Junction Vortex Research Articles

Three-Dimensional Turbulent Flow Characteristics Near the Leading Edge of a Longitudinal Structure in the Presence of an Inclined Channel Bank

The present work investigates turbulent flow structures and behavior near the leading edge of a longitudinal flow obstruction in an open channel with an inclined bank. A volumetric particle image velocimetry (VPIV) was employed to collect velocity data. The results indicate that a relatively moderate channel bank angle (θ=28∘) does not prevent the formation of a junction vortex (JV) system. Indeed, it is found that the JV system develops over the channel bank and extends to the leading edge of the flow obstruction. It is demonstrated that the main factor that strengthens the primary junction vortex (JV1) at the tip of the protrusion is the pronounced downward flow. The probability density functions (pdfs) of the velocity fluctuations specify that the JV1 oscillates aperiodically between the so-called zero-flow and back flow modes. This explains the amplification of TKE at its core. It is shown that the velocity difference between the peaks of the pdf increases from over the channel bank towards the tip of the flow obstruction. The aperiodic behavior of the JV1 is confirmed via the proper orthogonal decomposition (POD) technique. Specifically, it is demonstrated that the leading POD modes, associated with the JV system, contain lower energy content compared to periodic flows. The time-averaged vertical vorticity field verifies the presence of a tornado like vortex structure near the upstream face of the retaining wall. Finally, the results suggest that the largest bed shear stress values in the mean flow are located near the tip of the protrusion.

RANS-based optimization of a T-shaped hydrofoil considering junction design

Hydrodynamic lifting surfaces usually include junctions. High-fidelity simulations are necessary to capture critical physics near these regions, such as separation, junction vortices, and cavitation. We present RANS-based hydrodynamic optimizations of a T-shaped hydrofoil, including changes in the junction geometry. The optimized hydrofoils avoid separation and delay cavitation compared to the baseline. The full optimization design with planform, cross-section, and junction geometry variables yields a total drag reduction of 6.4%. The optimized results show that the relative locations of the maximum foil thickness and the maximum strut thickness significantly impact the junction cavitation. Including the translation between the strut and the foil and more strut geometric variables as design variables will provide further improvement. The comparison between optimized designs demonstrates that optimizing planform and detailed junction geometry provides further improvement in addition to designing the cross-sectional geometry. The hydrostructural analyses show that the optimized T-foils have lower stress at the junction than the baseline because of the resultant junction fairing. However, these hydrodynamic-only optimized T-foils have higher deformation and maximum stress, which could result in accelerated fatigue, highlighting the need for hydrostructural responses in design optimization. The results demonstrate that the developed methodology is useful for designing next-generation complex hydrodynamic lifting surfaces.

Vortex system around a step cylinder in a turbulent flow field

The vortex system around the step surface of a step cylinder with a diameter ratio D/d=2 at Reynolds number (ReD) 3900 was investigated by directly solving the three-dimensional Navier–Stokes equations. Formation mechanisms and vortex dynamics of the complex vortex system were studied by performing a detailed investigation of both the time-averaged and instantaneous flow fields. For the time-averaged flow, including the known junction and edge vortices, in total, four horseshoe vortices were observed to form above the step surface in front of the upper small cylinder. The crossflow width of the four horseshoe vortices varies differently as they convect downstream. Moreover, we captured a pair of base vortices and a backside horizontal vortex in the rear part of the step surface behind the small cylinder. For the instantaneous flow, hairpin vortices were found to form between the legs of two counter-rotating horseshoe vortices located on the same side of the step cylinder. Furthermore, in the small step cylinder wake, Kelvin–Helmholtz vortices were observed to shed at an unexpectedly high shedding frequency.

Analysis of vortices shed by a notional submarine model in steady drift and pitch advancement

Results of Reynolds-averaged Navier-Stokes simulations are reported on a towed fully-appended notional submarine model. Computations are validated against data in the literature, dealing with both global loads and the vortices originating from the sail. The analysis of the flow is focused on the coherent structures shed from the tip and the root of the appendages, defining the wake signature of the submarine and affecting the flow field over the propeller plane. Their positions and intensities are compared across four operative conditions: straight ahead, drift, and both positive and negative pitch. Computations demonstrate that the condition with negative pitch is the one producing the most complex topology of the flow in the stern region, where the vortices from the tip of the sailplanes are able to influence both the fins and the flow at the propeller plane. However, the widest and most intense vortex is generated in drift, from the tip of the sail, because of the strong gradient of pressure produced between the two sides of the fore appendages. The pattern of the coherent structures produced by the stern appendages is substantially dependent on the maneuvering conditions. In both pitch conditions their tip vortices have values of circulation very similar across the four fins. This is not the case in drift: the one most affected by the wake of the sail is the smallest and weakest. The most intense junction vortices from the stern appendages, which are able to influence more directly the inflow of the propeller, occur instead in drift.

