Time-averaged Velocity Profiles Research Articles

Numerical Analysis of Heat Transfer Enhancement in a Micro-Channel Due to Mechanical Stirrers

Abstract Using two-dimensional numerical simulations of the momentum, mass, and energy conservation equations, we investigate the enhancement of heat transfer in a rectangular micro-fluidic channel. The fluid inside the channel is assumed to be stationary initially and actuated by the motion imparted by mechanical stirrers, which are attached to the bottom of the channel. Based on the direction of the oscillation of the stirrers, the boundary conditions can be classified as either no-slip (when the oscillation is perpendicular to the length of the channel) or periodic (when the oscillation is along the length of the channel). The heat transfer enhancement due to the motion of the stirrers (with respect to the stationary stirrer situation) is analyzed in terms of the Reynolds number (ranging from 0.7 to 1000) and the Peclet number (ranging from 10 to 100). We find that the heat transfer first increases and then decreases with an increase in the Reynolds number for any given Peclet number. The heat transferred is maximum at a Reynolds number of 20 for the no-slip case and at a Reynolds number of 40 for the periodic case. For a given Peclet and Reynolds number, the heat flux for the periodic case is always larger than the no-slip case. We explain the reason for these trends using time-averaged flow velocity profiles induced by the oscillation of the mechanical stirrers.

CFD modelling of buoyancy driven flows in enclosures with relevance to nuclear reactor safety

Nuclear reactor containment flows are primarily driven by density differences in the long term of a severe accident. It is crucial to predict such flows from the perspective of reactor safety. The present work addresses the assessment of open-source CFD code OpenFOAM in modelling buoyancy-driven turbulent flows and heat transfer relevant for analyzing the flow and mixing in a nuclear reactor containment during severe accident scenarios. A fully compressible solver with the k-ω based shear stress transport (SST) turbulence model was utilized to simulate the two-dimensional turbulent flow of a differentially heated square cavity(h ~ 0.75, Ra ~ 109) and then against the technical scale THAI-TH21 natural convection test (60 m3, h ~ 9 m, Ra ~ 1012). The results from the current study, emphasize the importance of considering radiation phenomenon in buoyancy-driven flows and the grid resolution needed in the near-wall region to accurately predict wall heat flux. Overall, the simulation results of time-averaged temperature and velocity profiles in the quasi-steady state compare very well with experimental data. The predicted values of external wall heat transfer rates on the THAI vessel, the convective wall heat flux in the gas region, and the impact of buoyancy on turbulence are discussed.

Self-preserving characteristics in wall-wake flow downstream of an isolated bedform

The results of an experimental study on the turbulence characteristics in flow over and downstream of an isolated bedform are presented. The vertical profiles of time-averaged streamwise velocity, Reynolds stresses and turbulent kinetic energy (TKE) fluxes at different horizontal locations over and downstream of a bedform are shown to demonstrate their variations. The experimental results signify a flow acceleration, separation and deceleration over the stoss-side, at the crest and in the leeside of a bedform. In addition, in the wall-wake region (far downstream of the bedform), the defects (uninterrupted upstream value minus downstream value at a given vertical distance) in time-averaged streamwise velocity, Reynolds shear stress (RSS), turbulence intensities and TKE fluxes demonstrate the self-preserving characteristics in their individual vertical profiles when they are scaled by their individual peak defects. For scaling the vertical distance to achieve the self-preserving characteristics, the defect profiles of velocity were scaled by the half-width of peak velocity defect, those of RSS and turbulence intensities by the half-width of peak RSS defect and those of TKE fluxes by the half-width of individual peak TKE flux defects. The peak defects of all the parameters lessen, as one moves toward the downstream, recovering their uninterrupted upstream profiles.

Determination of Wall Friction Velocity of the Turbulent Boundary Layer by Two-Parameter Nonlinear Curve Fitting

Hot-wire anemometer is a fundamental tool for flow field measurement, and has been widely adopted in research of turbulent flow. A turbulent boundary layer was generated by a trip wire and a piece of sandpaper in the low-speed wind tunnel, and the time-averaged velocity profiles at three different streamwise stations in the boundary layer were measured with IFA-300 constant temperature anemometer. Targeting the dimensionless velocity profile model White Law of the turbulent boundary layer, a nonlinear curve fitting MATLAB program for two parameters, which were theoretical original point of wall coordinate y0 and wall friction velocity uτ, was developed. Based on the measured time-averaged velocity profiles, the program was adopted to determine y0 and uτ of the three streamwise stations. It is found that the initial search domain and search step size of y0 and uτ have no effect on the fitting results if the physical solutions are included in the domain. It is also found that the selected fitting velocities data are closely related to the results. The value of the friction velocity with high precision will be obtained by this nonlinear curve fitting method if the highest fitting velocity data point dose not fall into the wake region. The method given in this paper is simple in programming and reliable in fitting results, and it is of practical to obtain the wall friction velocity of the turbulent boundary layer.

