Periodic Wake Research Articles

Self-sustaining turbulent wake characteristics in fluid–structure interaction of a square cylinder

We present a numerical study on the fluid–structure interaction of a square cylinder at subcritical Reynolds numbers 1400≤Re≤10,000. Variation in spatial and temporal structures during the self-sustaining regeneration cycle are investigated via a three-dimensional Navier–Stokes flow coupled with a freely vibrating square cylinder at relatively low mass ratio. We employ a variational fluid–structure interaction formulation based on the recently developed partitioned iterative scheme and the dynamic subgrid-scale turbulence model. To begin, we assess the response amplitudes, the synchronization regimes and the vortex shedding patterns against the experimental measurements for the flow-induced vibration of a square cylinder at zero incidence angle. Of particular interest is to predict and analyze the synchronization regimes and the associated wake structures for a range of reduced velocity. The vibration of the cylinder provides an avenue for the merging of smaller eddies in the vicinity of the cylinder and there are relatively more clustered spanwise rollers and streamwise ribs as compared to the stationary counterpart. We provide a comparative assessment of Reynolds stress distributions in the near-wake region between the VIV lock-in case and its stationary counterpart. We find that the spatial symmetry of the shearing process in the wake shifts to the temporal symmetry of Reynolds stress when the cylinder is free to vibrate. Consequently, in the vibrating case, the competition between the mean shear growth and damping results in a relatively lower frequency shearing as compared to the stationary cylinder. We introduce a representative control volume in the near-wake region to assess the kinetic energy and enstrophy evolution for the stationary and vibrating configurations. We examine the core reason of the matching of periodic wake frequency with the vibrating cylinder frequency through the development of near-wake flow structures and the kinetic energy evolution. By combining these results with the self-sustained process of coherent vortex structure development, we finally explain the formation of intermediate hairpin-like structures in close proximity to the vibrating cylinder and the absence of them in the stationary cylinder.

Effects of periodic wakes on the endwall secondary flow in high-lift low-pressure turbine cascades at low Reynolds numbers

Periodic wakes affect not only the surface boundary layer characteristics of low-pressure turbine blades and profile losses but also the vortex structures of the secondary flow and the corresponding losses. Thus, understanding the physical mechanisms of unsteady interactions and the potential to eliminate secondary losses is becoming increasingly important for improving the performance of high-lift low-pressure turbines. However, few studies have focused on the unsteady interaction mechanism between periodic wakes and endwall secondary flow in low-pressure turbines. This paper verified the accuracy of computational fluid dynamics by comparing experimental results and those of the numerical predictions by taking a high-lift low-pressure turbine cascade as the research object. Discussion was focused on the interaction mechanisms between the upstream wakes and secondary flow within the high-lift low-pressure turbine. The results indicated that upstream wakes have both positive and negative effects on the endwall flow, where the periodic wakes can decrease significantly the size of the separation bubble, prevent the formation of secondary vorticity structures at relatively high Reynolds numbers (100,000 and 150,000), and reduce the cross-passage pressure gradient of cascade. In addition, periodic wakes can improve the cascade incidence characteristic in terms of reducing the overturning and underturning of the secondary flow at downstream of the cascade all of which are beneficial for decreasing the endwall secondary losses, whereas more endwall boundary layer is involved in the main flow passage due to the wake transport, resulting in increased strength of the secondary flow at low Reynolds number of 25,000 and 50,000. Compared with the results without wakes, the total pressure loss for unsteady condition at the cascade exit decreases by 2.7% and 6.1% at high Reynolds number of 100,000 and 150,000, respectively. However, the secondary loss at unsteady flow conditions increases at low Reynolds number of 25,000 and 50,000.

Modeling of Laminar-Turbulent Transition in Boundary Layers and Rough Turbine Blades

A local, intermittency-function-based transition model was developed for the prediction of laminar-turbulent transitional flows with freestream turbulence intensity Tu at low (Tu < 1%), moderate (1% < Tu < 3%), and high Tu > 3% levels, and roughness effects in a broad range of industrial applications such as turbine and helicopter rotor blades, and in nature. There are many mechanisms (natural or bypass) that lead to transition. Surface roughness due to harsh working conditions could have great influence on transition. Accurately predicting both the onset location and length of transition has been persistently difficult. The current model is coupled with the k–ω Reynolds-averaged Navier–Stokes (RANS) model, that can be used for general computational fluid dynamics (CFD) purpose. It was validated on the ERCOFTAC experimental zero-pressure-gradient smooth flat plate boundary layer with both low and high leading-edge freestream turbulence intensities. Skin friction profiles agree well with the experimental data. The model was then tested on ERCOFTAC experimental flat plate boundary layer with favorable/adverse pressure gradients cases, periodic wakes, and flows over Stripf's turbine blades with roughness from hydraulically smooth to fully rough. The predicted skin friction and heat transfer properties by the current model agree well with the published experimental and numerical data.

