T. Terry Ng

AERO-OPTICAL CHARACTERISTICS OF COMPRESSIBLE, SUBSONIC TURBULENT BOUNDARY LAYERS

An extensive experimental study of optical aberrations due to propagation through fully-developed turbulent boundary layers at high subsonic Mach numbers was performed. Time-resolved, high- bandwidth, direct optical measurements of the dynamic aberrations were made using a Malley probe. The probe was used to obtain the convective speeds of the optically-significant turbulence structures and to measure the optical path differences. Measurements were made over a range of boundary layer thicknesses and Mach numbers. Optical distortions were found to scale linearly with boundary layer thickness and freestream density, and to go as the square of the freestream Mach number.

The Optical Environment of a Cylindrical Turret with a Flat Window and the Impact of Passive Control Devices

Optical aberrations over a cylindrical turret with a flat window were measured using a 2-D wavefront sensor and a Malley probe as a function of laser beam elevation angle. It was found that depending on the window back-facing angle the flow either had a weak separation bubble followed by a reattached boundary layer or a strong separation with a large recirculation region behind the cylindrical turret. It was found that optical aberrations were high in the case of a weak separation bubble and at large look-back elevation angles. Different passive devices placed upstream from the turret were studied for their effectiveness in improving the optical-propagation environment. The results are presented and discussed.

Similarities and Differences in Aero-Optical Structure over Cylindrical and Hemispherical Turrets with a Flat Window

This paper discusses similarities and differences for aft-looking angles of the aerooptical environment over flat windows on a three-dimensional hemispherical turret and two-dimensional cylindrical turret. Both fluid-mechanic and aero-optical data for the baseline flows over the turrets and with two types of passive flow devices are presented and discussed for relatively-high, subsonic Mach numbers. Although the flow around the hemispherical turret was found to be far-more complex than over the cylindrical turret, the flow physics over the windows themselves were shown to be very similar; this similarity afforded an opportunity to study the flow over the window in more detail for the cylindrical turret and to easily examine a range of flow devices for improving its aero-optical environment, selecting two devices, presented here, for further study on the hemispherical turret. For both turret configurations the most aberrating flow environments were cased by separated flows. For small aft-looking angles the separated flows reattach over the windows and the devices were shown to be effective in preventing the separation altogether and greatly improving the aero-optical environment. Once fully separated, neither device improved the environment.

OPTICAL DISTURBANCES CAUSED BY TRANSONIC SEPARATED BOUNDARY LAYER BEHIND A 20-DEGREE RAMP: PHYSICS AND CONTROL.

In a previous paper [1], we documented the optical disturbance environment created by an attached turbulent boundary layer in high-Mach-number subsonic flow. In the earlier paper, an opticalaberration measurement instrument, the Malley probe, was described and used to collect a high-resolution time series of the optical aberrations for the boundary layer. In the present paper, results using the same instrument will be presented for a high-subsonic, turbulent boundary layer passing a downward 20 degree ramp. In this case, the flow becomes globally separated; the optical aberration environment upon separation starts at the level associated with the turbulent boundary layer but rapidly grows to a much higher level than for the upstream, attached boundary layer. Streamwise and spanwise measurements of Optical Path Difference (OPD) and local convective speeds of the opticallysignificant structures along the ramp were performed, along with velocity profiles. The objective of the work was to explore the use of passive devices to reduce the aberration environment created by the separating shear layer. A variety of

Hingeless Flight Control of a Smart Projectile Using Miniature Actuators

An experimental assessment of an innovative hingeless flight control system for low-cost smart projectiles is presented. The ultimate goal of the present work is to replace the conventional hinged control surfaces with a hingeless control system that provides significant reductions in weight, volume, and cost of the flight Control Actuation System (CAS), and an overall improvement in the projectile performance. The control system presented herein utilizes miniature mechanical devices as control surfaces in the projectile tail and nose regions for steering control. A compact CAS utilizing a light-weight pneumatic actuation mechanism for active deployment of actuators is also developed. The system enables active deployment of miniature actuators in both steady and unsteady modes of operation for on-demand force generation. In the retracted position the actuators do not use any power and are stowed flushed to the projectile surface, thus do not induce additional drag over the baseline. Exploratory wind tunnel investigations are conducted on a scaled 105mm projectile at angles of attack up to 18 deg at Mach numbers of 0.1, 0.56, 0.61, 0.76, 0.9, 0.95, 1.05, 2.4 and 3.0. The results presented here provide a comprehensive evaluation of the spoiler-based control concept in low-subsonic to supersonic flow conditions. Results show that the actuators are able to trim the projectile at moderate angles of attack in most cases, and are able to maintain a significant authority in pitch and yaw control for all cases examined.

