Compressible Shear Layers Research Articles

Numerical Simulation of Supersonic Turbulent Separated Flows Based on k–ω Turbulence Models with Different Compressibility Corrections

The accurate prediction of supersonic turbulent separated flows involved in aerospace vehicles is a great challenge for current numerical simulations. Based on the k–ω equations, several different compressibility corrections are incorporated in turbulence models to improve their prediction capabilities. Two benchmark test cases, namely the ramped cavity and the compression corner, are adopted for the numerical validation. Detailed comparisons between simulations and experiments are conducted to evaluate the effect of compressibility corrections on turbulence models. The computed results indicate that compressibility corrections have a significant impact on turbulence model performance. The compressibility correction, considering the effects of both dilatation dissipation and pressure dilatation, is suitable for the prediction of compressible free shear layers, but it may have a negative impact on the prediction of low-speed flows in the near-wall region due to the severe underprediction of the wall skin friction coefficient. In comparison, the compressibility correction only considering the effects of dilatation dissipation is conservative, with decreased predictability of free shear layers in supersonic flows, although it improves the predictions of the original models without corrections.

Quasi-two-dimensional ejector model for anode gas recirculation fuel cell systems

An anode gas recirculation ejector plays an important role in the performance and service life of fuel cells. This study establishes a new quasi-two-dimensional ejector model by considering the flow phenomena of the boundary layer, shock train, and mixing layer. The two-dimensional flow field of the mixing process of the primary and secondary flows is calculated using the compressible turbulent shear layer development theory. The effects of both compressible flow friction and nozzle flow separation caused by the boundary layer are considered. Furthermore, the dynamic pressure loss and mixing acceleration caused by the shock train are calculated in the model. The mixing-length theory proposed by Prandtl is used to calculate the momentum transfer between fluids, and a method for calculating the viscous dissipation of the mixing layer is presented. In addition, the effects of the flow phenomena are introduced into the governing equations through source terms. Finally, an experimental system is built to test the ejector performance under room-temperature, high-temperature, and wet-secondary-flow conditions. The performance and flow parameters obtained using the proposed model are verified through experiments and CFD simulations. The results show that the quasi-two-dimensional ejector model can accurately predict the overall performance and axial flow parameters of the ejector. The root-mean-square errors of the mass flow rates of the primary and secondary flows are 2.29% and 8.78%, respectively. The two-dimensional flow field obtained using the quasi-two-dimensional ejector model is in good agreement with the CFD simulation results. The development of the mixing layer affects the axial parameters and performance of the ejector because it influences the momentum transfer, friction loss, and viscous dissipation of the flow field. This study is of significance for accurately predicting the ejector performance as well as for optimizing the design of the ejector for anode gas recirculation fuel cell systems. This contributes to the efficient operation of fuel cells and simplification of supporting auxiliary systems, which has energy and economic benefits. The proposed ejector model can also be extended to ejectors in other fields.

Global and local modal characteristics of supersonic open cavity flows

Flows past cavities at high-speeds have become increasingly important in applications such as flame-holding and propulsion unstart control. Recently maturated linear techniques have helped discern the underlying mechanisms in the subsonic and low supersonic speed regime (M∞<1.4). Here, we combine these linear methods with fully non-linear two- and three-dimensional simulations to assimilate the significant changes observed when the Mach number is increased further to the 1.4≤M∞≤3 range. The resolvent method is first employed to analyze cavity-shear layer coupled oscillations and modal characteristics, which are found to differ in key respects from those reported at lower Mach numbers. At higher speeds, more 2D coupled modes are obtained with the dominant modes containing secondary waves displaying elaborate patterns. The role of the shear layer on the cavity dynamics is then examined with local spatial stability analyses. In addition to the well-known Kelvin–Helmholtz instability encountered in the subsonic and transonic regimes, forward-propagating (k+) supersonic shear layer instabilities are detected at higher speeds. These are associated with Mach wave reflections between the shear layer and the cavity floor and may introduce higher order coupled modes. Furthermore, 3D modal analysis indicates a shift toward the dominance of 3D modes compared to 2D modes; although consistent with compressible free shear layer observations, 2D cavity modes remain significant to higher convective Mach number. When the Reynolds number is increased, resolvent-based mode shapes and frequencies continue to compare favorably with Dynamic Mode Decomposition of large-eddy simulations because of inviscid instability dominance.