Influence of the transition of a laminar separation bubble on the downstream evolution of strong adverse pressure gradient turbulent boundary layers

The evolution of boundary layers subject to adverse pressure gradient (APG) is strongly influenced by the upstream history. Hence, the detailed understanding of APG flows in general or downstream of reattachment of a separation bubble requires a clear acquaintance of the relation between the upstream flow structures and the downstream ones. In this work the results for three different direct numerical simulations (DNS) of APG flows are analyzed to scrutinize the relaxation of these flows having distinct development histories. The three cases have the same overall characteristics – laminar separation, transition, and turbulent reattachment – with respect to the imposed APG and the Reynolds number Reδ990 at the inlet. However, in the first case no additional perturbations are imposed to trigger transition, in the second case the flow is tripped by a trip wire and in the third case a periodic wake is superimposed on the flow. The detailed information provided by DNS is critical to evaluate the current turbulence models and develop new ones. Hence, main points that are discussed are the Reynolds stress budgets, two-point spatial correlations, and spanwise spectra. Attention is in particular directed to a comparison between the three different cases. The most important conclusion is that the flow downstream of the reattachment decorrelates faster with the flow at the transition position if transition to turbulence completes within the separation bubble than in the case the transition completes after reattachment.

Two- and three-dimensional wake transitions of an impulsively started uniformly rolling circular cylinder

This paper presents the characteristics of the different stages in the evolution of the wake of a circular cylinder rolling without slipping along a wall at constant speed, acquired through numerical stability analysis and two- and three-dimensional numerical simulations. Reynolds numbers between 30 and 300 are considered. Of importance in this study is the transition to three-dimensionality from the underlying two-dimensional periodic flow and, in particular, the way that the associated transitions influence the fluid forces exerted on the cylinder and the development and the structure of the wake. It is found that the steady two-dimensional flow becomes unstable to three-dimensional perturbations at $Re_{c,3D}=37$, and that the transition to unsteady two-dimensional flow – or periodic vortex shedding – occurs at $Re_{c,2D}=88$, thus validating and refining the results of Stewart et al. (J. Fluid Mech. vol. 648, 2010, pp. 225–256). The main focus here is on Reynolds numbers beyond the transition to unsteady flow at $Re_{c,2D}=88$. From impulsive start up, the wake almost immediately undergoes transition to a periodic two-dimensional wake state, which, in turn, is three-dimensionally unstable. Thus, the previous three-dimensional stability analysis based on the two-dimensional steady flow provides only an element of the full story. Floquet analysis based on the periodic two-dimensional flow was undertaken and new three-dimensional instability modes were revealed. The results suggest that an impulsively started cylinder rolling along a surface at constant velocity for $Re\gtrsim 90$ will result in the rapid development of a periodic two-dimensional wake that will be maintained for a considerable time prior to the wake undergoing three-dimensional transition. Of interest, the mean lift and drag coefficients obtained from full three-dimensional simulations match predictions from two-dimensional simulations to within a few per cent.

Numerical study of vortex shedding suppression and aerodynamic characteristics of three obstacle configurations having two shapes

Two-dimensional incompressible fluid flows around a rectangular shape placed over a larger rectangular shape at low Reynolds numbers have been numerically analyzed in the present work. The vortex shedding is investigated at different arrangements of the two shapes allowing the investigation of three possible configurations. The calculations are carried out for several values of Reynolds numbers ranging from 1 to 200. The effect of the obstacle geometry on the vortex shedding is analyzed for crawling, steady, and unsteady regimes. A special attention is paid to compute the drag and lift forces acting on the rectangular shapes, which allowed determining the best shapes configuration in terms of both drag and lift. The unsteady periodic wake is characterized by the Strouhal number, which varies with the Reynolds number and the arrangements of the two shapes studied. Hence, the values of vortex shedding frequencies are calculated in this work.

