Flow over a cylinder
REPORT ON FLOW OVER A CYLINDER SIMULATION USING SOLIDWORKS
Created by Shahbaaz khan Deoda
On 18 June 2020
Aim: -Simulation of flow over a cylinder using solidworks.
Objective: -Calculate the baseline case Reynolds number.
- Re = rho*U_inlet*D/mu
- D = cylinder diameter
- mu = dynamic viscosity
- rho = density
- Run a baseline simulation and then calculate the Reynolds number. Then increase the Reynolds number by a factor of 20%, 40%, and 100% then run the transient flow simulation.
- Simulation end time should be calculated properly.
Physics: -The primary reason for the formation of the boundary layer is Coanda effect. It defines the tendency of a fluid jet to stay attached to a convex surface. ... This creates a thin layer of fluid near the surface in which the velocity changes from zero at the surface to the free stream value away from the surface.
Let’s explore the flow of air around the airfoil section of the wing. The air around the airfoil is flowing as shown in figure at a very small angle of attack with free stream velocity-V infinity. The flow of air very close to the surface of the body gets stick to the surface because of the friction between the layers of the air and the surface of the body. And also the adjacent layer of the air flow above the sticky layer of air closer to the surface experience friction and shear forces between the layers of the fluid which decelerates that layer of the flow. As the layers of the air are gradually becoming less tense or less restrained as we move above the surface of the airfoil because the friction between the decelerated layers keep on diminishing and the fluid then reaches to the free-stream flow above the airfoil where we can treat the flow to be frictionless. The region/layers of fluid near the surface of the airfoil are treated as viscous and this viscous flow adjacent to the surface of the body is separated (imaginably) by a boundary and thus known as the boundary layer. Above the boundary layer the flow can be treated as frictionless (inviscid) flow since there is no deceleration/acceleration because of the viscous nature of the flow and thus we can analyse this non-viscous flow in an inviscid region using inviscid theory of the flow.
Cause of Boundary layer formation-
The point of separation of the flow from the surface of the body depends upon the Reynolds number of the flow. The separation of the boundary layer over the surface of the airfoil is because of the deceleration of the flow along the viscous flow path which causes the downstream pressure at a point inside the boundary layer flow higher than the pressure of the upstream flow closer to the leading edge. This difference in pressure inside the boundary layer from upstream to downstream creates a flow inside the boundary layer opposite to the (upstream flow) direction of the free stream flow around the body. This movement of the downstream flow from higher pressure region to the upstream flow at lower pressure region inside the boundary layer creates a motion of the flow opposite to the motion of the free stream flow creating disturbance to the upcoming free stream flow which then propagates this disturbance along the surface of the airfoil and hence opposing of the upstream flow which interferes with the free stream flow creating a wake/turbulence inside the boundary layer that grows along the surface of the airfoil from the point of separation. This opposing of the upstream flow/deceleration of the flow increases the drag over the airfoil at large angle of attack which reduces the momentum of the flow.
It has been analysed experimentally that the pressure distribution for an inviscid flow around the body is same as the pressure distribution for a viscous flow around the body. It is because the pressure at a point acts normal to the surface and thus do not show any variation through the boundary layer perpendicular to the surface and the variation of the velocity from the surface of the body through the boundary layer is parabolic in nature. Since there is variation of velocity perpendicular to the surface of the body there will be shear forces because of velocity gradients which causes shear stresses inside the boundary layer.
Reynolds Number Formula
`(rho*V*d)/mu`
Where, rho = density
V = velocity
D = diameter of cylinder
Mu = dynamic viscosity
SIMULATION: -
Geometry of cylinder: -
Length of cylinder = 0.5 meter
Diameter of cylinder = 0.05 meter
Results: - We have evaluated the cut plot and animation of velocity and pressure successfully.
Simulation parameter: -
for baseline simulation: -1. Considering the baseline velocity of air to be 10m/s.
- On the basis of initial velocity, Reynolds number is calculated.
Parameters;
Velocity = 10 m/s
Density = 1.225 kg/m3
Dynamic Viscosity (18 .96/10^6) Ns/m2
By substituting the above values, Reynold's number (Re) = 25846.5
L domain = 0.72 meter
T flow through time =0.072 s
T simulation end time = 0.144 s
T output step time = 0.008 s
cut plot of pressure
cut plot of velocity
animation of pressure variation
animation of velocity variation
Similarly, Reynold's number at 20, 40 and 100 % will be calculated.
Case 1: 120% of baseline Reynolds number
Parameters;
Velocity = 12 m/s
Density = 1.225 kg/m3
Dynamic Viscosity (18 .96/10^6) Ns/m2
By substituting the above values, Reynold's number (Re) = 30968
L domain = 0.72 meter
T flow through time =0.06 s
T simulation end time = 0.12 s
T output step time = 0.006 s
cut plot of pressure
cut plot of velocity
animation of pressure variation
animation of velocity variation
Case 2: 140% of baseline Reynolds number
Parameters;
Velocity = 14 m/s
Density = 1.225 kg/m3
Dynamic Viscosity (18 .96/10^6) Ns/m2
By substituting the above values, Reynold's number (Re) = 36129
L domain = 0.72 meter
T flow through time =0.051 s
T simulation end time = 0.102 s
T output step time = 0.005 s
cut plot of pressure
cut plot of velocity
animation of pressure variation
animation of velocity variation
Case 3: 200% of baseline Reynolds number
Parameters;
Velocity = 20 m/s
Density = 1.225 kg/m3
Dynamic Viscosity (18 .96/10^6) Ns/m2
By substituting the above values, Reynold's number (Re) = 51613
L domain = 0.72 meter
T flow through time =0.036 s
T simulation end time = 0.072 s
T output step time = 0.004 s
cut plot of pressure
cut plot of velocity
animation of pressure variation
animation of velocity variation