Large Velocity Fluctuations Research Articles

Experimental and numerical study of turbulent penetrative Rayleigh-Bénard flow of water near 4 °C in a box-shaped container

Understanding the reversal process of the large-scale circulation will facilitate system recognition of turbulent penetrative Rayleigh-Bénard convection. The present work describes experimental and numerical investigations on the Rayleigh-Bénard flow of water near 4 °C in a box-shaped container. The results indicate that the revised Nusselt number is slightly higher than average Nusselt number with adiabatic assumption. The Nusselt numbers from the simulations show good agreement with the experiment values. Large-scale circulation reversal becomes less likely with the increasing Rayleigh number and density inversion parameter. Reversal is not observed at large density inversion parameters and Rayleigh numbers. Large vertical velocity and temperature fluctuations at the center point are found during the reversal process. However, the Nusselt number is slightly affected by the occurrence of the reversal.

Moho Depth and Crustal Velocity Structure Beneath Stations RTC and TAM from Teleseismic Receiver-Function Analysis

We apply the P-wave receiver-function technique to invert teleseismic data to investigate the S- wave velocity structure beneath stations RTC and TAM in West Africa. Teleseismic waveform receiver function analysis is a widely used technique for studying crustal structure beneath broadband seismic stations. Our results show that the crust beneath RTC is relatively complex with large velocity fluctuations. In the upper crust, a low velocity layer extends from 8 to 12 km deep. At Tamanrasset crustal structure east and west of the station differ. We find a high-velocity zone between 2 and 8 km to the east that we attribute to a high conductivity unit corresponding to intrusions described in the literature. We find no similar feature west of the station. Our velocity models for RTC and TAM are consistent with their differing tectonic environments. TAM, on a stable craton, displays a fairly homogeneous velocity structure while RTC, on thick sediments in an ocean-continent transition zone, has more heterogeneous structure. This structural difference is reflected as well by the depth to Moho, ~20 km at RTC and nearly 38 km at TAM, although both have gradational velocity increases with depth.

The dynamics of accretion flows near to the innermost stable circular orbit

ABSTRACT Accretion flows are fundamentally turbulent systems, yet are classically modelled with viscous theories only valid on length scales significantly greater than the typical size of turbulent eddies in the flow. We demonstrate that, while this will be a reasonable bulk description of the flow at large radii, this must break down as the flow approaches absorbing boundaries, such as the innermost stable circular orbit (ISCO) of a black hole disc. This is because in a turbulent flow large velocity fluctuations can carry a fluid element over the ISCO from a finite distance away, from which it will not return, a process without analogy in conventional models. This introduces a non-zero directional bias into the velocity fluctuations in the near-ISCO disc. By studying reduced random walk problems, we derive a number of implications of the presence of an absorbing boundary in an accretion context. In particular, we show that the average velocity with which a typical fluid element crosses the ISCO is much larger than is assumed in traditional theories. This enhanced velocity modifies the thermodynamic properties of black hole accretion flows on both sides of the ISCO. In particular, thermodynamic quantities for larger ISCO stresses no longer display pronounced cusps at the ISCO in this new formalism, a result with relevance for a number of observational probes of the intra-ISCO region. Finally, we demonstrate that these extended models reproduce the trans-ISCO behaviour observed in GRMHD simulations of thin discs.

Capillary imbibition of confined monodisperse emulsions in microfluidic channels.

We study imbibition of a monodisperse emulsion into high-aspect ratio microfluidic channels with the height h comparable to the droplet diameter d. Two distinct regimes are identified in the imbibition dynamics. In a strongly confined system (the confinement ratio d/h = 1.2 in our experiments), the droplets are flattened between the channel walls and move more slowly compared to the average suspension velocity. As a result, a droplet-free region forms behind the meniscus (separated from the suspension region by a sharp concentration front) and the suspension exhibits strong droplet-density and velocity fluctuations. In a weaker confinement, d/h = 0.65, approximately spherical droplets move faster than the average suspension flow, causing development of a dynamically unstable high-concentration region near the meniscus. This instability results in the formation of dense droplet clusters, which migrate downstream relative to the average suspension flow, thus affecting the entire suspension dynamics. We explain the observed phenomena using linear transport equations for the particle-phase and suspension fluxes driven by the local pressure gradient. We also use a dipolar particle interaction model to numerically simulate the imbibition dynamics. The observed large velocity fluctuations in strongly confined systems are elucidated in terms of migration of self-assembled particle chains with highly anisotropic mobility.

