Transcript Document

Flow Characterization of Inclined Jet in Cross
Flow for Thin Film Cooling via Large Eddy
Simulation
I.Z. Naqavi1, E. Savory1 & R.J. Martinuzzi2
1Advanced
Fluid Mechanics Research Group
Department of Mechanical and Materials Engineering
The University of Western Ontario
2Mechanical
and Manufacturing Engineering
University of Calgary
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:
Hot fluid
Cooling film
Cold fluid
Holes for film cooling on turbine
blade.
Thin film cooling (Durbin, 2000)
 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.
Background:
Counter rotating
vortex pair
Jet shear-layer
vortices
Horseshoe vortices
Wake vortices
Wall
 Four major structures have been identified i.e.
horse shoe vortex, jet shear-layer vortex, counter
rotating vortex pair and wake vortices.
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.
U i
 0 mass 
xi
Ui UiU j
p
1  2Ui  ij




2
t
x j
xi Re D x j
x j
momentum 
U i  Filtered velocity Re D  Re ynolds number
p  Filtered pressure  ij  Subgrid scale stress tensor ( sgs)
Large Eddy Simulation (cont.):
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 3rd order RungeKutta 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.):
ReD =3500
 Domain size [5D,12 D]  [0,7.5D]  [3D,3D]
 Grid size 171  71  64
 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 isosurfaces of pressure poisson.
p,ii

ii 

S
2
ij Sij
p,ii  Pr essure Poisson
i  Vorticity
Sij  Velocity Strain Tensor
Coherent structures for inclined jet in cross flow (Laminar)
Hairpin structures
Stream-wise structure
Coherent structures for inclined jet in cross flow (Turbulent)
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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