Presence Of Large-scale Vortices Research Articles

Investigation of Energy Loss Mechanism and Vortical Structures Characteristics of Marine Sediment Pump Based on the Response Surface Optimization Method

Marine sediment pumps are extensively applied in marine engineering fields with complex media and harsh flow conditions. Therefore, this study conducts a multi-factor optimization design for a marine sediment pump. The response surface optimization method is utilized to improve the efficiency by optimizing the number of impeller blades, the blade inlet angle, the blade outlet angle, and the blade wrap angle. Next, a response surface regression model is created, and the influence of geometric parameters on the efficiency is determined. Meanwhile, the energy loss mechanism and vortical structure characteristics after optimization are analyzed by applying entropy production and the method for identifying Omega vortices. The findings suggest a 6.33% efficiency enhancement in the optimized model under the design conditions. The impeller’s internal flow field is enhanced, and the entropy generation rate is significantly diminished. The fluid flow adhered more closely to the blade profile, and the velocity and pressure distribution exhibit better uniformity. The presence of large-scale vortices and occurrences of flow separation within the impeller passage experience a notable decrease, and the overall fluid pressure fluctuation amplitude decreased, resulting in a more stable flow. Therefore, the discoveries from the research offer references for the design and selection of marine sediment pumps.

Periodic Lagrangian Coherent Structures around a tidal inlet

We present an extensive experimental campaign dedicated to the identification of coherent trajectory patterns owing to flow occurring in tidal environments, characterized by a tidal inlet and a channel with lateral tidal flats. Single and multiple harmonics tides are here reproduced on a large-scale physical model. The study of the large scale macro-vortices, generated by vortex shedding during the flood phase from the inlet barrier, is performed employing the Lagrangian Average Vorticity Deviation (LAVD). The presence of large-scale vortices with a complex dynamics within a tidal period suggested a deeper understanding on the possible environmental implications in terms of transport connections or barriers. Finite Time Lyapunov Exponents are employed in order to recognize stable and unstable manifolds within the flow that are defined as preferred paths along which particles are repelled (forward integration) or attracted (backward).

Precessing spherical shells: flows, dissipation, dynamo and the lunar core

SUMMARYPrecession of planets or moons affects internal liquid layers by driving flows, instabilities and possibly dynamos. The energy dissipated by these phenomena can influence orbital parameters such as the planet’s spin rate. However, there is no systematic study of these flows in the spherical shell geometry relevant for planets, and the lack of scaling law prevents convincing extrapolation to celestial bodies. We have run more than 900 simulations of fluid spherical shells affected by precession, to systematically study basic flows, instabilities, turbulence and magnetic field generation. We observe no significant effects of the inner core on the onset of the instabilities. We obtain an analytical estimate of the viscous dissipation, mostly due to boundary layer friction in our simulations. We propose theoretical onsets for hydrodynamic instabilities, and document the intensity of turbulent fluctuations. We extend previous precession dynamo studies towards lower viscosities, at the limits of today’s computers. In the low viscosity regime, precession dynamos rely on the presence of large-scale vortices, and the surface magnetic fields are dominated by small scales. Interestingly, intermittent and self-killing dynamos are observed. Our results suggest that large-scale planetary magnetic fields are unlikely to be produced by a precession-driven dynamo in a spherical core. But this question remains open as planetary cores are not exactly spherical, and thus the coupling between the fluid and the boundary does not vanish in the relevant limit of small viscosity. Moreover, the fully turbulent dissipation regime has not yet been reached in simulations. Our results suggest that the melted lunar core has been in a turbulent state throughout its history. Furthermore, in the view of recent experimental results, we propose updated formulas predicting the fluid mean rotation vector and the associated dissipation in both the laminar and the turbulent regimes.

Time-Resolved PIV Technique for High Temporal Resolution Measurement of Mechanical Prosthetic Aortic Valve Fluid Dynamics

Prosthetic heart valves (PHVs) have been used to replace diseased native valves for more than five decades. Among these, mechanical PHVs are the most frequently implanted. Unfortunately, these devices still do not achieve ideal behavior and lead to many complications, many of which are related to fluid mechanics. The fluid dynamics of mechanical PHVs are particularly complex and the fine-scale characteristics of such flows call for very accurate experimental techniques. Adequate temporal resolution can be reached by applying time-resolved PIV, a high-resolution dynamic technique which is able to capture detailed chronological changes in the velocity field. The aim of this experimental study is to investigate the evolution of the flow field in a detailed time domain of a commercial bileaflet PHV in a mock-loop mimicking unsteady conditions, by means of time-resolved 2D Particle Image Velocimetry (PIV). The investigated flow field corresponded to the region immediately downstream of the valve plane. Spatial resolution as in "standard" PIV analysis of prosthetic valve fluid dynamics was used. The combination of a Nd:YLF high-repetition-rate double-cavity laser with a high frame rate CMOS camera allowed a detailed, highly temporally resolved acquisition (up to 10000 fps depending on the resolution) of the flow downstream of the PHV. Features that were observed include the non-homogeneity and unsteadiness of the phenomenon and the presence of large-scale vortices within the field, especially in the wake of the valve leaflets. Furthermore, we observed that highly temporally cycle-resolved analysis allowed the different behaviors exhibited by the bileaflet valve at closure to be captured in different acquired cardiac cycles. By accurately capturing hemodynamically relevant time scales of motion, time-resolved PIV characterization can realistically be expected to help designers in improving PHV performance and in furnishing comprehensive validation with experimental data on fluid dynamics numeric modelling.

