Cavitation in the rotational structures of a turbulent wake

1995 ◽  
Vol 287 ◽  
pp. 383-403 ◽  
Author(s):  
B. Belahadji ◽  
J. P. Franc ◽  
J. M. Michel
Keyword(s):  
Pressure Coefficient ◽  
Three Dimensional ◽  
Turbulent Wake ◽  
Streamwise Vortices ◽  
Near Wake ◽  
Absolute Value ◽  
Cavitation Inception ◽  
Wake Vortices ◽  
Mean Pressure ◽  
The Mean

Experiments show that cavitation, if moderately developed, makes three kinds of vortical coherent structures visible inside the turbulent wake of a two-dimensional obstacle: Bénard–Kármán vortices, streamwise three-dimensional vortices and finally the vortices which appear on the borders of the very near wake. The latter, which are called here near-wake vortices, result by successive pairing in the first ones and there is some indication that they are also the origin of streamwise vortices. Cavitation is not a passive agent of visualization, as can be established on the basis of fundamental arguments, and it reacts with the flow as soon as it appears; when it is developed, it breaks the connection between the elongation rate and the vorticity rate of the vortex filaments. Then the subsequent evolution of a cavitating vortex and its final implosion are rather complicated. Despite its active character, cavitation in rotational structures, if properly interpreted, can give information of interest on the basic non-cavitating turbulent flow. By adapting a simple model due to Kermeen & Parkin (1957) and Arndt (1976), and counting near-wake vortices, it is possible to accurately predict the conditions of cavitation inception: consideration of coherent rotational structures is probably the best approach to explain, in an almost deterministic way, the large difference between the absolute value of the mean pressure coefficient at the obstacle base and the incipient cavitation number.

An Investigation of the Forces on Three Dimensional Bluff Bodies in Rough Wall Turbulent Boundary Layers

1977 ◽  
Vol 99 (3) ◽  
pp. 503-509 ◽  
Author(s):  
B. E. Lee ◽  
B. F. Soliman
Keyword(s):  
Three Dimensional ◽  
The Body ◽  
Bluff Bodies ◽  
Drag Forces ◽  
Pressure Distributions ◽  
Mean Pressure ◽  
Flow Phenomena ◽  
The Mean ◽  
Building Ventilation ◽  
Group Density

A study has been made of the influence of grouping parameters on the mean pressure distributions experienced by three dimensional bluff bodies immersed in a turbulent boundary layer. The range of variable parameters has included group density, group pattern and incident flow type and direction for a simple cuboid element form. The three flow regimes associated with increasing group density are reflected in both the mean drag forces acting on the body and their associated pressure distributions. A comparison of both pressure distributions and velocity profile parameters with established work on two dimensional bodies shows close agreement in identifying these flow regime changes. It is considered that the application of these results may enhance our understanding of some common flow phenomena, including turbulent flow over rough surfaces, building ventilation studies and environmental wind around buildings.

Studies on high speed jets from nozzles with internal grooves

2004 ◽  
Vol 108 (1079) ◽  
pp. 43-50 ◽  
Author(s):  
S. Elangovan ◽  
E. Rathakrishnan
Keyword(s):  
High Speed ◽  
Cell Structure ◽  
Pressure Profile ◽  
Equivalent Diameter ◽  
Streamwise Vortices ◽  
Shock Cell ◽  
Underexpanded Jets ◽  
Pitot Pressure ◽  
Mean Pressure ◽  
The Mean

Experiments were carried out on jets issuing from circular nozzles with grooved exits and the results compared with those of the plain nozzle. The plain nozzle had an exit diameter of 10mm. Because of the introduction of semi-circular grooves at the exit, the effective or equivalent diameter of the grooved nozzles was 10·44mm. The groove lengths were varied as 3, 5 and 8mm. The nozzles were operated at fully expanded sonic and underexpanded exit conditions. The corresponding fully expanded Mach numbers were 1·0 and 1·41. The shock cell structure of the underexpanded jets from grooved nozzles appeared to be weaker than that of the plain nozzle, as indicated by lesser amplitudes of the cyclic variation of the Pitot pressure. The iso-Mach contours indicate that the jet spread along the grooved plane is significantly higher than that along the ungrooved plane. Off-centre peaks were observed in the mean pressure profile of underexpanded jets from grooved nozzles. They were probably due to the streamwise vortices shed from the grooves.

