Jet Flow Field Research Articles

Comparative study of spectral-element and finite-volume solvers for direct numerical simulation of synthetic jets

Direct Numerical Simulations of synthetic jet flow fields issuing in a quiescent environment, performed by means of two extensively used open-source computational fluid dynamics (CFD) codes, Nek5000 and OpenFOAM solver pimpleFoam, are reported and compared. While the former employs a high-order spectral-element method, the latter is based on finite-volume, lower-order schemes. The present paper specifically refers to SJ fields produced by slotted orifice piezo-driven actuators. The dimensionless flow parameters adopted to perform the simulations basically match the experimental data of Yao et al. (2006), whilst a reasonably simplified geometry of the actuator is adopted. Results are generally in good agreement with the experimental data for both codes, in terms of first- and second-order moments of the long-time and phase-averaged velocity field, as well as of global engineering quantities. The performances of the two codes are compared thoroughly, with regard to accuracy and computer load as well. Overall, when high-fidelity of simulations is required for the problem under study, Nek5000 is found to be computationally more efficient.

Properties of a sweeping jet emitted from a fluidic oscillator

This experimental study investigates the flow field and properties of a sweeping jet emitted from a fluidic oscillator into a quiescent environment. The aspect ratio of the outlet throat is 1. Stereoscopic particle image velocimetry is employed to measure the velocity field plane-by-plane. Simultaneously acquired pressure measurements provide a reference for phase correlating the individual planes yielding three-dimensional, time-resolved velocity information. Lagrangian and Eulerian visualization techniques illustrate the phase-averaged flow field. Circular head vortices, similar to the starting vortex of a steady jet, are formed repetitively when the jet is at its maximum deflection. The quantitative jet properties are determined from instantaneous velocity data using a cylindrical coordinate system that takes into account the changing deflection angle of the jet. The jet properties vary throughout the oscillation cycle. The maximum jet velocity decays much faster than that of a comparable steady jet indicating a higher momentum transfer to the environment. The entrainment rate of the spatially oscillating jet is larger than for a steady jet by a factor of 4. Most of the mass flow is entrained from the direction normal to the oscillation plane, which is accompanied by a significant increase in jet depth compared to a steady jet. The high entrainment rate results from the enlarged contact area between jet and ambient fluid due to the spatial oscillation. The jet’s total force exceeds that of an idealized steady jet by up to 30 %. The results are independent of the investigated oscillation frequencies in the range from 5 to 20 Hz.

Flow field and acoustics characteristics of elliptical jet

PurposeThe purpose of this paper is to present the results of mixing promotion and screech frequency of controlled elliptical supersonic jet.Design/methodology/approachFlow field characteristics of low-aspect-ratio elliptical jets are examined at over-expanded, under-expanded and correctly expanded conditions. The tabs are placed at elliptical jet exit along the major and minor axes.FindingsThe results show that the mixing done by the minor axis is superior to the tabs along major axis. At all pressure ratios, the content of jet noise and the frequency are high for the tabs along the major axis because of increase in the amplitude of screech frequency. Further the tabs along minor axis show a dominance of large-scale vertical structures. In under-expanded conditions, the shock cell shows the rapid change because of the presence of tabs. The tabs along minor axis are making the shock weaker, hence no evidence of axis switching.Practical implicationsTo achieve the greater performance of jet, the authors need to reduce the potential core length of the issuing jet. This can be achieved by implementing different types of tabs at the exit of the nozzle.Originality/valueThe present paper represents the flow of controlled jet using inverted triangular tabs. By achieving the controlled jet flow, the performance of propulsion systems can be improved. This can be used in systems such as combustion chamber, missile’s noise reduction and thrust vector control.

