scholarly journals Scalar structures around the mixing interface of a thermal mixing layer in multiscale-generated decaying turbulent flows visualized using DNS database

2021 ◽  
Vol 1978 (1) ◽  
pp. 012012
Author(s):  
Shotaro Jomura ◽  
Hiroki Suzuki ◽  
Shinsuke Mochizuki ◽  
Yasuhiko Sakai
Keyword(s):  
Turbulent Flows ◽  
Mixing Layer ◽  
Thermal Mixing

Investigation of turbulent flows via pseudo flow visualization part I: Axisymmetric jet mixing layer

1994 ◽  
Vol 9 (4) ◽  
pp. 391-404 ◽  
Author(s):  
D.P. Wick ◽  
M.N. Glauser ◽  
L.S. Ukeiley
Keyword(s):  
Flow Visualization ◽  
Turbulent Flows ◽  
Mixing Layer ◽  
Jet Mixing ◽  
Axisymmetric Jet

scholarly journals Optimal nonlinear eddy viscosity in Galerkin models of turbulent flows

2015 ◽  
Vol 766 ◽  
pp. 337-367 ◽  
Author(s):  
Bartosz Protas ◽  
Bernd R. Noack ◽  
Jan Östh
Keyword(s):  
Turbulent Flows ◽  
High Speed ◽  
Eddy Viscosity ◽  
Broad Class ◽  
Constitutive Relations ◽  
Body Height ◽  
Mixing Layer ◽  
The Body ◽  
Stream Velocity ◽  
Initial Vorticity

AbstractWe propose a variational approach to the identification of an optimal nonlinear eddy viscosity as a subscale turbulence representation for proper orthogonal decomposition (POD) models. The ansatz for the eddy viscosity is given in terms of an arbitrary function of the resolved fluctuation energy. This function is found as a minimizer of a cost functional measuring the difference between the target data coming from a resolved direct or large-eddy simulation of the flow and its reconstruction based on the POD model. The optimization is performed with a data-assimilation approach generalizing the 4D-VAR method. POD models with optimal eddy viscosities are presented for a 2D incompressible mixing layer at $\mathit{Re}=500$ (based on the initial vorticity thickness and the velocity of the high-speed stream) and a 3D Ahmed body wake at $\mathit{Re}=300\,000$ (based on the body height and the free-stream velocity). The variational optimization formulation elucidates a number of interesting physical insights concerning the eddy-viscosity ansatz used. The 20-dimensional model of the mixing-layer reveals a negative eddy-viscosity regime at low fluctuation levels which improves the transient times towards the attractor. The 100-dimensional wake model yields more accurate energy distributions as compared to the nonlinear modal eddy-viscosity benchmark proposed recently by Östh et al. (J. Fluid Mech., vol. 747, 2014, pp. 518–544). Our methodology can be applied to construct quite arbitrary closure relations and, more generally, constitutive relations optimizing statistical properties of a broad class of reduced-order models.

Reaction in a scalar mixing layer

1991 ◽  
Vol 233 ◽  
pp. 211-242 ◽  
Author(s):  
R. W. Bilger ◽  
L. R. Saetran ◽  
L. V. Krishnamoorthy
Keyword(s):  
Chemical Reaction ◽  
Mixing Layer ◽  
Reacting Flow ◽  
Scalar Mixing ◽  
Scalar Theory ◽  
Thermal Mixing ◽  
Fast Chemistry ◽  
The Mean ◽  
Reactive Scalars ◽  
Made In

Reaction in a scalar mixing layer in grid-generated turbulence is studied experimentally by doping half of the flow with nitric oxide and the other half with ozone. The flow conditions and concentrations are such that the chemical reaction is passive and the flow and chemical timescales are of the same order. Conserved scalar theory for such flows is outlined and further developed; it is used as a basis for presentation of the experimental results. Continuous measurements of concentration are limited in their spatial and temporal resolution but capture sufficient of their spectra for adequate second-order correlations to be made. Two components of velocity have been measured simultaneously with hot-wire anemometry. Conserved scalar mixing results, deduced from reacting and non-reacting measurements of concentration, show the independence of concentration level and concentration ratio expected for passive reacting flow. The results are subject to several limitations due to the necessary experimental compromises, but they agree generally with measurements made in thermal mixing layers. Reactive scalar statistics are consistent with the realizability constraints obtainable from conserved scalar theory where such constraints apply, and otherwise are generally found to lie between the conserved scalar theory limits for frozen and very fast chemistry. It is suggested that Toor's (1969) closure for the mean chemical reaction rate could be improved by interpolating between the frozen and equilibrium values for the covariance. The turbulent fluxes of the reactive scalars are found to approximately obey the gradient model but the value of the diffusivity is found to depend on the Damköhler number.

