Physical insight into a Mach 7.2 compression corner flow

High-order implicit Large Eddy Simulations were conducted to study shock-boundary layer interaction around a 33° compression corner at Mach 7.2 and Reynolds number of Reθ = 3,500 based on the momentum thickness. A grid-convergence study was performed to reduce the computational uncertainty and the results were compared with experiments and theoretical predictions. Furthermore, the turbulent flow properties were analysed with respect to the Reynolds normal stress, skewness and flatness, and conclusions were drawn regarding the shock boundary layer interaction behavior

Direct numerical simulation of supersonic flow and acoustics over a compression ramp

We present direct numerical simulations of the shock wave boundary layer interaction (SBLI) at Mach number 2.9 over a 24° ramp. We study both the numerical accuracy and flow physics. Two classes of spatial reconstruction schemes are employed: the monotonic upstream-centered scheme for conservation laws and the Weighted Essentially Non-Oscillatory (WENO) scheme, of accuracy ranging from 2nd- to 11th-order. Using the canonical Taylor–Green vortex test-case, a simple and computationally inexpensive rescaling of the candidate stencil values—within the context of the high-order WENO scheme—is proposed for reducing the numerical dissipation, particularly in under-resolved simulations. For the compression ramp case, higher-order schemes are shown to capture the size of the SBLI separation zone more accurately, a consequence of resolving much finer turbulence structures. For second- and fifth-order schemes, the energy of the unresolved small scale turbulence shifts the cascade of the turbulence kinetic energy (TKE) spectrum, thus resulting in more energetic large scale turbulent structures. Consequently, the λ-shock foot shifts further downstream, leading to a smaller separation bubble size. Nonetheless, other statistical quantities, such as the turbulence anisotropy invariant map and the turbulence kinetic energy budget terms, show little dependence on the type and order of the spatial reconstruction scheme. Finally, using the more accurate ninth-order WENO results, it is reasoned that the interaction of the λ-shock with the post-shock relaxation region drives the low-frequency oscillation of the λ-shock

Thermoacoustic effects in high-speed compressible transitional and turbulent boundary layers

A numerical investigation of the thermal and acoustic effects in high-speed compressible flows is presented. Two case studies are considered: i) transition to turbulence in supersonic flows over a flat plate, and ii) supersonic shock wave turbulent boundary layer interaction (SWTBLI) over a compression ramp. Implicit Large Eddy Simulations (iLES) are performed using the second and fifth order Monotone-Upstream Central Scheme for Conservation Laws (MUSCL) and the ninth order Weighted Essentially Non-Oscillatory (WENO) schemes. The aim of this study is twofold: i) to examine the acoustic and thermal effects associated with transitional and turbulent boundary layers, particularly in the near wall region; ii) to investigate the effects of numerical accuracy on acoustic and thermal loading. The results are compared with theoretical models, Direct Numerical Simulations (DNS) and experiments

Performance of high-order implicit large eddy simulations

The performance of parallel implicit large eddy simulations (iLES) is investigated in conjunction with high-order weighted essentially non-oscillatory schemes up to 11th-order of accuracy. Simulations were performed for the Taylor Green Vortex and supersonic turbulent boundary layer flows on High Performance Computing (HPC) facilities. The present iLES are highly scalable achieving performance of approximately 93% and 68% on 1,536 and 6,144 cores, respectively, for simulations on a mesh of approximately 1.07 billion cells. The study also shows that high-order iLES attain accuracy similar to strict direct numerical simulation (DNS) but at a reduced computational cost

Simulation of multi-mode transition to turbulence in compressible boundary layers using high-order methods

