Optical tomography applied to speckle photographic measurement of asymmetric flows with variable density

Optical tomography is applied to the speckle photographic measurement of an asymmetric flow field with variable fluid density. The convolution back projection algorithm is used for obtaining the 3-D density distribution. Noise in the experimental data is reduced by spline smoothing. The method is verified with a steady, laminar, axisymmetric helium jet exhausting vertically into the ambient air, and then applied to a non-axisymmetric helium jet for determining the helium concentration. It is found that speckle photographic recordings are very adequate for tomographic reconstruction, because they provide a high number of data points from each projection. The influence of the limited number of projections on the reconstruction quality is particularly investigated. Dedicated to Professor Dr.-Ing. J. Zierep on the occasion of his 60th birthday     

Turbulence microscale variation due to interaction with shock wave

A speckle photographic technique is used for visualizing the planar distribution of the refractive deflection angles of light transmitted through a compressible turbulent flow. Double exposure speckle photography is applied for the statistical analysis of such flows. Quantitative visualization of the planar distribution of the refractive deflection angles of the light transmitted through the compressible turbulent flow is performed. Turbulent structures are visible in the patterns of the deflection angles isolines. The 2-D correlation functions of these deflection angles are constructed and analyzed. A mathematical procedure for solving Erbeck-Merzkirch integral transformation is described and the 3-D density correlation functions are evaluated using 2-D correlation functions of deflection angles of the light passed through the turbulence. Received 14 November 1999 / Accepted 3 June 2000     

The effect of density ratio on the near field of a naturally occurring oscillating jet

The triangular oscillating jet nozzle generates a triangular jet partially confined within an axi-symmetric chamber to produce a large scale flow oscillation that has application in thermal processes. Particle image velocimetry and oscillation frequency measurements were conducted to investigate the influence of the jet fluid to ambient fluid density ratio on the resulting oscillating flow. The investigation was conducted with a jet momentum flux of 0.06 kg m s⁻² (Re = 7.3−47.2 × 10³) and density ratios ranging from 0.2 to 5.0. The initial spread and decay of the emerging jet was found to depend upon the density ratio but in a more complex way than does an unconfined jet. Both the spread and decay are strongly influenced by the instantaneous angle of jet deflection, with greater deflection leading to increased spreading and decay of the jet. Decreasing the density ratio below unity results in a rapid decrease in the deflection angle, while increasing the density ratio above unity results in an increase in the deflection angle, albeit with less sensitivity. The frequency of oscillation was also shown to depend on the density ratio with an increase in the density ratio causing a decrease in the dominant oscillation frequency.     

Rotation of the thrust vector of a two-dimensional nozzle by means of displacing the critical surface

The method of thrust vector deflection by means of displacing the critical surface of a nozzle is numerically and experimentally investigated. The displacement is realized at the expense of extending rotatable walls, or deflectors, into the flow; in this case, one of the edges of the critical surface is displaced from the throat onto the deflector surface. Two nozzle configurations, with short and long deflectors, are studied. The thrust vector deflection angles and the nozzle thrust coefficients are determined in the thrust vector deflection regime. For the configuration with long deflectors the angle of rotation of the thrust vector amounts to 30° and is determined by the effect of jet ejection toward the opposite wall, similar with the Coanda effect.     

