Search

The transition process within a Laminar Separation Bubble (LSB) that formed on a compressor blade surface was investigated using Large Eddy Simulations (LESs) at a Reynolds number of 1.5 × 105 and incidence angles of 0°, +3°, and +5°. The vortex dynamics in the separated shear layers were compared at various incidence angles and its effects on the loss generation were clarified through entropy analysis. Results showed that transition onset, which was accurately identified by the Linear Stability Theory (LST), was significantly promoted at the increased incidence angle. As such, the development of LSB was suppressed and the relative role of viscous instability played in the transition process was weakened. At the incidence angle of 0°, two-dimensional spanwise vortices detached from the blade surface and rolled up periodically, which were further stretched and eventually evolved into large-scale hairpin vortices. As time passed, the fully developed hairpin vortices broke down into small-scale eddies. Meanwhile, the flow near the wall reversely ejected into the outer separated shear layers and a sweeping process happened subsequently, forcing the separated shear layers to reattach and accelerating the generation of turbulent fluctuations. By comparison, the strength of vortex rolling-up was weakened at higher incidence angles, and the vortex pairing and breakdown of large-scale vortices were less pronounced. Therefore, the level of turbulent fluctuations that generated in the separated shear layers was reduced. Detailed entropy analysis showed that the turbulent dissipation effect related to the Reynolds shear stresses determined the largest amount of positive entropy generation, which declined to a lower level as the incidence angle increased from 0° to +5°. Correspondingly, the profile loss was reduced by 50.4%.

Laboratory experiments were performed to characterize the distinct features of hairpin-like vortical structures induced via pulsed, wall-normal jets in laminar (LBL), and turbulent boundary layers (TBLs). Hotwire anemometry was used to characterize the disturbed flow at multiple streamwise, transverse, and vertical locations. The dominant spatiotemporal structure of the induced motions was reconstructed using Taylor’s hypothesis and time-averaged, phase-locked data. Results support the concept that the meandering of very-long, low-momentum streaks observed in the logarithmic layer and lower part of wake regions of TBLs results from the upwash of hairpin vortices or packets. Comparing the induced hairpin-like vortices in the LBL and TBL reveals the modulation of the mean shear; using the second-order derivative of the mean streamwise velocity, we show a linkage between the mean shear gradient and appearance of low- and high-momentum streaks in the logarithmic layer. Streamwise velocity profiles with a pitot tube by Smith (Effect of Reynolds number on the structure of turbulent boundary layers, 1994) at ten Reynolds numbers from Reθ = 4601 to 13,189 are also inspected to evaluate such linkage in a broader range of Re. As Reynolds number increased, the vertical regions of the negative second derivative of mean streamwise velocity increased. The minimum value of mean shear gradient decreased, indicating that the mean shear stress gradient increased to form more noticeable low- and high-momentum streaks in the logarithmic layer. Besides, the Reynolds tensor’s principal stresses and principal axes angles associated with the hairpin vortex legs stretching are demonstrated by DNS data (Spalart, P. R. 1988) as Reynolds number increased.

We have studied the resonance curves of a quartz tuning fork of the fundamental frequency 32 kHz immersed in superfluid 4He and mixtures 3He–4He with 3He concentration of 5 and 15% in the temperature range of 0.35–2.5 K at saturated vapor pressure. Two types of experiments have been carried out, with a tuning fork both not covered by a bulb and coated by a bulb, i.e., in the restricted geometry. In both cases, the velocity-force dependences for the tuning fork showed a linear damping force at low peak velocities and extra drag due to the appearance of vortex lines accompanying the transition to turbulence under increasing peak velocity. These dependencies are mainly determined by the density of the normal helium component. There is a pronounced difference between superfluid 4He and mixtures of 3He in 4He, where 3He impurity particles provide a constant temperature-independent contribution to the normal component of the mixture. The extra contribution to the damping force, so-called “turbulent drag force”, decreases with concentration increase at the same peak velocity of the tuning fork that can be explained by the extension of the range of laminar flow with an increase in the concentration of 3He. We found that the drag coefficient in superfluid 4He and mixtures 3He–4He reaches a plateau at different peak velocities and different exciting forces and explained this fact by different conditions for vortex formation, depending on the different thickness of the near-wall viscous layer. The comparison between the data obtained in restricted and unrestricted geometries shows that there is an excessive dissipation of the tuning fork motion associated with the emission of the first sound wave in unrestricted geometry.

Natural vortex development in double diffusive confined wall jet flow is firstly analyzed as a basic state. A consecutive vortices production and shedding processes are observed at moderated Reynolds numbers. The influence of these phenomena on combined transfers is then discussed. Strouhal number evolution was determined by spectral analysis conducted in the near-field region of the laminar wall jet. Accordingly, two critical values were found indicating the transitions between stationary, non stationary and quasi-periodic states. Uniform amounts of water are then added or extracted through a thin slot placed on the horizontal flat plate in order to control large structures creation and growth. The main purpose is to assess the effectiveness of uniform suction or blowing on each flow state and to establish the appropriate conditions under which the natural instability disappears. An Average Nusselt and Sherwood numbers as well as skin friction coefficient dependent behavior were observed which implies that vortex dynamic directly affects thermo-solutal transfer.

A series of Direct Numerical Simulations (DNS) at low Reynolds number is performed to investigate the flow and the heat transfer in ducts with complex geometry, which are frequently encountered in many engineering fields, especially in aeronautics. A finite-difference approach and the immersed-boundary method are adopted to solve the governing equations in the computational domain. The flow and heat transfer phenomena in a circular pipe are studied in detail for laminar and turbulent regimes; thus the code is validated through a comparison of present DNS results with those provided by some trusted references. Straight ducts with regular polygonal cross sections represent the first type of ducts with complex geometry analyzed in this paper. The behavior of the pumping power expenditure is studied when the number of polygonal sides changes. The secondary motions are then investigated, and their effects on the mean velocity and temperature contours are studied. The shapes of vortices in secondary motions are compared with those provided by an analytical approach, which is based on the eigenfunctions of the Laplacian operator. Also the geometries pioneeringly studied by Nikuradse in 1930 are taken into account.

