Interaction of an oscillating vortex with a turbulent boundary layer

An oscillating vortex embedded within a turbulent boundary layer was generated experimentally by forcing a periodic lateral translation of a half-delta wing vortex generator. The objective of the experiment was to investigate the possibility that a natural oscillation, or meander, might be responsible for flattened vortex cores observed in previous work, which could also have contaminated previous turbulence measurements. The effect of this forced oscillation was characterized by comparison of measurements of the mean velocities and Reynolds stresses at two streamwise stations, for cases with and without forcing. The Reynolds stresses, especially w, were affected significantly by the forced oscillation, mainly through contributions from the individual production terms, provided the vortex was not too diffuse.List of Symbols a amplitude of forced vortex motion - f frequency of forced vortex generator motion - l vortex generator root chord - L flow length scale - R _Y , R _Z vortex core radial dimensions in vertical and spanwise directions, respectively - R_r vortex circulation Reynolds number R = / - u, v, w instantaneous velocity components in X, Y, Z directions - U, V, W mean velocities; shorthand notation for u, , w - X, Y, Z right-hand Cartesian streamwise, vertical, and spanwise coordinate directions - boundary-layer thickness - overall circulation - air kinematic viscosity - _x streamwise vorticity, _X = W/Y–V/d+t6Z - ( )₀ reference value (measured at X = 10 cm) - ( )c refers to vortex center - ( ) _max maximum value for a particular crossflow plane - ( ) (overbar) time average - ( ) (prime) fluctuating component, e.g., u=U+u     

The unsteady flow generated by an oscillating wall with transpiration

In this work, an analytical solution for the fluid behavior over flat plates with impulsive and oscillating motions, starting from rest, and with wall transpiration, is presented. The classical solution of this problem is given by Panton [7] and is found to be an especial case of the solution here presented. The analytical solution is obtained without the use of any special transformations, such as Laplace or Fourier transforms. Instead, an extension of the variable separation technique is used together with similarity arguments. A non-dimensional number—the transpiration rate—is used to take into account the injection or suction of fluid at the wall. This parameter is shown to be of great influence on the proposed velocity solution.     

On the behaviour of periodic disturbances introduced into a turbulent channel flow by an oscillating airfoil

This paper is on the propagation characteristics of disturbances in turbulent shear flow. Periodic disturbances are introduced into a fully developed turbulent channel flow by an airfoil executing pitching oscillations. Hot-wire measurements are done on the mean and fluctuating velocities, on the Reynolds stresses and their spectra. The spectral data are evaluated to obtain the decay characteristics of the induced disturbance in this flow. The results show that the disturbance propagation in this flow is characterised by different decay rates in the regions near and far from the disturbance source.     

Internal waves generated by the turbulent wake of a sphere

Internal waves generated by the turbulent wake of a sphere travelling horizontally through a linearly stratified fluid were studied using shadowgraph and particle-streak photography. The Reynolds and internal Froude number ranges considered were 2,000 Re 12,900 and 2.0 Fi 28.0, respectively. Two quite distinct flow regimes based on the structure of the turbulent wake were identified. In one, the wake is characterized by large-scale coherent structures. In the other, the wake, as viewed on a side-view shadowgraph, grows in a roughly symmetric fashion to a maximum height and then collapses slowly; such flows are termed the smallscale structures regime.Wave lengths and maximum wave heights of the internal waves were measured as functions of Nt and Fi, where N is the Brunt-Väisälä frequency and t the time. It was found that the wave lengths scale well with the streamwise dimension of the spiralling coherent structures. The maximum amplitude of the internal waves were found to scale with the vertical dimension of the turbulent wake, upon varying the internal Froude number.     