Influence of the initial conditions on axisymmetric jets in a parallel flow

An experimental investigation was made of the initial-section flow of axisymmetric helium, air, and freon-12 jets in a parallel air flow for two different velocity profiles at the nozzle exit near the boundary of the jet. In one case, the velocity profile was determined by boundary layers on the nozzle walls; in the other case, it was produced artificially by means of a honeycomb of tubes of variable length. Measurements were made of the profiles of the mean and the pulsation velocity and the temperature. The flow was also photographed. The investigations showed that, depending on the initial conditions, the intensity of mixing of the jets in the initial section at Reynolds numbers Re 10⁴ (calculated using the jet diameter) can change from the level determined by molecular diffusion to the level characteristic of developed turbulent flow. The flow structure in the annular mixing layer also depends strongly on the initial conditions. The observed ordered structures in the mixing layer are related to a section of development of perturbations near the nozzle. The ordered structures are strongly influenced by the effect on the jet of acoustic vibrations from an external source. When the initial velocity profile is produced by the honeycomb, the transition to developed turbulence may be due to the development of long-wavelength perturbations or to the development of small-scale turbulence generated by the flow over the end of the honeycomb.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 18–24, July–August, 1980.We thank V. M. levlev and K. I. Artamonov for assistance and for discussing the work.     

Density distribution in a supersonic jet exhausting into vacuum from a nozzle with an inclined exit

Fairly effective methods have been developed to calculate axisymmetric jets of an inviscid perfect gas exhausting into vacuum, and approximations have also been obtained for the density field in such jets at large distances from the nozzle exit (see, for example, [1—3] and the bibliography in them). If the plane of the exit of the nozzle is not perpendicular to its axis, the flow in the jet is three dimensional. In a number of cases one can take into account the influence of the inclined exit with sufficient accuracy by a correction in the density distribution. An expression for such correction is given in the present paper. It is obtained on the basis of earlier calculations of the author [4], in which flow from a source was specified at the nozzle exit.     

Velocity and Reynolds stresses in a precessing jet flow

 A novel fluid mixing device, described elsewhere, has been shown to have a dramatic effect on the combustion characteristics of a fuel jet. The main features of the flow are the deflection of the jet between 30° and 60° from the nozzle axis and its precession about that axis. Many of the factors governing the nozzle instabilities which drive the mixing in the external field are imprecisely defined. It is the aim of the present paper to examine, in isolation from the nozzle instabilities, the influence of precession on a deflected jet as it proceeds downstream from the nozzle exit. The fluid dynamically driven phenomena within the nozzle which cause the precession are in the present investigation replaced by a mechanical rotation of a nozzle from which is emerging a jet which is orientated at an angle from the nozzle axis. By this means the effect of precession on the deflected jet can be investigated independently of the phenomena which cause the precession. The experimental data reported here has been obtained from measurements made using a miniature, rapid response four-hole “Cobra” pitot probe in the field of the precessing jet. Phase-averaged three dimensional velocity components identify the large scale motions and overall flow patterns. The measured Reynolds stresses complement the velocity data and are found to be compatible with the higher entrainment rates of the jet found in earlier investigations. Received: 8 November 1995 / Accepted: 27 September 1996     

Control of a submerged jet in a thin rectangular cavity

An innovative method is presented for control of an oscillatory turbulent jet in a thin rectangular cavity with a thickness to width ratio of 0.16. Jet flow control is achieved by mass injection of a secondary jet into the region above the submerged primary jet nozzle exit and perpendicular to the primary nozzle axis. An experimental model, a 2-D and a 3-D computational fluid dynamics (CFD) model are used to investigate the flow characteristics under various secondary injection mass flow rates and injection positions. Two-dimensional laser Doppler anemometry (LDA) measurements are compared with results from the CFD models, which incorporate a standard k–ε turbulence model or a 2-D and 3-D realisable k–ε model. Experimental results show deflection angles up to 23.3° for 24.6% of relative secondary mass flow are possible. The key to high jet control sensitivity is found to be lateral jet momentum with the optimum injection position at 12% of cavity width (31.6% of the primary nozzle length) above the primary nozzle exit. CFD results also show that a standard k–ε turbulence closure with nonequilibrium wall functions provides the best predictions of the flow.     

Density-sensitive whole-field flow measurement by optical speckle photography

An optical whole-field, line-of-sight method is surveyed that allows the measurement of the refractive deflection angles of light transmitted through a flow with density changes. The method uses the principles of speckle photography. In comparison to classical optical methods such as interferometry, a much higher number of data values can be obtained from one record without the need of interpolating between experimental data. This high signal density is particularly advantageous for analyzing turbulent flows with density fluctuations. Such an analysis is, so far, restricted to the existence of axisymmetric turbulence. A number of applications to this type of turbulent flow are reported, and proposals are made for future developments that will allow this speckle technique to be applied to the study of turbulent shear flow.     

