Seiji Uzawa

Numerical and Experimental Study of Active Flutter Suppression With Piezoelectric Device for Transonic Cascade

Possibility of active suppression for transonic cascade flutter with piezoelectric device was studied both numerically and experimentally. In the numerical study, a previously proposed control method in which the blade trailing edges were actively oscillated was analyzed in detail toward realistic application by a developed numerical method with flow-structure coupling. From the results, the effect of the control was confirmed, and the suppression was revealed to come from the appropriate change in the oscillatory behavior of the passage shock. Experimental study was conducted in linear cascade wind tunnel under transonic flow condition to verify that the method realized substantial effect on stability of the blade oscillation. Unsteady aerodynamic forces induced by the active oscillation of a blade on which piezoelectric devices were glued were measured and superposed with the unsteady induced force causing flutter instability. The results showed a distinctive stabilization effect of flutter suppression in the case with appropriate phase difference between original blade vibration and the active oscillation of the piezoelectric device. The active oscillation was, however, found to generate destabilization effect if the phase was inappropriate.Copyright

Impinging Atomization Enhanced by Microjet Injection - effect, mechanism and optimization -

Impinging atomization, which has been widely utilized in liquid rocket propulsion systems, is able to produce fine drops at a rated operation. In contrast, the atomization characteristics deteriorate under off design conditions when injection velocity comes to be slower. In the present study, for improving atomization characteristics at off design conditions, an effective technique is verified utilizing small amount of gas (microjet) injection. The microjet is supplied from a pressurized reservoir and is injected from the center of the liquid nozzles toward the impingement point. To clarify the flow field and the mechanism of the effect, experimental visualizations, drop size measurements and corresponding numerical analyses are carried out. It is elucidated that Sauter Mean Diameter (SMD) becomes one-tenth of the original SMD by the microjet injection with the amount of only 1% of liquid mass flow rate. The dominant non-dimensional number is found to be the ratio of the dynamic pressure (microjet/liquid jet) at the impingement point. The optimized atomization efficiency is achieved when the dynamic pressure ratio is approximately two.

Consistent Theoretical Model of Mean Diameter and Size Distribution by Liquid Sheet Atomization

A consistent theoretical model is proposed and validated for calculating droplet diameters and size distributions. The model is derived based on the energy conservation law including the surface free energy and the Laplace pressure. Under several hypotheses, the law derives an equation indicating that atomization results from kinetic energy loss. Thus, once the amount of loss is determined, the droplet diameter is able to be calculated without the use of experimental parameters. When the effects of ambient gas are negligible, injection velocity profiles of liquid jets are the essential cause of the reduction of kinetic energy. The minimum Sauter mean diameter produced by liquid sheet atomization is inversely proportional to the injection Weber number when the injection velocity profiles are laminar or turbulent. A non-dimensional distribution function is also derived from the mean diameter model and Nukiyama-Tanasawa’s function. The new estimation methods are favorably validated by comparing with corresponding mean diameters and the size distributions, which are experimentally measured under atmospheric pressure.Copyright

Heat Exchange and Pressure Drop Enhanced by Sloshing

For the prediction of heat transfer coupled with sloshing phenomena in the propellant tanks of reusable launch vehicle, the pressure drop induced by heat transfer and the dynamic motion of liquid in sub-scale vessels were experimentally observed and numerically investigated. The correlation between the pressure drop and liquid motion was confirmed in the experiment. The mechanisms enhancing heat transfer were discussed based on the computation. It was suggested that splash and wavy surface induced by violent motion of liquid cause the pressure drop in the closed vessel. In addition, as the preliminary investigation, non-isothermal sloshing of liquid nitrogen and liquid hydrogen were successfully visualized and pressure drop depending on the gaseous species was discussed.

