Fundamentally excited flow past a surface-mounted rib. Part II: Kinetic energy budget details

This paper presents the detailed turbulent kinetic energy budget and higher order statistics of flow behind a surface-mounted rib with and without superimposed acoustic excitation. Pattern recognition technique is used to determine the large-scale structure magnitude. It is observed that most of the turbulence contributions after the reattachment region are from the large-scale structures contrary to the belief that mostly random turbulent structures are present after reattachment. The dissipation is not a small-scale phenomena only. It may result due to the interaction between large-scale structures. From the results of higher order moments, the outer edge of the shear layer is observed to be non-Gaussian in nature with significant deviation from the Gaussian skewness and flatness value. The kinetic energy budget results show positive intermodal production in the outer edge of the shear layer indicating the presence of back scattering. The non-Gaussian velocity distribution, ejection motions and back-scattering present in the outer shear layer may be conjectured to be correlated with each other.     

Turbulence characteristics of open channel flow over non-equilibrium 3-D mobile dunes

This paper reports velocity measurements over mobile dunes using an acoustic Doppler velocimetry (ADV). Experiments were conducted with two different flow conditions resulting in the formation of two different size mobile dunes. Dunes height, wavelength and velocity of dunes found to be increasing with increase in average flow velocity for a constant flow depth. The quasi-stationary bed condition was assumed while measuring the velocity distribution along the depth. The effect of the non-equilibrium mobile dunes on the flow characteristics and turbulence is examined by computing turbulent intensities, turbulent kinetic energy and Reynolds shear stresses using time averaged and time–space averaged velocity measurements. The magnitudes of transverse velocities are approximately 1/10 of streamwise velocities and vertical velocities are approximately half of the transverse velocities. The considerable magnitudes of transverse velocities over mobile bedforms necessitate measurement of 3-D velocity components to analyze the flow field. Computed turbulence intensities are found to be maximum in the region consisting of the trough and the reattachment point of the dunes. It is observed that streamwise turbulence intensities near the bed are twice the transverse turbulence intensities, and transverse turbulence intensities are twice the vertical turbulence intensities. Reynolds stresses (transverse fluxes of streamwise and vertical momentum) are observed to be high on mobile bedforms which shows mobile dunes reinforce the secondary currents. Peak values of turbulent kinetic energy (TKE) and Reynolds stresses are also found in the region consisting of the trough and the reattachment point. It is visually observed in the present experiments that maximum erosion takes place at the reattachment point and eroded sediment is carried as total load and dropped on the lee slope of the subsequent downstream dune. This phenomenon is caused by flow expansion in the separation zone, and which is also the main reason for mobility of dunes and associated bedload transport. Most importantly, it is found that turbulence anisotropy increases with increase in size of mobile bedforms and anisotropy is extended up to the free surface in the flows over mobile bedforms, which proves the entire depth of flow is being disturbed by the mobile dunes.     

基于PIV技术的平屋盖表面分离泡流动结构研究

该文利用粒子图像测速技术,通过风洞流场显示试验,观察了大跨平屋盖表面的分离泡现象,给出了不同来流工况下多个可视化平面的旋涡流线和涡量场分布。试验结果表明,当风向垂直于平屋盖迎风前缘时,屋盖顶面将出现典型的分离泡现象,涡量场的负向峰值出现在迎风前缘处,屋面风压力随离迎风前缘距离的增加而减小。均匀流场下流动将在迎风边缘产生分离而后再附,再附长度近乎横跨整个屋盖;而湍流场中的小尺度湍流促使分离剪切层较早地再附形成分离泡,且湍流度越大,旋涡再附长度越短。运用FLUENT模拟了平屋盖表面的分离泡,与流动显示试验结果吻合较好。通过多个可视化平面的综合分析得到分离泡的三维形态特征,建立了旋涡的演化、涡核位置与建筑物表面压力分布的内在联系,获得了若干有价值的结论。     

Hydrodynamic structures — elements of the self-sustained oscillation system in the flow

The character of hydrodynamic disturbances in the near-wall shear flow inducing self-oscillations in the channel has been investigated. Experiments have been performed in a small and a large wind tunnel at a flow velocity of 10–40 m / sec. It has been found that excitation of self-oscillations becomes possible as a result of narrow-band acoustic radiation generation by not only vortex formations but also large-scale coherent structures in the region of the laminar-turbulent transition and in the turbulent boundary layer.     