Engineering diagnostics for vortex-induced stay vanes cracks in a Francis turbine

Despite the fact that vortex-induced vibration (VIV) in hydraulic turbines components (especially in stay vanes) is a well-known phenomenon, it still remains challenging for operation and maintenance teams in several power plants around the world. Since the first publication of a similar problem in 1967, literature shows that at least 27 other turbines witnessed strong stay vane vibrations associated with vortex shedding. Recurrent stay vane cracks in a 250 MW Francis turbine in Brazil motivated an engineering study involving prototype measurements, structural and Computational Fluid Dynamics (CFD) analysis in order to determine a proper geometry modification that could eliminate the periodic vortex wake generated at the stay vanes trailing edge. First cracks appeared in 1978 just after the machine was put into operation. A study published in 1982 associated these cracks with dynamic excitations caused by the water flow at high flow conditions. New stay vane profiles were proposed and executed as well as improved welding recommendations. Cracks however, continued to appear requiring welding repairs roughly every two years. Although Voith Hydro was not the original equipment manufacturer for these units, the necessary information was available to study the issue and propose and execute new stay vane profiles. This paper details the approach taken for the study. First, indirect vibration measurements were used to determine vibration frequencies to help to characterize the affected mode shapes. These results were compared to finite element (FE) calculations. Strain gage measurements performed afterwards confirmed the conclusions of this analysis. Next, transient CFD calculations were run to reproduce the measured phenomenon and to serve as a basis for a new stay vane geometry. This modification was then implemented in the actual turbine stay vanes. A new set of indirect vibration measurements indicated the effectiveness of the proposed solution. Final confirmation will come from new strain gage measurements.

Effects of periodic wakes on boundary layer development on an ultra-high-lift low pressure turbine airfoil

The laminar-turbulent transition process in the boundary layer is of significant practical interest because the behavior of this boundary layer largely determines the overall efficiency of a low pressure turbine. This article presents complementary experimental and computational studies of the boundary layer development on an ultra-high-lift low pressure turbine airfoil under periodically unsteady incoming flow conditions. Particular emphasis is placed on the influence of the periodic wake on the laminar-turbulent transition process on the blade suction surface. The measurements were distinctive in that a closely spaced array of hot-film sensors allowed a very detailed examination of the suction surface boundary layer behavior. Measurements were made in a low-speed linear cascade facility at a freestream turbulence intensity level of 1.5%, a reduced frequency of 1.28, a flow coefficient of 0.70, and Reynolds numbers of 50,000 and 100,000, based on the cascade inlet velocity and the airfoil axial chord length. Experimental data were supplemented with numerical predictions from a commercially available Computational Fluid Dynamics code. The wake had a significant influence on the boundary layer of the ultra-high-lift low pressure turbine blade. Both the wake’s high turbulence and the negative jet behavior of the wake dominated the interaction between the unsteady wake and the separated boundary layer on the suction surface of the ultra-high-lift low pressure turbine airfoil. The upstream unsteady wake segments convecting through the blade passage behaved as a negative jet, with the highest turbulence occurring above the suction surface around the wake center. Transition of the unsteady boundary layer on the blade suction surface was initiated by the wake turbulence. The incoming wakes promoted transition onset upstream, which led to a periodic suppression of the separation bubble. The loss reduction was a compromise between the positive effect of the separation reduction and the negative effect of the larger turbulent-wetted area after reattachment due to the earlier boundary layer transition caused by the unsteady wakes. It appeared that the successful application of ultra-high-lift low pressure turbine blades required additional loss reduction mechanisms other than “simple” wake-blade interaction.

Characteristics of distinct flow regimes in the wake of an infinite span normal thin flat plate

The wake of an infinite, two-dimensional (2D), thin flat plate of height h was studied using Direct Numerical Simulation (DNS) for two Reynolds numbers, Re=1.2×103 and 2.4 × 103. The vortex shedding Strouhal number was 0.158. In addition to the high - and low - intensity shedding regimes (H and L) first reported by Najjar and Balachandar (1998), a distinct period of reorganization (M) is identified following regime H. The average flow characteristics for regime M are similar to those of the long-term mean flow. The regime-averaged recirculation length at Re=1.2×103 was 2.16h during H, 3.60h during L and 2.81h during M. Whereas the spanwise vortex rollers are well-organized during both regimes H and M, the arrangement of the streamwise ribs is much more regular and can be observed further downstream in regime H than M. The flow is highly three-dimensional during regime L, which includes less-coherent vortex rollers and spanwise randomly distributed ribs. The impact of this behavior on the roll-up, and thus the extension of the shear layer, is identified as a driving mechanism for generation and sustainability of the periodic wake behavior.

