Journal Of Applied Fluid Mechanics
Volume:15 Issue: 6, Nov-Dec 2022

This article introduces a novel Induction Blade (IB) prototype modeled by Blade Element Momentum (BEM) theory, which develops higher torque during the starting phase for Horizontal Axis Wind Turbines (HAWT), especially for micro-turbines. The IB is composed of two parallel blades joined at their tips and roots, forming a distinctive hole in the space between the blades that generates a Venturi effect as air passes through. This phenomenon in the IB hole together with the extra lift generated by the area of the second blade produce extra valuable torque during the starting phase. We used Computational Fluid Dynamics (CFD) analysis to evaluate the aerodynamic properties of this design compared with a traditional blade design of the same radius. The IB and traditional prototypes were built (50W, diameter 0.62m, λ=9 and at speed rated 8m/s) by additive manufacturing in a 3D printer and their aerodynamic behaviors tested in a small wind tunnel (square section 0.7m x 0.7m). Our results using CFD analysis show that this novel IB produces up to 65% extra torque without losing output power for low wind velocity (5-8 m/s). IMPI (Mexican Institute of Industrial Protection) protects this prototype shape.

The stall of an aircraft is one of the most dangerous phenomena in the aviation world, resulting in a sudden loss of lift because of boundary layer separation. This work aims to delay separation and to improve wing aerodynamic performances by introducing bumps and cavities on the upper surfaces of the wing. A numerical study on the effects of both cavities and bumps on flow structures and wing aerodynamics of NACA 0012 profile is conducted. The CFX code has been used to perform calculations of steady and uncompressible Reynolds Averaged Naviers-Stokes equations. The airfoil has been exposed to a free stream velocity of 5.616 m/s and chord based Reynolds number of 3.6 x 105 (chord length). A series of test on unmodified airfoil has been carried out for various turbulence models at angles of attack ranging from 0° to 15°. Then, the two-equation k-ω SST (Shear Stress Transport) has been retained for the further cases. Different configurations obtained through a modification of cavities and bumps shape, dimension, and position on the airfoil chord are investigated. Both the shapes considered are semi-spherical and semi-cylindrical, placed at two positions on the airfoil chord. The first location is in suction pick at X/C= 0.3 and the second one is at 0.7. Results show that the application of bumps delays the boundary layer separation and increase drag coefficient. A slight enhancement in lift and drag is observed at angle of attack of 15° for the cases where the cavities are placed at 0.7 m from the leading edge. In addition, calculations show that the stability of the vortex formed inside the cavities depends strongly on their shape and the cylindrical one has better performances.

The flow in the vertical long-axis fire pump exhibits complex, three-dimensional, unsteady flow features. In an attempt to understand the effects of turbulence models on the flow mechanism and performance characteristics of the pump, the ANSYS CFX software was used to carry out numerical studies on the vertical fire pump using URANS. The main objective of this study was to investigate the unsteady flow dynamics within the vertical fire pump and the influence of applying different computational turbulence models. The study then sought to conduct a brief analysis of the unsteady pressure pulsation characteristics of the pump. The reliability of the CFD model was validated with an external characteristic test. The transient pressure distribution, velocity field and external characteristics were analyzed. The results were compared to experimental results, where it was revealed that the SST k-ω model showed 1.82% and 0.81% improvements in efficiency and head, respectively, over the k-ε models. In terms of the power performance, however, the standard k-ε is less likely to over-predict the power used by the pump in overload conditions as compared to the other turbulence models. The pressure charts did not show significant reactions to varying turbulence models across all the studied flow rates. However, the velocity streamlines revealed that there were several disruptions in streamwise flow, where both the standard and RNG k-ε models exhibited more recirculation areas than the SST k-ω and standard k-ω models. Overall, for this type of application, SST k-ω was the best-performing turbulence model, while RNG k-ε showed the poorest performance. Nonetheless, the RNG k-ε also has its strengths. This investigation would serve as a theoretical reference for further research and development in fluid machinery.

