The Effect of Cross Flow and Isolated Roughness Elements on the Boundary Layer and Wall Pressure Fluctuations on Circular Cylinders
Abstract : In this paper pressure fluctuation measurements on smooth cylinders aligned with the flow have been extended to include the effect of slight cross flow and isolated roughness elements. A yaw angle of a few degrees is sufficient to produce severe asymmetry of the outer portion of the boundary layer as deduced from impact pressure measurements using computer controlled traversing gear. The root-mean-square surface pressure fluctuations are a maximum on the windward surface but only slightly larger than with no yaw. On the leeward surface they are much lower. The power spectrum of the pressure has greater energy density at high frequencies on the windward surface and at low frequencies on the leeward side. With no cross flow the root-mean-square surface pressure is greatly increased by a cylindrical collar placed around the cylinder. A collar which increases the diameter by 50% more than doubles the wall pressure three diameters downstream. The spectra of the pressure scale with collar thickness and the intensity of the surface pressure fluctuations scales with the ratio of collar thickness to distance downstream. (Author)
The Turbulent Boundary Layer on a Small Diameter Flexible Cylinder in the Wake of a Streamlined Towing Apparatus
Title of Dissertation: THE TURBULENT BOUNDARY LAYER ON A SMALL DIAMETER FLEXIBLE CYLINDER IN THE WAKE OF A STREAMLINED TOWING APPARATUS Deborah Ann Furey, Doctor of Philosophy, 2005 Dissertation directed by: Professor James Duncan Department of Mechanical Engineering Long thin flexible cylinders instrumented with acoustic sensors are used by the Navy for surveillance and detection. Acoustic transducers are mounted in a long linear array surrounded by an insulated jacketing material resulting in a long flexible array of sensors with a smooth outer surface. These cylinders, called towed arrays, are towed behind ships and submarines. As a result of the towing speed, a turbulent boundary layer develops along the outer boundary of the flexible cylinder which generates noise in the near field. Flow noise is critical to the measured acoustic signatures and therefore, to system performance. By understanding the nature of turbulent boundary layers on long, thin flexible cylinders, the performance of towed acoustic sensors used for surveillance and tactical operations can be improved. Towed array detection performance is a function of several factors including ambient acoustics, array gain, flow noise, and signal processing. To advance existing systems, the primary goal is to increase the system gain by introducing longer apertures, i.e. longer arrays. Longer arrays of conventional design lead to additional stowage requirements that are difficult to implement. The development of very small diameter towed array technology is required. Therefore, the results of this research will directly contribute to the work being done on small diameter arrays and multi-line array systems. This is an experimental study to evaluate the development of the boundary layer thickness, δ, and momentum thickness, θ, along long thin flexible cylinders (L/a=1.5*10 and 3.0*10 where δ/a>>1, a=radius). The experiments use conventional test methods in conjunction with Stereo Particle Image Velocimetry (SPIV) measurement techniques to evaluate the flow in the boundary region of a small diameter flexible cylinder towed in the high speed towing basin at the Naval Surface Warfare Center Carderock Division (NSWCCD). The flexible cylinders are approximately neutrally buoyant and have an initial length of 152 m and radii of 0.45 mm and 1.25 mm. The first objective for this experiment is to evaluate the streamwise development of wall shear stress (τw) and momentum thickness (θ) in axisymmetric turbulent boundary layers using drag measurements at 3.1, 5.2, 9.3 and 14.4 m/s for comparison to existing data. The second and primary objective for this experiment is to determine the streamwise development of the axisymmetric boundary layer flow and to evaluate relevant boundary layer parameters at 3.8, 7.7, 12.9 and 15.4 m/sec using SPIV images acquired over the entire length of the cylinders. Drag measurements reveal that the wall shear stress is large and that the momentum thickness grows slowly when compared to flat plate boundary layers. The velocity field data shows that the boundary flow remains turbulent over the entire length of the flexible cylinder and that the turbulent profile is different from that of flat plate boundary layers. THE TURBULENT BOUNDARY LAYER ON A SMALL DIAMETER FLEXIBLE CYLINDER IN THE WAKE OF A STREAMLINED TOWING APPARATUS By Deborah Ann Furey Dissertation submitted to the Faculty of the Graduate School of the University of Maryland, College Park in partial fulfillment of the requirements for the degree of Doctor of Philosophy 2005
Texture Feature Extraction of RGB, HSV, YIQ and Dithered Images using GLCM, Wavelet Decomposition Techniques
When changing the format of an image from simple RGB to HSV, YIQ and Dithered image, the characteristics of image also change. In this paper, the similar images in the above formats are retrieved using statistical and structural retrieving techniques i.e. GLCM (Gray Level Co-occurrence Matrix) and Wavelet Decomposition techniques. The best results are coming for dithered, HSV images by using GLCM technique for feature extraction and by using Wavelet decomposition; HSV images are giving the best results. While other formats also give the correct retrieval but from the accuracy point of view, HSV images are retrieved with better accuracy using both techniques.