Near-wall Velocity Research Articles

Thermochemical non-equilibrium hypersonic flow over a rectangular cavity embedded on a compression ramp

This paper reports a systematic computational investigation that elucidates the fundamental thermochemical non-equilibrium physics that occurs when air at Mach number of 11 encounters a rectangular cavity of aspect ratio L/D = 2.0 embedded on a 25° compression ramp. The mechanistic details of this highly complex flow phenomenon are obtained by solving the compressible form of the Navier–Stokes equations in two dimensions using a finite-volume open-source library. Chemical and thermal non-equilibrium processes are treated using a five-species, 12-reaction chemical kinetics, and a two-temperature model, respectively. Following a detailed validation and grid sensitivity study, two simulations are conducted, one with isothermal boundary conditions and the other with conjugate heat transfer (CHT) to identify the effect of energy transmission to the material on surface heat flux. Fast Fourier transforms and near-wall velocity profiles inside and in the neighborhood of the cavity are used to identify primary oscillatory modes and shear layer dynamics. Two new descriptive states defined as “states I and II,” representative of the minimum and maximum deflection of the shear layer, are used to discuss the dynamical behaviors in the cavity, including the separation region before the cavity, trailing edge effects, frequency analysis of probe data collected at several key locations, and the effect of CHT on surface heat flux. It is found that the flow features at the cavity's center strongly influence the separation upstream of the cavity, and the transrotational temperature near the cavity's trailing edge is strongly correlated with the oscillations of the shear layer.

Data-driven prediction of the equivalent sand-grain roughness

Surface roughness affects the near-wall fluid velocity profile and surface drag, and is commonly quantified by the equivalent sand-grain roughness {k}_{s}. It is essential to estimate {k}_{s} for accurate fluid dynamic problem modeling. While numerous roughness correlation formulas have been proposed to predict {k}_{s} in the fully rough regime, most of them are restricted to certain roughness types, with various geometric parameters considered in each case, leading to ongoing disagreements regarding its parameterization and lack of universality. In this study, a Particle Swarm Optimized Backpropagation (PSO-BP) method is proposed to predict {k}_{s} based on the selected surface parameters from previous DNS, LES, and experimental results for flow behavior over various surface roughness. The PSO-BP model’s ability to predict {k}_{s} in the fully rough region is evaluated and compared with both the existing roughness correction formulas as well as the traditional BP model. An optimized polynomial function is also proposed to serve as a ‘white box’ for predicting {k}_{s}. It turns out that the PSO-BP method has better performance in the evaluation metrics compared to other methods, yielding a Mean Absolute Error (MAE) of 0.0390, a Mean Squared Error (MSE) of 0.0026 and a Mean Absolute Percentage Error (MAPE) of 28.12%. This novel approach for estimating {k}_{s} has practical applicability and holds promise for improving the precision and efficiency of calculations related to equivalent sand-grain roughness, and thus provides more accurate and effective solutions for CFD and other engineering applications.

Accurate near-wall measurements in wall bounded flows with optical flow velocimetry via an explicit no-slip boundary condition

High fidelity near-wall velocity measurements in wall bounded fluid flows continue to pose a challenge and the resulting limitations on available experimental data cloud our understanding of the near-wall velocity behavior in turbulent boundary layers. One of the challenges is the spatial averaging and limited spatial resolution inherent to cross-correlation-based particle image velocimetry (PIV) methods. To circumvent this difficulty, we implement an explicit no-slip boundary condition in a wavelet-based optical flow velocimetry (wOFV) method. It is found that the no-slip boundary condition on the velocity field can be implemented in wOFV by transforming the constraint to the wavelet domain through a series of algebraic linear transformations, which are formulated in terms of the known wavelet filter matrices, and then satisfying the resulting constraint on the wavelet coefficients using constrained optimization for the optical flow functional minimization. The developed method is then used to study the classical problem of a turbulent channel flow using synthetic data from a direct numerical simulation (DNS) and experimental particle image data from a zero pressure gradient, high Reynolds number turbulent boundary layer. The results obtained by successfully implementing the no-slip boundary condition are compared to velocity measurements from wOFV without the no-slip condition and to a commercial PIV code, using the velocity from the DNS as ground truth. It is found that wOFV with the no-slip condition successfully resolves the near-wall profile with enhanced accuracy compared to the other velocimetry methods, as well as other derived quantities such as wall shear and turbulent intensity, without sacrificing accuracy away from the wall, leading to state of the art measurements in the region of the turbulent boundary layer when applied to experimental particle images.

