Philippe Lavoie

Effects of initial conditions in decaying turbulence generated by passive grids

The effects of initial conditions on grid turbulence are investigated for low to moderate Reynolds numbers. Four grid geometries are used to yield variations in initial conditions and a secondary contraction is introduced to improve the isotropy of the turbulence. The hot-wire measurements, believed to be the most detailed to date for this flow, indicate that initial conditions have a persistent impact on the large-scale organization of the flow over the length of the tunnel. The power-law coefficients, determined via an improved method, also depend on the initial conditions. For example, the power-law exponent m is affected by the various levels of large-scale organization and anisotropy generated by the different grids and the shape of the energy spectrum at low wavenumbers. However, the results show that these effects are primarily related to deviations between the turbulence produced in the wind tunnel and true decaying homogenous isotropic turbulence (HIT). Indeed, when isotropy is improved and the intensity of the large-scale periodicity, which is primarily associated with round-rod grids, is decreased, the importance of initial conditions on both the character of the turbulence and m is diminished. However, even in the case where the turbulence is nearly perfectly isotropic, m is not equal to −1, nor does it show an asymptotic trend in x towards this value, as suggested by recent analysis. Furthermore, the evolution of the second- and third-order velocity structure functions satisfies equilibrium similarity only approximately.

Feedback control of slowly-varying transient growth by an array of plasma actuators

Closed-loop feedback control of boundary layer streaks embedded in a laminar boundary layer and experiencing transient growth, which is inherent to bypass boundary layer transition, is experimentally investigated. Streaky disturbances are introduced by a spanwise array of cylindrical roughness elements, and a counter disturbance is provided by a spanwise array of plasma actuators, which are capable of generating spanwise-periodic counter rotating vortices in the boundary layer. Feedback is provided by a spanwise array of shear stress sensors. An input/output model of the system is obtained from measurements of the boundary layer response to steady forcing, and used to design and analyze a proportional-integral controller, which targets a specific spanwise wavenumber of the disturbance. Attention is directed towards a quasi-steady case in which the controller update is slower than the convective time scale. This choice enables addressing issues pertinent to sensing, actuation, and control strategy that are...

Dielectric material degradation monitoring of dielectric barrier discharge plasma actuators

It is a known phenomenon that some dielectric materials used to construct plasma actuators degrade during operation. However, the rate at which this process occurs, to what extent, as well as a method to monitor is yet to be established. In this experimental study, it is shown that electrical measurements can be used to monitor changes in the material of the plasma actuators. The procedure we introduce for monitoring the actuators follows from the work of Kriegseis, Grundmann, and Tropea [Kriegseis et al., J. Appl. Phys. 110, 013305 (2011)], who used Lissajous figures to measure actuator power consumption and capacitance. In the present study, we quantify changes in both the power consumption and capacitance of the actuators over long operating durations. It is shown that the increase in the effective capacitance of the actuator is related to degradation (thinning) of the dielectric layer, which is accompanied by an increase in actuator power consumption. For actuators constructed from layers of Kapton® polyimide tape, these changes are self-limiting. Although the polyimide film degrades relatively quickly, the underlying adhesive layer appears to remain intact. Over time, the effective capacitance was found to increase by up to 36%, 25%, and 11% for actuators constructed with 2, 3, and 4 layers of Kapton tape, respectively. A method is presented to prevent erosion of the Kapton dielectric layer using a coating of Polydimethylsiloxane oil. It is shown the application of this treatment can delay the onset of degradation of the Kapton dielectric material.

Scaling range of velocity and passive scalar spectra in grid turbulence

Isotropic velocity and scalar fluctuations are closely approximated by slightly stretching a heated grid flow through a short (1.36:1) contraction. The heating is such that temperature serves as a passive scalar, and the velocity/scalar time scale ratio is about one. At small values of Taylor microscale Reynolds number (10 102) indicate that, to obtain a 5/3 scaling range, Rλ must exceed 103. The ratio (5/3 + mu)/mθ is approximately 2, in close conformity with the proposal of Danaila and...

Scale-by-scale energy budget in fractal element grid-generated turbulence

Measurements were conducted downstream of a square-fractal-element grid at , where L0 is the size of the largest element in the grid. The scale-by-scale energy budget for grid turbulence is used to investigate the phenomenological change in the turbulence between the inhomogeneous and homogeneous regions downstream of the grid, providing greater insight into the evolution of the turbulence in these two regions. It is shown that in the far field, x/L0 ≥ 20, where the flow is approximately homogeneous and isotropic, the scale-by-scale energy budget for grid turbulence is well balanced. In the near field, x/L0 < 20, the same energy budget is not satisfied, with the imbalance of the budget occurring at scales in the range λ ≲ r ≲ L0. It is proposed that the imbalance is caused by non-zero transverse transport of turbulent kinetic energy and production due to transverse mean velocity gradients. Approach of the spectra to k−5/3 behaviour with a decade long scaling range in the inhomogeneous region is attributed...

