Near-wall Velocity Research Articles

Axial-flow-induced structural vibration in a 5×5 cylinder cluster

This work aims to experimentally study the incident turbulence intensity Tu effect on the flow-induced vibration of an elastic cylinder positioned at the center of a 9- or 25-cylinder cluster subjected to an axial flow. Tu is examined at 0.71% – 0.80% and 2.30% – 2.91%. The pitch-to-diameter ratio P* is 1.36 ∼ 1.64. Lateral vibrations along two orthogonal directions are simultaneously measured with the interstitial flow of the cylinder bundle. Two mechanisms are identified behind the elastic-cylinder vibration at low and high Tu. One is the presence of a varying velocity gradient within the cylinder bundle, and the other is incident flow fluctuations. At low Tu (0.71% – 0.80%), the root-mean-square vibration amplitude Arms* of the elastic cylinder exhibits strong dependence on the P* and cylinder number N. Increasing velocity gradient with decreasing P* or increasing N plays a key role in destabilizing the shear layers surrounding the elastic cylinder, inducing eddies to separate from the cylinder-wall and actively interact with those near the neighboring cylinder. Therefore, the near-wall velocity fluctuation urms* and Arms* are increased. At high Tu (2.3% – 2.91%), Arms* is weakly dependent on P* compared with that at low Tu. It is found that the shear-layer instability surrounding the elastic cylinder is mainly intensified by the incident flow fluctuations with a higher Tu, accounting for the enhanced eddy activities, while the velocity-gradient effect associated with a change in P* is of less importance.

Pulse electroosmotic flow of Newtonian fluids in parallel plate microchannels under triangular and half-sinusoidal pulse electric fields

Unlike the conventional electroosmotic flow (EOF) driven by direct current and alternating current electric fields, this study investigates the pulse EOF of Newtonian fluids through a parallel plate microchannel actuated by pulse electric fields. Specifically, the pulses considered encompass triangular and half-sinusoidal pulse waves. By applying the Laplace transform method and the residual theorem, the analytical solutions for the velocity and volumetric flow rate of the pulse EOF associated with these two pulse waves are derived, respectively. The influence of pulse width a¯ and electrokinetic width K on velocity is further considered, while the volumetric flow rate as a function of time t¯ and electrokinetic width K is examined separately. A comparison of the volumetric flow rates related to these two pulse waves under varying parameters is also conducted. The research findings indicate that irrespective of the pulse wave, a broader pulse width results in a prolonged period and increased amplitude of the velocity profile. Elevating the electrokinetic width yields higher near-wall velocities, with negligible effect on near-center velocities. It is noteworthy that regardless of the electrokinetic width, the near-wall velocity exceeds that of the near-center during the first half-cycle, while the situation reverses during the second half-cycle. The volumetric flow rate varies periodically with time, initially surging rapidly with electrokinetic width before gradually stabilizing at a constant level. More interestingly, independent of pulse width and electrokinetic width, the volumetric flow rates linked to the half-sinusoidal pulse wave consistently surpass those of the triangular pulse wave. For any pulse width, the volumetric flow rates corresponding to the two pulse waves grow with higher electrokinetic widths, especially prominent at alternating intervals of the two half-cycles within a complete cycle. These findings have important implications for improving the design and optimization of microfluidic devices in engineering and biomedical applications utilizing pulse EOF.

Small-scale Helmholtz resonators with grazing turbulent boundary layer flow

ABSTRACT Helmholtz resonators flush-mounted in a wall beneath turbulent boundary layer flow are studied by focusing on their flow-induced excitation and effect on the grazing turbulent flow. A particular focus lies on single resonators tuned to the most intense spatio-temporal fluctuations in the near-wall vertical velocity and wall-pressure, residing at a spatial scale of λ x + ≈ 250 (or temporal scale of T + ≈ 25 ). Resonators are examined in a boundary layer flow at R e τ ≈ 2280 . Two neck-orifice diameters of d + ≈ 68 and 102 are considered, and for each value of d + three different resonance frequencies are studied (corresponding to a period of T + ≈ 25 , as well as one lower, and one higher, period). The response of the TBL flow is analysed by employing velocity data from hot-wire anemometry and particle image velocimetry measurements. Passive resonance only affects streamwise velocity fluctuations in the region y + ≲ 25 , while vertical velocity fluctuations due to resonance reach up to y + ≈ 100 . A narrow-band increase in streamwise turbulence kinetic energy at the resonance scale co-exists with a more than 20% attenuation of lower-frequency energy. Current findings on single resonator cases will aid in the development of passive surfaces with distributed resonators for boundary-layer flow control.

