Main Flow Research Articles

Salinity dynamics under different water management plans coupled with sea level rise scenarios in the Red River Delta, Vietnam

In recent years, saltwater intrusion in river estuaries has become more severe and frequent worldwide. The common reasons lie in increasing freshwater withdrawal, river flow regulation and sea level rise due to global warming. In particular, the Red River Delta in northern Vietnam is facing a strong population growth worsening the pressure on freshwater resources for drinking water and irrigation needs. During the dry season, increasing conflicts and constraints in freshwater availability have already been experienced. Adverse combinations of river flow regulations and high sea levels lead to severe upstream propagations of salinity. This study takes advantage of a statistical characterization of discharges released from Hoa Binh reservoir and observed at Son Tay station, the main river flow control upstream of the river delta, along with downscaled and updated sea level rise scenarios to estimate the future extents of saltwater intrusion under different options of water release from reservoirs in the dry season. To do so, a 1D hydraulic model of the river delta network was implemented using MIKE11 software. The hydraulic and the quality modules were calibrated and validated with respect to the present scenario by using water stages and salinity concentrations observed in estuary branches. Sea level rise projections for 2050 and 2100 referred to RCP4.5 and RCP8.5 AR5 emission scenarios were then considered. Results show that river flow regulation can provide an effective mitigation measure. A 20–30% increase in the discharge released from the Son Tay station would be beneficial to push downstream the saltwater intrusion in the main Red River branch during the dry season. For instance, in 2050 the 1‰ salt concentration front is expected to be pushed back at least 6 km when the exceeding probability of the discharge released by Son Tay station decreases from 95% to 25%.

Aerothermal optimization of turbine cascade squealer tip with non-uniform squealer height

The squealer tip has significant influence on both the aerodynamic and heat transfer characteristics of the high-pressure turbine blade. However, due to the complexity of parameterization and meshing of the squealer and the complicated flow structure within the over-tip region, the existing squealer designs in the open literature have constant squealer heights. In this paper, the design space to the squealer height with non-uniform squealer height is extended and the new flow features it may bring are investigated. A parameterization system specifically designed for the non-uniform squealer height using five control parameters is implemented to automatically generate the geometry and hybrid meshes. Combining it with the multi-objective optimization system using genetic algorithms, a transonic turbine cascade squealer tip is optimized employing Reynolds-averaged Navier–Stokes k–ω shear stress transport model. The main objective of this study is to obtain a squealer configuration with the lowest total pressure loss coefficient and heat transfer coefficient. The optimum configuration with non-uniform squealer height achieves improvements in both the aerodynamic efficiency and the heat transfer performance, relative to the baseline conventional squealer tip geometry with the constant squealer height. Additionally, this work demonstrates that a flow structure in which the main flow forms a “blanket” below the leakage flow in the squealer is beneficial for aerothermal performance, especially reducing heat transfer losses, which provides valuable insight into the squealer tip design of advanced high-pressure turbines.

Influence of incoming turbulence and shear on the flow field and performance of a lab-scale roof-mounted vertical axis wind turbine

Flow conditions in an urban environment are complex, featuring varying levels of turbulence intensity and shear. The influence of these flow characteristics on the performance of a roof-mounted vertical axis wind turbine of the Savonius (drag) type is investigated at lab scale. Five different inflow conditions are generated with an active grid in a wind tunnel, covering turbulence intensities from 0.9% to 11.5% and relative vertical shear from 0% to 17%. The flow field is captured using particle image velocimetry, and the power output of the turbine is assessed through measurements of the converted power. The set-up consists of two-surface mounted cubes aligned with each other in the main flow direction, spaced apart by two cube heights. The turbine is placed on top of these model buildings at six different streamwise positions along the centerline and at two different heights. It was observed that the turbulence intensity in the inflow has a significant impact on the flow field and also on the power output of the turbine. The increasing turbulence intensity leads to smaller regions of recirculating flow. Thus, the turbine experiences higher flow velocities, which is reflected in the measured power. The influence of shear is comparably small on both the flow field and the turbine performance. The higher of the two turbine positions yields higher power output overall. Furthermore, it was shown that the impact of the turbine on the flow field is significant for all inflow conditions and can vary substantially depending on the inflow.