In the current numerical work, a 2D wave tank has been planned to explain the shared impacts among three different solitary waves and three different floating breakwaters by applying Reynolds-Averaged Navier-Stokes models and the volume of fluid method. Three dissimilar floating breakwaters (i.e., square breakwater, circular breakwater, and modified breakwater) were chosen. A total of eighteen cases were investigated, including three different floating breakwaters, a solitary wave (SW) with three different wave heights, and two different densities of floating breakwaters. We achieved the production of a solitary wave by moving a wave paddle (WP) and the motion of floating breakwater in two various directions by applying two different codes as user-defined functions. The dynamic mesh technique has been employed for re-forming mesh during the motion of the wave paddle and the floating breakwater. The numerical calculations have been confirmed by some numerical, analytical, and experimental case studies. First, the generation of a SW using the WP movement and the free motion of a heaving round cylinder on the free surface of motionless water were modeled and validated. Additionally, the effects of various parameters, including floating breakwater shape, floating breakwater density, and solitary wave height, on the hydrodynamic performances of the floating breakwater, the floating breakwater’s motions, and the free-surface elevation were considered under various conditions.

Primary nozzle is one of the most important factors which has a large influence on the performance of thermo-compressors. Most of studies carried out until now, have been performed on single-nozzle thermo-compressors. In this paper, an actual industrial thermo-compressor is considered and the purpose is to study the effect of increasing primary nozzle number on the performance of this thermo-compressor. For this purpose, a triple-nozzle thermo-compressor is simulated numerically and its performance is compared with single-nozzle thermo-compressor. Ideal gas thermodynamic properties are considered to simulate the compressible flow within the thermo-compressor and numerical result is validated using the experimental result. In addition, the effects of variation in mixing chamber convergence angle and position of nozzles at the radial direction in triple-nozzle thermo-compressor are investigated. The numerical results show that at the same condition, a triple-nozzle thermo-compressor is able to provide superior critical back pressure and entrainment ratio than single-nozzle thermo-compressor. The proximity of nozzles (with 34% changes in radial distance) increases the critical back pressure about 8% and decreases the entrainment ratio about 5%. By increasing mixing chamber convergence angle about 66%, the value of critical back pressure decreases about 29% and the value of entrainment ratio decreases about 16% in single nozzle-thermo-compressor and 10% in triple-nozzle thermo-compressor. Also, by 8% reduction of mixing chamber convergence angle, the critical back pressure decreases 6% but entrainment ratio decreases about 2% in single-nozzle thermo-compressor and increases about 3% in triple-nozzle thermo-compressor.

When combustion instability occurs, fluctuation in the release of heat couples with oscillating pressure, while the sensitivity of flame to acoustic disturbance restricts the oscillation intensity. This paper investigates the efficacy of helium in suppressing combustion instability. The flame structure, its sensitivity to acoustic disturbance and the inhibition of oscillating pressure with the addition of helium were studied by means of open tests, external-excited and self-excited combustion instability experiments. First of all, the addition of helium made larger flame surface area, which shaped the distributed flame, and the heat was such released over a broader space. Then, the external-exited combustion instability experiments confirmed that adding helium to fuel could decrease the sensitivity of flame to acoustic disturbance. Finally, Helium was used in the case of self-excited combustion instability to further investigate its effectiveness on the oscillation suppression. Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) methods were used to study flame fluctuation intensity. The results showed that the amplitudes of oscillating pressure were greatly reduced by the added helium. For 250Hz mode, adding helium with 20% of fuel flow could significantly reduce the flame pulsation and reduce the pulsation pressure by more than half. However, for the 160hz mode, more helium should be added to achieve better results. When the helium flow exceeded 80% of fuel flow, the combustion instability could be converted to stable combustion.

In this study, a closed-loop pulsating heat pipe experimental investigation was done with 1% wt concentration of cerium oxide/EG-water (60-40) nanofluid. The unsteady state measurement was done to determine the effect of heat input, filling ratio and evacuation pressure on the thermal performance of the closed-loop pulsating heat pipe. The thermal performance is assessed in terms of temperature variation, thermal resistance and effective thermal conductivity of the pulsating heat pipe. The most appropriate behaviour is observed at 0.0799 bar evacuation pressure with 50% FR. The lowermost thermal resistance of 0.116598K/W was observed at 0.0799 bar evacuation pressure and 50% FR. The effective thermal conductivity value was observed as 5078.34 W/mK at 50% FR, 0.0799 bar for 80W heat input which is 12 to 13 times better than pure copper. The pulsation action inside the pulsating heat pipe is verified with the power spectral density analysis. This study supports the better performance of the heat pipe at 50% FR with a lower evacuation pressure of 0.0799 bar.

