The document discusses various types of boundary conditions that are important to consider when performing computational fluid dynamics (CFD) simulations. It explains that boundary conditions reflect the real physical situation being modeled and are crucial for obtaining accurate results. Some of the key boundary conditions discussed include inlet/outlet conditions, wall conditions like no-slip and temperature, symmetry planes, and turbulence specifications for inlets. Setting boundary conditions correctly based on the actual physical problem is emphasized.

1. Dr Patrick Geoghegan
Book: H. Versteeg and W. Malalasekera An Introduction to
Computational Fluid Dynamics: The Finite Volume Method
FEA/CFD for
Biomedical
Engineering
Week 10: CFD

2. Boundary Conditions

3. • The same set of equations are used whether flow is over a fighter jet or
through a wind tunnel.
• But the flow fields are different. Why?
• Differences occur due to boundary conditions
Physical Boundary Conditions

4. • Important to set boundary conditions that accurately reflect the real
situation to obtain accurate results
Boundary Conditions

5. • Consider the Flow through a pipe
• Inflow boundary –typically provide velocity (or mass flow), temperature,
density
Inflow Boundary Condition

6. • Outflow boundary (1) can provide mass flow or pressure, (2) when flow
at outlet far away from any disturbances in flow domain (e.g. inlet, or
objects) so that the flow does not change across outlet boundary
Outflow Boundary Condition

7. • Most Robust
Inlet-Outlet Boundary Condition Combinations
Static Pressure Boundary has two forms:
(1)Average static pressure: allows pressure variation across outlet
boundary, average value is set
(2)Static pressure: set pressure across entire outlet boundary at a
constant value

8. • Relatively Robust
Inlet-Outlet Boundary Condition Combinations
Total Pressure=P+ρU2/2 (incompressible fluid, P is
the static pressure)

9. • Not robust
Inlet-Outlet Boundary Condition Combinations
More likely to lead to an un-converged solution
We will discuss this later

10. • Inlet Turbulence
– You specify Turbulence Intensity (I) in
ANSYS Fluent
• Velocity at a stationary point in turbulent
flow composed of
(1)Steady mean components: (𝑢𝑢, 𝑣𝑣, 𝑤𝑤)
(2)Time-varying fluctuating components:
(u’, v’, w’)
Inlet Boundary Condition
u′
u
u
u
t
u ′
+
=
)
(
time, t
velocity,
u
𝑢𝑢 𝑡𝑡 = 𝑢𝑢 + 𝑢𝑢𝑢 𝑡𝑡 𝑣𝑣 𝑡𝑡 = 𝑣𝑣 + 𝑣𝑣𝑣 𝑡𝑡 𝑤𝑤 𝑡𝑡 = 𝑤𝑤 + 𝑤𝑤𝑤 𝑡𝑡

11. • In simplified terms, Turbulence Intensity is more or less the fluctuating
velocity divided by the mean velocity
• Very Low Turbulence I=0.1%
• Low Turbulence I=1%
• Medium Turbulence I=5%
• High Turbulence I=10%
Turbulence Intensities normally lie between 1 and 5%
Inlet Boundary Condition
𝐼𝐼 =
𝑟𝑟. 𝑚𝑚. 𝑠𝑠 𝑜𝑜𝑜𝑜 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉
𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉
× 100%
We will discuss this later

12. • Important to set boundary conditions that accurately reflect the real
situation to obtain accurate results
Boundary Conditions

13. • Place outlet boundaries well
away from any zones of
influence
• Rule of Thumb: Place outlet at
downstream distance of 10x
height of last obstacle
WHERE SHOULD YOU PLACE AN OUTLET?
Ansys will create an
artificial wall to
prevent inflow out
an outlet

14. Considerations
(1) No slip -zero relative velocity between surface and gas immediately
at surface: u=v=w=0 (Most Common Boundary condition)
(2) Free slip -velocity of fluid near wall not retarded by wall friction effects
(shear stress τ = 0)
(3) Rotating and moving walls
(4) Wall roughness
Wall Conditions

15. • Under normal conditions, the layer of fluid right next to a solid surface
e.g. a pipe wall is static: it is not moving along with the rest of the flow.
• This is called the no-slip condition
• We will use it as a useful boundary condition.
No-slip condition

16. (1) Temperature of gas layer immediately in contact with wall is equal to wall
temperature T=TW
(2) If temperature of wall not known, can set a heat flux at the wall, based on
Fourier law of heat conduction:
• qw must be known – might have to solve thermal transport equation in solid
unless you can assume wall is adiabatic (i.e. qw =0)
Wall Conditions

17. • Can reduce the amount of computation required by assuming a
symmetry plane
– No flow of mass or heat across the symmetry plane
Symmetry Plane

18. • Warning!! A physical symmetrical geometry does not necessarily mean
the flow is symmetrical
– Coanda Effect: natural tendency of a fluid jet to follow the contour of a
wall when the jet is discharged adjacent to the wall surface
https://www.youtube.com/watch?v=AvLwqRCbGKY