Mainstream Flow Research Articles

Numerical simulation of transpiration cooling in porous nose cone under hypersonic conditions

Transpiration cooling in a two-dimensional porous nose cone under hypersonic flow have been numerically investigated. The numerical model intricately captures the interaction between compressible and incompressible flows in the boundary layer and the phase change of coolant within porous region. The validation of numerical method is performed by comparing the simulation results with experimental data. The simulation results are in reasonable agreement with the experimental data. The results show that the increase of blowing ratio can effectively improve the cooling efficiency of transpiration cooling，where the blowing ratio represents the mass flow rate ratio between the coolant of porous region and the mainstream free flow. Such as, the average cooling efficiency rises from 0.33 to 0.86 when the blowing ratio increases from 0.0459% to 0.2753%. Although the porous resistance force increases as the particle diameter and porosity decrease, the effect of these parameters and solid material of porous region on the cooling efficiency are marginal.

Hydraulic performance improvement of a two-way pumping station through bell mouth shape design

A two-way pumping station is a specialized device that facilitates bidirectional water pumping and drainage. The pressure pulsation characteristics of two-way pumping stations have emerged as a prominent research focus in the field of hydraulic engineering. In this work, with the aim of systematically proposing optimization measures to ensure operational stability, a transient numerical simulation is conducted to elucidate the influence mechanism of the suspension height of the bell mouth (SHb) on the internal flow field and pressure pulsation of a two-way pumping station. High-precision experiments are performed to compare time-frequency domain characteristics under different SHb using a continuous wavelet transform (CWT). The findings indicate that an appropriate reduction in SHb effectively reduces unstable flow and pressure pulsation within the inlet conduit, consequently reducing the pressure pulsation of the impeller. With a reduction in SHb, the influx of low-velocity backflow into the bell mouth is prevented and the generation and propagation of suction vortices are suppressed. However, the reduction amplifies the flow impact between the mainstream flow and the bell mouth wall. The spatial distribution of the pressure pulsation is also examined, and it is found that a reduction in SHb increases the pressure pulsation intensity on the side facing the incoming flow and on the rear side, while the mainstream area tends to exhibit stability. In terms of time-frequency domain characteristics, a reasonable reduction in SHb leads to improved circumferential uniformity of the impeller inflow and the effective suppression of low-frequency disturbances.

Supersonic cooling film flow evolution on a curved wall under hypersonic flow

Curvature plays a crucial role in evolving supersonic cooling film flow-field structures. Flow-field structural images were captured using nanotracer-based planar laser scattering,, and wall pressure values were obtained using experimentally validated numerical simulations. A supersonic cooling film is tangentially injected at the Mach number of Maj = 2.3 into a laminar boundary layer at a mainstream of Ma = 6. The supersonic cooling film inhibits mixing-layer instability on the convex curved wall (CV) and promotes it on the concave curved wall (CC). After increasing the total incoming pressure, the reduction ratio of static pressure (RSP) between the supersonic cooling film and the mainstream flow causes a delay in the position of the mixing-layer instability, smaller-scale vortex structures, and decreased flow velocity of the typical vortex structures on the CC and CV. The wall pressure increases for the CV and decreases for the CC, indicating that the supersonic cooling film suppresses the changes in wall pressure due to curvature. The supersonic cooling film suppresses the decrease in the impulses for bulk dilatation (Ip) due to convex curvature and the increase in Ip due to concave curvature. The growth rate of Ip on the CC increases from −15% to −8% and decreases on the CV from 31% to 12% in the bending impulse (IΦ) range of |IΦ| = 1.337–3.624 for a total inlet pressure of 0.5 MPa. Increasing the RSP could control the Ip values on curved surfaces more effectively. The results of this study can be applied to cooling the infrared optics window on hypersonic vehicles.