This study investigated the enhanced cooling of electronic components at high temperatures with cross-flow and jet-flow combinations. The cooling performance of four different model geometries (Models 1, 2, 3, and 4) of an electronic component was analysed by considering different jet-to-channel inlet velocity ratios (Vj/Vc) and ratios of the distance between the jet and impinging surface to jet diameter (H/D). The Vj/Vc and H/D ratios were varied in the 0–3 and 2–4 ranges, respectively, in the computational fluid dynamics analysis. The thermal and flow characteristics were revealed through a comparative result analysis, also considering results from the literature. The heat transfer improved, the Nusselt number increased, and the electronic surface temperature decreased with an increase in the Vj/Vc ratio. However, the Nusselt number decreased with an increase in the H/D ratio. Models 2 and 4 had higher heat transfer from the electronic component than the other models. A low H/D ratio and low Vj/Vc ratio yielded higher heat transfer in Model 3 than in Model 1.

We perform a thorough numerical analysis of the impact of inflow conditions on the aerodynamic performance of a tandem cascade. In particular, we investigate the effects of the incidence angle and the inlet boundary layer (IBL) thickness on the three-dimensional flow field structure and aerodynamic performance. Our results show that the gap flow strength of the tandem cascade decreases with the increase of incidence angle, and it can effectively reduce the mixing of the wakes of the forward blade (FB) and rear blade (RB). In turn, this prevents the passage vortex (PV) in the RB passage from developing along the circumferential direction. The occurrence of IBL does not modify the effects of the incidence angle on the tandem cascade, however, it reduces the load of the RB and the gap flow strength near the endwall. Under all incidence angles, IBL increases the total pressure loss of the tandem cascade, and decreases the static pressure rise (except for an incidence angle equal to -6°). The maximum loss increment is at 2° incidence angle, and the maximum static pressure rise decrement is at 6° incidence angle (Thick-IBL condition) or 7° incidence angle (Thin-IBL condition). Furthermore, we found that the presence of IBL changes the minimum loss condition from 0° (design condition) to -2° incidence angle. Our results thus indicate that in the practical engineering application of the tandem cascade, the reality that IBL degrades the tandem cascade performance in the full incidence angle range should be considered. And the strong endwall secondary flow effect caused by IBL should be considered in the tandem cascade three-dimensional design, so that the tandem cascade two-dimensional performance advantage can be better played.

To examine the flow characteristics of particles in the vertical tube corresponding to different curvature radius bends, the particle velocity is measured at the Minimum Pressure Drop (MPD) using a high-speed Particle Image Velocimeter (PIV). This experiment explores the effects of particle flow characteristics at different curvature radius bends and their corresponding vertical tubes with pressure drop, power consumption, the intensity of particle fluctuation velocity, power spectrum and time-frequency characteristics. It is observed that the pressure drop and power consumption can be reduced with the help of a large curvature radius bend. Besides, the reduction of particle velocity in the large curvature radius bend and its corresponding vertical tube is less, and the particle possesses a larger intensity of fluctuation velocity in its corresponding vertical tube. The particles in the vertical tube corresponding to the large curvature radius bend lead to large peaks of the power spectrum in the low-frequency region, which is closely linked to the pressure drop. Eventually, the dynamics of particles in a vertical tube are revealed from the perspective of time-frequency analysis by using a continuous wavelet transform.

In this work, the accuracy of the multiphase lattice Boltzmann method (LBM) based on the phase-field models, namely the Cahn-Hilliard (C-H) and Allen-Cahn (A-C) equations, are evaluated for simulation of two-phase flow systems with high-density ratios. The mathematical formulation and the schemes used for discretization of the derivatives in the C-H LBM and A-C LBM are presented in a similar notation that makes it easy to implement and compare these two phase-field models. The capability and performance of the C-H LBM and A-C LBM are investigated, specifically at the interface region between the phases, for simulation of flow problems in the two-dimensional (2D) and three-dimensional (3D) frameworks. Herein, the equilibrium state of a droplet and the practical two-phase flow problem of the rising bubble are considered to evaluate the mass conservation capability of the phase-filed models employed at different flow conditions and the obtained results are compared with available numerical and experimental data. The effect of employing different equations proposed in the literature for calculating the relaxation time on the accuracy of the implemented phase-field LBMs in the interfacial region is also studied. The present study shows that the LBM based on the A-C equation (A-C LBM) is advantageous over that based on the C-H equation in dealing with the conservation of the total mass of a two-phase flow system. Also, the results obtained by the A-C LBM is more accurate than those obtained using the C-H LBM in comparison with other numerical results and experimental observations. The present study suggests the A-C LBM as a sufficiently accurate and computationally efficient phase-field model for the simulation of practical two-phase flows to resolve their structures and properties even at high-density ratios.