Boundary layer measurements of the flow along a streamwise-oriented circular cylinder using particle tracking velocimetry

Particle tracking velocimetry (PTV) measurements of an axisymmetric turbulent boundary layer (ATBL) around a long circular cylinder in axial flow were carried out in a large-scale water tunnel at different velocities and at different measurement positions along the cylinder. A relatively thick ATBL developed with ratios of δ/a between 4.6 and 8.1 and a/δν between 313 and 1027, where δ is the boundary layer thickness, δν the viscous length scale and a the cylinder radius. At ratios of δ/a above unity and a/δν above 250, transverse curvature effects are known to influence the boundary layer in the outer region. This was observed in the measured mean velocity profiles as the profiles fall below the planar logarithmic law of the wall. Shape factors close to unity with a rather full velocity profile were determined that explain the relatively high skin friction coefficient known to be present for such ATBL flows. Additionally, measurements of the turbulence intensity and Reynolds stress profiles are presented. The non-intrusive PTV measurement approach allowed to gather near-wall velocity data down to a wall-distance below 10 wall-units. The present study provides valuable information regarding ATBL flows along streamwise-oriented cylinders in the flow regime indicated above by the ranges of δ/a and a/δν. In particular, it fills a gap in the literature as far as experimentally obtained higher-order statistical quantities are concerned, such as root-mean-square velocity fluctuations and Reynolds stresses.

Analysis of pulse electromagnetic electroosmotic flow of Jeffrey fluid through parallel plate microchannels under a constant pressure gradient

The purpose of this work is to investigate the pulse electromagnetic electroosmotic flow (EOF) behavior of a Jeffrey fluid in parallel plate microchannels under a constant pressure gradient. Analytical solutions for the velocity and volumetric flow rate are derived by employing the Laplace transform method and the residue theorem. In particular, the electric double layer (EDL) effect is considered, and an analysis is conducted on both the near-center and near-wall velocities. The flow periodicity is also noted, and these velocities are evaluated during the first and second half-cycles, respectively. Moreover, both 3D and 2D graphics are simultaneously utilized to visualize the impact of relevant parameters on velocity and volumetric flow rate. Our analysis yielded the following key findings: The effects of relevant parameters, such as pulse width a¯, pressure gradient Ω, Hartmann number Ha, electric field strength β, electrokinetic width K, relaxation time λ¯1 and retardation time λ¯2, on the near-center velocity during the first half-cycle align with the results of previous studies. During the second half-cycle, higher values of variables Ha and β lead to reduced velocities. Interestingly, during any half-cycle, the near-wall velocity demonstrates similar characteristics to the near-center velocity in response to variations in certain parameters, including Hartmann number Ha, electric field strength β, relaxation time λ¯1 and retardation time λ¯2. It is worth mentioning that the influence of relaxation time λ¯1 and retardation time λ¯2 on the near-wall velocity is largely independent of the Hartmann number, especially for the retardation time. Regardless of the values of the parameters λ¯1 and λ¯2, a higher volumetric flow rate is observed with an increased electrokinetic width K. Additionally, the volumetric flow rate profile of the Jeffrey fluid exhibits a comparatively smoother trend over time compared to that of the Maxwell fluid. The remarkable similarity between Jeffrey fluid and blood suggests that the results presented in this paper hold immense potential as a crucial reference for studying blood transport within the field of microfluidics.

