Stability Of Wake Research Articles

Wake structure of an array of cylinders in shallow flow

Although there is a range of approaches for classifying the wake structure behind an array of obstacles, these approaches provide inconsistent results across different array systems. This motivates the present study to integrate and reconcile these approaches into one that is consistent across different systems. This new, transferable classification approach is based on the dimensionless flow blockage of the array and the wake stability parameter. To demonstrate this approach, a series of laboratory experiments was conducted to characterise the wake structure behind an array of emergent cylinders across a practically relevant parameter space that has not previously been explored. Two arrays with the same values of flow blockage and wake stability parameters but different sizes display the same wake structure, demonstrating the controlling influence of these two parameters on the wake structure. This approach classifies four different wake structures, which are distinct in that they display differences in instantaneous and time-averaged flow fields, temporal velocity oscillations, shear layer growth and the length of the steady wake region. The dependence of the wake structure on the two parameters is a consequence of (i) the controlling influence of blockage on the fraction of incident flow passing through the array and (ii) the ability of shallowness to suppress wake instabilities and, to a lesser extent, also influence the velocity through the array. This paper provides a predictive framework for the wake structure based on knowledge of the array geometry, and the depth and velocity of incident flow across the entire relevant practical parameter space.

Experimental study on water-exit of cylinder

In this paper, the hydrodynamic characteristics of the cylinder water-exit are studied by high-speed camera experiments. The characteristics of cylindrical motion trajectory, velocity and acceleration are analyzed by image processing algorithm. The emission voltage (i.e. initial velocity) and emission depth are important influencing factors. Based on different emission voltage and depth, the hydrodynamic coefficient is compared and analyzed. The results show that the new research method has good adaptability to the experimental study of cylindrical water-exit. The emission voltage directly affects the initial velocity of the cylinder, which in turn affects the force changes during the water exit process. The increase in emission depth exacerbates the uncertainty of the moment when the cylinder exits the water, thereby affecting the changes in the wake and motion stability during the cylinder water exit process.

Performance and wake interaction between two aligned vertical axis turbines

Performance and wake interaction between two aligned vertical turbines are investigated by solving the three-dimensional Navier-Stokes equations with the k−ω SST IDDES turbulence model. Specifically, the impact of longitudinal spacing between the upstream and downstream turbines is examined. The numerical results show that as the longitudinal distance increases, the performance of the downstream turbine shows a quadratic growth. This is phenomenon is strongly correlated to the wake recovery of the upstream turbine. Based on the velocity and turbulence distribution, the wake of the upstream turbine could be divided into three distinct zones. However, the zone characterized by low axial velocity and turbulence level is not observed in the wake of the downstream turbine. This difference primarily arises from the high turbulence present in the incoming wake from the upstream turbine, combined with the turbulence generated by the downstream turbine. Interestingly, regardless of the longitudinal distances between the turbines, the downstream turbine displays similar wake patterns for different streamwise distances between turbines. This consistency is attributed to the influence of the upstream turbine's wake on the stability of the downstream turbine's wake. The downstream turbine is suggested to be positioned in the zone where the upstream turbine's wake has fully developed.

Active learning-based hydrodynamic shape optimization and numerical simulation of auxiliary structures for circular bridge piers

Cylinders as a common form of bridge piers are widely used in engineering practices, yet the hydrodynamic forces and local scouring of the cylinder due to the fluid flow cause significant concern about the functionality of circular bridge piers. In this paper, two novel auxiliary structural types are proposed for passive control of flow around bridge piers, and their optimal structural parameters are determined efficiently using a surrogate model-based hydrodynamic shape optimization framework. Specifically, a Kriging-based active learning algorithm is proposed for hydrodynamic shape optimization of auxiliary structures, which are placed at the rear and both the front and rear of a cylinder. The hydrodynamic performance of the circular bridge pier with the optimized auxiliary structures (F3 and F6) is investigated through 2D and 3D numerical simulations. The effectiveness of the optimized auxiliary structures in mitigating the adverse effect of local scouring around a cylinder is also revealed through a case study. Results indicate that the optimized auxiliary structures F3 and F6 can dramatically reduce the drag coefficient compared with the bare cylinder, i.e., with a reduction rate of 69.8% and 77.2%, respectively; and the reduction rate is more pronounced for the lift coefficient, i.e., with a reduction rate higher than 90% for both F3 and F6. The reduction of drag and lift coefficients is mainly attributed to the positive effects of the optimized auxiliary structures for adjusting the pressure distribution around the cylinder, elongating the free shear layer, improving the wake stability etc. Interestingly, the auxiliary structure F6 also exhibits a high potential to mitigate the scouring effect of fluid flow around a cylinder by decreasing the flow velocity and bed shear stress around the cylinder. However, it should be noted that placing the auxiliary structure at the rear of a cylinder (such as the auxiliary structure type F3) could introduce an adverse effect on reducing local scouring. This study provides valuable guidance for designing effective auxiliary structures for passive control of fluid flow around cylinders.

