scholarly journals USING TWO TIME SCALES OF TURBULENCE FOR BOUNDARY LAYER FLOWS

2017 ◽  
Vol 19 (2) ◽  
pp. 45-55
Author(s):  
Nawaf H. Saeid
Keyword(s):  
Experimental Data ◽  
Boundary Layer ◽  
Asymptotic Behavior ◽  
Turbulent Boundary Layer ◽  
Lower Limit ◽  
Time Scale ◽  
Present Model ◽  
Wall Region ◽  
Boundary Layer Flows ◽  
Near Wall

A near wall improvement of the k-ε model of turbulence is proposed and evaluated. The present model takes into account the asymptotic behavior of turbulent scales near wall region and for free turbulence region. The Kolmogorov turbulent time scale is introduced as a lower limit. The model is used for prediction of turbulent boundary layer flows. Predictions compared with experimental data of several flow cases, with encouraging results.

scholarly journals Active control of multiscale features in wall-bounded turbulence

2019 ◽  
Vol 36 (1) ◽  
pp. 12-21 ◽  
Author(s):  
Xiaotong Cui ◽  
Nan Jiang ◽  
Xiaobo Zheng ◽  
Zhanqi Tang
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Turbulent Energy ◽  
Streamwise Velocity ◽  
Discrete Wavelet ◽  
Wall Region ◽  
Mean Velocity ◽  
Pzt Actuator ◽  
The Impact ◽  
Near Wall

Abstract This study experimentally investigates the impact of a single piezoelectric (PZT) actuator on a turbulent boundary layer from a statistical viewpoint. The working conditions of the actuator include a range of frequencies and amplitudes. The streamwise velocity signals in the turbulent boundary layer flow are measured downstream of the actuator using a hot-wire anemometer. The mean velocity profiles and other basic parameters are reported. Spectra results obtained by discrete wavelet decomposition indicate that the PZT vibration primarily influences the near-wall region. The turbulent intensities at different scales suggest that the actuator redistributes the near-wall turbulent energy. The skewness and flatness distributions show that the actuator effectively alters the sweep events and reduces intermittency at smaller scales. Moreover, under the impact of the PZT actuator, the symmetry of vibration scales’ velocity signals is promoted and the structural composition appears in an orderly manner. Probability distribution function results indicate that perturbation causes the fluctuations in vibration scales and smaller scales with high intensity and low intermittency. Based on the flatness factor, the bursting process is also detected. The vibrations reduce the relative intensities of the burst events, indicating that the streamwise vortices in the buffer layer experience direct interference due to the PZT control.

Experimental observation of hairpin auto-generation events in a turbulent boundary layer

2016 ◽  
Vol 795 ◽  
pp. 611-633 ◽  
Author(s):  
Y. Jodai ◽  
G. E. Elsinga
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Shear Layer ◽  
Transition Process ◽  
Wall Region ◽  
Generation Model ◽  
Streamwise Vortices ◽  
Hairpin Vortices ◽  
Time Resolved ◽  
Near Wall

Time-resolved tomographic particle image velocimetry experiments show that new hairpin vortices are generated within a fully developed and unperturbed turbulent boundary layer. The measurements are taken at a Reynolds number based on the momentum thickness of 2038, and cover the near-wall region below $y^{+}=140$, where $y^{+}$ is the wall-normal distance in wall units. Instantaneous visualizations of the flow reveal near-wall low-speed streaks with associated quasi-streamwise vortices, retrograde inverted arch vortices, hairpin vortices and hairpin packets. The hairpin heads are observed as close to the wall as $y^{+}=30$. Examples of hairpin packet evolution reveal the development of new hairpin vortices, which are created upstream and close to the wall in a manner consistent with the auto-generation model (Zhou et al., J. Fluid Mech., vol. 387, 1999, pp. 353–396). The development of the new hairpin appears to be initiated by an approaching sweep event, which perturbs the shear layer associated with the initial packet. The shear layer rolls up, thereby forming the new hairpin head. The head subsequently connects to existing streamwise vortices and develops into a hairpin. The time scale associated with the hairpin auto-generation is 20–30 wall units of time. This demonstrates that hairpins can be created over short distances within a developed turbulent boundary layer, implying that they are not simply remnants of the laminar-to-turbulent transition process far upstream.

POD Investigation of Near-Wall Turbulent Structures

2002 ◽  
Author(s):  
Gaetano Maria Di Cicca ◽  
Angelo Iollo ◽  
Pier Giorgio Spazzini ◽  
Gaetano Iuso ◽  
Michele Onorato
Keyword(s):  
Experimental Data ◽  
Boundary Layer ◽  
Particle Image ◽  
Digital Particle Image Velocimetry ◽  
Orthogonal Decomposition ◽  
Wall Region ◽  
Turbulent Structures ◽  
Image Velocimetry ◽  
Proper Orthogonal ◽  
Near Wall

Experimental data of a turbulent boundary layer developing over a flat plate, obtained by Digital Particle Image Velocimetry (DPIV) technique, are analyzed making use of proper orthogonal decomposition (POD). Different POD definitions have been used in order to check their ability in educing the various structures dominating the near wall region. Results show a specific sensitivity depending on the POD definition adopted.