Impact of the Purge Flow Density Ratio on the Rim Sealing Effectiveness in Hot Gas Ingestion Measurements

Abstract The rim seals of gas turbines are used to control the ingestion of hot mainstream gas into the wheel space between the rotor disk and the stationary casing. Sealing air, which is generally used to pressurize the cavity space, flows through the seal clearance and then mixes with the flow path in the annulus. Predicting the correct quantity of purge flow necessary to prevent excessive ingestion of hot gases while, at the same time, minimizing the penalties in terms of engine efficiency and stage aerodynamics represents a great challenge for the designers and a crucial point for the design of reliable engines. Such estimate is governed by unsteady phenomena, and computational fluid dynamics (CFD) approaches are still expensive and time consuming, especially if 3D domains and unsteady conditions have to be simulated. Fundamental test cases, replicating actual engines geometries, are still a valid approach to calibrate correlations or simplified models such as the so-called orifice model. However, most of the experimental studies deal with test rigs at room temperature and do not take into account the effect of the density ratio (DR) between purge and main flows. To fill this gap, the present article reports the impact of the density ratio on the rim sealing effectiveness by performing a nonintrusive diagnostic based on the pressure-sensitive paint (PSP) technique on both the stator side and the rotor side. The analysis was performed on a cold rotating cavity rig, developed for the study of hot gas ingestion, where two different values of density ratios were tested by using N2 (DR = 1) and CO2 (DR = 1.52) as purge flow. The data extracted from the PSP seal effectiveness maps allowed to calibrate the orifice model for the stator side and to fit the coefficients of the buffer ratio model for the rotor surface at different flow conditions where the externally induced ingress is the dominant mechanism for gas ingestion. The results highlighted the impact of the DR on the seal effectiveness and on the low-order models considered for the data analysis. In the end, it is shown that the obtained results can be used to scale experimental data, generally collected at DR close to one, toward more representative engine values where the difference between the density of purge and main flows cannot be neglected.

Flow loss mechanism of a novel co-rotating scroll hydrogen recirculation pump for fuel cell systems

The hydrogen recirculation pump is a key component of fuel cell systems. The aerodynamic performance of a novel co-rotating scroll hydrogen recirculation pump with the array scroll structure was investigated in this study. The performance differences between the co-rotating and orbiting scroll pumps were obtained via computational fluid dynamics approach. The flow loss mechanism of the co-rotating scroll pump was revealed by the entropy production analysis and omega vortex identification. The results show that compared with the orbiting scroll pump, the co-rotating one has a better over-compression consistency between adjacent working chambers, and a higher isentropic efficiency with a lower direct dissipation entropy production. Main flow losses of co-rotating scroll pump originate from the turbulent and direct dissipations in scroll chambers, and mainly occur in the discharge process. The large-scale omega vortices flow is responsible for the pump flow losses. The strengthened large-scale omega vortex expands the area of high entropy production in central discharge region. The suction flow losses increase due to the strengthened suction flow separations. The total area of omega vortex in the surrounding and central regions contrarily change with the pump isentropic efficiency. The smallest area of omega vortex represents the minimum flow loss of the co-rotating scroll pump at 7500 r·min−1.

Highly accelerated compressed sensing 4D flow MRI in congenital and acquired heart disease: comparison of aorta and main pulmonary artery flow parameters with conventional 4D flow MRI in children and young adults.