Effects of Forebody Geometry on the Flow and Control of a Blunt-Nose Projectile at High Angles of Attack

Wind tunnel experiments are conducted on a projectile at high incidence to examine the effect of nose shapes and a passive strake control device on the side force and yawing moment. The study was also aimed at gaining some insights into the fluid mechanics of blunt-nose bodies of revolution at high angles of attack. Detailed flow visualization and force measurements are obtained on a projectile with a fineness ratio of 4 at a Reynolds number of 0.19 x 10, and 42 and 48 deg angles of attack. Offand on-surface flow visualization records are collected to study the effects of two blunt noses: a hemispherical nose and an elliptical nose with 33% ellipticity. It was found that the elliptical-nose results in flow behaviors typical of a blunt-nose, while the hemispheric-nose results in behaviors that are akin to a pointed-nose. An explanation for the contrasting behaviors is provided.

Range and Endgame Performance Assessment of a Smart Projectile Using Hingeless Flight Control

This paper presents ballistic trajectory and maneuver footprint predictions from flight simulations of a 105mm gun-fired projectile using hingeless tail-spoiler microactuators for directional control. An extensive database of aerodynamic coefficients was created for the 105mm smart projectile, with and without tail-spoilers, from wind tunnel tests at Mach numbers of 0.1, 0.56, 0.61, 0.76, 0.9, 0.95, 1.05, 1.15, 2.4, and 3.0. Based on the wind tunnel data and Missile Datcom predictions, 3-DOF and 6-DOF flight simulations were conducted by the Orbital Research Inc. and U.S. Army team to conduct an overall performance assessment of the hingeless spoiler control system for the guided projectile. Two endgame scenarios were investigated. The results in the form of ballistic trajectories and maneuver footprints show that range enhancement by over 7% using the improved tail-fi nd esign, and conrollable deflection of over 1.5 km over an uncontrolled ballistic flight trajectory of 10 km can be achieved using the tail-spoiler microactuators.

Effects of Periodically Distributed Surface Porosity on a Cylinder in Laminar Cross Flow - A Numerical Investigation

The effects of surface porosity on a cylinder in laminar cross-flow are modeled using a 3D Navier- Stokes flow solver, and the simulation results are compared with experimental results. Different porous configurations on a cylinder are examined and in each case, flow patterns are compared with that of a non-porous configuration. Results indicate that, in the presence of surface porosity, the flow exhibits strong three-dimensional flow characteristics which are not observed in the case of a smooth (non-porous) cylinder. The separation points and velocity distributions of a porous cylinder are found to be very different than that of a solid cylinder. The velocity fluctuations in the near wake region are also greatly reduced in the case of a porous cylinder. These findings strongly suggest that a distributed porosity approach can be used as a control parameter for lift and drag control on the cylinder, and other bluff bodies.

Distributed Mechanical Actuators for Design of a Closed-Loop Flow-Control System

Abstract : This report was developed under a SBIR contract. Active flow control experiments were conducted on a two-dimensional, single-element NACA 4312 airfoil to assess the performance of vortex generators and gurney flaps as lift-enhancing devices for the control of longitudinal dynamics of an air vehicle. The vortex-generators are shown to delay boundary layer separation and provide an increase in the lift coefficient for angles of attack above 12 deg, and the gurney flaps yield a constant shift in the lift curve for all angles of attack up to stall angle. The vortex-generators are most effective when used between 2 to 5% chord, and the gurney flap is most effective when used at the trailing-edge. Experiments from actuators distributed spanwise along the wing reveal that there is minimal interaction between individual gurney flaps at low-to-moderate angles of attack, while, the interactions between the individual vortex-generators are slightly more pronounced at high angles of attack. The effects of these individual actuators can be combined linearly to produce a desired net effect.

Coning Motion Control of a Blunt-Nose Projectile Using Small Deployable Strakes

A method to control the coning motion of a projectile at high angles of attack using a pair of deployable strakes is experimentally investigated. Low-speed wind tunnel tests were conducted on a fineness ratio 4 projectile with a cylindrical body and an elliptical, blunt nose forebody. The model was supported free to rotate at the center of gravity in a low speed wind tunnel. Tests were conducted at a Reynolds number of 0.19 × 10 6 and angles of attack of 50 and 52 deg. The strakes were placed near the start of the cylindrical section slightly downstream of the nose section. Open-loop tests were first conducted to characterize the effects of the strakes for coning motion control. A feedback controller was then designed based upon the results of the open-loop experiments, and validated via closed-loop dynamic experiments. Two methods of feedback control were examined; a singlestrake method and a dual-strake method. The dual-strake controller demonstrated improved control in achieving commanded coning angle control within a tolerance of ± 10 deg. Finally, the effect of strakes on the projectile pitching moment, which compromised the accuracy of the coning motion controller, was controlled and compensated using a pair of deflectable, horizontal, aft-fins.