Journal Of Applied Fluid Mechanics
Volume:14 Issue: 2, Mar-Apr 2021

The application of the high-order accurate schemes with multi-block domains is essential in problems with complex geometries. Primarily, accurate block-interface treatment is found to be of significant importance for precisely capturing discontinuities in such complex configurations. In the current study, a conservative and accurate multi-block strategy is proposed and implemented for a high-order compact finite-difference solver. For numerical discretization, the Beam-Warming linearization scheme is used and further extended for threedimensional problems. Moreover, the fourth-order compact finite-difference scheme is employed for spatial discretization. The capability of the high-order multi-block approach is then evaluated for the onedimensional flow inside a Shubin nozzle, two-dimensional flow over a circular bump, and three-dimensional flow around a NACA 0012 airfoil. The results showed a reasonable agreement with the available exact solutions and simulation results in the literature. Further, the proposed block-interface treatment performed quite well in capturing shock waves, even in situations that the location of the shock coincides with block interfaces.

The conventional method to promote mass, momentum and energy transport between fluid particles is to introduce a disturbance to the flow. An ultrasonic velocity profiler, UVP experiment was used to study the mean and fluctuating flow properties in the near wake of the rigid and flexible protruding surface in a water tunnel under the Reynolds number of 4000, 6000 and 8000. In the current study, circular finite cylinders (cantilevers) with various aspect ratios (AR = 10, 12, 14 and 16) and materials were used as the geometry of the rigid and protruding surface. The motion of the cylinder alters the fluid flow significantly. The increment of the wake region (~10% larger in the flexible cylinder compared to the rigid case, for AR = 16) is due to the weakening of the influence of downwash caused by the stream-wise deflection of the flexible cylinder. As a mean to quantify turbulence, the turbulent intensity, Ti, was studied. In general, the flexible cylinders show better capability in augmenting the turbulence than the rigid cylinder. The stream-wise turbulent intensity for AR = 16 and Re = 8000 can be as high as 97% for the flexible cylinder compared to only 26% for the rigid case. The normalized amplitude response graph, which records the cross-flow oscillation of the flexible cylinder was also analyzed. Under the same Reynolds number, the turbulence enhancement increases with the structural velocity. An organized oscillating motion is in favor of a higher performance of turbulence enhancement.

This study aims to explore equivalence between active and passive flow control techniques in reducing the wave drag and surface heat flux over a blunt cone model kept in Mach 8 stream. Computational investigations were carried out by using finite volume-based compressible flow solver. Throughout the study, the solution of governing equations is sought by assuming two dimensional-axisymmetric nature of the flowfield. Both counter flow-stagnation point injection and forward facing-physical spike are considered to mitigate the excess drag and heat flux experienced by a blunt body representing the nose cone section of a hypersonic vehicle. Eventually, based on identified drag reductions, the present study proposes equivalence cases between these two methods. It is shown that a pointed spike of L/D=1 provides almost the same drag reduction as the counterflow injection jet with a pressure ratio of 8.25. Similarly, other equivalence cases are identified and the physics behind them is explored. The identified equivalence is expected to help the designers in effectively replacing one technique with another according to the requirement. Equivalence matrix is presented for different spike cases in terms of injection ratios of counterflow injection.

To study the periodically time-varying dynamic characteristics of static eccentric squeeze film damper (SFD), two models of computational fluid dynamics were established by dynamic mesh method of transient flow field. Numerical investigation was carried out for periodically time-varying dynamic characteristics of static eccentric SFD and the results were verified by theoretical formulas. As shown by the study, due to the presence of static eccentricity, oil-film damping and stiffness present distinct periodically time-varying characteristics with precession angle and their peak values occur in different circumferential positions; excessive static eccentricity will significantly alter the distribution of oil film pressure and direction of radial force of oil film. For SFD in practice, when dynamic eccentricity is smaller, the periodically time-varying characteristics are subjected to the dual influence of oil supply hole and static eccentricity. With the augment of dynamic eccentricity, the influence from oil supply hole is reduced, and the periodically time-varying characteristics are mainly embodied as the influence of static eccentricity.