The stability of wakes of floating wind turbines

Floating offshore wind turbines (FOWTs) are subjected to platform motion induced by wind and wave loads. The oscillatory movement trigger vortex instabilities, modifying the wake structure and influencing the flow reaching downstream wind turbines. In this work, the wake of a FOWT is analyzed by means of numerical simulations and a comparison with linear stability theory. Two simplified models based on the stability of vortices are developed for all degrees of freedom of turbine motion. In our numerical simulations, the wind turbine blades are modeled as actuator lines and a spectral-element method with low dispersion and dissipation is employed to study the evolution of the perturbations. The turbine motion excites vortex instability modes predicted by the linear stability of helical vortices. The flow structures that are formed in the non-linear regime are a consequence of the growth of these modes and preserve some of the characteristics that can be explained and predicted by the linear theory. The number of vortices that interact and the growth rate of disturbances are well predicted by a simple stability model of a two-dimensional row of vortices. For all types of motion, the highest growth rate is observed when the frequency of motion is one and a half the frequency of rotation of the turbine that induces the out-of-phase vortex pairing mechanism. For lower frequencies of motion, several vortices coalesce to form large flow structures, which cause the high amplitude of oscillations in the streamwise velocities, which may increase fatigue or induce high amplitude motion on downstream turbines.

Aerodynamic interference and unsteady loads for a hovering intermeshing rotor

Synchropters are increasingly applied for precision work in placing suspended loads due to their unique flying characteristics. However, the special intermeshing rotor structural helicopter easily introduces rotor-and-rotor interference, leading to unsteady aerodynamic loads on the rotor blades in mission flight. Hence, we use the unsteady vortex lattice method to validate and analyze the aerodynamic characteristics of the intermeshing rotor in hover. In this method, a circulation suppression technique on the blade root is introduced to enhance the stability of the rotor wake in the steady-state hover. Results show that the meniscus truncated regions on the intermeshing rotor disks formed by the blade–vortex impact are nearly unchanged to collective pitches. The low-order frequency harmonic components and phase lag phenomenon universally found in dual rotor loads and additional oscillating roll moment become inherent characteristics of the intermeshing rotor in hover, especially for kΩ caused by interference of rotor downwash flow and 2 kΩ produced by the blade–vortex interaction at 10° and 190° azimuth. Finally, the left/right rotor loads exist harmonic components of 1, 2, and 3 kΩ. In contrast, the total thrust of the intermeshing rotor system eliminates the odd-order harmonic components but aggravates the even-order harmonic components.

Impact of Turbulent Time Scales on Wake Recovery and Operation

Enhancing wake recovery behind wind turbines has the potential to significantly improve the power production and efficiency of large wind farms. Rather than investigating turbine control strategies, floater motion or global turbulent quantities such as turbulence intensity, this work aims to study wake stability and recovery through a focus on the turbulent scales of the inflow. Using Large Eddy Simulations of a single turbine, sinusoidal streamwise forcing is applied to the inflow with a constant amplitude and mean flow velocity, but differing time scales between 80s and 140s. For all applied time scales the turbine wake is characterised by the rolling-up of the near wake into the periodic shedding of vortex rings, and an excitation of the applied forcing frequency resulting in velocity fluctuations in the wake several times larger than that at the inflow. For shorter time scales (80s - 90s) a more aggressive and earlier wake roll-up led to a shorter near wake region, faster overall recovery and significantly improved the expected power output from 6R downstream onwards. An inflow time period of 80s gave rise to more than a 50% increase in power output of a fictive downstream turbine placed at 14R downstream, compared to an inflow time scale of 140s.

