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Molecular Mixing in Rayleigh -Taylor Turbulence

#1739


Study of Molecular Mixing Effects and Heterogeneous Structure of Rayleigh -Taylor Turbulence

Tech Area / Field

  • PHY-NGD/Fluid Mechanics and Gas Dynamics/Physics

Status
3 Approved without Funding

Registration date
20.12.1999

Leading Institute
VNIITF, Russia, Chelyabinsk reg., Snezhinsk

Collaborators

  • Lawrence Livermore National Laboratory, USA, CA, Livermore

Project summary

The goal of the Project is to experimentally and theoretically investigate the effects of fluids molecular mixing and heterogeneous structure of turbulent flow that results from Rayleigh-Taylor instability development. Main objectives of the Project are to study molecular mixing rate in turbulent mixture; the effect of molecular diffusion on turbulent region growth, mean parameters of the flow, local and structural features of turbulence by conducting laboratory experiments at the special facilities using new measuring techniques and numerical simulations using hydrodynamic codes.

Rayleigh-Taylor (or gravitational) instability takes place at an interface between two fluids of different density, when pressure in less dense fluid is higher than that in a denser one. The most familiar example is a situation, when a layer of heavy fluid is resting above light one in external gravitational field. At the final stage of Rayleigh-Taylor instability development the flow becomes turbulent and a region of turbulent mixing forms. If the fluids are miscible, turbulence enhances mixing down to a molecular level. In its turn molecular mixing can effect inversely the overall evolution and structure of turbulent flow.

Gravitational turbulent mixing studies are of great scientific and practical value, since this phenomenon occurs in many natural and man-made processes. For instance, it takes place at supernova bursts and elevation of a hot cloud in atmosphere. It is one of the principal difficulties in implementing Inertial Confinement Fusion Program. When modeling atmospheric and oceanic stratified flows, it is required to parameterize vertical mixing process taking into account its molecular component. Quantitative data on molecular mixing rate are important when modeling chemically active turbulent flows and turbulent combustion processes.

Due to the great importance of all these problems gravitational turbulent mixing is being studied at the world leading laboratories. Not long ago only global properties of turbulent flow such as mixing region width and mean density shape were measured in most experiments. Data on growth rate of turbulent mixing region under constant and variable acceleration were obtained for different values of density drop. However, most of these experiments were conducted mainly using immiscible fluids, so the urgent task is to study the effects of molecular diffusion in experiments with miscible fluids.

Further progress in gravitational turbulent mixing investigation is closely connected with receiving more detailed and direct information about the features of turbulence itself based on improvement of experimental techniques and development of more precise diagnostic methods. Direct registration of instantaneous distributions of hydrodynamic parameters (density, concentration, and velocity) makes it possible to study local statistical characteristics of turbulence, turbulent transport, turbulence anisotropy, mixture heterogeneity. Purposeful studies in this direction have been started only recently and the lack of experimental data deters development of theoretical approaches and improvement of engineering models used for Rayleigh-Taylor turbulence description.

The research to be conducted under the Project proposed will enable to acquire new experimental information on the role of molecular phenomena in gravitational turbulent mixing, to investigate important structural features of turbulence and finally to verify computational models applied.

RFNC-VNIITF is one of the pioneers in Rayleigh-Taylor turbulence study. Experimental, theoretical and computational research in this direction has been performed here since 70-es. Today the Institute possesses a good experimental base for gravitational mixing investigation. Specialized facilities (gas guns EKAP and SOM) intended for acceleration of ampoules with liquids under study to create artificial gravity field are in operation.

Last years turbulent mixing studies at RFNC-VNIITF were fulfilled also under ISTC support (Project #177 “Investigations into Peculiarities of the Turbulent Mixing and Development of the Program Complexes for Its Description”). A large scope of experimental, theoretical and numerical research was performed and new interesting results were received.

Current efforts are focused on modernization of the existing experimental base, development of new diagnostic techniques and experimental data processing tools, further improvement of numerical models and codes.

Experimental research under the Project proposed will be performed by Experimental Division of RFNC-VNIITF at the EKAP and SOM facilities specially developed to model gravitational mixing.

EKAP facility is a vertical gas gun providing constant acceleration of an ampoule with fluids under study up to 10,000 g at the distance of 1,000 mm. It is used X-ray technique to measure turbulent mixing zone width and mean density profile at four moments.

