Time-averaged Velocity Profiles Research Articles

Orifice Versus Converging-Nozzle Grid Turbulence: A Wavelet Perspective

Grids such as perforated plates are of fundamental importance in flow turbulence study and are commonly utilised to promote mixing. An orificed perforated plate (OPP) and its reversed counterpart, the converging-nozzle perforated plate (CNPP), were applied to produce quasi-isotropic turbulence inside a wind tunnel. The three orthogonal velocity components were measured using a triple hotwire at 10D downstream of the perforated plate for Reynolds numbers, ReD, 18,700 and 28,400, where D is the diameter of the perforated holes. The statistics of the grid-generated turbulence was analysed using the time-averaged local velocity profile and turbulence intensity, which revealed a more homogeneous distribution of the flow field with a higher level of turbulence for the OPP. Fourier and wavelet analyses were employed to investigate the energy of the eddies as a function of frequency and multiscale characteristics of the fluctuating velocity, respectively. At ReD = 18,700, the turbulent energy remains prominently with large-scale vortical structures which are non-intermittently present in the flow for both perforated plates. The thickness of the converging channels of the CNPP appears to provide the venue for spawning intermittent fluctuations. At higher ReD 28,400, the effect of this intermittent behaviour becomes evident for the CNPP, leading to a multiscale distribution of turbulent energy.

Kármán Street-Boundary Layer Interactions In Turbulent Flow

Understanding the intricate interplay between turbulent boundary layers and wake structures caused by obstacles in the flow is key to assessing the local shear stress fields as well as the drag. These types of flows can be relevant to natural phenomena including sediment and nutrient transport. While existing literature predominantly explores the flow downstream of cylinders mounted perpendicular to bounding surfaces or downstream of cylinders positioned outside the boundary layer region, this work experimentally studies the turbulent boundary layer passing over a cylindrical obstacle positioned near and parallel to the surface of an underlying solid wall. Planar particle image velocimetry (PIV) was performed in an open recirculating water channel for three test cases at a friction Reynolds number Reτ = 1320. Key parameters investigated include the cylinder diameter relative to the boundary layer thickness (D/δ = 0.09 and 0.01) and the gap between the cylinder and the wall (G/δ = 0.09), where the range of D/δ used is smaller than most examples found in the literature. Instantaneous and averaged velocity and vorticity fields are examined based on the PIV measurements. The results in Fig. 1 show how von Kármán vortex streets and turbulent boundary layer structures interact to form unique flow features. The larger cylinder generates a clear separation downstream of its axis as well as a wake that remains coherent over a distance δ downstream. The smaller cylinder with D/δ = 0.01 generates a much narrower and weaker wake, but still shows a clear influence on time-averaged velocity profile. Future experiments will evaluate flow conditions further downstream as well as additional values of G/δ for the same D/δ values.

An Experimental Investigation Of Sweeping Jets Impinging Flow Field: Effects Of Nozzle-To-Plate Distance And Feedback Channel Minimum Cross-Sectional Area

