“Prediction is very difficult, especially if it’s about the future…” – Niels Bohr
Growing up as the child of two software engineers, mathematics and computers were an inseparable part of my life. My father always had these wonderful science-related stories and the patience to follow up on each and every question I had.
I remember when I was nine and my parents took us on a ski vacation at La Plagne.
La Plagne ski resort
We have encountered quite a bumpy flight on our way to France and being ever so curious (and somewhat frightened…) I asked my father: “why is the plane shaking like that?”. My dad relaxed me by saying: “don’t worry, there’s nothing to be afraid of, it’s just turbulence…”. I thought about what he just said for a minute or so and found myself a lot less frightened, but a question remained: “well, ok, just turbulence… but what is turbulence dad?”. My father looked at me with a tiny smile and replied: “well son, turbulence is all around us…”, then he got up and went to the toilets…
almost 30 years later I think to myself how lucky I was nature has taken its role by calling my father, as it left me with so much to blog about… 😉
The post shall present a fascinating forecast about the future incorporation of Scale-Resolving Simulations (SRS) in engineering design process . The forecast is based on a set of mixed references (U. Piomelli, P. Moin, F. Menter, P.G Tucker, NASA Report by J. Slotnick, etc’…), all of which aim to provide a road map of future CFD (sharpened with my own added spice… 😉).
Scale-Resolving Simulations (SRS)
Computational Fluid Dynamics (CFD) progress has been tremendous in the past half a decade. Moore’s Law vision of an exponential growth in computational resources lived up to its expectation and it’s predicted to keep doing so (at least) for the next 20 years.
Moore’s Law applied to CFD
Scientists and engineers have developed models of many levels of ﬁdelity for ﬂowﬁelds. On the ladder of CFD one may find many stages. Lifting-Surface Methods that model only the camber lines of lifting surfaces, not the thickness, vortex wakes that must of course be paneled. Linear Panel Methods that solve either the incompressible potential-flow equation or one of the versions applicable to compressible flow with small disturbances. Nonlinear Potential Methods where the velocity is represented as the gradient of a potential, as it is in incompressible potential flow, nonlinearity through effectively incorporating an entropic relation for the density as a function of the local Mach number. Euler Methods, solving the Navier-Stokes equations with the viscus and heat-conduction terms omitted. Coupled Viscous/Inviscid Methods solving the boundary-layer equations in the inner near wall region and matched to an outer region inviscid flow calculations.
One huge leap forward was achieved through the ability to simulate Navier-Stokes Methods Such as Reynolds-Averged Navier-Stokes (RANS).
RANS is based on the Reynolds decomposition according to which a flow variable is decomposed into mean and fluctuating quantities. When the decomposition is applied to Navier-Stokes equation an extra term known as the Reynolds Stress Tensor arises and a modelling methodology is needed to close the equations. The “closure problem” is apparent as higher and higher moments of the set of equations may be taken, more unknown terms arise and the number of equations never suffices.
Levels of RANS turbulence modelling are related to the number of differential equations added to Reynolds Averaged Navier-Stokes equations in order to “close” them.
0-equation (algebraic) models are the simplest form of turbulence models, a turbulence length scale is specified in advance through experimenting. 0-equations models are very limited in applications as they fail to take into account history effects, assuming turbulence is dissipated where it’s generated, a direct consequence of their algebraic nature.
1-equation and 2-equations models, incorporate a differential transport equation for the turbulent velocity scale (or the related the turbulent kinetic energy) and in the case of 2-equation models another transport equation for the length scale, subsequently invoking the “Boussinesq Hypothesis” relating an eddy-viscosity analog to its kinetic gasses theory derived counterpart (albeit flow dependent and not a flow property) and relating it to the Reynolds stress through the mean strain.
In this sense 2-equation models can be viewed as “closed” because unlike 0-equation and 1-equation models (with exception maybe of 1-equations transport for the eddy viscosity itself) these models possess sufficient equations for constructing the eddy viscosity with no direct use for experimental results.
