External Flow over cylinder for varying Reynolds number
Flow over cylinder
Initial flow parameters
Flow Velocity over cylinder – 1 m/s
Kinematic viscosity (air) – 1.562 * 10 to -5
Dia of cylinder - 0.05 m
Baseline Reynold number
Reynold number = 3201
Increase in Reynold number by 20 % , 40 % and 100 % ( i.e. 1.2 , 1.4 and 2)
Case 1 - Reynold number increase by 20%
Re 1 = 1.2 * 3201 V1 = 1.2 m/s Flow length = 1m
= 3841.2 Flow through time = 0.833 s
End time = 1.67 s
Velocity plot
Observation (velocity plot):
For case 1 reynold number is increased by 20% and thus the reynold number of the flow sums to 3841 and the resultant velocity is 1.2 m/s.
with a reynold flow of 3841 , the flow is transition +turbulent in nature. Areas showing the front of the cylinder ( yellow and green in colour) from flow direction have the least velocities , thus velocity profile in this region has a semi-circular bubble formation. Green area on the leading edge
The fluid being air has very low viscosity and thus the effect of viscosity in this flow study is very less ( Formation of boundary layer is very small.)
Velocity has started to increse 60 degrees from the leading edge and is maximum in the red coloured area , which then decreases a bit ( orange coloured area above and below the cylinder.) flow starts to seperate in this region.
The area after the highest velocity region, shows considerable drop in velocity , till a point with minimum velocity ( Area shown blue in colour). Green , yellow and blue colured region past the cylinder shows region of different velocity for the flow. The areas just behind the cylinder ( In direction of flow ) reaches the minimum point and thenafter the velocity starts to increase again.
Pressure plot
Observation (pressure plot):
Pressure plot for this flow ( with the conditions as mentioned above ) is almost same throughout; with very few areas of changes in pressure around the cylinder.
Pressure is maximum at the leading edge of the cylinder, and has built up in the front area of cylinder ( areas red in colour). 50 to 60 degrees from the leading edge pressure starts to lower and a wake pressure region is created just after the cylinder.
As this is a transient flow, values of pressure and velocity in different regimes have changed over time and thus profile of velocity and pressure has changed from start of the flow to end of this flow. All the changes in pressure and velocity are shown in the animation videos below.
Link to animation for pressure plot
https://drive.google.com/open?id=1RMt5Rj_WVGXSEiNdTVFZhuM2D8mLeQ3M
Link to animation for Velocity plot
https://drive.google.com/open?id=195El1IAilWbRIgEJszT_CHMuuTDx1Kd3
Observation
Case 2 - Reynold number increase by 40%
Re 2 = 1.4 * 3201 V1 = 1.4 m/s Flow length = 1m
= 4481.4 Flow through time = 0.714 s
End time = 1.43 s
Velocity plot
Changes in comparision to first case.
Reynold number of flow has been increased by 40% in comparision to baseline flow. The region of flow , past the cylinder is converging a at a faster rate compared to the first case. Velocity in this region ( flow next to cylinder on right.) seems to reach the free stream velocity at faster rate ( in comparision to first case.)
Grey areas showing the maximum velocity for this case , shows uniformity - which means the peak velocity is uniform thoroughout the grey regions. and this region with maximum velocity has widened up compared to case 1.
Regions with dark red and brown colour have the minimum velocity and this areas can be found right after the flow passes the cylinder.
Pressure plot
Changes in comparision to first case.
Pressure plot for the second case shows clear changes in the wake pressure region. Dark blue regions are more clearly visible in this case in compariosion to the first one. This dark blue regions are the regions with minimum velocity in whole pressure plot.
wake pressure regions in this case is staring at nearly 40 to 50 degees from the leading edge.
Orange and red region - shows the maximum values of pressure . In comparision to the first case; region of maximum pressure has decreased. Pressure of fluid directly impinging on the front face of cylinder has maximum value due to the pressure impressed by the flowing fluid around that region.
As this is a transient flow , all the changes in pressure and velocity can be seen in action in the animations below.
Link to animation for pressure plot
https://drive.google.com/open?id=14aGtYDIKLofWaBJIdiX1faPlUwz2dAK1
Link to animation for Velocity plot
https://drive.google.com/open?id=1ZVwBbRaTf0qrdWYXJuWWNHv_VjNAZvgG
Case 3 - Reynold number increase by 100%
Re 3 = 2 * 3201 V1 = 2 m/s Flow length = 1m
= 6402 Flow through time = .5 s
End time = 1 s
Velocity plot
Free stream velocity has increased considerably in comparision to the baseline flow. whole plot as shown above can be seen with a orange colour , which means the free stream velocity is close to the maximum velocity generated.
maximum velocity regions have started 35 to 40 degrees from the leading edge and the regions showing green and yellow colour at front of cylinders has narrowed up. A much more sharper regions of the minimum velocity regions ( blue coloured regions past the cylinder) means the flow is converging at much faster rate compared to the baseflow.
Pressure plot
Link to animation for pressure plot
https://drive.google.com/open?id=1uwV657dn-RkqpCDM6IaSAwvXFUGU8-Qp
Link to animation for Velocity plot
https://drive.google.com/open?id=1lu0_P7ZrzP_RiCeErCYtu7Oi2ObXFgfA
All the animations attached as above are for a transient flow case and thus shows the variation in flow over time.