Turbulence quantification and sediment resuspension in an oscillating grid chamber

An oscillating grid chamber has been developed to study sediment suspension, desorption of compounds from the resuspended sediment, and air–water mass transfer. The chamber is designed to allow researchers to study desorption of contaminants from cohesive sediments and the flux of those contaminants to the vapor phase. The chamber uses a single vertically oscillating grid driven by a DC motor and closed-loop controller. Sediment to be studied is placed in the bottom of the chamber and entrained into the water column by the turbulence generated by the oscillating grid. A two-component laser Doppler velocimeter (LDV) was used to measure the turbulent velocity field inside the chamber. Detailed mapping of the turbulent kinetic energy (TKE) produced by this grid arrangement was compared with established grid-stirred systems. At distances closer to the grid than two grid bar spacings, large lateral gradients exist in the TKE. The suspension of cohesive sediments was also studied using this chamber. Steady-state suspended sediment concentrations were achieved within 30 min for a variety of turbulence levels. By adjusting the grid operating parameters, the TKE can be set to simulate the turbulence found either at the bed or free surface in open-channel flow systems. With some care, the oscillating grid chamber can be used as a simple laboratory analogue to study various environmental processes within the flow or at either the sediment–water or air–water interface.     

Periodic solution of turbulent oscillating channel flows

The time-dependent turbulent Navier–Stokes equations are solved numerically by a finite element method with an algebraic eddy viscosity model (Baldwin–Lomax formulation) for oscillating turbulent channel flows. The method of averaging is used to analyse the resulting periodic motion of the fluid. Numerical results are obtained for various Strouhal numbers and relative amplitudes. A comparison is made between the numerical and published experimental results. It appears that for low relative amplitudes in a certain range of frequencies the agreement is satisfactory.     

Unsteady phenomena of an oscillating turbulent jet flow inside a cavity: Effect of aspect ratio

Self-sustained oscillatory phenomena in confined flow may occur when a turbulent plane jet is discharging into a rectangular cavity. An experimental set-up was developed and the flow analysis has been made using mainly hot-wire measurements, which were complemented by visualisation data. Previous studies confirmed that periodic oscillations may occur, depending on the location of the jet exit nozzle inside the cavity, and also the distance between the side-walls. The present study deals with the symmetrical interaction between a turbulent plane jet and a rectangular cavity and the influence of the geometrical characteristics of the cavity on the oscillatory motion. The size and aspect ratio of the cavity were varied together with the jet width compared to that of the cavity. The study is carried out both numerically and experimentally. The numerical method solves the unsteady Reynolds averaged Navier–Stokes equations (URANS) together with the continuity equation for an incompressible fluid. The closure of the flow equations system is achieved using a two-scale energy-flux model at high Reynolds number in the core flow coupled with a wall function treatment in the vicinity of the wall boundaries. The fundamental frequency of the oscillatory flow was found to be practically independent of the cavity length. Moreover, the oscillations are attenuated as the cavity width increases, until they disappear for a critical value of the cavity width. Contour maps of the instantaneous flow field are drawn to show the flow pattern evolution at the main phases of oscillation. They are given for several aspect ratios of the cavity, keeping constant values for the cavity width and the jet thickness. The proposed approach may help to investigate further the oscillation mechanisms and the entrainment process occurring in pressure driven jet–cavity interactions.     

Numerical prediction of turbulent flow structure generated by a synthetic cross‐jet into a turbulent boundary layer

A detailed numerical study using large‐eddy simulation (LES) and unsteady Reynolds‐averaged Navier–Stokes (URANS) was undertaken to investigate physical processes that are engendered in the injection of a circular synthetic (zero‐net mass flux) jet in a zero pressure gradient turbulent boundary layer. A complementary study was carried out and was verified by comparisons with the available experimental data that were obtained at corresponding conditions with the aim of achieving an improved understanding of fluid dynamics of the studied processes. The computations were conducted by OpenFOAM C++, and the physical realism of the incoming turbulent boundary layer was secured by employing random field generation algorithm. The cavity was computed with a sinusoidal transpiration boundary condition on its floor. The results from URANS computation and LES were compared and described qualitatively and quantitatively. There is a particular interest for acquiring the turbulent structures from the present numerical data. The numerical methods can capture vortical structures including a hairpin (primary) vortex and secondary structures. However, the present computations confirmed that URANS and LES are capable of predicting current flow field with a more detailed structure presented by LES data as expected. Copyright © 2011 John Wiley & Sons, Ltd.     

Numerical modeling of a turbulent flow behind a heated grid

A numerical model of the dynamics of turbulence and temperature fluctuations behind a heated grid located in a wind tunnel is constructed on the basis of closed Kármán-Howarth and Corrsin equations. Results calculated by this model are in reasonable agreement with available experimental data. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 50, No. 3, pp. 118–126, May–June, 2009.     

