Ahmed Naguib

Wall-pressure-array measurements beneath a separating/reattaching flow region

A database of wall-pressure-array measurements was compiled for studying the space–time character of the surface-pressure field within a separating/reattaching flow region. The experimental setup consisted of a long splitter plate located within the wake of a fence and instrumented with an array of flush-mounted microphones. Data were acquired for a Reynolds number of 7900, based on the fence height above the splitter plate. Two distinctive regions, defined based on their location relative to the position of the mean reattachment point (xr) of the shear layer, emerged from this investigation. Upstream, from the fence to 0.25xr, the surface-pressure signature was dominated by large time scale disturbances and an upstream convection velocity of 0.21U∞. Beyond 0.25xr, turbulent structures with smaller time scales and a downstream convection velocity of 0.57U∞ generated most of the pressure fluctuations. Interestingly, the low-frequency wall-pressure signature typically associated with the flapping of the sep...

Stochastic estimation of a separated-flow field using wall-pressure-array measurements

Concurrent, surface-pressure and planar, particle image velocimetry (PIV) measurements were obtained in the separating/reattaching flow region downstream of an axisymmetric, backward-facing step at a Reynolds number of 8081, based on step height. The surface-pressure and PIV measurements were used to investigate the evolution of coherent structures in the flow field by employing proper orthogonal decomposition (POD) and multipoint, linear, stochastic estimation (mLSE) analysis techniques. POD was used to determine the dominant modes in the pressure signature, while mLSE was used to estimate the dominant flow structures above the wall from the wall-pressure POD modes over a series of time steps. It was found that a large-scale, coherent structure develops in place (i.e., temporally) at approximately half the reattachment distance. Once this structure reaches a height equivalent to the step, it sheds and accelerates downstream. This growth in place, and then shedding, resembles the evolution of the flow str...

On wall-pressure sources associated with the unsteady separation in a vortex-ring/wall interaction

Molecular tagging velocimetery measurements from an earlier investigation are used to study the wall-pressure and the flow structures responsible for its generation in the flow field resulting from the impingement of an axisymmetric vortex ring on a flat wall. The velocity-field data are used to obtain the spatial distribution of the pressure sources, and the result is employed in conjunction with the solution to Poisson’s equation to yield the wall-pressure information. The outcome reveals a characteristic wall-pressure signature that is produced repeatedly whenever the primary vortex ring interacts with the wall to form vortex rings with opposite sense of vorticity. The details of the signature are analyzed and related to specific flow features and their mutual interaction at different phases of the generation/evolution cycle of the new rings. Finally, the flow mechanisms leading to the generation of substantial positive and negative wall pressure in the characteristic signature are clarified.

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...

High-frequency oscillating-hot-wire sensor for near-wall diagnostics in separated flows

A new high-frequency oscillating-hot-wire sensor for magnitude and direction measurements of the wall-shear stress in separated flows is presented. The stress direction is determined from the phase angle between the imposed and measured oscillation velocity, and the corresponding magnitude is obtained from the low-pass filtered signal of the sensor, after removal of the modulating influence of the oscillation. The sensor was used to conduct measurements downstream of an axisymmetric backward-facing step (BFS) at different streamwise locations ranging from 0.3 to 10 step heights downstream of the step. The results agreed qualitatively with existing one-point measurements, such as the mean/rms skin-friction distribution, forward flow probability, and power spectra, in planar BFS flows. However, some fundamental quantitative differences were found including a shorter reattachment length and a larger streamwise location for the peak rms wall-shear fluctuations. These differences were attributed to the axisymmetric nature of the present geometry, transverse curvature of the step, or differences in the measurement methods. Additionally, the power spectra of the fluctuating wall-shear stress revealed the existence of two characteristic frequencies: f ∗ = 0.1 and 0.65. The former is associated with low-frequency shear-layer flapping, and the latter corresponds to the passage of the separated shear-layer vortex structures.

An electrostatic microactuator system for application in high-speed jets

The development of an electrostatic microactuator system for the study and control of high-speed jet flows is presented. The electrostatic actuator is 1.3 mm wide, 14 /spl mu/m thick and has a head that overhangs a glass substrate, intruding into the flow by 200 /spl mu/m. The actuator has been fabricated using a bulk-silicon dissolved-wafer process to increase device thickness for increased stiffness in the flow direction. Characterization of the new actuators demonstrated their ability to oscillate with amplitudes of up to 70 /spl mu/m peak-to-peak at resonant frequencies of 5 and 14 kHz. This is a very large motion at such high frequencies when compared to existing macro or micro mechanical actuators. The full actuator system was mounted around the exit of a high-speed jet using several sector-shaped PC boards. This enabled detailed examination of the ability of the actuators to withstand the flow environment and generate substantial flow disturbances. The results showed that the microactuators functioned properly up to jet speeds of 300 m/s while generating disturbances in the shear layer surrounding the jet comparable to those produced by other macro-scale methodologies.