Long-Term Inertial Navigation Aided by Dynamics of Flow Field Features

A current-aided inertial navigation framework is proposed for small autonomous underwater vehicles in long-duration operations ( $>$ 1 h), where neither frequent surfacing nor consistent bottom tracking is available. We instantiate this concept through mid-depth underwater navigation. This strategy mitigates dead-reckoning uncertainty of a traditional inertial navigation system by comparing the estimate of local ambient flow velocity with preloaded ocean current maps. The proposed navigation system is implemented through a marginalized particle filter where the vehicle's states are sequentially tracked along with sensor bias and local turbulence that is not resolved by general flow prediction. The performance of the proposed approach is first analyzed through Monte Carlo simulations in two artificial background flow fields, resembling real-world ocean circulation patterns, superposed with smaller scale turbulent components with Kolmogorov energy spectrum. The current-aided navigation scheme significantly improves the dead-reckoning performance of the vehicle even when unresolved small-scale flow perturbations are present. For a 6-h navigation with an automotive-grade inertial navigation system, the current-aided navigation scheme results in positioning estimates with under 3% uncertainty per distance traveled (UDT) in a turbulent double-gyre flow field, and under 7.3% UDT in a turbulent meandering jet flow field. Further evaluation with field test data and actual ocean simulation analysis demonstrates consistent performance for a 6-h mission, positioning result with under 25% UDT for a 24-h navigation when provided direct heading measurements, and terminal positioning estimate with 16% UDT at the cost of increased uncertainty at an early stage of the navigation.

Effect of a Chevron Nozzle on Noise Radiation from a Compressible Jet

The effect of a chevron on a radiated (acoustic) component of near-field pressure fluctuations in a high-speed compressible jet is experimentally investigated for a nozzle exit Mach number of 0.8. The main focus in the present experimental study is to examine the effect of a chevron on noise sources, their location in the jet flowfield, and the noise radiation pattern in the far-field through simultaneous measurement of near-field and far-field pressure fluctuations using a cross-correlation technique. The formation of lobes in the mixing layer close to the chevron nozzle exit alters the near-field and far-field noise characteristics as compared to the base nozzle. The overall sound pressure level closer to the chevron nozzle exit increases, whereas downstream of the potential core, it reduces in comparison with the base nozzle. Furthermore, the position of the near-field/far-field cross-correlation coefficient peak suggests that the interaction of chevrons with the jet flow moves the dominant noise source upstream and closer to the nozzle exit. The chevron nozzle disintegrates large-scale flow structures, thereby reducing the noise levels radiating at a lower polar angle in the far-field. The near-field/far-field cross-correlations suggest that an acoustic component of the near-field pressure fluctuations, propagating from the jet to the higher polar angles in the far-field, is generated by fine-scale turbulent flow structures in the region close to the nozzle exit, and the chevron nozzle is found to strongly support this mechanism. An increase in the noise levels closer to the nozzle exit in the near-field and at a higher polar angle in the far-field are due to high-frequency noise sources generated by the chevron nozzle.

Effect of shrouding Mach number and ambient temperature on the flow field of coherent jet with shrouding Laval nozzle structure

ABSTRACTThe coherent jet technology was widely used to improve the stirring effect of molten bath in steelmaking field, and the key to this technology was to form a low-density zone around the main oxygen jet by a high-temperature shrouding flame. With this revelation, a shrouding nozzle was processed to a Laval nozzle structure fitted with a loop arrangement for increasing the velocity of shrouding jet. For further increasing the area of the low-density zone, the preheating method was also adopted in this new coherent lance structure. In this paper, the effect of Mach number of the shrouding nozzle on the flow field of the coherent jet was investigated at room and high ambient temperature using numerical simulation and experimental studies. The result represented the simulation model used in this research showed good agreement with the experimental data at the texted conditions. Although the shock wave formed by the shrouding jet removed more kinetic energy form the main oxygen jet, the impaction ability of the coherent jet was much bigger than that of conventional supersonic jet, and this phenomenon would be further strengthen if ambient temperature and Mach number of the shrouding nozzle increases.