In this article, the calm water resistance and dynamic instabilities of a semi-displacement catamaran fitted with a stern wedge is investigated using an experimental method and numerical technique. This is accomplished in order to probe into the effects of aft geometry modification on semi-displacement ship dynamic characteristics, especially at medium and high speeds. An advanced 6-DOF model that takes into consideration the dynamic mesh method has been utilized in open source code OpenFOAM. Reynolds-Average Navier-Stokes (RANS) equations are solved using standard k-ε turbulence model and VOF method. The accuracy of the current numerical method is investigated by the calm water test in National Persian Gulf Towing Tank. The resistance, trim and sinkage of the ship were monitored during the experiments. The experimental analysis was performed on the initial model and a modified model with 8º wedge at different Froude numbers. After that, the wedges were mounted at different angles at the transom of the vessel and the effect of the angle change for 4 different angles was evaluated using numerical solution. The results show that fitting a stern wedge to this type of ship causes an intense pressure at the stern bottom. Also, it decreases the dynamic trimming and forward resistance of the craft. As well as, stern wedge causes increasing the lift force which affects the reduction of dynamic instabilities. It is concluded that numerical model presented here is quite suitable for accurately predicting dynamic characteristics of a semi-planing twin-hull ships at medium and high Froude numbers. As a result, 14% reduction in total resistance was observed due to the installation of a 6 degree stern wedge.

The study of corrugated wings has become acquainted in the field of insect flight in recent times. Recent studies on the aerodynamic effects of a corrugated wing are based on insects like the Dragonfly; whereas the likes of Fruitfly (Drosophila Melanogaster) usually go unobserved due to their smaller size. Consequently, the behaviour of these corrugations is found to be anomalous especially in the low and ultra-low Reynolds number region. Therefore, the main aim of this study is to understand the aerodynamic effects of the corrugated airfoil present in the wing of a Fruitfly; by conducting a geometric parametric study during a static non-flapping flight at 1000 Re. In this computational study, a 2-D section of the corrugated wing along the chord is considered. The parametric study helps in understanding the effects of varying number of corrugations, angle of corrugations and the presence of a hump at the trailing edge. The dimensions were scaled to a suitable reference value to additionally compare the corrugated airfoil of Fruitfly to that of a Dragonfly. The present study shows that the aerodynamic performance of the corrugated wing in terms of cl and cd are predominantly governed by the subtle geometric variations that can largely impact the formation of bubbles, vortex zones, and their mutual interaction. The reduction in the number of leading edge corrugations improved the cl/cd ratio and reduction in the corrugation angle helped produce higher lift. The presence of a trailing edge hump also improved the stall angle with a better flow re-attachment. The presence of corrugation at the trailing edge proved to be more beneficial compared to the model with corrugations at the leading edge. This also helped in understanding, the aerodynamic superiority of the trailing edge corrugations present in the Dragonfly's wing when compared to the Fruitfly's.

Chilled water energy storage using thermal stratification technique currently used in the vast area because it contributes to reducing energy consumption and refrigeration capacity as well as its maintenance, operating and capital costs are low. In this paper, experimental tests were carried out on a small-scale vertical cylindrical storage tank equipped with an elbow-type conventional diffuser at inlet heights of 20, 170, 320 and 470mm for flow charging rates from 1.5-7.5l/min. in order to obtain a good thermal separation. The degree of stratification was estimated by means of temperature distributions and performance metrics, which involve thermocline thickness, the half-cycle figure of merit and equivalent lost tank height. The results show that the decrease in diffuser height above the tank floor tends to the steep thermocline or satisfactory thermal separation, the stratification and thermal performance were obtained at diffuser height of 20 mm within the limiting volume flow rates 1.5-4.5 l/min. better than those at volume flow rates ranging from 5.5-7.5l/min. and much better than at diffuser heights of 170, 320 and 470mm for various flow rates.

