| 1. |
|
|
| 2. |
- Talamelli, Alessandro, et al.
(författare)
-
CICLoPE-a response to the need for high Reynolds number experiments
- 2009
-
Ingår i: Fluid Dynamics Research. - : IOP Publishing. - 0169-5983 .- 1873-7005. ; 41:2
-
Tidskriftsartikel (refereegranskat)abstract
- Although the equations governing turbulent flow of fluids are well known, understanding the overwhelming richness of flow phenomena, especially in high Reynolds number turbulent flows, remains one of the grand challenges in physics and engineering. High Reynolds number turbulence is ubiquitous in aerospace engineering, ground transportation systems, flow machinery, energy production (from gas turbines to wind and water turbines), as well as in nature, e.g. various processes occurring in the planetary boundary layer. High Reynolds number turbulence is not easily obtained in the laboratory, since in order to have good spatial resolution for measurements, the size of the facility itself has to be large. In this paper, we discuss limitations of various existing facilities and propose a new facility that will allow good spatial resolution even at high Reynolds number. The work is carried out in the framework of the Center for International Cooperation in Long Pipe Experiments (CICLoPE), an international collaboration that many in the turbulence community have shown an interest to participate in.
|
|
| 3. |
- Österlund, Jens, M., et al.
(författare)
-
A note on the overlap region in turbulent boundary layers
- 2000
-
Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; :12, s. 1-4
-
Tidskriftsartikel (refereegranskat)abstract
- Two independent experimental investigations of the behavior of turbulent boundary layers with increasing Reynolds number were recently completed. The experiments were performed in two facilities, the Minimum Turbulence Level (MTL) wind tunnel at Royal Institute of Technology (KTH) and the National Diagnostic Facility (NDF) wind tunnel at Illinois Institute of Technology (IIT). Both experiments utilized oil-film interferometry to obtain an independent measure of the wall-shear stress. A collaborative study by the principals of the two experiments, aimed at understanding the characteristics of the overlap region between the inner and outer parts of the boundary layer, has just been completed. The results are summarized here, utilizing the profiles of the mean velocity, for Reynolds numbers based on the momentum thickness ranging from 2500 to 27 000. Contrary to the conclusions of some earlier publications, careful analysis of the data reveals no significant Reynolds number dependence for the parameters describing the overlap region using the classical logarithmic relation. However, the data analysis demonstrates that the viscous influence extends within the buffer region to y+≈200, compared to the previously assumed limit of y+≈50.Therefore, the lowest Reθ value where a significant logarithmic overlap region exists is about 6000. This probably explains why a Reynolds number dependence had been found from the data analysis of many previous experiments. The parameters of the logarithmic overlap region are found to be constant and are estimated to be κ=0.38, B=4.1 and B1=3.6 (δ=δ95).
|
|
| 4. |
|
|
| 5. |
- Stuck, Maxime, et al.
(författare)
-
Spectral-Element Simulation of the Turbulent Flow in an Urban Environment
- 2021
-
Ingår i: Applied Sciences. - : MDPI AG. - 2076-3417. ; 11:14
-
Tidskriftsartikel (refereegranskat)abstract
- The mean flow and turbulence statistics of the flow through a simplified urban environment, which is an active research area in order to improve the knowledge of turbulent flow in cities, is investigated. This is useful for civil engineering, pedestrian comfort and for health concerns caused by pollutant spreading. In this work, we provide analysis of the turbulence statistics obtained from well-resolved large-eddy simulations (LES). A detailed analysis of this database reveals the impact of the geometry of the urban array on the flow characteristics and provides for a good description of the turbulent features of the flow within a simplified urban environment. The most prominent features of this complex flow include coherent vortical structures such as the so-called arch vortex, the horseshoe vortex and the roof vortex. These structures of flow have been identified by an analysis of the turbulence statistics. The influence of the geometry of urban environment (and particularly the street width and the building height) on the overall flow behavior has also been studied. Finally, the well-resolved LES results were compared with an available experimental database to discuss differences and similarities between the respective urban configurations.
|
|
| 6. |
- Vinuesa, Ricardo, et al.
