Transient Simulation of Flow Over A Cylinder

Introduction:

External flow past around a circular cylinder has been extensively studied by the fluid community because of its practical applications. The flow of fluids over heat exchanger tubes and civil structures are such examples. The flow past a circular cylinder experiences a boundary layer separation and very strong flow oscillations in the region behind the body where recirculation happens. The flow pattern is dependent on the Reynold's number and is affected by factors such as surface roughness, the compressibility of the fluid, etc. In this project, we are going to simulate flow over a cylinder and understand how variations in the Reynold’s number impacts the recirculation that occurs behind the cylinder.

Objective:

To perform a transient flow over a circular cylinder and post-process the results in SolidWorks to understand the effect vortex shedding due to the change in the Reynold's number.

Software used: SolidWorks

Geometry:

A circular cylinder of diameter 50 mm was modeled in SOLIDWORKS and imported into SOLIDWORKS Flow Simulation add-ins.

The imported cylinder in SOLIDWORKS

Calculating Velocity Based on the Reynolds Number:

Formula Used:                         

Where,

Re – Reynolds Number,

ρ = density of the fluid,

V = velocity of the fluid,

μ = viscosity of fluid,

L = length or diameter of the object.

Calculation:

On calculating the Velocity of the Fluid, here are the corresponding values and their units

Constant Parameters	Value	Units
Reynolds Number	37	No unit
Diameter of Cylinder	0.05	m
Dynamic Viscosity of Air	1.729E-5	kg/ms
Kinematic Viscosity of Air	1.338E-5	sq.m/s
Density of Air	1.274	kg/cu.m

Velocity of fluid = 0.01 m/s.

Computational Domain:

A computational domain is a region which refers to the simplified form of the physical domain. This is the region where the physical equations of fluid flow are solved. Here, a 2-D computational domain has been created as the flow over the cylinder is symmetrical about the Z-axis. The size of the computational domain has been shown below with a negligible thickness of 1 mm on the Z-axis.

Meshing:

In SOLIDWORKS, the computational domain must be broken down into individual volumes called cells to solve the governing equation. The mesh can have any shape in any size and is discretized based on the model requirement. Here, a  fine mesh of 151 cells on the X-axis and 80 cells on the Y-axis has been created. The governing equations are solved in each and every cell to provide us with a solution.

Meshed Geometry

Results:

The results obtained from the simulation are as follows

Velocity Profile:

The figure below shows the velocity contour at a Reynolds number of 37. Here, the shape of the streamline is symmetrical around the cylinder.

Flow when Re=37

When we increased the Reynolds number to about 186846, the rear flow widens and the flow behind the cylinder oscillates to produce vortex shedding. In other words, re-circulation occurs behind the cylinder.

Flow when Re = 186846

When the Reynolds number reaches 1307922, the vortices are mixed together and the flow behind the cylinder are irregular and the behavior is termed as turbulent flow.

Flow when Re = 1307922

In the video here, you can see the change in the flow from laminar to turbulent due to the change in Reynolds number:

 

Conclusion:

Thus, we have understood the effect of the Reynolds number in the fluid flow around a circular cylinder by this transient flow simulation. We can employ a similar procedure to study vortex shedding due to changes in Reynold’s number around various geometries.