Hyo-Won Yeom

Investigation of Rocket Exhaust Diffusers for Altitude Simulation

The design and operational parameters of rocket exhaust diffusers equipped to simulate high-altitude rocket performance on the ground were investigated and characterized using a comprehensive approach (theoretical, numerical, and experimental). The physical model of concern includes a rocket motor, a vacuum chamber, and a diffuser, which have axisymmetric configurations. Further, the operational characteristics of a rocket exhaust diffuserwereanalyzed froma flowdevelopmentpointof view.Emphasiswasplacedondetailed flowstructure inthe diffuser, to observe the pressure oscillation in both the vacuum chamber and diffuser, which determines the minimum rocket-motor pressure required to start the diffuser. Numerical simulations were compared with experimental data on startup and in operational conditions to understand the effects of major design parameters, including the area ratio of diffuser to rocket-motor nozzle throat, the rocket-motor pressure, and the vacuumchamber size. Nomenclature Ad = inner cross-sectional area of diffuser Ade = exit cross-sectional area of diffuser Ae = exit cross-sectional area of rocket nozzle At = throat cross-sectional area of rocket nozzle

Numerical Analysis of a Scramjet Engine with Intake Sidewalls and Cavity Flameholder

A detailed three-dimensional numerical simulation was conducted to investigate the flow and H2-air mixing characteristics in a scramjet engine with two intake sidewalls and a cavity flameholder. Turbulence closure was achieved using a model that combines the low-Reynolds-number k-e two-equation model and Sarkar and Wilcox’s compressible turbulent-correction model. The governing equations were solved numerically by means of a finite volume, preconditioned flux-differencing scheme. Cases of with and without intake sidewalls were considered. Intake sidewalls were found to strongly affect the inlet flow structure, which became more complex in the nonuniform flowfield on the cross section perpendicular to the engine axis. The complex and nonuniform flow affected the H2-air mixing pattern inside the combustion chamber, unlike the pattern of the case of without sidewalls. To verify the accuracy of the simulation, the computed wall pressure was compared with the experimental data. Mixing efficiency and fuel-propa...

On the Assessment of Compressibility Effects of Two-Equation Turbulence Models for Supersonic Transition Flow with Flow Separation

An assessment of two-equation turbulence models, the low Reynolds k-e and k-ω SST models, with the compressibility corrections proposed by Sarkar and Wilcox, has been performed. The compressibility models are evaluated by investigating transonic or supersonic flows, including the arc-bump, transonic diffuser, supersonic jet impingement, and unsteady supersonic diffuser. A unified implicit finite volume scheme, consisting of mass, momentum, and energy conservation equations, is used, and the results are compared with experimental data. The model accuracy is found to depend strongly on the flow separation behavior. An MPI (Message Passing Interface) parallel computing scheme is implemented.

Control-Oriented Model for Intake Shock Position Dynamics in Ramjet Engine

R AMJET-POWERED engines have a history of over 100 years [1]. The secret to efficiency, safety, and performance of ramjet combustion systems has been the correct location and control of the terminal shock in the intake duct [2]. The position of the intake shock is affected by perturbations propagating upstream from the combustor [3] and from disturbances in the freestream [4]. These can lead to the familiar instability problems of unstart or buzz [5,6]. At the same time, these very instabilities can be controlled by pressure perturbations injected by suitably manipulating the exhaust nozzle throat area [7]. To properly evaluate such a control, it is necessary to obtain amodel for the ramjet engine including the intake, combustor, and exhaust nozzle. The issue of shock position control has always been an interesting one [8], but accurately sensing the position of the intake shock for the purpose of active control has been a challenge. In recent years, though, the problemhas attractedmuch attention [9,10], and building on the earlier work on the dynamics of shocks in ducts [3,11], models that may be used for designing intake shock position controllers have been obtained [12,13].However, itmay be noted that all thesemodels were limited to the intake alone and hence could not be directly used to study the effect of the exhaust nozzle throat area variation on the intake shock location. A model that couples the intake with the combustor and exhaust nozzle for a ramjet was first realized recently by our coworkers [14], and it was used [15] to derive a control law for the intake shock location using the nozzle throat area as input. A significant feature of the model in [14] was the use of time lags to capture the physics of upstream anddownstreampropagatingwaves between the intake and combustor. However, these time lags were applied to the primitive variables for simplicity instead of the specific pressure or entropy waves [12]. Second, despite the relatively low order of the global model in [14], the model for each component was multiparameter and nonlinear, in order to capture the various physical phenomena in the intake and combustor. Thus, in deriving the control law in [15], local linearized reduced-order models at several operating points had to be obtained using a system identification tool. The system identification is, unfortunately, a mathematical procedure that results in a set of states that cannot be easily related to the physical variables, thus masking the physical relationships inherent in the system. Control of the terminal shock positionwas obtained indirectly in [15] by defining a related parameter called intake backpressure margin. The present Note differs from these previous works in three significantways. First, wewrite an explicit equation for the dynamics of the intake shock location in a coupled model of the intake and the combustor plus nozzle. Second, a single time-lag parameter is obtained numerically, and applied directly to the shock position variable. Third, the various component models are written using standard quasi-one-dimensional flow relations in such a manner that the physical relationships they represent are apparent. These features make the present model more suitable for controller design with the objective of regulating the intake shock position; hence, it is called a control-oriented model.

