COMPUTATIONAL FLUID DYNAMICS IN INDUSTRY
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MSc course
A three-day course
Large Eddy Simulation (LES), hybrid LES-RANS, Detached Eddy Simulation (DES) and unsteady RANS.

 
LES is suitable for bluff-body flows or flows at low Reynolds numbers. To extend LES to cover industrial flows at high Reynolds numbers, new approaches (hybrid LES-RANS, DES, URANS, SAS, PANS, PITM) must be used. They are all based on a mix of LES and RANS. The course will give an introduction to LES and these new methods.
 
LES, or any of the new approaches, is the first step when performing accurate CAA (Computational Aero-Acoustics)
 
Lectures will be given in the mornings; in the afternoons there will be workshops using Matlab on PC Workstations.
 
In the workshops, the participants will use Matlab for analyzing SGS models, SAS, PANS, DES and DDES. Matlab scripts will be used to generate isotropic and anistropic (non-isotropic) synthetic turbulent fluctuations for inlet boundary conditions and embedded LES.
 
The number of participants is limited to 16.
 

 
 
BACKGROUND

The development of computers and Computational Fluid Dynamics (CFD) has made the numerical simulation of complex fluid flow, combustion, aero-acoustics and heat transfer problems possible. Turbulent flow in three-dimensional, complex geometries -- unsteady or steady -- can be dealt with.
 
Presently CFD methods can replace, or complement, many experimental methods; we can use a numerical wind tunnel instead of an experimental one.
 
Today, most CFD simulations are carried out with traditional RANS (Reynolds-Averaged Navier-Stokes). In RANS, we split the flow variables into one time-averaged (mean) part and one turbulent part. The latter is modelled with a turbulence model such as k-eps or Reynolds Stress Model.
 
For many flows it is not appropriate to use RANS, since the turbulent part can be very large and of the same order as the mean. Examples are unsteady flow in general, wake flows or flows with large separation. For this type of flows, it is more appropriate to use Large Eddy Simulation (LES). In order to extend LES to high Reynolds number flows new methods have recently been developed. These are called DES (Detached Eddy Simulation), URANS (Unsteady RANS), PITM (Partially Integrated Transport Model), PANS (Partially Averaged Navier-Stokes) or Hybrid LES-RANS. They are all unsteady methods and they are a mixture of LES and RANS.
 
In aero-acoustics the noise is generated by turbulence. The best way to accurately predict large-scale turbulence is to carry out an unsteady simulation of the flow field (i.e. LES, DES, hybrid LES-RANS or URANS). After that the noise is predicted separately in CAA (Computational Aero-Acoustics) in which the large-scale turbulence is used in analogy methods based on Lighthill, Kirchhoff or Ffowcs Williams.
 
In LES, DES, URANS and Hybrid LES-RANS the large-scale part of the turbulence is solved for by the discretized equations whereas the small-scale turbulence is modelled. The definition of ''large-scale'' varies in the different methods. Furthermore, the limit between ''large-scale'' and ''small-scale' is often not well defined. Since turbulence is three-dimensional and unsteady, it means that in all the methods the simulations must always be carried out as 3D, unsteady simulations.

 


 

THE COURSE
The course will give an introduction to LES, DES, hybrid LES-RANS and unsteady RANS. During the lectures we will discuss the theory and during the workshops we will use simple Matlab programs to gain detailed insight in various numerical and modelling aspects. There will be a PC workstation for each participant at the workshops. The number of participant is limited to 16
 
We will address questions like:
  • how should I make my mesh?
  • why should I in LES use a dissipative discretization scheme?
  • is it necessary to used central differencing in DES and URANS?
  • what is the difference between LES and unsteady RANS?
  • what turbulence models can I use in DES and unsteady RANS?
  • what fk values are suitable in PANS?
  • to enhance numerical stability, can a turbulence model with high dissipation be used?
  • how do I prescribe inlet boundary conditions?
  • inlet boundary conditions: can I use steady conditions? which is best, synthesized turbulence or a pre-cursor DNS? Download Matlab files for generating synhetic inlet fluctuations
In the workshop, we will learn how to interpretate results from an unsteady simulation. We will evaluate and compare the two types of turbulent stresses, i.e. the resolved stresses and the modelled stresses (more detail at the bottom). When doing LES-URANS/DES, you have to ask yourself similar questions as when doing measurements:
  • when is the flow fully developed so that I can start time-averaging?
  • for how long time do I need to time-average?
  • is it enough if I get accurate mean flow or do I also need accurate resolved turbulent stresses?
  • how do I estimate the quality of my LES or hybrid LES-RANS? Spectra? 2-point correlations? SGS dissipation? For more info, see QLES 2009 and references
     
The most important drawback/bottleneck of LES is the requirement to use very fine grid near walls. The grid must be fine in all directions, not only the wall-normal direction. Much of the research on LES is today focused in getting around this bottleneck. One approach is hybrid LES-RANS. In this method RANS is used near walls and LES is used in the remaining part of the domain. In the afternoon of Day 2 some hybrid LES-RANS methods (including the SAS model) will be presented and discussed.
 