Experimental investigation on a rotating detonation combustor with the pulse operating frequency of 10 Hz

The rotating detonation rocket engine has certain advantages in propulsion performance and is expected to develop into a new type of attitude/orbit control power device. In this paper, an experimental investigation on the detonation propagation characteristics during the short-time pulse operating process of rotating detonation combustor (RDC) was performed. A 60-mm RDC was employed, and the H2 and oxygen-rich air were used as the reactant. The results show that the experiment can successfully achieve rotating detonation mode when the RDC is operating with a 10-Hz pulse frequency. With the increase of oxidizer mass flow rate, the velocity and pressure of the detonation wave will increase. The average propagation frequency of the detonation wave shows a trend of first increasing and then slightly decreasing as increasing the equivalence ratio. There is a large velocity fluctuation in the RDC pulse operation, and the stability of the detonation wave is significantly affected by the equivalence ratio. The propagation velocity and stability of the detonation wave can be improved by increasing the supply mass flow rate or equivalence ratio with a certain condition. When the equivalence ratio is larger than 0.7, the establishment time of the detonation wave is less than 1 ms, which can meet the requirement of the fast response when the RDC is applied to attitude control engine.

Stenosis to stented: decrease in flow disturbances following stent implantation of a diseased arteriovenous fistula.

The arteriovenous fistula (AVF) is commonly faced with stenosis at the juxta-anastomotic (JXA) region of the vein. Implantation of a flexible nitinol stent across the stenosed JXA has led to the retention of functioning AVFs leading to the resulting AVF geometry being distinctly altered, thereby affecting the haemodynamic environment within it. In this study, large eddy simulations of the flow field within a patient-specific AVF geometry before and after stent implantation were conducted to detail the change in flow features. Although the diseased AVF had much lower flow rates, adverse flow features, such as recirculation zones and swirling flow at the anastomosis, and jet flow at the stenosis site were present. Larger velocity fluctuations (leading to higher turbulent kinetic energy) stemming from these flow features were apparent in the diseased AVF compared to the stented AVF. The unsteadiness at the stenosis created large regions of wall shear stress (WSS) fluctuations downstream of the stenosis site that were not as apparent in the stented AVF geometry. The larger pressure drop across the diseased vein, compared to the stented vein, was primarily caused by the constriction at the stenosis, potentially causing the lower flow rate. Furthermore, the WSS fluctuations in the diseased AVF could lead to further disease progression downstream of the stenosis. The change in bulk flow unsteadiness, pressure drop, and WSS behaviour confirms that the haemodynamic environment of the diseased AVF has substantially improved following the flexible stent implantation.

LES Investigation of a Piston-driven Synthetic Jet Actuator with Multiple Orifices

A piston-driven synthetic jet actuator has the potential for application in flow control and fundamental studies of turbulence, although the high-speed flow generated by this actuator is less investigated than a low-speed synthetic jet. The interaction of high-speed jets issued from a piston-driven synthetic jet actuator with multiple orifices is investigated with large eddy simulation (LES). The maximum jet Mach number is related to the maximum pressure inside the actuator regardless of the number of orifices. Temporal variations of the jet Mach number are almost identical for different cycles, and the jet formation in each cycle occurs under the same conditions despite the unsteady nature of the jet interaction. The phase-averaged statistics are used to examine the interaction of the synthetic jets. The converging, merging, and combined regions known for the interaction of continuous jets appear for the interaction of the high-speed synthetic jets slightly before the end of the blowing phase. However, the converging region is not clearly observed at the beginning of the blowing phase because the jets tend to be parallel to each other. Therefore, the combined region forms at a late stage of the blowing phase. Before the jets are combined, velocity fluctuations in the blowing phase become large near the furthest locations where the jets reach. Once the jets merge by their interaction, large velocity fluctuations are observed at the downstream end of the merging region. The probability density functions of velocity fluctuations in the blowing phase tend to deviate from a Gaussian distribution along the centerline of the jets. This deviation is more significant for the two-orifice model than for the four-orifice model under the same actuation frequency.