Exploration of transient flow perturbations in a bulb turbine diffuser using proper orthogonal decomposition

Hydraulic turbines with high efficiency over a wide range of operating conditions offer a much sought-after flexibility to electricity producers. However, some low-head turbines exhibit sharp losses of efficiency close to their peak efficiency discharge that are linked to draft tube flow separations whose causes remain misunderstood. This paper presents the latest results obtained in the scope of the BulbT project that focused on the flow dynamics related to the efficiency drop. The main objectives are to document the transient characteristics of the flow in the hub-wake region and investigate interactions between the core flow and wall separations to identify potential mechanisms explaining the efficiency drop. Using proper orthogonal decompositions of synchronized time-resolved velocity and pressure measurements, highly energetic modes representing stochastic perturbations across BulbT's draft tube are identified. These perturbations occur only for the discharges affected by the efficiency drop, past the best efficiency point. Despite the absence of near-wall velocity measurements, the modal decompositions provide evidence that the onset of the efficiency drop is the result of two independent types of flow separation that occur on opposite sides of the draft tube. Upstream separations are found to happen simultaneously with an asymmetric acceleration of the flow in a region surrounding the turbine's axis and the tip of the runner hub and become the most important contributor to the efficiency drop at the highest measured flow rate. Furthermore, the likeliness of observing one kind of flow separation increases after it has already occurred, pointing to a strong history effect.

Influence of morphological characteristics on the deposition of low-density lipoprotein in intracranial bifurcation aneurysms

The increasingly demonstrated association of wall enhancement (WE) measured by vascular wall imaging with the instability/rupture of intracranial aneurysms (IAs) implies the significance of investigating the transport and accumulation of blood substances [e.g., low-density lipoprotein (LDL), lipoprotein (a)] related to WE in IAs. In the present study, we perform numerical simulations to explore the relationships between the distribution/severity of LDL deposition on the lumen surface and the morphological characteristics of aneurysm and its adjacent arteries as well as the underlying fluid dynamic mechanisms. Bifurcation aneurysms located at the middle cerebral artery are selected as the subject of investigation and for which both idealized and patient-specific models are built. Studies on the idealized models reveal that the aneurysm–parent (AP) artery angle is more powerful than other morphological parameters [e.g., daughter–parent (DP) artery angle, nonsphericity index (NI) of aneurysm] in determining the severity of LDL deposition. A bleb (i.e., secondary sac) can enhance local LDL deposition, especially when located in an LDL deposition-prone aneurysm wall region. In patient-specific models, the dominant effect on the LDL deposition of the AP angle remains identifiable, whereas the effects of the DP angle and NI are overwhelmed by confounding factors. Enhanced LDL deposition prefers to appear in wall regions perpendicular to the intra-aneurysmal vortex centerline. In comparison with wall shear stress magnitude, the normal-to-tangential near-wall velocity ratio and particle residence time can better predict the localization of enhanced LDL deposition.

The hunt for the Kármán ‘constant’ revisited

The log law of the wall, joining the inner, near-wall mean velocity profile (MVP) in wall-bounded turbulent flows to the outer region, has been a permanent fixture of turbulence research for over hundred years, but there is still no general agreement on the value of the prefactor, the inverse of the Kármán ‘constant’ $\kappa$ , or on its universality. The choice diagnostic tool to locate logarithmic parts of the MVP is to look for regions where the indicator function $\varXi$ (equal to the wall-normal coordinate $y^+$ times the mean velocity derivative $\mathrm {d} U^+/\mathrm {d} y^+$ ) is constant. In pressure-driven flows, however, such as channel and pipe flows, $\varXi$ is significantly affected by a term proportional to the wall-normal coordinate, of order $O({Re}_{\tau }^{-1})$ in the inner expansion, but moving up across the overlap to the leading $O(1)$ in the outer expansion. Here we show that, due to this linear overlap term, ${Re}_{\tau }$ values well beyond $10^5$ are required to produce one decade of near constant $\varXi$ in channels and pipes. The problem is resolved by considering the common part of the inner asymptotic expansion carried to $O({Re}_{\tau }^{-1})$ , and the leading order of the outer expansion. This common part contains a superposition of the log law and a linear term $S_0 \,y^+{Re}_{\tau }^{-1}$ , and corresponds to the linear part of $\varXi$ , which, in channel and pipe, is concealed up to $y^+ \approx 500\unicode{x2013}1000$ by terms of the inner expansion. A new and robust method is devised to simultaneously determine $\kappa$ and $S_0$ in pressure-driven flows at currently accessible ${Re}_{\tau }$ values, yielding $\kappa$ values which are consistent with the $\kappa$ values deduced from the Reynolds number dependence of centreline velocities. A comparison with the zero-pressure-gradient turbulent boundary layer, further clarifies the issues and improves our understanding.

