Root Mean Square Velocity Research Articles

Understanding balance differences in individuals with multiple sclerosis with mild disability: an investigation of differences in sensory feedback on postural control during a Romberg task

A major presenting symptom in 'individuals with multiple sclerosis with mild balance disability' (IwMS) is poor postural control, resulting from slowed spinal somatosensory conduction. Postural control deficits in IwMS are most apparent when vision is removed and the base of support is reduced such is the case during tandem and single support stances. The current study used center of pressure (COP) measurements to determine whether postural control differences exist between IwMS and either 'healthy age-matched individuals' (HAMI) or 'community-dwelling older adults' (OA). Postural control was evaluated using a Romberg standing task, which required participants to stand with their feet together and hands by their sides for 45 s with either their eyes open or closed. Results revealed that COP velocity root mean square was greater in IwMS and their COP position was closer to their self-selected maximum stability limits (e.g., greater Standing Index proportion) when vision was removed compared to HAMI. Conversely, IwMS displayed similar postural control characteristics to OA. The current study highlights two novel findings: (1) the utility of novel COP measurements to assess differences in the level of postural control in IwMS; and (2) the benefit of assessing postural control levels in IwMS to not only a population with a fully intact and functional postural control system (HAMI) but also to another population that is thought to experience postural control deficits (OA).

The influence of gas phase velocity fluctuations on primary atomization and droplet deformation

The effects of grid-generated velocity fluctuations on the primary atomization and subsequent droplet deformation of a range of laminar liquid jets are examined using microscopic high-speed backlit imaging of the break-up zone and laser Doppler anemometry of the gas phase separately. This is done for fixed gas mean flow conditions in a miniature wind tunnel experiment utilizing a selection of fuels, turbulence-generating grids and two syringe sizes. The constant mean flow allows for an isolated study of velocity fluctuation effects on primary atomization in a close approximation to homogeneous decaying turbulence. The qualitative morphology of the primary break-up region is examined over a range of turbulence intensities, and spectral analysis is performed in order to ascertain the break-up frequency which, for a case of no grid, compares well with the existing literature. The addition of velocity fluctuations tends to randomize the break-up process. Slightly downstream of the break-up region, image processing is conducted in order to extract a number of metrics, which do not depend on droplet sphericity, and these include droplet aspect ratio and orientation, the latter quantity being somewhat unconventional in spray characterization. A turbulent Weber number \(We^{\prime}\) which takes into account gas phase fluctuations is utilized to characterize the resulting droplet shapes, in addition to a mean Weber number . Above a \(We^{\prime}>0.05\) a clear positive relationship exists between the mean aspect ratio of droplets and the turbulent Weber number where \(We^{\prime}\) is varied by altering all relevant variables including the velocity root mean square, the initial droplet diameter, the surface tension and the density.

PDPA Investigation on an Electrostatically-Assisted Twin-Fluid Atomization Flow

The Phase Doppler Particle Analyzer (PDPA) technique is utilized for characterizing the droplet sizes and velocity profiles in an electrostatic spray flow which is atomized using the aerodynamic forces assisted with electrostatic forces. An electrostatically-assisted twin-fluid atomization system is presented and spatial distributions of the droplet size and two-dimensional velocity components are measured simultaneously at each specified point. The droplets are firstly ejected from a liquid jet through the nozzle by aerodynamic shear forces, and the breakup mechanism of such an electrostatically-assisted twin-fluid atomization is investigated using the Rayleigh instability conditions through studying the specific surface charges on these droplets measured by a Faraday canister. The discrepancies between experiment data and Rayleigh limit for specific charges of droplets are presented and discussed due to energy transforming from electrical energy to surface energy, energy loss, Taylor instability breakup and nonexcess charge polarization. The measured average velocity profiles and their RMS (root mean square) velocity values for two different scale drops further reveal the effects of the aerodynamic and electrostatic forces on the atomization process.

A measurement system for the bicycle crank angle using a wireless motion sensor attached to the crank arm

In this study, we propose a system for measuring the rotational angle of a bicycle crank arm. This system consists of a wireless motion sensor with bi-axial acceleration and angular velocity sensors attached to the crank arm. Rotational angles measured by the motion sensor were compared with data from a motion capture system. We found the RMS (root-mean-square) error for the bicycle crank angle was 0.339 ± 0.115 degrees (mean ± SD), and the angular velocity RMS error normalized by the average angular velocity was 0.655 ± 0.217 %. Using the proposed motion sensor system, we were able to measure the rotational angle and angular velocity of the bicycle crank to the same accuracy and high frequency as the motion capture system. Furthermore, we demonstrate it is possible to precisely obtain the rotational crank angle to evaluate fluctuations of crank angular velocity.