Abrupt expansions are a very frequent geometry in mechanical engineering systems, i.e. in combustion chambers, valves, heat exchangers or impinging cooling devices. However, despite the large number of devices that use this geometry, the expanded flow behaviour still needs further research to understand and predict the full system performance. This paper presents the application of the non-uniform finite difference approximation method developed in Sanmiguel et al. for the numerical characterisation of a confined swirling laminar jet discharging with a large expansion ratio. This investigation can be considered an extension of previous work by Revuelta, but now a swirling flow is generated by a rotating pipe upstream the expansion. The structures found when a fully-developed rotating Hagen-Poiseuille flow discharges into a much larger pipe section are summarised in a bifurcation diagram, whose coordinates are the Reynolds number of the jet ( Rej) and the swirl parameter ( L), for which the time-dependent, axisymmetric and incompressible Navier-Stokes equations are integrated numerically. For values of the jet Reynolds number below 200, there is a critical value of the swirl parameter above which stable vortex breakdown appears. For values of the Reynolds number above 200, three different behaviours are observed, and each performance appears for a critical value of the swirl parameter. When increasing the swirl parameter from zero, the flow becomes axisymmetrically unstable, showing an oscillatory behaviour. If further increasing the swirl intensity, the oscillatory flow coexists with a vortex breakdown bubble and, finally, a steady vortex breakdown is reached. The expansion ratio ε considered in all the simulations is 1[Formula: see text]. In previous literature, the exactness of the limiting critical Rej and L values that define these behaviours has been found to be influenced by the variability in the inlet profile conditions, which affects the expanded flow. This enhances the importance in the present investigation to accurately simulate the discharge pipe inlet profiles.

The influence of laminar-to-turbulence transition of the laminar separation bubble (LSB) on the dynamic stall process of a pitching foil was investigated using transition SST k-ω turbulence model. As the LSB moves to the leading edge, the strong strain rate produces a pronounced laminar-to-turbulence transition, resulting in a turbulent mixing, faster turbulent reattachment, and shrinkage of LSB. This transition leads to the enhancement of the friction coefficient after the reattachment point and delays the suction collapse and evolution of secondary vortex. A baseline SST k-ω model was also applied. Without the transition mechanism, the suction collapse occurs too early, and the downstroke relaminarization is absent. The oscillation of the downstroke lift curve is severer using the SST model than that using the transition SST model. This study extends the detailed validations of transition model and helps improve the understanding of the influence of the transition on the dynamic stall process.

Slag entrainments in a continuous casting mold result in serious defects in steel products. This slag entrainment is directly connected with the meniscus vortex accompanying a large meniscus deformation. To exactly evaluate the air or slag entrainments, it is critical that numerical studies can reproduce the meniscus vortex deformation. In our study, we found that Reynolds-averaged Navier–Stokes (RANS) turbulent models failed to reproduce the meniscus vortex deformation, and furthermore, the laminar model underestimated the meniscus vortex depth at high-casting conditions over 1.4 m/min. In this study, we proposed the hybrid and large eddy simulation models to correctly predict the distortion for real physics of the meniscus vortex. In contrast to the laminar and RANS turbulent models, the proposed models show strong agreement with the experiment even in high-casting conditions.

In this article, we introduce new field equations for incompressible non-viscous fluids, which can be treated similarly to Maxwell’s electromagnetic equations based on artificial intelligence algorithms. Lagrangian and Hamiltonian formulations are used to arrive at field equations that are solved using convolutional neural networks. Four linear differential equations, which describe the two fields, namely, the dynamic pressure and the vortex fields, are derived, and these can be used in place of Euler’s equation. The only assumption while deriving this equation is that the dynamic pressure and vortex fields obey the superposition principle. The important finding to be noted is that Euler’s fluid equations can be converted into field equations analogous to Maxwell’s electromagnetic equations. We solve the flow problem for laminar flow past a cylinder, sphere, and cone in two dimensions similar to the conduction in a uniform electric field and arrive at closed-form expressions. These closed-form expressions, which are obtained for the potentials of fluid flow, are similar to the streamline potential functions in the case of fluid dynamics.

Numerical simulations of incompressible laminar flow past a circular cylinder using wavy wall confinement are performed in order to study drag reduction and vortex shedding suppression for Reynolds numbers (50 ≤ Re ≤ 280) and blockage ratios (0.5 ≤ D/H ≤ 0.9), where D is the cylinder diameter and H is the channel height. The optimum configuration of wavy wall confinement is identified to have an amplitude of 0.2D and wavelength of 4D which manifests the minimum drag coefficient and complete vortex shedding suppression (zero lift coefficient). The flow over the cylinder with optimized wavy wall confinement produces a stable vortex pair in the wake of the cylinder. The length of vortex pair in the wake increases by increasing the Reynolds number and decreasing the blockage ratio. The drag on the cylinder reduces when the length of vortex pair increases due to early flow separation and reduction in pressure drag. The drag coefficient decreases by 36% in comparison to the plane wall confinement at D/H = 0.5. The drag coefficient decreases for wavy wall configuration by about 67% for D/H = 0.7 and 94% for D/H = 0.9. The lift coefficient remains zero at all Reynolds numbers and blockage ratios which indicates complete suppression of vortex shedding.

The effects of jet pulsation intensity and excitation Strouhal number on the flow and dispersion characteristics of dual parallel plane jets at a low Reynolds number of 200 were experimentally studied so that the transition from laminar to turbulence was focused. A hot-wire anemometer was used to record the jet velocities. The flow patterns were obtained using the laser-light sheet assisted smoke flow visualization technique. Jet spread widths were derived from the long-exposure images using the binary edge detection method. The jet fluid dispersion characteristics were examined using the tracer-gas concentration detection technique. Four characteristic flow modes (discrete coherent vortices, contiguous coherent vortices, early vortex breakup, and lateral dispersion) were identified in the domain of the jet pulsation intensity and excitation Strouhal number. At small jet pulsation intensities, discrete and contiguous coherent vortices appeared at low and high excitation Strouhal numbers, respectively. The vortices appearing in the jet shear layers broke up slowly and caused small jet expansions and turbulent intensities, thus inducing a low efficiency of jet fluid dispersion. The early vortex breakup mode was induced by the low-frequency back-and-forth flow motions in the near region at large jet pulsation intensities and low excitation Strouhal numbers. Large turbulence fluctuations caused by the early vortex breakup increased the jet dispersion significantly. At large jet pulsation intensities and excitation Strouhal numbers, high-frequency back-and-forth flow motions in the region near the jet exits significantly reduced the time-averaged axial momentum and induced large lateral jet expansion and jet fluid dispersions.