Characterization of a turbulent flow with independent variation of Mach and Reynolds numbers

The Variable Density and Speed of Sound Vessel (VDSSV) produces subsonic turbulent flows that are both compressible and observable at all scales with existing instrumentation including hot wires and particle tracking. We realize this objective by looking at the flow of a heavy gas (sulfur hexafluoride $SF_6$), with a speed of sound almost three times lower than for air. By switching between air and $SF_6$, we isolate the influence of the turbulent Mach number (up to $M_t$ = 0.17) on turbulence statistics from the influences of changes in the Reynolds number (up to $R_{\lambda}$ = 1600), and boundary conditions, which we hold constant. A free shear flow is produced by a ducted fan, and we show that it behaves like a turbulent jet in that the mean velocity profiles approach self-similarity with increasing distance from the orifice (up to $x/D_f$ = 9). The jet responds like a compressible shear layer in that it spreads more slowly at higher Mach numbers (up to $M_j$ = 0.7) than at low Mach numbers. In contrast, the integral length scales and Kolmogorov constant of the turbulence are approximately invariant with respect to changes in either the Reynolds or Mach numbers. We briefly report on instrumentation under development that will extend the accessible Taylor-scale Reynolds and turbulent Mach numbers to 4000 and 0.3, respectively.

Assessment of turbulence models using DNS data of compressible plane free shear layer flow

The present paper uses the detailed flow data produced by direct numerical simulation (DNS) of a three-dimensional, spatially developing plane free shear layer to assess several commonly used turbulence models in compressible flows. The free shear layer is generated by two parallel streams separated by a splitter plate, with a naturally developing inflow condition. The DNS is conducted using a high-order discontinuous spectral element method (DSEM) for various convective Mach numbers. The DNS results are employed to provide insights into turbulence modelling. The analyses show that with the knowledge of the Reynolds velocity fluctuations and averages, the considered strong Reynolds analogy models can accurately predict temperature fluctuations and Favre velocity averages, while the extended strong Reynolds analogy models can correctly estimate the Favre velocity fluctuations and the Favre shear stress. The pressure–dilatation correlation and dilatational dissipation models overestimate the corresponding DNS results, especially with high compressibility. The pressure–strain correlation models perform excellently for most pressure–strain correlation components, while the compressibility modification model gives poor predictions. The results of an a priori test for subgrid-scale (SGS) models are also reported. The scale similarity and gradient models, which are non-eddy viscosity models, can accurately reproduce SGS stresses in terms of structure and magnitude. The dynamic Smagorinsky model, an eddy viscosity model but based on the scale similarity concept, shows acceptable correlation coefficients between the DNS and modelled SGS stresses. Finally, the Smagorinsky model, a purely dissipative model, yields low correlation coefficients and unacceptable accumulated errors.

Wall temperature effects on shock unsteadiness in a reattaching compressible turbulent shear layer

In this study, the effect of heat transfer on the compressible turbulent shear layer and shockwave interaction in a scramjet has been investigated. To this end, highly resolved Large Eddy Simulations (LES) are performed to explore the effect of wall thermal conditions on the behavior of a reattaching free shear layer interacting with an oblique shock in compressible turbulent flows. Various wall-to-recovery temperature ratios are considered, and results are compared to the adiabatic wall. It is found that the wall temperature affects the reattachment location and the shock behavior in the interaction region. Furthermore, fluctuating heat flux exhibits a strong intermittent behavior with severe heat transfer compared to the mean, characterized by scattered spots. The distribution of the Stanton number shows a strong heat transfer and complex pattern within the interaction, with the maximum thermal (heat transfer rates) and dynamic loads (root-mean-square wall pressure) found for the case of the cold wall. The analysis of LES data reveals that the thermal boundary condition can significantly impact the wall pressure fluctuations level. The primary mechanism for changes in the flow unsteadiness due to the wall thermal condition is linked to the reattaching shear layer, which agrees with the compressible turbulent boundary layer theory.