A Doppler sensor for high spatial resolution measurements of unsteady surface pressure

The Doppler frequency shift is used as the basic sensing mechanism for a new technique for measurement of unsteady surface pressure. The frequency shift is experienced by a focused laser beam reflected off the aluminized top of a flexible polymer diaphragm subjected to the unsteady pressure. Prototype sensors based on this concept, with different sizes and diaphragm materials and thicknesses, are constructed as well as evaluated. The results provide an understanding of the limits of the sensors sensitivity, bandwidth, resolution and noise level. Moreover, analysis of typical wall-pressure spectra beneath boundary layers of high and low Reynolds number in the light of these limits underlines the potential advantage of the new sensor in resolving the signature of small-scale turbulent structures at high Reynolds numbers.

A silicon micromachined microphone for fluid mechanics research

MEMS piezoresistive sound detectors have been fabricated using the dissolved wafer process for the first time. The sensors utilize stress compensated PECVD ultra-thin silicon-nitride/oxide membrane together with monocrystalline ion-implanted p++ silicon piezoresistors to achieve high sensitivity. Tests reveal that sensors with a diaphragm size of 710 μm have a static sensitivity of 1.1 μV VPa−1 with 2% non-linearity over an operating pressure range of 10 kPa. This sensitivity is substantially larger than that of commercially available microfabricated sensors. Furthermore, the new sensors dynamic response is found to be flat (within ±2.5 dB) over a frequency range extending up to 10 kHz. This paper contributes to existing literature in the field by demonstrating a new way of fabricating capable MEMS piezoresistive pressure sensors, hence adding to the overall versatility of the technology and associated range of applications.

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.

Effect of Cavity Width on the Unsteady Pressure in a Low-Mach-Number Cavity

T HE flow over a cavity (Fig. 1) has drawn the attention of many researchers (e.g., see the review articles by Rockwell and Naudascher [1] and Colonius [2]) because of its relevance to a range of engineering applications. These include the flow past windows and sunroofs in automobiles, airplane wheel wells, and dump combustors. The flow in these circumstances is characterized by selfsustained, high-pressure fluctuations, which can produce vibration and fatigue of the underlying structure, a high level of noise, and drastic increase in the drag force on the body containing the cavity. Hence, understanding and controlling the unsteadiness associated with cavity flows are important. Rossiter [3] identified the basic mechanism leading to strong, selfsustained oscillations in cavities. More specifically, he proposed that small disturbances in the shear layer separating at the upstream edge of the cavity are amplified, forming periodic vortex structures that travel downstream and interact with the aft edge of the cavity, generating strong pressurefluctuations. Thesefluctuations propagate (“feedback”) acoustically to the separation edge and reexcite the shear layer, hence, sustaining the cavity oscillations. A notable deviation from the Rossiter mechanism is the case where the vortex structures do not form upstream of the cavity’s aft edge. In this situation, self-sustained oscillations could still exist, but they are driven by convective waves, which cause large lateral motion (or “flapping”) of the shear layer near the downstream lip of the cavity (e.g., Sarohia [4] and Chatellier et al. [5]). Another important aspect of self-sustained oscillations of a cavity is the nature of the feedback. As M ! 0, the disturbance-edge interaction is inefficient in producing acoustic disturbances and the feedback is driven by the conservation of the fluid mass within the cavity (e.g., see Martin et al. [6] and Rockwell [7]). Although there is a large body of literature on cavity flows, very few studies have focused on three-dimensional aspects of the flow. Cell structures along the span of the cavity were found in the experiments of Maull and East [8]. Rockwell and Knisely [9] found that secondary, spanwise-periodic, streamwise vortices formed and distorted the primary spanwise vortex structures. More recently, three-dimensional instability analysis of a two-dimensional cavity flowwas carried out by Bres and Colonius [10]. It was found that the most amplified three-dimensional mode had a typical frequency that was an order of magnitude smaller than the frequency of the selfsustained cavity oscillations. This modewas linked to the centrifugal instability of the primary recirculation flow inside the cavity. The objective of this study is to examine the influence of the cavity width on the behavior of the unsteady pressure acting on the cavity floor and how this behavior depends on Reynolds number. A unique aspect of the present investigation is that an axisymmetric cavity geometry is employed to establish a “reference” condition that is free of any end-wall influences. The behavior of this cavity flow can then be compared against that of finite-width cavities byfilling portions of the axisymmetric cavity (see the next section for further details). It is significant to contrast the present work to the study ofMaull and East [8], who employed a rectangular cavity geometry that had to be terminated with end walls (even for the widest cavity). In addition, Maull and East did not report any quantitative measurements of the cavity unsteadiness. As will be seen, the present findings show that, asM ! 0, the accepted criteria, based on cavity depth and length as well as boundary layer thickness, for the occurrence of self-sustained oscillation are not sufficient to guarantee the establishment of the oscillation. The cavity width must also be accounted for; this offers a possible explanation for the discrepancy between recent (Grace et al. [11] and Ashcroft and Zhang [12]) and earlier studies at a low Mach number.