order instabilities, traveling surface waves, subharmonic bifurcations, and bubbling. We find that for particles of diameter d ≤ 1 mm, the pressure P of the surrounding gas plays a key role in the instabilities. We propose criteria to determine when gas effects are relevant.
Research Interests:
... Depf of Elecmcal Eng and Computer Science. University of Central Flonda, Orlando, FL 32826Ravi Palaniappan. Judd Tracy. ... rcra aaa .. Figure 6 Conclusion The results of the investigation on outdoor propagation of Ultra wide hand... more
... Depf of Elecmcal Eng and Computer Science. University of Central Flonda, Orlando, FL 32826Ravi Palaniappan. Judd Tracy. ... rcra aaa .. Figure 6 Conclusion The results of the investigation on outdoor propagation of Ultra wide hand ~ignals are presented. ...
Research Interests:
Guy Schiavone, Parveen Wahid, Eric Van Doom&amp;#x27;, Ravi Palaniappan, Judd Tracy, Department of Electrical and Computer Engineering, University of Central Florida, Orlando, FL Intelligent Automation Inc&amp;#x27;, Rockville, MD... more
Guy Schiavone, Parveen Wahid, Eric Van Doom&amp;#x27;, Ravi Palaniappan, Judd Tracy, Department of Electrical and Computer Engineering, University of Central Florida, Orlando, FL Intelligent Automation Inc&amp;#x27;, Rockville, MD 20855 guy@ist.ucf.edu Abstract One of the major ...
Research Interests:
We consider layers of non-cohesive granular materials lying on a vertically vibrating surface. Above a threshold acceleration, Γ = Γc internal convective motion begins, and the free surface of the granular material becomes unstable to the... more
We consider layers of non-cohesive granular materials lying on a vertically vibrating surface. Above a threshold acceleration, Γ = Γc internal convective motion begins, and the free surface of the granular material becomes unstable to the formation of a heap. As Γ increases further, there are several higher order instabilities, traveling surface waves, subharmonic bifurcations, and bubbling. We find that for particles of diameter d ≤ 1 mm, the pressure P of the surrounding gas plays a key role in the instabilities. We propose criteria to determine when gas effects are relevant.
Research Interests:
Research Interests:
Research Interests:
Cable-driven parallel robots (CDPRs) have been applied very well because of its advantages. Like other parallel robots, the forward kinematic solution of the parallel robot driven by cable is usually faced with complex nonlinear... more
Cable-driven parallel robots (CDPRs) have been applied very well because of its advantages. Like other parallel robots, the forward kinematic solution of the parallel robot driven by cable is usually faced with complex nonlinear equations, and its solution is always a difficult problem. The traditional numerical iteration method is heavily dependent on the selection of initial values, which is easy to fall into local convergence, leading to the result of calculation is not necessarily the ideal spatial pose. The neural network has good nonlinear fitting ability and can give correct spatial pose, but the solution accuracy needs to be improved. The convergence and precision of the forward kinematic solution of the CDPRs is solved by the improved algorithm combining neural network and Levenberg- Marquardt algorithm.
Research Interests:
ABSTRACT
Research Interests:
Research Interests:
ABSTRACT
ABSTRACT Much effort has been spent over the years on studying the decay of grid-generated turbulence. The following specific aspect is studied here in the case of a grid pulled through once in a box of otherwise still water. Homogeneity... more
ABSTRACT Much effort has been spent over the years on studying the decay of grid-generated turbulence. The following specific aspect is studied here in the case of a grid pulled through once in a box of otherwise still water. Homogeneity is usually assumed in this situation, but this will clearly not be correct near the wall. For the study of the final period of decay, for instance, some knowledge of the behavior of turbulence near the wall, as well as of the influence of this behavior on the core flow, is important. This aspect is studied using a two-color PIV system. Data were taken at different times for four spatial positions across the tank. The statistical treatment of the data includes the study the probability density functions (pdfs) of the velocity, turbulent intensity, decay laws, and the spatial structure. The exponents of the decay power law were lower in the vicinity of the wall than in the core. A time history for the effect of the wall can be extracted from the study of the turbulent intensity. Some ideas concerning irregularities in the pdfs will be discussed.