Wall-shear-stress measurements using volumetric µPTV

Accurately determining the wall-shear-stress, tau _textrm{w}, experimentally is challenging due to small spatial scales and large velocity gradients present in the near-wall region of turbulent flows. To avoid these resolution requirements, several indirect iterative fitting methods, most notably the Clauser chart method, exist for determining overline{tau }_{w} by fitting the mean velocity profile further away from the near-wall region in the log-law layer. These methods often require proper selection of fitting constants, assumptions of a canonical flow state, and other empirical-based generalizations. To reduce the amount of ambiguity, determining the near-wall velocity gradient by assuming a linear relationship between the mean streamwise velocity and wall normal distance in the viscous sublayer can be used. However, this requires an accurate unbiased measurement of the near-wall velocity profile in the region below five viscous spatial units, which can be less than 50 µm for high Reynolds number flows. Therefore, in this study a method for a volumetric defocusing microparticle tracking velocimetry method is presented that is capable of resolving the flow in the viscous sublayer of a turbulent boundary layer up to U_{textrm{e}}=44.7,m/s (Re_{theta }=27250). This method allows for the measurement of the near-wall flow through a single optical access for illumination and imaging and serves as an excellent complement of larger scale measurements that require near-wall information. The overline{tau }_textrm{w} values determined from the defocusing approach were found to be in good agreement values obtained from a simultaneous parallax PTV measurement. Furthermore, analysis of the diagnostic plot and cumulative distribution of measured fluctuations in the near-wall region, showed that both methods are capable of accurately determining mean velocity and fluctuation profiles in the self-similar viscous sublayer region.

Robust Eddy Viscosity Turbulence Modeling with Elliptic Relaxation and Compound Wall Treatment

This paper presents a holistic Reynolds-averaged Navier–Stokes (RANS) turbulence modeling framework for the computational fluid dynamics (CFD) simulations of complex wall-bounded turbulent flows. Based on the elliptic relaxation idea, the deployed eddy viscosity turbulence model reconstructs the near-wall stress anisotropy and nonviscous effects. The appropriate selection of the turbulent quantities that are being solved for, together with the zero value wall boundary condition for the related turbulent quantities, renders the model less sensitive to the near-wall grid nonuniformities and resolution. The unified near-wall velocity profile, obtained based on the boundary layer theory, is used to devise the compound near-wall treatment that ensures the robustness of the numerical simulation. The proposed turbulence modeling framework is implemented into the general-purpose open-source CFD code and validated against the generic test cases with satisfactory agreement.

Lie-group method solutions for a viscous flow in a dilating-squeezing permeable channel with velocity slip

The investigation focuses on the effects of wall velocity slip on the solution of a viscous, laminar, incompressible channel flow subjected to small-scaled contraction and expansion of the weakly permeable walls. The study of such flow systems is often contextual for fluid transport in biological organisms. In the considered flow configuration, the vertically moving porous walls enable the fluid to enter or exit with a constant rate. The tangential slip velocity of the flow at the porous walls is modeled with the Navier slip boundary condition. The flow dynamics inside the channel is governed by the full Navier–Stokes equations. The Lie symmetry analysis and the invariant method are adopted to reduce the number of independent variables in the system of governing equations. Consequently, a single fourth-order ordinary differential equation is obtained, which is solved analytically by the double perturbation method and the variation of iteration method. The solutions are compared for different arrangements. Furthermore, the approximated analytical solutions are likened to the numerical solutions obtained from a fourth-order Runge–Kutta solver embedding the Shooting method to check the accuracy. It is observed that the boundary layers are formed, and the flow rapidly turns near the walls, when suction and wall contraction coexist. Alternatively, if injection and wall expansion are paired, the flow adjacent to the walls is delayed. The existence of wall velocity slip advances the near-wall velocity and cuts down the speed of centerline velocity. It results in a change in the volumetric flow rate and shear rate. The overall pressure is also varied by higher wall velocity slip. The results are explored for different values of the permeation Reynold number and the dimensionless wall dilation rate to capture all possible impacts of the flow parameters. The current analysis rectifies the existing errors in the work of Boutros et al. [“Lie-group method solution for two-dimensional viscous flow between slowly expanding or contracting walls with weak permeability,”Appl. Math. Model. 31(6), 1092–1108 (2007)] with the no-slip boundary condition and discusses the overall influences of slip boundary condition on the Lie symmetry solution of the flow system.

