Below is a list of previous and ongoing investigations.

Flow Physics of the Onset of Dynamic Stall
Stalled Airfoil Oscillations and Hysteresis
Closed-Loop Control of Trailing-Edge Separation
Innovative Flow Control Actuators
Distributed Propulsion Testbed
Flow Physics of the Tiltrotor Fountain Effect
Design, Analysis, and Evaluation of a Novel Propulsive Wing Concept
Sensitivity Analysis of Hybrid-Electric Aircraft Systems
Analysis of Optimum Wing Spanloadings using Multi-Fidelity Methods
Aerodynamic Optimization of Hyperelliptic Cambered Wings

Flow Physics of the Onset of Dynamic Stall

Dynamic Stall Flow Field

Dynamic stall is a complex phenomenon that can occur in the flowfield of airfoils in unsteady motion. When dynamic stall occurs, the flowfield is dominated by a dynamic stall vortex, which acts to augment the lift and drag of the airfoil. However, there are several aspects about the emergence of this dynamic stall vortex that merit additional study. The current understanding about dynamic stall is focused on the flow features and unsteadiness that occurs at the same time scales as the unsteady motion of the body. This study seeks to understand the unsteadiness in the flowfield, leading up to the dynamic stall vortex, which occurs as sub-scale flow features at much shorter time scales. By understanding these aspects of the dynamic stall flowfield, this study opens up opportunities to develop new methods for dynamic stall control, or introduce significant improvement of existing control methods.

Support by AFOSR is gratefully acknowledged.

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Stalled Airfoil Oscillations and Hysteresis

Low-Frequency Oscillations

Separated flow fields associated with airfoil stall are very complex. Numerous sources of unsteadiness can be present all at once, making it difficult to characterize the physical mechanisms which lead to large-scale oscillations in pressure, velocity, and performance. Previous studies have identified a circulation-driven low-frequency oscillation which can occur in airfoil flows near stall. Recent experiments have revealed that these oscillations can also occur in stalled airfoil flow fields. The purpose of this investigation is to characterize this low-frequency unsteadiness in the flow field about an NACA 0012 airfoil, and to understand the physics associated with the unsteady flow field.

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Closed-Loop Control of Trailing-Edge Separation

Vortex Structures over Airfoil

In general, flow control acts to manipulate an existing flowfield through some type of actuation in order to achieve a more desirable flow state than would occur without forcing. If an active flow control method is used, this forcing can be provided through steady or unsteady actuation. When this actuation is unsteady, there tends to be a wide parametric space of variables that can dictate how the actuation is modulated. For example, for certain actuation methods the frequency, amplitude, duty cycle, and waveform of the forcing can all be varied independently. Active flow control is typically placed into two categories: open-loop and closed-loop. When open-loop flow control is used, the actuation parameters are provided a priori to the actuation system. When a closed-loop flow control architecture is used, however, measured sensory information of the flowfield is utilized to institute the desired effects on the flowfield. This study seeks to utilize a closed-loop flow control system, comprising of a set of pulsed blowing slots on an airfoil and a series of unsteady pressure transducers, in order to predict what actuation parameters provide the desired control of trailing-edge separation. Results from this study have utilized adaptive modal decomposition techniques in order to produce a system that predicts the necessary blowing amplitude and frequency to achieve a desired lift coefficient on an airfoil through manipulation of the trailing-edge separation.

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Innovative Flow Control Actuators

Cyclotronic Plasma Actuator

Turbulent separation remains one of the main limiting factors to the high-lift capabilities of modern aircraft. Active flow control provides a means to mitigate turbulent boundary-layer separation, though many actuation techniques either require complex, heavy infrastructure to be effectively feasible, or are limited to low speeds due to low actuation amplitudes. During this study, a new set of simple flow control actuators are being developed which leverage the formation of natural vortical flow structures to enhance flow mixing and manage turbulent boundary-layer separation. In addition, these devices can be actively deployed, providing on-demand performance with no perceptible cruise penalty, as opposed to passive devices.

Support by NASA is gratefully acknowledged. This study is also being conducted in collaboration with CU Aerospace.

