Separated nozzle flow

A separated turbulent flow in an axisymmetrical nozzle is studied numerically. Two configurations nozzle are investigated. The first one is the truncated ideal contour nozzle, DLR-TIC, is fed with nitrogen. The second configuration is called the thrust optimized contour nozzle or TOC type, ONERA-TOC, where the operating gas is a hot air. The classical pattern of a free shock separation is obtained for different values of the nozzle pressure ratio. The results are compared and validated using experimental data.     

Simulation of the starting flow in a wedge–like nozzle

The starting process of the flow in a wedge-like expansion nozzle of a shock tunnel is simulated by an unsplit 2-D GRP scheme on an unstructured grid. The scheme is briefly outlined and results are presented and discussed in comparison to the experimental (shadowgraph) findings obtained by Amann. The simulated pattern of reflected and transmitted shock waves in the nozzle inlet region and inside the nozzle is found to agree well with the experimental data. Received 5 April 1996 / Accepted 16 June 1997     

Slowly mixing cylinder in a cone-shaped nozzle

We report on a surprising phenomenon in a turbulent jet setup which uses a cone-shaped nozzle with an excentric inlet. Inside the nozzle a slowly mixing, rotating cylinder surface was observed. The speed of mixing on this cylinder surface is reduced by approximately a factor of 8 compared to the remaining flow field of the nozzle. The phenomenon seems to be independent of the Reynolds number and the pressure distribution inside the nozzle.     

The effect of nozzle layout on droplet ejection of a piezo-electrically actuated micro-atomizer

We study here effects of nozzle layout on the droplet ejection of a micro atomizer, which was fabricated with the arrayed nozzles by the MEMS technology and actuated by a piezoelectric disc. A theoretical model was first built for this piezoelectric-liquid-structure coupling system to characterize the acoustic wave propagation in the liquid chamber, which determined the droplet formation out of nozzles. The modal analysis was carried out numerically to predict resonant frequencies and simulate the corresponding pressure wave field. By comparing the amplitude contours of pressure wave on the liquid-solid interface at nozzle inlets with the designed nozzle layout, behaviors of the device under different vibration modes can be predicted. Experimentally, an impedance analyzer was used to measure the resonant frequencies of the system. Three types of atomizers with different nozzle layouts were fabricated for measuring the effect of nozzle distribution on the ejection performance. The visualization experiment of droplet generation was carried out and volume flow rates of these devices were measured. The good agreement between the experiment and the prediction proved that only the increase of nozzles may not enhance the droplet generation and a design of nozzle distribution from a viewpoint of frequency is necessary for a resonant related atomizer. The project supported by the National Natural Science Foundation of China (50405001).     

High enthalpy nozzle flow sensitivity study and effects on heat transfer

The sensitivity of the flow along the nozzle and in the test section of high enthalpy wind tunnels to the thermochemical response of the nozzle expansion process, as well as effects on the pressure and heat transfer distributions over the Electre blunt cone standard test model, are examined in the framework of properly characterizing the test section flow field in such facilities. Particularly sensitive to the thermochemical behaviour of the nozzle flow, in the facilities under consideration, are the static pressure, static temperature and Mach number, whereas stagnation point (pitot) pressure and heat transfer data or freestream velocity are inadequate for the characterization of the thermochemical state of the flow. The Electre and nozzle wall pressure data in the F4 arc jet wind tunnel suggest, in contrast to nonequilibrium computations, that the flow in the F4 nozzle is close to equilibrium. In the HEG and, to some extent, the T5 piston-driven shock tunnels, there are indications that significant heat losses occur in the reservoir. Lastly, simple semi-empirical formulations for stagnation point heating are shown to perform reasonably well in high enthalpy flow conditions.     

Delayed detached eddy simulation of the end-effect regime and side-loads in an overexpanded nozzle flow

The separated flow in an overexpanded nozzle featuring a restricted shock separation is investigated numerically using delayed detached eddy simulation and compared with the experimental data of Nguyen et al. (Int J Flow Turbul Combust 71(1):161–181, 2003). First, the enormous cost of a Large Eddy Simulation for such a nozzle flow is assessed before being performed to motivate the practical need for using an hybrid RANS/LES method. The calculation is then used to investigate the “end-effect” regime which involves a strong global unsteadiness with very large amplitude fluctuations of about 15–20% of nozzle divergent length. The flow regime is characterized by high wall pressure fluctuations which are hopefully nearly axisymmetric. The main properties (rms levels, amplitude of displacement of the separation) of the motion are rather well reproduced by DDES compared to the experiment. However, a major difference lies in the frequency of the computed motion which is higher than in the experiment. This major discrepancy is currently not explained by the author. The properties of the side-loads are also briefly discussed.      