A method of evaluating the vortex rope strength in draft tube of Francis turbine

A precessing vortex core in the draft tube of Francis turbine is considered as unstable flow conditions and the cause of many undesirable system instabilities for hydraulic power stations. The feature of vortex rope, pressure fluctuations and runner outlet velocity at part-load conditions are investigated experimentally and numerically in this paper. An obvious vortex rope could be observed at 0.88Q11BEP point and produces the most intensive periodic pressure fluctuation at the vortex rotation frequency fv along the whole passage of Francis turbine. At other conditions away from this point, the vortex rope becomes unobvious or even invisible, and the associated pressure fluctuation amplitude also reduces. This trend could be well represented by the new parameters G and Vs, which could be used to determine the strength of helical vortex field based on the gradient of time-averaged velocity on horizontal section of the draft tube cone. Transient simulations using VLES turbulence model could obtain satisfying results by comparing with experimental data, especially in terms of the time-averaged axial velocity profile resolution. Therefore, this new method enables the designers to evaluate vortex strength in engineering applications.

Velocity and thermal boundary layer equations for turbulent Rayleigh-Bénard convection

The authors derive a full system of boundary layer equations for turbulent Rayleigh-Benard convection that incorporates all the relevant physical effects, namely, buoyancy, fluctuations, and vanishing large-scale circulation velocity in the central bulk region of the convection cell. The theory predicts the time-averaged temperature and velocity profiles for fluids with a general Prandtl number.

A new analytical model for dip modified velocity distribution in fully developed turbulent open channel flow

This paper presents a new analytical model to predict the streamwise time-averaged velocity profile affected by the dip phenomenon in open channel flows. The novel approach of the present study is that the Finley wake law has been used instead of Coles’ wake law for the outer layer. To validate the new analytical model, six high quality experiments were conducted in a hydraulically rough bed open channel flow by considering variations of aspect ratio, defined as the ratio of the width of the channel to the depth of flow, from 2 to 4. In these controlled experiments, the time-averaged velocities were measured using a Nortek Vectrino-plus acoustic Doppler velocimeter. In addition, 14 sets of available experimental data, including five field experiments conducted across the globe were also used to test the performance of the proposed model. The proposed model, the Finley-dip-modified-log-wake law (FDMLWL), was used to develop a semiempirical equation to compute the dip position as a function of the dip correction factor and the wake parameter. In addition, using the experimental data and FDMLWL, an empirical equation was developed to compute the dip correction factor for hydraulically smooth open channel flows. The comparison of the FDMLWL model with the experimental data belonging to hydraulically smooth, transition, and rough regimes has consistently indicated better representation of the velocity dip phenomenon. The FDMLWL model has also been compared with other analytical models available in the literature and the superior performance of the proposed model is further observed. Finally, based on the satisfactory validation between experimental data and FDMLWL, it is inferred that the proposed model is better suited for modeling zero velocity gradient at the boundary layer edge, as in open channel flows with dip phenomenon.

Effects of red blood cell aggregation on the blood flow in a symmetrical stenosed microvessel.

In order to figure out whether red blood cell (RBC) aggregation is beneficial or deleterious for the blood flow through a stenosis, fluid mechanics of a microvascular stenosis was examined through simulating the dynamics of deformable red blood cells suspended in plasma using dissipative particle dynamics. The spatial variation in time-averaged cell-free layer (CFL) thickness and velocity profiles indicated that the blood flow exhibits asymmetry along the flow direction. The RBC accumulation occurs upstream the stenosis, leading to a thinner CFL and reduced flow velocity. Therefore, the emergence of stenosis produces an increased blood flow resistance. In addition, an enhanced Fahraeus-Lindqvist effect was observed in the presence of the stenosis. Finally, the effect of RBC aggregation combined with decreased stenosis on the blood flow was investigated. The findings showed that when the RBC clusters pass through the stenosis with a throat comparable to the RBC core in diameter, the blood flow resistance decreases with increasing intercellular interaction strength. But if the RBC core is larger and even several times than the throat, the blood flow resistance increases largely under strong RBC aggregation, which may contribute to the mechanism of the microthrombus formation.