Investigation of Microjet Injection for Reduction of Supersonic Jet Noise

Jet noise reduction is one of essential issues to realize environmentally-friendly and highly-efficient supersonic jet propulsion system. In the present study, experimental and numerical investigations were conducted in order to clarify the effect of microjet injection on supersonic jet noise. The experiments were focused on supersonic jet with Mach number up to 1.49, generated from a rectangular nozzle with high aspect ratio. The microjet injection angle was set to 90 degrees against the main jet axis. Far field measurements were conducted for the jet noise in the cases with and without microjet injection, and the noise reduction up to 7.5 dB was obtained. To study the mechanism of noise reduction, flow field visualization by schlieren technique and CFD analysis were conducted.Copyright

Experimental Study of Supersonic Jet Noise Reduction With Microjet Injection

Experimental study was conducted concerning active control of supersonic jet noise with a microjet injection technique. The microjets were injected into a rectangular main jet with Mach number up to 1.49. The nozzle lip of the main jet was equipped with 44 injection holes of the microjets, whose angles against the main jet were changed as 60 and 90 degrees. From far-field sound pressure data, a significant reduction of the jet noise by several dB was found in the cases with 60 and 90 degrees of injection angles. The microjet was found to affect all components of supersonic jet noise, namely, turbulent mixing noise, shock-associated broadband noise and screech tone noise. In the results of FFT analysis, the effect of the microjet was observed in the sound pressure level of the shock-associated broadband noise, the pressure level and frequency of the screech tone noise, and average level of the turbulent mixing noise. Schlieren visualization was also made for the jet flow, and the microjet was seen to change the shock structure and shear layer behavior of the supersonic jet.© 2009 ASME

Influence of Microjet Injection on Supersonic Jet Noise and Flow Field

Jet noise reduction is essential for realization of environmentally-friendly and highly-efficient supersonic jet engines for future civil transport. In the present study, experimental and numerical investigations were conducted to clarify the effect of microjet injection on supersonic jet noise. The experiments were focused on supersonic jet with Mach number up to 1.49 that was generated from a rectangular nozzle with high aspect ratio. Far field acoustic measurements were executed and the spectra and sound pressure data of jet noise were obtained. In order to understand the mechanism of noise reduction, flow field visualization was performed with shadowgraph technique. CFD analysis was conducted as well to observe the flow field and to estimate thrust loss due to the microjet injection.Copyright

Numerical and Experimental Investigation on Spray Flux Distribution Produced by Liquid Sheet Atomization

Eulerian-Lagrangian hybrid method is implemented for the prediction of liquid atomization phenomena produced by 2 liquid water jets impinging by an angle of 40 deg. in quiet ambient air. To calculate the flow fields with liquid/gas interface, Eulerian analyses are conducted inside a fixed computational grid system. After the atomization occurs, every droplet is converted to a spherical particle. The motion of particles are tracked in Lagrangian form. For the validation of the developed Eulerian-Lagrangian hybrid method, flow visualization by using a high-speed video camera is carried out. To obtain quantitative values of spray characteristics, the liquid mass flux distribution in space is measured by utilizing a patternator. Numerical and experimental results of atomization process and mass flux distribution of spray show a similarity, and thus the developed method is evaluated that it has potential to predict spray characteristics produced by liquid sheet atomization. The developed numerical method can calculate unsteady spray distributions not only at the plane close to the injector but also far downstream. The spray mass flux distribution in the transient state, which is hard to measure by experiment, is demonstrated.Copyright

Liquid Jet Dynamics and Primary Breakup Characteristics at Near-Field of Coaxial Type Injector

Aiming at elucidating the relationship between injection conditions and atomization characteristics of liquid jet at the coaxial type of injector, numerical analysis and theoretical analysis were carried out. For computing atomization phenomena, a numerical method has been developed and it was verified through quantitative comparisons with corresponding experiment of pinch off. It was confirmed that the method can compute inertia force, interfacial tension, viscous force and gravity force adequately, all of which generally affect atomization phenomena. Numerical analysis showed satisfactorily good agreement with corresponding experimental results of Kelvin-Helmholtz type instability due to interaction between slow liquid flow and fast gas stream. When the gaseous injection velocity became slow at constant mass flow rate, the atomization was suppressed, which was coincident with linear stability analysis of two dimensional liquid/gas parallel flow. It was clearly represented that installation of recess enhanced atomization due to straight gas flow guided by the recess.

