The recent progress in three-dimensional boundary-layer stability and transition is reviewed. The material focuses on the crossflow instability that leads to transition on swept wings and rotating disks. Following a brief overview of instability mechanisms and the crossflow problem, a summary of the important findings of the 1990s is given.

Crossflow-dominated swept-wing boundary layers are known to undergo a highly nonlinear transition process. In low-disturbance environments, the primary instability of these flows consists mainly of stationary streamwise vortices that modify the mean velocity field and hence the stability characteristics of the boundary layer. The result is amplitude saturation of the dominant stationary mode and strong spanwise modulation of the unsteady modes. Breakdown is not caused by the primary instability but instead by a high-frequency secondary instability of the shear layers of the distorted mean flow. The secondary instability has been observed in several previous experiments and several computational models for its behaviour exist. None of the experiments has been sufficiently detailed to allow either model validation or transition correlation. The present experiment conducted using a 45 • swept wing in the low-disturbance Arizona State University Unsteady Wind Tunnel addresses the secondary instability in a detailed fashion under a variety of conditions. The results reveal that this instability is active in the breakdown of all cases investigated, and furthermore, it appears to be well-described by the computational models.

Recent experiments on transient disturbance growth in boundary layers indicate that disturbances generated by surface roughness undergo suboptimal growth. The implication is that the receptivity mechanism governing the distribution of disturbance energy among the continuous spectrum of damped Orr-Sommerfeld/Squire modes produces energy distributions that are significantly different from the theoretical optimum. Experiments presented here are intended to investigate how the amplitude and diameter of cylindrical roughness elements arranged in a spanwise array affect various features of transient growth. The objective is to infer how or to what extent the roughness features affect the continuous spectrum and to use this information as a foundation for future receptivity models. The results show that the energy of stationary disturbances varies as Re k 2 and that the streamwise distance over which the disturbances grow increases slightly with increasing Re k . As the roughness diameter is varied, dramatic changes in the qualitative nature of the resulting transient growth occur. Both the variation of the growth length with Re k and the behavioral changes with roughness diameter indicate that the energy distribution among the continuous modes is a strong function of roughness features and that an accurate and sophisticated receptivity model will be necessary to accurately predict transient growth.

Theoretical and direct numerical simulation models of transient algebraic growth in boundary layers have advanced significantly without an adequate, parallel experimental effort. Experiments that feature disturbances excited by high levels of freestream turbulence or distributed surface roughness show behavior consistent with optimal-disturbance theories but cannot address the theories’ key predictions concerning the growth and decay of disturbances at specific spanwise wavenumbers. The present experiment seeks to provide such data for a flat plate boundary layer using a spanwise roughness array to excite controlled stationary disturbances. The results show that although general trends and qualitative behaviors are correctly captured by optimal-disturbance theories, significant quantitative differences exist between the theories’ predictions and the current experimental measurements. Discrepancies include the location of the wall-normal disturbance profiles’ maxima and the streamwise location of the maximum energy growth. While these discrepancies do not argue against the validity of transient-growth theory in general, they do indicate that correct modeling of receptivity to realistic disturbances is critical and that realistic stationary disturbances can exhibit strongly nonoptimal behavior.