Measurements versus Numerical Simulations for Slotted Synthetic Jet Actuator

In many studies concerning synthetic jet flow fields the analysis is usually restricted to simple configurations, such as a single diaphragm oscillating in a cylindrical cavity, which is linked to the external environment with only one orifice/slot. Nonetheless, in many applications the requirement of small sizes and weights leads to many implementation issues, such as asymmetric actuator geometries, presence of several slots and diaphragms and irregular cavity shapes. Therefore, the design of a synthetic jet actuator for a specific flow control problem requires a dedicated study in order to characterize its behavior even in quiescent conditions. The aim of this work is to investigate the behavior of a novel synthetic jet actuator, composed of three independent diaphragms, acting on a single cavity, and linked to the external environment through four slots per diaphragm. The device has been studied in quiescent conditions, both numerically and experimentally. The experimental investigation has been carried out by means of hot-wire measurements. In particular, the distribution of the phase-averaged streamwise velocity along the slot spanwise direction has been detected near the slot exit plane. From the computational side, incompressible direct numerical simulations have been carried out using the open-source OpenFOAM code. The diaphragm motion is mimicked by a inhomogeneous inlet boundary condition, whose amplitude is chosen to match the experimental velocity at the exit plane. A fair agreement between the numerical and the experimental results is achieved for both the velocity field at the slot exit and the main non-dimensional parameters of the synthetic jet. After the validation, the numerical results are finally processed, to obtain information about the vortex motion in the external environment.

Effect of a Lateral Jet on the Turbulent Flow Characteristics of an Open Channel Flow with Rigid Vegetation

Aquatic vegetation can purify polluted bodies of water and beautify the environment, as well as alter the structure of water flow and affect the migration and diffusion of pollutants in bodies of water. Vegetation can significantly change the original water flow, especially in cases in which aquatic vegetation interacts with a jet. Characteristics of jet flow open channels without vegetation have been studied, but research on the characteristics of open-channel flows under the action of lateral jets and in the presence of vegetation are rare. High-frequency particle image velocimetry (PIV) was used to measure a lateral jet in water with rigid vegetation and our results were compared to the lateral jet flow field in water without vegetation. The results show that the vegetation arrangement and vegetation resistance cause significant changes. The presence of vegetation increased the surface velocity of the water, and the flow velocity decreased in the interior area of the vegetation and near the bottom of the water tank. The changes in flow velocity with changes in water depth displayed “S” type and anti-“S” type distributions, and the flow velocity of free layer was approximately logarithmic. Due to vegetation resistance and jet pressure differences, the lateral jet trajectory in water with vegetation was more likely to bend than in water without vegetation. The turbulence intensity and Reynolds stress had different distributions for water with and without vegetation. At the top of the vegetation and near the water surface, turbulent mixing of the water flow was strong, the flow velocity gradient was large and the turbulence intensity and the Reynolds stress reached their maxima. The effects of a lateral jet on an open-channel flow were compared for different vegetation conditions, revealing that a rhombic vegetation arrangement has a stronger deceleration effect than other arrangements. The theoretical results can be applied to wastewater discharge into vegetation channels.

Sensitivity of acoustic predictions on the mesh size of large-eddy simulation of supersonic jet