Functionalized super-hydrophobic nanocomposite surface integrating with anti-icing and drag reduction properties

Anti-icing and drag reduction are two significant issues for aircraft, but functional surfaces integrating both properties are rarely reported. Here, we propose a functional super-hydrophobic nanocomposite surface with sharkskin-like groove structures, which achieves the combination of anti-icing and drag reduction properties. The nanocomposite surface was prepared by stripping polydimethylsiloxane doped with carbon nanotubes from a laser-prepared sharkskin-like structure mold and then spraying a layer of polytetrafluoroethylene hydrophobic coating. The microstructures and nanoparticles with low surface energy provide the surface with hydrophobicity and icephobicity, enabling droplet transitions between Cassie and Wenzel states during icing-melting process. Carbon nanotubes enhance the Joule heating performance when voltage is applied. Ice at interface melts into water film, significantly reducing adhesion and enabling efficient anti-icing. Under the influence of the bionic sharkskin riblet structure, the vortices are lifted and pinned above the riblet tips, which reduces the near-wall velocity gradient and total wall shear stress, ultimately leading to lower drag. In addition, the flexible surface undergoes adaptive deformation to more effectively conform to the fluid flow during motion. The riblet protrusions increase the contact area between the flexible surface and the fluid, thereby enabling the absorption of more turbulent energy and fluctuations, further reduces drag. This study presents a new method for manufacturing functional surfaces for practical anti-icing and drag reduction applications.

A transient formulation of entropy and heat transfer coefficients of Newton's cooling law with the unifying entropy potential difference in compressible flows

The original Newton's law of cooling is considered an ideal convection model of a point source ignoring conduction, but its unknown convection conductance and inconsistent thermal potential difference make it difficult to further elucidate the transient heat transfer mechanism of convection because of the no-slip condition at the interface between a body and a fluid. For this purpose, both the convective entropy transfer theory and the hypothesis of the isothermal velocity sublayer are developed to recast Newton's law of cooling into a standard Ohm's law form via the unified entropy driving force and the unambiguous entropy transfer coefficient. An explicit expression for the transient heat transfer coefficient is presented theoretically and experimentally in compressible flows by virtue of the product of the free-stream velocity, the volumetric thermal capacity of the fluid, and the near-wall velocity constant. The unified thermal potential difference is established between the body temperature and the total temperature (reduced to the ambient temperature for incompressible fluids). The physical mechanism of convection mode in the original Newton's cooling law is reasonably explained by the entropy transfer theory and entropy transfer efficiency, and the formulae for the entropy flux vector driven by the standard entropy potential difference and the rate of entropy generation are obtained. The analytical heat transfer coefficients as well as the standard thermal driving forces are validated by comparison with the forced convection experiment with a maximum error of 4.6 % in incompressible flows and the previous Newton's benchmark cooling experiments with the maximum error of 6.7 % in compressible flows. The effects of radiation on the total heat transfer coefficient are also analyzed quantitatively.

Wall Shear Stress Characteristics Of A Turbulent Boundary Layer Obtained With Multi-Aperture Defocusing Micro-PTV And High-Speed Stereo Profile-PIV

A multi-aperture 3d 3c micro PTV system is presented which relies on single window optical access for both high-speed tracer illumination and image recording. Similar to the “defocusing” concept described by C. Willert & Gharib (1992), the wall distance of individual particles is obtained from the size of projected particle image triplets formed by a triplet of apertures on the entrance pupil of the microscope lens. The present article extends upon previously published material Klinner & Willert (2022) by applying multi-aperture micro particle tracking velocimetry (MA-micro PTV) on a canonical turbulent boundary layer (TBL) to track the near-wall motion of tracer particles with the aim of estimating the unsteady wall-shear stress from particle tracks. The technique is validated with measurements of a TBL inside the closed test section of the 1 m wind tunnel of DLR in Göttingen (1mWK) at free- stream velocities of 5.2 to 20 m/s with corresponding shear Reynolds numbers between 560 and 1630. In the viscous sublayer, the probability density distributions of stream- and span-wise wall shear stress (WSS) components could be reliably captured down to probability densities of 10E−3. As far as we know, this has not been achieved in the past, particularly not for the span-wise component. For the stream-wise component the measured Reynolds-number dependency of the root mean squared fluctuations agrees to correlations by Örlü & Schlatter (2011). Furthermore, the skewness of the stream-wise WSS agrees well to values from DNS. On the other hand, the span-wise WSS fluctuations consistently underestimate the predicted DNS values, for which it is assumed that the deviation can be explained by the rapid decrease of the span-wise velocity fluctuations with increasing wall distance. Wall-normal profiles of 3c velocity statistics were obtained by bin averaging of particle velocities. Up into the buffer layer, these profiles agree well with profiles from stereo PIV as well as from DNS and LES, indicating an accurate determination of both near-wall velocities and inner scaling.

