scholarly journals Forces and Flow Structures on a Simplified Car Model Exposed to an Unsteady Harmonic Crosswind

2013 ◽  
Vol 136 (1) ◽  
Author(s):  
Valérie Ferrand
Keyword(s):  
Phase Shift ◽  
Transient Flow ◽  
Measurement Techniques ◽  
Flow Topology ◽  
Average Force ◽  
Ground Vehicle ◽  
Vortical Structures ◽  
Car Model ◽  
Image Velocimetry ◽  
Evolution Phase

A ground vehicle traveling along a road is subject to unsteady crosswinds in a number of situations. In windy conditions, for example, the natural atmospheric wind can exhibit strong lateral gusts. Other situations, such as tunnel exits or overtaking induce sudden changes in crosswinds, as well. The interaction of this unsteady oncoming flow with the vehicle and the resulting aerodynamic forces and moments affect the vehicle stability and comfort. The objectives of the current study are to improve the understanding of flow physics of such transient flow and ultimately to develop measurement techniques to quantify the vehicle’s sensitivity to unsteady crosswind. A square back simplified car model is exposed to a forced oscillating yaw and results are compared to static measurements. Tests are conducted at Reynolds number Re = 3.7 × 105 and reduced frequencies ranging from 0.265 × 10−2 to 5.3 × 10−2. Unsteady side force and yawing moment measurements are associated with particle image velocimetry flow fields to interpret dynamic loads in link with flow topology evolution. Phase average force and moment measurements are found to exhibit a phase shift between static and dynamic tests that increases with oscillating frequency. Velocity fields reveal that the phase-shift seems to originate from the rear part of the car model. Moreover, lateral vortical structures appearing on the lee side from β = 15 deg increase this phase-shift and consequently appear to be favorable to the lateral stability of the vehicle.

Forces and Flow Structures on a Simplified Car Model Exposed to a Transient Crosswind

2012 ◽  
Author(s):  
Valérie Ferrand ◽  
Bartlomiej Grochal
Keyword(s):  
Phase Shift ◽  
Flow Structures ◽  
Dynamic Tests ◽  
Average Force ◽  
Piv Measurements ◽  
Vortical Structures ◽  
Car Model ◽  
Phase Average ◽  
Flow Evolution ◽  
Static And Dynamic Tests

A squareback simplified car model is exposed to a forced oscillating yaw and results are compared to static measurements. Tests are conducted at Reynolds number Re = 3.7×105 and a maximum Strouhal number St=fLrefU0=0.053. Phase average force and moment measurements exhibit a phase shift between static and dynamic tests that increases with oscillating frequency. To gain better understanding of the origin of the phase shift phenomenon, this paper proposes to characterize the flow evolution around the model using PIV measurements. Phase-shift seems to originate from the pressure recovery area where velocity fields exhibit a time delay in their response to dynamic yawing. Moreover, lateral vortical structures appearing on lee side from β = 15° increase this phase-shift and consequently appear to be favourable to the lateral stability of the vehicle.

A STUDY OF VORTICAL STRUCTURES PAST THE LOWER PORTION OF THE AHMED CAR MODEL

2012 ◽  
Vol 19 (1) ◽  
pp. 81-95 ◽  
Author(s):  
Giancarlo Alfonsi ◽  
Agostino Lauria ◽  
Leonardo Primavera
Keyword(s):  
Lower Portion ◽  
Vortical Structures ◽  
Car Model

Characterization of the Surface Curvature Effect Using LES for a Single Round Impinging Jet

2017 ◽  
Author(s):  
Pierre Aillaud ◽  
Florent Duchaine ◽  
Laurent Gicquel ◽  
Sheddia Didorally
Keyword(s):  
Heat Transfer ◽  
Flat Plate ◽  
Temporal Distribution ◽  
Impinging Jet ◽  
Ambient Air ◽  
Surface Curvature ◽  
Flow Topology ◽  
Reference Case ◽  
Vortical Structures ◽  
Eddy Simulation

