An overview is provided of various direct numerical simulations (DNS) of transitional flows in turbine-related geometries. Two flow cases are considered: the first case concerns separating flow over a flat plate and the second case flows in turbine cascades. In the first case, in which $Re=60,000$, either an oscillating oncoming flow (1) or a uniform flow with and without oncoming turbulent free-stream fluctuations (2) is prescribed at the inlet. In both subcases (1) and (2), separation is induced by a contoured upper wall. In (1), the separated boundary layer is found to roll up due to a Kelvin-Helmholtz (KH) instability. This rolled-up shear layer is subject to spanwise instability and disintegrates rapidly into turbulent fluctuations. In (2), a massive separation bubble is obtained in the simulation without oncoming free-stream fluctuations. A KH instability is eventually triggered by numerical round-off error and is followed again by a rapid transition. With oncoming turbulent fluctuations, this KH instability is triggered much earlier and transition is enhanced, which leads to a drastic reduction in size of the separation bubble. The second case, concerning flow in turbine cascades, includes (1) flow in the T106 turbine cascade with periodically oncoming wakes at $Re=51,800$ and (2) flow and heat transfer in a MTU cascade with oncoming wakes and background turbulence at $Re=72,000$. In the simulation of flow in the T106 cascade with oncoming wakes, the boundary layer along the downstream half of the suction side is found to separate intermittently and subsequently rolls up due to a KH instability leading to separation-induced transition. At times when the wakes impinge separation is suppressed. In the simulations of flow around a MTU turbine blade, evidence of by-pass transition in the suction-side boundary-layer flow is observed while the pressure-side boundary layer remains laminar in spite of significant fluctuations present. In agreement with the experiments, the impinging wakes cause the heat transfer coefficient to increase significantly in the transitional suction-side region close to the trailing edge and by about 30% on the pressure side. The large increase in heat transfer in the pre-transitional suction-side region observed in the experiments could not be reproduced. The discrepancy is explained by differences in spectral contents of the turbulence in the oncoming wakes.

1.
Hourmouziadis
,
J.
, 2000, “
Das DFG Verbundvorhaben Instationäre Strömung in Turbomaschinen
,” in
Deutscher Luft und Raumfahrtkongress
,
Leipzig, Germany
, September 18–21, DGLR-JT2000-030.
2.
Alam
,
M.
, and
Sandham
,
N. D.
, 2000, “
Direct Numerical Simulation of ‘Short’ Laminar Separation Bubbles With Turbulent Reattachment
,”
J. Fluid Mech.
0022-1120,
410
, pp.
1
28
.
3.
Maucher
,
U.
,
Rist
,
U.
,
Kloker
,
M.
, and
Wagner
,
S.
, 2000, “
DNS of Laminar-Turbulent Transition in Separation Bubbles
,” in
High-Performance Computing in Science and Engineering
,
E.
Krause
and
W.
Jäger
, eds.,
Springer
,
Berlin
.
4.
Spalart
,
P. R.
, and
Strelets
,
M. Kh.
, 2000, “
Mechanisms of Transition and Heat Transfer in a Separation Bubble
,”
J. Fluid Mech.
0022-1120,
403
, pp.
329
349
.
5.
Wissink
,
J. G.
, and
Rodi
,
W.
, 2002, “
DNS of Transition in a Laminar Separation Bubble
,” in
Advances in Turbulence IX
,
I. P.
Castro
,
P. E.
Hancock
, and
T. G.
Thomas
, eds.,
CIMNE
,
Barcelona
, pp.
727
730
.
6.
Wissink
,
J. G.
, and
Rodi
,
W.
, 2003, “
DNS of a Laminar Separation Bubble in the Presence of Oscillating Flow
,”
Flow, Turbul. Combust.
1386-6184,
71
, pp.
311
331
.
7.
Wissink
,
J. G.
,
Michelassi
,
V.
, and
Rodi
,
W.
, 2003, “
Heat Transfer in a Laminar Separation Bubble Affected by Oscillating External Flow
,” in
Turbulence, Heat and Mass Transfer-4
,
K.
Hanjalic
,
Y.
Nagano
, and
M. J.
Tummers
, eds.,
Begell House
,
New York
, pp.
199
206
.
8.
Wissink
,
J. G.
, and
Rodi
,
W.