Initial boundary layer state of typical model-scale jet nozzles and its impact on noise

A set of 2-inch diameter nozzles is used to investigate the effect of varying exit boundary layer (BL) states on the radiated noise from high-subsonic jets. It is confirmed that nozzles involving turbulent boundary layers are the quietest while others, involving nominally-laminar BLs, are noisier. A turbulent BL is thicker and there is simply an effect of thickness on noise. A thicker BL results in a decrease in the sound pressure spectral amplitudes due to a less vigorous growth of instability waves in the jet’s shear layer. A nominally-laminar BL, besides being thinner, may also involve significantly higher turbulence intensities, much higher than that in a turbulent BL. Such a BL state, referred to as ‘highly disturbed laminar’, results in the largest noise amplitudes especially on the high-frequency side of the spectrum. This transitional state, often encountered with model scale nozzles, involves a ‘Blasius-like’ mean velocity profile but large velocity fluctuation intensities and intermittency. The higher initial turbulence adds to the increase in high-frequency noise. The results leave little doubt that an anomaly noted with subsonic jet noise databases in the literature is due to similar effects of differences in the initial boundary layer state.

Three-dimensional flow and turbulence structure around Sea Cucumber Apostichopus japonicus

The sea cucumber Apostichopus japonicus (A. japonicus) is a benthic invertebrate with a quasi-cylindrical body and several rows of papillae (conical prominence) on its body wall. As nutrient recyclers, A. japonicus plays a fundamental role in marine ecosystems and is one of the most important commercial aquaculture species in eastern Asia. Despite its ecological and economical importance, little is known concerning the flow characteristics surrounding A. japonicus on the organism scale. Research on the detailed flow structure at such a scale may serve as a significant step toward understanding the transport of sediment and nutrients in the vicinity of A. japonicus. This study sets up the D3Q27 multiple-relaxation-time lattice Boltzmann method to obtain the three-dimensional flow structure around an individual A. japonicus placed at the bottom of a flat-bed channel. The numerical model is validated by a water flume experiment. The flow patterns exhibit distinctive features at three different angles of attack (0°, 45°, 90°). The increasing inflow velocity (0.1m/s, 0.25m/s, 0.4m/s) brings no fundamental change to the structure of flow fields but induces more small-scale vortices and larger velocity fluctuations. This study investigates the complex turbulent flow fields around the A. japonicus and provides new insight for future studies on the interaction between water flow and A. japonicus.

Environmental fluid mechanics of minimum energy loss weirs: hydrodynamics and self-aeration at Chinchilla MEL weir during the November–December 2021 flood event

During the recent decades, a number of overflow embankment protection systems were implemented. One design is the Minimum Energy Loss (MEL) weir, developed to pass large flood events with minimum energy loss and low erosion. Several MEL weirs have successfully operated for decades in Australian catchments affected by heavy tropical and sub-tropical rainfalls with very flat gradients. Their historical performances are discussed and confirmed the design capability to pass large floods with small afflux and very small energy loss and little environmental impact including no significant erosion at the abutments. During a major flood, visual and quantitative observations were undertaken at the Chinchilla MEL weir. On the smooth converging chute, the observations showed that the inception of self‐aeration was a three‐dimensional process with a gradual change in free‐surface roughness, from a smooth glassy free-surface to a very-rough choppy surface. An optical technique (OF) was implemented to derive the contour maps of surface velocities based upon video movies taken from a sturdy tripod. The self-aerated flow velocity data were close to the backwater equation in the self‐aerated region, while highlighting regions of high- and low-velocities across the chute. The OF data showed large streamwise surface velocity fluctuations in the aerated flow region, consistent with the broad literature on self-aerated flow measurements. The ratio of transverse to streamwise surface turbulence intensity indicated a strong anisotropy of the free-surface turbulence.

