201 research outputs found
Vertical Coherence of Turbulence in the Atmospheric Surface Layer: Connecting the Hypotheses of Townsend and Davenport
Statistical descriptions of coherent flow motions in the atmospheric boundary
layer have many applications in the wind engineering community. For instance,
the dynamical characteristics of large-scale motions in wall-turbulence play an
important role in predicting the dynamical loads on buildings, or the
fluctuations in the power distribution across wind farms. Davenport (Quarterly
Journal of the Royal Meteorological Society, 1961, Vol. 372, 194-211) performed
a seminal study on the subject and proposed a hypothesis that is still widely
used to date. Here, we demonstrate how the empirical formulation of Davenport
is consistent with the analysis of Baars et al. (Journal of Fluid Mechanics,
2017, Vol. 823, R2) in the spirit of Townsend's attached-eddy hypothesis in
wall turbulence. We further study stratification effects based on two-point
measurements of atmospheric boundary-layer flow over the Utah salt flats. No
self-similar scaling is observed in stable conditions, putting the application
of Davenport's framework, as well as the attached eddy hypothesis, in question
for this case. Data obtained under unstable conditions exhibit clear
self-similar scaling and our analysis reveals a strong sensitivity of the
statistical aspect ratio of coherent structures (defined as the ratio of
streamwise and wall-normal extent) to the magnitude of the stability parameter
On ranked and bounded Kohnert posets
In this paper, we explore combinatorial properties of the posets associated
with Kohnert polynomials. In particular, we determine a sufficient condition
guaranteeing when such ``Kohnert posets'' are bounded and two necessary
conditions for when they are ranked. Moreover, we apply the aforementioned
conditions to find complete characterizations of when Kohnert posets are
bounded and when they are ranked in special cases, including those associated
with Demazure characters
Quantifying inner-outer interactions in non-canonical wall-bounded flows
We investigate the underlying physics behind the change in amplitude
modulation coefficient in non-canonical wall-bounded flows in the framework of
the inner-outer interaction model (IOIM) (Baars et al., Phys. Rev. Fluids 1
(5), 054406). The IOIM captures the amplitude modulation effect, and here we
focus on extending the model to non-canonical flows. An analytical relationship
between the amplitude modulation coefficient and IOIM parameters is derived,
which is shown to capture the increasing trend of the amplitude modulation
coefficient with an increasing Reynolds number in a smooth-wall dataset. This
relationship is then applied to classify and interpret the non-canonical
turbulent boundary layer results reported in previous works. We further present
the case study of a turbulent boundary layer after a rough-to-smooth change.
Both single-probe and two-probe hotwire measurements are performed to acquire
streamwise velocity time series in the recovering flow on the downstream smooth
wall. An increased coherence between the large-scale motions and the
small-scale envelope in the near-wall region is attributed to the stronger
footprints of the over-energetic large-scale motions in the outer layer,
whereas the near-wall cycle and its amplitude sensitivity to the superposed
structures are similar to that of a canonical smooth-wall flow. These results
indicate that the rough-wall structures above the internal layer interact with
the near-wall cycle in a similar manner as the increasingly energetic
structures in a high-Reynolds number smooth-wall boundary layer
Transportation Barriers to Healthcare in Adults 65+ in the Greater Burlington Area
Introduction. Missed appointments often lead to poorer health care outcomes for patients and pose a major economic burden on medical centers. Transportation is an obstacle to accessing medical care for elderly patients in Vermont and results in delayed medical appointments.
Methods. We surveyed senior citizens in Chittenden county to determine both the type of transportation barriers and medical care missed due to the lack of transpor- tation. An original survey assessing the impact of transportation to health care was distributed in person and through an online platform. Participants were asked to identify the following in the past year: how often transportation was an issue for healthcare, specific barriers to transportation, and which specific health care appointments were missed due to lack of transportation. Ninety-six surveys out of a total of 251 collected were included in the analysis. Respondents were grouped into either having high transportation barriers, n=43, (always, often, sometimes had issues in the past year), or low transportation barriers, n=53, (rarely had issues).
Results. The high barriers group reported more missed appointments, with eye appointments being the most frequent, and depended more on other modes of trans- portation. The low barriers group was able to drive themselves to their appointments more often.
Conclusion. The results suggest a trend between barriers to transportation and a lack of access to healthcare appointments. Although more than half of the survey respondents indicated that they do not currently experience transportation barriers, many expressed concern about the transportation difficulties they could encounter in the future.https://scholarworks.uvm.edu/comphp_gallery/1263/thumbnail.jp
Recovery of wall-shear stress to equilibrium flow conditions after a rough-to-smooth step change in turbulent boundary layers
This paper examines the recovery of the wall-shear stress of a turbulent boundary layer that has undergone a sudden transition from a rough to a smooth surface. Early work of Antonia and Luxton (J. Fluid Mech., vol. 53, 1972, pp. 737–757) questioned the reliability of standard smooth-wall methods for measuring wall-shear stress in such conditions, and subsequent studies show significant disagreement depending on the approach used to determine the wall-shear stress downstream. Here we address this by utilising a collection of experimental databases at Reτ≈4100 that have access to both ‘direct’ and ‘indirect’ measures of the wall-shear stress to understand the recovery to equilibrium conditions of the new surface. Our results reveal that the viscous region ( z+≲4 ) recovers almost immediately to an equilibrium state with the new wall conditions; however, the buffer region and beyond takes several boundary layer thicknesses before recovering to equilibrium conditions, which is longer than previously thought. A unique direct numerical simulation database of a wall-bounded flow with a rough-to-smooth wall transition is employed to confirm these findings. In doing so, we present evidence that any estimate of the wall-shear stress from the mean velocity profile in the buffer region or further away from the wall tends to underestimate its magnitude in the near vicinity of the rough-to-smooth transition, and this is likely to be partly responsible for the large scatter of recovery lengths to equilibrium conditions reported in the literature. Our results also reveal that smaller energetic scales in the near-wall region recover to an equilibrium state associated with the new wall conditions within one boundary layer thickness downstream of the transition, while larger energetic scales exhibit an over-energised state for several boundary layer thicknesses downstream of the transition. Based on these observations, an alternative approach to estimating the wall-shear stress from the premultiplied energy spectrum is proposed
Identifying regions of importance in wall-bounded turbulence through explainable deep learning
Despite its great scientific and technological importance, wall-bounded turbulence is an unresolved problem in classical physics that requires new perspectives to be tackled. One of the key strategies has been to study interactions among the energy-containing coherent structures in the flow. Such interactions are explored in this study for the first time using an explainable deep-learning method. The instantaneous velocity field obtained from a turbulent channel flow simulation is used to predict the velocity field in time through a U-net architecture. Based on the predicted flow, we assess the importance of each structure for this prediction using the game-theoretic algorithm of SHapley Additive exPlanations (SHAP). This work provides results in agreement with previous observations in the literature and extends them by revealing that the most important structures in the flow are not necessarily the ones with the highest contribution to the Reynolds shear stress. We also apply the method to an experimental database, where we can identify completely new structures based on their importance score. This framework has the potential to shed light on numerous fundamental phenomena of wall-bounded turbulence, including novel strategies for flow control
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