Note: All slides not commented upon here; many of
the word slides are self-explanatory.
Issues Addressed
- We ran mostly with the 1-to-1 grid, but also
included some results using the overset grid.
- Our primary focus was parametric studies.
- We ran only M=0.75 cases.
Grid Convergence
- We performed a grid study using the given grid
(with every other point removed, then again), but
these coarser grids are probably too coarse to be in
the asymptotic region for 2nd order convergence.
- For purposes of workshop, it would have been
best to have been given a family of grids, including
one level finer than the current fine grid.
Effect of Grid on Surface Pressures
- Comparing 1-to-1 grid with overset grid results
at alpha=0: 1-to-1 grid smears shock more than
overset, especially near tip. Also, 1-to-1
underpredicts LE suction CPs over much of wing.
Grid Comparison
- 1-to-1 has much coarser streamwise spacing
(153 points around vs. 257 points around).
- 1-to-1 also is of generally poorer quality:
- too coarse near LE.
- non-smooth interfaces.
- grid not orthogonal to wing surface!!
Effect of Grid on Forces & Moments (2 slides)
- In spite of differences in CPs for 1-to-1 vs.
overset, the forces and moments are fairly similar.
Quantitative comparisons will be given in a later
slide.
Efect of Turbulence Model on Surface Pressures
- All of the above was with SA (Spalart-Allmaras,
Version Ia).
- We now compare with SST (Menter's k-omega
shear-stress transport) and EASM (explicit
algebraic stress model, k-omega form).
- Biggest differences at alpha=2.
- There is separation at this condition, but in
spite of this, the 3 models yield very similar
results. But they're not identical - there are
small differences, for example, near TE.
Streamlines at Alpha=2 deg
- If look at surface streamlines, we see small
variations in the separation patterns. For
example, SA predicts the largest separation region
near TE, EASM the smallest. There is no way to
know which is "better" in this case.
Effect of Turbulence Model on Forces & Moments
(2 slides)
- Very little difference noticeable,
qualitatively.
- Biggest difference occurs in friction drag.
- < 20 count difference between models.
- SA has bigger friction drag than SST &
EASM.
- 3 counts on upper wing surface,
3 counts on lower wing surface,
12 counts on body.
- This difference may be smaller on finer
grids,
but because different models are
calibrated
for flat plate and there are some
differences
in the calibration, there will
likely be
skin friction drag differences
even on
fine grids.
- Quantitative differences given later.
Actual "Fully Turbulent" Transition Locations for
Different Turbulence Models
- "Fully turbulent" is a misnomer!
- Contrary to popular belief, running a code
"fully turbulent" does NOT mean the flow is actually
fully turbulent.
- Turbulence models transition on their own,
a function of Re (unless you FORCE it with
something akin to Spalart's forcing term).
- For low-ish Re (1-10 million), the delay can
be significant.
- Here are upper surface transition locations
for the 3 turbulence models at alpha=0. They are
all fairly close to the LE, but they are not the
same: SA is furthest forward, and EASM is furthest
back.
Effect of Forcing SA Transition to Match "Natural"
Transition of EASM
- We tested the transition effect by forcing
SA to transition later, near the "natural"
transition location of EASM.
- The effect is very small.
- (Note: we did NOT compare the effect of
transition by setting to the experimental trip
location - we only did a a study showing the
effects due to differences in "natural"
fully turbulent trip locations.)
Effect of SA Version on Transition
- Particular VERSION of SA model has an effect
on variability, also!
- Often, people claim to run a particular model
(e.g., "SA"), but there are multiple versions
and one is often not sure which version is being
used.
- SA-Ia version is official version from
Recherche Aerospatial journal article.
- SA-fv3 version resulted from an e-mail
from Spalart to code developers implementing
his model early in the model's development.
Effect of SA Version on Forces
- In spite of big variations in transition
location, effect on integrated forces and moments
is relatively small.
Summary of Effects at Alpha=0
- Quantitative comparisons for all the
parametric studies conducted are shown here,
at the angle of attack where most of the runs
were performed.
Comments on EASM
- Be cautioned that EASM is like many models:
there are a lot of versions out there, all with
the same or similar designation ("EASM").
- Current version is in k-omega form.ÿÿÿ