Excitation of self-oscillations in flow past a surface with a recess and the possibility of preventing them

The results of experimental investigations of acoustic radiation initiated by hydrodynamic perturbations at the inlet to a cavity on a surface with a stream flowing past it are analyzed. A comparison is made between the processes of formation of large-scale hydrodynamic vibrations in a shear flow shed from the leading edge of the junction of the recess with the surface past which a stream flows and the processes in the region of transition from a laminar boundary layer on the surface of a wing to a turbulent one. The advisability of division of the flow inhomogeneity in the zone of the junction in order to prevent or weaken self-oscillations in the flow is estimated. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 3, pp. 148–154, May–June, 2007.     

Edge states intermediate between laminar and turbulent dynamics in pipe flow

We studied the dynamics near the boundary between laminar and turbulent dynamics in pipe flow. This boundary contains invariant dynamical states that are attracting when the dynamics is confined to the boundary. These states can be found by controlling a single quantity, in our case the energy content. The edge state is dominated by two downstream vortices and shows intrinsic chaotic dynamics. With increasing Reynolds number the separation between the edge state and turbulence increases. We can track it down to Re=1900, where the turbulent lifetimes are short enough that spontaneous decay can also be seen in experiments.     

Wavelet diagnostics for detection of coherent structures in instantaneous turbulent flow imagery: A review

A review of work over the last decade shows that 2D wavelet techniques applied on flow imagery can provide powerful insights into the nature and lifecycle of coherent structures (the latter through wavelet movies) in turbulent shear flows. The advantage of wavelet techniques in often being able to infer the nature of coherent motion from a single image is emphasized. The techniques are first calibrated by using them on well-known results in the turbulent mixing layer. They are then applied to jets and plumes, and it is shown how off-source heating in such flows can disrupt the coherent structures in the unheated flow. A suitably reduced version of the present method, using discrete wavelet transforms on signals from a finite array of sensors, could be a useful diagnostic tool in near-real-time detection of coherent structures or patterns for the purpose of selecting appropriate control signals to the actuators in a flow-control system.     

Rotating-disk-type flow over loose boundaries

Rotating-disk-type flow of a liquid over a loose boundary, such as a layer of sand, is investigated. For this flow the formation of a new large-scale spiral pattern has been discovered. The new pattern is reminiscent of the Type-I spiral-vortex structures which characterize the laminar–turbulent transition region of boundary layers over rigid rotating disks. Flow visualizations reveal that the new pattern and the Type-I spiral vortices co-exist in the loose-boundary flow. The research investigating the origin of the new large-scale pattern is reviewed. Then photographs from flow visualizations are analysed to obtain estimates for the critical Reynolds number for which Type-I spiral vortices first appear for the loose-boundary flow and for the critical Reynolds numbers for the laminar–turbulent transition of the boundary layer. The results suggest that Type-I vortices appear at much lower Reynolds numbers over loose boundaries in comparison with flow over rigid rotating disks and that transition also appears to be advanced to much lower Reynolds numbers. The discussion of the results suggests that advanced transition arises from disturbances introduced into the flow after the loose boundary has been mobilized and not from disturbances associated with the roughness that the surfaces of the granular layer represents to the flow while grains are at rest.     

Experimental study of the spill and vaporization of a volatile liquid

Pool and vapor cloud characteristics of an acetone spill issuing from the downstream wall of a flow obstruction oriented perpendicular to a uniform flow were investigated experimentally. Data indicate that the spill event was largely governed by the temperature of the surface in relation to the boiling point of the spilled liquid. The free stream velocity (ranging from 0.75 to 3.0m/s) also impacted the spreading of the spill. Planar laser-induced fluorescence (PLIF) was used to measure acetone vapor concentrations during the transient pool spreading and vaporization in a window 60cm long by 50cm high and located downstream of the 16cm high obstruction. The recirculation region induced by the flow obstruction caused upstream transport of the acetone vapor along the spill surface, after which it was convected vertically along the obstruction wall before being entrained into the flow and convected downstream. The recirculating flow caused regions of vapor within the flammability limits to be localized near the flow obstruction. These regions moved into and out of the measurement plane by large three-dimensional flow structures. The flammable region of the evolved vapor cloud was observed to grow well past the downstream edge of the measurement domain. With decreasing wind speeds, both the mass of acetone vapor within the flammability limits and the total spill event time increased significantly. The data presented herein provides a basis for validating future spill models of hazardous chemical releases, where complex turbulent flow modeling must be coupled with spill spreading and vaporization dynamics.     

