MULTLAB
FEM-UNICAMP
UNICAMP
SETTING SOURCES & BOUNDARY CONDITIONS
USING VR
Section 7 of TR326 describes Sources and
B.C. 5n the form of object and its
attribute.
Sources and Boundary Conditions
• Sources may represent some external force, or heat
transfer inside the domain.
• Boundary Conditions are information given at the boundary
of the domain.
•For numerical purposes PHOENICS makes no distinction
between them. It handles both using a single procedure.
 PATCH & COVAL
 OBJECTS
Typical sources and units :
• Mass Equation:
• Momentum Equation:
• Energy Equation:
R1
- Mass - kg/s
U1,V1,W1 - Force - Newtons
TEM1
- Energy - Watts
• Mass sources may occur when there is a phase change (liquid
to vapor for example) or when for multiple chemical species
simulations where one specie is transformed into other.
• Momentum sources are regarded as the sources due (mV),
pressure, skin friction.
• Energy sources are associated when energy (W) is imposed
inside the domain or at its boundaries. Also when temperature
is set at an object and heat is transferred due to temperature
difference.
Source Terms Echoed by Phoenics
• For evaluation purposes phoenics informs the values of
some selected source terms. They are:
• Mass sources are typically associated with Inlets and Outlets
objects. Closed Cavities do not have mass source.
• Momentum sources are associated with flux of mass at the
Inlets and Outlets (mV) and also with the friction force at the
walls (Plate and Thin Plate objects). Blockages inside domain
which may have skin friction and pressure forces but
Phoenics do not reported them.
• Energy sources are associated with Blockages, Inlets and
Plates. It is possible to fix heat flux, to fix a temperature, to
set a solid object adiabatic (just allow heat conduction) or
even set a heat flux due to a heat transfer coefficient external
to the domain.
•The next workshops we will explore the use of echoed source
values.
Most Common Objectcs (VR EDITOR)
These objects usually introduces sources into the flow domain
•Blockage 3D, solid or fluid - Can apply heat and momentum
sources.
•Inlet 2D\ Angled-in 3D - fixed mass source.
•Outlet 2D\ Angled-out 3D - fixed pressure.
•Plate 2D - zero thickness obstacle to flow. May be porous.
•Pressure Relief - single cell fixed pressure point.
•Fan 2D - fixed velocity
•Point_History - single cell transient monitor point.
•Fine Grid Vol 3D - region of fine grid.
•Drag_lift 3D - region over which momentum imbalance
(force) will be calculated.
WORKSHOP#3 – Cylinder in Cross Flow 2D (XY plane)
• Consider a single cylinder in
cross flow on a XY plane.
• Problem data:
• Isothermal & Laminar flow
• Inlet UIN = 0.1 m/s
• YVLAST = 1m
• XULAST = 2m
• Download workshop q1
• Run case
Analysis of the Net Sources (see Result File)
ò r U ×U ×dA
[N]
inlet
ò
r U ×U ×dA [N]
outlet
ò
r U ×V ×dA [N]
outlet
ò r U ×dA
[kg/s]
intlet
ò
outlet
r U ×dA [kg/s]
• Net Source of R1 - is the mass flux evaluated at the objects
(INLET & OUTLET)
Mass Flux Inlet 
 U
IN
A
[KG/S]
A INLET
Mass Flux Outlet 

U OUTLET A [KG/S]
A OUTLET
• It means 1.189E-01 kg/s flow thru the inlet and exit by the
outlet
•The net sum coincide with mass imbalance. The result is
null (it is not a usual result, usually the net sum is a number orders of
magnitude below of the inlet mass flow).
r Vr ×n ×dA = 0
• The flow field GLOBALLY satisfies mass.
ò (
SC
)
• Net Source of U1 - is the momentum flux evaluated at the
objects (INLET & OUTLET)
U Momentum Flux Inlet 
  U
IN
A  U IN
[NEWTONS]
A INLET
U Momentum Flux Outlet 
  U
OUT
A  U OUT [NEWTONS]
A OUTLET
• It means 1.189E-02 N flows thru the inlet and -1.2589E-02 N
exit by the outlet.
• The momentum imbalance is not necessarily equal to the
momentum net sum.
• Usually sources like momentum inside the domain such as
blockages and also the pressure forces are not displayed on
RESULT file (unfortunately).
GLOBAL MOMENTUM IMBALANCES x NET SUM
• Consider the CS. The momentum
imbalance is the net sum of the LHS and
the RHS of the mom. eq. (including pressure,
shear, gravity and mechanical forces acting on the CS).
Mechanical support
ò r V (V ×n)×dA = - ò p ×n ×dA + ò t ×n ×dA + ò r g ×dV + F
r
SC
mec
SC
SC
SC
• Phoenics does not echoes: the net pressure, the net
buoyancy force and the mechanical force. Sometimes the
shear force at the CS is available.
• Nevertheless it is possible to extract valuable information
for some particular cases. The ‘cylinder in cross flow’ is one
of them.
GLOBAL MOMENTUM IMBALANCES x NET SUM
• The cylinder in cross flow has nearly
atmospheric pressure at the C.S., no shear at
the C.S. and no buoyancy force. Therefore the
momentum balance reduces to:
ò r U (V ×n)×dA =
r
SC
Drag
&
ò r V (V ×n )×dA =
r
Mechanical support
Lift
SC
• This case, in particular, the net sum of U1 and V1 will result
on the cylinder Drag and Lift forces!
• Nevertheless to get a precise estimate of D and L forces are
necessary further considerations on the domain size and on
the north and south boundaries.