Intense noise produced by the supersonic jet plume from a rocket can cause damage to its structural system and even a failure during launch. In order to predict the acoustic loading by rocket jet noise, large-eddy simulation (LES) is commonly employed for analysis of the jet flow field that is subsequently used as a source condition for predicting the acoustic field. Although desirable from the accuracy standpoint, LES of a supersonic jet is often burdened by an overwhelming computational cost in both runtime and memory. In this talk, we discuss the sensitivity of acoustic predictions on the spatial resolution of LES, and propose a guideline for reducing the mesh requirement. . Here, the flow field of a supersonic jet is calculated using a hybrid RANS/LES scheme on meshes of different sizes, and is fed into the Helmholtz-Kirchhoff integral for prediction of the acoustic farfield. Changes in overall sound pressure level (OASPL) and directivity of the Mach radiation are monitored as the LES mesh varies in size. The study suggests that (a) the accuracy of the acoustic prediction is not much affected by the use of a coarser mesh to a certain extent and (b) the resulting improvement in computation speed and economy thus outweighs the loss of details in LES, given the prediction of acoustic loading as the ultimate goal. (This work was conducted at High-Speed Vehicle Research Center of KAIST with the support of the Defense Acquisition Program Administration and the Agency for Defense Development under Contract UD170018CD.)Intense noise produced by the supersonic jet plume from a rocket can cause damage to its structural system and even a failure during launch. In order to predict the acoustic loading by rocket jet noise, large-eddy simulation (LES) is commonly employed for analysis of the jet flow field that is subsequently used as a source condition for predicting the acoustic field. Although desirable from the accuracy standpoint, LES of a supersonic jet is often burdened by an overwhelming computational cost in both runtime and memory. In this talk, we discuss the sensitivity of acoustic predictions on the spatial resolution of LES, and propose a guideline for reducing the mesh requirement. . Here, the flow field of a supersonic jet is calculated using a hybrid RANS/LES scheme on meshes of different sizes, and is fed into the Helmholtz-Kirchhoff integral for prediction of the acoustic farfield. Changes in overall sound pressure level (OASPL) and directivity of the Mach radiation are monitored as the LES mesh varies in s...

Prediction of fluid flow and acoustic field of a supersonic jet using vorticity confinement.

In this study, the numerical simulation of the fluid flow and acoustic field of a supersonic jet is performed by using high-order discretization and the vorticity confinement (VC) method on coarse grids. The three-dimensional Navier-Stokes equations are considered in the generalized curvilinear coordinate system and the high-order compact finite-difference scheme is applied for the space discretization, and the time integration is performed by the fourth-order Runge-Kutta scheme. A low-pass high-order filter is applied to stabilize the numerical solution. The non-reflecting boundary conditions are adopted for all the free boundaries, and the Kirchhoff surface integration is utilized to obtain the far-field sound pressure levels in a number of observer locations. Comparisons of the jet mean flow and jet aeroacoustics results with the other numerical and experimental data at similar flow conditions are made and show a reasonable agreement. The study shows that the proposed solution methodology based on the high-order compact finite-difference scheme in conjunction with the VC method can reasonably predict the near-field flow and the far-field noise of high Reynolds number jets with a fairly coarser grid than that used in the large eddy simulations and, thus, the computational cost can be significantly decreased.

Effect of cross wire tab orientation on twin jet mixing characteristics

The effect of the relative azimuthal orientation of the cross wires, placed at the exit of two identical converging-diverging nozzles, on the flow field of the twin jet is studied experimentally. The nozzle is designed for Mach number of 1.8, with the inter-nozzle spacing of 1.5 and 2.25 times the nozzle exit diameter. Pitot pressure surveys are conducted along the centerline and the cross-stream direction of the twin jet. Cross wire is shown to be successful in reducing the core length and enhancing the mixing characteristics at all the NPRs studied. The combinations of the horizontal and vertical orientation of the cross wire are found to significantly influence the development of the jet flow field for twin jets. Wires, both mounted vertically, are found to be superior in enhancing the mixing characteristics and reducing the core length. The shadowgraph images reveal that the wire bifurcates the jet and displaces the fluid away from the centerline. Strong shocks are formed near the cross wire, but the pressure losses and enhanced mixing of the jet fluid result in smaller supersonic core length with less number of shock-cells.

