Vortex Roll-up Research Articles

Prediction of vortex structures in pulsatile flow through S-bend arterial geometry with different stenosis levels

Arterial stenosis poses a high cardiovascular risk, and clinical intervention is needed when these stenoses grow beyond a specific limit. The study of vortex dynamics in these diseased arteries can be beneficial to understand its severity. Therefore, in the present work, we have investigated the flow structures in an S-bend arterial geometry with different levels of stenosis using a sharp interface immersed boundary method. We have observed an onset of Kelvin-Helmholtz-type vortex roll-up for higher degrees of stenoses. Fluctuations in the wall shear stress are observed for higher stenosis degrees. However, these fluctuations depend on the position and length of the stenosis. Newtonian and non-Newtonian Carreau fluids predict similar vortex structures, although minor differences in the Kelvin-Helmholtz vortex structures and associated fluctuations are observed in the diastolic phase. The Newtonian fluid predicts a slightly longer low time-averaged wall shear stress (≤0.5 Pa) region immediately after the stenosis compared with the Carreau fluid in the 58 % blockage S-bend artery.

Three-dimensional density measurements of a heated jet using laser-speckle tomographic background-oriented schlieren

Tomographic background-oriented schlieren (TBOS) is a promising optical technique for 3D density field measurement in turbulent flows. Path-integrated information on refractive index gradients is captured by cameras looking through the flow at textured background patterns in the form of apparent displacements of the patterns. A tomographic reconstruction of the three-dimensional refractive index gradients is performed, from which the instantaneous three-dimensional density field is obtained. Most TBOS measurements to date have been limited by large temporal blurring and/or large defocus blurring. Our 15-camera experimental set-up uses a pulsed laser for both illumination and creation of a laser-speckle background pattern. The high-power laser ensures adequate exposures in nanoseconds, so temporal integration is minimised in each measurement. The beam is spread to illuminate a surface that is observed by fifteen cameras that are evenly spaced circumferentially around the flow. Laser speckle patterns are observed by the cameras due to the diffuse reflection and interference of coherent light from roughness on the surface. We develop a methodology for selecting the optimal focal length, focus distance, and lens aperture by considering the compromise between displacement sensitivity, defocus blurring and speckle size. Low temporal- and spatial-blurring 3D temperature field measurements with excellent resolution of turbulent flow features are obtained and good agreement with thermocouple. Classical behaviours of the potential core instability such as vortex rollup and radial ejections are identified. The behaviour of the potential core is examined further, which reveals that the potential core undergoes considerable deformation and fragmentation during the transition to turbulence.

Mean Flow, Turbulent Structures, and SPOD Analysis of Thermal Mixing in a T-Junction with Variation of the Inlet Flow Profile

This paper investigates the effects of different inlet flow profiles on thermal mixing within a T-junction using CFD simulations with the IDDES-SST turbulence model. The different combinations of inlet flow profiles are related to different stage in the flow entry region. The effects of the inlet flow profile on the mean and transient flow behaviour are assessed, while a spectral proper orthogonal decomposition and power spectral density analysis are performed to assess the underlying flow structures and the predominant frequency modes. It is found that the vortical structures associated with the horseshoe and hovering vortex systems consist of a single roll-up vortex for cases with uniformly distributed boundary conditions (BCs) at the branch inlet whereas a double roll-up vortex is observed for the other cases. The double roll-up vortex enhances the mixing locally due to the entrainment of fluid from the branch pipe in these vortical structures, which then results in a lower mean temperature distribution. The appearance of the secondary vortex pair and the nested vortices is delayed for cases with uniformly distributed BCs at the branch inlet, which again results in lower thermal mixing and consequently higher values of mean temperature when compared with the other cases. It is also found that the vorticity related to the counter-rotating vortex pair as well as to the second pair of vortices rotating in the opposite direction is higher for cases with uniformly distributed BCs at the branch inlet. Lastly, the combinations of inlet flow profiles lead to different coherent structures, and the dominant frequencies are of a Strouhal number of around 0.7 for uniformly distributed profiles at the branch inlet and in the range 0.4–0.5 for the other cases.