New! -> Frictionless and Adiabatic Boundaries
• If one does not give any
attribute to a boundary,
like the North and South
walls, internally Phoenics
consider these
boundaries like a
frictionless and adiabatic
wall.
• Usually these boundaries
are known as symmetric
boundaries, i.e., the
normal derivative of the
variable is null.
Consequences of the use of Frictionless and Adiabatic
Boundaries to model the ‘Cylinder in Cross Flow’
• The cylinder to Y size domain
ratio is 1:7 D.
• Y size is too short to simulate
a free flow in a cylinder.
• The flow is influenced by the
symmetric boundaries.
• The Drag force should be
greater than the one observed
for a flow without boundaries.
• It would be necessary at least
a ratio of 1:20D
ADDING HEAT TO THE CYLINDER IN CROSS FLOW
• On the Menu/Models activate the Energy/Static Temperature
equation. The options for Energy Sources are:
•Energy Equation for
Temperature
1. Define solids: Aluminun
2. Automatically activates
conjugate heat transfer
3. Choose Total Heat Flux 100 W
Set a temperature at the surface
Set a heat flux (present case)
No heat flux
Q = hA(T-Tref)
UNICAMP
New->Fixed Heat Flux at the Cylinder
net source of TEM1 for (see result file)
MULTLAB
FEM-UNICAMP
Energy Eq. TEM1 (Watts)
1. Source INLET: 35011 W
2. Source CYL: 97.5 W (100W)
3. Source OUTLET: -35108 W
4. Net Sum: 0.5W
5. Net Sum: 0.5/35011 = 0.001%!
• Global energy balance satisfied!
• Source is 97W instead of 100W .
• Refining grid the reported source
approaches 100W.
• If you need download q1
Modify the cylinder heat source to
FIXED TEMPERATURE (97.36C)
and compare the results.
MULTLAB
FEM-UNICAMP
Fixed Tem1 =
97.36oC
Results ----------------------------------------------
Fixed Heat
Flux = 97.5 W
New-> Fixed Temperature at the Cylinder
net source of TEM1 for (see result file)
UNICAMP
1st case the heat imbalance is the added heat: 97.6 W. Since fixed temperature is
equal to the temp. from the 2nd case the dissipated heat are off only by 1:97Watts
Application of the Net Source of Energy
• The difference among the net sources of energy at the
INLETs and OUTLETs allows one to evaluate the amount of
energy dissipated or generated inside the domain.
• Blockages inside the domain, or Plates at the boundary of
the domain, may be subjected at a heat flux or fixed
temperature.
• Knowing the Heat dissipated by an object at fixed
temperature one can estimate the averaged heat transfer
coefficient
Q  hA T  Tref 
New-> DEFINING BOUNDARY WALLS
• Very often it is desired to specify friction (source of
momentum) and heat transfer (source of energy) at a wall
coincident with the domain surface.
• PLATE is the object to be used, it patches the cell’s
boundary area, therefore it has zero thickness in a direction
normal to the cell areas (likewise inlets or outlets).
• Workshop – add two plates, one at the north and the other
at the south boundaries.
NWALL
SWALL
New-> Plate; NWALL to the cylinder in cross flow
• Define wall roughness
(turbulent flow only)
• Select the friction law to be
used at the wall
New-> Plate; NWALL + LINEAR HEAT SOURCE
• The linear heat source is a
way to easily set up a heat
flow conditioned to an
external flow at given
external temperature and heat
transfer coefficient.
100oC
30 m/s
Qconv = h ×A (Text - Tplate )
SWALL
• When this heat transfer
model suffices to the thermal
simulation it avoids solving
the external flow and
simplifies the CFD model.
New-> Plate; SWALL + FIXED HEAT FLUX
• It is similar to fixing the
heat flux at the cylinder
surface; already done on the
last workshop
NWALL
Plate with fixed heat
flux of 250 W
How the Energy Sources Look Like
• There are two new sources at SWALL and NWALL.
• These sources add, respectively, 250 W and 343 W to the
domain.
• The net sum is 0.01 W, or 0.00003% of the inlet energy!
• Check yourself the RESULT file output
• If you need download q1
FLOW IN CAVITIES
• To simulate flow in cavities, i.e. geometries which do not
have inlets and outlets it is necessary to create a reference
pressure at the domain.
Temperature profile inside
a square cavity with a
circular heated cylinder
Streamlines in a
rectangular cavity
with top lid sliding
Streamlines in a
rectangular cavity with
two heated squares
WORKSHOP#5 - FAN<625>
Fan and Inlet Tutorial
•
•
•
•
•
If SARAH is not activated solution does not
converge for KE -> Numerics, Relaxation,
Auto=OFF, set U1 and W1 to 0.1. Alternatively
one may use KELOW Re or Laminar flow with
AUTO Convergency that it will converge.
Isothermal & Turb (KE)
Fan U = 1 m/s
ZWLAST = 0.6m
XULAST = 1m
Grid Auto
Discuss the
application of:
Pressure Relief,
Momentum
Sources
Additional Settings
• Replace the blockages by plates as suggested in the figure.
Compare your results and comment.
Make a Bigger Domain 6x10 m
•Fans (Momentum Sources) are commonly used in highway
tunnels as exhaust system for cars emissions.
• Place the fan at the middle of the domain and explain the
differences on the streamlines on the intake and discharge
of the fan. Hint: look at the movie link: source x sink
•Intake acts as a sink, the streamlines goes radially inward
•Discharge acts as a jet, streamlines are almost parallel.
END OF THE SOURCES WORKSHOP