Among all aquatic species, mantas and rays swim by flapping their pectoral fins; this motion is similar to other fishes in terms of efficiency, but it gives better maneuverability and agility in turning. The fin's motion is featured by a traveling wave going opposite to the forward motion, producing a force thanks to momentum conservation. This article aims at understanding the swimming dynamics of rays, focusing on energy efficiency. A computational fluid dynamics (CFD) model of the swimming motion of a cownose ray has been implemented in openfoam , simulating the acceleration of the fish from still to the steady-state velocity using an overset mesh. In this analysis, the one degree-of-freedom dynamics of forward swimming is solved together with the fluid velocity and pressure. The effect of frequency and wavelength of fin motion on thrust, power, and velocity has been investigated and an analysis of the vortices in the wake showed has been performed. The energy efficiency of a self-propelled body has been defined in a novel way and it has been calculated for different motion conditions. The results showed that batoid fishes swim with high energy efficiency and that they are a promising source of inspiration for biomimetic autonomous underwater vehicles.

A vortex pump with a specific speed of 76 was studied in its turbine mode by using fluent 6.3 based on the steady, three-dimensional, incompressible, Reynolds time-averaged Navier–Stokes equations, standard k - ε turbulence model, and nonequilibrium wall function in multiple reference frame system. The performance and flow structure of six liquids with different densities and viscosities were characterized, and the hydraulic, volumetric, and mechanical losses were discomposed. The correction factors of flow rate, head, shaft-power, efficiency, and disk friction power in turbine mode were correlated with impeller Reynolds number at three operational points. The conversion factors of flow rate, head, efficiency from the pump mode to the turbine mode were expressed with Reynolds number and compared with the counterparts of centrifugal pumps in the literature. It was indicated that the vortex pump can produce power as a turbine but becomes inefficient with increasing viscosity or decreasing impeller Reynolds number, especially as the number is smaller than 10 4 due to increased hydraulic, volumetric, and mechanical power losses. A vortex structure with radial, axial, and meridian vortices occurs in the impeller at different flow rates and viscosities. The incidence at blade leading edge and deviation angle at the blade trailing edge depend largely on flow rate and viscosity. The impeller should be modified to improve its hydraulic performance under highly viscous fluid flow conditions. The entropy generation rate method cannot demonstrate the change in hydraulic loss with viscosity when the Reynolds number is below 10 4 .

Unsteady three-dimensional flow simulations on a twin-screw pump are performed for an assessment of the radial, circumferential, and flank gap flow effect on the pump performance. By means of the overset grid technique, rigid computational grids around the counter-rotating spindles yield a high cell quality and a high spatial resolution of the gap backflow down to the viscous sublayer in terms of y + < 1 . An optimization of the hole-cutting process is performed on a generic gap flow and transferred to the complex moving gaps in the pump. Grid independence is ensured, and conservation properties of the overset grid interpolation technique are assessed. Simulation results are validated against measured pump characteristics. Pump performance in terms of pressure build-up along the flow path through the spindles and volume flow rate is presented for a wide range of spindle speed and pump head. Flow rate fluctuations are found to depend on head but hardly on speed. By a profound assessment of the respective radial, circumferential, and flank gap contribution to the total backflow, the importance of the most complex flank gap is pointed out. Backflow rate characteristics in dependence on the pump head and the pump speed are presented.

