Engineering topics

Reed, Helen L.

Publications and source records attributed to Reed, Helen L..

23 records · Page 2

NASA NTRS1988

Numerical simulation of transition in a decelerating boundary layer

Transition in a decelerating flat-plate boundary layer is numerically simulated up to the beginning of three-dimensional breakdown, and the results are compared with an experiment. The adverse pressure gradient induced by deceleration increases the growth rate of disturbances and allows transition at lower Reynolds numbers. The primary instability is characterized by a wave packet, which undergoes three-dimensional distortion. Lambda vortices are locally observed, but they are not aligned with respect to the flow direction.

Yang, Kyung Soo↗

NASA NTRS1987

Wave interactions in swept-wing flows

The leading-edge region of swept wings is dominated by the crossflow instability, resulting in vortices that all rotate in the same sense. The effect of these vortices on the behavior of other disturbances is examined and an interaction between these and disturbances of half the dominating crossflow wavelength is predicted. According to theory, the interaction is of crossflow-crossflow type. The effect explains the anomalies found in the experimental observations of Saric and Yeates (1985). Visually Saric and Yeates observe vortices at the wavelength predicted by linear theory; however, in their hot-wire measurements they find that the second harmonic dominates disturbance growth, with eventually three times the amplitude of the primary wave. In this case, the usual transition-prediction methods would fail, clearly indicating the importance of studying interactions of this sort.

Reed, Helen L.↗

NASA NTRS1987

Stability of three-dimensional boundary layers

The computational modeling of the transition process characteristic of flows over swept wings is discussed. Specifically, the crossflow instability and crossflow/Tollmien-Schlichting wave interactions are analyzed through the numerical solution of the full three-dimensional Navier-Stokes equations including unsteadiness, curvature, and sweep. This approach is chosen because of the complexity of the problem and because it appears that regular stability theory is insufficient to explain the discrepancies between experiments and between theory and experiment. The leading edge region of a swept wing will be considered in a three-dimensional spatial simulation with random disturbances as the initial conditions.

Reed, Helen L.↗

NASA NTRS1987

Local intermodal energy transfer of the secondary instability in a plane channel

A mathematical technique for analyzing local energy-transfer rates among wave-vector triads is developed and applied to the data generated by direct numerical simulations of waves with various types of initial conditions. Starting the simulation with a primary two-dimensional wave and random noise produced structures very similar to those which evolved from three-dimensional center modes. The local transfer rates determined in this case help to explain the eventual deformation of the primary two-dimensional wave which was observed in wind-tunnel experiments. Including weak streamwise vortices in the initial flow field results in large amounts of energy being transferred to the K-type modes early in the simulation. The later development of the waves (and hence the energy-transfer rates) is similar to the previous case.

Singer, Bart A.↗

NASA NTRS1987

Three-dimensional stability of boundary layers

The most recent efforts on the stability and transition of three-dimensional flows are reviewed. These include flows over swept wings, rotating disks, rotating cones, yawed bodies, corners, and attachment lines. The generic similarities of their stability behavior is discussed. It is shown that the breakdown process is very complex, often leading to contradictory results. Particular attention is paid to opposing observations of stationary and traveling wave disturbances.

Saric, William S.↗