Importance of the nozzle-exit boundary-layer state in subsonic turbulent jets

To investigate the effects of the nozzle-exit conditions on jet flow and sound fields, large-eddy simulations of an isothermal Mach 0.9 jet issued from a convergent-straight nozzle are performed at a diameter-based Reynolds number of $1\times 10^{6}$. The simulations feature near-wall adaptive mesh refinement, synthetic turbulence and wall modelling inside the nozzle. This leads to fully turbulent nozzle-exit boundary layers and results in significant improvements for the flow field and sound predictions compared with those obtained from the typical approach based on laminar flow in the nozzle. The far-field pressure spectra for the turbulent jet match companion experimental measurements, which use a boundary-layer trip to ensure a turbulent nozzle-exit boundary layer to within 0.5 dB for all relevant angles and frequencies. By contrast, the initially laminar jet results in greater high-frequency noise. For both initially laminar and turbulent jets, decomposition of the radiated noise into azimuthal Fourier modes is performed, and the results show similar azimuthal characteristics for the two jets. The axisymmetric mode is the dominant source of sound at the peak radiation angles and frequencies. The first three azimuthal modes recover more than 97 % of the total acoustic energy at these angles and more than 65 % (i.e. error less than 2 dB) for all angles. For the main azimuthal modes, linear stability analysis of the near-nozzle mean-velocity profiles is conducted in both jets. The analysis suggests that the differences in radiated noise between the initially laminar and turbulent jets are related to the differences in growth rate of the Kelvin–Helmholtz mode in the near-nozzle region.

Downhole Transient Flow Field and Heat Transfer Characteristics During Drilling With Liquid Nitrogen Jet

As a novel jet technology, liquid nitrogen jet (LNJ) is expected to effectively break rocks and further provide a high-efficiency method for drilling, especially geothermal drilling. Using this technology, rocks can be broken down by the coupled effects of cryogenic cooling and jet impingement. In this study, transient downhole jet flow field and heat transfer during drilling with LNJ were simulated. Then, the distributions of temperature (including LNJ and ambient rock), velocity, and pressure at different times were analyzed. Finally, the effects of the parameters on jet impingement and rock cooling performance were discussed. Results indicated that cryogenic LNJ could be efficiently generated in the downhole region. The temperature of the rock surface remarkably decreased as the LNJ reached the bottomhole. The high-speed LNJ caused axial impingement and radial shear effects on the bottomhole rock. The rock cooling performance caused by the LNJ was influenced by the initial rock temperature. With the increase of the initial rock temperature, the drop amplitude of the rock temperature also increased. The impingement capability of the LNJ was improved by increasing the nozzle diameter and the nozzle pressure drop. With the increase of standoff distance, the wall pressure and the radial velocity of the bottomhole decreased while increasing the impingement scope. The confining pressure hardly influenced the rock cooling performance and jet impingement capability, thereby indicating that LNJ could work even at high confining pressure conditions.

Numerical investigation on vorticity of rectangular turbulent jet enhanced by plasma

A three-dimensional plasma excitation model based on the phenomenological model was proposed and used as an excitation source of the jet. The large - eddy simulation method was used to simulate the flow field of a rectangular turbulent jet. The position deviation of the large eddy structures in the simulated flow field is within 8% compared with the experimental data. The model can accurately reflect the vortices distribution of the flow field and has a good accuracy. The variation trend of jet vortices is surfaces and vortices field is studied under different excitation voltages, which proves that the stream wise vortex is the key to the influence of the plasma exciter on the jet. The maximum value of velocity and vortices in the flow field enlarge with the voltage increasing while the turbulent position of the jet advances significantly, and the vortex shedding strength increases. The jet mixing characteristics are significantly enhanced.

Self-similar behavior of turbulent impinging jet based upon outer scaling and dynamics of secondary peak in heat transfer

The flow field of an impinging wall jet created by the impingement of a turbulent axisymmetric jet normal to a flat surface was characterized by the particle image velocimetry technique. Experimental data is analyzed to explore two basic features of the impinging jet: first, to bring out unexplored aspects which are responsible for secondary peak in heat transfer distribution and to understand the reason for discrepancies in the existing observations about the peaks in heat transfer. Second, to analyze the self-similarity of radial wall jet based upon outer scaling. Measurements of the cross-wise mean velocity and turbulence statistics were initially used to explain the dynamics of secondary peak. Our results show that flow separation/reattachment occurs along the surface. At the reattachment location an intense increase in crosswise velocity, normal stresses and higher mixing are evident, which would lead to a peak in heat transfer. The separation/reattachment location is further found to depend upon the specific stage of vortical structure and is a function of surface spacing. The location of maximum value of mean cross-wise velocity and normal stress is located at the intersection of inner and outer shear layers. While the maximum Reynolds shear stress location is shifted to the outer shear layer and is located between the location of maximum velocity and jet half-width. The impinging jet exhibits a self-similar behavior as evident by the collapse of mean velocity and turbulent stress profiles when scaled with appropriate parameters. The outer scaling is able to bring out the self-similar profile of mean velocity and normal stresses. However, the shear stress profile does not show the self-similar behavior by the use of outer scaling. Data in the inner shear layer show small scatter compared to the outer shear layer especially close to the surface. The results show that the outer scales are not suitable to scale the data in the inner layer. It is also observed that the presence of vortical structure in wall jet delays attainment of self-similarity and the location beyond which self-similarity is observed is a function of surface spacing. These results aid in interpretation of heat transfer behavior from a flat surface and provide comprehensive benchmark data for theoretical modeling of the flow.

