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N EAR-STALL three-dimensional separation patterns, which are also referred to as “stall cells,” can appear on the suction side of certain airfoils. An early report on this phenomenon was provided by Gregory et al. [1]. Detailed oilflow visualizations of stall cells were obtained by, e.g., Winkelmann and Barlow [2], Bippes et al. [3], and Schewe [4]. Yon and Katz [5] sampled the wall pressure in the separated flow region and observed large-amplitude but lowfrequency fluctuations at the beginning of the range of the angle of attack over which the stall cells occurred. Outside this range, the amplitude of the fluctuations was much lower and the low-frequency signal was absent. Broeren and Bragg [6] investigated several airfoils with different stalling characteristics. Stall for airfoils with trailingedge separation was characterized by the appearance of stall cells. The stall cells that were observed for angles of attack at or above stall were generally steady. Stall for airfoils with leading-edge separation (i.e., thin-airfoil stall) displayed low-frequency oscillations near stall but no stall cells. Stall cells can appear suddenly and, therefore, have a profound effect on the performance, maneuverability, and safety of aircraft. An improved understanding of the criteria determining the appearance of stall cellswouldmake flight at near-stall conditionsmore predictable, and thus safer. In addition, the mechanism responsible for the formation of stall cells may be exploited for high angle-of-attack flow control applications. Weihs and Katz [7] proposed a physical mechanism for the appearance of stall cells based on the Crow instability [8]. At stall, in the time mean, a spanwise vortex can be associatedwith the recirculating flow region on the suction side of the airfoil. Because this vortex is located close to the airfoil surface, a reflected image of the vortex with the opposite sense of rotation can be imagined. As shown by Crow [8], this steady vortex pair is unstable with respect to a symmetric mode. The growth of the symmetric mode may result in the breakup of the steady vortex and the formation of stall cells. Rodriguez and Theofilis [9] argued that stall cells are the consequence of a global instability. The linear superposition of the two-dimensional flow around a stalled airfoil with the leading stationary three-dimensional eigenmode resulted in skin-friction lines that were strongly reminiscent of stall cells. In the present Note, an explanation for the appearance of stall cells based on lifting-line theory is offered and a simple formula for estimating the stall cell spacing is derived.

A wave packet generated by a localized impulse can be defined in terms of an integral or sum of periodic wavetrains. This integral can be represented asymptotically in a number of different ways. Wave forms of a twodimensional wave packet defined by various expansions are evaluated numerically and compared over a range of distances from their point of origin.