Design of Rotary Wings with Passive Mitigation of Coherent Tip Vortex Roll-Up

A rotor blade radial loading profile is presented that aims to passively mitigate the formation of coherent tip vortex structures in the near-field of the rotor wake. The method leverages principles of Helmholtz’s theorems, where the strength of the vortex wake is proportional to radial gradients in the blade loading distribution. By avoiding such radial gradients, the near-field roll-up of the wake vortex can be mitigated. A pair of two-bladed rotor designs configured with the same thrust coefficient was produced using a finite-vortex rotary lifting line framework coupled to a constrained power optimization problem. A baseline rotor was designed based on a power-optimized approach, and a wake-optimized rotor was designed based on a power-optimized approach with an additional blade-root bending moment constraint. Phase-averaged stereoscopic particle image velocimetry data were acquired for each rotor operating in hover. The wake-optimized rotor blade produced a wake characterized by conical vortex sheets of significantly lower peak vorticity than the compact, helical tip vortices produced by the baseline design. The wake-optimized design exhibited increased axial velocities near the root that caused radial distortions of the wake flow structures, which were also found to decay at a faster rate than that for the baseline design.

Visualization of the flow behind a backward-facing step using a schlieren technique and tetrafluoroethane

This paper explores the possibility of visualizing the low-speed, near-wall flow using a schlieren imaging technique. In this investigation, wind tunnel testing is conducted on a backward-facing step at step Reynolds numbers of 2940, 5140, and 6120. Tetrafluoroethane is injected through a rectangular slot mounted flush to the wind tunnel floor and is introduced to the downstream region of the step as refractive tracers. The visualization acquired at the midsection suggests that the mass flux of tetrafluoroethane should be approximately 5%–10% lower than that of the freestream air to ensure that the injection is less intrusive to the flow field while still achieving the visibility required to characterize the flow field. For long-exposure photography, the reattachment length or the length scale behind the step can be estimated when the exposure time of a camera is 3.2–12 times longer than the timescale of vortex roll-up. For high-speed photography, the sequence of vortex shedding can be adequately visualized when the frame rate of a camera is at least four times higher than the vortex shedding frequency.

Numerical study on helicopter brownout with crosswind

In previous studies, helicopter brownout was investigated using various numerical methods, but the influence of a crosswind on the evolution of the dust cloud was not considered. In the present study, a method for analyzing helicopter brownout is developed that includes the effects of a crosswind on the unsteady flow field and the dynamics of the sand particles. The evolution of the dust cloud is simulated and compared with flight-test data obtained with no crosswind, and the differences in the dust clouds with and without a crosswind are investigated. Also, the influences of crosswind direction and velocity on the behavior of the dust cloud are analyzed. The results show that a crosswind has significant influence on the behavior of the dust cloud. A crosswind from the port side strengthens the dust cloud greatly, whereas a headwind weakens it. Compared with that when the crosswind is from the starboard side, the density of the dust cloud when the crosswind is from the port side is greater because the interaction between the crosswind and the rolled-up vortex on the retreating side is stronger than that on the advancing side. When the crosswind is from the port side, the density of the dust cloud increases initially and then decreases with increasing crosswind velocity.

Drag reduction of a slanted-base cylinder using sweeping jets

In this work, a pair of sweeping jet actuators is installed underneath the endplate of a slanted-base cylinder at ReD = 200 000. The sweeping jets form a 30° inclined angle with the endplate and are placed at different streamwise locations, and their strength is varied with a momentum coefficient, Cμ, ranging from 3.8 × 10−3 to 6.0 × 10−2. For all the cases examined in this paper, it is found that while a higher Cμ produces a higher drag reduction, the flow control energy efficiency decreases rapidly as Cμ increases. A net energy saving is achieved when Cμ is less than 0.01, and the highest energy efficiency obtained in the present study is 2.8% when the actuator pair is placed at the most upstream location tested. The drag reduction is attributed to the reaction force and an increase in the surface pressure force acting on the endplate produced by the jet pair. The contribution from the former constitutes an increasing proportion of the total drag reduction as Cμ increases leading to lower energy efficiency in flow control. Depending on the relative positions between the trajectory of the sweeping jet and afterbody vortex, sweeping jets are not only capable of altering the surface pressure distributions via directly imposing a footprint of high pressure on the surface, but also affecting the roll-up of the afterbody vortex and/or reducing its strength via injecting turbulence into the afterbody vortex.