A drawback evident in almost all eddy-viscosity models is the inability to inherently account for rotation and curvature. This drawback is resulted from relating the Reynolds stress to the mean flow strain and in fact is the major difference between such a modeling approach and a full Reynolds-stress model (RSM). The RSM approach accounts for the important effect of the transport of the principal turbulent shear-stress. On the other hand, RSM simulations are not computationally cost-effective, in as much that one does get an improved physical fidelity that is worth the time and computational resources consumed, not only that, they often do not converge.
RANS methodology strength has proven itself for wall bounded attached flows due to calibration according to the law-of-the-wall. For free shear flows however, especially those featuring a high level of unsteadiness and massive separation RANS has shown poor performance following its inherent limitations due to the fact that it’s a one-point closure and by that do not incorporate the effect of strong non-local effects and of long correlation distances characterizing many types of flows of engineering importance.
The turbulent boundary-layer and the “law of the wall”
Alleviation of the “one-point closure” issue, still under RANS framework, are found in second generation URANS Scale-Adaptive Simulation (SAS – F. Menter) and Partially-Averaged Navier-Stokes (PANS – S. Girimaji) turbulence models (a thorough evaluation of the models appears in the links).
Second Generation URANS – SAS and PANS – An Alternative to LES
An interesting methodology to simulate Large-Eddy Simulation (LES) like unsteadiness, lies in the midst of RANS and LES and is especially attractive for flows of which strong instabilities of the flow exist, is termed Scale Adaptive Simulation (SAS) (Menter and Egorov – available in the Fluent code).
SST-URANS Vs. SAS – Circular cylinder in a cross flow at Re=3.6⋅106
( Iso-surface of Q=S2-Ω2, coloured according to the eddy viscosity ratio)
In SAS formulation, two additional transport equations are solved for. The first is the turbulence kinetic energy transport equation (k) and the second for the square root of KL transport equation (hence the name kskl turbulence model).
What distinguishes the KSKL model from other 2-equation closures is the fact that in the last, the turbulence length scale (which may be defined on dimensional grounds by the transported variables) will always approach the thickness of the shear layer, while for KSKL model, the behavior is such that it allows the identification of the turbulent scales from the source terms of the KSKL model to a measure of both the thickness of the shear layer but also for non-homogenous conditions, as the Von-Karman length scale is related to the strain-rate, individual vortices have locally different time constants (inversely to turnover frequencies) and therefore from a certain size dependable upon the local strain rate, they may not be merged to a larger vortex. Meaning that the Von-Karman length scale gives a first order estimation for the spatial variation.
In PANS method, the so-called “partial averaging” concept is invoked, which corresponds
to a filtering operation for a portion of the fluctuating scales. This concept is based on the observation that the optimum resolved-to-modeled ratio will change from one engineering application to another depending on the reciprocal relations between the level of physical fidelity intended, geometry at hand and computational resources available.
The original PANS model is based on the 2-equation RANS modelling concept and solves two evolution equations for the unresolved kinetic energy and dissipation.
It is widely known and goes all the way back to Richardson and granted a more precise view by Kolmogorov, that in turbulence physics, large scales contain most of the kinetic energy and much of the dissipation occurs in the smallest scales, The smaller the unresolved kinetic energy is, the smaller is the modeled-to-resolved ratio and the greater are both computational effort and physical fidelity for a suited numerical resolution. moreover, the highest value that could be attained for the unresolved dissipation implies that RANS and PANS unresolved scales are the same.
Direct Numerical Simulation (DNS)
My first actual encounter with DNS was while researching for my thesis relating to the role of hairpins in transition and turbulence (specifically originating from bypass transition mechanism). ChannelFlow code as simple as it was made me feel ever so powerful in my direct confrontation with turbulence… 😉
Turbulence phenomena is very precisely described by a seemingly simple set of equations, the Navier-Stokes equations, their nature is such that analytic solutions to even the most simple turbulent flows can not be obtained and resorting to numerical solutions seems like the only hope.