Experiments have found substantial morphological differences between buoyancy-driven flames developing on the upper and lower surfaces of inclined burning plates. These differences cannot be explained on the basis of existing analytical solutions of steady semi-infinite flames, which provide identical descriptions for the top and bottom configurations. To investigate the potential role of flame instabilities in the experimentally observed flow differences, a temporal linear stability analysis is performed here. The problem is formulated in the limit of infinitely fast reaction, taking into account the non-unity Lewis number of the fuel vapor. The stability analysis incorporates non-parallel effects of the base flow and considers separately spanwise traveling waves and Gortler-like streamwise vortices. The solution to the stability eigenvalue problem determines the downstream location at which the flow becomes unstable, characterized by a critical value of the relevant Grashof number, whose value varies with the plate inclination angle. The results for the flame formed on the underside of the fuel surface indicate that instabilities emerge farther downstream than they do for a flame developing over the top of the fuel surface, in agreement with experimental observations. Increased buoyancy-induced vorticity production is reasoned to be responsible for the augmented instability tendency of topside flames.

This paper experimentally examined a feedback-loop mechanism generating tonal noise when a two-dimensional (2-D) protuberance as high as the boundary-layer thickness was placed in zero- and weak adverse-pressure-gradient laminar boundary layers. Onset of tonal noise required rapid disturbance growth due to strong instability in a small separation bubble formed just upstream of the protuberance, in addition to slow but long-distance amplification of Tollmien–Schlichting (T–S) waves in a 2-D form. The experiments clearly showed that the adverse pressure gradient, even if weak, appreciably affected amplification of T–S waves, and consequently reduced the threshold height of protuberance for the onset of tonal noise significantly. The result was supported by stability analyses (based on the PSE) of the boundary layers with and without a protuberance. The N -factor was found to be 11–12 for the threshold protuberance height in both boundary layers, which was large enough for T–S waves to evolve into strong vortices. When an additional receptivity region other than the leading edge region was introduced by gluing a thin 2-D roughness element much thinner than the boundary-layer displacement thickness at a location beyond the critical Reynolds number for linear instability, the threshold protuberance height decreased below 60% of the value in the absence of the roughness element. The frequencies of multiple discrete tones were explained by a simple feedback-loop model similar to the case of trailing-edge noise, assuming that the feedback-loop was established between the receptivity location and protuberance.

Bioinspired grooved surface structure design has been widely used as an efficient passive flow control method in drag reduction. The total drag of plate with grooved surface structure can be decomposed into friction and pressure drag. In this paper, the relationship between them and the distribution of vortex structure in flow field has been analyzed for obtaining the drag reduction mechanism of grooved surface structure. The concept of ‘Vortex-Driven Design’ is proposed to improve the performance of conventional periodic single-level grooved surface structure. A design scheme of micro-nano scale nested-grooved surface structure with better drag reduction performance is given. The authors conduct numerical simulation of rectangular nested-grooved surface structure in laminar flow considering rarefaction based on Lattice Boltzmann Method at high Knudsen number. The results show that the nested-grooved surface structure induces higher complexity in the secondary vortex structure, and that the changes of vorticity distribution and flow characteristics drive the changes of velocity distribution and shear stress in the grooved surface. Compared with conventional grooved surface structure, the average friction drag of the surface is further reduced. Two geometric optimization cases of the conventional grooved surface and nested-grooved surface are realized by Genetic Algorithm, showing that the maximum drag reduction rate of the optimal nested-grooved surface can reach 18.76% while that of the optimal conventional grooved surface only reaches 13.61%. Based on ‘Vortex-Driven Design’, an innovative material improvement method for drag reduction is proposed as a new direction for subsequent research on microstructure in terms of drag reduction and energy conservation.

The objective of this study is to construct a regime diagram for laminar flames stabilized behind flame holders with respect to the presence of a recirculation zone (RZ), trend of heat loss to the burner, and flow strain and flame curvature effects. This is achieved by varying the radius of the cylindrical flame holder and the mixture velocity between the flashback limit and the blow-off limit at a fixed equivalence ratio. It is found that for all flame holders, a RZ vortex is not present near the flashback limit. At flashback, flow strain is almost zero and the flame curvature is found to be the main contributor to flame stretch. With increasing mixture velocity, the heat loss to the flame holder decreases for smaller radii and a RZ does not appear till blow-off occurs. For flame holders with radii greater than twice the flame thickness, the heat loss to the flame holder first decreases with increasing mixture velocity without a RZ. A further increase in the mixture velocity does not result in blow-off but instead, a RZ appears behind the flame holder reversing the heat loss trend. In this scenario, flow strain is found to increase significantly and becomes the major contributor to flame stretch, although curvature effects are still present. With the RZ present, the blow-off limits are significantly extended and the stabilization mechanism is altered. The RZ vortex shields the flame base from intense pre-heating resulting from the increase in heat loss to the flame-holder while it provides support to the flame leading edge by recirculation of hot products. The results obtained from this study are used to construct a regime diagram, which offers a broader view of the whole flame stabilization process and its mechanisms.

Vertical velocity profiles are obtained experimentally for the boundary layer of a flow with an inverse pressure gradient in different phases of the periodic process of fluid deceleration. At the greatest drop in velocity, the profile has a local minimum with two inflection points at the upper boundary of the viscous layer. Chains of vortices then form at this location, testifying to a loss of stability of the laminar motion.