Spatial distribution and migration pattern analysis of Erythroculter erythropterus utilizing a telemetry approach in relation to water quality in the Nakdong River basin

The fish Erythroculter erythropterus has been introduced to the Nakdong River. It is a top carnivore (growing to up to 1.5 m long) but is inedible. The population of Erythroculter erythropterus tends to increase in areas where mainstream flow is slowed by weirs, likely due to the creation of more favorable habitat conditions. Erythroculter erythropterus dominates the fish community, reducing the number of endemic and commercial fish. The South Korean government sought to eliminate the species, but the spread was too rapid, resulting in top-down biological disturbance. The species distributions and movement patterns of Erythroculter erythropterus are not well known and no management strategy is available. Here, we determined the distribution and migration patterns of Erythroculter erythropterus in the tributaries and mainstream of the Nakdong River and analyzed how the water chemistry and physical habitat affected these variables. We used a submersible ultrasonic receiver to collect signals from transmitters attached to fish and analyzed the migration pattern. Massive aggregations of this species were evident at depth. Erythroculter erythropterus dominated all study sites but was significantly (p < 0.05) more common in the mainstream (relative abundance [RA] = 12.2%) than in tributaries (RA = 1.45%). Water quality parameters significantly regulated the population; total organic carbon was the highest one (R2 = 0.71). The transmitters revealed massive congregations in the deepest water (− 17 m) during winter hibernation, which sheds light on how the fish can best be removed.

Experimental and Numerical Investigation of Cylindrical and Shaped Cooling Holes With Forward and Reverse Injection

The present work proposes a suitable injection hole configuration of cylinder and laidback fan-shaped with forward and reverse direction for film cooling of a gas turbine blades application, based on experimental and numerical analysis. The experimental study is conducted for cylindrical hole at blowing ratio (1), injection angle (35°), and density ratio (1.2). The numerical study is performed for a wide range of operating parameters such as blowing ratios on (1-3), density ratios (2.42), mainstream flow Reynolds number as 4000 based on the hydraulic diameter of wind tunnel channel and, injection angle (35°) with the effect of forward and reverse injection of laidback fan shaped. The present study reveals that the formation of kidney vortices mitigated for reverse-shaped holes (secondary air is injected such that its axial velocity component is in the reverse direction to that of the mainstream) results in higher cooling performance with respect to forward-shaped holes. The coolant coverage is likewise more consistent and higher in the lateral direction compared to the forward injection.

TPIV Experimental Investigation of Film Coolant-to-Mainstream Interaction From Shaped Cooling Holes With Various Inlet Geometries

Abstract In this investigation, multiple sets of time-averaged tomographic particle imaging velocimetry measurements are completed for laid-back, fan-shaped film cooling holes with “racetrack” shaped inlets. Traditional 10-10-10, laid-back, fan-shaped holes are inclined 30 deg to the mainstream flow on a flat plate. The inlet cross section varies from round to two elongated racetrack shapes. The cross-sectional area and the outlet-to-inlet area ratio for all the geometries are held constant. The flat plate is installed in a low-speed wind tunnel with a mainstream turbulence intensity of 8% and an average velocity of 21.6 m/s. The blowing ratios of the film jets range from 0.6 to 1.5 and the density ratio is 1. The Reynolds number of the cooling jet varies from 2600 to 8400. The characteristics of the resulting flowfield are coupled with the detailed film cooling effectiveness distributions. It can be noted from the results that the counter-rotating vortex pair generated by the 2:1 inlet is the closest to the surface and weakest in strength, likely caused by the minimum peak jet momentum of the three. The Reynolds stresses downstream of the 2:1 and 4:1 inlets are significantly lower than those downstream of the shaped hole with a round inlet. An inverse relation between volumetric turbulence accumulation (TA), and surface effectiveness (η), can be correlated for the blowing ratios considered. The turbulence accumulation term can thus be used to evaluate the performance of a film cooling hole design with flowfield data.

Blade aerodynamic force and tip region vortex spatial-temporal evolution study of a compressor cascade

This paper utilized a numerical simulation method to get the vortex structure and aerodynamic force in a low-speed compressor cascade at 5-deg incidence with a hybrid unsteady Reynolds averaged Navier-Stokes/large eddy simulation (URANS/LES) turbulence model. The aerodynamic force, tip leakage vortex (TLV), and the mainstream flow interaction, especially in the blade tip region, are well analyzed. The instant flow pattern showed that the tip secondary leakage vortex and trailing edge backflow radial vortex occur by turning in the tip region. The spanwise flow pattern analysis exhibited that the dilation item dominates the tip region flow instability. The integral aerodynamic force at different spans showed that the tip region aerodynamic forces are far greater than that of mid-span and root-span. The correlation analysis between the aerodynamic force peak and the vortex structure showed that the float in and out of the trailing edge radial vortex closely relates to the aerodynamic force peak value change. The proper orthogonal decomposition (POD) mode results showed that the static pressure on the blade surface refers closely to the leakage vortex structures. The POD time coefficient curves and the aerodynamic force wave results showed that the tip secondary leakage vortex and the radial vortex at the trailing edge in the tip region dominate the aerodynamic force in the tip region. This research found a correlation between the POD mode time coefficient and blade aerodynamic force.