A numerical simulation on diffuser‐nozzle based piezoelectric micropumps with two different numerical models

This study has been conducted to compare two different numerical models for the evaluation of the performance characteristics of a diffuser‐nozzle based piezoelectric micropump. Here, the transient displacements of the membrane and the flow characteristics in the piezoelectric micropump have been closely investigated with the FSI model and the prescribed deformation model for two different frequencies. It has been found that the behaviour of the membrane computed with the FSI model is not in accordance with that with the prescribed deformation model, and that the net flow rate with the FSI model is larger than that with the prescribed deformation model. Therefore, the choice of numerical model is very important in conducting numerical analysis for piezoelectric micropumps. The results of this study can be utilized as basic data for the design and analysis of piezoelectric micropumps. Copyright © 2006 John Wiley & Sons, Ltd.     

Flows on the nozzle plate of an inkjet printhead

Flow patterns of ink layers on the nozzle plate of a piezo-driven printhead are investigated. Two different flow types are identified. First, a jet of droplets induces a radial airflow in the direction of the jet, which in turn causes a liquid flow towards the nozzle. Second, the movement of the meniscus in the nozzle causes an equally strong flow, but completely different flow patterns. The results are presented in a phase diagram with pulse amplitude and firing frequency as parameters.     

Aeroelastic stability analysis of flexible overexpanded rocket nozzle

The aim of this paper is to present a new aeroelastic stability model taking into account the viscous effects for a supersonic nozzle flow in overexpanded regimes. This model is inspired by the Pekkari model which was developed initially for perfect fluid flow. The new model called the “Modified Pekkari Model” (MPM) considers a more realistic wall pressure profile for the case of a free shock separation inside the supersonic nozzle using the free interaction theory of Chapman. To reach this objective, a code for structure computation coupled with aerodynamic excitation effects is developed that allows the analysis of aeroelastic stability for the overexpanded nozzles. The main results are presented in a comparative manner using existing models (Pekkari model and its extended version) and the modified Pekkari model developed in this work.     

Investigation of hypersonic nozzle flow uniformity using NO fluorescence

Planar laser-induced fluorescence visualisation is used to investigate nonuniformities in the flow of a hypersonic conical nozzle. Possible causes for the nonuniformity are outlined and investigated, and the problem is shown to be due to a small step at the nozzle throat. Entrainment of cold boundary layer gas is postulated as the cause of the signal nonuniformity. PACS 47.80.Jk, 47.40.Ki, 47.60.+i     

Investigation of the effect of nozzle shape on supersonic/hypersonic impactors designed for size discrimination of nanoparticles

In this study the flow field and the nanoparticle collection efficiency of supersonic/hypersonic impactors with different nozzle shapes were studied using a computational modeling approach. The aim of this study was to develop a nozzle design for supersonic/hypersonic impactors with the smallest possible cut-off size d₅₀ and rather sharp collection efficiency curves. The simulation results show that the changes in the angle and width of a converging nozzle do not alter the cut-off size of the impactor; however, using a conical Laval nozzle with an L/D_n ratio less than or equal to 2 reduced d₅₀. The effect of using a cap as a focuser in the nozzle of a supersonic/hypersonic impactor was also investigated. The results show that adding a cap in front of the nozzle had a noticeable effect on decreasing the cut-off size of the impactor. Both flat disks and conical caps were examined, and it was observed that the nozzle with the conical cap had a lower cut-off size.     

Numerical simulations of nozzle starting process

The starting process of two-dimensional nozzle flow is investigated both experimentally and numerically. Discussions are made on the comparison between experimental and numerical results. Performances of two numerical methods which are used in the present study of unsteady flow problem are also discussed and indications for future development of numerical tools to study nozzle problems are obtained. Received 16 June 1998 / Accepted 17 August 1998     

Dynamics of the rolling and sliding of an elongated cylinder

A snap of a finger on an elongated cylinder produces on it a surprising fast spinning motion during which its mass center rises with very large oscillations. After that, the mass center goes to a long quasi-stationary oscillations state and eventually goes down very slowly. To explain this behavior we present a theoretical and numerical analysis of the dynamics of a spinning elongated cylinder moving with a single point of contact on a horizontal plane under the action of gravity. The study has been made taking into account the rolling and sliding dissipation as well as the Kutta–Joukowski airflow effect. The results of the simulations are in agreement qualitatively with the observed real motion.     

Semiempirical method of calculating overexpanded turbulent separated flow in a conical laval nozzle

A mathematical model of separated nozzle flow is developed. The model takes into account the effect of the boundary layer and the pressure variation over the entire separation zone inside the nozzle. The effect of the geometric and gas dynamic factors on the separated flow pattern is investigated numerically.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 60–66, November–December, 1988.     

