Shock interaction phenomena

Aerodynamic

Analysis of improved digital filter inflow generation methods for compressible turbulent boundary layers

We propose several enhancements to improve the accuracy and performance of the digital filter turbulent inflow generation technique and assess their efficacy in the context of wall-resolved large-eddy simulations of a compressible turbulent boundary layer. Improvements of accuracy include a more realistic correlation function for the transversal directions, target length scales that vary with wall-distance, and a counter-intuitive approach that involves the suppression of streamwise velocity fluctuations at the inflow. For improving the computational performance, we propose to generate the inflow data in parallel in single precision and at a prescribed time interval based on the turbulence time scale, and not at every time-step of the simulation. Based on the results of 7 wall-resolved large-eddy simulations, we find that the new correlation functions and the considered performance improvements are beneficial and therefore desired. Suppressing streamwise velocity fluctuations at the inflow leads to the fastest relaxation of the pressure fluctuations; however, this approach increases the adaptation length defined in terms of compliance with the von Kármán integral equation. The adaptation length can be shortened by artificially increasing the wall-normal Reynolds stresses, thereby preserving the desired turbulence kinetic energy level. A detailed inspection of the Reynolds stress transport budgets reveals that the observed spurious spatial transients are largely driven by pressure-related terms. For instance, increased values of u′p′¯ are found throughout the computational domain when a physical Reynolds stress distribution is prescribed at the inflow. Therefore, efforts to enhance digital filter techniques should aim at modeling pressure fluctuations as well as their correlation with the velocity components.Aerodynamic

Dynamics of unsteady asymmetric shock interactions

The response of asymmetric and planar shock interactions to a continuous excitation of the lower incident shock is investigated numerically. Incident shock waves and centred expansion fans are generated by two wedges asymmetrically deflecting the inviscid free stream flow at Mach 3. The excitations mechanisms considered are (i)Â pitching of the lower wedge traversing the steady-state dual-solution domain (DSD) of regular interaction (RI) and Mach interaction (MI), (ii) a periodic (sinusoidal) oscillation of the lower wedge deflection with a mean value both within and outside of the steady-state DSD and (iii) a periodic (sinusoidal) streamwise oscillation of the lower wedge location with fixed wedge deflection. A detailed analysis of characteristic unsteady flow features, including the Mach stem growth, pressure evolution across the shock system and corresponding flow deflections and entropy rise, is presented with a focus on the bi-directional RIMI transition process. For fast pitching conditions, the MI pattern is maintained far inside the steady-state RI domain. The observed transition limit as the rotational velocity decreases does not fully match steady-state theory, however. This is attributed to geometry-related effects. In the opposite case, transition, good agreement with steady-state theoretical predictions is obtained for slow rotations, and a shock polar analysis applied in the (moving) frame of reference of the shock interaction location improves the agreement with fast pitching numerical data significantly. Furthermore, the MI pattern is found to be more robust against periodic perturbations than the corresponding RI configuration for mean flow conditions inside the steady-state DSD, which appears to be a consequence of the dynamics of the Mach stem during a period of excitation. This is not the case for mean flow conditions outside the steady-state DSD in the RI domain for which a periodic alternation occurs instead.Aerodynamic

Numerical investigation of the unsteady transition between asymmetric shock systems

The dynamic interaction of two planar and asymmetric shock waves at a free-stream Mach number M¥ = 3 is studied numerically in order to characterize the transition between the regular (RI) and Mach (MI) interaction patterns. Shock deflection disturbances are independently introduced in the form of a sinusoidal oscillation of the shock generator. Selected amplitudes of oscillations ensure that both boundaries of the theoretical dual solution domain (DSD) are crossed every period. The range of angular frequencies investigated resembles the dynamics of the separation shock in shock-wave/turbulent boundarylayer interactions. Computational results show that the MI unambiguously prevails regardless of the initial wave pattern disturbed, provided that the oscillation frequency is not too large. This holds for mean conditions embedded inside the DSD. For those outside, a RI_MI alternation is observed when the initial wave pattern is a RI, and no single event of a RI interaction occurs when the initial patter is a MI.Aerodynamic

