Masters Theses

Date of Award


Degree Type


Degree Name

Master of Science


Mechanical Engineering

Major Professor

Frank Collins

Committee Members

Ahmad Vikili, Roy Schulz


The Arnold Engineering Development Center (AEDC) offers a unique test capability for the evaluation of inlet-engine compatibility. The capability features a variable-attitude and variable-Mach number free-jet nozzle that subjects an aircraft propulsion system to a flow field approximating the flight environment. The free-jet nozzle provides a flow quality commensurate with inlet-engine compatibility test requirements over a wide range of pitch angles, yaw angles, and Mach numbers. The development of the nozzle centered on achievement of the required flow quality.

Initial flow quality experiments revealed a tendency for large, secondary vortical flows to develop in the free-jet nozzle flow field. These vortical flows severely degraded the flow quality delivered by the nozzle. The failure of the initial nozzle to achieve flow quality goals motivated research focused on preventing the formation of vortices in subsonic free-jet nozzles.

This thesis describes a comprehensive investigation that coupled water flow and airflow experiments to improve the understanding of the mechanisms leading to the formation of nozzle vortices, and to develop vortex suppression methods. Providing a unique flow visualization capability, the water tunnel revealed features of the complex flow field associated with the vortex formation and facilitated the identification of vortex suppression techniques. The airflow tests provided detailed flow-field measurements for validating water tunnel findings and verifying achievement of flow requirements.

The water flow and airflow experiments provided information that enabled the development of specifications for a subsonic free-jet nozzle to be applied in the AEDC Aeropropulsion Systems Test Facility (ASTF) free-jet test system. The experiments used two sub-scale models of the ASTF Test Cell C-2. The airflow model quantified nozzle exit flow quality through measurements of Mach number and flow angle distributions. The measurements were obtained at nominal Mach numbers ranging from 0.3 to 0.9 and nozzle pitch angles ranging from 0 to 50 deg. The water flow model, installed in the University of Tennessee Space Institute water tunnel, used dye injection to delineate streaklines of the flow entering and exiting free-jet nozzle configurations.

The development of the free-jet nozzle employed multiple entries in both the air flow and water flow facilities. The work progressed through the process of identifying flow anomalies, determining the critical parameters that dominate the secondary flow formation, identifying candidate flow quality improvement methods, selecting a method for application in the ASTF free-jet nozzle, and validating nozzle configurations prior to full-scale implementation.

This thesis provides both visual and measured free-jet nozzle flow characteristics. obtained during the investigation. Comparisons of airflow and water flow simulations illustrate the validation of the water tunnel as a tool for studying secondary internal flows. Parametric results reveal the nozzle and installation features that influence the secondary flow formation. Finally, results show the numerous methods investigated for preventing the formation of vortices.

The research yielded two successful vortex suppression methods. Each method modified the flow field in the vicinity of the vortex attachment point, near the nozzle inlet, to prevent vortex formation. The thesis describes the selection and application of one method in a nozzle configuration that meets the requirements for the ASTF free-jet test system.

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