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Distributed Propulsion Testbed

Distributed EDF Aircraft

In order to understand the vast potential of a distributed propulsion system, a UAV-scale testbed is currently being developed. This vehicle will represent a modified version of a dynamically-scaled Cessna SR22 aircraft. The aircraft will feature a series of electrical ducted fans distributed across the wing span, which enable significant improvements in propulsive efficiency and vehicle capabilities beyond the baseline aircraft system. This testbed will be utilized in order to understand how a distributed propulsion system can be utilized as control effectors in a fully propulsion-airframe integrated configuration.

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Flow Physics of the Tiltrotor Fountain Effect

Tiltrotor Fountain Effect

When a tiltrotor aircraft is situated in a hovering configuration, a complex flow interaction can be produced where the induced velocity across both rotors are deflected inboard along the span of the wing, with a sudden acceleration in the upward direction when these two streams meet. The subsequent jet that is produced can then be re-ingested by the rotors. This, so called “fountain effect”, introduces an increase in download of the aircraft, causing a subsequent increase in the power required for hover. The detailed flow physics associated with this flow field, however, are not very well understood. This study seeks to quantify the unsteady flow between the rotors using a basic wing and dual-rotor geometry in hover and in a cross-flow.

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Design, Analysis, and Evaluation of a Novel Propulsive Wing Concept

Propulsive Wing Concept

Several recent studies into alternative aircraft configurations and propulsion technologies have shown significant promise. However, outboard of the fuselage or main body of the aircraft, most of these configurations utilize a conventional wing surface. It is believed that further increases in efficiency can be achieved by combining the respective configuration’s merits with an advanced concept airfoil section on the main wing. The proposed study seeks to develop an advanced propulsive wing concept by introducing a transonic Griffith/Goldschmied airfoil as a method for introducing large, realizable runs of laminar flow on the wing surfaces, coupled with a significantly reduced pressure drag, with the additional benefit of wake filling.

Support by NASA is gratefully acknowledged. This study is also being conducted in collaboration with Rolling Hills Research Corporation.

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Sensitivity Analysis of Hybrid-Electric Aircraft Systems

Hybrid-Electric Aircraft System Analysis

In order to meet the ambitious goals in fuel burn reduction in place for the next generation of aircraft, the possibilities provided by hybrid-electric propulsion systems are currently being explored. Traditionally, hybrid-electric systems can be laid out into three system architectures. However, little is understood about how the respective efficiencies of these hybrid-electric architectures change from one aircraft platform, or mission, to another. In order to understand this sensitivity, this study seeks to develop a simulation system for a hybrid-electric aircraft propulsion system that can be used to analyze the various trade-offs associated with various components and hybrid architectures.

Support by NASA is gratefully acknowledged. This study is also being conducted in collaboration with Rolling Hills Research Corporation.

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Analysis of Optimum Wing Spanloads using Multi-Fidelity Methods

Elliptic Wing Wake

Several classic investigations have led to an improved understanding of how induced drag is produced and how it can be minimized. Undoubtedly, the most significant of these studies were those conducted by Prandtl and Munk, where the elliptical wing was identified as producing the minimum induced drag for a fixed value of lift and span. Several subsequent studies have also used various methods to determine wing spanloads that minimize induced drag for wings with fixed structural constraints, rather than a fixed span constraint. These efforts span from classic solutions derived from circulation theory to those resulting from the rise of modern multi-disciplinary design optimization methods. However, there currently exists little understanding on how the sensitivity of a minimum drag solution is tied to the limitations of the modeling method. In order to better understand this coupling, the focus of this study will be to develop a series of optimum spanload wing configurations that have been designed using tools of varying fidelity, and performing wind-tunnel experiments on the resulting designs. The result of this study will lead to an improved understanding of how design and analysis tools of varying fidelity can be used for conceptual wing design.

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Aerodynamic Optimization of Hyperelliptic Cambered Wings

The introduction of wingtip devices has allowed for unprecedented improvements in the aerodynamic efficiency of aircraft wings. Various wingtip devices, such as winglets, have now reached common use in commercial aircraft applications. This study seeks to understand how improved aerodynamic performance of a wing can be achieved by blending the non-planar wingtips into a continuous spanwise camber of the wing design. This continuous distribution of spanwise camber can be achieved using a hyperelliptic function, which is then optimized to produce a minimum drag solution under a set of fixed performance constraints.

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