Rotor Wakes; Formations, Interactions, and Design

Kopperstad, Tove Elisabeth

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https://hdl.handle.net/2142/132451 Copy

Description

Title

Rotor Wakes; Formations, Interactions, and Design

Author(s)

Kopperstad, Tove Elisabeth

Issue Date

2025-08-13

Director of Research (if dissertation) or Advisor (if thesis)

Ansell, Phil

Doctoral Committee Chair(s)

Ansell, Phil

Committee Member(s)

• Merret, Jason
• Saxton-Fox, Theresa
• Chamorro, Leonardo

Department of Study

Aerospace Engineering

Discipline

Aerospace Engineering

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Aerodynamics
• Rotors
• Propellors
• Wake augmentation
• RLLT
• Applied Aerodynamics
• Rotor Wind Tunnel Testing
• Wake roll-up
• Induced Velocities
• Rotor Design
• Aero-Propulsive Interactions
• Rotor Optimization
• Wake Characterization

Abstract

With the increasing demand for sustainable energy solutions in aviation, distributed electric propulsion (DEP) has emerged as a promising configuration to enable advanced capabilities such as electric short take-off and landing (eSTOL). Using two primary research objectives, this dissertation investigates the fundamental flow physics of rotor wakes and presents a rotor design methodology that enables the creation of mission-tailored wakes for improved aero-propulsive integration. These goals are addressed through a series of experimental campaigns involving both ducted and unducted rotors in static and advancing configurations. Advanced flow diagnostics tools, such as stereoscopic particle image velocimetry (stereo-PIV) to quantify the wake flow in the near-field and far-field, as well as surface pressure taps, and force balance measurements to capture aerodynamic loading. A rotor-test stand was used to monitor rotor performance throughout these experimental campaigns. The current program considers the swirl produced by three different propulsive devices: a ducted fan device with standard stators, a ducted fan device with swirling stators, and an open propeller. These devices permit an isolated comparison of the influences of jet axial velocity, tangential (swirl) velocity, and wake vortex features on wing/body integration considerations. Using the integrated axial thrust across the same radial volume it was found that peak axial velocity across each propulsive device was within ten percent of the reference velocity using momentum theory of an equivalent actuator disk, thus each propulsive device could be determined to have nominally dimensional equivalent thrust with wakes advecting at the same bulk velocity. An empirical model for viscous vortex cores was used to estimate the concentrated circulation strength produced by each propulsive device. It was found that compared to the baseline EDF, the EDF Swirl and the propeller produced five times higher vorticity at the same wake pitch normalized axial location. It was, however, found that the empirical model of the swirl velocity did not well match the experimental data outside the core region. This entails that the vortical flows produced by these propulsive devices cannot be adequately modeled as a concentrated vortex due to the lack of significant induction effect outside the propulsive streamtube. Integrating the propulsive wake into an aero-propulsive 3D model for distributed propulsion will require further consideration of the velocity profiles, and cannot simply be modeled using a vortex filament in its place. The minimum induced loss (MIL) rotor has long served as the conventional baseline in rotor design. However, with the rise of DEP configurations, blown-wing systems, urban air mobility, and the widespread use of drones in populated environments, rotor design must move beyond minimizing power consumption alone. Rotor wakes require careful consideration to address system-level aerodynamic and acoustic performance. To address this, a rotating lifting line theory (RLLT) formulation is implemented within a Lagrangian optimization framework, enabling blade circulation distributions to be prescribed that yield desired wake velocity profiles. This approach resolves the interdependency between blade-induced velocities and the shed vortex system, allowing for direct control over wake structure and vortex behavior. Owing to Helmholtz's Vortex Theorems and Kelvin's Circulation Theorem, the formation of coherent vortex structures in the wake of a rotating wing is directly attributable to radial gradients in lift production. To mitigate these features, a constrained optimization problem was formulated to attenuate radial gradients in the bound circulation of a rotary wing, local to the propeller tip region. Prototypes of baseline and circulation-optimized propellers were tested in a wind tunnel environment to verify design thrust characteristics and assess propeller wake flow physics. Phase-averaged stereoscopic particle image velocimetry data were acquired to obtain the wake velocity profiles across a range of wake phase angles. The baseline wake demonstrated typical coherent tip vortex roll-up behavior, resulting in a strong double helix wake, in direct contrast to the vortex-attenuated propeller, which featured a distributed sheet of wake vorticity arranged in a conical spiral pattern. Additionally, the wake vorticity of the vortex-attenuated propeller dissipated faster than that of the baseline configuration. The propulsive wake of the baseline configuration behaved as expected with a uniform axial flow distribution, whereas the vortex-attenuated configuration demonstrated significantly higher axial velocities near the axis of rotation, due to the axial velocity profile produced by the blade circulation distribution of this propeller configuration. In the final experimental campaign, this design framework was used to fabricate a prototype rotor with a prescribed induced axial velocity profile optimized to maximize kinetic energy in the wake for enhanced blown-wing performance. It was found that through an induced axial velocity profile, the novel rotor creates larger peaks in sectional lift by increasing the suction peak and lowering the stagnation point of the wing section within the rotor streamtube as reflected through surface tap measurements. Aerodynamic forces captured by a 3-component force balance also reflect increases in wing lift from both the conventional and novel rotors in a blown wing configuration; a significant difference between the rotor models was not observed at the macro scale, as the rotor to wing sizing was not optimized for the blown wing configuration. Instead, closer observation in the micro, or spanwise, surface pressure measurements reveals that the increases in induced axial velocity from the blade does propagate into increases in kinetic energy seen by the wing. These increases are further exaggerated by increased swirl through the induced blade-tangential velocity, which was found to increase the effective AoA seen by the wing, creating an even larger influence on the sectional lift distribution. A far-field stereoscopic particle image velocimetry data of the three velocity components in the Trefftz-plane allows for the characterization of the interactions seen between the rotor wake and the lifting wake. It was found that when the rotor wake spins in the same orientation as the wing-tip roll-up, the induced drag of the blown wing is increased by about 21\% for both rotor systems. When the rotor wake spins counter to the wing-tip roll-up, the conventional rotor reduces the induced drag by about 12\% whereas in the same configuration, the novel rotor reduces the induced drag by 23\%. These experiments show that an increased sectional lift can be successfully prescribed through rotor design, as well as detail the interactions of the blown wing system and its resulting aerodynamic effects. This dissertation presents a lower-order rotor design framework capable of prescribing mission-specific wake properties to enhance aerodynamic performance. The results emphasize the importance of incorporating wake dynamics into rotor design and illustrate the aerodynamic advantages of wake-optimized rotors in DEP and blown-wing applications.

Graduation Semester

2025-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/132451

Copyright and License Information

Copyright 2025 Tove Elisabeth Kopperstad

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