On the Origin and Control of Large Coherent Structures in Turbulent Shear Flow
Abstract
Inhibition of two-dimensional Tollmien-Schlichting waves by active mean is well known. Surface deformation, heat input and mechanical means were used to impose disturbances on the boundary layer which have an opposite phase to the waves existing in the flow. Thus, transition might be delayed by suppressing the amplitude of these waves. Natural disturbances in boundary layers start as three-dimensional wave packets, because they originate at surface imperfections or are precipitated by temporal disturbances in the incoming stream. The possibilities of controlling such disturbances are currently being investigated. In this context the spatial interaction among three dimensional wave trains, emanating from discrete point-sources, in a boundary layer are discussed. Only a local wave attenuation is feasible by activating two harmonic, point-source disturbances anywhere in the boundary layer. This was shown theoretically for a variety of locations, separation distances and phase delays between the two source and proven experimentally when the disturbance sources were displaced along the span. The spanwise phase gradients of the disturbances, linked to the streamwise distance from their point-source present a major obstacle to such a simple attenuation scheme. These difficulties cannot be foreseen by considering 3-D perturbations. Spatially distributed control mechanisms are therefore required for the purpose of delaying the amplification of concentrated three-dimensional disturbances.
Document Details
- Document Type
- Technical Report
- Publication Date
- Oct 31, 1991
- Accession Number
- ADA243727
Entities
People
- A. Seifert
- I. Wygnanski
Organizations
- Tel Aviv University