Large eddy simulation of the turbulent flow field around a submerged pile within a scour hole under current condition

ABSTRACT Coastal finite structures, bridge piers, and bank protection structures may become submerged during storm surges. In the present study, a turbulent flow field around a submerged pile within a scour hole is numerically simulated considering a very fine mesh in the order of 50 million cells distributed within a relatively small computational domain. Necklace vortices, including two horseshoe vortices (HVs) and a junction vortex (JV), are detected upstream from the cylinder. In many snapshots, the tails of the HVs stretch in the flow direction, while their size and length change considerably over time. Downstream of the cylinder, the influence of the trailing vortex from the cylinder roof is mainly limited to a zone near the cylinder top, whereas the effect of the separated shear layers (SSLs) from the cylinder sides is dominant within a larger region. The strength of vorticity associated with the trailing vortex is mainly notable within the “very near wake” area, where the strength of the upward velocity component of the wake vortices generated from the cylinder sides mitigates. This paper describes the distribution and origin of streaks and rollers on the scoured bed, i.e. the main cause of sediment transported downstream of the cylinder.

Flow development upstream of a fence

The effect of Reynolds number on the flow development upstream of a rigid, non-porous, static fence is investigated experimentally. The flow field is measured using time-resolved, two-component particle image velocimetry at Reynolds numbers based on fence height of 18000, 36000, and 54000. The results show that a laminar separation bubble forms upstream of the junction vortex at the base of the fence. The mean extent of the bubble decreases with increasing Reynolds number, with mean separation moving downstream and mean reattachment moving upstream. In the aft portion of the bubble, shear layer vortices form and are shed at scaled frequencies and wavelengths that are comparable to laminar separation bubble shedding in low Reynolds number airfoils and flat plates with an imposed adverse pressure gradient. The strong periodicity of the associated coherent structures and the proximity of shear layer roll-up relative to the fence should be taken into consideration in the relevant designs due to potential implications to structural loading. A simple flow separation prediction model combining inviscid fence flow solution with Thwaites’ method is introduced and shows good agreement with the experimental results for the Reynolds number range considered.

A numerical investigation about the effects of Reynolds number on the flow around an appended axisymmetric body of revolution

Large-eddy simulations for the case of an axisymmetric body of revolution with appendages are considered. The geometry is the benchmark case of the DARPA suboff body. The paper focuses on the effects of the Reynolds number on the structure of the boundary layer in the stern area as well as the near wake. For this purpose we compare results for two Reynolds numbers (based on the length of the body, , and the free-stream velocity, ): and . Results are in good agreement with published experiments at the same Reynolds numbers. The boundary layer thickness over the stern increases substantially at both simulated Reynolds numbers, due to the adverse pressure gradient at the rear of the body. However, for the high Reynolds number case, a weaker peak of turbulent kinetic energy develops in the outer layer over the stern. Nonetheless, the associated bimodal distribution of the turbulent stresses in the wake is already very similar a few diameters downstream of the tail. First- and second-order moments demonstrate that junction vortices at the stern bring higher velocities and turbulence at the root of the appendages for both Reynolds numbers, with a more evident signature at . An azimuthal readjustment of turbulent kinetic energy occurs in the wake, becoming more axisymmetric, with increasing values in the planes aligned with the stern appendages, due to turbulence coming from both the stern boundary layer and junction vortices.

Propeller wake evolution mechanisms in oblique flow conditions

In the present study the wake flow past an isolated propeller operating in oblique flow conditions is investigated experimentally. In particular, the investigation concerns a systematic topological comparison of the wake behaviour in axisymmetric and in oblique inflow conditions, for three inclination angles, and is focused on an analysis of the underlying mechanisms of wake evolution and instability. To this end, the experiment has been designed to investigate the dynamics of propeller vortical structures over a wide spatial extent covering the wake region from the propeller disk up to 4.5 diameters in the streamwise direction. Detailed flow measurements have been undertaken by particle image velocimetry (PIV), using a multicamera configuration with three cameras arranged side by side. This allowed simultaneous acquisition of a large flow extent at a spatial resolution adequate to resolve the smallest vortical structures involved in the process of propeller wake instability. The analysis has been based on both phase-locked averaged and instantaneous flow fields. The study extends the knowledge on the subject of propeller wake dynamics, highlighting the major hydrodynamic effects that non-axisymmetric propeller operating conditions exert on the mechanisms of wake evolution, instability and breakdown, such as asymmetric destabilization of the tip vortices on the leeward and windward sides of the wake, and the interference between the tip and the junction vortices, as well as the cause–effect relation between the breakdown of the blade trailing wake and the instability of the tip and hub vortices.

