Topological studies of circular and elliptic jets in a cross flow

Ph.DDOCTOR OF PHILOSOPH

Triple condensate halo from water droplets impacting on cold surfaces

Understanding the dynamics in the deposition of water droplets onto solid surfaces is of importance from both fundamental and practical viewpoints. While the deposition of a water droplet onto a heated surface is extensively studied, the characteristics of depositing a droplet onto a cold surface and the phenomena leading to such behavior remain elusive. Here we report the formation of a triple condensate halo observed during the deposition of a water droplet onto a cold surface, due to the interplay between droplet impact dynamics and vapor diffusion. Two subsequent condensation stages occur during the droplet spreading and cooling processes, engendering this unique condensate halo with three distinctive bands. We further proposed a scaling model to interpret the size of each band, and the model is validated by the experiments of droplets with different impact velocity and varying substrate temperature. Our experimental and theoretical investigation of the droplet impact dynamics and the associated condensation unravels the mass and heat transfer among droplet, vapor and substrate, offer a new sight for designing of heat exchange devices

Vortical structures and behaviour of an elliptic jet impinging upon a convex cylinder

A study on a Reh = 2100, AR = 3 elliptic jet impinging upon different convex cylinders at a jet-to-cylinder separation distance of H/dh = 4 has been conducted. Laser induced fluorescence (LIF) and digital particle image velocimetry (DPIV) techniques were utilized to investigate the effects of cylinder-to-jet diameter-ratio (i.e. D/dh = 1.15, 2.3 and 4.6) and jet orientation upon the vortical structures and their behaviour. Results show that comparable flow developments occur along both convex surfaces and straight-edges when the elliptic jet minor-plane is aligned with the cylindrical axis (i.e. EJ1 configuration), while more non-uniform flow behaviour occurs when the elliptic jet major-plane is aligned with cylindrical axis (i.e. EJ2 configuration). Additionally, significant vortex engulfment behaviour between adjacent ring-vortices upon impingement is observed along the elliptic jet minor-plane regardless of exact impingement configuration, which subsequently leads to different flow modes. Braid vortices play a surprisingly interesting role, where they lead to cross-stream and upstream vortical motions for D/dh = 1.15 and 2.3 cylinders under EJ1 configuration. In contrast, they interact with adjacent jet ring-vortices and rib structures for D/dh = 4.6 cylinder. Proper orthogonal decomposition (POD) analyses provided additional information on the unique vortical behaviour identified in the flow fields, while momentum thickness profiles characterize the mixing layers in relation to the different configurations. Lastly, wall shear stress distributions under the above-mentioned flow conditions have also been determined and related to the vortical structures and behaviour.Accepted versio

An investigation on supersonic bevelled nozzle jets

This paper reports upon a numerical and experimental study on supersonic jets exhausting from bevelled nozzles with 30° and 60° exit inclination angles. To begin with, a simple but effective method to design a shock-free convergent–divergent circular jet nozzle to produce a well-conditioned supersonic Ma = 1.5 jet based on simple circular fillets is presented. Subsequently Reynolds-Averaged Navier–Stokes simulations of circular non-bevelled and bevelled nozzle jets were performed at over-expanded, perfectly expanded and under-expanded conditions. Lastly, these supersonic jets were visualized experimentally using a modified Z-type Schlieren system. Results show that the shock cell structure within the jet potential core changes from a diamond pattern to triangular and rectangular patterns as the nozzle pressure ratio and inclination angle are varied. Furthermore, jet plumes are deflected differently when the bevelled nozzles were operated at off-design conditions, with changes to the supersonic jet potential core length. Finally, quantitative analysis of the results reveals that bevelled nozzles are able to reduce the intensity of the supersonic jet shock cell structure considerably, which is potentially useful for broadband shock-associated noise mitigation purposes.MOE (Min. of Education, S’pore)Accepted versio