(författare)
-
Alternative interpretation of the Superpipe data and motivation for CICLoPE : The effect of a decreasing viscous length scale
- 2016
-
Ingår i: European journal of mechanics. B, Fluids. - : Elsevier. - 0997-7546 .- 1873-7390. ; 58, s. 109-116
-
Tidskriftsartikel (refereegranskat)abstract
- Pressurization and cryogenic conditions have been used in some experiments to change the kinematic viscosity v of the flowing gas by many orders of magnitude in order to achieve high Reynolds number conditions in facilities of limited size. This leads to a substantial reduction of the viscous length scale l* = v/u(tau), as in the so-called Princeton "Superpipe" experiments, We demonstrate that the limited dimensions of the facilities and probes can lead to inaccuracies in the near-wall measurements for increasing Reynolds number. Specifically, a lack of accurate wall-normal probe positioning is simulated using three different datasets of wall-bounded turbulent flows. Relatively large errors in the overlap region parameters are observed for position errors of small physical magnitude that become greatly amplified in wall units as l* is reduced. This offers an alternative interpretation to some of the key findings reported by the Superpipe team, such as the increasing lower limit of the logarithmic region y(log,min)(+), the existence of a power law region between the wall and the logarithmic layer, and the "mixing transition" phenomenon in wall bounded turbulence.
|
|
| 7. |
- Vinuesa, Richardo, et al.
(författare)
-
Aspect ratio effects in turbulent duct flows studied through direct numerical simulation
- 2014
-
Ingår i: Journal of Turbulence. - : Informa UK Limited. - 1468-5248. ; 15:10, s. 677-706
-
Tidskriftsartikel (refereegranskat)abstract
- Three-dimensional effects in turbulent duct flows, i.e., sidewall boundary layers and secondary motions, are studied by means of direct numerical simulation (DNS). The spectral element code Nek5000 is used to compute turbulent duct flows with aspect ratios 1-7 (at Re-b,Re- c = 2800, Re-tau,Re- c similar or equal to 180) and aspect ratio 1 (at Re-b,Re- c = 5600, Re-tau,Re- c similar or equal to 330), in streamwise-periodic boxes of length 25h. The total number of grid points ranges from 28 to 145 million, and the pressure gradient is adjusted iteratively in order to keep the same bulk Reynolds number in the centreplane with changing aspect ratio. Turbulence is initiated via a trip forcing active during the initial stages of the simulation, and the statistical convergence of the data is discussed both in terms of transient approach and averaging period. Spanwise variations in wall shear, mean-flow profiles, and turbulence statistics are analysed as a function of aspect ratio, and also compared with the spanwise-periodic channel (as idealisation of an infinite aspect ratio duct). The computations show good agreement with experimental measurements carried out in parallel at the Illinois Institute of Technology (IIT) in Chicago, and highlight the relevance of sidewall boundary layers and secondary vortices in the physics of the duct flow. The rich array of secondary vortices extending throughout the upper and lower walls of the duct, and their dependence on Reynolds number and aspect ratio, had not been reported in the literature before.
|
|
| 8. |
- Vinuesa, Ricardo, et al.
(författare)
-
Convergence of numerical simulations of turbulent wall-bounded flows and mean cross-flow structure of rectangular ducts
- 2016
-
Ingår i: Meccanica (Milano. Print). - : Springer Netherlands. - 0025-6455 .- 1572-9648. ; 51:12, s. 3025-3042
-
Tidskriftsartikel (refereegranskat)abstract
- Convergence criteria for direct numerical simulations of turbulent channel and duct flows are proposed. The convergence indicator for channels is defined as the deviation of the nondimensional total shear-stress profile with respect to a linear profile, whereas the one for the duct is based on a nondimensional streamwise momentum balance at the duct centerplane. We identify the starting () and averaging times () necessary to obtain sufficiently converged statistics, and also find that optimum convergence rates are achieved when the spacing in time between individual realizations is below . The in-plane structure of the flow in turbulent ducts is also assessed by analyzing square ducts at and 360 and rectangular ducts with aspect ratios 3 and 10 at . Identification of coherent vortices shows that near-wall streaks are located in all the duct cases at a wall-normal distance of as in Pinelli et al. (J Fluid Mech 644:107-122, 2010). We also find that large-scale motions play a crucial role in the streamline pattern of the secondary flow, whereas near-wall structures highly influence the streamwise vorticity pattern. These conclusions extend the findings by Pinelli et al. to other kinds of large-scale motions in the flow through the consideration of wider ducts. They also highlight the complex and multiscale nature of the secondary flow of second kind in turbulent duct flows.
|
|
| 9. |
- Vinuesa, Ricardo, et al.