In order to study self-starting characteristics for H-type wind turbine, firstly, the effect of low Reynolds number and large separated flow on aerodynamic characteristics of airfoil were analyzed in detail, then two H-type wind turbines with different aerodynamic configurations were tested in a low speed wind tunnel for collecting the static torques at different phase angles and time-rotating speed curves in starting process. Based on theoretical analysis and experimental data, the cause of self-stating problem of H-type wind turbine has been revealed. The aerodynamic profile parameters of the wind turbine are closely related to the dependency of starting on initial phase position, and the minimum static torque determines whether the wind turbine has potential to start from rest. The time-rotating speed curves exhibit two different starting behaviour features, determined by the minimum dynamic torque in driving force conversion stage. Unless both the minimum static torque and minimum dynamic torque in driving force conversion stage are greater than the friction torque, the self-starting of the wind turbine cannot be realized. The typical self-starting behavior characteristics is that the time-rotating speed curve includes four different stages of initial linear acceleration, plateau, rapid acceleration and stable equilibrium with the final tip speed ratio more than 1.

Laminar, transient forced convection problem over a 2D backward facing step (BFS) at an inlet Reynolds number (Re) of 400 is investigated numerically using OpenFOAM. To increase the Nusselt number (Nu) along the bottom wall, active flow control is applied by zero-net-mass-flux (ZNMF) combinations of suction and injection through three thin slits which are placed on the top, step and the bottom walls in the vicinity of the BFS. The combinations of each jet velocity is determined by jet to inlet mean velocity ratios which are limited to integer numbers between -2 and 2 and satisfying ZNMF condition where negative and positive values indicate suction and injection, respectively. All 19 cases which satisfy these rules are investigated. Average Nusselt number, friction coefficient and recirculation zone lengths are calculated along the bottom wall from time averaged flow fields. Among 19 cases with each having different jet configuration, some cases converged to steady state solution while others indicated temporal effects and converged to periodic solutions. To understand these transient effects, velocity oscillation magnitude and Strouhal number which are monitored at a selected critical point are evaluated. It is shown that temporal interaction of chosen active flow control methodology has significant effect on enhancing mixing which results in an increase of Nusselt number. Among all cases, the best case concerning thermal improvement has an increase of 78.5% in Nu number while the best aerodynamic improvement is achieved for another case with a decrease of 81% in total recirculation zone length compared to the reference case where no control is applied.

In this research, natural convection of power law fluid in a square cavity, with a porous deposit in the shape of a semi-cylinder is studied numerically, using the multiple-relaxation-time lattice Boltzmann method. The modified Darcy-Brinkman model is applied for modelling the momentum equations in porous medium and the Boussinesq assumption is adopted to model the buoyancy force term. The influences of power law index (0.6 ≤ n ≤ 1.4), Darcy number (10−5 ≤ Da ≤ 10−2 ), Rayleigh number (103 ≤ Ra ≤ 106 ) and the radius ratio of the semi-cylindrical porous deposit (0.05 ≤ R ≤ 0.5) on hydrodynamic and heat transfer are studied. The obtained results show that these parameters have an important effect, on the structure of hydrodynamic and thermal transfer. The improvement of the power law index leads to a decrease in the heat transfer rate, illustrated by the average Nusselt number, and the augmentation in Darcy number induces an increase in that rate. Moreover, the variation of the Rayleigh number and the porous deposit radius has a significant effect on the transfer rate and convective structure. Besides, an unusual phenomenon is noticed for high Rayleigh numbers, where a better heat evacuation from the porous deposit is noticed for the dilatant fluid compared to the pseudoplastic one.

In the present study, the effect of the use of false-ceiling on fire-induced smoke flow characteristics in tunnels is investigated using a 3D developed computational fluid dynamics tool. The critical velocity, the minimum required tunnel ventilation velocity to stop the smoke flow from moving toward the tunnel inlet (toward the upstream of the fire source), and the maximum gas temperature beneath the ceiling are selected to evaluate the smoke flow control in presence of the false-ceiling. The hydraulic height of the cross-sectional geometry of the tunnel is used as the characteristic length in order to dimensional analyze and compare the nondimensional results. The results indicate that the use of the false-ceiling reduces the critical velocity and the smoke backlayering, while increases the maximum ceiling gas temperature. Reducing the critical velocity results in ventilation cost reduction (positive impact), while increasing the maximum gas temperature beneath the ceiling increases the risk of instrumental and life damages (negative impact). The detailed results and corresponding physical discussions are presented to clarify the reason for the significant differences between the results of the tunnels with and without false-ceiling.