Near-Bed Flow Mechanisms Around a Circular Marine Pipeline Close to a Flat Seabed in the Subcritical Flow Regime Using a k-ɛ Model

2011 ◽  
Vol 134 (2) ◽  
Author(s):  
Muk Chen Ong ◽  
Torbjørn Utnes ◽  
Lars Erik ◽  
Dag Myrhaug ◽  
Bjørnar Pettersen
Keyword(s):  
Experimental Data ◽  
Boundary Layer ◽  
Flow Regime ◽  
Boundary Layer Thickness ◽  
Friction Velocity ◽  
Lift Coefficient ◽  
Pressure Coefficient ◽  
Mean Pressure ◽  
Flow Mechanisms ◽  
The Mean

Flow mechanisms around a two-dimensional (2D) circular marine pipeline close to a flat seabed have been investigated using the 2D unsteady Reynolds-averaged Navier–Stokes (URANS) equations with a standard high Reynolds number k-ɛ model. The Reynolds number (based on the free stream velocity and cylinder diameter) ranges from 1 × 104 to 4.8 × 104 in the subcritical flow regime. The objective of the present study is to show a thorough documentation of the applicability of the k-ɛ model for engineering design within this flow regime by means of a careful comparison with available experimental data. The inflow boundary layer thickness and the Reynolds numbers in the present simulations are set according to published experimental data, with which the simulations are compared. Detailed comparisons with the experimental data for small gap ratios are provided and discussed. The effects of the gap to diameter ratio and the inflow boundary layer thickness have been studied. Although under-predictions of the essential hydrodynamic quantities (e.g., time-averaged drag coefficient, time-averaged lift coefficient, root-mean-square fluctuating lift coefficient, and mean pressure coefficient at the back of the pipeline) are observed due to the limitation of the turbulence model, the present approach is capable of providing good qualitative agreement with the published experimental data. The vortex shedding mechanisms have been investigated, and satisfactory predictions are obtained. The mean pressure coefficient and the mean friction velocity along the flat seabed are predicted reasonably well as compared with published experimental and numerical results. The mean seabed friction velocity at the gap is much larger for small gaps than for large gaps; thus, the bedload sediment transport is much larger for small gaps than for large gaps.

Quantitative measurements of three-dim ensional structures in the wake of a circular cylinder

1994 ◽  
Vol 270 ◽  
pp. 277-296 ◽  
Author(s):  
Hussein Mansy ◽  
Pan-Mei Yang ◽  
David R. Williams
Keyword(s):  
Circular Cylinder ◽  
Secondary Flow ◽  
Three Dimensional ◽  
Streamwise Vortices ◽  
Mean Flow ◽  
Near Wake ◽  
Three Dimensional Flow ◽  
Primary Flow ◽  
Flow Component ◽  
Dimensional Flow

The fine scale three-dimensional structures usually associated with streamwise vortices in the near wake of a circular cylinder have been studied at Reynolds numbers ranging from 170 to 2200. Spatially continuous velocity measurements along lines parallel to the cylinder axis were obtained with a scanning laser anemometer. To detect the streamwise vortices in the amplitude modulated velocity field, it was necessary to develop a spatial decomposition technique to split the total flow into a primary flow component and a secondary flow component. The primary flow is comprised of the mean flow and Strouhal vortices, while the secondary flow is the result of the three-dimensional streamwise vortices that are the essence of transition to turbulence. The three-dimensional flow amplitude increases in the primary vortex formation region, then saturates shortly after the maximum amplitude in the primary flow is reached. In the near-wake region the wavelength decreases approximately like Re−0.5, but increases with downstream distance. A discontinuous increase in wavelength occurs below Re = 300 suggesting a fundamental change in the character of the three-dimensional flow. At downstream distances (x/D = 10-20), the spanwise wavelength decreases from 1.42D to 1.03D as the Reynolds number increases from 300 to 1200.

scholarly journals Large-eddy simulation of three-dimensional dunes in a steady, unidirectional flow. Part 1. Turbulence statistics

2013 ◽  
Vol 721 ◽  
pp. 454-483 ◽  
Author(s):  
Mohammad Omidyeganeh ◽  
Ugo Piomelli
Keyword(s):  
Secondary Flow ◽  
Three Dimensional ◽  
Wall Stress ◽  
Two Dimensional ◽  
Streamwise Vortices ◽  
Mean Flow ◽  
Form Drag ◽  
Separated Shear Layer ◽  
Large Eddy ◽  
The Mean