(författare)
-
Enhancing the accuracy of measurement techniques in high Reynolds number turbulent boundary layers for more representative comparison to their canonical representations
- 2016
-
Ingår i: European journal of mechanics. B, Fluids. - : Elsevier BV. - 0997-7546 .- 1873-7390. ; 55, s. 300-312
-
Tidskriftsartikel (refereegranskat)abstract
- Existing differences between experimental, computational and theoretical representations of a particular flow do not allow one-to-one comparisons, prevent us from identifying the absolute contributions of the various sources of uncertainty in each approach, and highlight the importance of developing suitable corrections for experimental techniques. In this study we utilize the latest Pitot tube correction schemes to develop a technique which improves on the outcome of hot-wire measurements of mean velocity profiles in ZPG turbulent boundary layers over the range 11 500 < Re-theta < 21 500. Measurements by Bailey et al. (2013), carried out with probes of diameters ranging from 0.2 to 1.89 mm, supplemented by other data with larger diameters up to 12.82 mm, are used first to develop a somewhat improved Pitot tube correction which is based on viscous, shear and near-wall schemes (which contribute with around 85% of the effect), together with a turbulence scheme which accounts for 15% of the whole correction. The correction proposed here leads to similar agreement with available high-quality datasets in the same Reynolds number range as the one proposed by Bailey et al. (2013), but this is the first time that the contribution of the turbulence scheme is quantified. In addition, four available algorithms to correct wall position in hot-wire measurements are tested, using as benchmark the corrected Pitot tube profiles with artificially simulated probe shifts and blockage effects. We find that the kappa B-Musker correction developed in this study produces the lowest deviations with respect to the introduced shifts. Unlike other schemes, which are based on a prescribed near-wall region profile description, the kappa B-Musker is focused on minimizing the deviation with respect to the (kappa) over tilde(B) over tilde relation, characteristic of wall-bounded turbulent flows. This general approach is able to locate the wall position in probe measurements of the wall-layer profiles with around one half the error of the other available methods. The difficulties encountered during the development of adequate corrections for high-Re boundary layer measurements highlight the existing gap between the conditions that can be reproduced and measured in the laboratory and the so-called canonical flows.
|
|
| 10. |
- Vinuesa, Ricardo, et al.
(författare)
-
Experiments and Computations of Localized Pressure Gradients with Different History Effects
- 2014
-
Ingår i: AIAA Journal. - 0001-1452 .- 1533-385X. ; 52:2, s. 368-384
-
Tidskriftsartikel (refereegranskat)abstract
- The study of high-Reynolds-number wall-bounded turbulent flows has become a very active area of research in the past decade, where several recent results have challenged current understanding. In this study, four different localized pressure gradient configurations are characterized by computing them using four Reynolds-averaged Navier-Stokes turbulence models (Spalart-Allmaras, k-epsilon, shear stress transport, and the Reynolds stress model) and comparing their predictions with experimental measurements of mean flow quantities and wall shear stress. The pressure gradients were imposed on high-Reynolds-number, two-dimensional turbulent boundary layers developing on a flat plate by changing the ceiling geometry of the test section. The computations showed that the shear stress transport model produced the best agreement with the experiments. It was found that what is called "numerical transition" (a procedure by which the laminar boundary conditions are transformed into inflow conditions to characterize the initial turbulent profile) causes the major differences between the various models, thereby highlighting the need for models representative of true transition in computational codes. Also, both experiments and computations confirm the nonuniversality of the von Karman coefficient kappa. Finally, a procedure is demonstrated for simpler two-dimensional computations that can be representative of flows with some mild three-dimensional geometries.
|
|