Large- and very-large-scale motions in channel and boundary-layer flows

Large-scale motions (LSMs; having wavelengths up to 2-3 pipe radii) and very-LSMs (having wavelengths more than 3 pipe radii) have been shown to carry more than half of the kinetic energy and Reynolds shear stress in a fully developed pipe flow. Studies using essentially the same methods of measurement and analysis have been extended to channel and zero-pressure-gradient boundary-layer flows to determine whether large structures appear in these canonical wall flows and how their properties compare with that of the pipe flow. The very large scales, especially those of the boundary layer, are shorter than the corresponding scales in the pipe flow, but otherwise share a common behaviour, suggesting that they arise from similar mechanism(s) aside from the modifying influences of the outer geometries. Spectra of the net force due to the Reynolds shear stress in the channel and boundary layer flows are similar to those in the pipe flow. They show that the very-large-scale and main turbulent motions act to decelerate the flow in the region above the maximum of the Reynolds shear stress.     

A fundamental limit on the performance of correlation based phasecorrection and flow estimation techniques

Cross correlation and similar operations are used in ultrasonic imaging to estimate blood or soft tissue motion in one or more dimensions and to measure echo arrival time differences for phase aberration correction. These estimates are subject to large errors known as false peaks and smaller magnitude errors known as jitter. While false peaks can sometimes be removed through nonlinear processing, jitter errors place a fundamental limit on the performance of delay estimation techniques. This paper applies the Cramer-Rao Lower Bound to derive analytical expressions which predict the magnitude of jitter for 1-D and 2-D problems using both radio frequency (RF) and envelope detected data. One-dimensional simulation results are presented which closely match theoretical predictions. These results indicate that for typical clinical conditions axial jitter for detected data is approximately five times greater than that for RF data. Lateral jitter is approximately ten times greater than axial jitter for RF data. Examples are presented which utilize these results to predict the performance of phase aberration correction and flow estimation systems     

Free convection-radiation interaction from an isothermal plate inclined at a small angle to the horizontal

Summary The interaction of free convection with thermal radiation in boundary layer flow from an inclined isothermal plate is studied numerically. Introducing appropriate transformations the equations governing the flow are expressed in the form of local nonsimilarity equations valid near the leading edge as well as in the downstream region. A group of transformations is also introduced such that the flow near the leading edge and far downstream can be described. Heated upward facing plates with positive and negative inclination angles are investigated. When the inclination is negative the boundary layer separates from the surface and the numerical solutions can be extended downstream past the point of separation. From the present investigation it may be concluded that the position of the separation point moves away from the leading edge with the increase of either of the thermal radiation parameter or the surface temperature parameter of the heated surface.     

Simulation of Second Moments of Fluctuations of the Velocity and Temperature of Particles in Equilibrium Turbulent Flows

An analysis is performed of equilibrium solutions of equations for the second moments of fluctuations of the velocity and temperature of particles in a homogeneous shear flow, in a homogeneous flow subjected to tensile or compressive strain, and in a wall layer. The stability of equilibrium solutions to small perturbations is investigated. Algebraic models are given for turbulent stresses in the dispersed phase.     

Role of coherent structures in turbulent shear flows

A definition for the large-scale coherent structure is presented, and the nature and role of coherent structures in turbulent shear flows are examined. The equations governing the coherent motions and the experimental considerations as well as constraints in the investigations of coherent structures in wall-bounded and free turbulent shear flows are discussed. Results from a few of our recent and ongoing studies of coherent structures in excited and unexcited free turbulent shear flows are reviewed. These results show that coherent structures are dominant in transport in the early stages of their formation, but not in the self-preserving regions of turbulent shear flows.     