Measurement of tip penetration of confined high-density transient jets by laser beam deflection

The penetration of high density transient jets injected into a quiescent chamber containing air at atmospheric temperature and various pressures has been measured in terms of laser beam deflection. The sensitivity of the technique is quantified and the rate of jet penetration shown to increase with jet to chamber density ratio, decrease with chamber pressure and vary with downstream distance from the nozzle.     

基于主动流动控制的射流矢量偏转技术

本文介绍了一种基于主动流动控制技术的射流矢量偏转新方法和控制思路。通过在主射流出口两侧加装斜置扩张固壁板来降低射流两侧与固壁边界间的流体压力,将射流偏转由"不敏感-难控"转变成"敏感-易控",再在固壁板布置自行研制的斜出口合成射流激励器对主射流进行比例偏转控制。实验结果表明,射流最大偏转角可达15°。此外还研究了激励位置角度、激励频率、激励电压不同工作参数对射流矢量偏转控制的影响,实现了主射流偏转角的比例控制。当合成射流与主射流动量比为1∶43时,主射流偏转角可达13°,合成射流激励器消耗的能量为1.5W,初步实现了以小的能量消耗获取高的控制效益。     

Full-field measurements of self-excited oscillations in momentum-dominated helium jets

Flow structure of momentum-dominated helium jets discharged vertically into ambient air was investigated using a high-speed rainbow schlieren deflectometry (RSD) apparatus operated at up to 2,000 Hz. The operating parameters, i.e., Reynolds number and Richardson number were varied independently to examine the self-excited, flow oscillatory behavior over a range of experimental conditions. Measurements revealed highly periodic oscillations in the laminar region at a unique frequency as well as high regularity in the flow transition and initial turbulent regions. The buoyancy was shown to affect the oscillation frequency and the distance from the jet exit to the flow transition plane. Instantaneous helium concentration contours across the field of view revealed changes in the jet flow structure and the evolution of the vortical structures during an oscillation cycle. A cross-correlation technique was applied to track the vortices and to find their convection velocity. Time traces of helium concentration at different axial locations provided detailed information about the oscillating flow.     

Cavitation: a contributory factor in the transition from symmetric to asymmetric jets in cross-flow nozzles

The structure and evolution of cavitation and its influence on jet patterns from two transparent cross-flow nozzles with holes inclined at 90 degrees (nozzle A) and 80 degrees (nozzle B) to the nozzle axis have been investigated using high-speed motion pictures, flash photography and stroboscopic visualization. At the onset, cavitation inception was in the form of travelling bubbles, which were transported along the flow and clearly detached from the wall. As the flow was increased the bubbles grew and merged into a dense group of bubbles (cloud cavitation), partly unsteady and shedding. Further increasing the flow caused the cavitation at the entrance to transform mainly into a glassy appearance and at this stage the cavitation was well inside the hole and the spray appeared symmetric. When the flow was increased beyond this stage, cavitation extended to the exit of the hole, occupying a significant part of the hole on one side, resulting in a jet that atomized on the side where cavitation was most extensive and a non-atomizing jet on the side with less cavitation. The distribution of cavitation in the hole is very sensitive to the nozzle geometry and it substantially influences the spray dispersion.     