Effect of Macroscopic Turbulent Gust on the Aerodynamic Performance of Vertical Axis Wind Turbine

Vertical Axis Wind Turbines (VAWTs) have proven to be suitable for changing wind conditions, particularly in urban settings. In this paper, a 2D URANS (Unsteady Reynolds-Averaged Navier Stokes) numerical analysis is employed for an H-Darrieus VAWT. A turbulent domain is created through systemically randomising the inlet velocity to create macro-turbulence in front of the VAWT. The parameters for spatial and temporal randomisation of velocity and its effects on the turbine performance are studied for a mean free stream velocity, U∞ = 10 m/s, and a tip speed ratio (TSR) of 4.1. The mean Coefficient of power (Cp) for randomised fluctuation of 2 m/s and half-cycle randomisation update frequency is 0.411 and for uniform inlet velocity is 0.400. The Cp vs. Tip Speed ratio plot suggests that the optimal tip speed ratio for operation is around 4.1 for this particular wind turbine of diameter 1 m, chord 0.06 m, and NACA 0018 airfoils. The effect of randomisation for tip speed ratio λ = 2.5, 3.3, 4.1, and 5.3 on the performance of the turbine is studied. Turbine wake recovers at a faster rate for macro-turbulent conditions and is symmetric when compared to wake generated by uniform velocity inlet. The maximum velocity deficit for a distance behind the turbine, x/d = 8 at TSR (λ) = 4.1 is 46% for randomised inlet and 64% for uniform inlet. The effect of randomisation for λ = 2.5 to 5.3 on the performance of the turbine is analysed. A time-varying gust based on International Electrotechnical Commission (IEC) Extreme Operating Gust is used to study the effect of fluctuating wind conditions in a turbulent environment. Since real-time conditions often exceed gust factors mentioned by IEC, winds with large gust factors such as 1.50, 1.64, and 1.80 are analysed. With an increase in gust amplitude, Ugust = 6 m/s to Ugust = 12 m/s on a free stream velocity of U∞ = 10 m/s, the mean Cp decreases from 0.41 to 0.35 since the wind turbine operates under tip speed ratios outside optimal range due to large fluctuations in incoming velocity.

Experimental Analysis of the Annular Velocity of a Capsule When Starting at Different Positions of a Horizontal Bend Pipe

The study of the annular slit flow field is important for energy consumption, transport efficiency, and the force on the capsule for hydraulic capsule transportation. A combination of physical experiments and theoretical analysis was used to study the annular flow field around a capsule that was set in motion at different positions of a horizontal bend pipe. We study the flow velocity distribution of the gap flow field at different bend positions of the capsule by changing the position of the capsule at the bend. We found that the distribution of the flow field remained similar for different starting positions of the capsule, but the flow velocity increased suddenly and dramatically at the inflow section of the ring gap. We recorded different velocity distributions of the annular gap on the concave and convex sides of the pipe; on the convex side, the streamline of the gap was smooth, and the change in velocity was relatively small. The flow velocity of the slit flow varied more notably on the concave side of the pipe, and there was a greater fluctuation in the flow velocity distribution. Because the effects of the capsule and the pipe on water flow were not the same, we found large fluctuations in gap flow velocity at different measuring points on the concave side. Gap flow velocity was most influenced by axial flow velocity. We found that the axial flow velocity was about one order of magnitude greater than the radial flow velocity or circumferential flow velocity. In this paper, we analyze the changes in the ring gap flow field of the capsule at different bending positions and analyze the reasons for the flow field changes and the flow velocity distribution law. This is of great significance to the study of the transport efficiency and energy consumption of the capsule. The results of this paper complement the study of capsule initiation at different positions in the bend and provide a reference point in terms of transport efficiency, energy consumption, and capsule stress. The results of this study promote the development of hydraulic capsule transportation.

Aerodynamics of a wing in turbulent bluff body wakes

The aerodynamics of a stationary wing in a turbulent wake are investigated. Force and velocity measurements are used to describe the unsteady flow. Various wakes are studied with different dominant frequencies and length scales. In contrast to the pre-stall angles of attack, the time-averaged lift increases substantially at post-stall angles of attack as the wing interacts with the von Kármán vortex street and experiences temporal variations of the effective angle of attack. At an optimal offset distance from the wake centreline, the time-averaged lift becomes maximum despite of small amplitude oscillations in the effective angle of attack. The stall angle of attack can reach 20° and the maximum lift coefficient can reach 64 % higher than that in the freestream. Whereas large velocity fluctuations at the wake centreline cause excursions into the fully attached and separated flows during the cycle, small-amplitude oscillations at the optimal location result in periodic shedding of leading edge vortices. These vortices may produce large separation bubbles with reattachment near the trailing-edge. Vorticity roll-up, strength and size of the vortices increase with increasing wavelength and period of the von Kármán vortex street, which also coincides with an increase in the spanwise length scale of the incident wake, and all contribute to the remarkable increase in lift.

