Transcript Review of Valeo Aerofoil Testing - CDadapco.ppt

Review of the Valeo-CD Aerofoil
Tests
Clare Turner
Purpose
1) Extensive tests have been done on the T3 flat plate tests to
demonstrate abilities to predict transition under high freestream turbulence intensities ; i.e. testing bypass transition.
2) The 2-D Valeo aerofoil case exhibits transition which is not
induced by high free-stream disturbances, which most RANS
models cannot predict.
3) This is a step towards the final application of a three
dimensional rear wing which undergoes separation.
4) It is also an opportunity to test the models’ performance
under a pressure gradient and their abilities to predict
pressure coefficient.
The test case set-up: general
• The Valeo aerofoil is cambered and thin with an 8°
geometric angle of attack. The maximum thickness of
4.5% of the chord length.
• The experiments showed that turbulence from the
jet quickly dissipated away; so a very small value of k
was chosen at the inlet.
• The cells are reported as y+ < 1 and x+ ≤ 20, with one
cell in the z direction. There are 80,808 cells in total.
• The geometry and viscosity are scaled to give a unit
chord and a free-stream velocity of 1ms-1.
Test case set-up: inlet conditions
• Originally, the inlet conditions were determined from
RANS results from a full experimental domain. The
results provided with the mesh are from simulations
performed in Fluent using the above inlet conditions.
• In this set-up a profile is defined with the following
equations:
Validating the set-up
• Comparing the provided v2-f results with those from star-CD
and Code_Saturne is useful as validation of the different
codes and versions of the models
• The v2-f comparisons are mainly used here to indicate any
errors in the set-up
• Experimental, v2-f and LES results of pressure coefficients and
8 wake profiles were provided with the mesh
• The following results show that the v2-f results are similar
from all three codes, if anything the results from Saturne and
Star-CD are closer to the experimental than those provided
• Results from the two kT-kL-ω models in Saturne are also
included in the figures
Wake profiles: stream-wise velocity
x/C=0.057356
x/C=0.13129
Click here to animate
x/C=0.07528
x/C=0.16863
x/C=0.09395
x/C=0.11262
x/C=0.20597
x/C=1.01319
Click on figure to enlarge
Click again to return
Model Comparisons – wake profiles (U)
• There are two main differences between the v2-f models and
the experimental results for the wake profiles: the minimum
U-component of the velocity in the wake is noticeably lower;
the free-stream velocity is too high. The latter can be
explained by an over prediction of the inlet velocities.
• The kT-kL-ω models have the opposite problem to the v2-f
models in that the minimum velocity is too high.
• Other than this the Walters-Leylek profile is close to the
experimental results.
• The Walters-Cokljat model deviates both in the location of the
centre of the wake and the velocity at that point.
Wake profiles: cross-stream velocity
x/C=0.057356
x/C=0.13129
Click here to animate
x/C=0.07528
x/C=0.16863
x/C=0.09395
x/C=0.11262
x/C=0.20597
x/C=1.01319
Click on figure to enlarge
Click again to return
Model Comparisons – wake profiles (V)
• The outcome is similar, in that the magnitude of the velocity
deficit is too large for the v2-f models and too small for the kTkL-ω models. However, the Walters-Leylek model is very close
to the experimental values.
• The Walters-Cokljat model deviates from expected values
towards the free-stream.
Model Comparisons – wake profiles (SST)
• The SST model was also tested, to see if it is using ω as the
scale-determining variable which contributes to the different
shaped profile. The results are similar to that of the v2-f
model implying this is not the case.
• Click to view figures showing stream-wise velocities:
x/C=0.057356
x/C=0.07528
x/C=0.09395
x/C=0.11262
x/C=0.13129
x/C=0.16863
x/C=0.20597
x/C=1.01319
• Click to view figures showing cross-stream velocities:
x/C=0.057356
x/C=0.07528
x/C=0.09395
x/C=0.11262
x/C=0.13129
x/C=0.16863
x/C=0.20597
x/C=1.01319
Standard models Comparison – Cp
• The predictions for pressure coefficient are very close to the
experimental results. The profile for the RANS models appears
slightly shifted at the leading edge but the shape is correct.
kT-kL-ω models comparison – Cp
• The Walters-Leylek model gives similar values to the v2-f
models.