The flashing flow in a Moby_Dick converging-diverging nozzle under pressurized hot water from 460.5 to 483.5 K is simulated using a homogeneous compressible water-vapor two-phase flow model. The kinematic and thermodynamic mass transfer are accessed using the cavitation model based on the Hertz–Knudsen–Langmuir equation. Our simplified thermodynamic model is coupled with the governing equations to capture the phase-change heat transfer. This numerical method proved its reliability through a comparison with available experimental data of flow parameters inside the nozzle. Consequently, the present numerical method shows good potential for simulating the flashing flow under pressurized hot water conditions. The satisfying prediction of averaged flow parameters with a slight improvement compared to reference numerical data is reproduced. The results confirm a noticeable impact of the thermodynamic effect on the mechanism of flashing flow, resulting in a considerable decrease in the flow temperature and the saturated vapor pressure. The flashing nonequilibrium is significantly decreased, forcing the flashing flow to be classified as the usual cavitation behavior and better suited to homogeneous model. While the temperature drop is highly dependent on evaporation, the thermodynamic suppression is influenced by the condensation. The suppression effect, unobserved in water at a lower temperature in previous studies, is noticeable for the pressurized hot water flow characterized by the cavitation mechanism. The vapor void fraction decreased considerably in the radial and axial directions as the water temperature rose to 483.5 K in this study.

This work presents an analytical study of the flow and energy loss immediately downstream of rectangular sharp-crested weirs for free and submerged flows, using the theory of plane turbulent jets and the analysis of some relevant studies. The flow regimes downstream of the sharp-crested weir is characterized as the impinging jet and surface flow regimes. Based on the flow characteristics and the downstream tailwater depths, each flow regime is further classified, and the relative energy loss equation is developed. It is found that significant energy loss occurs for the regime of supercritical flow and the upper stage of impinging jet flow. The energy loss for the submerged flow regime is minimal.

In this study, we focus on the response of biological, rheological, and physical properties of dilute suspensions of cyanobacterium Synechocystis sp. CPCC 534 to shear induced by stirring. Experiments were carried out at three different stirring rates in well-controlled conditions, and the results are compared with stationary conditions where only molecular diffusion and cell motility govern the transport phenomena and cell growth. Our results show that the growth, biomass, total chlorophyll, and carotenoid production of Synechocystis sp. under various shear conditions were improved significantly, and the yield was nearly doubled. The viscosity of Synechocystis suspensions, subjected to different shear rates, was also measured. The data showed Newtonian behavior for suspensions at different cell concentrations. Cell concentration showed a noticeable increase in the viscosity of suspensions. However, we observed that this increase was smaller than the one predicted for a suspension of hard spheres. Addition of shear to the cyanobacterium Synechocystis sp. culture demonstrated a positive impact on the production of value-added products from the micro-organism. The obtained results can be used to improve the bioreactor design for better productivity.

The steam trap valve is used in thermal power systems to pour out condensate water and keep steam inside. While flowing through steam trap valves, the condensate water can easily reach cavitation, which may cause serious damage to the piping system. In this paper, in order to control cavitation inside steam trap valves, effects of sleeve parameters, including orifice diameter, installation angle and thickness, are investigated using a cavitation model. The pressure, velocity, and vapor distribution inside valves are analyzed and compared for different sleeve geometrical parameters. The total vapor volumes are also predicted and compared. The results show that the sleeve parameters have a significant influence on the cavitation intensity and cavitation vapor distributions. Specifically, the orifice diameter of the sleeve has much larger effect on each aspect than that of other two geometrical parameters of the sleeve. The improved geometrical parameters of the sleeve are determined to suppress the cavitation inside the valve. The sleeve with smaller diameter orifices, higher installation angle (maximum 80 deg), and higher thickness is recommended in practice for better anticavitation performance. The work is of significance for cavitation control and the optimization design of steam trap valves.

An experimental investigation on the flow separation of a hypersonic laminar boundary layer flow over a compression ramp with angles of 32 deg, 36 deg, and 40 deg is carried out in a Mach 5 hypersonic wind tunnel. The detailed structures are measured by particle image velocimetry (PIV), and some typical flow structures, such as a shear layer, separation shock, recirculation zone and reattachment shock, are clearly captured. In the 32 deg compression ramp flow, the hypersonic laminar flow does not experience flow separation, and the boundary layer always attaches to the ramp surface. When the ramp angle increases to 36 deg, a typical flow separation appears in the hypersonic laminar flow, and a shear layer and reattachment shock arise within the flow field. As the ramp angle increases to 40 deg, the separation shock moves upstream, the reattachment shock moves downstream, and the recirculation zone expands. Proper orthogonal decomposition (POD) analysis is performed on the velocity contours for three cases, revealing the spatial structure of the flow field. As the ramp angle increases, the coherent flow structures are captured more effectively by less POD modes, and there are more coherent structures in the flow field of a large-angle compression ramp. Finally, numerical investigations of the flow separation on three different compression ramps are carried out, and the simulation results are consistent with the measurement results.

