Engineering topics

Waleffe, F.

Publications and source records attributed to Waleffe, F..

3 records

NASA NTRS1992

Triad interactions in the dissipation range

Nonlocality of the triad interactions in the dissipation range of developed turbulence is investigated by numerical simulation and the quasi-normal theories. It is found that the energy transfer is dominated by nonlocal triad interactions over the wavenumber range 0.1 is less than k/k(sub d) is less than 4, where k(sub d) is the Kolmogorov wave number. The nonlocality of the interaction has a close relation with the power of an algebraic prefactor of the exponential decay of the energy spectrum in the far-dissipation range.

Kida, S.↗

NASA NTRS1990

Low order dynamical models for mixing layers

The goal of this project was to investigate the feasibility of developing useful low order dynamical systems to allow one to determine cheaply the effect of various initial conditions on the evolution of mixing layers. A mathematically rigorous approach for a time evolving base flow as found in a mixing layer seems to be the singular value decomposition of the linear operator transforming some initial perturbations into the solution at a given later time. Unfortunately, this is very costly, if at all feasible, and only deals with the linear evolution of a disturbance. A different approach based on weakly nonlinear analysis was attempted, but numerous difficulties were encountered. Several formulations were proposed which may be ill-posed. The reasons for this possible failure of the attempted approach is related to the fundamental limitations of weakly nonlinear analysis. The clarification of these issues is attempted in this report.

Riahi, D. N.↗

NASA NTRS1990

Organized motions underlying turbulent shear flows

The objective of this project is to determine the nature and significance of the organized motions underlying turbulent shear flow. There is considerable experimental evidence for the existence of such motions. In particular, one consistently observes longitudinal streaks with a spacing of about 100 in wall units in the near-wall region of wall-bounded shear flows. Recently, an analysis based on the direct resonance mechanism has predicted the appearance of streaks with precisely such a spacing. Also, the minimum channel simulations of Jimenez and Moin have given a strong dynamical significance to that spanwise length scale. They have shown that turbulent-like flows can not be maintained when the spanwise wavelength of the motion is constrained to be less than about that critical number. A critical review of the direct resonance ideas and the non-linear theory of Benney and Gustavsson is presented first. It is shown how this leads to the later mean flow-first harmonic theory of Benney. Finally, we note that a different type of analysis has led to the prediction streaks with a similar spacing. This latter approach consists of looking for optimum fields and directly provides deep insights into why a particular structure or a particular scale should be preferred.

Waleffe, F.↗