Flow over a cylinder

Flow-Over the Cylinder

Objective:

• Calculate the Reynolds number for the baseline simulation.
• Run a transient simulation for an increasing Reynolds number by a factor of 20% ,40% and 100% of baseline Reynolds no.
• Create animations for pressure-velocity for all the three Reynolds number.

Introduction:

For viscous fluids the flow pattern is much more complicated and the balance between inertia forces and viscous forces is important. The relative importance is expressed by the Reynolds no Re_D defined as

where D is the tube diameter and ν the kinematic viscosity of the fluid.

As the fluid approaches the front side of the tube, the fluid pressure rises from the freestream value to the stagnation point value. The high pressure forces the fluid to move along the tube surface and boundary layers develop on both sides. The pressure force is counteracted by viscous forces and the fluid cannot follow the tube surface to the rear side but separates from both sides of the tube and form two shear layers. The innermost part of the shear layers are in contact with the tube surface and moves slower than the outermost part. As a result, the shear layers roll up. The flow pattern is dependent on the Reynolds number and in figure-1 a principal description of the various occurring flow phenomena is provided.

Fig -1 Fig-2

Assumptions:

1. Temperature effects are no considered.
2. Cylinder diameter =0.04m
3. Cylinder length =0.05m
4. Surface of cylinder is smooth and roughness is considered zero.
5. Bounding box velocity in x-direction =10 m/s
6. Working Fluid is air

Model with Computational Domain

Solution Procedure:

Procedure:

1. Cylinder Model is initially created with above mentioned dimensions.
2. Set the external flow analysis in the wizard.
3. In the wizard, given total analysis time as the calculated end time for different scenarios.
4. The air is considered as the fluid.
5. Velocity in the X-direction is taken as calculated U-inlet.
6. Set the computational domain in X-direction as 0.25m.
7. Run the project and get the solutions.
8. The cut plot is created to visualize the pressure and velocity flow simulation.
9. Finally , animations are created for both the cut plots of velocity and pressure.

Results and Discussion:

1. ) Baseline Calculation:

Re =rho*U_inlet*D/mu = 27012

Where D =0.04m

mu =dynamic viscosity =1.82*10^(-5) kg/ms

rho =density = 1..225Kg/m^3

U-inlet velocity =10 m/s

Velocity plot and Animation @ 10 m/s

Animation Video Link - https://youtu.be/NsZmfE-idK0

Pressure plot and animation @ 10 m/s

Animation Video Link - https://youtu.be/eb1qTkLRoKM

b) For Re =120% *27012 = 32414

U-inlet = 32414*1.81*10^-5/(1.225*0.04)

U-inlet = 11.99m/s

Flow through time(FTT) =0.25/11.99 = 0.020

End time = FTT*2 = 0.041

Velocity plot and animation @ 12 m/s

Animation Video Link - https://youtu.be/WyzfdBud7EQ

Pressureplot and animation @ 12 m/s

Animation Video Link - https://youtu.be/6tWGWXMs53s

c) For Re =140%*27012 = 37816

U-inlet = 37816*1.81*10^-5/(1.225*0.04)

U-inlet = 13.99m/s

Flow through time(FTT) =0.25/13.99 = 0.017

End time = FTT*2 = 0.035

Velocity plot and animation @ 14 m/s

Animation Video Link - https://youtu.be/e_g0SZaPzkA

Pressure plot and animation @ 14 m/s

Animation Video Link - https://youtu.be/Tq1DcRK_6pU

d) For Re =200%*27012 = 54024

U-inlet = 54024*1.81*10^-5/(1.225*0.04)

U-inlet = 19.95m/s

Flow through time(FTT) =0.25/19.95 = 0.012

End time = FTT*2 = 0.025

Velocity plot and animation @ 20 m/s

Animation Video Link - https://youtu.be/TQeslgsc9o4

Pressure plot and animation @ 20 m/s

Animation Video Link - https://youtu.be/cIMARVpklCE

Conclusion:

1. As the Reynolds no increases the velocity increases and the subsequently the flow through time is decreased.
2. As the velocity increases the flow over the cylinder becomes steady, this can be illustrated by the steady velocity plots at 20 m/s when compared with the unsteady velocity plots at 12 m/s and 14 m/s.
3. The pressure is same at all the cases and it is acting mostly at the frontal area of the cylinder.

References:

1. http://www.mdp.eng.cam.ac.uk/web/library/enginfo/aerothermal_dvd_only/aero/fprops/poten/node37.html