Experimental studies are carried out on an axisymmetric cylindrical base body for six freestream Mach numbers between 0.54 and 1.41. Unsteady pressure is measured on the base surface using high-frequency response Kulite pressure transducers. The effect of passive flow control devices on the mean base pressure and the unsteady characteristics of base pressure have been studied. A blunt base, a conventional cavity device, and three different ventilated cavity devices have been tested along with four different rounded base lip devices. A total of 20 different base geometric modifications are tested at 6 freestream Mach numbers resulting in 120 test cases. The cavity devices improve the base pressure as compared to the blunt base case, particularly for freestream Mach numbers more than 0.98. Among all the cases considered, a maximum increase of 8.6% in the base pressure coefficient is noticed for the normal ventilated cavity device as compared to the blunt base case for freestream Mach number of 1.22. The power spectral density of base pressure fluctuations revealed the dominant peaks on the base surface. The Strouhal number associated with the coherent structures developing in the shear layer varies between 0.2 and 0.27 for the six freestream Mach numbers considered. In the presence of cavity devices, dominant peaks are observed for Strouhal numbers between 1 and 5. The root-mean-square, skewness, kurtosis of the base pressure fluctuations for all the cases are presented. Maximum reduction in base pressure fluctuation is observed for the normal and inclined ventilated cavity device configuration test cases.

Pump-turbines cope very well with modern electricity-market demand, having high operational flexibility and storage capabilities. Nevertheless, dynamic operation of these machines can lead to very challenging transient conditions, depending on the shape of the characteristic. Mechanical integrity can be correspondingly affected. Therefore, assessment of the characteristic during the design phase is of crucial importance. In the past years, different attempts to accurately compute the characteristic under steady and transient conditions have been undertaken using Reynolds-averaged Navier–Stokes (RANS) computational fluid dynamics. While the k– ω shear stress transport (SST) turbulence model has become the reference for machine design, it often fails for conditions close to or around instabilities. Under unstable conditions, which are characterized by continuous unsteady vortex formation, turbulence isotropy as assumed by linear two equation models is no longer the right choice. Accordingly, a turbulence model able to capture anisotropy, explicit algebraic Reynolds stress model (EARSM), has been implemented in an in-house code and used for the computation of the characteristic of various machines, stable and unstable, to assess the model performance. In this paper, computations of three different dynamic pump-turbine operating conditions are presented. Results using steady boundary conditions (BC) in the unstable region as well as transient BC like load-rejection and runaway are computed with EARSM, showing its superiority compared to linear two equation models. The model's capability to capture anisotropic effects—such as the influence of corners—produces more physical flow structures in the vaneless space, which lead to an overall improvement of the predicted stability characteristics.

Knowledge of the modal parameters of the guide vane is essential for evaluating the operating stability of pump-turbines. In the present investigation, experiments and simulations are designed to analyze the influence of submergence level and sidewall clearance on the vibration characteristics of a guide vane-like structure. The results show that the type of mode shape remains unchanged at different submergence levels, while the position of the node line (NL) demonstrates a slight shift. According to the angle of the NL and the free surface, the mode types are divided into parallel NL, vertical NL, and slanted NL modes. The added mass tends to increase with increasing submergence levels, while the slope of added mass in conjunction with the submergence level, is dependent on the mode type. In particular, in relation to the parallel NL mode, the slope is almost zero, if the free surface is close to the NL region; with regard to the slanted NL mode, the slope in the NL region is significantly smaller than that outside this region; in the case of the vertical NL mode, the slope remains approximately constant. The damping ratio increases with increasing submergence level for the vertical NL mode. While the damping ratios for the parallel and slanted NL modes are decreased if the free surface is close to the NL regions. In addition, as the side wall clearance increases, both the added mass and damping ratio tend to decrease.

