Velocity Decay Research Articles

Effect of side dilution jets on the velocity field and mixing in swirl and bluff-body stabilised annular confined flows

Stationary and mobile power generation systems, as well as those for aero propulsion, continue to rely heavily on gas turbines. Operational envelopes which critically depends on (flame) jet stabilisation mechanisms, such as swirl, are also affected by considerations for emissions control. In this context, the evolving swirling flow domains feature injection of side (peripheral) dilution air. Despite this, the role of side dilution jets on modifying the flow, turbulence, and mixing fields of turbulent bluff-body (BB) stabilised (swirling) annular flows has not been investigated. This study uses three-dimensional simulations, supported with experimental data to establish boundary conditions, to investigate the interaction of side dilution jets (Red = 18,000) with both non-swirling (S = 0) and swirling (S = 0.3) turbulent BB stabilised annular flows (Res = 35,500). Constant Temperature Anemometry (CTA) measurements are conducted to resolve inlet boundary conditions. The Reynolds Stress Model (RSM) and species transport model (isothermal) are used with validations against benchmark datasets. Results show that the introduction of side dilution jets into a confined domain, whether swirling or not, induces (asymmetrical) Peripheral Recirculation Zones (PRZ). However, the strength, length, and location of PRZ varies with swirl and the downstream positioning of side dilution jets (H = 2.6D, 6.6D). Additionally, side dilution jets increase centreline velocity decay for both, non-swirling and swirling BB flows. In terms of turbulence characteristics, addition of side dilution jets leads to significant gains in shear layer turbulence with an increase of 72% and 35% in turbulent kinetic energy for non-swirling and swirling BB flows, respectively. Analysis of the mixing characteristics shows negligible impact of side dilution jets in the upstream region, however, from x/D > 3 considerable improvement in central jet (fuel) mixing is observed.

How Peripheral Vestibular Damage Affects Velocity Storage: a Causative Explanation.

Velocity storage is a centrally-mediated mechanism that processes peripheral vestibular inputs. One prominent aspect of velocity storage is its effect on dynamic responses to yaw rotation. Specifically, when normal human subjects are accelerated to constant angular yaw velocity, horizontal eye movements and perceived angular velocity decay exponentially with a time constant circa 15-30s, even though the input from the vestibular periphery decays much faster (~ 6s). Peripheral vestibular damage causes a time constant reduction, which is useful for clinical diagnoses, but a mechanistic explanation for the relationship between vestibular damage and changes in these behavioral dynamics is lacking. It has been hypothesized that Bayesian optimization determines ideal velocity storage dynamics based on statistics of vestibular noise and experienced motion. Specifically, while a longer time constant would make the central estimate of angular head velocity closer to actual head motion, it may also result in the accumulation of neural noise which simultaneously degrades precision. Thus, the brain may balance these two effects by determining the time constant that optimizes behavior. We applied a Bayesian optimal Kalman filter to determine the ideal velocity storage time constant for unilateral damage. Predicted time constants were substantially lower than normal and similar to patients. Building on our past work showing that Bayesian optimization explains age-related changes in velocity storage, we also modeled interactions between age-related hair cell loss and peripheral damage. These results provide a plausible mechanistic explanation for changes in velocity storage after peripheral damage. Results also suggested that even after peripheral damage, noise originating in the periphery or early central processing may remain relevant in neurocomputations. Overall, our findings support the hypothesis that the brain optimizes velocity storage based on the vestibular signal-to-noise ratio.

On transient MHD heat transfer within a radiative porous channel due to convective boundary conditions

AbstractIn this paper, the steady/transient magnetohydrodynamics heat transfer within a radiative porous channel due to convective boundary conditions is considered. The solution of the steady‐state and that of the transient version were conveyed by the perturbation and finite difference methods, respectively. The heat transfer mechanism of the present work ascertain the influence of Biot number (), magnetizing parameter , radiation parameter , temperature difference, suction/injection , Grashof number , and time on velocity , temperature , skin friction , and Nusselt number . The results were established and discussed with the help of a line graph. It was found that the velocity, temperature, and skin friction decay with increasing suction/injection and magnetizing parameters while the Nusselt number upsurges with suction/injection at y = 0 and falls at y = 1. The steady‐state solution was in perfect agreement with the transient version for a significant value of time t. It is interesting to report that the Biot number has a cogent influence consequently, as its values upsurge the result of the present work slant the extended literature.

