THE IMMERSED BOUNDARY METHOD

• What is the Immersed Boundary method ?

The Immersed Boundary (IB) belongs to a class of mathematical methods which are used to integrate the Navier-Stokes (NS) system of equations on non-body conformal Cartesian grids. Rather than integrate the NS equations on grids which conform to the body shape (i.e. grid lines start and end on body walls), in the IB approach the body is immersed into a Cartesian grid and its effect on the flow field is enforced into the NS equations through ad hoc correction terms. This results into a dramatic simplification of the entire pre-processing phase (mesh generation) to the point that it is virtually non-existent. In different words, the IB method substitutes the time-consuming, inaccurate and necessarily interactive body conformal grid generation process with automatic and extremely fast algorithms based on geometrical, logical and topological issues.

• Why is the Immersed Boundary method so innovative ?

What makes the method so attractive to CFD analysts is the almost absence of any pre-processing phase. It is a well known fact that pre-processing can be the most time consuming and expensive part of any simulation process, especially when complex geometries are involved. Any method that could effectively eliminate or at least alleviate pre-processing, would have an edge on any other methods.

• Is the IB technique really mature and robust for producing reliable results ?

To be honest, the IB method is not that new. In fact, it was developed by Peskin in the 70s. Because of lack of computing power though, and probably because of the quest for the “perfect” grid generation method that involved the R&D CFD community in the 80s and 90s, the method has been neglected for long time until at the beginning of the years 2000, among which the fundamental works of Verzicco et al.

Since its re-discovery about a decade ago, the IB method has attracted the interest of many academicians the world over and a massive number of paper have been published since. Those works have demonstrated that IB method can effectively be applied to a vast class of fluid problems, ranging from external flows to automotive applications, to internal and biological flows. KARALIT came to the conclusion that time was ripe to bring IB out from the University to the industrial world.

• Why don't all of the CFD vendors offer the IB technique ?

This is probably due to the massive investment that would be needed to the already established CFD companies to basically re-develop their products from scratch. Moreover, some of those companies have invested heavily in pre-processing technologies and want to have a return on their investment.

• Is it true that the IB technique is not suited for turbulent simulations ?

The answer to this question is no. A plenitude of papers shows that the method is indeed very well suited for RANS and LES calculations. But there is a catch. Since the body’s surface cuts the Cartesian grid cells in a totally random way, the severe requirements on the grid spacing orthogonal to the wall have to be met in all directions (and not only in the direction normal to the surface), and that prevents the use of very high aspect ratio cells near the walls, which is a standard practice with body fitted grids. This results in grids with a far greater number of cells compared to body fitted grids, for obtaining a sufficient resolution of the boundary layer. KARALIT CFD offers a solution to these problems (the Immersed Mesh) that maintains the simplicity of the IB method, while ensuring the good wall behaviour typical of BF methods.

• What is the Immersed Mesh method offered by KARALIT CFD softwares ?

KARALIT CFD softwares offer what it is called the Immersed Mesh method, by allowing the users to generate a hyperbolic body fitted grid in the region immediately close to the body surface. This can be done in a very simple way because the construction of the BF grid is not constrained by any outer boundary, and the grid construction can proceed freely outwards. The BF mesh is then immersed into a Cartesian mesh the same way a body surface is immersed using the IB method (in fact it is the outermost grid line of the BF grid that is immersed).

As a matter of fact, the IM method is somewhere in between the CHIMERA method and the use of non-conformal grids. This way both advantages of the BF and IB methods are preserved, with a resulting improved accuracy and a great saving in computational cells and in the process turn around time.

• What are the advantages of the IB technique with respect to the BF technique ?

The main advantage of the IB method over the BF method is obviously the almost absence of the pre-processing stage (meshing). At pre-processing, users are only required to feed the software with the geometry to be immersed and to select a certain number of parameters. What can typically take hours or days can be resolved in a matter of minutes. But there is much more than this. The IB method will in fact be able to show its maximum and real strength when coupled to optimization algorithms and when dealing with bodies that have to be moved around the flow field.

• Does the IB technique require larger meshes compared to the BF approach ?

This is not true as long as the user is interested in inviscid simulation. In general terms, for turbulent simulations, the answer is yes, it does. And the reason is easy to understand if one remembers that IB methods work on Cartesian uniform grids, whereas BF methods can use cells with quite high aspect ratio at solid walls. There are possible solutions though to this drawback. And KARALIT CFD adopted the most convenient : it uses hybrid grids, that is what KARALIT CFD calls Immersed Mesh.

• Can RANS turbulent models be used in a IB framework ?

