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• Experimental and numerical studies of turbulent flow over rough surfaces in APG. • Effects of two-dimensional and three-dimensional surface topologies examined. • Effect of amplitude of two-dimensional roughness also examined. • Earlier flow detachment caused by increased roughness height. The effects of surface roughness and other surface topographical feature on turbulent boundary layer (TBL) development have been widely explored, especially for flow cases with a constant streamwise pressure gradient distribution. In the case where the TBL is exposed to the action of an adverse pressure gradient (APG), flow separation is expected to occur and the classical mechanisms responsible for energy transport between the inner and outer part of the TBL are invalid. The combined effect of roughness and APG on the TBL is not easy to predict, especially due to the lack of databases containing relevant research data. The main aims of the present study were to conduct experimental investigations and to perform large eddy simulations to determine the effects of different surface topographies (two-dimensional (2D) and three-dimensional (3D)) on turbulent flow development over a smooth wall under a strong APG downstream of the corrugation. The investigation was performed with a fixed value of the Reynolds number Re θ = 4900. In particular, the differences in the production and dynamics of near-wall vortical structures under different corrugation geometries were studied. It was found that the higher amplitude of 2D wavy surface the greater production of turbulence and the earlier separation of the TBL. The response of the TBL differed according to the 3D topography, where it was characterized by a greater streamwise velocity deficit, lower growth of all components of the velocity fluctuations, and a smaller increase in the normal velocity gradient relative to the 2D case with comparable amplitude. The results also demonstrated that the 3D case generated smaller and less energetic structures on the flat plate downstream of the corrugation compared with the 2D case. [ABSTRACT FROM AUTHOR]

The paper presents an extension of the diagnostic-plot scaling (Alfredsson et al., 2012) of the turbulence-intensity profiles for the outer region of adverse-pressure-gradient (APG) turbulent boundary layers (TBLs). An extended formula including the shape factor is proposed, which allows the diagnostic-plot scaling to be used in strong APGs at high Reynolds numbers. The validity of the new formulation is verified using several available databases. We demonstrate that the new formula allows to scale profiles in cases where the Clauser–Rotta pressure-gradient parameter β is below 14 even in presence of very strong flow-history effects. In order to extend the scaling to the near-wall region of TBLs the adapted complete difference function is proposed. The proposed scaling yields a unified description of the turbulence-intensity profiles regardless of their flow history (as opposed to other previously proposed scalings), and is valid for a wider range of cases not only for the outer but also for the inner region. • New diagnostic-plot scaling of the turbulence-intensity profiles is presented. • Validity of the new approach is examined using a number of available databases. • The complete difference function is proposed to reconstruct the inner region. • The method works under adverse pressure gradient conditions up to β = 14. • The new approach was found to be insensitive to the flow-history effects. [ABSTRACT FROM AUTHOR]

The convection velocity (U C) of turbulent structures has been studied in adverse-pressure-gradient (APG) turbulent boundary layers (TBLs) for a wide range of Reynolds numbers R e τ ≈ 1400 − 4000. The study is based on estimation of the convection velocity using decomposed streamwise skewness factor introduced in (Dróżdż A., Elsner W., Int. J. of Heat and Fluid Flow. 63 (2017) 67-74) and verified by means of two-point correlation method. It was shown that in the overlapping region of APG flows, the convection velocity profiles (when scaled in viscous units) reassemble the universal logarithmic law characteristic for the ZPG flows up to Clauser-Rotta pressure gradient parameter β ≲ 19 for the considered range of Reynolds number, what means that in the inner region of TBL the friction velocity in APG is not proportional to U (as in ZPG) but to U C instead. The physical mechanism that explains the impact of increased convection velocity on the mean flow is proposed. The difference between U C and U increases as a function of APG, which causes the stronger sweeping that enhances momentum transfer to the wall and compensates the weaker mean shear profile that is created by low vorticity near the wall. This effect is a result of an enhancement of amplitude modulation of the small scales by large-scale motion. The process becomes more pronounced as eddy density grows, so with increasing Re. The proposed model addresses a number of literature observations found in adverse pressure gradient flows which have been so far left without a well-founded explanation. [Display omitted] • The mean convection velocity in decelerating flow is substantially increased. • Universal logarithmic distribution of convection velocity is observed for β ¡ 19. • Increase in large-scale energy leads to increased convection velocity near the wall. [ABSTRACT FROM AUTHOR]

Turbulence behavior of non-zero pressure gradient flows departs significantly from what was known for canonical flows like flat plate zero pressure gradient turbulent boundary layers, channel or pipe flows. The paper presents experimental investigations performed for the pressure conditions representative for turbomachinery flows and for Reynolds number varying from R e θ ≈ 2300 – 6200 . By providing new data, scaling law characteristics can be better understood enabling the use of numerical modeling for flow prediction. The goal was to study and to evaluate available scaling laws using collected data. Particular attention was devoted to scaling of Reynolds stresses and the phenomenon of the outer peak of velocity fluctuation, which appears in the adverse pressure gradient conditions. At these conditions the self-similarity cannot be obtained using the scale, which is constant across the boundary layer thickness. In the paper we propose a modification of Alfredsson et al. (2012), expression, which is dedicated to ZPG flows. The proposed formulation, utilizing the shape factor H , allows to extend validity of Alfredsson et al. proposal to pressure gradient flows. [ABSTRACT FROM AUTHOR]