On optimal NMO and generalised Dix equations for velocity determination and depth conversion

Abstract The classical Earth model used to process seismic data assumes plane layers and a velocity analysis based on a hyperbolic approximation of the reflection events, where basically one parameter (the stacking velocity) is used to perform the normal move-out (NMO) correction to obtain the stacked section. We explore the possibility of using alternative approaches, other than the standard Dix equation based on the root-mean-square (RMS) velocities, to obtain the interval velocities and perform the depth conversion in order to locate the interfaces. Specifically, we consider traveltime equations as a function of offset using different NMO approximations, based on the average, RMS and root-mean-quartic (RMQ) velocities. A generalised form of Dix's equation is used for this purpose. Moreover, we analyse the model-dependency of the different NMO equations. We consider a simple 4-layer model and a model based on data from the Cooper basin, South Australia, to test the NMO equations. We build an elastic-velocity model and compute a common midpoint (CMP) synthetic seismogram. The reflection events are identified and traveltimes are picked to perform a non-linear inversion with the conventional (one parameter) hyperbolic approximation, the 3-term Taner and Koehler equation and approximations based on the average, RMQ and RMS velocities, also using two parameters. The model has a velocity inversion which poses a challenge to the approximations. The performance of the approximations is model dependent, so a-priori information of the velocity profile can be useful to perform a suitable inversion, or an optimal stack of the reflection events is required to test the NMO correction. Moreover, the results show that the inversion based on the RMS velocities yields better results than those based on the RMQ velocities, but this is not always the case. On the other hand, the inversion using the average velocity performs a worse velocity–depth estimation, even compared to the RMS results from the hyperbolic approximation.

Walking on ballast impacts balance

Railroad workers often perform daily work activities on irregular surfaces, specifically on ballast rock. Previous research and injury epidemiology have suggested a relationship between working on irregular surfaces and postural instability. The purpose of this study was to examine the impact of walking on ballast for an extended duration on standing balance. A total of 16 healthy adult males walked on a 7.62 m × 4.57 m (25 ft × 15 ft) walking surface of no ballast (NB) or covered with ballast (B) of an average rock size of about 1 inch for 4 h. Balance was evaluated using dynamic posturography with the NeuroCom® Equitest System™ prior to experiencing the NB or B surface and again every 30 min during the 4 h of ballast exposure. Dependent variables were the sway velocity and root-mean-square (RMS) sway components in the medial–lateral and anterior–posterior directions. Repeated measures ANOVA revealed statistically significant differences in RMS and sway velocity between ballast surface conditions and across exposure times. Overall, the ballast surface condition induced greater sway in all of the dynamic posturography conditions. Walking on irregular surfaces for extended durations has a deleterious effect on balance compared to walking on a surface without ballast. These findings of changes in balance during ballast exposure suggest that working on an irregular surface may impact postural control.

Contribution of vision to postural behaviors during continuous support-surface translations

During standing balance, kinematics of postural behaviors have been previously observed to change across visual conditions, perturbation amplitudes, or perturbation frequencies. However, experimental limitations only allowed for independent investigation of such parameters. Here, we adapted a pseudorandom ternary sequence (PRTS) perturbation previously used in rotational support-surface perturbations (Peterka in J Neurophysiol 88(3):1097-1118, 2002) to a translational paradigm, allowing us to concurrently examine the effects of vision, perturbation amplitude, and frequency on balance control. Additionally, the unpredictable PRTS perturbation eliminated effects of feedforward adaptations typical of responses to sinusoidal stimuli. The PRTS perturbation contained a wide spectral bandwidth (0.08-3.67 Hz) and was scaled to 4 different peak-to-peak amplitudes (3-24 cm). Root mean square (RMS) of hip displacement and velocity increased relative to RMS ankle displacement and velocity in the absence of vision across all subjects, especially at higher perturbation amplitudes. Gain and phase lag of center of mass (CoM) sway relative to the perturbation also increased with perturbation frequency; phase lag further increased when vision was absent. Together, our results suggest that visual input, perturbation amplitude, and perturbation frequency can concurrently and independently modulate postural strategies during standing balance. Moreover, each factor contributes to the difficulty of maintaining postural stability; increased difficulty evokes a greater reliance on hip motion. Finally, despite high degrees of joint angle variation across subjects, CoM measures were relatively similar across subjects, suggesting that the CoM is an important controlled variable for balance.