On the mass exchange mode controlled by high-speed compressive mainstream/cavity interaction in a laboratory dual-mode scramjet combustor

Stable and efficient mass exchange is crucial to the combustion stability of a dual-mode scramjet. In subsonic mode, the progression of mass exchange between the cavity fluid and the mainstream, which is controlled by the cavity shear layer, is different from the scramjet mode due to the weak compressibility effects. However, mass exchange mode studies in the subsonic mode are rare. In this study, experimental research has been conducted to obtain the flow structure of Mach 0.3 and 0.5 inflow over a rectangular cavity and reveal the properties of mass exchange using particle image velocimetry technology. The results reveal the weak compressibility effects and geometry effects on the mass exchange by analyzing the growth rate and turbulence characteristics of the cavity shear layer. The mass exchange between the cavity fluid and mainstream is determined based on the vertical velocity along the cavity lip line. When the flow Mach number increased from 0.3 to 0.5, the growth rate of the cavity shear layer decreases due to the weak compressibility effects. Also, the growth rate of the cavity shear layer is smaller than that of the compressible free shear layer due to the effects of recirculation within the cavity. For length-to-depth (L/D) ratios ranging from 0.8 to 1.2, the cavity shear-layer growth rate decreases with increasing L/D. However, at an L/D of 1.5, the shear-layer growth rate increases, which changes the mass exchange mode. Finally, the effects of increasing the Mach number (Mach = 0.3 versus 0.5) show that the L/D ratio at the “crossover” point of the mass exchange mode transition is reduced (L/D = 1.5 versus 1.2) due to the weak compressibility at a higher Mach number.

Compressibility effects on energy exchange mechanisms in a spatially developing plane free shear layer

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Internal regulation in compressible turbulent shear layers

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The role of the “monopole” instability in the evolution of two-dimensional turbulent free shear layers

The role of instability in the growth of a two-dimensional, temporally evolving, “turbulent” free shear layer is analyzed using vortex-gas simulations that condense all dynamics into the kinematics of the Biot–Savart relation. The initial evolution of perturbations in a constant vorticity layer is found to be in accurate agreement with the linear stability theory of Rayleigh. There is then a stage of non-universal evolution of coherent structures that is closely approximated not by Rayleigh stability theory, but by the Karman–Rubach–Lamb linear instability of monopoles, until the neighboring coherent structures merge. After several mergers, the layer evolves eventually to a self-preserving reverse cascade, characterized by a universal spread rate found by Suryanarayanan, Narasimha, and Hari Dass [“Free turbulent shear layer in a point vortex gas as a problem in nonequilibrium statistical mechanics,” Phys. Rev. E 89, 013009 (2014)] and a universal value of the ratio of dominant spacing of structures (Λf) to the layer thickness (δω). In this universal, self-preserving state, the local amplification of perturbation amplitudes is accurately predicted by Rayleigh theory for the locally existing “base” flow. The model of Morris, Giridharan, and Lilley [“On the turbulent mixing of compressible free shear layers,” Proc. R. Soc. London, Ser. A 431, 219–243 (1990)], which computes the growth of the layer by balancing the energy lost by the mean flow with the energy gain of the perturbation modes (computed from an application of Rayleigh theory), is shown, however, to provide a non-universal asymptotic state with initial condition dependent spread rate and spectra. The reason is that the predictions of the Rayleigh instability, for a flow regime with coherent structures, are valid only at the special value of Λf/δω achieved in the universal self-preserving state.

Mixing enhancement in a subsonic-supersonic shear layer with a cavity splitter plate

The development of more efficient ramjet engines has motivated the study of enhanced mixing in compressible shear layers. Cavity-mixer is a widely used passive flow control tool, but its mixing enhancement mechanisms in compressible subsonic-supersonic shear layers has yet to be comprehensively explored. In this work, a cavity-mixer is used to increase mixing in a subsonic-supersonic shear layer, and the underlying flow control mechanisms are examined. Parametric experiments were performed on a laboratory-scale blowdown wind tunnel and an extraction wind tunnel. Numerical simulations were performed in a large eddy simulation framework. Schlieren and nanoparticle-based planar laser scattering techniques were adopted for mixing flow visualization. It is found that the growth rate of the cavity (length-to-depth ratio of 5) disturbed shear layer is 31% higher than the benchmark configuration at convective Mach number of 0.49∼0.56. It is demonstrated that the compressibility and length-to-depth ratio affect the cavity-mixer performance. High convective Mach number suppresses the cavity disturbance and weakens its mixing enhancement efficiency. Cavity-mixer with a length-to-depth ratio of 3 shows the highest experimental and numerical growth rate, compared to 5 and 7 under a convective Mach number around 0.49. Large-scale coherent vortices are confirmed in the subsonic-supersonic mixing process. Cavity disturbance promotes the three-dimensional characteristics of the flow field, increases the vorticity of large coherent structures, and advances the K-H instability position of shear layer, which plays a key contribution to mixing enhancement.

