Streamwise Vortices Research Articles

On the wake deflection of vertical axis wind turbines by pitched blades

AbstractWake losses are a critical consideration in wind farm design. The ability to steer and deform wakes can result in increased wind farm power density and reduced energy costs and can be used to optimize wind farm designs. This study investigates the wake deflection of a vertical axis wind turbine (VAWT) experimentally, emphasizing the effect of different load distributions on the wake convection and mixing. A trailing vortex system responsible for the wake topology is hypothesized based on a simplified vorticity equation that describes the relationship between load distribution and its vortex generation; the proposed vorticity system and the resulting wake topology are experimentally validated in the wind tunnel via stereoscopic particle image velocimetry measurements of the flow field at several wake cross‐sections. Variations in load distribution are accomplished by a set of fixed blade pitches. The experimental results not only validate the predicted vorticity system but also highlight the critical role of the streamwise vorticity component in the deflection and deformation of the wake, thus affecting the momentum and energy recoveries. The evaluation of the various loading cases demonstrates the significant effect of the wake deflection on the wind power available to a downwind turbine, even when the distance between the two turbines is only three diameters.

On the drag reduction mechanism of hypersonic turbulent boundary layers subject to heated wall blowing

Turbulence drag reduction is of great significance for the range increase of hypersonic flight vehicles. The proposed velocity-temperature coupling control method (Liu et al, Phys Rev Fluids 6:044603, 2021) is further extended to the hypersonic turbulent boundary layer. Direct numerical simulation results of four comparative cases show that the heated wall blowing achieves a drag reduction rate of 10.58%, which is about the sum of wall blowing (5.27%) and wall heating (6.35%). By evaluating the control efficiency, however, it is found that heated wall blowing is not as good as wall blowing and cannot obtain net energy saving rate. The modified FIK decompositions of skin friction coefficient indicate that the cliffy decrease of the mean convection term is the primary contribution for the drag reduction. Effects of the proposed control measure on turbulence statistics and coherent structures are also analyzed. Streamwise vortex is found to be away from the wall, thus leading to a lower friction drag.

Active flow control of the airflow of a ship at yaw

This paper implements the steady Coanda effect active flow control (AFC) on the Chalmers ship model (CSM) to study its influence on the ship’s side force and airwake under the yaw effect. The study is conducted numerically using Large Eddy Simulation (LES) with Wall-Adapting Local-Eddy Viscosity (WALE) model. Numerical methods are validated by the experimental data acquired from the baseline CSM under 10∘ port-side wind. The model with AFC is created by modifying the square-shaped hanger base to the Coanda surface and added with injection slots along the base’s roof edge and two side edges. The results show that the base-shape modification significantly alters the vortex structure on deck from z-direction vortex (ZV) to streamwise vortex (SV), and the steady Coanda effect with a momentum coefficient (Cμ) of 0.02 further enhances the SV with the removal of port-side vortex (PV). The side force and yaw moment are reduced by 5.27% and 7.97%, respectively in the AFC case due to the reduction of port-side (windward) ship-surface pressure. Furthermore, the current AFC can suppress the low-speed region and alleviate the velocity gradient in the lateral direction, which mitigates the regions of high TKE (turbulent kinetic energy) and high shear stress along the port-side deck.

Enhancing the noise reduction capability of serrations usinglow-profile vortex generators

This paper presents an experimental investigation on improving the noise reduction capability of the trailing edge serrations using vortex generators. Experiments were conducted on a 150 mm chord flat plate at four angle of attack between 0° and 9° and flow speeds of 20-50 m/s. The Reynolds number based on the chord length ranges from 2.1×105 to 5×105. Vane type vortex generators with various heights of h/δ = 0.16-0.25 were placed along with the serration roots, where δ is the boundary layer thickness at the trailing edge without the vortex generators. The aeroacoustic measurements were made with a phased microphone array, and the aerodynamic measurement near the trailing edge was measured using a hot-wire. Additionally, the lift and drag forces were measured by load cells. The source integrated noise spectra showed a slight reduction in the trailing edge noise over a broadband frequency range. The generation of streamwise vortices by vortex generators can be identified in the wake measurement. These streamwise vortices were further found to counteract the cross-flow generated at serration roots, hence improving the performance of the serration. Moreover, the presence of low-profile vortex generators was found to have an insignificant effect on the lift and drag coefficients.

