Transient Flow Phenomena Research Articles

Development of digital particle imaging velocimetry for use in turbomachinery

Digital Particle Imaging Velocimetry (DPIV) is a powerful measurement technique, which can be used as an alternative or complementary approach to Laser Doppler Velocimetry (LDV) in a wide range of research applications. The instantaneous planar velocity measurements obtained with PIV make it an attractive technique for use in the study of the complex flow fields encountered in turbomachinery. The ability to acquire multiple measurement points of comparable accuracy to LDV results in reduced runtime and enables the study of both transient and steady state flow phenomena. Many of the same issues encountered in the application of LDV to rotating machinery apply in the application of PIV. Techniques for optical access, light sheet delivery, CCD camera technology and particulate seeding are discussed. Results from the successful application of the PIV technique to both the blade passage region of a transonic axial compressor and the diffuser region of a high speed centrifugal compressor are presented. Both instantaneous and time-averaged flow fields were obtained. The 95% confidence intervals for the velocity estimates were also determined.

An Investigation of Surge in a High-Speed Centrifugal Compressor Using Digital PIV

Compressor stall is a catastrophic breakdown of the flow in a compressor, which can lead to a loss of engine power, large pressure transients in the inlet/nacelle, and engine flameout. The implementation of active or passive strategies for controlling rotating stall and surge can significantly extend the stable operating range of a compressor without substantially sacrificing performance. It is crucial to identify the dynamic changes occurring in the flow field prior to rotating stall and surge in order to control these events successfully. Generally, pressure transducer measurements are made to capture the transient response of a compressor prior to rotating stall. In this investigation, Digital Particle Imaging Velocimetry (DPIV) is used in conjunction with dynamic pressure transducers to capture transient velocity and pressure measurements simultaneously in the nonstationary flow field during compressor surge. DPIV is an instantaneous, planar measurement technique that is ideally suited for studying transient flow phenomena in high-speed turbomachinery and has been used previously to map the stable operating point flow field in the diffuser of a high-speed centrifugal compressor. Through the acquisition of both DPIV images and transient pressure data, the time evolution of the unsteady flow during surge is revealed.

Ill-Posedness in a Thermomechanically Consistent Constrained Theory for Materials With Prescribed Temperature-Dependent Density

We examine the local dynamics of nonisothermal viscous flows in the neighborhood of the constant equilibria using the thermomechanically consistent constrained theory for materials with prescribed temperature-dependent density developed by Cao et al.. We discover that the linearized growth rate of small length scale, infinitesimal disturbances near the equilibria is proportional to the reciprocal of their wave length, a classical phenomenon known as the Hadamard instability, indicating the local ill-posedness of the constrained theory. Therefore, the use of the theory to model transient flow phenomenon is not advised. [S0021-8936(00)01901-2]

NUMERICAL STUDY ON TRANSIENT HEAT TRANSFER AND FLUID FLOW OF NATURAL CONVECTION IN AN ENCLOSURE WITH A HEAT-GENERATING CONDUCTING BODY

A comprehensive numerical study has been conducted to investigate transient heat transfer and flow phenomena of natural convection of three different fluids of sodium, air, and water in a vertical square enclosure within which a centered, squared, heat-conducting body generates heat. The physical model considered here is that a temperature difference exists across the enclosure (right cold wall and left hot wall) all the time and the body generates a constant amount of heat at an initial dimensionless time of tau0. At time tau0 the amount of heat generated by the body is suddenly increased to 10 times as large as that at time tau0. Under this situation, the flow inside the enclosure is driven by two temperature differences: a temperature difference across the enclosure and a temperature difference caused by the heat source. A ratio of these two temperatures is a key parameter in this study. We also consider the effects of Prandtl number and thermal conductivity ratio on the heat transfer and flow in the enclosure. The unsteady and two-dimensional governing equations are written in a dimensionless form with dimensionless parameters that decide the heat transfer and flow characteristics in this system. The analysis proceeds by observing variations of the streamlines and isotherms with respect to the dimensionless time for different Rayleigh numbers, temperature-difference ratios, Prandtl numbers, and thermal conductivity ratios. The variations of average Nusselt numbers on the hot and cold walls are also presented with respect to the dimensionless time, to show the transient behavior of overall heat transfer characteristics inside the enclosure.

