Mahmoud Pasandidehfard

This study experimentally investigates the impact of finlet integration on the aeroacoustic and aerodynamic characteristics of a T-MOTOR 18*6.1 carbon fiber propeller operating at low Reynolds number regimes. Far-field noise and thrust measurements were conducted under static conditions across rotational speeds ranging from 2000 to 6000 RPM and various sound radiation polar angles. Aerodynamic results indicated that finlet integration led to an average thrust performance reduction of 8% across the entire operating range, primarily attributed to increased profile drag. From an aeroacoustic perspective, findings revealed a strong dependency of finlet performance on both propeller rotational speed and noise propagation angle. Notably, finlets at 2000 RPM resulted in broadband noise reduction at high frequencies across all polar angles. Their optimal aeroacoustic performance was observed at lower rotational speeds (2000 RPM) and suction-side polar angles, particularly at 30°, where the broadband noise reduction averaged approximately 6 dB at low frequencies and 3.5 dB at high frequencies. While a tonal noise increase of approx. 1.5 dB occurred at the first blade passing frequency at this speed, the study indicates an influence on flow patterns and pressure fluctuations, inferred from the acoustic and thrust results. However, further optimization of finlet design and selection is essential to expand their beneficial impact across diverse operating conditions.

The numerical investigation of the water flow with partial cavitation around the NACA 0009 hydrofoil with bumps inspired by the mako shark (biomimicry) was performed. The simulation was done by ANSYS Fluent software for cavitation numbers of 1 and 1.5 at flow speeds of 5 and 10 m/s and various angles of attack from 4° to 12°. The effect of bumps and their related parameters, including number, thickness, and installation location, has been investigated in conditions without and with cavitation. The hydrofoil with the highest lift‐to‐drag ratio was selected for each angle of attack. The results show that the formation of a patterned bump inspired by the surface of the shark skin can affect the lift and drag coefficients by affecting the cavitation and separation phenomena. If these bumps are installed in the right number and place, they can reduce the drag coefficient and increase the lift coefficient. At a water flow speed of 10 m/s and a cavitation number of 1.5, the bumps had a good performance causing an increase in the lift‐to‐drag ratio from the angle of attack of 6°. However, at a flow speed of 5 m/s and a cavitation number of 1, the improvement of the lift‐to‐drag ratio starts from the angle of attack of 4°. Among the various parameters examined in this research, which were also confirmed by the Taguchi method and Minitab software, the most influential parameters are the installation location and the number of bumps. The hydrofoil with two bumps at a chord length position of 13% with a chord length thickness of 1.14 mm at a 6° angle of attack had the best performance that caused an increase in the lift‐to‐drag ratio up to 40% in a cavitation number of 1.5 and a flow speed of 10 m/s.

Improving the aerodynamic coefficients of airfoils is an important and practical issue that has been the subject of research and development over the years. The use of porous media is one of the passive methods in fluid mechanics aimed at enhancing the aerodynamic coefficients of wings. This study investigates the effect of porous media at the leading edge of the NACA 0012 airfoil on its performance in the stall region, utilizing numerical simulation of the porous medium with the assumption of a medium containing circular micro-grains at low Reynolds numbers. In this study, various parameters of the porous medium are examined individually for their effects on the aerodynamic performance of the airfoil. The results indicate that the use of porous media improves the aerodynamic performance of the airfoil in the stall region and delays the onset of the stall. Additionally, parametric studies on the variables of the porous medium show that reducing the length of the porous medium from 0.3c to 0.1c results in a 25% increase in the airfoil’s performance in the stall region. Furthermore, decreasing the depth of the porous medium from 0.05c to 0.01c leads to more than a 50% improvement in airfoil performance, while doubling the porosity leads to an increase of more than 80% in airfoil performance.

Noise pollution is a critical issue in urban and industrial areas, particularly those near airports. This study experimentally investigated brush and serration edge treatments as passive methods to control wake-leading-edge interaction noise in a tandem symmetric airfoil configuration using surface-mounted unsteady pressure sensors in a subsonic wind tunnel. Surface pressure fluctuations, a key source of aerodynamic noise, were examined on the rear airfoil surface at a relatively low angle of attack and a Reynolds number of 185,000. The results demonstrate that the brush increases the surface pressure fluctuation power spectral density (PSD) levels at low frequencies, whereas its performance at medium and high frequencies varies depending on the relative positioning of the airfoils. The highest PSD reduction (up to 5.5 dB) was achieved when the downstream airfoil leading-edge was aligned with the upstream airfoil chord line. In contrast, the serration performs satisfactorily at low frequencies for negative vertical distances between the rear airfoil leading-edge and the front airfoil trailing-edge (yg < 0); however, its effectiveness is minimal for positive distances, except when the rear airfoil leading-edge aligns with the front airfoil chord line, where it performs optimally. Although serration can sometimes increase PSD levels, it generally reduces pressure fluctuations over a broader frequency range than the brush. Both treatments can be effective for wake-leading-edge interaction noise control, particularly in configurations where the downstream airfoil leading-edge aligns with the upstream airfoil chord line, providing valuable insights for mitigating aerodynamic noise in practical applications.