The laminar–turbulence transition phenomenon widely exists on the surface of many energy equipment, which is deserved to be studied because of the complex mechanics and some induced undesirable consequences. The goal of present work is to investigate the transitional flows around the forward and reversed hydrofoils at different incidences using the SST γ - R e ˜ θ t transition model, with special emphasis on the dynamics of the transition. The effect of inflow turbulence condition is considered initially. Then, the difference between the original SST k- ω model and SST γ - R e ˜ θ t transition model is analyzed, in terms of the near-wall velocity profiles and flow morphology. Afterwards, the change of the transition with the incidence for the reversed hydrofoil is clarified in detail. The primary results show that the flow separation near the sharp leading edge where the reverse dynamic vortex (RDV) appears makes the contribution to the transition. The size of RDV is much larger than laminar separation bubble (LSB) over the forward hydrofoil and it forms near the leading edge earlier. Moreover, the transition locations are mapped both for the forward and reversed hydrofoils. Finally, the effect of Reynolds number on the transition process for the reversed hydrofoil is presented. It is believed that this work can deep the understandings of the transition, especially for the reversed hydrofoils.

This numerical investigation is focused on determining the structures of blue whirls, recently found to occur in laboratory investigations of fire whirls when the circulation becomes sufficiently large to produce a vortex breakdown that drastically shortens the fire whirl and correspondingly reduces residence times, so that the yellow flames turn blue. The computations address axisymmetric configurations for round pools of liquid fuels flush with and at the center of a larger solid horizontal disc, at the outer edge of which vanes of adjustable angles cause the entrained air to enter with a controllable azimuthal component of velocity. The nondimensionlized conservation equations employed include realistic Lewis numbers with temperature-dependent transport coefficients and a one-step chemical-kinetic approximation that correctly reproduces laminar burning velocities. Buoyancy and radiant energy transport from the flames to the liquid surface are both taken into account, the latter being found to be essential for the blue whirl. Along with the vaporization-equilibrium and energy-conservation boundary conditions at the fuel surface, inflow boundary conditions are provided by a recently developed solution for the boundary-layer flow over the solid disc, while zero-gradient outflow conditions are applied above the whirl. Controlling nondimensional parameters, besides Reynolds, Damkohler, and Froude numbers, are a ratio of radiant to convective energy flux and a ratio of azimuthal to inward radial flow velocity in the boundary layer at the edge of the disc. The computed conditions for the onset of the blue whirl, as well as the computed structure of the whirl itself, bear close resemblance to what was found experimentally.

Curved pipes flows are encountered in different areas such as heat transfer, chaotic mixing, separation of mixtures in pipes, or blood circulation among others and exhibit a variety of characteristics depending on the ranges of Dean numbers and pipe curvatures. Studies on curved pipes flows usually consider the cases of circular, elliptical, and rectangular shapes for the cross sections of the pipe. The present work extends the availability of asymptotic analytical solutions to new ranges of cross-sectional shapes while considering fully developed steady state flows at low Dean numbers. The new shapes are given by a polar equation R* (q) satisfying the relation 1-R^(*2) (q)+dR^(*y) (q)sin(yq)=0 where d and y are parameters. The zero-order azimuthal velocity profiles for various cross-sections are given by exact analytical solutions. Solutions for the nonhomogeneous biharmonic equation for the secondary flows are given by using exact expressions for the particular solutions. Furthermore, the Fourier series decomposition of the solution is adopted to determine the integration constants that allow satisfying the non-slip boundary conditions. Solutions are presented for semi triangular (y=3) , square (y=4), and pentagonal (y=5) cross sections shapes. It is found that the velocity distribution and the Dean’s vortexes intensities are modified in function of the cross-section shapes.

In this paper laminar free convection heat transfer from a horizontal isothermal elliptic tube placed in a micropolar fluid with its major axis vertical is investigated. The governing equations basd on the conservation of mass, linear momentum, angular momentum and energey are numerically solved using the Fourier Spectral method. The main controlling parameters, beside the material parameters of the micropolar fluid, are the Rayleigh number, the Prandil number and the elliptic section axis ratio The micropolar fluid parameters are the vortex Viscosity, micro-inertia density and Spin gradient viscosity The results, which were obtained for a range of these parameters, were carried Out. at two axis ratios of 0.5 and 0.025. The axis ratio of 0.025 approximates the geometry of the special case of vertical flat plate. The results for vertical plate placed in Newtonian fluid arc compared with the relevant results in the literature and very good agreement was obtained. The study has shown that the vortex viscosity is the frost influencing material parameter on heat transfer rate. The study showed that as the vortex viscosity increases the heat transfer rate decreases.

Numerical studies of laminar flow and heat transfer around a sharp 180° turn, of especial relevance to electronic systems, are made. Two-dimensional unsteady simulations are considered because of the high geometrical aspect ratios (AR > 5) occurring in the practical application. A high-order time-accurate finite volume scheme was used to solve unsteady incompressible Navier-Stokes and energy equations. Simulations are performed for 50 ≤ Re ≤1000. Predictions show that the sharp tum flow remains steady up to Re ≤ 600. In this steady regime, the re-attachment length increases gradually with the Reynolds number. However, a further increase in the Reynolds number gives rise to oscillatory flow because of the large-scale vortices emanating from the sharp edge. These vortices dominate the flow field. Oscillations cause a considerable reduction in the re-attachment length and a significant increase in convective heat transfer. The correlation between the local beat transfer and the flow field is examined. Instantaneous friction coefficient and Nusselt number Nu variations show that local heat transfer distributions correlate closely with the flow fields.

Solar air heater is used to heat air for space heating and drying process, but its low heat transfers between absorber plate and air account for its low thermal performance. Various surface geometries and fin modifications in a solar air heater have improved its thermal performance. The fabrication and performance evaluation of a solar air heater with an arched wavy absorber plate having porous aluminum wire mesh cylindrical fins attached have been investigated. The arched absorber plate design resulted in an increase in the air turbulence and vortex generation, which leads to the breaking of laminar sub-layer near the surface of the absorber plate and improved the heat transfer. In addition, cylindrical porous wire mesh aluminum fins resulted in an increase in the fin porosity which reduces the pressure drop and improved the thermal performance of the solar air heater. Comparative analysis has been carried for different arrangements at three different fluid flow velocities. The arched wavy absorber plate with fins resulted in better thermal performance with a temperature difference in the range of 55–70 °C approx. compared to the other arrangements at a fluid flow velocity of 5 m/s.

