Scale-up of an unsteady flow field for enhanced spatial and temporal resolution of PIV measurements: application to leaflet wake flow in a mechanical heart valve

2011 ◽  
Vol 51 (1) ◽  
pp. 161-176 ◽  
Author(s):  
Alessandro Bellofiore ◽  
Eilis M. Donohue ◽  
Nathan J. Quinlan
Keyword(s):  
Flow Field ◽  
Unsteady Flow ◽  
Heart Valve ◽  
Temporal Resolution ◽  
Scale Up ◽  
Mechanical Heart Valve ◽  
Wake Flow ◽  
Piv Measurements

High-Resolution Measurements of Velocity and Shear Stress in Leakage Jets From Bileaflet Mechanical Heart Valve Hinge Models

2015 ◽  
Vol 137 (11) ◽  
Author(s):  
Ewa Klusak ◽  
Alessandro Bellofiore ◽  
Sarah Loughnane ◽  
Nathan J. Quinlan
Keyword(s):  
Shear Stress ◽  
High Resolution ◽  
Flow Field ◽  
Heart Valve ◽  
Temporal Resolution ◽  
Mechanical Heart Valve ◽  
Small Scale ◽  
Bileaflet Mechanical Heart Valve ◽  
Viscous Shear Stress ◽  
Viscous Shear

In flow through cardiovascular implants, hemolysis, and thrombosis may be initiated by nonphysiological shear stress on blood elements. To enhance understanding of the small-scale flow structures that stimulate cellular responses, and ultimately to design devices for reduced blood damage, it is necessary to study the flow-field at high spatial and temporal resolution. In this work, we investigate flow in the reverse leakage jet from the hinge of a bileaflet mechanical heart valve (BMHV). Scaled-up model hinges are employed, enabling measurement of the flow-field at effective spatial resolution of 167 μm and temporal resolution of 594 μs using two-component particle image velocimetry (PIV). High-velocity jets were observed at the hinge outflow, with time-average velocity up to 5.7 m/s, higher than reported in previous literature. Mean viscous shear stress is up to 60 Pa. For the first time, strongly unsteady flow has been observed in the leakage jet. Peak instantaneous shear stress is up to 120 Pa, twice as high as the average value. These high-resolution measurements identify the hinge leakage jet as a region of very high fluctuating shear stress which is likely to be thrombogenic and should be an important target for future design improvement.

Two-Dimensional Experimental Investigation on the Effects of a Fowler Flap Gap and Overlap Size on the Wake Flow Field

2016 ◽  
Vol 139 (2) ◽  
Author(s):  
David Demel ◽  
Mohsen Ferchichi ◽  
William D. E. Allan ◽  
Marouen Dghim
Keyword(s):  
Experimental Investigation ◽  
Flow Field ◽  
Angle Of Attack ◽  
Wake Flow ◽  
Two Dimensional ◽  
Edge Region ◽  
Suction Surface ◽  
Trailing Edge ◽  
Piv Measurements ◽  
Image Velocimetry

This work details an experimental investigation on the effects of the variation of flap gap and overlap sizes on the flow field in the wake of a wing-section equipped with a trailing edge Fowler flap. The airfoil was based on the NACA 0014-1.10 40/1.051 profile, and the flap was deployed with 40 deg deflection angle. Two-dimensional (2D) particle image velocimetry (PIV) measurements of the flow field in the vicinity of the main wing trailing edge and the flap region were performed for the optimal flap gap and overlap, as well as for flap gap and overlap increases of 2% and 4% chord beyond optimal, at angles of attack of 0 deg, 10 deg, and 12 deg. For all the configurations investigated, the flow over the flap was found to be fully stalled. At zero angle of attack, increasing the flap gap size was found to have minor effects on the flow field but increased flap overlap resulted in misalignment between the main wing boundary layer (BL) flow and the slot flow that forced the flow in the trailing edge region of the main wing to separate. When the angle of attack was increased to near stall conditions (at angle of attack of 12 deg), increasing the flap gap was found to energize and improve the flow in the trailing edge region of the main wing, whereas increased flap overlap further promoted flow separation on the main wing suction surface possibly steering the wing into stall.