Compressibility effects on turbulence production

Turbulence in compressible flows plays a major role in a variety of applications such as air/fuel mixing strategies for scramjet combustors. Therefore, the ability of controlling its production is of particular interest. However, compressible turbulence is more challenging than its incompressible counterpart due to the fact that thermodynamic quantities also fluctuate. In particular, the compressible turbulent kinetic energy production is a function of both velocities and density fluctuations. The so-called Morkovin's hypothesis has been often used in the literature to express the density fluctuations as a function of the velocity ones and the local Mach number. In this work, we discuss the validity of this hypothesis, and we use its expression to reformulate the turbulence production as a function of velocity only. The resulting expression shows that the planar component of turbulence production is substantially unaffected by compressibility effects. This result validates a previous work aimed at the sustainment of turbulence production using streamwise vortices. On the other hand, the turbulence production terms aligned with the convective velocity are affected by compressibility, even at relatively low Mach numbers.

Wall temperature effects on the hypersonic boundary-layer transition over an inclined, blunt cone

Wall temperature is a crucial factor for the transition of hypersonic boundary layers. This paper investigates the wall temperature effects on the hypersonic boundary-layer transition over an inclined blunt cone with two wall temperatures by direct numerical simulations and instability analysis. The breakdown processes of leeward streamwise vortices and crossflow vortices are both studied. Inner modes with lower phase velocities and outer modes with higher phase velocities are responsible for the breakdown of leeward streamwise vortices, respectively, in cooled- and heated-wall cases. A higher wall temperature will promote the boundary layer transition in the vicinity of the leeward centerline due to the collision of nearby crossflow vortices with the leeward mushroom structure, instead of by destabilizing inner and outer modes. In the crossflow-dominated region, the transition is triggered by the interaction between second modes and crossflow vortices for the cooled-wall case. Increasing the wall temperature will promote the transition in this region even though second modes are suppressed.

Flow behavior analysis in boundary layer transition based on the Liutex–shear decomposition

The Liutex based vortex identification method is superior to previous methods in that it overcomes the issues of threshold problem, shear contamination, etc., with a clear physical meaning that the direction of the Liutex vector represents the local axis of rotation, while the magnitude is equal to twice the angular velocity of the rigid-rotation part of the flow. The current study focuses on the interaction between the Liutex represented rotation and the residual shear part during the development of Λ vortex and hairpin vortex in boundary layer transition. The temporal–spatial evolution and development of typical vortical structures are analyzed based on the Liutex–shear decomposition with particular attention paid to the position and strength changes of Liutex and shear. Vortex core lines are extracted to investigate the mutual interactions between rotation and shear inside vortices. It is demonstrated that for the Λ vortex, spanwise rotating motions are formed at the head region and can persist for a long time under the influence of surrounding shear, while the tail region of the Λ vortex stretches down near the wall but then becomes weaker due to dissipation. High-shear regions tend to be located on top or below the vortices. When the shear layer formed between the legs gets stronger, it will be rolled up to form new vortices. On the one hand, the vortex legs together with the newly formed spanwise vortex consist of a new hairpin vortex, which, in turn, leads to the generation of the second- and third-level hairpin vortices. On the other hand, it also generates many smaller streamwise vortices in the near wall region. The results show that the interaction between Liutex and shear is very important in the development of vortical structures during transition.

