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A global partner in engineering services and product information

A global partner in engineering services and product information. Advancement in RANS modelling of turbulent dispersion . Federico Ghirelli. Gothenburg region OpenFOAM user group meeting November 13, 2013. SIMPLEST POSSIBLE EXPERIMENT: Point source in steady homogeneous turbulence.

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A global partner in engineering services and product information

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  1. A global partner in engineering services and product information

  2. Advancement in RANS modelling of turbulent dispersion Federico Ghirelli Gothenburg region OpenFOAMusergroupmeeting November 13, 2013

  3. SIMPLEST POSSIBLE EXPERIMENT: Pointsource in steadyhomogeneousturbulence The fluid in position X0 is marked with a tracer at time t0. Severalrealizationsgive the averagetracerconcentrationfield (orpdf of particle position). The dispersion of the tracer is characterisedby the standarddeviationσ(t) of the concentration.

  4. It is widelyaccepted that in this experiment dispersion can be modelled as diffusion with diffusion coefficient: Effect of correlation (oftenneglected) Turbulent diffusion coefficient for the point source in steadyhomogeneousturbulence Dotted and dashedcurves: default RANS models in CFD codes

  5. Time scale of the vortexsheddingbehind a cylinder source

  6. Two-equations approach Point source in homogeneousturbulence or plume in homogeneousturbulence Plume in decayingturbulence

  7. My modelofpremixedflame, validation and comparison Moreau burner: flamestabilized by a pilot flame Volvo test: bluff bodystabilizedflame V-flame: Stabilized by a wire

  8. Multiple pointsources • Relevant becauseusingmanypointsources it is possible to representarbitrarysources • The two-equations approach losesaccuracywhennon-simultaneoussources are placedneareachother • The mostcriticalcase is the ”unsteadyplume in zeromeanvelocity”

  9. The unsteadyplume in zeromeanvelocity: A point source that is zero before a given time t0 and constant afterwards. The flow has zero mean velocity and homogeneous steady turbulence. This case is critical because diffusion occurs simultaneously at several regimes at the same position.

  10. New approach • Using a single diffusion coefficientseemsinadequate for modelling dispersion when dispersion occurs at several regimes simultaneously. • New approach: dispersion is modelled as a discrete number of processes each of which is solved by means of a transport equation. • The model will be referred to as DEMD (discrete • Eulerian model of dispersion)

  11. Results: singlepointsource • The two-equationmodel is the mostaccurate for modelling dispersion from a singlepointsource • The accuracy of the DEMD modelincreases with finerdiscretization

  12. Results: unsteadyplume The DEMD modeldoes not produce the qualitativeerror as the two-equationsmodeldoes.

  13. Results: singlepointsource in decayingturbulence . Compared to experimental results of Warhaft WARHAFT Z. 1984 The interference of thermal fields from line sources in grid turbulence. J. Fluid Mech. 144, 363–87

  14. Discussion • Dispersion is highly relevant for the transport of: • momentum • mass • turbulence • heat • The accuracy of the DEMD modelcannotexceed the accuracy of its input parameters • (i.e u’ and t).

  15. Conclusion • Accounting for the ”effect of correlation” leads to significantimprovements in dispersion modelling. • The DEMD approach predicts dispersion correctly in the case of multiple sources (i.e. genericsources).

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