Modeling of Transient Flow Phenomena in Continuous Casting of Steel

This paper describes initial efforts to develop and apply 3D finite-difference models to simulate transient flow in the mold. These transient flow phenomena include flow pattern oscillations caused by sudden changes in nozzle inlet conditions and rapid fluctuations in the molten steel/flux interface level at the top surface of the mold. The flow model incorporates interactions with other transport phenomena, including turbulence, superheat removal and argon gas bubble injection. Predictions are shown for the oscillatory evolution of the flow pattern from biased steady flow to symmetrical steady flow after a sudden change in inlet conditions. In addition, the predicted turbulent kinetic energy levels at steady state are shown to correlate with measured surface level fluctuations. The effect of processing conditions are consistent with experimental findings. Without argon, the greatest level fluctuations are found near the narrow face, while increased argon moves the maximum towards the center. Fluctuations decrease with deeper submergence and lower casting speed. These transient phenomena are important because they may lead to defects in the final steel product from entrainment of slag, disruption of solidification at the meniscus and non-uniform heat transfer. © 1998 Canadian Institute of Mining and Metallurgy. Published by Elsevier Science Ltd. All rights reserved.RésuméCe document décrit les éfforts initiaux de déveoppement et d'application de modèles aux différences finies en 3D à la simulation de l'écoulement transitoire dan le moulé. Ces phénoménés d'écoulement transitoiré incluent les oscillations du patron d'écoulement causées par des changements de conditions de la tuyere d'entrée ainsi que les fluctuations rapides du niveau de l'interface acier fondu/écoulement à la face superiéure du moulé. Le modéle d'écoulement incorpore des interactions avec d'autres phénoménés de transport, incluant la turbulénce, l'énlevement de la surchauffé et l'injection de bulle de gas d'argon. On predit l'evolution oscillatoire du patron d'écoulement, allant d'un écoulement permanént biaise à un écoulement permanent symétrique, après un changement soudain des conditions d'entrée. En plus, on montré que les niveaux predits d'energie cinetiqué turbulente en regime permanent sont correles avec les valeurs mesurées de fluctuations du niveau de la surface. L'effet des conditions du traitement est consistant avec les données experimentales. San argon, le niveau le plus élevé de fluctuations se situe prés de la face etroité alors que l'augmentation d'argon deplace le maximum vers le centre. Les fluctuations diminuent avec une immersion plus profonde et une vitesse de moulage plus lente. Ces phénoménés transitoires sont importants parce qu'ils peuvent conduire à des defauts dan le produit final en acier par entrainement de scories, par derangemént de la solidification au menisqué et par transfert de chaleur non uniforme. © 1998 Canadian Institute of Mining and Metallurgy. Published by Elsevier Science Ltd. All rights reserved.

Modeling of transient flow phenomena in continuous casting of steel

This paper describes initial efforts to develop and apply 3D finite-difference models to simulate transient flow in the mold. These transient flow phenomena include flow pattern oscillations caused by sudden changes in nozzle inlet conditions and rapid fluctuations in the molten steel⧹flux interface level at the top surface of the mold. The flow model incorporates interactions with other transport phenomena, including turbulence, superheat removal and argon gas bubble injection. Predictions are shown for the oscillatory evolution of the flow pattern from biased steady flow to symmetrical steady flow after a sudden change in inlet conditions. In addition, the predicted turbulent kinetic energy levels at steady state are shown to correlate with measured surface level fluctuations. The effect of processing conditions are consistent with experimental findings. Without argon, the greatest level fluctuations are found near the narrow face, while increased argon moves the maximum towards the center. Fluctuations decrease with deeper submergence and lower casting speed. These transient phenomena are important because they may lead to defects in the final steel product from entrainment of slag, disruption of solidification at the meniscus and non-uniform heat transfer.

Modeling of Transient Flow Phenomena in Continuous Casting of Steel

Modeling of Transient Flow Phenomena in Continuous Casting of Steel

Numerical modelling of unsteady flow in a twin side-by-side intake system

AbstractThe unsteady flowfield in a twin side-by-side intake arrangement arising as a result of a surge in one of the engines has been studied numerically by solving the Euler equations. Unsteadiness was introduced by prescribing a pressure disturbance at the exit plane of one of the intakes. The two cases considered correspond to static ground-running of the engines and a flight Mach number of 0.6 with a single rapidly increasing exit plane pressure disturbance representative of a pop surge. The amplitude of the imposed pressure disturbance varied between 100% and 200% of the mean exit static pressure. In the cases considered, the results indicate that static pressure attenuation of the propagating hammer shock occurs upstream of the intake entry plane, resulting in a relatively weak rarefaction wave travelling down the adjacent intake. No evidence of increased dynamic flow distortion likely to lead to a complementary surge in the adjacent engine was observed. These conclusions have been confirmed by available test data thus demonstrating the value of CFD techniques for modelling complex unsteady transient flow phenomena of this type.