Flapping foil energy harvesting systems are considered as highly competitive devices for conventional turbines. Several research projects have already been carried out to improve performances of such new devices. This paper is devoted to study effects of non-sinusoidal heaving trajectory, non-sinusoidal pitching trajectory, and the effective angle of attack on the energy extraction performances of a flapping foil operating at low Reynolds number (Re=1100). An elliptic function with an adjustable parameter S (flattening parameter) is used to simulate various sinusoidal and non-sinusoidal flapping trajectories. The flow around the flapping foil is simulated by solving Navier–Stokes equations using the commercial software Star CCM+ based on the finitevolume method. Overset mesh technique is used to model the flapping motion. The study is applied to the NACA0015 foil with the following kinetic parameters: a dimensionless heaving amplitude h0 = 1c, a shift angle between heaving and pitching motions  = 90°, a reduced frequency f * = 0.14, and an effective angle of attack αmax varying between 15° and 50°, corresponding to a pitching amplitude in the range 0 = 55.51° to 99.51°. The results show that, the non-sinusoidal trajectory affects considerably the energy extraction performances. For the reference case (sinusoidal heaving and pitching motions, Sh = S =1), best performances are obtained for the effective angle of attack, αmax = 40°. At small effective angle of attack αmax <30°, the non-sinusoidal pitching motion combined with a sinusoidal heaving motion, greatly improves energy extraction performances. For αmax = 15°, Sh = 1 and S = 2, energy extraction efficiency is improved by 52.22% and the power coefficient by 70.40% comparatively to sinusoidal pitching motion. At high effective angles of attack (αmax > 40°), non-sinusoidal pitching motion has a negative effect. Performances improvement is quite limited with the combined motions non-sinusoidal heaving/sinusoidal pitching.

Cardiovascular disorders are among the most important causes of sudden death and adult disability worldwide. Abdominal aortic aneurysm (AAA) is a critical clinical condition where the aorta dilates beyond 50% of its normal diameter and leads to a risk of rupture. In this study, we performed fluid-structure interaction (FSI) analysis on an eccentric computational AAA model in order to investigate the effects of wall thickness on AAA wall stresses, which are critically important to estimate the rupture risk. For this purpose, we modeled the problem domain using finite element analysis, and coupled the solutions of fluid and structure domains for improving the accuracy of results. ANSYS commercial finite element analysis software was used for modeling, solving, and post-processing the results. Expanded diameter in AAA sac resulted in altered hemodynamics. Wall shear stresses (WSS) caused by the flow are quite low on the AAA sac, which may deteriorate the endothelial cell regeneration and vascular remodeling in the long term. It is concluded that the most critical region for the rupture risk is the posterior distal end of AAA sac due to being exposed to peak mechanical stresses during the cardiac cycle. Obtained results shed light in understanding the rupture risk assessment of AAA.

Flow past a heated cylinder kept at constant surface temperature is computationally simulated and analyzed in the laminar regime at moderate buoyancy. In this study, we have restricted to moderate Reynolds numbers to completely eliminate the presence of mode-A and mode-B instabilities. The three dimensional transition due to the mode E instability is captured using a cell-centered finite volume method. The present study reveals the existence of two different kinds of coherent structures - the “surface plumes” and the “mushroom structures”. The role of these mushroom structures in the heat transfer mechanism and the changes that the Prandtl number would bring into this coherent structure are discussed. The mushroom structures observed show high dependency on the changes in Prandtl number whereas the surface plumes are found almost unaffected.

