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High flow velocity near the free surface in rivers is due to the presence of shear stress near the bed and its absence on the free surface. This phenomenon results in unsteadiness of the vertical velocity profile. Moreover, secondary flows in river bends cause velocity variations, accordingly leading to changes in shear stress near the bed. The present study evaluates and analyzes the effect of streamlines variations, maximum velocity distribution, and secondary flow strength on bed shear stress distribution along a 180 degree sharp bend built in the Hydraulic Laboratory of Persian Gulf University. Results suggest of the occurrence of maximum secondary flow strength at the second half of the bend. The evaluation of bed shear stress distribution using the TKE, modified TKE, and Reynolds methods at turbulent boundary layer demonstrated that the maximum shear stress occurred from the entrance of the bend to the bend apex area near the inner wall. Moreover, comparison of the Reynolds shear stress method at distances of 5 and 15% of the flow depth from the bed indicated that the maximum shear stress occurring at the lower layer moved from the 40 degree cross section to 60 degree cross section at the upper layer.

Purpose This study aims to minimize the pressure drop across wavy microchannels using secondary branches without compromising its capacity to transfer the heat. The impact of secondary flows on the pressure drop and heat transfer capabilities at different Reynolds numbers are investigated numerically for different wavy microchannels. Finally, different channels are evaluated using performance evaluation criteria to determine their effectiveness. Design/methodology/approach To investigate the flow and heat transfer capabilities in wavy microchannels having secondary branches, a 3D conjugate heat transfer model based on finite volume method is used. In conventional wavy microchannel, secondary branches are introduced at crest and trough locations. For the numerical simulation, a single symmetrical channel is used to minimize computational time and resources and the flow within the channels remains single-phase and laminar. Findings The findings indicate that the suggested secondary channels notably improve heat transfer and decrease pressure drop within the channels. At lower flow rates, the secondary channels demonstrate superior performance in terms of heat transfer. However, the performance declines as the flow rate increased. With the same amplitude and wavelength, the introduction of secondary channels reduces the pressure drop compared with conventional wavy channels. Due to the presence of secondary channels, the flow splits from the main channel, and part of the core flow gets diverted into the secondary channel as the flow takes the path of minimum resistance. Due to this flow split, the core velocity is reduced. An increase in flow area helps in reducing pressure drop. Practical implications Many complex and intricate microchannels are proposed by the researchers to augment heat dissipation. There are challenges in the fabrication of microchannels, such as surface finish and achieving the required dimensions. However, due to the recent developments in metal additive manufacturing and microfabrication techniques, the complex shapes proposed in this paper are feasible to fabricate. Originality/value Wavy channels are widely used in heat transfer and micro-fluidics applications. The proposed wavy microchannels with secondary channels are different when compared to conventional wavy channels and can be used practically to solve thermal challenges. They help achieve a lower pressure drop in wavy microchannels without compromising heat transfer performance.

The influence of partitioned profiling design based on a large-pitch highly loaded cascade is studied by numerical simulation. The partitioned profile is mainly composed of a pressure-side convex structure near the leading edge and a suction-side convex structure at the midstream and downstream sides of the passage. The influence of the change in the vertex axial position and peak value of the B-line on the secondary flow control is analyzed. In this paper, air (ideal gas) is selected as the flow media. The average static pressure at the outlet and the average total temperature at the inlet are kept constant. SST γ - θ is used as the turbulence model. The results show that the pressure-side convex structure suppresses the spanwise and pitchwise migration of the inlet flow by adjusting the static pressure distribution of the flow field, so the development of the pressure-side leg of the horseshoe vortex is effectively limited. The suction-side convex structure adjusts the static pressure distribution of the flow field and increases the included angle between the cross-flow and suction surface, so the accumulation of low-momentum fluid, the development of a corner vortex and the flow separation at the trailing edge of the suction-side surface are all suppressed near the endwall-suction corner. Consequently, the energy loss coefficient of the large-pitch highly loaded cascade is decreased from 0.0564 to 0.0485, representing a 25% reduction in secondary flow losses.

