Steady Jet Research Articles

Image-Based Analysis of Interactions Between a Steady Jet and an Impulsively Started Jet

Ignition overpressure (IOP) is a principal dynamic load experienced by a launch vehicle during liftoff. The IOP arises due to the sudden ignition of the solid rocket boosters (SRBs) of a launch vehicle. The present work attempts to simulate the ignition of an SRB with an impulsively started supersonic jet generated using a quick-open valve. High-speed schlieren flow visualization is used to document the evolution of an impulsively started jet flowfield that included the characteristic precursor shock wave (that simulates the blast wave causing the IOP) and the following vortex ring. The dynamic interactions between the impulsively started supersonic jet and an established steady supersonic jet from another nozzle () in close proximity are also investigated. Both the precursor shock wave and the vortex ring interact strongly with the established steady-jet column as they proceed downstream from the nozzle exit, thereby introducing new sources of acoustic waves. When a generic jet blast deflector is introduced downstream of the nozzles, the precursor shock wave reflected from it, propagating upstream toward the nozzle exit. The reflected shock wave is recognized as the source of IOP experienced by the launch vehicle. The results show that the quick-open valve presents a unique way to generate impulsively started jets for understanding transients associated with the ignition of the SRB leading to the IOP, without the complexity associated with the use of open-ended shock tubes for carrying out such simulations.

Dynamic wake of a square cylinder controlled with steady jet positioned at the rear stagnation point

We experimentally investigated the dynamic wake evolution of flow around a square cylinder controlled with steady jet ejecting from the rear stagnation point in a wind tunnel at Reynolds number of 1.33 × 104. For diversified jet momentum coefficients Cm, modal analyses, dynamic vortex shedding process, time-averaged turbulence kinetic energy (TKE) and Reynolds shear stress (RSS) were obtained from high-speed Particle Image Velocimetry (PIV) measurements, proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) analyses. As Cm grew, it was found that TKE distribution was restrained and RSS was shifted towards the outlet of the steady jet. Variations of dominant frequencies in POD and DMD modes were observed. The pattern of vortex shedding from the shear layer were transferred from antisymmetric to a symmetric one. The vortex sheets were also observed to be elongated downstream in the wake. Experimental results indicated that the dynamic wake of the square cylinder could be controlled significantly by the steady jet from the rear stagnation point.

On numerical investigation of heat transfer augmentation of flat target surface under impingement of steady air jet for varying heat flux boundary condition

The prediction of flow pattern for the proposed range of Reynolds number and nozzle–target spacing is carried out using SST + Gamma–Theta turbulence model. The simulations of the flow field in a computational domain are carried out using CFX as a base solver with $$10^{-6}$$ being the converging criteria. The present work aims to determine the local Nusselt number magnitude for varying heat flux input boundary conditions. Not only this, the impinging Reynolds numbers and nozzle–target spacing are also varied to record sufficient data, enough to predict a semi-empirical correlation. The proposed correlation for calculating the cooling characteristic (Nusselt number magnitude) under the impingement of air jet is presented in terms of profile heat flux parameter, impinging Reynolds number, and target to nozzle exit spacing. The corresponding mathematical parameter representing the profile heat flux boundary condition is the slope in heat flux magnitude versus the target surface's radial distance. The Nusselt number profile, which describes the cooling characteristic under different impinging Reynolds numbers and nozzle–target spacing, initially increases, takes a peek, and decreases. The rise in the cooling rate near the stagnation region is due to the turbulence palpitation, resulting from imbalance adverse pressure gradient and onset transition of Reynolds number. The local heat transfer under such boundary conditions increases with nozzle–target spacing and least depends on Reynolds number. However, the Nusselt profile for a constant heat flux magnitude but varying slope (non-uniform) shows an enhancement with a decrease in the slope from unit value.

Numerical modelling of the sand particle flow in pelton turbine injector

Pelton turbine is commonly employed high head impulse type turbine. Pelton turbine injector is an integrated part of the Pelton turbine machine which serves the purpose of converting entire pressure energy of water to kinetic energy and also regulates the water flowrate, with partial opening hence governing the power production. Severe erosion in Pelton turbine injector is reported from field setting research studies. Since the jet is atmospheric pressure jet, there is few chances of occurrence of cavitation hence it can be understood that impact of sand particles is the major cause of erosion. Furthermore, with turbine operating in partial flow condition, more erosion is reported in the needle of injector. For a long spear type injector, this study explores the cause of erosion by modeling the motion of the sand particle flow in steady state jet. For numerical modeling of the flow, the realizable k-epsilon model is used and for modeling the particle flow, the Discrete Phase Model (DPM) is used. Three different operating condition of the injector is considered and 77000 particles were injected to the flow domain. It is observed from the numerical simulations that the more sand particle hits the nozzle-needle surface with partial opening of the injector.

