Flow Over Cylinder
Aim: Simulate flow over a cylinder at different Reynolds number.
Objective:
- Run a baseline simulation and then calculate the Reynolds number.
- Increase the Reynolds number by a factor of 20%, 40%, and 100% then run the transient flow simulation.
- Create animations for pressure-velocity for all the three Reynolds number.
- Explain your observations.
Procedure:
In this flow simulation, we're required to alter the value of Reynolds no., which means we've three different inlet velocities. But first we need to run a Baseline simulation where we do not consider the Reynolds no. and depending on the inlet velocity we'll calculate the constant value later.
First we begin with the creation of the geometry i.e. our cylinder, using the 'Extrude boss/base' command. Here the geometrical parameters of the cylinder are:diameter D = 0.05m and length L = 0.1m. To start with the baseline simulation, we consider the inlet velocity in x-dir as 10m/s.
Click on Wizard option, give a proper name to the project, Select the units system and here the flow is over the cylinder hence, the type of analysis is External. After setting up the initial conditions, we have to fulfill a condition where the complete time of simulation must be in relation to the inlet velocity and the length of the Computational domain. Hence, with Vin = 10m/s and Computational domain length decided as 2m our flow through time is 0.02sec and the end time as 2x of flow through time i.e. 0.04sec. For mesh size, we're refining the mesh so as to get more accuracy.
After running the analysis we get the result. As we're interested in the Pressure and Velocity(x) also the animations for both. We create two cut plots one each for Velocity(x) and for Pressure.
Velocity:
Pressure:
For animation, Click on Animation option and insert a new animation. In the animation window click on wizard, select scenerio as we're interested in the flow as well. Now delete the plot that has been entered by the system and right click in the line just below that of the link created by the wizard that will be for Velocity. Now we can run the animation.
Similarily create an animation for Pressure also.
Now for changing the reynolds no. at the inlet, we first calculate it by using the formula
`Re = Rho*V*D/(mu)`
Where, Rho = Density of fluid (Air) = 1.225 Kg/m^3.
mu = Dynamic Viscocity = 1.81x10^-5 Pa.s
V = 10 m/s
D = 0.05 m
Hence, we get Re = 33839.779 , i.e. it is Turbulent flow and we calculate the increased Re and Velocity as:
- 20% increase Re = 33839.779 + (33839.779*0.2) = 40607.73, V = 12 m/s.
- 40% increase Re = 33839.779 + (33839.779*0.4) = 47375.69, V = 14 m/s.
- 100% increase Re = 33839.779 + (33839.779*1) = 67679.558, V = 20 m/s.
After determining the changed inlet velocities, we run the new simulation as new project and we get the following results:
V = 12 m/s:
Velocity:
Pressure:
V = 14m/s:
Velocity:
Pressure:
V = 20 m/s:
Velocity:
Pressure:
Conclusion:
- After observing the cut plots for Pressure and Velocity for different Reynolds no., it may be concluded that the Pressure plot remains unaffected though the magnitude of the is increasing at the outlet for different Re.
- The Velocity plot show a considerable change as in baseline simulation there's a low velocity region at the surface where the Air strike and just around that there's a high velocity region. As the Air leaves the outlet its produces the tail, where the velocity is gradually increasing. This may be made clear from the animation.
- With increase in the velocity, the region of velocity at inlet remains almost similar to that of Baseline condition. But we can observe that the air leaving the geometry is increased and so the plot is slightly changed w.r.t. baseline simulation.
- With V = 20 m/s, we observe a considerable change in the plot as in this case we've doubled the inlet velocity. The inlet portion shows an accumulation of the velocity and at the outlet the velocity is increased greatly.