In reversible pump-turbines, guide vane vibrations are considered to have potentially severe consequences of noise and structural damage. Unstable torsional mode self-excited vibrations of guide vanes have been reported at small guide vane openings during transient operations involving pump flow, such as pump starting and closing processes. In this study, coupling simulations were carried out under different operating conditions based on the unsteady computational fluid dynamics (CFD) method with a single-degree-of-freedom (1DOF) oscillator. The results show that the operating conditions, including the initial opening angle and the pressure difference between the runner side and the stay vane side, significantly affect the instability of guide vane torsional mode self-excited vibration. Energy-based analysis indicates that the positive cumulative work done by total hydraulic torque is responsible for unstable torsional mode self-excited vibration. Furthermore, the relatively small phase difference between total hydraulic torque and guide vane angular velocity, and the positive feedback between vibration amplitude and energy accumulation, are considered to be the root causes that eventually induce unstable self-excited vibrations under the operating conditions of small opening angles and high pressure differences.

After the deflection of the wing control surface, flow separation is easily generated at the trailing edge of the wing, which will reduce the lift coefficient and the control surface efficiency. The rudder of the wing is aileron. If the lift generated by the wing is used to improve the efficiency of the control surface, the flow separation caused by the deflection of the control surface must be restrained. Using synthetic jet to change the flow state of boundary layer is the main method to solve the problem of flow separation. Synthetic jet actuator (SJA) has the advantages of no energy loss and simple structure. In this paper, a method of using synthetic jet actuator to suppress the flow separation at the rear of the wing when the aileron deflects is proposed, and the lift coefficient is obtained. The increase of aileron efficiency is calculated by the change of lift coefficient. The EPPLER555 wing with aileron deflection angle of 3°~9° is simulated, and the changes of lift coefficient and aileron efficiency under corresponding working conditions are obtained. The results show that the average lift coefficient of the wing is 0.5 when the deflection angle of the aileron is 3°~9° without SJA. After SJA employed, the lift coefficient will be greatly improved, and the control surface efficiency of EPPLER555 wing will be effectively improved, the lift coefficient will increase by about 20% to 0.6-0.7. For example, when the deflection angle of aileron is 4°, using a SJA with a maximum outlet velocity of 200m/s and an excitation frequency of 400/2π, the effective lift coefficient generated by the wing is 0.5931. Under the effect of SJA, the control surface efficiency of EPPLER555 wing will be effectively improved. The lift coefficient is reflected by the ratio of the change of lift coefficient after SJA employed to the lift coefficient without synthetic jet actuator.

This paper aims to understand the effects of circumferential inlet distortion and tip injection on a transonic impeller performance and flow field. For distorted inflow, the impeller is subjected to a stationary 120-degrees circumferential total pressure distortion. Full annulus unsteady three-dimensional analysis has been used to study the inlet distortion and tip injection effects on the impeller performance, stability and flow field. The results show that the circumferential inlet distortion reduces the impeller total pressure ratio and adiabatic efficiency; however, it has no significant impact on the safe operating range. Unlike the inlet distortion, the tip injection considerably increases the operating range. According to the results, the distortion and tip injection effect on the compressor performance is mainly due to changes in tip leakage flow. The inlet distortion has unfavorable influences on the flow field, especially near the impeller tip; however, the tip injection ameliorates the flow field in this region. In both the clean and distorted inflow, the tip injection causes downstream shock transmission, weakening the shock-tip leakage interaction. Hence, stall inception is postponed, and the impeller stability is improved in the presence of the tip injection.

