Oblique Wave Angle Research Articles

Numerical simulations of wedge-induced oblique detonation waves in ammonia/hydrogen/air mixtures

In this study, two-dimensional numerical simulations of oblique detonation waves (ODWs) under a flight altitude of 10 km were performed. Because of their high importance in a zero-carbon economy, ammonia and hydrogen are considered as the fuel for aircrafts. Six cases with various hydrogen addition ratios in the fuel mixture were considered to explore the effects of hydrogen addition on the ignition structure, surface instability and NO emission of ODWs. It was found that as the hydrogen addition ratio decreases, the ODWs become more difficult to initiate with increased ignition length, and more complicated multi-wave system emerges with a second oblique detonation wave (SODW). The oblique wave angle is rarely influenced by hydrogen addition ratio. Two types of reaction surfaces, i.e. “saw-tooth” and “key-stone”, along with a series of transverse waves and vortices can be observed for the case with a hydrogen addition ratio of 20% by volume in the fuel mixture. The oscillation amplitude of the reaction zone length, which quantifies the surface instability, increases with decreasing hydrogen addition ratios. The mass fraction of NO is the highest in the case with a hydrogen addition ratio of 40%. NO concentration is higher in detonation compared to that in deflagration in all cases. The pathway analysis of NO indicates that the thermal NO is the primary contributor to NO production in the cases with high hydrogen addition ratios, while the HNO pathway becomes more significant in the cases with low hydrogen addition ratios.

The role of cuspate spits on wave attenuation and energy redistribution in a coastal lagoon, Lagoa dos Patos, Brazil

This research uses a process-based model (Delft3D), validated with measured wave data, to investigate the controls that a cuspate shoreline and its associated submerged morphology have on wave generation, propagation, and attenuation within a large elongated lagoon (Lagoa dos Patos, Brazil). A method based on the Energy-Flux-Method was applied to the historical wind dataset to define representative wind cases to be used as forcing conditions in the model. The results show that, during extreme wind conditions, the spits dissipate wave energy in the lagoon. This (wave attenuation and the wave sheltering effect) controls the stability of the adjacent coastline. The wave attenuation varies between 18% and 46% along the submerged spit depending on the crest width and the amplitude of the incident waves. Waves are mainly attenuated in the proximal and central portions of the spits where the spits are wider, resulting in a reduced transmitted energy to the adjacent coastline, while larger waves are also attenuated on the distal end of the spits. The degree of attenuation depends on the direction of wave generation, the respective fetch, the spit width, and the water depth. A strong relationship of mutual co-adjustment between the morphology and the wave field results in a very low occurrence of oblique wave angles of incidence, especially for waves propagating across the long lagoon axis. Furthermore, the wave attenuation over the spits is also responsible for its progressive erosion, which, from decades to centuries, may lead to an increase of changes on the lagoon shorelines.

Whistler mode waves and Hall fields detected by MMS during a dayside magnetopause crossing

AbstractWe present Magnetospheric Multiscale (MMS) mission measurements during a full magnetopause crossing associated with an enhanced southward ion flow. A quasi‐steady magnetospheric whistler mode wave emission propagating toward the reconnection region with quasi‐parallel and oblique wave angles is detected just before the opening of the magnetic field lines and the detection of escaping energetic electrons. Its source is likely the perpendicular temperature anisotropy of magnetospheric energetic electrons. In this region, perpendicular and parallel currents as well as the Hall electric field are calculated and found to be consistent with the decoupling of ions from the magnetic field and the crossing of a magnetospheric separatrix region. On the magnetosheath side, Hall electric fields are found smaller as the density is larger but still consistent with the decoupling of ions. Intense quasi‐parallel whistler wave emissions are detected propagating both toward and away from the reconnection region in association with a perpendicular anisotropy of the high‐energy part of the magnetosheath electron population and a strong perpendicular current, which suggests that in addition to the electron diffusion region, magnetosheath separatrices could be a source region for whistler waves.