Response characteristics of inflow-stimulated Kelvin-Helmholtz vortex in compressible shear layer

By numerically solving the Navier-Stokes equations, the response characteristics of inflow-stimulated Kelvin-Helmholtz vortex in compressible shear layer arestudied. The mixing characteristics and the unique growth mechanism of the vortex structure are clearly revealed. By employing the index of vorticity thickness, the mixing properties are quantitatively analyzed. Based on the flow visualization results, the spatial size and the structure angle of the flow coherent structure are investigated by utilizing spatial correlation analysis. The evolution mechanism of the vortex structure in supersonic mixing layer induced by inlet forcing is revealed by analyzing the dynamical performances of the flow structure under different frequency disturbances. The numerical results show that with low forcing frequency at <i>f</i> = 5 kHz, the mixing efficiency is remarkably increased in the near-field of the flow. Whereas, in the far-field downstream the flow, the size of the structure reaches saturation state and the vortex passage frequency is locked, which causes the vorticity thickness to stabilize from 12mm to 14mm. Meanwhile, in a free mixing layer, the pairing and merging process occur in the flow field to promote the growth of the vortex structure, while in mixing layer with inlet forcing, the growth mechanism is that the vortex core engulfs a string of vortices induced by Kelvin-Helmholtz instability. The process of engulfment contributes much to the growth of the vortex structure. The analysis of spatial correlation distribution shows that in the area where engulfment occurs, the contour line shows the property of long and narrow ellipse instead of full ellipse and the structure in the area possesses the characteristics of intense rotation and inclination. Besides, with high inlet forcing frequency at <i>f</i> = 20 kHz, the size of the vortices becomes full in the near-field, and the vorticity thickness stabilizes between 3mm and 4 mm downstream the flow field. Meanwhile, the size of the vortex in controlled supersonic mixing layer is dominated by the imposed high-frequency forcing. An equation describing the quantitative relationship between the vortex characteristics and the imposed forcing frequency is derived, that is, the size of the uniform distribution vortex is approximately equal to the ratio of the value of convective velocity to inlet forcing frequency.

Effects of eddy shocklets on the segregation and evaporation of droplets in highly compressible shear layers

Numerical studies of droplet-laden spatially developing shear layers are conducted with a high convective Mach number (Mc = 1.0), in which a high-order hybrid weighted essentially nonoscillatory scheme is used for the turbulence as well as shock capturing. The evaporating droplets are tracked as point mass in the Lagrangian manner, and the two-way coupling between the flow and droplets is considered by adding the source terms to the governing equations of the gas-phase. This research focuses on the preferential concentration of droplets and the interactions between droplets and eddy shocklets in the shear layers with high flow compressibility and analyzes the underlying mechanisms of momentum and thermal response behaviors of droplets to eddy shocklets. The segregation of droplets exhibits the strongest spatial preference in the highly compressible shearing vortices, and droplets tend to accumulate as stripes behind the shocklets, associated with the coherent structures. The high flow compressibility results in the strong spatiotemporal variations of pressure and temperature, and the distributions of the expansion zones with low temperature and the compression zones with high temperature occur alternately in the shear layer. The relaxation response behaviors of the droplets to the change of momentum and thermal features of the surrounding carrier gas result in the delay of droplet evaporation in the high temperature region and then the enhancement of droplet-vapors in the low temperature region. The present observations can be ascribed to the interactions between the dispersed droplets and the distributed eddy shocklets in the shear flows with high compressibility.

Compressibility effects on the transition to turbulence in a spatially developing plane free shear layer

