Cory Hendrickson

Transition to global instability in transverse-jet shear layers

In a recent paper (Megerian et al ., J. Fluid Mech. , vol. 593, 2007, pp. 93–129), experimental exploration of the behaviour of transverse-jet near-field shear-layer instabilities suggests a significant change in the character of the instability as jet-to-crossflow velocity ratios R are reduced below a critical range. The present study provides a detailed exploration of and additional insights into this transition, with quantification of the growth of disturbances at various locations along and about the jet shear layer, frequency tracking and response of the transverse jet to very strong single-mode forcing, creating a ‘lock-in’ response in the shear layer. In all instances, there is clear evidence that the flush transverse jets near-field shear layer becomes globally unstable when R lies at or below a critical range near 3. These findings have important implications for and provide the underlying strategy by which active control of the transverse jet may be developed.

Phase Compensation Strategies for Modulated-Demodulated Control With Application to Pulsed Jet Injection

Modulated-demodulated control is an effective method for asymptotic disturbance rejection and reference tracking of periodic signals, however, conventional static phase compensation often limits the loop gain in order to avoid sensitivity function peaking in a neighborhood of the frequencies targeted for rejection or tracking. This paper introduces dynamic phase compensation for modulated-demodulated control which improves disturbance rejection characteristics by inverting the plant phase in a neighborhood of the control frequency. Dynamic phase compensation is implemented at baseband which enables the use of low-bandwidth compensators to invert high frequency dynamics. Both static and dynamic phase compensation methods are used to demonstrate a novel application of repetitive control for pulsed jet injection. In this application pulsing an injectant has been shown to produce advantageous effects such as increased mixing in many energy generation and aerospace systems. The sharpness of the pulse can have a large impact on the effectiveness of control. Modulated-demodulated control is used to maximize the sharpness of a pulsed jet of air using active forcing by tracking a square wave in the jet’s temporal velocity profile.

Strategic Control of Transverse Jet Flows

*† ‡ § This experimental study explores the nature and control of shear layer instabilities associated with the single jet in crossflow or transverse jet, a flowfield widely used in propulsive devices. These studies take advantage of prior experimental findings which suggest that the character of the jet’s nearfield shear layer instabilities can be significantly different for the transverse jet as compared with the free jet, and that the instabilities transition further when the jet-to-crossflow velocity ratio R is reduced below approximately 3.5. The differences in the stability characteristics of the jet in crossflow suggest the necessity of a “two-pronged” approach to the control of transverse jet penetration and spread, depending on the values of R, which is explored in this paper. For the transverse jet where R > 3.5, the instabilities are initiated beyond one diameter of the jet orifice, with a type of spatial evolution that indicates a convectively unstable flow. Such instabilities are affected by even low level sinusoidal forcing of the jet, which can then be used to control jet penetration and spread. For the case where R lies below 3.5, however, there is a rapid initiation of strong, distinct modes and harmonics near the jet exit that do not evolve spatially and are not affected by even strong external sinusoidal forcing of the jet. In this instance, strong jet forcing with a prescribed time scale is required to impact jet behavior. It is demonstrated that when the applied jet forcing waveform is a square wave with a prescribed temporal pulsewidth, significant jet response can be obtained even under globally unstable conditions.

Scalar and Velocity Field Measurements in Acoustically Excited Variable Density Transverse Jets

This experimental study explores both the natural and acoustically forced behavior of variable density transverse jets. A recent study by Getsinger et al. determined that transverse jets (of Reynolds number Rej = 1800) likely transition to global instability in response to su cient lowering of the jet-to-cross ow density ratio S (below 0.45-0.40) or momentum ux ratio J (below 10). This transition is characterized by weak shear layer oscillations easily overcome by external forcing for the convectively unstable (high S and J) case, and strong pure-tone oscillations resistant to external forcing for the globally unstable (low S and J) case. Here, simultaneous PLIF and PIV measurements of jet uid concentration and velocity elds are obtained in the jet neareld, as well as standalone PLIF measurements with a larger eld of view. Neareld PLIF/PIV measurements show agreement with hotwire measurements in terms of jet structural features and the character of the shear layer instabilities. The e ect of this apparent transition on jet mixing is also examined here, with implications for control of jet behavior in practical applications.

Compensation of Nonlinear Harmonic Coupling for Pulsed-Jet-Velocity Shaping

This paper describes an approach to periodic reference tracking in a pulsed-jet-injection experimental study. The objective was to match the jet’s temporal velocity profile to a periodic reference. The challenge lies in controlling the highly nonlinear and poorly understood dynamics associated with the jet velocity. Although the actuator maintains good authority over the jet velocity, the nonlinear jet dynamics creates a high degree of coupling among neighboring harmonics that depends on the forcing level and the desired waveform. The coupling is quantified by demodulating the jet-velocity measurement into baseband components centered at the harmonic frequencies represented in the desired waveform. An empirical input–output relationship is developed by perturbing the baseband components and measuring their effect on neighboring harmonics, and it is shown that this relationship can be modeled as a linear multi-input/multi-output system. This knowledge is exploited to create stabilizing feedback controls th...

Identification and control of nonlinear harmonic coupling for pulsed jet injection

This paper describes an approach to periodic reference tracking in a fundamental pulsed jet injection experimental study. The objective is to match the jets temporal velocity profile to a periodic reference for purposes of studying the mixing dynamics between the jet and surrounding fluid. The challenge lies in controlling the highly nonlinear and poorly understood dynamics associated with the jet velocity. Although the actuator maintains good authority over the jet velocity, the nonlinear jet dynamics creates a high degree of coupling among neighboring harmonics that depends on the forcing level. We approach the problem by demodulating the jet velocity measurement at harmonic frequencies represented in the desired waveform into baseband components. In a neighborhood of a desired operating point, an empirical relationship is developed by perturbing the baseband components and measuring their effect on neighboring harmonics. We demonstrate that this relationship can be reasonably modeled as a linear MIMO gain for nearby operating points. This knowledge is exploited to create a stabilizing feedback control that asymptotically drives the jet velocity to its reference.