Non-uniform Velocity Research Articles

Object kinetics perception in auto-stereoscopic vision.

In this paper, we study the conditions for the perception of object kinetics, produced by a multiview auto-stereoscopic system and a set of still images. We assess the capabilities and performances of such an optical system to encode complex trajectories and kinetics of objects moving in depth. In particular, we set up rules to create motions with nonuniform velocities, when obtained by motion parallax induced by the observer. We establish a link with plenoptic systems from where we derive applicable scaling rules to ensure stereoscopic vision and to provide fluid motion perception with satisfying visual comfort. Finally, we scale the optical system, thanks to obtained parameters, to emulate the perception of object motions in depth with fluid kinetics and to create impressive motion effects.

Experimental visualization of flow structure inside subchannels of a 4 × 6 rod-bundle

A fuel element geometry frequently used in nuclear reactors is the rod-bundle. In it, coolant flows axially through subchannels formed between the rods. The mixing of cooling fluid in a rod-bundle reduces the temperature differences in the coolant and along the perimeter of the rods. To ensure thermal performance of a nuclear reactor, detailed information about heat transfer and turbulent mixing taking place within subchannels is required. To improve the prediction ability of the subchannel analysis using computer code, a considerable amount of experimental data should be utilized for developing a new model for subchannel analysis. Experimental work has been conducted in the PRIUS-I (in-PWR Rod-bundle Investigation of Undeveloped mixing flow across Subchannel) test facility, which has adopted the matched index-of-refraction (MIR) technique to visualize the multi-dimensional velocity and turbulence fields in an unheated 4 × 6 rod-bundle geometry as well as to provide benchmark data for computer code validation. The 4 × 6 rod-bundle array has the same pitch-to-diameter (P/D) as the prototype fuel assembly. PRIUS-I test facility can simulate the uniform and non-uniform inlet flow conditions in order to validate the effect in crossflow between the fuel assemblies of PWR. In this study, the crossflow and mixing phenomena that exist between two adjacent subchannels through the gaps are visualized in any cross section of the rod-bundle geometry using a MIR technique. For non-uniform inlet velocity condition, the flow characteristics near the inlet where the flow mixing occurs actively is compared with computational fluid dynamics (CFD) calculation results. The experimental data can be used as benchmark data for the system safety analysis code or CFD code validation.

공기유속 분포와 열전달 향상조건을 고려한 발전 플랜트용 공랭식 응축기(ACC)의 성능에 관한 수치적 연구

공기유속 분포와 열전달 향상조건을 고려한 발전 플랜트용 공랭식 응축기(ACC)의 성능에 관한 수치적 연구

Optimization of die design for thin-walled flat multi-port tube with the aid of finite element simulation

Abstract Die design plays a critical role in aluminum alloy profile production, which directly influences the quality of the final products and the service life of the die. In the present study, a porthole die for a multi-hole extrusion process of a thin-walled flat multi-port tube is designed and optimized through numerical simulations. The simulation results show that with the original die, the extruded tube has a non-uniform velocity distribution in its cross-section and bends outward when exiting the die orifice. Meanwhile, a severe stress concentration in the mandrel teeth causes the premature failure of the die, and an uneven deflection of the mandrel teeth and bearing causes the arc-shaped wall thickness of the tube to exceed the tolerance level, which is validated by corresponding extrusion experiments. To solve these problems, two periods of die structure modifications are proposed, and their extrusion processes are simulated. The optimized die is confirmed to exhibit better performance than the original one in terms of lowered maximum die stress, increased bearing pressure, and decreased distortion of the final products. The results of the extrusion experiments indicate that, with the optimized die, a highly qualified tube with good appearance, desired dimensional accuracy, fine-grained microstructure, and high-pressure bearing capacity is manufactured successfully, and the die failure is delayed effectively.

Dufour and Soret effects on unsteady heat and mass transfer for Powell-Eyring fluid flow over an expanding permeable sheet

In the present analysis, the Dufour and Soret effects on unsteady heat-mass transfer of a viscous incompressible Powell-Eyring fluids flow past an expanding/shrinking permeable sheet are reported. The fluid boundary layer develops over the variable sheet with suction/injection to the non-uniform free stream velocity. Under the symmetry group of transformations, the governing equations along with three independent variables, are converted into a system of PDEs with two independent variables. Finally, by employing the order-reduction technique the PDEs are transformed into ODEs, which are then solved numerically. The results are presented graphically and analyzed. The main advantage of this technique is that without any prior knowledge, one can easily find the scaling transformations, expanding velocity, suction/injection velocity, and free-stream velocity. From computed numerical results many important findings are obtained. Most importantly, thermal and concentration overshoots are found for larger values of Dufour and Soret numbers, respectively. Also, thermal and concentration crossing over found for different values of Soret and Dufour numbers, respectively.

