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.

Wall-Shear-Stress-Based Conditional Sampling Analysis of Coherent Structures in a Turbulent Boundary Layer

2020 ◽  
Author(s):  
Sangjin Ryu ◽  
Ethan Davis ◽  
Jae Sung Park ◽  
Haipeng Zhang ◽  
Jung Yoo
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Turbulent Boundary Layer ◽  
Coherent Structures ◽  
Detection Method ◽  
Streamwise Velocity ◽  
Shear Stresses ◽  
Conditional Sampling ◽  
Wall Shear

Abstract Coherent structures are critical for controlling turbulent boundary layers due to their roles in momentum and heat transfer in the flow. Turbulent coherent structures can be detected by measuring wall shear stresses that are footprints of coherent structures. In this study, wall shear stress fluctuations were measured simultaneously in a zero pressure gradient turbulent boundary layer using two house-made wall shear stress probes aligned in the spanwise direction. The wall shear stress probe consisted of two hot-wires on the wall aligned in a V-shaped configuration for measuring streamwise and spanwise shear stresses, and their performance was validated in comparison with a direct numerical simulation result. Relationships between measured wall shear stress fluctuations and streamwise velocity fluctuations were analyzed using conditional sampling techniques. The peak detection method and the variable-interval time-averaging (VITA) method showed that quasi-streamwise vortices were inclined toward the streamwise direction. When events were simultaneously detected by the two probes, stronger fluctuations in streamwise velocity were detected, which suggests that stronger coherent structures were detected. In contrast to the former two methods, the hibernating event detection method detects events with lower wall shear stress fluctuations. The ensemble-averaged mean velocity profile of hibernating events was shifted upward compared to the law of the wall, which suggests low drag status of the coherent structures related with hibernating events. These methods suggest significant correlations between wall shear stress fluctuations and coherent structures, which could motivate flow control strategies to fully exploit these correlations.

Pulsatile Flow Patterns and Wall Shear Stresses in Arch of a Turn-Around Tube With/Without Stenosis

2011 ◽  
Vol 27 (1) ◽  
pp. 79-94 ◽  
Author(s):  
R. F. Huang ◽  
C.-Y. Ho ◽  
J.-K. Chen
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Upstream Region ◽  
Shear Stresses ◽  
Recirculation Bubble ◽  
Diastolic Phase ◽  
Pulsatile Pressure ◽  
Systolic Phase ◽  
Wall Shear

ABSTRACTThe temporal/spatial evolution processes of the flow pattern, velocity distribution, and wall shear stress of pulsatile water flows in the arch of 180o turn-around tubes with/without stenosis were experimentally studied by using the particle image velocimetry (PIV). Three transparent tubes made of glass were used: A tube without stenosis in the arch, a tube with a 25% stenosis at the inner wall of arch, and a tube with a 50% stenosis at the inner wall of arch. Here the percentage of stensis denoted the ratio between the stenosis height to inner diameter of arch in the diametral cross section across mid-arch of the central plane. The flow was provided by a pump which approximately simulated the pulsatile pressure waves of human heart beats. The systole to diastole time period ratio is set at 35%:65%. The Womersley parameter, Dean number, and time-averaged Reynolds number were 14, 2348, and 3500, respectively. In the arch of the turn-around tube without stenosis, no boundary layer separation was found during the systolic phase. The reverse flow and recirculation bubble appeared in the arch only during the diastolic phase. The inner wall of the arch experienced lower wall shear stress during the diastolic phase due to the formation of recirculation bubble and secondary flow. In the arch with stenosis, the boundary layer separated from the inner wall and formed a recirculation bubble downstream the stenosis during the systolic phase. Lower stenosis (25%) did not cause drastic variation of the wall shear stresses. At higher stenosis (50%), however, the wall shear stress around the inner wall downstream the stenosis became extraordinarily low, whereas the wall shear stress around the upstream region of the outer wall of the downstream branch of the tube became anomalously large.

On the Effects of a Flexible Structure on Boundary Layer Stability and Transition

2011 ◽  
Vol 133 (7) ◽  
Author(s):  
Ashraf Al Musleh ◽  
Abdelkader Frendi
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Young Modulus ◽  
Boundary Layer Transition ◽  
Beam Equation ◽  
Flexible Structure ◽  
Boundary Layer Stability ◽  
Wall Shear ◽  
Fluid Wall Shear Stress

Delaying the onset of boundary layer transition has become a major research area in the last few years. This delay can be achieved by either active or passive control techniques. In the present paper, the effects of flexible or compliant structures on boundary layer stability and transition is studied. The Orr-Sommerfeld equation coupled to a beam equation representing the flexible structure is solved for a Blasius type boundary layer. A parametric study consisting of the beam thickness and material properties is carried out. In addition, the effect of fluid wall shear stress on boundary layer stability is analyzed. It is found that high density and high Young modulus materials behave like rigid structures and therefore do not alter the stability characteristic of the boundary layer. Whereas low density and low Young modulus materials are found to stabilize the boundary layer. High values of fluid wall shear stress are found to destabilize the boundary layer. Our results are in good agreement with those published in the literature.

