Periodic Velocity Fluctuations Research Articles

Tracking the Nearfield Evolution of an Initially Shallow, Neutrally Buoyant Plane Jet Over a Sloping Bottom Boundary

AbstractUnderstanding coastal plane jets which occur when a body of water discharges into an ocean or a lake through a channel or outlet is important, since they play a significant role in sediment, nutrient, and pollutant exchange. This study investigates the nearfield of initially shallow, neutrally buoyant plane jets, bounded by a free surface and a sloping bottom (Sloping Bottom Jet; SBJ) that issue into a laterally unconfined quiescent ambient both experimentally and numerically, and compares them with a plane jet flowing over a horizontal bottom (Horizontal Bottom Jet; HBJ). Results revealed that, different from the HBJ, the width and centerline velocity of SBJ decrease near the mouth. The SBJ width gradually increases after that as the transverse longitudinal velocity profile progressively transforms from a “top‐hat” into a Gaussian distribution. Once the Gaussian distribution is established, both jets diverge and centerline velocity decreases. Shear layers are generated on the sides of both jets with Kelvin Helmholtz‐type Coherent Structures (KHCS) developing inside. KHCS produce periodic velocity fluctuations with a Strouhal number of ∼0.079 and contribute significantly to momentum exchange and turbulent kinetic energy production. Since the thickness of the SBJ increases longitudinally, the vertical extent of KHCS also increases. When the two shear layers meet and merge at the centerline, they cause a flapping motion of the jet. This location is closer to the jet mouth for SBJs than for the HBJ. These findings demonstrate that a sloping bottom modifies the flow field from quasi‐2D for the HBJ to strongly 3D for SBJs.

Rough Surface Effect on CO2 Displacement at the Nanoscale

CO2 injection has great potential for enhancing shale oil recovery. Understanding the CO2 displacement in nanometer pores of shale is critical for developing effective CO2 injection techniques. In this work, we applied direct numerical simulation to study the effect of the rough surface on CO2 displacement in nanometer pores of shale. By quantifying CO2 displacement in rough nanochannels, we aimed to understand how surface roughness and morphology affect displacement progress. We found that the rough surface results in shrinkage of the CO2 displacement path, slowing overall displacement rate. Additionally, the pinch-point effect in a rough nanochannel impedes the smooth progression of the interface contact line, causing periodic velocity fluctuation that further hinders CO2 displacement. We also simulated the CO2 displacement in rough nanoporous media, finding that the rough surface leads to a substantial decrease in CO2 displacement efficiency, especially under low pressure gradient conditions. Our simulation results indicate that the rough surface of a shale nanometer pore has a nonnegligible effect on CO2 displacement.

Influence of aspect ratio on the wake dynamics of a pitching wing

The three-dimensional flow around a custom wing performing pitching oscillation is analyzed numerically using large-eddy simulation with the wall adapting local eddy-viscosity subgrid-scale model. The wake characteristics behind the pitching wing with three different aspect ratios, AR=2/3, 1, 4/3, is investigated in an upstream flow with Reynolds number, Re=2640. The Q−function is employed to identify the swirling structures in the flow field, and four primary vortices, namely, primary leading edge vortex (PLEV), secondary leading edge vortex, wing tip vortices, and trailing edge vortex, are identified. The evolution of vortices over the wing surface is presented in detail, and the arrangement of vortex rings in the wake is compared for three different aspect ratios. The PLEV develops into an arch vortex using the self-induction phenomenon, and the interlinking between the subsequent vortex rings depends on the arch vortex leaving the trailing edge. The force coefficient computed over the wing shows a positive thrust for AR=4/3. The mean velocity field contours indicate a spanwise loading on the wing, which is unaccountable in two-dimensional studies. The Reynolds stresses generated due to large-scale periodic velocity fluctuations in the wake are presented. The vorticity field in the midplane of the wing appears to be diffusive at higher aspect ratios. The comparison of vorticity from the three-dimensional and two-dimensional models suggests that a two-dimensional model will over predict the vorticity distribution and force coefficients in the wake.

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.

