Similar Reynolds Number Research Articles

Experimental Investigation of a Cavitating Water Flow With the Addition of Drag- Reducing Agents

Local reduction of the pressure in a flow field to levels lower than the saturation pressure triggers the production of tiny vaporous bubbles and causes the cavitation phenomenon. Higher pressure reductions significantly increase the cavitation bubbles’ population, which can coalesce and generate large-scale cavitation structures such as cloud cavitation that shed downstream of the flow channel. The chaotic collapse of cavities produces strong shockwaves in regions with a recovered pressure which causes serious erosion on solid surfaces and high levels of noise. Hence, there is a growing interest in cavitation control methods. In this study, drag-reducing polymer additives are utilized as cavitation reducing (CR) agents in a converging-diverging mesoscale nozzle to verify the applicability of these agents in the control of the cavitation process. Analysis of high-speed images of the cavitating flow fields reveals that the viscoelastic flow of a 400 ppm polymer solution reduced the cavitation intensity by nearly 60 % relative to the pure water flow at a similar Reynolds number. Ultra-high-speed imaging of single cavitating bubbles at the inception showed that in a viscoelastic flow, the collapse period of cavities is longer, and their sizes are shrunk at a lower rate relative to their counterparts in the puer water. Particle image velocimetry (PIV) was used to study the near-wall turbulent flow fields at the flow conditions close to the cavitation inception, at different flow locations with non-zero pressure gradients present on the curved surfaces. Preliminary analysis of the results reveals that viscoelasticity alters the near-wall turbulence and distribution of the pressure gradient fluctuations, which might link to the significant reduction of cavitation intensity in polymeric flows.

Numerical study of hydrodynamic and thermal behavior of Al2O3/Water nanofluid and Al2O3-Cu/Water hybrid nanofluid in a confined impinging slot jet using two-phase mixed model

This study utilizes the two-phase mixture model to conduct a numeric study of Al2O3/Water and Al2O3-Cu/Water (hybrid) nanofluid's hydrodynamic and thermal behavior in a laminar confined impinging slot jet in 50 ≤ Re ≤ 300 and nanoparticles volume fractions (NVF) ranging from 0 - 2%. This study considers various aspect ratios (H/W), including 2, 4, and 6, to investiate the confining effects. This paper gives a comparative analysis of nanofluids and hybrid nanofluids in terms of the parameters concerning the flow: Reynolds and local Nusselt number (Nux), average Nusselt number (Nuavg), flow lines' contour, and temperature distribution under similar geometric conditions and Reynolds number. In comparison with nanofluids, the hybrid nanofluids have higher local Nusselt number on the entire target surface, this advantage of hybrid nanofluid attribute to higher thermal conductivity of them. The average Nusselt numbers of nanofluids and hybrid nanofluids plotted at different Refor various aspect ratios(H/W=2,4), and the effect of aspect ratio and momentum are explained. Furthermore, pumping power of both fluid analysed for all nanoparticles volume fraction (0 - 2%) at different Reynolds number. The result shows that pumping power of hybrid nanofluid is higher than base fluid and nanofluid, because the dynamic viscosity of hybrid nanofluid is higher than base fluid (water) and nanofluid. Besides; the study identified some correlations in the hybrid nanofluids regarding the stagnation point and the average Nusselt numbers. Presumably, these correlations are valid under certain conditions: 50≤ Re ≤ 300, 2 ≤ H/W≤ 6, and volume fracture (0 - 2%).

Platelet concentrates preparation using a rotating membrane with Taylor vortices and axial flow

Taylor-Couette devices with a rotating filtration membrane are utilized to separate cellular components and to prepare platelet (thrombocytes, PLT) concentrate from whole blood. Due to the complicated nature of the working fluids (e.g. blood), systematic analysis is conducted to enhance the separation efficiency. Flow visualization and cell extraction techniques are applied to understand the cell distributions and classification in different flow regimes in the devices. Blood cells are found to be classified by Taylor vortices: erythrocytes (red blood cells, RBCs) and leukocytes (white blood cells, WBCs) moved toward the periphery of the vortices, while platelet remained close to the center of the vortices due to different sizes for these blood cells. The blood filtration efficiencies are evaluated using two devices at different radius ratios with a series of Reynolds number and flow rates. Operated at similar Reynolds numbers, the device can have higher filtration throughput by making a smaller inner cylinder, for comparable PLT recovery rates. An approximately 90 % recovery rate for PLT was obtained within 40 % volumetric ratio, while the ones for RBC and WBC were 75 % and 85 % respectively.

