Jet Distortion Research Articles

Evaluation of the jet distortion by sediment erosion in the needle in the Pelton turbines

Abstract In order to succeed in the transition to a more renewable energy sector, the hydropower industry must overcome multiple challenges in the coming decades, including the need for faster and reversible turbines to support emerging renewable energy sources. In addition, the location of new projects in tropical and semi-tropical areas poses unique considerations, as does the increase in sediment. Evaluating sediment erosion in turbines is a critical challenge, which may partly be addressed by analyzing the behavior of water jets from eroded nozzles in Pelton turbines. This paper presents an evaluation of the quality of the jet under various hydraulic conditions, focusing on needle erosion. The study is part of a research project conducted in the Waterpower Laboratory at NTNU, focusing on Pelton turbines. Furthermore, this paper investigates erosion patterns caused by sediments on the needle and explores the impact on jet performance. Through high-speed visualization and image processing, the behavior of a jet from the needle tip to a representative distance downstream has been investigated. The investigation revealed discernible differences between jets with eroded and uneroded needles in size and edge variation.

Extreme jet distortions in low-z radio galaxies

AbstractJets often display bends and knots at which the flows change character. Extreme distortions have implications for the nature of jet flows and their interactions. We present the results of three radio mapping campaigns. The distortion of 3CRR radio galaxy NGC 7385 is caused by a collision with a foreground magnetised gas cloud which causes Faraday rotation and free-free absorption, and is triggered into star formation. For NGC 6109 the distortion is more extreme, creating a ring-shaped structure, but no deflector can be identified in cold or hot gas. Similar distortions in NGC 7016 are apparently associated with an X-ray gas cavity, and the adjacent NGC 7018 shows filaments drawn out beyond 100 kpc. Encounters with substructures in low-density, magnetised, intergalactic gas are likely causes of many of these features.

Experimental Investigation of Inlet Distortion Effect on Performance of a Micro Gas Turbine

An experimental study has been carried out to determine how inlet total-pressure distortion affects the performance of a micro gas turbine. An inlet simulator is designed and developed to produce and measure distortion patterns at the inlet to the gas turbine. An air jet distortion generator (AJDG) is used to produce nonuniform flow patterns and total pressure probes are installed to measure steady-state total-pressure distribution at the inlet. A set of wind tunnel tests have been performed to confirm the fidelity of distortion generator and measuring devices. Tests are carried out with the gas turbine exposed to inlet flow with 60 deg, 120 deg, and 180 deg circumferential distortion patterns with different distortion intensities. The performance of the gas turbine has been measured and compared with that of clean inlet flow case. Results indicate that the gas turbine performance can be affected significantly facing with intense inlet distortions.

Numerical analysis of a Pelton bucket free surface sheet flow and dynamic performance affected by operating head

The present paper aims to find out the influence of operating head on the rotating bucket free surface sheet flow and hydrodynamics performance for a Pelton turbine. Three-dimensional unsteady air-water two-phase flow simulations in the rotating buckets were performed by adopting the shear stress transport curvature correction turbulence model and homogenous model. The sensitivities of the unsteady simulation results to moving mesh resolution and computational fluid dynamics solver time-step have been evaluated to discuss the effect of Courant–Friedrichs–Lewy conditions on the two-phase flow simulation. The accuracy of the numerical predicted flow pattern and the hydrodynamic performance results are reasonable when compared with the experimental data. The simulation results indicate that the remaining kinetic energy carried by the outflow under the nondesigned water head is the main reason for the efficiency loss in the Pelton turbine. Under low water head conditions, jet distortion caused by the Coanda effect and flow interference will lead to more severe efficiency deterioration.

An Air Jet Distortion Generation System

An air jet distortion generation system is developed to simulate the distorted flow field ahead of gas turbine engines in ground test facility. The flow field of a system of four jets arranged circumferentially and issuing into a confined counterflow was studied experimentally and numerically. The total pressure distortion parameters were evaluated at the Aerodynamic Interface Plane (AIP) for several values of mass flow ratios. Since the total pressure loss distribution at theAIPis characteristically “V” shaped, the number of jets was increased to obtain total pressure distributions as required for gas turbine engine testing. With this understanding, a methodology has been developed to generate a target total pressure distortion pattern at theAIP. Turbulent flow computations are used to iteratively progress towards the target distribution. This methodology was demonstrated for a distortion flow pattern typical of use in gas turbine engine testing using twenty jets, which is a smaller number than reported in the literature. The procedure converges with a root-mean-square error of 3.836% and is able to reproduce the target pattern and other distortion parameters.

