Two modal decomposition techniques are employed to analyse the stability of wind turbine wakes. A numerical study on a single wind turbine wake is carried out focusing on the instability onset of the trailing tip vortices shed from the turbine blades. The numerical model is based on large-eddy simulations (LES) of the Navier-Stokes equations using the actuator line (ACL) method to simulate the wake behind the Tjaereborg wind turbine. The wake is perturbed by low-amplitude excitation sources located in the neighbourhood of the tip spirals. The amplification of the waves travelling along the spiral triggers instabilities, leading to breakdown of the wake. Based on the grid configurations and the type of excitations, two basic flow cases, symmetric and asymmetric, are identified. In the symmetric setup, we impose a 120• symmetry condition in the dynamics of the flow and in the asymmetric setup we calculate the full 360• wake. Different cases are subsequently analysed using dynamic mode decomposition (DMD) and proper orthogonal decomposition (POD). The results reveal that the main instability mechanism is dispersive and that the modal growth in the symmetric setup arises only for some specific frequencies and spatial structures, e.g. two dominant groups of modes with positive growth (spatial structures) are identified, while breaking the symmetry reveals that almost all the modes have positive growth rate. In both setups, the most unstable modes have a non-dimensional spatial growth rate close to π/2 and they are characterized by an out-of-phase displacement of successive helix turns leading to local vortex pairing. The present results indicate that the asymmetric case is crucial to study, as the stability characteristics of the flow change significantly compared to the symmetric configurations. Based on the constant non-dimensional growth rate of disturbances, we derive a new analytical relationship between the length of the wake up to the turbulent breakdown and the operating conditions of a wind turbine.

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We use linear control theory to construct an output feedback controller for the attenuation of small-amplitude Tollmien-Schlichting (TS) wavepackets in a flat-plate boundary layer. We distribute evenly in the spanwise direction up to 72 localized objects near the wall (18 disturbances sources, 18 actuators, 18 estimation sensors and 18 objective sensors). In a fully three-dimensional configuration, the interconnection between inputs and outputs becomes quickly unfeasible when the number of actuators and sensors increases in the spanswise direction. The objective of this work is to understand how an efficient controller may be designed by connecting only a subset of the actuators to sensors, thereby reducing the complexity of the controller, without comprising the efficiency. We find that using a semi-decentralized approach -where small control units consisting of 3 estimation sensors connected to 3 actuators are replicated 6 times along the spanwise direction -results only in a 11% reduction of control performance. Our results reveal that the best performance is obtained for a control unit which (i) is sufficiently "wide" to account for the full spanwise scale of the wavepacket when it reaches the actuators and (ii) is designed to account for the perturbations which are coming from the lateral sides (crosstalk) of the estimation sensors. We have also found that the influence of crosstalk is not as essential as the spreading effect.

Objective: Busulfan is one of the most common chemotherapeutic drugs and has the ability to induce apoptosis in testicular germ cells, which leads to infertility. In this study, the effects of ozone therapy and melatonin were evaluated on testicular disorders induced by busulfan. Methods: In this study, we divided 24 male mice into four groups: control group, groups treated with busulfan, busulfan/melatonin, and busulfan/ozone. At the end of a 35-day period, blood samples were taken from the mice and their testosterone levels were measured. Both of the mice’s testes were removed and weighed, afterwards, each one of them was used for evaluation of morphology by Johnson’s score, as well as for measuring the diameter and thickness of seminiferous tubules. The other testis was homogenized for measuring Malondialdehyde (MDA) and antioxidant status using Catalase (CAT), Super Oxide Dismutase (SOD), and Total Antioxidant Capacity (TAC) levels. Epididymis spermatozoa were also used to evaluate motility, morphology, and sperm count. Results: Busulfan significantly reduced the testis quality (weight, sperm parameters, testosterone, CAT, SOD, and TAC levels) and increased MDA and destruction of seminiferous tubules compared to the control group. Ozone and melatonin treatments significantly increased testis quality, sperm parameters, MDA, and antioxidant status, but they did not affect the TAC level. Conclusions: This study showed that similar to melatonin, ozone can reduce the effect of busulfan toxicity on mice testis. However, further studies are needed to understand the precise mechanism of ozone function on testis.

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