Turbulent Jet Flow Research Articles

Data-driven modal decomposition methods as feature detection techniques for flow problems: A critical assessment

Modal decomposition techniques are showing a fast growth in popularity for their wide range of applications and their various properties, especially as data-driven tools. There are many modal decomposition techniques, yet Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) are the most widespread methods, especially in the field of fluid dynamics. Following their highly competent performance on various applications in several fields, numerous extensions of these techniques have been developed. In this work, we present an ambitious review comparing eight different modal decomposition techniques, including most established methods, i.e., POD, DMD, and Fast Fourier Transform; extensions of these classical methods: based either on time embedding systems, Spectral POD and Higher Order DMD, or based on scales separation, multi-scale POD (mPOD) and multi-resolution DMD (mrDMD); and also a method based on the properties of the resolvent operator, the data-driven Resolvent Analysis. The performance of all these techniques will be evaluated on four different test cases: the laminar wake around cylinder, a turbulent jet flow, the three-dimensional wake around a cylinder in transient regime, and a transient and turbulent wake around a cylinder. All these mentioned datasets are publicly available. First, we show a comparison between the performance of the eight modal decomposition techniques when the datasets are shortened. Next, all the results obtained will be explained in detail, showing both the conveniences and inconveniences of all the methods under investigation depending on the type of application and the final goal (reconstruction or identification of the flow physics). In this contribution, we aim at giving a—as fair as possible—comparison of all the techniques investigated. To the authors' knowledge, this is the first time a review paper gathering all these techniques have been produced, clarifying to the community what is the best technique to use for each application.

Investigation of dynamic mixedness characteristics of a transverse acoustically excited turbulent jet by high-repetition-rate acetone tracer planar laser-induced fluorescence technique

The dynamic mixedness characteristics of a bluff-body stabilized turbulent jet under transverse acoustic excitations were investigated using high-repetition-rate acetone planar laser-induced fluorescence (PLIF) at 7 kHz and multipoint scanning hot-wire measurements. Acetone mixedness imaging was applied for the turbulent jet to assess the interaction between the turbulent jet and the imposed transverse acoustic excitations at a driving frequency of 50 Hz. The high-repetition-rate acetone PLIF images showed that the acetone mixedness distribution swung left and right frequently under the transverse acoustic excitation, and the deflection angle could reach about 6°. The mixedness area of a turbulent jet flow could also be increased by 13.3% when excited by a transverse acoustic wave. Meanwhile, the sequence of instantaneous acetone PLIF images illustrated how the wrinkled edges were generated when acoustic excitations were imposed. The curvature of the acetone PLIF interface showed that the portion of large curvatures increased by 60% after applying an acoustic wave of 123 dB. Multipoint hot-wire measurements further stressed that the turbulent intensity at the transverse acoustic excitation of 123 dB increased to be about 1.3 times the natural turbulence. The proper orthogonal decomposition results showed that the large and small scales of the jet wrinkles both increased with the sound pressure level. RANS transient simulation also implied that distorted velocity streamlines could be achieved inside the turbulent jet due to the transverse acoustic excitation. They could further lead to inhomogeneous convection and increased mixing between the turbulent jet and the surrounding air.

Entrainment mechanism of turbulent synthetic jet flow

Synthetic jets have received extensive attention due to their superior mixing property. However, its mechanisms have not been investigated from the perspective of the turbulent/non-turbulent interface (TNTI). To shed new light on this issue, the entrainment and TNTI properties of a synthetic jet are experimentally investigated and compared with a continuous jet at $Re_j = 3150$ . The fuzzy clustering method is applied to select an appropriate vorticity threshold to detect the TNTI. Statistically, it is revealed that the entrainment coefficients of the two jets significantly differ in the near field, while they become almost identical in the far field. Instead of the vortex ring, the ‘breakdown of the vortex ring’ enhances the entrainment in the present synthetic jet. Instantaneously, the TNTI more violently fluctuates in the near field of the synthetic jet, which leads to a larger fluctuation in the TNTI radial position, a higher fractal dimension and enhanced local entrainment. Moreover, the transition of the probability density function of the TNTI orientation from the unimodal distribution in the near field to the bimodal distribution in the far field is found in both jets. The multi-scale analysis reveals a new mechanism for the bimodal distribution caused by the TNTI-thickness-scale structures.

