Roughness effects in turbulent forced convection

We conducted direct numerical simulations (DNSs) of turbulent flow over three-dimensional sinusoidal roughness in a channel. A passive scalar is present in the flow with Prandtl number $Pr=0.7$, to study heat transfer by forced convection over this rough surface. The minimal channel is used to circumvent the high cost of simulating high Reynolds number flows, which enables a range of rough surfaces to be efficiently simulated. The near-wall temperature profile in the minimal channel agrees well with that of the conventional full-span channel, indicating it can be readily used for heat-transfer studies at a much reduced cost compared to conventional DNS. As the roughness Reynolds number, $k^+$, is increased, the Hama roughness function, $\Delta U^+$, increases in the transitionally rough regime before tending towards the fully rough asymptote of $\kappa_m^{-1}\log(k^+)+C$, where $C$ is a constant that depends on the particular roughness geometry and $\kappa_m\approx0.4$ is the von K\'arm\'an constant. In this fully rough regime, the skin-friction coefficient is constant with bulk Reynolds number, $Re_b$. Meanwhile, the temperature difference between smooth- and rough-wall flows, $\Delta\Theta^+$, appears to tend towards a constant value, $\Delta\Theta^+_{FR}$. This corresponds to the Stanton number (the temperature analogue of the skin-friction coefficient) monotonically decreasing with $Re_b$ in the fully rough regime. Using shifted logarithmic velocity and temperature profiles, the heat transfer law as described by the Stanton number in the fully rough regime can be derived once both the equivalent sand-grain roughness $k_s/k$ and the temperature difference $\Delta \Theta^+_{FR}$ are known. In meteorology, this corresponds to the ratio of momentum and heat transfer roughness lengths, $z_{0m}/z_{0h}$, being linearly proportional to $z_{0m}^+$, the momentum roughness length [continued]...Comment: Accepted (In press) in the Journal of Fluid Mechanic

Evidence of large-scale amplitude modulation on the near-wall turbulence

The relationship between large- and small-scale motions remains a poorly understood process in wall-bounded turbulence. Such misunderstanding is perhaps, in part, due to the limited scale separation typical of many laboratory-scale facilities. A recent investigation performed by Hutchins and Marusic [11] in a high Reynolds number turbulent boundary layer has qualitatively shown the existence of a modulating influence of the large-scale log region motions on the small-scale near-wall cycle. For this study we build upon these observations, using the Hilbert transformation applied to the spectrally filtered smallscale component of fluctuating velocity signals, in order to quantitatively determine the degree of amplitude modulation imparted by the large-scale structures onto the near-wall cycle

Direct numerical simulation of open-channel flow over smooth-to-rough and rough-to-smooth step changes

Direct numerical simulations (DNS) are reported for open-channel flow over streamwise-alternating patches of smooth and fully rough walls. The rough patch is a three-dimensional sinusoidal surface. Owing to the streamwise periodicity, the flow configuration consists of a step change from smooth to rough, and a step change from rough to smooth. The friction Reynolds number varies from 437 over the smooth patch to 704 over the rough patch. Through the fully resolved DNS dataset it is possible to explore many detailed aspects of this flow. Two aspects motivate this work. The first one is the equilibrium assumption that has been widely used in both experiments and computations. However, it is not clear where this assumption is valid. The detailed DNS data reveal a significant departure from equilibrium, in particular over the smooth patch. Over this patch, the mean velocity is recovered up to the beginning of the log layer after a fetch of five times the channel height. However, over the rough patch, the same recovery level is reached after a fetch of two times the channel height. This conclusion is arrived at by assuming that an error of up to 5 % is acceptable and the log layer, classically, starts from 30 wall units above the wall. The second aspect is the reported internal boundary-layer (IBL) growth rates in the literature, which are inconsistent with each other. This is conjectured to be partly caused by the diverse IBL definitions. Five common definitions are applied for the same DNS dataset. The resulting IBL thicknesses are different by 100 %, and their apparent power-law exponents are different by 50 %. The IBL concept, as a layer within which the flow feels the surface underneath, is taken as the basis to search for the proper definition. The definition based on the logarithmic slope of the velocity profile, as proposed by Elliot (Trans. Am. Geophys. Union, vol. 39, 1958, pp. 1048–1054), yields better consistency with this concept based on turbulence characteristics

