“…2016; Hearst et al. 2021). Figure 7.The instantaneous field of the streamwise velocity gradient for figure 6( a ) in the lower half of the channel.…”
Section: Core Identificationmentioning
confidence: 99%
“…2018; Hearst et al. 2021; Laskari & McKeon 2021) as well as turbulent channel (Kwon et al. 2014; Yang, Hwang & Sung 2016; Jie et al.…”
Section: Introductionmentioning
confidence: 99%
“…(2017), Hearst, Dogan & Ganapathisubramani (2018), Hearst et al. (2021), Jooss et al. (2021) and, recently, using DNS, e.g.…”
Section: Introductionmentioning
confidence: 99%
“…Recently, Hearst et al. (2021) analysed the instantaneous structure of a ZPG-TBL under the influence of FST using the PIV data of Dogan et al. (2019).…”
The impact of inlet turbulence on the structure of turbulent channel flow is investigated using particle image velocimetry. Streamwise–wall-normal plane measurements are performed in a channel, where different turbulence intensities were generated at the inlet with an active grid. Four cases are tested with matched centreline mean velocities, while the centreline turbulence intensities ranged from 3.7 % for the reference case, up to 6.4 %. The friction velocity is found to be approximately constant with varying centreline turbulence intensities, resulting in a matched friction Reynolds number of
$Re_\tau \approx 770$
for all cases, which contrasts with similar experiments performed in a zero-pressure-gradient boundary layer. The log region remains intact for all cases. The so-called quiescent core of the turbulent channel flow is also investigated. In addition to increased core discontinuity, the increased fluctuations of the streamwise velocity give rise to new core states, which differ from the conventional ones in their characteristic velocity. They are associated with a bulk of low- or high-momentum fluid passing through the measurement domain, and their occurrence increases with turbulence intensity. Tracking the core boundaries indicates an overall tendency of the core to move closer to the wall for increased inlet turbulence intensities, resulting in an increased core thickness. Moreover, it is found that the low-momentum cores generally reside closer to the wall compared with the ordinary cores and appear to be thicker than them, whereas the opposite, i.e. residing farther from the wall and being thinner, is true for the high-momentum cores.
“…2016; Hearst et al. 2021). Figure 7.The instantaneous field of the streamwise velocity gradient for figure 6( a ) in the lower half of the channel.…”
Section: Core Identificationmentioning
confidence: 99%
“…2018; Hearst et al. 2021; Laskari & McKeon 2021) as well as turbulent channel (Kwon et al. 2014; Yang, Hwang & Sung 2016; Jie et al.…”
Section: Introductionmentioning
confidence: 99%
“…(2017), Hearst, Dogan & Ganapathisubramani (2018), Hearst et al. (2021), Jooss et al. (2021) and, recently, using DNS, e.g.…”
Section: Introductionmentioning
confidence: 99%
“…Recently, Hearst et al. (2021) analysed the instantaneous structure of a ZPG-TBL under the influence of FST using the PIV data of Dogan et al. (2019).…”
The impact of inlet turbulence on the structure of turbulent channel flow is investigated using particle image velocimetry. Streamwise–wall-normal plane measurements are performed in a channel, where different turbulence intensities were generated at the inlet with an active grid. Four cases are tested with matched centreline mean velocities, while the centreline turbulence intensities ranged from 3.7 % for the reference case, up to 6.4 %. The friction velocity is found to be approximately constant with varying centreline turbulence intensities, resulting in a matched friction Reynolds number of
$Re_\tau \approx 770$
for all cases, which contrasts with similar experiments performed in a zero-pressure-gradient boundary layer. The log region remains intact for all cases. The so-called quiescent core of the turbulent channel flow is also investigated. In addition to increased core discontinuity, the increased fluctuations of the streamwise velocity give rise to new core states, which differ from the conventional ones in their characteristic velocity. They are associated with a bulk of low- or high-momentum fluid passing through the measurement domain, and their occurrence increases with turbulence intensity. Tracking the core boundaries indicates an overall tendency of the core to move closer to the wall for increased inlet turbulence intensities, resulting in an increased core thickness. Moreover, it is found that the low-momentum cores generally reside closer to the wall compared with the ordinary cores and appear to be thicker than them, whereas the opposite, i.e. residing farther from the wall and being thinner, is true for the high-momentum cores.
“…Since the turbulence intensity is negligible in the free-stream region and the velocity is fairly constant, it is possible to accurately determine δ and U ∞ in a zero-pressure-gradient TBL. Hearst et al (2021) noticed issues with this approach and determined the boundary layer thickness using the streamwise Reynolds normal stress profile. In open channel flow, U ∞ is commonly taken as the maximum velocity near the free surface.…”
A fully developed approach flow is necessary in open channel studies to maintain commonality among datasets obtained from different facilities. Two-component planar particle image velocimetry is used to study the characteristics of fully developed smooth open channel flow at a constant Reynolds number of 3.9 × 104 based on the maximum velocity and flow depth. The near-bed boundary layer is tripped to achieve a fully developed state and compared with the under- and over-tripped cases. The Reynolds stresses and higher-order moments are used as indicators to establish the fully developed state. Flow properties are explored by identifying uniform momentum zones (UMZs) using the probability density function of streamwise velocities. The instances are grouped based on the number of UMZs (NUMZ) and conditional averaging of flow variables of each group is used to evaluate the difference in flow properties between the developed and the developing flow. Large-scale ejections are found in the logarithmic layer when NUMZ is higher, whereas a lower number indicates the existence of large-scale sweeping motions. The distribution of the conditionally averaged ratio of the shear contribution from ejections and sweeps and velocity deficits shows a vertical variability in the fully developed state. The large-scale and pointwise quadrant events are used simultaneously to depict variability in inner flow properties between developing and fully developed flow which cannot be recognized in the mean flow characteristics. The sweep events have much higher shear generation in the outer flow in the fully developed state whereas the shear stress contribution from ejection is lower than that in developing flow.
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