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Turbulent boundary layers under adverse pressure gradients are studied using well-resolved large-eddy simulations (LES) with the goal of assessing the influence of the streamwise pressure-gradient development. Near-equilibrium boundary layers were characterized through the Clauser pressure-gradient parameter $\\unicode[STIX]{x1D6FD}$ . In order to fulfil the near-equilibrium conditions, the free stream velocity was prescribed such that it followed a power-law distribution. The turbulence statistics pertaining to cases with a constant value of $\\unicode[STIX]{x1D6FD}$ (extending up to approximately 40 boundary-layer thicknesses) were compared with cases with non-constant $\\unicode[STIX]{x1D6FD}$ distributions at matched values of $\\unicode[STIX]{x1D6FD}$ and friction Reynolds number $Re_{\\unicode[STIX]{x1D70F}}$ . An additional case at matched Reynolds number based on displacement thickness $Re_{\\unicode[STIX]{x1D6FF}^{\\ast }}$ was also considered. It was noticed that non-constant $\\unicode[STIX]{x1D6FD}$ cases appear to approach the conditions of equivalent constant $\\unicode[STIX]{x1D6FD}$ cases after long streamwise distances (approximately 7 boundary-layer thicknesses). The relevance of the constant $\\unicode[STIX]{x1D6FD}$ cases lies in the fact that they define a ‘canonical’ state of the boundary layer, uniquely characterized by $\\unicode[STIX]{x1D6FD}$ and $Re$ . The investigations on the flat plate were extended to the flow around a wing section overlapping in terms of $\\unicode[STIX]{x1D6FD}$ and $Re$ . Comparisons with the flat-plate cases at matched values of $\\unicode[STIX]{x1D6FD}$ and $Re$ revealed that the different development history of the turbulent boundary layer on the wing section leads to a less pronounced wake in the mean velocity as well as a weaker second peak in the Reynolds stresses. This is due to the weaker accumulated effect of the $\\unicode[STIX]{x1D6FD}$ history. Furthermore, a scaling law suggested by Kitsios et al. (Intl J. Heat Fluid Flow, vol. 61, 2016, pp. 129–136), proposing the edge velocity and the displacement thickness as scaling parameters, was tested on two constant-pressure-gradient parameter cases. The mean velocity and Reynolds-stress profiles were found to be dependent on the downstream development. The present work is the first step towards assessing history effects in adverse-pressure-gradient turbulent boundary layers and highlights the fact that the values of the Clauser pressure-gradient parameter and the Reynolds number are not sufficient to characterize the state of the boundary layer.

The gold standard for assessing pressure gradients (PG) across coarctation involves measurements obtained through cardiac catheterization or surgical intervention. There has been ongoing discussion regarding the accuracy of non-invasive methods for estimating these gradients. This study sought to establish the correlation and agreement between the systolic blood pressure (SBP) gradient between the upper and lower extremities, as well as, the mean and maximum PG derived from echocardiography, in comparison to the peak-to-peak pressure gradient obtained from either cardiac catheterization or surgery. We conducted a retrospective study on patients < 18 years diagnosed with coarctation at Chiang Mai University Hospital from 2011 to 2022. The study involved the measurement of the SBP gradient between the upper and lower extremities, mean and maximum PG using echocardiography, peak-to-peak pressure gradient obtained from cardiac catheterization, and pressure gradient recorded during surgical procedures. The Spearman’s correlation and Bland-Altman analysis were employed to assess correlation and agreement. Fifty-four patients with aortic coarctation were enrolled. The mean PG measured by echocardiography showed a significantly moderate correlation (r = 0.78, p < 0.001) and the highest level of agreement according to Bland Altman plots, in comparison to the peak-to-peak pressure gradient measured during both cardiac catheterization and surgical procedure. The max PG demonstrated a notable overestimation compared to the gold standard (mean difference + 13.14 with a slope of biases + 0.64, p < 0.001). The mean PG obtained through echocardiography has more potential to be applied in practical application in predicting pressure gradient in patients with coarctation.