Hydrodynamic loads and wake dynamics of ducted propeller in oblique flow conditions

ABSTRACT The hydrodynamic loads and wake dynamics of the ducted propeller in oblique flow are investigated by detached eddy simulation (DES), with particular emphasis on the characteristics of wake fields and kinetic energy (KE) spectra. The influence of the duct, together with the strengthened induction of the propeller, cause the non-uniform velocity distributions in the inflow plane, resulting in unstable and periodic hydrodynamic loads on the blades. Shedding vortices from the leading edge of the duct dominate the leeward wake, and strengthen the self- and mutual induction of tip vortices. Large deformation of the tip vortices is observed due to the complex interference of the shear-layer vortices, shedding vortices and secondary vortices. The trajectories of the tip vortices are found distorted but still recognisable on the windward wake. The spiral-to-spiral distances of tip vortices increase and the re-alignment effect of the main flow enhances in the downstream wake.

Migration of a red blood cell in a permeable microvessel

One of the important features of the microvessels is with permeable vessels. The hyperpermeablility is especially prounounced due to the fast grow of the microvessel in the tumor tissues. Flow field induced by leakage often presents a different figure compared to that of the health vessel. Non-uniform velocity and pressure distribution are found in the simulation by using Lattice Boltzmann Method combined with Starling law. This non-uniform flow field has significant influence on the mass transport in permeable mocrovessel, such as the motion of red blood cell. Using 2D spring network model to express the red blood cell and immersed boundary method to implement the coupling of fluid and structure, numerical simulations were carried out to study the migration of a red blood cell in a straight permeable channel. The results show that the staying time of red blood cell in the leakage region increases with the intensity of leakage. We also found that the leakage can weaken the lateral migration of the red blood cell from the wall to the channel center, and even tune the migration towards the wall when the leakage is reasonably strong.

Combining DC and AC electric fields with deterministic lateral displacement for micro- and nano-particle separation

This paper describes the behavior of particles in a deterministic lateral displacement (DLD) separation device with DC and AC electric fields applied orthogonal to the fluid flow. As proof of principle, we demonstrate tunable microparticle and nanoparticle separation and fractionation depending on both particle size and zeta potential. DLD is a microfluidic technique that performs size-based binary separation of particles in a continuous flow. Here, we explore how the application of both DC and AC electric fields (separate or together) can be used to improve separation in a DLD device. We show that particles significantly smaller than the critical diameter of the device can be efficiently separated by applying orthogonal electric fields. Following the application of a DC voltage, Faradaic processes at the electrodes cause local changes in medium conductivity. This conductivity change creates an electric field gradient across the channel that results in a nonuniform electrophoretic velocity orthogonal to the primary flow direction. This phenomenon causes particles to focus on tight bands as they flow along the channel countering the effect of particle diffusion. It is shown that the final lateral displacement of particles depends on both particle size and zeta potential. Experiments with six different types of negatively charged particles and five different sizes (from 100 nm to 3 μm) and different zeta potential demonstrate how a DC electric field combined with AC electric fields (that causes negative-dielectrophoresis particle deviation) could be used for fractionation of particles on the nanoscale in microscale devices.

A field-segmentation-based variational optical flow method for PIV measurements of nonuniform flows

Air flow fields have nonuniform velocity distributions because of the compressibility of air. Current optical flow-based particle image velocimetry (PIV) methods cannot thoroughly solve these distributions because of the global optical flow formulation. In this study, we propose a novel field-segmentation-based variational optical flow (FS-VOF) method for preserving the spatially discontinuous characteristics of nonuniform flow fields. The proposed method is designed based on the level-set segmentation method, in which the particle image is segmented according to the velocity distribution of the fluid flow. Subsequently, a segmented smoothness constraint term built from the assumption that velocity varies continuously in each segmented particle image region and the discontinuous structures of the velocity field are preserved at the boundaries of the segmented regions. In addition, the data term of the proposed method is based on the segmented regions and derived from the physics-based optical flow equation. The proposed method is evaluated over synthetic and experimental images, and the comparison with advanced variational optical flow algorithms demonstrates the effectiveness of our method in preserving the spatially discontinuous characteristics of nonuniform flow fields.

