Jet Exit Velocity Research Articles

Control of a Shock/Boundary-Layer Interaction by Using a Pulsed-Plasma Jet Actuator

S HOCK wave/boundary-layer interactions (SWBLIs) are a common feature of supersonic/hypersonic flight, and the unsteadiness of strongly separated interactions can lead to rapid fatigue of structural panels as well as inlet instability and unstart. To mitigate these problems, there is interest in developing techniques for controlling the separatedflowunsteadiness by using both passive and active control techniques. Previous SWBLI control work has focused on reducing the size of the separated flow and/or shifting the frequency of the interaction unsteadiness to a band that does not coincide with the resonant frequency of structural panels. A detailed survey of the variousmeans used for controlling SWBLI until the late 1980s is given in [1]. Recently, plasma-based actuators have been used by researchers for active control of SWBLI, since these actuators have several inherent desirable features such as high bandwidth and no moving parts. For example, previous researchers have used surface-mounted arc discharges [2] and glow discharges with external magnetic fields [3,4] to achieve control of reflected SWBLI. Wang et al. [5] used surface-mounted arc discharges with external magnetic fields and demonstrated the weakening of the separation shock strength in front of a compression ramp. Recently, arc discharges have been employed by Grossman et al. [6] and subsequent researchers [7–11] to generate a pulsed synthetic jet, which they termed a spark jet. The spark-jet design was modified by Narayanaswamy et al. [12] to extend the pulsing frequency to the kilohertz range. They termed the actuator a pulsed-plasma jet since the term spark implies a thermal discharge, which was not the case at the pressures used in their study (and in the current work). Narayanaswamy et al. [12] performed a detailed parametric study of the velocity and temperature characteristics of the pulsed-plasma jets. They reported a jet-exit velocity of about 300 m=s and a bulk gas temperature in the range 600–1000 K for the range of discharge currents tested. The same pulsed-plasma-jet array actuator is used in the present work to control the separation shock of a SWBLI generated by a compression ramp in a Mach 3 flow.

Mini-Jet Controlled Turbulent Round Air Jet

We report an investigation of the active control of a round air jet by multiple radial blowing mini-jets. The Reynolds number based on the jet exit velocity and diameter is 8000. It is found that once the continuous minijets are replaced with pulsed ones, the centerline velocity decay rate K can be greatly increased as the pulsing frequency of mini-jets approaches the natural vortex frequency of the main jet. For example, the K value is amplified by more than 50% with two (or four) pulsed mini-jets blowing, compared with the continuous mini-jets at the same ratio of the mass flow rate of the mini-jets to that of the main jet.

A study of Mach wave radiation using active control

Mach wave radiation is one of the better understood sources of jet noise. However, the exact conditions of its onset are difficult to determine and the literature to date typically explores Mach wave radiation well above its onset conditions. In order to determine the conditions for the onset of Mach wave radiation and to explore its behaviour during onset and beyond, three ideally expanded jets with Mach numbers Mj = 0.9, 1.3 and 1.65 and stagnation temperature ratios ranging over To/T∞ = 1.0–2.5 (acoustic Mach number 0.83–2.10) were used. Data are collected using a far-field microphone array, schlieren imaging and streamwise two-component particle image velocimetry. Using arc filament plasma actuators to force the jet provides an unprecedented tool for detailed examination of Mach wave radiation. The response of the jet to various forcing parameters (combinations of one azimuthal mode m = 0, 1 and 3 and one Strouhal number StDF = 0.09–3.0) is explored. Phase-averaged schlieren images clearly show the onset and evolution of Mach wave radiation in response to both changes in the jet operating conditions and forcing parameters. It is observed that Mach wave radiation is initiated as a coalescing of the near-field hydrodynamic pressure fluctuations in the immediate vicinity of the large-scale structures. As the jet exit velocity increases, the hydrodynamic pressure fluctuations coalesce, first into a curved wavefront, then flatten into the conical wavefronts commonly associated with Mach wave radiation. The results show that the largest and most coherent structures (e.g. forcing with m = 0 and StDF ~ 0.3) produce the strongest Mach wave radiation. Conversely, Mach wave radiation is weakest when the structures are the least coherent (e.g. forcing with m = 3 and StDF > 1.5).

