Thomas M. Crittenden

A high-speed, compressible synthetic jet

The operation of a high-speed, compressible synthetic (zero net mass flux) jet is investigated experimentally using a small (nominally 6.5 cm3) piston/cylinder actuator driven by a variable speed motor. The actuator performance is characterized using phase-locked cavity pressure measurements, particle image velocimetry of the jet flow, and Schlieren flow visualization. Cylinder pressure ratios (relative to the ambient) measured over the experimental limits range as high as 8 and as low as 0.2, during the blowing and suction phases, respectively (producing sonic flow velocities at the orifice during both phases). Owing to compressibility effects, the shapes of the time-periodic pressure curves during the blowing and suction cycles are dissimilar (the suction duration is longer) and there is a clear phase shift between the peak piston displacements and the corresponding pressure extrema. It is shown that the performance is primarily affected by the characteristic velocity (defined by the product of the oper...

Transitory Separation Control over a Stalled Airfoil

Transitory attachment of the flow over a stalled, 2-D airfoil is investigated in wind tunnel experiments using pulsed actuation. The impulse of the momentary control jets is produced by combustion based actuators on characteristic time scales O[1 ms] that are an order of magnitude shorter than the convective time scale of the flow. A single actuation pulse results in transitory flow attachment that is manifested by rapid increase in the global circulation and aerodynamic forces and persists for about ten convective time scales before the flow becomes fully stalled again. Large-scale changes in vorticity accumulation that are associated with repetitive, burst-modulated actuation pulses are exploited for significant extension of the streamwise domain and duration of the attached flow with a corresponding increase in the peak circulation. High-resolution PIV measurements of the interaction between the pulsed jets and the cross flow reveal details of the severing and collapse of the separated flow domain and the dynamics of vorticity accumulation within the attaching boundary layer.

Combustion-Powered Actuation for Dynamic-Stall Suppression: High-Mach Simulations and Low-Mach Experiments

An investigation on dynamic-stall suppression capabilities of combustion-powered actuation (COMPACT) applied to a tabbed VR-12 airfoil is presented. In the first section, results from computational fluid dynamics (CFD) simulations carried out at Mach numbers from 0.3 to 0.5 are presented. Several geometric parameters are varied including the slot chordwise location and angle. Actuation pulse amplitude, frequency, and timing are also varied. The simulations suggest that cycle-averaged lift increases of approximately 4% and 8% with respect to the baseline airfoil are possible at Mach numbers of 0.4 and 0.3 for deep and near-deep dynamic-stall conditions. In the second section, static-stall results from low-speed wind-tunnel experiments are presented. Low-speed experiments and high-speed CFD suggest that slots oriented tangential to the airfoil surface produce stronger benefits than slots oriented normal to the chordline. Low-speed experiments confirm that chordwise slot locations suitable for Mach 0.3-0.4 stall suppression (based on CFD) will also be effective at lower Mach numbers.

Parylene-Insulated Ultradense Microfabricated Coils

This paper details the microfabrication and characterization of electrodeposited coils with high packing density. The process consists of electroplating a first sequence of metal microstructures, followed by conformal insulation of these conductors by a thin vapor-deposited layer of parylene, and subsequent electrodeposited metal filling between the first-layer conductors. Using this approach, the packing density limitation due to photoresist aspect ratio is overcome. The microcoils, which are fabricated onto a dummy substrate, are released and embedded into a parylene layer to reduce parasitic substrate losses at high frequencies, as well as to facilitate the device integration. Comblike test structures were designed and fabricated in order to validate the approach and to explore the electrical properties of such microconductors. Furthermore, ultradense parylene-insulated spiral windings were fabricated and electrically characterized. A large number of turns per volume can be fabricated because of this fabrication approach, which is a requirement for highly efficient small-scale magnetic actuators. Finally, an array of substrateless parylene-coated 2-D coils were built, then folded on top of each other, and electrically connected to form 3-D coil devices. A 14.6-mm-diameter 96-turn three-layer copper winding was fabricated and characterized. The packing density of the 3-D fabricated coil was 81%.

