Flow Visualizations and Extended Thrust Time Histories of Rotor Vortex Wakes in Descent
Abstract
An experimental study is performed on a three-bladed rotor model in two water tanks. The blade pitch, rotational velocity, descent angle, and descent speed are all varied in order to simulate a wide range of rotorcraft operating states, focusing on descent cases where the rotor is operating in or near vortex ring state--an area in which there is currently very little available data. Flow visualization is done by injecting air bubbles and fluorescent dye tangentially from the blade tips to mark the vortex core, showing the development of both short-wave ("sinuous") and long-wave ("leapfrogging") instabilities on the helical vortices in the wake. Strain gages are used to record transient loads, allowing a correlation between the rotor thrust performance and the development of the vortex wake. Reynolds numbers are of order 105 and test runs are performed for extended periods--up to 500 rotor revolutions-- demonstrating the repeatability of the patterns of thrust variation. The data indicate that as the instabilities develop, adjacent vortices merge and form thick vortex rings, particularly during descent. Periodic shedding of these rings from the wake associated with vortex ring state is observed, resulting in peak-to-peak thrust fluctuations of up to 95% of the mean and occurring at regular intervals of 20-50 rotor revolutions, depending on flow parameters.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jan 01, 2004
- Accession Number
- ADA529846
Entities
People
- Francis X. Caradonna
- James Stack
- Omer Savas
Organizations
- National Aeronautics and Space Administration