Steady Vs Unsteady flow over a cylinder
OBJECTIVE
(A) To Simulate the flow over a cylinder and explain the phenomenon of Karman vortex street.
PART-I
- Simulate the flow with the steady and unsteady case and calculate the Strouhal Number for Re= 100.
PART-II
- Calculation of the coefficient of drag and lift over a cylinder by varying the Reynolds number to 10,100,1000,10000 & 100000.
(B)
(1) How to Animate the result ?
(2) Comparison of Steady and Unsteady Case .
SCOPE:
(A) Basic understanding of Fluent Solver.
(B) Understanding of Von-karman effect.
SOFTWARE USED
- ANSYS SPACE CLAIM- To design the fluid flow problem
- ANSYS MESHING:- Generation of mesh.
- ANSYS FLUENT:- To setup the physics ; solution initialisation and solving the fluid flow problems.
- ANSYS CFD POST:- To document and compare the results
ABSTRACT
Karman vortex street is a repeating pattern of swirling vortices, caused by a process known as vortex shedding , which is responsible for the unsteady separation of flow of a fluid around blunt bodies i.e. the blunt bodies experiences "pressure drag". The pressure starts to increase in the rear half of the cylinder and the fluid experiences "an adverse pressure gradient . inevitably, the flow separates from the surface and creating a highly turbulent region behind the cylinder called the wake.
PRACTICAL IMPLICATION
- Design of tall structures.
- Designing of an Automobile Vehicles.
- By consider it as a standard fluid flow problem we can understand the implications of varioud turbulence models
GEOMETRIC DESCRIPTION:-
I have used the Space Claim to construct the Geometric Model of the problem statement. Cylinder is represented by the Circle in the 2D problem . Diameter of the circle is taken as 2 m. To examine the fluid flow around a cylinder a computational domain has been created as shown in the figure 2.1. Inlet is taken to be 10 times of the of the diamter of the cylinder .Outlet is taken at the distance of 20 times the diameter of cylinder .(To fully examine the problem)
PULLING OF THE SURFACE
MESHING
Parameters in meshing
- Element Size = 0.2 m
- Element Type: - Triangular Mesh.
- Sizing: - Edge Sizing.
- Number of divisions: - 36
- Inflation Layer: - First Layer Thickness.
- First layer Thickness height: - 5e-3
NAME SELECTIONS
EDGE SIZING
INLET NAME SELECTION
OUTLET NAME SELECTION
WALL
SYMMETRY BOUNDARY CONDITION
FLUENT SETUP/ PROCESSING :-
REYNOLDS NUMBER : `Re=ρuL/μ`
where `ρ`is density of the fluid
u is the velocity
L characherstic length (in case of cylinder it is the diameter)
`μ`is the dynamic viscosity.
SETUP :-
MATERIAL SPECIFICATION
INLET VELOCITY : 1 m/s
OUTLET CONDITION: Gauge Pressure =0 atm
SOLVER TYPE : Steady State Solver (Pressure Based)
PHYSICAL MODEL : Laminar
WALL B.C. : No Slip Condition
CASE 1:
FOR REYNOLDS NUMBER 100
STEADY CASE
INTIALISATION
RESIDUALS:
VELOCITY CONTOUR :
PRESSURE CONTOUR :
VERTEX AVERAGE OF THE VELOCITY :
DRAG COEFFICIENT : 2.135
LIFT COEFFICENT : 0.06135
STROUHAL NUMBER : The Strouhal number represents the ratio of inertia forces due to the local acceleration of the flow to the inertial forces due to the convective acceleration
ANIMATION :
CFD POST PROCESSING :
VELOCITY CONTOURS
CASE 2:-
RESIDUAL:
VERTEX MAGINUTE :
FOR RE=1000
RESIDUAL:-
VERTEX MAGINUTE :
VELOCITY CONTOUR :
PRESSURE CONTOUR :
STROUHAL NUMBER :
COEFICCIENT OF LIFT: -
ANIMATION
FOR RE 10:-
Cd = -1.54
RESIDUALS :
VERTEX MAGNITUDE :
PRESSURE CONTOUR :
VELOCITY CONTOUR :
COEFFICIENT OF LIFT:
ANIMATION :
FOR RE -10000
CD :- 9275.34
VERTEX MAGINITUDE:
VLEOCITY CONTOUR:
PRESSURE CONTOUR:
STROUHAL NUMBER :
COEFFICIENT OF LIFT :
ANIMATIONS :
CONCLUSION:-
- AS THE REYNOLDS NUMBER INCREASES DOMINANCE OF INERTIA FORCES INCREASES OVER VISCOUS FORCE INCREASES ,(FORM DRAG DECREASES) WHICH ULTIMATELY LEADS TO DECREASE IN COEFFICEIENT OF DRAG.
- FlOW SEPERATION STARTS AT THE REYNOLD NUMBER NEARLY =100
- NEGATIVE VALUE OF LIFT REPRESENTS THE AMOUNT OF FORCE IN NEGATIVE DIRECTION.