Control of jet structure utilizing the change in the outlet velocity distribution with time

Flow control using jets is widely implemented. In particular, the clarification of jet technology is indispensable in the control of the blade stall control of blades and for thermal management, such as refrigeration and air conditioning. As part of the fundamental research into the topics mentioned above, various studies of the flow characteristics of steady jets have long been conducted, and the structure of the jets investigated. However, studies on flow control by synthetic jets that essentially exhibit an unsteady flow have begun only recently. Synthetic jets have a flow wherein the jet structure forms downstream of the slot, although the net flow rate is zero, because of repeated ejection and suction. Attempts have been made to control of the jet’s direction of travel by adjusting only the jet’s frequency without any geometric changes. However, many aspects of the effect of oscillation characteristics on the jet’s structure are still unclear. Moreover, since the flow rate is low, it is not suitable for large-scale fluid control, such as that required for high-power fluid machinery. In this study, excited jets are proposed, wherein the velocity distribution change with time at the slot exit and the time-averaged flow rate are considered. An attempt was made to control the jet structure by means of the oscillation characteristics. The influences of the frequency and the phase of the slot outlet velocity distribution on the maximum jet velocity and the jet width were investigated by numerical calculations and experiments. Our research led us to conclude that the staggered vortex street is generated when the phase of the slot exit velocity distribution is π. Further, we concluded that the jet structure could be controlled by the frequency under a certain specific set of conditions.

Numerical simulation of falling film flow hydrodynamics over round horizontal tubes

The performance of a falling film heat exchanger depends significantly on the flow characteristics and the associated heat transfer in the liquid film flowing over its horizontal tube surfaces. In this study, three-dimensional numerical simulations based on the Volume of Fluid method are conducted to investigate the hydrodynamic behavior of a falling film over horizontal round tubes with diameter and tube-to-tube spacings each of 16 mm. Using a mixture of water and ethylene glycol, the simulations comprise a range of flow rates (Re = 15 to 210) that covers all basic intertube flow modes i.e., droplet, jet (in-line and staggered), and sheet mode. InterFoam, a two-phase incompressible flow solver in OpenFOAM, has been used to simulate the flow field. The computational model agrees well with the available experimental data. The numerical results present the variation of the liquid film thickness and the film interface velocity over the tube surface for all the flow modes. In the droplet mode, the impact of the droplet on the tube produces liquid waves that interact with the neighboring waves and are seen to travel on the tube surface downstream of the impingement region and the region between the impingement sites. The movement of these waves causes a significant change in the film thickness over the tube surface, quantitatively, by more than 350% during a short interval of the droplet mode cycle. After the droplet impact, the liquid bridge formed in between the tubes undergoes necking and finally bifurcates under the Raleigh-Plateau instability mechanism at a location of spacing-to-tube-diameter ratio of 0.8 from the top surface of the impinging tube and produces satellite droplets in between the tubes. In the steady in-line jet mode, the liquid from the neighboring jets do not interact with each other, and the bulk liquid flows downstream of the impingement region. A dimple around the jet impingement region is formed where the film thickness changes by around 40%. The base of the impinging jets was seen to possess ripples due to the Raleigh-Plateau instability with wavelengths ranging from 0.3 - 1.0 times the capillary length scale. For the steady staggered jet mode, the liquid jet impinges on the tube surface, which then spreads rapidly and interacts with the adjacent jet liquid to develop crest and stable segments along the tube length direction. The ratio of the film thickness between the crest and the stable segments was found to be 1.7. Finally, for the sheet mode, due to the formation of surface waves in the liquid sheet, interfacial waves are seen to travel along the tube periphery with amplitudes of about ±20% of the nominal film thickness. These flow mode characteristics alter the film thickness and flow velocity distribution over the tube surface and are expected to play an essential role in the surface heat transfer behavior over round horizontal tubes. A set of correlations have been presented to predict the film thickness and interfacial velocity over the tube surface for each of the studied modes of flow with reasonable accuracy.