Four-dimensional flow (4D flow) MRI has become a clinically utilized cardiovascular flow assessment tool. However, scans can be lengthy and may require anesthesia in younger children. Adding compressed sensing can decrease scan time, but its impact on hemodynamic data accuracy needs additional assessment. To compare 4D flow hemodynamics acquired with and without compressed sensing. Twenty-seven patients (median age: 13 [IQR: 9.5] years) underwent conventional and compressed sensing cardiovascular 4D flow following informed consent. Conventional 4D flow was performed using parallel imaging and an acceleration factor of 2. Compressed sensing 4D flow was performed with an acceleration factor of 7.7. Regions of interest were placed to compare flow parameters in the ascending aorta and main pulmonary artery. Paired Student's t-tests, Wilcoxon signed-rank tests, Bland-Altman plots, and intraclass correlation coefficients were conducted. A P-value of < 0.05 was considered statistically significant. Mean scan acquisition time was reduced by 59% using compressed sensing (3.4 vs. 8.2min, P < 0.001). Flow quantification was similar for compressed sensing and conventional 4D flow for the ascending aorta net flow: 47 vs. 49ml/beat (P = 0.28); forward flow: 49 vs. 50ml/beat (P = 0.07), and main pulmonary artery net flow: 49 vs. 51ml/beat (P = 0.18); forward flow: 50 vs. 55ml/beat (P = 0.07). Peak systolic velocity was significantly underestimated by compressed sensing 4D flow in the ascending aorta: 114 vs. 128cm/s (P < 0.001) and main pulmonary artery: 106 vs. 112cm/s (P = 0.02). For both the aorta and main pulmonary artery, compressed sensing 4D flow provided equivalent net and forward flow values compared to conventional 4D flow but underestimated peak systolic velocity. By reducing scan time, compressed sensing 4D flow may decrease the need for anesthesia and increase scanner output without significantly compromising data integrity.

Energy flow analysis of in Litopenaeus vannamei-Tegillarca granosa polyculture ponds based on EwE model

In order to characterize the energy flow and trophic structure of a polyculture pond that contained Litopenaeus vannamei and Tegillarca granosa, we used an Ecopath with Ecosim (EwE) model. We constructed the EwE model using 13 functional groups including L. vannamei, T. granosa, macrozooplankton, microzooplankton, microbenthos, bacterioplankton, benthic bacteria, micro-phytoplankton, nano-phytoplankton, pico-phytoplankton, artificial feed, detritus in water, and detritus in sediment. Our findings showed that this polyculture ecosystem contained five trophic levels and that two of them accounted for 99.41% of the total system throughput. L. vannamei, microbenthos, and detritus in sediment displayed high ecotrophic efficiency (EE) values, while artificial feed held relatively low EE implying that the artificial feed was used inefficiently. The bacteria including benthic bacteria and bacterioplankton was inefficiently used. The main energy flows were associated with artificial feed, phytoplankton, and detritus in this polyculture system. The compatibility of L. vannamei and T. granosa was high according to the mixed trophic impact analysis. In conclusion, the study showed that this polyculture ecosystem still needed to be improved in the aspects of energy consumption. It would be necessary for this polyculture ecosystem to perform further optimization and enhancement on the energy utilization efficiency and comprehensive benefits.

О моделировании гидродинамического режима течений в системах речных русел в период прохождения волны половодья на примере участка р. Оби

Рассмотрен пример численного моделирования прохождения волны половодья на участке верхней и средней Оби ниже плотины Новосибирской ГЭС. Расчеты выполнены с использованием численной модели для описания распространения и трансформации волн половодий и паводков в системе русел (STREAM), разрабо танной в ИВЭП СО РАН. Модель основана на одномерных уравнениях Сен-Венана. Система русел моделируется в виде графа типа дерево, в вершинах которого за даются условия сопряжения основных гидродинамических параметров. Приведена приближенная численная оценка порядка точности метода решения. Результаты моделирования хорошо согласуются c данными наблюдений. The goal of the study is to analyze the possibility of using the numerical model STREAM for performing research on the passage of a flood wave in a system of channels. The model was developed at IWEP SB RAS on the basis of one-dimensional Saint-Venant equations. It allows describing the flow regimes in channels, represented as a directed graph of the tree type. In the nodes of the graph (channel gates), the conjugation conditions for the main hydrodynamic parameters flow rates and water levels are set. The finite difference method is used to solve the system of equations. The difference scheme has a second order of accuracy in space and a first order in time. An approximate numerical estimate of the order of accuracy of the model using the Runge method is obtained. Numerical simulation of the Ob river section under the conditions of a high-water year 2014 has been carried out. To construct the morphometric model of the graph, data from the pilot map of the river and data of the expedition studies of IWEP SB RAS were used. The hydrological information is taken from the open access source “Automated information System of State monitoring of water bodies”. As a simulated object, a section of the Ob river from the Novosibirsk hydroelectric power station to Alexandrovskoye village is taken. The length of the site is 1210 km. The flow rates of six tributaries are taken into account. The graph consists of 12 segments. In the closing section, a boundary condition is set for water flow in the form of a flow rate versus level. To estimate the influence of the boundary condition on the simulation results, two water flow curves were constructed according to the data of 2013 and 2014 years. Qualitative and quantitative estimates of the difference in levels using two different curves, as well as comparison with the measurement data, were obtained. It is shown that the calculation method provides an acceptable agreement between the calculated values of water levels and the observational data. The possibility of using the water flow curve according to the data of the previous year as a boundary condition in the closing gate is demonstrated.

