1,721,034 research outputs found
Characteristics of vortex packets in turbulent boundary layers
No full textStereoscopic particle image velocimetry (PIV) was used to measure all three instantaneous components of the velocity field in streamwise–spanwise planes of a turbulent boundary layer at Reτ = 1060 (Reθ = 2500). Datasets were obtained in the logarithmic layer and beyond. The vector fields in the log layer (z+=92 and 150) revealed signatures of vortex packets similar to those proposed by Adrian and co-workers in their PIV experiments. Groups of legs of hairpin vortices appeared to be coherently arranged in the streamwise direction. These regions also generated substantial Reynolds shear stress, sometimes as high as 40 times —uw‾. A feature extraction algorithm was developed to automate the identification and characterization of these packets of hairpin vortices. Identified patches contributed 28% to —uw¯ while occupying only 4% of the total area at z+ = 92. At z+ = 150, these patches occupied 4.5% of the total area while contributing 25% to —uw¯. Beyond the log layer (z+ = 198 and 530), the spatial organization into packets is seen to break down.<br/
Energy transfer in turbulent channel flows and implications for resolvent modelling
No full textWe analyse the inter-scale transfer of energy for two types of plane Poiseuille flow: the P4U exact coherent state of Park & Graham (J. Fluid Mech., vol. 782, 2015, pp. 430-454) and turbulent flow in a minimal channel. For both flows, the dominant energy-producing modes are streamwise-constant streaks with a spanwise spacing of approximately 100 wall units. Since the viscous dissipation for these scales is not sufficient to balance production, the nonlinear terms redistribute the excess energy to other scales. Spanwise-constant scales (that is, Tollmien-Schlichting-like modes with zero spanwise wavenumber), in particular, account for a significant amount of net energy gain from the nonlinear terms. We compare the energy balance to predictions from resolvent analysis, and we show that it does not model energy transfer well. Nevertheless, we find that the energy transferred from the streamwise-constant streaks can be predicted reasonably well by a Cess eddy viscosity profile. As such, eddy viscosity is an effective model for the nonlinear terms in resolvent analysis and explains good predictions for the most energetic streamwise-constant streaks. It also improves resolvent modes as a basis for structures whose streamwise lengths are greater than their spanwise widths by counteracting non-normality of the resolvent operator. This is quantified by computing the inner product between the optimal resolvent forcing and response modes, which is a metric of non-normality. Eddy viscosity does not respect the conservative nature of the nonlinear energy transfer, which must sum to zero over all scales. Since eddy viscosity tends to remove energy, it is less effective in modelling nonlinear transport for scales that receive energy from the nonlinear terms.</p
On the different contributions of coherentstructures to the spectra of a turbulent round jetand a turbulent boundary layer
Full text link©Cambridge University Press. Nickels, T.B. and Marusic, Ivan (2001) On the different contributions of coherentstructures to the spectra of a turbulent round jetand a turbulent boundary layer. Journal of Fluid Mechanics, 448, 367-385. http://www.cambridge.org/This paper examines and compares spectral measurements from a turbulent round jetand a turbulent boundary layer. The conjecture that is examined is that both flows consist of coherent structures immersed in a background of isotropic turbulence. In the case of the jet, a single size of coherent structure is considered, whereas in the boundary layer there are a range of sizes of geometrically similar structures. The conjecture is examined by comparing experimental measurements of spectra for the two flows with the spectra calculated using models based on simple vortex structures.The universality of the small scales is considered by comparing high-wave number experimental spectra. It is shown that these simple structural models give a good account of the turbulent flows
Dual-plane PIV technique to determine the complete velocity gradient tensor in a turbulent boundary layer
No full textSimultaneous dual-plane PIV experiments, which utilized three cameras to measure velocity components in two differentially separated planes, were performed in streamwise-spanwise planes in the log region of a turbulent boundary layer at a moderate Reynolds number (Re 1100). Stereoscopic data were obtained in one plane with two cameras, and standard PIV data were obtained in the other with a single camera. The scattered light from the two planes was separated onto respective cameras by using orthogonal polarizations. The acquired datasets were used in tandem with continuity to compute all 9 velocity gradients, the complete vorticity vector and other invariant quantities. These derived quantities were employed to analyze and interpret the structural characteristics and features of the boundary layer. Sample results of the vorticity vector are consistent with the presence of hairpin-shaped vortices inclined downstream along the streamwise direction. These vortices envelop low speed zones and generate Reynolds shear stress that enhances turbulence production. Computation of inclination angles of individual eddy cores using the vorticity vector suggests that the most probable inclination angle is 35° to the streamwise-spanwise plane with a resulting projected eddy inclination of 43° in the streamwise-wall-normal plane.<br/
Coherent structures in the linearized impulse response of turbulent channel flow
No full textWe study the evolution of velocity fluctuations due to an isolated spatio-temporal impulse using the linearized Navier–Stokes equations. The impulse is introduced as an external body force in incompressible channel flow at Reτ=10000 . Velocity fluctuations are defined about the turbulent mean velocity profile. A turbulent eddy viscosity is added to the equations to fix the mean velocity as an exact solution, which also serves to model the dissipative effects of the background turbulence on large-scale fluctuations. An impulsive body force produces flow fields that evolve into coherent structures containing long streamwise velocity streaks that are flanked by quasi-streamwise vortices; some of these impulses produce hairpin vortices. As these vortex–streak structures evolve, they grow in size to be nominally self-similar geometrically with an aspect ratio (streamwise to wall-normal) of approximately 10, while their kinetic energy density decays monotonically. The topology of the vortex–streak structures is not sensitive to the location of the impulse, but is dependent on the direction of the impulsive body force. All of these vortex–streak structures are attached to the wall, and their Reynolds stresses collapse when scaled by distance from the wall, consistent with Townsend’s attached-eddy hypothesis