Research Interests:
ABSTRACT
Research Interests:
Research Interests:
In the study of turbulence, most attention has been given to the components of the velocity field, and their properties at different scales. However, the relation between the different components, i.e., the direction of the velocity... more
In the study of turbulence, most attention has been given to the components of the velocity field, and their properties at different scales. However, the relation between the different components, i.e., the direction of the velocity vector, has received less consideration by the turbulence community. Directional statistics play an important role in climatological models, and are of interest to those who study the spread of polutants. We study the statistics of wind direction fluctuations in the atmosphere for a wide range of length scales (1 mm to 10^4 m) and in both vertical and horizontal planes to the ground surface. We find a nearly universal probability density function for a surprisingly large range of length scales. We propose a simple model that captures most quantitative features of the directional statistics.
Research Interests:
An ultra wide band (UWB) signal is defined as any radiation in which the 3-dB bandwidth is greater than 25% of the center frequency. UWB signals are characterized by extreme low powers and large bandwidths, which can be used for data,... more
An ultra wide band (UWB) signal is defined as any radiation in which the 3-dB bandwidth is greater than 25% of the center frequency. UWB signals are characterized by extreme low powers and large bandwidths, which can be used for data, voice and video communication. Since UWB waveforms have very short time duration, they are relatively immune to multi-path cancellation effects. In this paper we test the performance of a simulation to model the propagation of an UWB signal in outdoor forested environment. The simulation uses a combination of Finite Difference Time Domain and ray tracing methods to simulate the UWB wave propagation. The model takes into consideration the dielectric constants of the materials of the trees and measures the signal strength for vertical and horizontal polarizations of the UWB antennas placed at various heights and distances from each other. The results of the simulation are compared to measurements obtained from tests conducted at a wooded area in Seneca Creek State Park, Gaithersburg, Maryland. It was observed that upto 150 ft distance between the transmitter and receiver, the horizontally polarized antenna system gave better signal-to-noise ratio, but at greater distances the vertically polarized antenna system gave a better signal-to-noise ration performance. Three dimensional plots of the signal strengths and the signal-to-noise ratio for various transmitter and receiver distances are plotted for the system. These are compared with experimental results and the simulation closely matched the experimental data. The results of the simulation and measurements will be used for further developing an UWB location and tracking system for outdoor environments.
Research Interests: Antennas, Radiation, Video, Signal Analysis, Simulation, and 14 moreRay Tracing, Polarization, Wave propagation, GNSS receivers, Dielectric Constant, Three Dimensional, Radio Transmitters, TRANSMITTERS, Ultrawideband, Bandwidth, Signal to Noise Ratio, Finite Difference Time Domain, Antenna measurements, and Tracking system
We investigate the processes leading to phase-space equilibration of parton distributions in nuclear interactions at collider energies. We derive a set of rate equations describing the chemical equilibration of gluons and quarks including... more
We investigate the processes leading to phase-space equilibration of parton distributions in nuclear interactions at collider energies. We derive a set of rate equations describing the chemical equilibration of gluons and quarks including medium effects on the relevant QCD transport coefficients, and discuss their consequences for parton equilibration in heavy ion collisions.
Research Interests:
We investigate the processes leading to phase-space equilibration of parton distributions in nuclear interactions at collider energies. We derive a set of rate equations describing the chemical equilibration of gluons and quarks including... more
We investigate the processes leading to phase-space equilibration of parton distributions in nuclear interactions at collider energies. We derive a set of rate equations describing the chemical equilibration of gluons and quarks including medium effects on the relevant QCD transport coefficients, and discuss their consequences for parton equilibration in heavy ion collisions.
