Pair Of Longitudinal Vortices Research Articles

Heat transfer enhancement of a Stirling engine heating tube with three-pronged slant rods under oscillatory flow

Heaters are crucial components in Stirling engines, where the working medium adsorbed heat from an external heat source. Enhancing heat transfer within the heater is essential for boosting the operational efficiency of the Stirling engine. In this study, a tube equipped with three-pronged slant rods was employed to improve the heat transfer within the heater under conditions of oscillatory flow. Three pairs of dynamic longitudinal vortices were generated and exhibited regular movement patterns with distinct phase angles. In comparison to the smooth tube configuration, the enhanced tube outlet exhibited a notable increase in the average working-medium temperature by 62 K and 46 K during the entry and return stages, respectively. The degree of heat transfer enhancement was found to be contingent upon the pitch, height, and inclination angle of the slant rod. Optimal heat transfer enhancement was achieved with slant rods featuring a pitch of 26 mm, a height of 3.5 mm, and an inclination angle of 45°. The performance evaluation criterion ranged from 1.29 to 1.81 of enhanced tube compared to smooth tube. These findings underscore the effective enhancement of heat transfer facilitated by the three-pronged slant rods within the Stirling engine's heater.

Numerical simulation study of enhanced convective heat transfer in the tube based on winglet vortex generator

In order to enhance the heat transfer effect and improve the energy utilization efficiency of the circular tube under turbulent flow, rhombic and triangular winglet vortex generators are proposed on the basis of rectangular winglet vortex generator. The effects of three vortex generators on flow and heat transfer in the tube are investigated by numerical simulations. The results indicate that, the rectangular and triangular winglets generate two pairs of longitudinal vortices and the triangular winglets generate four pairs of longitudinal vortices due to additional shrink-age regions. The multiple longitudinal vortices increase the mixing between the fluids, raise the temperature in the central region of the tube, and enhance the heat transfer effect. Furthermore, the triangular winglet enhanced tube has the largest direct flow area and the least effect on resistance. The triangular winglet enhanced tube had the best comprehensive performance, followed by the rhombic winglet enhanced tube, and finally, the rectangular winglet enhanced tube. The performance evaluation criterion reached 1.04-1.13, 1.05-1.15, and 1.08-1.21 for the three enhanced tubes in the given flow rate interval. In addition, the effect of inclination angle on the triangular winglet enhanced tube was further investigated, and the results show that the best comprehensive performance of enhanced heat transfer is achieved at an inclination angle of 45?, with a maximum performance evaluation criterion of 1.25.

Numerical simulation study of enhanced convective heat transfer in the tube based on winglet vortex generator

In order to enhance the heat transfer effect and improve the energy utilization efficiency of the circular tube under turbulent flow, rhombic and triangular winglet vortex generators are proposed on the basis of rectangular winglet vortex generator. The effects of three vortex generators on flow and heat transfer in the tube are investigated by numerical simulations. The results indicate that, the rectangular and triangular winglets generate two pairs of longitudinal vortices and the triangular winglets generate four pairs of longitudinal vortices due to additional shrinkage regions. The multiple longitudinal vortices increase the mixing between the fluids, raise the temperature in the central region of the tube, and enhance the heat transfer effect. Furthermore, the triangular winglet enhanced tube has the largest direct flow area and the least effect on resistance. The triangular winglet enhanced tube had the best comprehensive performance, followed by the rhombic winglet enhanced tube, and finally, the rectangular winglet enhanced tube. The performance evaluation criterion (PEC) reached 1.04-1.13, 1.05-1.15, and 1.08-1.21 for the three enhanced tubes in the given flow rate interval. In addition, the effect of inclination angle on the triangular winglet enhanced tube was further investigated, and the results show that the best comprehensive performance of enhanced heat transfer is achieved at an inclination angle of 45?, with a maximum PEC of 1.25.