Hydroelastic analysis of oblique irregular waves with a pontoon-type VLFS edged with dual inclined perforated plates

The fluid-structure interaction of oblique irregular waves with a pontoon-type very large floating structure (VLFS) edged with dual horizontal/inclined perforated plates has been investigated in the context of the direct time domain modal expansion theory. For the hydroelastic analysis, the boundary element method (BEM) based on time domain Kelvin sources is implemented to establish water wave model including the viscous effect of the perforated plates through the Darcy’s law, and the finite element method (FEM) is adopted for solving the deflections of the VLFS modeled as an equivalent Mindlin thick plate. In order to enhance the computing efficiency, the interpolation-tabulation scheme is applied to assess rapidly and accurately the free-surface Green function and its partial derivatives in finite water depth, and the boundary integral equation of a half or quarter VLFS model is further established taking advantage of symmetry of flow field and structure. Also, the numerical solutions are validated against a series of experimental tests. In the comparison, the empirical relationship between the actual porosity and porous parameter is successfully applied. Numerical solutions and model tests are executed to determine the hydroelastic response characteristics of VLFS with an attached anti-motion device. This study examines the effects of porosity, submerged depth, inclined angle and gap distance of such dual perforated anti-motion plates on the hydroelastic response to provide information regarding the optimal design. The effects of oblique wave angle on the performance of anti-motion and hydroelastic behavior of VLFS are also emphatically examined.

Spatial direct numerical simulation of transitional boundary layer over compliant surfaces

An iterative implicit fractional step method is developed and employed for the simulation of transitional boundary layer over compliant surfaces. The three-dimensional perturbation Navier–Stokes equations are discretized by curvilinear finite volumes on a collocated grid system. A multigrid procedure is used for computations associated with the pressure-Poisson equation, while simulation is carried out by MPI-based parallel computation with domain decomposition. Results in the literature for oblique linear waves and the non-linear breakdown of a wave triad over a rigid wall are repeated to check the accuracy of the codes that had been developed. Oblique linear TS (Tollmien–Schlichting) waves over finite-length compliant membranes are generally found to coexist with CIFI (compliance induced flow instability) or FISI (flow induced surface instabilities) waves. The latter waves usually possess longer wavelengths and thus propagate at a larger oblique wave angles than the TS waves. Simulation reveals that compliant surfaces may slow down the development of secondary instabilities during the early stages of laminar–turbulent transition. However, during the later stages of fundamental wave breakdown, interactions with CIFI and edge-generated waves may increase the amplitudes of the original 2D and 3D TS waves, leading to an earlier breakdown on compliant surfaces. Linear interaction between the flow and compliant membranes has been assumed.

The interaction of periodically generated cylindrical detonations in a simulated hypersonic flow

A new experimental method of establishing oblique detonation waves in stationary detonable mixtures was demonstrated. This method of simulating oblique detonations in hypersonic flow involved the periodic initiation of multiple cylindrical detonation fronts, which upon interaction quickly transitioned to an oblique detonation wave. Its axisymmetric analog was the point ignition of multiple spherical detonation waves forming a conical oblique detonation. The two-dimensional system was chosen here to focus on the interaction of the waves and the motion of the triple points, as well as to determine how rapidly the interacting fronts form a single oblique wave. A stoichiometric mixture of acetylene and oxygen was utilized at pressures between 30 and 50 kPa in these experiments. Ignition was accomplished by branching an established normal detonation wave into five waves propagating in small tubes and time delaying then with respect to one another to initiate cylindrical detonations in the test section. The simulated hypersonic flow speeds were 1.5, 2, and 3 times the Chapman-Jouguet (C-J) velocity. The corresponding initiation frequencies were 1.2×10 5 , 1.6×10 5 , and 2.4×10 5 Hz. The measured oblique wave angles at these three simulated speeds were in good agreement with the expected values based on the wave front kinematics. Interrogation of the wave front velocities indicated that the undisturbed regions propagate at near C-J velocities, whereas the triple point regions propagate at velocities up to 4% to 13% greater than the undisturbed regions, helping to smooth the combined wave front. These results suggest that the periodic strong ignition of a detonable mixture in a hypersonic flow can be a viable method of sustaining detonative combustion without the need of a “flame holder.”

