Endwall Effects Research Articles

Detached-Eddy Simulation of Supersonic Turbulent Flow over a Cylinder/Skewed Flare Configuration

The computational campaign reported in this article focuses on a series of experiments at Mach 2.85 carried out in the 1980s at NASA Ames Research Center on a set of cylinder/skewed flare configurations designed to produce highly three-dimensional shock-wave/boundary-layer interactions in the absence of end-wall effects. Computations carried out in that era were unable to match the experimental results using the numerical techniques, turbulence models, and grid resolution available at the time. In the present work, newer Reynolds-averaged Navier–Stokes and detached-eddy simulation methods have been applied to these flows, and relatively good agreement has been obtained with the experimental data. Difficulty in capturing the correct separation-bubble size was encountered with initial detached- eddy simulations, but the introduction of resolved turbulence via a boundary-layer trip produced much better results. The present paper reports on results obtained for four inclination angles (0, 5, 10, and 23 deg) of the skewed 30-deg flare. Detached-eddy simulation is seen to be an economical alternative to large-eddy simulation for capturing many features of large-scale separation unsteadiness over long time intervals at true Reynolds number.

Investigation of vaned diffuser endwall contouring technology for improving the stable operating range of a centrifugal compressor with an asymmetric volute

AbstractIn this study, nonaxisymmetric endwall contouring technology is employed as a passive control strategy to enhance the stable operating range of a centrifugal compressor with an asymmetric volute. The endwall contouring method is applied to the hub‐side wall of the vaned diffuser within a centrifugal compressor stage. Instead of utilizing a flat hub as the baseline diffuser, various diffuser configurations featuring hub‐side contoured endwalls are explored, with adjustments in the peak radial position and peak height of the convex and concave profiles. Numerical simulations are conducted to evaluate the performance of the centrifugal compressor stage with both the baseline and contoured vaned diffusers. The investigation explores the underlying flow control mechanisms and establishes the effective endwall contouring guidelines. The findings highlight the effectiveness of endwall contouring in enhancing the stable operating range of centrifugal compressors. Notably, a substantial enhancement of 15.1% in the stable operating range is achieved by employing the contoured diffuser with the peak radial position near the vane leading edge (LE) and the peak height at 20% of the vane height. The endwall contoured diffusers reduce the positive LE incidence angles to direct the fluid toward the suction side (SS), and increase the radial velocity near the SS to suppress the hub‐suction corner separations in the diffuser passages near the volute tongue. These improvements collectively contribute to the enhancement of diffuser flow characteristics. Finally, a set of effective endwall contouring guidelines is proposed to guide the endwall contouring design for enhancing the stable operating range of centrifugal compressors.

Numerical study on the smoke temperature distribution beneath the ceiling of a single-ended sealing tunnel under the condition of double fire sources

As a special tunnel structure, the single-ended sealing tunnel has only one evacuation port, which increases greatly the risk of fire. Numerical modeling of a double source fire in a single-ended sealed tunnel was developed based on FDS 6.7. The effects of double-source spacing and end walls on ceiling smoke temperature were analyzed and discussed. The results show that when the double fire sources are considered as a whole, there are varying degrees of smoke backflow throughout the tunnel, with the upstream smoke backflow phenomenon being more pronounced under the influence of the end wall. In this study, the dimensionless separation distance φ is introduced and divided into two regions to quantify the effect of the spacing of the double fires. When 0 ≤ φ < 0.625, the maximum smoke temperature beneath the ceiling falls exponentially with increasing separation distance. On the other hand, when 0.625 < φ ≤ 1, the maximum smoke temperature under the ceiling rises exponentially. Furthermore, we suggest empirical formulas for forecasting the longitudinal distribution of downstream temperatures as well as the maximum smoke temperature of the ceiling. The predictive equations’ dependability was further confirmed by contrasting the predicted equations with simulated and historical experimental data. The study results serve as a reference for fire prevention and rescue in similar special tunnel structures, such as single-end sealing.