Analysis of general unified MHD boundary-layer flow of a viscous fluid - a novel numerical approach through wavelets

The common boundary-layer equations are derived in which the boundary-layer forms either due to the flow of a viscous fluid over a moving wedge or due to the stretching of the surface with a non-uniform velocity using the concept of the velocity ratio (free stream velocity to the stretching surface velocity). The extreme values of the velocity ratio parameter then characterize both former and latter boundary-layer flows. The model also considers the effect of uniform magnetic field which is applied normal to the flow. Once the mainstream flow is approximated either in the form of the power of distance along the wedge surface or as zero, the self-similar solutions exist. Using suitable transformations, the boundary-layer equations have been converted into the Falkner–Skan-type equation over the unbounded domain that accounts the velocity ratio parameter. The governing problem over an unbounded domain is solved using a novel numerical scheme which makes use of the power of Haar wavelets coupled with collocation method and quasilinearization technique. The wavelet solutions have been verified to be very accurate when compared with numerous previously observed results. Several interesting physical aspects of the considered problem are focused and justified through both theoretical as well as numerical approach. The results show that there are over- and under-shoots in the solutions. Also, the flow divides into near-field (the solutions are confined to a viscous region) and far-field region (inviscid region). It is noticed that the boundary-layer thickness decreases for increasing pressure gradient and magnetic field parameters. Further, interestingly, our study leads to dual solutions for some range of physical parameters, which is explored for the first time through wavelet method in the literature. The physical hydrodynamics is explored and discussed.

Effect of velocity non-uniformity of supply air on the mixing characteristics of push-pull ventilation systems

The aim of this paper is to investigate the effect of the supply air velocity profile and direction on the mixing characteristics of push-pull ventilation systems. An experiment was set up to validate the numerical simulation. Then, numerical simulations were performed to investigate five velocity profile models (uniform, parabolic, linear-down, linear-up and side) and four flow direction models. Mixing fraction ε and mixing rate‾ε were proposed as evaluating indexes of the mixing amount of ambient air. The simulation results indicated that the mixing fraction distribution can visually show the mixing characteristics of the ambient air under different air supply conditions. Secondly, when there is airflow escape at the pull hood, the effect of the supply air velocity profile and direction on the mixing amount should be taken seriously. On the other hand, when there is no airflow escape, the mixing amount along the flow path increases linearly, and the effect of the velocity profile and direction on the mixing amount is small. Finally, the mixing amount along the flow path has a minimum value at a certain flow ratio. The results of the present study have certain guiding significance for the design of push-pull ventilation systems.

Statistics of overpressure fluctuations behind a weak shock wave interacting with turbulence

The overpressure fluctuations behind a weak shock wave interacting with turbulence are studied by wind tunnel experiments, where a spherical shock wave propagates in grid turbulence. The experiments are conducted for various values of the shock Mach number MS0 of the shock wave and turbulent Mach number MT of the grid turbulence. The experimental results show that the root-mean-squared peak-overpressure fluctuation divided by the averaged peak-overpressure, σΔp/⟨Δp⟩, where the inherent noise caused by the experimental facility is removed, follows a power law of MT2/(MS02−1). The probability density functions of the overpressure fluctuations are close to the Gaussian profile for a wide range of MT2/(MS02−1). A shock deformation model based on the deformation due to nonuniform fluid velocity is proposed for the investigation of the influences of turbulence on the shock wave. The deformation changes the cross-sectional area of the ray tube, which is related to the shock Mach number fluctuation of the area. The model for a weak shock wave yields the relation σΔp/⟨Δp⟩≈(1/3)[MT2/(MS02−1)]1/2, which agrees well with the experimental results. The model also predicts the Gaussianity of the peak-overpressure fluctuations behind the shock wave interacting with Gaussian velocity fluctuations. Good agreements between the model and experiments imply that the change in the shock wave characteristics by the interaction with turbulence is closely related to the shock wave deformation caused by the fluctuating turbulent velocity field.

Nonreciprocal acoustic propagation and leaky-wave radiation in a waveguide with flow.

