Time resolved investigations on flow field and quasi wall shear stress of an impingement configuration with pulsating jets by means of high speed PIV and a surface hot wire array

2009 ◽  
Vol 30 (5) ◽  
pp. 877-885 ◽  
Author(s):  
Timm Janetzke ◽  
Wolfgang Nitsche
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Flow Field ◽  
High Speed ◽  
Wire Array ◽  
Hot Wire ◽  
Time Resolved ◽  
Wall Shear ◽  
Pulsating Jets

Modeling Reichardt’s Formula for Eddy-Viscosity in the Fluid Film of Tilting Pad Thrust Bearings

2017 ◽  
Author(s):  
Xin Deng ◽  
Brian Weaver ◽  
Cori Watson ◽  
Michael Branagan ◽  
Houston Wood ◽  
...  
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Turbulence Model ◽  
High Speed ◽  
Eddy Viscosity ◽  
The Other ◽  
Fluid Film ◽  
Suitable Model ◽  
Velocity Gradients ◽  
Wall Shear

Oil-lubricated bearings are widely used in high speed rotating machines such as those used in the aerospace and automotive industries that often require this type of lubrication. However, environmental issues and risk-adverse operations have made water lubricated bearings increasingly popular. Due to different viscosity properties between oil and water, the low viscosity of water increases Reynolds numbers drastically and therefore makes water-lubricated bearings prone to turbulence effects. The turbulence model is affected by eddy-viscosity, while eddy-viscosity depends on wall shear stress. Therefore, effective wall shear stress modeling is necessary in producing an accurate turbulence model. Improving the accuracy and efficiency of methodologies of modeling eddy-viscosity in the turbulence model is important, especially considering the increasingly popular application of water-lubricated bearings and also the traditional oil-lubricated bearings in high speed machinery. This purpose of this paper is to study the sensitivity of using different methodologies of solving eddy-viscosity for turbulence modeling. Eddy-viscosity together with flow viscosity form the effective viscosity, which is the coefficient of the shear stress in the film. The turbulence model and Reynolds equation are bound together to solve when hydrodynamic analysis is performed, therefore improving the accuracy of the turbulence model is also vital to improving a bearing model’s ability to predict film pressure values, which will determine the velocity and velocity gradients in the film. The velocity gradients in the film are the other term determining the shear stress. In this paper, three approaches applying Reichardt’s formula were used to model eddy-viscosity in the fluid film. These methods are for determining where one wall’s effects begin and the other wall’s effects end. Trying to find a suitable model to capture the wall’s effects of these bearings, with aim to improve the accuracy of the turbulence model, would be of high value to the bearing industry. The results of this study could aid in improving future designs and models of both oil and water lubricated bearings.

scholarly journals Impinging jets array: an experimental investigation and numerical modeling

2019 ◽  
Vol 85 ◽  
pp. 05004
Author(s):  
Nilesh Dhondoo ◽  
Ştefan-Mugur Simionescu ◽  
Corneliu Bălan
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Stagnation Point ◽  
High Speed ◽  
Jet Impingement ◽  
Impinging Jets ◽  
Stagnation Region ◽  
Jet Mixing ◽  
Point Pressure ◽  
Wall Shear

This paper reports on the measurements of wall shear stress and static pressure along a smooth static wall upon which jet impingement occurs. The effect of a single circular jet, respectively an array of jets is studied using a high speed/resolution camera. The areas of interest are the stagnation region and the wall jet region, where the jet is deflected from axial to radial direction. The effect of increasing the distance between the inlets is also investigated. The results are obtained by performing direct flow experimental visualizations and CFD numerical simulations, using the Reynolds averaged Navier-Stokes (RANS) approach with the commercial software ANSYS Fluent. The findings suggest that the smaller the nozzle-to-wall distance is, the higher the pressure peak. The wall shear stress has a bimodal distribution; at stagnation point, the wall shear stress is 0. An increase in the number of inlets produces the effect of a decrease in the stagnation point pressure. The greater the inter-inlet distance is, the greater the stagnation point pressure (there is less inter-jet mixing, less energy is lost in vortices formed between jets).

Comparison in High-Speed Droplet Impact Between Single and Multiple Collisions Against a Wall Covered With a Liquid Film

2019 ◽  
Author(s):  
Yoichiro Fukuchi ◽  
Tomoki Kondo ◽  
Keita Ando
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
High Speed ◽  
Water Layer ◽  
Liquid Jet ◽  
Surface Erosion ◽  
Single Droplet ◽  
Cleaning Efficiency ◽  
Droplet Impingement ◽  
Wall Shear