In this paper, wall resolved Large Eddy Simulation is used to study the effect of the surface curvature for two impinging jet configurations. The reference case is a single round jet impinging on a flat plate at a Reynolds number (based on the bulk velocity Ub and the pipe diameter D) Re = 23 000 and for a nozzle to plate distance H = 2D. The results on this configuration have been previously analyzed and validated against experimental results. This paper compares for the same operating point, the flat plate impingement to an impinging jet on a concave hemispherical surface with a relative curvature d/D = 0.089 where d is the concave surface diameter. Mean and Root Mean Square (RMS) quantities are compared to highlight differences and similarities between the two cases. In addition high order statistic such as Skewness of the temporal distribution of wall heat flux is analyzed. Probability density functions (PDF) are also built to further characterize the effect of surface curvature. It is shown that the surface curvature has a destabilizing effect on the vortical structures present in such a flow leading to a modification of the wall heat transfer compared to the flat plate case. The flow topology in the concave case is dominated by a large toroidal stationary vortex. This vortex generates a natural confinement that causes the increase of the mean temperature of the ambient air around the jet. The main effect is the reduction of the capacity of the vortical structures to enhance heat transfer. Finally, the confinement effect combined with the destabilization due to the concave curvature lead to an alleviation of the secondary peak in the Nusselt distribution and a reduction of the heat transfer at the wall.

scholarly journals A New Particle Image Velocimetry Technique for Turbomachinery Applications

2016 ◽  
Vol 138 (12) ◽  
Author(s):  
Sayantan Bhattacharya ◽  
Reid A. Berdanier ◽  
Pavlos P. Vlachos ◽  
Nicole L. Key
Keyword(s):  
Particle Image Velocimetry ◽  
Particle Image ◽  
Laser Sheet ◽  
Measurement Techniques ◽  
Leakage Flow ◽  
Tip Leakage Flow ◽  
Particle Image Velocimetry Technique ◽  
Optical Access ◽  
Tip Leakage ◽  
Image Velocimetry

Nonintrusive measurement techniques such as particle image velocimetry (PIV) are growing in both capability and utility for turbomachinery applications. However, the restrictive optical access afforded by multistage research compressors typically requires the use of a periscope probe to introduce the laser sheet for measurements in a rotor passage. This paper demonstrates the capability to perform three-dimensional PIV in a multistage compressor without the need for intrusive optical probes and requiring only line-of-sight optical access. The results collected from the embedded second stage of a three-stage axial compressor highlight the rotor tip leakage flow, and PIV measurements are qualitatively compared with high-frequency response piezoresistive pressure measurements to assess the tip leakage flow identification.

Time-Resolved DPIV Analysis of Vortex Dynamics in a Left Ventricular Model Through Bileaflet Mechanical and Porcine Heart Valve Prostheses

2004 ◽  
Vol 126 (6) ◽  
pp. 714-726 ◽  
Author(s):  
Olga Pierrakos ◽  
Pavlos P. Vlachos ◽  
Demetri P. Telionis
Keyword(s):  
Mitral Valve ◽  
Vortex Formation ◽  
Flow Patterns ◽  
Left Ventricular ◽  
Time Resolved ◽  
Vortical Structures ◽  
Jude Medical ◽  
Image Velocimetry ◽  
Valve Prostheses ◽  
Physical Phenomena

The performance of the heart after a mitral valve replacement operation greatly depends on the flow character downstream of the valve. The design and implanting orientation of valves may considerably affect the flow development. A study of the hemodynamics of two orientations, anatomical and anti-anatomical, of the St. Jude Medical (SJM) bileaflet valve are presented and compared with those of the SJM Biocor porcine valve, which served also to represent the natural valve. We document the velocity field in a flexible, transparent (LV) using time-resolved digital particle image velocimetry (TRDPIV). Vortex formation and vortex interaction are two important physical phenomena that dominate the filling and emptying of the ventricle. For the three configurations, the following effects were examined: mitral valve inlet jet asymmetry, survival of vortical structures upstream of the aortic valve, vortex-induced velocities and redirection of the flow in abidance of the Biot–Savart law, domain segmentation, resonant times of vortical structures, and regions of stagnant flow. The presence of three distinct flow patterns, for the three configurations, was identified by the location of vortical structures and level of coherence corresponding to a significant variation in the turbulence level distribution inside the LV. The adverse effect of these observations could potentially compromise the efficiency of the LV and result in flow patterns that deviate from those in the natural heart.