The mixing layer of turbulent flows in homogeneous nonconducting and conducting fluids

1973 ◽  
Vol 6 (1) ◽  
pp. 41-53
Author(s):  
V. G. Lushchik ◽  
S. A. Regirer
Keyword(s):  
Turbulent Flows ◽  
Mixing Layer ◽  
Conducting Fluids

Evolution of a non-self-preserving thermal mixing layer

1986 ◽  
Vol 29 (12) ◽  
pp. 3976 ◽  
Author(s):  
J. L. Lumley
Keyword(s):  
Mixing Layer ◽  
Thermal Mixing

Turbulent time scales and the dissipation rate of temperature variance in the thermal mixing layer

1983 ◽  
Vol 26 (9) ◽  
pp. 2415 ◽  
Author(s):  
S. E. Elghobashi
Keyword(s):  
Time Scales ◽  
Dissipation Rate ◽  
Mixing Layer ◽  
Temperature Variance ◽  
Thermal Mixing

Consistent subgrid scale modelling for lattice Boltzmann methods

2012 ◽  
Vol 700 ◽  
pp. 514-542 ◽  
Author(s):  
Orestis Malaspinas ◽  
Pierre Sagaut
Keyword(s):  
Lattice Boltzmann Method ◽  
Turbulent Flows ◽  
Lattice Boltzmann ◽  
Stokes Equations ◽  
Mixing Layer ◽  
Velocity Distribution Function ◽  
Theoretical Point ◽  
Subgrid Scale ◽  
Hermite Expansion ◽  
Boltzmann Method

AbstractThe lattice Boltzmann method has become a widely used tool for the numerical simulation of fluid flows and in particular of turbulent flows. In this frame the inclusion of subgrid scale closures is of crucial importance and is not completely understood from the theoretical point of view. Here, we propose a consistent way of introducing subgrid closures in the BGK Boltzmann equation for large eddy simulations of turbulent flows. Based on the Hermite expansion of the velocity distribution function, we construct a hierarchy of subgrid scale terms, which are similar to those obtained for the Navier–Stokes equations, and discuss their inclusion in the lattice Boltzmann method scheme. A link between our approach and the standard way on including eddy viscosity models in the lattice Boltzmann method is established. It is shown that the use of a single modified scalar relaxation time to account for subgrid viscosity effects is not consistent in the compressible case. Finally, we validate the approach in the weakly compressible case by simulating the time developing mixing layer and comparing with experimental results and direct numerical simulations.

scholarly journals Randomness Representation in Turbulent Flows with Kolmogorov Complexity (In Mixing Layer)

2013 ◽  
Vol 8 (3) ◽  
pp. 407-422 ◽  
Author(s):  
Masashi ICHIMIYA ◽  
Ikuo NAKAMURA
Keyword(s):  
Turbulent Flows ◽  
Kolmogorov Complexity ◽  
Mixing Layer

Orthonormal wavelet decomposition of turbulent flows: intermittency and coherent structures

1997 ◽  
Vol 348 ◽  
pp. 177-199 ◽  
Author(s):  
R. CAMUSSI ◽  
G. GUJ
Keyword(s):  
Turbulent Flows ◽  
Coherent Structures ◽  
Velocity Component ◽  
Wavelet Transforms ◽  
Wavelet Decomposition ◽  
Transverse Velocity ◽  
Mixing Layer ◽  
Grid Turbulence ◽  
Jet Mixing ◽  
Vortex Tubes

Experimental data obtained in various turbulent flows are analysed by means of orthogonal wavelet transforms. Several configurations are analysed: homogeneous grid turbulence at low and very low Reλ, and fully developed jet turbulence at moderate and high Reλ. It is shown by means of the wavelet decomposition in combination with the form of scaling named extended self-similarity that some statistical properties of fully developed turbulence may be extended to low-Reλ flows. Indeed, universal properties related to intermittency are observed down to Reλ≃10. Furthermore, the use of a new conditional averaging technique of velocity signals, based on the wavelet transform, permits the identification of the time signatures of coherent structures which may or may not be responsible for intermittency depending on the scale of the structure itself. It is shown that in grid turbulence, intermittency at the smallest scales is related to structures with small characteristic size and with a shape that may be related to the passage of vortex tubes. In jet turbulence, the longitudinal velocity component reveals that intermittency may be induced by structures with a size of the order of the integral length. This effect is interpreted as the signature of the characteristic jet mixing layer structures. The structures identified on the transverse velocity component of the jet case turn out on the other hand not to be affected by the mixing layer and the corresponding shape is again correlated with the signature of vortex tubes.

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