The role of surface roughness in boundary layers continues to be a topic of significant interest, especially with regard to how controlled roughness might be used to delay laminar-to-turbulent transition. Although it may be useful for control, large-amplitude roughness may itself lead to transition. In an effort to understand the breakdown mechanics associated with large-amplitude surface roughness, experiments are conducted to investigate the steady and unsteady disturbances generated by three-dimensional roughness elements whose amplitudes are close to the critical roughness-based Reynolds number Re k for roughness-induced transition. Measurements are obtained in a flat-plate boundary layer downstream of a spanwise array of cylindrical roughness elements at both subcritical and supercritical values of Re k . The steady disturbance field has strong shear in the wall-normal and spanwise directions, and the unsteady streamwise velocities in the roughness elements' wake show evidence of hairpin vortices. The locations of maximum fluctuation intensity correspond to the locations of inflection points in the steady flow streamwise velocity, and this suggests that the fluctuations may result from a Kelvin-Helmholtz-type instability. Temporal power spectra indicate an unstable band of frequencies from 300 to 800 Hz. The Strouhal number associated with the 650-Hz fluctuations that are often observed to be the strongest give Sr = 0.15, a value that is in good agreement with previous findings. At supercritical Re k , rapid transition takes place when the unsteady disturbances reach nonlinear amplitudes. The disturbance growth rates indicate that in this situation transition can be understood as a competition between the unsteady disturbance growth and the rapid relaxation of the steady flow that tends to stabilize these disturbances. NomenclatureD = roughness diameter E = steady disturbance energy e f = unsteady disturbance energy in a frequency band centered at f f = frequency H = shape factor, δ * /θ k = roughness height N = number of samples Re k = roughness-based Reynolds number,Ū (k)k/ν Re = unit Reynolds number, U ∞ /ν Sr = Strouhal number of unsteady vortex shedding, f δ * /U ∞ U = spanwise-invariant streamwise basic state velocity U = steady streamwise disturbance velocity U ∞ = freestream velocity u = unsteady streamwise disturbance velocity x, y, z = streamwise, wall-normal, and spanwise coordinates x k = streamwise location of the roughness array α = spatial growth rate δ = boundary-layer length scale, [(x − x vle )/Re ] 1/2 δ * = displacement thickness λ k = roughness spacing η = Blasius coordinate, y/δ θ = momentum thickness ν = kinematic viscosity Subscripts c = centerline crit = critical rms = root-mean-square

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The cross-flow instability that arises in swept-wing boundary layers has resisted attempts to describe the path from disturbance initiation to transition. Following concerted research efforts, surface roughness and free-stream turbulence have been identified as the leading providers of initial disturbances for cross-flow instability growth. Although a significant body of work examines the role of free-stream turbulence in the cross-flow problem, the data more relevant to the flight environment (turbulence intensities less than 0.07 %) are sparse. A series of recent experiments indicates that variations within this range may affect the initiation or growth of cross-flow instability amplitudes, hindering comparison among results obtained in different disturbance environments. To address this problem, a series of wind tunnel experiments is performed in which the free-stream turbulence intensity is varied between 0.02 % and 0.2 % of free-stream velocity, U ∞ . Measurements of the stationary and travelling mode amplitudes are made in the boundary layer of a 1.83 m chord, 45 • swept-wing model. These results are compared to those of similar experiments at higher turbulence levels to broaden the current knowledge of this portion of the crossflow problem. It is observed that both free-stream turbulence and surface roughness contribute to the initiation of unsteady disturbances, and that free-stream turbulence affects the development of both stationary and unsteady cross-flow disturbances. For the range tested, enhanced free-stream turbulence advances the transition location except when a subcritically spaced roughness array is employed.

Performance data for 261 NCAA Division 1A collegiate football players were analyzed to determine if player position, body weight, body fat, and training time were correlated with changes in performance in the following events: power clean (PC), bench press (BP), squat (SQ), vertical jump (VJ), 40-yd dash (40yd), and 20-yd shuttle (20yd). Individual positions were combined into the following groups: (A) wide receivers, defensive backs, and running backs, (B) linebackers, kickers, tight ends, quarterbacks, and specialists, and (C) linemen. Increases in body weight were positively correlated with increases in BP and PC performance for all groups. Increases in body fat were negatively correlated with performance in the PC and VJ for all groups. For group C, increases in body fat were also negatively correlated with performance in the 40yd and 20yd. Group and training time exhibited no linear relationship with performance in any of the tested events. No linear relationships were observed between the independent variables and performance in the SQ. When individual training data were analyzed longitudinally, a nonlinear increase in performance in the PC, BP, and SQ was observed as training time increased, with the greatest rate of change occurring between the first and second semesters of training.