A study of two-phase annular flow using unsteady numerical computations

Flow behavior of thin liquid films in an annular flow regime is an important element of the thermal hydraulics of a BWR – it controls the heat transfer from the fuel rods to the coolant. Despite its relevance, the flow behavior of dynamic liquid films is not well understood even in adiabatic conditions. To investigate it, unsteady numerical simulations with an interface tracking model were performed in a double subchannel geometry with a P/D ratio of 1.325. Different turbulence models, namely the large eddy simulation (LES) and linear/nonlinear eddy viscosity unsteady-RANS (URANS) models, were tested. A novel approach for generating turbulent inlet conditions in a periodic flow domain was developed to reduce computational efforts. Validation against experimental data revealed shortcomings of the linear eddy-viscosity RANS model in predicting key flow parameters. By capturing the effects of the secondary flow structures in a subchannel geometry, improved predictions were obtained with a non-linear SST k–ω (QCR) turbulence model. Time-averaged liquid film thickness (LFT) and tracer distribution obtained with LES were found to have the best agreement with experimental data. Instantaneous and time-averaged velocity profiles were analyzed to understand the influence of the gas-liquid interface. Secondary flow structures in the subchannel gap region were found to enhance the turbulent mixing of the passive scalar in the liquid film. This finding is relevant towards the prediction of thermal-hydraulic parameters in a multichannel flow assembly by accounting for inter-channel mixing phenomena.

DDES와 IDDES를 이용한 Aerospatiale A-airfoil 주위 천이 유동 해석

The transitional flow over Aerospatiale A-airfoil at critical angle of attack is simulated using DDES and IDDES models. This airfoil features relatively thick boundary layer and a mild separation around trailing edge. Simulation results based on three different grid system and reference data from LESFOIL project are compared. Time averaged velocity profiles, Reynolds stress and vortical structures are presented. The results of DDES model show the consistency on different grid system but fail to predict the fluctuation of flow field. IDDES model is able to resolve the fluctuation but overpredict the resolved quantities than experimental data. Also different grid system with different length scale for LES model have much effect on IDDES results.

On the flow physics and vortex behavior of rectangular orifice synthetic jets

Synthetic jet actuators possess a continuous jet like behavior in its far region and have found wide-scale engineering applications since it allows momentum transport to the flow system without any net mass transfer across the flow boundaries. The case of a non-axisymmetric synthetic jet is particularly significant since it is affected by the differential shear layer at the orifice exit, that depends on its aspect ratio. However, despite exhaustive research on both continuous and synthetic jets, very few studies have experimentally investigated the case of rectangular orifice synthetic jets, focusing on the effect of aspect ratio of the orifice as well as the actuation frequency upon the vortex behavior and the flow physics. In particular, the intriguing phenomenon of vortex bifurcation has mostly been reported only for an individual vortex or for a plain jet. Yet, in a train of vortex rings, such as that obtained in a synthetic jet, the occurrence of vortex bifurcation can be expected, although the flow physics in the wake of individual vortex rings is significantly different. The present study experimentally investigates a rectangular orifice synthetic jet at different orifice aspect ratios and actuation frequencies, focusing on exploring the conditions at which vortex bifurcation occurs, through LIF imaging and Hot-film measurements. The primary objective of these experiments is to provide a qualitative physical insight into the synthetic jet ejected from a rectangular orifice (through LIF imaging), as well as to quantitatively explore the experimental conditions that promote different flow structures (through velocity time trace, time-averaged velocity profiles and power spectral density measurements), particularly the bifurcation of vortex rings. Our experiments indicate that the phenomenon of vortex bifurcation is observed during the axial switching of vortex rings, but only in a narrow range of experimental conditions. Further, the velocity measurements have ascertained that the two prominent reasons behind this bifurcation process are a large disparity in the velocities of the vortex core and the center of vortex ring, as well as the time lag in which the separation distance between the counter-rotating vortices decrease gradually to zero.

