Projects

While it is known that electrification of aircraft propulsion can produce several benefits across the aircraft system, the aeronautics community does not currently have component-level technologies and a thorough understanding of fundamental approaches to electrified propulsion integration for large, commercial vehicles. As a part of the Center for High-Efficiency Electrical Technologies for Aircraft (CHEETA), technologies and design methods for far-term, fully-electric aircraft are currently being developed. This includes design principles for distributed propulsion system integration into an aircraft platform, assessing the feasibility of hydrogen fuel cell power systems for commercial aviation, and developing distortion-tolerant ducted fans for boundary-layer ingestion.

Support by NASA is gratefully acknowledged.

Blade-vortex interactions (BVIs) are critical components in rotor design and aircraft configurations in everything from tactical aircraft to commercial and hobby drone operation. When tip vortices from rotor blades interact with rotor blades during shallow descent, a sudden change in pressure loading produces significant sound in the system. In order to mitigate this effect, Helmholtz vortex theorems where leveraged using blade root bending moments as a surrogate variable to attenuate coherent vortex structures that manifest due to gradients in circulation at the blade tip. This design process was used for both advancing and hovering propellers, and it was shown using stereoscopic particle image velocimetry (stereoPIV) that the resulting 3D wakes created a weaker conical wake structure compared to a conventional power-optimized propeller which produces a strong helical wake structure.

Support from the US Air Force is gratefully acknowledged as well as collaboration with CU Aerospace and Infinity Labs.

Recent interest into distributed electric propulsion (DEP) in aeronautics has motivated exploration of novel approaches for integrating this technology into fixed-wing aircraft. While the benefits of DEP are well known for lift-augmentation in STOL/VTOL aircraft, details of how lifting flows on an aircraft wing integrate with swirling flows from a DEP system are still to be investigated. Using both low-order computational simulations and wind-tunnel based experiments, characterization of the flow field around an airfoil system with integrated DEP and thrust vectoring capabilities has been accomplished. Particle-image velocimetry (PIV) techniques have also been utilized to distinguish the swirling velocity features between ducted and unducted propulsion systems, the results of which further support specific design practices for aero-propulsive integrated systems.

Support from the Office of Naval Research (ONR) is gratefully acknowledged.

In 2023, researchers at the University of Illinois were among recipients of an AFOSR Multi-University Research Initiative (MURI) award to investigate fluid-metamaterial interaction (FMI). This study is a project within the MURI that focuses on resonant phononic materials (RPMs) – a type of metamaterial whose frequency-dependent structural dynamics provide opportunities to target interaction with specific flow modal instabilities to achieve improved performance. For instance, one objective of this research is to determine if by tailoring an RPM’s frequency response to match the shedding frequency of a cylinder in cross flow, the amplitude of the Kármán vortex street can be passively amplified or attenuated. Through wind tunnel experiments this project seeks to better understand the nature of this specific interaction, enabling this passive control method to be harnessed in other critical flow conditions such as transition to turbulence. Thus, the findings of this study will not only deepen our understanding of fundamental mechanisms governing FMI but have the potential to help revolutionize the use of passive flow control to improve air vehicle performance.

Support from the US Air Force Office of Scientific Research is gratefully acknowledged.

Influence of Laminar Separation Bubbles on Low-Re Airfoil Aerodynamics

In order to improve the flight efficiency of future commercial transport vehicles, this study seeks to develop and mature a series of slotted, natural laminar flow (SNLF) airfoil sections. The Aerodynamics and Unsteady Flows Research Group is responsible for providing experimental diagnostics of the SNLF airfoil to understand the airfoil performance and flow physics. Additionally, a new SNLF configuration will be developed that incorporates the benefits of active flow control for low-speed, high-lift applications. Additional experimental characterization of the transonic flow field about the SNLF airfoil is also being performed in the Illinois 6"×9" transonic wind tunnel.

Support by NASA is gratefully acknowledged.

After its initial development in the late 1990's, the Empirical Mode Decomposition (EMD) has been used and extensively advanced across a broad range of applications. The fundamental advantage of EMD is its ability to represent nonlinear processes in nonstationary signals. While typically used with point-based measurements or simulation data, the extension of EMD to handle n-Dimensional full-field data has many advantages to understanding complex fluid flow interactions. Furthermore, when used with a multivariate approach, modal content can be represented that retains coherence between flow field variables (e.g., three components of velocity vector). This multivariate, multi-dimensional EMD methodology is currently under development at Illinois. After development, this novel approach to signal analysis will be utilized to understand the nonlinear interactions associated with laminar-turbulent boundary layer transition. The emergence of a turbulence spectrum by way of these nonlinear interactions has long stood as an important problem in fluid flows. Through the use of adaptive signal analysis, the spatio-temporal evolution of instabilities can be characterized through a series of amplitude/frequency (wavenumber)-modulated contributions.

Support by ARO is gratefully acknowledged.

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.

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.

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.

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.

In order to understand the vast potential of a distributed propulsion system, a UAV-scale testbed has been developed. This vehicle represents a modified version of a dynamically-scaled Cessna SR22 aircraft. The aircraft features a series of 8 electrical ducted fans distributed across the wing span, which enables significant improvements in propulsive efficiency and vehicle capabilities beyond the baseline aircraft system. This testbed is currently being leveraged in order to understand how a distributed propulsion system can be utilized as control effectors in a fully propulsion-airframe integrated configuration.

Support by NASA is gratefully acknowledged. This study is also being conducted in collaboration with ES Aero.

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.

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.

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.

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.

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.