The optimization of biomimetic sharkskin riblet for the adaptation of drag reduction

The biomimetic sharkskin riblet has attracted more attention in engineering and research owing to the economic benefit of drag-reducing and antifouling properties. Although several works have optimized the parameters for a number of riblet designs, there is not yet a study dedicated to optimizing for rectangular cuboidal riblet parameters. Hence, this work proposes a new numerical optimization of riblet orientations and arrangements of a three-dimensional rectangular cuboidal riblet. The shear-stress transport k-omega was adopted as an appropriate turbulence model for the simulation. The results showed that the riblets efficiently improved drag reduction regardless of the flow state. Riblets oriented perpendicularly to the flow direction showed the greatest performance of 11.3% and 6% in the laminar and turbulent flow, respectively. Moreover, the analysis of the flow field characteristics near the wall revealed a significant improvement in terms of reduction in near-wall velocity and shear stress. This work offers new perspectives on the role of the intricate multifunctional architecture of shark-scale structures in improving swimming speed and opening new doors for marine and underwater applications.

Separating large-scale superposition and modulation in turbulent channels

The presence of very-large-scale motions in wall-bounded turbulent flows is commonly associated with their footprint in the form of the superposition of the large scales at the wall and the additional amplitude modulation of small-scale near-wall turbulence. These two phenomena are currently understood to be interlinked, with the superposed large-scale velocity gradient causing the modulation of small-scale activity in the proximity of the wall. To challenge this idea, we devise a numerical strategy that selectively suppresses either superposition or amplitude modulation, in an effort to isolate and study the remaining phenomenon. Results from our direct numerical simulations indicate that a positive correlation between the amplitude of the small scales in the near-wall region and the large-scale signal in the outer flow persists even when near-wall large-scale motions are suppressed – i.e. in absence of superposition. Clearly, this kind of correlation cannot be caused by the near-wall large-scale velocity or its gradients, as both are absent. Conversely, when modulation is blocked, the near-wall footprints of the large scales seem to disappear. This study has been carried out on channel flows at friction Reynolds number $Re_\tau =1000$ in both standard simulation domains and minimal streamwise units (MSUs), where the streamwise fluctuation energy is enhanced. The consistency of the results obtained by the two approaches suggests that MSUs can capture correctly this kind of scale interaction at a much reduced cost.

Confined Boundary-Layer Flashback Flame Dynamics in a Turbulent Swirling Flow

Understanding boundary-layer flashback is critical to the design of safe and efficient gas turbines, especially as the addition of highly reactive hydrogen to these devices becomes a prevailing trend for reduction of carbon dioxide emissions. In this work, the boundary-layer flashback of lean hydrogen–air mixtures is studied using large-eddy simulations based on a generic turbulent swirl burner previously investigated experimentally. Simulations at increasing equivalence ratios are used to identify the flashback limits, showing reasonable qualitative agreement with experimental measurements. Flame dynamics during flashback are compared to previous experimental studies, indicating that important physics are captured in the simulations even if the exact flashback limits are not. In particular, a change in the swirl vane angle is shown to dramatically change the flame dynamics, with flame propagation occurring in the direction of swirl at high angles and against the direction of swirl at low angles, consistent with experimental observations. The near-wall nonreacting mean velocity field is shown to be controlled by a temporally stationary flow instability, which is directly responsible for the counterswirl flame flashback exhibited at low swirl angles. Finally, the interaction of local axial flow velocity and flame propagation speed of the leading flame point is investigated with varying swirl angles both before and after the onset of flashback, elucidating the differences in flame dynamics in all four of these cases. In particular, the importance of local flame extinction on flashback limits is emphasized.