Superhydrophobic surfaces have attracted great attention owing to their capacity of reducing fluid resistance. Most of the previous numerical simulations on drag reduction of the superhydrophobic surfaces have concentrated on the rectangular microstructures, whereas few studies have focused on the continuous V-shaped microstructures. Based on the gas–liquid two-phase flow theory and volume-of-field model, combined with the semi-implicit method for pressure-linked equations algorithm, the effects of laminar drag reduction for superhydrophobic surfaces with continuous V-shaped microstructures were numerically studied. Three different sizes of superhydrophobic microchannels with continuous V-shapes were simulated according to the experimental data. Results showed that the drag reduction effects of continuous V-shaped microstructures were mainly determined by the width of adjacent microstructures, with the height of the microstructures only having minimal influence. At the same time, the effects of drag reduction for superhydrophobic surfaces with continuous V-shaped microstructures were compared with those with V-shaped and rectangular microstructures. The results indicated that the effects of drag reduction for superhydrophobic surfaces with continuous V-shaped microstructures were obviously better than for those with V-shaped microstructures, whereas the superhydrophobic surfaces with rectangular microstructures were more effective in reducing their drag than those with V-shaped microstructures under the condition of the same shear-free air–water ratios. Therefore, in the preparation of superhydrophobic materials, the continuous V-shaped microstructures are recommended; in addition, increasing the microstructure width should be emphasized in the preparation of superhydrophobic materials with continuous V-shaped microstructures.

The dynamic erosion characteristics of pipe bends exposed to gas-solid two-phase flow are investigated by using an erosion-coupled dynamic meshing method to elucidate the erosion failure phenomenon that is common in pipe elbows, transporting coal fly ash and subjected to particle erosion. The static mesh is compared with the erosion-coupled dynamic mesh method by CFD. The dynamic erosion characteristics of bends with different r/D ratios, D and r are investigated before and after surface deformation under gas-solid two-phase flow. The results lead to the following conclusions Improved performance of the erosion-coupled dynamic mesh by taking full consideration of the coupling between the erosion-induced surface deformation and the particle motion under prolonged erosion. The erosion rate at the elbow changes significantly upon surface deformation, and the sites with a high risk of erosion shift downstream. With increasing of deformation, the larger the r/D ratio, the more obvious the concentration of erosion location evolving downstream. As D decreases, the high-risk erosion areas become more concentrated. In particular, the emergence of the “bending increase” phenomenon leads to a different perception of how r/D ratio and the diameter affect erosion in static-grid simulations: a larger r/D ratio of the elbow makes it more sensitive to surface deformation and increases the erosion rate. This study leads us to consider the coupled deformation of erosion in the context of erosion problems, which has important implications for predicting the service life of overflow components.

In the present work, the effects of modifying the tongue geometry of a centrifugal pump on pressure pulsations under the design and off-design conditions are carried out numerically by the unsteady analysis of fluid flow. Numerical modeling based on the Re-Normalization Group (RNG) k-ε turbulence model using a Mosaic mesh structure, a technology which can easily, quickly and formally connects any type of mesh for complex geometries and flow regimes, is applied to simulate the flow within the modeled pump, which is validated with the available experimental results. The flow is simulated through a commercial Computational Fluid Dynamics (CFD) software that solved Reynolds-Averaged Navier-Stokes (RANS) equations for a three-dimensional unsteady flow. In addition to choosing Qd (the design flow rate), 0.4 Qd and 1.2Qd  are also taken into account as the inlet flow rates. Besides, pressures of 101KPa and 13KPa are considered as additional inlet conditions for this investigation. This unsteady simulation employing different inlet conditions is used to investigate the impacts of various volute tongue angles on the pressure coefficient (cp ). Results indicate that, overall, by changing the angle from 40° to 85°, the value of the pressure coefficient at the pump outlet grows by about 10% where it also causes a rise in the amplitude of pressure fluctuations. By the same token, a decrement to the inlet flow rate up to 40% of the nominal value brings about the amplitude of pressure fluctuations at the pump outlet to be increased significantly.

This study describes the large eddy simulations of a centrifugal pump impeller considering a sinusoidal flow rate and a constant rotation speed. Five different oscillation amplitudes of flow rate (A = 0.1Qd, 0.15Qd, 0.2Qd, 0.25Qd, and 0.3Qd, Qd indicates the design flow rate) are selected to determine the influence of oscillation amplitude on the internal flow characteristics. The simulation results show that, with increasing oscillation amplitude, the alternating stall phenomenon weakens or even disappears during the dropping stage, whereas the opposite trend is observed during the rising stage. The total mean normal vorticity is insensitive to changes in the oscillation amplitude. Moreover, the difference in pressure fluctuations between adjacent passages decreases with increasing oscillation amplitude. The first and second dominant frequencies of the pressure fluctuations are mainly affected by the oscillation amplitude in the non-stall passage. The internal flow exhibits a clear hysteresis effect, and the lag time of the head increases with the oscillation amplitude. Additionally, the average head is approximately 2.38 m, regardless of the oscillation amplitude.