Effect of non-axisymmetric endwall contouring and swirling inlet flow on film cooling performance of turbine endwall

Non-axisymmetric endwall contouring (NEC) is one of the verified approaches to suppress secondary flows and improve aerodynamic performance. However, the design of NEC brings significant challenges to the design of endwall cooling structures. Herein, a pressure-sensitive paint experimental approach was used to obtain the film cooling effectiveness of the NEC endwall with a purge slot in this study. Three NEC types were adopted: NEC (COS), NEC (SIN), and NEC (−SIN). In addition, lean premixed combustion technology was used to achieve lower levels of NOx emissions. The turbine inlet was characterized by high turbulence and strong swirling. The effects of different swirling angles (±10, ±20, and ±30°) and densities were further explored. Due to the NEC profiling changing the secondary flow near the endwall area, coolant from the purge slot was better attached to the slot exit position, leading to a significant increase in the size of the high-cooling-efficiency region. With the mass flow ratio (MFR) varying from 0.5 to 2%, the film cooling effectiveness of the flat and NEC endwalls had similar variation characteristics. When the MFR = 0.5%, the area-averaged cooling efficiencies of the NEC (COS), NEC (SIN), and NEC (−SIN) endwalls could be improved by 2, 12.5, and 20%, respectively. Positive swirling and smaller negative swirling inflow could improve the film cooling effectiveness inside the channel. The case of SA = +20° had the best improvement, where the film cooling effectiveness of the NEC (COS), NEC (SIN), and NEC (−SIN) endwalls could reach up to 29, 35, 36, and 34%, respectively. The NEC (−SIN) endwall was less sensitive to the effects of the swirling inflow.

Development of Curved Shape Diffuser Duct for Aero Engines With a Backend Centrifugal Compressor

In the present study, a design of the curved shape diffuser-duct is proposed to apply to the compressor stage for aero-engines with a backend centrifugal compressor. For these configurations, the flow exiting radially from the centrifugal impeller needs to be changed from radial-to-axial direction to meet downstream combustion chamber requirements. Hence, the specially designed exit duct with a curved shape to manage the flow from the radial-to-axial direction was later diffused using the diffuser-shaped exit duct. This study aims to design and develop an efficient diffuser-duct for the adequate deceleration of flow in the exhaust diffuser and, as a result, a uniform and low-velocity flow at the exit. This configuration comprises a radial-to-axial turning annular passage at the centrifugal impeller exit. Then, the passage is directed to either a purely axial direction or is inclined to the axial direction by a slight angle. The results indicate a uniform total pressure at the stage exit with marginal variation along the span. It shows equivalent end wall effects near both the shroud and hub surfaces. This endwall effect may be occurring mainly due to the growth of the boundary layer across the diffuser passage.

Non-axisymmetric Endwall film cooling characteristics considering the influences of cylindrical holes and laidback fan-shaped holes

Flow fields near the turbine vane endwall are complicated due to the endwall cross flows. The use of a non-axisymmetric endwall is regarded as an efficient technique to reduce the lateral pressure difference, decreasing the endwall cross flow. Numerical analysis was performed to determine how the non-axisymmetric endwall affected the vortex structure and heat transfer level. The cooling performance was investigated with cylindrical and laidback fan-shaped holes (7–7–7), which were arranged in rows aligned in the axial direction. The results showed that the non-axisymmetric endwall could significantly reduce the circumferential pressure difference and suppress the growth of the passage vortex, and the area-averaged heat transfer coefficient was reduced by 3.34%. The outlet area of the film hole was altered by the non-axisymmetric endwall, and the over-cooled regions may have appeared as a result of the excessive area increase. The influence of the non-axisymmetric endwall was concentrated at 0.4 < Z/Cax < 1.0 for the cylindrical hole. With the increase in M, the film cooling effectiveness of the non-axisymmetric endwall attained a higher level than that of the flat endwall. For the laidback fan-shaped hole, the effect of the non-axisymmetric endwall was confined within 0.25 < Z/Cax < 1.0. The half-period trigonometric function of the non-axisymmetric endwall (HTFN) achieved the optimal cooling performance for three blowing ratios. However, the periodic trigonometric function of the non-axisymmetric endwall (PTFN) only outperformed the flat endwall when M= 1.5.