But the resourcefulness of the plea to a direct numerical description of the equations is a mixed blessing as it seems the availability of such a description is directly matched to the power of a dimensionless number reflecting on how well momentum is diffused relative to the flow velocity (in the cross-stream direction) and on the thickness of a boundary layer relative to the body – The Reynolds Number.
It is found that the computational effort in Direct Numerical Simulation (DNS) of the Navier-Stokes equations rises as Reynolds number in the power of 9/4 which renders such calculations as prohibitive for most engineering applications of practical interest and it shall remain so for the foreseeable future, its use confined to simple geometries and a limited range of Reynolds numbers in the aim of supplying significant insight into turbulence physics that can not be attained in the laboratory.
Saying all that, it is not expected that DNS will take on vital role in the engineering design process, where many designs are to be evaluated working through a repetitive cycle of obtaining a CAD geometry–> grid generation–>Solving the equation–>post-processing the results–>optimization decisions.
Nonetheless, DNS shall find its place in the industrial CFD community for specialized research as it does in the academy, where on the line of an academic study which lasts up to approximately 5 years only a few high-fidelity simulations are conducted
The above presents a three dimensional direct numerical simulation using high-order methods has been performed to study the flow around the asymmetric NACA-4412 wing at a moderate chord Reynolds number (Rec = 400,000), with an angle of attack of 5 degrees. This flow regime corresponds approximately to the flow around a small glider. In addition to providing highly accurate data, high-order methods produce massive amount of data enabling proper flow visualization. For instance, in this study vortical structures emerging from tripping the flow to turbulence are visualized using the lambda2 criterion. It is interesting to see how interaction of such vortical structures from the turbulent boundary layer and the turbulent wake creates a natural art of its own.
Large-Eddy Simulation (LES) and hybrid RANS-LES
In LES the large energetic scales are resolved while the effect of the small unresolved scales is modeled using a subgrid-scale (SGS) model and tuned for the generally universal character of these scales. LES has severe limitations in the near wall regions, as the computational effort required to reliably model the innermost portion of the boundary layer (sometimes constituting more than 90% of the mesh) where turbulence length scale becomes very small is far from the resources available to the industry. Anecdotally, best estimates speculate that a full LES simulation for a complete airborne vehicle at a reasonably high Reynolds number will not be possible until approximately 2050…
Modeling of LES is formally described by the application of spatially filtering NSE. An explicit approach would explicitly apply a filter with some kind of shape (may it be cutoff, top hat, etc…). subsequently, a model is devised to capture the effect of under-resolved length-scales. The most common representation, is a linear stress-strain relation relying on the Boussinesq hypothesis and the eddy viscosity concept. The first and possibly still the most popular is the Smagorinsky model. Applying the Smagorinsky model to flows other than those it was tuned for, shall prove out of its range of applicability consequence of its many shortcomings, fully explained in my former post That’s a Big W(H)ALE as well as the remedies to overcome these shortcomings from a purely physical perspective.
Models such as these are termed “explicit SGS Models” as the filter and its shape are “clearly” defined (Its effect not quite though…). Other popular explicit modelling procedures include:
Another route for modelling the effect of unresolved scales is found through the utilization of higher order numerical schemes to take the role of the explicit filter in the aim of adding dissipation only in the high wave number range (small and unresolved scales) – termed Implicit LES (ILES). The first of such method was MILES (F. Grinshtein, also followed by a good book on the subject of ILES).
Returning to Moore’s law prediction it could be assumed that LES is going to take more and more of a vital role in engineering design process, being ever so attractive as its level of fidelity is such that it combines the advantages of simulations along with reliability features of experiments. This allows the engineer to build up his confidence while extracting high fidelity realizable results, such that the margin of safety could be tighten for the few more percentages of optimality which are the hardest to achieve.