Isolators, devices with unidirectional wave transmission, are integral components in computing networks, enabling a one-way division of a large system into independent subunits. Isolators are created by breaking the inversion symmetry between a source and a receiver, known as reciprocity. In acoustics, a steady flow of the background medium in which sound travels can break reciprocity, but significant isolation is typically achieved only for large, often impractical speeds. This article proposes acoustic isolator designs enabled by duct flow that do not require large flow velocities. A basic isolator design is simulated based on the acoustic analogue of a Mach-Zehnder interferometer, with monomodal entry and exit ports. The simulated device footprint is then reduced by using bimodal ports. Further, a nonuniform velocity profile combined with a grating to induce phononic transitions is considered, which, combined with filters, can provide significant isolation. By coupling a waveguide with flow to free space through an array of small apertures, largely nonreciprocal leaky-wave radiation is demonstrated, breaking the symmetry between reception and transmission patterns of an acoustic linear aperture array. These investigations open interesting pathways towards efficient acoustic isolation, which may be translated into integrated acoustic and surface acoustic waves, as well as phononic technology.

Direct-drive measurements of laser-imprint-induced shock velocity nonuniformities.

Perturbations in the velocity profile of a laser-ablation-driven shock wave seeded by speckle in the spatial beam intensity (i.e., laser imprint) have been measured. Direct measurements of these velocity perturbations were recorded using a two-dimensional high-resolution velocimeter probing plastic material shocked by a 100-ps picket laser pulse from the OMEGA laser system. The measured results for experiments with one, two, and five overlapping beams incident on the target clearly demonstrate a reduction in long-wavelength (>25-μm) perturbations with an increasing number of overlapping laser beams, consistent with theoretical expectations. These experimental measurements are crucial to validate radiation-hydrodynamics simulations of laser imprint for laser direct drive inertial confinement fusion research since they highlight the significant (factor of 3) underestimation of the level of seeded perturbation when the microphysics processes for initial plasma formation, such as multiphoton ionization are neglected.

Stall Inception in a Boundary Layer Ingesting Fan

Jet engines with boundary layer ingestion (BLI) could offer significant reductions in aircraft fuel burn compared with podded turbofans. However, the engine fans must run continuously with severe inlet distortion, which is known to reduce stability. In this paper, an experimental study has been completed on a low-speed rig fan operating with a BLI-type inlet distortion. Unsteady casing static pressure measurements have been made at multiple locations during stall events. Steady-state, full-annulus area traverses have also been performed at rotor inlet and exit at a near-stall operating point. The reduction in stability caused by BLI is found to be small. It is found that with BLI the fan can operate stably despite the presence of localized regions where the rotor operating point lies beyond the stability boundary measured in clean flow. With the BLI-type distortion applied, the measured rotor incidence varies around the annulus due to nonuniform upstream velocity and swirl. The measured amplitude of unsteady casing pressure fluctuations just prior to stall is found to correlate with the circumferential variation of rotor incidence, suggesting that rotor incidence is a key variable affecting the creation and growth of flow disturbances. In regions of high incidence, disturbances resembling local flow separations are initiated. However, in regions of low or negative incidence, any disturbances decay rapidly. Full rotating stall with BLI occurs when high incidence regions are widespread enough to sustain disturbances which can propagate around the entire annulus.

Effect of Orifice Geometry on Synthetic Jet Evolution

The present work discusses an experimental investigation into the effect of orifice geometry on the formation and advection of vortical structures associated with a synthetic jet issued into a quiescent flow. Three orifice geometries (namely, rectangular, trapezoidal, and triangular shapes) having the same aspect ratio () and area were tested at a hydraulic diameter-based Reynolds number and a Strouhal number of 7000 and 0.066, respectively. Using a single hot wire, the spanwise distribution of the velocity at the jet orifice plane was first measured. The trapezoidal and triangular orifices exhibited a nonuniform velocity distribution along the orifice, where the velocity was 25 and 130% higher at the smaller cross-section side as compared to the large section side, respectively. Stereoscopic particle image velocimetry was employed to investigate the velocity and vorticity fields downstream of the synthetic jet’s orifice. Vortical, ringlike structures were created at each orifice, where the trapezoidal and triangular orifice geometries resulted in vectoring of these structures toward the larger cross-sectional area of the shape. Axis switching was also observed by each orifice; however, it occurred closer to the orifice for the triangular and trapezoidal orifices when compared to the rectangular orifice. Jet spreading and vorticity dissipation are also presented and were shown to be highly dependent on the orifice geometry.