Implicit Subgrid-Scale Modeling of a Mach 2.5 Spatially Developing Turbulent Boundary Layer.

We employ numerically implicit subgrid-scale modeling provided by the well-known streamlined upwind/Petrov–Galerkin stabilization for the finite element discretization of advection–diffusion problems in a Large Eddy Simulation (LES) approach. Whereas its original purpose was to provide sufficient algorithmic dissipation for a stable and convergent numerical method, more recently, it has been utilized as a subgrid-scale (SGS) model to account for the effect of small scales, unresolvable by the discretization. The freestream Mach number is 2.5, and direct comparison with a DNS database from our research group, as well as with experiments from the literature of adiabatic supersonic spatially turbulent boundary layers, is performed. Turbulent inflow conditions are generated via our dynamic rescaling–recycling approach, recently extended to high-speed flows. Focus is given to the assessment of the resolved Reynolds stresses. In addition, flow visualization is performed to obtain a much better insight into the physics of the flow. A weak compressibility effect is observed on thermal turbulent structures based on two-point correlations (IC vs. supersonic). The Reynolds analogy ( vs. ) approximately holds for the supersonic regime, but to a lesser extent than previously observed in incompressible (IC) turbulent boundary layers, where temperature was assumed as a passive scalar. A much longer power law behavior of the mean streamwise velocity is computed in the outer region when compared to the log law at Mach 2.5. Implicit LES has shown very good performance in Mach 2.5 adiabatic flat plates in terms of the mean flow (i.e., and ). iLES significantly overpredicts the peak values of , and consequently Reynolds shear stress peaks, in the buffer layer. However, excellent agreement between the turbulence intensities and Reynolds shear stresses is accomplished in the outer region by the present iLES with respect to the external DNS database at similar Reynolds numbers.

Properties of the inertial sublayer in adverse pressure-gradient turbulent boundary layers

The inertial sublayer of adverse pressure-gradient (APG) turbulent boundary layers is investigated using new experimental measurements ( $7000 \lesssim \delta ^+ \lesssim 7800$ ), existing lower Reynolds number experimental ( $\delta ^+ \approx 1000$ ) and computational ( $\delta ^+<800$ ) data sets, where $\delta ^+$ is the friction Reynolds number. In the present experimental set-up the boundary layer is under modest APG conditions, where the Clauser PG parameter $\beta$ is ${\leq }1.8$ . Well-resolved hot-wire measurements are obtained at the Flow Physics Facility at the University of New Hampshire in the region of an APG ramp. Comparisons are made with zero pressure-gradient turbulent boundary layer (ZPG TBL) experimental data at similar Reynolds number and numerical simulation data at lower Reynolds number. The main aims of the present study centre on the inertial sublayer of the APG TBL and the degree to which its characteristics are similar to those of the ZPG TBL. This investigation utilizes equation-based analyses and empirical approaches. Among other results, the data suggest that even though the APG TBL streamwise variance does not exhibit a logarithmic profile (unlike the ZPG TBL) both ZPG and APG TBLs exhibit distance-from-the-wall scaling on the inertial sublayer. Theoretical arguments suggest that wall-distance scaling resulting from a self-similar dynamics is consistent with both a single velocity scale leading to a log-law in mean velocity profile as well as multiple velocity scales leading to a power-law mean velocity profile.

