models.cfd.rotating disk

Solved with COMSOL Multiphysics 4.2a

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Sw i r l F l ow A r ound a Ro t a t i n g D i s k

Introduction

This example models a rotating disk in a tank. The model geometry is shown in Figure 1. Because the geometry is rotationally symmetric, it is possible to model it as a 2D cross section. However, the velocities in the angular direction differ from zero, so the model must include all three velocity components, even though the geometry is in 2D.

Figure 1: The original 3D geometry can be reduced to 2D because the geometry is rotationally symmetric.

Model Definition

D O M A I N E Q U A T I O N S

The flow is described by the Navier-Stokes equations:

(1)ρ t∂

∂uρ u ∇⋅( )u+ ∇ pI– η ∇u ∇u( )T

+( )+[ ]⋅ F+=

∇ u⋅ 0=

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In these equations, u denotes the velocity (m/s), ρ the density (kg/m3), η the dynamic viscosity (Pa·s), and p the pressure (Pa). For a stationary, axisymmetric flow the equations reduce to (Ref. 1):

(2)

Here u is the radial velocity, v the rotational velocity, and w the axial velocity (m/s). In the model you set the volumetric force components Fr, Fϕ , and Fz to zero. The swirling flow is 2D even though the model includes all three velocity components.

B O U N D A R Y C O N D I T I O N S

Figure 2 below shows the boundary conditions.

Symmetry

No slipAxial symmetry

Sliding wall

Figure 2: Boundary conditions.

On the stirrer, use the sliding wall boundary condition to specify the velocities. The velocity components in the plane are zero, and that in the angular direction is equal to the angular velocity, ω, times the radius, r:

(3)

ρ u∂u∂r------ v2

r-----– w

∂u∂z------+⎝ ⎠

⎛ ⎞ ∂p∂r------+ η

1r--- ∂

∂r----- r

∂u∂r------

⎝ ⎠⎛ ⎞ u

r2-----–

∂2u

∂z2---------+ Fr+=

ρ u∂v∂r------ uv

r------- w

∂v∂z------+ +⎝ ⎠

⎛ ⎞ η1r--- ∂

∂r----- r

∂v∂r------

⎝ ⎠⎛ ⎞ v

r2-----–

∂2v

∂z2---------+ Fϕ+=

ρ u∂w∂r------- w

∂w∂z-------+⎝ ⎠

⎛ ⎞ ∂p∂z------+ η

1r--- ∂

∂r----- r

∂w∂r-------

⎝ ⎠⎛ ⎞ ∂2w

∂z2----------+ Fz+=

ww rω=

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At the boundaries representing the cylinder surface a no slip condition applies, stating that all velocity components equal zero:

(4)

At the boundary corresponding to the rotation axis, use the axial symmetry boundary condition allowing flow in the tangential direction of the boundary but not in the normal direction. This is obtained by setting the radial velocity to zero:

(5)

On the top boundary, which is a free surface, use the Symmetry condition to allow for flow in the axial and rotational directions only. The boundary condition is mathematically similar to the axial symmetry condition.

PO I N T S E T T I N G S

Because there is no outflow boundary in this model, where the pressure would be specified, you need to lock the pressure to some reference pressure in a point. In this model, set the pressure to zero in the top right corner.

Results

The parametric solver provides the solution for four different angular velocities. Figure 3 shows the results for the smallest angular velocity, ω = 0.25π rad/s.

u 0 0 0, ,( )=

u 0=



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Figure 3: Results for angular velocity ω= 0.25π rad/s. The surface plot shows the magnitude of the velocity field and the white lines are streamlines of the velocity field.

The shape of the two recirculation zones, which are visualized with streamlines, changes as the angular velocity increases. Figure 4 shows the streamlines of the velocity field for higher angular velocities.

Figure 4: Results for angular velocities ω = 0.5π, 2π, and 4π rad/s. The surface plot shows the magnitude of the velocity and the white lines are streamlines of the velocity field.

Figure 5 and Figure 6 show isocontours of the rotational velocity component together with surface plots of the velocity magnitude for different angular velocities.



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.

Figure 5: Isocontours for the azimuthal velocity component for angular velocity ω = 0.25π rad/s. The surface plot shows the magnitude of the velocity.

