Vorticity generation due to cross-sea

2014 ◽  
Vol 744 ◽  
pp. 286-309 ◽  
Author(s):  
M. Postacchini ◽  
M. Brocchini ◽  
L. Soldini
Keyword(s):  
Ad Hoc ◽  
Flow Characteristics ◽  
Cross Flow ◽  
Shallow Waters ◽  
Wave Forcing ◽  
Flow Length ◽  
Nonlinear Shallow Water Equations ◽  
Flow Evolution ◽  
Wave Trains ◽  
Vorticity Generation

AbstractSimilarly to shore-parallel waves interacting with submerged obstacles, two wave trains, approaching the shore with different angles, generate breakers of finite cross-flow length and an intense vorticity at their edges. The dynamics of crossing wave trains in shallow waters is studied by means of a simple theoretical approach that is used to inspect the flow characteristics at breaking. The post-breaking dynamics, with specific focus on the vorticity generation and evolution processes, is described on the basis of the analytical results of Brocchini et al. (J. Fluid Mech., vol. 507, 2004, pp. 289–307). Ad hoc numerical simulations, performed by means of a nonlinear shallow-water equations (NSWE) solver, are used to support the analytical findings and detail the post-breaking flow evolution. Comparisons between numerical and analytical findings confirm that: (i) the cross-sea theory successfully predicts the breaking position when a finite-length breaker stems from two crossing wave trains and (ii) the dynamics induced by such a breaking (i.e. vorticity generation, mutual-advection and self-advection mechanisms) is similar to that occurring after the breaking event of a shore-parallel wave over a submerged obstacle: vortices generated at the breaker edges are first subjected to wave forcing and self-advection, these pushing the vortices shoreward; then, oppositely-signed vortices pair and the mutual interaction enables them to invert the motion and move seaward. Useful relationships have been found to describe the main features of such a dynamics (i.e. breaker length, vortex trajectories, etc.).

scholarly journals BORE-INDUCED MACROVORTICES OVER A PLANAR BEACH: THE CROSS-SEA CONDITION CASE

2012 ◽  
Vol 1 (33) ◽  
pp. 18
Author(s):  
Matteo Postacchini ◽  
Maurizio Brocchini ◽  
Luciano Soldini
Keyword(s):  
Shallow Water Equations ◽  
Wave Breaking ◽  
Ad Hoc ◽  
Rip Currents ◽  
Positive Interaction ◽  
Wave Ray ◽  
Nonlinear Shallow Water Equations ◽  
Fair Comparison ◽  
Wave Trains ◽  
Vorticity Generation

Wave breaking over submerged topographic obstacles leads to vorticity generation and, at times, to the generation of strong offshore-directed rip currents. The generation of finite-length breakers may also be induced by the positive interaction of wave trains propagating to shore with a relative angle. Such an interaction gives rise to a short-crested system, this, in turn, generating both breakers of finite crossflow length and an intense associated vorticity. We here analyze such a vorticity generation mechanism specifically focusing on the location where wave breaking occurs. To this purpose we use both a simple theoretical approach, based on the well-known theory of wave ray propagation, and ad-hoc numerical simulations, by means of a NSWE (Nonlinear Shallow Water Equations) solver. A fair comparison between such preliminary theoretical and numerical results suggests that the present work can be used as the basis for future analyses of vorticity generation by cross seas.

scholarly journals Large-scaled simulation on the coherent vortex evolution of a jet in a cross-flow based on lattice Boltzmann method

2015 ◽  
Vol 19 (3) ◽  
pp. 977-988 ◽  
Author(s):  
Yanqin Shangguan ◽  
Xian Wang ◽  
Yueming Li
Keyword(s):  
Lattice Boltzmann Method ◽  
Lattice Boltzmann ◽  
Flow Characteristics ◽  
Cross Flow ◽  
Hairpin Vortex ◽  
Graphic Processing Units ◽  
Computational Performance ◽  
Good Ability ◽  
Secondary Vortices ◽  
Boltzmann Method

Large eddy simulation (LES) is performed on a jet issued normally into a cross-flow using lattice Boltzmann method (LBM) and multiple graphic processing units (multi-GPUs) to study the flow characteristics of jets in cross-flow (JICF). The simulation with 8 1.50?10 grids is fulfilled with 6 K20M GPUs. With large-scaled simulation, the secondary and tertiary vortices are captured. The features of the secondary vortices and the tertiary vortices reveal that they have a great impact on the mixing between jet flow and cross-flow. The qualitative and quantitative results also indicate that the evolution mechanism of vortices is not constant, but varies with different situations. The hairpin vortex under attached jet regime originates from the boundary layer vortex of cross-flow. While, the origin of hairpin vortex in detached jet is the jet shear-layer vortex. The mean velocities imply the good ability of LBM to simulate JICF and the large loss of jet momentum in detached jet caused by the strong penetration. Besides, in our computation, a high computational performance of 1083.5 MLUPS is achieved.

