Time-averaging Technique Research Articles

Self-stabilized holographic interferometry for the measurement of two-dimensional vibrations

In this paper, we focus on the features of a self-stabilized holographic recording system with a photorefractive crystal as a vibration optical sensor. With it and the time-average holographic interferometry technique, high quality vibration maps of a thin plate were obtained. The results show that the system is capable of phase control in low diffraction efficiency conditions and under high disturbances.

Benchmark Experiments for Turbulent Mixing in the Scaled-Down Upper Plenum of High-Temperature Gas-Cooled Reactors Under Accident Scenario

The time-averaged particle image velocimetry technique has been applied to measure flow mixing in the Michigan Multi-jet Gas-mixture Dome (MiGaDome) facility, a 1/12th, scaled-down model of the high-temperature gas-cooled reactor upper plenum. Measurements were first conducted with one jet injection into the upper plenum for various Reynolds numbers (Re = 1022, 2038, 4097, and 6021). The experimental region of interest includes a plane within the dome located above one of the jet inlets of interest. First- and second-order statistics are presented and discussed to analyze the local mixing process and turbulent characteristics under the effects of jet spreading and jet impingement. Results have shown that the normalized statistics of the jet reach asymptotic behavior as the inlet Reynolds number is increased. By investigating the two-dimensional budgets for the momentum equation on the measurement plane, it was concluded that the contribution of turbulent diffusion is minor near the enclosure surface where strong convection is present due to impingement. An additional measurement on a triple-jet injection case has shown that jet spreading is suppressed by a recirculation zone, which causes a redistribution of turbulent fluctuations. The detailed local fluctuation patterns/coherent structures have been examined through a proper orthogonal decomposition analysis.

INVESTIGATION OF OVERSTABILITY ANALYSIS OF OSCILLATORY THERMO-BIOCONVECTION WITH ARBITRARY DIRECTIONS OF VIBRATION

The vibrations under the temperature gradient in the fluidic system could induce a thermo-vibrational force even in the absence of gravity, which helps to develop the bioconvective flow in the system. The present study aims to explore the effect of arbitrary directional vibrations on the overstability of the fluidic system. Flow governing equations are framed using the Kuznetsov model to study the effect of vibrations on the suspension's overstability. The secular equation is obtained from the flow governing equations utilizing the time-averaged technique, linear stability analysis, and finally the Galerkin method for rigid-rigid boundary conditions. The study shows that the vibrations stabilize the system when applied at an angle ranging from (0, π/6) ∪ (2π/3, π) but destabilize it when applied at an angle ranging from (π/6, 2π/3). Furthermore, the gyrotactic microorganisms slowly diffuse in the horizontal direction and quickly diffuse in the vertical direction.

On the turbulent boundary layer over a flat plate at moderate Reynolds numbers

Two separate experimental campaigns of a spatially developing turbulent boundary layer under approximately zero-pressure-gradient at moderate Reynolds numbers (1700<Reθ<3400) are conducted with stereoscopic Particle Image Velocimetry (PIV) and one component Hot Wire Anemometry. This range of Reynolds numbers is found to be of particular interest for turbulent boundary layer control investigations. The motivations behind this work rely on the lack of recent studies that provide a rigorous experimental database on a flat plate turbulent boundary layer, openly available online. This is critical as, in most of the cases, the modification of the statistics resulting from turbulent boundary layer control strategies are compared with a smooth baseline reference. The statistics of the velocity fields, obtained with the two techniques, show a good match with the direct numerical simulation in literature results. We focused on the skin friction evaluation by means of Clauser's chart technique. The near wall turbulence activity and the associated coherent structures are investigated by means of the Variable Interval Time Averaging technique using the hot wire signal. The influence of the acquisition and algorithm parameters as well as the effect of the Reynolds number are reported. The logarithmic and outer structures are investigated by applying the Uniform Momentum Zones technique to the PIV dataset. The hierarchical distribution of the uniform momentum zones as a function of the wall distance as well as their variation with the Reynolds number confirm the validity of the attached eddy model even at the moderate Reynolds numbers of the current investigation.