Experimental and Numerical Study of Heat Transfer and Turbulent Flow in a Rotating channel with V Rib-Dimple Hybrid Structures

Heat transfer and turbulent flow structure of novel hybrid structures with V rib-dimple arrays in channels with a sudden contraction have been investigated through experimental and numerical studies for the Reynolds number (Re) from 10,000 to 200,000 and Rotation number (Ro) from 0 to 0.413, which are also compared with those of a smooth and a dimpled only channel. For rotating smooth channels, sudden contraction can significantly enhance heat transfer in the entrance region for the first 4 times Dh of the channel, and heat transfer rates for trailing and leading walls are higher compared with those of static channels. For the rotating V rib-dimple channel at Re = 10,000, heat transfer rates of the trailing wall significantly increase by about 39.13% at Ro = 0.413 compared with those under the non-rotating state, which is because the longitudinal vortex on the trailing side becomes stronger with the actions of the Coriolis force produced by the rotation, thus significantly enhancing heat transfer there. It is also noted that strong longitudinal vortex pairs from inside the dimples of the V rib-dimple structure can partially offset influences of the rotation effects. Therefore, the maximum heat transfer difference between leading and trailing sides of rotating V rib-dimple channels is just 22.57%, which is much smaller than the difference of 107.25% of the rotating dimpled only channel.

Drag reduction using longitudinal vortices on a flat-back Ahmed body

The paper presents different cavity wall designs at the rear of a flat-back Ahmed body to achieve drag reduction. The wake balance is assessed using the variance of the base pressure gradient and correlated to the obtained drag reduction. Without the rear cavity, the flat-back Ahmed body is subject to a steady instability producing a strong wake imbalance in the horizontal direction. The low drag rear design consists of a 5^circ inclined spoiler at the bottom (refered to as a diffuser) that first provokes a vertical imbalance and a non-inclined top spoiler with central imprints that re-balance the wake. The drag reduction is 11.5%, the wake structure is observed to respect the symmetry of the body base in both lateral and vertical directions suggesting a full stabilization of the initial steady instability. Both the wake balance and the stabilization are associated with the formation of a longitudinal vortex pair initiated at the imprint edges and penetrating the recirculating area.

Experimental study on the structure in turbulent wall jet managed by pair of longitudinal vortices

Experimental study on the structure in turbulent wall jet managed by pair of longitudinal vortices

An optimized natural convection Y-shape-shifted heat sink design problem

An optimized design problem for a natural convection three-dimensional Y-shape-shifted heat sink is investigated numerically and experimentally in the present study. The software package CFD-ACE+ and the Levenberg-Marquardt method (LMM) are utilized as the numerical solution solver and design tool, respectively. The objective of this work is to design the optimal fin stem height, fin branch length, branching angle, and shift distance of the Y-shaped fin under fixed Y-shaped fin volume conditions to minimize the base plate temperature and enhance the cooling performance of the heat sink. The thermal radiation effect between the air and Y-shaped fin is considered in this work to improve the accuracy of the numerical solution of temperatures. The result indicates that in the optimized Y-shaped heat sink, the stem height and branching angle become shorter and larger, respectively, than those of the original design. When the shift distance is considered as a design variable, i.e., the optimized Y-shape-shifted heat sink with an up-up-up fin displacement arrangement, the chimney-flow effect due to the buoyant plume is much more pronounced than that of the original design, and a pair of longitudinal vortices also occur; therefore, better heat dissipation performance can be achieved. Experiments were performed on manufactured heat sinks to check the validity of the design results. The results indicated that the measured temperatures of the heat sinks are very close to the computed data since the maximum error between the computed and measured base temperatures is less than 0.81%. In addition, design B1 (a vertical heat sink) has the smallest thermal resistance Rth among all designs at 2.6% smaller than that of the original vertical heat sink, which confirms the validity of the present work in designing the optimal design of the Y-shape-shifted heat sink.