Competing flow and collision effects in a monodispersed liquid–solid fluidized bed at a moderate Archimedes number

We study the effects of fluid–particle and particle–particle interactions in a three-dimensional monodispersed reactor with unstable fluidization. Simulations were conducted using the immersed boundary method for particle Reynolds numbers of 20–70 with an Archimedes number of 23 600. Two different flow regimes were identified as a function of the particle Reynolds number. For low particle Reynolds numbers ( $20 < Re_p < 40$ ), the porosity is relatively low and the particle dynamics are dominated by interparticle collisions that produce anisotropic particle velocity fluctuations. The relative importance of hydrodynamic effects increases with increasing particle Reynolds number, leading to a minimized anisotropy in the particle velocity fluctuations at an intermediate particle Reynolds number. For high particle Reynolds numbers ( $Re_p > 40$ ), the particle dynamics are dominated by hydrodynamic effects, leading to decreasing and more anisotropic particle velocity fluctuations. A sharp increase in the anisotropy occurs when the particle Reynolds number increases from 40 to 50, corresponding to a transition from a regime in which collision and hydrodynamic effects are equally important (regime 1) to a hydrodynamic-dominated regime (regime 2). The results imply an optimum particle Reynolds number of roughly 40 for the investigated Archimedes number of 23 600 at which mixing in the reactor is expected to peak, which is consistent with reactor studies showing peak performance at a similar particle Reynolds number and with a similar Archimedes number. Results also show that maximum effective collisions are attained at intermediate particle Reynolds number. Future work is required to relate optimum particle Reynolds number to Archimedes number.

Direct numerical simulation of a turbulent plane Couette-Poiseuille flow with zero-mean shear

Direct numerical simulations of a turbulent Couette-Poiseuille flow with zero-mean-shear at the moving wall (SL-flow) is performed to examine flow features compared to those for a turbulent pure Poiseuille flow (P-flow). Profiles of the streamwise mean velocity, indicator function and ratio of production to dissipation show that the logarithmic region is significantly elongated for the SL-flow compared to that for the P-flow at a similar Reynolds number. In addition, the magnitudes of the Reynolds stresses are found to be larger in both inner and outer layers for the SL-flow than those for the P-flow. The spanwise spectra of the production term in the turbulent kinetic energy equation are examined to provide a structural basis for explaining the statistical behaviors. In addition, because the growth of the energy-containing motions extends to the outer layer further for the SL-flow due to the presence of a positive mean shear throughout the entire wall layer, the self-similar behavior of the energy balance between the production and transport terms with respect to the self-similar wavenumber is found far from the wall. We also find the increase in the number of uniform momentum zones in the SL-flow, revealing the hierarchical distribution of the energy-containing eddies which are composed of multiple uniform momentum zones. These coherent motions lead to the elongation of the logarithmic region for the SL-flow. Finally, investigation of the turbulent energy transfer process in a spectral domain for the SL-flow demonstrates importance of outer layer very-long structures, and these structures attribute to the energy transport process in an entire flow field.

Instability and transition in the boundary layer driven by a rotating slender cone

Abstract

Intense Reynolds-stress events in turbulent ducts

The aim of the present work is to investigate the role of intense Reynolds shear-stress events in the generation of the secondary flow in turbulent ducts. We consider the connected regions of flow where the product of the instantaneous fluctuations of two velocity components is higher than a threshold based on the long-time turbulence statistics, in the spirit of the three-dimensional quadrant analysis proposed by Lozano-Durán et al. (J. Fluid Mech., vol. 694, 2012, pp. 100–130). We examine both the geometrical properties of these structures and their contribution to the mean in-plane velocity components, and we perfom a comparison with turbulent channel flow at similar Reynolds number. The contribution to a certain mean quantity is defined as the ensemble average over the detected coherent structures, weighted with their own occupied volume fraction. In the core region of the duct, the contribution of intense events to the wall-normal component of the mean velocity is in very good agreement with that in the channel, despite the presence of the secondary flow in the former. Additionally, the shapes of the three-dimensional objects do not differ significantly in both flows. In the corner region of the duct, the proximity of the walls affects both the geometrical properties of the coherent structures and the contribution to the mean component of the vertical velocity. However, such contribution is less relevant than that of the complementary portion of the flow not included in such objects. Our results show that strong Reynolds shear-stress events are affected by the presence of a corner but, despite the important role of these structures in the dynamics of wall-bounded turbulent flows, their contribution to the secondary flow is relatively low, both in the core and in the corner.