Without requiring any moving part or piezoelectrical elements, fluidic oscillators are actuators able to convert a steady jet into an oscillating one, only due to internal fluid dynamic mechanisms. Since the oscillations of such jets are self-induced and self-sustained, the interest in fluidic oscillators has grown over the last years, especially for film cooling, flow control and heat transfer applications. The oscillation frequency, the sweeping angle and the flow field characteristics of sweeping jets greatly depend on various geometric and fluid dynamic parameters. Therefore, in the current work, through the application of the planar PIV technique, the effects of nozzle-to-plate distance and feedback channel minimum cross-sectional area on the external flow field of impinging sweeping jets are investigated. Non-dimensional nozzle-to-plate distances H/w equal to 2, 4, 6, 8 and 10 are taken into account, being w the exit nozzle throat section width. To analyze the effects of the feedback channel minimum cross-sectional area, a fluidic oscillator having a mixing chamber length Lf/w = 4.5 is provided with threaded holes in correspondence of the feedback channels, in which a micrometric screw is fastened to vary the parameter g/w, being g the feedback channel minimum passage width; values of g/w equal to 0.17, 0.33, 0.50, 0.67 and 1 are considered. Time-averaged and phase-averaged analyses are performed; indeed, the pressure signal measured between the inlet and outlet sections of one feedback channel is used to identify the phase of each velocity snapshot, despite the velocity measurements are not time-resolved. Firstly, the configuration with g/w=1 is taken as reference case, thus analyzing the nozzle-to-plate distance effect. The phase-averaged velocity and fluctuating kinetic energy fields show that the jet sweeps in the external flow field, with a sweeping angle approximately equal to 25° and a frequency of 50 Hz. Due to this behavior, the time-averaged velocity field exhibits a twin-jet pattern, which can also be identified from the time-averaged Phase-correlated Kinetic Energy (PKE) distributions (see Figure 1, left). Indeed, the extreme positions of the oscillating motion are the zones in which the coherent oscillation of the velocity values is greater and, therefore, the PKE values are the greatest. Furthermore, while at H/w=2 the jet sweeps in an almost rigid-like way between the two extreme positions reached during its oscillating motion, at greater nozzle-to-plate distances a phase delay between the motion of the jet near the impinging plate and near the exit nozzle section can be observed. Considering the effects of the feedback channel minimum cross-sectional area, by reducing g/w the sweeping angle of the jet decreases, while the oscillation frequency increases. Still, a twin-jet pattern can be identified for g/w≥0.33, as it can be seen from the crosswise profiles of the dimensionless time-averaged axial velocity (see Figure 1, right), while for g/w=0.17 a single-jet velocity structure can be identified. Nevertheless, as g/w decreases the maximum values of the velocity increase while the oscillation intensity decreases, resulting in a decrease of the PKE values.

Data acquisition in a simplified turbine model for prediction of unsteady vortex phenomena

Abstract The utilization of machine learning in finding decisions of engineering problems is the optimal way. This study presents a new tool that applies machine learning algorithms, to predict the frequency response of an unsteady vortex phenomenon known as the precessing vortex core (PVC) that appears in a conical draft tube behind a runner. The basic values involved in Linear Support Vector Classification model training are the two components of the time-averaged velocity profile at the cone diffuser inlet and cone angle which should be accurately measured. The paper introduces the approach to accumulating an experimental database and conducting primary analysis of the implemented regimes of swirling flow in a simplified hydraulic turbine model. It was obtained that it is necessary to clearly identify the zone of recirculation flow. The presence of this zone is a necessary, but not sufficient condition for the formation of the PVC in the flow. Injection of an axial jet in a situation with moderate swirl flow allows to shift the PVC frequency about by 10% relative to the PVC frequency without an additional jet.

Liquid flow field and residence time distribution in a baffleless oscillatory flow coil reactor

Baffleless coils can operate as continuous oscillatory flow reactors to produce pharmaceutical products where process intensification is intended. A method to evaluate mixing performance and its impact on reaction conversion is residence time distribution (RTD) analysis. Liquid macromixing in a long baffleless coil reactor (L = 24 m, dt = 4.57 mm) at ranges of oscillation amplitude (11.75–58.75 mm), frequency (0–3.68 Hz), and net flow rate (30–120 g⋅min−1) was investigated based on measured RTDs fitted to the skewed normal distribution model. The impact of operating conditions on the RTD was supported by analyzing the liquid flow field derived from CFD simulations. The RTD variance under different oscillation conditions suggests a near plug flow pattern, although deviations from ideality such as skewness and long tailing were always observed. The model parameters, along with the RTD variance, were correlated to the operating conditions lumped in an oscillatory Dean number that used the flow amplitude as characteristic length (Deox). Values of Deox varied between 150 and 11000, and the RTD variance at the different net flow rates was minimized for 300 < Deox < 600, as also evidenced from the minimum cross-sectional variance of the simulated time-averaged axial velocity profile. Normalizing the data allowed comparison to similar coil studies with different geometrical dimensions and operating conditions, leading to generalized correlations for the RTD model parameters. Finally, chemical conversion was assessed for first- and second-order single phase reactions at different Damköhler numbers using the segregation and maximum mixedness micromixing models combined with the fitted RTDs. The performance of the coil remained consistent with a plug flow reactor despite its departures from ideal flow.

