15:00
Wall-bounded flows 2
Chair: Stefan Hickel
15:00
15 mins
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EXPERIMENTAL CHARACTERISATION OF LARGE SCALE STRUCTURES IN A HIGH REYNOLDS NUMBER TURBULENT BOUNDARY LAYER
Sricharan Srinath, Christophe Cuvier, Jean-Marc Foucaut, Jean-Philippe Laval
Abstract: A very large field of view (4δ x 1δ) with a good spatial resolution owing to the use of four 2k x 2k pixel cameras was conducted in a flat plate boundary layer at two Reynolds numbers (Reθ ≈7,500 and 20,000). Comparing the flow statistics with previously obtained hot-wire data under similar flow conditions show good agreement. The goal of this experiment is to detect and characterise the large scale motions which develop in the log region of a high Reynolds number turbulent boundary layer.
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15:15
15 mins
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Paper cancelled/moved
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Abstract: Paper cancelled/moved
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15:30
15 mins
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EXPERIMENTAL INVESTIGATION OF GEOMETRY ON TORQUE HYSTERESIS BEHAVIOUR OF TAYLOR-COUETTE FLOW
Melika Gul, Gerrit E. Elsinga, Jerry Westerweel
Abstract: This study investigates the effect of the Taylor-Couette geometry, namely the aspect ratio, Γ, and the gap width ratio, ƞ, on the torque hysteresis reported by [1]. Measurements were taken at two shear Reynolds numbers, Res, of 5.5x104 and 9x104.The torque was recorded while decreasing or increasing the rotation frequency ratio a=-fo/fi, keeping the rotation frequency difference fi-fo constant, where fo and fi are the outer and inner cylinder rotation frequencies, respectively. Results obtained for Γ=22 and ƞ=0.917 at Res of 5.5x104 and 9x104 show hysteresis occurring in the torque data. Interestingly the hysteresis behaviour is opposite to what was reported before [1], ie. torque is highest for decreasing a. Further studies will be performed to understand whether this hysteresis is due to the geometry itself and whether there is a relation between the this phenomenon and the flow structures of the Taylor-Couette flow.
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15:45
15 mins
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Recent PIV Studies in Fully Developed Pipe Flow at High Reynolds Numbers
Emir Onguner
Abstract: Investigating the coherent structures incl. large-scale and very large-scale motions is still progressing and the definition for the sizes of these structures at high Reynolds numbers in fully developed pipe flow is not clear. Recent studies with intrusive measurement techniques show that the streamwise extension of these structures is highly dependent on the Re-number. They are usually represented in wave length (λ) or wave number (k). It can be observed that the sizes of these coherent structures can reach even couple of meters along the pipe axis. This phenomenon is going to be investigated with Particle Image Velocimetry (PIV) as a non-intrusive measurement technique to validate and compare the results of Hot-Wire Anemometry (HWA).
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16:00
15 mins
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Turbulent structures in unsteady wall-bounded flow subject to temporal acceleration
Zhixin Wang, Tariq Talha, Yongmann Chung
Abstract: Direct numerical simulations (DNS) of a transient turbulent channel flow subject to constant temporal acceleration have been performed with a final Reynolds number of $\Retau=800$. The response of turbulent structures to the temporal acceleration is investigated. A significant delay in the response of turbulent flow is observed in various turbulent properties. It is found that the response of turbulent flow to temporal acceleration consists of two stages: the destruction of the initial \emph{old} turbulence, followed by the generation of \emph{new} turbulence associated with a higher $Re$ number. The \emph{new} turbulence is much stronger than the \emph{old} turbulence.
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16:15
15 mins
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Turbulent separation in lower curved wall channels
Jean-Paul Mollicone, Francesco Battista, Carlo Massimo Casciola
Abstract: Turbulent boundary layer separation in channels with a lower curved wall is studied using direct numerical simulations (DNS). Turbulence dynamics are studied through classical statistical tools such as the turbulent kinetic energy budget for varying lower curved wall dimensions. The geometry features are expected to have a significant effect on the fluid flow structures and the characteristic scales of separation. The separation bubble behind the bump is studied in terms of its size, turbulent kinetic energy production mechanisms and transfer and scale-by-scale energy budget. New innovative data-analysis techniques will be used based on the generalisation of the Kolmogorov equation to anisotropic and spatially non-homogeneous flow configurations.
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16:30
15 mins
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NUMERICAL SIMULATION OF NON-PREMIXED LEAN METHANE-AIR TURBULENT COMBUSTION IN A HIGH SWIRL BURNER
Javier Martin, Teresa Parra, Francisco Castro
Abstract: Obtaining high efficiency while keeping low levels of NOx emissions is a problem in burner technology that has attracted a lot of attention recently. A swirling flow in combustion ensures a fixed position of a compact flame. In this paper we present simulations of the reacting flow of a non-premixed lean methane/air mixture confined in a high swirl burner. RANS equations are solved with a finite volume method and a SIMPLE algorithm for pressure-velocity coupling. The mesh is composed of more than one million cells. Turbulence was modeled using a RNG K-epsilon using the swirl-dominated version. A PDF was used to model the combustion.
Local properties of the mixture were calculated based on the temperature and local composition. These calculations were developed with the commercial software ANSYS Fluent. The results obtained from this study show that the Inner Recirculation Zone (IRZ) plays a major role in stabilizing the location of the flame in the shear layer between the IRZ and Outer Recirculation Zone (ORZ). In conclusion, the swirling motion allows a more stable combustion of lean mixtures, reducing the fuel consumption and NOx emissions.
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16:45
15 mins
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Detached coherent structures in channel revisited: comparison with homogeneous shear turbulence
Siwei Dong, Adrián Lozano Durán, Atsushi Sekimoto, Javier Jiménez
Abstract: Coherent structures in statistically-stationary homogeneous shear turbulence (HST) are compared with those of the detached family in channels. Similarly to attached ones, detached Qs in channels form streamwise trains of side-by-side groups of a Q2 and a Q4. This is also true in HST. Contrary to attached structures, but similarly to those in HST, detached Q4s in channels are comparable in size to their related high-streamwise velocity streaks. Vortex clusters tend to associate with Q2s and Q4s, equally distributed between them in HST but more closely with Q2s in channels. The results strongly suggest that coherent structures in channels are not particularly associated with the wall, or even with a given shear profile.
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