The transitional flow around the SD7003 airfoil at $$\alpha = 8^\circ$$ , $${\text { Re }}= 60,000$$ and $${\text { Ma }}= 0.2$$ was investigated employing p-adaptive large eddy simulations in a Discontinuous Galerkin (DG) framework. This test case is particularly challenging since a laminar separation bubble (LSB) with transition and reattachment of the turbulent boundary layer has been observed. The results obtained with the dynamic anisotropic model have been compared with experimental measurements and other numerical results available in literature. The polynomial adaptivity technique confirmed its capability to correctly represent the flow with a great saving in the computational cost. The results are close to experimental measurements thanks to the capability of the subgrid model to capture the energy backscatter from the subgrid scales. Starting from the statistically steady state flow field obtained, the viscous, parallel, Blade-Vortex Interaction (BVI) has been studied in the same numerical framework. A modelled vortex has been superimposed to the developed flow around the SD7003 airfoil and the dynamic p-adaptivity has been successfully employed to capture its advection over time. The loads have been recorded and compared with a reference simulation without the vortex. While the drag rapidly comes back to its steady-state value, the transients for lift and moment coefficients are longer because of the effect of the vortex on the LSB.

This paper characterizes the hydrodynamics of laminar flows through circular ducts equipped with plain square woven meshes/screens in their role as static mixers. The CFD model was used to investigate the effect of screen geometry, number of screens, inter-screen spacing, and operating conditions on the velocity and pressure fields. This work therefore presents an analysis of the velocity contours, streamlines, and profiles at various locations within the pipe in addition to the pressure profiles to obtain a better understanding of the complex behavior of the flow. The presence of screens normal to the flow was found to alter the fully developed parabolic profile and flatten it both upstream and downstream, hence simultaneously decelerating and accelerating regions in the cross-sectional area of the flow. Downstream of screens, the velocity profile was characterized by a sinusoidal shape that quickly decays, and vortices were observed under certain operating and design conditions. The study of the pressure field helped delineate the contribution of static and dynamic pressures in addition to identifying a minimum inter-screen distance if maximum pressure recovery is desired. Predictions of the pressure drop for various mesh geometries and under different operating conditions were validated using empirical correlations and the results reflected a very good accuracy with the maximum relative error falling within ± 7.3%.

The study is devoted to the flames instabilities that occur during the transition from the normal gravity to zero one. The experiments were carried out at the Bremen Drop tower. A stoichiometric methane-air flame was investigated during the first second after the capsule release. High amplitude oscillations with periodic flame flashback inside the burner and detaching of the hot spots during the transition period were found. The similarity of oscillation spectra for stoichiometric flames with different flow rates was observed. The coincidence of the first and second fundamental harmonics of the oscillations frequencies for all the studied flows indicates instability mechanism independence from the Reynolds number for the investigated range of Re: 600–2000 (laminar and weak turbulent regimes). It was found that the oscillations logarithmic decrements for all flow Reynolds numbers under investigation have a similar value. Based on a linear approximation, the hydrodynamic instability of a flame was considered and it was shown that the transition from conditions of normal terrestrial gravity to microgravity leads to the flame instability region extension which was confirmed experimentally. A following instability occurrence model was proposed. The buoyant motion of Kelvin-Helmholtz vortices arising at the air/combustion products interface slows down due to gravity decrease. Therefore the vortices deform not only the flame front tip but side part as well, leading to flame front deformations and constrictions, forming separating hot spots.

Image-based Computational Fluid Dynamic (CFD) simulations of anatomical models of human arteries are gaining clinical relevance in present days. In this study, CFD is used to study flow behaviour and hemodynamic parameters in aneurysms, with a focus on the effect of geometric variations in the aneurysm models on the flow dynamics. A computational phantom was created using a 3D modelling software to mimic a spherical aneurysm. Hemodynamic parameters were obtained and compared with the available literature to validate. Further, flow dynamics is studied by varying the dome size of the aneurysm from 3.75 mm to 6.25 mm with an increment of 0.625 mm keeping the neck size constant. The aneurysm is assumed to be located at a bend in the arterial system. Computational analysis of the flow field is performed by using Navier – Stokes equation for laminar flow of incompressible, Newtonian fluid. Parameters such as velocity, pressure, wall shear stress (WSS), vortex structure are studied. It was observed that the location of the flow separation and WSS vary significantly with the geometry of the aneurysm. The reduction of WSS inside the aneurysm is higher at the larger dome sizes for constant neck size.

Pulsar glitches offer an insight into the dynamics of superfluids in the high density interior of a neutron star. To model these phenomena, however, one needs to have an understanding of the dynamics of a turbulent array of superfluid vortices moving through a pinning lattice. In this paper we develop a theoretical approach to describe vortex mediated mutual friction in a pinned, turbulent and rotating superfluid. Our model is then applied to the study of the post glitch rotational evolution in the Vela pulsar and in PSR J0537-6910. We show that in both cases a turbulent model fits the evolution of the spin frequency derivative better than a laminar one. We also predict that the second derivative of the frequency after a glitch should be correlated with the waiting time since the previous glitch, which we find to be consistent with observational data for these pulsars. The main conclusion of this paper is that in the post-glitch rotational evolution of these two pulsars we are most likely observing the response to the glitch of a pinned turbulent region of the star (possibly the crust) and not the laminar response of a regular straight vortex array., Submitted to MNRAS

The influence of a local change in surface temperature of a contoured nozzle corresponding to the Mach number M = 6 on the boundary layer stability and laminar-turbulent transition is numerically studied. The profiles of the laminar boundary layer are obtained by solving the Navier-Stokes equations with the use of the Ansys Fluid software system. N-factors of the growth rates of the Goertler vortices and also disturbances of the first and second Mack modes are calculated in the approximation of the linear stability theory. It is demonstrated that local heating ensures lower growth rates of the amplitudes of the Goertler vortices and the first Mack mode as compared to the base case; the more intense the heating, the more expressed this effect. The growth rate of the amplitude of the second-mode disturbances decreases during local heating of the nozzle to a temperature close to the stagnation temperature and increases at higher temperatures of local heating. It is found that local cooling leads to an increase in the growth rates of the amplitudes of the Goertler vortices and second Mack mode. The amplitude of the first Mack mode in the cooling region is smaller than that in the base case; however, further downstream, it is much greater than that in the base case. It is found that the surface of contoured nozzles should be heated in the region of the maximum growth rates of the amplitudes of the Goertler vortices; the higher the temperature, the more pronounced the expected effect. However, the maximum possible temperature is determined by the growth of the second Mack mode. The optimal option is to use the temperature of local heating of the surface at which the growth rate of the amplitude of the second mode is smaller than that of the Goertler vortices.

