Large-scale vortical structure of turbulent separation bubble affected by unsteady wake

The large-scale vortical structure of a turbulent separation bubble under the influence of an unsteady wake was investigated. The unsteady wake was generated by a spoked-wheel type wake generator installed in front of the separation bubble. This wake generator was rotated either clockwise or counter-clockwise at Re _H=5.600. The mechanism of vortex shedding from the separation bubble was analyzed in detail by taking a conditional average as well as a phase average. Spatial box filtering (SBF) was used to extract the large-scale vortical structure from the turbulent separation bubble affected by the unsteady wake. To elucidate the influence of the unsteady wake on the large-scale vortical structure, conditional averages of the velocity, vorticity and turbulent kinetic energy were calculated. The nature of the convection of the vortical structure under the influence of an unsteady wake was analyzed. The dipole acoustic pressure level was predicted using Curle's integral of wall-pressure fluctuations.     

Phase sampling in the analysis of a propeller wake

A phase sampling procedure is used for the analysis of the non-steady, periodic flow field in the near wake of a marine propeller. This method allows to obtain a true ensemble averaging of the experimental measurements. The average is made over a large number of repeated experiments each of which is taken during a complete revolution of the propeller. The measurements are carried out in a recirculating water tunnel with a two-channel laser Doppler velocimeter. The computer-aided evaluation of the experimental results visualizes the following characteristic features of the wake: (1) the vortex sheet developing from the trailing edge; (2) a sudden increase of the axial velocity in the core of the tip vortex; (3) a boundary layer effect near the shaft of the propeller. From the analysis of the direction of vortex rotation along the radial direction of the blade, it is possible to derive information on the working conditions of the propeller.     

Multi-resolution analysis of the large-scale coherent structure in a turbulent separation bubble affected by an unsteady wake

Multi-resolution analysis (MRA) was applied to the large-scale coherent structure in a turbulent separation bubble affected by an unsteady wake. The unsteady wake was generated using a spoked-wheel type wake generator, which was installed in front of the separation bubble. The wake generator was rotated either clockwise (CW) or counter-clockwise (CCW) with a normalized passing frequency of St_H=0.2. The Reynolds number based on the half-thickness of the blunt body was Re_H=5600. To show the unsteady dynamic flow structures between the ‘cutting’ and ‘wrapping’ regimes, a MRA using the maximal overlap discrete wavelet transform (MODWT) was performed. This method enabled delineation of the coherent structure of the turbulent separation bubble through a scale-resolved analysis. Reconstruction of the flow field in combination with conditional averaging was attempted. Flapping motions as well as sawtooth movements of the unsteady separation bubble were analyzed using the MODWT. The unsteady wakes decayed faster in the system with CCW rotation than in that with CW rotation.     

“Analysis of the propeller wake evolution by pressure and velocity phase measurements”

In the present study an experimental analysis of the velocity and pressure fields behind a marine propeller, in non-cavitating regime is reported. Particle image velocimetry measurements were performed in phase with the propeller angle, to investigate the evolution of the axial and the radial velocity components, from the blade trailing edge up to two diameters downstream. In phase pressure measurements were performed at four radial and eight longitudinal positions downstream the propeller model at different advance ratios. Pressure data, processed by using slotting techniques, allowed reconstructing the evolution of the pressure field in phase with the reference blade position. In addition, the correlation of the velocity and pressure signals was performed. The analysis demonstrated that, within the near wake, the tip vortices passage is the most important contribution in generating the pressure field in the propeller flow. The incoming vortex breakdown process causes a strong deformation of the hub vortex far downstream of the slipstream contraction. This process contributes to the pressure generation at the shaft rate frequency.     

Coherent vortical and straining structures in the finite wall-mounted square cylinder wake

Topological aspects of the turbulent wake of a finite, surface-mounted, square-cross-section cylinder of h/d = 4 are addressed by decomposing the velocity field into a quasi-periodic coherent part and the unresolved incoherent fluctuations. The three-dimensional large scale structure is educed through a reconstruction of planar phase-averaged PIV measurements using the simultaneously sampled surface pressure difference on opposing sides of the obstacle as a phase reference. A topological model for the vortex structure is educed and mean streamwise wake vorticity is explained in terms of the connections between initially vertical structures shed alternately from either side of the obstacle, rather than previously proposed ‘tip’ vortex structures generated at the obstacle free-end. The coherent structure educed accounts for a significant portion of the fluctuating energy in the wake. The turbulent field is further analyzed by finding Lagrangian straining structures that form by induction of the coherent vorticity field, and these structures are related to the energy transfer from the base phase-averaged flow since they act to stretch incoherent vorticity fluctuations in their neighbourhood.     