Thrust misalignments of fixed-nozzle solid rocket motors

Major sources of thrust misalignments are described, and both ground test and flight data for several different upper stage solid rocket motors are presented. Special test data and flight data statistics are presented to show the known sources, such as 1) manufacturing mechanical tolerances, 2) motor case distortions and nozzle cant angles as a result of pressurization, 3) nonsymmetrical nozzle erosion, 4) transient gas flow phenomenon from internal ballistics, 5) center of mass radial offset as a result of static unbalance, and 6) static unbalance produced by propellant bore concentricity errors. Nozzle canting as a result of pressurization has been measured during hydrotest, and static firing was found to be the largest source of thrust misalignment in several composite case motors. Transient moments or side forces have been observed early in the burn period and during rapid thrust tailoff. A brief set of guidelines is presented for developing and testing new solid rocket designs to estimate the disturbing torque.

Comparison of planar fluorescence measurements and computational modeling of shock-layer flow

Planar laser-induced fluorescence is used to image transient and quasisteady flow phenomena associated with an impulsively started supersonic jet incident on a circular cylinder. The transient phenomena observed are consistent with established theoretical work on the development of nozzle flows in a shock tunnel and with well-understood shock reflection processes. The technique is also used to measure the rotational temperature in the jet and in the shock layer on the cylinder after the establishment of quasisteady flow conditions. The inviscid flow between the bow shock and the edge of the boundary layer and the viscous flow within the boundary layer are modeled numerically using an iterative scheme. Good agreement is achieved between the computational and experimental results for most of the imaged field, except for the region near the shock vertex, where flow nonuniformities near the centerline perturb the shock from the shape expected for a uniform incident flow.

Three-dimensional particle image velocimetry for use in three-phase fluidization systems

A three-dimensional particle image velocimetry (3-DPIV) system is developed to measure the three-dimensional local flow properties of gas-liquid and gas-liquid-solid fluidization systems. The 3-DPIV system requires one camera to simultaneous record orthogonal views of the flow field created by a special optical arrangement. The 3-DPIV has been successfully calibrated and is capable of providing qualitative and quantitative flow information including three-dimensional, full-field, instantaneous velocities, accelerations and holdups of each phase. In this study, sample results of the application of the 3-DPIV technique to a three-dimensional gas-liquid-solid fluidization system operating in the dispersed bubble flow regime demonstrate that the 3-DPIV technique is an effective instrument in studying the local, transient flow phenomena in multiphase systems.

Transient flow structure in the entrance region of a bubble column using particle image velocimetry

The transient flow phenomena in the entrance region of a three-dimensional bubble column system were investigated over the dispersed bubble and coalesced bubble regimes. Two different gas distributors (porous plate and perforated plate) were studied to elucidate their effects on the overall hydrodynamic behavior of the system and on the hydrodynamic behavior of the entrance region. The instantaneous and averaged flow information was obtained through the use of a Particle Image Velocimetry (PIV) system. The PIV system is a non-intrusive technique capable of simultaneously providing the full-field flow information of both the gas and liquid phases. The overall gas holdup of the bubble column system was used to demarcate the flow conditions, in order of increasing gas velocity, into the dispersed bubble regime and the coalesced bubble regime which consisted of the vortical—spiral and turbulent flow conditions. The instantaneous flow fields and averaged liquid velocity and gas holdup profiles obtained through the PIV system provide a better understanding of the mechanisms responsible for the development of the flow in each regime. The averaged results were also used to determine the length required for the flow to become fully developed. The results show that the entrance effect are significant in the dispersed bubble regime but diminish with increasing velocity in the coalesced bubble regime, represented by the vortical—spiral and turbulent flow conditions.