Simulation analysis of energy characteristics of flow field in the transition process of pump condition outage of pump-turbine

The pump-turbine is the heart of a pumped storage power plant. This study combines numerical simulations with experiments to investigate the flow stability, energy loss in the main flow area and their interconnection in the transition process of the double-row cascade and runner chamber of the pump-turbine, when the pump-turbine is operated under pumping conditions with a sudden power failure and the guide vane refuses to close. The results show that the overall flow pattern of the transition process is extremely unstable, and the flow state is presented as smooth-disturbed-smooth-disturbed, with the maximum vorticity difference(Δω) of 696756 s−1 in the neighboring working conditions; the maximum entropy production rate of the double-row cascade and runner chamber in the braking process has a similar trend in the vorticity change trend, and the rapid change of the flow pattern of the internal fluid causes the sharp change of the entropy production in the pump-turbine; The maximum entropy production rate in the runner chamber is of the order of e6, which is much higher than that in the other regions, and the poor flow phenomenon caused by the runner is the main reason for the energy loss.

Influence of Rotation Speed and Gas Content on the Transient Gas–Liquid Two-Phase Flow of an Electric Submersible Pump

In order to study the internal flow characteristics of the electric submersible pump (ESP) when the gas–liquid two-phase flow is conveyed by the variable frequency variable speed operation and the change of the imported gas content, the impeller of the Q10# ESP is taken as the research object, based on the Eulerian-Eulerian non-homogeneous phase. The flow model, the unsteady Reynolds time-averaged N-S equation, and the standard k-ε turbulence model are used for transient simulation calculations of the gas–liquid two-phase flow in the impeller of the ESP. Calculations show that with the rotation of the impeller, the gas phase is unevenly distributed in the flow channel. The gas phase is mainly concentrated on the inlet side of the flow channel near the front cover, and the gas phase exhibits periodic aggregation and diffusion in the flow channel. When the impeller speed increases, the period of periodic accumulation and diffusion of gas in the flow channel is shortened and the gas concentration in the impeller decreases, the overall flow velocity in the flow channel increases, and the pressure difference between the inlet and outlet increases. The pressure difference between the two sides of the blade is proportional to the speed of the impeller, and the fluctuation frequency of the blade surface also increases. As the gas content increases, the maximum concentration of gas phase in the flow channel increases. The area occupied by the high concentration of gas phase in the flow channel expands toward the blade’s working surface, and periodically accumulates, diffuses, and grows. The gas-liquid splitting area shrinks toward the front cover side and the pump. The internal pressure increases slightly, the main flow velocity increases, and the vortex action range increases.