Oil-Based Drilling Fluid's Cuttings Bed Removal Properties for Deviated Wellbores

Abstract Results from cuttings transport tests in the laboratory using different field-applied oil-based drilling fluids with similar weight and varying viscosities are presented in this paper. The fluids are designed for highly deviated wells, and the cuttings transport performance at relevant wellbore inclinations was investigated. The experiments have been performed in a flow loop that consists of a 10-m-long test section with 50.4 mm (2″) diameter freely rotating steel drill string inside a 100-mm (≈4″) diameter wellbore made of cement. Sand particles were injected while circulating the drilling fluid through the test section. Experiments were performed at three wellbore inclinations: 48, 60, and 90 deg from vertical. The applied flow loop dimensions are designed so that the results are scalable to field applications; especially for the 12 ¼″and 8 ½″ sections. The selected setup provides correct shear rate ranges and similar Reynolds numbers to the field application when the same fluids are applied. Results show that hole cleaning abilities of the tested fluids vary significantly with well angle, drill string rotation, and flowrate. Results support field experience showing that low viscous fluids are more efficient than viscous fluids at higher flowrates and low drill string rotation. As well as per field experience, more viscous fluids are efficient in combination with high drill string rotation rates. The results show the effect of cuttings transport efficiency as a function of hydraulic frictional pressure drop, demonstrating methods to achieve a more optimal hydraulic design in the tested conditions. The key findings have direct relevance to drilling operations.

Three-dimensional analysis of precursors to non-viscous dissipation in an experimental turbulent flow

Abstract

Swirling turbulent pipe flows: Inertial region and velocity–vorticity correlations

Swirling pipe flows are studied here with an aim towards understanding the onset of the inertial region — where the turbulent-inertia term in the mean momentum equation is balanced by pressure gradient and viscous term is sub-dominant — as well as the clarifying the velocity–vorticity correlations that make up the turbulent inertia. To this end, we first manipulate the mean momentum equation in both axial and azimuthal directions and find some exact results in the inertial region, and carry out direct numerical simulations of swirling pipe flows at axial friction Reynolds numbers of 170 and 500. The swirl number considered in our simulations is S≈0.3, and we compare our results to non-swirling pipe flows at similar Reynolds numbers. We find that swirling produces a drag increase and an influence on the turbulence statistics similar to increasing the Reynolds number except for the streamwise turbulence intensity. An analysis on the axial and azimuthal mean momentum equations shows that swirling shifts the beginning of the inertial region wall-normal location closer to the wall. The turbulent inertia decomposition reveals that the near-wall region the velocity–vorticity correlations of the axial direction are similar to a 2D channel flow and interpreted as vorticity stretching/reorientation and dispersion, whereas in the new correlations in the azimuthal direction can also be given a similar physical meaning in the near-wall region. In the outer-region, however, the pipe axial correlations are different to the 2D-channel, and so are the azimuthal correlations. We find that the pipe has new a ‘geometric’ contribution in both axial and azimuthal directions that play an important role in contributing towards vorticity dispersion in the outer core region of a swirling pipe flow.

Experimental investigation of the influence of Reynolds number and buoyancy on the flow development of a plane jet in the transitional regime

Heated horizontal plane jets find wide applications in engineering appliances such as air curtains and discharge of industrial effluents. In the present study, experimental investigations are conducted on a heated horizontal plane jet with the Reynolds numbers in the transitional regime, using a hotwire anemometer. In the far to very far-field (20 < x/d < 100) centreline velocity decay and jet spread increases faster with the decrease of Reynolds number. This is because, with the increase of Reynolds number, the turbulent kinetic energy is distributed on a broadband of scales. As a result, larger scales, which are responsible for increased entrainment, get weaker. The shifting of the centre plane generally occurs in the far region for low Reynolds number jets. A comparison with the result of an isothermal jet at similar Reynolds numbers from the literature at identical conditions shows that the turbulence intensity is decreased due to heating. Centreline velocity decays slowly and half-width increases marginally for a heated jet when compared with an isothermal jet. The effect of heating is prominent for low Re jets. Spectral development shows a delayed transition due to heating. Probability density function plots reveal lack of equilibrium and presence of large-scale eddies in the flow field.