AbstractWe performed large-eddy simulations of flow over a series of three-dimensional dunes at laboratory scale (Reynolds number based on the average channel depth and streamwise velocity was 18 900) using the Lagrangian dynamic eddy-viscosity subgrid-scale model. The bedform three-dimensionality was imposed by shifting a standard two-dimensional dune shape in the streamwise direction according to a sine wave. The statistics of the flow are discussed in 10 cases with in-phase and staggered crestlines, different deformation amplitudes and wavelengths. The results are validated qualitatively against experiments. The three-dimensional separation of flow at the crestline alters the distribution of wall pressure, which in turn may cause secondary flow across the stream, which directs low-momentum fluid, near the bed, toward the lobe (the most downstream point on the crestline) and high-momentum fluid, near the top surface, toward the saddle (the most upstream point on the crestline). The mean flow is characterized by a pair of counter-rotating streamwise vortices, with core radius of the order of the flow depth. However, for wavelengths smaller than the flow depth, the secondary flow exists only near the bed and the mean flow away from the bed resembles the two-dimensional case. Staggering the crestlines alters the secondary motion; the fastest flow occurs between the lobe and the saddle planes, and two pairs of streamwise vortices appear (a strong one, centred about the lobe, and a weaker one, coming from the previous dune, centred around the saddle). The distribution of the wall stress and the focal points of separation and attachment on the bed are discussed. The sensitivity of the average reattachment length, depends on the induced secondary flow, the streamwise and spanwise components of the channel resistance (the skin friction and the form drag), and the contribution of the form drag to the total resistance are also studied. Three-dimensionality of the bed increases the drag in the channel; the form drag contributes more than in the two-dimensional case to the resistance, except for the staggered-crest case. Turbulent-kinetic energy is increased in the separated shear layer by the introduction of three-dimensionality, but its value normalized by the plane-averaged wall stress is lower than in the corresponding two-dimensional dunes. The upward flow on the stoss side and higher deceleration of flow on the lee side over the lobe plane lift and broaden the separated shear layer, respectively, affecting the turbulent kinetic energy.

Vortical structures in the near wake of tabs with various geometries

2017 ◽  
Vol 825 ◽  
pp. 167-188 ◽  
Author(s):  
A. M. Hamed ◽  
A. Pagan-Vazquez ◽  
D. Khovalyg ◽  
Z. Zhang ◽  
L. P. Chamorro
Keyword(s):  
Large Scale ◽  
Three Dimensional ◽  
Three Dimensions ◽  
Incoming Flow ◽  
Mean Flow ◽  
Near Wake ◽  
Hairpin Vortices ◽  
Vortical Structures ◽  
The Mean ◽  
Hairpin Structures

The vortical structures and turbulence statistics in the near wake of rectangular, trapezoidal, triangular and ellipsoidal tabs were experimentally studied in a refractive-index-matching channel. The tabs share the same bulk dimensions, including a 17 mm height, a 28 mm base width and a $24.5^{\circ }$ inclination angle. Measurements were performed at two Reynolds numbers based on the tab height, $Re_{h}\simeq 2000$ (laminar incoming flow) and 13 000 (turbulent incoming flow). Three-dimensional, three-component particle image velocimetry (PIV) was used to study the mean flow distribution and dominant large-scale vortices, while complementary high-spatial-resolution planar PIV measurements were used to quantify high-order statistics. Instantaneous three-dimensional fields revealed the coexistence of a coherent counter-rotating vortex pair (CVP) and hairpin structures. The CVP and hairpin vortices (the primary structures) exhibit distinctive characteristics and strength across $Re_{h}$ and tab geometries. The CVP is coherently present in the mean flow field and grows in strength over a significantly longer distance at the low $Re_{h}$ due to the lower turbulence levels and the delayed shedding of the hairpin vortices. These features at the low $Re_{h}$ are associated with the presence of Kelvin–Helmholtz instability that develops over three tab heights downstream of the trailing edge. Moreover, a secondary CVP with an opposite sense of rotation resides below the primary one for the four tabs at the low $Re_{h}$. The interaction between the hairpin structures and the primary CVP is experimentally measured in three dimensions and shows complex coexistence. Although the CVP undergoes deformation and splitting at times, it maintains its presence and leads to significant mean spanwise and wall-normal flows.

Interference effects of three consecutive wall-mounted cubes placed in deep turbulent boundary layer

2014 ◽  
Vol 756 ◽  
pp. 165-190
Author(s):  
Hee Chang Lim ◽  
Masaaki Ohba
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Pressure Coefficient ◽  
Azimuth Angle ◽  
Pressure Variation ◽  
Interference Effects ◽  
Detached Eddy Simulation ◽  
Eddy Simulation ◽  
Mean Pressure ◽  
The Mean

AbstractIn this study we undertook various calculations of the turbulent flow around a building in close proximity to neighbouring obstacles, with the aim of gaining an understanding of the velocity and the surface-pressure variations with respect to the azimuth angle of wind direction and the gap distance between the obstacles. This paper presents the effects of flow interference among consecutive cubes for azimuth angles of $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}\theta = 0$, 15, 30, and $45^{\circ }$ and gap distances of $G = 0.5{h}, 1.0{h}, 1.5{h}$, and $\infty $ (i.e. a single cube), where $h$ is the cube height, placed in a turbulent boundary layer. A transient detached eddy simulation (DES) was carried out to calculate the highly complicated flow domain around the three wall-mounted cubes to observe the fluctuating pressure, which substantially contributes to the suction pressure when there is separation and reattachment around the leading and trailing edges of the cubes. In addition, the results indicate that an increasing azimuth angle increases the pressure variation on the centre cube of the three parallel-aligned cubes. The mean pressure variation can even change from negative to positive on the side face. Owing to interference effects, the mean pressure coefficient of the centre cube of the three parallel-aligned cubes was generally lower than the coefficient of the single cube and tended to increase depending on the gap distance. Furthermore, when the three consecutive cubes are in a tandem arrangement, the gap distance has little influence on the first cube but results in significant interference effects on the second and third cubes.