As LES shall remain too expensive in the following few decades for the ever increasing number of engineering complexities (e.g. complete aircraft wing), researchers have shifted much of the attention and effort to hybrid formulations incorporating RANS and LES in certain ways. Due to the fact that computational cost of LES is practically independent on the Reynolds number for free shear ﬂows, only weakly dependent on the Reynolds number for the outer portion of the turbulent boundary layer, but becomes strongly dependent on the Reynolds number for the innermost layer (the viscous sublayer, the buffer layer and the initial part of the log layer), in most hybrid RANS-LES methods RANS is applied for an inner portion of the boundary layer and large eddies are resolved away from these regions by an LES (e.g. WMLES).
Detached Eddy Simulation (DES)
One of the most popular hybrid RANS-LES models is Detached Eddy Simulation (DES) devised originally by Philippe Spalart. The term DES is based on the Idea of covering the boundary layer by RANS model and switching the model to LES mode in detached regions thereby cutting the computational cost significantly yet still offering some of the advantages of an LES method in separated regions.
The formulation of the hybridization of the model is fairly straight forward:
This means that as Δ is max(ΔX, ΔY, ΔZ) this modification of the S-A model, changes the interpretation of the model as the modified distance function causes the model to behave as a RANS model in regions close to walls, and as an eddy-viscosity based LES (Smagorinsky, WALE, etc’…) manner away from the walls.
The original DES is set to Spalart-Allmaras eddy-viscosity transport equation to achieve an eddy viscosity (see the link for an in-depth evaluation of the turbulence model) for RANS mode and an eddy-viscosity based LES model (such as WALE for example).
The actual formulation for a two-equation model is (the turbulence kinetic energy equation of a k-ω model):
In subsequent improvements to the DDES formulation, RANS are applied to the innermost portion of the boundary layer and large eddies are resolved away from these regions. In such formulation LES is confined to the rest of the boundary layer or to regions where flow is detached which provides a Wall-Modelled Large-Eddy Simulation (WMLES) of attached flows at high but fair computational cost.
Improved-DDES for the flow behind a circular cylinder
Another subtlety concerns that concerns the “grey area”, specifically the region of transition between RANS and LES models. DES utilizes a model parameter very similar to the one in Smagorinsky LES model which is found deficient in the ability to handle laminar-turbulent transition (among other deficiencies). The same is observed in DES as high levels of eddy viscosity attenuate the transition process which contribute to the “grey area” problem, specifically the RANS to LES transition by interfering with “turbulence content” arising from shear layer instability. This is an ongoing issue with DES and some options to overcome this “grey area” phenomena incorporating local formulation (so as they can be straightforwardly implemented in an OpenFOAM code) have been proposed such as processing the local velocity gradient to distinguish between situations of which the eddy viscosity is low (such as plane shear) to regular turbulence, where the subgrid-scale model of the LES can be in use.
Grid Induced Separation
Being so popular, some of the natural DES (P. Spalart 1997) inherent limitations were often overlooked in simulations as practitioners often apply the model in order to increase physics fidelity without dwelling on subtle issues. The following paragraphs address some of these subtleties (following references from P. Spalart et al. 2006 and F. R. Menter 2000).
In DES the hybrid formulation has a limiter switching from RANS to LES as the grid is reduced. The problem with natural DES is that an incorrect behavior may be encountered for flows with thick boundary layers or shallow separations. It was found that when the stream-wise grid spacing becomes less than the boundary layer thickness the grid may be fine enough for the DES length scale to switch the DES to its LES mode without proper “LES content”, i.e. resolved stresses are too weak (“Modeled Stress Depletion” or MSD”), which in turn shall reduce the skin friction and by that may cause early separation. The phenomenon is termed Grid Induced Separation (GIS).
mean velocity in different types of grids in a boundary layer –
top: natural DES, left: ambiguous grid spacing, right: LES
As a consequence of the original DES deficiencies an advancement to the model was devised, termed Delayed-DES (DDES). In the Fluent DES-SST formulation a DES limiter “shield” is added to maintain RANS behavior in the boundary layer without grid dependency.