Study of Slanted Perforated Jets

This paper presents the numerical simulation of the subsonic jets controlled by slanted perforated tabs and its performance of mixing efficiency is compared with the jet controlled by solid tab and free jet. The objective of this paper is to study the performance of slanted perforation geometry tabs in controlling high speed jets to enhance the mixing of jet with the ambient air, to suppress the noise level and to minimize the thrust loss. In this paper the simulations have been carried out using the commercial meshing and analysis software. Due to the effect of tabs the potential core decay occurs and velocity reduces drastically because of enhanced mixing produced by the tabs. From the results it is found that in slanted perforated tab the main jet interacts with the slanted perforated jet which causes in effective mixing, instability in jets and lower thrust loss when compared with the free jet. The decay of the potential core and velocity reduction is computed by simulation for 0.4 Mach number. Velocity plots are obtained at both near field and far field downstream locations to study the jet distortion with slanted perforated tabs and solid tabs. The results obtained for perforated tabs for 0.4 Mach number are also compared with various other Mach numbers. They have also been validated with experimental results which show good agreement with the computational results.

Visualization and structure of confined, milliscale, unsteady impinging slot jets and associated vortices

Flow characteristics of confined, laminar milliscale slot jets are investigated from visualizations, as they impinge upon a flat target plate, with a fully developed velocity profile at the nozzle exit. The effects of Reynolds number Re and normalized nozzle-to-plate distance H/B are considered for a nozzle width B of 1.0 mm. Transition from a stable symmetric jet to an unsteady oscillating jet is observed as the Reynolds number increases (with H/B constant), where the Reynolds number associated with this transition decreases as the normalized nozzle-to-plate distance H/B increases. Instantaneous visualizations show unsteady lateral distortions of jet columns at experimental conditions corresponding to the presence of continuous sinusoidal oscillations, intermittent oscillating motion of the jet column, and jet flow fluctuation/flapping motion. Also apparent in flow visualization sequences are smoke signatures associated with instantaneous vortex structures, which form as secondary flows develop in fluid which, initially, is just adjacent to and within the jet column. Associated jet and vortex structural changes are described as different modes of unsteadiness are present, including characterization of jet column unsteadiness using jet column oscillation frequency, and lateral and streamwise extents of jet distortion.

비활성 요소의 방호 메커니즘 분석

In this study, a series of ballistic experiments have been performed to investigate the protection mechanism of some inert reactive cassettes against shaped charge jet. Three kinds of material were tested as a core material of the inert cassettes, i.e. one of rubber materials, a high modulus and high strength composite material used for ballistic protection and a mixture of energetic materials. Parameters such as deformation of the cassettes, occurrence time of jet distortion, leading jet length and residual penetration depth were investigated from the experiments and they were compared to each other quantitatively according to the jet incidence angles. The results show that the increment of cassette deformation caused jet distortion to occur early and jet distortion brought decrease of the length of leading jet. Then the decrease of the length of leading jet accompanied the decrease of residual penetration depth.

Zonal Detached Eddy Simulation of a Controlled Propulsive Jet

This paper presents zonal detached-eddy simulation of a round underexpanded sonic jet exhausting from a realistic aircraft afterbody and controlled by four radial injections. The comparison with a former experiment proves the capability of zonal detached-eddy simulation to reproduce mean flow induced by the complex interaction between the compressible waves and the turbulent jet shear layer. However, the very near-field jet structure indicates that the shear-layer transition to a fully turbulent state is delayed. Therefore, a modification of the characteristic length scale used in the modeling is proposed which is shown to improve the prediction of the shear-layer growth. The control injectors are aimed to enhance jet mixing. Experiment indicated the efficiency of this additional device but also expressed the need for more data to get a deeper and more complete insight into the whole flowfield. Both spatial and temporal information are provided by zonal detached-eddy simulation which is then a good candidate. This method is shown to correctly simulate streamwise vortices generated by secondary injections, their action in jet distortion, as well as their subsequent decay. Finally, computational results allow us to evaluate the action of the hypermixer on turbulent activity and its consequences on jet dilution.