Improving turbine endwall overall cooling effectiveness using curtain cooling and redistributed film-hole layouts: an experimental and computational study

Abstract To enhance the cooling deficiency that occurs in a baseline endwall using axially-arranged cooling holes, this paper proposes a new locally-enhanced hole layout using curtain cooling and fan-shaped film holes being arranged on iso-Mach lines. The objective of cooling hole re-design is to minimize secondary flows and thus to provide better film coverage. In experiments, Infrared thermography techniques are applied to validate overall cooling effectiveness of the newly-designed endwall, and aero-thermal fields at the cascade exit are detected by five-hole and thermocouple probes. Additionally, computational fluid dynamic simulations are performed to provide complementary flow insights. A comparison with the baseline hole layout reveals that for a given total coolant flow rate, the newly-designed endwall significantly improves the cooling performance by up to 27% without a noticeable aerodynamic penalty, resulting in a lower and more uniform temperature field. Curtain coolant effectively suppresses the development of horseshoe vortex and provides adequate thermal protection for leading-edge junctures and pressure-side corner regions. The redistribution of fan-shaped film holes reinforces the cooling performance in the passage throat and trailing edge regions. At low and high total mass flow rates, the coolant split between various cooling sources has a substantial impact on cooling performance.

Numerical investigation on film cooling and aerodynamic performance for gas turbine endwalls with upstream vane-type and cascade-type slots

To further improve the thermal efficiency of gas turbine, the turbine inlet temperature (TIT) increases up to 2000 K, leading to an extremely high heat load on the first-stage vane. Taking into consideration that there are complex endwall secondary flow and the passage vortices which have significant effect on the flow field and thermal behavior near the endwall, it is essential to study the aero-thermal characteristics of endwall and develop efficient endwall cooling approach. In this work, two novel upstream slot structures, referred to as the vane-type slot and cascade-type slot, are proposed, and a comprehensive numerical analysis is conducted to reveal their mechanism and superiority in improving the film cooling and aerodynamic performance of first-stage vane compared with the conventional slot structure. The results indicate that compared with the conventional rectangular straight slot, the coolant jets from two novel slots have a more significant inhibition effect on the development of the endwall secondary flow due to their lower outflow angle and thus higher momentum in streamwise direction, which contributes to reducing the strength of the endwall secondary flow. This is beneficial to provide a lower cascade aerodynamic loss and a higher film cooling effectiveness of the endwall near the suction side leading edge and the higher film cooling effectiveness along the trajectory of the horseshoe vortex on the more downstream region. Moreover, the vane-type slot provides the largest coolant coverage and cooling effectiveness of endwall and the lowest total pressure loss within slot and cascade aerodynamic loss, followed by the cascade-type slot and conventional slot.

Effects of inherent surface roughness of additively manufactured lattice frame material on flow and thermal transport

Additively manufactured (AM) fiber-based lattice topologies provide superior thermal and mechanical performance. For true characterization of flow and thermal transport of AM lattices based on different unit cell topologies, it is imperative to understand the role of inherent surface roughness of AM parts on the same. To this end, the effect of inherent roughness on the endwalls and fibers of a tetrakaidecahedron unit cell-based lattice on the flow and thermal transport has been computationally studied here for Reynolds number of 9,806. The predictions were compared to the heat transfer coefficient values obtained from the experiments conducted on tetrakaidecahedron coupon additively manufactured in Stainless steel 420 (SS 420) using Binder-Jetting process. The AM parts were characterized using X-ray computed tomography (CT) scan and optical profilometer. For the computational study, artificial roughness was generated such that the roughness characteristics were similar to the AM parts. First set of analysis includes the as-designed smooth CAD geometry with desired lattice porosity of 0.886 and as-manufactured geometry with the rough endwalls and fibers having manufactured porosity of 0.856. The as-designed geometry underpredicted the velocity magnitudes and turbulent kinetic energy levels as compared to the as-manufactured counterpart. The overall heat transfer coefficient values were under-predicted by 26.8% and 13.9% from the experimental data, respectively. The second set of analysis included two geometries-(i) smooth endwall and rough fiber, and (ii) rough endwall and smooth fiber, both having manufactured lattice porosity of 0.856. The overall heat transfer coefficient values in the rough fiber-only and rough endwall-only deviated by 14.2% and 18.9% respectively. Amongst all the four geometries simulated, the as-manufactured part with roughness on both the endwalls and fibers provided the best agreement with the experimental results suggesting that the roughness effects cannot be overlooked for accurate thermal performance predictions. The rough fiber-only case provided better results than the rough endwall-only case suggesting that the roughness of the fibers accounting for the porosity deviations is important in dictating the overall lattice heat transfer performance. However, the endwall effects can become increasingly significant if the roughness scales become comparable to the global unit cell dimensions.