Systematic assessment of computational models for the prediction of flow fields in bubble columns

Systematic changes in interfacial forces, turbulence models for the continuous and dispersed phases, lift force coefficient, and bubble size distribution were used to evaluate, in terms of complexity, the performance of nine computational models in the simulation of the time-averaged axial liquid velocity and holdup profiles in a bubble column. The oscillation period of the gas plume was also included in this evaluation. The mesh used for the simulations was chosen after a convergence analysis carried out with four meshes. The finest mesh displayed the highest divergence from the experimental data. Such behavior indicates that the proper mesh for a simulation of a multiphase flow must consider other factors in addition to element size and the quality indicators. The standard, RNG, and realizable κ-ε models were used for the liquid phase, while an algebraic and the standard κ-ε were the models for the dispersed phase. The isotropic RNG model for the liquid phase achieved accurate predictions in the anisotropic region near the gas distributor. The algebraic approach for the gas phase produced data similar to those calculated with the standard model. The simultaneous presence of the lift and virtual mass forces led to reliable predictions, and a lift coefficient of 0.2 displayed better performance than 0.14. This coefficient produced an excessive dampening of the lift force and caused the simulations to lose accuracy. The population balances did not improve the predictions.

Determining the flow transition from laminar to turbulence using simple spin-echo magnetic resonance techniques

We have recently introduced a methodology to determine the average velocity and flow behavior index of laminar pipe flow of a power-law fluid using simple magnetic resonance (MR) techniques. In general, MR techniques are noninvasive and capable of working on optically opaque fluids. Knowledge of the average velocity and flow behavior index provides the information needed to reconstruct the flow velocity profile. However, as the flow velocity increases, the flow will begin to develop turbulence. For pipe flow of a particular fluid, the velocity profile is flatter in the center of the pipe at turbulent flow rates compared with laminar flow. An effective flow behavior index can approximate the time-averaged velocity profile, as the Reynolds number increases, as a fluid transitions from laminar to turbulent flow. Here, we show the results of testing the utility of such a simplification in monitoring that transition. For the present study, Reynolds numbers ranged from approximately 490 to 6800, which corresponds to flow rates of 200 to 2750 ml/min and average velocity of 5 to 80 cm/s. We found that visual inspection of the data would be sufficient to determine the state of the flow. With some external knowledge of the flow rate, the shape of the time-averaged velocity profile and eddy diffusivity can be estimated (and potentially also an average fluid particle acceleration).

Modelling shear stress distribution in ice-covered streams

Abstract Distribution of bed shear stress is the critical factor in regulating the meandering of single-thread rivers. However, the impact of ice cover on bed shear stress is largely unknown. In this study, we develop a theoretical model of cross-stream momentum balance to examine the distribution of bed shear stresses in ice-covered meandering rivers. To validate the theoretical model, field surveys were carried out in a river reach of the Red River in Fargo, North Dakota. Data monitoring was completed using an Acoustic Doppler Current Profiler to obtain time-averaged velocity profiles. Our theoretical model indicates that an ice covering develops high-shear zones near both the inner and outer banks, which might exacerbate sediment transport and enhance bank erosion. Velocity measurements confirm the results of the proposed model and demonstrate a clear impact of meandering river banks on velocity profiles and secondary flow patterns under ice cover. Based on our results, we hypothesize that ice cover increases turbulent stresses near banks, which in turn lead to the enhancement of the bed shear stress. Our work provides new insights into the impact of ice cover on bed shear stress distribution, which could play an important role in driving sediment-transport processes and the long-term morphodynamic evolution of meandering rivers seasonally covered by ice.