Electric Armor Against Shaped Charges: Analysis of Jet Distortion With Respect to Jet Dynamics and Current Flow

Shaped charges are a warhead technology often applied to rocket propelled grenades and represent a dangerous threat for armored vehicles in combat as well as in peace-keeping operations. Their armor piercing performance rests upon an explosively induced collapse of a metallic liner to a stretching jet with very high particle velocities. A copper jet produced by a shaped charge can be distorted by high electric currents injected into the jet by means of spaced electrode plates connected to a high voltage capacitor. In tests carried out at Fraunhofer EMI, a shaped charge with a well characterized jet was used for the experiments in order to examine the current flow through the jet and its effect on the jet evolution. The measured current flow is related to the jet dynamics and the distortion pattern observed by multiple flash X-ray images. As expected, the current flow starts when the jet tip reaches the back electrode plate. No significant change of the current flow is observed at the characteristic jet break-up time. The current flow continues after the tail of the copper jet has left the electrodes and resembles a damped sinusoidal. A distortion of the jet is observed where the jet particles are not aligned along the jet axis. Instead the particles are stretched orthogonally to the jet axis with increased separation along the jet axis. The tip part of the jet is hardly affected. The jet distortion is analyzed with respect to jet dynamics and current flow which allows formulating criteria for the design of electric armor systems. The current injection effective for jet distortion is limited to a time slot of a magnitude of 60 mus for the 44-mm caliber-shaped charge used in the experiments. To a first approximation, the current flow can be modeled by an electric arc. An electric circuit model can describe the current flow behavior with respect to the electric impedance and allows designing an electrical circuit adequate for the time slot. By the analogy of a wire explosion the necessary current magnitude for an effective jet disruption with respect to the interaction time slot can be estimated to begin at 300 kA. For the tip portion of the shaped charge jet, the time of effective current injection is very short. When the current starts to set in, the jet tip is already passing the back electrode plate. For this reason, an effective distortion of the jet tip represents a challenge that has to be mastered

Flow instability in polymer solutions and melts

The existing experimental data concerning the problem of flow instability in polymer solutions and melts is considered and critically discussed. The instability is understood as both regular distortions of the jet surface shape and turbulence of the flow as such. The visual manifestations and physical mechanisms determining the development of flow instability are analyzed and classified. The following principal forms of instability are distinguished: small-scale regular surface defects, periodic oscillations with the scale of the jet diameter, the slip—stick periodic transition phenomenon, self-oscillations of the stream, jet spurt, and large-scale distortions passing into stream discontinuities. In all cases, the instability of the jet is due to rubber elasticity of polymer fluids, a property which causes storage of elastic energy during deformation with its subsequent release in the form of stream distortions. Therefore, the general criterion for the onset of instability is a certain critical value of the Weissenberg number. The key factors determining the loss of the flow stability are concentration of stresses at the channel outlet, transition from laminar flow to slip along a solid wall (adhesive ruptures) under certain critical conditions, and mechanical fracture (cohesive ruptures) of a material. In the appearance of hysteresis oscillations, bulk elasticity and compressibility of the melt also play a certain role. Alternative mechanisms proposed in the literature are also discussed. Examples illustrating the possibility of suppressing jet distortions are given; this suppression is important for many industrial applications in polymer processing.

The Effect of External Winds on Relativistic Jets

Relativistic jets in Galactic superluminals and extragalactic active galactic nuclei may be surrounded by a wind near the central engine. Theoretical analysis and numerical simulation reveal considerable stabilization of relativistic jet flow by a wind to helical and higher order asymmetric modes of jet distortion. When velocities are measured in the source (inlet) frame, reduction in the absolute velocity difference between jet and wind, Δv = vjet - vwnd, provides stabilization in addition to stabilization provided by a high jet Lorentz factor, but a high Lorentz factor wind is not needed to stabilize a high Lorentz factor jet. However, the fundamental pinch mode is not similarly affected, and knots with spacing a few times the jet radius are anticipated to develop in such flows. Thus, we identify a mechanism that can suppress large-scale asymmetric structures while allowing axisymmetric structures to develop. Relativistic jets surrounded by outflowing winds will be more stable than if surrounded by a stationary or backflowing external medium. Knotty structures along a straight jet like that in 3C 175 could be triggered by pinching of an initially low Mach number jet surrounded by a suitable wind. As the jet enters the radio lobe, suppression of any surrounding outflow or backflow associated with the high-pressure lobe triggers exponential growth of helical twisting.