Numerical Research on the Jet-Mixing Mechanism of Convergent Nozzle Excited by a Fluidic Oscillator and an Air Tab

Unsteady numerical simulations, coupled with the SST (Shear Stress Transport) k-ω turbulence model, were conducted to study the mixing-enhancement characteristics of the excited jet generated by the fluidic oscillator and the air tab in a single channel convergent nozzle with an inlet total pressure of 140–200 kPa. Compared with the steady air-tab jet, the sweeping jet generated by the fluidic oscillator has roughly the same penetration in the main flow, but it can induce streamwise vortices and planar vortices of larger scale and longer duration, which is beneficial to enhance jet mixing efficiency in the range of 1.0 DN (DN represents the diameter of the main nozzle outlet) downstream from the main nozzle. When x > 1.0 DN, the jet mixing is mainly dominated by the shear layer between the main jet and the ambient. As the sweeping jet suppresses the expansion of the main jet, which reduces the contact area between the main jet and the ambient, its mixing efficiency is less than that of the air tab in this region. With the increasing inlet total pressure of the fluidic oscillator, the influence range of the sweeping jet is increased, but its mixing efficiency does not increase significantly. In general, the fluidic oscillator can use a small jet flow (<5%) to achieve a high mixing efficiency (i.e., 60% at x = 2.0 DN) at the expense of low total pressure loss (<2.3%), which indicates that it has good engineering applicability.

Spatial and temporal alterations due to vortex generators in a flare induced shock–boundary layer interaction

This paper describes the modifications instigated in a flare induced shock–boundary layer interaction flowfield by two vortex generators viz. Baseline Ramps (BRs) and Ramped Vanes (RVs). Experimental and computational investigations were conducted at Mach 2, on a cone–cylinder–flare model, with a flow turning angle of 24° at the cylinder–flare juncture. The vortex generators were placed 50 mm (10δ) upstream of the compression corner and their height was 50% of the local boundary layer thickness (h = 2.1 mm = 0.5δV G). The counter-rotating vortices emanated by Baseline Ramps (BRs), remained in close proximity to one another and interacted strongly along the upwash regions. Hence, they quickly dissipated and moved away from the model surface, causing only a nominal impact on the interaction region. Alternatively, the vortices produced by Ramped Vanes (RVs) were amply separated and therefore, largely retained their core strength and were not rapidly advected away from the near-wall region. As a result, the separation occurred a little prematurely in the upwash regions and was significantly delayed along the downwash regions. Additionally, tornado-like vortices were formed, due to the streamwise vortices interacting with the separation bubble. The delay in separation along the downwash regions, also brought the separation shock closer to the compression corner. This was accompanied by a suitable increase in the shock’s oscillating frequency, in such a way that the Strouhal number based on the interaction length (distance between the corner and the mean position of the separation shock foot) remained more or less the same.

Numerical Investigation on the Effect of Section Width on the Performance of Air Ejector with Rectangular Section

Due to its simple structure and lack of moving parts, the supersonic air ejector has been widely applied in the fields of machinery, aerospace, and energy-saving. The performance of the ejector is influenced by the flow channel structure and the velocity of the jet, thus the confined jet is an important limiting factor for the performance of the supersonic air ejector. In order to investigate the effect of the confined jet on the performance of the ejector, an air ejector with a rectangular section was designed. The effects of the section width (Wc) on the entrainment ratio, velocity distribution, turbulent kinetic energy distribution, Mach number distribution, and vorticity distribution of the rectangular section air ejector were studied numerically. The numerical results indicated that the entrainment ratio of the rectangular section air ejector increased from 0.34 to 0.65 and the increment of the ER was 91.2% when the section width increased from 1 mm to 10 mm. As Wc increased, the region of the turbulent kinetic energy gradually expanded. The energy exchange between the primary fluid and the secondary fluid was mainly in the form of turbulent diffusion in the mixing chamber. In addition to Wc limiting the fluid flow in the rectangular section air ejector, the structure size of the rectangular section air ejector in the XOY plane also had a limiting effect on the internal fluid flow. In the rectangular section air ejector, the streamwise vortices played an important role in the mixing process. The increase of Wc would increase the distribution of the streamwise vortices in the constant-area section. Meanwhile, the distribution of the spanwise vortices would gradually decrease.