Experimental study on the combustion characteristics of square fire

In this paper, the combustion characteristics of square fire are studied experimentally, with emphasis on the changes in flame structure during the flow evolution. A series of fire tests with different fuel types, heat release rates, and fire scales are performed. The time-averaged temperature and velocity profiles and the flame length are measured and discussed. Experimental results show that as the square fire progresses downstream from the flame base, its cross-sectional shape undergoes a square-to-round transition. In this process, the self-induced axis-switching phenomenon intensifies the square fire's turbulence, manifested as more significant axial and radial temperature fluctuations than the round fire. This enhanced turbulent mixing promotes flame burning, resulting in a greater axial temperature and velocity of the square fire than the round fire. At different fire scales, the critical height at which a square fire completes the axis-switching first increases and then slows down with increasing heat release rate, which can be well correlated by the dimensionless heat release rate normalized by the side length. Experimental data verify that Quintiere and Grove's classic model can well normalize the flame lengths of square fires at different scales. A square fire's three-dimensional flow and enhanced turbulent mixing cause a higher constant representing the near-field entrainment strength than a round fire.

Altering the wake dynamics of a circular cylinder with harmonic forcing

The time-averaged velocity profile in the wake of a circular cylinder resembles a “U” type or a “V” type profile due to a velocity defect. Bhattacharya and J. W. Gregory [“The effect of spatially and temporally modulated plasma actuation on cylinder wake,” AIAA J. 58, 3808–3818 (2020)] showed experimentally that the wake dynamics could be altered with harmonic forcing in such a way that instead of a velocity defect, a “jet-like” profile emerged with a characteristic “W” profile. The harmonic forcing was created by modulating the waveform of a dielectric barrier discharge plasma actuators' supply signal with a frequency twice that of the shedding frequency. However, the reason for the appearance of the “W” profile was not clear in S. Bhattacharya and J. W. Gregory [“The effect of spatially and temporally modulated plasma actuation on cylinder wake,” AIAA J. 58, 3808–3818 (2020).] In this paper, we use numerical simulation to recreate the test conditions implemented by Bhattacharya and J. W. Gregory [“The effect of spatially and temporally modulated plasma actuation on cylinder wake,” AIAA J. 58, 3808–3818 (2020).] We apply large-eddy simulation to study the impact of the pulsed, harmonic forcing on the wake of a circular cylinder at a subcritical Reynolds number of 4700. The plasma actuators are modeled with a body-force approach. The frequency of the driving signal of the plasma actuator is modulated at twice the shedding frequency. The amplitude of the signal is set at 6 kV peak to peak to create a blowing ratio of 0.8. The goal is to understand how the wake changes in three dimensions and the impact on separation on the cylinder surface due to the harmonic forcing. Results show that pulsed forcing causes vortices from one side of the wake to cross the centerline. This crossing creates an effective jet-like velocity along the centerline, resulting in a W velocity profile. Such a W profile is observed at least up to a streamwise distance of five cylinder diameters. Additionally, the pulsed actuation significantly increases the magnitude of primary and secondary frequencies throughout the wake. Forcing caused a 50% increase in the transverse velocity fluctuations at the centerline of the wake at the streamwise location of x/d=5. There was a similar increase in 33% at the centerline in the streamwise velocity fluctuations at the same location.

Experimental and Analytical Investigations of Wire-Partially Insulated Parallel Plate Electrode Type Electrohydrodynamic Fan