Figure 6: Magnitude of the velocity field (surface) and isocontours for the azimuthal velocity component for angular velocities (left to right) ω = 0.5π, 2π, and 4π rad/s.

Figure 7 shows the turbulent viscosity and flow fields for the angular velocity to ω = 500π rad/s and turbulent flow in the mixer volume.



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Figure 7: Results for angular velocity ω = 500π rad/s. The surface plot shows the turbulent viscosity and the white lines are streamlines of the velocity field.

Reference

1. P.M. Gresho and R.L. Sani, Incompressible Flow and the Finite Element Method, vol. 2, p. 469, John Wiley and Sons Ltd, 1998.

Model Library path: CFD_Module/Single-Phase_Tutorials/rotating_disk

Modeling Instructions

M O D E L W I Z A R D

1 Go to the Model Wizard window.

2 Click the 2D axisymmetric button.

3 Click Next.



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4 In the Add physics tree, select Fluid Flow>Single-Phase Flow>Laminar Flow (spf).

5 Click Add Selected.

6 Click Next.

7 Find the Studies subsection. In the tree, select Preset Studies>Stationary.

8 Click Finish.

G L O B A L D E F I N I T I O N S

Parameters1 In the Model Builder window, right-click Global Definitions and choose Parameters.

2 Go to the Settings window for Parameters.

3 Locate the Parameters section. In the Parameters table, enter the following settings:

G E O M E T R Y 1

Rectangle 11 In the Model Builder window, right-click Model 1>Geometry 1 and choose Rectangle.

2 Go to the Settings window for Rectangle.

3 Locate the Size section. In the Width edit field, type 0.02.

4 In the Height edit field, type 0.04.

5 Click the Build All button.

6 Click the Zoom Extents button on the Graphics toolbar.

Rectangle 21 In the Model Builder window, right-click Geometry 1 and choose Rectangle.




5 Locate the Position section. In the z edit field, type 0.014.


Rectangle 31 In the Model Builder window, right-click Geometry 1 and choose Rectangle.


NAME EXPRESSION DESCRIPTION

omega 0.25*pi[rad/s] Angular velocity



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5 Locate the Position section. In the z edit field, type 0.017.


Difference 11 In the Model Builder window, right-click Geometry 1 and choose Boolean

Operations>Difference.

2 Select the object r1 only.

3 Go to the Settings window for Difference.

4 Locate the Difference section. Under Objects to subtract, click Activate Selection.

5 Select the objects r2 and r3 only.

6 Go to the Settings window for Difference.

7 Locate the Difference section. Clear the Keep interior boundaries check box.


9 Click the Zoom Extents button on the Graphics toolbar.

M A T E R I A L S

Material 11 In the Model Builder window, right-click Model 1>Materials and choose Material.

2 Go to the Settings window for Material.

3 Locate the Material Contents section. In the Material contents table, enter the following settings:

L A M I N A R F L O W

1 In the Model Builder window, click Model 1>Laminar Flow.

2 Go to the Settings window for Laminar Flow.

3 Locate the Physical Model section. From the Compressibility list, choose Incompressible flow.

4 Select the Swirl flow check box.

PROPERTY NAME VALUE

Density rho 1e3

Dynamic viscosity mu 1e-3



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5 In the Model Builder window’s toolbar, click the Show button and select Discretization in the menu.

6 Click to expand the Discretization section.

7 From the Discretization of fluids list, choose P2 + P1.

This setting gives quadratic elements for the velocity field.

Wall 21 Right-click Model 1>Laminar Flow and choose Wall.

2 Select Boundaries 3–5 and 7 only.

3 Go to the Settings window for Wall.

4 Locate the Boundary Condition section. From the Boundary condition list, choose Sliding wall.

5 In the vw edit field, type omega*r.

Symmetry 11 In the Model Builder window, right-click Laminar Flow and choose Symmetry.

2 Select Boundary 6 only.

Pressure Point Constraint 11 In the Model Builder window, right-click Laminar Flow and choose Points>Pressure

Point Constraint.

2 Select Point 8 only.

S T U D Y 1

Step 1: Stationary1 In the Model Builder window, expand the Study 1 node, then click Step 1: Stationary.