Education of coherent structures in a numerically simulated plane wake

Coherent structures (CS) are educed using a conditional sampling technique involving alignment of vorticity patches of largest size and strength; hence we educe dominant CS. A numerically simulated spatially evolving wake of a thick flat plate is used as the database, and the inflow condition for the simulated wake includes random velocity perturbations which emulate turbulent conditions at a plate exit in the laboratory. In addition to previously educed properties such as coherent vorticity and production, and incoherent Reynolds stress and turbulence intensity, other measures such as coherent pressure and passive scalar distributions are also studied. In spite of the geometry difference, the near-wake dynamics of the plate seem quite similar to that of a cylinder. For example, turbulence is mostly produced by vortex stretching of the ribs at the saddle and then advected to the structure center, where it accumulates, and is balanced by incoherent dissipation. The distribution of coherent passive scalar indicates that mixing occurs in the saddle regions and that the mixed fluid is advected into the structure center.     

Three-component velocity field measurements of propeller wake using a stereoscopic PIV technique

A stereoscopic PIV (Particle Image Velocimetry) technique was used to measure the three-dimensional flow structure of the turbulent wake behind a marine propeller with five blades. The out-of-plane velocity component was determined using two CCD cameras with an angular displacement configuration. Four hundred instantaneous velocity fields were measured for each of four different blade phases, and ensemble averaged in order to find the spatial evolution of the propeller wake in the region from the trailing edge up to one propeller diameter (D) downstream. The influence of propeller loading conditions on the wake structure was also investigated by measuring the velocity fields at three advance ratios (J=0.59, 0.72 and 0.88). The phase-averaged velocity fields revealed that a viscous wake formed by the boundary layers developed along the blade surfaces. Tip vortices were generated periodically and the slipstream contracted in the near-wake region. The out-of-plane velocity component and strain rate had large values at the locations of the tip and trailing vortices. As the flow moved downstream, the turbulence intensity, the strength of the tip vortices, and the magnitude of the out-of-plane velocity component at trailing vortices all decreased due to effects such as viscous dissipation, turbulence diffusion, and blade-to-blade interaction.     

Effect of the number of blades on propeller wake evolution

The effect of the number of blades on wake evolution was investigated on three propellers having the same blade geometry but different numbers of blades. The experiments concerned velocity measurements along nine transversal planes of the wake by LDV phase-sampling techniques. The study was performed with all the propellers having the same tip vortex intensity. In addition, high-speed visualizations were carried out to analyze the main features of propeller wake evolution in the transition and in the far wake. Aspects concerning wake evolution were pointed out, with particular emphasis on the instability mechanism of the propeller slipstream and on its correlation with the blade-to-blade interaction phenomenon.     

Flow of gas with a cellular vortical structure through a nozzle

The operation of rocket motors is often accompanied by the development of powerful secondary vortices in the combustion chamber [1–3], The superposition of the secondary vortices on the main flow leads to the formation of a cellular flow structure. Each of the cells represents a three-dimensional vortical circulation of the gas, and this causes a change in the working conditions of the nozzle. The model of helical motion [4] is used in this paper in considering the influence of the three-dimensional behavior of an adiabatic flow on the flow and traction characteristics of the nozzle.     

Fine structure of fluctuating fields in a wake

A semiempirical calculation is made of the root-mean-square values, scales, and spectra of fluctuating fields in the wake behind a body. The spectra and scales are represented by using the well-known results obtained for Isotropie turbulence in an incompressible fluid. Closure of the problem is achieved through the use of turbulent energy balance equations and root-mean-squares of fluctuating scalar fields. The wake parameters are calculated by means of an explicit finite-difference scheme. In contrast to standard computational methods, down-stream does not involve an increase in the number of nodes in the computational mesh in the radial direction because special variables are introduced into the equations. The calculated results for the rms values and the spectra of fluctuating density and electron-density fields in a hypersonic wake are compared to well-known experimental data.     

Cut-off analysis of coherent vortical structure identification in a three-dimensional external flow

Vortical structure identification has more recently been applied in the study of the transport of vortical structures in low Reynolds number three-dimensional complex geometry flows. An important issue in this identification procedure is to choose an appropriate cut-off value ${\lambda }_2$ which takes into consideration the finite precision vortex interfaces. This cut-off choice is studied in this Note and applied to an external flow around a curved cylinder. The vortex identification technique at different cut-off values is compared to the threshold of the vorticity field showing the efficiency of choosing the optimal tolerance gap. The computations are performed with a fully three-dimensional spectral/hp element method. To cite this article: A. Miliou et al., C. R. Mecanique 333 (2005).     