Intraventricular pressure gradients or hemodynamic forces, which are their global measure integrated over the left ventricular volume, have a fundamental importance in ventricular function. They may help revealing a sub-optimal cardiac function that is not evident in terms of tissue motion, which is naturally heterogeneous and variable, and can influence cardiac adaptation. However, hemodynamic forces are not utilized in clinical cardiology due to the unavailability of simple non-invasive measurement tools. Hemodynamic forces depend on the intraventricular flow; nevertheless, most of them are imputable to the dynamics of the endocardial flow boundary and to the exchange of momentum across the mitral and aortic orifices. In this study, we introduce a simplified model based on first principles of fluid dynamics that allows estimating hemodynamic forces without knowing the velocity field inside the LV. The model is validated with 3D phase-contrast MRI (known as 4D flow MRI) in 15 subjects, (5 healthy and 10 patients) using the endocardial surface reconstructed from the three standard long-axis projections. Results demonstrate that the model provides consistent estimates for the base-apex component (mean correlation coefficient r=0.77 for instantaneous values and r=0.88 for root mean square) and good estimates of the inferolateral-anteroseptal component (r=0.50 and 0.84, respectively). The present method represents a potential integration to the existing ones quantifying endocardial deformation in MRI and echocardiography to add a physics-based estimation of the corresponding hemodynamic forces. These could help the clinician to early detect sub-clinical diseases and differentiate between different cardiac dysfunctional states.

This paper investigates the effect of corrugated surfaces on the wind turbines power output for both laminar and turbulent flows. Conservation principles including continuity and momentum equations, wind turbine power equations, and the corrugated surface equation have been implemented to build up a theoretical model then which has been solved using MATLAB. This model simulates wind turbines power output and analyzes several case studies implementing different parameters such as air pressure wave amplitude (Po), air wave fluctuation frequency (n), and wind layer turbulence (b). Also, different complex terrains in two main scenarios representing two different positions (X) of the wind turbine are analyzed. This analysis indicates the importance of wind turbines micro siting. In addition, it is found that increasing the pressure ratio increased wind turbine power output, while increasing the frequency decreased the power ratio of the wind turbines for both laminar and turbulent flow conditions. Increasing turbulence for the turbulent model increased the power ratio.

The riblets surface is a passive turbulence drag reduction technology that holds promising application prospects in drag reduction for large aircraft. Currently, most research on the drag reduction characteristics of riblets is limited to medium and low Reynolds number environments with zero pressure gradient, and the effects of adverse pressure gradient on the drag reduction rate remain controversial. The inconsistency between the local flow direction on the fuselage surface and the arrangement direction of the riblets can lead to cross-flow effects. To investigate the drag reduction performance of riblets under flow conditions more representative of actual aircraft surfaces, this study establishes an adverse pressure gradient environment at moderate-to-high Reynolds numbers. Hot-wire anemometry is employed to measure the drag reduction rate of the riblet surface, and the fundamental turbulent boundary layer statistics are observed. The measurement results indicate that the adverse pressure gradient and cross-flow effects contribute positively and negatively to the drag reduction rate of the riblets, respectively, while the increase in Reynolds number in this experiment has no substantial effect. The changes in the basic statistics of the turbulent boundary layer on the surface of the riblets are consistent with existing literature.

This paper presents THEMIS measurements of two substorm events to show how the substorm current wedge (SCW) is generated. In the late growth phase when an earthward flow burst in the near‐Earth magnetotail brakes and is diverted azimuthally, pressure gradients in the X‐ and Y‐directions are observed to increase in the pileup and diverting regions of the flow. The enhanced pressure gradient in the Y‐direction is dawnward (duskward) on the dawnside (duskside) where a clockwise (counter‐clockwise) vortex forms. This dawn‐dusk pressure gradient drives downward (upward) field‐aligned current (FAC) on the dawnside (duskside) of the flow, which, when combined with the FACs generated by the clockwise (counter‐clockwise) vortex, forms the SCW. Substorm auroral onset occurs when the vortices appear, Near‐Earth dipolarization onset is observed by the THEMIS spacecraft (probes) when a rapid jump in the Y‐component of pressure gradient is detected. The total FACs from the vortex and the azimuthal pressure gradient are found to be comparable to the DP‐1 current in a typical substorm. Key Points Two dimensional pressure gradient could be estimated using three satellites Dawn‐dusk pressure gradient was generated after flow diversion FAC generated by azimuthal pressure gradient is enough for SCW formation