Effect of mainstream oxygen concentration on supersonic film cooling and friction reduction performance using hydrocarbon fuel

One of the effective methods to simultaneously achieve the thermal protection and friction reduction requirement of a scramjet engine is the usage of fuel film cooling. As the combustion process inside the combustor proceeds, the oxygen concentration in the burnt gas gradually decreases and will influence the thermal and friction characteristics of the hydrocarbon film. Therefore, the effects of oxygen concentration in the mainstream on the hydrocarbon film are evaluated by numerical simulations based on RANS framework. The results indicate that the cooling performance of the hydrocarbon film is not sensitive to the oxygen concentration, although the fuel film burns more intense as the oxygen concentration increases. While the friction reduction performance is sensitive to the oxygen concentration. Specifically, about 10% friction reduction can be obtained for 10% oxygen in the mainstream compared to that of inert cases, and the wall friction can be further reduced by 30% for 20% oxygen. Further analysis reveals that the mechanism of the above two drag reductions is different. For weak boundary layer combustion, the decrease of the molecular viscosity contributes to the reduction of friction. While for significant boundary layer combustion, a lower near-wall velocity gradient is the main reason.

Assessment of Computational Fluid Dynamic Modeling of Multi-Jet Impingement Cooling and Validation With the Experiments

Abstract The current study involves numerical and experimental investigations of circular in-line jets impinging on a heated flat plate. The generic configuration is characterized by nine jets, each with a diameter of D = 0.0152 m. The jets are influenced by a self-generating crossflow and are positioned at a nozzle-to-plate distance (H/D) of 5 and a jet pitch (p/D) of 5. The steady Reynolds-averaged Navier–Stokes (RANS) simulations are performed for turbulent jet Reynolds numbers with the in-house CFD code TRACE. The Menter k–ω shear stress transport (SST) model is applied for turbulence modeling and the turbulent scalar fluxes are modeled based on the Reynolds analogy for a constant turbulent Prandtl number. To gain a closer insight into the impingement jet physics, high-resolution near-wall velocity and thermal fields are obtained through large eddy simulations (LESs) and measurements from particle image velocimetry (PIV). Focus is laid on the comparison of RANS results with the LES data and the experimental data. The results exhibit a qualitative similarity between the simulations and the experiments. Furthermore, correlations of the Nusselt number from the literature are used to validate the simulation results.

Abstract WMP118: Sex-specific And Dose-dependent Effects Of Drag-reducing Polymers On Post-traumatic Ischemia In Rats After Traumatic Brain Injury Of Different Severity

Introduction: Previously, we showed that drag-reducing polymers (DRP) improve hemodynamics in rat models of brain injury (TBI). Here we examined the dose- and sex-dependent DRP effects on post-TBI ischemia. Methods: In-vivo 2-photon microscopy over the rat parietal cortex was used to monitor the effect of DRP on microvascular perfusion and tissue oxygenation (NADH) and blood-brain barrier permeability. Lateral fluid-percussion TBI (1.5 ATA-moderate or 2.5 ATA-severe, 100 ms) was induced after baseline imaging and followed by 4 hours of monitoring. DRP injected at 1, 2, or 4 ppm. Data analysis was done by GraphPad Prism 7. Results: Moderate TBI progressively decreased microvascular circulation and hypoxia in the pericontusion zone (p<0.05). The i.v. injection of DRP increased near-wall flow velocity and flow rate in arterioles, leading to an increase in the number of erythrocytes entering capillaries, enhancing capillary perfusion in a dose-dependent manner without distinguishable difference between males and females (p<0.01). The severe TBI resulted in intracranial pressure increase (31±3.2 mmHg, p<0.05), leading to microcirculation redistribution to non-nutritive microvascular shunt (MVS) flow and stagnation of capillary flow. Tissue hypoxia was more prominent, especially in male rats; the pericontusion zone was 25±5.2% larger than after moderate TBI (p<0.01). Both were reverted by DRP in a dose-related manner with better efficiency in females (p<0.01). After severe TBI, BBB degradation was faster and more prominent. DRP was more efficient in attenuating the progression of permeability increase in moderate TBI (p<0.05). Conclusion: DRP at 4 ppm was most efficient, with a better effect on female rats. Supported by NIH/NINDS R01NS112808

Characteristics of the shedding vortex around the Coanda surface and its impact on circulation control airfoil performance