Turbulent flow over a long flat plate with uniform roughness

For turbulent boundary-layer flow under a uniform freestream speed U∞ over a plate of length L, covered with uniform roughness of nominal sand-grain scale k_s, the physical behaviors underlying two distinguished limits at large Re_L≡U∞L/ν are explored: the fully rough wall flow where k_s/L is fixed and the long-plate limit where Re_k≡U∞k_s/ν is fixed. For the fully rough limit it is shown that not only is the drag coefficient C_D independent of Re_L but that a universal skin-friction coefficient C_f and normalized boundary-layer thickness δ/k_s can be found that depends only on ks_/x, where x is the downstream distance. In the long-plate limit, it is shown that the flow becomes asymptotically smooth at huge Re_L at a rate that depends on Re_k. Comparisons with wind-tunnel and field data are made

The Influence of Spatial Resolution due to Hot-Wire Sensors on Measurements in Wall-Bounded Turbulence.

Reassessment of compiled data reveal that recorded scatter in the hot-wire measured near-wall peak in viscous-scaled streamwise turbulence intensity is due in large part to the simultaneous competing effects of Reynolds number and viscous-scaled wire-length l ( lUt n, where l is the wirelength, Ut is friction velocity and n is kinematic viscosity). These competing factors can explain much of the disparity in existing literature, in particular explaining how previous studies have incorrectly concluded that the inner-scaled near-wall peak is independent of Re. We also investigate the appearance of the, so-called, ‘outerpeak’ in the broadband streamwise intensity, found by some researchers to occur within the log-region of high Reynolds number boundary layers. We show that this ‘outer-peak’ is most likely a symptom of attenuation of small-scales due to large l . Fully mapped energy spectra, obtained with two different l , are presented to demonstrate this phenomena. The spatial attenuation resulting from wires with large l effectively filters small-scale fluctuations from the recorded signal

Influences affecting decision-making regarding use of pre-exposure prophylaxis among black men who have sex with men in the USA: a systematic review and meta-synthesis

AIM: To systematically identify, evaluate, and synthesise qualitative research examining positive and negative influences affecting decision-making behaviour among black men who have sex with men (BMSM) in the USA regarding use of pre-exposure prophylaxis (PrEP). BACKGROUND: Used correctly, PrEP is highly efficacious in preventing HIV infection and is available via healthcare services throughout the USA. BMSM are a key target population for HIV prevention services, however their engagement with these services is low. With potential barriers to access ranging from systemic to personal, a phenomenological perspective on the influences affecting individuals' decision-making is essential, helping to better understand the needs of this target population and guide development and delivery of more effective future policy and intervention services. DESIGN: Qualitative meta-synthesis with meta-aggregation. DATA SOURCES: The electronic databases Medline, CINAHL, APA PsycInfo, Embase and Ovid Emcare were comprehensively searched from inception to 21 January 2022. REVIEW METHODS: Systematic identification, quality assessment and synthesis of existing qualitative research according to protocols of meta-aggregation. This included identifying salient study findings and corroborating illustrations from the data, sorting like findings into descriptive themed categories and developing transformative synthesised statements from aggregate appraisal of category findings. RESULTS: Seventeen studies met the inclusion criteria and were assessed to be of acceptable quality. Synthesis of study data yielded 30 categories grouped under five themes: Stigma, Discrimination, Mistrust, PrEP positivity and PrEP negativity. Twelve synthesised statements were produced to provide a summary of the results and suggest improvements to the delivery of future PrEP services and interventions. CONCLUSION: A more targeted approach focused on advocacy and ambassadorship outside of clinical settings may be more influential in positive decision-making regarding use of PrEP in BMSM populations than relying on traditional outreach methods via institutions and their representatives where stigma, mistrust and structural inequalities perpetuate