Purpose The loading mechanism of textures considering turbulence has not been fully covered. This paper aims to investigate the effect of turbulence on the textured loading capacity under water lubrication and to analyze the causes of the turbulence effect. Design/methodology/approach Computational fluid dynamic models with different textured shapes are established after validation. The transition shear stress transport (SST) model, which is suitable for predicting the transition process of fluid from laminar state to turbulent state, is adopted in the present study. To illustrate the effect of turbulence, the loading capacity of textures predicted by transition SST model and laminar model is compared. Findings The loading capacity is higher after considering turbulence because more lubricant enters into textures and the flow rate of lubricant to textured outlet increases. There exists an optimal textured depth ratio and density for loading capacity and the change of flow state would not affect the optimal values. The degree of fluid blockage at textured outlet has a dominant influence on loading capacity. As the textured shape changes to triangle or ellipse from rectangle, the vortices at the textured bottom move forward and the blockage at a textured outlet is enhanced, which makes loading capacity improved under the action of blocking effect. Originality/value The enhancement of the blocking effect is found to be crucial to the improvement of textured loading capacity after considering turbulence. Present research provides references to understand the loading mechanism of textures under turbulent conditions. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-04-2020-0149/

In the present study, a two-dimensional phase field model coupled with Lattice Boltzmann method (PF-LBM) is proposed to predict the dendritic growth and motion in the melt of Fe-C binary alloy, where the phase field method (PF) is used to calculate the dendritic growth, including the phase field and the concentration field, and the lattice Boltzmann method (LBM) is used to calculate the flow field. The dendrite motion is determined by Newton’s Second Law and tracked by Lagrangian point in a Cartesian coordinate system. Later, the model validations were performed with the benchmark of a solid particle settlement in a stagnant fluid and particle motion in a shear flow, and the results show that the present model is capable of predicting the solid particle motion in the fluid flow. Finally, the model is adopted to investigate the dendritic growth and motion in a forced fluid flow (laminar flow or rotational flow), and the dendrite settlement in a terrestrial environment. The results show that when the forced fluid flow is a laminar flow, the free dendrite would be driven to translate, and the relative velocity between the dendrite and flow fluid decreases, resulting in weak influence of fluid flow on the dendritic growth. When the forced fluid flow is a rotational fluid flow, the dendrite would centrifugally rotate on the domain center with a counterclockwise self-spinning, and the rotation radius becomes larger and larger. For the case of dendrite settlement in a terrestrial environment, the relative movement between the dendrite and melt promotes the downward branch growth, but inhibits the upward branch growth, and two vortices form at the wake region of dendrite. Therefore, the settling dendrite shows a significant asymmetrical morphology.

The effect of basic process conditions and the initial distribution of mixed liquids on mixing on the molecular scale was studied experimentally for helical and twisted bends reactors. The selectivity of two competitive-parallel reactions, acid-base neutralization and alkaline ester hydrolysis, carried out in aqueous solutions between initially unmixed reactants, was used to quantify micromixing in steady-state, isothermal and laminar flow. Faster micromixing is achieved in the twisted bends mixer than in the helical mixer due to the periodic reorientation of Dean vortices, present in curved conduits. The difference in micromixing performance of the two reactors decreases for high Reynolds numbers (Re > 1000) when ester hydrolysis is no longer controlled by mixing. Micromixing for lower Reynolds numbers can be improved by changing the ratio of volumetric flows of the reactant solutions and by using a distribution head, which at the reactor entrance creates the contact surface between mixed liquids perpendicular to the curvature plane of the first pipe bend. When micromixing in the reactor becomes so slow that the final selectivity approaches its maximum value, changing the ester to a slower reacting one helps to adjust the sensitivity of the test reaction system to the deteriorating mixing conditions.

Atherosclerosis preferentially occurs near the junction of branching vessels, where blood recirculation tends to occur (Malek et al. in J Am Med Assoc 282(21):2035-2042, 1999, https://doi.org/10.1001/jama.282.21.2035 ). For decades, CFD has been used to predict flow patterns such as separation and recirculation zones in hemodynamic models, but those predictions have rarely been validated with experimental data. In the context of verification and validation (VV), we first conduct a CFD benchmark calculation that reproduces the vortex detection experiments of Karino and Goldsmith (1980) with idealised branching blood vessels (Karino and Goldsmith in Trans. Am. Soc. Artif. Internal Organs 26:500-506, 1980). The critical conditions for the formation of recirculation vortices, the so-called critical Reynolds numbers, are the main parameters for comparison with the experimental data to demonstrate the credibility of the CFD workflow. We then characterise the wall shear stresses and develop a surrogate model for the size of formed vortices.An automated parametric study generating more than 12,000 CFD simulations was performed, sweeping the geometries and flow conditions found in the experiments by Karino and Goldsmith. The flow conditions were restricted to steady-state laminar flow, with a range of inflow Reynolds numbers up to 350, with various flow ratios between the main branch outlet and side branch outlet. The side branch diameter was scaled relative to the main branch diameter, ranging from 1.05/3 to 3/3; and the branching angles ranged in size from [Formula: see text] to [Formula: see text]. Recirculation vortices were detected by the inversion of the velocity vector at certain locations, as well as by the inversion of the wall shear stress (WSS) vector.The CFD simulations demonstrated good agreement with the experimental data on the critical Reynolds numbers. The spatial distributions of WSS on each branch were analysed to identify potential regions of disease. Once a vortex is formed, the size of the vortex increases by the square root of the Reynolds number. The CFD data was fitted to a surrogate model that accurately predicts the vortex size without the need to run computationally more expensive CFD simulations.This benchmark study validates the CFD simulation of vortex detection in idealised branching vessels under comprehensive flow conditions. This work also proposes a surrogate model for the size of the vortex, which could reduce the computational requirements in the studies related to branching vessels and complex vascular systems.