Direct visualization of phase separation between superconducting and nematic domains in Co-doped CaFe2As2 close to a first-order phase transition

We show that biaxial strain induces alternating tetragonal superconducting and orthorhombic nematic domains in Co substituted CaFe2As2. We use Atomic Force, Magnetic Force and Scanning Tunneling Microscopy (AFM, MFM and STM) to identify the domains and characterize their properties, finding in particular that tetragonal superconducting domains are very elongated, more than several tens of micron long and about 30 nm wide, have the same Tc than unstrained samples and hold vortices in a magnetic field. Thus, biaxial strain produces a phase separated state, where each phase is equivalent to what is found at either side of the first order phase transition between antiferromagnetic orthorhombic and superconducting tetragonal phases found in unstrained samples when changing Co concentration. Having such alternating superconducting domains separated by normal conducting domains with sizes of order of the coherence length opens opportunities to build Josephson junction networks or vortex pinning arrays and suggests that first order quantum phase transitions lead to nanometric size phase separation under the influence of strain.

Vortex breakdown, linear global instability and sensitivity of pipe bifurcation flows

Pipe bifurcations are common flow configurations in both natural and man-made systems. This study follows our previous report (Chen et al., Phys. Fluids, vol. 27, 2015, 034107) by describing three aspects of flows through junction angles of $70^{\circ }$, $90^{\circ }$ and $110^{\circ }$, with a square cross-section. First, the inflow creates tightly spiralling vortices in four quadrants of the junction. For sufficiently large Reynolds number $Re$, these vortices undergo behaviour resembling steady near-axisymmetric breakdown. With increasing $Re$, the flow through the $90^{\circ }$ junction remains steady and stable until the first Hopf bifurcation. Beyond the Hopf bifurcation, the vortices undergo a helical instability. The $70^{\circ }$ and $110^{\circ }$ junctions, however, first exhibit pitchfork bifurcations leading to asymmetric solutions. Second, the direct eigenmodes of the linearised flow are large in vortices in the outlet pipes, whereas the adjoint eigenmodes primarily reside in a small region in the inlet and the junction, near the front and back walls. Third, the sensitivities of the eigenvalues to spatially localised feedback and base flow modifications are greatest in and near the junction vortices. We highlight the regions of high growth rate and frequency sensitivity, as well as regions where the production and transport of perturbations by modifications of the base flow contribute most to the base flow sensitivity. The flow separation at the corners of the junction does not coincide with the eigenmodes or sensitivity regions.

Manipulating Josephson junctions in thin-films by nearby vortices

It is shown that a vortex trapped in one of the banks of a planar edge-type Josephson junction in a narrow thin-film superconducting strip can change drastically the dependence of the junction critical current on the applied field, Ic(H). When the vortex is placed at certain discrete positions in the strip middle, the pattern Ic(H) has zero at H=0 instead of the traditional maximum of ‘0-type’ junctions. The number of these positions is equal to the number of vortices trapped at the same location. When the junction–vortex separation exceeds ∼W, the strip width, Ic(H) is no longer sensitive to the vortex presence. The same is true for any separation if the vortex approaches the strip edges.

Interaction of Josephson junction and distant vortex in narrow thin-film superconducting strips

The phase difference between the banks of an edge-type planar Josephson junction crossing the narrow thin-film strip depends on wether or not vortices are present in the junction banks. For a vortex close to the junction this effect has been seen by Golod, Rydh, and Krasnov, \prl {\bf 104}, 227003 (2010), who showed that the vortex may turn the junction into $\pi$-type. It is shown here that even if the vortex is far away from the junction, it still changes the 0-junction to $\pi$-junction when situated close to the strip edges. Within the approximation used, the latter effect is independent of the vortex-junction separation, a manifestation of topology of the vortex phase which extends to macroscopic distances of superconducting coherence.

Effects of Sweep Angles and Angles of Attack on Junction-Flow Patterns

This study utilized the NACA 0012 finite wings to investigate the effects of wing sweep angle (Λ) and angle of attack (α) on the junction vortex at Re = 8 × 10^4. The junction-vortex structures were visualized using the surface oil-flow visualization. The junction vortex is classified as-separation, attached, bubble, and bluff-body wake modes. The separation mode occurs at Λ ＜ 12° and α ＜ 5°. The attached mode occurs at low α for a swept-back wing (Λ ＞ 0°) and the bluff-body wake mode occurs at high α for a forward-swept wing (Λ ＜ 0°). Furthermore, the bubble mode occurs at high sweep angle (i.e., high backward sweep angle) and high angle of attack. Furthermore, the properties of velocity vectors, normal stress and shear stress are also detected and analyzed using an X-type hot-wire anemometer.

