Numerical Investigation of Compressibility Effect on Bubble Bursting for a Helicopter Blade Section
Abstract
Dynamic stall on helicopters limits the maximum airspeed, increases the power consumption and vibratory loads, and is generally undesirable. For flight Mach numbers as low as 0.2, the flow near the suction peak of the retreating blade can become sonic. Compressibility effects promote leading-edge stall and the formation of the dynamic-stall vortex at lower angles of attack compared to the incompressible case. Leading-edge stall is initiated by the bursting of a laminar separation bubble. The compressibility effects on the stability of the laminar separation bubble and bubble bursting are not well understood. The delicate interplay of observed low-frequency lambda-shockwaves with instabilities of the laminar separation bubble has not been explained. Furthermore, it is unclear to what extent low-Mach number dynamic stall control strategies carry over to compressible Mach numbers. In summary, a need exists for new research on the onset and control of dynamic stall at compressible Mach numbers. The objectives of the proposed research are to: (1) carry out high-fidelity simulations that provide detailed insight into the compressibility effects on bubble bursting and leading-edge stall; (2) develop a new understanding of the physical mechanisms that explain the compressibility effects on bubble bursting; (3) investigate active flow control strategies for delaying bubble bursting at compressible Mach numbers. To meet the research objectives, a combined investigative approach of high-fidelity simulations, advanced data-driven modal decomposition tools, and linear stability theory analyses will be employed. Due to the high complexity of the helicopter flow field, instead of simulating an entire rotor blade, implicit large-eddy simulations of a blade section in dynamic stall with a relatively narrow spanwise extent will be performed. This deliberate simplification will allow for a higher grid resolution and an easier identification of the relevant underlying flow physics. The Reynolds number for the simulations will be in the range Re=200,000 to 400,000; the Mach number will be in the range 0.1 to 0.4. The simulations will first be validated with existing wind tunnel measurements. Then, ÒclassicalÓ local and bi-global linear stability theory analyses as well as advanced data-driven modal decomposition tools will be employed to investigate the underlying flow physics. The data-driven methods are more suitable for investigating nonlinear processes such as those associated with shockwaves. The stability investigations will provide a critical new understanding of the compressibility effects on bubble bursting and dynamic stall. Based on this new understanding, active flow control strategies will be developed that exploit underlying flow instabilities for an efficient and effective delay of the stall onset. The data obtained from the proposed simulations will aid in the interpretation of the wind tunnel measurements and support the development and validation of turbulence modeling strategies for transitional flows. The new improved understanding of the flow physics will inform the development of criteria for the onset of leading-edge stall and support the design of airfoils that are more resistant to bubble bursting and leading-edge stall. Both the new airfoils and the active flow control strategies, if successful, will motivate follow-up blade section experiments. Confirmation of the active flow control results in experiments will open the door towards their implementation on full-size rotors. The potential performance gains and other benefits are numerous (higher advance ratios, larger maximum airspeed, reduced fuel consumption, lower vibration and noise, reduced maintenance cost, increased passenger comfort, etc.). The project will also educate and train students in areas that are of high relevance to the Army.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Feb 09, 2023
- Source ID
- W911NF2310071
Entities
People
- Andreas Gross
Organizations
- Army Contracting Command
- New Mexico State University
- United States Army