Research Interests:
We used a mechanical shaker to subject granular material and the ambient air to vertical vibrations. The purpose of this study is twofold: To distinguish mean flow properties from fluctuations, and to differentiate gas and wall effects.... more
We used a mechanical shaker to subject granular material and the ambient air to vertical vibrations. The purpose of this study is twofold: To distinguish mean flow properties from fluctuations, and to differentiate gas and wall effects. The container has an annular geometry and the direction of the vibration and gravity are parallel to the cylinder axis. The container is filled to height h with non-cohesive particles of size d. The vibration amplitude, a, peak acceleration Γ = aω^2/g, where ω is the vibration frequency, and the gas pressure P are key control parameters. Above Γc ≈ 1 and h/d >> 1 the surface of the granular material becomes unstable to a steady convective motion. This motion evolves in the form of two counter rotating rolls, to a single heap making an angle θd with the horizontal(a dynamical angle of repose). Here we report on velocimetry measurements of tracer particles in the flowing sand. The mean flow rate increases with Γ but saturates for Γ ≈ 2 . The magnitude of the velocity fluctuations scale roughly with aω. Near the onset of the traveling wave instability, a novel slow velocity fluctuation in the bulk flow has been observed. This work supported by the NSF through grants DMR-9321791 and DMS-9504577.
Research Interests:
We consider layers of noncohesive granular materials of mean height h subject to vertical vibration z = Acos\(omegat\). Above gamma≡Aomega2/g~=1, convection begins, creating a heap of height L. We find that for particles of diameter... more
We consider layers of noncohesive granular materials of mean height h subject to vertical vibration z = Acos\(omegat\). Above gamma≡Aomega2/g~=1, convection begins, creating a heap of height L. We find that for particles of diameter d<1 mm the pressure P of the surrounding gas plays a key role: L = btanh\(KP\)+c, where b and K depend on h, the gas viscosity, d, gamma, and A. Convection is significantly suppressed if the shaker sidewalls are permeable, showing that gas trapped in the granular material is a major source of heaping. We consider criteria to determine when gas effects are relevant.
Research Interests:
A novel small wavelength instability occurs in experiments for vertically shaken (z = a cos(ωt)) sand, for small particle diameters d and when gas effects are important. At onset, the energy (aω)2/(gd) is proportional to a/(d), as... more
A novel small wavelength instability occurs in experiments for vertically shaken (z = a cos(ωt)) sand, for small particle diameters d and when gas effects are important. At onset, the energy (aω)2/(gd) is proportional to a/(d), as previously found for traveling waves. The instability begins as a surface perturbation with wavelength λ0 ~ 6 mm; subsequently λ grows exponentially in time.
Research Interests:
We report measurements of the dilation, δh, of a vertically shaken sand heap in the presence of air. These measurements were carried out for dimensionless accelerations 1.0 <= Γ <= 2.7, for particle diameters of 0.05 mm <= d<=1.0 mm, for... more
We report measurements of the dilation, δh, of a vertically shaken sand heap in the presence of air. These measurements were carried out for dimensionless accelerations 1.0 <= Γ <= 2.7, for particle diameters of 0.05 mm <= d<=1.0 mm, for layer height to particle diameter ratios of 80 <= h0/d <= 140, and for shaking amplitudes of 0.66 mm <= a<=6.65 mm. δh is mostly limited to a thin avalanching surface layer. The vertical motion of this layer is well described by a simple model of an inelastic bouncing ball, independent of other complex motion elsewhere.
Research Interests:
We used a mechanical shaker to subject granular material and the ambient air to vertical vibrations. The container has an annular geometry and the direction of the vibration and gravity are parallel to the cylinder axis. The container is... more
We used a mechanical shaker to subject granular material and the ambient air to vertical vibrations. The container has an annular geometry and the direction of the vibration and gravity are parallel to the cylinder axis. The container is filled to height h with non-cohesive particles of size d. The vibration amplitude, a, peak acceleration Γ = aω^2/g, where ω is the vibration frequency, the gas pressure P, and the particle shape are key control parameters. Above Γc ≈ 1 and for h/d >> 1 the surface of the granular material becomes unstable to a steady convective motion. In the radial direction, this motion evolves in the form of two counter rotating rolls driven by the walls when particles are smooth. For the same size rough particles, these radial rolls are suppressed, leaving flow along the azimuthal direction. This azimuthal flow leads to a single heap making an angle θd with the horizontal(a dynamical angle of repose). For fixed Γ, θd is function of both a and P. In the absence of gas, θd is small for a >> d, but remains substantial for small amplitude, indicating another mechanism for convection, in addition to wall friction and gas effects. We also present dilation and velocimetry data. This work supported by NSF grants DMS-9504577 and DMR-9321791.