• There are obviously errors in the Walters-Cokljat model
Contours of turbulence
• The turbulent kinetic energy should be present on the suction
side of the aerofoil after the separation bubble and in the
wake, as presented in the Walters-Leylek contours.
• Transition occurs much later than expected in the WaltersCokljat model.
Walters-Cokljat
Walters-Leylek
Monitoring points
• The main variables in the models in the Code_Saturne
simulations (U-velocity, V-velocity, dissipation, pressure,
turbulent kinetic energy and laminar kinetic energy - where
applicable) are monitored at 13 points. There is a monitor at
the 3 inlets and the outlet and one in the boundary layer of
the aerofoil. The remaining 8 are at y=0 for the 8 x-values of
the profiles.
• Residuals are shown in star-CD, however it was unable to
converge using MARS with a blending factor of 0.5 and small
under-relaxation factors .
• There was no information on convergence etc. For the
provided v2-f results from Fluent.
Monitoring points – v2-f
Click on figure to enlarge
Click again to return
Monitoring points – Walters-Leylek
Click on figure to enlarge
Click again to return
Monitoring points – Walters-Cokljat
Click on figure to enlarge
Click again to return
Laminar kinetic energy monitoring points in the wake
Walters-Cokljat
Walters-Leylek
Laminar kinetic energy monitoring points in the wake
• Laminar kinetic energy is the result of energy from large
length-scales being deflected from a wall. Therefore far away
from the wall the values should be close to zero.
• The values predicted by the Walters-Cokljat model are far too
high (of the order of 0.1 rather than 0.001).
• The values for kL become consistent very quickly for the
Walters-Leylek model but the predictions continue to oscillate
for the Walters-Cokljat model.
• The amplitude of the oscillations are very large close to the
aerofoil but become smaller further down-stream.
Laminar kinetic energy monitoring points –
at the boundaries
• kL is zero at the inlets because this was the boundary
condition. As expected, the value of kL at the outlet is zero
• As with the wake profiles, the Walters-Cokljat model does not
give consistent values for non-zero laminar kinetic energy.
Walters-Cokljat
Walters-Leylek
Differences between the kT-kL-ω models
• The main difference between the two kT-kL-ω models is the
method of determining the bypass transition threshold. Also,
the Walters-Cokljat model includes a “shear-sheltering” term.
Both of these differences involve the calculation of
Ω = sqrt(2Ωij Ωij) where Ωij=0.5*(dUi/dxj – dUj/dxi).
• Bypass transition is triggered when the critical threshold value
is reached, which is defined differently for the two models:
Walters-Leylek
Walters-Cokljat
Differences between the kT-kL-ω models
• The calculation of bypass transition should not be an issue as
there is not sufficient disturbance in the free-stream to
initiate bypass transition, however the shear-sheltering term
does make a large difference to the production term in the
turbulent kinetic energy equation.
where fW is the ratio of effective length-scale and turbulent
length-scale; CSS is a constant.
Effect of curvature – set-up
• Nothing in the transport equations suggest curvature is
considered more in one model than the other, however this is
a significant difference between the flat plate tests and the
aerofoil.
• Adding a “hump” to a flat plate case is a simple way of
checking for any un-physical results due to added curvature.
Effect of curvature – high Tu∞
• Below are the turbulent kinetic energy contours over the
hump. Although there is a larger concentration of turbulence
at the rear of the hump in the Walters-Cokljat model, the
contours are similar.
Walters-Leylek
Walters-Cokljat
Effect of curvature – low Tu∞
• This case has a turbulence intensity of < 0.5% which is
generally too low to initiate bypass transition.
• The curvature does not trigger any transition in the 2nd model.
