Calculation of Turbulent Boundary Layer for a Slot Jet Impingement on a Flat Surface

2014 ◽  
Author(s):  
H. Arabnejad ◽  
A. Mansouri ◽  
S. A. Shirazi ◽  
B. S. McLaury
Keyword(s):  
Experimental Data ◽  
Boundary Layer ◽  
Flat Surface ◽  
Jet Impingement ◽  
Turbulent Jets ◽  
Integral Form ◽  
Shear Stresses ◽  
Submerged Jet ◽  
Flow Parameters ◽  
Wall Shear

The objective of this study is to characterize flow parameters for two-dimensional turbulent jets impinging on a flat surface. An integral form of the momentum equation has been used to obtain a hydrodynamic solution. The boundary layer was divided into three regions, stagnation zone, developing zone and fully developed zone for free-surface and free shear, and into two regions, stagnation and wall jet zone for submerged jet configurations. A nonlinear ordinary differential equation has been obtained for frictional velocity at each zone using a logarithmic velocity profile with Coles’s law of the wake and solved numerically to predict wall shear stress as well as boundary layer and momentum thicknesses. The proposed method is more straightforward and computationally less expensive in calculating the main flow parameters as compared to turbulent flow models such as RANS and LES. Predicted wall shear stresses for a submerged jet were compared to experimental data for different cases and showed agreement with experimental data.

Critical Heat Flux During Submerged Jet Impingement Boiling of Saturated Water at Sub-Atmospheric Conditions

2011 ◽  
Author(s):  
Ruander Cardenas ◽  
Vinod Narayanan
Keyword(s):  
Experimental Data ◽  
Heat Flux ◽  
Critical Heat Flux ◽  
Jet Impingement ◽  
Atmospheric Conditions ◽  
Predictive Tool ◽  
Submerged Jet ◽  
Fluid Saturation ◽  
Saturated Water ◽  
Jet Impingement Boiling

Experimental data for critical heat flux (CHF) during submerged jet impingement boiling of saturated water at sub-atmospheric conditions is presented. Experiments are performed at three sub-atmospheric pressures of 0.176 bar, 0.276 bar, and 0.477 bar with corresponding fluid saturation temperatures of about 57.3 °C, 67.2 °C, and 80.2 °C. Jet exit Reynolds numbers ranging from 0 to 14,000 are considered for two different heater surface finishes at a fixed nozzle to surface spacing of six nozzle diameters. CHF correlations from literature on jet impingement boiling are compared against the experimental data and found to poorly predict CHF under the conditions considered. A CHF correlation that captures the entire experimental data set within an average error of ±3 percent and a maximum error of ±13 percent is developed to serve as a predictive tool for the range of conditions examined.

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.

Impact of the artery diameter and the surgical patch geometry on the boundary layer thickness and wall shear stresses distribution

Energy ◽  
2020 ◽  
Vol 197 ◽  
pp. 117216
Author(s):  
Natalia Lewandowska ◽  
Michał Ciałkowski ◽  
Bartosz Ziegler ◽  
Joanna Jójka
Keyword(s):  
Boundary Layer ◽  
Layer Thickness ◽  
Boundary Layer Thickness ◽  
Shear Stresses ◽  
Wall Shear ◽  
Patch Geometry

Thermal Boundary Layer Response to Periodic Fluctuations for Turbulent Flow

2018 ◽  
Vol 141 (3) ◽  
Author(s):  
J. Saavedra ◽  
G. Paniagua ◽  
O. Chazot
Keyword(s):  
Boundary Layer ◽  
Thermal Boundary Layer ◽  
Low Frequency ◽  
Shear Stresses ◽  
Periodic Flow ◽  
Mean Flow ◽  
Flow Topology ◽  
Flow Acceleration ◽  
Periodic Fluctuations ◽  
Wall Shear

The detailed characterization of the thermal boundary layer under periodic fluctuations is vital to improve the performance of cooled turbine airfoils, as well as to assess noise thermal and structural fatigue. In the present contribution, we performed detailed unsteady Reynolds-averaged Navier–Stokes (URANS) simulations to investigate wall heat flux response to periodic flow velocity fluctuations over a flat plate. We also investigated the boundary layer response to sudden flow acceleration including periodic flow perturbations, caused by inlet total pressure variations. During a flow acceleration phase, the boundary layer is first stretched, resulting in an increase of the wall shear stress. Later on, due to the viscous diffusion, the low momentum flow adjusts to the new free stream conditions. The behavior of the boundary layer at low frequency is similar to the response to an individual deceleration followed by one acceleration. However, at higher frequencies, the mean flow topology is completely altered. One would expect that higher acceleration rates would cause a further stretching of the boundary layer that should cause even greater wall shear stresses and heat fluxes. However, we observed the opposite; the amplitude of the skin friction coefficient is abated, while the peak level is a full order of magnitude smaller than at low frequency.