The performance of TBM disc cutter in soft strata: A numerical simulation using the three-dimensional RBD-DEM coupled method

Though disc cutters are widely used in mixed strata, eccentric wear of disc cutters in soft part of mixed strata is still an important factor influencing the efficiency and cost of TBM tunnelling. The eccentric wear of disc cutters is closely related to their motion state, however, in-depth research on this issue are still less. This paper introduces a three-dimensional RBD-DEM coupled method, adopting a newly developed RBD coupling calculation program in which the mutual interaction between disc cutters and strata was considered. Based on this method, the cutting process of disc cutter was simulated and the performance of disc cutter was investigated. The results show that the slip ratio decreases as the depth of penetration depth increases, however, the degree of influence is not obvious in terms of a critical value of penetration depth, on the contrary, if the penetration depth is too large, the slip ratio fluctuation increases. At the same penetration, linear velocity does not have significant effects on slip ratio, but it can have great effects on the torque of the disc cutter, especially the torque at the start-up phase. Existing eccentric wear would lead to the periodic fluctuations of angular velocity and enlarge both the rolling force and thrust force, therefore, existing wear will accelerate eccentric wear.

Transient startup characteristics of a drag-type hydrokinetic turbine rotor

The present study aims to reveal startup characteristics of a drag-type rotor as it is used to harness kinetic energy of flowing water. The rotational speed of the rotor and the hydraulic loads exerted on the rotor were correlated. After being validated through experimental results, a numerical scheme was used to investigate instantaneous flow parameter distributions and the torque on the rotor during the startup process. Startup processes were analyzed at different initial azimuthal angles. The results show that a successful startup of the rotor is featured by two consecutive stages, an acceleration stage and a transition stage, which is then followed by periodic fluctuations of the angular velocity. The startup failure is proved at two initial azimuthal angles of 70° and 150°. The former one is associated with positive static torque coefficient. The two failed startup processes share similar rotation positions where the rotor eventually halts. During the startup process, instantaneous flow patterns differ significantly from those obtained in static state. Small torque coefficients lead to an overall attenuation of the rotational speed and are ascribed to uniform pressure distributions over each side of the blade surface.

Unsteady turbulence structure in and downstream of a short elbow at post-critical Reynolds numbers

Unsteady turbulence structures in and downstream of a short elbow (radius to pipe diameter ratio 1.0) were studied experimentally for sub-to-post-critical Reynolds numbers (Re = 1 × 105 to 3 × 105). Measurement of the velocity field was carried out using particle image velocimetry (PIV) combined with proper orthogonal decomposition (POD) analysis. The mean velocity, turbulence intensity, power spectrum, spatial variation of POD modes, and time variation of instantaneous velocity and vorticity fields were evaluated. The experimental results indicate that a separation region forms around the elbow exit, which is almost independent of the Reynolds number for the post-critical Reynolds numbers. The power spectrum analysis indicates that the periodic oscillation is observed at Strouhal number 0.5, and that it originates from interaction of the secondary flow in the elbow and the unsteady separating flow behavior near the exit of the inner elbow wall. Furthermore, POD analysis was conducted to clarify the variation of POD modes in the elbow flow. It was found that the periodic velocity fluctuation is generated by the oscillation of the flow in spanwise direction downstream of the elbow, which consists of a secondary flow structure directed to the inner elbow wall, and a streamwise longitudinal structure along the centerline of the elbow. The POD reconstructed velocity and vorticity fields indicate periodic oscillation of the entire secondary flow arising from unsteady oscillation of a pair of vortices in the cross section of the elbow; this condition prevails in and downstream of the elbow.

Heat Transfer Enhancement in the Fields of Mass Forces, Pressure Gradients, Velocity Fluctuations, and Impinging Jets

Results are discussed of the experimental investigations into the effect of mass forces, longitudinal pressure gradient, and other factors on convective heat transfer enhancement in a turbulent flow. Heat transfer intensifiers include protrusions on and dimples in a heat transfer surface. It has been demonstrated that, depending on the intensifier type, the affecting factors have different effects on the convective heat transfer. The investigated combinations of affecting factors and intensifiers yield enhancement, deterioration, or no change in heat transfer. In a flow over concave or convex surfaces of moderate curvature with semispherical dimples, the centrifugal mass forces have the same effect on heat transfer as they do in the flow over smooth curvilinear surfaces. At the same time, in a flow over concave or convex surfaces with semispherical protrusions or transverse ribs, the mass forces do not affect the heat transfer. The investigation of the effect of a streamwise pressure gradient in a separation flow over spherical dimples has demonstrated that relaxation processes in the return flow in a spherical dimple results in a change in the of heat transfer rate opposite in sign compared to the effect pressure gradient in a flow without heat transfer intensifiers. It has been established that with the same relative dimensions of dimples and the same experimental conditions, the array of diffuser-type dimples can enhance heat transfer by a factor of 1.4 as compared with heat transfer in a flow over dimples with separation. The same heat transfer augmentation can be obtained in the array of spherical dimples from the imposition of periodic velocity fluctuations on a turbulent flow. It is shown that calculation of heat transfer for a flow over a surface with heat transfer intensifiers under the effect of disturbing factors sometimes requires correction to the used correlations that consider the effect of these factors.