Mixing characteristics in two innovative variants of coiled flow inverter, namely, symmetrical compact coiled flow inverter (SCCFI) and asymmetrical compact coiled flow inverter (ACCFI) are assessed against that of standard coiled flow inverter (CFI), through experimental and numerical analysis conducted under diffusion-free laminar flow condition ( 2.35 ≤ N D e ≤ 11.5 ; 7.55 ≤ N Re ≤ 36.96 ) . The experimental study involves the comparison of step input residence time distribution (RTD) curves to examine (i) the narrowing effects with the increase in Dean number until a unique RTD is achieved, and (ii) the effect of bends on narrowing of RTD. Results show that maximum narrowing of RTD is achieved in SCCFI, followed by ACCFI and standard CFI, and the unique RTDs obtained comply with the narrowing trends: SCCFI at NDe = 5.3, ACCFI at NDe = 3.5 and standard CFI at NDe = 3. The experimental trends are interpreted through an elaborate CFD analysis that predicts the secondary flow features as well as RTD curves. The CFD results show Dean vortices and flow inversion intensifying with Dean number that explains the narrowing effect with the increase in Dean number. RTDs obtained at intermediate bends in all the geometries, show the sequential narrowing with increasing number of bends, thus establishing the beneficial role of complete flow inversion on radial mixing.

A comprehensive review of the local scour due to vortical flow around a cylindrical bridge pier under steady current is presented in this paper. The mechanism of the formation of vortices, the size, velocity and strength of horseshoe vortex (HSV), formation of the HSV by the separation of laminar and turbulent boundary layer and the scour around a cylindrical pier due to vortices have been presented. The complexity involved in the scour-related calculations, and the scope for future research are discussed in the last section.

Numerical simulation of mixed convection and surface radiation in a horizontal rectangular channel with five discrete heat sources protruded from the bottom wall has been carried out. Air is considered as working fluid and flow to be laminar, steady, and incompressible. The parameters varied are Reynolds number (Re) = 100–750, the spanwise position of vortex generator (VG) x/H = 1.8, 2.8, 3.8 4.8 and 5.8, the pitchwise position of VG, y/H = 0.5, 0.6 and 0.7 and emissivity of the heat source, ec = 0.1–0.9, while emissivity of the VG and channel walls is fixed as 0.9. The governing equations are solved based on SIMPLE algorithm using ANSYS 16.2. The results show that the Reynolds number, the position of vortex (spanwise and pitchwise), and emissivity of heat source have significant effects on heat transfer. It is also noticed that the maximum non-dimensional temperature (θmax) is 47.81% with VG at Re = 250 with change in emissivity of the heat sources from 0.1 to 0.9. Finally, a correlation has been developed for average non-dimensional temperature (θavg) using response surface methodology.

Current effort is devoted to investigate the thermophysical aspects of viscous material in a triangular enclosure interacting with a heated square cylinder placed at central position. To view thermal aspects in explicit manner laminar, steady, incompressible viscous material for two different thermal situations are considered bottom wall is (a) uniformly heated (b) non-uniformly heated. Mathematical modelling of phenomenon is attained in the form of dimensional partial differential expressions. To handle manipulated partial differential system an efficient finite element method is implemented. Discretization of domain at multiple levels is generated in the form hybrid grid containing triangular and quadrilateral elements. Velocity, pressure and temperature are approximated by LBB-stable element. Impact of flow controlling parameters on velocity and thermal profiles for spacious range of Rayleigh number (Ra) and Prandtl number (Pr) are considered. Impact of the sundry parameters on coefficient of convective heat transfer, vortex centres and kinetic energy is also computed and shown in tabular format. It is deduced from heat is more efficiently and effectively supplied for non-uniform heating base wall than the uniformly heated wall. It is worthwhile to mention that discontinuity appears in case of uniform heat supply whereas this singularity is removed in the situation where non-uniform heat is provided. Positive trends in Nusselt number and kinetic energy are disclosed for uplifting magnitude of Rayleigh number. It is also found that velocity magnitude at all locations in cavity increments for intensifying values of Rayleigh number.

The objective of this paper is to study the flow regime and hydrodynamic performance for a pitching Clark-Y hydrofoil. The aims are to (1) improve the understanding of the interplay between the hydrodynamic performance, unsteady flow structures and dynamic motion of the hydrofoil, (2) study the influence of the pitching rate on the transition of different flow regimes. The experimental investigations were conducted in the looped cavitation tunnel, and the dynamic moment measurement system was applied to obtain the hydrodynamic forces. The pitching hydrofoil is controlled to rotate from α+ = 10° to α+ = 15° firstly, then goes from α+ = 15° to α− = 5°, finally goes back to α+ = 10° from α− = 5°. The pitching rate is varying with the Reynolds number Re = 4.4 × 105. The numerical investigations were performed by solving the incompressible URANS equations using the coupled k-ω SST turbulence model and γ-Reθ transition model. The numerical results agree well with the experimental measurements. The pitching motion affects the turbulence kinetic energy distribution around the hydrofoil, leading to the delay or acceleration of the transition between different flow patterns. During the pitching process, higher level of turbulence kinetic energy distribution causes earlier transition from laminar to turbulence. Moreover, hysteresis effect of the hydrodynamic force is observed. For the upstroke stage, the higher pitching rate promotes the laminar separation slightly and intensifies the delay of turbulence separation. For the downstroke stage, the higher pitching rate promotes the turbulence separation extensively. The first leading edge vortex (LEV) and anticlockwise trailing edge vortex (TEV) are delayed with the increase of pitching rate, which is responsible to the delay of dynamic stall. The lower pitching rate shrinks the hysteresis loops and intensifies the fluctuation of the dynamic force.