Heat transfer process for penetration flow into a branch pipe causing thermal fatigue at a T-junction

The heat transfer process between a fluid and a wall at a T-junction was investigated for impinged penetration flow in which the hot mainstream flow impinged on the branch pipe wall and penetrated into the branch pipe on the downstream side. The temperature distributions of the fluid near the branch pipe wall and the pipe inner surface were simultaneously measured using thermocouples. The fluid temperature close to the branch pipe wall fluctuated as a result of the hot mainstream flow penetrating into the branch pipe, and these fluctuations caused temperature fluctuations on the pipe inner surface. However, the temperature fluctuation range on the inner surface did not exceed 40% of the range of the fluid temperature fluctuation, because fluctuations were attenuated due to the heat transfer process from fluid to structure. The power spectrum method was used to the heat transfer coefficient on the pipe inner surface. The evaluated heat transfer coefficient was 1.62 times larger than that calculated using the Dittus-Boelter equation. The temperature fluctuation range on the inner surface was successfully estimated by leveraging the derived heat transfer coefficient and the measured fluid temperature.

A Numerical Study on the Energy Dissipation Mechanisms of a Two-Stage Vertical Pump as Turbine Using Entropy Generation Theory

Utilizing a two-stage vertical pump as turbine (TVPAT) is an economically method for constructing small-scale pumping and storage hydropower stations at high head-low discharge sites, such as underground coal mines. The energy dissipation mechanisms in flow passages are theoretically important for performance prediction and geometric parameter optimization. In this paper, the energy dissipation within the TVPAT has been studied using entropy generation theory, which can be applied to visual, locate and quantify energy dissipation. The numerical solution of entropy dissipation components was extracted on turbine modes in different flow rates using the steady-state single-phase SST k-ω turbulence model. The numerical results show that the energy dissipation in TVPAT mainly comes from turbulent fluctuation (43.6%-72.1%) and blade surface friction (27.8%-58.2%). The runners are the main source of turbulent entropy (SD′ ) generation (47.2%-83.3%). The contribution of the return channel and spiral case to the generation under overload conditions is significant, accounting for 33.6% and 14.3 at 1.3QBEP, respectively. Flow field analysis reveals that high generation within a runner are located in the striking flow region of the leading edge, the flow squeezing region in the blade channel, and the wake region of tailing edge. The mismatch between the placement angle of the blades or guide vanes and the liquid flow angle is an important incentive for SD′ generation. Moreover, hydraulic energy is consumed through the interaction between mainstream and local inferior flows such as separation and vortices, as well as the striking and friction between local fluid and wall surfaces.

Accuracy Investigations of Dynamic Characteristic Predictions of Tip Leakage Flow Using Detached Eddy Simulation

The accurate prediction of tip leakage flow is the premise for flow mechanism analysis and compressor performance optimization. The detached eddy simulation (DES) method, which compromises cost and accuracy, has excellent potential for a high Reynolds flow, like a compressor.However, in the case of tip leakage flow, especially when there are multiple wall boundary layers and strong shear between the mainstream and leakage flow, the DES method exhibits accuracy deficiencies. This paper explores the resolution of the critical detailed structures using the DES method and its correlation with the accuracy of time-averaged aerodynamic parameter predictions. Based on this, we propose the necessary conditions for the DES method to accurately predict the leakage flow from the perspective of the detailed structure of the flow field. A simplified model is proposed to emphasize the characteristics of tip leakage flow with “multiple walls + narrow tip gap”, and the high-fidelity flow field of the WALE LES method is used as a benchmark. With the main fluctuation structures obtained by the SPOD method, it is concluded that the DES method is unable to resolve the Kelvin–Helmholtz instability at the initial position of the leakage, which leads to the generation of the secondary leakage vortex upstream of the leakage and the breakdown of the induced vortex, two critical flow structures, being incorrectly estimated. This can lead to misestimationsof the force direction on the tip leakage vortex and the main fluctuation on the flow field. As a result, the tip leakage vortex trajectory evolves toward the middle of the passage along the tangential direction and away from the upper wall downstream of the leakage compared with the LES results. Predictions of losses in the upstream and midstream regions are underestimated, whereas they are overestimated downstream of the leakage and outside the passage.Therefore, the accurate resolution of these two critical detailed structures is an essential prerequisite for the precise prediction of tip leakage flow using DES series methods.