Experimental investigations of turbulent decaying behaviors in the core-flow region of a propeller wake

This work investigates the turbulent decaying behaviors downstream of a propeller in the core-flow region. Both axial and tangential velocity fluctuations behind a two-bladed propeller were measured using a stationary hot-wire probe. Unexpectedly, the complex near-wake core-flow of the propeller is found to show a similar decay characteristic of homogeneous turbulence, such as grid turbulence. The decay of turbulence intensity is found to be dominated by the level of periodic velocity fluctuations, showing a similar behavior of the homogenous and isotropic turbulence. This turbulent decaying behavior of the core-flow can be adopted for future turbulent modeling techniques.

A Study on Flow Characteristic due to the Periodic Velocity Fluctuation of Upstream at Single Tube

A Study on Flow Characteristic due to the Periodic Velocity Fluctuation of Upstream at Single Tube

Study of Combustion Oscillation Mechanism and Flame Image Processing

In the pursuit of low emissions and large propulsion in the fields of civil and military aviation, combustion oscillation has been attracting increasing attention. In the present study, the coupling mechanisms of combustion oscillations in a model combustor are investigated. The oscillation characteristics and dynamic flame structure are studied for better comprehending their intrinsic nature. Current surveys show that inlet parameters, nozzle mounting positions, and fuel supply have important effects on combustion oscillation. The coupling and decoupling boundary composed of the factors mentioned above divides combustion into stable, transition, and instable regions. The effects of the operating conditions on combustion present a certain complexity in terms of studying oscillations. For the model combustor, periodic velocity fluctuation with large amplitudes is regarded as “bridge” that links pressure wave and heat release fluctuation, and it further leads to coupling processing. The acoustic simulation results suggest that the oscillation frequency is in accordance with the acoustic resonance frequency of the exhaust pipes. Finally, to investigate the internal spatiotemporal fluctuation mechanisms of the flame structure, the proper orthogonal decomposition method is used to decompose the dynamic flame structure. The phase relationship between the pressure wave and heat release fluctuation is investigated herein as well.

Thermal Boundary Layer Response to Periodic Fluctuations for Turbulent Flow

The detailed characterization of the thermal boundary layer under periodic fluctuations is vital to improve the performance of cooled turbine airfoils, as well as to assess noise thermal and structural fatigue. In the present contribution, we performed detailed unsteady Reynolds-averaged Navier–Stokes (URANS) simulations to investigate wall heat flux response to periodic flow velocity fluctuations over a flat plate. We also investigated the boundary layer response to sudden flow acceleration including periodic flow perturbations, caused by inlet total pressure variations. During a flow acceleration phase, the boundary layer is first stretched, resulting in an increase of the wall shear stress. Later on, due to the viscous diffusion, the low momentum flow adjusts to the new free stream conditions. The behavior of the boundary layer at low frequency is similar to the response to an individual deceleration followed by one acceleration. However, at higher frequencies, the mean flow topology is completely altered. One would expect that higher acceleration rates would cause a further stretching of the boundary layer that should cause even greater wall shear stresses and heat fluxes. However, we observed the opposite; the amplitude of the skin friction coefficient is abated, while the peak level is a full order of magnitude smaller than at low frequency.

Explicit finite-difference solution of two-dimensional solute transport with periodic flow in homogenous porous media

AbstractThe two-dimensional advection-diffusion equation with variable coefficients is solved by the explicit finitedifference method for the transport of solutes through a homogenous two-dimensional domain that is finite and porous. Retardation by adsorption, periodic seepage velocity, and a dispersion coefficient proportional to this velocity are permitted. The transport is from a pulse-type point source (that ceases after a period of activity). Included are the firstorder decay and zero-order production parameters proportional to the seepage velocity, and periodic boundary conditions at the origin and at the end of the domain. Results agree well with analytical solutions that were reported in the literature for special cases. It is shown that the solute concentration profile is influenced strongly by periodic velocity fluctuations. Solutions for a variety of combinations of unsteadiness of the coefficients in the advection-diffusion equation are obtainable as particular cases of the one demonstrated here. This further attests to the effectiveness of the explicit finite difference method for solving two-dimensional advection-diffusion equation with variable coefficients in finite media, which is especially important when arbitrary initial and boundary conditions are required.