Numerical Investigation on the Effect of Geometric Shape and Outlet Angle of a Bladeless Fan for Flow Optimization using CFD Techniques

A three dimensional numerical study has been carried out to investigate the effect of various geometric shapes and slit angle on performance of the bladeless fan for various aerodynamic profiles. Aerofoils considered for the present study are E169, E473 and E479 which are then reformed into a typical bladeless fan arrangement. The numerical model of bladeless fan encompassing the outer domain is discretized tetrahedrally through finite volume approach using ANSYS ICEMCFD 20.0 and the necessary boundary conditions are specified in the ANSYS Fluent 20.0 pre-processor. The three dimensional fluid flow variations through and across the aerofoil have been simulated by solving the appropriate governing equations namely continuity and Reynolds Averaged Navier-Stokes equations (RANS). The turbulence induced in the fluid is modelled using SST k-ωmode of closure. Numerically predicted results of lift, drag and streamwise velocity decay along the jet centreline of a cylindrical channel are compared with literature and a very close agreement exists between the two. Upon validation, geometric shapes - circular and square cross section with aspect ratios of 1, 1,5 and 2 and slit angles of 20, 40, 60 and 80 degree for all the above three aerofoil configurations are analysed numerically for various inlet Reynolds number. Pressure, velocity contours and stream line across the bladeless fan are presented and discussed. From the study it is observed that Eppler 473 aerofoil profile with slit thickness of 1 mm and slit angle of 80° provided the maximum discharge ratio of 34.17 for an inlet mass flow rate of 80 LPS.

Entropy optimized radiative flow of viscous nanomaterial subject to induced magnetic field

Here we examine the time-dependent viscous nanofluid flow with induced magnetic field. Energy expression comprising dissipation and radiation is accounted. Additionally Brownian movement and thermophoresis have been addressed. Thermal expression examines the thermodynamical system performance. Flow subject to first order reaction is chemically reactive. Dimensionless system is obtained through adequate variables. Dimensionless differential systems are solved employing finite difference scheme. Variations in velocity, induced magnetic field, thermal field and concentration against influential parameters are explored. Entropy generation is designed. Computational results of drag force, thermal transport rate and Sherwood number have been organized. Decay in velocity is seen for suction variable. Larger magnetic Prandtl number rises the thermal field. An increment in induced magnetic field is noticed through magnetic Prandtl number. Reverse results of entropy and thermal field for radiation are observed. Larger approximation of thermophoresis variable has reverse impact on concentration and thermal field. Higher estimation of Brinkman number correspond to an increase of entropy rate.

Influence of sidewalls on three-dimensional turbulent wall jet: An experimental and numerical approach

In this study, with the help of with and without sidewall configurations, experimental and numerical approaches are utilised to investigate the sidewall influence on a three-dimensional turbulent wall jet. The mean flow profile of the three-dimensional turbulent wall jet is measured through experimental method and compared with the numerical results. In addition, the numerical method is used to characterise the turbulence characteristics of the wall jet for both the configurations. A 200 mm square nozzle (height h=20 ± 0.5 mm) is used to generate the developing jet exit profile for the experimental results. The Reynolds number based on the jet exit velocity and nozzle height is 25,000. The numerical results are obtained by solving the Reynolds Average Navier stokes (RANS) equations with low Reynolds number k−ε turbulence models proposed by Yang and Shih and Launder and Sharma. The experimental results are obtained by a single probe hotwire anemometer and a K-type thermocouple. It is observed that the sidewall affects the temperature distribution just after the potential core region x/h=5 whereas the velocity distribution is affected in the fully developed region after the downstream location x/h=22.5. Sidewalls drastically influenced the thermal and velocity decay in wall-normal and lateral directions. It is found from the numerical simulation that the decay of maximum streamwise velocity is increased by 9%, whereas centerline temperature decay is decreased by 25% in sidewall configuration as compared to without sidewall configuration. The contour plots of temperature and velocity also exhibit the sidewall effect on the whole domain. The Reynolds shear stress <u′v′> dominates in the vertical jet centerline plane atz=0, whereas <u′w′> dominates in the lateral direction at ymax plane in both the configurations. On the vertical jet centerline plane at z=0, Reynolds shear stress <u′v′> is nearly increased by 10% in the presence of sidewall compared to without sidewall configuration. The entrainment of the ambient fluid initially decreases, but after the downstream location x/h=22.4, it increases for the case of the sidewall compared to the corresponding case without the sidewall. This happens owing to an increase in maximum turbulent kinetic energy generation inside the flow domain by 10%. The turbulent heat flux <T′V′> dominates in lateral and wall-normal shear layers in both the configurations. The correlations of decay of temperature and velocity are also suggested through numerical techniques.