Effect of Rocket Engine Layouts on Jet Flowfield Inside a Launch Pad

A numerical analysis is conducted to understand the flowfield inside a launch pad affected by the layouts of clustered rocket engines for the first stage. The Japanese H3 launch vehicle with typical rocket engines and a launch pad is selected for analysis. In a configuration having three clustered liquid-propellant rocket engines at a core stage, the exhaust jets from adjacent engines are found to interact with each other after impinging on the flame deflector, resulting in side jet reversal in the upstream direction along the flame deflector. An investigation of the layouts of solid rocket boosters around the core stage reveals that exhaust jets from the solid rocket boosters interfere with those from liquid-propellant rocket engines spreading on the flame deflector. This interference also causes reversed jets. Guidelines for suppressing such jet reversal are proposed for designing a launch vehicle and a launch pad for clustered rocket engines at the first stage.

Jet Flow Control Using Semi-Circular Corrugated Tab

Abstract This investigation aims to present the jet mixing characteristics and thrust variations of the subsonic jet employed with plain triangular tab and semi-circular corrugated tab by numerical simulation. A triangular tab with semi-circular corrugations is used in this regard at the exit plane of a convergent nozzle, to study the behavior of the jet and its structure. The near jet flow field is studied for different Mach numbers of 0.6, 0.8 and 1, and the comparisons were done for the jet employed with plain triangular tab. To validate the numerical results, experimental validation is carried out for 0.6 Mach jet. The thrust and the potential core length of any jet depend mainly on the percentage of blockage ratio. Since the relationship between the thrust and blockage ratio is such that, the blockage ratio increases, the thrust and the potential core length decreases and vice-versa. The blockage ratio is kept 8.27 % for both the corrugated and plain triangular tabs. From the results, it is found that the Potential core length of the free jet is cut down to 66 % by the jet employed with plain triangular tab, whereas it is 64.5 % for the corrugated tab enabled jet. It is also concluded that the corrugated tab enhances the thrust by 4.43 % for the same blockage ratio and increases potential core length by 3.33 % when compared with the plain triangular tab. This increase in thrust is there by an added advantage of this investigation.

Analysis of the stress wave effect during coal breakage by a high-pressure abrasive air jet