Reconstructing three-dimensional wake topology based on planar PIV measurements and pattern recognition analysis

The present study presents a new technique for reconstructing the salient aspects of three-dimensional wake topology based on time-resolved, planar, two-component particle image velocimetry data collected in multiple orthogonal planes. The technique produces conditionally averaged flow field reconstructions based on a pattern recognition analysis of velocity fields. It is validated on the wake of a low-aspect ratio dual step cylinder geometry, consisting of a large diameter cylinder (D) with small aspect ratio (L/D) attached to the mid-span of a small diameter cylinder (d). For a dual step cylinders with D/d = 2, and L/D = 1, numerical and experimental data are considered for ReD = 150 (laminar wake) and for ReD = 2100 (turbulent wake). The results show that the proposed technique successfully reconstructs the dominant periodic wake vortex interactions and can be extended to a wide range of turbulent flows.

Vortex synchronization in the cylinder wake due to harmonic and non-harmonic perturbations

This paper reports a numerical study of two-dimensional periodically perturbed flow past a cylinder. Both harmonic and non-harmonic perturbation waveforms of the inflow velocity are considered for a maximum instantaneous Reynolds number of 180. Phase portraits of the lift force are employed to identify the dynamical state of the cylinder wake and determine the range of kinematical parameters for which primary synchronization occurs, that is the regime where vortex formation is phase-locked to the subharmonic of the perturbation frequency. The effect of different perturbation waveforms on the synchronization range and on patterns of vortex formation is examined in detail over the normalized amplitude–frequency space. It is shown that systematic shifts of the synchronization range, towards either higher or lower frequencies, can be attained by imposing different perturbation waveforms. As a consequence, in certain regions of the parameter space it is possible to obtain multiple periodic and/or quasi-periodic wake states for waveforms of exactly the same amplitude and frequency. For the range of parameters where synchronization occurs, different vortex patterns are attained in the wake involving the shedding of solitary and binary vortices, or mixtures thereof, which can be controlled by the perturbation waveform. The phenomenology of these patterns, which result from modification of the basic Kármán mode in the unperturbed wake, is discussed and explained in terms of the generation of circulation that is induced by perturbations in the relative velocity. Vortex patterns from cylinders oscillating either in line with or transverse to a free stream are recast in the framework of the relative velocity.

Formation and dynamics of vortices in a Bose–Einstein condensate with a moving impenetrable disk-shaped potential

We consider a quasi-two-dimensional Bose–Einstein condensate with a moving impenetrable disk-shaped potential. By performing direct numerical simulations, we investigate the formation and dynamics of the vortices in the wake as a function of the moving velocity and the radius of the potential. For moderate moving velocity, we observe the formation of a bound vortex pair in the wake of the potential. Its shedding period and associated patterns depend on the radius of the potential, showing V-shaped, periodic, or complex wakes. More interestingly, at high velocity both the vortex pair and vortex dipole can be formed simultaneously. Furthermore, the activation time for vortex formation is investigated for different velocities. We also give an experimental protocol to observe these phenomena in future experiments.

Acceleration feature points of unsteady shear flows

A framework for extracting features in 2D transient flows, based on the acceleration field to ensure Galilean invariance is proposed in this paper. The minima of the acceleration magnitude (a superset of acceleration zeros) are extracted and discriminated into vortices and saddle points, based on the spectral properties of the velocity Jacobian. The extraction of topological features is performed with purely combinatorial algorithms from discrete computational topology. The feature points are prioritized with persistence, as a physically meaningful importance measure. These feature points are tracked in time with a robust algorithm for tracking features. Thus, a space-time hierarchy of the minima is built and vortex merging events are detected. We apply the acceleration feature extraction strategy to three two-dimensional shear flows: (1) an incompressible periodic cylinder wake, (2) an incompressible planar mixing layer and (3) a weakly compressible planar jet. The vortex-like acceleration feature points are shown to be well aligned with acceleration zeros, maxima of the vorticity magnitude, minima of the pressure field and minima of λ 2 .