Characteristics of Tip Clearance Flow Instability in a Transonic Compressor

2010 ◽  
Author(s):  
Chunill Hah ◽  
Melanie Voges ◽  
Martin Mueller ◽  
Heinz-Peter Schiffer
Keyword(s):  
Flow Field ◽  
Unsteady Flow ◽  
Flow Instability ◽  
Tip Clearance ◽  
Piv Measurements ◽  
Tip Clearance Flow ◽  
Transonic Compressor ◽  
Compressor Rotor ◽  
Clearance Flow ◽  
Tip Clearance Vortex

In the present study, unsteady flow phenomena due to tip clearance flow instability in a modern transonic axial compressor rotor are studied in detail. First, unsteady flow characteristics due the oscillating tip clearance vortex measured with the particle image velocimetry (PIV) and casing-mounted unsteady pressure transducers are analyzed and compared to numerical results with a large eddy simulation (LES). Then, measured characteristic frequencies of the unsteady flow near stall operation are investigated. The overall purpose of the study is to advance the current understanding of the unsteady flow field near the blade tip in an axial transonic compressor rotor near the stall operating condition. Flow interaction between the tip leakage vortex and the passage shock is inherently unsteady in a transonic compressor. The currently applied PIV measurements indicate that the flow near the tip region is unsteady even at the design condition. This self-induced unsteadiness increases significantly as the compressor operates toward the stall condition. PIV data show that the tip clearance vortex oscillates substantially near stall. The calculated unsteady characteristics from LES agree well with the PIV measurements. Calculated unsteady flow fields show that the formation of the tip clearance vortex is intermittent and the concept of vortex breakdown from steady flow analysis does not seem to apply in the current flow field. Fluid with low momentum near the pressure side of the blade close to the leading edge periodically spills over into the adjacent blade passage. The spectral analysis of measured end wall and blade surface pressure shows that there are two dominant frequencies near stall. One frequency is about 40–60% of the rotor rotation and the other dominant frequency is about 40–60% of the blade passing frequency (BPF). The first frequency represents the movement of a large blockage over several consecutive blade passages against the rotor rotation. The second frequency represents traditional tip flow instability, which has been widely observed in subsonic compressors. The LES simulations show that the second frequency is due to movement of the instability vortex.

Numerical Investigation of the Unsteady Flow Fields in Centrifugal Compressor Diffusers

2010 ◽  
Author(s):  
Ahmed Abdelwahab
Keyword(s):  
Flow Field ◽  
Unsteady Flow ◽  
Centrifugal Compressor ◽  
Flow Characteristics ◽  
Tip Clearance ◽  
Wake Flow ◽  
Vortical Structures ◽  
Centrifugal Stage ◽  
Unsteady Wake ◽  
Flow Interactions

The performance of radial diffusers in centrifugal compressor stages is influenced by the impeller exit flow characteristics as well as the vaneless and semi-vaneless space flow characteristics. Both steady and unsteady flow interactions are present due to the propagation of pressure fields upstream and downstream between the impeller and diffuser. Furthermore, unsteady flow interactions occur when the impeller moving wakes developed due to secondary and tip clearance flows propagate through the diffuser passages. The present study aims at presenting a model that describes the unsteady wake propagation in the centrifugal compressor diffuser using vorticity principles. 3D unsteady Reynolds-Averaged Navier Stokes simulations are performed for both a vaned and a vaneless diffuser centrifugal stage. The simulations are used to examine the mechanism for the unsteady wake flow interactions in the diffuser. The unsteady streamwise vortical structures present in the impeller wakes and their propagation through the diffuser flow field is presented. The effect of the unsteady flow on loss production in the compressor stage is investigated. The velocity perturbations due to these vortical structures are presented. The present study indicates that the impeller wake propagation in the diffuser can be represented by a series of unsteady streamwise vortices superimposed on a uniform flow field. These vortices result in velocity perturbations that take the form of both positive and negative jets in the diffuser.

scholarly journals On the challenge of five-hole-probe measurements at high subsonic Mach numbers in the wake of transonic turbine cascades

