ALBERT

Description: The method of apparent masses is utilized to compute the initial lift and drag of an airfoil that starts impulsively from rest. Analytical solutions are obtained for in viscid incompressible flow past a slightly cambered airfoil at a small angle of attack. For a Joukowski airfoil with a cusped trailing edge, it is found that increasing camber or angle of attack will cause increases in both initial lift and drag, whereas increasing thickness will result in an opposite effect. Effects of trailing-edge angle are examined by considering the symmetric Kármán-Trefftz airfoil. The result shows that both lift and drag vanish at the initial instant if the airfoil has a finite trailing-edge angle. © 1982, Cambridge University Press. All rights reserved.

Description: The general triple-deck theory of laminar viscous-inviscid interaction is extended to axisymmetric bodies. With body radius/length ratios scaled in terms of Reynolds number as Re−1/8β(β 〉 0), it is found that for β 〈 3 the only three-dimensional effect is that on the incoming undisturbed boundary-layer profile as accounted for by the Mangler transformation. When β = 3, however, an explicit axisymmetric effect on the interaction equations also enters: the upper-deck flow is governed by the equation of axisymmetric potential disturbance flow, whereas the middle and lower decks are still governed by equations of two-dimensional form. When β 〉 3, the body is so slender that transverse curvature effects become important and the lower decks too are explicitly influenced by three-dimensional effects. A detailed example application of this theory is given for weak interactions on a flared cylinder and cone in supersonic flow with β ≤ 3. The three-dimensional effects on the interactive pressure and shear-stress distributions are shown to relieve the strength of the interaction and reduce its upstream influence, as expected. Correspondingly, it is found that the smallest flow deflection angle provoking incipient separation increases with increasing axisymmetric body slenderness. These results are shown to be in qualitative agreement with several experimental studies. © 1983, Cambridge University Press. All rights reserved.

Description: The SESAME-AVE IV (9–10 May 1979) rawinsonde data were analyzed to uncover possible triggering mechanisms for severe storms that developed over western Oklahoma and Texas. The high frequency of observations (at 3 h intervals) and high vertical resolution of reported data (at 25 mb intervals) at all stations permitted investigation of the diurnal variation of the planetary boundary layer on the synoptic scale. Thunderstorms developed first just ahead of a stationary front over the Texas panhandle on the afternoon of 9 May. This area was characterized by the absence of a strong inversion (or “lid”) that represented an interface between very warm and dry air aloft, and warm moist tropical air below. Apparently, mesoscale low-level ascending motion associated with frontal lifting and/or the inland sea breeze effect led to the removal of the lid. Another noteworthy feature in this storm event was the strong vertical wind shear at low and middle levels over the storm area. When combined with the development of a deep boundary layer with weak stratification during the daytime, the Richardson number became less than one in the boundary layer in the prestorm situation. The results of our numerical linear stability analysis indicate that the observed basic states were indeed symmetrically unstable. This may suggest that the triggering processes were argumented by symmetric instability. Although a well defined dry line was present, it does not seem to have contributed directly to initiation of storms in this case. It also was observed that, as the thermal low began to weaken in the early evening, the cold air behind a stationary front started advancing eastward and helped to extend the line of thunderstorms deep into central Texas. This may be another process whereby some storms prefer to develop in the late afternoon or early evening.