Numerical Analysis Versus Experimental Investigation of a Distributor-Type Diesel Fuel-Injection System

A production distributor-type fuel-injection system for diesel engines has been extensively investigated via computer-assisted simulation and experimentation. The investigation was mainly aimed at assessing and validating a sophisticated computational model of the system, developed with specific attention given to the pump and to some important aspects concerning the injection pressure simulation, such as the dynamic effects of the injector needle lift, the flow unsteadiness, and compressibility effects on the nozzle-hole discharge coefficient. The pump delivery assembly was provided with a valve of the reflux type. This presented a flat in the collar, forming a return-flow restriction with the seat, and had no retraction piston. A single-spring injector, with a reduced sac volume, was fitted to the system. The numerical analysis of transient flow phenomena linked to the mechanical unit dynamics, including possible cavitation occurrence in the system, was performed using an implicit finite-difference algorithm, previously set up for in-line injection equipment. Particular care was exercised in modeling the distributor pump so as to match the dynamics of the delivery-valve assembly to the pressure wave propagation in the distributor and its outlets. The so-called minor losses were also taken into account and it was ascertained that sudden expansion and contraction losses were significant for the type of pump examined. The experimental investigation was performed on a test bench at practical pump speeds. Pressures were measured in the pumping chamber, at two different pipe locations, and upstream to the needle seat opening passage. This last measurement was taken in order to evaluate the nozzle-hole flow coefficient with the support of the simulation, using experimental values of the needle lift, injection rate, and injected fuel quantity as known variables. The numerical and experimental results were compared and discussed, showing the validity of the model. The injection pressure time history and the influence of the delivery return-flow restriction on the system performance were numerically examined.

The transient magnetohydrodynamic phenomena in electromagnetic levitation processes

This paper presents an analytical study on the transient electrodynamic and fluid flow phenomena in a magnetically-levitated liquid sphere. Using the method of separation of variables, analytical solutions are derived for the transient electromagnetic field inside and outside the sphere in the form of the magnetic vector potential, and the fluid flow field in terms of the stream function, respectively. Both asymptotic and detailed analyses are described. It is found that the decay of a levitation system is characterized by three quite different time scales: one for the electromagnetic field, one for the fluid flow field, and one for the thermal field. The electromagnetic field decays several orders of magnitude faster than either the fluid flow field or the temperature field, and therefore its effect can be well ignored when the flow and/or thermal fields are considered. The ratios of these time scales may be related to the nondimensional system numbers that characterize the system. Detailed calculations show that when started from a symmetric velocity field, the decay results in a reduction of the velocity level but the flow pattern remains unchanged, whereas the evolution from an asymmetric flow field involves both a diminution in the velocity level and a change in the flow pattern. The two decaying processes are also characterized by different time scales, with the one for the asymmetric case being larger. Application of the analyses to design and control the experiments of undercooling and/or free surface oscillations under microgravity is also discussed. It is found that an undercooling of a desired degree may be obtained free from the internal flow influence if an appropriate starting temperature is chosen for the experiment. It is also possible to carry out the experiments of free surface oscillation without the interference from the internal flow; however, careful control is needed to maintain the liquid at a relatively constant temperature.

Numerical study of transient flow phenomena in shock tunnels

Computational fluid dynamics (CFD) was used to study some transient flow features that can occur during the startup process of a shoch tunnel. The investigation concentrated on two areas: (1) the flow near the endwall of the driven tube during shock reflection and (2) the transient flow in the nozzle. The driven tube calculations were inviscid and focused on the study of a vortex system that was seen to form at the driven tube's axis of symmetry. The nozzle flow calculations examined viscous and inviscid effects during nozzle startup. The CFD solutions of the nozzle flows were compared with experimental data to demonstrate the effectiveness of the numerical analysis.

Fast Correlation Technique of Particle Image Velocimetry.

Particle image velocimetry (PIV) is a useful tool in the study of transient fluid flow phenomena, The light of a laser sheet scattered by seeding particles generates a moving particle-image pattern. This pattern is recorded using a multiple-exposure photographic technique, where the distance between corresponding particle images in the interrogation region is proportional to the local displacement. To determine the direction of the velocity vector from the multiple-exposure photograph, a velocity bias technique is used. The processing techniques for PIV data have traditionally been tedious and time-consuming. In this paper, a new approach to PIV data processing is presented. The 2-D intensity distribution array in the interrogation region is replaced by two orthogonal 1-D distribution arrays using integration. Each velocity component is extracted by autocorrelation of the 1-D array. This system has been applied to a quantitative measurement of the vortical structure behind a circular cylinder.