The role of coherent vortices near the turbulent/non-turbulent interface in a planar jet

The role of coherent vortices near the turbulent/non-turbulent (T/NT) interface in a turbulent plane jet is analysed by a direct numerical simulation (DNS). The coherent vortices near the jet edge consist of large-scale vortical structures (LSVSs) maintained by the mean shear and intense vorticity structures (IVSs) created by the background fluctuating turbulence field. The radius of the LSVS is equal to the Taylor micro-scale R(lsvs)≈λ, while the radius of the IVS is of the order of the Kolmogorov micro-scale R(ivs)~η. The LSVSs are responsible for the observed vorticity jump at the T/NT interface, being of the order of the Taylor micro-scale. The coherent vortices in the proximity of the T/NT interface are preferentially aligned with the tangent to the T/NT interface and are responsible for the viscous dissipation of kinetic energy near the T/NT interface and to the characteristic shape of the enstrophy viscous diffusion observed at that location.     

Numerical analysis of unsteady behavior of cloud cavitation around a NACA0015 foil

Three-dimensional unsteady cavitating flow around a NACA0015 hydrofoil fixed between the sidewalls was simulated and the mechanism of U-shaped cloud cavity formation was clarified. A local homogeneous model was used for the modeling of the vapor–liquid two-phase medium. The compressible two-phase Navier–Stokes equations as the governing equations were solved. To describe the phase change between water and vapor, the mass transfer model based on the theory of evaporation/condensation on a plane interface was introduced. The cell-centered finite volume method was employed to discretize the governing equations. Assuming turbulent flow, the turbulent eddy viscosity coefficient was computed by using the Baldwin–Lomax model with the Degani–Schiff modification. As a result, even in the case of cavitating flow without sidewalls, the shed cloud cavities has slightly 3D structure, which was not so much large as extending across the whole spanwise direction. On the other hand, in the case of cavitating flow with sidewalls, the end of sheet cavities bows in the spanwise direction because of the development of boundary layer near both sidewalls. After that, due to the occurring of the reentrant jet towards the mid-span region, the sheet cavities breaks off from mid-span region near the leading edge of the hydrofoil, and became the vortical cloud cavities, which have the large-scale U-shaped structure.     

外伸肋板对高层建筑流场影响的PIV试验研究

研究表明：高层建筑表面附属物,如外伸水平和竖向肋板,能大幅降低结构风荷载。然而,此类气动优化措施的作用机理尚不清楚。针对这一问题最有效的方法是对气动优化前后的建筑周围流场进行对比分析以明确优化措施对流场的作用效果。该文通过粒子图像测速(PIV)试验研究了5种不同布置形式的外伸肋板对高层建筑周围流场的影响,通过对比不同类型外伸肋板对平均和脉动流场的影响,分析了外伸肋板的作用效果,揭示了外伸肋板气动优化的工作机理。结果表明：水平连续横板可以明显限制流场的竖向流动,降低剪切层内的湍流强度。相比水平横板,外伸竖板能更明显地修正模型周围的脉动流场。外伸竖板接近建筑角部时可以改变来流的流动分离过程,减小建筑两侧剪切层曲率,明显减弱建筑周围顺风向和横风向湍流强度。水平和竖向外伸肋板使尾涡沿顺风向和横风向尺寸明显增大。以上流场特征的变化直接影响了作用于建筑上的平均和脉动风荷载。     

Wind pressure features of large-span flat roof in different wind fields induced by conical vortex

The wind pressure features on a large-span flat roof in uniform flow field and turbulent field induced by conical vortex were studied, through wind tunnel tests. From the comparison of the mean and fluctuating wind pressure distributions on a flat roof in different wind fields induced by conical vortex, results indicate that the mean suction dominates in the smooth flow, whereas the fluctuating suction is more obvious in the turbulent flow. The probability density function for the pressure fluctuations under different approaching flows is analyzed. The two-peaked distribution, peculiar to turbulent flow field, is observed on the curve of probability density. The fluctuating pressures at reattachment points are larger under the turbulent flow. This indicates a more intense reattachment, which may cause overturning moment for roof-mounted items. Point vortex, RanKine vortex, and simplified Cook expression are applied to fit the pressure profiles beneath conical vortices, respectively. The results have shown that the RanKine vortex model and simplified Cook expression were applicable to forecast the wind pressure profiles beneath conical vortices, while point vortex underestimated the real wind suction. The wind pressure distributions in turbulent fields induced by different wind angles were contrasted, when the approaching flow is along the diagonal of the roof, the intensity of the vortex pairs is almost equal, with obvious reattachment. When the approaching flow deviate from the diagonal of the roof, the lateral turbulent component spins the vortex more quickly; this induces larger mean suctions beneath windward vortices. Smaller suctions are observed beneath the leeward vortex, due to less vorticity being converted to vortex motion from the freestream.     