3D structures on relativistic jets

The properties of wave-like helically twisted normal mode structures on steady relativistic jets are summarized. Wave speeds are a function of the wavelength and less than the jet speed. However, normal mode interference can lead to both stationary and superluminal phase effects. A maximum pressure fluctuation criterion suggested by numerical simulations of axisymmetric relativistic jets is used to find the maximum asymmetric jet distortions and velocity fluctuations. Cyclic transverse velocity fluctuation can lead to variation in the flow direction on the order of the relativistic beaming angle. Resulting variation in the Doppler boost factor can lead to significant brightness asymmetries as helical structures twist around the jet beam. Growth of these structures is reduced as the jet density, Lorentz factor or Mach number are increased. Maximum jet distortion is reduced as the Lorentz factor increases and this suggests a reduction in mass entrainment or other non-linear disruptive processes that influence the morphological development of radio sources.

On Three‐dimensional Structures in Relativistic Hydrodynamic Jets

The appearance of wavelike helical structures on steady relativistic jets is studied using a normal mode analysis of the linearized —uid equations. Helical structures produced by the normal modes scale relative to the resonant (most unstable) wavelength and not with the absolute wavelength. The resonant wave- length of the normal modes can be less than the jet radius even on highly relativistic jets. High-pressure regions helically twisted around the jet beam may be con—ned close to the jet surface, penetrate deeply into the jet interior, or be con—ned to the jet interior. The high-pressure regions range from thin and ribbon-like to thick and tubelike depending on the mode and wavelength. The wave speeds can be sig- ni—cantly diUerent at diUerent wavelengths but are less than the —ow speed. The highest wave speed for the jets studied has a Lorentz factor somewhat more than half that of the underlying —ow speed. A maximum pressure —uctuation criterion found through comparison between theory and a set of relativistic axisymmetric jet simulations is applied to estimate the maximum amplitudes of the helical, elliptical, and triangular normal modes. Transverse velocity —uctuations for these asymmetric modes are up to twice the amplitude of those associated with the axisymmetric pinch mode. The maximum ampli- tude of jet distortions and the accompanying velocity —uctuations at, for example, the resonant wave- length decreases as the Lorentz factor increases. Long-wavelength helical surface mode and shorter wavelength helical —rst body mode generated structures should be the most signi—cant. Emission from high-pressure regions as they twist around the jet beam can vary signi—cantly as a result of angular variation in the —ow direction associated with normal mode structures if they are viewed at about the beaming angle h 1/c. Variation in the Doppler boost factor can lead to brightness asymmetries by factors up to 6 as long-wavelength helical structure produced by the helical surface mode winds around the jet. Higher order surface modes and —rst body modes produce less variation. Angular variation in the —ow direction associated with the helical mode appears consistent with precess- ing jet models that have been proposed to explain the variability in 3C 273 and BL Lac object AO 0235)164. In particular, cyclic angular variation in the —ow direction produced by the normal modes could produce the activity seen in BL Lac object OJ 287. Jet precession provides a mechanism for trig- gering the helical modes on multiple length scales, e.g., the galactic superluminal GRO J1655(40. Subject headings: BL Lacertae objects: individual (OJ 287) ¨ galaxies: activegalaxies: jets ¨ hydrodynamicsinstabilitiesrelativity

On the Dynamics and Structure of Three‐dimensional Trans‐Alfvenic Jets

Three-dimensional magnetohydrodynamical simulations of strongly magnetized "light" conical jets have been performed. An investigation of the transition from sub-Alfvénic to super-Alfvénic flow has been made for nearly poloidal and for helical magnetic fields. The jets are stable to asymmetric modes of jet distortion provided they are sub-Alfvénic over most of their interior but destabilize rapidly when they become on average super-Alfvénic. The jets are precessed at the origin, and the resulting small-amplitude azimuthal motion is communicated down the jet to the Alfvén point, where it couples to a slowly moving and rapidly growing helical twist. Significant jet rotation can contribute to destabilization via increase in the velocity shear between the jet and the external medium. Destabilization is accompanied by significant mass entrainment, and the jets slow down significantly as denser external material is entrained. Synchrotron intensity images satisfactorily reveal large-scale helical structures but have trouble distinguishing a large-amplitude elliptical jet distortion that appears as an apparent pinching in an intensity image. Smaller scale jet distortions are not clearly revealed in intensity images, largely as a result of the relatively small total pressure variations that accompany destabilization and growing distortions. Fractional polarization is high as a result of the strong ordered magnetic fields except where the intensity image suggests cancellation of polarization vectors by integration through twisted structures.

