DNS 1-3 Description
Introduction
Give a brief overview of the test case. Describe the main characteristics of the flow. In particular, what are the underlying flow physics which must be captured by the computations ? Give reasons for this choice (e.g. poorly understood flow physics, difficulty to predict the flow with standard turbulence models, ...). Detail any case-specific data that needs to be generated.
Review of previous studies
The only high-fidelity data available is the DNS performed by Ohlsson et al. (2010). This DNS had a Re=10000 and a simplified diffuser and was solved using a spectral element code with 11th order polynomials, resulting in approximately 220 million degrees of freedom (DoF). With a stretched grid, the maximum grid resolution in the duct centre is reported at Δz + = 11.6, Δy + = 13.2 and Δx += 19.5. Correspondingly, the wall resolution (in terms of the first grid point) is reported as z += 0.074, y + = 0.37 in the spanwise and normal directions, respectively. This resolution was deemed sufficient to compute the flow in the diffuser. Their calculations were performed on the Blue Gene/P at ALCF, using 32768 cores and 8 million core hours. Another computation was performed on the cluster “Ekman” at PDC, Stockholm, Sweden, using 2048 cores and a total of 4 million core hours, also at the reduced Reynolds number 10000 and with the simplified geometry. The flow was computed for 13 flow-through-times (based on the bulk inlet velocity and diffuser length) before gathering statistics. The statistics were gathered over an additional 21 flowthrough-times.
Description of the test case
The diffuser studied is the UFR_4-16_Test_Case, Diffuser 1 provided in the ERCOFTAC database.
Geometry and flow parameters
The diffuser shape, dimensions and the coordinate system are shown in Fig. 1 (reproduced from UFR 4-16 Test Case).
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| Figure 1: Geometry of the 3-D diffuser 1 considered (not to scale), Cherry et al. (2008); see also Jakirlić et al. (2010a) |
For the current diffuser, the upper-wall expansion angle is 11.3° and the side-wall expansion angle is 2.56°. The flow in the inlet duct (height h=1 cm, width B=3.33 cm) corresponds to fully-developed turbulent channel flow. The L=15h long diffuser section is followed by a straight outlet part (12.5h long). Downstream of this the flow goes through a 10h long contraction into a 1 inch diameter tube. The curvature radius at the walls transitioning between diffuser and the straight duct parts are 6 cm. The bulk velocity in the inflow duct is in the x-direction resulting in the Reynolds number based on the inlet channel height of 10000. The origin of the coordinates (y=0, z=0) coincides with the intersection of the two non-expanding walls at the beginning of the diffuser's expansion (x=0).
Boundary conditions
Specify the prescribed boundary conditions, as well as the means to verify the initial flow development. In particular describe the procedure for determining the in flow conditions comprising the instantaneous (mean and fluctuating) velocity components and other quantities. Provide reference profiles for the mean flow and fluctuations at in flow - these quantities must be supplied separately as part of the statistical data as they are essential as input for turbulence-model calculations. For checking purposes, these profiles should ideally also be given downstream where transients have disappeared; the location and nature of these cuts should be specified, as well as the reference result.
References
- Ohlsson, J., Schlatter, P., Fischer P.F. and Henningson, D.S. (2009): DNS of three-dimensional separation in turbulent diffuser flows. In Advances in Turbulence XII, Proceedings of the 12th EUROMECH European Turbulence Conference, Marburg. Springer Proceedings in Physics, Vol. 132, ISBN 978-3-642-03084-0
Contributed by: Oriol Lehmkuhl, Arnau Miro — BSC
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