A random simulation of droplet distribution in nozzle and plume flows

A simple entrainment model is used to estimate droplet streamlines, velocity and mass flux in rocket exhaust plumes. Since droplet mass flux constitutes only about 1% of the exhaust mass flux, the effect of droplet entrainment on the gas flow is neglected. The novelty of the present model is in obtaining the droplet distribution within the nozzle by assuming a small radial random velocity component for droplets at the throat. Gas flow in the nozzle is approximated as isentropic plus a correction for the boundary layer. The computed distribution of droplet mass flux is found to be in good agreement with experimental data. Received 15 January 1996 / Accepted 11 September 1996     

Numerical investigation of transient nozzle flow

Abstract. The starting process of two-dimensional and axisymmetric nozzle flows has been investigated numerically. Special attention has been paid to the early phase of the starting process and to the appearance of a strong secondary shock wave. For both cases, shock intensities and velocities are obtained and discussed. The flow evolution in the axisymmetric case is proved to be more complex and the transient starting process is slower than in the plane case. Finally, the effects of changing the nozzle angle and the incident shock wave Mach number on the transient flow are addressed. It is shown that a faster start-up can be induced either by decreasing the nozzle angle or increasing the Mach number of the incident shock wave. Received 16 November 2001 / Accepted 24 September 2002 / Published online 4 December 2002 Correspondence to:A.-S. Mouronval (e-mail: mouronv@coria.fr)     

Air flow and pressure inside a pressure-swirl spray and their effects on spray development

Air flow and pressure inside a pressure-swirl spray for direct injection (DI) gasoline engines and their effects on spray development have been analyzed at different injector operating conditions. A simulation tool was utilized and the static air pressure at the centerline of the spray was measured to investigate the static pressure and flow structure inside the swirl spray. To investigate the effect of static air pressure on swirl spray development, a liquid film model was applied and the Mie-scattered images were captured. The simulation and experiment showed that recirculation vortex and air pressure drop inside the swirl spray were observable and the air pressure drop was greater at high injection pressure. At high fuel temperature, the air pressure at the nozzle exit showed higher value compared to the atmospheric pressure and then continuously decreased up to few millimeters distance from the nozzle exit. The pressure drop at high fuel temperatures was more than that of atmospheric temperature. This reduced air pressure was recovered to the atmospheric pressure at further downstream. The results from the liquid film model and macroscopic spray images showed that the air pressure started to affect the liquid film trajectory about 3 mm from the nozzle exit and this effect was sustained until the air pressure recovered to the atmospheric pressure. However, the entrained air motion and droplet size have more significant influence on the spray development after the most of the liquid sheet is broken-up and the spray loses its initial momentum.     

Numerical investigation of overexpanded nozzle flows

The flow in a planar overexpanded nozzle with a slope discontinuity is studied numerically by means of two- (2D) and three-dimensional (3D) Reynolds-averaged Navier–Stokes simulations and is compared to experimental results. The nozzle pressure ratios (NPR) vary from 1.6 to 10. A good agreement is found between experimental and numerical results and two configurations are observed: under a certain critical NPR, the flow is shown to be asymmetrical with respect to the nozzle axis, while it is perfectly symmetrical for higher NPRs. The value of the critical NPR is found to be very dependent on the turbulence model. Finally, an hysteresis phenomenon is evidenced since the NPR at which the change of flow configuration occurs is different whether the NPR is increasing or decreasing in the nozzle.     

3D tangentially injected swirling recirculating flow in a nozzle with a slotted‐tube—Effects of groove parameters

A numerical prediction for 3D swirling recirculating flow in an air‐jet spinning nozzle with a slotted‐tube is carried out with the realizable k–ε turbulence model. The effects of the groove parameters on the flow and yarn properties are investigated. The simulation results show that some factors, such as reverse flow upstream of the injector, vortex breakdown downstream of the injector, corner recirculation zone (CRZ) behind the step and vortex ring in the groove caused by the groove geometric variation, are significantly related to fluid flow, and consequently to yarn properties. With increasing groove height, the length of the CRZ increases, while the initial vortex ring in the groove decreases and a same direction rotating vortex forms in the bottom of the groove. Similarly, as the groove width increases, the extent of both vortex breakdown in downstream of the injectors and the vortex ring in the groove increases slightly, whereas the CRZ lengths in stream‐wise direction decrease. Some factors, such as the negative tangential velocities, the size of the vortex rings in the grooves and the CRZ, are constant for nozzles with different groove lengths. Copyright © 2009 John Wiley & Sons, Ltd.