Turbulent flow structure and heat transfer mechanisms over surface vortex structures of micro V-shaped ribs and dimples

Turbulent flow and heat transfer characteristics in the channels with surface vortex structures of micro V-shaped ribs, dimples, and their hybrid structures have been investigated by carrying out delayed detached eddy simulations (DDES) at the Reynolds number of 50,500. It is found that, for the micro V-shaped rib, lower turbulent mixing is produced behind the leeward rib region, while the stronger longitudinal vortex pairs induced by the V-shaped ribs enhance the convective transport of heat. For the dimple, the shed vortices and the shear-layer vortices increase the near-wall turbulent mixing. The complex secondary flows induced by the flow separation, attachment and ejection increase the convective transport of heat. However, the flow separation produces a dead-air zone limiting the further increase of heat transfer enhancement. For the micro V-shaped rib-dimple hybrid structure, the strong downwash flow induced by the micro V rib breaks the recirculation zone inside the downstream dimple and the dimple here works as a vortex amplifier, which produces much stronger longitudinal vortex pairs. This process enhances both the turbulent mixing and convective transport of heat. Therefore, the micro V-shaped rib-dimple hybrid structure provides even more pronounced heat transfer enhancement.

Effects of alternating elliptical chamber on jet impingement heat transfer in vane leading edge under different cross-flow conditions

ABSTRACTThis paper proposes an alternating elliptical impingement chamber in the leading edge of a gas turbine to restrain the cross flow and enhance the heat transfer, and investigates the detailed flow and heat transfer characteristics. The chamber consists of straight sections and transition sections. Numerical simulations are performed by solving the three-dimensional (3D) steady Reynolds-Averaged Navier–Stokes (RANS) equations with the Shear Stress Transport (SST) k– $\omega$ turbulence model. The influences of alternating the cross section on the impingement flow and heat transfer of the chamber are studied by comparison with a smooth semi-elliptical impingement chamber at a cross-flow Velocity Ratio (VR) of 0.2 and Temperature Ratio (TR) of 1.00 in the primary study. Then, the effects of the cross-flow VR and TR are further investigated. The results reveal that, in the semi-elliptical impingement chamber, the impingement jet is deflected by the cross flow and the heat transfer performance is degraded. However, in the alternating elliptical chamber, the cross flow is transformed to a pair of longitudinal vortices, and the flow direction at the centre of the cross section is parallel to the impingement jet, thus improving the jet penetration ability and enhancing the impingement heat transfer. In addition, the heat transfer in the semi-elliptical chamber degrades rapidly away from the stagnation region, while the longitudinal vortices enhance the heat transfer further, making the heat transfer coefficient distribution more uniform. The Nusselt number decreases with increase of VR and TR for both the semi-elliptical chamber and the alternating elliptical chamber. The alternating elliptical chamber enhances the heat transfer and moves the stagnation point up for all VR and TR, and the heat transfer enhancement is more obvious at high cross-flow velocity ratio.

Influence of a Microramp Array on a Hypersonic Shock-Wave/Turbulent Boundary-Layer Interaction

Experiments were performed to study the effects of an array of microramp sub-boundary-layer vortex generators on a hypersonic shock/turbulent boundary-layer interaction. Two staggered rows of microramps were installed upstream of a 33 deg compression-corner interaction with large-scale separation at Mach 7.2, and the influence of these devices on the mean and turbulent flowfield, and the separation region in particular, was studied with particle image velocimetry. The microramps strongly altered the mean flow topology: The previously two-dimensional interaction region with a large separated zone in the vicinity of the ramp corner is broken up into a three-dimensional interaction, where the mean velocity at the spanwise locations downstream of the microramp centerlines is decreased, whereas it is increased at the spanwise locations in between the microramp vertices. The mean overall separation length is reduced, most strongly at spanwise locations in between microramps. A narrow local increase occurs directly downstream of the device vertices. The turbulence behavior across the interaction is not fundamentally altered under the influence of vortex-generator control, but a superposition of effects is observed. The microramp-induced longitudinal vortex pairs increase turbulent mixing and, by adding momentum into the near-wall region, support a faster decrease of turbulence intensity and return to equilibrium conditions downstream of the interaction.