Helheim Glacier's Terminus Position Controls Its Seasonal and Inter‐Annual Ice Flow Variability

AbstractOver the past decade, one of the largest contributors to total ice discharge across the Greenland ice sheet, Helheim Glacier, has experienced large fluctuations in ice velocity. In this study, we simulate the dynamics of Helheim, from 2007 to 2020, using the Ice‐sheet and Sea‐level System Model to identify the drivers of these large changes in ice discharge. By quantifying the impact of individual external forcing and model parameters on Helheim's modeled velocity, we find that the position of the calving front alone explains the dynamic variability of the glacier, as it has a direct and large impact on Helheim's ice velocity. The seasonal to inter‐annual variability of Helheim Glacier is, however, relatively insensitive to the choice of friction law or ice rheology factor. This study shows that more research on calving dynamics and ice–ocean interactions is required to project the future of this sector of Greenland.

Joint motion planning of industrial robot based on hybrid polynomial interpolation

To plan an industrial robot's point-to-point joint motion that has velocity constraints, the cubic polynomial interpolation planning method has angular acceleration discontinuity. The quintic polynomial interpolation planning method needs to set in advance the angular acceleration values of target points and easily causes angular velocity fluctuation. Because of the existence of quintic polynomials, hybrid polynomial planning 3-5-…-5-3 also easily causes large angular velocity fluctuation. To solve these problems, a hybrid polynomial interpolation planning method based on cubic and quartic polynomial interpolation is proposed. Cubic polynomial interpolation planning is applied to the first planning, and quartic polynomial interpolation planning is used for the rest of planning. The planning method can not only specify the angular velocities of intermediate target points but also ensure the continuity of angular acceleration. In addition, there is no need to set in advance the angular acceleration values of target points so as to avoid the angular velocity fluctuation caused by the unreasonable presetting of angular acceleration. Furthermore, in order to avoid excessive peaks of angular velocity during the joint motion planning, a calculation method for finding out reasonable motion time is put forward, under which the larger speed at two target points is taken as maximum, thus making the planned angular velocity fluctuation small. A case study is performed to verify the rationality and effectiveness of the planning method. Compared with the other two planning methods used for the case study, our planning method can effectively solve the problems of point-to-point joint motion planning with velocity constrains, therefore being beneficial to the working performance and service life of an industrial robot.

On air entrapment onset and surface velocity in high-speed turbulent prototype flows

In a free-surface spillway, the upstream flow is non-aerated and the flow becomes a strong air-water mix downstream of the onset location of air entrapment. Field observations were conducted over a steep spillway chute, and detailed quantitative measurements were undertaken in real-world high-speed flows with strong turbulence and very high Reynolds numbers within the range 107 to 108. The data showed that the onset of air entrapment is a complicated transient three-dimensional process in high-speed strongly-turbulent flows. A robust optical flow (OF) technique was applied and provided physically-meaningful surface velocities in the non-aerated flow region. The streamwise velocities were reasonably close to ideal fluid flow calculations, with large streamwise surface velocity fluctuations, in the non-aerated flow region. Overall, the study demonstrated the application of optical techniques to prototype spillway flows, provided that some careful validation was undertaken.

Statistical properties of two-dimensional elastic turbulence.