Research Interests:
We describe a coarsening transition which occurs for granular materials subject to vertical vibration, z = A \cos (ω t). The traditional control parameter is Γ = Aω^2/g, where g is the acceleration of gravity. When a layer of granular... more
We describe a coarsening transition which occurs for granular materials subject to vertical vibration, z = A \cos (ω t). The traditional control parameter is Γ = Aω^2/g, where g is the acceleration of gravity. When a layer of granular material is shaken with Γ > 1, a heap will form (for layer heights large compared to the particle size, d). An alternative control parameter is E = Γ (A/d). In a typical rectangular container, a single heap forms by a complex process involving several mechanisms; at this point the convective ``wavelength'', λ is set by the container length, L---λ ~= L. We have observed and characterized a new short-λ transition to granular convection in which the initial instability has λ ~= w << L, where w is the container width. Over time, λ grows until a single heap, λ ~= L remains. The coarsening is characterized by exponential decay of the number crests on the surface of the layer, and the rate for the decay shows critical slowing down as Γ arrow Γ_onset^+. The onset of the short-λ transition is such that E_onset is a universal linearly increasing function of A/d for 0.09mm <= d <= 0.4 mm. This is similar to the onset rule for traveling waves in this system, and may be related to an instability observed in fluidized beds. Specifically, the present instability vanishes if the surrounding air is evacuated.
Research Interests:
Research Interests:
The heaping of a granular material subject to vertical vibration vanishes abruptly as the pressure of the surrounding gas, P, is lowered below a critial value ∼10 Torr, depending on particle diameter etc. We show that the vanishing of... more
The heaping of a granular material subject to vertical vibration vanishes abruptly as the pressure of the surrounding gas, P, is lowered below a critial value ∼10 Torr, depending on particle diameter etc. We show that the vanishing of the heap is consistent with two different effects. One of these is the onset of a Knudsen regime where the mean free path of a gas molecule becomes comparable to or larger than the typical distance to a grain. The usual Darcian gas flow models fail in this regime, and a Knudsen replacement predicts a vanishing of gas effects as P → 0. The other is that at low enough pressures, there is not enough gas to sustain flow under the usual linearized flow scenario. A detailed description of this regime is beyond the present analysis.
Research Interests:
We consider layers of noncohesive granular materials of mean height h subject to vertical vibration z = Acos\(ωt\). Above γ≡Aω2/g~=1, convection begins, creating a heap of height L. We find that for particles of diameter d<1 mm the... more
We consider layers of noncohesive granular materials of mean height h subject to vertical vibration z = Acos\(ωt\). Above γ≡Aω2/g~=1, convection begins, creating a heap of height L. We find that for particles of diameter d<1 mm the pressure P of the surrounding gas plays a key role: L = btanh\(KP\)+c, where b and K depend on h, the gas viscosity, d, γ, and A. Convection is significantly suppressed if the shaker sidewalls are permeable, showing that gas trapped in the granular material is a major source of heaping. We consider criteria to determine when gas effects are relevant.
Research Interests:
... (Paris), 50 (1989) 699. [12] Duran J., Mazozi T., Clément E. and Rajchenbach J., Phys. Rev. E, 50 (1994) 3092. ... Rev. Lett., 65 (1990) 393. [19] Brennen CE, Ghosh S. and Wassgren CR, Transactions of the ASME, 63 (1996) 156. [20]... more
... (Paris), 50 (1989) 699. [12] Duran J., Mazozi T., Clément E. and Rajchenbach J., Phys. Rev. E, 50 (1994) 3092. ... Rev. Lett., 65 (1990) 393. [19] Brennen CE, Ghosh S. and Wassgren CR, Transactions of the ASME, 63 (1996) 156. [20] Menon N. and Durian D., preprint (1996).