Experimental analysis of transverse jet using various decomposition techniques

This experimental study reports on testing of a single circular jet in crossflow (JICF) for three different jet-to-crossflow velocity ratios: R=0.5, 1.0, and 1.5. Velocity measurements were made using a hot-wire anemometer. Experimentally obtained JICF data were processed using Fourier, wavelet, and proper orthogonal decomposition (POD) techniques. Power spectrum obtained by Fourier analysis shows that flow at the center of nozzle has better laminar property than subsequent stations. The principle observation is that when jet exit velocity increases, the energy of the flow decreases. The center of the nozzle jet acts as an obstacle and damps the effect of the crossflow. Wavelet analysis reveals the dominant frequencies of the flow to 45 Hz for R=0.5 and 85–105 Hz for R=1.0. Energy distribution of the flow was also calculated by using the POD technique; results show that lower velocity ratio carries more energy of the flow.

Study of jets from rectangular nozzles with square grooves

AbstractAn experimental study has been carried out to understand jet flow development from plain and grooved rectangular nozzles of aspect ratio 2:1 using two-component hotwire anemometry. Grooves of square configuration (side 4mm) and length 5mm were introduced in the (i) minor-axis, (ii) major-axis and, (iii) in both minor- and major-axes directions. The equivalent diameter of the plain rectangular nozzle is 37·5mm. Studies were carried out for a nominal jet exit velocity of 20ms−1 and for Reynolds number based on equivalent diameter of 54,000. The introduction of grooves in either plane does not show any influence on the potential-core length but results in faster jet-decay thereafter. It is observed that the grooves when introduced in minor-axis direction inhibit the jet growth in that plane while promoting the jet growth along major-axis plane and hence, delays the phenomena of axis-switching. However when introduced in major-axis direction, the grooves promote jet growth along major-axis plane while inhibiting jet-growth in minor-axis plane. Cross-sectional contours of mean-velocity suggest that the grooves modify the process of overall jet development significantly in the plane in which they are introduced relative to the plain jet and hence, significantly affect the axis-switching location in each case.

Characteristics of Turbulent Three-Dimensional Wall Jets

Three-dimensional turbulent wall jet was investigated using a particle image velocimetry technique. Three Reynolds numbers based on the jet exit velocity and diameter of 5000, 10,000, and 20,000 were studied. Profiles of the mean velocities, turbulence intensities, and Reynolds shear stresses as well as two-point velocity correlations and proper orthogonal decomposition analyses were used to document the salient features of the wall jets. The decay and spread rates are independent of Reynolds numbers in the self-similar region. The estimated values of 1.15, 0.054, and 0.255 for the decay rate, wall-normal spread rate, and lateral spread rate, respectively, are within the range of values reported in the literature. The two-point correlation analysis showed that the inclination of the streamwise velocity correlation contours in the inner layer is 11±3 deg in the wall region, which is similar to those of canonical turbulent boundary layers. The results from the proper orthogonal decomposition indicate that low-order modes contribute more to the turbulence statistics in the self-similar region than in the developing region. The Reynolds shear stresses are the biggest benefactors of the low-order mode contribution while the wall-normal turbulence intensities are the least.

Modelling tunnel jet emissions with LASAT: evaluation study with two Austrian data sets (Ehrentalerbergtunnel and Kaisermuhlentunnel)

Two comprehensive data sets are used to investigate the ability of the Lagrangian particle diffusion model LASAT to simulate the dispersion of plumes emitted from tunnel jets. The data sets differ in traffic volume, tunnel geometry and temporal resolution of the measurement data. In the framework of the measurement campaign at the Ehrentalerbergtunnel in Carinthia, seven trace gas experiments with SF6 were conducted in 2001. Short term averages (30 minutes) of concentrations were measured at 25 air quality stations in the vicinity of the tunnel portal during different meteorological conditions. In general the dispersion of the plume depends on the meteorological conditions (wind, stability) and the modification of the flow by terrain and buildings in the vicinity of the portal. The influence of the exit velocity of the tunnel jet must also be considered as well as the difference between the exhaust temperature and the ambient air temperature to account for buoyancy effects. The temperature increment cannot be provided directly as input parameter to LASAT as in case of the tunnel jet velocity although it is an important parameter. With LASAT, the model user can adjust two empirical input parameters to the tunnel specifications. Relationships between these model parameters and the tunnel parameters are developed in this study. They are based on the data set Ehrentalerbergtunnel and provide reasonable input values for the model user. The simulations with LASAT show that the model is able to reproduce the location and the height of the observed peak concentrations very well. The second data set was generated from January to October 2001 at the Kaisermuhlentunnel in Vienna. Measurements of NO x at four air quality stations near the portal are available. Because of uncertainties in the emission data caused by vehicle counts in only one direction, only long term averages of concentrations are compared for this data set. The functions between tunnel and model parameters derived for the Ehrentalerbergtunnel are also applied to this site. It is shown that LASAT is able to simulate the position of the plume and that the modelled and the measured concentration values do not deviate more than 30 %. This study reveals that LASAT can be applied to tunnel portal emissions. The model simulations for both the Ehrentalerbergtunnel and Kaisermuhlentunnel meet the requirements specified in the Austrian design guideline RVS 9.263 Ventilation Systems - Pollutant burden at portals.

