Anupam Sharma | Argonne National Laboratory



Anupam Sharma


Argonne National Laboratory

Award Title: 

Unraveling Silent Owl Flight to Develop Ultra-­Quiet Energy Conversion Machines




Acoustic  emission  (noise)  from  wind  turbines  is  curtailing  the  growth  of  wind  energy,  which  is  currently  the  primary  renewable  energy  source  in  the  US  and  in  the  world.  A majority of the noise radiated from wind turbines is generated aerodynamically – due to interaction of wind with blade surfaces. Aerodynamic sound (aeroacoustics) is an issue not just  for  wind  turbines  but  also  for  aircraft,  jet  engines,  combustion  turbines  used  for  electricity generation, cooling fans, and ventilation systems.

A solution to the problem of aerodynamic noise generation is available in nature but has not yet been leveraged to develop silent machines. The nocturnal owl is known to have a silent  flight  both  when  gliding  and  flapping.  This  has  been  known  for  decades,  but  the  physical  mechanisms  enabling  its  silent  flight  are  not  well  understood.  Previous investigations  have  identified  three  feather  features  that  are  unique  to  the  owl.  Experimental  investigations  have  demonstrated  that  these  unique  feather  features  are responsible for the owl’s acoustic stealth.  However, these experiments alone are unable to identify  the  reasons/mechanisms  behind  noise  reduction.

This project supports very high resolution simulations to bridge the scientific gap between experimental results and theoretical understanding. A systematic numerical investigation of  the  unique  owl  feather  features  is  proposed  to  answer  key  questions  that  will  help  unravel  the  mystery  behind  owl’s  silent  flight.  The  extremely  high  spatial  and  temporal resolution offered by high-­fidelity numerical simulations will enable source diagnostics to identify  how  the  unique  feather  features  curb  noise  generation.  The  knowledge  and  understanding  gained  from  these  simulations  can  empower  us  to  design  nearly  silent energy conversion-­‐ and various other engineering machines.

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Sharma spends summer at Wright Patterson Air Force Base

Professor of aerospace engineering, Anupam Sharma spent two months at the prestigious Air Force Research Laboratory (AFRL) at the Wright Patterson Air Force Base in Ohio this summer. As part of the 2016 AFOSR (Air Force Office of Scientific Research) summer faculty fellowship, Sharma investigated the fluid dynamic phenomenon that occur at the onset of dynamic stall.Sharma_Grey

Rapidly maneuvering aircraft, helicopter- and wind turbine rotors, flying insects, etc. experience the phenomenon of dynamic stall. Dynamic stall is a nonlinear flow phenomenon associated with large increase in aerodynamic loads (lift, drag, and moment), which are much greater than what the lifting body would experience under quasi-steady conditions (static stall). With the exception of insect flight, where dynamic stall can potentially augment lift, the increased loads associated with dynamic stall are detrimental. Mechanisms to suppress dynamic stall in engineering machines are therefore desirable. Such suppression mechanisms are most effective when applied just before the onset of stall.

Sharma is using high-fidelity computational fluid dynamics (CFD) simulations to examine the role of geometry, specifically thickness of the lifting body (an airfoil) on incipience of dynamic stall. Dynamic stall can be triggered by different mechanisms, e.g., gradual progression of the turbulent boundary layer separation from the trailing edge to the leading edge of the airfoil, “bursting” of the laminar separation bubble (small region of recirculating flow), etc. The simulations show that airfoil thickness can change the mechanism that triggers dynamic stall – thick airfoils stall via the turbulent boundary layer separation mechanism while the laminar separation bubble burst mechanism is observed in relatively thin airfoils. Sharma’s group is continuing this research and expect to publish their findings in the next few months.

The top animation shows the out-of-plane component of vorticity for the NACA 0018 airfoil pitching about its quarter chord point at a constant rotation rate. The bottom animation shows contours of negative blade-relative flow velocity (highlighting regions of flow reversal) in the frame of reference rotating with the airfoil. Turbulent separation reaches the leading edge for this airfoil when stall occurs. Dynamic stall onset is marked by the formation of a large scale vortex that is formed very near the airfoil leading edge.