1,720,977 research outputs found
Interacting synthetic jets: fundamentals and applications
No full textSynthetic jets are zero-net-mass-flux devices that create a train of vortical structures from alternating blowing and suction cycles through an orifice. In this thesis the interaction of synthetic jets with three different flows is investigated: neighbouring synthetic jets interacting with each other, a synthetic jet interacting with the separated flow over a backward-facing step and the interaction of a synthetic jet with a turbulent boundary layer. In general, for these different flows, it is proposed that the vortical structures of the jet are the leading driver behind observed interactions. This supplements theliterature where interactions are often explained based on the low pressure behind jet pulses, viscous blockage of jet pulses or frequency modes amplified by the jet frequency. Using the physical mechanisms presented in this thesis, a higher level of control over interacting synthetic jets can be obtained. This includes control over the direction of two parallel synthetic jets, control over the reattachment length of the separated flow behind a backward-facing step and control over the trajectory and the wake of a synthetic jet issuing into a turbulent boundary layer. The focus in the present thesis is on the physical mechanisms behind observed interactions rather than on optimal control. The underlying idea is that the obtained physical understanding can be used in subsequent studies to develop optimal control
Effects of vortex-induced velocity on the development of a synthetic jet issuing into a turbulent boundary layer
No full textA synthetic jet issuing into a cross-flow influences the local velocity of the cross-flow. At the jet exit the jet is oriented in the wall-normal direction while the cross-flow is oriented in the streamwise direction, leading to a momentum transfer between the jet and the cross-flow. Streamwise momentum transferred from the cross-flow to the jet accelerates the pulses created by the jet. This momentum transfer continuous up to some point downstream where these pulses have the same velocity as the surrounding flow and are no longer blocking the cross-flow. The momentum transfer from the cross-flow to the jet leads to a momentum deficit in the cross-flow far downstream of the viscous near field of the jet. In the literature this momentum-flux deficit is often attributed to viscous blockage or to up-wash of low-momentum fluid. The present paper proposes and quantifies a third source of momentum deficit: a velocity induced opposite to the cross-flow by the vortical structures created by the synthetic jet. These vortical structures are reconstructed from measured data and their induced velocity is calculated using the Biot-Savart law. The three-dimensional three-component induced velocity fields show great similarity to the measured velocity fields, suggesting that this induced velocity is the main contributor to the velocity field around the synthetic jet and viscous effects have only a small influence. The momentum-flux deficit induced by the vortical structures is compared to the measured momentum-flux deficit, showing that the main part of this deficit is caused by the induced velocity. Variations with Strouhal number (frequency of the jet) and velocity ratio (velocity of the jet) are observed and discussed. An inviscid-flow model is developed, which represents the downstream evolution of the jet in cross-flow. Using the measured data as an input, this model is able to predict the deformation, (wall-normal) evolution and qualitative velocity field of the jet. The present study presents evidence that the velocity induced by the vortical structures forming a synthetic jet plays an important role in the development of and the velocity field around the jet.</p
Dataset and Vortex model code for paper titled "Effects of vortex-induced velocity on the development of a synthetic jet issuing into a turbulent boundary layer"
No full textThis dataset presents the experimental data described in:
Tim Berk and Bharathram Ganapathisubramani, "Effects of vortex-induced velocity on the development of a synthetic jet issuing into a turbulent boundary layer", Journal of Fluid Mechanics, 2019. DOI:10.1017/jfm.2019.279
Please cite the above-mentioned paper if you use the data or the matlab code that provides the vortex model for the synthetic jet in cross-flow.