Experimental Characterization of an Isothermal Swirler Flowfield

A method for characterizing the flowfield associated with an ensemble of production swirl cups before their implementation into an aircraft engine is presented. LDV is used to acquire mean, rms, and skewness velocity profiles downstream of the swirlers' venturi. The skewness of the streamwise velocity data is used to locate the fluid boundary between the primary and secondary corotating streams. The data reduction technique utilizes the modality transformation of the streamwise velocity histograms as the LDV probe volume is translated from the low-speed secondary stream to the high-speed primary stream. By locating this boundary, primary and secondary flowfield metrics such as swirl number and primary-secondary mass flow split compliment the conventional effective area measurements. To corroborate the LDV measurements, digital particle image velocimetry images are used to qualify the size and position of the recirculation zone created at the venturi exit as well as highlight the unsteady nature of the feature. Instantaneous side- and end-view digital particle image velocimetry images provide evidence of coherent radial movement, that is, wagging of the recirculation zone and the presence of large-scale vortices in the secondary stream.

Triggering the Formation of Halo Globular Clusters with Galaxy Outflows

We investigate the interactions of high-redshift galaxy outflows with low-mass virialized (Tvir 20% of the cloud. Such estimates ignore the likely presence of large-scale vortices, however, which would further enhance turbulence generation. Thus quantitative mixing predictions must await more detailed numerical studies.

Investigation of the flow field downstream of an artificial heart valve by means of PIV and PTV

Measurements of the velocity field downstream of an artificial heart valve are performed by using particle image velocimetry (PIV) and particle tracking velocimetry (PTV). The investigated field corresponds to the region immediately downstream of the valve outlet i.e. the initial ascending part of the aorta. The aim of the paper is to investigate the evolution of the flow field in time in such inhomogeneous, anisotropic, and unsteady conditions. To do this, a high-speed video camera is used to acquire images of the seeding particles illuminated by a continuous infrared laser. high seeding density conditions are investigated using PIV to perform phase-sampled Eulerian averages, whereas low seeding conditions are used to determine particle trajectories and Lagrangian statistics using PTV. Both are needed for the complete description of the magnitude and duration of the stress on blood cells. The following features are described: The very high inhomogeneity and unsteadiness of the phenomenon The presence of large scale vortices within the field especially in the sinuses of Valsalva and in the wake of the valve leaflets The strong stress and strain rates at the jet–wake interface downstream of the leaflets and close to large-scale vortices The non-negligible time spent by fluid particles in some of the high stress and strain regions

An experimental investigation of thermal mixing and combustion in supersonic flows

A radially lobed nozzle (petal nozzle) is being increasingly recognised as a potential candidate for promoting mixing in compressible flows. An experimental investigation has been conducted to study its effectiveness in improving thermal mixing and combustion in supersonic flow. A hot gas jet issuing supersonically from a lobed nozzle mixes with a cold supersonic jet in a circular mixing tube. The two jets issue coaxially. A detailed survey of the flow field inside the mixing duct reveals that nearly complete thermal mixing (as exemplified by the nearly uniform temperature distribution) could be achieved in a short distance when a lobed nozzle is employed. The results also indicate the presence of large-scale vortices in the flow field downstream of the lobed nozzle. Having thus created a field in which mixing is good, supersonic combustion was then attempted. Kerosene was introduced into the hot stream issuing from the lobed nozzle and it burned mainly in the mixing tube, which served as a supersonic combustor. Resulting temperature and pressure rises were measured and the supersonic combustion efficiency was found to be of the order of 60%. The performance of a conventional conical nozzle was found to be much inferior to that of the petal nozzle under identical conditions.

Simulating moth wing aerodynamics - Towards the development of flapping-wing technology

The mechanization of flapping-wing flight is addressed. A tethered moth's flapping wings are simulated using an unsteady aerodynamic panel method and accounts for wing flexibility using a finite element model. The resultant simulation code delineates both the aerodynamic and inertial forces acting on flapping, flexible wings undergoing arbitrary motion in the presence of large-scale vortices and establishes the importance of including the wake in the unsteady analysis of flapping flexible wings. A switching pattern is discovered where the magnitude and direction of the aerodynamic force are decoupled, thereby pointing to a means whereby control is achieved. Overall, important groundwork necessary for the establishment of the principles of flapping-wing flight is laid, leading to the development of a highly agile, alternative flight technology.