I.Z. Naqavi 1, E. Savory 1 & R.J. Martinuzzi 2 1 Advanced Fluid Mechanics Research Group Department of Mechanical and Materials Engineering The University of Western Ontario 2 Mechanical and Manufacturing Engineering University of Calgary Flow Characterization of Inclined Jet in Cross Flow for Thin Film Cooling via Large Eddy Simulation

Overview:  Jets in Cross Flow  Thin Film Cooling  Background  Current Work  Large Eddy Simulation  Results  Conclusions

Jets in Cross Flow:  A flow configuration representing a variety of industrial and environmental flows.  A jet is introduced from the wall at a certain angle to the main stream.  Used in VTOL, thin film cooling, pollutant dispersion etc.

Thin film cooling (Durbin, 2000) Cold fluid Holes for film cooling on turbine blade. Thin Film Cooling:  Separation of a hot fluid from a wall by a cold fluid, in form of a thin layer ejecting from wall, is called thin film cooling. Hot fluid Cooling film

Background:  Four major structures have been identified i.e. horse shoe vortex, jet shear-layer vortex, counter rotating vortex pair and wake vortices. Horseshoe vortices Jet shear-layer vortices Counter rotating vortex pair Wake vortices Wall

Current Work:  In this work LES is performed for inclined jet in cross flow.  Effort is being made to introduce a cross flow with true turbulence.  Previous LES simulations lack effective turbulence specification at the inlet. In this work a real turbulent field is specified at the inlet.  This will enhance the understanding of the effect of background turbulence on the jet in cross flow.

Large Eddy Simulation:  In LES spatially filtered unsteady Navier Stokes equation are solved numerically.

 A fractional step scheme (Moin, 1982) is used to solve Navier Stokes equations.  A semi implicit time advancement scheme is used where convection terms are discretized explicitly with 3 rd order Runge- Kutta scheme and diffusion terms are discretized implicitly with Crank-Nicolson scheme.  Resulting set of linear system is approximately factorized and solved using Tri-diagonal matrix algorithm.  To solve pressure poisson equation fourier decomposition is applied in span-wise direction and resulting system of equation is solved using cyclic reduction method. Large Eddy Simulation (cont.):

 Re D =3500  Domain size  Grid size  At inlet a true turbulent velocity field is specified for that purpose a separate channel flow code is run and velocities are saved at a plane for some 150 flow through time.

Results

Average Vorticity Field: Average stream-wise vorticity at different y-z planes

Streamlines overlaid on average stream-wise vorticity on a y-z plane at x=5D showing counter rotating vortex pair.

Average wall normal vorticity at the bottom x-z plane Average span-wise vorticity at the central x-y plane

Instantaneous Vorticity Field: Instantneous stream-wise vorticity at different y-z planes

Instantaneous wall normal vorticity at the bottom x-z plane

Instantaneous span-wise vorticity at the central x-y plane

Coherent Structure:  Coherent structures can be represented by iso- surfaces of pressure poisson.

Coherent structures for inclined jet in cross flow (Laminar)

Coherent structures for inclined jet in cross flow (Turbulent) Hairpin structures Stream-wise structure

Conclusions:  Instantaneous flow picture is presenting a very strong interaction of cross flow with jet.  Vortical structures coming from upstream interact with the jet.  Such interactions can have strong influence on heat transfer.

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