Evaluating Large-Eddy Simulation (LES) and High-Speed Particle Image Velocimetry (PIV) with Phase-Invariant Proper Orthogonal Decomposition (POD)

This study is part of a program to understand the stochastic variations in IC engine flows; in particular, it is a comparison of measured (PIV) and computed (LES) velocity from multiple cycles of the same motored engine. Comparison procedures included traditional RANS (Reynolds Averaged Navier-Stokes) decomposition (ensemble-averaged and RMS (Root Mean Square) velocity), phase-dependent, and phase- invariant POD. Phase-dependent POD was performed on the PIV and LES samples separately and on the combined samples, thus creating separate or a single POD mode sets, respectively. The phase- invariant POD was performed both on normalized snapshots and on snapshots where the original energy was conserved. Initial comparisons of the mass-specific kinetic energies of the ensemble average and RMS velocities revealed that the PIV and LES data sets differed significantly during most of the intake stroke. This discrepancy was quantifiedfirst by comparing the relevance indices calculated between ensemble average velocity fields and, second, using phase-dependent POD, which quantified cycle-to-cycle flow variations of the RANS average and turbulence. Phase-dependent POD was applied separately to the PIV and LES data sets during the intake stroke (76° ATDCE, After Top Dead Center Exhaust), where the intake-valve jet is strong and the PIV and LES data were earlier found to be significantly different. The cyclic variability of the LES ensemble average was estimated to be significantly higher than that of the PIV data set. POD was also applied to the combined sample of LES and PIV snapshots for quantitative comparison creating a single set of modes, so that comparison could be made with POD coefficients alone. Example comparisons were made at again for data at 76° and also 330° ATDCE, which is a viable spark timing in a fired engine. The results at 76° ATDCE were similar to those obtained with the POD analysis of the separate samples. At 330° ATDCE, the PIV ensemble average and RMS velocities showed somewhat more cyclic variability. Phase-invariant POD was applied to the combined PIV and LES velocity data from all crank angles to study the flow evolution over the crank angle range. Conclusions derived from the two different energy transformations are contrasted. When the energy of each velocity field is normalized, the phase-invariant POD results focus on differences in flow structures and their evolution. On the other hand, when the energy of each velocity field is conserved, the phase-invariant POD results also take into account differences in energy between the PIV and LES data.

Feeding efficiency of Chaoborus flavicans (Insecta, Diptera) under turbulent conditions

Turbulence can affect predator–prey interactions. The effect of turbulence on the feeding efficiency of an ambush predator was tested with laboratory experiments. The experiments were conducted in 100-L aquaria in which ten individuals of fourth instar Chaoborus flavicans larvae were placed as predators. Two prey densities (3 and 10 ind. of Daphnia pulex L−1) and two durations (30 and 120 min) were tested in a nonturbulent treatment and five different turbulence levels [average root-mean-square (RMS) velocities ranging from 0 to 7.3 cm s−1, corresponding dissipation rates from 7.2 × 10−7 to 1.3 × 10−3 m2 s−3]. We hypothesized that the feeding rate of C. flavicans would be enhanced by turbulence due to increasing encounter rates up to a turbulence level above which a disturbance in post-encounter processes would lead to reduced feeding efficiency. However, the results showed no significant increase in the feeding rate of C. flavicans at intermediate turbulence. At high turbulence we found the expected significant negative response in the feeding rate of Chaoborus larvae. The feeding rate declined below the rates at nonturbulent and intermediate turbulence conditions as the average RMS velocity exceeded 3.1 cm s−1 (dissipation rate 9.9 × 10−5 m2 s−3, respectively).