The evolutionarily conserved miRNA-137 targets the neuropeptide hypocretin/orexin and modulates the wake to sleep ratio

Hypocretin (Hcrt), also known as orexin, neuropeptide signaling stabilizes sleep and wakefulness in all vertebrates. A lack of Hcrt causes the sleep disorder narcolepsy, and increased Hcrt signaling has been speculated to cause insomnia, but while the signaling pathways of Hcrt are relatively well-described, the intracellular mechanisms that regulate its expression remain unclear. Here, we tested the role of microRNAs (miRNAs) in regulating Hcrt expression. We found that miR-137, miR-637, and miR-654-5p target the human HCRT gene. miR-137 is evolutionarily conserved and also targets mouse Hcrt as does miR-665. Inhibition of miR-137 specifically in Hcrt neurons resulted in Hcrt upregulation, longer episodes of wakefulness, and significantly longer wake bouts in the first 4 h of the active phase. IL-13 stimulation upregulated endogenous miR-137, while Hcrt mRNA decreased both in vitro and in vivo. Furthermore, knockdown of miR-137 in zebrafish substantially increased wakefulness. Finally, we show that in humans, the MIR137 locus is genetically associated with sleep duration. In conclusion, these results show that an evolutionarily conserved miR-137:Hcrt interaction is involved in sleep–wake regulation.

Altered sleep intensity upon DBS to hypothalamic sleep-wake centers in rats.

Deep brain stimulation (DBS) has been scarcely investigated in the field of sleep research. We hypothesize that DBS onto hypothalamic sleep- and wake-promoting centers will produce significant neuromodulatory effects and potentially become a therapeutic strategy for patients suffering severe, drug-refractory sleep–wake disturbances. We aimed to investigate whether continuous electrical high-frequency DBS, such as that often implemented in clinical practice, in the ventrolateral preoptic nucleus (VLPO) or the perifornical area of the posterior lateral hypothalamus (PeFLH), significantly modulates sleep–wake characteristics and behavior. We implanted healthy rats with electroencephalographic/electromyographic electrodes and recorded vigilance states in parallel to bilateral bipolar stimulation of VLPO and PeFLH at 125 Hz and 90 µA over 24 h to test the modulating effects of DBS on sleep–wake proportions, stability and spectral power in relation to the baseline. We unexpectedly found that VLPO DBS at 125 Hz deepens slow-wave sleep (SWS) as measured by increased delta power, while sleep proportions and fragmentation remain unaffected. Thus, the intensity, but not the amount of sleep or its stability, is modulated. Similarly, the proportion and stability of vigilance states remained altogether unaltered upon PeFLH DBS but, in contrast to VLPO, 125 Hz stimulation unexpectedly weakened SWS, as evidenced by reduced delta power. This study provides novel insights into non-acute functional outputs of major sleep–wake centers in the rat brain in response to electrical high-frequency stimulation, a paradigm frequently used in human DBS. In the conditions assayed, while exerting no major effects on the sleep–wake architecture, hypothalamic high-frequency stimulation arises as a provocative sleep intensity-modulating approach.

The pitfalls of investigating rotational flows with the Euler equations

Small viscous effects in high-Reynolds-number rotational flows always accumulate over time to have a leading-order effect. Therefore, the high-Reynolds-number limit for the Navier–Stokes equations is singular. It is important to investigate whether a solution of the Euler equations can approximate a real flow at large Reynolds number. These facts are often overlooked and, as a result, the Euler equations are used to simulate laminar rotational flows at large Reynolds number. Based on the Fredholm alternative, an asymptotic perturbation theory is described to establish secularity conditions determined by viscosity for an inviscid solution to approximate a real viscous fluid. Four important classical inviscid solutions are investigated using the theory with the following conclusions. The Stuart cats’ eyes and Mallier–Maslowe vortices are inconsistent with any real fluid at high Reynolds number; whereas Hill's spherical vortex is confirmed to be consistent with a steady state in the spherical core region and the Lamb–Chaplygin dipole is found to be consistent with a quasi-steady state in the circular core region. These solutions have been widely used for analysing the stability of vortex flows and wakes, and their interactions with shock waves or bubbles. Serendipitously, we have revealed an original exact solution of the Navier–Stokes equations which is time dependent, has non-zero nonlinear convective terms and is restricted to a finite domain with the decay rate depending on dipole radius.

