In the context of building a framework for active flow control of turbulent boundary layers in wings, a set of large-eddy simulation (LES) are implemented in OpenFOAM. The flow around a NACA4412 wing profile is simulated at 5° angle of attack and Re_c = 400˙000. Validation of the uncontrolled flow results is performed with respect to the dataset generated by Vinuesa et al. (2018) at the same aerodynamic configuration. Afterwards, two different flow control strategies are analyzed over the suction side (SS) of the wing to yield skin friction drag reduction and an overall improvement of the aerodynamic efficiency. The region subject to the actuation spans 0.25 x_ss/c to 0.:86 x_ss/c, where c is the chord length of the wing. In the current setup, uniform blowing (BLW) and suction (SCT) control schemes show close agreement with the trends presented by Atzori (2021). Indeed, BLW decreases the viscous drag, but increases its pressure contribution and penalizes the lift, thus lowering the global efficiency of the wing, while SCT has an opposite effect. Thus, these methods behave similarly to pressure gradients (PGs) conditions, as BLW enhances the APG, whereas SCT damps it. The streamwise travelling waves strategy is then assessed for three set-ups characterized by different phase speeds. A consistent skin friction drag reduction and efficiency improvement are observed for two cases, while milder benefits are recorded even when drag increase was expected. Trends which have already been reported in the literature by Quadrio et al. (2009) and Skote (2014) are identified, i.e. the effects of this actuation to be mainly enclosed in the viscous sub-layer and the gross amount of drag reduction to be dependent on the wave relative speed; however, it is believed that the PGs conditions over the SS of the wing significantly alters the outcomes of the chosen parameters. Eventually, Reynolds averaged Navier-Stokes (RANS) simulations are performed to assess their accuracy with respect to the generated LES set-up, in the effort to enable a multi-fidelity approach for future works.

No description available.

This thesis presents a series of direct numerical simulations (DNS) performed in order to understand the discrepancy in the literature regarding turbulent asymptotic suction boundary layers (TASBLs) at low Reynolds numbers. The hypothesis to be tested is that the main reason for higher turbulence intensities observed in experiments compared to DNS is that the latter exhibits intermittent patches of laminar flow, developing both temporally and spatially. This hypothesis is confirmed here by comparing simulations with and without tripping, where the former removed patches of laminar flow thereby establishing a fully developed turbulent state with higher turbulence intensities compared to its naturally developing counterpart. The DNS were performed at different suctions rates corresponding to Reynolds numbers above the critical value of 270. The statistics taken from the simulations at different streamwise positions also support the developing character of the flow with increasing intermittency further downstream. Thus, it can be concluded that the actual flow state at these marginal Reynolds numbers is indeed an intermittent one, with lower fluctuation values as the experimental data would indicate.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1980. / Microfiche copy available in Archives and Science. / Bibliography: leaves 92-98. / by Eric Arthur D'Asaro. / Ph.D.

Research presented herein involved the use of a smart skin treatment used for the attenuation of turbulent boundary layer induced interior noise. The treatment consisted of several Smart Foam actuators each having a reference and error sensor along with a feed forward, filtered-x controller. Studies were performed to determine if the use of multiple instances of single input, single output (SISO) control systems could be implemented with success given the difficulty of actively suppressing turbulent boundary layer induced interior noise. Further, this research will lead to the development of an integrated Smart Foam element consisting of a Smart Foam actuator, reference sensor, error sensor and SISO controller in one complete, stand--alone unit. Several topics were studied during this effort: reference sensing, error sensing, actuator design, controller causality, correlation of turbulent flow and resulting plate vibration, and coherence between plate vibration and the interior noise field. Each study was performed with the goal of improving the performance of active attenuation of turbulent boundary layer induced interior noise. Depending on the configuration of the control system, control was performed using either experiments or simulations based on experimental data. Within the desired control band of 400--800~Hz, attenuation of up to -3.1~dB$_A$ was achieved at the error sensors and up to -1.4~dB$_A$ within the observer plane relative to the uncontrolled case. However, over a band of greater coherence from 480--750~Hz, attenuation of up to -4.8~dB$_A$ was achieved at the error sensors and up to -2.6~dB$_A$ within the observer plane. Further, peak attenuation of up to -12~dB$_A$ was achieved within the observer plane. Studies were also conducted to increase the low frequency performance of the Smart Foam treatment. These experiments used tuning masses placed on the tops of the integrated Smart Foam elements to tune them to the fundamental mode of the vibrating plate. This treatment was used to reactively attenuate plate vibration such that the radiated acoustic field would be minimized. These experiments resulted in -6~dB$_A$ global attenuation at the plate fundamental resonance. Further, it was shown that the reactive treatment did not inhibit active control. / Ph. D.