Abstract In semiconductor industry, liquid jet cleaning plays an important role because of its high cleaning efficiency and low environmental load. However, its cleaning mechanism is not revealed in detail because the experimental observation of high-speed and sub-micron droplets is challenging. Furthermore, higher impact velocity may give rise to surface erosion due to water-hammer shock loading from the impingement. To study cleaning mechanisms and surface erosion, numerical simulation of droplet impingement accounting for both viscosity and compressibility is an effective approach. In the previous study, wall-shear-flow generation has evaluated from the simulation of high-speed single droplet impingement. To evaluate more practical model of jet cleaning application, simulation of two droplets simplifying mono-dispersed splay of droplet train is favorable. Here, we numerically simulated impingement of two droplets, which allows for evaluating water-hammer pressure and wall shear stress. We consider the case of two water droplets (200 μm in diameter) that collides continuously, at speed 50 m/s, at the inter-droplet distance from 250 to 400 μm, with a no-slip rigid wall covered with a water layer (100 μm in thickness). The simulation is based on compressible Navier-Stokes equations for axisymmetric flow and the mixture of two components appears in numerically diffusion interface expressed by the volume average and advection equation. The simulation is solved by finite-volume WENO scheme that can capture both shock waves and material interface. In our simulation, the impingement of second droplet impingement gain higher shear stress than the single droplet impingement. At the case that the inter-droplet distance is 300 μm, maximum shear stress is 30.22 kPa (at the second droplet impingement), which is much larger than at the first droplet impingement (8.42 kPa). This result indicates how the second droplet impingement make wall shear flow induced by first droplet impingement stronger. From the parameter study of the inter-droplet distance, we can say that wall shear stress gets stronger as water layer thickness decreases. Furthermore, the maximum wall pressure is 1.96 MPa at the second droplet impingement, which is larger than at the first droplet impingement (1.46 MPa). From this study, the evaluation of surface erosion caused by jet cleaning is expected. The simulation suggests that multiple droplets impingement continuously may gain higher cleaning efficiency, which will give us a fundamental insight into liquid jet cleaning technologies. For further study, simulation of water column impingement and comparing the result of impingement of two droplets are expected.

Fluctuating Shear Stress Calibration Method Using a Channel Flow

2010 ◽  
Author(s):  
Daniel C. Cole ◽  
Michael L. Jonson ◽  
Kendra V. Sharp
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Flow Field ◽  
Calibration Method ◽  
Direct Shear ◽  
Radiated Noise ◽  
Calibration Device ◽  
Wall Shear ◽  
Oscillating Plate ◽  
Temporal Field

Fluctuating wall shear stress causes vibration and radiated noise from a structure. In the past wall shear stress has been measured indirectly using hot wires and hot films. Recently direct shear sensors have been developed. In this paper a calibration device consisting of a 305 mm × 60 mm × 5 mm channel filled with glycerin is used to calibrate a direct shear stress sensor with amplitudes up to 10 Pa of shear stress over a frequency range from 10 Hz to 1 kHz. The analytically known flow field caused by an oscillating plate 5 mm from the sensor is verified using laser Doppler velocimetry (LDV). The flow field is derived using a frequency-wavenumber approach thereby allowing for a known spatial and temporal field to be generated by specifying a derived plate vibration.

scholarly journals Wall shear stress from jetting cavitation bubbles

2018 ◽  
Vol 846 ◽  
pp. 341-355 ◽  
Author(s):  
Qingyun Zeng ◽  
Silvestre Roberto Gonzalez-Avila ◽  
Rory Dijkink ◽  
Phoevos Koukouvinis ◽  
Manolis Gavaises ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
High Speed ◽  
Temporal Distribution ◽  
Surface Cleaning ◽  
Shear Stresses ◽  
Rigid Boundary ◽  
Equivalent Radius ◽  
Wall Shear

The collapse of a cavitation bubble near a rigid boundary induces a high-speed transient jet accelerating liquid onto the boundary. The shear flow produced by this event has many applications, examples of which are surface cleaning, cell membrane poration and enhanced cooling. Yet the magnitude and spatio-temporal distribution of the wall shear stress are not well understood, neither experimentally nor by simulations. Here we solve the flow in the boundary layer using an axisymmetric compressible volume-of-fluid solver from the OpenFOAM framework and discuss the resulting wall shear stress generated for a non-dimensional distance, $\unicode[STIX]{x1D6FE}=1.0$ ($\unicode[STIX]{x1D6FE}=h/R_{max}$, where $h$ is the distance of the initial bubble centre to the boundary, and $R_{max}$ is the maximum spherical equivalent radius of the bubble). The calculation of the wall shear stress is found to be reliable once the flow region with constant shear rate in the boundary layer is determined. Very high wall shear stresses of 100 kPa are found during the early spreading of the jet, followed by complex flows composed of annular stagnation rings and secondary vortices. Although the simulated bubble dynamics agrees very well with experiments, we obtain only qualitative agreement with experiments due to inherent experimental challenges.

In Vitro, Time-Resolved PIV Comparison of the Effect of Stent Design on Wall Shear Stress

2009 ◽  
Vol 37 (7) ◽  
pp. 1310-1321 ◽  
Author(s):  
John Charonko ◽  
Satyaprakash Karri ◽  
Jaime Schmieg ◽  
Santosh Prabhu ◽  
Pavlos Vlachos
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Stent Design ◽  
Time Resolved ◽  
Wall Shear
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