Yes, RANS models can be used also on a IB framework. It is generally necessary to make use of appropriate wall functions to enforce the right values of turbulent properties at walls.

• Some People say that IB is better suited for rapid design evaluation only, but that it would be advisable to switch to the body fitted approach for high fidelity analysis; what is your position regarding this?

We totally disagree. Cartesian uniform grids provide the most accurate solution for a given numerical scheme, as no errors are introduced into the solution by cells' distortion. Cells’ distortion is on the contrary inevitable when constructing body fitted grids in complex geometries, and introduces a level of degradation in the order of accuracy of the algorithms used, and so in the computed solution. From this point of view, the Immersed Mesh option offered by KARALIT CFD guarantees a highly accurate solution at solid walls while preserving the simplicity and flexibility of the IB method, especially when the proposed Immersed Mesh approach is used.

• Is the IB method suited for modeling moving objects ?

It is not only suited, it would probably be the method of choice. No more complicated (and quite often disappointing) morphing processes, or actual total re-meshing of the computational domain. Processes that always require trade-offs between time required and accuracy sought. The grid stays the same, only the geometrical information needed to the IB method are to be re-computed as the object is moving around in a transparent and effortless way from the user's standpoint.

• Is IB method suited for optimization processes?

There is no better method than IB for optimization problems. The IB method is automated by definition, while the standard grid generation process is interactive, at least partially if not entirely.

• What about near wall accuracy issues with IB method?

The only reason why one wouldn't use IB methods at walls is not related to a loss of accuracy of the method per se. The method is numerically accurate, as it works on Cartesian grids. Problems which arise at solid walls are due to the enormous number of grids cells that would be required to achieve the same wall resolution of an equivalent body fitted grid. It is in fact not unusual to have body fitted grids with aspect ratio of up to 1000 or more at walls. To maintain the same wall distance by using a Cartesian grid, that would translate into a factor of thousand in the number of computational cells. This is the only reason why IB methods are rarely used to solve the wall problem without doing some special treatment. These special treatments could be either local mesh refinements or, as offered by the current version of KARALIT CFD, the use of hybrid grids, namely the Immersed Mesh method.

• Is the IB method equally suited for viscous calculations as the body fitted one?

Yes it is. Special treatments must be used at solid walls to avoid using too many computational cells. IB methods are in fact those which provide the lesser grid numerical dissipation, as they work on Cartesian grids.

• Does KARALIT CFD make use of wall functions at walls whenever IB method is used?

Yes the current KARALIT CFD version implements a wall function for turbulent quantities at wall whenever the pure IB method is used.

• Is the IB method implemented in KARALIT CFD an evolution of the “Chimera” approach?

No it is not. Chimera methods are completely different algorithms.

KARALIT CFD SOFTWARE

• What is the Free Download KARALIT CFD product I could download in June ?

The free version of KARALIT CFD which will is downloadable from our website starting June 2011, is a fully functional 2D CFD software.

It will have initially a number of applications (external aerodynamics, non reactive flows in turbomachineries, a class of internal flows and heat transfer) that will be expanded in next releases, with no limitations whatsoever. The software is intended to make possible for users to enter the revolutionary approach to CFD offered by KARALIT. Academicians will get a great free teaching tool that can be also used for more advanced research projects. Although the restriction to 2D we do encourage people from industry to use the software to appreciate the revolutionary features offered by KARALIT CFD and become familiar with them, ready for the 3D commercial version of KARALIT CFD.

• Is your software optimized to be run on HPC systems ?

KARALIT CFD uses MPI for parallel computations and doesn't have any limitation on the number of CPUs.

• What are the layouts ?

Layouts are sort of parametric templates which help users to setting up their calculations in a straightforward and easy way. KARALIT CFD already provides layouts for external flows, wind tunnel like flows and periodic flows in turbo-machinery, and many more are in the pipeline and will be offered to users in the next weeks to come. Users will only have to adapt those parametric templates to their special needs, by defining a few geometric information and relevant physical quantities at boundaries. The entire process could take a few minutes at most, as opposed to hours or even days needed for a traditional CFD pre-processing stage.

• Which numerical schemes are available, both in space and time?

KARALIT CFD implements three different spatial discretization schemes (first order, 2^nd order symmetric TVD and 2^nd order upwind TVD) and both explicit and implicit time marching schemes. Time accurate computations can be performed when an implicit method is used via a dual time stepping method.

? It is plenty of ongoing analysis that use standard body fitted meshes. Can they be used with KARALIT CFD ?

Yes, in principle KARALIT CFD can be used with BF grids, as well. The current version of the software requires the use of multi-block, structured meshes, whose formats are given in the accompanying documentation. We encourage our customers to let us know about their owns meshing and BC's format so that we could make possible to them to keep running their old cases.