Federal employment law designed to assure equal employment opportunity for faculty has only been applicable to higher education since 1972. Prior to 1972, the higher education world, moreover, was immune from the most comprehensive federal employment law, Title VII of the Civil Rights Act of 1964. However, Title VII was amended in 1972 to include education institutions. Ever since the protection of the civil rights law was extended to higher education, faculty employment discrimination litigation has increased. The reality of this phenomenal growth in litigation is clear, the potential for judicial intervention in academic decision making is undeniable, and reliance on the judicial process is increasingly becoming common. Thus, no institution of higher education may consider itself immune from the possibilities of litigation, nor immune from the decisions handed down by the courts. The main focus of this study was a legal one, which necessitated a heavy concentration upon the historical and current state of employment discrimination law, specifically, Title VII of the Civil Rights Act of 1964. The study was conducted by using a combination of legislative analysis and legal research methods. The legal research methods used in this study included the same problem-solving processes as other traditional research methods: (1) collecting data; (2) analysis; and (3) interpretation. The main purpose of this study was to examine, analyze, and summarize legislative history and case law relevant to Title VII, and sex discrimination in higher education. In summary, although Title VII prohibits discrimination on the basis of race, color, religion, sex and national origin, the issues surrounding women faculty and sex discrimination is probably the fastest growing area of litigation for administrators on the university campus. Therefore, this study was an attempt to examine the employment discrimination issues and developments pertaining to sex discrimination only. College and university administrators may find this study useful for: (1) examining Title VII, and its amendments; (2) examining sex discrimination case law; and (3) utilizing the research for developing procedures, policies and guidelines to minimize potential lawsuits. / Ph. D.

Measurements were made on three forward steps with step height to boundary layer ratio of approximately 3.8%, 15% and 60% and Reynolds number based on step height ranging from 6640 to 213,000. The measurements included mean wall pressure, single and 2 point wall pressure fluctuations, single and 2 point velocity fluctuations and, oil flow visualization. Pressure fluctuation measurements were made 5 boundary layer thicknesses upstream of step to 22 boundary layer thickness (or 600 step heights for smallest step size) downstream of the step. The results show that the steps remarkably enhance the wall pressure fluctuations that scale on the step height in the vicinity of the step and far downstream of the step. The decay of wall pressure fluctuations post reattachment is a slow process and elevated levels can be seen as far as 150 step heights downstream for the mid step size. The enhanced pressure fluctuations come from the unsteady reattachment region on top face of the step which was found to be a strong function of flow geometry and flow parameters such as Reynolds number. The 2 point pressure and velocity space-time correlations show a quasi-periodic structure which begins to develop close to the reattachment and grows in intensity and scale further downstream of reattachment and is responsible for the elevated pressure fluctuations downstream of the step. However, the velocity correlations lack in scale reflecting the fact that large scales reflected in pressure are masked by smaller scales that exist within them. / Master of Science