Modal analysis of tip vortex cavitation with an insight on how vortex roll-up enhances its instability

The instability mechanism of tip vortex cavitation (TVC) is investigated in the present paper, which is found to be significantly influenced by the roll-up of secondary vortices. The three-dimensional proper orthogonal decomposition (3D-POD) analysis is utilized to identify the coherent structures around the TVC, and a modal turbulence transport theory is introduced to further measure the modal contribution to the TVC instability. According to the dynamic evolution of coherent structure identified by the 3D-POD analysis, the fluctuating component is gradually introduced into the tip vortex together with the roll-up of secondary vortices. Moreover, the dynamic evolution of coherent structure is always accompanied by a significant distribution of modal turbulence kinetic energy (TKE), implying that the TVC instability is promoted along with the merging of secondary vortices into the tip vortex. A deeper understanding is gained with the assistance of the modal turbulence transport theory. It is found that the modal Reynolds stress introduced by the vortex roll-up is the trigger for the local TVC instability, because it can significantly enhance the energy input from mean flow to fluctuating flow.

Analysis of flow-induced performance change of cycloidal rotors: Influence of pitching kinematic and chord-to-radius ratio

As a new type of the propulsive devices, cycloidal rotor has been attracting more attention recently. The present work concentrates on the analysis of the performance and internal flow field of a two-bladed cycloidal rotor operating at low Reynolds numbers, with special emphasis on understanding how the flow structure affects the performance at different working conditions. The symmetrical/asymmetrical pitching motions are evaluated initially and the primary results show that the asymmetrical pitching with a mean pitching angle of 5° improves the efficiency of the rotating system. Then, the effect of the chord-to-radius ratio at three different conditions is discussed, which shows that the chord-to-radius ratio of 0.45 is the best value to maximise the performance. The flow pattern, involving the blade-wake interaction, wake-wake interaction, three vortex structures on the blade surface, roll-up vortices inside the laminar boundary layer, separation bubble, leading-edge vortex, trailing-edge vortex and flow separation vortex, are discussed in detail under various operating conditions. The study on the performance of a single blade, the forces (lift and drag) distributions, the blade loadings and the near-wall flows on two blades, provide a new understanding of the global performance change of the propeller.

Reconstruction of the deterministic turbulent boundary layer for the study of aerofoil self-noise mechanisms

In the experimental aeroacoustics, it is always a challenge to study the far-field radiation and near field hydrodynamics simultaneously, and be able to firmly establish the causality between them. The main objective of this paper is to present an experimental technique that can exploit the deterministic turbulent boundary layer generated under a base flow of either mildly separated or laminar boundary layer to either disrupt an existing acoustic scattering mechanism, or reconstruct a new acoustic scattering scenario to enable the ensemble-averaging and wavelet analysis to study the aerofoil trailing edge noise source mechanisms in the spatial, temporal and frequency domains. One of the main attractions of this technique is that the experimental tool does not need to be extremely high fidelity as a priori in order to fully capture the pseudo time-resolved boundary layer instability or turbulent structures. In one of the case studies presented here, roll-up vortices of the order of ∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim$$\\end{document} kHz can be captured accurately by a 15-Hz PIV. A single hot-wire probe is also demonstrated to be capable of reconstructing the turbulent/coherent structures in a spatio−temporal domain. The proposed experimental technique can be extended to other self-noise scenarios when the aerofoil trailing edge is subjected to different flow control treatments, such as the porous structure, surface texture, or finlet, whose mechanisms are largely not understood very well at present.Graphical abstract

Tracer-Based High-Speed Laser-Induced Fluorescence Measurements Of Temperature And Fuel Concentration Fields In A Rapid Compression Expansion Machine

Tracer planar laser-induced fluorescence (PLIF) using 1-methylnaphthalene (1-MN) is applied to study temperature and fuel courses during compression in a rapid compression expansion machine (RCEM). A burst mode Nd:YAG-laser at 266 nm is utilized for excitation of tracer fluorescence at a repetition rate of 7.5 kHz. A high-speed intensified CMOS camera equipped with an image doubler is applied for 2-color LIF (2c-LIF). The measurements were conducted in nitrogen atmosphere for which a calibration curve is presented that was generated in a flow cell. The temperature and fuel concentration fields are very homogeneous at early points in time during compression, however, inhomogeneities in terms of millimeter-sized hot and cold gas regions can be found especially near top dead center (TDC). These inhomogeneities were also visible in the fuel partial density field and are due to the distinct heat transfer between the hot gas and the cool walls, and probably also because of roll-up vortices induced by the piston movement. Especially at TDC the minimal gas temperature is about 300 K lower than the peak temperature near the cylinder head. These temperature differences are much larger than in piston engines and in other RCEMs reported in the literature at comparable conditions. This is because of the special design of the present layout of the machine. A relatively large difference to the adiabatic temperature can be overserved particularly at TDC. About 70 K lower temperatures were measured, which can be explained by these temperature inhomogeneities in the cylinder.