Electroosmotic mixing of non-Newtonian fluid in an optimized geometry connected with a modulated microchamber

The main objective of this work is to enhance the micromixing of different species transported through the electrokinetic mechanism applicable in lab-on-a-chip devices used in BioMEMS. In this process, it is essential to predict the efficiency and precision of the micromixture for the quick and correct mixing. In this paper, a numerical study is conducted to investigate the mixing quantification of the electroosmotic micromixer with a nozzle–diffuser shaped channel connected to reservoirs located at both ends of the channel with a microchamber located in the middle of the channel modulated with an inner rectangular obstacle. Since enhancing mixing quality is the paramount factor, this study examines how the design of the mixing chamber (circular and triangular), the size of the inner obstacle, the conical angle of the nozzle–diffuser channel, and the electric double layer height influence the flow inside the electroosmotic micromixer. Numerical simulations have been performed by using the Poisson–Nernst–Planck based Cauchy momentum equations for a non-Newtonian power-law fluid. This study focuses on both the mixing enhancement and the performance evaluation factor by lowering the pressure drop with variation of geometric modulation. The reservoir end wall effects are considered for the flow rate and mixing of the power-law fluids with variation of different flow parameters. After obtaining the optimal values of the effective parameters used in the micromixers for the experiments, regardless of the geometry of the obstacles, the present model is formulated and validated, and the results are presented. According to the findings, it is observed that the height and width of the inner obstacle, Debye–Hückel parameter, and the slope of the channel have a significant role in the overall mixing quality. The mixing efficiency is improved up to 90% for Newtonian fluid and 96% for shear thickening fluid by using obstacle fitted in the microchamber of the system. In addition, the results demonstrate that shear thickening fluids have better mixing performance than shear thinning fluids, which can be helpful in the fabrication of advanced micromixers.

Aspect-ratio dependence of heat and angular momentum transport in turbulent Taylor-Couette flows with axial thermal forcing

• Scaling law of transports determined in finite systems can be extrapolated to large-scale flows. • Coherent turbulent structures in form of Taylor vortices promote heat and angular transports. • Magnitudes of both heat and angular momentum transport depend on domain aspect-ratio. We report numerical studies of the effects of geometry factors, i.e. the aspect ratio Γ and radius ratio η , on the axial heat transport N u and the radial angular momentum transport N ω by turbulent Taylor-Couette flows that are subjected to a vertically destabilizing temperature gradient. For a given Reynolds number R e the magnitudes of both N u and N ω exhibit a pronounced aspect-ratio dependence. The angular momentum transport is reduced in long cylinders, since endwall effects are weakened as Γ increases. For the heat transfer in large- Γ cylinders, a significant N u -reduction appears for low- R e regime, whereas N u can be greatly enhanced in the regime of turbulent Taylor vortex (TV). The former N u -reduction is associated with the process that the thermal plumes are swept away by the Ekman vortices near the boundaries, and the fluid exchange is severely restricted between adjacent vortices in the bulk region. The latter N u -enhancement is attributed to the strengthened shears arising in large- Γ cylinders, resulting in a more turbulent boundary layer. Moreover, the pronounced inter-mixing of turbulent TVs continuously pumps the hot (cold) fluids into the bulk, intensifying the transport processes. It is found that although the constraints from the endwalls impact the global heat and angular momentum transport in the low- R e regime, they hardly affect their scaling properties in the high- R e regime with large Γ . We thus expect that the scaling relationship of N u ( R e ) and N ω ( R e ) determined with finite Γ can be extrapolated to large-scale, and even vertically unbounded industrial and geophysical flows.