Direct numerical simulation of ellipsoidal particles in turbulent channel flow

This paper investigates the behaviour of elongated, axi-symmetric ellipsoidal particles, their interaction with turbulence, and the effects of the ellipsoids on turbulence in a turbulent channel flow with Reτ = 150. The simulations are carried out with full four-way coupling using the point-source approach: the particles are affected by the fluid, the particles affect the fluid, and the particles can collide with each other or the wall using a realistic collision algorithm. The trajectories of the ellipsoids are tracked by solving the translational and rotational equations of motion in a Quaternion framework and are closed with hydrodynamic drag and torque laws. To specifically identify the effect of particle shape, simulations of single phase channel flow are compared to simulations with spherical particles and to simulations with ellipsoids. In all cases, the driving pressure drop, to establish a flow with Reτ = 150, is kept constant. Both the spherical particles and the ellipsoidal particles have a Stokes number of 5. Although the volume fraction is very low, 0.00725 and 0.0219 % for the spheres and ellipsoids, respectively, there is some effect of the particles and the ellipsoids on the turbulence. Although the transport terms in the turbulent kinetic energy equation of the fluid are hardly affected, the turbulence kinetic energy itself decreases by 6.0 % for the flow laden with spherical particles and 4.8 % for the ellipsoidal particles. The homogeneous dissipation of turbulence kinetic energy by the fluid decreases due to the addition of particles, and the production also decreases. The particles dissipate turbulence kinetic energy of the fluid phase, predominantly in the near-wall region. Because there is a high average slip velocity in the stream-wise direction between the particles and the fluid in the near-wall region, the root mean square of the particle velocity is higher than that of the fluid velocity in this direction. In the other directions, the root mean square velocities of the particles are significantly lower than of the fluid. There is, however, a positive slip velocity between the particles and the fluid in the wall-normal direction, indicating that the particles move towards the wall with a higher momentum than that they return to the centre of the channel with. As a result, there is a 4–5 times higher concentration of particles near the wall than in the centre of the channel. As both the spherical and the ellipsoidal particles are very small, there is no major difference in their overall behaviour. However, in the near-wall region, there are some profound differences. The collision mechanism of ellipsoids with the walls is significantly different compared to spheres, the former predominantly inducing rotation resulting from a collision and the latter predominantly moving away from the wall after colliding. This is confirmed by the strong rotation as well as large root mean square of rotation of the ellipsoidal particles in the near-wall region. This results in a slight inward shift of the peak of the root mean square velocities of the fluid and the ellipsoidal particles as well as the peak in slip velocity, driving the momentum transfer, compared to the simulations with the spheres. Finally, the statistics of the orientation show that the ellipsoids align in the stream-wise direction in the near-wall region, because of the fluid boundary layer as well as the particle–wall collisions, but that there is no significant orientation of the ellipsoids outside of the near-wall region.

Turbulence disturbs vertical refuge use byChaoborus flavicanslarvae and increases their horizontal dispersion

Summary Small-scale turbulence is a key factor in increasing encounter rates but also dispersing prey patches of planktonic organisms and can thus contribute to their distribution. Our hypotheses were that turbulence can (i) disturb the vertical refuge use of fourth instar Chaoborus flavicans larvae and (ii) also disperse them horizontally. Both were tested experimentally, across a gradient of turbulence. The vertical experiments were conducted in 46-L cylinders, providing the larvae a vertical refuge by darkening the bottom layer. The horizontal experiments were conducted in a 200-L aquarium, providing them a horizontal refuge by darkening one end of the aquarium. With both set-ups, two light treatments were tested: uniform darkness as a control and the bottom layer of the cylinder or either end of the aquarium darkened to provide a refuge. In addition to non-turbulent treatment, five different root-mean-square (RMS) velocities were generated, ranging from 0.6 to 10.2 cm s−1 in the vertical and to 10.9 cm s−1 in the horizontal experiment. Increasing turbulence had a significant effect on the distribution of C. flavicans larvae in both vertical and horizontal directions and increased the proportion of larvae in the non-preferred illuminated environment. In the vertical direction, the proportion of larvae in the refuge decreased linearly with increasing turbulence, showing a decline from 84% under calm conditions to 61% at the highest RMS velocity (10.2 cm s−1). The effect of turbulence was more prominent in the horizontal direction; without turbulence, 86% of the larvae occupied the refuge, but the proportion declined to 32% at 5.4 cm s−1 RMS velocity. In experiments with uniform darkness, the distribution of chaoborids was not affected by turbulence either in the vertical or in the horizontal direction. Our results indicate that turbulence has substantial effects on refuge use and distribution of C. flavicans larvae. Increasing turbulence can greatly diminish the ability of prey species to use their vertical refuges. Once away from their vertical refuge, chaoborids are highly vulnerable to horizontal dispersion, thus upsetting their horizontal escape behaviour. Small-scale turbulence is therefore likely to have considerable effects on food-web dynamics in lakes.