Ratio-cut background removal method and its application in near-wall PTV measurement of a turbulent boundary layer

Optical contamination due to wall reflection creates limitations for near-wall velocity field measurement via either particle image velocimetry (PIV) or particle tracking velocimetry (PTV). In this paper, a simple image pre-processing method, i.e. the ratio cut method, is proposed to deal with this problem. It is based on the ratio between the grayscale intensities of tracer particles and those of the laser-illuminated background, on which a direct minimum cut is applied on the basis of a non-dimensional threshold for background removal. To evaluate its performance in near-wall measurement, this ratio cut method, along with two other typical pre-processing methods, i.e. the minimum removal method and the proper orthogonal decomposition (POD) filtering method, are applied to particle images in the near-wall region of turbulent boundary layers over an opaque roughness wall (ORW), whose characteristic roughness height is small enough to be regarded as hydraulically smooth, but still gives rise to severe wall reflection. Results for a case involving a transparent smooth wall, which suffers less from wall reflection issues, and direct numerical simulation (DNS) data at a similar Reynolds number are employed as reference baselines for performance evaluation. The examination of pre-processed particle images, as well as the probability density function (PDF) of grayscale intensities, indicates that the ratio cut method is capable of eliminating time-dependent flare, reducing noise level, and retaining low-intensity particles in the ORW case. These features are almost completely absent in both the minimum removal method and the POD filtering method. In addition, PTV-obtained velocity statistics for an ORW, pre-processed by the ratio cut method, including data relating to fluctuating intensity and the PDF distribution of fluctuating velocity, are shown to be more consistent with those relating to baseline cases than data obtained by either of the the other two methods used for comparison. Moreover, evidence is also provided regarding the superiority and robustness of this approach, in terms of estimating the mean skin friction from the near-all mean velocity profile.

Corrugation Assisted Enhancement of Aerodynamic Characteristics of Delta Wing for Micro Aerial Vehicle

Some of the existing challenges of fixed-wing micro aerial vehicle designs at low Reynolds number are low lift and reduced aerodynamic efficiency. Delta wings, despite their characteristic maneuverability, are rarely used for micro aerial vehicles, perhaps due to the limited lifting capacity because of the reduced aspect ratio. Corrugations on insect wings are ubiquitous and augment the aerodynamic performance in a similar Reynolds number regime. Herein, corrugations are applied to enhance the lift and aerodynamic efficiency of a delta wing. Three delta-wing geometries are examined using numerical simulations at a Reynolds number of : a nonslender delta wing with 53 deg leading-edge sweep and two variants with corrugations in the rear planform. The simulations reveal that the variants potentially offer approximately 11 and 27% increments in the aerodynamic efficiency and the lift, respectively, over the conventional delta wing at an angle of attack of 5 deg. Moreover, the corrugations strengthen the leading-edge vortices and generate an additional nosedown moment about the leading edge. The introduction of corrugations on micro aerial vehicle wings could be a passive technique to potentially improve the aerodynamic characteristics, without any significant weight or drag penalty.

Backflow events under the effect of secondary flow of Prandtl's first kind

The occurrence of backflow events, defined as negative wall-shear stress, is rare in turbulent flows. It is found that these backflow events are further suppressed in a toroidal flow when compared to a straight pipe flow at a nominally similar Reynolds number. The reduction of backflow is due to the secondary flow of Prandtl's first kind occurring in the toroidal flow.

Characterisation of aeroelastic harvester efficiency by measuring transient growth of oscillations

With growing demand for small autonomous electrical devices, such as those in wireless sensor networks, energy harvesting has attracted interest with the promise of low maintenance and sustainable power sources. Galloping energy harvesters utilise the fluid-structure interaction to transform kinetic energy in fluid flow into electrical energy. The performance of galloping energy harvesters depends on the geometry of the tip, with the structure of the flow around the tip defining the nature of fluid-structure interaction and hence the potential efficiency of the device.In this work the curved blade tip geometry is investigated. To experimentally characterise the performance of the harvester, a method utilising the free oscillation transient is developed. The method avoids implementation of a transduction mechanism and hence optimisation of the associated parameters. The developed method is generic and can be applied to other energy generators.The power coefficient of curved blades of different curvatures is measured and the optimal range identified. The maximum coefficient of performance of the curved blade harvester occurs at tip speed ratios from 0.32 to 0.74 and reaches 0.08, which is 3 to 4 times lower than Savonius turbines, the best performing devices at similar Reynolds numbers. The square prism geometry is used as a comparator and found to have a coefficient of performance 10 times less than the curved blade. Flow visualisations confirm the curved blade to act as an airfoil in the highest performing cases, hence future tip shapes should be developed to promote flow attachment.