A liquid jet impinging on stationary and rotating superhydrophobic and hydrophilic convex surfaces is experimentally investigated. The effects of the rotation and wettability of the surface and the inertia and impingement rate of the jet on the flow, and the reflection and deflection behavior of the impinging jet are examined. This study examines the effect of air film formation at the constantly regenerating interface between a superhydrophobic surface and a liquid jet. For this purpose, two copper pipes and one plexiglass pipe, which had outer diameters of 8, 22, and 50 mm, were used for the convex surfaces. The copper pipes were coated with a superhydrophobic coating with a 157° apparent contact angle. The uncoated plexiglass pipe had a 73° apparent contact angle. The Reynolds and Weber numbers ranged from 1082 to 3443 and from 3.90 to 35.12, respectively. The liquid jet was sent to the rotating convex surfaces at different impingement rates. The experimental results show that the impinging liquid jet is reflected off the stationary superhydrophobic surface. This reflection behavior is not nearly distributed from the rotation of the superhydrophobic convex surface. The distribution increases slightly with an increase in the Reynolds or Weber numbers, the diameter of the convex surface, and the impingement rate. Nevertheless, the impingement liquid jet is deflected off the stationary hydrophilic surface. This deflection increases considerably with the rotation of the convex surface. The renewal of the air film between the superhydrophobic surface and the liquid significantly reduces the viscous drag force. Therefore, the impinging liquid jet cannot be dragged by the rotating superhydrophobic convex surface.

The smoothed particle hydrodynamics (SPH) method is based on the kernel particle approximation, which is sensitive to the uniformity of the SPH particle distribution in the computational domain; that is, all SPH particles must be distributed evenly in the computational domain. These factors significantly influence the practical application of the SPH method. Meanwhile, calculating the sum near the boundaries of the computational domain may cause boundary defect problems since there are insufficient particles in the support domain, thus often resulting in relatively high errors in numerical simulation results near boundaries. To address these problems, the kernel particle approximation discrete process was corrected based on the traditional SPH method, and the corrected SPH method, the Godunov-type corrective smoothed particle method (CSPM), was formulated by introducing Riemann decomposition. In this study, the traditional SPH method and Godunov-type CSPM method were applied in a comparative study of discontinuous function problems, 1D shock tubes and 1D detonation waves. According to the analysis results, the Godunov-type CSPM method can not only effectively improve the calculation accuracy and compatibility of the traditional SPH method in discontinuous shock wave problems but also increase the accuracy of the traditional SPH method in capturing strong discontinuities.

There has been much recent research on high-speed projectiles entering water, but research on the selection of the material for supercavitating projectiles is limited. Some important properties of such projectiles—mass and moment of inertia, for example—are related to the material density, so the projectile’s density has an important effect on the performance of the supercavitating projectile. This study, using Ansys fluent 19.0 simulation software, studied the details of water entry of four high-speed projectiles of the same shape but made of different materials: aluminum (2.7 g/cm3), steel (7.85 g/cm3), brass (8.5 g/cm3), and tungsten alloy (17.5 g/cm3). The cavity shape, ballistic and hydrodynamic characteristics, and cavity flow field characteristics of projectiles with different densities were analyzed for a water-entry velocity of 600 m/s. The results show that within 3 ms, the velocity of a projectile with a density of 2.7 g/cm3 drops to 171.8 m/s, and the velocity of a projectile with a density of 17.5 g/cm3 drops to 433.1 m/s. Increasing the density of the projectile evidently reduces the deceleration of the projectile. The drag coefficient depends, primarily on the size and shape of the projectile, only slightly on its density. Just after water-entry time, the higher the density of the projectile, the faster the expansion of its cavity wall. As time after water entry increases, the expansion velocity of the cavity wall gradually decreases. The simulation results show that the projectile head experiences the greatest pressure, producing a sharp peak, at the moment when it touches the water surface. During the flow stabilization phase, the lower the density of the projectile, the lower the pressure on the head of the projectile. The results of this study will help to guide the selection of material for supercavitating projectiles.