A model for particles deposition in turbulent inclined channels

A computational model for studying the deposition of particles in duct flows in the presence of thermal force under turbulent flow condition was developed. The V2F turbulence model was used to evaluate the mean flow and temperature fields, as well as the root mean-square fluctuation velocities. The V2F model was selected due to its potential for accurate evaluation of the intensity of turbulent velocity fluctuations normal to the wall. The instantaneous velocity field was simulated with the use of the Kraichnan continuous Gaussian random field model and anisotropic turbulence intensities generated by the V2F model. A Lagrangian particle trajectory model was used for evaluating the transport and deposition of particles. The Stokes drag, lift, Brownian, gravity and thermophoretic forces were included in the particle equation of motion. The predicted deposition velocities for vertical and horizontal ducts in the absence of thermophoresis effects were compared with the available experimental data and earlier empirical models, and good agreements were found. A series of simulations for particle deposition on the lower surface of an inclined duct for different imposed temperature gradient fields was performed. The earlier developed models for particle deposition in vertical and horizontal ducts were extended to inclined ducts in the presence of thermophoresis. The simulation results for particle deposition in turbulent inclined duct flows in the presence and absence of thermophoresis were shown to be in good agreement with the empirical model predictions.

Experimental investigation of the interaction between incoming wakes and instability mechanisms in a laminar separation bubble

The present paper reports the results of a detailed experimental study on the interaction process between incoming wakes and a laminar separated boundary layer. Experimental investigations have been carried out, for a low Reynolds number case, over a flat plate installed within a double contoured test section designed to produce an adverse pressure gradient typical of Ultra-High-Lift turbine blade profiles. Measurements have been performed by means of a single-sensor hot-wire anemometer. The phase-locked ensemble averaging technique, synchronized with the wake frequency, has been adopted to reconstruct the boundary layer space–time evolution in terms of ensemble-averaged velocity and velocity root mean square. These experimental data have been exploited in order to investigate how the unsteady incoming flow interacts with the instability mechanisms of the boundary layer to drive transition. Two main effects of wake passage were found. The low frequency disturbance associated with the wake passage triggers the Kelvin–Helmholtz instability process developing in the shear layer in the time interval between wakes. Moreover, the temporary reduction/suppression of the separation bubble observed when the incoming wake reaches the separated flow region has been found to be driven by linear stability mechanisms, which promote a quick amplification of the high frequency velocity fluctuations carried by wakes.

Higher order statistical analysis of Eulerian particle velocity data in CFB risers as measured with high speed particle imaging