Nevertheless, we are confident that customers will soon abandon the old way to do things to embrace the amazingly easy way offered by KARALIT CFD.

• Do I need to install on my computer additional libraries ?

No, KARALIT CFD doesn't make use of any external library. This ensures the full portability of the software on different platforms/operative systems. At any rate, the software is distributed with an installing program, which automatically sets everything needed to KARALIT CFD to work.

• Which are the Operating Systems KARALIT CFD is ported to ?

KARALIT CFD offers version for the most popular Linux distributions as well as for Windows 7 and MAC OS. Both 32 and 64 bit versions are available.

• What formats can I export my results to?

KARALIT CFD provides results in Tecplot format, as Tecplot appears to be the best option to visualise IB results. Nevertheless, on inputs received by customers, KARALIT CFD will make available more export formats in the near future.

• Tecplot or Para-View or other scientific visualization tools for post-processing KARALIT CFD results can be used?

KARALIT CFD offers a default output already optimised for Tecplot. Nevertheless, as part of KARALIT philosophy, we are going to increase the number of output formats for different visualization tools, according to the requests that we will receive from our customers.

• Do you have an integrated pre-processor?

KARALIT CFD comes with a GUI that helps users in setting up their problem. Because of the terrific simplification offered by KARALIT CFD in solving CFD problems, users do not need complicated pre-processing tools anymore.

• In what programming language was the code written ?

KARALIT CFD is entirely written in F90. The GUI is developed in Python.

• Can I use KARALIT CFD in batch mode ?

Yes. The GUI provided together with the software basically writes the five input files that KARALIT CFD uses, plus a binary file where all inputs given to the GUI are saved. All those files but the binary .sav file of the GUI, can be easily edited and modified by the user. Users should be aware that any modification made via an external editor to the files needed by KARALIT CFD outside the GUI environment, will not be acknowledge by the GUI if that case is open via GUI at a later stage.

• What mesh import formats can KARALIT CFD accept?

The FD version (FD o la prima a pagamento)? of KARALIT CFD makes uses of structured, multi-block meshes for BF calculation. No mesh is needed for IB computations. The first commercial release will …. (aggiungere se sara’ diverso)

• Can I import the mesh we use with other solvers into KARALIT CFD ?

• Can I import unstructured meshes in KARALIT CFD?

• Which type of mesh format can I import into KARALIT CFD?

The FD version of KARALIT CFD only accept structured, multi-block meshes for BF calculation. By following requests coming from our customers, other formats will be made available soon. The first commercial release will …. (aggiungere se sara’ diverso)

• Can I import my geometry into KARALIT CFD directly?

• Which CAD format do you handle?

The FD version of KARALIT CFD, being 2D, needs to import geometrical definitions of curves as geometric inputs. At this stage, curves must be represented as a matrix of x,y coordinates, which is probably the simplest and more general available format. Users are thus required to have as outputs form their CAD programs in x,y text format. The exact content of those files is given in the accompanying documentation.

The first commercial3D version of KARALIT CFD will import geometries in STL format, which is a standard output format available from any CAD system. Other formats will may eventually become available upon specific requests from our customers.

• Can I use KARALIT CFD for combustion/acoustic/multiphase/real gas analysis ?

The first commercial version of KARALIT CFD will focus on external aerodynamics, turbomachinery, internal flows, heat transfer and buoyancy. Combustion, multiphase, real gas and more will be available in next releases of KARALIT CFD. We invite all interested users to fill in the questionnaire that can be found in our login area. Our development timeline will be also based on users’ desires and requests.

• Can I implement my own models/methods in KARALIT CFD ?

Can I customise some models through UDF or UDS (User Defined Functions or Subroutines) ?

Yes, in the commercial versions of KARALIT CFD. The functionality is not available only with free-download version of the software.

• Did you benchmark KARALIT CFD against other CFD codes? How do results compare?

KARALIT CFD is continuously tested in each and every phase of its development against well known and documented test cases for the validation of CFD codes available in the literature.

As soon as we will have new documentation available it will be published on our website. Moreover we encourage our customers who are willing to do so, to share their experiences and benchmark results.

• Does KARALIT CFD offer LES modeling?

Both LES and DDES capabilities are in the pipe line and will be offered in the commercial 3D release of the software within 2012.

• Is it possible to couple a Monte Carlo algorithm in KARALIT CFD ?

Such methods will be offered in later versions of the software to treat stochastic dominated as well as inherently Lagrangian problems, such as sprays for instance.