Meshing strategy for bifurcation arteries in the context of blood flow simulation accuracy

The study presents a mesh dependency study for a carotid artery bifurcation geometry of a real-life specimen. The results of time-averaged velocity profiles at artery control surfaces and wall shear stresses are compared between a set of structured and unstructured meshes, with varying non–dimensional boundary layer first element thickness (y+) parameter. A set of four meshes in total is considered: a full–hexagonal structured mesh, an unstructured tetrahedral mesh with prism inflation layer, both created for y+=1 and y+=30. Apart from numerical results, overall mesh creation work time, overall analysisstability are compared with the mesh quality results: cell non–orthogonality, cell skew and aspect ratio. Numerical results are validated against results of real–life CT examination performed in Poznań Medical University.

Large eddy simulations of the turbulent flows of twin parallel jets

Turbulent jet interactions play a significant role in terms of momentum and heat transfer. For example, in next generation nuclear reactors, high temperature flow mixing in the lower plenum and mixing fluctuations in the coolant may influence power oscillations. Thus, the estimation of the mixing condition needs to be accurate. Recent benchmark experiments using particle image velocimetry (PIV) technique provided high-fidelity experimental data that could be used in comparative studies. In the present study, large eddy simulation (LES) numerical methodology is used on a parallel jet system and validated with benchmark PIV experiments. The results indicate good agreement in terms of the merging point and time averaged velocity profile. Spectral analyses via Welch’s power spectral density functions are used to analyze frequency information in turbulence jets. The proper orthogonal decomposition (POD) analysis method is applied using a snapshot method. The POD analysis indicates vortex structures similar to those in the benchmark PIV experiment.

Simultaneous measurements of turbulent Reynolds stresses and particle flux in both parallel and perpendicular directions in a linear magnetized plasma device.

We report temporally resolved simultaneous measurements of the turbulent Reynolds stresses in both the parallel and perpendicular directions and the corresponding particle fluxes in the fusion relevant cylindrical magnetized plasma device Controlled Shear Decorrelation eXperiment (CSDX). CSDX simulates the plasma conditions of multiple plasma instabilities that can arise in the scrape-off layer of fusion devices. In this study, we designed and used a 6-tip Langmuir probe in a novel yet simple design to simultaneously measure all the three dimensional components (radial, azimuthal, and axial) of fluctuations in velocity from the floating potentials and plasma densities with high temporal resolution. From these, we calculated the parallel and perpendicular Reynolds stress and the particle fluxes in addition to the density and potential spectra and the cross phase between different quantities. We can obtain radial profiles of all the aforementioned plasma quantities, which are extremely useful for studying plasma turbulence due to multiple instabilities. We have also cross-checked the time averaged velocity profiles from the probe with laser induced fluorescence measurements of the mean plasma velocity for some common plasma source parameters.

Turbulent Flow Characterization over the Gravel Bed

The double-averaged momentum equations are used as a natural basis for the hydraulics of rough-bed open channel flows, especially with small relative submergence. To study the double-averaged turbulence characteristics in flows over a gravel-bed, a uniform flow over a gravel-bed was run in a laboratory flume and the velocities were measured at different verticals by a vectrino probe. The result reveals that the local time-averaged velocity profiles can be split into three typical classes, namely log, S-shaped and accelerated. In the log class velocity profile fits a logarithmic curve over almost the entire measured profile. S Shaped refers to profiles that can be encountered in wake of macro-roughness bed elements due to flow separation on the lee side. The accelerated profile located in the vicinity of top of the macro-roughness elements and the velocities are slightly higher than log classes but rapidly drop to zero below crest. It never extends to zero bed level above the crest they are not influenced by the bed heterogeneity. The logarithmic law of velocity profile is preserved above the roughness crest and velocity below the crest follows a polynomial. The field of gravel-bed appears to be near Gaussian matches with the work of previous researchers. The Reynolds shear stress is the main governing stress across the flow depth with a damping in the vicinity of gravel bed due to decrease in turbulence level. The form induced stress is prominent below the roughness crest.

Particle Image Velocimetry Experiment and Computational Fluid Dynamics Simulation of Flow Around Rigid Cylinder

This work focuses on the study of the flow around a rigid cylinder with both particle image velocimetry (PIV) experiment and computational fluid dynamics (CFD) simulation. PIV measurements of the flow field downstream of the cylinder are first presented. The boundary conditions for CFD simulations are measured in the PIV experiment. Then the PIV flow is compared with both Reynolds-averaged Navier–Stokes (RANS) two-dimensional (2D) and large eddy simulation (LES) three-dimensional (3D) simulations performed with ANSYS fluent. The velocity vector fields and time histories of velocity are analyzed. In addition, the time-averaged velocity profiles and Reynolds stresses are analyzed. It is found that, in general, LES (3D) gives a better prediction of flow characteristics than RANS (2D).

