Fluid_ENG

8/13/2019 Fluid_ENG

1/13

Determining the drag force with CFD methodANSYS Workbench 11.00

Ott Pabut

Tallinn

2010

8/13/2019 Fluid_ENG

2/13

2

Task 1

Determine analytically and with CFD method the moment which is generated on the

foundation of the water tower. The mass of the tower can be ignored.

Fig 1 The water tower

Data

D1 = 15 m

D2 = 5 m

b = 20 m

U = 30 m/s

Csphere = 0,42

Ccylinder= 1,15

air = 1,2401 kg/m3

1.1Analytical solutionThe drag force for each element can be determined separately withReyleighequation.

, 1

where FD drag force generated by the body, density of the fluid,

U speed of the body or the fluid,

Cd experimentally determined drag force coefficient,

A surface area of the body perpendicular to the flow.

For the sphere:

12

4 12 1,240130

0,42 3,1415

4 41397,17 N 41,4 kN

For the cylinder:

12 12 1,240130 1,15 20 5 64175,18 N 64,2 kN

D1

D2 bb/2

b+D1/2

U

M

8/13/2019 Fluid_ENG

3/13

3

Force applied to the water tower

41,4 64,2 105,6 kNIn order to find the moment at the foundation the forces must be applied to their centers of

area:

2 2 41,4 20

152 64,2

202 1780,5 kNm

Answer: Foundation must resist bending moment M=1780,5 kNm.

1.2CFD solutionIn this case the model for Ansys Workbench would have been with dimensions 45x15x15 m.

With these figures the flow analysis would have taken up much time and computer resource.

To decrease the calculation time element count could have been made smaller, but that would

have also taken down accuracy of the results.

For a better solution the hydrodynamic similarity laws are used. This enables to conduct

experiments on smaller models but ensures that the results are transferable to reality.

As we are dealing with incompressible flow,Reynlods similarity criteria is used. Two systems

are similar, when theirReynolds numbers are equal.

, 2

where D system dimension,

dynamical viscosity of the fluid,

kinematical viscosity of the fluid.

From the equation 2 it can be seen that in order to reduce the dimensions of the system, speed

of the fluid U must be increased or kinematic viscosity must be decreased. In our task

dimensions must be reduced at least 10 times, which means that the speed of the flow wouldgo up to 300 m/s. This is unfavorable since it is close to the speed of sound 343 m/s. Systems

with sonic flows are not stable and our results might be inadequate. To solve the problem we

will change the viscosity by choosing a different fluid water instead of air.

Table 1 Physical parameters of air and water

air water

density 1,2041 kg/m3 998,19 kg/m3

dynamical viscosity 1,8410-5Pa/s 1,0110-3Pa/s

kinematical viscosity 1,5310-5m2/s 1,0110-6m2/s

8/13/2019 Fluid_ENG

4/13

4

Proportional coefficient:

1,53 10

1,0110 15,15

Therefore the dimensions of the new model must be 15,15 smaller.

Table 2 New dimensions of the system

D1 = 0,990 m

D2 = 0,330 m

b = 1,320 m

U = 30 m/s

1.2.1 Creating a new projectOpen program ANSYS Workbench

1. Click Empty project2. Inside the wizard choose File, Save As...3. Save the project as Veetorn4. From the left menu choose Advanced CFD, New Simulation

Workbench offers possibilities for different flow analysis General, Turbomachinery, QuickSetup, Library Template

5. Choose General as the simulation typeWorkbench opens the preprocessing unit CFX-pre

6. Inside the wizard choose File, Save Simulation As...Save the project as Veetorn.cfx

1.2.2 Importing the mesh1. Choose File, Import mesh... and upload the file Veetornmesh.cfx2. Save the simulation

8/13/2019 Fluid_ENG

5/13

5

Fig 1 Imported mesh

1.2.3 Creating the computational domain

It is assumed that the flow inside the computational domain is turbulent and isothermal.

These are the most similar conditions to those in the real environment. For these simulations

the Shear Stress Transport (SST) turbulence model with automatic wall condition analysis is

used. This enables for very precise separation of flow particles when there are at least 10

mesh nods in the boundary layer. Currently the mesh is more robust to save calculation time.

1. From the upper taskbar click Create a Domain and name it Veetorn.2. Apply the following settings

Tab Settings Value

General

options

Basic Settings > Fluids List Water

Domain Models > Pressure > Reference Pressure 1 [atm]

Fluid Models Heat Transfer > Option IsothermalHeat Transfer > Fluid Temperature 288 [K]

Turbulence > Option Shear Stress Transport

In order to get realistic results the boundary conditions must be similar to those in the reality.

For that reason atmospheric pressure and possible temperatures of the fluid are determined.

3. Click OK

8/13/2019 Fluid_ENG

6/13

6

1.2.4 Determining the boundary conditionsThe imported mesh contains predefined 2D regions, which make it easier to apply boundary

conditions. For the simulation following conditions are needed: inlet, outlet and walls (no slipand slip condition).

Inlet

1. From the upper taskbar click Create aBoundary Condition2. Name itInlet3. Apply following settings

Tab Settings Value

Basic Settings Boundary Type Inlet

Location Inlet

Boundary

Details

Flow Regime > Option Subsonic

Mass and Momentum > Option Normal Speed

Mass and Momentum > Normal speed 30 [m s ^-1]

Turbulence > Option Low (Intensity = 1%)

Turbulence intensity is similar to the average wind tunnel where it is approximately 1-2 %.