SOM facility is a vertical gas accelerator capable to accelerate an ampoule up to 1,000 g within 600 mm. Transparent ampoules and fluids are used in the experiments. The facility is equipped with an optical 24-channel pulse system to measure turbulent layer width.

EKAP and SOM facilities allow to govern acceleration mode, in particular, to reverse acceleration direction during ampoule travel.

In the course of the Project available experimental techniques will be improved and the new ones will be developed to measure intensity of molecular mixing, distributions of concentration, density and velocity in turbulent region:

· At the EKAP facility a “tomography” technique will be developed based on simultaneous registration of X-ray images of the turbulent region from four directions with subsequent numerical reconstruction of 3D density distribution. An “electrocontact” method will be created to measure mass concentrations of mixture components by conductivity registration at the fixed space points.

· The SOM facility will be equipped with an optical “laser sheet” technique to obtain 2D density distribution within a fixed plane based on measurements of visualizing material concentration. A “chemical reaction” technique will be developed to measure fractions of substances mixed to molecular level by quantifying products of chemical reaction between reagents added to fluids beforehand.

· At both facilities it will be developed techniques to measure large-scale mass velocity distribution by “particles-markers” paths shadow graphs registration.

Comparative experimental research of Rayleigh-Taylor turbulence using miscible and immiscible fluids will be conducted.

Integral properties of the process will be studied using the available diagnostics:

· Growth rate of mixing zone and mean density profile will be measured in a self-similar mode of constant acceleration at different density ratios of fluids.

· Turbulence decay after change of acceleration direction will be studied; width and density measurements of the molecular mixing zone remained after separation of heterogeneous component of mixture will give an estimate of molecular diffusion integral rate.

Molecular component of mixture and other features of turbulence will be studied using new techniques to be developed for measuring:


· fluid concentration (“electrocontact” and “chemical reaction” techniques);
· instant mass density field (“tomography” and “laser sheet” techniques) and
· instant velocity field (“particles-markers” techniques).

Using special software for mathematical analysis and processing of experimental information there will be obtained data on intensity of molecular diffusion, local parameters of turbulence such as r.m.s. fluctuations of concentration, density and velocity, kinetic turbulent energy. Two-point correlation functions of density and velocity which characterize heterogeneous structure and anisotropy of Rayleigh-Taylor turbulence will be calculated.

Theoretical and computational research will be fulfilled by the scientists of Theoretical and Mathematical Divisions of RFNC-VNIITF. Activities under the Project will result in improvement of the existing theoretical models and computer codes and development of the new ones. Numerical modeling of Rayleigh-Taylor turbulence will be performed to interpret the experimental results.

Direct numerical simulation (DNS) by hydrodynamic code MAH will be used for detailed 2D and 3D calculations of interface perturbation growth and turbulent mixing evolution.

Large eddy simulation (LES) technique including subgrid viscosity to treat the effect of unresolvable vortexes will be developed basing on 3D MAH code (this technique requires less number of grid-points making calculations cheaper).

DNS and LES will make it possible to study turbulence characteristics that cannot be measured in the experiments such as cross-correlation of pressure and velocity fluctuations.

Semi-empirical turbulence models used in 1D and 2D codes will be improved and extended basing on new experimental data and 3D numerical results.

So, complex and complementary investigations based on laboratory and numerical experiments, theoretical and computational interpretation of new experimental data, comparative analysis of results for miscible and immiscible fluids will give rich scientific information on the basic turbulence features, and enable verification of theoretical assumptions and closure hypothesis used in engineering turbulence models.

The research proposed logically continues the activities performed at RFNC-VNIITF under ISTC Project #177 “Investigations into Peculiarities of the Turbulent Mixing and Development of the Program Complexes for Its Description”. The specialization established, scientific expertise and experience in joint activities of experimental, theoretical and mathematical groups provide a good basis for successful solution of scientific problems in the realm of turbulence.

In the course of the Project the scientists previously involved in nuclear weapons development would contribute their efforts to peaceful research. At moderate total cost of the Project its successful implementation would provide new data on the basic features of turbulence useful in many spheres of human activities.

Foreign collaborators are expected to take part in joint theoretical and numerical activities related to set-up of the experiments and interpretation of the results. Arrangement of discussions and exchange of scientific information with foreign scientists would facilitate deeper understanding of issues under study.


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