We numerically investigate the spatial and temporal statistical properties of a dilute polymer solution in the elastic turbulence regime, i.e., in the chaotic flow state occurring at vanishing Reynolds and high Weissenberg numbers. We aim at elucidating the relations between measurements of flow properties performed in the spatial domain with the ones taken in the temporal domain, which is a key point for the interpretation of experimental results on elastic turbulence and to discuss the validity of Taylor's hypothesis. To this end, we carry out extensive direct numerical simulations of the two-dimensional Kolmogorov flow of an Oldroyd-B viscoelastic fluid. Static pointlike numerical probes are placed at different locations in the flow, particularly at the extrema of mean flow amplitude. The results in the fully developed elastic turbulence regime reveal large velocity fluctuations, as compared to the mean flow, leading to a partial breakdown of Taylor's frozen-field hypothesis. While second-order statistics, probed by spectra and structure functions, display consistent scaling behaviors in the spatial and temporal domains, the third-order statistics highlight robust differences. In particular the temporal analysis fails to capture the skewness of streamwise longitudinal velocity increments. Finally, we assess both the degree of statistical inhomogeneity and isotropy of the flow turbulent fluctuations as a function of scale. While the system is only weakly nonhomogenous in the cross-stream direction, it is found to be highly anisotropic at all scales.

Effect of cusp magnetic field on the turbulent melt flow and crystal/melt interface during large-size Czochralski silicon crystal growth

The influences of cusp magnetic field on the turbulent melt flow and the crystal/melt interface are studied by three-dimensional unsteady simulations for 300 mm industrial Czochralski silicon crystal growth. Detailed comparisons are implemented for the melt flow pattern, temperature and velocity oscillations as well as the crystal/melt interface with and without magnetic field. It is found that the melt flow is complex and irregular in this system, accompanied with rich flow structures and large temperature and velocity fluctuations. Under the application of cusp magnetic field, the turbulent melt flow is significantly inhibited with the simple flow structure as well as the periodic temperature and velocity fluctuations. The anticlockwise periodical flow in the melt induced by cusp magnetic field can help to reduce the impurity in the crystal and suppress the crystal/melt interface deformation. In particular, the thermal waves on the melt free surface are firstly observed with cusp magnetic field for large-size Czochralski silicon crystal growth, traveling in the direction of crucible rotation.

Effect of Temperature and Multichannel Stopper Rod on Bubbles in Water Model of a Steel Continuous Caster

Herein, effects of the water temperature and a multichannel stopper rod with various gas hole diameters on the size and distribution of bubbles are investigated using a 0.5‐scale water model of a steel slab continuous casting mold. The breakage ratio of bubbles increases with the increasing of the water temperature. The mean diameter of bubbles decreases from 2.11 to 1.32 mm with the water temperature increasing from 25 to 34 °C. The smaller dynamic viscosity and surface tension of the water at a higher temperature result in large turbulence velocity fluctuation at the bottom of the submerged entry nozzle. The effect of water temperature on the bubble size and distribution can hardly be ignored in the water model experiment. With the increase of the hole diameter of the gas channel (Dh) of the stopper rod, the number of bubbles first decreases to the minimum value atDh = 1.5 mm and then increases, and bubble mean diameters increase, and the horizontal dispersion and penetration depth increase when the bubble diameter is larger than 1 mm. The multichannel stopper rod with the gas hole diameter of 1.5 mm is recommended under the current specific continuous casting condition.

Blowing-only opposition control: Characteristics of turbulent drag reduction and implementation by deep learning

Opposition control is an effective active control strategy for drag reduction, which has been extensively investigated. In the current work, the essential characteristics of drag reduction by the blowing-only opposition control scheme (i.e., opposition blowing) in turbulent channel flow are investigated. It is demonstrated that, under the condition of constant wall-normal mass flux, the drag reduction achieved by the opposition blowing scheme is almost independent of the allocation of the blowing velocity among all the effective blowing points. This feature simplifies the complexity of the control scheme and provides great convenience for the application of the convolutional neural network (CNN) to implement the opposition blowing scheme, i.e., only the direction of the wall-normal velocity at the detection plane needs to be predicted. In this paper, both the streamwise and spanwise wall shear stresses are taken as the input of the CNN model, and the reasonability of the CNN model is verified from a statistical perspective. It is found that as long as the directions of the large wall-normal velocity fluctuations are accurately predicted, the opposition blowing scheme can be successfully implemented, in which the CNN model is able to ensure a high prediction accuracy. Furthermore, applying the trained network model to a flow at a higher Reynolds number than the training set can still accurately predict the directions of the large wall-normal velocity fluctuations, which generalizes the applicability of the CNN model.