Controlling secondary flow in high-lift low-pressure turbine using boundary-layer slot suction

The design of high-lift Low-Pressure Turbines (LPTs) causes the separation of the boundary layer on the suction side of the blade and leads to a strong secondary flow. This present study aims to minimize secondary losses through endwall slot suction and incoming wakes in a front-loaded high-lift LPT cascade with Zweifel of 1.58 under low Reynolds number of 25000. Two slotted schemes for the boundary layer of the endwall were designed (Plan A and Plan B), and the effects of suction mass flow on secondary flow were studied. The underlying physics of the endwall boundary layer of the suction and secondary flow under unsteady wakes was discussed. The results show that slot suction at the endwall boundary layer can significantly suppress the secondary flow by removing low-momentum fluids. Plans A and B significantly reduced the secondary kinetic energy by 44.2% and 36.9%, respectively, compared with the baseline cascade at the suction mass flow ratios of 1%. With an increase in the mass flow ratio of suction, the secondary flow was gradually reduced in both Plans A and B. It is more beneficial to control the secondary flow to destroy the intersection of the pressure side and suction side of the horseshoe vortex before it develops into a passage vortex. Under unsteady wakes, the combined effects of incoming wakes and endwall boundary layer suction can further suppress the secondary flow at the suction mass flow ratios of 2% for Plan A, because the positive and negative vorticity inside upstream wakes accelerated the mixing of the main flow and secondary flow and thus increased the energy of secondary vortices.

Flood frequency and flood forecasting analysis of Krishna basin Andhra Pradesh

Floods are occurrences of natural hazards, frequent during the year in many rivers across the globe. Every year, several rivers in India are vulnerable to flooding, causing loss of property and life. Krishna is one of the major rivers in India which is vulnerable to flooding in every monsoon. In this study, flood analysis was conducted for the year 2019. Rainfall data from AWS was used to estimate discharge levels in dams during the monsoon of 2019. It was observed that moderate rainfall occurred in the months of August and September, corresponding to low rainfall and that extreme flooding occurred in the same month. Compared to the flood inundation map of the satellite, it shows a close relationship with the flood map of 2019 and the affected area. The study shows that proper analysis of rainfall will be helpful in predicting downstream floods. To evaluate the flood control situation with appropriate data management in the Krishna basin, the usage of flood water is strong. Such types of studies would help to provide reliable and prompt flood forecasts and advance warning to redirect the main river flow to small canals, which will help to mitigate, excavate and remediate flooding in any area.

Modelling secondary circulations in stratified bi-directional flows: A potential mechanism for flux transfer from lower to upper fluid layers

Results are presented for new laboratory-scale numerical simulations of bi-directional stratified flows generated within an idealized trapezoidal sill-channel topography, focussing on the lateral (or secondary) flow structure across the channel both in non-rotating and rotating frames of reference. The simulations utilise the non-hydrostatic Bergen Ocean Model (BOM), a three-dimensional general ocean circulation model. The results from the BOM simulations are verified by large-scale experimental data obtained in the LEGI Coriolis rotating platform in Grenoble, within which velocity and density fields for bi-directional stratified flows were measured through particle image velocimetry (PIV) and micro-conductivity density probes, respectively. The BOM simulations reproduce the main dynamic flow patterns and structure of the large-scale exchange flows generated through the trapezoidal sill-channel, with the lower layer saline intrusion flux shown to reduce, due to partial blockage, as the upper freshwater flow is increased (i.e. net barotropic forcing). Non-rotational BOM simulations show upward (and downward) flow along the centreline of the trapezoidal sill-channel in the upper (and lower) layers, with symmetrical counter-rotating circulation cells forming both above and below the density interface. By contrast, equivalent BOM simulations within a rotating frame of reference show the secondary flow circulation in the lower dense water layer to be dominated by Ekman dynamics, while two co-rotating cells are formed in the upper freshwater layer above the inclined density interface. In these rotational cases, significant upward and downward velocities (i.e. strong upwelling and downwelling) are generated adjacent to the inclined sill-channel side walls (in comparison to non-rotating cases). This secondary flow effect may be especially relevant for exchange flows in estuaries, fjords, deep-ocean channels and sea straits, where the channel width exceeds the Rossby radius of deformation and, thus, Earth rotation effects become important dynamically. In particular, any potential vertical transport pathway associated with this upwelling and downwelling at the inclined channel side slopes may have strong managerial implications in coastal areas, whereby tracers (e.g. contaminants, salinity, reduced DO waters, fine sediments) can be transferred from deep waters to the near surface.