Spectral analysis of the evolution of energy-containing eddies

Energy-containing eddies (energy-eddies) are the elementary structures of wall turbulence that carry most of the kinetic energy and momentum. Despite the consensus that energy-eddies can self-sustain at each relevant length scale, their precise origin and spatial evolution are currently not well understood. In this study, we examine the spatial evolution of energy-eddies by quenching them at the inflow of a turbulent channel flow. Our study shows that the eddies involved in the energy cascade cannot be sustained without the energy-eddies. The streamwise velocity spectra of the evolving flow start to recover at a spanwise wavelength of $\lambda _z^+ \simeq 100$ , equal to the near-wall spacing of streaks in the buffer layer located at $y^+ \simeq 15$ , whereas there are no active vortical motions in the streamwise vorticity spectra until the energy at the streak location is re-established. Hence, the present study demonstrates that in a spatially evolving flow, the formation of near-wall streaks is the primary process necessary in the recovery of energy-eddies.

Impact of the Streamwise Vorticity Current on Low‐Level Mesocyclone Development in a Simulated Supercell

AbstractResults from a large eddy simulation of a tornadic supercell developing in a horizontally homogeneous environment are presented which clearly illustrate a connection between low‐level mesoyclone development and the development of a streamwise vorticity current (SVC). Although the environment supports tornadic supercells, a strong low‐level mesocyclone (LLM) does not develop until a well‐defined SVC forms in the storm's forward flank. As the streamwise vorticity in the SVC flows southward and is tilted into the storm updraft creating updraft helicity, the LLM strengthens and lowers toward the surface. The SVC also focuses LLM development in a confined storm‐relative position favorable for converging/stretching preexisting vertical vorticity. Tornadogenesis occurs within ∼5 min of the establishment of a strong LLM. These results illustrate a possible mode of internal storm variability that may be an important factor in explaining why some supercells produce tornadoes while others do not in similar favorable environments.

Numerical Investigation of the Effect of Supersonic Air Temperature on the Mixing Characteristics of Liquid Fuel

The effect of supersonic air temperature on the mixing characteristics of liquid hydrocarbon fuel injected into three different supersonic airflows elevated in three steps from 373 K to 673 K was investigated numerically. Compressible Reynolds-averaged Navier–Stokes (RANS) equations were solved together with species conservation equation using ANSYS Fluent for two-phase flow simulations including fuel droplet breakup and evaporation. The turbulence model needed to close the RANS equations used the Shear Stress Transport (SST) k-ω model. The Eulerian–Lagrangian model was employed to track fuel droplets in mainstream air, and the Kelvin–Helmholtz and Rayleigh–Taylor (KH-RT) models were used to simulate the droplet breakup process. Numerical solutions were validated using experimental data. The higher the air temperature, the stronger the streamwise vortices downstream of the pylon. When the air temperature was 373 K, the liquid fuel hardly evaporated, but as the air temperature increased, and the mass fraction of the vaporized fuel and the mixing efficiency increased linearly downstream of the pylon. At air temperatures of 523 K and 673 K, the mixing efficiencies were 10% and 51% at the combustor outlet, respectively. The total pressure loss decreased slightly due to droplet evaporation as the temperature increased from 373 K to 673 K.

Secondary flow and streamwise vortices in three-dimensional staggered wavy-wall turbulence

The present paper simplifies the naturally formed dunes (riverbeds) as large-scale three-dimensional staggered wavy walls to investigate the features of the accompanying secondary flows and streamwise vortices via large-eddy simulation. A comparison between the swirling strength and the mean velocities suggests where a secondary flow induces upwash or downwash motions. Moreover, we propose a pseudo-convex wall mechanism to interpret the directionality of the secondary flow. The centrifugal instability criterion is then used to reveal the generation of the streamwise vortices. Based on these analytical results, we found that the streamwise vortices are generated in the separation and reattachment points on both characteristic longitudinal–vertical and horizontal cross-sections, which is related to the curvature effect of the turbulent shear layer. Furthermore, the maximum Görtler number characterized by the ratio of centrifugal to viscous effects suggests that, for fixed ratio of spanwise- to streamwise-wavelength cases, the strongest centrifugal instability occurring on the longitudinal–vertical cross-section gradually dominates with the increases in amplitude. A similar trend for the cases with varied spanwise wavelength can also be found. It is also found that the streamwise vortices are generated more readily via transverse flow around the crest near the separation and reattachment points when the ratio of spanwise- to streamwise-wavelength equals 1.

