The wake behind a rectangular forebody with variable inlet conditions is investigated. The perforated surface of the two-dimensional rectangular forebody, with a smooth leading edge and a blunt trailing edge, allows for boundary layer modification by means of wall suction. The test section, of which the rectangular forebody is the main part, is experimentally evaluated with a series of hot-wire and Prandtl tube measurements in the boundary layer and the wake. For a suction coefficient of Γ&gt;9, corresponding to 0.9% suction of the free stream velocity, the asymptotic suction boundary layer (ASBL) is obtained at the trailing edge of the forebody for laminar boundary layers (Rex=1.6×105−3.8×105). The key feature of the ASBL, a spatially invariant boundary thickness which can be modified independent of the Reynolds number, is used to perform a unique parametrical study. Turbulent boundary layers (Rex=4.5×105−3.0×106) subject to wall suction are also investigated. For a critical suction coefficient Γcrit, which depends on Rex, the boundary layer relaminarizes. Strong evidence is found to support the hypothesis that turbulent boundary layers will ultimately attain the ASBL as well, provided that the wall suction is strong enough. The effect of the modulated laminar and turbulent boundary layers on the wake characteristics is studied. The shape of the mean wake velocity profile, scaled with the velocity deficit U0and the wake half width ∆y1/2, is found tobe independent of x/h, for x/h&gt; 6 and Reh &gt;6.7×103. The wake width is shown to scale with the effective thickness of the body h+2δ1, where the ratio is expected to vary with the downstream location. A decrease of the displacement thickness leads to a decrease of the base pressure, with Cp,b = −0.36 in the ASBL limit. The Strouhal number based on the effective thickness becomes Sth+2δ1 ≈ 0.29 in the ASBL limit and independent of the plate thickness (h) Reynolds number, in the range Reh = 2.9×103 − 6.7×103. For the turbulent boundary Sth+2δ1 is found to be 25% lower, which shows that the wake characteristics depend on the state of the boundary layer at the trailing edge. The total drag is found to be reduced by as much as 30% for Reh = 2.7×104 when a wall normal velocity of only 3.5% of the free stream velocity is applied. Wall suction successively reduces the total drag with increasing wall suction, at least in the Reynolds number rangeReh = 8.0×103−5.5×104. / <p>QC 20140312</p>

Wind storms pose dangerous threats to human lives and are an enormous drain on the economy. Their damage to buildings usually starts with the failure of structural components that are subjected to excessive wind loads. In this dissertation, we investigate the characteristics of extreme loads on low-rise structures through analysis of full-scale and numerical data. We also use numerical simulations to evaluate different approaches to control the separated flow over a surface-mounted prism with the objective of reducing extreme pressure coefficients or loads on its surface. In the first part, we use a probabilistic approach to characterize peak loads as measured on a subject house during Hurricane Ivan on 2004. Time series of pressure coefficients collected on the roof of that house are analyzed. Rather than using peak values, which could vary due to the stochastic nature of the data, a probabilistic analysis is used to determine the probability of non-exceedence of specific values of pressure coefficients and associated wind loads. The results show that the time series of the pressure coefficients follow a three-parameter Gamma distribution, while the peak pressure follows a two-parameter Gumbel distribution. The results of the analysis are contrasted with the design values. In the second part, we perform numerical simulations of the flow over a surface-mounted prism as a simplified example for the flow over a low-rise structure. A Direct Numerical Simulation (DNS) code is developed to solve the unsteady two-dimensional incompressible Navier-Stokes equations of the flow past the prism. The pressure coefficients are then computed on the prism surface in order to assess the wind loads. The code is written on a parallel platform using the Message Passing Interface (MPI) library. We use the simulations to study the effects of inflow disturbances on the extreme loads on structures. The sensitivities of peak loads on a surface mounted prism to variations in incident gust parameters are determined. Latin Hypercube Sampling (LHS) is applied to obtain different combinations of inflow parameters. A non-intrusive polynomial chaos expansion is then applied to determine the sensitivities. The results show that the gust enhances the destabilization of the separation shear layer, forces it to break down and moves it closer to the roof of the prism. As for the sensitivities, the results show that the extreme loads are most sensitive to the transverse amplitude of the disturbance. Because the separated flow over sharp edges is responsible for the extreme pressure peaks, we investigate the use of active and passive control strategies to reduce wind loads. The studied active flow control strategies include blowing, suction, and synthetic jets. We implement them by using different flux injections, different slot locations and different angles. Investigation of the possible peak pressure reduction for two Reynolds numbers is performed. For Re = 1000, a reduction by nearly 50% of the peak pressure is obtained. For Re = 10, 000, the highest achieved reduction is nearly 25%. For passive control, we mount a flexible membrane on the top of the prism. In a two-dimensional framework, the membrane equation is modeled by a forced string equation. This mechanical equation is coupled with the DNS solver and integrated in time using a fourth order Hamming predictor corrector scheme. The results show that this strategy is as efficient as the active control approach, in terms of reducing extreme loads, for Re = 10, 000. / Ph. D.