Flow in the Initial Region of Axisymmetric Turbulent Jets

1980 ◽  
Vol 102 (1) ◽  
pp. 85-91 ◽  
Author(s):  
S. M. N. Islam ◽  
H. J. Tucker
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Nonlinear Function ◽  
Turbulent Jets ◽  
Integral Form ◽  
Eddy Diffusivity ◽  
Axial Distance ◽  
Free Shear Layer ◽  
Initial Region ◽  
Mean Velocity

In the initial region of axisymmetric turbulent jets a core of uniform velocity is assumed to exist, bounded by an annular free shear layer. An empirical model for axial mean velocity is found from experimental measurements using a length scale which forces self-preservation in the central part of the free shear layer. This model is applied to the integral form of the momentum and energy equations, subject to the boundary layer simplifications, to obtain an approximate solution for the development of jets where the thickness of the mixing layer at the nozzle exit is assumed negligible. The differential form of momentum and continuity equations are also solved by a finite difference technique of DuFort-Frankel type using a typical boundary layer type of velocity profile at the exit of the nozzle. The results of this method are compared with those of the empirical velocity method, and the present and existing experimental results. Prandtl’s mixing length is shown to be a slightly nonlinear function of the axial distance and is used to define the eddy diffusivity for this region.

CFD for Jet Impingement Heat Transfer With Single Jets and Arrays

2005 ◽  
Author(s):  
Mounir B. Ibrahim ◽  
Bejoy J. Kochuparambil ◽  
Srinath V. Ekkad ◽  
Terrence W. Simon
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Flat Surface ◽  
Turbulent Transport ◽  
Jet Impingement ◽  
Target Plate ◽  
Plate Spacing ◽  
Multiple Jets ◽  
Single Jet ◽  
Jet Axis

CFD experiments were conducted for heat transfer with jet impingement over solid surfaces. The parameters include: 1) Jet Reynolds number from 3,000 to 23,000, 2) Jet-to-target-plate spacing (z/d), from 2 to 14 (single jet), d is jet diameter, 3) Target plate shape: 3a) flat, 3b) concave, 3c) convex, (single jet), 4) One row of seven jets impinging on a flat surface, the channel has one end closed (at 24d away from the most upstream jet axis), 5) Three rows of seven jets each in-line arrangement impinging on a flat surface, the channel has one end closed (at 24d away from the most upstream jet axis). Four CFD models (utilizing FLUENT commercial code) have been considered: 1) laminar flow (no turbulent transport), and turbulent flow with turbulence modeling by 2) the standard k–ε model, 3) the k–ω model, and 4) the v2–f model. The predictions of Nu number for each case were compared with experimental data available from the literature. It is shown that the v2–f model gives the best overall performance, though the k–ω model gives good predictions for most of the flow, with the exception of near the stagnation zone for some cases. The models are in much better agreement (with the data) as z/d grows and at larger radial locations from the jet axis, as expected. For multiple jets in one row (z/d = 2), again the v2–f showed the best overall agreement with the experimental data. The k–ω model is not as good while k–ε clearly overpredicts the Nusselt numbers. For multiple jets in three inline rows (z/d = 5), all the three models were in overall agreement with the experimental data. However, k–ε and k–ω exhibit an important phenomenon, reported by the experiments: a decrease of the stagnation Nu from the upstream jet to the downstream ones. The v2–f model did not reproduce this feature.

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.

Pulsatile Flow in an End-to-Side Vascular Graft Model: Comparison of Computations With Experimental Data

2000 ◽  
Vol 123 (1) ◽  
pp. 80-87 ◽  
Author(s):  
M. Lei ◽  
D. P. Giddens ◽  
S. A. Jones ◽  
F. Loth ◽  
H. Bassiouny
Keyword(s):  
Fluid Dynamics ◽  
Experimental Data ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Intimal Hyperplasia ◽  
Vascular Graft ◽  
Small Diameter ◽  
Shear Stresses ◽  
Wall Shear

Various hemodynamic factors have been implicated in vascular graft intimal hyperplasia, the major mechanism contributing to chronic failure of small-diameter grafts. However, a thorough knowledge of the graft flow field is needed in order to determine the role of hemodynamics and how these factors affect the underlying biological processes. Computational fluid dynamics offers much more versatility and resolution than in vitro or in vivo methods, yet computations must be validated by careful comparison with experimental data. Whereas numerous numerical and in vitro simulations of arterial geometries have been reported, direct point-by-point comparisons of the two techniques are rare in the literature. We have conducted finite element computational analyses for a model of an end-to-side vascular graft and compared the results with experimental data obtained using laser-Doppler velocimetry. Agreement for velocity profiles is found to be good, with some clear differences near the recirculation zones during the deceleration and reverse-flow segments of the flow waveform. Wall shear stresses are determined from velocity gradients, whether by computational or experimental methods, and hence the agreement for this quantity, while still good, is less consistent than for velocity itself. From the wall shear stress numerical results, we computed four variables that have been cited in the development of intimal hyperplasia—the time-averaged wall shear stress, an oscillating shear index, and spatial and temporal wall shear stress gradients—in order to illustrate the versatility of numerical methods. We conclude that the computational approach is a valid alternative to the experimental approach for quantitative hemodynamic studies. Where differences in velocity were found by the two methods, it was generally attributed to the inability of the numerical method to model the fluid dynamics when flow conditions are destabilizing. Differences in wall shear, in the absence of destabilizing phenomena, were more likely to be caused by difficulties in calculating wall shear from relatively low resolution in vitro data.

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