The extended micro- and macroscopic model of laminar and stationary non-equilibrium turbulent processes of gas motion, which includes cascade cooperative processes of molecules energy self-organization, is suggested here. The model is based on the energy conservation law taking into account the fact that emission and absorption of energy are occurring during change of phases. Potential energy of thermodynamic potential of gas layers velocities difference is transformed into gas molecules rotational motion by overcoming the dynamic viscosity of laminar movement process as a result of the first self-organizing process and non-equilibrium phase change on micro-level. The second process of self-organization phase, which leads to the macroscopic dynamic spiral-vortex structure formation, is initiated when the heat conduction parameter (energy flux density) has reached its critical value

For small-scale pool fires, Vali et al. [1] showed a pair of vortices in the liquid pool. The first vortex appeared just close to the sidewall of the container, and the second one emerged slightly away from the first vortex. Large-eddy simulations of small methanol pool fires coupled with liquid fuel convective flow were conducted using an in-house version of FireFOAM to investigate the above phenomenon. In this study, a three-dimensional liquid phase model is newly developed. The model incorporates the effects of thermocapillary Marangoni convection, buoyancy, shear stress, and evaporation. For the gas phase, the combustion model is the extended eddy dissipation concept model coupled with the laminar combustion model. This combustion model uses the viscous diffusion rate to consider laminar-turbulent transition. The predictions were in reasonably good agreement with the measured local mass burning rate, flame height and distributions of liquid temperature. The error of the mass burning rate was within 4%. The present predictions captured a pair of vortices in line with Vali et al.'s experiment [1]. Their sizes increased with increasing the liquid temperature. The Reynolds analogy could explain the sensible reason behind this trend. Shear stress and thermocapillary force caused convection in the liquid pool, and this convection formed a pair of vortices. Thermocapillary force was due to the different distributions of convective and radiative heat transfer. Sensitivity test for sub-models for the liquid phase demonstrated that their effects on the mass burning rate were all less than 5.1%. Conversely, the simulation assuming zero gravity only in the liquid phase resulted in almost 64% reduction in the mass burning rate.

Direct numerical simulations were performed to investigate crossflow transition control by the three-dimensional steady upstream flow deformation technique under the influence of low-frequency fluc...

An axisymmetric laminar swirling jet of a viscous incompressible fluid flowing from a rotating semi-infinite tube in space filled with the same fluid is explored. The inner surface of the tube rotates with a constant angular velocity, and the outer surface is stationary or rotates with the same angular velocity. It is shown that in the first case, the flow field far from the tube orifice is described by the Loitsyansky asymptotic solution, and in the second case (with weak coflow), it is described by the Long-Gol’dshtik-Zubtsov self-similar solution. The Gol’dshtik hidden invariant is generalized to arbitrary axisymmetric swirling jets, and its influence on the jet asymptotics is studied. Strongly swirling jets are calculated, and the dependence of the parameters of the recirculation zone (vortex breakdown in a swirling jet) on the swirl number and the Reynolds number is examined.

We report on the results of numerical simulation of a supersonic laminar flow past the junction of a blunt-fin body and a plate. We analyze the gasdynamic and vortex structures of the flow, which are determined by the viscous–inviscid interaction. We consider the results indicating the qualitative and quantitative influence of key parameters such as the Mach number, the Reynolds number, the temperature factor, and the plate length on the flow structure and heat transfer.

This article presents descriptions of theoretical models and numerical methods for simulating turbulent thermal plasma flow with nanopowder growth. Turbulence models must express turbulent and laminar states because both states co-exist with thermal plasmas showing large density variation and transport properties. Time-dependent 3D simulations are conducted based on Large Eddy Simulation using a dynamic Smagorinsky model. Results show significant difference depending on temporal and spatial discretization schemes and velocity–pressure coupling algorithms. Simulation results demonstrate that advanced numerical methods with high-order accuracy should be used for long and robust computations capturing steep gradients of nanopowder concentration and plasma temperature and 3D dynamic motions of multiscale vortices, which are turbulent features of thermal plasma flows with low Mach numbers. A thermal plasma jet generates a double-layer structure of inner high-temperature thick vortex rings and outer low-temperature thin vortex rings near the nozzle exit. Flowing downstream, these vortices interact, deform, and break up. Consequently, plasma transits to a complex thermal flow. The widely spreading distribution of multiscale vortices agrees with experimental observations, which are not simulated using conventional methods. Nanopowder is generated from material vapour by nucleation and condensation at interfacial regions between plasma and cold gas. Those regions include numerous vortices. Therefore, the vortices convey the nanopowder, producing a complex distribution of nanopowder. Simultaneously, the nanopowder diffuses and increases in size, decreasing in number by interparticle coagulation. Cross-correlation analysis suggests that a nanopowder distribution distant from a plasma jet can be controlled through temperature fluctuation control at the upstream plasma fringe.

Diffusion of momentum gives rise to viscosity. This article presents a solution in the explicit form of the equation of the momentum diffusion for a viscous fluid flowing around a plate taking into account deceleration. Three characteristic regions of a viscous flow have been described: the mantle, the body of the boundary layer, the viscous sublayer. In the mantle, the effect of viscosity is significant at a considerable distance from the plate. The momentum diffusion is focused in the body of the boundary layer. The diffusion force that produces the momentum of force giving rise to eddies is localized in the viscous sublayer. At the beginning of the plate, a moment of force twists the liquid along the flow, creating eddies that roll along the plate. For this reason, they are pressed against the surface of the plate. But at some distance from the beginning of the plate, the moment of force changes its orientation to the opposite and twists the vortices in the opposite direction, causing the vortices to roll along the plate against the flow. This causes the liquid to detach from the surface of the plate. This is the beginning of turbulence. The diameter of the vortex produced in the viscous sublayer is small being of the order of the thickness of the viscous sublayer. The vortex possesses a large angular velocity. Due to the momentum diffusion and the effect of the eddies combined in passing along the plate, its diameter increases up to the size of the thickness of the boundary layer and even more, whereas its angular velocity decreases down to the values really observed. The value of the critical Reynolds number of the transition from the laminar flow to the turbulent one has been found, and it agrees with the experimental data. The value of the shear stress produced by the viscous fluid on the plate surface has also been obtained. The way of measurement of the friction coefficient characterizing the effect of the plate on the flow has been proposed. It has been shown that the boundary condition of adhesion to the surface of a body flown around, that is applied in the estimation of viscous flows, contradicts the real processes of the flow.