The compressibility effects on the transition to turbulence in a spatially developing, compressible plane free shear layer are investigated via direct numerical simulation using a high-order discontinuous spectral element method for three different convective Mach numbers of 0.3, 0.5, and 0.7. The location of the laminar–turbulent transition zone is predicted by the analyses of vorticities, Reynolds stresses, and the turbulent dissipation rate. In the turbulence transition and self-similar turbulence regions, the effects of compressibility on the flow properties, such as the velocity autocorrelation function, integral time scale, momentum thickness, Reynolds stress, and turbulent kinetic energy budget, are investigated. The compressibility effects on the onset and length of the turbulence transition zone are studied based on the analyses of such flow properties. The mean velocity, momentum thickness, and Reynolds stress profiles compare well with published experimental data. Vorticity contours and iso-surface of the second invariant of velocity gradient tensor identify the characteristic of flow structures. The two-point correlation functions of velocity components, the one-dimensional (1D) spanwise energy spectrum, and the balance of the turbulent kinetic energy transport equation validate the domain size and resolution of the adopted grid for turbulence simulation. An increase in the convective Mach number leads to a reduction in the sizes of the largest-scale structures, resulting in a significant decrease in Reynolds stresses and turbulence production. The onset of turbulence transition and the location where the transition completes shift downstream, while the length of the transition zone increases with increasing convective Mach number.

An in-depth numerical investigation of a supersonic cavity-ramp flow with DDES method

Delayed detached eddy simulation (DDES) studies of the highly unsteady supersonic cavity-ramp flow have been successfully carried out. It is generally believed that accurate prediction of the compressible unstable shear layer coupled with the followed reattaching process is a big challenge for DDES method. In the current DDES studies based on an engineering affordable mesh, significant improvements are achieved assessed by the agreements with experimental data. Effects of various crucial parameters related to DDES have been analyzed in detail. First of all, it is found that a fine mesh with sufficient computation domain is required to obtain accurate results, and the time step needs to be properly set. Secondly, effect of the definition of subgrid length scale Δ is observed, especially in the initial region of shear layer. The original Δ form of DDES method tends to depress the development of instabilities in the shear layer. Analysis on the physical causes is presented. Moreover, unexpected performance of DDES' shield function fd for the current case is observed and analyzed, whose influence on the simulated characteristic in the downstream region has also been discussed in particular.

On the role of bulk viscosity in compressible reactive shear layer developments

Abstract Despite 150 years of research after the reference work of Stokes, it should be acknowledged that some confusion still remains in the literature regarding the importance of bulk viscosity effects in flows of both academic and practical interests. On the one hand, it can be readily shown that the neglection of bulk viscosity (i.e., κ = 0 ) is strictly exact for mono-atomic gases. The corresponding bulk viscosity effects are also unlikely to alter the flowfield dynamics provided that the ratio of the shear viscosity μ to the bulk viscosity κ remains sufficiently large. On the other hand, for polyatomic gases, the scattered available experimental and numerical data show that it is certainly not zero and actually often far from negligible. Therefore, since the ratio κ ∕ μ can display significant variations and may reach very large values (it can exceed thirty for dihydrogen), it remains unclear to what extent the neglection of κ holds. The purpose of the present study is thus to analyze the mechanisms through which bulk viscosity and associated processes may alter a canonical turbulent flow. In this context, we perform direct numerical simulations (DNS) of spatially-developing compressible non-reactive and reactive hydrogen–air shear layers interacting with an oblique shock wave. The corresponding flowfield is of special interest for various reactive high-speed flow applications, e.g., scramjets. The corresponding computations either neglect the influence of bulk viscosity ( κ = 0 ) or take it into consideration by evaluating its value using the EGlib library. The qualitative inspection of the results obtained for two-dimensional cases in either the presence or the absence of bulk viscosity effects shows that the local and instantaneous structure of the mixing layer may be deeply altered when taking bulk viscosity into account. This contrasts with some mean statistical quantities, e.g., the vorticity thickness growth rate, which do not exhibit any significant sensitivity to the bulk viscosity. Enstrophy, Reynolds stress components, and turbulent kinetic energy (TKE) budgets are then evaluated from three-dimensional reactive simulations. Slight modifications are put into evidence on the energy transfer and dissipation contributions. From the obtained results, one may expect that refined large-eddy simulations (LES) may be rather sensitive to the consideration of bulk viscosity, while Reynolds-averaged Navier–Stokes (RANS) simulations, which are based on statistical averages, are not.