Influences of upstream floor roughness and aerodynamic parameters on swept-wing junction flow

This study examines several finite length NACA 0012 airfoils to explore how the angle of attack (α), the sweep angle (Λ) and the Reynolds number (Re) affect the junction vortex and horseshoe vortex. Upstream floor roughness and turbulence intensity (T.I.) influence the wing-junction flow was also studied. The junction-flow structures at low Reynolds numbers were visualized using the smoke-wire technique. The smoke-streak flow patterns were classified into two characteristic modes — horseshoe vortex and non-horseshoe vortex. The horseshoe-vortex patterns were further categorized as the junction-vortex mode and non-junction-vortex mode. The velocity vectors were measured using the particle-image velocimetry (PIV), and the data was utilized to calculate the junction vorticity (Ω). Experimental results indicate that the straight wing has the maximum junction vorticity. The Ω decreases with increasing α and Λ and with decreasing Re. The Ω decreases with increasing T.I. The upstream T.I. generated by the mesh fences was more significant than that produced by sandpapers.

Heat transfer effects and topology of the viscous interaction induced by surface protuberances in hypersonic flow

Surface protuberances with a height of the order of the boundary layer thickness are frequently unavoidable in the design of hypersonic vehicles. These protuberances interfere with the flow and induce complex interactions which may lead to notable increases in the local surface heating rates and cause a significant damage to the vehicle. This computational and experimental study investigated hypersonic interference heating in the vicinity of simple surface protuberances. Perfect gas and Reynolds averaged Navier–Stokes simulations were carried out based on benchmark experimental cases using the compressible commercial code Cobalt. The freestream conditions were M∞ = 8.2 and Re∞/ m = 9.35 × 106. The investigated protuberances were three-dimensional compression corners with different deflection angles (30°, 60°, and 90°) and with a relatively low profile where the height was approximately the same as the local boundary layer thickness. A comparison between numerical and experimental data shows a reasonable agreement, although there are areas where there are notable differences between the predictions and measurements. In the low-deflection case (30°), good agreement is observed and the location of the hot spot is well predicted by the different turbulence models. In the higher deflection cases (60° and 90°), in which the incoming boundary layer is separated ahead of the protuberance, the maximum heating rates are reasonably well predicted by the shear stress transport and k–ω models, but systematically overpredicted by the Spalart–Allmaras model. In terms of flow structure, the simulations enable a good understanding of the flow topology, which is marked by the occurrence of junction and separation vortices to the sides and upstream of the protuberance.

Tip and Junction Vortices Generated by the Sail of a Yawed Submarine Model at Low Reynolds Numbers

Results are presented on the behavior of the tip and junction vortices generated by the sail of a SUBOFF submarine model at yaw angles from 6 deg to 17 deg for a Reynolds number of 94×103 based on model length. The measurements were conducted in a water channel on a spanwise plane 1.3 chord lengths downstream from the trailing edge of the sail. In the vicinity of the sail hull junction, the presence of streamwise vortices in the form of horseshoe or necklace vortices locally dominates the flow. As the yaw angle is increased from 6 deg to 9 deg, the circulation of the sail tip vortex increases, and is in good accordance with predictions from finite wing theory. However, as the yaw angle is further increased, the sail boundary layer separates with an overall drop in circulation. In contrast, the circulation value for the junction vortex increases with yaw angle, and only drops slightly at the highest yaw angle.

Coherent Structures in the Flow Field around a Circular Cylinder with Scour Hole

Large-eddy simulation (LES) and laboratory-flume visualizations were used to investigate coherent structures present in the flow field around a circular cylinder located in a scour hole. The bathymetry corresponds to equilibrium scour conditions and is fixed in LES. The flow parameters in the simulation correspond to the experimental conditions in which the approach flow is fully turbulent. Detailed consideration is given to the interaction of the horseshoe vortex (HV) system within the scour hole with the detached shear layers formed from the cylinder, and the near bed turbulence. It is found that the overall structure of the HV system varies considerably in space and time, though a large, relatively stable, primary necklace vortex is present at practically all times inside the scour hole. The simulation captures the presence of bimodal chaotic oscillations inside the HV system, as well as the sharp increase in the resolved turbulent kinetic energy levels and pressure fluctuations reported in prior experimental investigations. High levels of the mean bed shear stress are observed beneath the primary necklace vortex, especially over the region where the bimodal oscillations are strong, as well as beneath the small junction vortex at the base of the cylinder. It is also found that the detachment and advection of patches of vorticity from the downstream part of the legs of the necklace vortices can induce large instantaneous bed shear stress values. When the critical bed shear stress value for sediment entrainment on a flat surface is adjusted for bed slope effects, the LES simulation correctly predicts that the distribution of the mean bed shear stress is consistent with equilibrium scour conditions.