High-Reynolds number effects in shock-wave/turbulent boundary-layer interactions

We investigate Reynolds number effects in shockwave/turbulent boundary-layer interactions (STBLI) with strong mean flow separation. Three wall-resolved large-eddy simulations (LES) are performed for this purpose, with different Reynolds number but otherwise equal flow parameters and simulation setup. The resulting LES data covers more than a decade of friction Reynolds number. The high-Reynolds case, with friction Reynolds number Reτ = 5118 and momentum Reynolds number Reθ = 26438 at the virtual impingement point without the shock, features a turbulent boundary layer with clear inner and outer scale separation. All STBLI simulations exhibit substantial flow reversal and have been integrated for a very long time (90 flow-through times of the full domain length) to properly resolve low-frequency dynamics. Instantaneous and mean flow as well as spectral features are described in detail, together with a modal analysis of the three-dimensional streamwise velocity, streamwise vorticity and pressure fields.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Aerodynamic

Shock-wave/turbulent boundary-layer interaction with a flexible panel

The dynamic coupling between a Mach 2.0 shock-wave/turbulent boundary-layer interaction (STBLI) and a flexible panel is investigated. Wall-resolved large-eddy simulations are performed for a baseline interaction over a flat-rigid wall, a coupled interaction with a flexible panel, and a third interaction over a rigid surface that is shaped according to the mean panel deflection of the coupled case. Results show that the flexible panel exhibits self-sustained oscillatory behavior over a broad frequency range, confirming the strong and complex fluid-structure interaction (FSI). The first three bending modes of the panel oscillation are found to contribute most to the unsteady panel response, at frequencies in close agreement with natural frequencies of the mean deformed panel rather than those for the unloaded flat panel. This highlights the importance of the mean panel deformation and the corresponding stiffening in the FSI dynamics. The time-averaged flow shows an enlarged reverse-flow region in the presence of mean surface deformations. The separation-shock unsteadiness is enhanced due to the panel motion, leading to higher wall-pressure fluctuations in the coupled interaction. Spectral analysis of the separation-shock location and bubble-volume signals shows that the STBLI flow strongly couples with the first bending mode of the panel oscillation. This is further confirmed by dynamic mode decomposition of the flow and displacement data, which reveals variations in the reverse-flow region that follow the panel bending motion and appear to drive the separation-shock unsteadiness. Low-frequency modes that are not associated with the fluid-structure coupling, in turn, are qualitatively similar to those obtained for the rigid-wall interactions, indicating that the characteristic low-frequency unsteadiness of STBLI coexists with the dynamics emerging from the fluid-structure coupling. Based on the present results, unsteady FSIs involving STBLIs and flexible panels are likely to accentuate rather than mitigate the undesirable features of STBLIs.Aerodynamic

Experimental study of symmetric and asymmetric shock-shock interactions with variable inflow Mach numbers

An experimental investigation of shock-shock interactions has been conducted with the aim of studying the transition between Regular (RI) and Mach (MI) interactions induced by a variation of the inflow Mach number. The experiments were conducted in the TST-27 wind tunnel at Delft University of Technology. For all cases, the wind tunnel runs were initialized in the RI domain after which the Mach number was slowly decreased to the MI domain, thereby traversing the whole dual solution domain. The process was then inverted to reach again the RI domain in order to investigate a possible transition hysteresis. Both conventional Schlieren and Focusing Schlieren systems were used to visualize the shock wave patterns. The recorded Schlieren images allow accurate transition detection together with quantitative measurements of the Mach Stem Heigth (MSH). The results show no hysteresis effects. All transitions are recorded to occur at the von Neuman line, for both RI to MI and MI to RI cases.Aerospace EngineeringAerodynamic

Experimental investigation of shock–shock interactions with variable inflow Mach number

Experiments on shock–shock interactions were conducted in a transonic–supersonic wind tunnel with variable free-stream Mach number functionality. Transition between the regular interaction (RI) and the Mach interaction (MI) was induced by variation of the free-steam Mach number for a fixed interaction geometry, as opposed to most previous studies where the shock generator angles are varied at constant Mach number. In this paper, we present a systematic flow-based post-processing methodology of schlieren data that enables an accurate tracking of the evolving shock system including the precise and reproducible detection of RI⇄ MI transition. In line with previous experimental studies dealing with noisy free-stream environments, transition hysteresis was not observed. However, we show that establishing accurate values of the flow deflections besides the Mach number is crucial to achieve experimental agreement with the von Neumann criterion, since measured flow deflections deviated significantly, up to 1. 2 ∘, from nominal wedge angles. We also report a study conducted with a focusing schlieren system with variable focal plane that supported the image processing by providing insights into the three-dimensional side-wall effects integrated in the schlieren images.Aerodynamic