Delayed Detached-Eddy Simulation (DDES) Formulation
The main corner stone for the DDES hybrid RANS-LES model is the Spalart-Allmaras Turbulence Model. One transport equations for the eddy-viscosity based models such as Spalart-Allmaras don’t have an internal length scale as far as a measure of the mean shear rate is concerned, but do incorporate a ratio (squared) of a model length scale to the wall distance. The parameter is modified in the DDES formulation to support any eddy viscosity based model (a straightforward procedure to extract an eddy viscosity transport model from a two transport equations model )
where νt is the kinematic eddy viscosity, ν the molecular viscosity, Ui,j the velocity gradients, κ the Kármán constant and d the distance to the wall.
As the length scale is 1 in the logarithmic layer and gradually goes to zero in the boundary layer edge the kinematic viscosity is added to the formulation to ensure its stays correct in high proximity to the wall such that the length scale remains away from zero (exceeding 1).
A function is defined to ensure that the solution will be a RANS solution even if the grid spacing is smaller than the boundary layer thickness (so it will be 1 in the LES region where the length scale defined above is much smaller than 1, and 0 elsewhere while not sensitive in situations of high proximity to the wall when the length scale exceeds 1.
Now an alteration to the DES length scale is proposed such that under specific coefficient values (which the above function is not so sensitive to even in the case of a different formulation of DES other than spalart-Allmaras, say the k-ω SST Model – we shall see such a formulation shortly)
In this formulation, when the function is 0, the length scale dictates RANS mode to operate, and when the function is 1 natural DES (P. Spalart 1997) applies. The difference lies in the fact that on contrary to natural DES formulation where the length scale depends solely on the grid, in the DDES formulation it depends also on the eddy-viscosity. This means that the revised formulation will “insists” upon remaining on RANS mode if the grid is inside the boundary layer and if massive separation is encountered, the functions value will switch to LES mode a much more abrupt manner than the switch in the natural DES formulation, rendering the “grey area” narrower which is highly desirable.
The original DDES is set to Spalart-Allmaras eddy-viscosity transport equation to achieve an eddy viscosity (see the link for an in-depth evaluation of the turbulence model) for RANS mode and an eddy-viscosity based LES model (such as WALE for example).
Vorticity isosurfaces in a circular cylinder simulation (F. Spalart 2009)
For two-equation models, the dissipation term in the turbulence kinetic energy equation is formulated as follows:
It is worth mentioning that DES and its variants are termed and essentially are global hybrid methods.
Global hybrid methods are based on a continuous treatment of the flow variables at the interface between RANS and LES and by that introduce a ‘grey area’ in which the solution is neither pure RANS nor pure LES since the switch from RANS to LES does not imply an instantaneous change in the resolution level. These methods can be considered as weak RANS–LES coupling methods since there is no mechanism to transfer the modelled turbulence energy into resolved turbulence energy.
In the above formulation The function FDDES is designed as to reach unity inside the wall boundary layer and zero away from the wall. The definition of this function is intricate as it involves a balance between proper shielding and not suppressing the formation of resolved turbulence as the flow separates from the wall. As the function FDDES blends over to the LES formulation near the boundary layer edge, no perfect shielding can be achieved. The limit for DDES is typically in the range of the maximum edge length of the local computational cell is less then 20% of the boundary layer thickness which allows for meshes where the maximum edge length of the local computational cell is of 20% than for natural DES. However, even this limit
is frequently reached so the GIS phenomena is not fully prevented with DDES.