The purpose of this work isto investigate transient aerodynamic characteristics of the coach under the crosswind in straight-line situations with different uniform speeds and uniform accelerations. The transient aerodynamics caused by different speed changes are analyzed using the real-time interaction between aerodynamic simulation and dynamic simulation. The target model is a simplified coach on a full scale. The SST (Menter) K-Omega Improved Delayed Detached Eddy Simulation and overset mesh technique are used to predict the transient aerodynamic loads. The accuracy of the turbulence model is verified by a wind tunnel experiment of the 1/7th scaled coach model. The present results show that the transient aerodynamic loads have different locations of maximum side force and the holding duration of yaw moment for different constant speeds. The speed becomes larger, and the position where the side force is maximum becomes farther away. The holding duration of the top yaw moment is larger simultaneously. Moreover, proper acceleration for low initial driving speed and crosswind of small influence range could build up stability. High speed driving in gust wind is not suggested for unskilled drivers.

In this study, uncertainty analysis of the vortex-induced vibration (VIV) tests, using a VIV test rig is presented. The VIV test rig is set up on the circulation channel in Ata Nutku Ship Model Testing Laboratory at Istanbul Technical University (ITU). The tests are performed using an elastically mounted rigid and smooth circular cylinder in low mass-damping and high Reynolds numbers conditions. The cylinder has one-degree-of freedom. It is allowed to move perpendicular to the flow while inline vibrations are constrained. The aim of the study is to demonstrate and establish a repeatable procedure to predict the uncertainty of VIV tests, utilizing some example applications of existing ITTC recommendations. Within this aim, five distinct VIV tests are carried out following ITTC guidelines and procedures measuring the amplitude (A * ) and frequency response (f * ) data. Uncertainty analysis study is performed for three different flow velocities, chosen from VIV tests and total uncertainty is calculated by root mean square values of precision and bias uncertainties. The precision uncertainty is predicted using response amplitude values obtained from five sets of VIV tests. The bias uncertainty is predicted utilizing the basic measurements and test results of the components of response amplitude for the cylinder. The results have demonstrated that the current test rig has low uncertainty level. Additionally, it has succeeded to reflect the characteristics of VIV phenomenon in the studied Reynolds number range, which is in the Transition Shear Layer 3 (TrSL3) flow regime. Consequently, it is believed that this study would help in spreading the application of the uncertainty analysis for VIV tests in the future.

As a cost-effective option for subsea gas and oil fields development, multiphase pump has been widely used and plays an important role in promoting field production especially for those brownfields. The objective of this research is to study the variations of flow parameters at the inlet of each stage in a five-stage helico-axial pump under design conditions by using commercial CFD packages. The numerical results show that inlet volume flow rate, gas volume fraction (GVF) and flow angle of each stage both decreases from the first stage to the fifth stage because of the compression. The variations of these parameters along the flow direction are susceptible to the effect of rotational speed and initial gas volume fraction at inlet of the first stage. These inlet flow parameters from inlet to outlet of the five-stage pump just change little when the initial inlet GVF is lower than 10% or higher than 90%. While the variation is obvious when inlet GVF is from 20% to 80%. Based on numerical simulation results, the stage-by-stage design method is proposed and geometry parameters such as inlet impeller hub diameter and inlet angle of blade in the second stage are modified according to its inlet flow conditions. The comparison results between the modified pump and original pump show that pressure distribution at leading edge of impeller blades in each stage becomes uniform in the modified multistage pump. The inlet incident flow loss of each stage is diminished and internal flow conditions has been significantly improved. Thus, the pump’s boosting capacity is enhanced. These research results in this paper are instructive for the performance optimization of multistage pumps used in gas oil fields.

In the oil-gas mixture transportation system of offshore oilfields, it is of great theoretical and practical significance to study the flow characteristics of the slug flow and its suppression or elimination. In this paper, the characteristics of the slug flow and microbubbles in a laboratory-scale rig are experimentally studied and analyzed by a multi-parameters measurement system including electrical resistance tomography (ERT), highspeed camera, and traditional pressure sensor. The suppression of the microbubbles on the slug flow formation is further investigated. Experimental results showed that the bubbles with different sizes from the microbubble generator have different aggregation and dispersion characteristics. The microbubbles can suppress the formation of the slug flow by increasing the liquid slug pressure and further affect the motion of the slug by enhancing the disturbance effect of the boundary layer, so as to achieve the suppression on slug flow.