This paper examines the effect of coupling several equations for vertical velocity profiles in a depth-integrated model to clarify the roles of three-dimensional flow structures minimizing the errors from the two-dimensional calculation model. In addition, this paper develops a numerical discretization method for the dispersion terms in the horizontal momentum equations. It is revealed that the use of the 2DC model underestimates the water surface elevation through the comparisons with the experimental datasets, advanced 2DC and 3DC results. The prediction of water surface elevation is considerably improved by taking into account of secondary flow effect with vorticity equations. It is clarified that the accuracy in predicting the water surface elevation is increased with increasing the degree of the function for vertical velocity profiles in advanced 2DC models or the number of vertical grids in 3DC model, while non-hydrostatic pressure and variation in vertical velocity have a second importance.

The influence of channel side slope on flow in strongly curved channel bends is studied numerically. The performances of five different turbulence models are investigated. Comparison to experimental measurements demonstrates that the fully 3D numerical model can reliably simulate a channel bend flow field. Among the tested turbulence models, the realizable k − ε model performed best. The present study also demonstrates that the realizable k − ε model can satisfactorily predict smaller flow features in bend flow, such as the outer-bank circulation cell. The validated model is employed to carry out additional computations for channel bends with different side slopes. It is found that the number, position, and strength of secondary flow cells varies with the channel side slope, with corresponding influence on flow distribution and flow vorticity.

Aircraft secondary flow systems are small-flow circulation devices that are used for thermal and cold management, flow control, and energy generation on aircraft. The aerodynamic characteristics of main-flow-based inlets have been widely studied, but the secondary-flow-based small inlets, jets, and blowing and suction devices have seldom been studied. Two types of secondary flow systems embedded in a supersonic aircraft wing, a ram-air intake and a submerged intake, are researched here. Firstly, wind tunnel tests under subsonic, transonic, and supersonic conditions are carried out to test the total pressure recovery and total pressure distortion. Secondly, numerical simulations are used to analyze the flow characteristics in the secondary flow systems. The numerical results are validated with experimental data. The calculating errors of the total pressure recovery on the ram-air and submerged secondary flow systems are 8% and 10%, respectively. The simulation results demonstrate that the total pressure distortion tends to grow while the total pressure recovery drops with the increasing Mach number. As the Mach number increases from 0.4 to 2, the total pressure recovery of the ram-air secondary flow system decreases by 68% and 71% for the submerged system. Moreover, the total pressure distortion of the ram-air and submerged secondary flow systems is increased by 19.7 times and 8.3 times, respectively. Thirdly, a detailed flow mechanism is studied based on the simulation method. It is found that the flow separation at the front part of the tube is induced by adverse pressure gradients, which primarily determine the total pressure recovery at the outlet. The three-dimensional vortex in the tube is mainly caused by the change in cross-sectional shape, which influences the total pressure distortion.

This work presents an extensive experimental and numerical analysis, aimed at investigating the impact of shelf-like fences applied on the suction side of a turbine nozzle guide vane. The cascade is constituted of vanes characterized by long chord and low aspect ratio, which are typical features of some LPT first stages directly downstream of an HPT, hence presenting high channel diffusion, especially near the tip. In particular, the present study complements existing literature by highlighting how blade fences positioned on the suction side can reduce the penetration of the large passage vortex. This is particularly effective in applications where flow turning is limited, the blades are lightly loaded at the front, and the horseshoe vortex is weak. The benefits of the present fence design in terms of losses and flow uniformity at the cascade exit plane have been demonstrated by means of a detailed experimental campaign carried out on a large-scale linear cascade in the low-speed wind tunnel installed in the Aerodynamics and Turbomachinery Laboratory of the University of Genova. Measurements mainly focused on the characterization of the flow field upstream and downstream of straight and fenced vane cascades using a five-hole pressure probe, to evaluate the impact of the device in reducing secondary flows. Furthermore, experiments were also adopted to validate both low-fidelity (RANS) and high-fidelity (LES) simulations and revealed the capability of both simulation approaches to accurately predict losses and flow deviation. Moreover, the accuracy in high-fidelity simulations has enabled an in-depth investigation of how fences act mitigating the effects of the passage vortex along the blade channel. By comparing the flow fields of the configurations with and without fences, it is possible to highlight the mitigation of secondary flows within the channel.