The generation of shock waves and their repercussions in high-speed vehicles are inevitable. Particularly, the hot plume from the aircraft exhaust ejecting at a high speed, as well as the emitted aeroacoustic noise, have several consequences. Besides, the occurrence of the supersonic core length and the emission of high screech noise are mostly due to the shock cells, prevailing in high-speed jets. Therefore, understanding the shock cell structures developed at the exit of aircraft or rocket nozzles is vital in improving mixing and thereby the noise characteristics. Essentially, non-circular nozzle shapes are well known for enhancing entrainment characteristics and mitigating the noise due to their differential spreading over the nozzle's perimeter. The current study examines the shock structures of circular, elliptic, and square jets at various sonic underexpansion levels. In this investigation, the nozzle geometries are considered to have the same exit area. The Nozzle Pressure Ratio (NPR) was adjusted to 3, 4, and 5 to achieve moderate and highly underexpansion conditions. The shadowgraph visualization method is used to study the development of shock cells from axisymmetric and asymmetric nozzles. It is interesting to observe that the incident and reflected shock structures exist only at moderate and high underexpansion levels. Besides, the elliptic and the square jets have distinctive flow patterns along their different axis planes. The intercepting shock appears on the elliptic jet in the minor axis rather than the major axis direction. The curvature of the intercepting shock wave was found to be greater for the elliptic jet than that for the circular jet. In addition, the square jet in the symmetry plane diverges from the jet centerline, but the jet in the diagonal direction converges. Moreover, the estimated shock cell lengths using shadowgraph images were compared to a theoretical model where the experimentally obtained results are in good agreement with the theoretical values.

The passive suppression of combustion instability by quarter wavelength tube was hereby studied to absorb the oscillation pressure with large amplitudes caused by combustion instability. The suppression effects of quarter wavelength tube on combustion instability were systematically analyzed by combining the acoustic numerical simulation and the experimental research methods. Firstly, the influence of quarter wavelength tube on the acoustic characteristics of the system was analyzed using acoustic numerical simulation; and then, the acoustic absorption characteristics to external acoustic disturbance and the suppression effects on the self-excited combustion instability were experimentally studied. The results show that the quarter wavelength tube can effectively absorb the acoustic pressure when the dominant frequency of acoustic pressure is close to the resonance frequency of the system, and can effectively suppress the combustion instability under acoustic resonance. However, given that the quarter wavelength tube adds adjoint dominant frequencies after eliminating the original resonant frequency of the system, and the combustion instability is stabilized on the adjoint dominant frequencies, combustion instability suppression is different from noise suppression. In addition, the diameter of wavelength tube exercises obvious effects on the above characteristics. All these make it necessary to determine the best parameters and the maximum suppression efficiency by combining numerical simulation and experiments. The research results of this paper provide theoretical and technical supports for the suppression of combustion instability by the quarter wavelength tube.

The improved delayed detached eddy simulation (IDDES) method was employed based on the shear–stress transport (SST) k-w two-equation turbulence model to simulate the slipstream distribution characteristics of a high-speed train traversing a tunnel. The accuracy of the numerical simulation method was verified through a full-scale test. First, the wake vortex structure and the distribution of slipstreams of the train with a streamlined nose length of 7 m running in a tunnel were analyzed. Then, the influence of the streamlined nose length on the wake dynamics and slipstream was compared and analyzed. The slipstream positive peak decreased with increasing distance from the top of the rail and center of the track. As the streamlined nose length increases, the vortex intensity in the wake area weakens; moreover, the influence ranges of the wake vortex and the slipstream positive peak value become smaller. Compared with the results of a train having a streamlined nose length of 5 m, the slipstream positive peak value at 1.4 m from the top of the rail and 100 m from the tunnel entrance decreased by 46.6% from that of a train with a streamlined nose length of 9 m.

This paper discusses the experimental studies performed on a single closed loop pulsating heat pipe (CLPHP) to evaluate its thermal performance. The pulsating heat pipe is brass which has a single closed loop. Aluminium oxide (Al2O3) and deionized (DI) water nanofluid were utilized as working fluids, with different volume concentrations of aluminium oxide nanoparticles of 0.05 %, 0.5 % and 1%. The aluminium oxide particles are mixed with water in the two-step method to produce a stable suspension. Experiments are carried out in the horizontal mode with watt loads of heat inputs ranging from 10 w to 100 w. The temperature differences between the evaporator and condenser portions, thermal resistance, heat transfer coefficient and thermal conductivity are the parameters used to determine thermal performance. The thermal resistance of aluminium oxide and DI water nanofluid was the lowest, having 48 % less than that of water. The effective thermal conductivity of the heat pipe improves as the concentration of nanoparticles increases. The comparison between experimental results and computational fluid dynamics (CFD) simulation results CLPHP was carried out under the same condition.