Reservoir end wall effects on bivariate ion and fluid transport in micro/nano-nozzles for effective electroosmotic mixing

The aim of this study was to develop an efficient convection diffusion-based mathematical model for the species transport and mixing in different shaped (i.e., nozzle, diffuser, diffuser–nozzle, nozzle–diffuser) micro/nano-channels connected to large reservoirs. Both analytical and numerical studies are performed to illustrate the impact of inertial and contact angles for the generation of complex flow patterns due to different aspect ratio specified transformations. The hydrodynamics of the ion and fluid transport are analyzed through the Poisson–Nernst–Plank-based Navier–Stokes model subjected to specified system of forces endured by the reservoir fluids. The numerical results for pressure velocity correlations are obtained when the transport mechanism of the domain is changed from nozzle to diffuser. Mixing efficiency is evaluated for different geometric configurations and compared with a rectangular slit channel when the parallel reservoirs are connected. The role of Debye–Hückel parameter, conical angles or slope, and reservoir height/width on the transport of ions and enhancement of mixing are discussed. The mixing efficiency is found to attain a higher value after considering the reservoir connected to a nozzle without involving any hurdles or heterogeneous zeta potential along the channel wall. Closed-form analytical solutions of the electric potential are obtained through the linearized Poisson–Boltzmann model and further incorporated for the pressure evaluation. The axial and transverse velocities are evaluated from the modified Navier–Stokes equation including electric body force term and are validated with the experimental results. Effective nonlinear coupling responses of ion transport are found to be more pronounced in nozzle compared with diffuser resulting a higher mixing. Also, the solutions of velocity resulting in a low torque satisfy the equilibrium conditions and are optimized in terms of adversion of frictional factor and viscous dissipation resulting in an effective mixing. The findings manifest the species patterns with high accuracy and versatility, which could possibly help to handle the technical challenges associated with the design of pumpless actuated microfluidic devices.

Study on Inlet Flow Field Structure and End-Wall Effect of Axial Flow Pump Impeller under Design Condition

The existing experimental technology cannot accurately and quantitatively measure the flow field structure and the wall boundary layer displacement effect in the axial flow pump. Based on SST k-ω turbulence model, a three-dimensional unsteady numerical simulation of the whole flow field of an axial flow pump was presented at the designed operating point to overcome the weakness of traditional measurement methods in measuring the flow field of the axial flow pump. The flow field structure of the axial flow pump inlet was studied quantitatively and the result was compared with the theoretical design value. It was found that there is an obvious impeller rotation effect and end-wall effect in the flow field of the axial flow pump inlet. The distribution law of the impeller inlet flow field and the crowding coefficient caused by the wall boundary layer were obtained. The pump inlet measurement point in the experiment and calculation domain inlet in the simulation should be kept at a distance of more than 0.5 Ds away from the impeller inlet to eliminate the influence of the impeller rotation effect. Through contrastive analysis, it was found that there is an obvious difference between the calculated value and the design value of the flow field structure due to the end-wall effect. The crowding coefficient should be taken into account when designing an axial flow pump. This study has certain reference significance for further understanding the flow field structure at the inlet of the axial flow pump impeller and improving the design theory of the axial flow pump.

Novel Optimization Design Methods of Highly Loaded Compressor Cascades Considering Endwall Effect.

The endwall effect has a great impact on the aerodynamic performance of compressor blades. Based on three conventional near-endwall blade modeling methods of bowed blade, endbend blade and leading-edge strake blade (LESB), two combined optimization design methods of highly loaded blades have been developed considering the endwall effect in the current study, i.e., the bowed blade combined with the LESB (bowed LESB blade) and the endbend blade combined with the LESB (endbend LESB blade). Optimization designs were conducted for a compressor cascade with low solidity by using the two combined modeling methods and the three conventional modeling methods, and the optimization results were compared and analyzed in detail. The results showed that the five optimization modelling methods could all improve the performance for the original cascade, and the optimized cascade with the bowed LESB modeling method has the best aerodynamic performance. The total pressure loss of the optimal bowed LESB cascade was only 40.3% of that in the original cascade while reducing the solidity of the original cascade from 1.53 to 1.25 and keeping the static pressure rise and diffusion factor at the same level as the original one. Among the optimal cascades, the radial migration height of the low-energy fluid and the corresponding vortex have great effects on the aerodynamic performance, and the optimal bowed LESB cascade is superior to the other optimal cascades in this aspect.