Coherent Structures in Shallow Water Jets

This paper reports an experimental investigation of a round jet discharging horizontally from a vertical wall into an isothermal body of water confined in the vertical direction by a flat wall on the bottom and a free surface on top. Specifically, this paper focuses on the effects of vertical confinement on the characteristics of large vortical structures. The jet exit velocity was 2.5 m/s, and the exit Reynolds number was 22,500. Experiments were performed at water layer depths corresponding to 15, 10, and 5 times the jet exit diameter (9 mm). The large-scale structures were exposed by performing a proper orthogonal decomposition (POD) analysis of the velocity field obtained using a particle image velocimetry system. Measurements were made on vertical and horizontal planes—both containing the axis of the jet. All fields-of-view were positioned at an axial location in the range 10<x/D<80. The number of modes used for the POD reconstruction of the velocity fields was selected to recover ∼40% of the turbulent kinetic energy. A vortex identification algorithm was then employed to quantify the size, circulation, and direction of rotation of the exposed vortices. A statistical analysis of the distribution of number, size, and strength of the identified vortices was carried out to explore the characteristics of the coherent structures. The results clearly reveal the existence of numerous vortical structures of both rotational senses in the jet flow, and their number generally decreases in the axial direction while their size increases. The size of vortices identified in the vertical plane is restricted by the water depth, while they are allowed to increase in size in the horizontal plane. Moreover, the results show a significant decrease in the number of small vortices for the shallowest case in the horizontal plane, with a corresponding increase in the number of large vortices and a significant increase in their size. This behavior was accompanied with an increase in the vortex circulation in the horizontal plane and a reduction in the circulation in the vertical plane. This is indicative of the dominance of the pairing process due to shallowness. Moreover, the balance between the positive and negative vortices in the vertical plane changed because of the formation of negative (clockwise) vortices near the solid wall at downstream locations.

噴出ガス密度と出口速度分布の影響によるプリファード周波数と噴流拡散の関係

噴出ガス密度と出口速度分布の影響によるプリファード周波数と噴流拡散の関係

Effects of Freestream Turbulence on an Axisymmetric CO2 Turbulent Jet Diffusion Field

Effects of freestream turbulence on an axisymmetric CO2 turbulent jet diffusion field is experimentally investigated. The composite probe consisting of two concentration-sensitive I-type hot-wires is used for the simultaneous measurement of instantaneous axial velocity and concentration. To measure grid turbulence without the jet, I- and X-type hot wire probes are used. The fractal square grid is used to generate freestream turbulence. The Taylor Reynolds number is Reλ = 80 - 160. The results are compared with previous measurements in which weak freestream turbulence at Reλ-35 is generated by the classical biplane grid (regular grid). For both flows, the mesh Reynolds number based on the grid mesh size M and freestream speed U0 is 6,000 and the jet Reynolds number based on the diameter d (= 3 mm) of jet exit and relative velocity between freestream and exit velocity, UJ-U0, is 5,000. The results show that profiles of normalized mean velocity and concentration in the CO2 jet do not change at the present Reλ in the freestream flow, while growth of half widths of mean velocity and mean concentration, turbulent intensity, turbulent mass flux and eddy diffusivity in the CO2 jet are significantly increased under the effect of strong freestream turbulence. The eddy diffusivity in the CO2 jet becomes approximately three times of that in the regular grid turbulence.

Experimental and Numerical Study of Laminar Round Jet Flows Along a Wall

In the present study, both experimental and numerical techniques were employed to study three-dimensional laminar wall jet flows. The wall jet was created using a circular pipe of diameter 7×10−3 m and flows into an open water tank. The inlet Reynolds numbers based on the pipe diameter and jet exit velocity were 310 and 800. A particle image velocimetry (PIV) was used to conduct detailed measurements at various streamwise-transverse and streamwise-spanwise planes. The complete nonlinear incompressible Navier–Stokes equation was also solved using a collocated finite volume based in-house computational fluid dynamics (CFD) code. The computation was performed for three inlet Reynolds numbers, namely, 310, 420, and 800. From the PIV measurements and CFD results, velocity profiles and jet half-widths were extracted at selected downstream locations. It was observed that the numerical results are in reasonable agreement with the experimental data. The distributions of the velocities, jet spread rates, and vorticity were used to provide insight into the characteristics of three-dimensional laminar wall jet flows.