No description available.

An aerodynamic investigation was performed to assess the effectiveness of combined boundary layer suction and trailing edge blowing at reducing the blade profile losses and the wake momentum deficit of a cascade of tandem IGV's operating at realistic flow conditions. Two trailing edge blowing designs were tested: metal-angle blowing, which oriented the blowing jets very near to the blade exit angle, and deviation-angle blowing, which oriented the blowing jets at a significant deviation angle from the blade exit angle. Both blowing designs used the same boundary layer suction arrangement. A linear cascade of five IGV's was tested with a flap deflection angle of 40 degrees and an inlet Mach number of 0.3. The Reynolds number based on the overall IGV chord length for these experiments was greater than 500,000. The inlet and exit angles of the IGV at this flap setting were 0 degrees and 55 degrees, respectively. Tests performed with no flow control showed significant suction surface flow separation that generated large wakes with high losses and large momentum deficits. The application of boundary layer suction reduced the baseline pressure loss coefficient and wake momentum thickness by 22%. A suction mass flow of 0.4% of the passage flow was used to obtain these results. The addition of metal-angle blowing with the suction resulted in total reductions of 48% and 38% for the pressure loss coefficient and wake momentum thickness. A blowing mass flow of 3.1% of the passage flow was used in addition to 0.4% suction mass flow to obtain these results. The application of the deviation-angle blowing was detrimental to the aerodynamics of the IGV, as both the pressure loss coefficient and wake momentum thickness increased slightly over their suction-only values. This was attributed to a manufacturing defect which distorted the flow of the blowing jet. The results of the deviation-angle blowing experiments were not considered representative of the design intent and reinforced the importance of the hole design for creating a proper blowing jet. While low speed tests of this cascade showed results and trends very similar to those of previous research, the application of flow control proved to be less effective at higher speeds due to the generation of significantly larger wakes. / Master of Science

No description available.

Flow measurements were made on two highly loaded, low pressure turbine blade configurations in a low-speed, linear cascade facility. The L1M blade has a design Zweifel coefficient of 1.34 with a peak cp near 47% cx (mid-loaded) and the Pack B blade has a design Zweifel coefficient of 1.15 with a peak cp at 63% cx (aft-loaded). Flow velocity and surface pressure measurements were taken for Rec=20,000 and 3% inlet freestream turbulence. For these operating conditions, a large separation bubble forms on the blade suction surface, beginning at 59% cx and reattaching at 86% cx on the L1M blade and a non-reattaching bubble beginning at 68% cx on the Pack B. A spanwise row of discrete vortex-generating jets located at 59% cx on the Pack B and 50% cx on the L1M were used as a separation control device and were pulsed at a frequency of 5 Hz with a duty cycle of 25%. The Pack B with its open separation bubble proved to be a better candidate for VGJ control than the L1M with its closed separation bubble. Further studies were made on the Pack B blade comparing wake and VGJ effects. A wake generator was used to simulate the periodic passing of upstream wakes through the blade passage for the Pack B configuration. The wake passing frequency of 4.5Hz was set to match a typical engine flow coefficient for a low pressure turbine. Data were taken using PIV and a hot-film anemometer mounted on a blade following device. Velocity, turbulence, and intermittency measurements were made along the suction surface of the blade to characterize the bubble dynamics and transitional behaviors for both the presence of unsteady wakes and pulsing VGJs. The wakes caused early breakdown of the separated free shear layer resulting in a thinning of the separation region. The VGJs caused an upstream disturbance which convects downstream, temporarily pushing off the separation bubble. Overall, both wakes and VGJs suppress the size of the steady-state separation bubble, though through different mechanisms. Three-dimensional aspects of the jet disturbance are studied by investigating the effects of the VGJs at two spanwise locations.