Improving cooling performance and robustness of NGV endwall film cooling design using micro-scale ribs considering incidence effects

Modern lean-burn combustors generate aggressive swirling flow at the entrance of high-pressure turbine nozzle guide vane (NGV), and lead to cooling performance deterioration of the NGV film cooling system. Improving the cooling performance of NGV becomes a delicate task for the designers. In this study, micro-scale ribs with novel arrangement method were applied to improve the cooling effectiveness and robustness of NGV endwall cooling system at varying incidence conditions. Numerical simulations were employed to perform comprehensive studies on film cooling performance of smooth and ribbed endwall with double-row staggered film holes. Three incidence angles −30°, 0° and 30° were specified. Endwall adiabatic film cooling efficiency, heat transfer coefficient, net heat flux reduction (NHFR) and flow field structures were analyzed. The results indicated that the designed ribs weaken the intensity of the horse shoe vortex and passage vortex effectively. Coupled with the suppression of cross migration of coolant and weakening of heat transfer intensity, endwall cooling performance is thoroughly improved. Area-averaged film cooling efficiency and NHFR are increased by 46.33% and 108.43%, respectively, in the 30° incidence condition. In addition, the discrepancies of film cooling performance at different incidence conditions are attenuated, and the robustness of endwall cooling design is improved.

Effects of end-walls on flows in a highly loaded compressor cascade with double-circular-arc blades

A numerical study is carried out to understand the flows in a highly loaded compressor cascade made of double-circular-arc blades, which were measured by Zierke and Deutsch in the late 1980s. A two-dimensional (2D) cascade with periodic boundary conditions in both pitch-wise and span-wise directions and a three-dimensional (3D) cascade with two end-walls that are far away from each other are accounted for in the study. For the incidence angle α=−8.5°, the numerical results of the 2D-cascade flow are in excellent accordance with the experimental data. This not only validates the numerical method used in the study but also suggests that a 2D and periodic flow was successfully generated in the experiment for this incidence angle. However, the numerical results of 2D-cascade flows for α=−1.5° and 5° deviate from the experiment considerably because the strong effects of the end-walls on the wake are neglected in the simulation. By contrast, the simulation of 3D-cascade flows predicts an accurate pressure coefficient at the blade surface, the pressure increase coefficient, and the total pressure loss coefficient for all three incidence angles. This means that, to generate experimental data for validating numerical simulation, it is important to consider the effect of end-walls when the incidence angle is large. The numerical results also show that, for 2D-cascade flows with a low inlet turbulence intensity, the laminar-turbulent transition on the pressure surface is determined by the interaction of the Klebanoff distortions and T-S waves. The Klebanoff distortions are also clearly identified on the suction surface for α=−8.5°. The end-walls induce span-wise elongated disturbances, which suppress the stream-wise disturbances. The transition in 3D-cascade flows generally follows the mechanism of natural transition.

Mechanisms for recirculation cells in granular flows in rotating cylindrical rough tumblers.

Friction at the endwalls of partially filled horizontal rotating tumblers induces curvature and axial drift of particle trajectories in the surface flowing layer. Here we describe the results of a detailed discrete element method study of the dry granular flow of monodisperse particles in three-dimensional cylindrical tumblers with endwalls and cylindrical wall that can be either smooth or rough. Endwall roughness induces more curved particle trajectories, while a smooth cylindrical wall enhances drift near the endwall. This drift induces recirculation cells near the endwall. The use of mixed roughness (cylindrical wall and endwalls having different roughness) shows the influence of each wall on the drift and curvature of particle trajectories as well as the modification of the free surface topography. The effects act in opposite directions and have variable magnitude along the length of the tumbler such that their sum determines both direction of net drift and the recirculation cells. Near the endwalls, the dominant effect is always the endwall effect, and the axial drift for surface particles is toward the endwalls. For long enough tumblers, a counter-rotating cell occurs adjacent to each of the endwall cells having a surface drift toward the center because the cylindrical wall effect is dominant there. These cells are not dynamically coupled with the two endwall cells. The competition between the drifts induced by the endwalls and the cylindrical wall determines the width and drift amplitude for both types of cells.