On factors influencing arc filament plasma actuator performance in control of high speed jets

Localized arc filament plasma actuators (LAFPAs) have been developed and used at The Gas Dynamics and Turbulence Laboratory for the purpose of controlling high-speed and high Reynolds number jets. The ability of LAFPAs for use in both subsonic and supersonic jets has been explored, and experiments to date have shown that these actuators have significant potential for mixing enhancement and noise control applications. While it has been established that the actuators manipulate instabilities of the jet, the exact nature of how the actuation couples to the flow is still unclear. All of the results previously reported have been based on a nozzle extension that has an azimuthal groove of 1 mm width and 0.5 mm depth along the inner surface approximately 1 mm upstream of nozzle extension exit. The ring groove was initially added to shield the plasma arcs from the high-momentum flow. However, the effect of the ring groove on the actuation mechanism is not known. To explore this effect, a new nozzle extension is designed, which relocates the actuators to the nozzle extension face and eliminates the ring groove. Schlieren images, particle image velocimetry and acoustic results of a Mach 0.9 jet of Reynolds number ~6.1 × 10⁵ show similar trends and magnitudes with and without a ring groove. Thus, it is concluded that the ring groove does not play a primary role in the LAFPAs’ control mechanism. Furthermore, the effect of the duty cycle of the actuator input pulse on the LAFPAs’ control authority is investigated. The results show that the minimum duty cycle that provides complete plasma formation has the largest control over the jet.     

Investigation on the generation process of impact-driven high-speed liquid jets using a CFD technique

High-speed liquid jets have been applied to many fields of engineering, science and medicine. It is therefore of benefit to all these areas to investigate their characteristics by modern and inexpensive methods using a computational fluid dynamics (CFD) technique. Previously, high-speed liquid jets have been studied experimentally using a momentum exchange method, called the “impact driven method (IDM)”, by which the impact of a high-velocity projectile on the liquid package contained in the nozzle cavity produced the jet. The shock pulse reflections in the cavity caused by the impact then drove a multiple pulsed jet from the nozzle exit. In this study, a two-fluid simulation consisting of liquid and air can be successfully calculated by using a two-phase flow mixture model and a moving mesh for the projectile motion. The CFD results show good agreement to the results of previous experimental studies, both quantitatively and qualitatively. For the first time, the wave propagation within the liquid in the nozzle has been captured and analyzed, thereby demonstrating the dynamic characteristics of multiple pulsed high-speed liquid jets initiated by the IDM. This provides a breakthrough in the simulation of the supersonic injection of a liquid into air by using a well-known and user-friendly CFD software. It is useful fundamental knowledge for future studies of high-speed injection with applications in all its related fields.     

A procedure for the design of nozzles used for the production of turbulent liquid jets

Where turbulent liquid jets are used for cutting and mining purposes the pressure generated by impact must be maximized. Initial jet behaviour has an important influence on subsequent jet impact pressures at medium range. Nozzle wall boundary layer history has a strong influence on the initial jet, and certain boundary layer features can be linked to poor jet performance. The procedure outlined in this paper was developed to eliminate new nozzle designs or changes in operating conditions on the grounds of badly behaved nozzle boundary flow. The design procedure consists of a potential flow analysis and a boundary layer analysis coupled to empirical correlations for boundary layers in accelerated flows. The procedure is exemplified by application to the design of a nozzle to be used for the specific purpose of mining china clay.     

Space–time correlation measurements of high-speed axisymmetric jets using optical deflectometry

An optical deflectometry system is used to provide unique space–time correlation measurements at two positions separated by varying axial distances within a high-speed jet shear layer. The measurements were made for both pure air and for helium/air mixture jets at Mach numbers M=0.9 and M=1.5. The jets issue from round nozzles and the sensing volumes at the two measurement positions consist of small light filaments along spanwise lines that are tangential to the annular jet shear layer. Applying this technique to obtain measurements detailing the level of correlation, spectral content, and convection velocity for jet flows in these flow regimes near the end of the potential core is particularly important in the understanding and prediction of jet noise. Measurements near the end of the potential core along the jet lip line exhibit distinct cross-correlation curves for the pure air jet cases. However, helium/air mixture jets display much lower levels of correlation and little evidence of large-scale structure in the measured spectra. It is believed that the thick visual density gradients dominated by smaller scales throughout the shear layer of the helium/air mixture jets effectively mask the large-scale structure, thus, reflecting a limitation of this optical deflectometer. Finally, a decrease in normalized convection velocity with helium addition is observed.     

