assignment for the course cfd with opensource …hani/kurser/os_cfd_2012/assignment1/...assignment...

Solver: icoFoam Solver: interFoam/laminar Solver: simpleFoam Solver: pisoFoam

Assignment for the courseCFD with OpenSource Software

OF21x

Ardalan Javadi

Applied Mechanics/Fluid Dynamics,Chalmers University of Technology,

Gothenburg, Sweden

2012-09-09

Ardalan Javadi Assignment 1 2012-09-09 1 / 15


cavity

Mesh, Velocity field and Streamlines

Mesh andVelocity field

Streamlines coloredby velocity mag.

The flow over a cavity with 1 m/s velocity at the top is laminarlysimulated. The velocity is not too high, so the flow structure inside the

cavity is symmetric.



cavityFine




The same geometry is simulated using finer cells. The flow structure islarge enough to be captured with previous coarse mesh.The results of twomeshes are very similar. The only visible difference would be the two small

vortices nested at the two bottom corners of the cavity.



cavityHighRe




The same geometry is simulated by decreasing the kinematic viscosity by afactor of 10. The flow structure inside of the cavity is not symmetric

anymore.Ardalan Javadi Assignment 1 2012-09-09 4 / 15


cavityGrade




Regarding the basic point which the structures near the walls are smaller,the cells in the near wall regions are got smaller.



cavityClipped




An asymmetric cavity is simulated using the coarse mesh and the sameboundary conditions. The primary vortex is a little pushed up and two

large secondary vortices are nested at the two bottom corners of the cavity.



damBreak

A Two-Phase Flow For 1 Sec.

Mesh and Phases att=0s

Mesh and Phases att=0.35s


A bulk of liquid is freed on left corner of the domain at time zero and dueto the gravity, splashes over the bottom of the domain. The process issimulated for first 1 second. The two-phase flow is captured at three

moments.



damBreakFine

A Two-Phase Flow For 1 Sec. with Fine Mesh


Mesh and Phases att=0.35s


The same geometry is simulated using fine grid. Due to directcamparioson of two grids, the computation of fine grid is conducted to 1

second. The splash process makes the flow structures small, so thedifference between two grids are ostensible.



pitzDaily

Flow over Backward-Facing Step

Mesh

Turbulent Kinetic Energy

Streamlines coloredby Velocity Mag.

Streamwise Velocity

Q-Criterion



pitzDaily

ε, close up at step edge

A flow over backward-facing step is studied which separates at step edge andreattachs about 5H downstream of the step, H is step height. The reattachmentpoint is emphesised by a line. The turbulence is developing with spreading shearlayer that strarts at separation point and extends over recirculation region. The ε

shows the development of shear layer. Q-criterion (see next slide) shows thestructure of the flow. The separation makes turbulence with the length scale ofthe step height. This worth mentioning that there is a large vortex above the

separation point, near the top wall. The pressure fluctuations of the separation atthe step edge might cause this vortex.



pitzDaily

Q-Criterion

Q is a proper orthogonal tensor, For a three-dimensional smooth velocityfield v(x, t), available Galilean-invariant vortex criteria use the velocitygradient decomposition:

∇v = S + Ω

where S = 12

[∇v + (∇v)T

]is the rate-of-strain tensor, and

Ω = 12

[∇v − (∇v)T

]is the vorticity tensor. Historically, the first

three-dimensional vortex criterion using above equation called Q-criterionwhich defines a vortex as a spatial region where:

Q = 12

[|Ω|2 − |S|2

]Ardalan Javadi Assignment 1 2012-09-09 11 / 15


ductFlow

Duct Flow over Roughness

The computationaldomain, h=0.1mm

Coarse Mesh Turbulent Kinetic Energy

Streamwise velocity Pressure Vorticity

A turbulent flow in a duct with rough bottom wall is simulated with k-εturbulece model. The Reynolds number at inflow based on the roughness

hieght is 1000. The flow is water with ν = 10−5.



ductFlowHighRe

Duct Flow over Roughness with higher Reynolds Number

Streamwise velocity

Pressure

Vorticity


The Reynolds number at inflow based on the roughness hieght is 2000.



ductFlowHighReFine

Duct Flow over Roughness with higher Reynolds Number and Finer Grid

Fine Mesh

Streamwise velocity

Pressure

Vorticity


The twice finer grid in each direction is used to assess the grid effectiveness in ourcomputations. The initial condition is mapped from the coarse grid. The Reynolds number at

inflow based on the roughness hieght is 2000.Ardalan Javadi Assignment 1 2012-09-09 14 / 15


ductFlowHighReFine

The flow in the duct is using pisoFoam solver with two meshes and twoReynolds numbers. A new blockMesh file is prepared and the geometry isblocked and meshed using what is learnt in first week. Owing to lenght

scale of the flow (roughness hieght) the results of two meshes are different.The roughness make the flow more turbulent as expected, the structures

around fourth and fifth roughness is very similar.


assignment for the course cfd with opensource …hani/kurser/os_cfd_2012/assignment1/...assignment...

Documents