Turbulent Thermal Convection At A Rough Surface

We present measurements of the near-wall velocity field in highly turbulent Rayleigh-Bénard(RB) convection with rough horizontal surfaces. The measurements have been undertaken in a large-scale RB experiment which is called "Barrel of Ilmenau". We used the technique of stereoscopic Particle Image Velocimetry to measure the three components of the velocity field in horizontal planes parallel to the heated bottom plate. The measurements reported here comprise two Rayleigh numbers Ra=8.6×10^{10} and Ra=6.3×10^{11}. The Prandtl number of the working fluid was Pr=0.7. We used a chess board-like pattern of cuboids with a base area of 30 mm by 30 mm and a height of h=12 mm. Our measurements confirm the observation from previous experiments that the relation Nu=f(Ra) remains unchanged with respect to RB convection with smooth surfaces as long as the thickness of the thermal boundary layer \delta_{th}=H/(2Nu) exceeds the height of the roughness elements h. If \delta_{th} falls below h, Nu becomes larger and grows faster with increasing Ra then it does in RB convection with smooth plates. We also could identify the valleys in between the cuboids as the main contribution for the increase of Nu

Turbulent Channel Flow Diagnostics By Means Of Shake-The-Box (STB): Flow Structures And Sublayer Profiles

Time-resolved Shake-the-Box (STB) experiments have been conducted in the channel-flow facility at the Institute of Fluid Mechanics (ISTM) at f = 30 kHz repetition rate to study the near-wall velocity field at a viscous Reynolds number of Reτ = 350 (centerline velocity Ucl = 8.4 m/s, channel half-height h = 12.6 mm, viscous unit δv ≈ 36 μm) to testify the STB method for the applicability to provide high-resolution velocity information of the given flow field including the sublayer region of the channel flow. In addition to the extraction of Lagrangian particle tracks and corresponding Eulerian iso-surfaces of vortical structure the major emphasis of the study centers around an evaluation of the resulting near wall velocity profile. Particularly, it is discussed how the choice of data-processing steps and strategies affects the resulting velocity estimates in the immediate vicinity of the wall, where the major impact for the current study has been identified to result from image-arithmetic efforts during data pre-processing. The achieved results are discussed and further compared with direct numerical simulations (DNS) of similar Reτ and earlier experimental efforts at ISTM of the same flow configuration by means of stereoscopic particle image velocimetry (stereo PIV) and a laser-Doppler velocimetry profile sensor (LDV-PS). Finally, future perspectives of the experimental efforts for near-wall shear flow measurements are outlined on the grounds of the achieved insights.

Accurate Method for Estimating Wall-Friction Based on Analytical Wall-Law Model

A novel method is proposed for accurately determining the local wall friction through the near-wall measurement of time-average velocity profile in a Type-A turbulent boundary layer (TBL). The method is based on the newly established analytical wall-law in Type-A TBL. The direct numerical simulations (DNS) data of turbulence on a zero-pressure-gradient flat-plate (ZPGFP) is used to demonstrate the accuracy and the robustness of the approach. To verify the reliability and applicability of the method, a two-dimensional particle image velocimetry (PIV) measurement was performed in a ZPGFP TBL with a low-to-moderate Reynolds number (Re). Via utilizing the algorithm of single-pixel ensemble correlation (SPEC), the velocity profiles in the ZPGFP TBL were resolved at a significantly improved spatial resolution, which greatly enhanced the measurement accuracy and permitted us to accurately capture the near-wall velocity information. The accuracy of the approach is then quantitatively validated for the high Reynolds number turbulence using the ZPGFP TBL data. The research demonstrates that the current method can provide the precise estimation of wall friction with a mean error of less than 2%, which not only possesses the advantage of its insensitivity to the absolute wall-normal distance of the measuring point, but also its capability of providing an accurate prediction of wall shear stress based on fairly sparse experimental data on the velocity profile. The current study demonstrates that the wall shear stress can be accurately estimated by a velocity even at a single-point either measured or calculated in the near-wall region.