There are a number of DDES models available in ANSYS Fluent/CFX. They follow the same principal idea with respect to switching between RANS and LES mode. The models differ therefore mostly by their RANS capabilities and should be selected accordingly.
Shielded Detached Eddy Simulation (SDES)
The SDES formulation is yet another variation of DES. The improvement is in the shielding function and the interaction with the grid scale. This is emphasized in the turbulence model by an additional sink term in the turbulence kinetic energy equation:
The shielding function in the SDES formulation (namely – fs) provides more shielding then the corresponding shielding function in the DDES formulation (F-DDES), this means that the original shielding based on the mesh length scale can be reduced and is therefore defined in SDES as:
The first part in the above is the conventional LES mesh length scale, the second is again based on the maximum edge length as in the DES formulation and the 0.2 in the above ensures that for highly stretched meshes the grid length scale is a fifth of that of DDES and another implication is the reduction of the eddy-viscosity in LES mode by a factor of 25 as it is dependent quadratically upon the grid size. This is an important artifact as it improves the RANS to LES transition of DES models.
In engineering flows, flow characteristics of shear flows is much more encountered than that of decaying isotropic turbulence (DIT). The last is the basis for the calibration of the DES/DDES constant. Shear flows the Smagorinsky constant is reduced and this is achieved by setting the constant in SDES to 0.4.
Now if we combine the above explained effect of the grid scale on the eddy viscosity with the modified constant a reduction by a factor of nearly 60 is achieved for separated flows on stretched grids which is favorably affects the RANS to LES transition.
Stress-Blended Eddy Simulation (SBES)
SBES is not a new hybrid RANS-LES model, but a modular approach to blend existing models to achieve optimal performance. In this sense SBES is a modular approach which allows the CFD practitioner to use a pre-selected RANS and another pre-selected LES model instead of the mix of both formulations within one set of equations.
This becomes handy in certain fields of which the modeling sophistication is to be extended from what was originally practiced with a specific and validated LES to include parts of the domain which can only be covered by RANS models without having to replace the trusted LES.
SBES model concept is built on the SDES formulation. In addition, SBES is using the shielding function to explicitly switch between different turbulence model formulations in RANS and LES mode.
For the general case one of the (RANS or LES) models is not based on the eddy viscosity concept the general formulation is presented either in modeled stress tensor:
For the case where both RANS and LES models are based on the eddy viscosity concepts, the formulation simplifies to:
The strong shielding is important for such a formulation to work in order to maintain a zero pressure gradient RANS boundary layer in any grid.
The intention of the SBES methodology is to resolve the following issues (F. R. Menter 2016):
- Exhibit an asymptotic shielding of the RANS boundary layers.
- perform an explicit switch to user-specified LES model in LES region.
- Allowance of rapid ‘transition’ from RANS to LES regions Allow practitioners to be able to clearly distinguish regions where the models run in RANS and regions where the model runs in LES mode.
- Allow Wall-modeled LES capability once in regions of sufficient numerical resolution and an upstream trigger into LES-mode for WMLES simulations.
Summing up all of the above, the following SRS models are available in the ANSYS CFD codes:
- Scale-Adaptive Simulation (SAS) models:
a. SAS-SST model (Fluent, CFX)
- Detached Eddy Simulation (DES) Models:
a. DES-SA (DDES) model (Fluent)
b. DES-SST (DDES) model (Fluent, CFX)
c. Realizable k-ε-DES model (Fluent)
- Shielded Detached Eddy Simulation (SDES):
a. All ω-equation based 2-equation models in Fluent and CFX.
- Stress-Blended Eddy Simulation (SBES):
a. All ω-equation based 2-equation models in Fluent and CFX.
