4 results
The formulation of the RANS equations for supersonic and hypersonic turbulent flows
- H. Zhang, T.J. Craft, H. Iacovides
-
- Journal:
- The Aeronautical Journal / Volume 125 / Issue 1285 / March 2021
- Published online by Cambridge University Press:
- 12 October 2020, pp. 525-555
-
- Article
- Export citation
-
Accurate prediction of supersonic and hypersonic turbulent flows is essential to the design of high-speed aerospace vehicles. Such flows are mainly predicted using the Reynolds-Averaged Navier–Stokes (RANS) approach in general, and in particular turbulence models using the effective viscosity approximation. Several terms involving the turbulent kinetic energy (k) appear explicitly in the RANS equations through the modelling of the Reynolds stresses in such approach, and similar terms appear in the mean total energy equation. Some of these terms are often ignored in low, or even supersonic, speed simulations with zero-equation models, as well as some one- or two-equation models. The omission of these terms may not be appropriate under hypersonic conditions. Nevertheless, this is a widespread practice, even for very high-speed turbulent flow simulations, because many software packages still make such approximations. To quantify the impact of ignoring these terms in the RANS equations, two linear two-equation models and one non-linear two-equation model are applied to the computation of five supersonic and hypersonic benchmark cases, one 2D zero-pressure gradient hypersonic flat plate case and four shock wave boundary layer interaction (SWBLI) cases. The surface friction coefficients and velocity profiles predicted with different combinations of the turbulent kinetic energy terms present in the transport equations show little sensitivity to the presence of these terms in the zero-pressure gradient case. In the SWBLI cases, however, comparisons show that inclusion of k in the mean flow equations can have a strong effect on the prediction of flow separation. Therefore, it is highly recommended to include all the turbulent kinetic energy terms in the mean flow equations when dealing with simulations of supersonic and hypersonic turbulent flows, especially for flows with SWBLIs. As a further consequence, since k may not be obtained explicitly in zero-equation, or certain one-equation, models, it is debatable whether these models are suitable for simulations of supersonic and hypersonic turbulent flows with SWBLIs.
Influence of birth cohort on age of onset cluster analysis in bipolar I disorder
- M. Bauer, T. Glenn, M. Alda, O.A. Andreassen, E. Angelopoulos, R. Ardau, C. Baethge, R. Bauer, F. Bellivier, R.H. Belmaker, M. Berk, T.D. Bjella, L. Bossini, Y. Bersudsky, E.Y.W. Cheung, J. Conell, M. Del Zompo, S. Dodd, B. Etain, A. Fagiolini, M.A. Frye, K.N. Fountoulakis, J. Garneau-Fournier, A. Gonzalez-Pinto, H. Harima, S. Hassel, C. Henry, A. Iacovides, E.T. Isometsä, F. Kapczinski, S. Kliwicki, B. König, R. Krogh, M. Kunz, B. Lafer, E.R. Larsen, U. Lewitzka, C. Lopez-Jaramillo, G. MacQueen, M. Manchia, W. Marsh, M. Martinez-Cengotitabengoa, I. Melle, S. Monteith, G. Morken, R. Munoz, F.G. Nery, C. O’Donovan, Y. Osher, A. Pfennig, D. Quiroz, R. Ramesar, N. Rasgon, A. Reif, P. Ritter, J.K. Rybakowski, K. Sagduyu, A.M. Scippa, E. Severus, C. Simhandl, D.J. Stein, S. Strejilevich, A. Hatim Sulaiman, K. Suominen, H. Tagata, Y. Tatebayashi, C. Torrent, E. Vieta, B. Viswanath, M.J. Wanchoo, M. Zetin, P.C. Whybrow
-
- Journal:
- European Psychiatry / Volume 30 / Issue 1 / January 2015
- Published online by Cambridge University Press:
- 15 April 2020, pp. 99-105
-
- Article
- Export citation
-
Purpose:
Two common approaches to identify subgroups of patients with bipolar disorder are clustering methodology (mixture analysis) based on the age of onset, and a birth cohort analysis. This study investigates if a birth cohort effect will influence the results of clustering on the age of onset, using a large, international database.