Investigating the Linear Dynamics of the Near-Field of a Turbulent High-Speed Jet Using Dual-Particle Image Velocimetry (PIV) and Dynamic Mode Decomposition (DMD)

The quest for the physical mechanisms underlying turbulent high-speed jet flows is underpinned by the extraction of spatio-temporal coherent structures from their flow fields. Experimental measurements to enable data decomposition need to comprise time-resolved velocity fields with a high-spatial resolution—qualities which current particle image velocimetry hardware are incapable of providing. This paper demonstrates a novel approach that addresses this challenge through the implementation of an experimental high-spatial resolution dual-particle image velocimetry methodology coupled with dynamic mode decomposition. This new approach is exemplified by its application in studying the dynamics of the near-field region of a turbulent high-speed jet, enabling the spatio-temporal structure to be investigated by the identification of the spatial structure of the dominant dynamic modes and their temporal dynamics. The spatial amplification of these modes is compared with that predicted by classical linear stability theory, showing close agreement, which demonstrates the powerful capability of this technique to identify the dominant frequencies and their associated spatial structures in high-speed turbulent flows.

Effect of Offset Ratio and Plate Motion on Conjugate Heat Transfer in a Turbulent Offset Jet Flow Over the Heated Plate

Abstract The present investigation focuses on studying the conjugate heat transfer behavior for a turbulent offset jet flow over a heated plate moving uniformly in a positive axial direction. Heat transfer from heated walls to flowing fluids is a subject of great scientific interest and practical importance due to many industrial applications like cooling hot rolling sheets and slabs, cooling continuous casting, etc. The conjugate technique adopted in the present investigation requires consideration of conduction in the solid and the convection from the solid surface to the fluid. For the simulation, a low-Re modified k−ε model developed by Yang and Shih (YS model) is adopted. The YS turbulence model uses the integration-to-wall (ITW) technique which enables it to interpret the variations in fluid and thermal parameters in the near-wall regions without adopting the wall functions. The simulation involves a turbulent offset jet ejecting out of a nozzle at a Reynolds number of 15000, submerged into a stationary environment of Prandtl number 7. The plate at its bottom is heated by constant flux. The various parameters taken into account for observing the thermal behavior are offset ratios (OR=3, 7, and 11), plate velocity ratios (Up=0, 0.5, 1.0, 1.5, and 2), plate thickness ratios (S=0.5, 1.0, and 1.5), and conductivity ratios (K=500, 1000, 1500, and 2000). The temperature distribution diagrams reveal the effect of the parameters mentioned above. The interface temperatures and the Nusselt number suggest better cooling can be achieved at lower offset ratio cases for low-velocity ratios. The contour diagrams and variation of average and local Nusselt numbers suggest the dominating effect of plate motion. A more effective cooling rate is achieved at higher plate velocity, irrespective of the offset ratios.

Flow and surface loading induced by a supersonic jet over an aft deck

Aircraft designs may include an engine that exhausts over the upper fuselage of the aircraft. This design has many benefits including reduced noise below the aircraft. However, the scrubbing of the high-speed turbulent jet flow over the fuselage has the potential to cause structural fatigue. This paper describes a joint experimental/computational study of the surface pressure fluctuations induced by the supersonic jet flow. The model configuration consists of a rectangular convergent-divergent nozzle that exhausts over a flat plate. The plate is a continuation of the lower lip of the nozzle. Pressure measurements are made at several locations on the plate surface. In addition, schlieren visualization is used to identify the jet's shock cell structure. Numerical simulations are performed using Large Eddy Simulation (LES). Results are compared between the simulations and the experiments. The importance of shock turbulent boundary layer interaction to the surface loading is identified.

NUMERICAL SIMULATION OF THREE-DIMENSIONAL CIRCULAR FREE TURBULENT JET FLOW USING DIFFERENT REYNOLDS AVERAGE NAVIER-STOKES TURBULENCE MODELS

The present research work shows the numerical investigation of free turbulent jet behavior as well as thermal characteristics of the jet being issued from the circular nozzle outlet and it is performed by solving the different Reynolds averaged Navier-Stokes (RANS) equation. In this research work, central jet velocity, and turbulent intensity of the jet along with thermal characteristics are calculated along the jet axis to check the coherency, heat-exchanging capability, and widening of the jet. Simulation results of the jet are first validated with the experimental data of previous literature work. The numerical experiment provides a better potential for future improvements in the field of jet flow and exhibits a great deal of resemblance with experimental data with minimum error. Computational fluid dynamics (CFD) is a capable tool for depicting the various flow behaviors and selecting the suitable turbulence model is always required for an accurate prediction of the jet-flow regime. Most of the turbulence models investigated in this research work show good results for turbulence intensity and predicted the near- and far-field region as per the experimental data. From the thermal analysis of the free turbulent jet, it is seen in the present results utilizing the various turbulence RANS models that <i>Nu</i><sub>x</sub> is rather high near the nozzle exit. The standard <i>k-ε</i> model describes the near field and long field of the free jet very well and it is also easy and inexpensive to execute. Furthermore, for the observed free turbulent jet, the most appropriate model is recommended.