2018 ◽  
Vol 2 ◽  
pp. JPRQQM
Author(s):  
Marcel Boerner ◽  
Martin Bitter ◽  
Reinhard Niehuis
Keyword(s):  
Flow Field ◽  
Flow Regimes ◽  
Reynolds Numbers ◽  
Sensitivity Study ◽  
Wake Flow ◽  
Piv Measurements ◽  
Image Velocimetry ◽  
Measurement Results ◽  
Transonic Turbine ◽  
Probe Measurements

Five-hole-probes are common use in turbomachinery flow investigations, even though, inserting a probe into a flow field inevitably induces perturbations to the flow which can falsify the measurement results, especially when exposed to transonic flows. The objective of the investigations presented here is to evaluate the Mach number measurements of a five-hole-probe (5HP) in the wake flow of a transonic turbine cascade at engine relevant Reynolds numbers by comparing them to the results of particle image velocimetry (PIV). Furthermore, PIV measurements were performed with inserted probe to investigate the influence of the probe on the wake flow field. Together with a sensitivity study of 5HP measurements in flow regimes close to Ma = 1, the results demonstrate how the measurement uncertainty can be improved in high subsonic flow regimes.

A Comparative Study of the Dynamic Wind Loads and Wake Flow Characteristics of a Turbine Sited in Onshore and Offshore Wind Farms

2014 ◽  
Author(s):  
Wei Tian ◽  
Ahmet Ozbay ◽  
Hui Hu
Keyword(s):  
Flow Field ◽  
Wind Turbine ◽  
Wind Farms ◽  
Field Measurements ◽  
Wake Flow ◽  
Offshore Wind ◽  
Wind Loads ◽  
Offshore Wind Farms ◽  
Piv Measurements ◽  
Turbine Model

An experimental study was conducted to compare the characteristics of the dynamic wind loads and evolution of the unsteady vortex and turbulent flow structures in the wake of a wind turbine sited in onshore and offshore wind farms. A scaled three-blade Horizontal Axial Wind Turbine (HAWT) model was placed in Atmospheric Boundary Layer (ABL) winds with different mean and turbulence characteristics to simulate the wind conditions in onshore and offshore wind farms. In addition to measuring dynamic wind loads acting on the wind turbine model by using a high-sensitive force-moment sensor unit, a high-resolution digital Particle Image Velocimetry (PIV) system was used to achieve flow field measurements to quantify the characteristics of the turbulent flow in the wake of the wind turbine model. Besides conducting “free-run” PIV measurements to determine the ensemble-averaged statistics of the flow quantities such as mean velocity, Reynolds stress, and Turbulence Kinetic Energy (TKE) distributions in the wake, “phase-locked” PIV measurements were also performed to elucidate further details about evolution of the unsteady vortex structures in the wake flow in relation to the position of the rotating turbine blades. The detailed flow field measurements are correlated with the dynamic wind loads measurements to elucidate underlying physics in order to gain further insight into changes of the dynamic wind loads and wake characteristics behind the wind turbine operating in either onshore or offshore wind farms.

scholarly journals An Investigation of the Response of Temperature Sensing Probes to an Unsteady Flow Field

1985 ◽  
Author(s):  
B. Agnew ◽  
R. L. Elder ◽  
M. Terrel
Keyword(s):  
Flow Field ◽  
Unsteady Flow ◽  
Temperature Sensor ◽  
Sensor Response ◽  
Temperature Sensing ◽  
Pulsating Flow ◽  
Wake Flow ◽  
Main Stream ◽  
Experimental Examination ◽  
Measuring Devices

The response of temperature measuring devices to pulsating flow fields has been a source of concern to compressor designers. A conventional temperature sensing device is known to respond to the highly energetic wake flow leaving a rotor and due to the long thermal time constant of the probe a temperature lying between the hot wake temperature and the relatively cooler main stream temperature tends to be indicated. This indicated temperature can be in serious error if included in a calculation to define the energy flux. This work is concerned with a theoretical and experimental examination of temperature sensor response to an unsteady pulsating flow typical of that occuring in a compressor.

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