Effect of artificial large-scale structures on bursting phenomenon in turbulent boundary layer

In this study, the effect of artificial very-large-scale motions (AVLSMs) generated by a dielectric-barrier-discharge plasma actuator (DBD-PA) array on the bursting phenomenon in the near-wall region (y+≤40 in the present study) was experimentally investigated. The DBD-PA array was embedded in the wall where the turbulent boundary layer (TBL) was fully developed. A hot-wire rake consisting of nine I-type probes was used to measure the streamwise fluctuation velocity throughout the TBL at two positions downstream from the DBD-PA array. At both measurement positions in the streamwise direction, it was observed that the negative artificial very-large-scale motions (nAVLSM) flanked on two sides by positive motions (pAVLSM) could extend to above 0.3δ height from the wall. The bursting phenomenon was detected using the variable-interval time-averaging technique. It was observed that the bursting phenomenon in pAVLSM was suppressed, whereas it was enhanced in nAVLSM. Further investigation showed that the bursting frequency normalized by the inner variables in nAVLSMs is the same as that of pAVLSMs only at the secondary measurement position, which is far from the DBD-PA array. These results suggest that the quasi-steady quasi-homogeneous theory only holds when the TBL is sufficiently developed.

Opposition control of turbulent spots

Opposition control of artificially initiated turbulent spots in a laminar boundary layer was carried out in a low-turbulence wind tunnel with the aim to delay transition to turbulence by modifying the turbulent structure within the turbulent spots. The timing and duration of control, which was carried out using wall-normal jets from a spanwise slot, were pre-determined based on the baseline measurements of the transitional boundary layer. The results indicated that the high-speed region of the turbulent spots was cancelled by opposition control, which was replaced by a carpet of low-speed fluid. The application of the variable-interval time-averaging technique on the velocity fluctuation signals demonstrated a reduction in both the burst duration and intensity within the turbulent spots, but the burst frequency was increased.

Modification of a turbulent boundary layer by circular cavities

It is shown how well-chosen perforations in a wall flow can locally reduce skin friction drag by modifying the generation of bursts in the boundary layer. For this purpose, a detailed hot wire boundary layer experimental investigation of the flow past a perforated plate, complemented with large eddy simulations, is carried out and compared to the smooth case. The perforated plate is obtained with an array of flush-mounted circular cavities. These cavities are disposed in a periodic staggered arrangement. For the three tested flow velocities, the momentum thickness-based Reynolds number varies from Reθ=1830 to 3380 and the cavity diameter and spacing in wall units, respectively, from d+=130 to 250 and L+=587 to 1075, the latter being identical in both spanwise and streamwise directions. The mean velocity profiles evidence a thickening of the viscous sublayer and a decrease in the friction velocity as compared to the smooth wall case. The application of the Variable Interval Time Averaging technique highlights an upward shift of the bursts from the wall and an attenuation of the average burst intensity and duration. Spanwise measurements evidence an overall bursts attenuation despite the lack of spanwise uniformity. The three-dimensional (3D) mean flow topology arising from the large eddy simulations provides evidence for the qualitative similarities between the current setup and the spanwise wall oscillations.

The meandering bend features of large-scale structures and the related coherent structures

The present study aimed to investigate the meandering bend features of large-scale structures, including large-scale motions (LSMs) and very-large-scale motions (VLSMs) and the relationship between these meandering features and small-scale hairpin vortex structures in a turbulent boundary layer (TBL). A multichannel hot-wire probe was used to measure the streamwise fluctuation velocity in the log-region. The LSMs and VLSMs were extracted based on their streamwise length, and the position of the hairpin vortex was determined using the Variable-Interval Time-Averaging (VITA) technique. The meandering bend features of LSMs and VLSMs were calculated from each extracted meandering segment of the structure. In addition, the meandering of the LSMs and VLSMs was highly related to the distribution of the VITA events, indicating that the LSMs and VLSMs were formed from the hairpin vortex packets. Further investigation revealed that the meandering bend direction of LSMs/VLSMs was independent of the asymmetry of the VITA events, suggesting that the asymmetry of the detected small-scale hairpin vortex has nearly no effect on the meandering bend of the LSMs/VLSMs.