This study investigates the characteristics of a shedding vortex around the Coanda surface and its impact on circulation control (CC) performance. Delay detach-eddy simulation based on the two-equation k−ω shear stress transport (SST) turbulence model is performed to solve the flow field. The simulation results show that near the Coanda trailing edge, large vortices periodically shed from the lip above the jet slot, and the shedding vortex is surrounded by a series of strip vortices during its downstream movement. The shedding vortex transfers the disturbance to the jet boundary layer and affects the near-wall velocity. Dynamic mode decomposition shows that the shedding vortex and its high-order harmonic flow structures attenuate very slowly and dominate the flow field. The mode with sub-harmonic frequency of the shedding vortex displays the strip vortices dragged out by the shedding vortex. A larger shedding vortex can intensify the mixing of the jet shear and boundary layers, making the jet velocity decay faster. The size of the shedding vortex can be reduced by decreasing the lip height, thus slowing down the attenuation of the jet velocity and improving the CC performance.

Sex-Specific and Dose-Dependent Effects of Drag-Reducing Polymers on Microcirculation and Tissue Oxygenation in Rats After Traumatic Brain Injury.

Traumatic brain injury (TBI) ultimately leads to a reduction in the cerebral metabolic rate for oxygen due to ischemia. Previously, we showed that 2ppm i.v. of drag-reducing polymers (DRP) improve hemodynamic and oxygen delivery to tissue in a rat model of mild-to-moderate TBI. Here we evaluated sex-specific and dose-dependent effects of DRP on microvascular CBF (mvCBF) and tissue oxygenation in rats after moderate TBI. In vivo two-photon laser scanning microscopy over the rat parietal cortex was used to monitor the effects of DRP on microvascular perfusion, tissue oxygenation, and blood-brain barrier (BBB) permeability. Lateral fluid-percussion TBI (1.5 ATA, 100ms) was induced after baseline imaging and followed by 4h of monitoring. DRP was injected at 1, 2, or 4ppm within 30 min after TBI. Differences between groups were determined using a two-way ANOVA analysis for multiple comparisons and post hoc testing using the Mann-Whitney U test. Moderate TBI progressively decreased mvCBF, leading to tissue hypoxia and BBB degradation in the pericontusion zone (p<0.05). The i.v. injection of DRP increased near-wall flow velocity and flow rate in arterioles, leading to an increase in the number of erythrocytes entering capillaries, enhancing capillary perfusion and tissue oxygenation while protecting BBB in a dose-dependent manner without significant difference between males and females (p<0.01). TBI resulted in an increase in intracranial pressure (20.1±3.2mmHg, p<0.05), microcirculatory redistribution to non-nutritive microvascular shunt flow, and stagnation of capillary flow, all of which were dose-dependently mitigated by DRP. DRP at 4ppm was most effective, with a non-significant trend to better outcomes in female rats.

An improved pressure drop correlation for modeling localized effects in a pebble bed reactor

Advances in the development of pebble bed reactors (PBRs) has created a desire for accurate and cost-effective simulation tools for design scoping studies and safety analysis. The current state-of-the-art for these simulations is the use of porous media models, although these models rely on correlations to capture the effects of flow features that are not explicitly modeled. One of the areas where correlation accuracy is currently lacking is in the near-wall region of the bed. In this region, the presence of the wall causes the pebbles to pack more orderly, drastically changing the geometry and flow behavior in this region.This work presents a new generalized pressure drop correlation for PBRs based on the KTA equation. A high-to-low methodology is applied, where large eddy simulation (LES) is performed on two beds of 1568 and 1700 pebbles to generate a high-fidelity dataset. The flow fields are then averaged in time and separated into concentric rings of 0.05 Dpeb width. Average porosity, velocity, and pressure drop are extracted for each ring and the friction and form losses are calculated. The Reynolds number range for this study is 625–10,000, and thus the form losses are dominant over the friction losses. The form losses across the rings are investigated, and a correction term for the form loss calculation is determined and applied to the KTA equation to drastically improve the capability of modeling localized porosity effects in a porous media code. The improved correlation reduces near-wall velocity prediction error from over 50% with the KTA correlation to around 5%. Agreement in pressure drop prediction between LES and porous media simulations is also improved.