Research on the Hydraulic Characteristics of Active Ship Collision Avoidance Devices for Hydrodynamic High-Energy Beam Bridges under Relatively Optimum Deployment Conditions

To address the limitations of existing bridge anti-ship collision devices, which cannot protect both ships and bridges, this study introduced a hydraulic high-energy beam for inland navigation safety. Using a bridge as the technical basis and a typical ship in a navigable river section as the research object, the reasonable deployment angle of the device was investigated and the optimal jet ratio of the device R (the ratio of the high-energy beam jet to the mainstream flow velocity) was clarified through combined numerical simulations and a generalized model test. The ship’s motion response state was subsequently validated when the device was reasonably deployed. The results showed that the reasonable deployment angles of the device were 0°, 15°, and 30°. R = 4 served as the optimal jet ratio. Furthermore, the peak value of turbulence intensity in the Y direction was noticeably smaller than in the other three groups, with a stable change. The coordinate error of the key positions in the numerical simulations and generalized model test of ship motion response was less than 10%, the maximum error of the transverse coordinate of the deflection position was −9.8% and the maximum error of the longitudinal coordinate was −7.0%. The maximum error of the transverse coordinate of the maximum deflection position was −6.8% and the maximum error of the longitudinal coordinate was 3.7%. The numerical simulations and generalized model tests of ship motion response fit well.

Swirl Enhancement Effect on Turbine Rim Seal Performance

Abstract In gas turbines, sealing flow is extracted from the high-pressure compressor and supplied into the turbine wheel-space to suppress hot gas ingestion. Such hot gas ingestion is thought to be driven by the difference in either the static pressure or swirl between the mainstream and wheel-space flows. For the first time, experiments have been conducted in a low-speed single-stage axial turbine with a single-radial clearance rim seal and a “swirler” on the rotor disk. The objective is to evaluate the impact of enhanced wheel-space flow swirl on the rim seal performance. The swirler does not affect the mainstream flow. In the wheel-space, however, the swirler reduces static pressure; increases swirl; and improves rim seal performance (i.e., reduces the minimum sealing flowrate needed for hot gas ingestion prevention). Thus, the seal performance improvement occurs with increased difference in pressure and reduced difference in swirl between the mainstream and wheel-space flows. Therefore, it can be inferred that the rim seal performance depends more strongly on swirl than pressure. Preliminary gas turbine cycle performance studies indicate that net cycle efficiency benefits can be obtained.

A Model of Mass and Heat Transfer for Disc Temperature Prediction in Open Compressor Cavities

Abstract Accurate prediction of heat transfer in compressor cavities is crucial to the design of efficient and reliable aircraft engines. The heat transfer affects the thermal expansion of the compressor rotor and, in turn, the tip clearance of the compressor blades. This article presents a novel, physically based predictive theoretical model of heat transfer and flow structure in an open compressor cavity, which can be used to accurately calculate disc temperatures. The radially higher region of the cavity is dominated by buoyancy effects created by the temperature difference between the hot mainstream flow and the axial throughflow used to cool the turbine. Strong interaction between the air in the cavity and this throughflow creates a mixing region at low radius. For a given geometry, the heat transfer and flow physics are governed by four parameters: the rotational Reynolds number Reϕ, the buoyancy parameter βΔT, the compressibility parameter χ, and the Rossby number Ro. The model quantifies both the buoyancy- and throughflow-induced mass and heat transfer, producing a reliable prediction of the disc and air temperatures. The model takes into account a twofold effect of the throughflow: being entrained into the cold radial plumes directly and creating a toroidal vortex in the radially lower region of the cavity. The exchange of mass between the cavity and throughflow is related to the mass flowrate in the radial plumes in the buoyancy-induced region, considering the effect of flow reversal at low Ro. The model is validated using data collected in the Bath compressor cavity rig and can be incorporated in engine design codes to robustly compute the thermal stress and expansion of the compressor rotor, contributing to more efficient engine designs.