2 Go to the Settings window for Stationary.

3 Click to expand the Extension section.

4 Select the Continuation check box.

5 Under Continuation parameter, click Add.

6 Go to the Add dialog box.

7 In the Continuation parameter list, select omega (Angular velocity).

8 Click the OK button.




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10 Locate the Extension section. In the Parameter values edit field, type 0.25*pi 0.5*pi 2*pi 4*pi.

11 In the Model Builder window, right-click Study 1 and choose Compute.

R E S U L T S

To create Figure 3 do the following steps:

Velocity (spf)1 Go to the Settings window for 2D Plot Group.

2 Locate the Data section. From the Parameter value (omega) list, choose 0.785398.

3 Click the Plot button.

4 In the Model Builder window, expand the Velocity (spf) node, then click Surface 1.

5 Go to the Settings window for Surface.

6 Locate the Expression section. From the Unit list, choose mm/s.

7 In the Model Builder window, right-click Velocity (spf) and choose Streamline.

8 Go to the Settings window for Streamline.

9 Locate the Streamline Positioning section. From the Positioning list, choose Uniform

density.

10 In the Separating distance edit field, type 0.02.

11 Locate the Coloring and Style section. From the Color list, choose White.


To produce the series of snapshots of the velocity and streamlines of the velocity field shown in Figure 4, proceed with the following steps:

13 In the Model Builder window, click Surface 1.


15 Locate the Coloring and Style section. Clear the Color legend check box.

16 In the Model Builder window, click Velocity (spf).

17 Go to the Settings window for 2D Plot Group.



20 From the Parameter value (omega) list, choose 6.283185.



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2D Plot Group 4To plot the isocontours for the azimuthal velocity component Figure 5, proceed with the following steps.

1 In the Model Builder window, right-click Results and choose 2D Plot Group.



4 Right-click Results>2D Plot Group 4 and choose Surface.

5 Right-click 2D Plot Group 4 and choose Contour.

6 Go to the Settings window for Contour.

7 Click to expand the Quality section.


9 In the upper-right corner of the Expression section, click Replace Expression.

10 From the menu, choose Laminar Flow>Velocity field>Velocity field, phi component (v).

11 Locate the Levels section. In the Total levels edit field, type 15.

12 Locate the Coloring and Style section. From the Coloring list, choose Uniform.

13 From the Color list, choose White.

14 Locate the Quality section. From the Resolution list, choose Finer.

15 From the Recover list, choose Within domains.

To reproduce Figure 6, do the following steps.

16 In the Model Builder window, click Surface 1.


18 Locate the Coloring and Style section. Clear the Color legend check box.

19 In the Model Builder window, click 2D Plot Group 4.










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L A M I N A R F L O W

Now, modify the model to simulate turbulent swirl flow.

The Pseudo Time Stepping enhances the stability of stationary turbulence simulations.

1 In the Model Builder window’s toolbar, click the Show button and select Advanced

Physics Interface Options in the menu.

2 In the Model Builder window, click Model 1>Laminar Flow.

3 Go to the Settings window for Laminar Flow.

4 Locate the Physical Model section. From the Turbulence model type list, choose RANS.

5 Click to expand the Advanced Settings section.

6 Select the Use pseudo time stepping for stationary equation form check box.

M E S H 1

1 In the Model Builder window, click Model 1>Mesh 1.

2 Go to the Settings window for Mesh.

3 Locate the Mesh Settings section. From the Element size list, choose Fine.

S T U D Y 1

Step 1: Stationary1 In the Model Builder window, click Study 1>Step 1: Stationary.


3 Locate the Extension section. In the Parameter values edit field, type range(pi*100,pi*200,pi*500).

4 In the Model Builder window, right-click Study 1 and choose Compute.

R E S U L T S

Velocity (spf)Next, define a surface plot visualizing the turbulent viscosity and the streamlines of the velocity field (Figure 7).

Velocity (spf) 11 In the Model Builder window, right-click Results>Velocity (spf) and choose Duplicate.



4 In the Model Builder window, expand the Velocity (spf) 1 node, then click Surface 1.



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6 In the upper-right corner of the Expression section, click Replace Expression.

7 From the menu, choose Turbulent Flow, k-ε>Turbulent dynamic viscosity (spf.muT).

8 Locate the Coloring and Style section. Select the Color legend check box.



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models.cfd.rotating disk

Documents

2d plot group

height edit

width edit

model builder

physical model

zoom extents

azimuthal

click model