Experimental Investigation on Flow Past an Isolated Micro Pin Fin Embedded in a Microchannel

ABSTRACT Micro pin fin heat sink is a very attractive cooling technique for high power density microelectronics. Optimization of its cooling performance requires insightful understanding on fundamental physics of flow inside it, especially around a single pin fin. In the present study, an experimental investigation on flow past an isolated low aspect ratio (height-to-diameter ratio) pin fin embedded in a microchannel is conducted using Micro-PIV. The flow field and vorticity distribution at different channel heights under various Reynolds numbers are obtained. Endwall effect is found to play an important role in flow past the pin fin in the microchannel, and the critical Reynolds numbers are larger than that at the conventional scale. Vorticity concentrations are formed on both sides of the pin fin along the shear layer and intensify with the increase in Reynolds number. Flow fields and vorticity distributions at different heights exhibit different characteristics, especially at higher Reynolds numbers, indicating three-dimensionalities of the flow. Viscous resistance of the endwalls leads to lower overall velocity, smaller extent of the recirculation zone and weaker vorticity in flow layer closer to the top and bottom channel walls. Pressure drop and flow resistance characteristics in the microdevice is analyzed. The effect of aspect ratio of the pin fin on the wake flow is also studied, and the results show that three-dimensionalities increase but critical Reynolds numbers decrease with larger aspect ratios. A comparison with flow across micro pin fin arrays is conducted and differences are observed in velocity field and wake flow features.

Laboratory-scale investigation of a periodically forced stratified basin with inclined endwalls

We present results of numerical simulations of a stratified reservoir with a three-layer stratification, subject to an oscillating surface shear stress. We investigate the effect of sloped endwalls on mixing and internal wave adjustment to forcing within the basin, for three different periods of forcing. The simulations are carried out at a laboratory scale, using large-eddy simulation. We solve the three-dimensional Navier–Stokes equations under the Boussinesq approximation using a second-order-accurate finite-volume solver. The model was validated by reproducing experimental results for the response of a reservoir to surface shear stress and resonant frequencies of internal waves. We find interesting combinations of wave modes and mixing under variation of the forcing frequencies and of the inclination of the endwalls. When the frequency of the forcing is close to the fundamental mode-one wave frequency, a resonant internal seiche occurs and the response is characterized by the first vertical mode. For forcing periods twice and three times the fundamental period, the dominant response is in terms of the second vertical mode. Adjustment to forcing via the second vertical mode is accompanied by the cancellation of the fundamental wave and energy transfer to higher-frequency waves. The study shows that the slope of the endwalls dramatically affects the location of mixing, which has a feedback on the wave field by promoting the generation of higher vertical modes.

Investigation of endwall effect on transitional flow inside compressor cascade passage at low Reynolds number

Laminar-to-turbulent transitional flow plays a key role in determining the overall aerodynamic performance of turbomachinery. In this paper, the physical mechanisms concerning transitional flow inside compressor cascade passages at low Reynolds number condition are investigated based on the large eddy simulations. Two categories of cascade flow simulations are conducted: one is for the quasi three-dimensional (3D) cascade flow without endwalls, i.e., the translational periodic boundary conditions are employed for hub and shroud surfaces, while the other focuses on the fully 3D cascade passage flow. Special emphasis is placed on the effect of endwalls on the laminar-to-turbulent transitional flow inside the compressor cascade passage. In addition, two levels of freestream turbulent intensity are set in these simulations. It is concluded that the endwall boundary layer flow has a non-ignorable influence on transitional flows in the lower-span region. Under the condition of low freestream turbulence intensity, the original laminar separation-induced transition pattern dominating the mid-span suction surface evolves to become natural and bypass transition flow when approaching the endwall region. With increase in the incoming turbulent intensity, the natural transitional flow disappears, and the cascade suction surface is dominated by the bypass transitional flow. Moreover, the blade loading near endwall is reduced and the aerodynamic loss on lower spanwise airfoil sections is substantially increased when compared to the cascade without endwall. The physical mechanisms concerning transitional flow described in this paper might provide some meaningful guidance toward developing advanced turbomachinery.