Though a simple daily observation, evaporation of drops is still poorly understood due to the complex nature that involves hydrodynamic effects in the bulk fluids and transport phenomena at the liquid-vapor interface. This paper reports on the evaporation of single component droplets (water, ethanol, acetone, and glycerol) levitated in a single-axis non-resonant levitator. It was observed that the acetone and ethanol evaporated faster than water, although the acetone is the most volatile. The estimated lifetime of acetone is less than 5min, which is much shorter as compared to 56min for ethanol or about 90min for water droplets. On the other hand, glycerol showed no tendency to evaporate. With increasing the evaporation time, the ratio of large and small semi-axis decreases and tends to 1 corresponding to changes in drops shape from oblate ellipsoid to a sphere. Based on the classical D2 -law, the surface regression rates have been estimated.

The rotor-stator interaction between the impeller and the volute is the main reason for the pump pressure pulsation and vibration. This work aims at designing a new type of tongue to minimize pressure pulsation, reduce vibration noise and energy loss. Inspired by the humpback pectoral fin, four volute tongues are investigated in this paper, three of which are sinusoidal tubercle volute tongues (STVT) and one is the original volute tongue (OVT). Based on the detached-eddy simulation (DES) turbulence model, the influence of the sinusoidal tubercle volute tongues on the pressure pulsation was investigated, and the flow structure and enstrophy of the four pumps were analyzed, aiming to minimize pressure pulsation, maximize hydraulic performance and reduce the energy dissipation in centrifugal pumps. The results show that the pressure pulsations of the STVT profiles are all lower. The reductions of the average pressure pulsation at the monitoring points are 13.3% (STVT-1), 20.6% (STVT-2), and 16.2% (STVT-3), respectively. The difference in pressure pulsation at the monitoring points closer to the tongue is more obvious. At the design flow rate, the efficiencies of the three bionic pumps are increased by 0.5% (STVT-1), 1.5% (STVT-2), 0.9% (STVT-3), respectively. The STVT profiles change the vortex structure near the tongue and minimized the vortex strength. Meanwhile, the flow in the pump with the STVT profiles have lower enstrophy. The enstrophy of the flow with the STVT-2 profile is the lowest, which is reduced by about 8%. This reduces the dissipation of mechanical energy. The results can be used as a guide for pump design optimization.

The Buoyancy-driven flow of two immiscible liquids having varying density and viscosity is studied in a threedimensional inclined confined channel. Initially, the heavier/lighter liquids occupy the upper/lower parts of the channel, respectively, which is an unstable configuration. The numerical simulations are performed using a multiphase lattice Boltzmann method (LBM) that is further implemented on the graphics processing unit (GPU). The three-dimensional flow dynamics and the associated physics are studied based on various parameters such as viscosity ratios (m), Atwood numbers (At) and Reynolds numbers (Re). The results were presented in the form of iso-surface/contour plots, average density profiles, and lengths of interpenetration. It is observed that larger interpenetration occurs with iso-viscous liquids having higher density gradients (higher At). The Reynolds number had a non-monotonic effect on the axial lengths of interpenetration (Lp∗); Lp∗ increases till Re = 500 and then decreases for Re = 1000. At larger Re, due to the development of KelvinHelmholtz instabilities higher transverse interpenetration is observed.

Excess Flow Valves (EFV) for gas-stop systems is generally used in natural gas pipelines to prevent possible damages or destruction due to gas leakage. It can be used in a wide operating range of pressure, but the shutoff flow rate could be in various values at different pressures since natural gas can easily be compressed and can reach higher density. In this study, shut-off and nominal gas flow which effect on a spring force attached to an EFV system simulation by using Fluid Solid Interaction (FSI) strategy was studied. Furthermore, User Define Function (UDF) adapted to simulation to obtain the time-dependent deformation of the spring. The simulations were repeated at five different operating pressures (1-5 bar) with changing flow rates to show if EFV can shut-off the system or not. Results were validated against experimental data of the EFV to show the consistency of the FSI strategy. Moreover, detailed behaviour information of the EFV obtained by means.