Ad hoc wireless networks are increasingly being used to provide connectivity where a wired networking infrastructure is either unavailable or inaccessible. Many deployments utilize group communication, where several senders communicate with several receivers; multicasting has long been seen as an efficient way to provide this service. While there has been a great deal of research on multicast routing in ad hoc networks, relatively little attention has been paid to the design of multicast transport protocols, which provide reliability and congestion control. In this dissertation we design and implement a complete multicast transport architecture that includes both routing and transport protocols. Our multicast transport architecture has three modules: (a) a multicast routing and state setup protocol, (b) a mobility detection algorithm, and (c) a hop-by-hop transport protocol. The multicast routing and state setup protocol, called ASSM, is lightweight and receiver-oriented, making it both efficient and scalable. A key part of ASSM is its use of Source Specific Multicast semantics to avoid broadcasting when searching for sources. ASSM also uses routes provided by the unicast protocol to greatly reduce routing overhead. The second module, MDA, solves the problem of determining the cause of frame loss and reacting properly. Frame loss can occur due to contention, a collision, or mobility. Many routing protocols make the mistake of interpreting all loss as due to mobility, resulting in significant overhead when they initiate a repair that is not required. MDA enables routing protocols to react to frame loss only when necessary. The third module is a hop-by-hop multicast transport protocol, HCP. A hop-by-hop algorithm has a faster response time than that of an end-to-end algorithm, because it invokes congestion control at each hop instead of waiting for an end-to-end response. An important feature of HCP is that it can send data at different rates to receivers with different available bandwidth. We evaluate all three components of this architecture using simulations, demonstrating the improved performance, efficiency and scalability of our architecture as compared to other solutions.

No description available.

This work is concerned with the study of flow stability and turbulence control - two old but still open problems of fluid mechanics. The topics are distinct and are (currently) approached from different directions and with different strategies. This thesis reflects this diversity in subject with a difference in geometry and, consequently, flow structure: the first problem is approached in the study of the flow in a toroidal pipe, the second one in an attempt to reduce the drag in a turbulent channel flow. The flow in a toroidal pipe is chosen as it represents the common asymptotic limit between spatially developing and helical pipes. Furthermore, the torus represents the smallest departure from the canonical straight pipe flow, at least for small curvatures. The interest in this geometry is twofold: it allows us to isolate the effect of the curvature on the flow and to approach straight as well as helical pipes. The analysis features a characterisation of the steady solution as a function of curvature and the Reynolds number. The problem of forcing fluid in the pipe is addressed, and the so-called Dean number is shown to be of little use, except for infinitesimally low curvatures. It is found that the flow is modally unstable and undergoes a Hopf bifurcation that leads to a limit cycle. The bifurcation and the corresponding eigenmodes are studied in detail, providing a complete picture of the instability. The second part of the thesis approaches fluid mechanics from a different perspective: the Reynolds number is too high for a deterministic description and the flow is analysed with statistical tools. The objective is to reduce the friction exerted by a turbulent flow on the walls of a channel, and the idea is to employ a control strategy independent of the small, and Reynolds number-dependent, turbulent scales. The method of choice was proposed by Schoppa &amp; Hussain [Phys. Fluids 10:1049-1051 (1998)] and consists in the imposition of streamwise invariant, large-scale vortices. The vortices are re-implemented as a volume force, validated and analysed. Results show that the original method only gave rise to transient drag reduction while the forcing version is capable of sustained drag reduction of up to 18%. An analysis of the method, though, reveals that its effectiveness decreases rapidly as the Reynolds number is increased. / <p>QC 20161004</p>

An experiment was performed in the Cal Poly Mechanical Engineering 2x2 ft wind tunnel to quantify the effect of spanwise synthetic jet actuation (SJA) on the drag of a NACA 0015 semispan wing. The wing, which was designed and manufactured for this experiment, has an aspect ratio of 4.20, a span of 0.427 m (16.813”), and is built around an internal array of piezoelectric actuators, which work in series to create a synthetic jet that emanates from the wingtip in the spanwise direction. Direct lift and drag measurements were taken at a Reynolds Number of 100,000 and 200,000 using a load cell/slider mechanism to quantify the effect of actuation on the lift and drag. It was found that the piezoelectric disks used in the synthetic jet actuators cause structural vibrations that have a significant effect on the aerodynamics of the NACA 0015 model. The experiment was performed in a way as to isolate the effect of vibration from the effect of the synthetic jet on the lift and drag. Lift and drag data was supported with pressure readings from 60 pressure ports distributed in rows along the span of the wing. Oil droplet flow visualization was also performed to understand the effect of SJA near the wingtip. The synthetic jet and vibration had effects on the drag. The synthetic jet with vibration decreased the drag only slightly while vibration alone could decrease drag significantly from 11.3% at α = 4° to 23.4% at α = 10° and Re = 100,000. The lift was slightly increased with a slight increase due to the jet and showed a slight increase due to vibration. Two complete rows of pressure ports at 2y/b = 37.5% and 85.1% showed changes in lift due to actuation as well. The synthetic jet increased the lift near the wingtip at 2y/b = 85.1% and had little to no effect inboard at the 37.5% location, hence, the synthetic jet changes the lift distribution on the wing. Oil flow visualization was used to support this claim. Without actuation, the footprint of the tip vortex was present on the upper surface of the wing. With actuation on, the footprint disappeared suggesting the vortex was pushed off the wingtip by the jet. It is possible that the increased lift with actuation can be caused by the vortex being pushed outboard.