Schlieren and BOS velocimetry of a round turbulent helium jet in air

Seedless velocimetry is gaining interest in many industrial and research applications. We report on a comparative study of time-resolved optical velocimetry using traditional, mirror-type knife-edge schlieren optics versus Background-Oriented Schlieren (BOS) of subsonic round turbulent helium jets in air at Red = 5890 and 11,300. Digital images with 1024 pixels streamwise resolution (0 ≤ x/d ≤ 200) were captured at 6000 frames/s in large ensembles. Velocimetry was performed on these results by digital image correlation (DIC) using OpenPIV software, and by streak-schlieren analysis of x-t diagrams (Kymography). Limited PIV data were also collected for verification of the schlieren velocimetry results. Both BOS and traditional schlieren show partial success in measuring the mean-flow helium jet self-similarity in terms of the 1/x decay of centerline velocity, Gaussian-shaped radial velocity profiles, and linear spreading rate of the jet. Visualized turbulent eddies, used as tracers in schlieren velocimetry, are observed to last longer than is necessary for this purpose in the present helium jets. Also, the measured convective velocity appears to be sufficiently robust to sum to the jet mean velocity in some of the results. Kymography yields better overall results than DIC, which we attribute to Kymography's spatiotemporal “spectrum” of jet velocities, enabling the discrimination of fast eddies near the jet centerline from slower ones near the jet periphery. DIC and other analysis methods suffer from a path-averaging bias which negatively affects the results. The reduction of kymographic data for velocimetry was done manually and also by a Fourier-transform image-feature-orientation code, both yielding equivalent results.

Wake rotation impacts on wake decay

This study considers the behavior of a well-conditioned swirling turbulent wake. A custom swirling wake generator was used to produce a swirling wake absent of a tower that complicates the flow regime. Four swirling wakes were studied with swirl numbers ranging from 0.19 to 0.37 and compared to a non-swirling counterpart. Two-component Laser-Doppler Anemometry was used to measure mean and turbulent quantities in the wake. Measured profiles were analyzed based on similarity theory applied to a swirling wake. The results show that wake behavior was impacted by the initial swirl strength. Generally, wake growth rate and axial deficit decay rate increased with higher levels of swirl, and tangential velocity decay rate increased with lower levels of swirl. However, the wake that diffused fastest was the case with a swirl number of 0.27.

Numerical and analytical solutions for unsteady nanofluid flow capturing magneto-hydrodynamic and radiation effects

This study elaborates porous medium impact on boundary-layer nanofluid flow by a permeable surface. The Buongiorno's nanofluid model that captures simultaneous attributes of Brownian and thermophoretic diffusions is introduced for modeling and analysis. In addition, magneto-hydrodynamic and radiation aspects are also part of this research. The partial differential equations of considered nonlinear problems are reduced to the ordinary ones by similarity variables. Kellerbox and homotopic algorithms are utilized to solve the nonlinear differential problems. The obtained results from both techniques are compared. Besides, the pertinent parameters are addressed graphically. We witnessed a decay in fluid velocity subjected to an increment in the porosity parameter.

