Abstract

This dissertation explores the aerodynamic behavior and interactions in the flow-fields of dual rotor systems. The experiments used Particle Image Velocimetry (PIV) combined with pressure and force measurements to characterize the flow with an emphasis on side-by-side rotor flows at small separation distances. This work is separated into six main chapters: first, the topic is introduced with a thorough literature review in Chapter 1, the methodology and details of the experiments are discussed in detail in Chapter 2; the effects of phase offset on dual-rotor rotor flows is explored in Chapter 3; the influence of separation distance on the side-by-side rotors in Chapter 4; the use of modern methods to solve the Pressure Poisson Equation (PPE) to explore the impact of separation distance on the pressure in dual rotor flows in Chapter 5; and finally, the summary and conclusions are shown in Chapter 6. In the methodology chapter (Chapter 2), the experimental measurement methods, facilities, test stands, and other measurement systems are presented in detail. This includes a discussion of 2D2C and 3D2C (stereoscopic) PIV systems, including seeding, illumination, recording, pre- and post-processing, and evaluation. The specifics of the experimental procedures and common issues and challenges are also considered. The uncertainty for each system is also presented and includes uncertainty values for velocity, vorticity, momentum flow rate, thrust, and pressure (from both pitot probe measurements and from solving the PPE equations). Chapter 3 presents an experimental investigation of the interactions of two DJI 9443 rotors in a side-by-side configuration. The rotors were rotating at 4860 RPM with a spacing of 5% of the propeller diameter. Rotor phase-locking was implemented so that the SPIV measurement images were synchronized with the rotor rotation. In this chapter, three scenarios were considered: a) an isolated rotor, b) dual, counter-rotating rotors rotating in-phase, and c) dual, counter-rotating rotors rotating 90 degrees out of phase. The tip vortices, wake velocities, and momentum flow rates were analyzed for the three scenarios. Adjacent rotor interactions led to a 55% increase in the peak vorticity decay rate for the tip vortices. When comparing the two dual-rotor configurations with differing phase offsets, minimal differences were observed in the peak vorticity magnitude within the vortex cores, the dissipation behavior of the tip vortices, and the downstream axial velocity distributions. On average, the momentum flow rate for both dual configurations exceeded that of the single rotor case by 2.2%, with peak values showing an increase of up to 5.2%. The experiments in Chapter 4 describe an investigation of the interactions of dual rotors in a side-by-side configuration with varying rotor spacings. Similar to Chapter 3, the same counter-rotating DJI 9443 rotors rotating at 4860 RPM (81 Hz) were analyzed using a SPIV system. Rotor phase-locking was also employed for rotation rate synchronization with the captured PIV images. The test cases included an isolated rotor case and four tip-to-tip separation distances (��/�� = 0.5, 0.2, 0.1, and 0.05). The shed tip vortices, the rotor wake velocity profiles, and momentum flow rate measurements were analyzed in the wakes of the rotors at each case. Reducing the tip-to-tip spacing led to a 28% increase in the contraction ratio of the wake boundary and up to three times faster decline in tip vortex strength at the smallest spacing compared to the single rotor. The presence of the adjacent rotor also caused a 47% reduction in the initial tip vortex vorticity and a 35% drop in circulation for the closest spacing. Rotor wake positions were displaced by as much as 0.1�� toward the adjacent rotor as spacing decreased. Additionally, similar to the findings of the previous chapter, the momentum flux was increased in all dual rotor configurations relative to the isolated case, with the greatest increase of 5.1% occurring at the minimum spacing. Finally, Chapter 5 presents the pressure fields obtained by solving the Pressure Poisson Equation (PPE) using a Minimal Norm Least Squares (MNLS) method with Neumann boundary conditions. The pressure was reconstructed using PIV velocity data from the measurements in Chapter 4. As before, the rotors were in hover condition (�� = 0) rotating at 4860 RPM (81 Hz) and phase-locked to synchronize the PIV image captures with rotor rotation rate. The same separation distance configurations including an isolated rotor scenario and the four tip-to-tip separation distances of ��/�� =0.5, 0.2, 0.1, and 0.05 were also considered. Pitot tube measurements were also performed for comparison. Phase-locked pressure fields showed regions of low pressure corresponding to tip vortex shedding. As the separation distance between rotors decreased, increased interaction led to a more rapid dissipation of these low-pressure regions. Although both vorticity and pressure weakened with reduced spacing, the pressure experienced a more substantial decline—an 86% drop compared to 47% for vorticity when contrasting the isolated rotor with the smallest separation case. On the second measurement plane, a ring of negative pressure surrounding the wake was observed to weaken and eventually dissipate due to rotor-rotor interference. Pitot tube comparisons supported the overall pressure behavior but revealed that PIV-based reconstructions yielded larger magnitudes in the pressure, particularly in the isolated rotor case. Using profiles from pitot and PIV, the pressure force was found and combined with the momentum flow rate and was able to accurately predict the thrust to within 1.5% for the pitot-measured pressure and 4.3% for the PIV-measured pressure. These findings highlight the need for volumetric velocity data to fully resolve pressure forces, indicating that future work should employ tomographic or multi-plane PIV for improved accuracy in pressure estimation.

Degree

PhD

College and Department

Ira A. Fulton College of Engineering; Mechanical Engineering

Rights

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