scholarly journals Investigating the In-Flow Characteristics of Multi-Operation Conditions of Cross-Flow Fan in Air Conditioning Systems

Processes ◽  
2019 ◽  
Vol 7 (12) ◽  
pp. 959
Author(s):  
Weijie Zhang ◽  
Jianping Yuan ◽  
Qiaorui Si ◽  
Yanxia Fu
Keyword(s):  
Pressure Distribution ◽  
Flow Rate ◽  
Total Pressure ◽  
Air Conditioning ◽  
Pressure Pulsation ◽  
Flow Characteristics ◽  
Cross Flow ◽  
Operating Conditions ◽  
Aerodynamic Noise ◽  
Cross Flow Fan

Cross-flow fans are widely used in numerous applications such as low-pressure ventilation, household appliances, laser instruments, and air-conditioning equipment. Cross-flow fans have superior characteristics, including simple structure, small size, stable airflow, high dynamic pressure coefficient, and low noise. In the present study, numerical simulation and experimental research were carried out to study the unique secondary flow and eccentric vortex flow characteristics of the internal flow field in multi-operating conditions. To this end the vorticity and the circumferential pressure distribution in the air duct are obtained based on the performed experiments and the correlation between spectral characteristics of multiple operating conditions and the inflow state is established. The obtained results show that when the area of the airflow passage decreases while the area of the eccentric vortex area gradually increases, then the airflow of the cross-flow fan decreases, the outlet expands, and the flow pattern uniformity reduces. It was found that wakes form in the vicinity of the blade and the tail of the volute tongue, which generate pressure pulsation, and aerodynamic noise. The pressure distribution along the inner circumference shows that the total minimum pressure appears in the eccentric vortex near the volute tongue and the volute returns near the zone. Moreover, it was found that the total pressure near the eccentric vortex is significantly smaller than that of the main flow zone. As the flow rate decreases, the pressure pulsation amplitude of the eccentric vortex region significantly increases, while the static and total pressure pulsation amplitudes are gradually increased. Close to the eccentric vortex on the inner side of the blade in the volute tongue area, total pressure is low, total pressure on the outside of the blade is not affected, and pressure difference between the inner and outer sides is large. When the flow rate of the cross-flow fan is 0.4 Qd, there is no obvious peak at the harmonic frequency of the blade passage frequency. This shows that the aerodynamic noise is caused by the main unstable flow.

Numerical Prediction of Flow and Heat Transfer Past a Circular Tube With Annular Fins

2003 ◽  
Author(s):  
D. Chakraborty ◽  
G. Biswas ◽  
P. K. Panigrahi
Keyword(s):  
Heat Transfer ◽  
Circular Tube ◽  
Flow Characteristics ◽  
Cross Flow ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Stagnation Line ◽  
Flow And Heat Transfer ◽  
Annular Fin ◽  
Annular Fins

A numerical investigation was carried out to study the flow and heat transfer behavior of a vertical circular tube, which is situated between two annular fins in cross-flow. The flow structure of the limiting streamlines on the surface of the circular tube and the annular fins was analysed. A finite volume method was employed to solve the Navier-Stokes and energy equations. The numerical results pertaining to heat transfer and flow characteristics were compared with the available experimental results. The following salient features were observed in this configuration. A horseshoe vortex system was formed at the junction of the stagnation line of the circular tube and the annular fin. The separation took place at the rear of the tube. The influence of the horseshoe vortices on local heat transfer was substantial. The ratio of the axial gap between two annular fins (L) to the radial protrusion length of the annular fin (LR) was identified as an important parameter. The flow and heat transfer results were presented for different L/LR ratios for a Reynolds number of 1000.

Hydrodynamic Coefficients for Riser In-Line VIV in Sheared Currents

2007 ◽  
Author(s):  
Djoni E. Sidarta ◽  
Kostas F. Lambrakos ◽  
Carl M. Larsen
Keyword(s):  
Fatigue Damage ◽  
Laboratory Tests ◽  
Ad Hoc ◽  
Science And Technology ◽  
Cross Flow ◽  
Hydrodynamic Coefficients ◽  
Simply Supported ◽  
Hydrodynamic Damping ◽  
Uniform Current ◽  
Mode Response