As the first endeavor, we have analyzed the pulsatile flow of Oldroyd-B viscoelastic fluid where the combined effects of fluid elasticity and pulsation parameters on the flow characteristics are numerically studied at a low Reynolds number. Computations are performed using a finite volume-based open-source solver OpenFOAM™ by appending the log-conformation tensor approach to stabilize the numerical solution at high Deborah number. Significant flow velocity enhancement is achieved by increasing the viscoelastic behavior of the fluid. High-velocity gradient zones and high polymeric stress regions are observed near the channel wall. The magnitude of axial velocity attenuates with increasing pulsation amplitude or pulsation frequency, and the extent of this attenuation is highly dependent on the Deborah number or the retardation ratio. This work finds application in the transport of polymeric solutions, extrusion, and injection molding of polymer melts in several process industries.

In this study, computational fluid dynamics analysis was performed on a three-dimensional (3D) model of a Libellulidae wing to determine aerodynamic performance in gliding flight. The wing is comprised of various corrugated features alongside the spanwise and chordwise directions, as well as twist. The detailed features of real 3D dragonfly wing models, including all the corrugations through both span and chord, have not been considered in the past for a detailed aerodynamic analysis. The simulations were conducted by solving the Navier–Stokes equations to demonstrate gliding performance over a range of angles of attack at low Reynolds numbers. The numerical model was validated against experimental data obtained from a fabricated corrugated wing model using particle image velocimetry. The numerical results demonstrate that bio-inspired wings with corrugations compared to flat profile wings generate more lift with lower drag, trapping the vortices in the valleys of wing corrugation leading to delayed flow separation and delayed stall. The experimental and numerical results demonstrate that the methodology presented in this study can be used to measure bio-inspired 3D wing flow characteristics, including the influence of complex corrugations on aerodynamic performance. These findings contribute to the advancement of knowledge required for designing an optimized bio-inspired micro-air vehicle.

This work deals with the effects of suction and injection on Casson nanofluid around a moving wedge under the influence of gyrotactic micro-organisms along with viscous and ohmic dissipation. The governing system of highly coupled nonlinear partial differential equations together with assisting boundary conditions is converted by applying suitable similarity transformations, into a set of nonlinear ordinary differential equations. The obtained flow model is solved numerically by bvp4c ( matlab ) procedure. The accuracy of the flow model under consideration is validated by employing another well-known mathematical technique Runge–Kutta-Fehlberg (RKF) having good agreement while comparing the numerical results obtained by bvp4c for both suction and injection cases. Impacts of various pertinent parameters active in the flow model such as thermophoresis and Brownian motion, moving wedge, magnetic field, viscous and ohmic dissipation are numerically calculated for both suction and injection flow situations and also presented graphically. It is observed that the increase in casson parameter enhances the velocity but declines the density of motile organism, concentration and temperature for suction as well as injection flow case. The impacts of mass transfer rate of gyrotactic micro-organisms, Nusselt and Sherwood numbers for various fluid parameters are numerically presented in tabular form, separately for both suction and injection. One of the important observations of this study is that the suction or injection plays a key role in controlling boundary layer flow and brings stability in the flow. Moreover, rate of heat and mass transfer get enhanced in the existence of gyrotactic micro-organisms. Further, it would be worth mentioning that physical behavior of this flow problem coincides very well with already published literature either graphically or in tabular representation.