Lip thickness effect on high bypass co-flowing jet mixing

PurposeThe purpose of this study is to analyse numerically and experimentally the effects of lip thickness (LT) and bypass ratio on co-flowing nozzle under subsonic and correctly expanded sonic jet decay at different Mach numbers.Design/methodology/approachCo-flowing jets from co-flowing nozzles of different LTs, 0.2, 1 and 1.5 Dp (where Dp is the primary nozzle exit diameter = 10 mm), with an annular gap of 10 mm at main jet exit Mach numbers 0.6 have been studied experimentally and the other cases have been performed numerically. The co-flowing jet with 2 mm LT was used for comparison.FindingsCo-flowing jet axial pitot pressure decay, axial static pressure decay, axial velocity decay, radial velocity decay and streamline velocity contours were analyzed. The results illustrate that the mixing of the co-flowing jet with profound LT is prevalent to the co-flowing jet with 2 mm LT, at all Mach numbers of the current study. Also, the LT of the co-flowing jet has a strong impact on jet mixing. Co-flowing jets with 10 mm and 15 mm LT with a constant co-flow width of 10 mm experience a considerably advanced mixing than co-flowing jets with 2 mm LT and a co-flow width of 10 mm.Practical implicationsThe application of bypassed co-flow jet is in turbofan engine operates efficiently in modern civil aircraft.Originality/valueAll subsonic jets are considered correctly expanded with negligible variation in axial static pressure. However, in the present study, static pressure along the centerline varies sinusoidally up to 9% and 12% above and below atmospheric pressure, respectively, for primary jet exit Mach number 1.0. The sinusoidal variation is less for primary jet exit Mach numbers 0.6 and 0.8 in static pressure decay.

Non-Cooperative Space Target De-Tumbling Approach Using Multiple High-Temperature Superconducting Coils

Aiming at de-tumbling a non-cooperative space target safely and efficiently, an eddy brake approach utilizing multiple high-temperature superconducting (HTS) coils is proposed in this paper. The configuration design criteria of HTS coil are constructed first to guarantee the validity of the eddy brake method. Then, considering that each HTS coil is mounted on a servicing satellite, the desired position and attitude of each servicing satellite is calculated to meet the design criteria. Next, the eddy brake process is analyzed, where how to generate and keep the desired magnetic field is considered, and then the angular velocity decay process of the target is presented. Concretely, a nonlinear control law utilizing the consensus theory is adopted to achieve and keep the desired formation of the multi-satellite system. Finally, numerical simulations are carried out to testify the proposed method using different number servicing satellites, and the simulation results verify the effectiveness of the proposed method.

Large-eddy simulation of a planar offset wall-jet with heat transfer: Characterization, turbulent kinetic energy and Reynolds shear stress budgets

Planar wall-attaching offset jets can be found in a variety of engineering applications. In the present work, a planar turbulent offset wall-jet, at a Reynolds number of 7500 and an offset-ratio of 5, with heat transfer is numerically studied using large-eddy simulation (LES). First, a grid sensitivity study is performed and the quality of the mesh used is assessed using LES index of quality of resolution and an appropriate mesh is selected. Next, the solver is validated for mean streamwise velocity, streamwise Reynolds normal stress, and the evolution of coefficient of pressure on the wall, using reference experimental data from the literature. Thereafter, the unsteady flow features of the jet, maximum velocity decay and jet-spread, statistical features of flow and temperature, and turbulent kinetic energy (TKE) budgets and in-plane component of Reynolds shear stress are studied. The streamwise evolution of Nusselt number on the wall shows three distinct peaks and the location of these peaks correlates with a change-of-sign of the wall skin-friction coefficient. The domain can be categorized into three distinct regions namely, the recirculation, impingement and wall-jet regions. Distinct flow and thermal characteristics are observed depending on the region of interest. The peak magnitudes of Reynolds normal and shear stresses increase as one moves from the recirculation region to the impingement region and decreases thereafter attaining a lowest value in the wall-jet region. The heat transfer from the wall is observed to be most effective in the recirculation region. The production of TKE is dominant in all the three regions of the flow, whereas the Reynolds shear stress budgets indicate that the production and velocity–pressure-gradient terms are dominant and balance each other in all the three regions of the flow.