The data is presented for each figure containing important data in the paper (filenames are given accordingly). Data for figures 2, 3, 7, 13, 15, 20, 21 and 22 is provided in XML spreadsheet files (.xlsx). Using the data provided and normalisation parameters (where applicable), the plots in the paper could be reproduced. Units of the data are given in the data files. Velocity fields, as used in figures 5, 6, 9, 10, 11, 12, 17, 18 and 19 are provided as Matlab files (.mat). Units of this data are given in each variable name. 'Velocity_fields_x1delta.mat' provides velocity in a cross-stream - wall-normal plane located at x/delta = 1. 'Velocity_fields_x3delta.mat' provides velocity in a cross-stream - wall-normal plane located at x/delta = 3. 'Velocity_fields_z0.mat' provides velocity in a streamwise - wall-normal plane located at z/delta = 0.</span
Dataset for Entrainment effects in periodic forcing of the flow over a backward-facing step
No full textDataset supporting 'Entrainment effects in periodic forcing of the flow over a backward-facing step' by Berk et al.</span
Entrainment effects in periodic forcing of the flow over a backward-facing step
No full textThe effect of the Strouhal number on periodic forcing of the flow over a backward-facing step (height, H) is investigated experimentally. Forcing is applied by a synthetic jet at the edge of the step at Strouhal numbers ranging from 0.21 < StH < 1.98 (StH = f H/U∞) at a Reynolds number of ReH = HU∞/ν = 41000. In the literature, the effect of Strouhal number on the reattachment length is often divided into low- and high frequency actuation, referring to specific frequencies present in the unforced flow. In the present study, variations with Strouhal number are explained based on a continuous variation of entrainment of momentum into the recirculation region rather than on specific frequencies. The reattachment length is shown to decrease linearly with entrainment of momentum. Vertical momentum flux is driven by vortices generated by the forcing and locally vertical momentum flux is shown to be qualitatively similar to circulation for all cases considered. Total circulation (and therewith entrainment of momentum and the effect on the reattachment length) is shown to decrease with Strouhal number whereas this is not predicted by models based on specific low- and high frequencies. An empirical model for the (decay of) circulation is derived by tracking vortices in phase-locked data. This model is used to decipher relevant scaling parameters that explain the variations in circulation, entrainment of momentum and reattachment length. Three regimes of Strouhal number are identified. A low-Strouhal-number regime is observed for which vortices are formed at a late stage relative to the recirculation region, causing a decrease in effectiveness. For high Strouhal numbers vortices are being re-ingested into the actuator or are packed so close together that they cancel each other, both decreasing the effectiveness of forcing. In the intermediate regime a vortex train is formed of which the decay of circulation increases for increasing Strouhal number. The scaling of this decay fully explains the observed variation in reattachment length. The observations on entrainment of momentum made in this study are expected to also hold for periodic forcing of other bluff-body flows
Vectoring of parallel synthetic jets: a parametric study
Full text linkThe vectoring of a pair of parallel synthetic jets can be described using five dimensionless parameters: the aspect ratio of the slots, the Strouhal number, the Reynolds number, the phase difference between the jets and the spacing between the slots. In the present study, the influence of the latter four on the vectoring behaviour of the jets is examined experimentally, using particle image velocimetry. Time-averaged velocity maps are used to give a qualitative description of the variations in vectoring for a parametric sweep of each of the four parameters independently. A diverse set of vectoring behaviour is observed in which the resulting jet can be merged or bifurcated and either vectored towards the actuator leading in phase or the actuator lagging in phase. Three performance metrics are defined to give a quantitative description of the vectoring behaviour: the included angle between bifurcated branches, the vectoring angle of the total flow and the normalized momentum flux of the flow. Using these metrics, the influence of changes in the Strouhal number, Reynolds number, phase difference and spacing are quantified. Phase-locked maps of the swirling strength are used to track vortex pairs. Vortex trajectories are used to define three Strouhal number regimes for the vectoring behaviour. In the first regime, vectoring behaviour is dominated by the pinch-off time, which is written as function of Strouhal number only. In the second regime, the pinch-off time is invariant and the vectoring behaviour slightly changes with Strouhal number. In the third regime, given by the formation criterion, no synthetic jet is formed. Vortex positions at a single phase, shortly after creation of the lagging vortex pair, are used to propose a vectoring mechanism. This vectoring mechanism explains the observed qualitative and quantitative variations for all four parameters
Dataset for Vectoring of parallel synthetic jets: A parametric study
No full textData supporting the publication titled "Vectoring of parallel synthetic jets: a parametric study" in Journal of Fluid Mechanics, DOI: http://dx.doi.org/10.1017/jfm.2016.559.</span
Simultaneous surface pressure and high-speed PIV measurements in stalled airfoil
No full textThe time-resolved velocity field (2D2C high-speed PIV) and surface pressure (pressure taps) of a stalled NACA0012 airfoil have been simultaneously measured. These measurements enable cross-evaluation of the velocity field and pressure time series. The present paper evaluates the flow field and surface pressure using POD, cross-correlation and conditional averaging. First, the flow and surface pressure are analysed independently. Both the time-average and the dynamic structures of the flow are presented. Distinct frequency bands are observed in the energy spectra of the surface-pressure signal. It is shown that a high-pressure event at the foremost pressure port (at x/c = 0.34) is followed by high-pressure events at the other pressure ports (x/c = 0.51-0.93), indicating a decaying pressure wave that travels over the surface with roughly half the free-stream velocity. Next, the link between the flow field and the surface pressure is explored. Using cross-correlation, it is shown that the pressure signal correlates with specific regions in the flow. Conditional averages of the flow fields, conditioned to high-pressure events at the surface, indicate that these pressure events are caused by coherent structures in the flow. These structures consist of a train of alternating vortices that induce velocity components toward and away from the surface, causing high-and low-pressure events at this surface. By filtering the pressure signal with different frequency bands, the flow structures responsible for different peaks in the energy spectra are recovered. A POD analysis indicates that the energy of the coherent flow structures leading to surface-pressure fluctuations only contain a fraction of the total energy of the flow. While POD and cross-correlation are applied to a full time-series, the conditional averages show that coherent structures in the flow can be identified in real-time using the surface pressure. Identification of coherent structures in the flow using only the surface pressure signal enables real-time control of such structures
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
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