Research on blending combustion characteristics of coaxial injector of oxygen/methane engine

Aiming at the demand of oxygen/methane rocket engine for future reusable spacecraft, the design of oxygen/methane coaxial injector engine and the study of flow mixing and combustion characteristics were carried out. The flow field characteristics, flame structure size and heat release state changes under different fuel-oxygen momentum ratio of coaxial injector were investigated. The results show that the increase of fuel-oxygen momentum ratio in the cold flow state increases the influence of the mainstream flow on the flow in the combustion chamber, and the length of the vorticity shear layer also increases. With the increase of fuel-oxygen momentum ratio, the flame's lifting distance gradually decreases, the rate of methane needed to stabilize the flame in the shear layer is over twice that of oxygen. At this time, as this ratio increases, the flame expansion angle increases while its length decreases. Rich combustion in the return zone is suppressed, but there is still some impact on component distribution ahead of main heat release. With the increase of fuel-oxygen momentum ratio, the starting position of the high temperature zone gradually advances, and the increase of fuel-oxygen momentum ratio makes the mainstream temperature change significantly intensified. Within a fuel-oxygen momentum ratio range of 0.5–2, the alteration of this ratio has a significant impact on the temperature profile of the gas close to the wall. However, as the ratio increases, the effect of this change on the gas temperature diminishes significantly.

Numerical Study of Endwall Modification with Micro-Scale Ribs in a Turbine Cascade

A novel modification method, the ‘micro-scale’ rib, is proposed to expand cooling coverage for turbine endwalls. However, the introduction of the rib will inevitably affect the flow in the near-wall region. Therefore, the variation in the flow pattern for the traditional model of secondary flow needs further exploration. In this paper, to gain a clearer understanding of the micro-scale rib, the original endwall and three types of ribbed endwalls were adopted to numerically present the detailed flow, film cooling, and heat transfer information for the endwall surface and phantom cooling on the suction side (SS) of the blade. The Ansys code CFX was utilized to solve the 3D Reynolds-averaged Navier–Stokes (RANS) equations, and the SST k-ω was selected as the turbulence model after the verification. The results show that the rib-like vortex changed the flow of the coolant and had various impacts on the cooling characteristics. Although the cooling performance of the ribbed endwall improved, it also had a negative impact on heat transfer in most cases. Compared with the original, the vertical rib cases provided optimal film cooling, with increases of 26.9% and 17.4% for rib spacing values of 8 mm and 10 mm, respectively, with little difference in heat transfer (less than 1%). In addition, the horizontal rib cases presented the worse performance for both film cooling and heat transfer, which indicates that the rib layout should consider a mainstream flow direction for future designs.

Experimental Analysis of a Solar Air Heater Featuring Multiple Spiral-Shaped Semi-Conical Ribs

Abstract To improve the thermal and hydraulic performance of artificially roughened solar air heaters (SAHs), the current study analyzes the thermal-hydraulic performance or thermal enhancement factor of artificially roughened solar air heaters. In present experimental research on a solar air heater's absorber plate, newly designed spiral-shaped semi-conical ribs have been explored. The spiral-shaped semi-conical ribs have been designed with the aim of reducing the pressure drop across the rib so that thermal performance may be improved with a little increase in pressure drop after integrating the ribs into the SAH mainstream flow. The higher value of thermal-hydraulic performance indicates an increased heat transfer rate with a minimum increase in pumping power. In order to achieve the highest possible thermal enhancement factor, this experimental study intends to analyze the effects of different geometrical parameters on the heat transmission and friction behavior of numerous spiral-shaped semi-conical ribs. Multiple experiments were conducted using different levels of roughness heights to optimize the rib profile parameters. The Reynolds number (Re) ranges from 3358.65 to 18,095.59, the relative roughness height (e/Dh) 0.09 to 0.227, and relative roughness pitch (P/e) 3.7 to 5.5. These multiple spiral-shaped semi-conical ribs give the maximum thermal enhancement factor of 2.85 at (e/Dh) 0.182 and P/e of 4.1 at Reynolds number 18,095.59. It has been found that current rib geometry can increase the thermal performance of solar air heaters with minimum increased pumping power with reference to rib explored by earlier researchers.