Although supersonic turbulent boundary layers (TBL) have been extensively studied both numerically and experimentally, hypersonic TBLs are not yet well understood, particularly transition to turbulence. Recently, Ritos et al. [1, 2] presented implicit Large Eddy Simulations (iLES) results of a supersonic TBL showing that high-order iLES can provide accurate and detailed description of TBL directly comparable to available DNS, while utilising significantly less computational resources. In the present paper, we have used a modified (for transitional flows) 9th order accurate WENO scheme to simulate hypersonic transition (and turbulence) over a flat plate. Most importantly, the present simulations have been conducted for an atmospheric (von Kármán) multimode energy spectrum instead of simple single mode perturbations. The computational study has been performed in the framework of implicit Large Eddy Simulations (iLES). Simulation results are presented for Mach numbers 4, 6 and 8 and different inflow turbulence intensities. Figure 1 shows that even a relatively small value of turbulent intensity triggers bypass transition and turbulence at a downstream location. We have investigated the effects of grid resolution, Mach number, and turbulent intensity on the transition point, turbulent structures, and pressure fluctuations over a flat plate. Further insight into the transition mechanism and the underline physical processes is provided along with numerical evidence on the accuracy of the modified 9th-order WENO method in transitional hypersonic flows

Physical insight into the accuracy of finely-resolved iLES in turbulent boundary layers

This paper investigates the numerical accuracy of implicit Large Eddy Simulations (iLES) in relation to compressible turbulent boundary layers (TBL). iLES are conducted in conjunction with Monotonic Upstream-Centred Scheme for Conservation Laws (MUSCL) and Weighted Essentially Non-Oscillatory (WENO), ranging from 2nd to 9th-order. The accuracy effects are presented from a physical perspective showing skewness, flatness and anisotropy calculations, among others. The order of the scheme directly affects the physical representation of the TBL, especially the degree of asymmetry and anisotropy in the sub-layers of the TBL. The study concludes that high-order iLES can provide an accurate and detailed description of TBL directly comparable to available DNS and experimental results

Computational aeroacoustics beneath high speed transitional and turbulent boundary layers

This paper concerns a study of pressure fluctuations beneath hypersonic shock-wave turbulent boundary layer interactions and the associated acoustic loading on a compression/expansion ramp. Using high-order methods, we have performed Direct Numerical Simulations at Mach 7.2. We compare the spectral analysis of the pressure fluctuations at various locations of the compression/expansion ramp with the spectra calculated beneath a hypersonic transitional boundary layer. Similarities and differences between the two hypersonic boundary layers, in the context of acoustic loading, are drawn. Extremely high values of pressure fluctuations are recorded after the shock re-attachment where we also observe the maximum pressure gradients indicating that acoustic loading is correlated with areas of high-pressure gradients. Finally, we discuss the impact of the boundary layer state (attached flow, turbulence bursts, recirculations, shock oscillations, shock re-attachment and expansion fans) on the frequency spectrum of the pressure fluctuations

Wavelet analysis of high-speed transition and turbulence over a flat surface

This paper presents a study of high speed boundary layers using the wavelet method. We analyze direct numerical simulation data for high-speed, compressible transitional, and turbulent boundary layer flows using orthogonal anisotropic wavelets. The wavelet-based method of extraction of coherent structures is applied to the flow vorticity field, decomposed into coherent and incoherent contributions using thresholding of the wavelet coefficients. We show that the coherent parts of the flow, enstrophy spectra, are close to the statistics of the total flow, and the energy of the incoherent, noise-like background flow is equidistributed. Furthermore, we investigate the distribution of the incoherent vorticity in the transition and turbulent regions and examine the correlation with the near-wall pressure fluctuations. The results of our analysis suggest that the incoherent vorticity part is not a random "noise"and correlates with the actual noise emanating from inside the boundary layer. This could have implications regarding our understanding of the physics of compressible boundary layers and the development of engineering models

Implicit large eddy simulation of acoustic loading in supersonic turbulent boundary layers

This paper investigates the accuracy of implicit Large Eddy Simulation in the prediction of acoustic phenomena associated with pressure fluctuations within a supersonic turbulent boundary layer. We assess the accuracy of implicit Large Eddy Simulation against Direct Numerical Simulation and experiments for attached turbulent supersonic flow with zero-pressure gradient, and further analyze and discuss the effects of turbulent boundary layer pressure fluctuations on acoustic loading both at the high and low frequency regimes. The results of high-order variants of the simulations show good agreement with theoretical models, experiments, as well as previously published Direct Numerical Simulations

Multichannel blind deconvolution methods for source separation in convolutive mixtures of speech

EThOS - Electronic Theses Online ServiceGBUnited Kingdo