Dynamics of three-dimensional vortical structures behind a barchan dune based on tomographic particle image velocimetry

In this study, tomographic particle image velocimetry (PIV) was used to measure three-dimensional (3D) flow structures behind a fixed-bed barchan dune model at a Reynolds number of 2528 in a circulation water tunnel. The topological evolution of the 3D instantaneous vortex structures and their dynamic characteristics in the dune wake were analyzed. 3D instantaneous arch-shaped vortex “chains” shedding from the barchan and typical quasi-streamwise vortex structures induced by the development of an internal boundary layer were found to be located both before and after the reattachment region, which differs a little from the results of previous studies. Nevertheless, both conditionally averaged and typical instantaneous 3D flow fields revealed that the arch-shaped vortex system dominates the barchan dune wake, and the inclination angle of these spanwise-oriented structures was found to change as it propagates downstream. Additionally, the dynamic characteristics of the dune wake were found to be similar to a flapping phenomenon such as oscillation of the recirculation in a backward-facing step flow. To provide complementary insight into this phenomenon, the separated and reattached shear flow were also examined using two-dimensional time-resolved planar PIV. It was found that the basic unsteady flapping motion, vortex roll-up, and complex vortex interactions can all be analyzed with ease in a Lagrangian framework.

Simplified vortex methods to model wake vortex roll-up in real-time simulations for fuel-saving formation flight

One possibility for reducing fuel consumption is to fly in the upwind field of the wake vortex generated by an aircraft that is flying ahead. Migratory birds use this principle. Manually flying an aircraft at the point of optimal fuel reduction is not suited for routine flight operations as the pilot workload is excessively high. Hence, an autopilot function has to carry out this task. For designing the autopilot, a flight mechanical simulation with a wake vortex velocity model is required that has the ability to calculate the vortex-induced velocity fields. This paper contributes to the choice of a real-time simulation method for modelling vortex-induced velocities that the wake vortex of a leading aircraft generates and that the trailing aircraft shall use during fuel-saving formation flight.Two different wake vortex velocity models are introduced and compared during steady, horizontal flight. One model is based on the Lifting Line Method (LLM) and the other on the unsteady Vortex Lattice Method (VLM). Both models are able to calculate the wake vortex roll-up phase for arbitrary lift distributions, whereas the commonly used Single Horseshoe Vortex Model (SHVM) ignores the near-field roll up. The differences in the induced upwind distribution and vortex filament position are analysed for coarse spatial and temporal discretisation that the real-time constraint requires. Despite the more stringent simplifications of LLM, both methods yield similar filament positions and similar velocity fields for the same discretisation of the lifting surfaces. Finally, the influence of the discretisation parameters is discussed and parameter values are recommended for using VLM and LLM in real-time flight simulations.

Direct numerical simulation of turbulent round jet released in regular waves

Direct numerical simulation of the round jet released in regular waves has been reported. Evolution of the vortical structures during initial and long term evolution of the jet has been studied with the help of the swirling strength criterion. During an initial evolution of the jet, the vortex roll-up and emergence of secondary instability have been studied. The frequency of vortex roll-up in the case of wave environment was found to be lower as compared to the corresponding frequency of the free jet. The counter-rotating vortex pairs in the case of jet in regular waves was found to be asymmetric due to deflection of the jet as opposed to symmetrical structures observed in the free jet. As a long term evolution of the jet, the wave tractive mechanism has been studied. The wave tractive mechanism provided the jet, an additional opportunity to entrain fresh fluid in the jet apart from the jet momentum and the jet deflection. The analysis of the preferred mode reveals that the preferred mode in the case of the jet in regular waves is helical as opposed to the varicose mode in the free jet. The mean quantities of the jet in regular waves showed a significant spreading and entrainment in the jet as compared to the free jet. The turbulence quantities showed an elevated magnitude both axially and laterally as compared to the free jet.