Suppression of thermoacoustic instability by targeting the hubs of the turbulent networks in a bluff body stabilized combustor

Abstract

Experimental study on flow characteristics and heat transfer of an oscillating jet in a cross flow

In the present paper, flow characteristics of an oscillating jet in a cross-flow (OJICF) emitted from a double feedback fluidic oscillator were investigated. The results were compared with a steady jet in a cross-flow (JICF) from a jet nozzle with length and diameter equal to those of the fluidic oscillator. Smoke visualization in a low-speed wind tunnel was used. The measurements were done in the symmetry plane with 3 cross sections for 4 different blowing ratios (BR) of 1 to 3.33. The OJICF shows high potential to enhance the effectiveness of film cooling since it penetrates to a lesser height and leads to considerable spanwise penetration. Then, heat transfer behaviors of an impinging OJICF (IOJICF) and impinging JICF (IJICF) were compared via thermographic phosphor thermometry (TPT). The cross-flow Reynolds number based on the hydraulic diameter of the jet exit and cross-flow velocity was varied between 510 and 2,700, and the spacing between the jet and target surface was 3 times the jet throat's hydraulic diameter. The results show that for the IOJICF, heat transfer is dominated by the cross flow, and changing the BR does not have a significant effect. However, the IJICF promotes heat transfer around 2 times more than the IOJICF, and the more the BR is, the greater the Nu number. Therefore, the OJICF could be applied for film cooling rather than impinging applications.

ALMA/NICER observations of GRS 1915+105 indicate a return to a hard state

Context. GRS 1915+105 is a transient black hole X-ray binary consistently emitting 10–100% of the Eddington luminosity in the X-ray band over the last three decades until mid-2018 when the source luminosity suddenly decreased by an order of magnitude. This phase was followed by a change to a state with even lower average X-ray fluxes never seen before during the outburst but presenting renewed flaring activity at different wavelengths, albeit with mean fluxes still in decline. Aims. GRS 1915+105 has the longest orbital period known among low-mass X-ray binaries, the largest accretion disk size, and therefore the largest mass supply for accretion. The high inclination of the disk allows the study of geometrical effects of the accretion flow such as changes in the height-to-radius ratio or the effect of accretion disk winds on the intrinsic emission that is expected during the outburst decay. In addition, the transient jet is expected to change to a compact, self-absorbed, steady jet. Methods. We conducted two full polarization Atacama Large Millimeter Array observations to study the jet properties during the outburst decay by analyzing the spectral, polarization, and intra-epoch variability for both observation epochs. In addition, we analyzed almost daily Neutron Star Interior Composition Explorer pointing observations, modeling X-ray power spectral densities, spectral energy distributions, and light curves with a physically motivated model to follow the changing accretion disk properties throughout the outburst decay and relating them to the jet emission. Results. We show that the X-ray and millimeter (mm) spectral, timing, and polarization properties are consistent with those of a typical decaying X-ray binary outburst and that GRS 1915+105 has descended into the low-luminosity hard X-ray state. The jet emission in the mm is consistent with a compact, steady jet with ∼1% linear polarization, and the magnetic field is likely aligned with the jet position angle. Relating the mm emission to the X-ray emission reveals that the source has changed from a higher radio/X-ray correlation index to a lower one; Lradio ∝ LX0.6.

VLBA Discovery of a Resolved Source in the Candidate Black Hole X-Ray Binary AT2019wey

Abstract AT2019wey is a Galactic low-mass X-ray binary with a candidate black hole accretor first discovered as an optical transient by ATLAS in 2019 December. It was then associated with an X-ray source discovered by SRG/eROSITA and SRG/ART-XC instruments in 2020 March. After a brightening in X-rays in 2020 August, VLA observations of the source revealed an optically thin spectrum that subsequently shifted to optically thick, as the source continued to brighten in the radio. This motivated us to observe AT2019wey with the VLBA. We found a resolved source that we interpret to be a steady compact jet, a feature associated with black hole X-ray binary systems in hard X-ray spectral states. The jet power is comparable to the accretion-disk X-ray luminosity. Here, we summarize the results from these observations.

Investigation of noise characteristics of periodic jet impinging on a flat plate

Noise characteristics of periodic jet impinging on a flat plate are investigated. The numerical model is established by large eddy simulation and FW-H acoustic analogy method. Compared with experimental data obtained by noise measurement experiment, the effects of acoustic integral surface and different subgrid-scale models on noise simulation results are studied. The accuracy of the optimized numerical model is also validated by verification experiment. Further, the present numerical model is applied to numerical prediction of noise directivity and spectrum distribution with different waveforms and periodic frequency. Compared with the steady impinging jet, noise characteristic of periodic jet impinging on a flat plate are presented and the mechanism of acoustic source is analyzed. The present work provides a reference value for the analysis and control of noise of periodic jet impinging on a flat plate.

