WHAT VWT NOW DOES

Objective: -

To provide a general overview of the functions and operations of

VWT - a Special Purpose Program for Virtual Wind Tunnel applications.

Contents: -

1. Applications and Capabilities
2. Modelling features
3. Problem-specification features
4. Post-processing features
5. Remote computing via Internet
6. Sources of additional information

2.1 APPLICATION AND CAPABILITIES

• Capabilities
• Applications
• What VWT does not do
• A list of strong features

• 2D and 3D
• steady and transient
• laminar and turbulent
• forced, natural and mixed convection
• conduction, convection and radiation
• conjugate heat transfer

1. System level analysis for electronics enclosures and their contents, eg. an electronic tower rack.
2. Sub-system analysis for sub-units such as sub-rack of a telephone exchange.
3. Printed Circuit Boards analysis for thermal behaviour within PCB and its interaction within the cooling air, for example convective cooling of two populated PCBs.
4. Packaging level analysis of electronic components with internal structure and with attached heat sinks and interaction with cooling air.
5. Component level: performance analysis of the single component models as for thermal model of a Plastic Quad Flat Pack.
6. Environment level analysis of internals and surroundings on thermal performance.
7. Efficient cooling techniques for high power dissipation units, such as counter-current air-water flow through modules.

Click here for HOTBOX-99 Applications Album

• Variables
• Grid system
• Physical-modelling features
• Numerical features

It operates in Cartesian coordinates - X, Y and Z.

Calculations are performed in a rectangular cuboid solution domain.

Solutions requires calculation throughout the domain of the variables:

• fluid(air) velocity components in X, Y and Z directions - U, V and W
• fluid static pressure - P
• temperature of the fluid and, if required, in solid regions - T.

A solution grid is made up of lines in X, Y and Z directions. These define grid cells.

For good representation the grid should fit any internal surfaces. VWT arranges this automatically, if required, for any object shape.

Grid can affect the accuracy of solution - exact requirements depend on the problem.

That is why, apart the matching internal surfaces, grid distribution and spacing are entirely under user control.

• Turbulence models
• Gravitational effects - buoyancy
• Thermal radiation
• Anisotropic conductivity
• Variable property options
• Distributed sources and resistances
• Advanced fan modeling

The LVEL model is the default - this is the model which is especially useful for conjugate heat transfer in conjested spaces.

Two further options available are:

• user-prescribed effective-viscosity model and
• well-known K-Epsilon model.

• Select laminar or turbulent flow
• Then, for turbulent flow:
  • select effective-viscosity or K-EP models,
  • otherwise retain LVEL model,
  • for critical cases test sensitivity of results to model selection.

Buoyancy or gravitational effects leading to natural convection are represented by either:

• the 'Boussinesq model' for constant density:

  Momentum source = density*expansivity*(T - Tambient)*gravacc , or

• the 'density difference' using the Ideal Gas Law:

  Momentum source = (reference density - density)*gravacc ,

User actions:

• activate gravitational effects if required (inactive by default),
• set gravity components in X, Y or Z, positive or negative,
• set, if required, expansion coefficient and
• reference conditions.

In VWT, the transfer of heat by thermal radiation is handled by IMMERSOL model. It requires solution of the energy transport in solids by conduction and that within the space between solids by radiation.

This equation is formulated in terms of radiosity, which is the temperature of the solid phase in solids and the radiation temperature in gaps separating solids.

All related formulations are activated automatically by switching on the IMMERSOL from within Main menu.

User actions:

• activate radiation effects if required (inactive by default),
• set absorption and scattering coefficient of the media,
• set the emissivity of each solid object, as appropriate,
• switch on external radiation links, if required, and
• set the temperature of surroundings.

• isotropic PCB and
• through-plane/in-plane coductivity ratio equals to 100.

• PCB/connector/module combination
• Flow distribution
• Temperature distribution

• select the PCB object,
• set object material and
• set through/in-plane conductivity ratio as required.

Resistances cover any kind of flow resistance (ie. pressure drop) caused by porous items within the flow domain.

The pressure drop is calculated as:

pres.drop = 0.5*Coef*Vel**n,

where Coef is the resistance coefficient and Vel is either the free area velocity - it takes the porosity factor into account, or 'approach' velocity, which is not.

It is applied to set up the planar resistances (eg. perforated plates, thin filters) and/or volume resistances (eg. thick filters, heat sink pin bundles).

User sets:

• resistance coefficient and
• porosities - free area and/or volume ratio.

It does so by automatic calculation of matching point between fan characteristic and system characteristic.

The computed fan mass flow and fan static pressure are reported, after solution, back to the user through tabulated output.

The user defines the fans by setting:

• fan locations and
• input fan data in terms of fan static pressure vs total mass flow.

In addition, an option exists for the system hydraulic response, system characteristic curve, to be calculated in a single simulation run.