Stability and optimal forcing analysis of a wind turbine wake: Comparison with POD

Understanding the dynamics and generation of coherent structures in wind-turbine wakes is crucial for efficiency improvement of wind farms, which will most probably represent one of the main renewable power generation sources in 2050. In this paper, we investigate the origin of such coherent structures by performing modal and non-modal stability analysis of the mean flow downstream of a wind-turbine rotor. The database consists of large-eddy-simulation results. Bi-local linear-stability and optimal-forcing analyses are performed at several wake's cross-sections. The most unstable perturbations are compared with the most energetic coherent structures recovered by the proper orthogonal decomposition (POD) analysis, showing a good agreement close to the rotor. Further downstream, these modes are overtaken by others with wavenumbers departing from those of the main POD modes. However, optimal-forcing analysis shows that asymptotically stable modes can be amplified by more than one order of magnitude via quasi-resonance mechanisms, bypassing the growth of the most unstable modes in the far wake. This suggests that the most energetic structures are originated by modal instabilities, which trigger quasi-resonance mechanisms in the far wake, determining the emergence of specific frequencies in the turbulent flow. These findings are crucial for designing efficient control systems to optimize wind farm performance.

Investigation into Yaw Motion Influence of Horizontal-Axis Wind Turbine on Wake Flow Using LBM-LES

The dynamic yaw motion of the wind turbine will affect the overall aerodynamic performance of the impeller and the corresponding wake flow, but the current research on this issue is inadequate. Thus, it is very necessary to study the complicated near-wake aerodynamic behaviors during the yaw process and the closely related blade aerodynamic characteristics. This work utilized the multi-relaxation time lattice Boltzmann (MRT-LBM) model to investigate the integral aerodynamic performance characteristics of the specified impeller and the dynamic changes in the near wake under a sine yawing process, in which the normalized result is adopted to facilitate data comparison and understanding. Moreover, considering the complexity of the wake flows, the large eddy simulation (LES) and wall-adapting local eddy-viscosity (WALE) model are also used in this investigation. The related results indicate that the degree of stability of tip spiral wake in the dynamic yaw condition is inversely related to the absolute value of the change rate of yaw angular speed. When the wind turbine returns to the position with the yaw angle of 0 (deg) around, the linearized migration of tip vortex is changed, and the speed loss in the wake center is reduced at about the normalized velocity of 0.27, and another transverse expansion appeared. The directional inducing downstream of the impeller sweep surface for tip vortex is clearly reflected on the entering side and the exiting side. Additionally, the features of the static pressure on the blade surface and the overall aerodynamic effects of the impeller are also discussed, respectively.

317 Sleep-wake survival dynamics in people with insomnia

Abstract Introduction Assessing objective measures of sleep fragmentation could yield important features reflecting impaired sleep quality in people with insomnia. Survival analysis allows the specific examination of the stability of NREM sleep, REM sleep and wake. The objective of this study was to assess the differences between survival dynamics of NREM sleep, REM sleep and wake between people with insomnia and healthy controls. Methods We analyzed polysomnography recordings from 88 people with insomnia and 92 healthy controls. For each participant, survival dynamics of REM sleep, NREM sleep and wake were represented using Weibull distributions. We used lasso penalized linear regression to analyze the difference between participant groups with respect to the Weibull scale and shape parameters, while correcting for age, sex, total sleep time and relevant interaction effects. Because comparisons were done for scale and shape parameters of REM sleep, NREM sleep and wake, a Bonferroni correction was applied, resulting in an alpha value of 0.05/6 = 0.0083. Results Significant effects of group were found for the NREM scale parameter (unstandardized model coefficient B=-0.79, t=-3.0, p=0.0035), and for the scale and shape parameters of wake (scale parameter B=7.6, t=2.8, p=0.0065; shape parameter B=0.20, t=2.9, p=0.0048). Results indicated that people with insomnia had less stable NREM sleep and more stable wake after sleep onset compared to healthy controls. Additionally, the altered distribution of wake segment lengths indicated an increased difficulty to fall asleep after longer awakenings in the insomnia group. However, these differences were mainly observed in younger participants. Significant effects of group for the survival parameters of REM sleep were not found. Conclusion As illustrated by our results, survival analysis can be very useful for disentangling different types of sleep fragmentation in people with insomnia. For instance, the current findings suggest that people with insomnia have an increased fragmentation of NREM sleep, but not necessarily of REM sleep. Additional research into the underlying mechanisms of NREM sleep fragmentation could possibly lead to a better understanding of impaired sleep quality in people with insomnia, and consequently to improved treatment. Support (if any):

Stability of finite and infinite von Kármán vortex-cluster streets.