Experimental Study of Large-Scale Flow Structures in an Aneurysm

2018 ◽  
Author(s):  
Paulo Yu ◽  
Vibhav Durgesh
Keyword(s):  
Large Scale ◽  
Flow Structures ◽  
Pump System ◽  
Vortical Structures ◽  
Abnormal Growth ◽  
Image Velocimetry ◽  
Large Scale Flow ◽  
The Impact ◽  
Inflow Conditions ◽  
Aneurysm Model

An aneurysm is an abnormal growth in the wall of a weakened blood vessel, and can often be fatal upon rupture. Studies have shown that aneurysm shape and hemodynamics, in conjunction with other parameters, play an important role in growth and rupture. The objective of this study was to investigate the impact of varying inflow conditions on flow structures in an aneurysm. An idealized rigid sidewall aneurysm model was prepared and the Womersley number (α) and Reynolds number (Re) values were varied from 2 to 5 and 50 to 250, respectively. A ViVitro Labs pump system was used for inflow control and Particle Image Velocimetry was used for conducting velocity measurements. The results showed that the primary vortex path varied with an increase in α, while an increase in Re was correlated to the vortex strength and formation of secondary vortical structures. The evolution and decay of vortical structures were also observed to be dependent on α and Re.

scholarly journals Importance of Sub-Grid Scale Modeling for Accurate Aerodynamic Simulations

2020 ◽  
Vol 143 (1) ◽  
Author(s):  
Petter Ekman ◽  
James Venning ◽  
Torbjörn Virdung ◽  
Matts Karlsson
Keyword(s):  
The Body ◽  
Small Scale ◽  
Piv Measurements ◽  
Vortical Structures ◽  
Scale Modeling ◽  
Simple Geometry ◽  
Image Velocimetry ◽  
Large Eddy ◽  
Near Wall ◽  
Ahmed Body

Abstract The Ahmed body is one of the most well-investigated vehicle bodies for aerodynamic purposes. Despite its simple geometry, the flow around the body, especially at the rear, is very complex as it is dominated by a large wake with strong interaction between vortical structures. In this study, the flow around the 25 deg Ahmed body has been investigated using large eddy simulations and compared to high-resolution particle image velocimetry (PIV) measurements. Special emphasis was put on studying three commonly used sub-grid scale (SGS) models and their ability to capture vortical structures around the Ahmed body. The ability of the SGS models to capture the near-wall behavior and small-scale dissipation is crucial for capturing the correct flow field. Very good agreement between simulations and PIV measurements were seen when using the dynamic Smagorinsky-Lilly and the wall-adopting local eddy-viscosity SGS models, respectively. However, the standard Smagorinsky-Lilly model was not able to capture the flow patterns when compared to the PIV measurements due to shortcomings in the near-wall modeling in the standard Smagorinsky-Lilly model, resulting in overpredicted separation.

scholarly journals Experimental Research and Numerical Analysis of Flow Phenomena in Discharge Object with Siphon

Water ◽  
2020 ◽  
Vol 12 (12) ◽  
pp. 3330
Author(s):  
Milan Sedlář ◽  
Pavel Procházka ◽  
Martin Komárek ◽  
Václav Uruba ◽  
Vladislav Skála
Keyword(s):  
Experimental Research ◽  
Turbulence Models ◽  
Free Surface Flow ◽  
Stationary Flow ◽  
Surface Flow ◽  
Flow Structures ◽  
Complex Flow ◽  
Flow Topology ◽  
Image Velocimetry ◽  
Flow Phenomena