As an alternative to conventional mechanical fans, EHD fans and/or EHD gas pumps, which generate less noise, were investigated for cooling systems, such as in electronic equipment and automobiles. Wire-parallel plate electrode type EHD fans, which have greater design freedom and potential for practical application, have been suggested. This study clarifies the characteristics of a wire-partially insulated parallel plate electrode type EHD fan under DC positive applied voltage. In order to understand the characteristics of the EHD fan more deeply, visualizations of the air flow in the flow channel and the exit area were conducted by using PIV and CFD analyses. In the experiment, air at atmospheric pressure and room temperature was used as a working fluid. The experimental results for fan characteristics of the EHD fan, such as the velocity profile, cross-sectional average velocity, volumetric flow rate in the flow channel or at the exit area, power, and so on, are considered in detail. In addition, the flow visualization and the instantaneous and time-averaged velocity profiles from the PIV analysis are discussed. A comparison with the experimental results described above, and differences of flow regime for different locations, are also presented and discussed. Furthermore, a two-dimensional steady state flow simulation by means of CFD analysis was conducted and its experimental results analyzed.

Experimental study of Stokes drift in combined wave-current flows using Particle Image Velocimetry

In the present study, Eulerian velocities were obtained using the PIV system to investigate the problem of Stokes drift in combined wave-current flows. Time-averaged mean Eulerian velocity profiles were presented after the flow has achieved its equilibrium status. Waves with various wave periods and amplitudes were generated and propagated with the turbulent current possessing a Reynolds number of 32000. Results of the time-averaged mean velocity profiles suggest the absence of a return flow in an open flume. The discrepancies of the depth-averaged mean velocity with and without waves superimposed were found to be in good agreements with the return flow speeds as predicted by Kim (1984). This further confirmed the applicability of their solution for calculating the return flow under the condition of waves propagating with currents.

Influence of channel bend angle on the turbulent statistics in sharply bent channel flows

Turbulence in wall-bounded flows shows a wide range of regimes, where the interaction between scales significantly occur. Reynolds number is used to characterize the dynamics of fluid corresponding to single phase channel flows. Meanwhile, different flow behavior exists in curved channels even at fixed Reynolds number, where the curvature varies as shown by Brethouwer [“Turbulent flow in curved channels,” J. Fluid Mech. 931, A21 (2022)]. In the present study, we show how wall-bounded turbulent flow behaves on sharply bending the channel by investigating the time averaged velocity profiles at the straight section, at the bend, and in the inclined section. The well-known logarithmic behavior of the time averaged normalized velocity profile is retained, where the von Kármán and the additive constants assume altered values depending on the sharp bend inclination. The near-wall fluctuations at the bend are enhanced, which is due to diffusion of counter-rotating vortices leading to increased turbulent activity. In terms of spatial structure of the random fluctuating field, the two-point correlation statistics suggest that multiple high speed and low speed streak pairs are generated, and there are multiple streamwise vortices of different sizes when bend inclination is increased. Budgets of turbulent kinetic energy are presented for various inclinations of the bend at different sections of the bend channel, which depicts that turbulent kinetic energy is modulated at the bend.

Data-Driven Prediction of Unsteady Vortex Phenomena in a Conical Diffuser

The application of machine learning to solve engineering problems is in extremely high demand. This article proposes a tool that employs machine learning algorithms for predicting the frequency response of an unsteady vortex phenomenon, the precessing vortex core (PVC), occurring in a conical diffuser behind a radial swirler. The model input parameters are the two components of the time-averaged velocity profile at the cone diffuser inlet. An empirical database was obtained using a fully automated experiment. The database associates multiple inlet velocity profiles with pressure pulsations measured in the cone diffuser, which are caused by the PVC in the swirling flow. In total, over 103 different flow regimes were measured by varying the swirl number and the cone angle of the diffuser. Pressure pulsations induced by the PVC were detected using two pressure fluctuations sensors residing on opposite sides of the conical diffuser. A classifier was constructed using the Linear Support Vector Classification (Linear SVC) model and the experimental data. The classifier based on the average velocity profiles at the cone diffuser inlet allows one to predict the emergence of the PVC with high accuracy (99%). By training a regression artificial neural network, the frequency response of the flow was predicted with an error of no more than 1.01 and 5.4% for the frequency and power of pressure pulsations, respectively.