Structure and dynamics of a laminar separation bubble near a wing root: towards reconstructing the complete LSB topology on a finite wing

The influence of the wing root junction on the laminar separation bubble forming on the suction surface of a semispan NACA 0018 wing cantilevered from the wind tunnel test section wall is studied using surface flow visualisations, particle image velocimetry and surface pressure measurements at a chord Reynolds number of 125 000 and an angle of attack of 6 $^\circ$ . The test section wall boundary layer upstream of the wing is turbulent, and the spanwise influence of the junction on the separation bubble extends well beyond the test section wall boundary layer thickness. Substantial three-dimensionality is seen in the separation bubble flowfield near the wing root, where earlier transition and a reduction in separation bubble thickness is observed. In contrast with the wing tip, earlier transition and a reduction in separation bubble length occurs near the wing root. Outside of the junction affected region, the separation bubble is similar to separation bubbles forming on two-dimensional geometries, and displays mild spanwise waviness. The transition process away from the end affected regions is characterised by the formation of spanwise roll-up vortices that are shed in a nearly two-dimensional manner across the span. The analysis of the results shows that, near the wing root, the increased level of perturbations leads to earlier vortex roll-up and spanwise flow contributes to more rapid vortex breakdown. The results in the wing root region are complemented by the analysis of data from Toppings and Yarusevych (J. Fluid Mech., vol. 929, 2021, A39) in the wing tip region to provide a more holistic outlook on the laminar separation bubble topology and dynamics on the entire finite wing.

Numerical investigations on unsteady vortical flows and separation-induced transition over a cycloidal rotor at low Reynolds number

The unsteady vortical flows and separation-induced transition over a 2-bladed cycloidal rotor are investigated numerically at two advance coefficients, with special emphasis on the influence of two different turbulence models, namely the original SST k-ω model and SST γ-Reθt transition model. Firstly, the applicability of the transition model in predicting the transitional flows over a single airfoil at different incidences is evaluated, and the main results show a good agreement with the experiments, in terms of the mean pressure coefficients and transition locations. Then, the global performance and detailed internal flow structures of the cycloidal rotor are also used to compare with the existing numerical and experimental data. The primary results show that increasing the advanced coefficient can’t change the transition location of the performance of the single blade, but the magnitudes of these variables (vertical force, propulsive force and power coefficients). Then, combining the forces (lift and drag) acting on two blades, the blade loadings and the detailed near-wall flows, the difference of the vertical force and propulsive force of the rotating system and single blade are clarified clearly for two turbulence models. Finally, at advancing side, the transition and its evolution on a single blade is elaborated. It shows that the SST γ-Reθt transition model is superior in predicting the overall performance, but is highly subjected to the disturbances, characterized by the large-scale vortex structures and massive flow separation, compared with SST k-ω model. Simultaneously, it has the capability to capture the transition process, from growing waves of the laminar boundary layer induced by the roll-up vortices to the fully generation of the separation bubble. It believes that this work can deepen the understandings of underlying flow physics inside the cycloidal rotor at low Reynolds number.

Computational analysis of compressibility effect on flow field and aerodynamics at low Reynolds numbers

The flow around the wing of exploration aircraft flying in the very dilute Martian atmosphere experiences very specific low-Reynolds-number compressible flows. A fundamental understanding of compressibility effects, especially on a separation bubble, is an important issue in the aerodynamic wing design. We investigated the effect of compressibility on the laminar separation bubbles and aerodynamic properties of a blunt flat plate with a thickness of 5% of the chord length in unsteady two-dimensional laminar and large-eddy simulations at chord-based Reynolds numbers (Re) of 6100 and 11 000 in the Mach number (M) range from M = 0.1 to 0.8. In the incompressible flow below M = 0.2 at Re = 11 000, the separated shear layer rolled up due to the Kelvin–Helmholtz (KH) instability reattaches to the surface with turbulent transition and then forms a laminar separation bubble. In contrast, with increasing Mach number, the transition and reattachment are delayed, and the separation bubble is stretched. In particular, at M = 0.8, the rolled-up vortex is not distorted in the span direction and advects downstream with a strong spanwise two-dimensionality. This is due to the compressibility effect that suppresses the KH instability. On the other hand, at Re = 6100, the separated shear layer is stable without spanwise distortion even in the incompressible flow due to the stronger influence of viscous forces. With increasing Mach number, such stable separated shear layer reattaches without rolling up further downstream, and the Strouhal number of the vortex also gradually decreases. This leads that the large lift oscillation owing to periodic and two-dimensional spanwise vortex shedding is significantly suppressed with an increasing Mach number. This is also due to the weakening of the KH instability due to the compressibility effects. However, at Re = 11 000, the reattachment points move downstream more significantly with the increasing Mach number, indicating that the suppression of the KH instability due to the compressibility effect is more pronounced at Re = 11 000 than at Re = 6100. Consequently, the negative skin friction shear stress owing to the reverse flow inside the expanded separation bubble becomes relatively large, decreasing the total drag with the increasing Mach number.