Dispersion of Molecular Patterns Written in Turbulent Air.

We study the dispersion of tiny molecular clouds in turbulence by writing patterns in turbulent air and following their deformation in time. The writing is done by fusing O_{2} and N_{2} molecules into NO in the focus of a strong ultraviolet laser beam. By crossing several of these laser beams, patterns that have both small and large scales can be painted. The patterns are visualized a while later by inducing fluorescence of the NO molecules with a second UV laser and registering the image. The width of the lines that make the pattern is approximately 50 μm, a few times the Kolmogorov length η, the smallest length scale in turbulence, while the overall size of the patterns (≈4 mm) is inside the inertial range of the used turbulent jet flow. At small scales molecular clouds disperse under the joint action of molecular diffusion and turbulence. The experiments reveal for the first time this subtle, yet very important interaction. At macroscales (≈200 η) we verify the Batchelor dispersion of objects whose size is inside the inertial range; however, the expected influence of molecular diffusion is smaller than the accuracy of the experiments.

Numerical simulation of a flat turbulent near-wall jet in the OpenFOAMv7 package

In this research, the numerical simulation of a planar stationary near-wall turbulent jet using the OpenFOAMv7 package at Reynolds number, Re, equal to 9600 has been performed. Comparison of numerical results with corresponding experimental data on longitudinal velocity profiles shows the adequacy of the used mathematical model of the problem under consideration. This model should be used in further studies of turbulent jet flows.

Direct numerical simulation of planar turbulent jets: Effect of a pintle orifice

The effects of a pintle-shaped orifice on a planar turbulent jet flow at Reynolds number 4000, based on the inlet bulk mean velocity and the jet width, are studied using direct numerical simulations. Flapping of the jet along with a low-frequency modulation of the Kelvin–Helmholtz (KH) instability, in the presence of a pintle-shaped orifice, is observed. To compare the pintle-jet behavior, a free-jet is simulated as a reference case. The effects of the near-field region on the far-field flow characteristics have been investigated. In both the cases, the KH instability in the near-field influences the far-field jet, whereas the pintle-jet also exhibits a low-frequency flapping. In addition, oblique vortex pattern has been observed in the case of pintle-jet. The far-field flow statistics of the pintle-jet with a top-hat inlet interestingly agree with those of the free-jet with a hyperbolic tangent inlet. Temporal variation of the jet characteristics has been analyzed using spatiotemporal plots. In addition, the large- and small-scale turbulent motion have been studied using three anisotropic invariant maps (turbulence triangles, eigenvalue, and barycentric maps). Moreover, that the barycentric map gives a better visualization of the anisotropic behavior has been observed in the current study.

An Analysis of Jet Noise Characteristics in the Compressible Turbulent Mixing Layer of a Standard Nozzle

In this paper, we seek to understand the influence of TPS standard nozzle shape on jet noise and improve the accuracy of flow and pressure ratio of aeronautical simulation. Through the structural design of an equal-diameter nozzle and a standard nozzle, a concentric dichotomy monitoring point arrangement scheme is proposed for its flow field and sound field, and the free flow of the jet is used to realize k − ε. The model is solved by hydrodynamics to analyze the flow characteristics of the turbulent boundary layer, and the near-field and far-field characteristics of jet noise are analyzed by using the Lighthill equation excited by a quadrupole under fluid input, revealing the turbulent velocity and turbulent kinetic energy and other relevant fluid characteristics of the two nozzles under a single variable, and analyzing the near-field and far-field noise pressure levels and directivity of the two nozzles at a specific frequency. When the boundary conditions are consistent, the change trends and laws of the two kinds of nozzles in the turbulent jet flow field are almost the same, and the eddy viscosity extreme value of the standard nozzle is 0.02 higher than that of the equal-diameter nozzle, while the static temperature extreme value is 8 K, but they have a great impact in the noise field, mainly reflected in the noise decreasing law, noise directivity and low-frequency noise distribution. The mean value of noise extremes of standard nozzles at four frequencies is 2 dB higher than that of equal-diameter nozzles. The relative characteristics of the two nozzles can be used to design through the actual demand conditions of the flow field and the sound field by providing a reference for the customized parameter design of the standard nozzle.