Wall pressure fluctuations of a turbulent separated and reattaching flow affected by an unsteady wake

The spatio-temporal characteristics of the wall-pressure fluctuations in separated and reattaching flows over a backward-facing step were investigated through pressure-velocity joint measurements carried out using multiple-arrayed microphones and split-film probes. A spoke-wheel-type wake generator was installed upstream of the backward-facing step. The flow structure at the effective forcing frequency (St _f=0.2) was found to be well organized in terms of wall pressure spectrum, cross-correlation, wavenumber-frequency spectrum, and wavelet auto-correlation. Introduction of the unsteady wake (St _f=0.2) reduced the reattachment length by 10%. In addition, the unsteady wake enhanced the turbulence intensity near the separation edge and, as a consequence, enhanced the quadrupole sound sources; however, the turbulence intensity near the reattachment region was weakened and the overall flow noise was attenuated. The greater organization of the flow structure induced by the unsteady wake led to a weakening of the dipole sound sources, which are the dominant sound sources in this system. The dipole sound sources generated by wall pressure fluctuations were calculated using Curles integral formula.Abbreviations AR Aspect ratio - SBF Spatial box filtering Roman symbols C _p Wall pressure fluctuation coefficient, p/0.5U ² - H Step height of backward-facing step (mm) - H _s Shape factor (H _s = ^*/) - R _s Distance from acoustic source point to observation point (m) - Re _H Reynolds number, U H/ - St The reduced frequency, fH/U - St _f Normalized forcing frequency by unsteady wake, f _p H/U - T Vortex shedding period (s) - U Free-stream velocity (m/s) - a Speed of sound (m/s) - f Frequency (Hz) - f _p Wake passing frequency (Hz) - k Turbulent kinetic energy (m²/s²) - k _x Streamwise wave number (1/m) - k _z Spanwise wave number (1/m) - l _j Cosine of angle - p Instantaneous wall pressure (Pa) - p _rms Root-mean-square of wall pressure (Pa) - p _SBF Spatial box filtered wall pressure (Pa) - p _d Dipole sound source (Pa) - p _w Conditionally-averaged wall pressure (Pa) - q Dynamic pressure, 0.5U ² (Pa) - r Distance from origin to observation point (mm) - u _c Convection velocity (m/s) - u_max Root-mean-square of streamwise velocity (m/s) - x _R Time-mean reattachment length (mm) Greek symbols _p Forward-flow time fraction - Auto-correlation of pressure at x ₀ - Two-dimensional cross-correlation of pressure with streamwise separation interval , spanwise separation interval , and time delay , at (x ₀, z ₀) - Boundary layer thickness (mm, _99%) - ^* Displacement thickness (mm, ) - _ij Kroneckers delta function - Phase angle (°) - Wavelength (mm) - Momentum thickness (mm, ) - Angle between vertical axis and observation point (°) - Density (kg/m³) - Time delay (s) - Streamwise separation interval (m) - Spanwise separation interval (m) - _p (f; x ₀) Autospectrum of pressure measured at x ₀ (Pa² s) - _pp (, ; x ₀) Streamwise cross spectrum of pressure at x ₀ (Pa² s) - _pp (, , ; x ₀, z ₀) Streamwise and spanwise cross spectrum of pressure at (x ₀, z ₀) (Pa² s) - _pp (kx, ; x ₀) Streamwise wavenumber-frequency spectrum of pressure at x ₀ (Pa² s) - _pp (kx, kz, ; x ₀, z ₀) Two-dimensional wavenumber-frequency spectrum of pressure at (x ₀, z ₀) (Pa² s)     

The near wake structure of a square cylinder

The near wake structure of a square cross section cylinder in flow perpendicular to its length was investigated experimentally over a Reynolds number (based on cylinder width) range of 6700–43,000. The wake structure and the characteristics of the instability wave, scaling on θ at separation, were strongly dependent on the incidence angle () of the freestream velocity. The nondimensional frequency (St_θ) of the instability wave varied within the range predicted for laminar instability frequencies for flat plate wakes, jets and shear layers. For = 22.5°, the freestream velocity was accelerated over the side walls and the deflection of the streamlines (from both sides of the cylinder) towards the center line was higher compared to the streamlines for = 0°. This caused the vortices from both sides of the cylinder to merge by x/d 2, giving the mean velocity distribution typical of a wake profile. For = 0°, the vortices shed from both sides of the cylinder did not merge until x/d 4.5. The separation boundary layer for all cases was either transitional or turbulent, yet the results showed good qualitative, and for some cases even quantitative, agreement with linearized stability results for small amplitude disturbances waves in laminar separation layers.     

基于结构化网格的导管螺旋桨敞水性能计算和艇桨干扰分析

本文采用结构化网格建模求解RANS方程,建立了导管桨和艇桨组合体的水动力性能数值计算方法.计算中以JD75简易导管与Ka4-70螺旋桨组合的导管螺旋桨及其与SUBOFF艇体模型的组合体为研究对象,利用RNG k-ε湍流模型封闭RANS方程得到控制方程.通过滑移网格方法计算了导管桨敞水性能,然后将该桨置于全附体SUBOFF潜艇模型后,计算螺旋桨与主艇体间的干扰特性,文中对流场等结果进行了分析比较.