Aerodynamics of a wing in turbulent bluff body wakes

The aerodynamics of a stationary wing in a turbulent wake are investigated. Force and velocity measurements are used to describe the unsteady flow. Various wakes are studied with different dominant frequencies and length scales. In contrast to the pre-stall angles of attack, the time-averaged lift increases substantially at post-stall angles of attack as the wing interacts with the von Kármán vortex street and experiences temporal variations of the effective angle of attack. At an optimal offset distance from the wake centreline, the time-averaged lift becomes maximum despite of small amplitude oscillations in the effective angle of attack. The stall angle of attack can reach 20° and the maximum lift coefficient can reach 64 % higher than that in the freestream. Whereas large velocity fluctuations at the wake centreline cause excursions into the fully attached and separated flows during the cycle, small-amplitude oscillations at the optimal location result in periodic shedding of leading edge vortices. These vortices may produce large separation bubbles with reattachment near the trailing-edge. Vorticity roll-up, strength and size of the vortices increase with increasing wavelength and period of the von Kármán vortex street, which also coincides with an increase in the spanwise length scale of the incident wake, and all contribute to the remarkable increase in lift.

Multi-scale observations of the magnetopause Kelvin–Helmholtz waves during southward IMF

In this study, we present the first observations from the Magnetospheric Multiscale (MMS) mission of the Kelvin–Helmholtz instability (KHI) at the dusk-flank magnetopause during southward interplanetary magnetic field conditions on September 23, 2017. The instability criterion for the KHI was fulfilled for the plasma parameters observed throughout the event. An analysis of the boundary normal vectors based on the application of the timing method onto the magnetic field and the electron density data and the minimum directional derivative method onto the magnetic field data shows signatures of surface waves in the plane parallel to the velocity shear. A comparison to 2D fully kinetic simulations demonstrates reasonable consistencies with the formation of surface waves generated by the KH instability, as well as the structures of rolled-up KH waves. The observations further indicated low density faster than sheath plasma as an indicator of rolled-up vortices, which is also consistent with the simulations. All of these results show that the observed waves and vortices are most likely generated by the KH instability. High-time resolution MMS measurements further demonstrate kinetic-scale electric field fluctuations on the low-density side of the edges of surface waves. Detailed comparisons with the simulations suggest that the observed fluctuations are generated by the lower-hybrid drift instability excited by the density gradient at the edges of these surface waves. These secondary effects can lead to a flattening of the edge layers, indicating the connection between kinetic and larger scales within the KH waves and vortices.

Impact of a jet orifice on the hydrodynamics and the oil droplet size distribution

Experiments were performed to understand the impact of the orifice on droplet size in the shear breakup region. Oil jet was released vertically from two orifice types: a pipe and a converging nozzle. The roll-up vortices were observed clearly near the edge of the jet with nozzle orifice while elongated oil ligaments in the jet direction were mostly observed near the edge of the jet with pipe orifice. The size of the droplets shed from the near field shear layer increased along the jet direction for both orifice types. At different distances from the orifice, the characteristic shed droplet size (d50) was up to 40% larger with pipe orifice. Multiphase, large-eddy simulations (LES) with two orifice types were conducted to interpret the droplet size distribution (DSD) observed in the experiments. A thicker boundary layer that was initially formed inside the pipe dominated the near field of the jet with the pipe. The turbulent kinetic energy dissipation rate (ε) was higher with pipe orifice and the peak values of ε were found closer to the jet centerline with the pipe where oil holdup is larger. The viscous shear stress with the nozzle was higher than that of the pipe orifice within the initial two diameters from the orifice and slightly lower or higher at the following distances studied. The Reynolds shear stress was significantly larger with the nozzle at different distances from the orifice. The tangential vorticity within one diameter from the orifice was also dramatically higher with the nozzle. The key parameter in the shed droplet size of a liquid jet in liquid was found to be the shear layer thickness at the orifice, similar to prior works of a liquid jet in gas. A thicker shear layer at the pipe orifice induced a larger droplet size. The increase in the instabilities and shear layer thickness in the jet direction increased the droplet size in the jet direction.