Luminous Late-time Radio Emission from Supernovae Detected by the Karl G. Jansky Very Large Array Sky Survey (VLASS)

We present a population of 19 radio-luminous supernovae (SNe) with emission reaching L ν ∼ 1026–1029 erg s−1 Hz−1 in the first epoch of the Very Large Array Sky Survey (VLASS) at 2–4 GHz. Our sample includes one long gamma-ray burst, SN 2017iuk/GRB 171205A, and 18 core-collapse SNe detected at ≈1–60 yr after explosion. No thermonuclear explosion shows evidence for bright radio emission, and hydrogen-poor progenitors dominate the subsample of core-collapse events with spectroscopic classification at the time of explosion (79%). We interpret these findings in the context of the expected radio emission from the forward shock interaction with the circumstellar medium (CSM). We conclude that these observations require a departure from the single wind–like density profile (i.e., ρ CSM ∝ r −2) that is expected around massive stars and/or from a spherical Newtonian shock. Viable alternatives include the shock interaction with a detached, dense shell of CSM formed by a large effective progenitor mass-loss rate, M ̇ ∼ 10 − 4 – 10 − 1 M ⊙ yr−1 (for an assumed wind velocity of 1000 km s−1); emission from an off-axis relativistic jet entering our line of sight; or the emergence of emission from a newly born pulsar-wind nebula. The relativistic SN 2012ap that is detected 5.7 and 8.5 yr after explosion with L ν ∼ 1028 erg s−1 Hz−1 might constitute the first detections of an off-axis jet+cocoon system in a massive star. However, none of the VLASS SNe with archival data points are consistent with our model off-axis jet light curves. Future multiwavelength observations will distinguish among these scenarios. Our VLASS source catalogs, which were used to perform the VLASS cross-matching, are publicly available at https://doi.org/10.5281/zenodo.4895112.

Shock interaction induced heat flux augmentation in hypersonic flows

This paper gives a summary of dedicated experiments on the shock interaction induced heat flux augmentation, by means of tests carried out in the hypersonic wind tunnel H2K. The first test case is devoted to the shock boundary layer interaction on a flat plate. The interaction impact has been varied by changing the free stream parameters and the position of the shock generator, i.e. shock impingement point on the plate. The heat flux distribution has been determined using surface temperature data measured by an infrared camera. The heat flux data combined with free stream flow parameters allow calculation of the Stanton number evolution. The second test case is a double sphere configuration with a variable axial and lateral distance between the spheres. This allowed measurements of the heat flux augmentation induced by a shock-shock interaction along the complete frontal surface of the second sphere, which was hit by the bow shock of the first sphere. Shock-shock and shock-boundary layer interaction effects are studied by means of experiments on the IXV flight configuration with double control flaps. Depending on the test configuration and flow parameters, shock interaction induced heat flux augmentation factors up to seven have been measured. Graphical abstract

Control algorithm for an elastic-viscoplastic model to study processes of shock interaction of bodies

Introduction. In engineering practice, dynamic processes, with the help of which mechanics of interaction of machine components and structural elements are described and studied, are of great importance. These dynamic processes are the cause of large deformations leading to the destruction. The research objective is to develop a more accurate shock simulation algorithm through the controlled transformation of the mechanorheological shock process model from elasticviscous to elastic-viscoplastic.Materials and Methods. Differential equations of the model movement are proposed. The conditions for the transformation of the model during the transition from elastic to plastic deformations, from the stage of loading the model to the stage of unloading under the shock interaction with the surface, are considered. When calculating deformations, the assumption is made that elastic and plastic deformations occur simultaneously from the very onset of the impact. The model functioning method is considered in detail, the algorithm of the model operation is developed, the logic of its functioning is described in detail.Results. To study shock processes, a mechanoreological elastic-viscoplastic model was developed. An important parameter of the model is the force corresponding to the onset of plastic deformation. As a result of the research, a more perfect algorithm was created, and a new computer program was developed to study the shock process using an elasticviscoplastic model with an adjustable elastic-plastic transformation. Discussion and Conclusions. The results obtained can be used to improve the accuracy and reliability of simulation of shock processes in order to further develop the techniques for determining the physical and mechanical characteristics of materials by shock methods. Knowledge of the mechanical characteristics of materials is required when solving various research problems through mathematical modeling of vibration and shock processes. At the same time, an important task is to adapt the design model to the real shock process, for which it is required to develop appropriate methods and techniques.

Numerical Investigation On the Influences of Boundary Layer Ingestion On Tip Leakage Flow Structures and Losses in a Transonic Axial-Flow Fan

In a boundary layer ingesting (BLI) propulsion system, the fan is continuously exposed to inflow distortions. The distorted inflows lead to non-uniform loss distributions along the radial and circumferential directions. Since the rotor tip suffers from higher intensive distortion, the local loss increment is a major contributor to the BLI fan performance penalty. To explore the effects of distorted inflows on tip leakage flow evolutions and associated mechanisms for increased loss in a BLI fan, three-dimensional full-annulus unsteady simulations are conducted. Results show that about 54% of total additional losses due to distortion are formed in tip region and more than 80% of tip entropy generation is related to the tip leakage flow. The intensities of leakage vortex-shock interactions vary at different annulus locations. When the rotor moves into distorted region, the vortex-shock interaction is weaker than the undistorted locations due to attenuated leakage flow. At the locations where the rotor is moving out from distorted region, the vortex-shock interaction is notably enhanced because the front part of blade tip airfoil suffers a higher load, resulting in a rapid vortex core expansion and eventually vortex breakdown. The increase of flow blockage in the front section of blade tip passages at local circumferential positions leads to a corresponding rise of flow loss. The findings in this study highlight the impacts of tip leakage flow on aerodynamic loss of fan working under BLI inflow distortion and provide improved understandings of loss mechanisms in a BLI fan.