10:30
15 mins
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A NEW SCALING FOR ADVERSE PRESSURE GRADIENT TUBULENT BOUNDARY LAYERS
Flint Thomas, David Schatzman
Abstract: A new scaling for strong adverse pressure gradient (APG) turbulent boundary layers (TBL) is presented. The new scaling is applied to both the author’s unsteady and steady APG TBL experiments as well as several previously published studies. The scaling is shown to provide a remarkable collapse of the mean velocity profiles in each case. The new scaling is motivated by the recognition that the physics of the strong APG TBL is dominated by the inflectional instability of an embedded shear layer within the boundary layer. The implications of the scaling on the physics of APG TBL flows in general is also discussed.
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10:45
15 mins
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Combined effects of pressure gradient and buoyancy in the boundary layer of a turbulent convection flow
Mikhail Ovsyannikov, Dmitry Krasnov, Mohammad Emran, Jörg Schumacher
Abstract: The new method for approximating the velocity and temperature of a flow within the boundary layers is developed by applying the combination of the Falkner-Skan approach and perturbation theory.
The former enables to include non-zero pressure gradient along a heated horizontal plate where the flow is considered and the latter gives an opportunity to take into account buoyancy effects caused by the temperature difference between the hot plate and the flow above it. It is assumed that buoyancy effects are small. The mathematical model of the developed method includes four ordinary differential equations which are solved numerically. The approach is adapted to Rayleigh-Benard convection considered in a cylindrical cell at aspect ratio one. The results obtained by the mathematical model and by direct numerical simulations of Rayleigh-Benard convection are compared and are presented together with the conclusions made. The simulations were conducted for a closed cylindrical cell of aspect ratio one at the Rayleigh number Ra=3x10^9 and the Prandtl number Pr=0.7 and Pr=7.0.
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11:00
15 mins
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Relation of skewness factor and convection velocity in turbulent boundary layer
Artur Drozdz, Witold Elsner
Abstract: The paper is devoted to prove the relation between skewness factor and convection velocity in turbulent boundary layer. It appears that skewness factor can be used as an indicator of convection velocity of coherent structures, which is not always equal to the average flow velocity. The analysis has been performed based upon velocity profiles measured with hot-wire technique in turbulent boundary layer with pressure gradient corresponding to turbomachinery conditions. The results show that the cross product term of skewness factor decomposed by spectral filtering, which is also alternative measure of amplitude modulation, describes the convection velocity in zero pressure gradient turbulent boundary layer.
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11:15
15 mins
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CHARACTERISTIC DISTRIBUTION AND SCALE INTERACTION OF TURBULENCE IN A BOUNDARY LAYER
Patrick Bechlars, Richard D. Sandberg
Abstract: This work revisits the concept of turbulent boundary layers from a novel perspective on scale transfer. Turbulence production
and dissipation together with the energy budgets are analyzed in the velocity gradient invariant phase space. In combination with
filtering, the mechanism of scale coupling is investigated and illustrated for different characteristic flow topologies. The understanding
of the scale coupling is important to model turbulence. Turbulence models describe the complex interaction of the scales of motion
in a simplified form. The essential task of turbulence modeling is to capture the coupling of the modeled and unmodeled scales as
well as the evolution of the modeled scales within the unmodeled flow. This work characterizes the scale coupling by focusing on the
interfaces between modeled and unmodeled flow such as production and dissipation. The mechanisms that govern the evolution of the
modeled quantities are investigated for their core properties and universal features. Direct numerical simulation (DNS) is carried out
to obtain data of a compressible zero pressure-gradient flat plate turbulent boundary layer flow. This flow topology allows to unveil the
effect of a wall on the coupling of scales and evolution of turbulence.
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11:30
15 mins
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Characteristics of overlap region in high-Reynolds number turbulent channel flow
Yoshinobu Yamamoto, Yoshiyuki Tsuji
Abstract: Direct numerical simulation of the fully developed turbulent channel flows have been carried out at the Reynolds number based on the friction velocity and the channel half width, 2000, 4000 and 8000. A hybrid 10th order accurate finite difference scheme in the stream and spanwise directions, and a second-order scheme in the wall-normal direction is adapted as the spatial discretization method. We observed the plateau profiles in the indicator function corresponded to the von Karman constant. Furthermore, second peak of streamwise pre-multiplied spectra were appeared in the same wall normal height, 300 < y+ < 600, in case of Re = 4000. Nevertheless, the effects of the lager than the channel half height scale on the streamwise turbulent intensity are fixed contributions without dependence on Reynolds number. These results suggested that the new streamwise vortexes are formed between buffer layer and outer layer with increasing of Reynolds number.
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11:45
15 mins
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Experiments on the interaction between hydrodynamic turbulence and free-surface waves
Timothée Jamin, Michael Berhanu, Eric Falcon
Abstract: We investigate interaction between hydrodynamic turbulence and a free surface in a meter scale basin. An intense, homogeneous and isotropic turbulence is generated by an 8x8 array of jets pointing upwards at the bottom of the basin. We study free-surface deformations induced by hydrodynamic turbulence. In a second stage, we will study the mutual interaction between hydrodynamic turbulence and waves generated by an electromechanical shaker at the free surface: attenuation or amplification of free-surface waves by hydrodynamic turbulence and energy exchange in particular between hydrodynamic turbulence and wave turbulence.
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12:00
15 mins
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Effect of viscosity and density gradients on turbulent channel flows
Ashish Patel, Jurriaan Peeters, Bendiks Jan Boersma, Rene Pecnik
Abstract: We perform Direct Numerical Simulations (DNS) of a turbulent channel flow with temperature dependent density and viscosity. The Navier-Stokes equations are solved using their low Mach number formulation. In the simulations performed, the fluid is internally heated and the temperature at the walls is fixed. The friction Reynolds number based on half channel height and wall friction velocity is $Re_\tau = 395$. The modulation of turbulence, which is caused by the density and viscosity gradients, is characterized using the semi-local scaling of Huang et al. [1995, JFM].
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12:15
15 mins
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ON THE INSTABILITY OF FLOW IN A GROOVED CHANNEL
A. Mohammadi, J.M. Floryan
Abstract: It is shown that flow in a channel with longitudinal grooves is subject to two types of instabilities. The modified classical TS instability represents the first class while an inviscid instability associated with the groove-induced flow modulations represents
the second class. The second instability dominates for grooves with the appropriate wave numbers and amplitudes.
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