Numerical method for an assessment of steady and motion-excited flowfields in a transonic cascade wind tunnel

This paper presents a numerical method and its application for an assessment of the flow field ins ide a wind tunnel. A structured CFD solver with overset mesh t echnique is developed in order to simulate geometrically com plex configurations. Applying the developed solver, a wh ole transonic cascade wind tunnel is modelled and simul ated by a two-dimensional manner. The upstream and downstream periodicity of the cascade and the effect of the tu nnel wall on the unsteady flow field are focused on. From the st eady flow simulations, the existence of an optimum throttle p osition for the best periodicity for each tailboard angle is sh own, which provides appropriate aerodynamic characteristics of ideal cascades in the wind tunnel environment. Unsteady simulations with blade oscillation is also conducte d, and the difference in the influence coefficients between id eal and wind tunnel configurations becomes large when the pressu amplitude increases on the lower blades. INTRODUCTION A detailed knowledge of the characteristics of moti onexcited aerodynamic force is essential for understa nding and predicting an aeromechanical behaviour in turbomach inery. In order to measure motion-excited aerodynamic force ( often referred to as unsteady aerodynamic force), a numbe r of researches have been conducted so far using linear cascade wind tunnels [1]. A typical way for obtaining unste ady aerodynamic force is to measure the responses of fl ow field and aerodynamic force acting on the airfoils under pr scribed blade motion. Such data are used for validations of numerical model for predicting and optimizing blade vibration characteristics during the design stage of turbomac hinery [2]. The operating conditions of the wind tunnels are car fully controlled to realize flow field similar to ideal i nf nite cascade (i.e. pitchwise periodicity) before detailed aerody namic measurement [3-4]. Therefore, establishing a guideli ne for controlling the wind tunnel is beneficial for gathe ring data over a wide range of flow conditions. In addition, the cascade wind tunnels often have different geometrical detai ls from an ideal infinite cascade, such as finite number of ai r oils, tailboards, and suction mechanisms. Thus, the differ ences that can arise from these real geometrical features shou ld be known in detail when a comparison is made between w ind tunnel measurement and numerical simulation results . Some past studies focuses attention on the effects of wind tunnel geometry on steady flow fields and unsteady aerodynamic force characteristics. Lepicovsky et al. [5] showed the importance of tailboard geometry on the periodicity of steady flow field through experiment al and numerical assessments. Buffum and Fleeter [6] discu ssed the deterioration of uniformity in unsteady pressure co fficient for traveling-wave-mode oscillation, with focusing on propagating wave direction and its interaction with the wind tunnel wall. Later they reported the effect of acous tic mode in the wind tunnel on the measured aerodynamic influen ce coefficients [7]. Ott et al. [8] conducted a numeri cal study for extracting the effect of tailboard on the steady an d u steady flow field in a transonic turbine cascade. All the studies reported the tailboard or wind tunnel wall have sig nif cant effect on the steady and motion-excited flow fields . The aim of this study is to develop a numerical meth od for an assessment of the flow field inside a cascad e wind tunnel for establishing a basic procedure for contr olling its flow field. The modelling of whole wind tunnel and parametric study of its geometrical setup are enabl ed y using overset mesh technique. Using the developed method, t e steady and unsteady simulations of the whole wind t u nel are conducted with focusing on the periodicity of the c as ade section and the effects of tunnel wall on the motio n-excited flow field. TRANSONIC CASCADE WIND TUNNEL An analysis target for this study is a transonic ca scade wind tunnel in the University of Tokyo. This wind t unnel is designed for aeroelastic investigations of fan or c ompressor cascade, and it had been used for fundamental resea rches on