A methodology for analyzing risers for in-line VIV fatigue damage has been developed that is based on the code SHEAR7, and laboratory in-line VIV coefficients. The in-line VIV fatigue in many instances governs the design of the riser since in-line VIV starts at a reduced velocity of about 1 whereas the threshold reduced velocity for cross flow VIV is about 4. The methodology can treat sheared currents on the basis of the cross flow VIV modeling in SHEAR7. Through the SHEAR7 modeling, the methodology removes conservatism implicit in the present ad hoc procedures for calculating riser in-line VIV response on the basis of the DNV-RP-F105 code. The reduction in conservatism is due to accounting properly for the power-in region in the VIV excitation, the hydrodynamic damping, and competing modal excitation (multiple mode response). The inline VIV coefficients have been derived from laboratory tests at the Norwegian University of Science and Technology (NTNU). The paper presents the in-line VIV coefficients, and examples to demonstrate the methodology for riser in sheared currents. The coefficients derived from the NTNU tests are functions of both the in-line VIV response amplitude and the reduced velocity. The coefficients presented in the paper are scaled test coefficients. The scaling of the NTNU coefficients assures that the methodology calculates in-line VIV amplitudes that are consistent with the response amplitudes in DNV-RP-F105 for the case of a simply supported riser in uniform current. This DNV code, although written for pipelines, has been extended to risers in sheared currents on the basis of conservative approaches.

Strouhal Numbers for Heat Exchanger Tube Arrays in Cross Flow

1987 ◽  
Vol 109 (2) ◽  
pp. 219-223 ◽  
Author(s):  
D. S. Weaver ◽  
J. A. Fitzpatrick ◽  
M. ElKashlan
Keyword(s):  
Heat Exchangers ◽  
Flow Characteristics ◽  
Cross Flow ◽  
Acoustic Resonance ◽  
Peak Frequency ◽  
Heat Exchanger Tube ◽  
Turbulence Spectrum ◽  
Tube Arrays ◽  
Exchanger Tube ◽  
Array Geometry

The prediction of tube or acoustic resonance due to cross-flow in heat exchangers is dependent upon knowledge of the flow characteristics for a given tube array geometry. For this, a Strouhal number relating a peak frequency in the turbulence spectrum to the velocity of the flow is required. The data available in the literature for this are rather confusing and the prediction methods appear somewhat contradictory. This paper reports the results from experiments conducted to determine Strouhal numbers for eight tube array models. These results together with the data available in the literature are then compared and appropriate conclusions drawn.

Numerical Investigation of a Banki Turbine in Transient State: Reaction Ratio Determination

2012 ◽  
Author(s):  
Sergio D. Croquer ◽  
Jesus de Andrade ◽  
Jorge Clarembaux ◽  
Freddy Jeanty ◽  
Miguel Asuaje
Keyword(s):  
Numerical Models ◽  
Flow Characteristics ◽  
Cross Flow ◽  
Turbulence Models ◽  
Transient State ◽  
Major Influence ◽  
Small Scale ◽  
Complex Flow ◽  
Impulse Turbine ◽  
Reaction Ratio

Cross-Flow Turbines (CFT) also known as Banki Turbines, are often considered for small scale hydroelectric generation. They are known for their simple construction, maintenance and operation, which means they incur in lower CAPEX and OPEX when compared to other types of turbines. However, they also tend to have a modest efficiency (82% [1–3]), hence they are not considered for big scale operations. Little is known about the flow characteristics inside the runner of the CFT. The objective of this investigation is to better understand the flow inside CFTs using Computational Fluid Dynamics (CFD) tools. Steady and Transient State simulations were performed for a CFT at an specific speed NS = 45. SST and κ–ε turbulence models were compared in terms of simulation requirements and obtained results. A proposed runner-nozzle interface, considering real CFT existent gap between these two components (free space) was evaluated as well. Results were compared to available experimental data. Maximum, numerically calculated efficiency deviation from reported experimental global efficiency was 15%. Pressure and velocity profiles along nozzle outlet, energy transfer stages location and CFT reaction ratio values were addressed. Results were compared in terms of runner-nozzle interface (gap vs no-gap), turbulence model (SST vs κ–ε) and calculation regime (steady vs transient regime). Only calculation state (steady vs transient) was found to have major influence over results. Transient state calculations better representing complex flow inside the CFT. Obtained degrees of reaction (no runner-nozzle gap, SST, transient state) were 0.12 and 0.08, for 1st and 2nd stages respectively. Hence the CFT is defined, according to this numerical models, as an impulse turbine.

The Influence of Non-Uniform Impeller on the Cross Flow Fan Noise

2016 ◽  
Vol 693 ◽  
pp. 251-256
Author(s):  
Zhi Qiang Yang ◽  
C.J. Wu
Keyword(s):  
Pressure Fluctuations ◽  
Flow Characteristics ◽  
Cross Flow ◽  
Internal Flow ◽  
Broadband Noise ◽  
Aerodynamic Noise ◽  
Air Conditioner ◽  
Fan Noise ◽  
Power Spectral ◽  
Cross Flow Fan

The aerodynamic noise of a cross flow fan with uneven blade spacing in room air-conditioner was simulated by computational aerodynamic acoustics (CAA) method. It is detailed to analyze the vorticity distribution of the flow field and the power spectral density of measured points’ pressure fluctuations, and the results demonstrate the non-uniform impeller used in this paper can significantly improve internal flow characteristics. Thus the broadband noise got reduced.

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