In various engineering applications, the phase change from steam to water droplet during rapid expansion under supersonic flow conditions is inevitable. The occurrence of condensation waves due to the latent heat released during phase change and aerodynamic shocks reduces equipment performance. Given the complexities of two-phase flow, researchers have always been interested in developing an accurate numerical method that can be used in different flow regimes while reducing computation costs. This research has developed a two-dimensional code using the simple low-dissipation advection upstream splitting method (SLAU) and high-resolution simple low-dissipation advection upstream splitting method (HR-SLAU) in the complex geometry of the steam turbine blades and the convergent–divergent nozzles. The performance of these two schemes was examined using the Eulerian–Lagrangian model under different boundary conditions. The governing equations are investigated using the Eulerian model, which incorporates the SLAU and HR-SLAU schemes, and the equations for determining the wet flow parameters are solved by the Lagrangian approach to predict the liquid mass in the grid cell. Comparative to the experimental data, regarding the simplicity of these two schemes, and in the absence of any tuning flow-dependent variable, the determination of the position and intensity of shocks has been improved considerably, while the droplet radius prediction has been significantly enhanced. Notably, the HR-SLAU scheme, in which the dissipation component of the pressure flux according to a cell-interface orientation angle and flow characteristics is locally controlled by the dissipation coefficient for modeling two-phase flows, leads to fewer numerical errors.

The phenomenon of artificial cavitation is a significant and practical aspect of reducing drag forces on devices interacting with water. Among various regimes, the supercavity regime, where the entire body or a substantial portion of it is enveloped by a cavity, plays a crucial role. This study investigates the cavitation phenomenon using a combination of numerical experimental methods and machine learning techniques. Specifically, the Support Vector Machine (SVM) classification model is employed to identify the type of artificial cavitation regime and to determine the occurrence of the supercavity regime based on input variables. The findings indicate that the Radial Basis Function (RBF) model outperforms other machine learning models in accurately detecting the cavity regime type, achieving an accuracy of over 95% in identifying the supercavity regime. Additionally, results from the RBF model demonstrate that the supercavity regime is observed at low cavitation numbers, exhibiting the longest cavity length.

Ventilated supercavitation is a hydrodynamic phenomenon where injected air forms a large bubble around a high-speed submerged body, reducing drag to enhance its movement. The study of cavitating flows is crucial near the free surface due to significant interactions between the supercavity and surface waves. In this study, the behavior of the ventilated supercavity and free surface waves at high Froude numbers and varying submersion depths has been investigated using a numerical simulation approach. The accuracy of the predicted results is verified by comparing them with the available experimental data. The results indicate that the supercavity size is simultaneously affected by submersion depth and Froude number. Additionally, as the Froude number increases, the submersion depth at which the free surface effect on the supercavity shape becomes negligible also increases. Moreover, near the free surface, unlike in fully submerged conditions, the supercavity length increases with depth. The results also show that three types of free surface wave patterns are formed, depending on the intensity of the first and second wave systems.

Minimizing drag on the cavitator is essential in hydrodynamic research and for improving the performance of objects used in marine environments. This study focuses on optimizing a parabolic cavitator and analyzing the cavities it generates in detail. First, the key factors for optimizing the cavitator were identified using the Taguchi method. Based on these factors, the three-dimensional shape of the cavitator was numerically simulated, and the hydrodynamic forces acting on it were calculated with consideration of cavitation. The optimized cavitator shape was then identified through further analysis using the Taguchi method and was experimentally tested to confirm its real-world performance. Subsequently, the characteristics of artificial cavitation behind the improved cavitator were examined both experimentally and numerically across various ventilation coefficients. The experiments included high-speed imaging and pressure measurements to capture the dynamics of cavity formation and collapse, while numerical simulations were performed using a k-omega shear stress transport turbulence model and a volume of fluid approach to accurately predict the phase interface. The results highlight the importance of the cavitator's incidence angle and the distance from its nose to its base in the optimization process. Moreover, the analysis shows that pressure fluctuations are significantly more intense at the point where the cavity closes than within the cavity itself. Additionally, the findings indicate that the supercavity characteristics generated by this optimized cavitator are 10% better than those produced by other cavitators, contributing to reduced drag and improved hydrodynamic efficiency.