Influence of Strouhal number on pulsating methane–air coflow jet diffusion flames

Four periodically time-varying methane–air laminar coflow jet diffusion flames, each forced by pulsating the fuel jet's exit velocity U j sinusoidally with a different modulation frequency w j and with a 50% amplitude variation, have been computed. Combustion of methane has been modeled by using a chemical mechanism with 15 species and 42 reactions, and the solution of the unsteady Navier–Stokes equations has been obtained numerically by using a modified vorticity-velocity formulation in the limit of low Mach number. The effect of w j on temperature and chemistry has been studied in detail. Three different regimes are found depending on the flame's Strouhal number S = aw j /U j , with a denoting the fuel jet radius. For small Strouhal number (S = 0.1), the modulation introduces a perturbation that travels very far downstream, and certain variables oscillate at the frequency imposed by the fuel jet modulation. As the Strouhal number grows, the nondimensional frequency approaches the natural frequency of oscillation of the flickering flame (S ≃ 0.2). A coupling with the pulsation frequency enhances the effect of the imposed modulation and a vigorous pinch-off is observed for S = 0.25 and S = 0.5. Larger values of S confine the oscillation to the jet's near-exit region, and the effects of the pulsation are reduced to small wiggles in the temperature and concentration values. Temperature and species mass fractions change appreciably near the jet centerline, where variations of over 2 % for the temperature and 15 % and 40 % for the CO and OH mass fractions, respectively, are found. Transverse to the jet movement, however, the variations almost disappear at radial distances on the order of the fuel jet radius, indicating a fast damping of the oscillation in the spanwise direction.

Leading-Edge Flame Fluctuations in Lifted Turbulent Flames

Studies are presented that examine the fluctuations in liftoff height of lifted methane flames in the presence of air coflow. At a certain jet exit velocity, a flame will lift from the burner exit and stabilize at some downstream position. The partially-premixed flame front of the lifted flame oscillates in the axial direction, with the fluctuations becoming greater in flames stabilized further downstream. These fluctuations are also observed in flames where blowout is imminent. This work investigates the role of fuel velocity and air co-flow on flame fluctuations in both stable and unstable regimes. The results of video imaging of a lifted methane-air diffusion flame are presented and discussed. Images are used to ascertain the changes in the reaction zone that influence these fluctuations and relate the movement to blowout.

Near-Field Acoustic Characteristics of a Turbulent Axisymmetric Pulsed Jet

Near-Field Acoustic Characteristics of a Turbulent Axisymmetric Pulsed Jet

Dynamics of impinging nanoscale jets

We use molecular dynamics simulations to describe the collision of two opposed liquid argon nanojets and examine the subsequent stagnation flow. The streamwise and transverse velocity profiles in the proximity of stagnation point are in agreement with macroscale continuum relations u∗ = αx∗, v∗ = − αy∗ for impinging laminar streams. Jet exit velocities are highest for non-wetting surfaces and decrease as solid–liquid interactions are enhanced. The work done to produce the nanojet dissipates at jet exits, increasing the local temperature. The fluid temperature is lowered through cooling as jets exit through the nonperiodic boundaries. In comparison, water nanojets have higher exit velocities with relatively lower pressure drops across the reservoir and cavity.

マイクロ拡散火炎の消炎限界

This paper presents a theory to predict the extinction limit of jet diffusion microflames. The constant-density approximation is introduced to separate flow field from combustion reaction; flow field then depends only on the jet Reynolds number, Re. An exact solution of the Navier-Stokes equation is adopted; the predicted flame shape based on the exact solution well agrees with experimental observation. An Arrhenius-type one-step global reaction is considered, adding the Damköhler number, Da, as an additional parameter to the Reynolds number. The extinction limit is obtained using an activation-energy asymptotics technique. The obtained extinction criterion, Da Re2 ≈ constant, reproduces experimental results that the jet exit velocity under the extinction condition is proportional to d-2, where d is the burner diameter.