A wake of vortices with sufficiently spaced cores may be represented via the point-vortex model from classical hydrodynamics. We use potential theory representations of vortices to examine the emergence and stability of complex vortex wakes, more particularly the von Kármán vortex street composed of regular polygonal-like clusters of same-signed vortices. We investigate the existence and stability of these streets represented through spatially periodic vortices. We introduce a physically inspired point-vortex model that captures the stability of infinite vortex streets with a finite number of procedurally generated vortices, allowing for numerical analysis of the behavior of vortex streets as they dynamically form.

Sleep-Wake Survival Dynamics in People with Insomnia

IntroductionAssessing objective measures of sleep fragmentation could yield important features reflecting impaired sleep quality in people with insomnia. Survival analysis allows the specific examination of the stability of NREM sleep, REM sleep and wake. The objective of this study was to assess the differences between survival dynamics of NREM sleep, REM sleep and wake between people with insomnia and healthy controls.MethodsWe analyzed retrospective polysomnography recordings from 86 people with insomnia and 94 healthy controls. For each participant, survival dynamics of REM sleep, NREM sleep and wake were represented using Weibull distributions. We used lasso penalized parameter selection in combination with linear regression to analyze the difference between participant groups with respect to the Weibull scale and shape parameters, while correcting for age, sex, total sleep time and relevant interaction effects.ResultsSignificant effects of group were found for the NREM scale parameter, and for the wake scale and shape parameters. Results indicated that people with insomnia had less stable NREM sleep and more stable wake after sleep onset compared to healthy controls. Additionally, the altered distribution of wake segment lengths indicated an increased difficulty to fall asleep after longer awakenings in the insomnia group. However, these differences were mainly observed in younger participants. Significant effects of group for the survival parameters of REM sleep were not found.ConclusionAs illustrated by our results, survival analysis can be very useful for disentangling different types of sleep fragmentation in people with insomnia. For instance, the current findings suggest that people with insomnia have an increased fragmentation of NREM sleep, but not necessarily of REM sleep. Additional research into the underlying mechanisms of NREM sleep fragmentation could possibly lead to a better understanding of impaired sleep quality in people with insomnia, and consequently to improved treatment.

Stability of nocturnal wake and sleep stages defines central nervous system disorders of hypersomnolence.

We determine if young people with narcolepsy type 1 (NT1), narcolepsy type 2 (NT2), and idiopathic hypersomnia (IH) have distinct nocturnal sleep stability phenotypes compared to subjectively sleepy controls. Participants were 5- to 21-year old and drug-naïve or drug free: NT1 (n = 46), NT2 (n = 12), IH (n = 18), and subjectively sleepy controls (n = 48). We compared the following sleep stability measures from polysomnogram recording between each hypersomnolence disorder to subjectively sleepy controls: number of wake and sleep stage bouts, Kaplan-Meier survival curves for wake and sleep stages, and median bout durations. Compared to the subjectively sleepy control group, NT1 participants had more bouts of wake and all sleep stages (p ≤ .005) except stage N3. NT1 participants had worse survival of nocturnal wake, stage N2, and rapid eye movement (REM) bouts (p < .005). In the first 8 hours of sleep, NT1 participants had longer stage N1 bouts but shorter REM (all ps < .004). IH participants had a similar number of bouts but better survival of stage N2 bouts (p = .001), and shorter stage N3 bouts in the first 8 hours of sleep (p = .003). In contrast, NT2 participants showed better stage N1 bout survival (p = .006) and longer stage N1 bouts (p = .02). NT1, NT2, and IH have unique sleep physiology compared to subjectively sleepy controls, with only NT1 demonstrating clear nocturnal wake and sleep instability. Overall, sleep stability measures may aid in diagnoses and management of these central nervous system disorders of hypersomnolence.