, 2004, “
DNS of a Laminar Separation Bubble Affected by Free-Stream Disturbances
,” in
Direct and Large-Eddy Simulation V
,
R.
Friedrich
,
B. J.
Geurts
, and
O.
Métais
, eds.,
Kluwer Academic Publishers
,
Dordrecht
.
9.
Lou
,
W.
, and
Hourmouziadis
,
J.
, 2000, “
Separation Bubbles Under Steady and Periodic-Unsteady Main Flow Conditions
,” in
Proceedings of the 45th ASME International Gas Turbine & Aeroengine Technical Congress
,
Munich, Germany
, May 8–11.
10.
Wu
,
X.
, and
Durbin
,
P. A.
, 2001, “
Evidence of Longitudinal Vortices Evolved From Distorted Wakes in a Turbine Passage
,”
J. Fluid Mech.
0022-1120,
446
, pp.
199
228
.
11.
Wissink
,
J. G.
, 2003, “
DNS of Separating, Low Reynolds Number Flow in a Turbine Cascade With Incoming Wakes
,”
Int. J. Heat Fluid Flow
0142-727X,
24
, pp.
626
635
.
12.
Michelassi
,
V.
,
Wissink
,
J. G.
, and
Rodi
,
W.
, 2002, “
Analysis of DNS and LES of Flow in a Low Pressure Turbine Cascade With Incoming Wakes
,”
Flow, Turbul. Combust.
1386-6184,
69
, pp.
295
329
.
13.
Kalitzin
,
G.
,
Wu
,
X.
, and
Durbin
,
P. A.
, 2002, “
DNS of Fully Turbulent Flow in a LPT Passage
,” in
Engineering Turbulence Modelling and Experiments 5
,
W.
Rodi
, and
N.
Fueyo
, eds.,
Elsevier
,
New York
.
14.
Raverdy
,
B.
,
Mary
,
I.
,
Sagaut
,
P.
, and
Liamis
,
N.
, 2003, “
High-Resolution Large-Eddy Simulation of Flow Around Low-Pressure Turbine Blade
,”
AIAA J.
0001-1452,
41
(
3
), pp
390
397
.
15.
Rogers
,
M. M.
, 2002, “
The Evolution of Strained Turbulent Plane Wakes
,”
J. Fluid Mech.
0022-1120,
463
, pp.
53
120
.
16.
Stadtmüller
,
P.
, and
Fottner
,
L.
, 2001, “
A Test Case for the Numerical Investigation of Wake Passing Effects on a Highly Loaded LP Turbine Cascade Blade
,” ASME Paper 2001-GT-311.
17.
Liu
,
X.
, and
Rodi
,
W.
, 1994, “
Velocity Measurements in Wake-Induced Unsteady Flow in a Linear Turbine Cascade
,”
Exp. Fluids
0723-4864,
17
, pp.
45
58
.
18.
Liu
,
X.
, and
Rodi
,
W.
, 1994, “
Surface Pressure and Heat Transfer Measurements in a Turbine Cascade With Unsteady Oncoming Wakes
,”
Exp. Fluids
0723-4864,
17
, pp.
171
178
.
19.
Wu
,
X.
,
Jacobs
,
R. G.
,
Hunt
,
J. C. R.
, and
Durbin
,
P. A.
, 1999, “
Simulation of Boundary Layer Transition Induced by Periodically Impinging Wakes
,”
J. Fluid Mech.
0022-1120,
398
, pp.
109
153
.
20.
Jeong
,
J.
, and
Hussain
,
F.
, 1995, “
On the Identification of a Vortex
,”
J. Fluid Mech.
0022-1120,
285
, pp.
69
94
.
21.
Dullenkopf
,
K.
, and
Mayle
,
R. E.
, 1995, “
An Account of Free-Stream-Turbulence Length Scale on Laminar Heat Transfer
,”
ASME J. Turbomach.
0889-504X, Vol.
117
, pp.
401
406
.
22.
Zaki
,
T. A.
,
Durbin
,
P. A.
,
Wissink
,
J. G.
, and
Rodi
,
W.
, 2006, “
Direct Numerical Simulation of By-Pass and Separation-Induced Transition in a Linear Compressor Cascade
,” ASME Paper GT2006-90885.
You do not currently have access to this content.