Density field of supersonic separated flow past an afterbody nozzle using tomographic reconstruction of BOS data

The background oriented Schlieren (BOS) technique has been applied to determine the density field in an oblique shock-separated turbulent boundary flow. Measurements were made for two cases, namely, with/without jet flow from the afterbody which is a nozzle. In addition, oil flow and Schlieren visualizations were carried out—the results show certain upstream features of interest including shock excursions. The mean density field from BOS is discussed along with results from conventional Schlieren flow visualization. The data extracted from the mean density field obtained through BOS have been compared for the jet-off and jet-on cases. The data obtained also show the mean density in the base region (jet-off case) to be about 50% of the freestream density and match the isentropic values for the underexpanded jet at the exit. The study involving shock–boundary interaction, movement of freestream shock over the afterbody in the presence of a jet plume provides understanding of flow physics in a flow regime where whole field velocity measurements are extremely difficult.     

Investigation of supersonic isobaric submerged turbulent jets

Turbulent supersonic submerged air jets have been investigated on the Mach number interval M_a = 1.5–3.4 and on the interval of ratios of the total enthalpies in the external medium and the jet i₀ = 0.01 – 1. Oxyhydrogen jets with oxidizer ratios = 0.3–5 flowing from a nozzle at Mach numbers M_a = 1 and 2.4 have also been investigated. When < 1 the excess hydrogen in the jet burns up on mixing with the air. Special attention has been paid to obtaining experimental data free of the influence on the level of turbulence in the jet of the initial turbulence in the nozzle forechamber, shock waves occurring in the nozzle or in the jet at the nozzle exit, and the external acoustic field. The jet can be divided into two parts: an initial part and a main part. The initial part extends from the nozzle exit from the section x, in which the dimensionless velocity on the jet axis u_m = u_x/u_d = 0.75. Here, u_x is the velocity on the jet axis at distance x from the nozzle exit, and u_a is the nozzle exit velocity. The main part of the jet extends downstream from the section x. For the dimensionless length of the initial part x_m = x/d_a, where d_a is the diameter of the nozzle outlet section, empirical dependences on M_a and i₀ are obtained. It is shown, that in the main part of the jet the parameters on the flow axis — the dimensionless velocity and temperature — vary in inverse proportion to the distance, measured in units of length x, and do not depend on the flow characteristics which determine the length of the initial part of the jet. The angles of expansion of the viscous turbulent mixing layer in the submerged heated or burning jet increase with decrease in i₀ and M_a and are practically independent of the afterburning process.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza. No. 4, pp. 56–62, July–August, 1988.     

A two-equation turbulence model for jet flows laden with vaporizing droplets

A two-equation turbulence model for steady incompressible two-phase flows including phase change has been recently developed by Mostafa & Elghobashi (1984). This model is tested for the flow of a turbulent axisymmetric gaseous jet laden with evaporating liquid droplets. To avoid the problem of density fluctuations of the carrier phase at this stage, only isothermal flow is considered and vaporization is assumed to be due to the vapor concentration gradient. The continuous size distribution of the droplets is approximated by finite size groups. Each group is considered as a continuous phase interpenetrating and interacting with the carrier phase. Two test cases have been predicted by the model. The first is for a Freon-11 spray issuing from a round nozzle, where experimental data are available at distances equal to or greater than 170 nozzle diameters. Good agreement between the data and the predictions was achieved. The second is for a methanol spray where no experiments are available yet and the predictions consider the flow region close to the nozzle ($\text{z/D < 40}$). The results of the methanol spray include distributions of the mean velocity, volume fractions of the different phases, concentration of the evaporated material in the carrier phase, turbulence intensity and shear stress of the carrier phase, droplet diameter distribution, and the jet spreading rate. In this case the results are analyzed based on a qualitative comparison with the corresponding single phase jet flow.