VWT also allows the user to define the swirl from the fans and represent their hobs of rectangular and/or circular geometry, if required.

• Solver methodology
• Higher-order convection-diffusion schemes
• 'Cut-cell' solid-fluid boundaries
• Fine-grid embedding

The above quantities are calculated by VWT at each grid cell for each solution iteration - and summations are displayed graphically during solution.

VWT provides user-set 'monitor points' to see how the progress of the solution moves towards the convergence. Positioned with the mouse, they are, in effect, the probes measuring the characteristics of the process and recording time histories, when required.

Click here for full account of their origin and performance.

By default, the selected differencing scheme applies to all solved variables. the default scheme is the Hybrid scheme.

Clicking on 'Set schemes individually' button in [Main menu/Numerics/Differencing schemes] allows different schemes to be set for different variables.

VWT can make use very accurate, though more time consuming, representation of sloping and curved surfaces of the true object geometries on Cartesian grid.

This is the Partial Solid method, PARSOL.

In this method, sometimes known as the 'cut-cell' technique, the areas and volumes of partially-blocked cells are calculated to a high degree of accuracy, and the equation formulation is modified to account for the local non-orthogonality.

The user activates PARSOL by setting Partial Solids Treatment to ON in the [Geometry] panel of the Main Menu.

The PARSOL is recommended when good prediction of turbulent boundary layers near to wall surfaces is required. Otherwise, the default 'stair-case' representation will provide entirely satisfactory results.

Fine-frid regions can be used to increase the mesh density near a surface, and thus further improve the resolution.

The series of fine-grids are automatically generated from a object associated grid constraints and user-positioned fine grid volumes.

The attributes dialog box sets the grid refinement ratios in each of the coordinate directions.

• VR Environment: the main window
• Use of the menu
• Methods of input
• Problem specifications
• Solution

• [File] - to select, manipulate and save data files;
• [Edit] - to edit some of the domain attributes;
• [View] - to control the visibility of control panel and tool bars;
• [Run] - to activate solution, display and on-line information modules;
• [Options] - to control running options, file formats, working directories and fonts;
• [Compile] - to activate the re-compilation of program modules (entry to this is prevented in standard VWT installation);
• [Build] - to activate re-linking and building executables (entry to this is prevented in standard VWT installation);
• [Demos] - to activate view-results-only demonstrations;
• [Tool bars] - to activate directly some of the dialog boxes;

• activation of command icons using mouse, eg [Run]
• selection of options using 'buttons'
• input of data fields (numerical or character) via keyboard - field to be se is selected by mouse
• scrolling and selection from the list

Following input, when appropriate:

• [Apply] confirms settings
• [Reset] resets data

Problem specifications

• Domain-related specifications
• Object-related specifications

The Top Level menu of VR Editor is reached by clicking the Main Menu button on the hand-set. This brings up the Main Menu top panel.

The Main Menu is where all the domain-related problem specification settings are made.

• Geometry: To set domain size, to set the grid and time-steps.
• Models: To select of variables solved, turbulence models etc.
• Properties: To set fluid properties - defaulted to air at atmospheric conditions.
• Initialisation: To set starting conditions.
• Help: Help on that panel.
• Top menu: Go to the top menu.
• Sources: To activate buoyancy, set orientation, global roughness and near-solid global treatment .
• Numerics: To set solution control parameters - under-relaxation and convergence criteria.
• GROUND: Inactive for standard VWT installation.
• Output: To define print-out and field dumping controls.
• Debug: To switch on debug printout from the Earth solver.
• OK: Return to VR environment.

All problem specifications related to external and internal conditions are represented by objects of certain types.

They may reprsent for example:

• blockages,
• inlets/fans,
• outlets/vents or
• heat sources.

Solution

During the solution - selected by [Run/Earth] - iteration progress is displayed on the screen.

• The left-hand graph shows the variation of pressure and velocities at the monitoring point.
• The right-hand graph shows the variation of errors as the solution progresses.
• Horizontal scale is iteration number.
• Vertical scale on the right-hand graph is residual error in the solution for each variable divided by automatic or user set termination residual.
• Vertical scale on the left-hand graph is magnitude of each variable at the monitoring point.

2.4 POST-PROCESSING FEATURES

• geometry,
• grids,
• attributes of boundary conditions,
• contour plots of temperatures,
• scalar quantities (contaminants) isosurfaces,
• velocity vectors and
• streamlines

The provision also exists to produce and dump animated pictures of, say, transient field developments.

2.5 SOURCES OF ADDITIONAL INFORMATION

Further information on the material covered in this overview is contained in VWT documents.

Users are advised to work through the examples of the above documents and the first tutorial exercise of the VWT Tutorial Exercises.

If a difficulty is encountered, the users are advised to:

• first - examine local on-line Help;
• next - consult relevant part of the VWT Reference Guide, where the contents of every menu are explained in detail;
• then - if necessary, consult CHAM or your supplier.

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