Effect of Mach number on the absolute/convective stability of compressible planar wakes

In this paper, the influence of the Mach number on the stability of two-dimensional compressible planar wakes is studied to gain physical insight into the turbine wakes. Two-dimensional instabilities of compressible wakes are studied using local spatiotemporal instability analyses. The absolute/convective boundary of a family of wake profiles is obtained at different Mach numbers. Then, local stability analyses of compressible wakes behind flat plates are performed. It is found that in the subsonic region increasing Mach number acts in two ways to modify the instability characteristics in the near wake region: it lengthens the recirculation zone and it reduces the absolute growth rate. These two mechanisms work against each other, which explains why the linear global growth rate varies slightly as Mach number changes in the subsonic region. Further increasing Mach number to 1 would greatly reduce the length of the absolute unstable region due to the occurrence of expansion waves around the trailing edge corner. As a result, the wake is strongly stabilized at $$\hbox {Ma}=1$$ .

Assessing sleep-wake survival dynamics in relation to sleep quality in a placebo-controlled pharmacological intervention study with people with insomnia and healthy controls

RationaleThe mechanisms underlying impaired sleep quality in insomnia are not fully known, but an important role for sleep fragmentation has been proposed.ObjectivesThe aim of this study is to explore potential mechanisms of sleep fragmentation influencing alterations of perceived sleep quality.MethodsWe analyzed polysomnography (PSG) recordings from a double-blind crossover study with zopiclone 7.5 mg and placebo, in elderly participants with insomnia complaints and age-matched healthy controls. We compared survival dynamics of sleep and wake across group and treatment. Subsequently, we used a previously proposed model to estimate the amount of sleep onset latency (SOL) misperception from PSG-defined sleep fragmentation. Self-reported and model-estimated amount of SOL misperception were compared across group and treatment, as well as model prediction errors.ResultsIn the zopiclone night, the average segment length of NREM sleep was increased (group F = 1.16, p = 0.32; treatment F = 8.89, p< 0.01; group x treatment F = 0.44, p = 0.65), while the segment length of wake was decreased (group F = 1.48, p = 0.23; treatment F = 11.49, p< 0.01; group x treatment F = 0.36, p = 0.70). The self-reported and model-estimated amount of SOL misperception were lower during the zopiclone night (self-reported group F = 6.08, p< 0.01, treatment F = 10.8, p< 0.01, group x treatment F = 2.49, p = 0.09; model-estimated F = 1.70, p = 0.19, treatment F = 16.1, p< 0.001, group x treatment F = 0.60, p = 0.55). The prediction error was not altered (group F = 1.62, p = 0.20; treatment F = 0.20, p = 0.65; group x treatment F = 1.01, p = 0.37).ConclusionsImpaired subjective sleep quality is associated with decreased NREM stability, together with increased stability of wake. Furthermore, we conclude that zopiclone-induced changes in SOL misperception can be largely attributed to predictable changes of sleep architecture.

Influence of limiting the projection region on coarse Large Eddy Simulation-Actuator Line Model simulations

Wind energy has developed worldwide, becoming a mature technology. Nevertheless, major advances regarding wind turbine aerodynamics and control as well as wind farm control and its integration in the power grid are being foreseen in the near future. To accomplish that, different simulation tools have emerged. The Actuator Line Model (ALM) in the frame of Large Eddy Simulation method has been used to study different related topics, from wake stability to wind turbine interaction and wind farm control, and it is considered the state of the art to simulate with high fidelity the wind flow through wind turbines and wind farms. Despite that, the ALM results, particularly loads and power production, have shown to be dependent on the numerical setup, like the projection function and its smearing parameter. The present paper aims to contribute to the latter by showing the influence of limiting the projection region. It is found that projecting the aerodynamic forces onto a cylinder that confines the rotor disk improves the computed power and loads compared to a Blade Element Momentum code and affects the wake development.

On the instability mechanisms of ship propeller wakes

This paper evaluates the propeller wake characteristics of the benchmark ship propeller INSEAN E779A using large eddy simulations and identifies instability mechanisms that lead to wake breakdown. The effects of propeller blade loading on wake stability are considered by conducting simulations at different advance coefficients (J = 0.85, J = 0.65 and J = 0.45). The hydrodynamic results capture the propeller wake vortical structures and flow features from the onset and evolution of the instability to the far wake. The simulation results are verified and validated against experimental measurements from the literature. A new instability mechanism concerning mutual interaction among adjacent helical vortex sheets in the transition region is identified. Further, the formation of short wave instabilities in the vortex filaments is found to be initiated by the same mutual interactions among sheets. Lastly, it is shown that interaction of sheets with the preceding tip vortex induces the onset of tip vortical filaments breakup, mutual-interaction and long wave instability.