Disentangling the Influences of Storm-Relative Flow and Horizontal Streamwise Vorticity on Low-Level Mesocyclones in Supercells

Abstract Sufficient low-level storm-relative flow is a necessary ingredient for sustained supercell thunderstorms and is connected to supercell updraft width. Assuming a supercell exists, the role of low-level storm-relative flow in regulating supercells’ low-level mesocyclone intensity is less clear. One possibility considered in this article is that storm-relative flow controls mesocyclone and tornado width via its modulation of overall updraft extent. This hypothesis relies on a previously postulated positive correspondence between updraft width, mesocyclone width, and tornado width. An alternative hypothesis is that mesocyclone characteristics are primarily regulated by horizontal streamwise vorticity irrespective of storm-relative flow. A matrix of supercell simulations was analyzed to address the aforementioned hypotheses, wherein horizontal streamwise vorticity and storm-relative flow were independently varied. Among these simulations, mesocyclone width and intensity were strongly correlated with horizontal streamwise vorticity, and comparatively weakly correlated with storm-relative flow, supporting the second hypothesis. Accompanying theory and trajectory analysis offers the physical explanation that, when storm-relative flow is large and updrafts are wide, vertically tilted streamwise vorticity is projected over a wider area but with a lesser average magnitude than when these parameters are small. These factors partially offset one another, degrading the correspondence of storm-relative flow with updraft circulation and rotational velocity, which are the mesocyclone attributes most closely tied to tornadoes. These results refute the previously purported connections between updraft width, mesocyclone width, and tornado width, and emphasize horizontal streamwise vorticity as the primary control on low-level mesocyclones in sustained supercells. Significance Statement The intensity of a supercell thunderstorm’s low-level rotation, known as the “mesocyclone,” is thought to influence tornado likelihood. Mesocyclone intensity depends on many environmental attributes that are often correlated with one another and difficult to disentangle. This study used a large body of numerical simulations to investigate the influence of the speed of low-level air entering a supercell (storm-relative flow), the horizontal spin of the ambient air entering the thunderstorm (streamwise vorticity), and the width of the storm’s updraft. Our results suggest that the rotation of the mesocyclone in supercells is primarily influenced by streamwise vorticity, with comparatively weaker connections to storm-relative flow and updraft width. These findings provide important clarification in our scientific understanding of how a storm’s environment influences the rate of rotation of its mesocyclone, and the associated tornado threat.

Evolution characteristics of streamwise vortex of crossing shock wave/turbulent boundary layer interaction

The streamwise vortex generated by the crossing shock wave/turbulent boundary layer interaction seriously affects the performance of the hypersonic inlet. The shock structure, vortex core, and vortex intensity in the flowfields of weak, moderate, and strong strength interaction are experimentally and numerically investigated at Mach 2.95. The evolution characteristics of the streamwise vortex are explored. The reflection and intersection of shock waves in the interaction flowfield divide the evolution process of the streamwise vortex into four stages. The typical conical vortex evolutes into two vortexes. The vortex close to the symmetric plane further evolutes into two vortexes in the weak and strong strength flowfields. A separated flow from the focus on the flat plate produces a new streamwise vortex in the strong strength flowfield. The streamwise vortex core trace is almost parallel to the symmetric plane after the pair of streamwise vortexes collide at the symmetric plane. The vorticity intensity of the streamwise vortex increases and then decreases along the flow direction, which is greatly influenced by the interaction strength.

