Flow over a cylinder

ABSTRACT

In this Project, CFD analysis of a cylinder in which external flow takes place and running a baseline simulation and then calculating the Reynolds number and also the simulation end time. Then increase the Reynolds number by a factor of 20%, 40%, and 100% then run the transient flow simulation. Animations for pressure-velocity for all the three Reynolds number created.

1.INTRODUCTION

Computational Fluid Dynamics

CFD has fascinated researchers, as it is a fast, accurate and reliable method of analyzing the variation of hydrodynamic properties of the flow over and within a body. Computational Fluid Dynamics (CFD) is a branch of fluid mechanics that uses numerical methods and algorithms to solve complex problems involving fluid flow.

Reynold's Number

Reynolds number can reveal whether the flow is laminar or turbulent. The transition from laminar to turbulent flow depends on the surface roughness, flow velocity, geometry, surface temperature, and type of fluid, among others.

The present study aims to analyze the fluid flow over a cylinder and calculate Reynold's number for baseline simulation and increase the calculated Reynolds number to a factor of 20%, 40%,100% and perform the simulation for all the three values of Reynold's number and create an animation file for velocity-pressure.

2.GEOMETRY OF THE MODEL

In this project, a cylinder is created in SOLIDWORKS CAD software. The design parameters are as follows

Diameter of the cylinder = 0.06 m

Length of the cylinder = 0.1 m

3.BASELINE SIMULATION PARAMETERS

Initially, I considered the baseline velocity of air to be 10m/s. On the basis of initial velocity, Reynolds number is calculated.

Re = rho*U_inlet*D/mu

D = cylinder diameter

mu = dynamic viscosity

rho = density

by Substituting the above values we get, the baseline case Reynolds number,

Re = 40518.1

Similarly, Reynold's number at 20, 40 and 100 % is calculated,

At 20% Re = 48621.83

At 40% Re = 56725.46

At 100% Re = 81036.38

Conducting the Baseline simulation

4. CALCULATiNG SMULATION END TIME

Velocity = Distance / time

Time = Distance / velocity

i.e. Time = 0.33/10 = 0.033 sec

The length of the domain along the X axis is 0.33 m

When the Velocity is 10 m/s for Re = 40518.19 then Simulation end time = 0.066 sec

Given that the end time should be 2 times the flow through time

At Baseline For V = 10 m/s , Re = 40518.19 , Simulation end time = 0.066sec

At 20% For V = 12 m/s , Re = 48621.83 , Simulation end time = 0.055sec

At 40% For V = 14 m/s , Re = 56725.46 , Simulation end time = 0.0471sec

At 100% For V = 20 m/s , Re = 81036.38 , Simulation end time = 0.033 sec

From this we can see that the Velocity increases the time decreases.

MATLAB PROGRAM TO REYNOLDS NUMBER FOR DIFFERENT INPUT VELOCITY AND ALSO TO FIND SIMULATION END TIME

% TO FIND REYNOLDS NUMBER FOR DIFFERENT INPUT VELOCITY AND ALSO TO FIND THE SIMULATION END TIME
clear all
close all
clc

%BASE-LINE SIMULATION
RHO = 1.225;          %density of air in (kg/m3)
u_input = 10;         %input velocity in (m/s)
d = 0.06;             %diameter of pipe in (m)
mu = 18.14*10^(-6);   %dynamic viscosity in (N-s/m2)
%reynolds number formula

re = (RHO*u_input*d)/(mu); 

%now at 20% of re
re_20 = re*0.2+re;
%now at 40% of re
re_40 = re*0.4+re;
%now at 100% of re
re_100 = re*1+re;

%NOW TO FIND SIMULATION END TIME
x_base = 0.33; %The length of the domain along the X axis is 0.33 m
v_base = 10;   %BASE-LINE velocity
v1 = 12;       %velocity at 20%     
v2 = 14;       %velocity at 40%
v3 = 20;       %velocity at 100%

%simulation end time
%note - Given that the end time should be 2 times the flow through time 
%hence multiplied by 2
time_10 = (x_base/v_base)*2 ; % simulation end time at BASE-LINE velocity
time_12 = (x_base/v1)*2;      % simulation end time at velocity at 20%
time_14 = (x_base/v2)*2;      % simulation end time at velocity at 40%
time_20 = (x_base/v3)*2;      % simulation end time at velocity at 100%

output:

5. RESULTS

1. Flow over a cylinder at Baseline Velocity10m/s

• Cut Plot of Velocity contour

ANIMATION VIDEO :

CUT PLOT FOR PRESSURE:

ANIMATION VIDEO:

Now We consider the all three Reynold's number and the flow which calculated from the baseline simulation.

Case 1: Flow over a cylinder at 20% of Reynolds Number i.e. @12m/s

Cut Plot of Velocity contour

CUT PLOT FOR PRESSURE

ANIMATION VIDEO:

Case 2: Flow over a cylinder at 40% of Reynolds Number i.e. @14m/s

Cut Plot of Velocity contour

CUT PLOT FOR PRESSURE:

ANIMATION VIDEO:

Case 3: Flow over a cylinder at 100% of Reynolds Number i.e. @20m/s

CUT PLOT FOR VELOCITY:

ANIMATION VIDEO:

CUT PLOT FOR PRESSURE:

ANIMATION VIDEO:

6. CONCLUSION(OBSERVATION)

• As Reynold's number increases the Velocity increases and the flow through time decreases.
• As the velocity or Renold's number increase the flow over the cylinder becomes steady and the velocity plot at 20% and 40% exhibit slight unsteady plot whereas at 100% the flow is steady and we can draw a clear understanding of the flow when velocity is increased.
• There is no significant change in the pressure plot and pressure is mainly acting of the frontal area of the cylinder.