Transient flow phenomena and reaction forces during blowdown of gas at high pressures through relief lines behind rupture discs

AbstractIn general, rupture discs are designed to give a virtually instantaneous pressure relief opening to a relief system at a predetermined pressure. For safe design of pressure relief systems, consideration must be given to the sizing and the specification of the rupture disc but also to the reaction forces that result when a gas is discharged. These reaction forces can be excessive in the case of venting at high pressures.When a disc ruptures, a shock wave propagates through the gas in the piping behind the rupture disc. This exerts a transient reaction force on the pressure relief system. In the case of steady supersonic flow choking often occurs at one or more points in the pressure relief line and a shock is formed in the pipe. The pressure in the relief line may increase considerably during blowdown as a result of choking at the end of the pipe. Depress‐surization experiments were conducted with nitrogen at high pressures.The results of the pressure measurements in the piping during blowdown are compared with exploratory calculations. The reaction forces exerted on a pressure relief system can be assessed from the pressures and the mass flow rate of the gas as a function of time. For safe design of pressure relief systems, both steady‐state flow reaction forces and the initial transient forces should be considered.

Comparing Simulated and Experimental Hysteretic Two‐Phase Transient Fluid Flow Phenomena

A hysteretic model for two‐phase permeability (k)‐saturation (S)‐pressure (P) relations is outlined that accounts for effects of nonwetting fluid entrapment. The model can be employed in unsaturated fluid flow computer codes to predict temporal and spatial fluid distributions. Consideration is given to hysteresis in S‐P relations caused by contact angle, irregular pore geometry, and nonwetting fluid entrapment effects and to hysteresis in k‐S relations caused by nonwetting fluid entrapment effects. An air‐water flow experiment is conducted with a 72‐cm vertical soil column where the water table is fluctuated to generate scanning S‐P paths. Water contents are measured via a gamma radiation system, and water pressures are measured via pressure transducers connected to ceramic tensiometers inserted in the soil column. Computer simulations of the experiment employing the hysteretic k‐S‐P model and a nonhysteretic k‐S‐P are compared with measured water contents and pressures. Close agreement is found between experimental water contents and those predicted by a numerical code employing the hysteretic k‐S‐P relations. When nonhysteretic k‐S‐P constitutive relations are utilized, there is poor agreement between measured and predicted water saturations of the scanning paths. Only one more parameter is needed to model two‐phase hysteretic fluid behavior than to model nonhysteretic behavior. Results of this study suggest that consideration should be given to effects of hysteresis in k‐S‐P relations to accurately predict fluid distributions.

Color schlieren methods in shock wave research

Schlieren methods are widely known and well established to visualize refractive index variations in transparent media. The use of color allows one to obtain more data and previously inaccessible information from a picture taken with this technique. In general, a hue can be related to a certain strength or a certain direction of a refractive index gradient. While the first case essentially corresponds to the usual black- and-white system the latter correlation cannot be made adequately evident without the use of color. Two color schlieren techniques are presented here, which reach or even exceed the quality and sensitivity range of conventional black- and-white methods. Using a powerful short duration light source these methods are applied to visualize transient flow phenomena in a shock tube.

Transient shear flow of nematic liquid crystals: Manifestations of director tumbling

Calculations of a variety of transient flow phenomena based on the Leslie–Ericksen model for nematic liquid crystals are presented. Emphasis is placed on the behavior of nematics subject to director tumbling. A wide range of complicated transient phenomena such as oscillatory responses are predicted when the Ericksen number is sufficiently high to cause significant rotation of the director at steady state. Particular attention is given to relaxation processes such as constrained elastic recoil and stress relaxation that arise from the interaction of the relaxing director profile with the viscous response of the nematic. These relaxation processes are dramatically different in tumbling and flow aligning systems, due to the saturation of the director profile at high Ericksen number for the flow aligning case. A qualitative physical model of a textured polymer liquid crystal is presented in which the dynamics on a local scale governed by the distance between defects are assumed to be similar to the macroscopic dynamics presented in these calculations. By assuming (i) director tumbling and (ii) the existence of a limiting Ericksen number in polymer systems, a wide range of published observations in lyotropic liquid crystalline polymer solutions may be qualitatively reproduced.