There have been very few studies investigating the rough wall pressure spectra under fully rough flows, which are relevant to many common engineering applications operating within this regime. This investigation uses the Virginia Tech Stability Wind Tunnel to perform experiments on a series of high Reynolds number zero pressure gradient turbulent boundary layers formed over rough walls in an effort to better understand and characterize the behavior of the rough wall pressure spectrum. The boundary layers were fully rough, and the boundary layer height remained sufficiently larger than the height of the roughness elements. Two rough surfaces were tested. One consisted of an array of 1-mm ordered hemispherical elements spaced 5.5-mm apart, and the other contained 3-mm hemispherical elements randomly spaced, but with the same element density as 1/3 of the 1-mm ordered roughness. The wall pressure spectrum and its scaling were then studied in detail, and it was found that the rough wall turbulent pressure spectrum at vehicle relevant conditions is defined by three scaling regions. One of which is a newly discovered high frequency scaling defined by viscosity, but controlled by the friction velocity adjusted to exclude the pressure drag on the roughness elements. Based on these three scaling regions an empirical model describing the wall pressure spectra for hydraulically smooth, traditionally rough, and fully rough flows was explored. Two point wall pressure fluctuations were also analyzed for each surface condition, and it was found that the roughness inhibits the convective velocities within the inner portions of the boundary layer. / Master of Science

In this work, the potential for a smart foam actuator in controlling interior cabin noise due to a turbulent boundary layer excitation has been experimentally demonstrated. A smart foam actuator is a device comprised of sound absorbing foam with an embedded distributed piezoelectric layer (PVDF) designed to operate over a broad range of frequencies. The acoustic foam acts as a passive absorber and targets the high frequency content, while the PVDF serves as the active component and is used to overcome the limitations of the acoustic foam at low frequencies. The fuselage skin of an aircraft was represented by an experimental test panel in an anechoic box mounted to the side of a wind tunnel. The rig was used to simulate turbulent boundary layer noise transmission into and aircraft cabin. An active noise control (ANC) methodology was employed by covering the test panel with the smart foam actuators and driving them to generate a secondary sound field. This secondary sound field, when superimposed with the panel radiation, resulted in a reduction in overall sound in the anechoic box. An adaptive feedforward filtered-x Least-Mean-Squared (LMS) control algorithm was used to drive the smart foam actuators to reduce the sound pressure levels at an array of microphones. Accelerometers measured the response of the test panel and were used as the reference signal for the feedforward algorithm. A detailed summary of the smart foam actuator control performance is presented for two separate low speed wind tunnel facilities with speeds of Mach 0.1 and Mach 0.2 and a single high speed tunnel facility operating at Mach 0.8 and Mach 2.5. / Master of Science

This study investigates the influence of wall roughness on large-scale structures within turbulent boundary layers, using wall-parallel stereoscopic Particle Image Velocimetry (PIV) at a Reynolds number of 1.3 × 106m−1. The data was collected at a distance of = 2.8 from the wall under adverse = 0.98, small = −0.04, and favorable = −0.60 pressure conditions. It was observed that coherent structures larger than the field of view (FOV) modify their length scales, streamwise orientation and spanwise distribution in response to changes in surface roughness and pressure gradients. The study also examines implications wall similarity hypothesis by comparing the development of these large-scale structures over smooth and rough wall flows under small pressure gradient and identical test conditions. Notably, the results reveal that existing models may not accurately capture the observed dynamics, as evidenced by discrepancies with previous studies, thereby enhancing our understanding of turbulent flow dynamics in non-equilibrium conditions. / Master of Science / Understanding the behavior of turbulent air and water flows around objects is crucial for improving engineering designs and various environmental applications. This study investigates how surface roughness, such as on ship hulls or aircraft wings, affects the movement of large-scale structures in turbulent flows. Advanced flow visualization techniques were employed to observe these turbulent structures in detail over homogeneous roughness and varying pressure gradients. The findings demonstrate that rough surfaces significantly alter the movement and interaction of these large-scale turbulent structures. Specifically, favorable pressure gradients result in organized flow structures, whereas adverse pressure gradients cause reorganization, altering the size and distribution of these structures within the flow. By comparing smooth and rough surfaces under identical conditions, the study aims to evaluate how well existing hypotheses predict these changes. The results highlight that rough surfaces and pressure variations critically impact turbulent flow characteristics, emphasizing the need for more sophisticated criteria to improve predictions and designs in real-world applications. This research provides valuable insights into the dynamics of turbulent flows over rough surfaces and stresses the importance of refining current models for better accuracy in practical engineering applications.