Temporal and spatial transients in turbulent boundary layer flow over an oscillating wall

Direct numerical simulations have been performed to study the effect of an oscillating segment of the wall on a turbulent boundary layer flow. Two different oscillation amplitudes with equal oscillation period have been used, which allows a direct comparison between a relatively weak and strong forcing of the flow. The weaker forcing results in 18% drag reduction while the stronger forcing, with twice the amplitude, yields 29% drag reduction. The downstream development of the drag reduction is compared with earlier simulations and experiments. In addition, a simulation with identical oscillation parameters as in previous numerical and experimental investigations allows for an estimation of the effect of the Reynolds number on the drag reduction.Reductions in the Reynolds stresses and the important role that the edge of the Stokes layer has is explained.An estimation of the idealized power consumption shows that a positive energy budget is only possible for the weaker wall velocity case.Spatial and temporal transients are investigated and a transformation between spatial and temporal coordinates via a convection velocity is shown to facilitate a comparison between the two transients in a consistent manner. The streamwise shear exhibits a similar monotonic behavior in the spatial and temporal transients, while the non-monotinic temporal transient of the longitudinal Reynolds stress has no counterpart in the spatial development. Furthermore, the evolution in time of the spanwise Reynolds stress is very similar to previously reported channel flow data.The instantaneous spanwise velocity profile (only averaged in the homogeneous spanwise direction) will for the first time be presented from a boundary layer over an oscillating wall, and comparisons with the analytical solution to the laminar Navier–Stokes equations show very good agreement.     

Mechanism of drag reduction by spanwise oscillating Lorentz force in turbulent channel flow

Numerical simulations and experimental research are both carried out to investigate the controlled effect of spanwise oscillating Lorentz force on a turbulent channel flow. The variations of the streaks and the skin friction drag are obtained through the PIV system and the drag measurement system, respectively. The flow field in the near-wall region is shown through direct numerical simulations utilizing spectral method. The experimental results are consistent with the numerical simulation results qualitatively, and both the results indicate that the streaks are tilted into the spanwise direction and the drag reduction utilizing spanwise oscillating Lorentz forces can be realized. The numerical simulation results reveal more detail of the drag reduction mechanism which can be explained, since the spanwise vorticity generated from the interaction between the induced Stokes layer and intrinsic turbulent flow in the near-wall region can make the longitudinal vortices tilt and oscillate, and leads to turbulence suppression and drag reduction.     

Numerical investigation of turbulent channel flow controlled by spatially oscillating spanwise Lorentz force

A formulation of the skin-friction drag related to the Reynolds shear stress in a turbulent channel flow is derived. A direct numerical simulation (DNS) of the turbulent control is performed by imposing the spatially oscillating spanwise Lorentz force. Under the action of the Lorentz force with several proper control parameters, only the periodically well-organized streamwise vortices are finally observed in the near-wall region. The Reynolds shear stress decreases dramatically, especially in the near-wall area, resulting in a drag reduction.     

Surface pressure and flow field behind an oscillating fence submerged in turbulent boundary layer

Phase-locked PSP and PIV measurements were used to study the evolution of three-dimensional disturbances produced by an oscillating fence actuator immersed in a zero pressure gradient turbulent boundary layer. For the single fence frequency studied, strong three-dimensionality is observed in the vortical structure that varies along the span of the fence soon after the fence enters the flow. At the midspan, the structure grows, weakens, and convects faster than at other locations. As the fence height increases, the data indicate that the vortical structure terminates near the edge of the fence. In contrast, the vortex structure terminates on the plate surface adjacent to the fence edge as the fence descends, similar to a wake vortex of a stationary obstacle. This study demonstrates that the combined use of surface and flow-field diagnostics provide a link between flow field and surface features, yielding an understanding of the flow that would have not been possible with any one technique.     