- Large Eddy Simulation (LES):
a. Smagorinsky-Lilly model (+dynamic) (Fluent, CFX)
b. WALE model (Fluent, CFX)
c. Kinetic energy subgrid model dynamic (Fluent)
d. Algebraic Wall Modeled LES (WMLES) (Fluent, CFX)
- Embedded LES (ELES) model:
a. Combination of all RANS models with all non-dynamic LES models (Fluent)
b. Zonal forcing model (CFX)
A review of ANSYS Fluent different approaches to turbulence modeling could be found in the presentation below.
For a comprehensive best practice guidelines reference I would recommend the following review – Best Practice: Scale-Resolving Simulations in ANSYS CFD (F. R. Menter 2015).
The incorporation of SRS in engineering process
In order for SRS to be best incorporated in engineering design process there are some challenges to overcome, most of which are related to LES rather than second generation URANS, based on RANS methodology which is very mature and well-tested as RANS has truly been the work horse for most large-scale engineering applications, in contrast with LES closures which are mostly algebraic and suffer from lack of complex engineering applications validity.
OPTIMIZATION AND SENSITIVITY ANALYSIS
Engineering design process is based on an iterative design achieving the best product through assessing a current design by optimization methodologies such as local sensitivity analysis, by which gradients of design parameters are calculated subsequently to be employed in gradient-based optimization algorithms.
In order to being able to use LES in such quantifications of design parameters it needs to be incorporated with tools of sensitivity analysis to measure how uncertainty factors affect the performance of the design.
The problem is that LES is a non-linear dynamical system, hence suffers from chaotic behavior. Local calculations of quantities of interest of which initial conditions slightly depart, exponentially diverge as time advances. A robust methodology to avoid uncertainty calculations divergence is mandatory if LES is to participate in the engineering design process.
A chance to add one of these beautiful (Lorenz System-like)
non-linear dynamical systems pictures 😉
GEOMETRY, GRID GENERATION AND NUMERICAL SCHEMES
In order for LES to come forth on its future vital role, many adjustments and advancements to current dominating LES approaches should be conducted. In essence, what differs practical engineering applications from their academic counterparts is the level of geometry complexity. Unstructured meshing for complex geometries has been dominating industrial CFD and from an LES standpoint this means that large errors due to commutation of non-commutative operations may hamper results accuracy substantially.
Advancements of Immersed Boundary Method (IBM), in which the boundaries of the body do not conform to the grid, the governing equations are discretized on fixed meshes and applying boundary conditions requires modifying the equations in the vicinity of the solid boundary by means of a forcing function that reproduces the effect of the boundary, are promising as far of high fidelity simulations of complex geometry and especially for moving meshes are concerned.
A snapshot of Large Eddy Simulation of a 5-bladed
rotor wake in hover with a novel multiblock IBM
(by Technion CFD Lab – S. Frankel)
Some new advancements in mixed-models (such dynamic and Smagorinsky) based on the integral formulation of the LES equation (F. M. Denaro) alleviate some of commutation problematic issues and allow for a much more accurate filtering.
Moreover, of true importance is the increasing the level of automation. As HPC shall keep obeying Moore’s law in its advancement, CFD workflows shall suffer tremendously from the “human-in-the-loop” syndrome, where the practitioner is too much involved, especially in the geometry accommodation and grid generation phases of the design and analysis.
Adaptive grid-generation for SRS are also a challenge. While in RANS grid adaptation is aimed only on reducing numerical error, for LES it is intended also to improve SGS model errors and increase the fraction of resolved motions. Suggestions to alleviate the difficulty are strongly related to the fact that standard algebraic eddy-viscosity modeling approach render LES as unclosed in the sense that the filter to be applied is not grid-independent. One exciting route of such by SB Pope suggesting adaptation aiming on resolving a user-deﬁned fraction of the kinetic energy, and also presented an incorporation of such in dynamic modeling.
The incorporation of higher order numerical methods in commercial CFD packages (by high I mean third order and above) shall also possibly be on the focus as the increase in computational power shall make them quite attractive for problems of which highly dissipative schemes are problematic such as vortex dominated flows and problems of wave propagation conducted in large scale and exploiting SRS.