Linking wall shear stress and vorticity topologies: Toward a unified theory of cardiovascular flow disturbances

Broadening current knowledge about the complex relationship at the blood-vessel wall interface is a main challenge in hemodynamics research. Moving from the consideration that wall shear stress (WSS) provides a signature for the near-wall velocity dynamics and vorticity is considered the skeleton of fluid motion, here we present a unified theory demonstrating the existing link between surface vorticity (SV) and WSS topological skeletons, the latter recently emerged as a predictor of vascular disease. The analysis focused on WSS and SV fixed points, i.e., points where the fields vanish, as they play a major role in shaping the main vector field features. The theoretical analysis proves that: (i) all SV fixed points on the surface must necessarily be WSS fixed points, although with differences in nature and stability and (ii) a WSS fixed point is not necessarily a SV fixed point. In the former case, WSS fixed points are the consequence of flow patterns where only shear contributes to vorticity; in the latter case, WSS fixed points are the consequence of flow impingement to/emanation from the vessel wall. Moreover, fluid structures interacting with the wall characterized by zero or non-zero rotational momentum generate WSS fixed points of different nature/stability. High-fidelity computational fluid dynamics simulations in intracranial aneurysm models confirmed the applicability of the theoretical considerations. The presented unified theory unambiguously explains the mechanistic link between near-wall flow disturbances and the underlying intravascular flow features expressed in terms of vorticity, ultimately facilitating a clearer interpretation of the role of local hemodynamics in vascular pathophysiology.

Study of soot dynamic behavior and catalytic regeneration in diesel particulate filters

Catalytic diesel particulate filters (CDPFs) are key technologies for controlling particulate matter emissions. Nevertheless, it is challenging to experimentally observe the internal working states of the CDPF, which hinders comprehensive research on the flow, capture, and regeneration characteristics. In this study, we utilized quartet structure generation set to generate two-dimensional porous walls, lattice Boltzmann method to simulate CDPF flow and diffusion, cellular automata to simulate soot motion, and global reaction kinetics to simulate soot oxidation. The accuracy of the model is validated through flow, combustion, and pressure drop experiments. We propose a new concept of local porosity, where the porosity of a partially porous wall is used to represent the filtering parameters of that part. These findings indicate that porosity dominates filtering. A higher local porosity results in increased near-wall velocity and greater deposition of soot particles. Similarly, alterations in local porosity and deposition mutually influence each other. The velocity distribution and pressure difference at the inlet and outlet are directly influenced by the upstream velocity. Compared with velocity and porosity, particle diameter has less effect on deposition location, but more effect on deposition efficiency. Active regeneration (800 K) occurs approximately 31.17 times more quickly than passive regeneration (600 K) in the absence of a catalyst. The platinum-based catalyst significantly enhanced the soot oxidation rate by oxidizing NO to NO2, resulting in a 3.62-fold increase in passive regeneration and an estimated 1.17-fold increase in active regeneration. These findings are crucial for developing efficient CDPF systems and for reducing particulate emissions.

Numerical study on the influence of riblet yaw-angle on drag reduction rate with different immersion heights