Threshold pressure gradient has great importance in efficient tight gas field development as well as for research and laboratory experiments. This experimental study is carried out to investigate the threshold pressure gradient in detail. Experiments are carried out with and without back pressure so that the effect of pore pressure on threshold pressure gradient may be observed. The trend of increasing or decreasing the threshold pressure gradient is totally opposite in the cases of considering and not considering the pore pressure. The results demonstrate that the pore pressure of tight gas reservoirs has great influence on threshold pressure gradient. The effects of other parameters like permeability and water saturation, in the presence of pore pressure, on threshold pressure gradient are also examined which show that the threshold pressure gradient increases with either a decrease in permeability or an increase in water saturation. Two new correlations of threshold pressure gradient on the basis of pore pressure and permeability, and pore pressure and water saturation, are also introduced. Based on these equations, new models for tight gas production are proposed. The gas slip correction factor is also considered during derivation of this proposed tight gas production models. Inflow performance relationship curves based on these proposed models show that production rates and absolute open flow potential are always be overestimated while ignoring the threshold pressure gradients.

The principle of selection of surface roughness level of thermal barrier coatings (TBCs) at turbine film-cooled vanes is unclear. In this work, the universal turbine vane flow environment was characterized by the proper combinations of different wall curvatures and streamwise pressure gradients. To achieve the direct measurements of overall effectiveness, a conjugate film cooling model with simulated TBCs was established by matching the hot-side Biot numbers of actual materials. The surface roughness levels were set in a range for TBC design stage. The measurements displayed the detailed trends in overall effectiveness of metal and TBC with the surface roughness level, wall curvature, streamwise pressure gradient, and cooling air flowrate. The roughness effect on overall effectiveness was complicated by the wall curvature, due to generating different jet mechanisms. When the engine Ra changes from 0.2 to 14 μm, the variations of metal overall effectiveness can be controlled below 15% at different walls. Overall, to improve the protection ability to metal, the smoother TBC is suggested to spray at the flat wall, while, at the convex and concave walls, the proper roughness of TBC can be considered in the near-hole region under the acceptable stress level.

The widely used Darcy’s law specifies a linear relation between the Darcy velocity of fluid flow and the pressure gradient that drives the flow. However, studies have shown that Darcy velocity can exhibit a nonlinear dependence on the pressure gradient in low-permeability porous media such as clay and shale when the pressure gradient is adequately low. This phenomenon is referred to as low-velocity non-Darcian flow or pre-Darcy flow. This paper provides a comprehensive review of the theories, experimental data, and modeling methods for pre-Darcy flow in low-permeability porous media. The review begins by outlining the fundamental mechanisms underlying pre-Darcy flow that regulate the unique characteristics such as nonlinear dependence of the Darcy velocity on the pressure gradient and its relevance to fluid–rock interactions. The review then proceeds to present a thorough compilation of experimental investigations performed in various low-permeability geomaterials including tight sandstones, shales, and clays. Next, empirical and theoretical models and simulation methods that have been developed to fit and interpret experimental data are reviewed. Finally, the review underscores the challenges encountered in conducting and interpreting pre-Darcy flow experiments and suggests future research directions. By analyzing previous experimental investigations, this review aims to offer a valuable resource for researchers and practitioners seeking to enhance their understanding of fluid dynamics in low-permeability geomaterials. This provides insights into the application of pre-Darcy flow in numerous natural and engineered processes such as shale oil and gas recovery, contaminant transport in low-permeability aquifers, and geological disposal of nuclear waste.

Portal hypertension (PH) is the initial and main consequence of liver cirrhosis. Hepatic venous pressure gradient (HVPG) measurement has been widely used to estimate portal pressure gradient (PPG) and detect portal hypertension. However, some clinical studies have found poor correlation between HVPG and PPG, which may lead to the misdiagnosis of portal hypertension. In this study, we provided a method to evaluate patients’ PPG based on clinically measured HVPG with computational fluid dynamics (CFD). Twenty-five patients who underwent HVPG measurement were recruit for analysis. Results show that HVPG significantly correlates with PPG (R = 0.7499, P < 0.0001), with an accuracy to distinguish clinically significant portal hypertension (CSPH) as high as 92 %. However, PH severity classification was underestimated for 36 % patients, especially for patients with hepatic venous collateral formation and presinusoidal portal vein occlusion. It is concluded that HVPG is a relatively reliable diagnostic method for PH when PPG cannot be directly measured. For patients who have clinical symptoms of PH but their HVPG are within a normal range, numerical evaluation of PPG with CFD is an excellent way for their diagnosis.