Heuy Dong Kim

Aerodynamics of high-speed railway train

Abstract Railway train aerodynamic problems are closely associated with the flows occurring around train. Much effort to speed up the train system has to date been paid on the improvement of electric motor power rather than understanding the flow around the train. This has led to larger energy losses and performance deterioration of the train system, since the flows around train are more disturbed due to turbulence of the increased speed of the train, and consequently the flow energies are converted to aerodynamic drag, noise and vibrations. With the speed-up of train, many engineering problems which have been neglected at low train speeds, are being raised with regard to aerodynamic noise and vibrations, impulse forces occurring as two trains intersect each other, impulse wave at the exit of tunnel, ear discomfort of passengers inside train, etc. These are of major limitation factors to the speed-up of train system. The present review addresses the state of the art on the aerodynamic and aeroacoustic problems of high-speed railway train and highlights proper control strategies to alleviate undesirable aerodynamic problems of high-speed railway train system.

Impulse noise and its control

Abstract The current understanding of the mechanism of origin and propagation of impulse noise from sources such as blasting, high speed train in a tunnel, sonic boom, muzzle blast and engine exhaust is reviewed. The state of the art on prediction methods and control techniques available are described. The need to develop a further understanding of the impulse noise is stressed.

Starting Transient Flow Phenomena in Inert Simulators of Solid Rocket Motors with Divergent Ports

The basic idea behind a solid rocket motor (SRM) is simple but its design is a complex technological problem requiring expertise in diverse subdisciplines to address all of the physics involved. The design optimization of high-performance rockets is more complex when the mission demands dual thrust. The motivation for the present study emanates from the desire to explain the phenomena or mechanism(s) responsible for the high ignition peak pressure (pressure peak), pressure-rise rate, instabilities, and pressure oscillations often observed during the static tests and the actual flights of certain class of high-performance SRMs with nonuniform ports [1–9]. In the SRM industry many dual-thrust motors (DTMs) are known to have experienced abnormal high ignition peak pressure often on the order of 5 times the steady state value [6]. Various measures were taken to eliminate the peak pressure, but none of the conventional remedies seemed to help. Nevertheless, through the empirical techniques increasing the port area of the motor has been proposed as one of the remedies for reducing the unusual ignition peak of the DTM. Although such a remedy could negate the unacceptable peak pressure, it has affected the high-performance nature of the motor. Hence the elimination of the unusual ignition peak and the pressure-rise rate without sacrificing the basic grain configuration or the volume loading became a meaningful objective for further studies.

On the near-field aerodynamics of a projectile launched from a ballistic range

A computational fluid dynamics method has been applied to simulate the unsteady aerodynamics of the projectile launched from a ballistic range. A moving coordinate scheme for a multi-domain technique was employed to investigate the unsteady flow with moving boundary. The coordinate system fixed to each moving domain was applied to the multi-domains, and the effect of virtual mass was added in the governing equations for each domain. The unsteady, axisymmetric Euler equation systems were numerically solved using the third order Chakravarthy-Osher total variation diminishing scheme, with MUSCL approach. The projectile mass and configuration effects on the unsteady aerodynamics were investigated based on the computational results. The present computations were validated with results of some other CFD works available. The computed results reasonably capture the major flow features, such as shock waves, blast waves, shear layers, vertical flows, etc. which are generated in launching a projectile up to a supersonic speed. The present computational method properly predicts the velocity, acceleration and drag histories of the projectile.

An Experimental Study of the Nozzle Lip Thickness Effect on Supersonic Jet Screech Tones

It is well known that screech tones of supersonic jet are generated by a feedback loop driven by the instability waves. Near the nozzle lip where the supersonic jet mixing layer is receptive to external excitation, acoustic disturbances impinging on this area excite the instability waves. This fact implies that the nozzle lip thickness can influence the screech tones of supersonic jet. The objective of the present study is to experimentally investigate the effect of nozzle-lip thickness on screech tones of supersonic jets issuing from a convergent-divergent nozzle. A baffle plate was installed at the nozzle exit to change the nozzle-lip thickness. Detailed acoustic measurement and flow visualization were made to specify the screech tones. The results obtained obviously show that nozzle-lip thickness significantly affects the screech tones of supersonic jet, strongly depending on whether the jet at the nozzle exit is over-expanded or under-expanded.

Investigation on onset of shock-induced separation

A great number of experimental data indicating shock wave/boundary layer interactions in internal or external supersonic flows were reviewed to make clear the mechanism of the interaction and to decide the onset of shock-induced separation. The interesting conclusions were obtained for the considerably wide range of flow geometries that the onset of separation is independent of the flow geometries and the boundary layer Reynolds number. It is found that the pressure rise necessary to separate the boundary layer in supersonic external flows could be applied to such internal flows as overexpanded nozzles or diffusers. This is due to the fact that the separation phenomenon caused by shock wave/boundary layer interactions is processed through a supersonic deceleration. The shock-induced separation in almost all of interacting flow fields is governed by the concept of free interaction, and the onset of shock-induced separation is only a function of the Mach number just upstream of shock wave. However, physical scales of the produced separation are not independent of the downstream flow fields.

Plume interference effects on missile bodies

Several features of plume flow and its interference effects on aft-mounted control surfaces were defined by CFD analysis. Plume pressure ratio has a significant effect on the interference.

The self-induced oscillations of the under expanded jets impinging upon a cylindrical body

The present study addresses the flow characteristics involved in the self-induced oscillations of the underexpanded jet impinging upon a cylindrical body. Both experiment and computational analysis are carried out to elucidate the shock motions of the self-induced oscillations and to find the associated major flow factors. The underexpanded sonic jet is made from a nozzle and a cylindrical body is placed downstream to simulate the impinging jet upon an obstacle. The computational analysis using TVD scheme is applied to solve the axisymmetric, unsteady, inviscid governing equations. A Schlieren system is employed to visualize the self-induced oscillations generated in flow field. The data of the shock motions are obtained from a high-speed video system. The detailed characteristics of the Mach disk oscillations and the resulting pressure variations are expatiated using the time dependent data of the Mach disk positions. The mechanisms of the self-induced oscillations are discussed in details based upon the experimental and computational results.

Experimental Study of the Shock Wave Dynamics in Micro Shock Tube

Micro shock tubes are now-a-days used for a variety engineering applications such as in the field of aerospace, combustion technology and drug delivery systems. But the flow characteristics of micro shock tube will be different from that of well established conventional macro shock tube under the influence of very low Reynolds number and high Knudsen number formed due to smaller diameter. In present study, experimental studies were carried out to a closed end (downstream) Micro Shock Tube with two different diameters were investigated to understand the flow characteristics. Pressure values were measured at different locations inside the driver and driven section. The results obtained show that with the increase in diameter the shock propagation velocity increases as well as the effect of reflected shock wave will be more significant under the same diaphragm rupture pressure.

A new method of controlling cavity-induced pressure oscillations using sub-cavity

A new passive control technique of cavity-induced pressure oscillations has been investigated numerically for a supersonic two-dimensional flow over open rectangular cavities at Mac h number 1.83 at the cavity entrance. A subcavity on the front wall of the cavity covered by a flat plate was evaluated for the effectiveness of controlling cavity-induced acoustic oscillations. The results showed that sub-cavity is very effective in reducing cavity-induced pressure oscillations. The results also showed that the resultant amount of attenuation of cavity-induced pressure oscillations was dependent on the length and thickness of the flat plate, and also on the depth of the sub-cavity used as an oscillation suppressor.