Seminar on "Experimental Investigations on Vortex Aerodynamics of Rotors at High Advance ratios and Bluff Body Aerodynamics"
Speaker: Dr. Nandeesh Hiremath.
Title: Experimental Investigations on Vortex Aerodynamics of Rotors at High Advance ratios and Bluff Body Aerodynamics
Rotor operation at high advance ratios is important for high-speed and compound rotorcraft concepts. The operation of helicopter rotors in reverse flow has taken on new significance in the context of co- axial rotors. A rotor moving edgewise at a high advance ratio, encounters reverse flow on parts of the retreating portion of the rotor disc. Predicting rotor stability and pitch link loads is complicated by the presence of unsteady pitch, yaw and rotation effects. Predictions using comprehensive codes have shown large differences from full-scale experimental data. Prior approaches have modeled these using flow separation with airfoil data modified for yaw, vortex shedding, dynamic pitch oscillations and reverse dynamic stall of an airfoil. However, the highly 3-dimensional flow phenomena do not conform to approaches based on 2-dimensional airfoil aerodynamics. The present work delineates the nature of flow around a rotating blade in reverse flow by integrating the results from fixed wing experiments with rotating wing experiments. The work focuses on a strong 3-D vortex similar to those seen on delta wings that would develop over the sharp edge at high yaw, providing an avenue for vortex lift aerodynamic analyses. The fixed wing and rotating wing experiments were performed on a tethered rotor blade with NACA0013 profile. Fixed-wing results from load measurements and flow visualization showed that the sharp-edge vortex (SEV) is a primary feature in reverse flow when the blade is yawed either forward or backward. The aerodynamic loads conform with analytical model using Polhamus Suction Analogy, thus showing significant contributions from vortex-induced lift and pitching moments. The study resulted in better understanding of the physics of reverse flow region. A semi-empirical model based on the delta wing concept was provided as a plug-in module for existing computational tools.
The second part of the talk would be focused on the studies carried out in understanding the Bluff body aerodynamics and its application to slung load aeromechanics. Unlike few streamline objects that have some analytical formulation for predicting the aerodynamic loads, the bluff body aerodynamics is a vastly unexplored area. Prior to our studies, in the literature the only experimental studies pertaining to bluff bodies were restricted to 2-D canonical shapes. In practice, a 3-D bluff body would have a highly three dimensional flow and the process of analyzing a complex shape based on its 2-D sectional profiles would greatly limit the out-of-plane components. The work systematically started with developing a Continuous Rotation Technique that would enable in obtaining aerodynamic load maps for for all 360 degree angle orientation on all three principal axes, with 0.5 degree precision. Studies were performed on canonical shapes involving circular cylinders and rectangular prisms for aspect ratios ranging from 0.05 to 4. The studies showed drastic similarities in the aerodynamic loads not only for the same class of object but even between two different classes of canonical shapes. Although the overall flow seemed to be chaotic in nature, a definite and predictable trends in aerodynamic loads started to appear for these canonical shapes. For very low aspect ratio cylinders (AR —> 0) a analytical model based on slender wing concept which is generally used for streamlined objects was found to be a apt solution. In contrast to the widely known 2-D circular cylinder flows in the literature (AR —> infinity), the analytical formulation for a really low aspect ratio cylinder followed by the empirical model for intermediate aspect ratios gave a better understanding of the loads. The results were directly applied to predicting divergent oscillations of a slung load object from a helicopter’s belly. The physics based modeling of slung load a compound pendulum followed by the incorporation of aerodynamic load maps, not only predicted the initial divergent oscillations but also the absolute divergence.
Speaker Bio: Nandeesh Hiremath, born in Dharwad, Karnataka, completed his Bachelors degree in Mechanical Engineering from the PES University, Bangalore in 2013. He was awarded MRD scholarship for maintaining high academic standards. He then joined Georgia Institute of Technology. He completed his Masters degree in Aerospace Engineering in 2015 while working with Dr. Narayanan Komerath. He later completed his Ph.D. in Aerospace Engineering in 2018 under his guidance. Nandeesh has accumulated an impressive portfolio in and out of engineering. For his MS, he developed an analysis and computational model for hypersonic aero-thermo-elasticity, to go with his analysis of liquid-air-cycle engines for air-breathing trans-atmospheric vehicles. His thesis work on rotor blades in reverse flow, and extracting static pressure in complex flows without pressure sensors, is only a part of the story: he has done pioneering work on generalizing bluff-body aerodynamics, while guiding a team of over 10 undergraduates, in numerous projects. His low-key leadership style shuns the limelight while bringing common sense problem-solving and superb organizational skills. He got elected as the Graduate President of Georgia Techs India Club (2016-2017). And India Club under his leadership won Georgia Tech’s Best Organizational Culture award, a first in their long history. He is the recipient of Ray Prouty Scholarship from Vertical Flight Foundation by American Helicopter Society. He was also nominated by Georgia Tech for AIAA/Aviation Week 20 Twenties award.
Event Date: 18th September, 2019(Wednesday)
Event Time: 11:30 AM