FINITE DIFFERENCE SOLUTION OF TWO-DIMENSIONAL SOLUTE TRANSPORT WITH PERIODIC FLOW IN HOMOGENOUS POROUS MEDIA

Two-dimensional advection-diffusion equation with variable coefficients is solved by the explicit finite-difference method for the transport of solutes through a homogeneous, finite, porous, two-dimensional, domain. Retardation by adsorption, periodic seepage velocity, and a dispersion coefficient proportional to this velocity are permitted. The transport is from a pulse-type point source (that ceases after a period of activity). Included are the first-order decay and zero-order production parameters proportional to the seepage velocity, periodic boundary conditions at the origin and the end of the domain. Results are compared to analytical solutions reported in the literature for special cases and a good agreement was found. The solute concentration profile is greatly influenced by the periodic velocity fluctuations. Solutions for a variety of combinations of unsteadiness of the coefficients in the advection-diffusion equation are obtainable as particular cases of the one demonstrated here. This further attests to the effectiveness of the explicit finite difference method for solving two-dimensional advection-diffusion equation with variable coefficients in a finite media, which is especially important when arbitrary initial and boundary conditions are required.

FINITE DIFFERENCE SOLUTION OF TWO-DIMENSIONAL SOLUTE TRANSPORT WITH PERIODIC FLOW IN HOMOGENOUS POROUS MEDIA

Two-dimensional advection-diffusion equation with variable coefficients is solved by the explicit finite-difference method for the transport of solutes through a homogeneous, finite, porous, two-dimensional, domain. Retardation by adsorption, periodic seepage velocity, and a dispersion coefficient proportional to this velocity are permitted. The transport is from a pulse-type point source (that ceases after a period of activity). Included are the first-order decay and zero-order production parameters proportional to the seepage velocity, periodic boundary conditions at the origin and the end of the domain. Results are compared to analytical solutions reported in the literature for special cases and a good agreement was found. The solute concentration profile is greatly influenced by the periodic velocity fluctuations. Solutions for a variety of combinations of unsteadiness of the coefficients in the advection-diffusion equation are obtainable as particular cases of the one demonstrated here. This further attests to the effectiveness of the explicit finite difference method for solving two-dimensional advection-diffusion equation with variable coefficients in a finite media, which is especially important when arbitrary initial and boundary conditions are required.

Experimental investigation of influence of Reynolds number on synthetic jet vortex rings impinging onto a solid wall

Time-resolved particle image velocimetry was employed to study the effect of Reynolds number (Re sj) on synthetic jet vortex rings impinging onto a solid wall. Four Reynolds numbers ranging from 166 to 664 were investigated for comparison while other parameters were kept constant. It is found that the Reynolds number has a significant impact on the spatial evolution of near-wall vortical structures of the impinging synthetic jet. Velocity triple decomposition reveals that periodic Reynolds shear stresses produced by both impinging and secondary vortex rings agree well with a four-quadrant-type distribution rule, and the random velocity fluctuations are strengthened as Re sj increases. For radial wall jet, radial velocity profiles exhibit a self-similar behavior for all Re sj, and this self-similar profile gradually deviates from the laminar solution as Re sj is increased. In particular, the self-similar profile for low Re sj (166) coincides with the laminar solution indicating that periodic velocity fluctuations produced by vortex rings have little effect on the velocity profile of the laminar wall jet. This also provides evidence that the impinging synthetic jet is more effective in mixing than the continuous jet for the laminar flow. For the high Re sj, the mean skin friction coefficient has a slower decay rate after reaching peak, and the radial momentum flux has a higher value at locations far away from the impingement region, both of these can be attributed to the enhanced random fluctuations.