Wetland Management in Recharge Regions of Regional Groundwater Flow Systems with Water Shortage, Nyírség Region, Hungary

Climate change and increasing human impacts are more emphasised in recharge regions, where the main flow direction is downward, resulting in negative water balance. Two wetlands located in the recharge position of regional groundwater flow systems were investigated in the Nyírség region, Hungary, as pilot areas for representing wetlands in similar hydraulic positions. Hydraulic data processes, chemical data evaluations, and numerical simulations revealed that the wetlands are fed via local flow systems, superimposing regional-scale recharge conditions in the area. The wetlands are discharge and flow-through types in connection with local flow systems. Nevertheless, in the case of significant regional water table decline—due to the high vulnerability of recharge areas to climate change—local flows are degraded, so they are not able to sustain the wetlands. To preserve the groundwater-dependent ecosystems in the areas, water retention at the local recharge areas of the wetlands may help in the mitigation of water level decline under present-day conditions. If the regional water table continues to decline, comprehensive water retention solutions are needed in the whole region. The results highlight that understanding the natural wetland–groundwater interactions at different scales is crucial for the preservation of wetlands and for successful water retention planning.

ON THE USE OF PSP TO DETERMINE THE RIM SEALING EFFECTIVENESS

Abstract The control of cavities sealing has been a challenging problem since early gas turbine development stages and several aspects regarding the flow physics and the modelling of rim seal flows still represent an open question. Fundamental test cases have been used in the open literature to characterize the level of ingestion by varying the main flow parameters and the geometrical features. In most of them, the effectiveness is measured by using taps connected to a gas analyzer used to sample the concentration of a foreign gas on the stator surface used to seed the purge flow. Consequently, the results are usually single point measurements, unsteady effects are inevitably neglected, the intrusiveness of the approach must be carefully checked and the application on the rotor side demands for complex slip-rings or telemetry. To overcome these limitations, the current work presents the application of the PSP technique to the study of hot gas ingestion phenomena on a single stage rotating cold rig. The 2D distributions of partial pressure of oxygen collected through the wide optical accesses present in the rig were then related to the seal effectiveness. The proposed methodology was firstly validated through a comparison with the data obtained from standard gas sampling and then applied as main experimental technique. The analysis of the results highlighted the capabilities of the PSP to fast collect data on both stator and rotor side, including the tip of the seal tooth where non uniform efficiency distributions in the circumferential direction have been detected.

Groundwater potentiality in hard-rock terrain of southern Saudi Arabia using electrical resistivity tomography approach

To detect groundwater-bearing zones in southern Saudi Arabia, 10 profiles of 2D electrical have been constructed resistivity tomography at five selected sites. The collected data were corrected and processed using the RES2DINV software. The results demonstrated that site-1 contains two groundwater-bearing zones; the first zone is situated 13 m beneath the ground surface, while the depth of the second zone varies from 4 m to 15 m. A promising 15-m-deep groundwater-bearing zone was identified at site 2. The groundwater-bearing zones exhibit depths of 40, 10, and 10 m at sites 3, 4, and 5, respectively. The detected groundwater-bearing zones are notably shallow, with a depth of 20 m below the ground surface and with lateral extension up to 200 m. These aquifers prevail in the alluvial deposits, where the weathered/fractured or fresh basement rocks underlying these aquifers. Moreover, rainfall recharges these unconfined aquifers, as evidenced by the increasing water levels along the main Wadi channels flow during rainy seasons. Sites 2, 3, and 5 are considered as groundwater-bearing sources with high potential. Because this unconfined aquifer can be swiftly replenished by the 300–360 mm of annual rainfall in the southern Saudi lowlands, it is a promising area for groundwater supply for future urbanization plans and agricultural projects in neighboring areas.