In view of the defects of borehole collapse, inhibition of gas desorption and migration of gas existing in hydraulic fracturing and other hydraulic permeability–increasing measures for soft coal seams with low-permeability technology is proposed for coal breakage by a high-pressure abrasive gas jet for relieving pressure and increasing permeability. The comparative analysis of gas jet flow field structure between convergent nozzle and Laval nozzle has been given by numerical simulation. For Laval nozzle, the expansion wave and compression wave alternate and move forward steadily in gas jet and vanish when potential core length reaches maximum. So, the Laval nozzle can form more stable flow filed structure of gas jet and avoid shock wave in gas jet. Furthermore, a high-speed camera is adopted to analyze the jet structure and verify the conclusion of numerical simulation. Based on thermodynamic theory, this article calculates and analyzes the critical local sound velocity and pressure generated from the stress wave during the process of coal breakage by the gas jet. Furthermore, experimental coal breakage by a high-pressure abrasive gas jet is carried out. The high-pressure abrasive gas jet impacts the coal body as a quasi-static load and a dynamic load and forms corrosion pits on the surface of the coal body. Penetrating cracks are formed within the coal in the pattern of the loaded stress wave which leads to coal breakage. The effects of porosity and permeability on the propagation of the stress wave in coal are analyzed by establishing the dispersion equation for the spread of the stress wave in coal. The results show that porosity has a significant effect on wave velocity and that the attenuation of the stress wave is intensified with an increase in porosity. Moreover, the stress wave attenuation is more obvious at high frequency. The effect of permeability on the wave velocity is not significant at low frequencies. In contrast, at high frequency and relatively low permeability, the wave velocity increases with the permeability, and the attenuation of the wave velocity initially increases and then decreases. When the permeability is greater than 10−11 m2, the wave velocity is not affected by the permeability. However, the stress wave is not attenuated.

Grid sensitivity of flow field and noise of high-Reynolds-number jets computed by large-eddy simulation

Three isothermal round jets at a Mach number of 0.9 and a diameter-based Reynolds number of 105 are computed by large-eddy simulation using four different meshes in order to investigate the grid sensitivity of the jet flow field and noise. The jets correspond to two initially fully laminar jets and one initially strongly disturbed jet considered in previous numerical studies. At the exit of a pipe nozzle of radius r0, they exhibit laminar boundary-layer mean-velocity profiles of thickness [Formula: see text] and [Formula: see text], respectively. For the third jet, a peak turbulence intensity close to 9% is also imposed by forcing the boundary layer in the nozzle. The grids contain up to one billion points, and, compared to the grids used in previous simulations, they are finer in the axial direction downstream of the nozzle and in the radial direction on the jet axis and in the outer region of the mixing layers. The main flow field and noise characteristics given by the simulations, including the mixing-layer thickness, the centerline mean velocity, the turbulence intensities on the nozzle lip line and the jet axis, spectra of velocity and far-field pressure obtained from the jet near field by solving the isentropic linearized Euler equations, are presented. With respect to those from previous studies, the results are very similar for the initially laminar jet with thick boundary layers, but they differ significantly for the initially laminar jet with thin boundary layers and for the initially disturbed jet. For the latter two jets, using a finer grid leads to a faster flow development, to higher turbulence intensities in the shear layers and at the end of the potential core, to stronger large-scale structures, and to the generation of more low-frequency noise. Moreover, very small mesh spacings appear to be necessary all along the jet mixing layers, and in particular during their early stages of growth, to properly capture the formation and dynamics of the flow coherent structures and thus obtain results in good agreement with measurements available for high-Reynolds-number jets.

Transient Flow Patterns of Multiple Plasma Synthetic Jets Under Different Ambient Pressures

The plasma synthetic jet is a new active flow control technique, which has great potentials for supersonic flow control A plasma synthetic jet actuator (PSJA) for supersonic flow control which operates under low ambient pressure is designed In order to explore the transient jet flowfield, high-speed Schlieren and electronic measurement systems are utilized to test the single-shot operating characteristics of the actuator under two ambient pressure conditions. The evolution of the jet boundary, which depicts the transient jet flowfield and can be used to estimate the flow control capability, is captured. It is found that the ambient pressure is the primary reason to affect the arc energy deposition, which directly determines the velocity of blast wave and jet front. In addition, the flow patterns of PSJA under various ambient pressures show some similarities. The jet evolution can be divided into three stages, i.e. pressure dominant stage, the inertia dominant stage and vortex ring dominant stage. During the pressure dominant stage, the PSJA could be treated as jet. For the inertia dominant stage, the Froude number decreases from 2263.82 to 21.73, indicating the inertia effect gets weakened with an intensified buoyancy effect, but the inertia effect still holds the dominant position, and this stage may be considered as the end mark of the injection process. As for the vortex ring dominant stage, large scale vortex ring becomes significant and the jet front velocity is very low but with apparent fluctuation.