3D printable self-propelling sensors for the assessment of water quality via surface tension

Water contamination is a serious global challenge and an on-site and out-of-lab method of assessment of contamination level is highly needed. In this study, we report the potential of using printable and biodegradable propelling sensors as indicators of water contamination in sewage wastewater. We used reactive 3D inkjet printing technology to fabricate self-propelling sensors which can quickly indicate the lowering of surface tension value caused by sewage contamination, and other surface tension lowering pollutants. The Z-shaped sensors were fabricated, with the dimensions of 2.0 ​mm at the longest side and 0.1 ​mm in thickness, from regenerated silk fibroin which is an environmentally friendly and biodegradable material. Inkjet printing has the advantage of high resolution and precise deposition of materials allowing the fabrication of small millimetre-sized sensors doped with a surface tension modifying polymer which acts as the ‘fuel’ to drive the sensors on the water surface via surface tension gradient. Our results showed that the sensor's propulsion velocity decay rate is an excellent metric to indicate the presence and approximate level of sewage contamination.

Characterizing the biomechanical differences between novice and expert point-of-care ultrasound practitioners using a low-cost gyroscope and accelerometer integrated sensor: A pilot study.

Point-of-care ultrasound (POCUS) has become an important diagnostic tool in acute care medicine; however, little is known about the biomechanical differences between novice and expert practitioners. A low-cost ($50 CAD) gyroscope and accelerometer integrated sensor was assembled and affixed to an ultrasound probe. Seventeen participants, nine novices and eight experts, were recruited to perform three abdominal and four cardiac scans on a standardized patient. Participant demographics, time per scan, average acceleration, average angular velocity, decay in acceleration and angular velocity over time, and frequency of probe movements were analyzed. Video capture with blinded video review was scored. On video review, experts had higher image optimization and acquisition scores for both abdominal and cardiac scans. Experts had shorter scan times for abdominal (7s vs. 26s, p=0.003) and cardiac (11s vs. 26s, p<0.001) scans. There was no difference in average acceleration (g) between novices and experts performing abdominal (1.02 vs. 1.01, p=0.50) and cardiac (1.01 vs. 1.01, p=0.45) scans. Experts had lower angular velocity (°/s) for abdominal scans (10.00 vs. 18.73, p<0.001) and cardiac scans (15.61 vs. 20.33, p=0.02) There was a greater decay in acceleration over time for experts performing cardiac scans compared to novices (-0.194 vs. -0.050, p=0.03) but not for abdominal scans or when measuring angular velocity. The frequency of movements (Hz) was higher for novices compared to experts for abdominal (16.68 vs. 13.79, p<0.001) and cardiac (17.60 vs. 13.63, p=0.002) scans. This study supports the feasibility of a low-cost gyroscope and accelerometer integrated sensor to quantify the biomechanics of POCUS. It may also support the concept of "window shopping" as a method by which experts obtain abdominal and cardiac views, where sliding is used to find an acoustic window, then smaller rocking and tilting probe movements are used to refine the image.

Space-time behavior of the solution to the Boltzmann equation with soft potentials

In this paper, we get the quantitative space-time behavior of the full Boltzmann equation with soft potentials (−2<γ<0) in the close to equilibrium setting, under some velocity decay assumption, but without any Sobolev regularity assumption on the initial data. We find that both the large time and spatial behaviors depend on the velocity decay of the initial data and the exponent γ. The key step in our strategy is to obtain the L∞ bound of a suitable weighted full Boltzmann equation directly, rather than using Green's function and Duhamel's principle to construct the pointwise structure of the solution as in [25]. This provides a new thinking in the related study.

NUMERICAL MODELLING OF THE JET FLOW

In this work, plane turbulent jets are simulated numerically using a three-dimensional computational model. The simulated jets exclude the influence of the free surface and solid boundaries on the jets. The accuracy of turbulent schemes is assessed by analyzing the decay of the average velocity, the rate of growth of the jet, and the similarity of the profiles of the longitudinal and vertical velocities. The results of the two turbulent closure schemes are compared with experimental and theoretical studies to determine their accuracy.

Relationship Between the Mobility of Aggregates and Fluid Penetration Depth Across a Range of Fractal Dimensions Using Stokesian Dynamics.