Velocities of individual particles have been measured in gas-particle flow fields within the risers of two circulating fluidized bed (CFB), one with a 0.305 m diameter riser at the National Energy Technology Laboratory (NETL) and one with a 0.20 m diameter riser at Particle Solid Research Inc. (PSRI). The risers were operated at moderate to high particle concentrations (solid fluxes up to 400 kg/m2s). The NETL riser was operated in the core–annulus regime. The PSRI riser was operated in both the core–annulus and dense up-flow regimes. HDPE particles with a mean diameter of 800 μm were used in the NETL riser and FCC particles with a mean diameter of 80 μm were used in the PSRI riser. Particle velocities were measured with a high speed particle imaging velocimetry (HSPIV) system developed by the NETL. The HSPIV measurement technique has the ability to measure the velocities and trajectories of thousands of particles simultaneously in flows of high particle concentration. In this study, particle velocities are measured in a small two-dimensional field-of-view with dimensions in the range of 1–5 mm wide by 1–10 mm high, with a depth of about 1 mm. The size of the field-of-view is chosen to be similar to the size of CFD grid cells in models used by NETL and small enough that gradients of the mean particle velocity are small over the field-of-view, but large enough to achieve high data sample rates (at least ten velocity vectors per camera frame). In this study sample rates for particle velocity vectors were in the range of 0.1 to 1 million per second. This sample rate provides the high temporal resolution necessary to resolve the complete temporal domain of particle velocity. Particle velocities in each camera frame (at each point in time) are averaged to yield a pointwise instantaneous particle velocity. Using a recently developed particle velocity decomposition technique (Gopalan and Shaffer, 2011 [15]) the pointwise particle velocity time series is decomposed into a varying mean Eulerian component and a random fluctuating component. Statistics of the Eulerian velocity, namely the mean, RMS, skewness and kurtosis, and the granular temperature of the random fluctuating component are presented in this study. Results show that the vertical component of the overall mean Eulerian velocity decreases with increasing mass flux in both the core–annulus and dense up-flow regimes. The root mean square (RMS) of the Eulerian velocity in the horizontal direction is independent of the radial location in the NETL riser. In the PSRI riser, for both the core–annulus and dense upflow regime, the RMS of the horizontal Eulerian velocity decreases monotonically from the center of the riser to the wall. The radial profile of the RMS of the vertical Eulerian velocity for the PSRI riser is parabolic with a peak near r/R ~ 0.5–0.6 for the dense upflow regime. For the core–annulus regime the radial profile of the RMS of the vertical Eulerian velocity is relatively flat for both the NETL and PSRI risers, with a slight decrease near the wall in the PSRI riser. The skewness of the PDF of Eulerian velocity is near zero in the horizontal direction for the dense upflow regime in the PSRI riser, the only Eulerian velocity distribution for which the Gaussian approximation is appropriate. The skewness trends of the vertical velocity distribution are more complex and require further experimental confirmation. The kurtosis of the PDF of the Eulerian velocity is always higher in the horizontal direction than the vertical direction except at the wall of the riser. The 80 μm FCC particles in the PSRI riser showed much higher granular temperature than the 800 μm particles in the NETL riser. The granular temperature decreases monotonically for all conditions from the center of the riser to the wall. granular temperature is anisotropic for all conditions in both risers. The radial profile of anisotropy of granular temperature is relatively flat over most of the NETL and PSRI risers with values in the range of 0.3 to 0.6. Near the wall it decreases for the PSRI riser, while increasing for the NETL riser.

Consecutive tracking for ballistic missile based on bearings-only during boost phase

This paper proposes a modified centralized shifted Rayleigh filter (MCSRF) algorithm for tracking boost phase of ballistic missile (BM) trajectory with a highly nonlinear dynamical model based on bearings-only. This paper contributes three folds. Firstly, the mathematical model of an MCSRF for multiple passive sensors is derived. Then, minimum entropy based one-dimensional optimization search to adaptively adjust the probability of the different filters for real time state estimation is deployed. Finally, the unscented transform (UT) is introduced to resolve the asymmetric state estimation problem. Simulation results show that the proposed algorithm can consecutively track the BM precisely during the boost phase. In comparison with the unscented Kalman filter (UKF) algorithm, the proposed algorithm effectively reduces the tracking position and velocity root mean square (RMS) errors, which will make more sense for early precision interception.

Passive heat transfer in a turbulent channel flow simulation using large eddy simulation based on the lattice Boltzmann method framework

Abstract In this paper, a large eddy simulation based on the lattice Boltzmann framework is carried out to simulate the heat transfer in a turbulent channel flow, in which the temperature can be regarded as a passive scalar. A double multiple relaxation time (DMRT) thermal lattice Boltzmann model is employed. While applying DMRT, a multiple relaxation time D3Q19 model is used to simulate the flow field, and a multiple relaxation time D3Q7 model is used to simulate the temperature field. The dynamic subgrid stress model, in which the turbulent eddy viscosity and the turbulent Prandtl number are dynamically computed, is integrated to describe the subgrid effect. Not only the strain rate but also the temperature gradient is calculated locally by the non-equilibrium moments. The Reynolds number based on the shear velocity and channel half height is 180. The molecular Prandtl numbers are set to be 0.025 and 0.71. Statistical quantities, such as the average velocity, average temperature, Reynolds stress, root mean square (RMS) velocity fluctuations, RMS temperature and turbulent heat flux are obtained and compared with the available data. The results demonstrate great reliability of DMRT–LES in studying turbulence.