Transition processes for junction vortex flow

The details of the start-up transient vortex structure that forms near the junction of an impulsively started plate and a stationary plate where a step jump in velocity occurs at the plate surfaces are investigated. Numerical simulations have been conducted in a geometry representative of recent experiments of this flow. The experiments did not have access to data at very early times following the impulsive start, but they did suggest that the flow undergoes transitions from a viscous-dominated phase to an inertia-dominated phase. The numerical simulations presented here are designed to explore the early viscous-dominated transients. The simulations show that when the non-dimensional time, τ = tU2/ν (t is the time that the plate has been in motion and ν is the kinematic viscosity), is less than 100, the development process is dominated by viscous forces. In this regime similarity scaling is used to collapse the data, which scale as $\sqrt{t}$. The simulation results at low τ, when evaluated using entrainment diagrams, show an unsteady transition process consisting of the following stages. Initially, the flow consists of a non-rotating vorticity front with a single critical point for τ < 40. For 40 < τ < 50, the flow has three critical points, two nodes and a saddle. A rotational leading jet head develops for τ > 50 as the outermost node evolves into a spiral focus. The simulations span the viscous range to the inertial range. In the inertial range, for τ > 103, the flow structure scales as t5/6, as was observed in the experiments.

Dual Location Separation Control on a Semispan Wing

Separation is controlled on a semispan wing via nominally two-dimensional zero mass-flux perturbations that are introduced at 1) the leading edge, 2) the shoulder of deflected flaps (20-40 deg deflection), and 3) both locations simultaneously. Leading-edge perturbations mimic the effect of a leading-edge device and flap-shoulder perturbations simulate additional flap deflection while simultaneously reducing drag. When perturbations are introduced simultaneously, at reduced frequencies of O(1) at poststall angles of attack, their combined effect on the wing's lift exceeds each individual contribution. When the physical frequencies are the same, lift is strongly dependent on the difference in phase between the two perturbations and an optimum is reached when the suction portion of flap-shoulder control coincides with the minimum shear layer proximity to the flap surface. At the optimum phase difference, increases in lift are observed over the entire semispan, resulting in an overall increase in the wing maximum lift coefficient between 0.24 and 0.44 depending on the flap deflection configuration and angle. Flap-shoulder control effectiveness does not diminish at the extremities of the finite inboard flap; neither inboard in the vicinity of the junction vortex, nor outboard in the vicinity of the flap-edge vortex. With a large flap deflection across the entire span, flap-shoulder control perturbations produce a powerful tip vortex that may be exploited for lift enhancement on low aspect ratio wings. The sensitivity to perturbation phase difference, however, diminished significantly toward the wing tip.

Scaling and instability of a junction vortex

This paper details experiments in the region where an impulsively started moving wall slides under a stationary wall. The experiments were conducted over a Reynolds number range of ReΓ=5×102–5×105. The length scale for the Reynolds number is defined as the distance the wall has moved from rest and increases during an experiment. Experiments show that for ReΓ>103 a vortex forms close to the junction where the moving wall meets the stationary one. The data shows that while the vortical structure is small, in relation to the fixed-apparatus length scale, the size of the vortex normalized with respect to the wall speed and viscosity scales in a universal fashion with respect to ReΓ. The scaling rate is proportional to t5/6 when the Reynolds number is large. The kinematic behaviour of the vortex is related to the impulse that the moving wall applies to the fluid and results in a prediction that the transient structure should grow as t5/6 and the velocity field should scale as t−1/6. The spatial-growth prediction is in good agreement with the experimental results and the velocity scaling is moderately successful in collapsing the experimental data.For ReΓ>2×104 three-dimensional instabilities appear on the perimeter of the vortical structure and the flow transitions from an unsteady two-dimensional flow to a strongly three-dimensional vortical structure at ReΓ≃ 4 × 104. The instability mechanism is centrifugal. The formation and growth of these instability structures and their ingestion into the primary vortex core causes the three-dimensional breakdown of the primary vortex. Two movies are available with the online version of the paper.