The hydrodynamic behavior of fractal aggregates plays an important role in various applications in industry and the environment, and has been a topic of interest over the past several decades. Despite this, crucial aspects such as the relationship of the mobility radius, Rm, with respect to the fractal dimension, df, and the fluid penetration depth, δ, have largely remained unexplored. Herein, we examine these aspects across a wide range of df's through a Stokesian dynamics approach. It takes into account all orders of monomer-monomer interactions to construct the resistance matrix for the entire cluster, which is assumed to be rigid. Statistical fractals created using algorithms such as diffusion limited aggregation (DLA), cluster-cluster aggregation (CCA), tunable Monte Carlo algorithm, and a deterministic Vicsek fractal, with df varying from 1.76 to 3, and the number of monomers ranging from 20 to 10 240 are considered. While confirming the expected asymptotic cluster-size independence of the hydrodynamic ratio, β = Rm/Rg (where Rg is the radius of gyration of the cluster), this study reveals a monotonically increasing trend for β with increasing df. The decay of the fluid velocity within the aggregate is quantified via the concept of penetration depth (δ). Analysis shows that the dimensionless penetration depth (δ* = δ/Rg) approaches asymptotic constancy with respect to cluster size in contrast to a weak dependency of the form δ* ∼ (Rg/a)-(df-1)/2, predicted by the mean-field theory (a being the monomer radius). Furthermore, the penetration depth is found to decrease rapidly, in an exponential manner, with increasing β. This establishes a quantitative relationship between the resistance experienced by the cluster and the degree of penetration of fluid into it. The implications of these results are further discussed.

Dynamic mechanical response and damage evolution of cemented tailings backfill with alkalized rice straw under SHPB cycle impact load

Dynamic disturbances such as blasting excavations cause damage to backfills. To ascertain and improve the dynamic mechanical response of backfill under frequent disturbance, single and cyclic impact load tests were conducted on cemented tailings backfill mixed (CTB)with different contents of alkalized rice straw (ARS) using a split Hopkinson pressure bar. The evolution laws of the dynamic mechanical properties and damage characteristics of CTB with the impact times and the ARS content were analyzed. The results showed that (1) The dynamic compressive strength (DCS) of CTB was improved by the ARS under single impact load with different strain rates. (2) Under the cyclic impact loads, the DCS, dynamic elastic modulus, and peak strain of CTB decreased with the increase in the cyclic impact times. Under the same impact times, the DCS of the backfill first increased and subsequently decreased with the increase in the ARS content, and 0.3% was the best content. (3) The unit volume absorbed energy of CTB was increased by the ARS and the longitudinal wave velocity (LWV) decay rate of CTB was decelerated under the cyclic impact loads. (4) The established damage evolution equation of CTB based on the LWV showed that its cumulative damage degree was increased by the cyclic impacts. Moreover, the incorporation of ARS was beneficial for decelerating and reducing the damage evolution rate and degree. The ARS was wrapped by hydration products in the CTB matrix, and its adhesion with the matrix promoted the ARS to play a bridging role, and inhibited and slowed down the development, propagation degree and propagation rate of cracks in CTB under cyclic impact. When the content of ARS was further increased, the probability of overlapping was significantly improved, which was not conducive to the adhesion between ARS and CTB matrix, resulting in the decline of the mechanical properties of CTB. The research results provide a reference for the understanding of the dynamic mechanical response of CTB with ARS under frequent blasting disturbances.

Thermal pollution level reduction by sweeping jet-based enhanced heat dissipation: A numerical study with calibrated Generalized k-ω (GEKO) model