DNS of unsteady effects on the control of laminar separated boundary layers

The main factor that determines the efficiency of a high-lift low-pressure-turbine operating at low Reynolds numbers is the size of the laminar separation bubble that forms on the suction side of the turbine blade. In this study, the suction side of a low-pressure gas-turbine blade is modeled as a flat plate with a streamwise pressure distribution. The combined effects of discrete surface roughness and periodic large-scale wake forcing on separation bubble transition and control are investigated numerically by direct numerical simulation. The laminar separation bubble is controlled by positioning the roughness element upstream of the separation bubble and introducing a low-frequency large-scale wake forcing. The three-dimensional discrete roughness with varying roughness height in the spanwise direction is modeled using immersed boundary method. The wake effect is numerically modeled as the mean wake deficit profile created by a linear row of cylinders moving in a direction perpendicular to the flat plate. Results indicate that the location of transition and hence the shape and extent of the separation bubble, largely depend on which of the above-noted passive control mechanisms dominates the unsteady transition process. It is shown that considering the mean flow properties, the roughness effect is found to be negligible compared to the individual wake effect.

A Quasi-Steady Lifting Line Theory for Insect-Like Hovering Flight.

A novel lifting line formulation is presented for the quasi-steady aerodynamic evaluation of insect-like wings in hovering flight. The approach allows accurate estimation of aerodynamic forces from geometry and kinematic information alone and provides for the first time quantitative information on the relative contribution of induced and profile drag associated with lift production for insect-like wings in hover. The main adaptation to the existing lifting line theory is the use of an equivalent angle of attack, which enables capture of the steady non-linear aerodynamics at high angles of attack. A simple methodology to include non-ideal induced effects due to wake periodicity and effective actuator disc area within the lifting line theory is included in the model. Low Reynolds number effects as well as the edge velocity correction required to account for different wing planform shapes are incorporated through appropriate modification of the wing section lift curve slope. The model has been successfully validated against measurements from revolving wing experiments and high order computational fluid dynamics simulations. Model predicted mean lift to weight ratio results have an average error of 4% compared to values from computational fluid dynamics for eight different insect cases. Application of an unmodified linear lifting line approach leads on average to a 60% overestimation in the mean lift force required for weight support, with most of the discrepancy due to use of linear aerodynamics. It is shown that on average for the eight insects considered, the induced drag contributes 22% of the total drag based on the mean cycle values and 29% of the total drag based on the mid half-stroke values.

A validation of wrist actigraphy against polysomnography in patients with schizophrenia or bipolar disorder.

PurposeSleep disturbances are frequent in patients with schizophrenia or bipolar disorder. Actigraphy has been established as a generally reliable method to examine these disturbances across varying time spans, but the validity against polysomnography (PSG) is not well investigated for this population. We validated wrist-worn actigraphy against PSG in a population of chronic, medicated patients with schizophrenia or bipolar disorder.Patients and methodsFrom a clinical trial, we derived data from 37 patients with schizophrenia and five patients with bipolar disorder who were examined with one-night PSG and concomitant actigraphy. The following sleep variables were compared between the two methods: total sleep time, sleep efficiency, sleep latency, number of awakenings, and time awake after sleep onset. The degree of consistency between the two methods was evaluated using the intra-class correlation coefficient and Bland–Altman plots. Subgroup analyses included splitting the analyses according to sex, diagnosis, and duration of wakefulness after sleep onset. PSG was considered the gold standard.ResultsThe intraclass correlation coefficient was high for total sleep time, moderate for the number of awakenings, and low or zero for the other examined sleep variables. These findings were reproduced in the subgroup analyses that compared men and women, as well as patients with bipolar versus schizophrenia spectrum disorders. When excluding patients with extensive periods of wakefulness after the initial sleep period (wake after sleep onset > 100 minutes), the reliability of the actigraphy-derived sleep variables markedly improved.ConclusionActigraphy reliably measures the total sleep time in this specific patient population. For patients without extensive periods of wakefulness after sleep onset, actigraphy might provide a useful measure of sleep efficiency, sleep latency, and number of awakenings.

Identifying children's nocturnal sleep using 24-h waist accelerometry.