Impact of tab location relative to the nozzle exit on jet distortion

Cursory observations during the course of the present study revealed that the flowfield distorsion produced by the tab changes drastically if the location of a tab is slightly varied relative to the nozzle exit.

Effects of pulmonary venous flow direction on mitral regurgitation jet area as imaged by color Doppler flow mapping. An in vitro study.

Although the effects of adjacent walls and left atrial pressure on mitral regurgitation (MR) jet area imaged by color Doppler have been examined, few data exist regarding the influence of pulmonary venous (PV) filling flow on regurgitant jets. Therefore, we designed a left atrial model to examine the relation between PV flow direction and MR jet area. The left atrial chamber (7.6 cm in diameter) was built with a PV inflow (1.0 cm in diameter) and mitral valve regurgitant orifice in the same plane. The MR jet was simulated as fixed in volume and velocity (3.5 m/s) and directed with a pulsatile pump into the left atrial model. PV flow with a constant velocity (30 cm/s) was driven by gravity (83 cm H2O). With left atrial mean pressure at either 10, 30, or 50 mm Hg, four flow patterns were examined: (1) PV flow away from the mitral valve, MR jet toward the pulmonary vein; (2) PV flow toward the mitral valve, MR jet toward the pulmonary vein; (3) PV flow away from the mitral valve, MR jet away from the pulmonary vein; and (4) PV flow toward the mitral valve, MR jet away from the pulmonary vein. MR color Doppler images were recorded with a 3.5-mHz frequency transducer and at 7-kHz pulse repetition frequency. For each condition, we compared jet area, length, and width of the MR signal. MR jet areas for conditions 3 and 4 were larger at 10 mm Hg than at 30 or 50 mm Hg left atrial pressure. Especially at the lower pressures, PV flow diminished the MR jet area in condition 4 compared with that in condition 3, such that MR jets were smaller in condition 4. In conditions 1 and 2, the jets were imaged at an oblique angle and were smaller than in conditions 3 and 4 (P < .001), but they were not significantly different from each other as imaged. In this model, factors including the direction of PV flow, the direction of MR as relates to the angle of interrogation, and the level of left atrial pressure influenced the size of MR jets. The effect of PV flow direction was diminished by increased left atrial pressure. PV flow directed away from the mitral valve was associated with larger MR jets than when PV flow was directed toward it (condition 4), probably because of jet distortion and flattening.

Three-Dimensional Wing/Jet Interaction Analysis including Jet Distortion Influences

Analysis of the interaction of wings with regions of high-energy flow in a uniform stream is motivated by the requirement to predict the aerodynamic characteristics of powered lift systems. The present formulation of the wing/jet problem includes jet deflection and distortion effects. Each jet boundary and wing is replaced by equivalent flow singularities embedded in a uniform stream. The singularity strengths and locations cannot be obtained directly since the jet position is unknown; therefore, an iterative solution technique is employed. Example calculations are presented and comparisons of current solutions are made with previous analyses and experiments.

Velocity-Feedback Combustion Oscillation

The low-frequency combustion, oscillation occurring in the combustion system comprising a combustion duct, inlet and outlet orifices and a capacity chamber was analyzed as a dynamic instability of "closed loop". From the analysis of experimental facts it was concluded that the exhaust temperature fluctuation as the result of distortion and necking of flame would be the chief feedback mechanism, and such distortion is caused by velocity fluctuation. This view was confirmed by a forced oscillation experiment applying a fluctuating velocity on a flame and fuel jet. Theoretical analysis was made of a jet placed in fluctuating velocity field. The theory predicts the necking and distortion of jet. The theoretical result was easily interpreted as the relation of exhaust temperature to velocity fluctuation. The dynamics of combustion thus obtained was used together with the dynamics of fluid flow to discuss the nature of self-exciting oscillation.