Numerical Investigation of the Compressible Flat-Plate Turbulent Boundary Layer with Extended GAO-YONG Turbulence Model

2013 ◽  
Vol 444-445 ◽  
pp. 416-422
Author(s):  
Yang Yang Tang ◽  
Zhi Qiang Li ◽  
Yong Wang ◽  
Ya Chao Di ◽  
Huan Xu ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Flat Plate ◽  
Turbulence Model ◽  
Turbulence Models ◽  
Wall Region ◽  
Flow And Heat Transfer ◽  
Empirical Coefficients ◽  
Near Wall

The extended GAO-YONG turbulence model is used to simulate the flow and heat transfer of flat-plate turbulent boundary layer, and the results indicate that GAO-YONG turbulence model may well describe boundary layer flow and heat transfer from near-wall region to far outer area, without using any empirical coefficients and near-wall treatments, such as wall-function or modified low Reynolds number model, which are used widely in all RANS turbulence models.

Mechanisms for particle transfer and segregation in a turbulent boundary layer

2002 ◽  
Vol 468 ◽  
pp. 283-315 ◽  
Author(s):  
CRISTIAN MARCHIOLI ◽  
ALFREDO SOLDATI
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Coherent Structures ◽  
Wall Region ◽  
Heavy Particles ◽  
Low Speed ◽  
Regeneration Cycle ◽  
Low Speed Streaks ◽  
Transfer Mechanisms ◽  
Near Wall

Particle transfer in the wall region of turbulent boundary layers is dominated by the coherent structures which control the turbulence regeneration cycle. Coherent structures bring particles toward and away from the wall and favour particle segregation in the viscous region, giving rise to non-uniform particle distribution profiles which peak close to the wall. The object of this work is to understand the reasons for higher particle concentration in the wall region by examining turbulent transfer of heavy particles to and away from the wall in connection with the coherent structures of the boundary layer. We will examine the behaviour of a dilute dispersion of heavy particles – flyashes in air – in a vertical channel flow, using pseudo-spectral direct numerical simulation to calculate the turbulent flow field at a shear Reynolds number Reτ = 150, and Lagrangian tracking to describe the dynamics of particles. Drag force, gravity and Saffman lift are used in the equation of motion for the particles, which are assumed to have no influence on the flow field. Particle interaction with the wall is fully elastic. As reported in several previous investigations, we found that particles are transferred by sweeps – Q2 type events – in the wall region, where they preferentially accumulate in the low-speed streak environments, whereas ejections – Q4 type events – transfer particles from the wall region to the outer flow. We quantify the efficiency of the instantaneous realizations of the Reynolds stresses events in transferring different size particles to the wall and away from the wall, respectively. Our findings confirm that sweeps and ejections are efficient transfer mechanisms for particles. In particular, we find that only those sweep and ejection events with substantial spatial coherence are effective in transferring particles. However, the efficiency of the transfer mechanisms is conditioned by the presence of particles to be transferred. In the case of ejections, particles are more rarely available since, when in the viscous wall layer, they are concentrated under the low-speed streaks. Even though the low-speed streaks are ejection-like environments, particles remain trapped for a long time. This phenomenon, which causes accumulation of particles in the near-wall region, can be interpreted in terms of overall fluxes toward and away from the wall by the theory of turbophoresis. This theory, proposed initially by Caporaloni et al. (1975) and re-examined later by Reeks (1983), can help to explain the existence of net particle fluxes toward the wall as a manifestation of the skewness in the velocity distribution of the particles (Reeks 1983). To understand the local and instantaneous mechanisms which give rise to the phenomenon of turbophoresis, we focus on the near-wall region of the turbulent boundary layer. We examine the role of the rear-end of a quasistreamwise vortex very near to the wall in preventing particles in the proximity of the wall from being re-entrained by the pumping action of the large, farther from the wall, forward-end of a following quasi-streamwise vortex. We examine several mechanisms for turbulence structures near the wall and we find that the mechanism based on the archetypal quasi-streamwise structures identified by Schoppa & Hussain (1997), the parent–offspring regeneration cycle for near-wall quasi-streamwise vortices discussed by Brooke & Hanratty (1993), and the mechanism based on coherent packets of hairpin vortices, the fundamental super-structure characterized by Adrian, Meinhart & Tomkins (2000), all depict the same characteristic pattern which is responsible for particle trapping very near to the wall.

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