EXACT DYNAMIC LOCALIZATION CASES IN THE SEMICLASSICAL APPROXIMATION

In this work we apply results from the semiclassical method and time-average techniques to a one-dimensional lattice described by a tight-binding Hamiltonian with long-range interactions to all neighbors. The charged particle moves in the lattice in the presence of an external homogeneous and rapidly oscillating electric field. Such a semiclassical application corresponds to the Dignam and de Sterke theorem referred to as the phenomenon of “exact dynamic localization” (EDL). This theorem has been deduced within the quantum formalism by the authors. To illustrate the validity of EDL, several external electric fields were chosen in this work. The results indicate that the semiclassical and the quantum formalism are equivalent and yield the same conditions for EDL.

Experimental Study of Energy Contribution by Coherent Structures in a Turbulent Boundary Layer

Kang, Y.-D. and Koo, B., 2021. Experimental study of energy contribution by coherent structures in a turbulent boundary layer. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 579–583. Coconut Creek (Florida), ISSN 0749-0208. An experimental study was carried out to investigate a contribution of large coherent structures in turbulent flow. Both dynamometer and Clauser fitting method were adopted to calculate the frictional velocity near the wall having uncertainty. In order to quantify coherent structures, Particle Image Velocimetry (PIV) system was used to reveal their characteristics connecting through boundary layer. Variable Interval Time Averaging (VITA) technique scanned through flow fluctuations at the edge of boundary layer, whether there was a large deviation from the background flow. According to the presence of large scale structures, the flow field was divided into weak and strong cases to validate their existence. Changing a free-stream velocity from 0.5 m/s to 1.0 m/s increased the Reynolds stress more than 3 times. Proper Orthogonal Decomposition (POD) analysis was also applied to compare the energy budgets between them.

A neural network predicting the amplitude of the N2pc in individual EEG datasets

Objective. The N2pc is a small amplitude transient interhemispheric voltage asymmetry used in cognitive neuroscience to investigate subject’s allocation of selective visuo-spatial attention. N2pc is typically estimated by averaging the sweeps of the electroencephalographic (EEG) signal but, in absence of explicit normative indications, the number of sweeps is often based on arbitrariness or personal experience. With the final aim of reducing duration and cost of experimental protocols, here we developed a new approach to reliably predict N2pc amplitude from a minimal EEG dataset. Approach. First, features predictive of N2pc amplitude were identified in the time-frequency domain. Then, an artificial neural network (NN) was trained to predict N2pc mean amplitude at the individual level. By resorting to simulated data, accuracy of the NN was assessed by computing the mean squared error (MSE) and the amplitude discretization error (ADE) and compared to the standard time averaging (TA) technique. The NN was then tested against two real datasets consisting of 14 and 12 subjects, respectively. Main result. In simulated scenarios entailing different number of sweeps (between 10 and 100), the MSE obtained with the proposed method resulted, on average, 1/5 of that obtained with the TA technique. Implementation on real EEG datasets showed that N2pc amplitude could be reliably predicted with as few as 40 EEG sweeps per cell of the experimental design. Significance. The developed approach allows to reduce duration and cost of experiments involving the N2pc, for instance in studies investigating attention deficits in pathological subjects.

Adaptive step size EASI algorithm based on nonlinear correlation for on-line separation of satellite micro-vibration sources

The on-line blind separation of satellite micro-vibration source signals can provide the basis for the on-line suppression of satellite micro-vibration from the perspective of vibration source control, which has important academic significance and remarkable engineering application value for improving the performance of satellite positioning accuracy. Focusing on the inherent contradiction between convergence rate and steady-state error in on-line separation, a novel adaptive step size EASI (Equivariant Adaptive Separation via Independence) algorithm is proposed. Since nonlinear uncorrelation of Blind Source Separation (BSS) results displays its independence equivalently, minimizing nonlinear correlation also marks the optimal state of separation. According to this basic BSS estimation principle, a separation index based on nonlinear correlation coefficients is constructed to represent the separation degree, and on-line updating equations of separation index is derived by time average technique. Then, a suitable nonlinear monotone function is adopted to map separation index to the step size, and the step size can be adaptively adjusted according to separation degree. Finally, the novel adaptive blind separation algorithm in this paper is formed by introducing the adaptive step size to traditional EASI algorithm. Compared with fixed step size EASI algorithm (FS-EASI) and several variable step size EASI algorithms, the proposed algorithm greatly improves convergence rate while ensuring high separation accuracy. Effectiveness and superiority of the proposed algorithm are verified by simulation case analysis and satellite cabin structure experimental study.