Preston-Static Tubes for the Measurement of Wall Shear Stress

1994 ◽  
Vol 116 (3) ◽  
pp. 645-649 ◽  
Author(s):  
Josef Daniel Ackerman ◽  
Louis Wong ◽  
C. Ross Ethier ◽  
D. Grant Allen ◽  
Jan K. Spelt
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Convenient Method ◽  
Pipe Flow ◽  
Total Pressure ◽  
Static Pressure ◽  
Shear Stresses ◽  
Streamline Curvature ◽  
Syringe Needle ◽  
Wall Shear

We present a Preston tube device that combines both total and static pressure readings for the measurement of wall shear stress. As such, the device facilitates the measurement of wall shear stress under conditions where there is streamline curvature and/or over surfaces on which it is difficult to either manufacture an array of static-pressure taps or to position a single tap. Our “Preston-static” device is easily and conveniently constructed from commercially available regular and side-bored syringe needles. The pressure difference between the total pressure measured in the regular syringe needle and the static pressure measured in the side-bored one is used to determine the wall shear stress. Wall shear stresses measured in pipe flow were consistent with independently determined values and values obtained using a conventional Preston tube. These results indicate that Preston-static tubes provide a reliable and convenient method of measuring wall shear stress.

A Superposition Analysis of the Turbulent Boundary Layer in an Adverse Pressure Gradient

1951 ◽  
Vol 18 (1) ◽  
pp. 95-100
Author(s):  
Donald Ross ◽  
J. M. Robertson
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Turbulent Boundary Layer ◽  
Velocity Profile ◽  
Pressure Gradient ◽  
Adverse Pressure Gradient ◽  
Pressure Recovery ◽  
Stress Coefficient ◽  
Wall Shear

Abstract As an interim solution to the problem of the turbulent boundary layer in an adverse pressure gradient, a super-position method of analysis has been developed. In this method, the velocity profile is considered to be the result of two effects: the wall shear stress and the pressure recovery. These are superimposed, yielding an expression for the velocity profiles which approximate measured distributions. The theory also leads to a more reasonable expression for the wall shear-stress coefficient.

Requirements for Mesh Resolution in 3D Computational Hemodynamics

2000 ◽  
Vol 123 (2) ◽  
pp. 134-144 ◽  
Author(s):  
Sujata Prakash ◽  
C. Ross Ethier
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Adaptive Mesh Refinement ◽  
Mesh Refinement ◽  
Adaptive Mesh ◽  
Velocity Fields ◽  
Shear Stresses ◽  
Large Artery ◽  
Mesh Independence ◽  
Wall Shear

Computational techniques are widely used for studying large artery hemodynamics. Current trends favor analyzing flow in more anatomically realistic arteries. A significant obstacle to such analyses is generation of computational meshes that accurately resolve both the complex geometry and the physiologically relevant flow features. Here we examine, for a single arterial geometry, how velocity and wall shear stress patterns depend on mesh characteristics. A well-validated Navier-Stokes solver was used to simulate flow in an anatomically realistic human right coronary artery (RCA) using unstructured high-order tetrahedral finite element meshes. Velocities, wall shear stresses (WSS), and wall shear stress gradients were computed on a conventional “high-resolution” mesh series (60,000 to 160,000 velocity nodes) generated with a commercial meshing package. Similar calculations were then performed in a series of meshes generated through an adaptive mesh refinement (AMR) methodology. Mesh-independent velocity fields were not very difficult to obtain for both the conventional and adaptive mesh series. However, wall shear stress fields, and, in particular, wall shear stress gradient fields, were much more difficult to accurately resolve. The conventional (nonadaptive) mesh series did not show a consistent trend towards mesh-independence of WSS results. For the adaptive series, it required approximately 190,000 velocity nodes to reach an r.m.s. error in normalized WSS of less than 10 percent. Achieving mesh-independence in computed WSS fields requires a surprisingly large number of nodes, and is best approached through a systematic solution-adaptive mesh refinement technique. Calculations of WSS, and particularly WSS gradients, show appreciable errors even on meshes that appear to produce mesh-independent velocity fields.

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).

scholarly journals Assessing Bioinspired Topographies for their Antifouling Potential Control Using Computational Fluid Dynamics (CFD)

2018 ◽  
Vol 152 ◽  
pp. 02004 ◽  
Author(s):  
Jacky Ling ◽  
Felicia Wong Yen Myan
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Extended Period ◽  
Slip Boundary Condition ◽  
Shear Stresses ◽  
Cfd Simulations ◽  
Potential Control ◽  
Slip Boundary ◽  
Computational Fluid Dynamics Cfd ◽  
Wall Shear

Biofouling is the accumulation of unwanted material on surfaces submerged or semi submerged over an extended period. This study investigates the antifouling performance of a new bioinspired topography design. A shark riblets inspired topography was designed with Solidworks and CFD simulations were antifouling performance. The study focuses on the fluid flow velocity, the wall shear stress and the appearance of vortices are to be noted to determine the possible locations biofouling would most probably occur. The inlet mass flow rate is 0.01 kgs-1 and a no-slip boundary condition was applied to the walls of the fluid domain. Simulations indicate that Velocity around the topography averaged at 7.213 x 10-3 ms-1. However, vortices were observed between the gaps. High wall shear stress is observed at the peak of each topography. In contrast, wall shear stress is significantly low at the bed of the topography. This suggests the potential location for the accumulation of biofouling. Results show that bioinspired antifouling topography can be improved by reducing the frequency of gaps between features. Linear surfaces on the topography should also be minimized. This increases the avenues of flow for the fluid, thus potentially increasing shear stresses with surrounding fluid leading to better antifouling performance.

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