Axial fans with a small hub-to-tip diameter ratio are used in many branches of industry. Optimization of their aerodynamic performance is important, for which using sweep, dihedral, and skew of the blades' stacking line form an important method. Investigations on axial fans with medium to high hub-to-tip diameter ratio have shown that forward sweep of blades can give an improved aerodynamic performance, especially the total-to-total efficiency. However, only a few studies for fans with a small hub-to-tip diameter ratio have been reported. For such fans, extensive regions of backflow are present behind the fan near the hub. Based on a validated computational fluid dynamics simulation method, the effects of a sweep, dihedral and skew in axial and circumferential directions (in forward and backward direction) on the aerodynamic performance of small hub-to-tip ratio fans are investigated, with a linear stacking line. Current results show that forward sweep and circumferential skew are beneficial for higher total-to-total efficiency and that higher total-to-static efficiency can be obtained by forward dihedral and axial skew. The backward shape variety generally gives negative aerodynamic effects. Forward sweep and circumferential skew shorten the radial migration path, but more flow separation is present near the hub. With forward dihedral and axial skew, the backflow region is reduced in size and axial extent, but a more significant hub corner stall region is found. The pressure reduction due to sweep and dihedral is more limited than what could be expected from wing aerodynamics.

An experimental study on the effect of sidewalls on the flow characteristics of a three-dimensional turbulent square wall jet is carried out at a Reynolds number of 25,000. The sidewalls are defined as the two parallel plates along the vertical jet centerline. Four different sizes of sidewall enclosure (here after referred to as ‘SWE’) are placed at the lateral positions (z) of ± 3.5 h , ± 4 h , ± 4.5 h , and ± 5 h from the vertical jet centerline plane, where h is the height of square jet. The mean characteristics of fluid flow in wall normal (y) and lateral (z) directions at different downstream locations ( x / h = 0.2 − 45 ) are measured using a hotwire anemometer. The velocity measurements are also performed in the z – y lateral plane at four downstream locations ( x / h = 30 , 35 , 40 , and 45 ) . Results indicate that the mean velocity profile in lateral and wall normal directions behaves differently depending on the size of SWEs. The decay rate of the maximum mean velocity increases with decrease in size of SWEs after the downstream location ( x / h ≥ 20 ) . The decay rate of the maximum mean velocity increases about 5.6% in 140 mm SWE as compared to 200 mm SWE. It is noted that spread of the jet in wall normal and lateral directions increases with decrease in size of SWEs after the attachment of the flow stream on the sidewalls. In the far-field location of the present case, the smaller size of SWE (140 mm SWE) has 10.7% and 23.1% higher spread rate as compared to larger size of SWE (200 mm SWE) in wall-normal and lateral directions, respectively. It is also seen that the self-similar profile gets delayed in wall-normal direction as compared to lateral direction for all the cases. The wall-normal self-similar profile is obtained early with increase in the size of SWEs and it is obtained at x / h = 30, 27, 24, and 20 for 140 mm, 160 mm, 180 mm, and 200 mm SWEs, respectively. The flow stream seems to climb the sidewall and this tendency increases with increase in size of SWEs.

This paper investigates and compares four commonly used flow transport equation-based cavitation models and their applicability to predict the cavitation performance of an industrial centrifugal pump with a helical inducer. The main purpose of this study is to identify, for this specific application, the most appropriate cavitation model and the associated empirical constants. Each cavitation model is reviewed in detail and the uniqueness of each model is outlined. Each cavitation model is incorporated in a computational fluid dynamics code to study the vaporization and condensation transport rate of the fluid. Experimental tests are conducted on a pump system to determine the true cavitation performance in terms of net positive suction head (NPSH). Experimental results are compared to simulation results for different cavitation models to validate cavitation performance predictions, along with the empirical constants. Lastly, bubble formation, cavitation inception, and bubble growth predicted by each cavitation model are compared with the experimental results. A sensitivity analysis is conducted in order to determine the impact of each set of empirical constants to the condensation and the vaporization rate in the centrifugal pump. Results show that two of the cavitation models exhibit high dependency on the empirical constants in terms of change in vaporization rate. Modifications to empirical constants for two of the four cavitation models are suggested to obtain agreement with the experimentally observed cavitation behavior and better predict NPSH performance for the industrial pump studied.