This article presents results of the experimental research and numerical simulations of the flow in a pumping system’s discharge object with the welded siphon. The laboratory simplified model was used in the study. Two stationary flow regimes characterized by different volume flow rates and water level heights have been chosen. The study concentrates mainly on the regions below and behind the siphon outlet. The mathematical modelling using advanced turbulence models has been performed. The free-surface flow has been carried out by means of the volume-of-fluid method. The experimental results obtained by the particle image velocimetry method have been used for the mathematical model validation. The evolution and interactions of main flow structures are analyzed using visualizations and the spectral analysis. The presented results show a good agreement of the measured and calculated complex flow topology and give a deep insight into the flow structures below and behind the siphon outlet. The presented methodology and results can increase the applicability and reliability of the numerical tools used for the design of the pump and turbine stations and their optimization with respect to the efficiency, lifetime and environmental demands.

scholarly journals PIV-Based Acoustic Pressure Measurements of a Single Bubble near the Elastic Boundary

Micromachines ◽  
2020 ◽  
Vol 11 (7) ◽  
pp. 637
Author(s):  
Qidong Yu ◽  
Zhicheng Xu ◽  
Jing Zhao ◽  
Mindi Zhang ◽  
Xiaojian Ma
Keyword(s):  
Flow Structure ◽  
Acoustic Pressure ◽  
Transient Flow ◽  
Single Bubble ◽  
Rigid Boundary ◽  
Inverted Cone ◽  
Migration Direction ◽  
Elastic Boundary ◽  
Image Velocimetry ◽  
Spark Bubble

The objective of this paper was to investigate acoustic pressure waves and the transient flow structure emitted from the single bubble near an elastic boundary based on the particle image velocimetry (PIV). A combination of an electric-spark bubble generator and PIV were used to measure the temporal bubble shapes, transient flow structure, as well as the mid-span deflection of an elastic boundary. Results are presented for three different initial positions near an elastic boundary, which were compared with results obtained using a rigid boundary. A formula relating velocity and pressure was proposed to calculate the acoustic pressure contours surrounding a bubble based on the velocity field of the transient flow structure obtained using PIV. The results show the bubbles near the elastic boundary presented a “mushroom” bubble and an inverted cone bubble. Based on the PIV-measured acoustic pressure contours, a significant pressure difference is found between the elastic boundary and the underside of the bubble, which contributed to the formation of the “mushroom” bubble and inverted cone bubble. Furthermore, the bubbles had opposite migration direction near rigid and elastic boundaries, respectively. In detail, the bubble was repelled away from the elastic boundary and the bubble was attracted by the rigid boundary. The resultant force made up of a Bjerknes force and buoyancy force dominated the migration direction of the bubble.

Dispersion and Deposition of Particles in Rayleigh-Be´nard Turbulent Flows

2006 ◽  
Author(s):  
Paolo Oresta ◽  
Antonio Lippolis ◽  
Roberto Verzicco ◽  
Alfredo Soldati
Keyword(s):  
Aspect Ratio ◽  
Turbulent Flows ◽  
Convective Flow ◽  
Cylindrical Domain ◽  
Flow Topology ◽  
Vortical Structures ◽  
Dispersed Particles ◽  
Large Eddy ◽  
Prandtl Numbers ◽  
Rayleigh Numbers

In this paper, Direct Numerical Simulation (DNS) and Lagrangian Particle Tracking are used to investigate dispersion and deposition of particles swarms in convective flow confined in a cylindrical domain with aspect ratio (diameter over height) 0.5. The numerical simulations are carried out with Prandtl and Rayleigh numbers respectively equal to Pr = 0.7 and Ra = 2108. For these values of aspect ratio, Rayleigh and Prandtl numbers the flow is turbulent and time-dependent. In such flow, three sets of particles with Stokes numbers, based on the large eddy time scale, equal to Stf = 0.01, Stf = 0.005 and Stf = 0.001 are randomly dispersed. Particles distribution in turbulent convective flow is highly inhomogeneous and shows the clustering correlated with the vortical structures. The level of clustering is computed with the deviation of particles probability density function (PDF) from Poisson distribution. With this technique is available the size of the cluster but their geometry is unknown. The organisation along lines, planes and surfaces was investigated using the fractal dimension of the cluster. Finally, the flow topology is studied to relate the particles dispersion to coherent flow structures.

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