Methods:The database includes 4037 patients with a diagnosis of bipolar I disorder, previously collected at 36 collection sites in 23 countries. Generalized estimating equations (GEE) were used to adjust the data for country median age, and in some models, birth cohort. Model-based clustering (mixture analysis) was then performed on the age of onset data using the residuals. Clinical variables in subgroups were compared.
Results:There was a strong birth cohort effect. Without adjusting for the birth cohort, three subgroups were found by clustering. After adjusting for the birth cohort or when considering only those born after 1959, two subgroups were found. With results of either two or three subgroups, the youngest subgroup was more likely to have a family history of mood disorders and a first episode with depressed polarity. However, without adjusting for birth cohort (three subgroups), family history and polarity of the first episode could not be distinguished between the middle and oldest subgroups.
Conclusion:These results using international data confirm prior findings using single country data, that there are subgroups of bipolar I disorder based on the age of onset, and that there is a birth cohort effect. Including the birth cohort adjustment altered the number and characteristics of subgroups detected when clustering by age of onset. Further investigation is needed to determine if combining both approaches will identify subgroups that are more useful for research.
Computational modelling of the flow and heat transfer in dimpled channels
- Part of
- K. Abo Amsha, T.J. Craft, H. Iacovides
-
- Journal:
- The Aeronautical Journal / Volume 121 / Issue 1242 / August 2017
- Published online by Cambridge University Press:
- 17 July 2017, pp. 1066-1086
-
- Article
- Export citation
-
The flow and heat transfer characteristics over a single dimple and an array of staggered dimples have been investigated using the Reynolds Averaged Navier-Stokes (RANS) approach. The objective is to determine how reliably RANS models can predict this type of complex cooling flows. Three classes of low-Reynolds number RANS models have been employed to represent the turbulence. These included a linear Eddy Viscosity Model (EVM), a Non-Linear Model (NLEVM) and a Reynolds Stress transport Model (RSM). Variants of the k-ε model have been used to represent the first two categories. Steady and time-dependent simulations have been carried out at a bulk Reynolds number of around 5,000 with dimple print diameter to channel height ratios of D/H = 1.0, 2.0 and ratios of dimple depth to channel height of δ/H = 0.2, 0.4. The linear EVM and the RSM tested both produce symmetric circulations in the dimples, while the NLEVM produces an asymmetric pattern. The mean velocity profiles predicted numerically are generally in good agreement with the data. The main flow characteristics are reproduced by the RANS models, but some predictive deviations from available data point to the need for further investigations. All models report an overall enhancement in heat transfer levels when using dimples in comparison to those of a plane channel.
Some numerical experiments on developing laminar flow in circular-sectioned bends
- J. A. C. Humphrey, H. Iacovides, B. E. Launder
-
- Journal:
- Journal of Fluid Mechanics / Volume 154 / May 1985
- Published online by Cambridge University Press:
- 20 April 2006, pp. 357-375
-
- Article
- Export citation
-
The paper reports numerical solutions to a semi-elliptic truncation of the Navier–Stokes equations for the case of developing laminar flow in circular-sectioned bends over a range of Dean numbers. The ratios of bend radius to pipe radius are 7:1 and 20:1, corresponding with the configurations examined experimentally by Talbot and his co-workers in recent years. The semi-elliptic treatment facilitates a much finer grid than has been possible in earlier studies. Numerical accuracy has been further improved by assuming radial equilibrium over a thin sublayer immediately adjacent to the wall and by re-formulating the boundary conditions at the pipe centre.
Streamwise velocity profiles at Dean numbers of 183 and 565 are in excellent agreement with laser-Doppler measurements by Agrawal, Talbot & Gong (1978). Good, albeit less complete, accord is found with the secondary velocities, though the differences that exist may be mainly due to the difficulty of making these measurements. The paper provides new information on the behaviour of the streamwise shear stress around the inner line of symmetry. Upstream of the point of minimum shear stress, our numerical predictions display a progressive shift towards the result of Stewartson, Cebici & Chang (1980) as the Dean number is successively raised. Downstream of the minimum, however, in contrast with the monotonic approach to an asymptotic level reported by Stewartson, the numerical solutions display a damped oscillatory behaviour reminiscent of those from Hawthorne's (1951) inviscid-flow calculations. The amplitude of the oscillation grows as the Dean number is raised.