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Home / Examples / Fluid Analysis [Bernoulli] / Example 3: Flow around Cylinder
Example 3: Flow around Cylinder
General
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The steady-state analysis is applied to the flow around the cylinder.
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The flow velocity distribution, the fluid velocity vectors, the streamlines, and the force on the wall face are solved.
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Unless specified in the list below, the default conditions will be applied.
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The above analysis can also be performed by using a circular sheet body as the model and applying [Automatic Fluid Creation].
The setting for the analysis is described at the end of this explanation. -
Obtain this session's project file. (Right-click and choose 'Save link as')
The project has two analysis models.
・Analysis Model: Create fluid manually
・Analysis Model_Automatic Creation: Create fluid automatically -
Results will vary depending on Femtet version and the PC environment.
Analysis Space
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Item |
Settings |
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Analysis Space |
2D |
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Model Unit |
mm |
Analysis Conditions
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Item |
Tab |
Settings |
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Solver |
Solver Selection |
Fluid analysis [Bernoulli] |
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Analysis Type |
Fluid Analysis |
Steady-state Analysis |
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Laminar Flow/Turbulent Flow |
Fluid Analysis |
Select Laminar flow |
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Flow Type |
Fluid Analysis |
Select External Flow. |
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Layer Mesh Setting for Wall Surface (General Settings) |
Fluid Analysis |
Specify mesh height of 1st layer Height of 1st Layer Mesh: 0.5 [mm] |
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Meshing Setup |
Mesh |
General mesh size: 10 [mm] |
Model
The Air (000_Air) is set to a rectangular sheet body. The boundary conditions of inlet and outlet are set on the left edge and the right edge respectively.
The model is a circle sheet body and the material is iron (007_Fe).
The edges surrounding a circle is a boundary of solid and fluid. Solid wall is automatically set to them.
The outer boundary condition is automatically applied to the top and bottom edges where the boundary condition is not set.
The outer boundary condition will be [Slip Wall] automatically, because [External Flow] specified in the analysis condition comes with one-way forced convection
Body Attributes and Materials Setting [Automatic Fluid Creation]
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Body Number/Type |
Body Attribute Name |
Material Name |
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0/Solid |
Air |
000_Air(*) |
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1/Solid |
007_Fe * |
* Available from the material DB
Boundary Condition
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Boundary Condition Name/Topology |
Tab |
Boundary Condition Type |
Setting |
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Inlet/Edge |
Fluid |
Inlet |
Forced Inflow |
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Outlet/Face |
Fluid |
Outlet |
Natural Outflow |
The outer boundary condition will be [Slip Wall] automatically, because [External Flow] specified in the analysis condition comes with one-way forced convection
If the Reynolds number exceeds 100, the time dependency of the turbulence occurs. It makes calculation difficult in the steady-state analysis.
The Reynolds number calculated from this model form, material property, and flow velocity is about 25.2. The steady-state analysis of the laminar flow can be executed.
Viscosity μ=1.816e-5 [Pa s]
Density ρ=1.144[kg/m3]
Kinematic viscosity v=μ/ρ=1.816e-5/1.144=1.587e-5 [m2/s]
Flow velocity V=0.01 [m/s]
Diameter of cylinder D=0.04 [m]
Reynolds number Re = V*L/ν=0.01*0.04/1.587e-5 = 25.2
Results
The vectors of the flow velocity distribution around the cylinder are shown below.
The vectors are adjusted to the same length in the graphics setup.
It is known that the so-called twin whirlpools occur behind the cylinder when the Reynolds number is around 30.
Two vortexes appear at the back of the cylinder.
The streamlines are shown below.
The pitch of the startpoints is adjusted in the graphics setup.
The streamlines are generated.
The force on the wall face is shown by the table.
[Column] shows the force which the cylinder receives from the fluid.
Drag F:The force in the flow direction (X-direction) is 6.58 x 10 ^ -9 [N].
The coefficient of drag CD can be calculated from the drag F.
Thickness in depth direction t = 1 [mm]
Cross sectional area S = D * t = 0.04 * 0.001 = 4e-5 [m2]
Dynamic pressure Pk = 0.5 * ρ*V^2 = 0.5 * 1.144 * 0.01 * 0.01 = 5.720e-5 [Pa]
Coefficient of Drag CD = F / Pk / S = 6.58e-9 / 5.720e-5 /4e-5 = 2.88
Apply [Automatic Fluid Creation]
For reference, the settings for analysis with [Automatic Fluid Creation] enabled are described below.
Analysis Conditions [Automatic Fluid Creation]
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Item |
Tab |
Settings |
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Solver |
Solver Selection |
Fluid analysis [Bernoulli] |
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Analysis Type |
Fluid Analysis |
Steady-state Analysis |
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Laminar Flow/Turbulent Flow |
Fluid Analysis |
Select Laminar flow |
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Flow Type |
Fluid Analysis |
Select External Flow. |
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Flow Type Automatic Fluid Creation |
Fluid Analysis |
Deselect [Set the mesh size of the fluid domain automatically]. External Flow: Forced convection in the +X direction) External Flow Velocity: 0.01 m/s Scale Factor for Fluid Domain: 5 Scale Factor for Back Domain of Fluid: 10 |
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Layer Mesh Setting |
Fluid Analysis |
Specify mesh height of 1st layer Height of 1st Layer Mesh: 0.5 [mm] |
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Meshing Setup |
Mesh |
General Mesh Size: 2 [mm] |
Model [Automatic Fluid Creation]
The model is a circle sheet body and the material is iron (007_Fe).
Body Attributes and Materials Setting [Automatic Fluid Creation]
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Body Number/Type |
Body Attribute Name |
Material Name |
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1/Solid |
007_Fe * |
Fluid material 000_Air * is set in the [Automatic Flow Creation] setting.
* Available from the material DB
Boundary Conditions [Automatic Fluid Creation]
Set no boundary condition. The external flow direction and velocity are set in the [Automatic Flow Creation] setting.