Transient Flow Simulation Over Cylinder

The flow of a fluid is transient or unsteady if its flow parameters (i.e. velocity and pressure) are dependent not only on the position in the coordinate system used to describe the field of flow, but also on time. A distinction is made between three different types of transient flow phenomena.

Transient flow phenomena

• Stochastically irregular phenomena, e. g. turbulent fluctuations (see Fluid mechanics)
• Run-up or run-down phenomena, e. g. when starting or stopping a centrifugal pump (see Start-up process and Starting torque)
• Periodic phenomena, e. g. pulsations caused by surge pressures in piping, unstable H/Q curves of centrifugal pumps or the influence of the rotating impeller

As a result of velocity changes at a fixed point in space, local accelerations or decelerations create additional mass forces in a transient flow which cause corresponding changes in pressure. These can manifest themselves as marked short-term pressure increases resulting from the sudden closing of a shut-off elementin a long, fluid-filled pipe, or as increased pressure losses caused by pulsating flow.

Periodically transient flow phenomena can, given a sufficiently low frequency of changes, often be treated as quasi-steady phenomena. On an averaged basis, they are at any given point in time subject to the same flow conditions as steady flow.

The flow through a rotating set of vanes (see Impeller) is, strictly speaking, always a transient flow if viewed from a stationary coordinate system (see Absolute velocity). At fixed point in space, both velocity and pressure change periodically as the vanes pass.

However, the flow in an impeller and its immediate vicinity can be considered steady as long as it is described using a system of coordinates which rotates with the impeller (see Relative velocity). The centrifugal and Coriolis forces arising in this relative system must be taken into account.

Equation To Calculate Reynolds Number:

Re=ρ*u*d/μ

where

ρ = Density of Air(1.1834kg/m3)

u = Velocity of the Air (m/s)

d = Diameter of the Cylinder

μ = Dynamic Viscosity (1.81*10⁻⁵kg/ms)

BaseLine Case for Calculating Reynolds Number

Diameter of the Cylinder = 0.02m

Length of the Cylinder = 0.05m

Velocity = 10m/s

Therfore, Re = 1.1834*10*0.02/1.81*10⁻⁵

^{Re = 13076.24}

^{TRANSIENT FLOW SIMULATION FOR RE = 20,40 AND 100}

^{Note: Diameter Of the Cylinder is considered to be = 0.08m}

^Conditions:

• If the inlet velocity is 1m/s and if the length of the domain is 5 m then the flow through time is 5 seconds. That is, it takes 5 seconds for the flow from the inlet to go to the outlet.
• This 5 seconds is called the flow through time
• Make sure that end time is set to 2 times flow through time. According to the previous example, the end time would be 10 seconds.

For Re = 20

Using Reynolds Number Equation

u = 0.0388m/s

For Flow Simulation Animation Refer:

https://www.youtube.com/watch?v=c8Wk0iSKU7U

For Re = 40

Using Reynolds Number Equation

u = 0.076m/s

For Flow Simulation Animation Refer:

https://www.youtube.com/watch?v=wEmDOmEsBJU

For Re = 100

Using Reynolds Number Equation

u = 0.0194m/s

For Flow Simulation Animation Refer:

https://www.youtube.com/watch?v=RcYDwSM26ns

Conclusion

After observing the cutplots it is observed that as reynolds number is increased the inlet velocity is increased and at outlet the velocity is gradually decreasing .