• Turbulent boundary layer development under adverse pressure gradient is studied. • Effect of Reynolds number on the flow statistics is examined. • Enhanced amplitude modulation with increasing Re is observed. • The higher Re the higher convection velocity of small-scales. • Turbulent separation is postponed due to increasing amplitude modulation with Re. The effect of Reynolds number on turbulent boundary layer under adverse pressure gradient has been barely investigated in the available literature. So far, experimental or numerical research on that issue were performed either for low Re or for low range of Re to capture its clear effect on flow statistics. Hence, the purpose of the present paper is to examine whether the effect of Reynolds number can be observed when much higher Re values and its much wider range, than previously investigated, are considered. Herein, the momentum loss thickness based Reynolds number Re θ was changed from 4900 to 14600. Such a wide range of Re θ appeared to be sufficient to capture an evident footprint of Re θ on the flow. It was found that the separation point is shifted downstream with increasing Reynolds number due to increased convection velocity close to the wall. More detailed studies of the skewness factor and premultiplied energy spectra revealed that this phenomenon can be attributed to enhanced, with increasing Re , large- and small-scale interactions commonly known as the amplitude modulation. [ABSTRACT FROM AUTHOR]

In this work, a comprehensive insight into the new passive flow control method, i.e. a two-dimensional streamwise waviness (recently proposed by Dróżdż et al. [1]) is provided using Large Eddy Simulations. This approach was demonstrated to be effective in postponing turbulent boundary layer separation at high Re , which is out of reach for other commonly known passive flow control strategies. Although the effectiveness of the wavy wall in delaying flow separation has been confirmed, the physical mechanisms standing behind the skin friction enhancement are not fully understood which opens a space for future work. Of particular interest is an insight into the fluid flow in the near-wall region just above the waviness and this paper is aimed at doing so. To separate the effect of the global adverse pressure gradient, the results were confronted with the ones obtained under zero pressure gradient conditions for the identical wavy wall geometry and the same R e τ = 2500. Also additional simulations of the turbulent boundary layer building up on the flat wall were conducted to have an additional reference for the flow fields obtained above the wavy wall under zero and adverse pressure gradients respectively. A detailed inspection of the streamwise velocity and its fluctuation profiles indicated that the contribution of the global adverse pressure gradient to the flow statistics begins when the Rotta-Clauser pressure gradient parameter β ≥ 3. The paper confirms an enhancement of the wall shear stress τ w downstream the corrugation, which is the necessary condition to postpone the separation. Interestingly, the effect of increased τ w was not observed for the case with the same waviness and without pressure gradient. [ABSTRACT FROM AUTHOR]

• Effect of streamwise waviness on turbulent separation is studied. • Impact of amplitude, period and length of wall undulation are examined. • A substantial growth in skin friction due to streamwise waviness is achieved. • Increase in convection velocity in the near-wall region is observed. It is commonly known that for a sufficiently high Reynolds number, for flow over a rough surface and other previously examined wall topologies, turbulent boundary layer separation generally occurs earlier, and the separation bubble is noticeably larger due to a greater momentum deficit caused by wall irregularities. The recently discovered amplitude modulation effect has provided an improved understanding of the momentum transport that occurs from the outer to the inner part of the boundary layer and its subsequent effect on skin friction. At sufficiently large Reynolds numbers, large-scale motion first causes an increase and then decrease in the flow velocity near the wall. This results in an increased net convection velocity of small structures in the streamwise direction, and consequently an increase in wall shear stress. In this work, we demonstrate that a wavy wall, with a streamwise waviness with carefully selected amplitude and period, can effectively enhance the amplitude modulation effect and ensure an increase in the wall shear stress, thereby postponing turbulent separation. The experimental results are presented in normalized general form using suitable flow scaling. The research goal is to find values of the amplitude, period, and length (always with the total number of periods) of the wavy wall for which the highest rise in skin friction is gained at a fixed point downstream of the wavy wall. The most effective wavy wall geometry investigated ensured a 13% increase in skin friction (relative to the value at zero-pressure-gradient inlet flow) at the location where flow separation occurred on an unmodified surface (i.e. on a flat plate). [ABSTRACT FROM AUTHOR]

• New method for estimating friction velocity in adverse pressure gradient is proposed. • The proposed method can be used for the Clauser–Rota pressure parameter β up to 28. • Uncertainty of the approach in estimation of friction velocity is not higher than 2.5%. • The method utilises confirmed relation between large-scale motion and friction velocity. Clauser-chart is commonly regarded as the only indirect method capable for estimation of the local mean skin friction in a turbulent boundary layer based on velocity profile in the overlapping layer, where the logarithmic relation is observed. This approach suffers, however, from the following limitation: it is applicable only for zero and weak pressure gradient flows. However, as is known, in a number of practical configurations the flow has a non-canonical character and is affected, in particular, by the adverse pressure gradient – here airfoils, diffusers and turbines may serve as examples. Hence, with this paper we would like to present a new method which can yield an accurate estimation of wall shear-stress under the strong pressure gradient conditions. In this approach the skin friction can be directly estimated based on the location of intersection point between the inner-scaled streamwise velocity profile and the newly found logarithmic line in the location which covers with the point of the local maximum of large scale motion activity in the outer region of the turbulent boundary layer. It was verified that the proposed method allows for estimation of the skin friction velocity with uncertainty lower than 2.5% for the Clauser–Rota pressure parameter β up to 28 and for 1800 < Re τ < 3900 using data from available literature. [ABSTRACT FROM AUTHOR]

This paper provides characteristics of extrahepatic manifestations of HBV (hepatitis B virus) infection. Their clinical symptomatology may be extremely rich including glomerulonephritis, polyarteritis nodosa, mixed cryoglobulinemia, peripheral neuropathies and other. Various regimens of treatment of these disorders were proposed and efficacy of IFN alpha in children suffering from glomerulonephritis has been stressed. [ABSTRACT FROM AUTHOR]