Research Interests:
To gain insight into the turbulent mixing processes at small scales, we study the time evolution of an initially regular passive scalar pattern in three-dimensional homegeneous turbulence. We generate turbulence (R_λ <= 100) by towing a... more
To gain insight into the turbulent mixing processes at small scales, we study the time evolution of an initially regular passive scalar pattern in three-dimensional homegeneous turbulence. We generate turbulence (R_λ <= 100) by towing a grid (mesh size M=1.27cm, solidity σ=0.44) down a 120 cm tall tank of cross-section 30cm× 30cm containing water. The initial pattern consists of a vertical line of highly concentrated KMnO4 solution with an approximate initial width w=0.5 mm, and is formed by dropping a small crystal of the dye in the tank after the grid has passed. Using back lighting and a high resolution digital camera, we resolve the integrated concentration field at scales ranging from 40μ m to 40 mm. We present data on the probability density functions (pdf) of both the concentration and concentration gradient fields. In some range, the pdf for the concentration falls off exponentially and the exponent decreases as the turbulence decays. The concentration gradient along the vertical direction, as well as the concentration gradient along the projected line of highest concentration have approximately exponential pdfs.
Research Interests:
The decay of turbulence behind a grid in polymer and surfactant solutions is studied with the use of Particle Image Velocimetry. The decay rate of the turbulent energy is significantly lower for both polymer and surfactant solutions as... more
The decay of turbulence behind a grid in polymer and surfactant solutions is studied with the use of Particle Image Velocimetry. The decay rate of the turbulent energy is significantly lower for both polymer and surfactant solutions as compared to water. As the turbulence decays, anisotropy is sustained longer for polymer and surfactant solutions. Both findings suggest a significant change in turbulent dynamics without the effect of the wall. However, there is a dramatic difference in the qualitative nature of the flow fields of polymer and surfactant solutions, strongly suggestive of distinct mechanisms for polymer and surfactant interaction with turbulence. The implications for drag reduction are discussed.
We study some elementary statistics of wind direction fluctuations in the atmosphere for a wide range of time scales (10-4 sec to 1 h), and in both vertical and horizontal planes. In the plane parallel to the ground surface, the direction... more
We study some elementary statistics of wind direction fluctuations in the atmosphere for a wide range of time scales (10-4 sec to 1 h), and in both vertical and horizontal planes. In the plane parallel to the ground surface, the direction time series consists of two parts: a constant drift due to large weather systems moving with the mean wind speed, and fluctuations about this drift. The statistics of the direction fluctuations show a rough similarity to Brownian motion but depend, in detail, on the wind speed. This dependence manifests itself quite clearly in the statistics of wind-direction increments over various intervals of time. These increments are intermittent during periods of low wind speeds but Gaussian-like during periods of high wind speeds.
Research Interests:
Research Interests:
Much effort has been spent over the years on studying the decay of grid-generated turbulence. The following specific aspect is studied here in the case of a grid pulled through once in a box of otherwise still water. Homogeneity is... more
Much effort has been spent over the years on studying the decay of grid-generated turbulence. The following specific aspect is studied here in the case of a grid pulled through once in a box of otherwise still water. Homogeneity is usually assumed in this situation, but this will clearly not be correct near the wall. For the study of the final period of decay, for instance, some knowledge of the behavior of turbulence near the wall, as well as of the influence of this behavior on the core flow, is important. This aspect is studied using a two-color PIV system. Data were taken at different times for four spatial positions across the tank. The statistical treatment of the data includes the study the probability density functions (pdfs) of the velocity, turbulent intensity, decay laws, and the spatial structure. The exponents of the decay power law were lower in the vicinity of the wall than in the core. A time history for the effect of the wall can be extracted from the study of the turbulent intensity. Some ideas concerning irregularities in the pdfs will be discussed.