Resonant growth of three-dimensional modes in Falkner–Skan boundary layers with adverse pressure gradients

This work documents the spatial development of a triad of instability waves consisting of a plane TS mode and a pair of oblique modes with equal-opposite wave angles which are undergoing subharmonic transition in Falkner-Skan boundary layers with adverse pressure gradients. The motivation for this study is that for wings with zero or moderate sweep angles, transition is most likely to occur in the adverse pressure gradient region past the maximum thickness point and, starting with low initial amplitudes, subharmonic mode transition is expected to be the predominant mechanism for the first growth of of three-dimensional modes. The experiment follows that of Corke & Mangano (1989) in which the disturbances to produce the triad of waves are introduced by a spanwise array of heating wires located near Branch I. The initial conditions are carefully controlled. These include the initial amplitudes, frequencies, relative phase and oblique wave angles. The basic flow consisted of a Falkner-Skan (Hartree) boundary layer with a dimensionless pressure gradient parameter in the range -0.06 ≤ β H ≤ -0.09. The frequency of the TS wave was selected to be near the most amplified based on linear theory. The frequency of the oblique waves was the subharmonic of the TS frequency. The oblique wave angles were set to give the largest secondary growth ( 60°). Compared to similar conditions in a Blasius boundary layer, the adverse pressure gradient was observed to lead to an extra rapid growth of the two- and three-dimensional modes. In this there was a relatively larger maximum amplitude of the fundamental mode and considerably shortened amplitude saturation region compared to zero pressure gradient cases. Analysis of these results includes frequency spectra, the wall-normal distributions of each mode amplitude, and mean velocity profiles. Finally, the streamwise amplitude development is compared with the amplitude model from the nonlinear critical layer analysis of Goldstein & Lee (1992).

Acoustic forcing of oblique wave resonance in the far wake

In this paper, we investigate to what extent the far-wake ‘signature’ of the near-wake vortex dynamics of a nominally two-dimensional bluff body is affected by the character of the free-stream noise. We confirm the existence of an oblique wave resonance (at frequency, fK–fT), which is caused by nonlinear ‘quadratic’ interactions between primary oblique shedding waves (fK) and secondary two-dimensional waves (fT), which are amplified from free-stream disturbances. In this work, oblique wave resonance is induced by acoustic forcing of two-dimensional waves. The use of acoustic forcing reveals a set of higher-order oblique wave resonances corresponding to frequencies (fK–nfT), where n is an integer. We find from visualization that, even when the secondary two-dimensional waves have the same frequency as the oblique waves, it is the oblique waves that are preferentially amplified. Oblique wave angles up to 74° have been observed. The response of the wake to a large range of forcing frequencies shows a broad region of peak response, centred around F = (fT/fK) = 0.55, and is in reasonable agreement with predictions from linear stability analysis. A similar broad response is found for each of the higher-order oblique wave modes. Simple equations for the oblique waves yield approximate conditions for maximum wake response, with a frequency for peak response given by Fmax = 1/2n = 1/2, 1/4, 1/6,…, and an oblique wave angle given by θmax = 2θK, where θK is the angle of oblique vortex shedding. An increase in forcing amplitude has the effect of bringing the nonlinear wave interactions, leading to oblique wave resonance, further upstream. Paradoxically, the effect of an increase in amplitude (A) of the two-dimensional wave forcing is to further amplify the oblique waves in preference to the two-dimensional waves and, under some conditions, to inhibit the appearance of prominent two-dimensional waves where they would otherwise appear. With a variation in forcing amplitude, the amplitude of oblique wave response is found to be closely proportional to A½. In summary, this investigation confirms the surprising result that it is only through the existence of noise in the free stream that the far wake is ‘connected’ to the near wake.