Inlet Buzz and Combustion Oscillation in an Axisymmetric Ramjet Engine

A unified numerical analysis was conducted to investigate the inlet buzz and combustion oscillation in an axisymmetric ramjet engine. The inlet buzz phenomenon in the subcritical operation arises large pressure oscillation, combustion instability, engine surge, and thrust loss, etc. The physical model of concern includes the entire engine flow path, extending from the leading edge of the inlet center-body through the exhaust nozzle. The theoretical formulation is based on the Farve-averaged conservation equations of mass, momentum, energy, and species concentration, and accommodates finite-rate chemical kinetics and variable thermo-physical properties. Turbulence closure is achieved using the combined model of a low-Reynolds number k-e two-equation model and Sarkar’s compressible turbulence model. The detail flow structures such as buzz shock train, shock/boundary layer interaction, and flame fluctuation are observed. Both the driving source to the inlet buzz and buzz effects on both flow and flame evolutions are studied.

A Numerical Analysis of Supersonic Intake Buzz in an Axisymmetric Ramjet Engine

A numerical analysis was conducted to investigate the inlet buzz and combustion oscillation in an axisymmetric ramjet engine with wedge-type flame holders. The physical model of concern includes the entire engine flow path, extending from the leading edge of the inlet center-body through the exhaust nozzle. The theoretical formulation is based on the Farve-averaged conservation equations of mass, momentum, energy, and species concentration, and accommodates finite-rate chemical kinetics and variable thermo-physical properties. Turbulence closure is achieved using a combined scheme comprising of a low-Reynolds number k-e two-equation model and Sarkar’s compressible turbulence model. Detailed flow phenomena such as inlet flow aerodynamics, flame evolution, and acoustic excitation as well as their interactions, are investigated. Mechanisms responsible for driving the inlet buzz are identified and quantified for the engine operating at subcritical conditions.

Study on Design- and Operation- Parameters of Supersonic Exhaust Diffuser

A comprehensive approach (theoretical, numerical, and experimental approach) has been conducted to study the designand operationparameters of supersonic exhaust diffusers simulating high altitude condition on the ground. A physical model of concern includes a rocket motor, a vacuum chamber, and a diffuser, which have axisymmetric configurations, using nitrogen gas as a driving fluid. An analysis has been conducted to investigate operation characteristics of a supersonic exhaust diffuser from a flow-development point of view. Emphasis is placed on physical phenomena and several designand operationparameters of the diffuser such as the area ratio of the diffuser to the rocket nozzle, the vacuum chamber size, and the minimum starting pressure of the rocket motor to start the diffuser.

Numerical Analysis of a Model Scramjet Engine with Two Intake Side Walls and a Cavity Flame-Holder

A detailed 3D numerical simulation of the flow and H2-air mixing characteristics in a model scramjet engine with two intake-sidewalls and a cavity flame-holder was conducted. Turbulence closure was achieved by a model combining the low-Reynolds-number k-e twoequation model and Sarkar and Wilcox’s compressible turbulent correction model. The governing equations were solved numerically by means of a finite-volume, preconditioned flux-differencing scheme. Cases with and without intake side walls were considered. Intake side walls were found to strongly affect the inlet flow structure, which became more complex in the non-uniform flow field on the cross section perpendicular to the engine axis. The complex and non-uniform flow affected the H2-air mixing pattern inside the combustion chamber, unlike the pattern of the case without side walls. Mixing efficiency and fuel propagation rate were evaluated for the two cases with and without side walls. To verify the accuracy of the simulation, the computed wall pressure was compared with experimental data.