4. Click OKOutlet

1. Create a new boundary condition Outlet2. Apply following settings

Tab Settings Value

Basic Settings Boundary Type Outlet

Location Outlet

Boundary

Details

Flow Regime > Option Subsonic

Mass and Momentum > Option Static pressure

Mass and Momentum > Relative Pressure 0 [Pa]

Relative pressure defines the difference between the outlet and inlet pressure, currently the

same pressure applies for both and therefore the relative is 0 Pa.

3. Click OK

8/13/2019 Fluid_ENG

7/13

7

Fig 2 Inlet and Outlet

For sides and the upper plane of the rectangle slip wall condition is suitable. For those walls

the shear stress value is 0 and the flow of the fluid is not interrupted. If wind tunnel is used,

the dimensions of the rectangle should be equal to the tunnel. Computational domain foratmospheric simulations must be big enough to ensure that streamlines exiting the area are

straight.

FreeWalls

1. Create new boundary conditionFreeWalls2. Apply following settings

Tab Settings Value

Basic Settings Boundary Type Wall

Location Free1

Boundary

Details

Wall Influence On Flow > Option Free Slip

3. Click Ok

8/13/2019 Fluid_ENG

8/13

8

Noslip wall

1. Create a new boundary conditionBody2.

Apply following settings

Tab Settings Value

Basic Settings Boundary Type Wall

Location Body

Boundary

Details

Wall Influence On Flow > Option No Slip

3. Click OKProperties for the remaining 2D regions (in this case the lower XZ plane) are determined bydefault. Currently the adiabatic no slip wall condition is suitable. Name of the default

conditions is Default Boundary. Even though Body and Default Boundary conditions are

identical, the Body condition was applied separately to allow easier post processing of the

results.

1.2.5 Determining the initial conditions1. Click Define the Global Initial Conditions2. Apply following settings

Tab Settings Value

Global

Settings

Initial Conditions > Cartesian Velocity

Components > Options

Automatic With Value


Components > U

0 [m s ^-1]


Components > V

0 [m s ^-1]


Components > W

30 [m s ^-1]

Initial Conditions > Turbulence Eddy Dissipation (Selected)

3. Click OK

8/13/2019 Fluid_ENG

9/13

9

1.2.6 Setting the output dataAs Workbench does not automatically calculate forces and moments, the parameters in

question must be set manually.

1. Click Create Output Files and Monitor Points2. Apply following settings

Tab Settings Value

Results Output Boundary Flows (Selected)

Output Boundary Flows > Boundary Flows All

3. Click OK1.2.7 Calculating the drag forceTo find the drag force on a surface, moduleExpressions must be used. It will apply CFX

expression language ((CEL) to find suitable parameters.

1. Click Create Expression2. Name itFFlow andclick OK3. Under Definitionwrite the following code:

force_z()@Body

4. Click OK

1.2.8 Modifying the solver control1. Click Solver Control2. Apply following settings

Tab Settings ValueBasic settings Convergence Control > Max. Iterations 15

Convergence Criteria > Residual Target 1e-05

3. Vajuta OKIn normal condition the number of iterations must be at least 100, but in order to reduce the

calculation time it has been reduced.

8/13/2019 Fluid_ENG

10/13

10

1.2.9 Getting the solution1. Click Write Solver File2. Name it Veetorn.def3. Click Save

Workbench opens theRun Definition window

4. Click Start RunProgress of the calculations can be observed form the Momentum and Mass charts. If the

results appear to go into the wrong direction, we can stop the calculations and enforce

necessary changes. On the right, info about the progress of the calculation and iterations

is displayed.

Fig 3 Progress of the calculations

When the number of iterations has been reached or results have converged Workbench will

issue a message. To display and process results, questionPost-process results now? must be

answered Yes.To study the simulation progress, the answer should be No.The results

can also be viewed by selecting CFX-Postform the lower taskbar.

8/13/2019 Fluid_ENG

11/13

11

1.3 Visualization and processing of resultsTo display results a base plane with sampling points must be created. This will determine the

starting points of the velocity vectors.

1. From the upper taskbar click Insert, Location, Plane2. Name itBaseplane 13. Apply following settings

Tab Settings Value

Geometry Definition > Method Point and Normal

Definition > Point 0, 2.5, 0

Definition > Normal 1, 0, 0Plane Bounds > Type Rectangular

Plane Bounds > X size 5 [m]

Plane Bounds > Y size 10 [m]

Plane Type Sample

Plane Type > X Samples 50

Plane Type > Y Samples 50

Render Draw Faces (Selected)

Draw Lines (Selected)

4. Click ApplyDisplay the pressure distribution on the base plane

1. Double-click Baseplane2. Apply following settings

Tab Settings Value

Color Mode Variable

Variable Pressure

3. Click ApplyNext the velocity vectors on the base plane are visualized. This helps to determine the

directions of flow particles and helps to display the recirculation zones.

1. Click Insert, Vector2. Name it velocityvectors3. Apply following settings

8/13/2019 Fluid_ENG

12/13

12

Tab Settings Value

Geometry Definition > Locations BaseplaneDefinition > Sampling Vertex

Symbol Symbol Size 0.5

4. Click ApplyNext streamlines are displayed to observe the flow.

1. Click Insert, Streamline2. Name it Streamlines3.

Apply following settings

Tab Settings Value

Geometry Type 3D Streamline

Definition > Start From Inlet

Definition > # of Points 200

4. Click Apply

Fig 4 Streamlines

To find the drag force, clickQuantitativeon the Outlinetoolbar and then click Fflow

Result is force_z()@Body = 211 370 [N], computational time0:8:42

8/13/2019 Fluid_ENG

13/13

13

Drag force determined analytically was Fsum=105,6 kN and with CFD method Force @ Z =

211, 4 kN. Analytical and CFD results differ about 2 times. The reason for big difference can

be found in the size of the generated mesh and in the small number of iterations.

Fluid_ENG

Documents