Turbulence generation and decay in the Taylor–Couette system due to an abrupt stoppage

Abstract

Effect of large-scale structures on bursting phenomenon in turbulent boundary layer

This study investigated the effect of large-scale structures including large-scale motions (LSMs) and hairpin vortex in the log-region on the bursting phenomenon in the buffer layer using the simultaneous measurement of the streamwise velocity fluctuation in the log-region and buffer layer of the turbulent boundary layer (TBL) with a multi-channel hot-wire probe. The Reynolds number in the present study, calculated from the friction velocity uτ, was Reτ=δuτ/ν=1260, where δ is the boundary layer thickness and ν is the kinematic viscosity. The LSMs were classified as positive large-scale motions (pLSMs) and negative large-scale motions (nLSMs) based on their streamwise velocities. Their effect on the bursting that was detected by the Variable-Interval Time-Averaging (VITA) technique was visualized. As a result, it was evident that the bursting became active with the existence of the pLSMs, while it was suppressed with the nLSMs. In addition, hairpin vortex was observed by applying the VITA technique to the velocity fluctuation of the log-region. Further investigation revealed that the hairpin vortex can also be observed in the log-region with the condition that the bursting was detected in the buffer layer, suggesting that the existence of a hairpin vortex in the log-region slightly increased the bursting phenomenon near the wall.

Phase Shift Strategies in Phase Shifting Time Averaging Interferometry

Classical time-averaging interferometry technique is widely used for MEMS/MOEMS dynamic behavior investigations. It provides useful information on resonant vibration frequencies and mode shapes in the form of Bessel fringe images. To evaluate the information on objects’ vibration amplitude distribution, one needs to further process Bessel fringe images (besselograms). Among different available approaches to besselograms processing, Temporal Phase Shifting is of particular interest since it provides the most accurate results. However, it requires an additional cumbersome correction routine via specially designed look-up-tables fitted to the calculation algorithm used. In this paper, we numerically investigate the possibility of reducing Bessel images argument (phase) evaluation error by different phase shifting strategies. Two different 5 step algorithms are investigated for that purpose. Additionally, a quick and robust correction procedure based on evaluated phase distribution is presented. It allows us to speed up the calculations significantly. Moreover, such an approach is much less susceptible to noise in comparison to a look-up-table solution.

Characteristics of array of distributed synthetic jets and effect on turbulent boundary layer

An array of distributed round synthetic jets was used to control a fully developed turbulent boundary layer. The study focused on the related skin friction drag reduction and mechanisms involved. The control effects were analyzed by measuring the streamwise velocities using a hot-wire anemometer downstream of the array. A reduction in the skin friction was observed both in the regions downstream of the orifices and in the regions between two adjacent orifices. A statistical analysis with the variable-interval time-averaging (VITA) technique demonstrated a weakened bursting intensity with synthetic jet in the near-wall region. The streamwise vortices were lifted by the upwash effect caused by synthetic jet and induced less low-speed streaks. The control mechanism acted in a way to suppress the dynamic interaction between the streamwise vortices and low-speed streaks and to attenuate the turbulence production in the near-wall region. The forcing frequency was found to be a more relevant parameter when synthetic jet was applied in turbulent boundary layer flow control. A higher forcing frequency induced a higher reduction in the skin friction. The power spectral density and autocorrelation of the fluctuating velocities showed that the synthetic jets gradually decayed in the streamwise direction, having an effect as far as 34.5 times the displacement thickness that was on the trailing edge of the distributed synthetic jets array.