Dynamics of laminar circular jet impingement upon convex cylinders

Flow dynamics associated with a laminar circular jet impinging upon a convex cylinder has been investigated by laser-induced fluorescence and digital particle-image velocimetry techniques. Cylinder-to-jet diameter ratios of 1, 2, and 4 were investigated, while the jet-to-cylinder separation distance was kept at four jet diameters throughout. Flow visualization and λ2 criterion results show that once the jet ring-vortices impinge upon the cylindrical surface, they move away from the impingement point by wrapping themselves partially around the surface. As the cylinder diameter increases, wall boundary layer separation, vortex dipole formation, and separation locations are initiated earlier along the cylindrical surface, producing significantly larger wakes. Along the cylinder straight-edges, ring-vortex cores are significantly smaller after impingement. This is due to accentuated vortex-stretching caused by partial wrapping around the cylindrical surface by the ring-vortices, on top of their movement away from the impingement point. Interestingly, vortex dipoles demonstrate a strong tendency to travel upstream and interact with other upstream vortex dipoles, instead of moving downstream gradually seen for flat-surface jet-impingements. Wall shear stress results are also presented to quantify the effects of cylinder diameter-ratio on surface skin friction distribution. Finally, these preceding observations are corroborated and explained in a three-dimensional flow dynamics model presented here.Published versio

Aerodynamic Performance and Surface Flow Structures of Leading-Edge Tubercled Tapered Swept-Back Wings

Effects of leading-edge tubercles on the aerodynamic performance and surface flow structures for tapered swept-back wings have been determined experimentally across a range of Reynolds numbers (Re). The orientation of SD7032 profile and tubercles were also particularly evaluated at Re=2.2×105. Lift and drag curve behavior of the baseline wing with SD7032 airfoil profile aligned in the streamwise direction do not vary significantly when Reynolds number exceeds Re=8.2×104. Results also indicate that the gross surface flow structures are not too sensitive towards the orientation of the SD7032 airfoil profile and tubercles. Nevertheless, compared with its baseline counterpart, the wing with tubercles normal to the leading-edge can slightly enhance the aerodynamic performance over a lower angle-of-attack (AOA) range of 2°<α<7°. The two different tubercle orientations are also observed to improve stall behavior with lift enhancements and drag reductions at AOAs larger than 20°. Surface flow patterns show that highly complicated surface vortex structures and regular critical points are produced at moderate AOAs, where the distribution of critical points is dependent upon the leading-edge tubercle orientation and presence of sweep-angle. Surface vortices are revealed to either modify the laminar separation bubbles (LSBs) or disrupt large-scale recirculating region at smaller or larger AOAs respectively. At moderate AOAs however, surface vortices produced downstream of troughs are speculated to lead to poor drag performance.Accepted versio

Application of an Eulerian granular numerical model to an industrial scale pneumatic conveying pipeline

In this paper, an Eulerian granular numerical model is applied in the modelling of an industrial scale pneumatic-based cement conveying system. Steady-state simulation results are found to match pressure and outlet flowrate values with actual system data. By modifying the inlet pressure and material feed rate, data that predicts the performance of the conveying system have been obtained within the present study. Transient simulations have also been conducted and the results reveal intricate details of the cement flows along the pneumatic pipes and pipe bends. In particular, particle roping behaviour is observed to follow the sides of the wall before, during and after the pipe bends. A sloshing-like cement flow motion is also observed after the cement exits the bend. The concentration distribution of the cement particles is found not only to be partly due to gravitational effects but also the pneumatic pipe configuration. Lastly, close inspection of the secondary flows within the pneumatic pipe shows that their directional changes lead to a corresponding change in the particle roping direction, indicating that particle roping is closely associated with the secondary flow structures induced by the exact pipe configuration.Accepted versio

On the application of an Eulerian granular model towards dilute phase pneumatic conveying