KARALIT CFD : DISCOVER THE REVOLUTION

• Why is the technology offered by KARALIT CFD so revolutionary ?

Because it is the first time that a commercial CFD software based on IB method is offered as a real CFD tool on the market. For the first time IB leaves the academic world to enter the real one. It is a real revolution in the way CFD problems are solved. No more painful work on mesh generation. In a matter of a few clicks and a little bit of data entry, CFD analysts will finally be able to focus their attention on results, rather than on pre-processing. Moreover, the use of the IB technique makes it possible to radically change the approach to the definition of the CFD problem, as it allows the use of a series of built-in parametric domain layouts, available in our easy-to-use GUI.

Each layout is designed to address a class of CFD problems and more new layouts can be easily developed and added in order to satisfy all of our customers’ needs

• Why should I choose KARALIT as my new CFD provider ?

Because of the revolutionary product that KARALIT is offering and for something even more important. KARALIT is a Company that really puts customers' needs at the core of its developing process. We want to listen to our customer and bring in our software, in due time, what customers want. This customer driven development is real and effective in KARALIT, where people in charge of R&D strategy have been CFD developers and analysts for the most part of their professional life and do understand very well what other fellow analysts dream of.

Our policy is such that whenever a customer ask us for a new development, it enters in the next release of the software and become available to everybody else as well. This is the reason why we seek and encourage a continuous dialogue with our customer base. We want our software to be able to grow through you and be the software you always wanted to have in your PC.

• Why is the pricing structure revolutionary ?

• What do you mean by NO ADD-ONS policy ?

• What do you mean by NO Multiprocessor Limitation ?

We couldn't be true to our policy if our customers had to hire an extra engineer to understand how much would cost them running their CFD analysis. We decided to be indeed very much innovative in this respect, with a transparent and simple pricing scheme.

We have a price for 1, 3 or unlimited users, 1 year lease or perpetual and for Academic or Industrial purposes.

We do not charge for any number of processors, and for add-ons and new releases.

Once one chooses a given licensing type, the cost is certain with no fringe attached. No more hidden costs for this or that model, or for any extra CPU for parallel computations.

• Can an Academic licence be used for industrial analysis?

KARALIT CFD licence scheme gives academic and research centers the right to legally use the software for economic and industrial consultancy purposes, without additional costs.

KARALIT CFD FD : SUPPORT and LEGAL

• Is there any obligation in downloading the FREE download version of the code ?

Download of KARALIT CFD FD comes with no obligation from the downloader and it is absolutely free of charge. It can be openly used for profit, provided that mention to KARALIT will be given.

• How long does the license last ?

The FD version of KARALIT CFD is licensed under a free and perpetual license agreement. Registered downloaders will enjoy access to free eventual future software upgrades, as well as to tutorials and documentation whenever available for download.

• Is any support provided for the FD version?

KARALIT CFD will not officially offer an on-line help service for the free version of the code, though we want to support our first users with sufficient documentation and tutorials that we will continue to make available on our website www.KARALIT.com.

Moreover we do want to listen and appropriately answer to our users’ problems, criticisms and requests, and we have a specialized email to address these kind of issues, This e-mail address is being protected from spambots. You need JavaScript enabled to view it

• Will any free upgrades be available?

Yes, registered users will be entitled to receiving any future update to the free KARALIT CFD software version. You will be notified whenever a new version or new documentation will become available.

• Can simultaneously two or more instances of the software be run ?

Yes, it is possible to run simultaneously as many instance of the code as one want. There is no limitation whatsoever on the use of the free version of KARALIT CFD.

• Is the software covered by the GNU licensing ? Is the source code accessible ?

No, KARALIT CFD (both the FD and the commercial version) is not covered by a GNU type license agreement and source code won’t be available to users.

• Can KARALIT CFD free download version be installed on a server and used over the network ?

Yes, it can. There is no technical issue with that.

• Can KARALIT CFD free download version be used on multiple sites type or organization ?

KARALIT CFD free download version doesn't have any limitation whatsoever. It is fair though that big organizations could register each and any copy of the software that they intend to use, to help KARALIT in its developing activity.

• Why register into your website private area?

Registered users will be able to download the free version of KARALIT CFD and to get access to all the future free upgrades and documentation, tutorials included. Registered users will also be encouraged to fill in our users survey form and to provide us their feedbacks.

• After downloading the free download version of the product, should be needed pay for it after a first trial period ?

No, KARALIT will never ask for any payment for the free version of KARALIT CFD. The software is distributed as it is and with no obligations of sort from the users. We just consider fair and ask our downloaders to acknowledge KARALIT whenever the software is used for publications or for paid consultancy projects.