Walters-Leylek
Walters-Cokljat
Effect of curvature downstream – low Tu∞
• The Walters-Cokljat model predicts transition much further
downstream, as is shown here.
Profiles of Ω - over the aerofoil
• There is no numerical data to compare these values of Ω to;
however Ω should only be large very close to the wall
• Profiles at 10 equidistant points at the rear of the aerofoil
show the Walters-Leylek model is qualitatively correct but the
Walters-Cokljat implementation gives un-physical values.
Click on figure to enlarge
Click again to return
Profiles of Ω – in the wake
• In the wake, the two models’ prediction of Ω are similar.
Although their differences are greater further down-stream.
Click on figure to enlarge
Click again to return
Profiles of fSS – in the wake
• The shear sheltering function, that damps the production of
small scale turbulent kinetic energy in regions of high vorticity
in the Walters-Leylek model, is shown to have quite an effect
in the wake immediately behind the aerofoil.
• Is the production of kT being damped too much?
x/C=0.057356
x/C=1.01319
Comparisons with Fluent
• The Walters-Cokljat model is implemented in the latest
version of Fluent.
• The resulting wake profiles are very similar to the WaltersLeylek implementation. For example at x/C = 0.07528 look
how close the magenta line (the Walters-Cokljat model in
Fluent) and the blue line (Walters-Leylek in Saturne) are:
Comparisons with Fluent (2)
• This trend continues in the wake profiles down-stream
U-Velocity:
x/C=0.09395
x/C=0.11262
x/C=0.13129
x/C=0.16863
x/C=0.20597
x/C=0.11262
x/C=0.13129
x/C=0.16863
x/C=0.20597
V-Velocity:
x/C=0.09395
Click on figure to enlarge
Click again to return
Comparisons with Fluent - kT
• The turbulence begins at a similar point in the two models
however the greatest concentration of turbulence for the
Fluent Walters-Cokljat model is at the leading edge, while the
greatest concentration of turbulence is in the wake.
• Additionally the maximum values are very different.
Comparisons with Fluent - kL
• The laminar kinetic energy is qualitatively correct. Contours
show laminar kinetic energy is only in the boundary layer for
the Fluent simulation, and tends to zero as the turbulence
develops.
Conclusions: initial observations
• The comparisons between the different codes and the
experimental results for the wake profile and pressure
coefficient imply the set-up used in Code_Saturne is valid.
• Fluent and Star-CD show convergence is difficult to achieve;
the residuals stop reducing after a normalised value of
approximately 0.001.
• Despite this, all of the models, with the exception of the
Walters-Cokljat model, give very good predictions of CP and
good predictions of the wake profiles.
• The monitoring points in Code_Saturne show the variables
keeping a consistent value after about 500 iterations, again
with the exception of the Walters-Cokljat model.
Conclusions: further
• The wake results from Fluent are very similar to those from
the Walters-Leylek run, which implies the problem lies only
within the implementation of the Walters-Cokljat model.
• Curved surfaces appear to cause a large delay in transition
with the Walters-Cokljat model when there is a low freestream turbulence intensity.
• This could be associated with the calculation of the
magnitude of the rotation tensor or its effect on the shearsheltering term, fSS.
Current and further work
• Data from the point of divergence in the Walters-Cokljat
model (43 iterations) is being examined closely to look for any
particular triggers.
• A simple channel flow, with a known velocity profile, is being
set up to check the Ω values calculated by the sub-routine.
• As curvature appears to affect the value of the shear
sheltering function, an investigation is required to determine
whether this is a weakness; if so, how to correct it.
• Since the point of transition is of importance, further analysis
is being made on the contours of turbulence around the
aerofoil for the different models (including comparisons in the
same software)
END
The following slides are additional
details and full size figures
The following slides are animations
of the profiles as you progress
down the wake
Wake profiles – experimental
results
Click here to return
Wake profiles comparison:
stream-wise velocity
Click here to return
Wake profiles comparison
cross-stream velocity
The following slides are profiles
with all important models
Wake profiles with just omega
based models