Fuel efficiency becomes very important for new vehicles. Therefore, improving the aerodynamics of tires has started to receive increasing interest. While the experimental approaches are time-consuming and costly, numerical methods have been employed to investigate the air flow around tires. Rotating boundary and contact patch are important challenges in the modeling of tire aerodynamics. Therefore, majority of the current modeling approaches are simplified by neglecting the tire deformation and contact patch. In this study, a baseline computational fluid dynamics (CFD) model is created for a tire with contact patch. To generate mesh efficiently, a hybrid mesh, which combines hex elements and polyhedral elements, is used. Then, three modeling approaches (rotating wall, multiple reference frame, and sliding mesh) are compared for the modeling of tire rotation. Additionally, three different tire designs are investigated, including smooth tire, grooved tire, and grooved tire with open rim. The predicted results of the baseline model agree well with the measured data. Additionally, the hybrid mesh shows to be efficient and to generate accurate results. The CFD model tends to overpredict the drag of a rotating tire with contact patch. Sliding mesh approach generated more accurate predictions than the rotating wall and multiple reference frame approaches. For different tire designs, tire with open rim has the highest drag. It is believed that the methodology presented in this study will help in designing new tires with high aerodynamic performance.

In a boundary layer ingesting (BLI) propulsion system, the fan is continuously exposed to inflow distortions. The distorted inflows lead to nonuniform loss distributions along the radial and circumferential directions. Since the rotor tip suffers from higher intensive distortion, the local loss increment is a major contributor to the BLI fan performance penalty. To explore the effects of distorted inflows on tip leakage flow evolutions and associated mechanisms for increased loss in a BLI fan, three-dimensional full-annulus unsteady simulations are conducted. Results show that about 54% of total additional losses due to distortion are formed in the tip region and more than 80% of tip region entropy generation is related to the tip leakage flow. The intensities of leakage vortex–shock interactions vary at different annulus locations. When the rotor moves into the distorted region, the vortex–shock interaction is weaker than the undistorted locations due to attenuated leakage flow. At the locations where the rotor is moving out from the distorted region, the vortex–shock interaction is notably enhanced because the front part of the blade tip airfoil suffers a higher load, resulting in a rapid vortex core expansion and eventually vortex breakdown. The increase of flow blockage in the front section of blade tip passages at local circumferential positions leads to a corresponding rise of flow loss. The findings in this study highlight the impacts of tip leakage flow on aerodynamic loss of fan working under BLI inflow distortion and provide improved understandings of loss mechanisms in a BLI fan.

In this paper, the effects of modifying the blade pressure side (PS) on unsteady pressure pulsation and flow structures in a low specific speed centrifugal pump are carried out by experimental and computational fluid dynamics (CFD). Seven monitor points are arranged in the circumferential direction of the impeller outlet to capture the pressure signals in the volute at flow rate of 0.2–1.6Q d . Results show that the blade PS modification introduced here can significantly alleviate the amplitude of pressure pulsation at blade passing frequency ( f BPF ) in all concerned operation conditions. To study the effects of blade modification on the internal flow field, the volute domain is replaced by an even outlet region for CFD analysis. The shear stress transfer (SST) turbulence model is adopted for steady-state numerical simulation while the delayed detached eddy simulation (DDES) based on the SST approach is adopted for transient simulation. Results show that local unsteady velocity fluctuation is the dominant reason for pressure pulsation in the volute. After blade PS modification, the relative velocity distribution at the impeller outlet is more uniform and the intensity of shedding vortex at the blade trailing edge (TE) decreases significantly. The change of internal flow structure improves the uniformity of circumferential velocity distribution the downstream of the impeller outlet, which leads to the decrease of pressure fluctuation amplitude in the volute. Meanwhile, the local Euler head (LEH) distribution at the impeller outlet and the blade loading of PS are presented and compared. It can be concluded that the reduction of pressure pulsation attributes to a more uniform energy distribution at the impeller outlet which is achieved by actively unloading the PS of the modified blades.