Water surface wave radiation generated by multiple cylinders oscillating with identical frequency

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An experimental analysis of the turbulent structures generated by the intake port of a DISI-engine

An essential task in the optimization of combustion processes for DISI (Direct Injection Spark Ignition) engines is the generation of a suitable in-cylinder flow, leading to easy ignition conditions and low pollutant emissions. Therefore, the determination of the transient flow behaviour generated in the cylinder by the intake port and the identification of the origin of flow fluctuations are equally important. A better insight into the time-dependent behaviour of in-cylinder flow is necessary to avoid unwanted flow variations and enhance the fuel-mixture preparation. Suitable information is provided here by the experimental measurement of instantaneous flow fields in a model cylinder flow, as obtained from High Speed Particle Image Velocimetry. The investigated flow fields are generated by a four-valve DISI production engine cylinder head on a steady-state test-bed. The present paper presents a procedure based on Singular Value Decomposition (SVD) in order to filter out measurement errors and to obtain information about the transient behaviour of in-cylinder flows. First, the procedure is presented and analyzed by considering generic vector fields, demonstrating that information concerning the transient behaviour is detectable in this manner. Next, the transient behaviour of the in-cylinder flow is investigated by reconstructing flow fields with the SVD procedure. The reconstruction employs a specified number of SVD spatial modes φ _i (x) and corresponding SVD time coefficients a _Di (t), which are reduced to their deterministic parts. Afterwards, the reduced SVD time coefficients a _Di (t) are used to determine the main fluctuation frequencies of the in-cylinder flow and to identify the origin of these fluctuations.     

Asymptotic expansions for the surface waves generated by a submerged body moving with rapidly oscillating velocity

Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 138–145, May–June, 1985.     

Flow structures behind a vertically oscillating fence immersed in a flat-plate turbulent boundary layer

A phase-averaging technique was employed to study the evolution of flow behind an oscillating bluff plate immersed in a flat-plate turbulent boundary layer. The experiments were performed for a reduced frequency of 0.0044. The large-scale disturbance generated by the plate developed to an organized form over 20 maximum plate height and then diffused rapidly, as quantified by the ratio of Reynolds stress of the phase-averaged fluctuation to that of the total fluctuation. The small-scale fluctuations embedded in the large-scale disturbance were almost removed by phase averaging. However, their contributions in Reynolds stress and kinetic energy were pronounced along the path of the core of the large-scale structure.A version of this paper was presented at the 11^th Symposium on Turbulence, University of Missouri-Rolla, Sept. 1988     

Experimental study of turbulent thermal diffusion in oscillating grids turbulence

We have experimentally detected a new effect, turbulent thermal diffusion, as predicted theoretically by Elperin et al. (Phys Rev Lett (1996) 76:224–228) and associated with the turbulent transport of inertial particles. The essence of this effect is an appearance of a non-diffusive mean flux of particles in the direction of the mean heat flux. This results in formation of large-scale inhomogeneities in the spatial distribution of inertial particles that are accumulated in regions of minimum mean temperature in the surrounding fluid. The experiments were performed in oscillating grids turbulence with an imposed mean temperature gradient. We used Particle Image Velocimetry to determine the turbulent velocity field, and an Image Processing Technique based on the analysis of the intensity of the Mie scattering to determine the spatial distribution of tracer particles. Analysis of the intensity of laser light Mie scattering by tracer particles showed that the tracer particles accumulate in the vicinity of the minimum of the mean temperature. The latter finding confirms the existence of the effect of turbulent thermal diffusion.     

Fluctuations of a spray generated by an airblast atomizer

This paper is devoted to the study of the aerodynamic instability of the spray generated by an airblast atomizer. As a result of this instability the spray shape and its velocity fluctuate with a certain frequency, which depends on the operational parameters of the atomizer. The effect of three parameters, namely; chamber pressure, liquid phase flow rate and the gas phase flow rate on the spray fluctuating frequency are investigated. The velocity vector of the drops in the spray and the arrival times to the detection volume are measured using the laser Doppler instrument. The slotting technique is applied to the data of axial velocity and arrival times of the drops in order to estimate the dominating spray frequencies. Additionally, the shape of the spray has been observed using the high-speed video system. The frequencies of the shape fluctuations are estimated using proper orthogonal decomposition of the time-resolved images of the spray. We show that the frequencies of the spray velocity and those exhibited by spray shape coincide over a wide range of spray parameters. Finally, a simple scaling for the spray frequency is proposed and validated by the experimental data.