Measurement of turbulent supersonic steam jet flow characteristics using TDLAS

Ejectors have no moving parts and are preferable to mechanical compressors in many applications, but ejectors typically have a relatively low efficiency. To aid in the ejector design process, thorough understanding of the turbulent mixing of multi-phase compressible jets is beneficial.This paper reports experimental results for Tunable Diode Laser Absorption Spectroscopy (TDLAS) measurements derived from an axisymmetric supersonic steam jet apparatus.In this experimental work, a supersonic steam jet nozzle exit of a diameter 13.6 mm was surrounded by a low-speed flow of dry nitrogen. The TDLAS system was traversed through the flow at three different planes downstream from the ejector nozzle exit: 15, 20, and 30 mm distance. At each of the three planes, line-of-sight measurements were made with the laser passing through locations between 0 and 15 mm from the jet centreline.Through the analysis of the TDLAS data and application of the Abel inversion method, the radial distribution of the pressure, temperature, and the concentration of the water-vapour were obtained. The key findings are that it is possible to determine key physical parameters using experimental TDLAS measurements when combined with a suitable numerical optimization approach.

Assessment of subgrid dispersion models for large-eddy simulations of turbulent jet flows with dilute spray droplets

High-fidelity simulations for polydispersed sprays in the Eulerian–Lagrangian framework need to incorporate subgrid-scale effects in the particle evolution equations. Although the quasi-linear evaporation rate formulation captures evaporating droplet statistics, further improvement is required when subgrid-scale velocity effects become essential. The subgrid dispersion model strongly affects droplets spatial distribution, and subsequently net evaporation rate, depending on how rapidly they are dispersed into the dry air region. The main original contribution of this study is to assess the performances of a number of commonly used dispersion models in a consistent manner, against a reference direct simulation results. The models considered are (i) discrete random walk, (ii) approximate deconvolution method, (iii) stochastic model based on the Langevin equation, and (iv) combined approximate deconvolution method with the Langevin equation. Mass and enthalpy transfer source terms together with droplet diameters and particle distributions were compared against corresponding direct numerical and large-eddy simulations without a model as reference cases. Numerical results at low Stokes and moderate Reynolds numbers indicate that the dispersion model choice does not affect Eulerian field averages or fluctuations. However, proper dispersion models are essential to capture droplet distributions in the far-field region after jet breakup for Stokes number smaller than unity. The unclosed Lagrangian momentum equation without any dispersion model most accurately reproduces direct numerical simulation in the near field.

The effect of plate motion on heat transfer enhancement using turbulent offset jet flow: A conjugate approach

The present research is focused on computational analysis of conjugate heat transfer behavior of a turbulent offset jet flow over a moving flat plate in a quiescent environment. The plate of definite thickness and conductivity is subjected to isothermal condition at the plate bottom surface. The computation is carried out with a low-Reynolds number (LRN) turbulence model proposed by Yang and Shih which involves integration to wall (ITW) approach to capture the thin viscous sub-layer region within the boundary layer. The variation of offset ratio (ranging as 3, 7 and 11) and plate velocity (Up = 0 − 2) plays quite a major role in the distribution of interface temperature and Nusselt number. The conduction heat flow rate in solid depends upon the plate thickness ratio (S) which is varied as 0.5 − 1.5 and the conductivity ratio (K) ranging as 500 − 2000. The variation of average Nusselt number gives evidence of better cooling rate for smaller offset ratios and higher plate to jet velocity ratio.

Numerical investigation of particle spray size distribution in turbulent jet flow

Numerical investigation of particle spray size distribution in turbulent jet flow

Dependence of wall jet phenomenology on inlet conditions and near-field flow development

In this paper, the three-dimensional turbulent wall jet flow is investigated for three different developing initial velocity profiles. The developing initial velocity profiles at the nozzle exit are generated by three different lengths (, 50 and 90) of the square nozzle . The velocity profiles at the nozzle exit are measured with the single probe hot-wire anemometer. The Reynolds number based on the bulk mean velocity and nozzle height is 25,000 for all the cases. The measured velocity profiles at the nozzle exit are used as the inlet conditions for the numerical simulations. The results show that the initial velocity profile affects the flow field of the wall jet in near and far-field regions. It is found that the contours of streamwise velocity and turbulent kinetic energy exhibit the effect of initial conditions in the near field. The Reynolds shear stress component dominates in the vertical jet centreline plane, and it increases with a decrease in the nozzle length. The Reynolds shear stress component dominates in the lateral plane, and also exhibit the dependency on initial conditions.