Thermal pollution is commonly the damage to natural ecosystems caused by industrial water discharge into the environment above ambient temperature. The highly efficiently dissipative sweeping jet actuator is thus considered as the solution to the reduced thermal pollution level. The thermal dissipation capacities of free steady, pulsed, and sweeping jets with a Reynolds number of 49,761 and a Strouhal number of 0.0152 are compared by numerical simulation. The Reynolds stress model (RSM) is used as the turbulence model, with the Generalized k-ω (GEKO) model innovatively used to calibrate jet dissipation rates. The temperature fields are first compared to confirm the overwhelming temperature dilution effect of the sweeping jet in both streamwise and normalwise directions. Then the reasons for the enhanced heat dissipation by sweeping jet are explained by numerous time-averaged and time-resolved contours of the distributions of flow-related physical quantities. The time-averaged flow fields directly show that the sweeping jet has the fastest streamwise velocity decay and the most uniform spanwise momentum distribution due to the continuous transverse oscillation, and hence has the moderate turbulence kinetic energy (TKE) over a wide spanwise range due to the destruction of the potential core commonly seen in a steady jet. The relative turbulence kinetic energy fields, excluding the contribution to turbulence generation from velocity magnitudes, show that the alternating formation and dissipation of shear layer-induced vortices most contribute to the inherent unsteady patterns. In the planes perpendicular to the streamwise direction, the spacings between adjacent velocity contour lines of the sweeping jet increase at the fastest rate with the streamwise distance, indicating the excellent entrainment capacity. The difference among the three primary Reynolds stresses of the sweeping jet is smaller than that of the other two jets, indicating that the sweeping jet converts more streamwise momentum into spanwise and normalwise velocity and turbulence and thus has higher spatial homogeneity. Pathlines of massless particles dispersed in the near-field show that the sweeping jet has the most substantial entrainment capability due to the longest motion distance of the traces originating outside the jet shear layer, although it is difficult for the ambient fluids to penetrate the central jet impact region. Finally, three mixing potential metrics, i.e., jet decay rate, jet spreading rate, and entrainment factor, are used to evaluate the three jets' more general mixing characteristics qualitatively. To extract the sweeping jet's centerline velocity and spreading width, the Lagrangian transformation, which invariably aligns the jet ejection direction with x-axis, is performed on the phase-averaged velocity flow fields. The near-field results show that compared with the steady jet, the sweeping jet boasts a 176% increase in the jet decay rate, a 241% increase in the jet spreading rate, and a 369% increase in the entrainment factor. In the far-field, the entrainment capacity of the sweeping jet is not as good as that of the pulsed jet but is still 68% higher than that of the steady jet. In all, this paper establishes a theoretical basis for the application of fluid oscillators to heat pollution level reduction and more general scenarios involving requirements for enhanced heat and mass transfer by proposing the standard mixing indicators of sweeping jets similar to those of conventional jets, and also serves as a pioneering example for the future possible extension of the GEKO model to a broader range of industrial CFD simulations.

Flow characterization in the uphill region of pulsed oblique round jet

An experimental study to investigate the effect of jet pulsations on the wall jet development in the uphill region of an obliquely inclined round water jet has been performed using particle image velocimetry technique. The study has been performed at a constant nozzle to target wall distance, L/D = 4 (D is the diameter of the nozzle) by varying the jet impingement angle θ (=60o, 45o, and 30o), Reynolds numbers (ReD = 1900 and 3280; based on nozzle diameter and average nozzle exit velocity Uavg), and Strouhal number (0 ≤ St ≤ 0.9; St=fDUavg, where f is the frequency of external pulsation). It is observed that the pulsations have no significant effect on the jet in the free jet region when the target plate is kept at a distance less than the potential core length (the potential core extends up to 4D–6D from the nozzle exit toward the impinging plate), and the jet impingement region extends up to 1D from the plate. The location of the stagnation point is observed to depend on all three parameters: the jet pulsation, the Reynolds number, and the jet impingement angle. An increase in Reynolds number creates an adverse pressure gradient toward the downstream direction in the uphill region, resulting in an intrusion of ambient fluid toward the wall jet. The distance between the geometric center and the stagnation point is observed to be minimum for St = 0.44 at both the Reynolds numbers. The wall jet that develops in the uphill region exhibits a maximum velocity decay rate and a jet half width growth rate corresponding to St = 0.44. These parameters are also observed to increase with the increase in the Reynolds number and decrease in the jet impingement angle. The velocity fields reconstructed using proper orthogonal decomposition reveal the dominant modes in the upstream location for St = 0.44 than the other pulsations. Furthermore, we observed that the jet after impingement deviates entirely in the downhill region for θ=30o irrespective of the jet pulsation, suggesting a non-dependence of the critical angle of inclination on jet pulsations for L/D = 4.