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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: R1 - Mass - kg/s• Momentum Equation: U1,V1,W1 - Force - Newtons• Energy Equation: 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)

• Download workshop q1• Run case

• 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

Analysis of the Net Sources (see Result File)

inlet

U U dA [N]r × ×ò

outlet

U U dA [N]r × ×ò

outlet

U V dA [N]r × ×ò

outlet

U dA [kg/s]r ×ò

intlet

U dA [kg/s]r ×ò

• Net Source of R1 - is the mass flux evaluated at the objects (INLET & OUTLET)

INLET

OUTLET

INA

OUTLETA

Mass Flux Inlet U A [KG/S]

Mass Flux Outlet U A [KG/S]

• 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). • The flow field GLOBALLY satisfies mass. ( )r

SC

V n dA 0r × × =ò

• Net Source of U1 - is the momentum flux evaluated at the objects (INLET & OUTLET)

INLET

OUTLET

IN INA

OUT OUTA

U Momentum Flux Inlet U A U [NEWTONS]

U Momentum Flux Outlet U A U [NEWTONS]

• 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).

• 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.

( )r mec

SC SC SC SC

V V n dA p n dA n dA g dV Fr × × =- ×× + t ×× + r × +ò ò ò ò

Mechanical support

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:

• 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.

( ) ( )r r

SC SC

U V n dA Drag & V V n dA Liftr × × = r × × =ò ò

Mechanical support

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: Aluminun2. Automatically activates

conjugate heat transfer3. 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)

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New->Fixed Heat Flux at the Cylinder net source of TEM1 for (see result file)

Energy Eq. TEM1 (Watts)1. Source INLET: 35011 W2. Source CYL: 97.5 W (100W)3. Source OUTLET: -35108 W4. Net Sum: 0.5W5. 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

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New

-> F

ixed

T

emp

erat

ure

at

th

e C

ylin

der

ne

t so

urce

of T

EM

1 fo

r (s

ee r

esul

t fil

e) Modify the cylinder heat source to

FIXED TEMPERATURE (97.36C) and compare the results.

Fix

ed T

em1

=

97.3

6oC

Fix

ed H

eat

Flu

x =

97.

5 W

Results ----------------------------------------------

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

refQ hA T T

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.

• When this heat transfer model suffices to the thermal simulation it avoids solving the external flow and simplifies the CFD model.

SWALL

100oC30 m/s ( )conv ext plateQ h A T T= × -

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 cylinderStreamlines 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

• Isothermal & Turb (KE)• Fan U = 1 m/s• ZWLAST = 0.6m• XULAST = 1m• Grid Auto

Discuss the application of: Pressure Relief, Momentum Sources

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.

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