Trajectory of a synthetic jet issuing into high Reynolds number turbulent boundary layers
No full textSynthetic jets are zero-net-mass-flux actuators that can be used in a range of flow control applications. For some applications, the scaling of the trajectory of the jet with actuation and cross-flow parameters is important. This scaling is investigated for changes in the friction Reynolds number, changes in the velocity ratio (defined as the ratio between the mean jet blowing velocity and the free-stream velocity) and changes in the actuation frequency of the jet. A distinctive aspect of this study is the high-Reynolds-number turbulent boundary layers (up to Re휏 = 12 800 ) of the cross-flow. To our knowledge, this is the first study to investigate the effect of the friction Reynolds number of the cross-flow on the trajectory of an (unsteady) jet, as well as the first study to systematically investigate the scaling of the trajectory with actuation frequency. A broad range of parameters is varied (rather than an in-depth investigation of a single parameter) and the results of this study are meant to indicate the relative importance of each parameter rather than the exact influence on the trajectory. Within the range of parameters explored, the critical ones are found to be the velocity ratio as well as a non-dimensional frequency based on the jet actuation frequency, the cross-flow velocity and the jet dimensions. The Reynolds number of the boundary layer is shown to have only a small effect on the trajectory. An expression for the trajectory of the jet is derived from the data, which (in the limit) is consistent with known expressions for the trajectory of a steady jet in a cross-flow
Flow Visualization Using Natural Textures
No full textThe use of natural textures provides a richly diverse set of possibilities for the visualization of flow data. In this paper, we present methods that utilize the qualities and attributes of natural textures to visualize multiple scalar distributions and multiple vector fields obtained across a 2D domain in a turbulent boundary layer flow. First, we illustrate how different attributes of textures can represent scalar quantities along streamlines. We then present a technique that allows for the perception of two separate vector fields within the same image by utilizing different textures. Finally, we illustrate how textures have the ability to indicate specific regions of interest within flow images.Urness, Timothy Matthew; Interrante, Victoria; Longmire, Ellen; Marusic, Ivan. (2005). Flow Visualization Using Natural Textures. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/215655
Strategies for the Visualization of Multiple Co-located Vector Fields
No full textFluids research often involves developing theories about the complex relationships between multiple scalar and vector quantities. We discuss strategies for effectively visualizing co-located vector fields, enabling the key physical structures of one vector field to be clearly understood within the context of a related vector field. We describe the range of effects that can be obtained by combining several existing flow visualization techniques for the purposes of analyzing multiple vector fields. Results are shown through two distinctly different scientific applications: the visualization of velocity and vorticity fields in experimentally acquired turbulent boundary layer flow data, and the visualization of velocity and magnetic fields in computational simulations of astrophysical jets.Urness, Timothy Matthew; Interrante, Victoria; Longmire, Ellen; Marusic, Ivan; O'Neill, Sean; Jones, Thomas W.. (2005). Strategies for the Visualization of Multiple Co-located Vector Fields. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/215673
Use of eddy viscosity in resolvent analysis of turbulent channel flow
No full textThe predictions obtained from resolvent analysis with and without an eddy viscosity model for turbulent channel flow at Reτ=550 are compared to direct numerical simulation data to identify the scales and wave speeds for which resolvent analysis provides good predictions. The low-rank behavior of the standard resolvent identifies energetic regions of the flow whereas the eddy resolvent is low rank when the resulting projection of the leading eddy resolvent mode onto the leading mode from spectral proper orthogonal decomposition is maximum. The highest projections are obtained for structures that are associated with the near-wall cycle and structures that are energetic at z=±0.5. It is argued that these types of structures are likely to be correctly predicted for any friction Reynolds number due to the inner and outer scaling of the Cess eddy viscosity profile. The eddy resolvent also correctly identifies the most energetic wave speed for these two scales. For all other scales, neither analysis reliably predicts the most energetic wave speed or mode shapes. The standard resolvent tends to overestimate the most energetic wave speed while the eddy resolvent underestimates it. The resulting eddy resolvent modes are overly "attached" to the wall since the wall-normal gradient of the eddy viscosity overestimates the transport of energy towards the wall. These observations have direct implications for future work towards estimating turbulent channel flows using resolvent analysis and suggest that the Cess profile can be further optimized for individual scales to provide better low-order models of turbulent channel flows.</p
Experimental investigation of vortex properties in a turbulent boundary layer
Full text linkDual-plane particle image velocimetry experiments were performed in a turbulent boundary layer with Re? = 1160 to obtain all components of the velocity gradient tensor. Wall-normal locations in the logarithmic and wake region were examined. The availability of the complete gradient tensor facilitates improved identification of vortex cores and determination of their orientation and size. Inclination angles of vortex cores were computed using statistical tools such as two-point correlations and joint probability density functions. Also, a vortex identification technique was employed to identify individual cores and to compute inclination angles directly from instantaneous fields. The results reveal broad distributions of inclination angles at both locations. The results are consistent with the presence of many hairpin vortices which are most frequently inclined downstream at an angle of 45? with the wall. According to the probability density functions, a relatively small percentage of cores are inclined upstream. The number density of forward leaning cores decreases from the logarithmic to the outer region while the number density of backward-leaning cores remains relatively constant. These trends, together with the correlation statistics, suggest that the backward-leaning cores are part of smaller, weaker structures that have been distorted and convected by larger, predominantly forward-leaning eddies associated with the local shea
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