Particulate Deposition Effects on Internal Swirl Cooling of Turbine Blades

Abstract Particulate deposition effects on flow and heat transfer in an internal swirl tube subjected to fly ash ingestion were investigated by constructing an unsteady simulation framework, in which a particle–wall interaction model and a mesh morphing technique were implemented. Swirling flows in the swirl tube were induced by two tangential jet nozzles. Particles having a mean diameter of 6.5 μm were released from the nozzle inlets to model an exposure duration of 4500 h for engine operation in real fly ash environment using scale factors in the unsteady simulations. Particle deposition and its dynamic process were examined, and the effects of deposition on the swirling flow were quantified by comparing time-averaged velocity profiles, vorticity, pressure loss, and heat transfer with those from a clean tube without deposition. Results reveal that the most upstream section of the swirl tube captures the majority of the particles and the deposition distributions show a spiral pattern over the tube wall. The total mass of the deposits within the tube linearly increases, while local deposition thickness has a nonlinear relationship with the exposure time due to the interaction of the particles with the swirling flow. The deposition can generate a maximum of 15% reduction in cross-sectional area of the tube within the exposure duration, resulting in a reduced swirl number, because of the accelerated axial velocity and the decreased circumferential velocity, and further lower heat transfer in the downstream section of the tube relative to the clean tube case. However, as the heat transfer in the upstream deposition section is enhanced by the roughness due to the deposition, area-averaged heat transfer throughout the entire swirl tube is slightly improved by 4.0% but simultaneously a 179% higher pressure loss is observed, leading to an overall thermal performance value of 0.79 (relative to 1.0 for a clean tube), indicating substantial degradation of cooling performance in the fouled swirl tube.

Effect of particle shape on packing fraction and velocity profiles at outlet of a silo

Many studies on how the particle shape affects the discharge flow mainly focus on discharge rates and avalanche statistics. In this study, the effect of the particle shape on the packing fraction and velocities of particles in the silo discharge flow are investigated by using the discrete element method. The time-averaged packing fraction and velocity profiles through the aperture are systematically measured for superelliptical particles with different blockinesses. Increasing the particle blockiness is found to increase resistance to flow and reduce the flow rate. At an identical outlet size, larger particle blockiness leads to lower velocity and packing fraction at the outlet. The packing fraction profiles display evidently the self-similar feature that can be appropriately adjusted by fractional power law. The velocity profiles for particles with different shapes obey a uniform self-similar law that is in accord with previous experimental results, which is compatible with the hypothesis of free fall arch. To further investigate the origin of flow behaviors, the packing fraction and velocity field in the region above the orifice are computed. Based on these observations, the flow rate of superelliptical particles is calculated and in agreement with the simulated data.

Formation and breakup of twisting ligaments in a viscous swirling liquid jet

We analyze the successive steps of the breakup morphology of a swirling liquid jet. Three-dimensional numerical simulations are carried out using the Volume of Fluid method with adaptive mesh refinement for axial Reynolds numbers of 50 and swirl numbers of 0.50≤S≤1.55. We present fundamental flow features of the swirling jet in terms of time-averaged axial and azimuthal velocity profiles for the considered range of swirl numbers. The provision of a swirl induces helical disturbance at the interface of the jet, which exhibits an azimuthal mode number of m = 4. We identified that viscous forces are the most dominant force in the flow, which causes the suppression of Kelvin–Helmholtz instability at the interface. In contrast, we found the existence of centrifugal instability, which destabilizes the helical rim developing at the interface. As a result, centrifugally induced corrugations in the form of tiny protrusions develop along each of the helical rims, which triggers Rayleigh–Taylor instability. Subsequently, these tiny protrusions get stretched in the radially outward direction and transform into twisting ligaments that break into droplets. We have elucidated the mechanism for the twisting of ligaments and its further disintegration into first-generation droplets, which has not been reported in previous studies.

