Abstracts - KTH Mechanics

Numerical Simulations of Transient Turbulent Flows
Y. M. Chung ∗
A detailed numerical study of transient turbulent wall-bounded flow for investigating
unsteady coherent near-wall structure are performed. Temporal acceleration
(and also deceleration) of turbulent boundary layer is studied using DNS, LES and
URANS. The main focus of the study is to develop a high-quality DNS database for
transient turbulent flow, which is then used in testing and assessing popular turbulence
models and LES wall models for unsteady turbulence for industrial use. The
test cases described below have a high importance in engineering and will improve
the understanding of the flow physics of transient turbulent flow.
Direct numerical simulations are performed for a temporally accelerating (and also
deceleration) turbulent pipe flows. The calculations are started from a fully-developed
turbulent pipe flow. For the acceleration cases, the Reynolds number increases due
to flow acceleration from Re0 = 7000 to Re1 =45, 200, and the Reynolds numbers
based on the friction velocity (uτ ) are Reτ = 230 and 1190, respectively. Preliminary
LES simulations have been performed with a 128 × 193 × 256 grid system 1 and DNS
is also applied to a lower Re number.
The responses of the turbulence quantities (e.g., turbulence intensities, Reynolds
shear stress, and vorticity fluctuations) and the near-wall turbulence structure to the
pressure gradient change are investigated. It is found that there are two different
relaxations: a fast relaxation at the early stage and a slow one at the later stage. The
early response of the velocity fluctuations shows an anisotropic response of the nearwall
turbulence 2 . Four turbulence models are tested in this study: the S-A model,
the k − ε model, the k − ω model, the Baldwin-Lomax model. The preliminary DNS
results of transient channel flow are shown in Figure 1.
∗ School of Engineering, University of Warwick, Coventry CV4 7AL, U.K.
1 Chung, Workshop on Dynamics Systems, Fluid Dynamics and Turbulence (2005).
2 Chung, Int. J. Num. Meth. Fluids 47, 925 (2005).
+
∆Uc 0.5
0
-0.5
-1
t +
-1.5
80 100 120 140 160
U
20
15
10
5
SA
t=0
t=4
t=8
t=12
t=16
t=20
Steady state
DNS
0
0 0.5 1 y
1.5 2
Figure 1: (a) Time history of the centreline velocity Uc using DNS. Symbols represents
steady channel flow, solid lines represent transient cases. (b) Streamwise velocity at
several time instants using DNS and the S-A turbulence model.
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