Study of Flow Separation on a Rotating Wing Using Volumetric Velocimetry in the Rotating Frame of Reference
Abstract
Predicting the onset of three-dimensional (3D) boundary layer separation on a rotating wing undergoing unsteady motion is a challenging problem. The spatio-temporal characteristics of the onset and progression of flow separation and the dynamics of the separated flow are not well understood. The primary question here is how do physical parameters that are unique to the rotating environment (such as Coriolis and centrifugal forces) govern these processes. An adequate understanding of this process has eluded researchers due to the inability to measure time-resolved volumetric velocity fields in the rotating frame of reference (rotating 3D velocimetry - R3DV). Hence, as a first step towards the long-term goal of R3DV at realistic flight conditions, this study proposes to implement a novel flow diagnostic methodology to experimentally study flow separation on a rotating wing undergoing unsteady motion. The flow diagnostic technique will leverage the state-of-the-art in plenoptic camera based volumetric velocimetry, a hub-mounted rotating mirror, and volumetric illumination to enable, for the first time, time-resolved volumetric velocimetry in the rotating frame of reference. The technique will be used here to bridge the gap in our current understanding of unsteady flow separation on a rotating wing. The experiments will be conducted at Auburn University in a hover tank facility consisting of a wing instrumented with force sensors and actuated by independent motors to simultaneously rotate and pitch in water, the working fluid. The effect of physical parameters that are unique to the rotating environment will be evaluated by studying the competing effects of inertial with viscous forces Reynolds number and with Coriolis forces Rossby number on the 3D flow separation process. In addition, the influence of unsteady forces reduced frequency on the spatio-temporal dynamics of the process will be studied. The volumetric velocity fields in the rotating frame and the associated aerodynamic loads on the wing undergoing unsteady motion will be measured. The primary aims of this study are to: (a) implement and validate the method for rotating 3D velocimetry; (b) identify the spatial location and timing of unsteady flow separation inception and progression; (c) investigate the cause for cycle-to-cycle variations in the onset of flow separation on the rotating wing; and (d) characterize the spatio-temporal dynamics of 3D vorticity transport, spanwise flow and pressure gradient on the rotating wing after separation. Initial experiments will focus on developing the general methodology for R3DV using existing plenoptic camera hardware. The work will address the fundamental flow physics of unsteady separation on rotating wings and will be conducted in a lower Reynolds number facility to minimize non-essential experimental complexity and improve overall data quality. This relatively low-risk approach will allow for the work to focus on the core components of the methodology and related development of 3D flow analysis methods for such high-fidelity velocimetry. Additionally, these measurements will be validated using an independent stationary volumetric velocimetry system. As such, this effort is expected to complement the ongoing development of kHz rate plenoptic imaging techniqu enabling future work in more complex systems associated with high Reynolds numbers and realistic flight conditions.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Feb 14, 2019
- Source ID
- W911NF1910052
Entities
People
- Vrishank Raghav
Organizations
- Army Contracting Command
- Auburn University
- United States Army