Inlet boundary conditions are much more difficult in LES and hybrid LES-RANS than in RANS. In RANS it is sufficient to prescribe time-averaged profiles. In LES and hybrid LES-RANS unsteady, turbulent fluctuations must be supplied. One alternative is to do a pre-cursor DNS and store data at a cross-sectional plane on disc which can be read in the subsequent LES or hybrid LES-RANS simulation. Another alternative is to use synthesized turbulence. The participants will during the last workshop (Day 3) have the opportunity to learn how to to create synthesized turbulence using Matlab.
 
Synthetic turbulent fluctuations are also important in Embedded LES in which an LES/DES region is embedded in a steady or unsteady RANS region. Usually the upstream region is a RANS region and the downstream region is LES or DES. Turbulent fluctuations must be added at the interface between RANS and LES to ensure a rapid transition from steady RANS where all turbulence is modelled to LES/DES where most of the turbulence is resolved.


 

OBJECT

The participants will be given an introduction to LES, DES, hybrid LES-RANS and unsteady RANS. We expect many participants to be first-year PhD students or users of in-house CFD codes or commercial CFD packages for traditional RANS simulations. This course will give the required knowledge to do CFD predictions using also unsteady methods.
 


 

PARTICIPANTS

We believe that the course will be useful for engineers and PhD students working with problems including pure fluid flow, aero-acoustics, combustion and heat transfer in industry as well as at universities.
The number of participants is limited to 16.

 

 

LECTURER

The lecturer at the course (both during lectures and workshops) will be Prof. Lars Davidson from the Division of Fluid Dynamics, Dept of Applied Mechanics, Chalmers University of Technology.
 

 


 

COURSE MATERIAL


 

COURSE LANGUAGE

The course material is in English and the lectures will be given in English.
 


 

LOCATION

The course will be held 29-31 October, 2014, at Chalmers University of Technology, Göteborg.
 

 

 
REGISTRATION

Registration form should be submitted no later than October 10, 2014.


 
The price is 14,700 SEK (excl. VAT) which includes course material, lunches, course dinner, coffee. For PhD students the fee is reduced to 11,400 SEK (excl. VAT). No refunding after October 10. The number of participants is limited to 16.
 


 

QUESTIONS & FURTHER INFORMATION

Please contact
 
Lars Davidson
tel. +46 (0) 31-772 14 04,
E-mail: lada@chalmers.se
 
or
 
Anna Stupak
Chalmers Professional Education
tel. +46 (0) 707 615185
E-mail:anna.stupak@chalmers.se
 


 

PROGRAM

DAY 1 (10.00 -- 19.00)
  • Introduction to Large Eddy Simulation
  • Filtering of the equations; discretization convection schemes for LES, SGS models.
  • Workshop: interpretation of results from a LES and unsteady RANS. Time-averaging, evaluation of various forms of turbulent stresses etc.
DAY 2 (8.00 -- 17.00)
  • Dynamic SGS models, scale-similarity models, transport equations for SGS stresses
  • Introduction to DES, URANS, PANS and SAS
  • Introduction to hybrid LES-RANS
  • Workshop cont'd: explicit filtering, SGS models, spectra, two-point correlations, viscous and SGS dissipation, scale-similarity models, DES, DDES and SAS
DAY 2 (19.30)
  • Course dinner

DAY 3 (8.00 -- 17.00)
  • Inlet boundary conditions: prescribed spectra, integral length scale, integral time scale, implementation of inlet boundary conditions in your LES/DES code
  • Workshop: generate synthetic isotropic and anistropic (non-isotropic) inlet boundary conditions with Matlab scripts
  • PANS and embedded LES
  • Synthetic isotropic and anistropic (non-isotropic) turbulent fluctuations for embedded LES
  • Workshop: analysis of PANS and embedded LES