Effect of flow behavior in extra-column volumes on the retention pattern of proteins in a small column

Experimental and theoretical analysis of deformation of band profiles in extra-column volumes (ECV) was performed, and its influence on the retention pattern of proteins in a small chromatographic column was quantified. Several macromolecule and small-molecule compounds, and their mixtures were eluted from a chromatographic system in the absence and presence of the column. The peak deformation in ECV was attributed to non-uniform velocity distribution in the radial direction in connecting capillaries. The phenomenon enhanced with increasing molecular weight of the model compound, when radial diffusion dominated the mechanism of band spreading. The band shape was also affected by the geometry of the injection system used, i.e., an injection loop capillary or a superloop. The phenomenon vanished for a small molecule compound, for which plug flow conditions could be established. The difference in flow behaviour of the macromolecule and small-molecule compounds caused them to migrate with different velocities in ECV, which resulted in partial separation of their bands. The ECV effect influenced the retention behaviour of macromolecules in a small column; it caused tailing of peaks and asymmetry of breakthrough curves. To describe the elution profiles in ECV and in the column, a mathematical model was used which accounted for non-ideality of the flow pattern. The model reproduced accurately band profiles of macromolecules within a range of relatively low velocities, typical however for protein chromatography.

Estimating the non-uniform air velocity distribution for the optimal design of a heat exchanger

The maldistribution of flow in a heat exchanger usually has negative effects on the heat exchanger specifications such as the effectiveness, annual cost, and pressure drop. This paper examines the issue to improve the thermoeconomic performance. For this purpose, a plate-fin heat exchanger was selected and optimized by considering both the effectiveness and total annual cost as objective functions. The design parameters were the parameters of the heat exchanger and fin, as well as the non-uniform inlet flow profiles for both sides of the hot and cold streams. To generalize the results, the optimization was applied at different mass flow rates. The optimum results of the non-uniform profile were compared with those obtained with a uniform profile of the inlet flow. The optimum results reveal that thermoeconomic improvement occurred in the case of non-uniform flow compared with the case of uniform flow. For example, a 12.7% reduction for the total annual cost was found in the case of non-uniform flow in comparison to the uniform flow for a fixed effectiveness of 0.92. At low effectiveness, no significant improvement was found in the case of non-uniform flow. Nevertheless, the suggested profiles could be considered as an alternative to uniform inlet profiles.

Numerical investigation of the jet velocity profile and its influence on the Pelton turbine performance

Numerical investigation of the unsteady flow patterns around the bucket can be helpful to improve the Pelton turbine efficiency. In fact, by studying the loss mechanisms in the flow, one can optimize the bucket design. For this purpose, an accurate investigation of the water jet is also necessary as an inlet condition for the bucket flow. The water jet ejected from the nozzle is actually non-uniform, due to the upstream turbulence and secondary flow in the distributor, bending pipes, needle and supports. This non-uniformity causes a deviation of the water jet from the ideal jet center, which is tangent to the jet circle, and it directly affects the flow patterns around the bucket. Whereas water splashing makes experimental observations very challenging, a numerical approach is effective and convenient to study the unsteady flow pattern in a Pelton turbine. In the present research, numerical analysis of the two-phase flow through the distributor, bending pipe and nozzle, has been performed. The jet velocity profile has been verified with experimental results, which are measured in the model test equipment, and good agreement is obtained for both velocity profile and jet deviation. In addition, the analysis of the bucket unsteady flow is performed using the aforementioned non-uniform jet velocity profile and the calculated flow pattern around the bucket is compared to the previous calculation using a uniform velocity profile at the inlet boundary. The difference between the two cases is discussed based on the quantitative and qualitative evaluation from the numerical analysis.

Research and optimization of performances of a pump turbine in pump mode

For reversible pump turbines, a sufficient hump safety margin has to be provided under small-flow and high-head conditions to avoid instability resulting from operations in the hump-shaped zone. On the other hand, under large flow conditions, cavitation easily occurs on the low-pressure edge of runner blades due to effects of low pressure and flow separation, sharply reducing the efficiency and lift head. Therefore, the hydraulic design of the pump turbine in pump mode is crucial for the properties of energy, cavitation and stability of the whole unit. A model runner (runner A) designed for a pumped storage power plant in China was demonstrated with insufficient hump safety margin and poor cavitation performance. To improve those deficiencies, optimization design was carried out on the geometric dimensions of the runner A, based on the computational fluid dynamics (CFD) analyses and model test results. The final model runner B was designed with less blade number Z and large blade wrap angle θ, to both alleviate the flow separation around the lower pressure edge of the blade and the non-uniform velocity distribution in the outlet area around high pressure edge. The model test results show that in comparison with the original runner A, the performances, such as efficiency, hump safety margin, cavitation and stability are improved in runner B.