Engineering topics

Landahl, Marten T.

Publications and source records attributed to Landahl, Marten T..

4 records

NASA NTRS1990

Hydrodynamic instability and coherent structures in turbulence

The evolution of a coherent structure in the near wall region is examined in the context of linear theory. In accordance with this theory, a three-dimensional inviscid disturbance is shown to grow into a streamwise elongated streaky structures in a manner similar to what has been observed in experiments and numerical simulations. It is found that spanwise asymmetry is necessary for the structures to evolve into long streaks. The implications of the results for turbulent drag reduction through active or passive means are discussed.

Landahl, Marten T.↗

NASA NTRS1988

Theoretical model for VITA-educed coherent structures in the wall region of a turbulent boundary layer

Experiments on wall-bounded shear flows (channel flows and boundary layers) have indicated that the turbulence in the region close to the wall exhibits a characteristic intermittently formed pattern of coherent structures. For a quantitative study of coherent structures it is necessary to make use of conditional sampling. One particularly successful sampling technique is the Variable Integration Time Averaging technique (VITA) first explored by Blackwelder and Kaplan (1976). In this, an event is assumed to occur when the short time variance exceeds a certain threshold multiple of the mean square signal. The analysis presented removes some assumptions in the earlier models in that the effects of pressure and viscosity are taken into account in an approximation based on the assumption that the near-wall structures are highly elongated in the streamwise direction. The appropriateness of this is suggested by the observations but is also self consistent with the results of the model which show that the streamwise dimension of the structure grows with time, so that the approximation should improve with the age of the structure.

Landahl, Marten T.↗

NASA NTRS1987

The simulation of coherent structures in a laminar boundary layer

Coherent structures in turbulent shear flows were studied extensively by several techniques, including the VITA technique which selects rapidly accelerating or decelerating regions in the flow. The evolution of a localized disturbance in a laminar boundary layer shows strong similarity to the evolution of coherent structures in a turbulent-wall bounded flow. Starting from a liftup-sweep motion, a strong shear layer develops which shares many of the features seen in conditionally-sampled turbulent velocity fields. The structure of the shear layer, Reynolds stress distribution, and wall pressure footprint are qualitatively the same, indicating that the dynamics responsible for the structure's evolution are simple mechanisms dependent only on the presence of a high mean shear and a wall and independent of the effects of local random fluctuations and outer flow effects. As the disturbance progressed, the development of streak-like-high- and low-speed regions associated with the three-dimensionality.

Breuer, Kenny↗

NASA NTRS1987

Leading-edge vortex solutions with large total pressure losses

Computations are presented for a Lambda = 75 deg delta wing in a supersonic freestream under two conditions which lead to leading-edge vortices. For one condition, analysis of the computed vortical flow reveals a closed streamline in the core. From varying computational parameters, it appears that this is due to truncation error of the convective derivatives. For the other condition, comparisons are made with wind-tunnel data, and good agreement is noted for pitot pressure distributions, flow angles on the symmetry plane, and the position of an embedded shock. Many of the aerodynamic parameters are shown to be insensitive to grid spacing.

Murman, Earll M.↗