Local heat transfer enhancement induced by a piezoelectric fan in a channel with axial flow

The present work experimentally and numerically investigates the local heat transfer enhancement induced by a piezoelectric fan interacting with a cross flow in a local heated channel. The piezoelectric fan is placed along the flow direction and tested under different amplitudes and flow rates. In the simulations, a spring-based smoothing method and a local remeshing technique are used to handle the moving boundary problems. Hybrid mesh is used to reduce the size of dynamic mesh domain and to improve computational efficiency. The experimental and numerical values of the time-averaged mean Nusselt number are found to be in good agreement, with deviations of less than 10%. The experimental result shows that the heat transfer performance of the heated surfaces is substantially enhanced with a vibrating piezoelectric fan. The numerical result shows that the heat transfer enhancement comes from the strong longitudinal vortex pairs generated by the piezoelectric fan, which significantly promote heat exchange between the main flow and the near-wall flow. In the case of a=0.66 (a is the dimensionless amplitude) and Re=1820, the enhancement ratio of the time-averaged mean Nusselt number reaches 119.9%.

Flow Symmetry and Heat Transfer Characteristics of Winglet Vortex Generators Arranged in Common Flow up Configuration

The generation of longitudinal vortices is an effective method for promoting thermal performance with a relative low-pressure penalty in heat exchangers. The winglet pair can generate symmetrical longitudinal vortices on the cross-section of the channel. The heat transfer and pressure-loss characteristics of a pair of winglet vortex generators with different transverse pitches are numerically studied in this paper. The winglet pair arranged in a common flow up configuration generates a pair of symmetrical longitudinal main vortices with counter-rotating directions. The symmetrical flow structure induces fluid to flow from the bottom towards the top of the channel in the common flow region between the longitudinal vortices. The flow symmetry of the longitudinal vortices and the heat transfer performance are strongly affected by the transverse pitch of the winglet pair owing to the interaction between the longitudinal vortices. The optimal transverse pitch of the studied winglet pair with the best thermal performance is reported. The increments in the vortex intensity and the Nusselt number for the optimal pitch are increased by up to 21.4% and 29.2%, respectively.

Thrust and drag scaling of a rigid low-aspect-ratio pitching plate

The flow around a rigid rectangular pitching plate immersed in a free stream is numerically investigated, addressing the force and drag generated by the oscillatory motion. Several aspect ratios (plate’s span to plate’s length) lower than 0.5 are considered, for a Reynolds number based on the plate’s length and the incoming flow velocity of 2000. The validity of the scaling law for viscous drag production, previously established for finite-span plates in uniform flapping motion, is investigated for the pitching motion, which is more representative in the context of propulsion modeling. The time averaged pressure force is shown to decompose into a propulsive part, scaling linearly with the aspect ratio and induced by the plate’s movement, and an opposite pressure force deficit, often interpreted as vortex induced drag and generally associated with the pair of longitudinal vortices at the plate’s lateral edges. A scaling for the time averaged pressure deficit is proposed, by analyzing the pressure drop along the span in terms of a Bernoulli-type effect induced by the transverse flow velocity. It is shown, that the pressure thrust is reduced, compared to what would be predicted by the elongated body theory, by more than 30% for the aspect ratios considered.