AVIA: Adaptive Virtual Aerosurface

Abstract : One can change the effective shape of a surface without any change in its physical moldlines through the transient injection of momentum to control unsteady flow phenomena, such as separation. This research program consists of a system level evaluation of the potential benefits and costs of active flow control as applied to UAVs for separation control, development of innovative actuator concepts, and proof-of-concept experiments showing aerodynamic effectiveness. System-level benefits include increased gust margin, reduced takeoff field length, increased payload capacity, and greater maneuverability. Actuator research has resulted in two major innovations: the use of pulsed/modulated waveforms to increase the effectiveness of synthetic jet actuators and the development of compact, high power, combustion-driven actuator modules. The general performance of the combustion actuators are characterized in isolation, embedded in cross-flows up to M=0.7, and integrated on a two-dimensional wing model. In addition, since conventional machining would not be suitable for low-cost, large volume production, a simpler, MEMS-based, batch fabrication

Combustion-Driven Jet Actuators in Reversed Flow Configurations

The utility of small-scale, high power combustion–based pulsed jet actuators for flow control is investigated in an annular-return flow geometry that is generated by a primary round jet which impinges normally on a bounded end wall and subsequently fully reverses direction and exits the system in counterflow to the primary jet. The actuator jet is a momentary (pulsed) jet that is produced by the ignition of a mixture of gaseous fuel and oxidizer in a small (cubic centimeter scale) combustion chamber. The actuator generates a brief (msec scale), high impulse momentary jet where the operating frequency and the phase can be continuously varied by independently controlling the flow rate of the fuel/oxidizer and the ignition frequency. The flow characteristics of the unactuated flow are investigated, and the actuators (mounted into the wall of the annular return) are used to trigger flow transients that alter the global flow through strong feedback.

Rotorcraft Fuselage Drag Reduction using Combustion Powered Actuators

Separation control of the 3-D flow over the aft body of a scale model of the NASA ROBIN mod7 rotorcraft fuselage is investigated in wind tunnel experiments. Pulsed actuation is effected by arrays of momentary, combustion-based actuator jets having a characteristic time scale O[1 ms] that is an order of magnitude shorter than the convective time scale of the flow. The actuators are placed upstream of the transition region between the fuselage and the tail boom and their interactions with the massively separated cross flow in this domain and effects on the global aerodynamic forces and moments are measured using an onboard six-axis load cell and high resolution PIV that is acquired phase-locked to the actuation waveform. The present investigations have demonstrated that the actuation can significantly mitigate separation, and lead to a reduction in drag (although flow attachment is accompanied by some lift penalty). It also is shown that transitory aft flow attachment using burst-modulated actuation can be exploited for effecting significant steering aerodynamic side forces for improved flight maneuverability.

Characterization of the Internal Flow Dynamics of Combustion Powered Actuators

The internal flow properties of small-scale combustion-powered fluidic actuator for flow control applications are characterized over a range of combustion chamber geometries and gas mixture flow conditions. Momentary (pulsed) actuation jets are produced by the ignition of a mixture of gaseous fuel and oxidizer within a cubic-centimeter-scale chamber. The combustion process yields a high pressure burst and the ejection of a high-speed exhaust jet. Modular chambers were constructed specifically to allow optical access to the chamber for phase-locked Particle Image Velocimetry (PIV) measurements over both the baseline and combusting flows as well as dynamic pressure measurements. It was observed that for low flow inlet velocities (in both remixed and non-premixed actuator configurations) an essentially laminar flame propagation process is observed. At increased mean inlet flow velocities for the non-premixed geometry, a transition quickly occurs to produce significant wrinkling of the flame front and an apparently irregular flame propagation process through the chamber. Information obtained during this and future similar studies will be used to create actuator designs with improved refill and combustion processes.

Microfabricated, ultra-dense, three-dimensional metal coils

This paper focuses on the fabrication of three-dimensional coils with high packing density. The process consists of electroplating a first sequence of metal microstructures, followed by conformal insulation of these conductors by a vapor-deposited parylene layer, and subsequent electrodeposited metal filling between the first-layer conductors. The windings are microfabricated as an array of two-dimensional coils, which are later released from the substrate, folded and electrically connected to form a three-dimensional coil device. The MEMS-fabricated, ultra-dense coils exhibited a 62% improvement in packing density over conventional wire-wound coils.

Combustion Powered Actuators for Separation Control (Invited)

An overview is presented of research involving the development and application of combustion powered actuation (COMPACT) for active control of separated flows. This actuation approach produces high-speed, pulsed actuation jets through the ignition of a mixture of gaseous fuel and air within a cubic-centimeter-scale combustion chamber. The basic characterization of the actuator including several geometric and chemical factors that affect the actuator performance is described. Dynamic pressure measurements and PIV show how these factors alter the dynamic pressure pulse within the combustor and the ensuing actuation jet. Environmental testing performed on the actuator is discussed for several harsh environmental conditions with sample results presented for rain exposure. Aerodynamic applications of COMPACT to mitigate both 2-D and 3-D separated flows are also described for airfoils and fuselage integration into a generic rotorcraft body. Flow field measurements show that reattachment of large-scale flow separation is accomplished for each of these applications with the reattachment taking place over a much larger time scale than that of the actuation pulse or the convective time over the surface, resulting in enhanced transitory aerodynamic performance.