Fluid Dynamics Android Application: An Efficient Semi-Implicit Solver for Compressible Flows

The computing power of smartphones has not received considerable attention in the mainstream education system. Most of the education-oriented smartphone applications (apps) are limited to general purpose services like Massive Open Online Courses (MOOCs), language learning, and calculators (performing basic mathematical calculations). Greater potential of smartphones lies in educators and researchers developing their customized apps for learners in highly specific domains. In line with this, we present Fluid Dynamics, a highly accurate Android application for measuring flow properties in compressible flows. This app can determine properties across the stationary normal and oblique shock, moving normal shock and across Prandtl $-$ Meyer expansion fan. This app can also measure isentropic flows, Fanno flows, and Rayleigh flows. The functionality of this app is also extended to calculate properties in the atmosphere by assuming the International Standard Atmosphere (ISA) relations and also flows across the Pitot tube. Such measurements are complicated and time-consuming since the relations are implicit and hence require the use of numerical methods, which give rise to repetitive calculations. The app is an efficient semi-implicit solver for gas dynamics formulations and uses underlying numerical methods for the computations in a graphical user interface (GUI), thereby easing and quickening the learning of concerned users. The app is designed for the Android operating system, the most ubiquitous and capable surveillance platform, and its calculations are based on JAVA based code methodology. In order to check its accuracy, the app's results are validated against the existing data given in the literature.

Confinement effects on regular–irregular transition in shock-wave–boundary-layer interactions

An oblique shock wave is generated in a Mach 2 flow at a flow deflection angle of$12^{\circ }$. The resulting shock-wave–boundary-layer interaction (SWBLI) at the tunnel wall is observed. A novel traversable shock generator allows the position of the SWBLI to be varied relative to a downstream expansion fan. The relationship between the SWBLI, the expansion fan and the wind tunnel arrangement is studied. Schlieren photography, surface oil flow visualisation, particle image velocimetry and high-spatial-resolution wall pressure measurements are used to investigate the flow. It is observed that stream-normal movement of the shock generator downwards (towards the floor and hence the point of shock reflection) is accompanied by (1) growth in the streamwise extent of the shock-induced boundary layer separation, (2) upstream movement of the shock-induced separation point while the reattachment point remains nearly fixed, (3) an increase in separation shock strength and (4) transition between regular and irregular (Mach) reflection without an increase in incident shock strength. The role of free interaction theory in defining the separation shock angle is considered and shown to be consistent with the present measurements over a short streamwise extent. An SWBLI representation is proposed and reasoned which explains the apparent increase in separation shock strength that occurs without an increase in incident shock strength.

Oblique shockwaves and expansion fans

External supersonic gas flow in which changes in the fluid and flow properties are brought about by direction change is analysed in this chapter. In addition, it is shown that flow over a corner between two flat surfaces resulted in an oblique shockwave if the angle between the two surfaces is less than 180° (a concave corner). The analysis of flow through an oblique shockwave is based upon the superposition of the flowfield for a normal shock onto a uniform flow parallel to the shock. It is also shown that both weak and strong oblique shocks can occur. For an angle in excess of 180° (a convex corner), the flow is turned through an isentropic Prandtl-Meyer expansion fan. Analysis of a Prandtl-Meyer expansion fan starts from consideration of an infinitesimal flow deflection through a Mach wave.

Synthesis of Observations from the Precision Atmospheric Marine Boundary Layer Experiment (PreAMBLE)

Research flights during the Precision Atmospheric Marine Boundary Layer Experiment (PreAMBLE) in Southern California during May–June 2012 focused on three main features found in the nearshore marine boundary layer (MBL): the coastal jet (10 flights), the Catalina eddy (3 flights), and the initiation of a southerly surge (1 flight). Several topics were examined with case studies, but results from individual events may not represent typical conditions. Although these flights do not constitute a long-term set of data, observations from PreAMBLE are used to find common features. Two main topics are addressed: the MBL collapse into the expansion fan, and the subsequent transition into the Santa Barbara Channel (SBC). The midmorning to late afternoon flights occur during moderate to strong northerly wind. Slope of the MBL in the expansion fan varies and wave perturbations can be embedded within the expansion fan. As the cool MBL flow turns into the SBC, it moves underneath a deeper and warmer MBL that originates from the southeast over the warmer ocean. The temperature inversion between the cool and warm MBL erodes toward the east until there is only the inversion between the warm MBL and free troposphere. The dissipation of the lower layer into the SBC observed by the aircraft differs from previous conceptual models that depict a continuous MBL that thins and then deepens again in the SBC, which was inferred from sparse observations and numerical simulations. Only one flight within the SBC detected a hydraulic jump from 100 to 200 m above the surface.

Size and shape of shock waves and slipline for Mach reflection in steady flow

For Mach reflection in steady supersonic flow, the slipline and reflected shock wave from the triple point are disturbed by secondary Mach waves generated over the slipline and by the expansion fan from the rear wedge corner. Analytical expressions for the shape of the curved slipline and reflected shock wave are derived in this paper. It is found that, due to transmitted expansion waves from the expansion fan, the slipline has a slope discontinuity at the turning point, i.e., the intersection point of the slipline and the leading characteristics of the transmitted expansion wave. The hypothetical shock wave calculated by considering this slope discontinuity as flow deflection angle matches a similar wave observed in numerical results by computational fluid dynamics, suggesting the existence of a weak shock wave from this turning point. The effects of the secondary Mach waves upstream of the turning point and of the turning point weak shock wave mutually cancel out approximately so that the transmitted Mach waves can be approximated as straight characteristic lines. This simplification leads to a fast analytical model which can predict the Mach stem height and shape of the slipline and reflected shock wave with increasing accuracy for the decreasing deflection angle of the slipline at the triple point. The slipline slope discontinuity at the turning point and the hypothetical turning point weak shock wave are new phenomena found in this work.