Compressible mixing layer in shock-induced separation

Unsteadiness in separated shock–boundary layer interactions have been previously analysed in order to propose a scenario of entrainment–discharge as the origin of unsteadiness. It was assumed that the fluid in the separated zone is entrained by the free shear layer formed at its edge, and that this layer follows the properties of the canonical mixing layer. This last point is addressed by reanalysing the velocity measurements in an oblique shock reflection at a nominal Mach number of 2.3 and for two cases of flow deviation ($8^{\circ }$ and $9.5^{\circ }$). The rate of spatial growth of this layer is evaluated from the spatial growth of the turbulent stress profiles. Moreover, the entrainment velocity at the edge of the layer is determined from the mean velocity profiles. It is shown that the values of turbulent shear stress, spreading rate and entrainment velocity are consistent, and that they follow the classical laws for turbulent transport in compressible shear layers. Moreover, the measurements suggest that the vertical normal stress is sensitive to compressibility, so that the anisotropy of turbulence is affected by high Mach numbers. Dimensional considerations proposed by Brown & Roshko (J. Fluid Mech., vol. 64, 1974, 775–781) are reformulated to explain this observed trend.

Construction and validation of a statistical model for the nonlinear Kelvin-Helmholtz instability under compressible, multimode conditions

A new model for the evolution of compressible, multimode Kelvin-Helmholtz (KH) instability is presented. The model is built upon compressible single vortex evolution and two-vortex interaction, resulting in a statistical description of the compressible KH mixing zone evolution. These two building blocks, which, due to complicated compressibility effects and the presence of shock waves, cannot be derived using simple flow models, are validated by novel supersonic high-energy-density physics experiments. The model was validated against numerical simulations, experimental results, and previous phenomenological models, confirming the compressible KH scaling law in the self-similar regime in good agreement with simulations and a compilation of experimental data. Moreover, the model extends and confirms the logical validity of previous work, done in the incompressible regime. Therefore, it sheds new light on the evolution of compressible shear layers up to the self-similar regime.

A localised dynamic closure model for Euler turbulence

ABSTRACTIn this work, we present a localised form of the dynamic eddy viscosity model for computationally efficient and accurate simulation of the turbulent flows governed by Euler equations. In our framework, we determine the dynamic model coefficient locally using the information from neighbouring grid points through a test filtering process. We then develop an optimised Gaussian filtering kernel, using a consistent definition with respect to the test filtering ratio, which gives full attenuation at the grid cut-off wave number. A systematic a-posteriori analysis of our model is performed by solving two 3D test problems: (i) incompressible Taylor–Green vortex flow and (ii) compressible shear layer turbulence induced by Kelvin–Helmholtz instability to show the wide range of applicability of the proposed localised dynamic model. We demonstrate that the proposed dynamic model is robust and provides a better estimation of the inertial range turbulence dynamics than other numerical models tested in this study.

Analysis of compressible free shear layers with finite-time Lyapunov exponents

Finite-time Lyapunov Exponents (FTLE) can, through suitable choice of integration parameters, capture not only convecting coherent structures in compressible turbulent flows but also propagating dilatational components. This semi-Lagrangian approach thus provides an implicit sliding decomposition technique to connect hydrodynamic and acoustic features, and furthermore facilitates a direct examination of individual time-local events such as intermittency, which are often obscured by statistical techniques. The FTLE method yields attracting (also designated stable or backward-integrated) and repelling (unstable or forward-integrated) structures. In this paper, we show that FTLE offers substantial additional analysis capability in compressible flows over corresponding incompressible situations. Not only does the method establish a connection between attracting structures and the acoustic field, it also offers an effective means to elucidate finer details of the governing dynamics. This is achieved by leveraging repelling structures, as well as negative FTLE magnitudes, which complement the positive values in incompressible flows. The test bed considered is a free shear layer, specifically a validated Large-Eddy Simulation of a Mach 0.9 jet, where hydrodynamic and acoustic components interact with each other in an intricate manner. The implications of attracting and repelling FTLE structures are found to depend on whether they arise in the core turbulent region or in regions where the acoustic energy dominates. In turbulent regions, attracting structures correlate well with lower frequency acoustic components propagating downstream but repelling structures isolate the higher frequency components of the principal vortex dynamics. On the other hand, away from the highly turbulent regions, attracting structures continue to be associated with the (relatively weaker) coherent vortices while repelling structures gradually capture more of the acoustic energy and show higher correlation with the Eulerian dilatation. The joint FTLE field, defined as the difference between attracting and repelling FTLE coefficients, also reveals crucial aspects of the underlying physics. In particular, it captures hydrodynamic features at time scales representative of convection, which are shown to induce and modulate acoustic events within the potential core as they relate to entrainment and vortex interactions. In compressible flows, therefore, proper interpretation of the different components of the FTLE enables a rich, time-accurate connection between different temporal events.