Deep learning combined with singular value decomposition to reconstruct databases in fluid dynamics

Fluid dynamics problems are characterized by being multidimensional and nonlinear. Therefore, experiments and numerical simulations are complex and time-consuming. Motivated by this, the need arises to find new techniques to obtain data in a simpler way and in less time. In this article, we present a novel methodology based on physical principles to reconstruct databases with three, four and five dimensions, from sparse databases formed by sensor measurements. The methodology consists of combining Single Value Decomposition (SVD), which can extract the main flow dynamics, with neural networks. The neural network used is characterized by a simple architecture based on combining two autoencoders that work in parallel and are joined in the last layer. This new algorithm has been proved with three databases with different dimensions and complexities: in an Atmospheric Boundary Layer (ABL) with a turbulence model and in the flow past a two- and a three-dimensional cylinder. By applying this methodology, it has been achieved to reconstruct databases of different dimensions obtaining errors of the same order as those obtained in simulations. Summarizing, this work proposes a new hybrid physics-based machine learning model with a simple, robust and generalizable architecture, which allows reconstructing databases from very few sensors and with a very low computational cost.

Rotating Instabilities in Shrouded Low Pressure Turbine at Design and Off-Design Conditions

Abstract The present study aims to analyze the rotating instabilities that may occur inside shroud cavities above rotors of low-pressure turbine configurations. Unsteady simulations have been carried out at design and off-design conditions on two configurations, one being a multi-stage configuration. Unsteady flow structures, uncorrelated from blade passing frequencies and depending on operating points, are identified in every rotor tip shroud cavity. Similarities regarding flow patterns and interactions with the main flow are observed: hot spots of gas having different azimuthal extend come from the tip shroud cavity and rotate at the interface with the main flow path. The influence on the main flow and the origin of these instabilities are discussed. The study at off-design operating points deepens the analysis and allows the identification of physical parameters which drive the instability.

Wall friction and heat transfer in turbulent pulsating flow in a square duct

For the first time, a DNS study of pulsating turbulent flows and heat transfer in a square duct is performed. Calculations were carried out at Re=2200 in the current-dominance mode for eight values of the oscillation period and three values of the Prandtl number. It was found that the time-mean friction coefficient in the studied pulsating flows has a lower value than in the stationary flow. The difference reaches −14.7%. The values of the Nusselt numbers are also decreasing, albeit by a smaller amount. The features of secondary flows of Prandtl’s 2nd kind at different phases of the flow oscillations are studied. The profiles of the wall friction stress and heat transfer coefficients and their evolution over time are determined. The time-oscillations of the wall friction are entirely determined by the oscillations of the main flow, being ahead of it in phase by π/4, which coincides with the behavior of laminar pulsating flows at high frequencies. The oscillations in heat transfer on the wall are determined by oscillations in the intensity of turbulent heat exchange, and the change in the latter occurs almost in phase with a change in the volume-average intensity of the transverse velocity fluctuations.

Analysis of flow characteristics and cooling performance of a novel impingement/effusion structure with bypass hollow holes

As a promising cooling technique, the double-wall cooling structure has been integrated into the design of advanced aeroengine high-temperature components. However, its widespread application in configurations with low pressure ratios between secondary flow and the main flow is hindered due to significant internal flow resistance. To address this issue, a novel low-resistance hollow pillar double-wall structure (NHDW) is developed. This study conducts conjugated heat transfer numerical simulations of NHDW and the traditional solid pillar double-wall structure (TSDW) for comparative analysis. Flow characteristics are examined to understand the sources of internal flow resistance and the coupling mechanism between internal and external heat transfer. The results demonstrate that the NHDW exhibits substantially lower flow resistance than the TSDW, with the total pressure loss dropping to approximately 1/3 of the corresponding TSDW under hole inclination angles of 30°, 60°, and 90°. The reduced internal flow resistance of the NHDW is attributed to the parallel bypass flow within the hollow pillar. Moreover, the overall cooling effectiveness (ϕ) of NHDW is enhanced by 9.2%–16.9% for different inclination angles at a blowing ratio of M = 1.0. Additionally, the interaction vortex structure on the mainstream side surface of the NHDW significantly improves the external cooling effectiveness, contributing to the overall enhancement of the cooling performance. Furthermore, a one-dimensional thermal resistance analysis method is introduced to distinguish the contributions of internal cooling and external film cooling. This analysis highlights the importance of external cooling enhancements in the novel structure.