This paper uses the Large-eddy simulation method to investigate the influence of riblet yaw angle on drag reduction rate with different immersion heights, revealing that the vortex structure is gradually transformed from riblet-tip vortex pairs to boundary vortex forms with the increase of yaw angle, and analyzing the influence mechanism of immersion height on the critical yaw angle. The result shows that the drag-reduction rate of riblets decreases as the yaw angle increases, and the critical angle of the riblet varies with the different immersion heights. When the immersion height is 0.7, 0.5, and 0.3, the critical yaw angle is close to 30, 40, and 50°, respectively. As the immersion height increases, the cross-flow velocity at the riblet tip is more accessible to reach the critical velocity at which a boundary vortex can be formed, resulting in a smaller critical yaw angle for larger immersion heights. Furthermore, with the yaw angle increases, the normal diffusivity of normal vorticity and spanwise vorticity increase and decrease, respectively, which indicates that the lateral flow fluctuations decrease beyond a critical yaw angle, while the Reynolds stress component due to energy dissipation induced by the streamwise velocity decreases. Meanwhile, with the formation of a boundary vortex, the ability of the riblet to reduce viscous shear stresses increases due to the reduction of near-wall velocity gradient, and the emergence of a high-pressure region on the windward side of the riblet and a low-pressure region on the leeward side, consequently augmenting the differential pressure drag. This work provides valuable insights for designing riblets with heightened effectiveness at larger yaw angles, thereby extending their applicability under diverse flow conditions.

Near-wall streamwise turbulence intensity as Reτ→∞

In this study, asymptotic scaling of near-wall streamwise turbulence intensity u′u′¯/uτ2 (uτ is the friction velocity) is theoretically explored. The three scalings previously proposed are first reviewed with their derivation and physical justification: (1) u′u′¯/uτ2∼lnReτ (Reτ is the friction velocity); (2) u′u′¯/uτ2∼1/U∞+ (U∞+ is the inner-scaled freestream velocity in boundary layer); (3) u′u′¯/uτ2∼Reτ−1/4. A new analysis is subsequently developed based on velocity spectrum, and two possible scenarios are identified based on the asymptotic behavior of the outer-scaling part of the near-wall velocity spectrum. In the former case, the outer-scaling part of the spectrum is assumed to reach a nonzero constant as Reτ→∞, and it results in the scaling of u′u′¯/uτ2∼lnReτ, both physically and theoretically consistent with the classical attached eddy model. In the latter case, a sufficiently rapid decay of the outer-scaling part of the spectrum with Re is assumed due to the effect of viscosity, such that u′u′¯/uτ2<∞ for all Reτ. The following analysis yields u′u′¯/uτ2∼1/lnReτ, asymptotically consistent with the scaling of u′u′¯/uτ2∼1/U∞+. The scalings are further verified with the existing simulation and experimental data and those from a quasilinear approximation [Holford ], the spectra of which all appear to favor u′u′¯/uτ2∼1/lnReτ, although new datasets for Reτ≳O(104) would be necessary to conclude this issue. Published by the American Physical Society 2024

On the shear-induced lift in two-way coupled turbulent channel flow laden with heavy particles

The importance of shear-induced lift, i.e. Saffman lift, is investigated in two-way coupling point-particle direct numerical simulation of turbulent channel flow laden with heavy particles. A new criterion is established for the critical particle diameter normalized by wall viscous length, and the effect of Saffman lift on particle motion can be neglected when the particle diameter is smaller than the critical value. The new criterion agrees well with the numerical results. We further study the influence of Saffman lift on particle motion and turbulence modulation. The results suggest that the Saffman lift will increase near-wall particle velocity and reduce particle accumulation therein. By integrating the particle motion equation, it is demonstrated that the mean particle streamwise velocity is dependent on the particle wall-normal velocity. This theoretically explains why the presence of a wall-normal force alters the streamwise velocity of particles. A theoretical profile of mean particle streamwise velocity at different Stokes numbers is then obtained by introducing the diffusive gradient hypothesis and Prandtl mixing length model. In addition, the present results reveal that the Saffman lift tends to augment the level of turbulence attenuation. The spectral analysis shows that the Saffman lift reduces the turbulent energy of small scales in the buffer region and large structures in the logarithmic region. When the Saffman lift is absent, near-wall particle streaks can be observed in the buffer region, which tend to separate fluid streaks. By contrast, the Saffman lift tends to prevent the formation of near-wall particle streaks.