The steady incompressible jet flows issuing from a finitely long nozzle

This paper mainly involves with the well-posedness of two-dimensional irrotational, non-viscous, incompressible jet flows through a finitely long nozzle. In comparison with the significant results on incompressible jet flows issuing from an infinitely long nozzle in [2] and [3], we propose a set of physically reasonable boundary conditions on the inlet and the outlet of the finitely long nozzle and establish the existence and uniqueness of the incompressible jet issuing from the finite nozzle. More precisely, given a horizontal velocity in the inlet and the atmospheric pressure patm, there exists a unique incompressible jet flow issuing from the given finite nozzle.

Effect of Froude Number on Submerged Gas Blowing Characteristics.

The flow behavior of gas in compressible and incompressible systems was investigated at an ambient temperature in an air–water system and at an operating process temperature in the IronArc system, using computational fluid dynamics. The simulation results were verified by experiments in the air–water system and established empirical equations to enable reliable predictions of the penetration length. The simulations in the air–water system were found to replicate the experimental behavior using both the incompressible and compressible models, with only small deviations of 7–8%. A lower requirement for the modified Froude number of the gas blowing to produce a jetting behavior was also found. For gas blowing below the required modified Froude number, the results illustrate that the gas will form large pulsating bubbles instead of a steady jet, which causes the empirical equation calculations to severely underpredict the penetration length. The lower modified Froude number limit was also found to be system dependent and to have an approximate value of 300 for the studied IronArc system. For submerged blowing applications, it was found that it is important to ensure sufficiently high modified Froude numbers of the gas blowing. Then, the gas penetration length will remain stable as a jet and it will be possible to predict the values using empirical equations.

Jet sweeping angle control by fluidic oscillators with master-slave designs

The fluidic oscillator is an instrument that can continuously generate a spatially sweeping jet entirely based on its internal geometry without any moving parts. However, the traditional fluidic oscillator has an inherent limitation, that is, the spreading angle cannot be controlled independently, rather by the jet volume flow rate and internal geometry. Accordingly, two types of fluidic oscillators based on the master-slave design are developed in current study to decouple this correlation. In both designs, the master layer inherits the similar oscillation mechanisms of a sweeping jet, and the slave layer resembles a steady jet channel. The difference between the two designs is that Design A has a short diverging exit in the slave layer, but Design B adds a long interaction chamber in the exit channel to intensify flow instability. The external flow fields and governing oscillation properties of these two designs are experimentally explored with time-resolved Particle Image Velocimetry (PIV), while the internal flow dynamics and driving oscillation mechanisms are numerically investigated. By fixing the total volume flow rate, the jet spreading angle of Design A can be increased smoothly from 0° to above 100° by increasing the proportion of master layer's flow rate from 0 to 100%. For Design B, the control authority of the master layer is significantly enhanced by adding the interaction chamber in the slave layer. In addition, the added chamber causes notable jet oscillation even when the master layer has none input.

Steady tangential control jet for improving the effectiveness of a rudder under one-engine inoperative condition

The aim of the research is to numerically investigate the use of a steady tangential control jet (CJ) at the hinge of the vertical stabilizer as an active flow control (AFC) technique to improve the effectiveness of a rudder on a subsonic aircraft during one-engine inoperative (OEI) condition by keeping the flow attached to the control surface. The goals are in two folds: firstly to investigate the effects of CJ parameters, specifically the jet-to-freestream velocity ratio (rV ) and the jet slot height ratio (rH ), on the vertical stabilizer performance, and secondly to find the optimum CJ condition that yields the improved and optimal performance. In this regard, the optimal condition of the CJ is prescribed as the condition that gives the maximum side force to drag ratio under the requirements that the required minimum trim side force can be achieved and there is an improvement in control surface margin for yawing motion. The investigation is carried out through computational fluid dynamics (CFD) using the Spalart-Allmaras turbulent model. Under the optimal CJ condition, its implementation can provide the required minimum rudder side force coefficient (Cy, trim ) of 1.19 at a rudder drag coefficient (Cd ) of 0.0013, a decrease in drag coefficient of 95% over the baseline case of plain flap without CJ. In addition, there is an improvement in rudder margin with the maximum side force coefficient increase by 42%, and the corresponding drag force coefficient decrease by 25% for the aircraft to yaw in a direction of the still-active engine.