ε-NTU analysis of turbulent flow in a corrugated double pipe heat exchanger: A numerical investigation

The present study numerically investigates a double pipe heat exchanger with a simple and corrugated tube assuming three different wave amplitudes. The simulation is performed using ANSYS package, considering turbulent flow and k-ω SST turbulence model. Accordingly, the heat exchanger type is considered to be water-to-water and the corresponding flow is parallel so that the hot and cold fluids pass through the inner tube and the shell, respectively. The results show that, in the similar Reynolds number, corrugating increases Nusselt number so that at the maximum state the average Nusselt number of the corrugated heat exchanger is about 1.75 times as compared to the simple heat exchanger. Furthermore, the maximum value of local Nusselt number and local skin friction coefficient occur at the bottleneck and the minimum value occur during reverse flow. Moreover, from effectiveness and efficiency of the second law of thermodynamics, the corrugation of the tube also improves the corresponding performance. Hence, compared to the simple heat exchanger, the ratio of effectiveness and second law efficiency of thermodynamics of the corrugated heat exchanger is 1.73 and 1.17, respectively.

Numerical and experimental investigation of a finite cylinder wake

Following a thorough time-resolved 2-D particle image velocimetry experimental investigation of the wake of a square wall-mounted finite cylinder, at Reynolds Re = 5.4 × 104 with aspect ratio H/d = 3, a 3-D DNS numerical investigation was performed under similar flow conditions and comparable Reynolds number (Re = 4.0 × 104). In this paper, the simulation results in four 2-D planes are compared to the experimental case using proper orthogonal decomposition (POD), spectral analysis and mean flow. Numerical and experimental POD modes were similar, with a few differences in mean flow values and main vortex emission frequency that can be explained by differences in boundary layer thickness. The investigated horizontal planes were z/H = 0.3, z/H = 0.5 and z/H = 1.0 planes, as well as the vertical x–z symmetry plane. While time-resolved experimental results were acquired independently for each investigated plane, numerical results were acquired concomitantly in all four planes, with full 3-D wake data available. The results in this paper confirm that the simulation is able to reproduce the main dynamic features of the flow, establishing the basis for deeper analysis of the full numerical data set in the future.

Generally-valid optimal Reynolds and Dean numbers for efficient liquid-liquid mixing in helical pipes

This work documents optimal flow conditions in terms of Reynolds number Re (and Dean number De) for mixing of two miscible liquids in helical pipes. Extending our previous investigations (Mansour et al., 2017, 2018), many more configurations have been considered with widely different dimensions; as a consequence, generally-valid values for Reopt and Deopt have been obtained. A full-factorial design of experiment (DOE) has been carried out by considering all possible combinations of geometrical parameters studied in Mansour et al. (2018). This leads to a total of 27 different coiled configurations with a wide variety of curvature ratio (0.033–0.214) and dimensionless pitch (0.034–0.273). Additionally, 47 different values of the Reynolds number have been considered within the laminar range of 5–4000, which corresponds to a Dean number range of De = 2–1850. This study involves at the end 1269 distinct numerical simulations. The results reveal that the mixing efficiency as function of the Reynolds number always exhibits two peaks at nearly similar Reynolds numbers. Average values of the two optimal ranges of Reynolds number and Dean number have been deduced from all simulations, leading to Reopt,1≈35 (Deopt,1≈10), and Reopt,2≈650 (Deopt,2≈185). Those can be used to get excellent mixing within any coiled configuration. The first optimal value leads to a much lower pressure drop and a slightly better mixing efficiency, and is therefore recommended to get optimal mixing and power consumption.