The Wake Behind Two Intersecting Flat Plates

2014 ◽  
Author(s):  
Fatemeh Hoseini Dadmarzi ◽  
Vagesh D. Narasimhamurthy ◽  
Helge I. Andersson ◽  
Bjørnar Pettersen
Keyword(s):  
Reynolds Number ◽  
Flat Plate ◽  
Three Dimensional ◽  
Wake Flow ◽  
Flat Plates ◽  
Total Drag ◽  
Vortex Streets ◽  
Mean Pressure ◽  
The Mean ◽  
Plate Width

The wake flow behind two intersecting flat plates forming a cross is studied by means of direct numerical simulation (DNS) at low Reynolds number. The Reynolds number based on the plate width, d, and the inflow velocity, U0, is 100. The flat plate structure is in one plane. Away from the intersecting center part, vortex streets can be observed similar to the wake flow behind a single normal flat plate. On the other hand, the flow is completely three-dimensional in the vicinity of the intersecting region where the wakes from each of the two normal flat plates interact with each other. The mean pressure distribution on the structure is evaluated in order to study the total drag force as well as the local force distribution.

Field Measurement of Boundary Layer Wind Characteristics and Wind Loads on Super-Tall Building

2011 ◽  
Vol 243-249 ◽  
pp. 5128-5135
Author(s):  
Wen Hai Shi ◽  
Zheng Nong Li
Keyword(s):  
Pressure Coefficient ◽  
Tall Buildings ◽  
Tall Building ◽  
Recent Past ◽  
Comprehensive Investigation ◽  
Pressure Fields ◽  
Mean Pressure ◽  
Wind Characteristics ◽  
The Mean

Significance of full-scale experiments, analyzing wind and pressure fields in the proximity or on tall buildings, is evident from the attention that has been dedicated by researchers to these programs in the recent past. In the south and southeastern regions of China this problem is of particular relevance due to the presence of Typhoons. This paper presents some recent results measured from a super tall building located near the coast of Xiamen, Fujian province, China. In the first part of this study, attention is devoted to the characterization of the wind field atop a super-tall building; a comprehensive investigation on wind velocity and turbulence characteristics during the passage of Typhoon Fanapi is summarized. In the second part, results associated with the mean pressure and mean pressure coefficient were concentrated on the identification of direction-dependent pressure characteristics is analyzed.

Mechanisms for generating coherent packets of hairpin vortices in channel flow

1999 ◽  
Vol 387 ◽  
pp. 353-396 ◽  
Author(s):  
J. ZHOU ◽  
R. J. ADRIAN ◽  
S. BALACHANDAR ◽  
T. M. KENDALL
Keyword(s):  
Channel Flow ◽  
Three Dimensional ◽  
Wall Layer ◽  
Critical Threshold ◽  
Streamwise Vortices ◽  
Mean Flow ◽  
Hairpin Vortices ◽  
Threshold Strength ◽  
Upstream Process ◽  
The Mean

The evolution of a single hairpin vortex-like structure in the mean turbulent field of a low-Reynolds-number channel flow is studied by direct numerical simulation. The structure of the initial three-dimensional vortex is extracted from the two-point spatial correlation of the velocity field by linear stochastic estimation given a second-quadrant ejection event vector. Initial vortices having vorticity that is weak relative to the mean vorticity evolve gradually into omega-shaped vortices that persist for long times and decay slowly. As reported in Zhou, Adrian & Balachandar (1996), initial vortices that exceed a threshold strength relative to the mean flow generate new hairpin vortices upstream of the primary vortex. The detailed mechanisms for this upstream process are determined, and they are generally similar to the mechanisms proposed by Smith et al. (1991), with some notable differences in the details. It has also been found that new hairpins generate downstream of the primary hairpin, thereby forming, together with the upstream hairpins, a coherent packet of hairpins that propagate coherently. This is consistent with the experimental observations of Meinhart & Adrian (1995). The possibility of autogeneration above a critical threshold implies that hairpin vortices in fully turbulent fields may occur singly, but they more often occur in packets. The hairpins also generate quasi-streamwise vortices to the side of the primary hairpin legs. This mechanism bears many similarities to the mechanisms found by Brooke & Hanratty (1993) and Bernard, Thomas & Handler (1993). It provides a means by which new quasi-streamwise vortices, and, subsequently, new hairpin vortices can populate the near-wall layer.

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