The present study considered the application of a multiphase model with Eulerian approach for the solids phase in dilute-phase conveying, where the results are compared against previously published experimental results based on 42 μm nominal diameter glass particles. In particular, the Favre-Averaged Drag turbulent dispersion model is studied where it is found to have greater effects on the particle concentration distribution as compared to the gas phase velocity. While certain discrepancies are observed between simulations and published experimental data, the flow characteristics are adequately captured after addressing the underlying cause of inaccuracies. Inaccuracies in the particle concentration distributions along a vertical pipe section result from the difficulty in capturing the transitional zone where the particle rope starts to disperse. On the other hand, particle diameter variations underpin the mismatches along a horizontal pipe section. Interestingly, increasing the particle diameter leads to the successful capturing of the particle concentration distribution along the horizontal pipe section. The accuracy of employing an Eulerian approach for solids phase is demonstrated, provided that effects due to the particle diameter are accounted for.Accepted versio

Leading-edge tubercles delay flow separation for a tapered swept-back wing at very low Reynolds number

Flow separation characteristics for two tapered swept-back wings, one with straight leading-edge (LE) and the other with tubercled LE, were investigated in a water tunnel using time-resolved particle image velocimetry (TR-PIV) technique. The two wings were based on the SD7032 aerofoil profile, with Reynolds number Re = 1.4 × 104, close to the working condition for common underwater gliders. The LE tubercles were designed such that the amplitude decreased linearly from the wing root to wing tip, while retaining constant wavelength. Results indicate that the baseline wing shows significantly separated flow in the outboard region at pitch angle of 10° and 20°, and the flow remains attached in the inboard region due to relatively larger local Reynolds number. Implementation of LE tubercles can mitigate flow separation downstream of both troughs and peaks. At higher pitch angles, the separated flows cover most of the baseline wing surface, whereas flow remains attached downstream most of tubercle peaks. Streamwise aligned counter-rotating vortex pairs (CVPs) formed over the tubercles are significantly tilted and asymmetrical due to the sweep and amplitude difference between the two sides of tubercle. Consequently, weaker vortices in CVPs close to the wing root are rapidly dissipated, allowing the CVPs to evolve into a series of co-rotating vortices (CVs), which exerted significant impact on flow separation characteristics downstream of the tubercles.Nanyang Technological UniversityAccepted versionThis study was supported by the National Natural Science Foundation of China (grants 11702173 and 41527901). The authors also thank Nanyang Technological University, Singapore, for providing the PIV facilities and water tunnel for the present experiments

A large-eddy simulation study on vortex-ring collisions upon round cylinders

A large-eddy simulation based numerical study was conducted on head-on collisions between vortex-rings and round cylinders. The vortex-ring Reynolds number was Re = 4000, while the ratio of the cylinder diameter to vortex-ring diameter (i.e., diameter ratio, D/d) was varied from 4 to 1. Vortical behavior predicted by the present simulations is observed to agree well with an earlier experimental study [New, T. H., and Zang, B., “Head-on collisions of vortex rings upon round cylinders,” J. Fluid Mech. 833, 648 (2017)]. The present simulations also reveal additional flow details on the vortex dynamics and vortex-core trajectories, which have not been observed previously. First, vortex-dipoles produced by D/d ≤ 2 cylinders are cross sections of elliptic vortex-ringlets formed via vortex disconnection/reconnection of secondary vortex-ring segments. Second, the aspect ratio of the elliptic vortex-ringlets increases when a smaller diameter-ratio cylinder is used, and finally, they undergo axis-switching behavior. Furthermore, up to three sets of tertiary vortex-ring cores are formed along the D/d = 2 and 1 cylinder straight-edges where they subsequently merge with the secondary vortex-ring cores within the confines of the primary vortex-ring cores. This merged vortex core moves toward the collision axis and forms an inner vortex-dipole with a wall separated vortex. Along the convex surface, up to two sets of tertiary vortex-ring cores are observed for D/d = 2 and 1 cylinders, and trajectories of the vortex-dipoles agree well with the past experimental results. These observations support the notion that higher vortex-stretching levels resulting from the use of small diameter-ratio cylinders with higher surface curvatures underpin the wide range of vortical behavior observed here.Submitted/Accepted versio