Secondary flow phenomena have a significant influence on the generation of losses and the propagation of coolant on the turbine end walls. The majority of film cooling studies are carried out on linear rather than annular cascades due to the structural simplicity and ease of measurement integration of the former. This approach neglects the effects of the radial pressure gradient that is naturally imposed on the vortex flow in annular cascades. The first part of this paper numerically investigates the effect of the radial pressure gradient on the secondary flow under periodic flow conditions by comparing a linear and an annular case. It is shown that the radial pressure gradient has a significant influence on the propagation of the secondary flow induced vortices in the wake of the nozzle guide vanes (NGV). In the second part of the paper, a novel approach of a five-passage annular sector cascade is presented, which avoids the hub boundary layer separation, as is typical for this type of test rig. To increase the periodicity, a benchmark approach is introduced that includes multiple pointwise and integral flow quantities at different axial positions. Based on the optimized best-case design, general design guidelines are derived that allow a straightforward design process for annular sector cascades.

This paper presents an experimentally validated numerical analysis of the influence of volute aspect ratio (VAR) on the fluid flow characteristics of a volute in a mixed flow turbine. The geometry of the volute subjects the flow to centrifugal forces that affect the vertical motion of the fluid, commonly known as secondary flow, which is fundamentally opposed in straight channels commonly associated with the primary flow. The flow characteristics were analysed for four-volute designs with constant volute area-to-centroid radius ratio ( A / r ) but with different VARs ranging from 0.5 to 2.0 at selected circumferential positions under steady state at different operating conditions. Secondary flow structure characteristics were identified based on pressure contours, velocity contours and streamlines. The internal volute flow structure was found to be dependent on the VAR. The results show corner and counter-rotating Dean effect-type vortices at higher volute aspect ratios. In addition, the results also show that as the pressure ratio increases, the deflection of primary flow increases and at certain positions, flow separation occurred. The resulting secondary flow structures that exist in the volute are strongly influenced by the VAR at different operating conditions, thus affecting turbine performance.

Micro-electrical discharge machining (micro-EDM) has an issue of uneven tool electrode wear that seriously affects the micro-hole accuracy. However, the mechanism of uneven tool electrode wear remains unclear. In this study, the uneven tool electrode wear mechanism has been studied both theoretically and experimentally. It was first discovered that the ultrafine debris particles produced by the EDM spark play a critical role in uneven tool electrode wear. A theoretical model was established to reveal the movement and the distribution of the debris by employing Einstein’s tea leaf paradox i.e., classic secondary flow theory and the electrophoretic theory. According to this model, when the polarity is positive, the ultrafine debris aggregates gradually and adheres onto  the bottom of the micro-hole, thereby  a debris layer of a parabolic profile is formed progressively. This dynamic debris layer shields the material to be removed by the EDM spark. As a result, the tip of the tool electrode is unevenly worn into a conical concavity shape. Conversely, under negative polarity, the tip of the tool electrode is unevenly worn into a conical shape. A set of experiments was performed to verify the model and the results agreed well with the predicted  phenomena. Subsequently, a novel approach is  proposed to eliminate the uneven tool electrode wear by reversing pulse polarity in a repetitive manner. Using this method, uneven tool electrode wear can be avoided and high accuracy micro-holes without the features of a cone and/or conical concavity can be obtained.