Case Series for Gamma Knife Surgery for Arteriovenous Malformation Associated Intracranial Aneurysms

Abstract Objective The incidence of aneurysms coexisting with arteriovenous malformations (AVMs) ranges between 2.7% and 16.7%. The anatomical relationship between AVM and aneurysm is critical in deciding the best management. Methods Between October 1994 and August 2017, gamma knife surgery (GKS) was performed in six patients with AVMs and associated aneurysms. The patients consisted of four men and two women with a mean age of 37.8 years (range, 18−57 years). The mean follow-up was 34.2 months (range, 13−84 months). The mean maximal dose was 35.9 Gy and the mean margin dose to AVM was 18 Gy. Coil embolization was performed in one of the aneurysms prior to GKS. In our study, GKS was performed in six AVM-associated aneurysms. Of the six aneurysms, four were intranidal and two were pedicular. The mean volume of AVMs was 3.6 cm3 (range, 1.6−6.5 cm3). Results The locations of aneurysms are as follows: four on posterior cerebral artery (PCA), one on posterior inferior cerebellar artery (PICA), and one on middle cerebral artery (MCA). Sublocation sites were MCA M3 above, PCA P3 above, and PICA distal. There were no GKS-related complications. Complete obliteration of AVM and aneurysm was documented in all four patients with intranidal aneurysm-associated AVMs. Both the aneurysm and AVM were completely obliterated in the two patients with proximal pedicular aneurysms. Conclusion GKS is a possible treatment for AVM with associated intranidal or pedicular aneurysms located above P3 or M3, etc., in which there is less turbulent flow and jet flow.

Analysis of Turbulent Boundary Layer Jet Flow Pattern of Laval Nozzle

The rationality of the structure design of the Laval nozzle directly affects its work efficiency. The simulation software is used to simulate and analyze the structure and operating conditions of the Laval nozzle, and to find the relationship between the change of the turbulent boundary layer jet flow pattern. From the simulation results, it can be seen that the outlet diameter under a single variable cannot directly determine the value and range of the jet Mach number, but it produces a huge change in the mixing zone of the turbulent boundary layer, which directly affects the turbulent kinetic energy.

An analytical investigation of transient imperfectly expanded turbulent jet

Supersonic turbulent high-pressure jet flows, which are discharging in low-pressure quiescent ambient, are recognized as imperfectly expanded turbulent jet. Steady-state imperfectly expanded jet flow has been already studied analytically; however, the transient flow has not been thoroughly studied. In the present study, the transient imperfectly expanded jet flow with focus on fuel spray in combustion is investigated analytically employing two-step separation of variables method and Fourier-Bessel expansion. The results are validated using available experimental data. The effects of different parameters such as eddy viscosity and pressure ratio on the behavior of the jet are studied. Results show that increasing the eddy viscosity decreases the velocity magnitude and required time to reach fully developed jet. Increasing the pressure ratio almost linearly increases the required time to reach steady state. The density distribution which affects the combustion performance is reported for different axial and radial positions. In the transient region, tip penetration is obtained and validated with the experimental results in the literature, and the velocity profile at different times is presented. The simplicity and accuracy are key advantages of the developed method compared with the experimental and numerical methods. The analytical method proposed in the present research helps to understand the behavior of jet flows from transient to steady state condition without using expensive and time-consuming numerical or experimental techniques.

Comparison of the mean skin friction with the heat transfer and the flow development in turbulent planar wall-attaching jet flows.

Measurements of the mean skin friction below turbulent planar wall-attaching offset jet flows were performed using oil-film interferometry for jets initially offset (Hs) 0.2 to 1 times the jet height (Hj) from the wall. The results were compared with turbulent flow field measurements and with previous heat transfer measurements. The profiles of the mean skin friction coefficient below the offset or preattachment region were similar when compared relative to the distance to the reattachment location (Xr). The initial recovery length of the mean skin friction downstream of the reattachment location was proportional to the reattachment length similar to the decrease in the wall pressure fluctuations in this region. The longer recovery and initial decay of the mean skin friction coefficient appeared to collapse in terms of (x-Xr)/Hj; as did the initial change in the mean static pressure, the measures of the mean flow development, and the Stanton number downstream of the reattachment point. The decay in the maximum of the turbulent stresses associated with the attached shear layer also collapsed in terms of (x-Xr)/Hj, unlike the fluctuating pressure and initial recovery of the mean skin friction. The change in the location of the minimum and zero in the turbulent shear stress as the flows transitioned to the wall jet region collapse in terms of x-Xr/Hj similar to the longer recovery and initial decay of the mean skin friction. The mean skin friction for flows with Hs of Hj and jet Reynolds numbers (Rej) of 21800 to 54500, varied as Rej-0.2 below the attaching jet, in the reattachment region, and in the downstream wall jet region.