Experimental and numerical investigation of flow structure of grooved cylinder

Making modifications to a cylinder is one of the most effective ways to change the flow in the cylinder wake region. This study aimed to investigate experimentally and numerically the flow structure of the wake region of a smooth cylinder and rectangular grooved cylinders (roughness coefficients are ϵ/D = 0.02, 0.04, and 0.06, ϵ is the roughness height, and D is the diameter of the cylinder) by using the Particle Image Velocimetry (PIV) and the Computational Fluid Dynamics (CFD). Numerical studies were conducted with the Reynolds-averaged Navier-Stokes (RANS) equations. The experimental studies were performed at Re = 5000 in an open water channel, while the numerical studies were performed at Reynolds numbers in the range of 5000 ≤ Re ≤ 20000. A good agreement was observed between the experimental and the numerical results in terms of the time-averaged patterns, time-averaged velocity profiles, and Strouhal numbers. The saddle points of the rectangular grooved cylinders were closer to the surface compared to those of the smooth cylinder. The results revealed that the time-averaged drag coefficient reduced by 30% for the largest groove roughness (ε/D = 0.06) at Re = 20000. In conclusion, the present study shows that rectangular grooved cylinders are effective in reducing the time-averaged drag coefficient.

Application of a Coupled Reynolds Stress Model to a Swirl-Driven Diffuser Flow

Swirling flow is a dominant feature in a significant number of technical applications. Hydraulic turbines at part-load are strongly affected by the related vortex rope phenomenon. Its dynamic behavior has a negative impact on the operating performance and durability of the machine.CFD can be used to get additional insight in this complex phenomenon but requires a valid simulation model able to capture the relevant flow physics, which is driven by highly anisotropic turbulent structures. The simulation results are therefore strongly affected by the turbulence modeling. A swirl apparatus (AC6-14), for which extensive experimental data is available, is used in this work for the assessment and validation of different turbulence models. The state-of-the-art SST k-ω model, with and without curvature correction, is compared to a coupled full Reynolds stress model. All models are integrated into a pressure-based coupled flow solver.The investigation revealed that both, SST k-ω with curvature correction and the full Reynolds stress model better predict the time-averaged velocity profiles in the diffuser compared to standard SST k-ω. The swirl component is thereby best captured with the Reynolds stress model. All models deliver a reasonable frequency spectrum for the dynamic behavior of the vortex rope. However, flow visualization shows that standard SST k-ω is not capable of predicting the shape and size of the vortex rope accordingly. Both, SST k-ω with curvature correction and the full Reynolds stress model, can be used in the future for more detailed flow investigations, which include also the assessment of flow control measures.

Large Eddy Simulation of a supersonic air ejector

This paper presents a study on the flow topology in the mixing chamber of a supersonic ejector using Large Eddy Simulation (LES). To this end, a supersonic air ejector of squared crossed-section was modelled using a specialized finite-element code. Comparisons with experimental data showed good agreement, both in terms of the primary jet shock cell structures and wall pressure measurements (mean deviation of 12%). Results have been discussed both in terms of time averaged profiles and instantaneous structures in the mixing layer. The general flow features have been identified by means of instantaneous temperature fields and pressure profiles through the device. Results show that, under the assessed conditions, the mixing layer is laminar at first and transitions towards turbulence in the first quarter of the mixing chamber, where Λ vortices have been identified. These evolve into hairpin vortices and finally break down around half of the mixing chamber. Time-averaged velocity profiles show self-similarity in this section. In comparison with an unconfined mixing layer (Fang et al., 2018), the supersonic ejector mixing layer grows slower first but then develops at a similar rate after the transition region. A shock train occurs towards the end of the mixing chamber, which enhances mixing. Given its location, it generates a recirculation bubble in the diffuser which narrows the main flow passage and breaks the flow vertical symmetry. This pioneer study shows the enormous potential that LES offers for the optimization and detailed analysis of supersonic ejectors. • Pioneer Large Eddy Simulation of an air supersonic ejector. • Turbulent structures are identified along the shear mixing layer. • An asymmetric shock wave/recirculation is observed in the diffuser. • Application of compound choke theory to study the choking phenomenon.