Flow speed has little impact on propulsive characteristics of oscillating foils

Experiments are reported on the performance of a pitching and heaving two-dimensional foil in a water channel in either continuous or intermittent motion. We find that the thrust and power are independent of the mean freestream velocity for two-fold changes in the mean velocity (four-fold in the dynamic pressure), and for oscillations in the velocity up to 38\% of the mean, where the oscillations are intended to mimic those of freely swimming motions where the thrust varies during the flapping cycle. We demonstrate that the correct velocity scale is not the flow velocity but the mean velocity of the trailing edge. We also find little or no impact of streamwise velocity change on the wake characteristics such as vortex organization, vortex strength, and time-averaged velocity profile development---the wake is both qualitatively and quantitatively unchanged. Our results suggest that constant velocity studies can be used to make robust conclusions about swimming performance without a need to explore the free-swimming condition.

Assessment of gas-particle flow models for pseudo-2D fluidized bed applications

ABSTRACTThe aim of this work is to provide more insight into the general modeling criteria for simulating pseudo-2D bubbling fluidized beds. For this purpose, two experimental-based problems are studied. First, a fluidized bed with a high-speed central jet problem is analyzed. A qualitative study of the first bubble indicates that the bubble shape prediction is highly sensitive to the frictional model adopted. The most accurate results in terms of bubble shape and detachment time are given by a frictional model that relates the strain-rate fluctuations with the granular temperature. Second, a uniformly fluidized bed problem in bubbling regime is considered. For this case, the drag models and boundary conditions for the particulate phase are investigated. Time-averaged solid phase velocity profiles are compared with the results of the literature where it is found that no-slip conditions (or partial slip with a high specularity coefficient) are more appropriate than slip conditions at the walls for these regimes. Regarding the drag force, although none of the models presented could match the experimental velocity predictions for low gas velocities at the lower region of the bed, the Di Felice model produces the most accurate results for the whole range of regimes considered.

Turbulent jet from a slender annular slot ventilated by a self-induced flow through the open core

The dynamics of an incompressible turbulent air jet from an annular source into otherwise quiescent surroundings are investigated. Focussing on a slender annulus with an open core, the development of the jet is examined using planar and stereoscopic particle image velocimetry (PIV) from the source to 16 (outer) source diameters downstream. Unique to these annular sources, the jet induces a flow through the open core - in other words, the core is `ventilated'. Our measurements indicate that the volume flux drawn through the core exceeds that from the source by approximately 20%. Based on the streamwise development of the jet, we identify four distinct regions: (i) an internal region of induced flow that is bounded by the jet; (ii) a near-field planar-jet-like region; (iii) a transition region; and, ultimately, (iv) a far- field round-jet-like region. We explore the evolution of the jet towards self-similar behaviour based on cross-stream profiles of time-averaged velocity and turbulence statistics within these different regions. Four distinct length scales are shown to characterise the streamwise extents and behaviours within the regions identified. Finally, to highlight the role of the open-core (ventilated) annular geometry on jet development, comparisons are made with PIV measurements of turbulent jets issuing from circular sources. These comparisons reveal that the ventilated geometry significantly enhances dilution in the near field.

Simulation of turbulent flow in MATIS-H rod bundle with split-type mixing vanes

Turbulent flow simulations of 5 × 5 rod bundle with the split-type mixing grid are presented. The simulations were performed by the Jožef Stefan Institute (JSI) in the frame of its participation in the OECD/NEA CFD Benchmark exercise. The experimental data from the MATIS-H benchmark experiment located at the Korean Atomic Energy Research Institute (KAERI) were used for posteriori comparison with simulation results. To predict the flow between the rods and mixing vanes, a complex, fully hexagonal flow-oriented mesh was constructed, taking into account the geometry of mixing vanes. High resolution numerical methods and a scale-resolving turbulence model was used in ANSYS CFX simulation. Time-averaged velocity profiles, profiles of velocity fluctuations and integral velocity circulation at the specified downstream locations show good agreement with experimental data. In addition, the simulated flow patterns over the entire rod bundle cross-section are presented and discussed.