Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

20010022635 Stanford Univ., Center for Turbulence Research, Stanford, CA USA
Control and Optimization of Turbulent Jet Mixing
Hilgers, Angela, Stanford Univ., USA; Annual Research Briefs - 2000: Center for Turbulence Research; December 2000, pp.
45-54; In English; See also 20010022631; No Copyright; Avail: CASI; A02, Hardcopy; A03, Microfiche
The control of turbulent jet flows has applications in various fields such as combustion, aerodynamic noise, and jet propulsion.
Control of mixing can be achieved by manipulation of the large-scale, global instabilities of the flow. In combustors it is
important to enhance turbulent mixing of the chemical species in order to achieve high combustion efficiency and to reduce the
emission of pollutants. In jet engines, aerodynamic noise can be reduced by controlling flow unsteadiness that produces noise.
One aim of enhanced mixing in jet propulsion applications is to decrease the plume temperature and suppress infrared radiation.
The mixing rate of a turbulent jet can be significantly altered by applying a suitable excitation at the jet orifice. Since the external
forcing interacts with the natural modes of the jet in a nonlinear way, it is not possible to predict which kind of actuation is optimal
to increase mixing. Various experiments have been carried out that study the reaction of jets to the nozzle geometry and to external
forcing. Different types of actuators such as piezoelectric devices and synthetic jets have been tested. It has been shown that a large
spreading of the jet can be achieved with a small mass flow actuation if suitable frequencies are chosen. Numerical simulations
of compressible and incompressible jet flows have been carried out that confirm many observations made in experiments with
periodically forced jets. So far it is difficult to carry out a systematic search for the optimal forcing because the simulations are
very computationally intensive. Koumoutsakos et al. showed that evolution strategies are capable of finding suitable actuations
for a vortex model and direct numerical simulations of compressible jets. In this paper we describe a systematic search for combined
axial and helical forcing of a round jet that maximize mixing. While stochastic search strategies and direct numerical simulation
(DNS) of a jet have been combined before, this work concentrates on the optimization of jets at higher Reynolds numbers
using large eddy simulation (LES). Different search strategies are introduced and their performance for the optimization of jet
mixing is compared. The strategies are used to search for an actuation that maximizes mixing in jets at Reynolds numbers Re =
6000 and Re 100,000.
Author
Jet Mixing Flow; Large Eddy Simulation; Turbulent Jets; Helical Flow; Axial Flow
20010022637 Universidad Politecnica de Madrid, Madrid, Spain
Low-Dimensional Dynamics of Near-Wall Turbulence
Jimenez, Javier, Universidad Politecnica de Madrid, Spain; Simens, Mark, Universidad Politecnica de Madrid, Spain; Annual
Research Briefs - 2000: Center for Turbulence Research; December 2000, pp. 67-78; In English; See also 20010022631; M.S.
is supported by the TMR program.
Contract(s)/Grant(s): CT98-0175; No Copyright; Avail: CASI; A03, Hardcopy; A03, Microfiche
Wall-bounded turbulent shear flows are perhaps the last area in ’classical’ incompressible turbulence in which there still are
open questions about basic physical mechanisms. There are two competing conceptual models. In the first one, wall turbulence
is just a modification of ordinary shear turbulence occurring when the latter is a near wall, and is therefore dependent on the prior
existence of an outside turbulent flow. In the second, it is an essentially different phenomenon which coexists with the outer flow
and merges into it when the distance from the wall is large enough. Jimenez took the latter view and argued that the dynamics of
near-wall turbulence is essentially different from the Kolmogorov mechanism. While the latter is fundamentally isotropic and the
energy is dissipated locally by cascading to smaller length scales, wall-bounded flows are intrinsically inhomogeneous and anisotropic,
and a substantial part of their energy diffuses from the wall into the outer flow, increasing, rather than decreasing, its length
scales in the process. In this paper we examine the dynamics of the structures of the viscous and buffer layers in very simplified
situations in which their interaction with the outer flow is severely restricted. Even natural flows scale in this region approximately
in wall units, defined in terms of the kinematic viscosity v and of the friction velocity u(sub tau) = (v (diff(y)) U)(exp 1/2), where
U is the mean velocity profile. In that approximation, and if we admit that near-wall turbulence is not a just a modification by the
wall of the outside turbulent flow, only local quantities such as dimensionless distance to the wall, y(sup +) = u(sub tau)y/v, should
matter, while global parameters such as the Reynolds number of the flow should be irrelevant. In the reduced systems considered
here, the outer flow is effectively removed, and the scaling in wall variables should be strict. No bulk Reynolds number may be
relevant because no bulk turbulent flow exists. Since the local wall-normal Reynolds numbers are low, we may expect quasi-laminar
structures whose behavior can be understood deterministically. In this sense this region corresponds to the Kolmogorov viscous
range of isotropic turbulence, but we will see that, while the latter is a sink for the energy cascading from the larger scales,
the structures studied here are not only self-sustaining, but actually export energy to the rest of the flow. The organization of this
paper is as follows. The energy balance is briefly examined in the next section. The wall region and the numerical experiments
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