• Towards a Semi-Analytical Tool for the Prediction of Dynamic Stall,
  Patrick H. Reisenthel
  [available as AIAA Paper 99-0537]

ABSTRACT: Direct numerical simulations are used to analyze in detail the vorticity dynamics of the leading edge region of a NACA0012 airfoil pitched about its 1/4 chord. The results presented in this paper illustrate how the formalism of indicial theory can be used to predict the integrated vorticity fluxes and the vorticity accumulation during unsteady maneuver. In particular, the flow response to large amplitude non-linear motions is shown to be predicted reasonably accurately, provided that the indicial functions of the flow are inferred in the Laplace domain and stretched to account for quasi-static non-linearity. The implication of this work is the possibility of developing a fast semi-analytical prediction method for incipient leading edge stall, which will be accurate within certain classes of maneuvers.
 

• A Study of Reynolds Number Effects on Incipient Leading Edge Stall,
  Patrick H. Reisenthel and Robert E. Childs
  [available as AIAA Paper 94-2339]

ABSTRACT: It has been suggested that eruptive plumes of vorticity might play a critical role in the physics of vortex formation during leading edge dynamic stall. Numerical simulations at low Reynolds number do not seem to adequately predict this phenomenon. To explore the possibility of a "bifurcation" in Reynolds number, we investigate in detail the scaling of incipient laminar separation, vortex formation, and shedding with respect to Reynolds number. Numerical simulations are used to study a model problem in which a two-dimensional airfoil remains stationary at angle-of-attack, but for which the leading edge flow separates as a result of an impulsively applied no-slip boundary condition. The calculations are laminar (50,000 <= Re <= 400,000), and are performed for alpha = 15 degrees and Mach number = 0.2. The resulting surface flow topology is analyzed as a function of Reynolds number. The results obtained thus far appear to contradict the hypothesis that a form of bifurcation takes place at some intermediate laminar Reynolds number. Furthermore, times and locations for the onset of separation bubbles, vortex formation, and feeding sheet rupture are found to scale according to various power laws of the Reynolds number, Re^eta, with 0.11 <= eta <= 0.45.
 

• Further Results on the Reynolds Number Scaling of Incipient Leading Edge Stall,
  Patrick H. Reisenthel
  [available as AIAA Paper 95-0780]

ABSTRACT: The events leading to the incipient formation of a dynamic stall vortex are investigated in detail using grid-converged solutions of the Navier-Stokes equations up to chord Reynolds numbers Re = 800,000. The results indicate the existence of self-similar behavior over the range 50,000 <= Re <= 800,000, thus invalidating a previous hypothesis concerning bifurcating solutions of high Reynolds number laminar flow separation.