Orifice plates in a shock tube

This note points out some interesting aspects of a recent study of the flow resulting from the use of area change near the diaphragm of a shock tube. (1) Earlier studies (2,3) have been directed toward the use of area change for increasing the available shock speed, and thus have considered geometries where A4/A1 > 1 and A*/A1 = 1 [see notation on Fig. 1(a)]. In the current work, shock tubes with arbitrary values of these parameters are considered over the complete range of Ms (the shock Mach number)

Aerodynamics of vortex generators

An experimental and theoretical study was undertaken of the separation delay and dramatic boundary-layer thinning that can occur in vortex-generator installations. Wind tunnel measurements of the dynamic-pressure profile downstream of a vortex generator were found to compare under certain conditions with that downstream of a suction slit, while water-tunnel visualization studies of vortex-generator height and geometry suggested optimum configurations, and only a minor effect of base porosity. A series of progressively more complex inviscid flow models was developed to be applied to a 3-D integral boundary-layer code. This code predicted layer thinning downstream of the suction site of the vortex models, and other observed features. Thin-layer Navier-Stokes equations are now being used with the ultimate goal of clarifying the physical processes involved in vortex generator performance and developing calculational procedures capable of predicting it

Non-invasive quantification of lower limb mechanical alignment in flexion

Objective: Non-invasive navigation techniques have recently been developed to determine mechanical femorotibial alignment (MFTA) in extension. The primary aim of this study was to evaluate the precision and accuracy of an image-free navigation system with new software designed to provide multiple kinematic measurements of the knee. The secondary aim was to test two types of strap material used to attach optical trackers to the lower limb. Methods: Seventy-two registrations were carried out on 6 intact embalmed cadaveric specimens (mean age: 77.8 ± 12 years). A validated fabric strap, bone screws and novel rubber strap were used to secure the passive tracker baseplate for four full experiments with each knee. The MFTA angle was measured under the conditions of no applied stress, valgus stress, and varus stress. These measurements were carried out at full extension and at 30°, 40°, 50° and 60° of flexion. Intraclass correlation coefficients, repeatability coefficients, and limits of agreement (LOA) were used to convey precision and agreement in measuring MFTA with respect to each of the independent variables, i.e., degree of flexion, applied coronal stress, and method of tracker fixation. Based on the current literature, a repeatability coefficient and LOA of ≤3° were deemed acceptable. Results: The mean fixed flexion for the 6 specimens was 12.8° (range: 6–20°). The mean repeatability coefficient measuring MFTA in extension with screws or fabric strapping of the baseplate was ≤2°, compared to 2.3° using rubber strapping. When flexing the knee, MFTA measurements taken using screws or fabric straps remained precise (repeatability coefficient ≤3°) throughout the tested range of flexion (12.8–60°); however, using rubber straps, the repeatability coefficient was >3° beyond 50° flexion. In general, applying a varus/valgus stress while measuring MFTA decreased precision beyond 40° flexion. Using fabric strapping, excellent repeatability (coefficient ≤2°) was observed until 40° flexion; however, beyond 50° flexion, the repeatability coefficient was >3°. As was the case with precision, agreement between the invasive and non-invasive systems was satisfactory in extension and worsened with flexion. Mean limits of agreement between the invasive and non-invasive system using fabric strapping to assess MFTA were 3° (range: 2.3–3.8°) with no stress applied and 3.9° (range: 2.8–5.2°) with varus and valgus stress. Using rubber strapping, the corresponding values were 4.4° (range: 2.8–8.5°) with no stress applied, 5.5° (range: 3.3–9.0°) with varus stress, and 5.6° (range: 3.3–11.9°) with valgus stress. Discussion: Acceptable precision and accuracy may be possible when measuring knee kinematics in early flexion using a non-invasive system; however, we do not believe passive trackers should be mounted with rubber strapping such as was used in this study. Flexing the knee appears to decrease the precision and accuracy of the system. The functions of this new software using image-free navigation technology have many potential clinical applications, including assessment of bony and soft tissue deformity, pre-operative planning, and post-operative evaluation, as well as in further pure research comparing kinematics of the normal and pathological knee

The average magnetic field draping and consistent plasma properties of the Venus magnetotail

A new technique has been developed to determine the average structure of the Venus magnetotail (in the range from −8 Rv to −12 Rv) from the Pioneer Venus magnetometer observations. The spacecraft position with respect to the cross-tail current sheet is determined from an observed relationship between the field-draping angle and the magnitude of the field referenced to its value in the nearby magnetosheath. This allows us statistically to remove the effects of tail flapping and variability of draping for the first time and thus to map the average field configuration in the Venus tail. From this average configuration we calculate the cross-tail current density distribution and J × B forces. Continuity of the tangential electric field is utilized to determine the average variations of the X-directed velocity which is shown to vary from −250 km/s at −8 Rv to −470 km/s at −12 Rv. From the calculated J × B forces, plasma velocity, and MHD momentum equation the approximate plasma acceleration, density, and temperature in the Venus tail are determined. The derived ion density is approximately ∼0.07 p+/cm³ (0.005 O+/cm³) in the lobes and ∼0.9 p+/cm³ (0.06 O+/cm³) in the current sheet, while the derived approximate average plasma temperature for the tail is ∼6×106 K for a hydrogen plasma or ∼9×107 K for an oxygen plasma

Transformation Optics with Photonic Band Gap Media

We introduce a class of optical media based on adiabatically modulated, dielectric-only, and potentially extremely low-loss, photonic crystals. The media we describe represent a generalization of the eikonal limit of transformation optics (TO). The foundation of the concept is the possibility to fit frequency isosurfaces in the k-space of photonic crystals with elliptic surfaces, allowing them to mimic the dispersion relation of light in anisotropic effective media. Photonic crystal cloaks and other TO devices operating at visible wavelengths can be constructed from optically transparent substances like glasses, whose attenuation coefficient can be as small as 10 dB/km, suggesting the TO design methodology can be applied to the development of optical devices not limited by the losses inherent to metal-based, passive metamaterials.Comment: 4 pages, 4 figure