Effect of Plasma Actuator Excitation for Controlling Bypass Transition in Boundary Layers

The response of a zero-pressure-gradient laminar boundary layer to forcing by a spanwise array of plasma actuators is investigated experimentally. The plasma actuators used in this study are designed to produce counter-rotating vortex pairs in a spanwise periodic arrangement inside the boundary layer. It was demonstrated by Hanson et al., in related studies, that this plasma actuator array can be successfully used to control the transient growth instability occurring in a Blasius boundary layer. However, it was also demonstrated that the control effectiveness could be greatly affected by the actuator array geometry, which affects the modal content and energy distribution of the resulting disturbance. The focus of the present work is on studying the effect of excitation signal parameters, including the driving frequency and voltage, on the disturbance introduced by the plasma actuators. It is demonstrated that the disturbance energy increases with increasing frequency and voltage, consistent with previous studies of the thrust force or induced velocity from simpler actuator geometries. A crucial finding of the present study is that the individual modes produced by the actuator array are not affected in the same proportional manner by varying the voltage. The control disturbance approached a pure spanwise-harmonic disturbance for increasing voltage. In contrast, the excitation frequency did not have a discernible effect on the relative magnitude of each mode produced by the actuator array. These results highlight the complexity of the response of the boundary layer to the various actuation parameters and have critical implications for the practical integration of these actuators as part of a closed-loop control system.

Distributed forcing flow control in the wake of a blunt trailing edge profiled body using plasma actuators

A modern flow control technique for reducing the drag associated with the periodic shedding of von Karman vortices in the wake of a blunt trailing edge profiled body is presented. The technique involves distributed forcing of the wake flow using an array of dielectric barrier discharge plasma actuators, with a spanwise spacing matched to the spanwise wavelength of the dominant secondary wake instability. The experiments include measurement of the velocity field in multiple vertical and horizontal planes in the wake using particle image velocimetry, as well as base pressure, at Reynolds numbers of 2000, 3000, and 5000 based on trailing edge thickness. The flow control technique causes elongation of the vortex formation region across the span, and significant reduction of the fluctuating and total drag forces, up to a maximum of 94% and 18%, respectively. The effectiveness of the flow control technique is shown to be dependent on the induced momentum coefficient. Proper orthogonal decomposition analysis is ...

The effect of Reynolds number on the scaling range along the centreline of a round turbulent jet

In this paper, the effect of Reynolds number on the scaling range of the velocity spectra and structure functions along the centreline in a round turbulent jet is considered. Hot-wire measurements were taken over a range of exit Reynolds numbers (). Both axial (u) and transverse (v) energy spectra in the scaling range demonstrate considerable departure from the k−5/3 (Kolmogorov inertial range scaling). Over the present range of Reynolds numbers, the scaling wave number exponent for axial, m1, and transverse, m2, spectra exhibit monotonic changes with Taylor Reynolds number: , . On the jet centreline, the data are in close agreement with the earlier findings for grid-generated turbulence. In addition, the structure functions are investigated. The scaling exponent for the second-order structure functions increases with in a power-law fashion () similar to the spectra. The peak value of the normalised third-order structure function C3 asymptotically approaches 4/5 as the Reynolds numbers increases: . The present round jet data confirm that a proper inertial range is unlikely to be established along the jet axis unless a very high Reynolds number of can be reached.

Velocity derivative skewness in fractal-generated, non-equilibrium grid turbulence

The evolution of the velocity derivative skewness, S(∂u/∂x), is investigated along two streamwise axes and four transverse positions in the wake of a square-fractal-element grid. In the near-field, the produced turbulence exhibits non-equilibrium characteristics including Cϵ∼ReMα/ReLβ. In the far-field, the turbulence agrees with canonical grid turbulence results and Cϵ is approximately constant. It is found that in the non-equilibrium region, the value of −S(∂u/∂x) is dependent on both streamwise and transverse positions, but after a sufficient decay period, it takes on a near constant value in the far-field. It is demonstrated that the evolution Cϵ approximately corresponds to that of −S(∂u/∂x), which is suggestive that some of the non-equilibrium properties are likely a result of residual strain from the turbulence generating conditions.

Laminar Boundary-Layer Response to Spanwise Periodic Forcing by Dielectric-Barrier-Discharge Plasma-Actuator Arrays

An extensive parametric study of the response of a Blasius boundary layer to forcing by spanwise arrays of plasma actuators was performed. The actuator arrays are designed to produce streamwise-oriented vortex pairs, which cause streaks of alternating low and high streamwise velocity. This work is motivated by the closed-loop control of bypass transition. The streaks are measured using hot-wire anemometry over a streamwise distance of approximately 100 boundary-layer thicknesses. The resulting spanwise periodic disturbance velocity is decomposed into spanwise Fourier modes. Typically, at least 90% of the disturbance energy is contained within a mode related to the spanwise spacing of the exposed electrodes and its first three harmonics. Along the streamwise direction, the growth and decay characteristics of the disturbance are considered for various actuator geometries, as well as excitation voltages and frequencies for a freestream velocity of 5  m/s. The total disturbance energy is shown to scale with t...