Thesis (M.S.)--Brigham Young University. Dept. of Mechanical Engineering, 2005. / Includes bibliographical references (p. 99-100).

Thesis (Ph. D.)--University of Texas at Austin, 2004. / Supervisor: David B. Goldstein. Vita. Includes bibliographical references. Also available from UMI.

Die vorliegende Arbeit widmet sich der, vorwiegend experimentellen, Untersuchung der Wirkung wandparalleler Lorentzkräfte in Strömungsrichtung auf Grenzschichtprofile und Körperumströmungen. Die Themen - Beeinflussung der Grenzschicht an ebenen Platten mit stationären Lorentzkräften - Kontrolle von Strömungsablösungen an Zylindern und symmetrischen Profilen mit stationären Lorentzkräften - Beeinflussung von Zylindernachläufen und abgelösten Tragflügelumströmungen mit zeitlich periodischen Lorentzkräften werden behandelt.

In order to improve the state of turbulence modeling for three-dimensional flows, more detailed information on the fundamental physics of the flow is required. It has been recognized for some time now that organized motions or coherent structures in the flow play a large part in determining the flow characteristics, and there is now a large body of literature dealing with various aspects of coherent structures. However, almost all of the existing literature deal with mean two-dimensional flows with very little reported for mean three-dimensional flows. In the present study, measurements were performed in a three-dimensional, pressure-driven turbulent boundary layer (<i>Re</i>_θ = 5936) in the flow around a wing-body junction with a variety of multiple-sensor probes, to examine the features of the coherent structures in the flow. This test flow has a number of practical applications and was selected because of its strong three-dimensional nature and the availability of an extensive set of mean-flow measurements from previous investigations. The measurements were carried out with a hot-wire rake with sixteen sensors spaced approximately logarithmically over 25.4 mm (1 inch), a parallel-sensor probe with two parallel sensors spaced approximately 4.8 mm apart, a rotatable wall-sensor probe with two wall-mounted hot-film sensors spaced 6.93 mm apart and a traversable wall-sensor probe with two variable-spacing wall-mounted hot-film sensors. The hot-wire rake was used to examine the structure of the flow in both the Y (normal to the wall) and Z (spanwise) directions. The parallel and rotatable wall-sensor probes were used to look at the angular characteristics of the coherent structures in the flow and at the wall, respectively, and the spanwise structure of the flow at the wall was examined through the traversable wall-sensor probe. The results of the measurements show that the spectral characteristics of the flow are affected by three-dimensional effects. The direction of motion of the coherent structures lags behind the local mean-velocity vectors in the X-Z plane (parallel to the wall) with very little variation with frequency (structure size). Unlike two-dimensional boundary layers, the spectral variation of the convective wave speed does not collapse when normalized with the local mean velocity and friction velocity in the outer and inner regions, respectively. In the outer region of the boundary layer, the distribution of the intermittency with Y appears to agree quite closely with previously reported results for two-dimensional boundary layers. The mean ejection frequency in the near-wall flow and the frequency at the peak of the first moment of the wall shear-stress power spectrum show fairly close agreement, consistent with previously reported results for a two dimensional boundary layer. The measurements with the traversable wall-sensor probe indicate the presence of an organized structure, probably low-speed streaks in the near-wall region, with a preferred spanwise spacing. This spanwise spacing was found to be Î Î * = 85 and 135 at two different measurement stations. somewhat different from the well accepted value of Î Î * = 100 for two-dimensional boundary layers. Time-delayed correlations of the velocity signal over a range of Y locations reveal an inclined linear wavefront similar to previously reported results for a two-dimensional boundary layer. / Ph. D.

The effect of humps on the stability of subsonic boundary layers over an airfoil is investigated. The mean flow is calculated by using an interacting boundary-layer solver which accounts for strong viscous/inviscid interaction and separation bubbles. The code is capable of solving compressible as well as incompressible flows. Then, the two-dimensional mean flow is fed into a stability program which is capable of doing two-and three-dimensional analysis. The output of this stability program is the growth rates which are integrated along a prescribed path to yield the amplification factor (i.e., N-factor), which is used to predict transition from laminar to turbulent flow. The analysis is performed for different heights and locations of the hump and for different Mach numbers. The results show that compressibility stabilizes the flow and that the most dangerous frequency decreases as the Mach number increases for a fixed location of the hump. Also this most dangerous frequency decreases as the hump is moved downstream. Moreover, the amplification factor increases as the hump height increases and as the hump is moved downstream. The influence of suction and heat-transfer strips on controlling the destabilizing influence of the hump is investigated. The results show that cooling and suction strips stabilize the flow and therefore delay transition from laminar to turbulent flow. Moreover, a heating strip destabilizes the flow in the presence of a hump. Applying suction through multiple strips can be as effective as continuous suction. Also the total flow rate required using multiple strips is less than that required using a single strip. We optimize the locations of these strips for a certain hump location. Moreover, cooling through multiple strips is as effective as cooling through a single strip. We optimize the locations and levels of these cooling strips for a certain hump location. / Master of Science