Experimental study on a jet issuing from an elliptic nozzle with cylindrical tabs

An elliptic jet with mixing tabs was investigated using the digital particle-image-velocimetry and two-component hotwire anemometry. Two tabs of cylindrical configuration were placed on the minor-axis sides of a 2:1 elliptic nozzle. Because of their sideways pressure-relieving effect, a cylindrical tab relieves the constraint on the flow relative to a thin tab with flat surfaces which can have important implications on tab drag reduction but with similar mixing characteristics. Measurements were made for a jet exit velocity of 20 m s−1 and for Reynolds number based on the nozzle equivalent diameter (De ) of 5.08 × 104. Furthermore, the effect of tab height (h/d= 1.0 and 1.67) is also investigated. The study shows the evolution of a mushroom structure behind each tab that distorts the jet flow into a two-finger structure. The tab wake mainly comprises of base vortices and tip vortices that are enveloped from the sides by spanwise vortical structures. While the strong upwash from the base vortices causes an...

On the Stability of a Turbulent Non-Premixed Methane Flame

This paper presents an experimental qualitative assessment of the stability of a turbulent, non-premixed methane flame. The burner consists of a central fuel nozzle surrounded by an annulus of co-airflow with varying swirl strength. Two distinct nozzle geometries having similar exit cross-sectional areas but different internal/orifice geometry—a rectangular and a contracted circular nozzle—were tested. They are referred to in this paper as RN and CCN, respectively. The main focus of the present study was on determining the flame liftoff and blowout velocities as well as the liftoff height and flame length, all of which can be used as indicators of the stability of non-premixed methane flame. The experimental data revealed that the blowout velocity of the RN nozzle's flame is remarkably higher than that of the CCN nozzle, and the liftoff velocity of the CCN is only slightly higher than that of the RN nozzle for identical swirl strength. In addition, the flame length of the RN nozzle is overall shorter than that of the CCN nozzle for identical test conditions, and the liftoff height of the CCN flame is higher than that of the RN flame. LDV velocity measurements were performed to determine the reacting flow overall dynamic along the centerline plane for typical jet and co-flow exit velocities. These results aimed at helping to explain the difference in flame stability between the two different nozzles' geometries in conjunction with the co-flow swirl strength.

Control of flames by tangential jet actuators in oxy-fuel burners

The active control of oxy -fuel flames from burners with separated jets is investigated. The control system consists of four small jet actuators, placed tangential to the exit of the main jets to generate a swirling flow. These actuators are able to modify the flow structure and to act on mixing between the reactants and consequently on the flame behavior. The burner (25 kW) is composed of separated jets, one jet of natural gas and one or two jets of pure oxygen. Experiments are conducted with three burner configurations, according to the number of jets, the jet exit velocities, and the separation distance between the jets. OH chemiluminescence measurements, particle image velocimetry, and measurements of NO x emissions are used to characterize the flow and the flame structure. Results show that the small jet actuators have a significant influence on the behavior of jets and the flame characteristics, particularly in the stabilization zone. It is shown that the control leads to a decrease in lift-off heights and to better stability of the flame. The use of jet actuators induces high jet spreading and an increase in turbulence intensity, which improves the mixing between the reactants and the surrounding fluid. Pollutant measurements show important results in terms of NO x reductions (up to 60%), in particular for low swirl intensity. The burner parameters, such as the number of jets and the spacing between the jets, also impact the flame behavior and NO x formation.

An experimental study of an axisymmetric turbulent pulsed air jet

An experimental study is carried out to elucidate the structure of a high Reynolds number (~105) turbulent pulsed jet. Particle image velocimetry measurements showed that the near flow field is dominated by a series of vortex rings with jet-like flows in between. The data show that the vortex rings convect at nearly constant speed of 0.6Uj(Uj: mean jet exit velocity) and the spacing between the rings assumes a value of about 0.6/St(St: Strouhal number=fd/Uj, wherefis the pulsing frequency anddis the nozzle exit diameter). With increasing Strouhal number, the rings are closely spaced and the flow tends to assume a steady jet character at five diameters downstream of the nozzle exit. At lower Strouhal numbers there is a distinct region of jet flow in between the rings. Many of the global characteristics, entrainment, mass and momentum flux are essentially determined by the strength and spacing of the rings which, in turn, depend onSt. We show that the increase in momentum is due to both increased momentum flux and overpressure at the exit in accordance with Krueger & Gharib (AIAA J., vol. 43 (4), 2005, p. 792). This increase in momentum comes at the expense of higher energy required to produce the jet. We also present results of organized and random components of the fluctuations and production of the random turbulence in a pulsed jet. The two regions of dominant turbulence production are identified with the ring and the trailing jet shear layers.