Heat transfer enhancement of water flow over a heating flat plate using 20 kHz ultrasonic waves irradiated from submerged horn-type transducer

The enhancement of heat transfer of water flow over a flat plate under 20 kHz ultrasound irradiated from a submerged horn-type transducer was investigated experimentally in a water tunnel. To examine the ultrasonic effects, the transducer emitted waves perpendicular to the mainstream direction, and the freestream velocity varied between 0.12 and 0.17 m/s. The transducer position was fixed at 0.45 m from the plate leading edge in a streamwise direction. This position was altered in a heightwise direction as dimensionless heightwise distances (Z) that were divided by the diameter of the horn-tip transducer (12 mm) at 1.5, 3.0, and 4.5 from the plate surface. A hot film sensor and thermochromic liquid crystals coated on the test surface were used to demonstrate the hydrodynamic and thermal characteristics, respectively. The maximum temperature reduction of 3.5 °C was observed in the case of Z = 1.5. By employing a horn-type transducer, the acoustic jet and cavitation phenomena played a key role in destabilizing the thermal and velocity boundary layers, which resulted in a decrease in surface temperature. The incident wave shape was triangular, with an enlarging lateral zone along the flow stream. When the waves were released at Z = 1.5, the local Nusselt number was increased up to 2.55 compared to without waves. Under ultrasonic effects, the near-wall flow velocity was detected to be decreasing upstream prior to the incident zone while increasing in the incident region. When waves existed, the highest turbulent intensity was up to 29 times more intense than it was in the absence of waves. Furthermore, a predictive formula for the Nusselt number with ultrasound was established depending on the local Reynolds number and the distance between the ultrasonic source and test surface.

A new wall function method for hypersonic laminar boundary layers

A new wall function method for hypersonic laminar boundary layers (HLBLs) is proposed to reduce the near-wall grid dependence of skin friction $c_f$ and wall heat flux $q_w$ in numerical simulations, aiming for fast and accurate predictions. First, an analytic laminar velocity law of the wall is derived, which achieves a universal scaling of the near-wall velocity of HLBLs. Then an accurate temperature–velocity relation is deduced by introducing the general recovery factor to address the invalidation of the Walz relation under the cold wall effect. Based on the laminar laws of the wall, a new wall function method for HLBLs is proposed. To avoid introducing the boundary layer edge quantities, the laminar laws of the wall are reformed by modifying the outer boundary conditions of the differential equation in deriving the temperature–velocity relation. Unlike the wall function method in turbulence, the new wall function obtains directly the accurate $c_f$ and $q_w$ by post-processing without being involved in the simulation iteration. The numerical experiments of a Mach 8 HLBL over the flat plate show that effectively, the new wall function can enlarge the distance of the first grid point off the wall $\Delta y_1$ from $10^{-6}$ m to $10^{-3}$ m, which brings a 50 times enhancement of the simulation efficiency. Meanwhile, the simulation errors of $c_f$ and $q_w$ of the mesh with $\Delta y_1=10^{-3}$ m are reduced significantly from 24.2 % and 18.5 % to 0.5 % and 0.1 %, respectively. Due to the new wall function removing the boundary layer edge quantities, success is also achieved under the curved walls.

Analysis of the relationship between near-wall velocity distribution and wall heat flux under diesel spray flame impingement in an RCEM

To clarify the relationship between the near-wall flow and the heat flux through the wall, the velocity distribution of the diesel spray flame and the heat flux at the combustion chamber wall were measured in a rapid compression and expansion machine (RCEM), which has a twodimensional combustion chamber. The velocity distribution of the diesel spray flame near the wall was measured by the particle image velocimetry (PIV), and the heat flux through the wall where the diesel spray flame impinges was measured using a newly developed multi-point heat flux sensor. Furthermore, the velocity gradient tensor was calculated based on the measured velocity distribution results, and the effect of the shear flow characteristics on wall heat transfer was analyzed. As a result, the higher the injection pressure leads the higher the peak value of the heat flux. A fluctuation of the heat flux exhibits the same trend as the fluctuations of the velocity, and a high heat flux appeared in the high-velocity gradient tensor region, which suggests that the characteristics of shear flow near the wall can affect the heat flux.

Abstract WP297: Time-Dependent Effects of Drag-Reducing Polymers Administration on Permanent Middle Cerebral Artery Occlusion Injury in Rats