This present investigation inspects the mixing promoting the efficacy of two short delta tabs which is axissymmetric, mounted circumferentially antipodal at the end for a Mach number 1.8 convergent-divergent circular nozzle computationally with the nozzle pressure ratios (NPRs) ranging from 4 to 8 with a unit step of one that covers all the critical states of the jet i.e., the overexpanded, the correctly-expanded and the underexpanded states of the jet. In order to minimize thrust loss, the geometric blockage offered by each delta tab is kept within 2.5%. The computational assessment is conducted by adopting and employing ANSYSFLUENT which is a comprehensive engineering simulation software. Further, the entire steady flow computations are carried out on a three-dimensional numerical enclosure by implementing Reynolds-averaged Navier-Stokes equations along with the κ − omega shear stress transport turbulence model. Interestingly, vital plots including the centerline pressure decay as well as the pressure profiles are depicted for uncontrolled and controlled jets accordingly. Also, numerically obtained schlieren illustrations are adopted for visualization of the shock cell structure, expansion fan, and the Mach wave structure existing in the stream field. Furthermore, Mach variations are also depicted for the varied nozzle pressure ratios in the form of contours. The shockstrength, shock-length, and the progressive disparity found in the shock structures are reasonably demonstrated by the Mach contours. The results of this research are discovered to be in sensible concurrence with the earlier established exploratory results. A maximum core length reduction of 70.81% is observed in underexpanded condition at the nozzle pressure ratio of 6. Absorbingly, a controlled jet has been seen to get split in equal proportion along the succeeding direction of the nozzle exit at a distance of approximately 5De, De indicating the exit diameter of the nozzle. Moreover, it was appealing to detect the development of jet dispersion along the succeeding direction of the nozzle exit periphery. The short delta tabs also performed satisfactorily in diminishing the waves and reducing the shock cell length as depicted via numerical schlieren images

Computational analysis is performed on a centrifugal compressor fitted with tapered vaneless diffuser in order to increase the rate of diffusion. The main parameter involved in the present study is the wall taper angle of the diffuser, which is varied from 1° to 6° in the interval of 1°. Simulations are performed for the stationary as well as rotating diffuser at a speed of 79,000rpm, by using ANSYS CFX 17.2. By considering the geometry with stationary parallel wall diffuser as the base case, the performance enhancement in the characteristics such as static pressure recovery coefficient, stagnation pressure loss coefficient, isentropic efficiency, energy coefficient and torque coefficient are reported. The flow features in the compressor having various diffuser geometries are studied with the help of static pressure, radial velocity, static entropy, and contours of velocity streamlines at the design point. Of all the cases of stationary tapered diffusers, the diffuser with 3° taper angle showed optimum performance: the increase in isentropic efficiency (η) is by 1.5%, the increase in static pressure recovery coefficient (CP) is by about 9% and the decrease in stagnation pressure loss coefficient (CPOL) by 10.7%. On the other hand, it was found that in the case of rotating diffuser optimum performance: an increase of about 40% in CP and decrease of about 32% in CP0L occurred for a taper angle of 6°. However, its efficiency decreased by 2.9% with rotating diffuser in comparison with the base case, due to increased energy losses

The development of high-performance aero engines put forward higher performance requirements for compressor components, and the improvement of aerodynamic performance of compressors has important engineering application value. The blade tip recess has great potential and advantages in improving the aerodynamic performance of compressors. In order to better understand the effect of the blade tip recess on the compressor aerodynamic performance, in this paper, the influence mechanism of the blade tip recess on the aerodynamic performance of the isolated rotor of a transonic axial compressor stage is discussed. Under the premise that the numerical method's results are almost consistent with the experimental test results, the full three-dimensional unsteady numerical results show that the main reason for the original blade rotor stall is the leading edge of the blade tip blockage, which is caused by blade tip clearance leakage vortex breakage. After adopting the measures of the blade tip recess, the study shows that the blade tip recess can increase the rotor stall margin by 2.10% without reducing the rotor efficiency and the total pressure ratio. A detailed analysis of the blade tip flow field shows that the blade tip recess can reduce the intensity of the tip clearance leakage flow by increasing the turbulence intensity of the blade tip near the casing wall, and reduces the leading edge of blade tip blockage, improves the rotor blade tip flow field, thereby achieving the purpose of enhancing rotor stability.