Numerical investigation of energy and Reynolds stress distribution for a turbulent flow in an orifice

The turbulent heavy water and light water flows through an orifice, which characterize different reactor systems in nuclear power plants, are studied. The aim was to reveal the influence of process fluid on the turbulence parameters by considering heavy water and light water flows under the unique flow accelerated corrosion (FAC) conditions. The heavy water and light water are referenced based on their density values. The change in density values may have an effect on the flow dynamics and hence on FAC. These effects are brought out in this study and they can be extended to other cases for example, the change in the density of light water due to the chemical additions for controlling the pH values. The flow details at the downstream of orifice were studied extensively with the aid of computational modelling for different Reynolds numbers. Also structural development of the entire vortical flow field which could immensely enhance the knowledge about vortical structures occurring in the recirculation regions at the upstream and downstream of orifice is investigated. This study has been started with the exploration of flow topology of the velocity field by checking the topological consistency. The kinetic energy and dissipation rate were predicted by the modelling of turbulence using the Realizable k–ɛ model. Also the Reynolds stresses were calculated using the Reynolds stress model. The recirculation region showed maximum value for these parameters near the center line of the elliptic point, but for the dissipation rate this maximum value is observed at the wall. The maximum values of kinetic energy and wall shear stress are observed at the periphery of the orifice in comparison with that of the recirculation region. The predicted turbulent parameters have higher values in the recirculation region for heavy water flow and at the periphery of the orifice for light water flow with respect to each other flows. Also, the Sh distribution has been analyzed to estimate the FAC rate along the solid surface. The predicted peak values of these parameters will help to locate the locations which are susceptible to FAC.     

The non-linear evolution of modulated waves in a boundary layer

The series of experiments by Schubauer and Skramstad (1948) provided the first experimental evidence of the role that the instability of Tollmien-Schlichting waves played in the transition of a zero pressure-gradient flat plate boundary layer. The initial experiments studied the oscillations in the boundary layer excited by the freestream fluctuations. This was only possible after the background disturbances in the wind tunnel had been reduced to a very low level. The background wind tunnel environment excited a broad band of amplitude modulated disturbance waves that grew as they propagated downstream, eventually leading to the formation of turbulent spots. Further experiments used artificial two-dimensional harmonic excitation to produce regular wavetrains that could be directly compared with linear theory. Unfortunately, two-dimensional harmonic excitation of this type has also been used in many of the subsequent nonlinear transition investigations; the modulation of the disturbance waves, essential in nonlinear studies, has been largely ignored. Gaster and Grant (1975) used a short duration acoustic pulse to excite the boundary layer and found that the modulated wavepacket that was created admitted bursts of high frequency oscillations. These occurred at amplitudes that were insufficient to generate non-linear behaviour in purely periodic wavetrains. Gaster (1980) suggested that the modulation of the wavepacket played an important role in the non-linear region of transition. This investigation used computer generated deterministic white noise to excite the boundary layer on a flat plate through a single buried loudspeaker. This type of excitation produced amplitude modulated T-S waves at some point downstream from the source. By repeatedly exciting the boundary layer with the same white noise sequence it was possible to map the entire flow-field with a single hot-wire probe and so study the evolution of the modulated wavetrains and the eventual development of turbulent spots. The modulated wavetrains were found to grow initially according to linear theory. But downstream, departures from the linear pattern were observed at isolated time instants. The amplitude of the irregular portions of the signal increased rapidly with downstream distance until bursts of oscillations of frequencies five or six times the basic T-S frequency were observed. These regions developed even higher frequency bursts until a turbulent spot could be considered to have formed. Excitation signals of various amplitudes with different phase relations between the spectral components were used in these experiments. It was found that the phases between the Fourier components played an important role in the highly non-linear behaviour that is the precursor to a turbulent spot. Novel signal processing techniques, such as the wavelet transform and Singular Value Decomposition were used to investigate the fine structure and the propagation characteristics of the high frequency disturbances.