Flapping motion of a permeable flag in uniform flow

The flapping motion of a permeable square-shaped flag is studied using an incompressible smoothed particle hydrodynamics method together with a newly proposed permeable flag model. Wind tunnel experiments are also performed, and the results are compared. The present model is successful in simulating two characteristic states depending on the uniform velocity, namely the ‘stretched-straight state’ and the ‘flapping state.’ Both the oscillation periods and the tail-end trajectories quantitatively agree with the experiments. However, the bistable region, which appears in the experiments, is not seen in the computational results. Except for the high-permeability case, the flags oscillate two dimensionally and no flow separation is observed irrespective of the permeability conditions. This might be due the flexibility of the flag in which the flag can change its shape in response to the low fluid pressure acting on the surface. A pair of longitudinal vortices rotating in opposite directions is generated at both spanwise ends of the flapping flag. Their strengths vary in accordance with the instantaneous lift force. When the flag is impermeable, these longitudinal vortices are connected to the spanwise vortex shed from the trailing edge and deform into a V-shaped vortex downstream. Conversely, the longitudinal vortices generated from a permeable flag simply move downstream without reconnecting. In addition, it is found that fluid permeation takes place at the location where the work by the fluid on the flag is concentrated.

Transient interaction between a reaction control jet and a hypersonic crossflow

This paper presents a numerical study that focuses on the transient interaction between a reaction control jet and a hypersonic crossflow with a laminar boundary layer. The aim is to better understand the underlying physical mechanisms affecting the resulting surface pressure and control force. Implicit large-eddy simulations were performed with a round, sonic, perfect air jet issuing normal to a Mach 5 crossflow over a flat plate with a laminar boundary layer, at a jet-to-crossflow momentum ratio of 5.3 and a pressure ratio of 251. The pressure distribution induced on the flat plate is unsteady and is influenced by vortex structures that form around the jet. A horseshoe vortex structure forms upstream and consists of six vortices: two quasi-steady vortices and two co-rotating vortex pairs that periodically coalesce. Shear-layer vortices shed periodically and cause localised high pressure regions that convect downstream with constant velocity. A longitudinal counter-rotating vortex pair is present downstream of the jet and is formed from a series of trailing vortices which rotate about a common axis. Shear-layer vortex shedding causes periodic deformation of barrel and bow shocks. This changes the location of boundary layer separation which also affects the normal force on the plate.

Thermal performance of heat exchanger tube inserted with curved-winglet tapes

The paper deals with the effect of curved-winglet (CW) inserts on thermal and flow behaviors in a constant heat-fluxed tube. A straight tape is used to support the 45° CWs mounted repeatedly on both tape sides to generate two pairs of longitudinal counter-rotating vortices along the test tube in order to assist the chaotic flow mixing and to disrupt the boundary layer leading to faster rate of heat transfer. The airflow and heat transfer behaviors in the tube are examined for Reynolds number (Re) in the range of 4150–25,400. The curved-winglet tape (CWT) parameters involved are the winglet attack angle of 45°, three relative winglet heights (b/D = BR = 0.1, 0.2 and 0.3) and winglet pitches, (P/D = PR = 0.5, 1.0 and 2.0). The investigation reveals that the maximum thermal enhancement factor (TEF) of the CWT is about 1.62 at BR = 0.1 and PR = 1.0. For further improvement, the CWT at BR = 0.1 and PR = 1.0 is modified by punching the CW to be the perforated-curved-winglet tape (P-CWT) to reduce the pressure loss. The P-CWT characteristics include five different punched hole diameters (d = 1.0, 1.5, 2.0, 2.5 and 3.0 mm). The experimental results show that TEF of all the P-CWTs is higher than that of the CWT and the maximum TEF of 1.76 higher than the CWT around 9% is found for d = 1.5 mm. To understand the flow pattern and heat transfer mechanism, a three-dimensional CFD investigation is also performed and for validation, the good agreement between numerical and experimental results is found. For experimental data, empirical correlations for Nu, f and TEF for the CWT and P-CWT inserts are also determined.