Numerical optimization of pulse frequency and heat transfer for a subsonic impinging jet array

Jet impingement technique with pulsed jet arrays has proved to be one of the most efficient heat transfer technologies for cooling applications. Detailed investigations are required to optimize the different parameters to achieve maximum heat transfer. Present study is an effort in this direction by analyzing the heat transfer from a flat heating film of dimension 60 × 60 × 0.1 mm. The chip generates a heat of 30 W which is to be carried away by means of air jets. The nozzle plate at the top generating the air jet has a set of 9 nozzles arranged in a 3x3 inline configuration. Two sides of the flow arrangement are confined and flow is only along one direction. The nozzle diameter is 5 mm and pitch length from center to center X/D and Y/D are 3. The nozzle plate, heating plate gap Z/D is 4. The Reynolds number values are in the range 3500 to 14000. The cross-flow approaches in a direction perpendicular to the jets and mixes with the jet flow. The results obtained predict a dominance of pulsed jets over steady jets, if heat transfer is considered. Among pulsed jets, those with higher amplitudes and lower frequencies showcase better heat transfer and more uniform distribution throughout the heated plate.

Comparison of the Pulsating and Steady Jets on FIV and Thermal Behavior of a Sprung Cylinder Inside an Isothermal Channel

Comparison of the Pulsating and Steady Jets on FIV and Thermal Behavior of a Sprung Cylinder Inside an Isothermal Channel

Jet evolution and fiber formation mechanism of amylopectin rich starches in centrifugal spinning system

AbstractThis article reports the jet evolution process and fiber formation mechanism of the amylopectin rich starch solution in centrifugal spinning system, and the linear polyvinylpyrrolidone (PVP) is applied to compare with starch solution. The results indicate that jet evolution processes of starch solution include steady state and Rayleigh–Taylor instability over the whole concentration range for the spinnability investigation, which are due to the hyperbranched molecular of amylopectin. As a compression, the linear molecular chain PVP solution expresses a steady state jet under the spinning concentration. The spinnability results show that the obtained starch fibers are always containing the beads due to the Rayleigh–Taylor instability of jet, but can be effectively controlled by solution concentration and amylose/amylopectin. Instead, the PVP fibers show a rapid decreasing of beads till to almost disappear with the increasing of solution concentration, which due to the improvement of chain entanglement increase the steady state portion of jet with the increasing of concentration. The thermal properties of obtained fiber show that fibers obtained from amylose with more thermal stability.

Effect of pulsating injection and mainstream attack angle on film cooling performance of a gas turbine blade

Relatively few studies have examined the effects of pulsating unsteadiness in turbine cooling blades. This unsteadiness can be a result of compressor blades and vane interaction. In addition, there is a particular lack of data of full turbine blades at various angles of attack. The effects of pulsation frequency (f = 2 Hz, 50 Hz, and 100 Hz) and the angle of attack (α = 0°, 15°, and 30°) on the film cooling effectiveness of a row film jet at the leading edge of a modified NASA C3X blade for two blowing ratios (M = 0.5 and 1.0) in comparison with the steady state experimentally investigated on pressure and suction sides of the blade and the flow field are obtained by simulation. Three-dimensional transient Reynolds-averaged Navier–Stokes equations coupled with the shear stress transport turbulence model (SST k − ω) are used in this research. Square waves are considered to pulse the injection air. Results show that the distribution of the instantaneous film cooling effectiveness is affected by frequency, angle of attack, blowing ratio, and curvature of the blade. With an increase in the angle of attack and the pulsing frequency, the averaged film cooling effectiveness increased. Pulsation and angle of attack have different effects on the performance of the injection jet toward the pressure and suction sides of the blade. Mixing of injection air with the mainstream at the pressure side is more than that at the suction side. Under certain conditions, the averaged film cooling effectiveness of pulsation flow is greater than the steady jet.

Active aerodynamics control of simplified vehicle body in a crosswind condition

In this study, the authors used a steady jet from a slit nozzle to control the bi-stable aerodynamics of a simple vehicle model under crosswind conditions. In general, the aerodynamic coefficients of an automobile change continuously with variations in the relative wind direction (i.e. yaw angle). However, some vehicles show a significant change in their aerodynamics at a certain yaw angle because their wake flow structure changes at that yaw angle. Furthermore, around that critical yaw angle, their aerodynamics sometimes show bi-stability in which both the wake structures before and after the change are stable. The investigated model is the famous simplified model in vehicle aerodynamic research known as the Ahmed model. Its slant angle was set at 32° as a configuration that causes bi-stability at a specific yaw angle range larger than 6°. The active aerodynamic control method was demonstrated in both the model scale wind tunnel test and computational fluid dynamics analysis. The slit jet with 18% of the main flow velocity successfully changed the wake flow structure. This showed that it is possible to improve the aerodynamic characteristics.