Statistical Analysis of Periodic Oscillations in LASCO Coronal Mass Ejection Speeds

A large set of coronal mass ejections (CMEs, 3463) has been selected to study their periodic oscillations in speed in the Solar and Heliospheric Observatory (SOHO) missions Large Angle and Spectrometric Coronagraph (LASCO) field of view. These events, reported in the SOHOLASCO catalog in the period of time 19962004, were selected based on having at least 11 height-time measurements. This selection criterion allows us to construct at least ten-point speed distance profiles and evaluate kinematic properties of CMEs with a reasonable accuracy. To identify quasi-periodic oscillations in the speed of the CMEs a sinusoidal function was fitted to speed distance profiles and the speed time profiles. Of the considered events 22 revealed periodic velocity fluctuations. These speed oscillations have on average amplitude equal to 87 kms(exp -1) and period 7.8R /241 min (in distance-time). The study shows that speed oscillations are a common phenomenon associated with CME propagation implying that all the CMEs have a similar magnetic flux-rope structure. The nature of oscillations can be explained in terms of magnetohydrodynamic (MHD) waves excited during the eruption process. More accurate detection of these modes could, in the future, enable us to characterize magnetic structures in space (space seismology).

Using travel times to simulate multi-dimensional bioreactive transport in time-periodic flows

In travel-time models, the spatially explicit description of reactive transport is replaced by associating reactive-species concentrations with the travel time or groundwater age at all locations. These models have been shown adequate for reactive transport in river-bank filtration under steady-state flow conditions. Dynamic hydrological conditions, however, can lead to fluctuations of infiltration velocities, putting the validity of travel-time models into question. In transient flow, the local travel-time distributions change with time. We show that a modified version of travel-time based reactive transport models is valid if only the magnitude of the velocity fluctuates, whereas its spatial orientation remains constant. We simulate nonlinear, one-dimensional, bioreactive transport involving oxygen, nitrate, dissolved organic carbon, aerobic and denitrifying bacteria, considering periodic fluctuations of velocity. These fluctuations make the bioreactive system pulsate: The aerobic zone decreases at times of low velocity and increases at those of high velocity. For the case of diurnal fluctuations, the biomass concentrations cannot follow the hydrological fluctuations and a transition zone containing both aerobic and obligatory denitrifying bacteria is established, whereas a clear separation of the two types of bacteria prevails in the case of seasonal velocity fluctuations. We map the 1-D results to a heterogeneous, two-dimensional domain by means of the mean groundwater age for steady-state flow in both domains. The mapped results are compared to simulation results of spatially explicit, two-dimensional, advective-dispersive-bioreactive transport subject to the same relative fluctuations of velocity as in the one-dimensional model. The agreement between the mapped 1-D and the explicit 2-D results is excellent. We conclude that travel-time models of nonlinear bioreactive transport are adequate in systems of time-periodic flow if the flow direction does not change.

Heat transfer and fluid flow characteristics of impinging jet using combined device with triangular tabs and synthetic jets

The fluid flow and heat transfer characteristics of an impinging jet with a combined active–passive device were investigated. The combined device consisted of triangular tabs and a synthetic jet array that provided periodic disturbance to the jet’s shear layer. The Reynolds number based on the nozzle diameter (D) was 3.9×104, and the nozzle-to-wall spacing (L/D) was 2. Four triangular tabs were attached to the nozzle outlet. The synthetic jets were issued at an angle of 30° against the primary jet. Under the triangular tab operation, the longitudinal vortices were generated and the transverse vortex of the primary jet collapsed. Moreover, the velocity of the primary jet increased because of contracted flow. Under the synthetic jet operation, the development of the transverse vortex was promoted due to the increased instability of the shear layer. The jet’s periodic disturbances strengthened because of the use of synthetic jets. The combined device operation led to increased velocity magnitude and periodic disturbance of the primary jet; thus, the peak RMS value of the velocity fluctuation was enhanced and the peak local Nusselt number near the stagnation point increased by 35% compared with cases where the device was not used. Additionally, the area-averaged Nusselt number was 10% larger than that obtained without the device. The heat transfer was governed by the jet’s velocity, RMS, and periodic velocity fluctuation. These results indicate that the fluid flow and heat transfer characteristics of an impinging jet could be controlled by optimizing the actuation conditions of the combined active–passive device.

Jet Flapping Control with Acoustic Excitation

The dynamics of flapping motion of a rectangular jet under acoustic excitation is studied experimentally by means of hot-wire measurement and flow visualization with smoke method. The excitation sufficiently enables“phase-lock”, which permitted us to extract the organized wave motion from a background field of finite turbulent fluctuations. The mean and fluctuation velocity are investigated and focused on the excitation frequency and the Reynolds number. As the excitation frequency decreases, it was found that the jet flapping and the jet spread were enhanced. The excitation with sub-harmonic frequency has significant effects on the rectangular jet behavior. The maximum value of the periodic velocity fluctuation strongly depends on the excitation frequency.