The purposes of this study were 1) to add layers and features to a previously published fully automated algorithm designed to identify children's nocturnal sleep and to exclude episodes of nighttime nonwear/wakefulness and potentially misclassified daytime sleep episodes and 2) to validate this refined sleep algorithm (RSA) against sleep logs. Forty-five fourth-grade school children (51% female) participants were asked to log evening bedtime and morning wake time and wear an ActiGraph GT3X+ (ActiGraph LLC, Pensacola, FL) accelerometer at their waist for seven consecutive days. Accelerometers were distributed through a single school participating in the Baton Rouge, USA, site of the International Study of Childhood Obesity, Lifestyle, and the Environment. We compared log-based variables of sleep period time (SPT), bedtime, and wake time to corresponding accelerometer-determined variables of total sleep episode time, sleep onset, and sleep offset estimated with the RSA. In addition, SPT and sleep onset estimated using standard procedures combining sleep logs and accelerometry (Log + Accel) were compared to the RSA-derived values. RSA total sleep episode time (540 ± 36 min) was significantly different from Log SPT (560 ± 24 min), P = 0.003, but not different from Log + Accel SPT (549 ± 24 min), P = 0.15. Significant and moderately high correlations were apparent between RSA-determined variables and those using the other methods (r = 0.61 to 0.74). There were no differences between RSA and Log + Accel estimates of sleep onset (P = 0.15) or RSA sleep offset and log wake time (P = 0.16). The RSA is a refinement of our previous algorithm, allowing researchers who use a 24-h waist-worn accelerometry protocol to distinguish children's nocturnal sleep (including night time wake episodes) from daytime activities.

J0520203 周期的後流通過の影響を受ける翼面境界層の流動解析

J0520203 周期的後流通過の影響を受ける翼面境界層の流動解析

Wind turbine wake stability investigations using a vortex ring modelling approach

In the present study, a simple inviscid vortex ring (VR) modelling approach isused to represent the developing rotor wake. This allows a straightforward investigation andcomparison of the impact of uniform, yawed and sheared flow conditions on the developmentof the rotor wake, with the additional possibility of including ground effect. The effect ofinstabilities on the development of the wake is manually introduced in the form of perturbationsof strength, ring position and size. The phenomenon of vortex filament interaction orleapfrogging, could play a role in the observation of unsteady phenomena and is therefore alsoaddressed. Such a study is hence performed in light of recent conflicting views on the causesof wake meandering: is the observed dynamic wake behaviour a result of large scale turbulentforcing or do more subtle and intrinsic wake instabilities play a role?This study concludes that the presence of the ground and external perturbations, mostnotably changes in the wake pitch and the rotor thrust coefficient, can significantly affectthe steady development of the wake. The mutual vortex pairing instability, whilst displayinginteresting periodic behaviour, does not correlate with periodic wake behaviour reported byMedici et al. [1]. However, in the absence of unsteady inflow, it is shown that the wake ofa Horizontal Axis Wind Turbine (HAWT) is certainly prone to displaying unstable, dynamicbehaviour caused by these additional factors.

Effect of Pin Tip Dual Clearance on Flow and Heat Transfer at Low Reynolds Numbers

This paper examines the augmentation of heat and mass transfer due to dual clearances on cylindrical pin fins, relative to a channel between parallel plates, in mini/microchannel reactors at low Reynolds numbers. In this work, diffusion limitations to heat and mass transfer in smooth-walled mini/microchannel reactors were minimized by the implementation of microcylinder pin fins with dual clearances that, (1) promote the production of instabilities in the wakes that enhance mixing and (2) reduce the viscosity dominated regions at pin-wall interfaces. A smooth catalyst coating is assumed on all exposed surfaces of the microchannel interior walls and pin fins. Due to the analogy of heat and mass transfer, augmentation of the Nusselt number is equivalent to the augmentation of the Sherwood number. Heat transfer augmentation is investigated in air (Pr = 0.705) at dual clearances ranging from 0 to 0.4 of the channel height and Reynolds numbers from 10 to 600. The pin fins and the clearance augmented the heat transfer coefficient by a factor of 4.0. The combination of the augmentation of the heat transfer coefficient and the increase in the surface area, by the clearances, results in an increase in the conductance over a plane channel, by a factor of 7.1. The results are extendable to overcoming laminar diffusion with laminar periodic wakes of fuel vapors such as methanol vapor in air where Scfuel ∼ Prair. For turbulent wakes impinging upon downstream pins, the results can be extended to fuel vapors with (Scfuel)turb ∼ (Prair)turb. A large eddy simulation (LES) approach was used in this study.