Offset continuation (OCO) ray tracing using OCO trajectories

Offset continuation (OCO) is a seismic configuration transform designed to simulate a seismic section as if obtained with a certain source-receiver offset using the data measured with another offset. Since OCO is dependent on the velocity model used in the process, comparison of the simulated section to an acquired section allows for the extraction of velocity information. An algorithm for such a horizon-oriented velocity analysis is based on so-called OCO rays. These OCO rays describe the output point of an OCO as a function of the Root Mean Square (RMS) velocity. The intersection point of an OCO ray with the picked traveltime curve in the acquired data corresponding to the output half-offset defines the RMS velocity at that position. We theoretically relate the OCO rays to the kinematic properties of OCO image waves that describe the continuous transformation of the common-offset reflection event from one offset to another. By applying the method of characteristics to the OCO image-wave equation, we obtain a raytracing-like procedure that allows to construct OCO trajectories describing the position of the OCO output point under varying offset. The endpoints of these OCO trajectories for a single input point and different values of the RMS velocity form then the OCO rays. A numerical example demonstrates that the developed ray-tracing procedure leads to reliable OCO rays, which in turn provide high-quality RMS velocities. The proposed procedure can be carried out fully automatically, while conventional velocity analysis needs human intervention. Moreover, since velocities are extracted using offset sections, more redundancy is available or, alternatively, OCO velocities can be studied as a function of offset.

Parametric vibration of submerged floating tunnel tether under random excitation

For the study of the parametric vibration response of submerged floating tunnel tether under random excitation, a nonlinear random parametric vibration equation of coupled tether and tube of submerged floating tunnel is set up. Subsequently, vibration response of tether in the tether-tube system is analyzed by Monte Carlo method. It may be concluded that when the tube is subjected to zero-mean Gaussian white noise random excitation, the displacement and velocity root mean square responses of tether reach the peak if the circular frequency of tube doubles that of tether; the displacement and velocity root mean square responses of tether increase as the random excitation root mean square increases; owing to the damping force of water, the displacement and velocity root mean square responses of tether decrease rapidly compared with tether in air; increasing the damping of the tether or tube reduces the displacement and velocity root mean square responses of tether; the large-amplitude vibration of tether may be avoided by locating dampers on the tether or tube.

Direct numerical simulation of turbulence in injection-driven three-dimensional cylindrical flows

Incompressible turbulent flow in a periodic circular pipe with strong injection is studied as a simplified model for the core flow in a solid-propellant rocket motor and other injection-driven internal flows. The model is based on a multi-scale asymptotic approach. The intended application of the current study is erosive burning of solid propellants. Relevant analysis for easily accessible parameters for this application, such as the magnitudes, main frequencies and wavelengths associated with the near-wall shear, and the assessment of near-wall turbulence viscosity is focused on. It is found that, unlike flows with weak or no injection, the near-wall shear is dominated by the root mean square of the streamwise velocity which is a function of the Reynolds number, while the mean streamwise velocity is only weakly dependent on the Reynolds number. As a result, a new wall-friction velocity $\(u_\tau{\,=\,}\sqrt{\tau_w/\rho}\)$, based on the shear stress derived from the sum of the mean and the root mean square, i.e. $\(\tau_{w,inj} {\,=\,} \mu |{\partial (\bar{u}+u_{rms})}/{\partial r}|_w\)$, is proposed for the scaling of turbulent viscosity for turbulent flows with strong injection. We also show that the mean streamwise velocity profile has an inflection point near the injecting surface.

Prestack Kirchhoff time migration of 3D coal seismic data from mining zones

Conventional seismic data processing methods based on post-stack time migration have been playing an important role in coal exploration for decades. However, poststack time migration processing often results in low-quality images in complex geological environments. In order to obtain high-quality images, we present a strategy that applies the Kirchhoff prestack time migration (PSTM) method to coal seismic data. In this paper, we describe the implementation of Kirchhoff PSTM to a 3D coal seam. Meanwhile we derive the workflow of 3D Kirchhoff PSTM processing based on coal seismic data. The processing sequence of 3D Kirchhoff PSTM includes two major steps: 1) the estimation of the 3D root-mean-square (RMS) velocity field; 2) Kirchhoff prestack time migration processing. During the construction of a 3D velocity model, dip moveout velocity is served as an initial migration velocity field. We combine 3D Kirchhoff PSTM with the continuous adjustment of a 3D RMS velocity field by the criteria of flattened common reflection point gathers. In comparison with post-stack time migration, the application of 3D Kirchhoff PSTM to coal seismic data produces better images of the coal seam reflections.