Regional contributions of climate change and human activities to altered flow of the Lancang-mekong river

Study regionThe Lancang-Mekong River (LMR) in Southeast Asia. Study focusThe LMR provides unparalleled economic and ecological resources for six Asian countries. However, the intensified impact of human activities, coupled with the effects of climate change, has led to alterations in its natural flow, thereby disrupting the river's flow regime. Ensuring the future sustainable management and downstream ecological integrity of this transboundary river requires understanding the regional contributions to the river's altered flow at the sub-basin level. We conducted an analysis for the last four decades using a hydrological model, which divided the entire basin into eight sub-basins, each associated with one of the eight main hydrological stations. New hydrological insights for the regionOur analysis showed significant regional differences in precipitation variation across the LMR, from its headwaters to the lower regions, during both periods. Furthermore, a spatial variation was observed for the impacts of human activities on the natural runoff in both seasons, indicating a lack of effective coordinated management efforts among riparian countries. Notably, our findings indicated that agricultural expansion and the construction of tributary dams in certain sub-basins had a more significant impact on the reduction of the mainstream flow compared to the mainstream dams. These findings underscore the enormous potential of regional water governance to alleviate the impacts of changing climate, offering valuable insights for addressing this critical issue.

Large eddy simulation of film cooling for a forward expansion hole under main flow oscillation

This study conducted a large eddy simulation (LES) to predict the flow structure and effectiveness of film cooling in a gas turbine. The study focused on a forward expansion hole on a flat plate with an inclined injection angle of 35° and spanwise injection angles (orientation angles) of 0° and 30° under main flow oscillation. In gas turbines, the mainstream flow can exhibit unsteadiness owing to stator–rotor interactions or freestream turbulence. Here, sinusoidal functions are used to approximate the flow oscillations, and their effects are evaluated at frequencies of 0, 8, and 32 Hz, corresponding to Strouhal numbers of 0, 0.603, and 2.413, respectively. The study aims to characterize the film cooling at two time-averaged blowing ratios: 0.5 and 1.0. The range of Strouhal numbers considered in this study is relevant to actual transonic gas turbines. The computational results obtained through LES are validated using experimental data and compared with the Reynolds-averaged Navier–Stokes simulation results. The findings demonstrate the efficiency of LES to predict accurately the distribution of spanwise film cooling effectiveness and spanwise-averaged effectiveness for forward expansion holes on turbine blades under flow oscillations. Comprehensive analyses of various aspects such as the instantaneous turbulent structures of the film cooling flow fields, time-averaged temperature, and root-mean-squared values of the coolant temperature and a proper orthogonal decomposition analysis for velocity vectors are performed. The results show that when the oscillation frequency of the main flow varied from 0 Hz to 32 Hz for a time-averaged blowing ratio of 0.5, the film cooling effectiveness for the forward expansion hole reduced significantly. However, when the frequency increased under a time-averaged blowing ratio of 1.0, the film cooling effectiveness for the hole was only slightly affected by the oscillations. Moreover, an orientation angle of 30° enhanced the film-cooling performance even under flow oscillations. Thus, this paper reports the development of highly effective film-cooling strategies that can guide researchers aiming to mitigate the adverse effects of flow oscillations, thereby enhancing the reliability and efficiency of gas turbine engines.

Debunking the lethal triad and delineating damage control surgery.

The last three decades of literature in trauma have been erroneously hammered by the belief the lethal triad (LT) of acidosis, hypothermia and coagulopathy was, as claimed originally, the statutory rationale underlying for the application of damage control surgery (DCS) strategy. As a matter of fact, the LT is not all lethal: only acidosis is lethal and a reliable hard sign for DCS, indicating severe levels of hypoxemia and tissues hypoxia. The mainstream flow of events leading to exitus in hemorragic shock pass through macro and microcirculation dynamics, oxygen, acidosis, and ischemia-reperfusion toxemia. It is solely by interfering or manipulating these variables dynamics that we can decrease morbidity and mortality. A solid synoptic list of the indications and timing of DCS has been elaborated.

Large eddy simulation of supersonic flow in ducts with complex cross-sections

Large Eddy Simulation (LES) has been employed for the investigation of supersonic flow characteristics in five ducts with varying cross-sectional geometries. The numerical results reveal that flow channel configurations exert a considerable influence on the mainstream flow and the near-wall flow behavior. In contrast to straight ducts, square-to-circular and rectangular-to-circular ducts exhibit thicker boundary layers and a greater presence of vortex structures. Given the same inlet area, rectangular-to-circular ducts lead to higher flow drag force and total pressure loss than square-to-circular ducts. Characterized by the substantial flow separation and shock waves, the “S-shaped duct shows significant vertically-asymmetric characteristics.