Introduction: Current treatments for ischemic stroke are not focused directly on cerebral microcirculation. We previously showed that 2 μg/ml of drag-reducing polymers (DRPs) restore cerebral microcirculation and oxygenation in rats subjected to permanent middle cerebral artery occlusion (pMCAO). Here we evaluated the dependence of DRPs efficiency on stroke outcome from the time of application. Methods: DRPs (2 μg/ml) or saline was i.v. injected 0.5, 3, and 6 hrs after filament pMCAO in rats (n=10/group). In-vivo 2-photon laser scanning microscopy (2PLSM) was used to assess the acute effects of DRPs on cerebral microcirculation, hypoxia (NADH), and blood-brain barrier (BBB). Cerebral infarction, blood flow (CBF), and neurological outcomes were evaluated by magnetic resonance imaging (MRI) and behavioral testing, respectively, at 24 hours, 1 and 3 weeks after pMCAO. Differences between groups were determined using a two-way analysis of variance for multiple comparisons and posthoc testing with the statistical significance level set at p<0.05. Results: In vivo 2PLSM showed that pMCAO progressively decreased cerebral microcirculation leading to hypoxia and BBB disruption (p<0.05 from baseline). DRPs (0.5-6 hrs) increased near-wall blood flow velocity and flow rate in arterioles, leading to an increase in the number of erythrocytes entering capillaries, capillary perfusion and oxygenation while protecting BBB compared to saline (p<0.05). MRI demonstrated reduced infarct expansion (T2) and mitigated CBF disturbances (arterial spin labeling) in all DRPs-treated groups compared to saline (p<0.05). Neurobehavioral testing revealed more preserved neurological function in all DRPs groups than in the saline group (p<0.05). The obtained results showed that DRPs efficacy reduces with the time of application from 0.5 h to 6 hours after pMCAO but remains significant (p<0.05). Conclusions: DRPs (effectively restore cerebral microcirculation after pMCAO reducing tissue hypoxia and BBB damage, leading to reduced infarct size and improved neurological outcome in a time-dependent manner. DRPs can be used as a new effective adjunct therapy for ischemic stroke even without reperfusion and after delayed administration following the stroke onset.

On the evolution of turbulent boundary layers during flame–wall interaction investigated by highly resolved laser diagnostics

The turbulent boundary layer behavior in the presence of flame–wall interactions (FWI) has an important role on the mass and energy transfer at the gas/solid interface. Detailed experiments resolving the turbulent boundary layer evolution in the presence of FWI are lacking, which impedes knowledge. This work presents a combination of particle image velocimetry (flow field), dual-pump coherent anti-Stokes Raman spectroscopy (gas temperature), and OH laser induced fluorescence (flame topology) measurements to study the evolution of the boundary layer structure in the presence of FWI. Experiments are conducted in a side-wall quenching (SWQ) burner. Findings reveal that the reacting boundary layer flow adheres to the linear scaling law u+=y+ in the viscous sublayer until y+ = 5. Beyond y+ = 5, the flame modifies the velocity and temperature field such that the uz+ streamwise velocity deviates from the viscous sublayer and the law-of-the-wall scaling in the log-layer with uz+ being smaller than that of the non-reacting flow (the subscript z refers to the streamwise coordinate and is used throughout this manuscript). As the fluid approaches the flame impingement location at the wall, the gas temperature increases significantly, causing a threefold increase in kinematic viscosity, ν. Although the near-wall streamwise velocity gradient d〈Uz〉/dy|y=0mm decreases, the larger increase in ν reduces uz+ and leads to the deviation from the law-of-the-wall. Downstream the flame impingement location, ν is relatively constant and uz+ values begin to approach those of the law-of-the-wall. Trends are presented for SWQ and head-on quenching flame topologies, and are intended to help development of more accurate wall models.

Assimilating experimental data of a mean three-dimensional separated flow using physics-informed neural networks

In this article, we address the capabilities of physics-informed neural networks (PINNs) in assimilating the experimentally acquired mean flow of a turbulent separation bubble occurring in a diffuser test section. The training database contains discrete mean pressure and wall shear-stress fields measured on the diffuser surface as well as three-component velocity vectors obtained with particle image velocimetry throughout the volumetric flow domain. Imperfections arise from the measurement uncertainty and the inability to acquire velocity data in the near-wall region. We show that the PINN methodology is suited to handle both of these issues thanks to the incorporation of the underlying physics that, in the present study, are taken into account by minimizing residuals of the three-dimensional incompressible Reynolds-averaged Navier–Stokes equations. As a result, measurement errors are rectified and near-wall velocity profiles are predicted reliably. The latter benefits from the incorporation of wall shear-stress data into the PINN training, which has not been attempted so far to the best of our knowledge. In addition to demonstrating the influence of this novel loss term, we provide a three-dimensional, highly resolved, and differentiable model of a separating and reattaching flow that can be readily used in future studies.