Memory effects on two-dimensional overdamped Brownian dynamics

We consider the effects of memory on the stationary behavior of a two-dimensional Langevin dynamics in a confining potential. The system is treated in an overdamped approximation and the degrees of freedom are under the influence of distinct kinds of stochastic forces, described by Gaussian white and colored noises, as well as different effective temperatures. The joint distribution function is calculated exactly by means of time-averaging techniques, and the long-term behavior is analyzed. We determine, by using the stochastic thermodynamics formalism, the influence of noise temporal correlations on the energetics in the steady-state regime. As a result, we find that non-Markovian effects lead to a decaying heat exchange with spring force parameter, which is in contrast to the usual linear dependence obtained when only Gaussian white noises are presented in overdamped treatments. Also, the memory time-scale affects in a nontrivial fashion the entropy production rate associated with stationary states.

A time-average filtering technique to improve the efficiency of two-layer wall models for large eddy simulation in complex geometries

Two-Layer wall models have been recurrently studied since they represent a good physical model for Large Eddy Simulations with underresolved wall regions. Specifically, those based on the Reynolds Averaged Navier-Stokes equations are of special interest, since they can be applied to a wide range of conditions including non-equilibrium flows. Nonetheless, these models are affected by two recurrent problems, the “log-layer mismatch” and the resolved Reynolds stresses inflow, which until now, have been dealt with separated techniques. In this work, a time-filtering methodology is applied to tackle both issues at once with a single and low-computational-cost step, easily applicable to complex three-dimensional geometries. The time-filtering technique has already been applied to other types of wall models to mitigate the “log-layer mismatch.” Now, it is applied for the first time in the Two-Layer wall model context, showing its ability not only in avoiding the mismatch issue but also in blocking the resolved Reynolds stress inflow, dramatically improving the wall model performance and generality compared to other existing implementations. A methodology to determine the necessary temporal filter length is proposed and validated in equilibrium and non-equilibrium conditions. Additionally, the filter size influence on large-scale unsteady flow motions is assessed. Good results are obtained in steady and unsteady flow regimes by suppressing the LES highest frequencies while taking into account large-scale temporal effects.

Probing RNA structure and interaction dynamics at the single molecule level

RNA structures and their dynamic fluctuations lie at the heart of understanding key biological process such as transcription, splicing, translation and RNA decay. While conventional bulk assays have proven to identify and characterize key pathway intermediates, the generally dynamic nature of RNA structures renders the information obtained from time and ensemble averaging techniques necessarily lacking in critical details. Here we detail Single-Molecule Kinetic Analysis of RNA Transient Structure (SiM-KARTS), a method that readily monitors structural fluctuations of single RNA molecules through the repetitive interaction of fluorescent probes with an unlabeled, surface-immobilized RNA target of virtually any length and in any biological context. In addition, we demonstrate the broad applicability of SiM-KARTS by kinetically fingerprinting the binding of cognate tRNA ligand to single immobilized T-box riboswitch molecules. SiM-KARTS represents a valuable tool for probing biologically relevant structure and interaction features of potentially many diverse RNA metabolic pathways.

TIME-AVERAGED SPRAY ANALYSIS IN THE NEAR-FIELD REGION USING BROADBAND AND NARROWBAND X-RAY MEASUREMENTS

The characterization of a spray in the near-field region is challenging because of its high optical density in this region. X-ray based techniques, with weak scatter and strong penetration properties, can provide better characterization than optical assessment techniques in this region. In this work, the effects of various operating parameters on the optical depth (defined as the accumulated liquid thickness in the beam path times the X-ray attenuation coefficient) and spray profile of an atomizing spray in the near-field region are evaluated based on time-averaged X-ray analysis techniques. Controlling parameters in the spray structure include swirl ratio, liquid phase Reynolds number, and gas phase Reynolds number. Data from the broadband X-ray radiographs obtained using a tube source at Iowa State University and from focused beam measurements at the Advanced Photon Source at Argonne National Laboratory are compared. The X-ray tube source at Iowa State University was operated at two different energy levels, which reveals that the X-ray tube source energy influenced the magnitude of the optical depth but did not change the shape of the distribution. For the no swirl condition, gas flow rate and liquid flow rate had opposite effects on the spray profile, where the spray widens as the gas flow rate increases and narrows as the liquid flow rate increases. As the swirl ratio increases from 0 to 1, the spray widens and then narrows. The critical swirl ratio at which the spray reaches its widest spread differs at different flow conditions.