The impinging wall effect on flame dynamics and heat transfer in non-premixed jet flames

The impinging jet flame is studied experimentally and numerically accounting for the complex flame-wall interactions in practical combustion devices. Flame dynamics and heat transfer with the effect of impinging wall are analyzed. The 3-D large eddy simulation coupled with detailed chemical reaction mechanism and particle image velocimetry experiment based on cross-correlation measurement principle are performed for verification and further analysis. Results show that vortices are generated due to the Kelvin-Helmholtz instability originated from velocity gradient. The 3-D vortex interactions involving vortex rings and spirals are also indicated by vorticity and the convection of streamwise vorticity is responsible for the effect of vortex spirals associated with turbulent flow transition. In addition, results calculated from four wall thermal conditions are compared and analyzed. Dirichlet condition is inferred to be more suitable for the case of wall materials with higher thermal conductivity. It is indicated that wall thermal condition mainly affects the heat transfer in the near-wall region, but has little effect on the momentum transfer. This study provides references for the adoption of wall conditions in numerical simulation and near-wall treatment in combustion systems.

A Numerical Study on Supersonic Combustion Optimization Based on the Streamwise Vortex-Couple Method

In this paper, some typical methods to promote mixing in supersonic combustion are reviewed, and the fluid-dynamic mechanism underpinning the development of the supersonic shear layer in the presence of a streamwise vortex is analyzed through computational fluid dynamics. It is proven that the streamwise vortex-couple method is an excellent approach to enhance mixing. A specific combustor design is proposed accordingly.

Passive control of vortex-shedding past finite cylinders under the effect of a free surface

Passive technologies for the control of the flow past bluff bodies have been widely studied. Most of these works have focused on high-aspect-ratio, wall-mounted, or infinitely long cylinders, leaving a gap for low-aspect-ratio bodies fully submerged or under the effect of a free water surface. This is the object of the present work. Detached-eddy simulations at a Reynolds number of 1000 have been carried out for infinitely long (case i) and low-aspect-ratio bodies. For the finite case, two configurations were investigated: bodies fully submerged in the flow, away from surface effects (case ii) and that involving a free surface, here represented by a fixed slip-allowing plane (case iii). These computations were conducted for the bare cylinder and the system is comprised of the same main body fitted with eight wake-control rods uniformly distributed around its perimeter. Results showed that the low-aspect-ratio cases relative to infinitely long structures (1) lowered mean drag and root mean square lift, (2) presented a less coherent wake topology, (3) had frontal rods that concentrated most of the hydrodynamic loads, (4) extended the formation length, and (5) although all cases developed larger hydrodynamic loads when the rods were fitted to the main body compared to the plain cylinder, the finite cases produced a lower increase. We show that these results are intrinsically related to end effects and associated with a less correlated wake lacking coherent vortical structures. Furthermore, we analyze the more accentuated streamwise vortices produced by the presence of the free surface in case iii compared with case ii.

Effect of streamwise vortex induced by streamwise grooves on drag reduction and heat transfer performance

Based on the speed streaks spacing of 100 times the dimensionless viscous length. In a rectangular channel, the streamwise grooves were arranged at the corresponding positions where the low-speed streaks are likely to occur. The influence of streamwise grooves on flow structure and the comprehensive thermal performance are numerically investigated. The results show that the streamwise grooves inhibited the spanwise movement of the speed streaks and destroyed the self-sustainability of the turbulence, which results in the achievement of the drag reduction rate of 11.7%–16.78%. The velocity gradient inside the streamwise grooves reduced, which contributed to the obtaining of drag reduction rate of 75.53%, in spite of occupying only 20.7% of channel bottom area. In addition, the effect of the evolution of the hairpin vortex on the shear stress is paid attention as well. It's found that the occurrence of head and leg of hairpin vortex would lead to the reduction of shear stress, while the dramatic development of hairpin vortex would lead to sharp increase in shear stress. Furthermore, by quantifying the frequency of turbulent motion, it's found that the redistribution of Reynolds stress increases the ejection by 58% and decreases sweep by 45% at Reynolds number of 3745. Consequently, the heat transfer increases by 7.69%, the drag reduction rate reaches 11.7%, and the comprehensive thermal performance coefficient reaches 1.122. With the increase of Reynolds number, the influence of large-scale vortex induced by grooves on heat transfer is gradually weakened, but the drag reduction could still be obtained, and the comprehensive thermal performance evaluation indexes are greater than 1.