LCS Fast 5th order accurate transient
simulation of the Elemental Rp1 track car
CONSISTENCY OF SUB-GRID SCALE MODELS
It is highly desirable and possibly a step towards increasing the physical fidelity if SGS models are consistent with the Navier-Stokes equations in a mathematical and physical standpoint. Properties like symmetry requirements, near wall scaling (such as eddy-viscosity cubed), Realizability, production of turbulence kinetic energy, zero subgrid dissipation for laminar ﬂow, consistency with the second law of thermodynamics and some others are to be explored while developing new or revised SGS methodologies.
Consistency of existing SGS models with regards to some
important mathematical and Physical features
The challenge for future consistency is to match physical and mathematical consistency while also preserving important features such as locality for example, to match expected sharp increase in parallelism and to support hierarchical memory architectures having numerous graphical processing units (GPUs) and co-processors.
HIGH POWER COMPUTING (HPC)
The effectiveness and impact of CFD on the engineering design process is extremely dependent on the power and availability of modern HPC systems. During the last decades, CFD codes were formulated using message passing (MPI) software models which match nowadays parallelism efficiently. As future route and prevailing computing hardware, memory architecture (hierarchical not supported by MPI) and network connecting is not a-priori known new algorithms have to be supportive and advance hand to hand with computing resources.
Numerical schemes such must also support tremendous parallelism in future exascale computing. Schemes involving global operations shall not prevail do to obvious bottlenecking.
Worldwide top HPC and its utilization
The second issue relates to the fact that in order to utilize such computational advancements, methodologies for SRS should be developed as to be also used outside the academy. As much as it is important that novel modelling techniques shall be validated and tested on simple canonical problems (e.g. ZPGBL/couette/channel/pipe flows) which lend themselves to detailed assessment, they should be developed to be also applied to real engineering problems.
It is no coincidence that the k-ω SST 2-equation turbulence model (F. Menter) Detached-Eddy Simulation (DES) (P. Spalart) and WALE LES model (F. Nicoud and F. Ducros) have gained such popularity. It is the fact that each of them was devised intentionally to perform well for industrial applications, that made them such. A good example is non-local operations which find their way to many LES formulations. Their use in commercial CFD code environment is near to impossible.
DEMOCRATIZATION OF SRS
I read quite an interesting post (by Keith Hanna – Mentor Graphics) about the “Democratization of CFD“. Referring to SRS, it is quite obvious that in order for LES to be widespread in the design process, it clearly needs to be much more accessible to non-proficient practitioners. In my post “Let’s LES” I have reviewed some of LES mandatory set of tools without which the credibility of the simulation is doubtful at best.
When a non-proficient practitioner tries to perform an LES, there are many instances of which being able to construct an animation of a time-varying flow that looks like a turbulent flow seems very satisfying. However, this offers no guarantee that the appropriate grid resolution has been used, spatial and temporal schemes have been selected, boundary conditions (especially time-varying, turbulent containing inflow conditions) are proper, etc’… CFD practitioners have to be educated to control a much different set of tools than those they were used to with RANS (and other low fidelity methodologies) to actually achieve the added benefit that LES could provide.
There are many aspects I have left out because I thought of them as too related to nowadays CFD practice while I believe that it is somewhat impossible to predict the one route that shall prevail, so alternatives should always keep advancing as well.
Nonetheless even though it’s safe to predict that SRS (and especially LES) shall not replace RANS in the near future, the level of physical fidelity achieved by SRS shall have a growing impact on engineering design process.
For this forecast to become reality, the one conclusion shared by all authors was that sincere confrontation with SRS challenges should be conducted while also taking under consideration its practicality to engineering design process.
So to conclude, although an answer to the question left unanswered as nature called is not included in the forecast, it is exciting to see if it shall live up to the expectation… 🙂