The behavior of a junction vortex formed around an obstacle in a boundary layer flow was studied experimentally in a water tunnel for two low speed cases. A wing consisting of a 3 : 2 elliptical nose and an NACA 0020 tail was used to simulate the junction vortex. A visual study using a hydrogen bubble technique was extensively conducted to investigate the flow structures in the stagnation region of the wing. It was observed that a multiple vortex system exists in this region and shows an acyclic flow pattern. LDV measurements were performed in the plane of symmetry upstream of the wing. The general behavior of the flow agrees with an earlier wind tunnel test of Devenport and Simpson which was conducted at higher speed. A low frequency, bistable flow structure was observed as in the wind tunnel measurements. The switching between two flow modes (a backfiow mode and a zero flow mode) was analyzed using LDV signals in the zone of a bimodal structure. A dimensionless frequency group (StT) was found to represent the average frequency of successive switches from a given mode to the other. The visual evidence of acyclic flow pattern was consistent with the LDV measurements, and revealed that aperiodic stretching of the junction vortex appears responsible for the bimodal (double-peaked) structure in the velocity histograms. An attempt to measure the three-dimensional instantaneous velocity field in this region was made. A unique PIDV (particle image displacement velocimetry) technique was developed using a multiple wire hydrogen bubble method and a high speed video system. A stereo vision approach was implemented to capture two orthogonal views simultaneously for the three-dimensional motion analysis. / Ph. D.

It was found in this investigation that the testing of supersonic airfoils at subsonic speeds to obtain pressure distributions will result in considerable error if the models are tested at very low Reynolds Numbers. The thickness of the boundary layer is critical at low Reynolds Numbers and causes a decrease in the overpressure region. This, coupled with the fact that the overpressure region increases more with Mach Number than predicted by the theory, could lead to erroneous calculations of the drag. / Master of Science

In this thesis, the benefits of boundary layer control (BLC) in improving aerodynamic performance and engine stability were examined in a compact, serpentine inlet exhibiting flow separation. A 1/14-scale turbofan engine simulator provided the flow through the inlet. The inlet's mass flow was measured to be 759 scfm (0.939 lbm/s) with an average throat Mach number of 0.23 when the simulator speed was 40 krpm. Boundary layer suction, blowing, and their combination were used to minimize the inlet's flow separation. The effectiveness of the suction alone and the blowing alone was shown to be approximately equivalent, and the effectiveness of the combined use of both was seen to be better than either one by itself. With blowing and suction flowrates around 1% of the simulator's core flow, the inlet's distortion was lowered by 40.5% (from 1.55% to 0.922%) while the pressure recovery was raised by 9.7% (from 87.2% to 95.6%). With its reduction in distortion, BLC was shown to allow the simulator to steadily operate in a range that would have otherwise been unstable. Minimizing the flow separation within the inlet was shown to directly relate to measurements from flush-mounted microphones along the inlet wall: as the exit distortion decreased the microphone spectrum also decreased in magnitude. The strong relationship between the aerodynamic profiles and the microphone signal suggests that microphones may be used in an active flow control scheme, where the BLC effort can be tailored for different engine operating conditions. Unfortunately, the sensing scheme used in this experiment showed the microphone signal to continue to decrease even when the separation is overly compensated; therefore refinements must be made before it would be practical in a real application. / Master of Science

Super-tall buildings located in high velocity wind regions are highly vulnerable to large lateral loads. Designing for these structures must be done with great engineering judgment by structural professionals. Present methods of evaluating these loads are typically by the use of American Society of Civil Engineers 7-10 standard, field measurements or scaled wind tunnel models. With the rise of high performance computing nodes, an emerging method based on the numerical approach of Computational Fluid Dynamics has created an additional layer of analysis and loading prediction alternative to conventional methods. The present document uses turbulence modeling and numerical algorithms by means of Reynolds-averaged Navier-Stokes and Large Eddy Simulation equations applied to a square prismatic prototype structure in which its dynamic properties have also been investigated. With proper modeling of the atmospheric boundary layer flow, these numerical techniques reveal important aerodynamic properties and enhance flow visualization to structural engineers in a virtual environment. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2015. / FAU Electronic Theses and Dissertations Collection

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