Comparative investigation on the aerodynamic effects of combined use of underbody drag reduction devices applied to real sedan

To reduce the aerodynamic drag, the performance of the underbody aerodynamic drag reduction devices was evaluated based on the actual shape of a sedan-type vehicle. An undercover, under-fin, and side air dam were used as the underbody aerodynamic drag reduction devices. In addition, the effects of the interactions based on the combination of the aerodynamic drag reduction devices were investigated. A commercial sedan-type vehicle was selected as a reference model and its shape was modeled in detail. Aerodynamic drag was analyzed by computational fluid dynamics at a general driving speed on highway of 120 km/h. The undercover reduced the slipstream area through the attenuation of the longitudinal vortex pair by enhancing the up-wash of underflow, thereby reducing the aerodynamic drag by 8.4 %. The under-fin and side air dam showed no reduction in aerodynamic drag when they were solely attached to the actual complex shape of the underbody. Simple aggregation of the effects of aerodynamic drag reduction by the individual device did not provide the accurate performance of the combined aerodynamic drag reduction devices. An additional aerodynamic drag reduction of 2.1 % on average was obtained compared to the expected drag reduction, which was due to the synergy effect of the combination.

Detached-Eddy Simulation of Ground Effect on the Wake of a High-Speed Train

The influence of ground effect on the wake of a high-speed train (HST) is investigated by an improved delayed detached-eddy simulation. Aerodynamic forces, the time-averaged and instantaneous flow structure of the wake are explored for both the stationary ground and the moving ground. It shows that the lift force of the trailing car is overestimated, and the fluctuation of the lift and side force is much greater under the stationary ground, especially for the side force. The coexistence of multiscale vortex structures can be observed in the wake along with vortex stretching and pairing. Furthermore, the out-of-phase vortex shedding and oscillation of the longitudinal vortex pair in the wake are identified for both ground configurations. However, the dominant Strouhal number of the vortex shedding for the stationary and moving ground is 0.196 and 0.111, respectively, due to the different vorticity accumulation beneath the train. A conceptual model is proposed to interpret the mechanism of the interaction between the longitudinal vortex pair and the ground. Under the stationary ground, the vortex pair embedded in a turbulent boundary layer causes more rapid diffusion of the vorticity, leading to more intensive oscillation of the longitudinal vortex pair.

Optimally growing boundary layer disturbances in a convergent nozzle preceded by a circular pipe

We report the findings from a theoretical analysis of optimally growing disturbances in an initially turbulent boundary layer. The motivation behind this study originates from the desire to generate organized structures in an initially turbulent boundary layer via excitation by disturbances that are tailored to be preferentially amplified. Such optimally growing disturbances are of interest for implementation in an active flow control strategy that is investigated for effective jet noise control. Details of the optimal perturbation theory implemented in this study are discussed. The relevant stability equations are derived using both the standard decomposition and the triple decomposition. The chosen test case geometry contains a convergent nozzle, which generates a Mach 0.9 round jet, preceded by a circular pipe. Optimally growing disturbances are introduced at various stations within the circular pipe section to facilitate disturbance energy amplification upstream of the favorable pressure gradient zone within the convergent nozzle, which has a stabilizing effect on disturbance growth. Effects of temporal frequency, disturbance input and output plane locations as well as separation distance between output and input planes are investigated. The results indicate that optimally growing disturbances appear in the form of longitudinal counter-rotating vortex pairs, whose size can be on the order of several times the input plane mean boundary layer thickness. The azimuthal wavenumber, which represents the number of counter-rotating vortex pairs, is found to generally decrease with increasing separation distance. Compared to the standard decomposition, the triple decomposition analysis generally predicts relatively lower azimuthal wavenumbers and significantly reduced energy amplification ratios for the optimal disturbances.

Experimental investigation of high speed granular flows down inclines

We report on laterally confined granular flow experiments on steep slopes. We provide evidences for the existence of di_erent flow regimes with secondary flows. At moderate mass flow, we observe a first flow regime with a pair of longitudinal vortices which are localized close to the lateral walls and span progressively over the whole flow width with increasing flow rate. They are counter-rotative and induce a vertical upward motion of the grains at the wall. Upon a further increase of the mass flow rate, a transition is evidenced by a reversal of the rotation direction of the vortices which trigger in contrast a downward motion of the grains close to the lateral walls and an upward motion at the center of the channel. We argue that these flows bear some resemblance with the flow regimes reported recently in discrete element simulations.