Abstract:The unsteadiness of a shock-wave/turbulent-boundary-layer interaction induced by an axisymmetric step (cylinder/$90^{\circ }$-disk) is investigated experimentally at Mach 3.9. A large-scale separation of the order of previously reported incoming turbulent superstructures is induced ahead of the step ${\sim}30\unicode[STIX]{x1D6FF}_{o}$ and followed by a downstream separation of ${\sim}10\unicode[STIX]{x1D6FF}_{o}$ behind it, where $\unicode[STIX]{x1D6FF}_{o}$ is the incoming boundary-layer thickness. Narrowba… Show more
“…A very similar correlation magnitude was also observed in earlier studies with similar separation scales (e.g. Chandola, Huang & Estruch-Samper (2017) in forward-facing step, Priebe & Martín (2012) in compression ramp and Brusniak & Dolling (1994) in blunt fin interactions). Furthermore, Poggie & Porter (2019) demonstrated that the peak correlation between the incoming boundary layer velocity fluctuation and separation shock location occurs several characteristic boundary layer convection times earlier, consistent with the present measurement.…”
The large-scale pulsations of shock-induced separation with length scale that significantly exceeds the incoming boundary layer thickness are investigated. The shock–boundary layer interaction (SBLI) unit is generated by an inward-turning axisymmetric compression ramp at an inflow Mach number of 2.5. A substantial region surrounding the centre azimuth exhibited mean and dynamic flow features that are consistent with two-dimensional separation. Two-dimensional highly resolved maps of surface pressure field are obtained using fast-response pressure-sensitive paint fluorescence imaging at 40 kHz repetition rate. The measurement domain covered significant regions of the incoming boundary layer through the relaxing boundary layer downstream of the reattachment as well as over 25 boundary layer thicknesses in the azimuthal direction. These measurements provide new insights into the spanwise coupling of the SBLI unit in addition to its inherent dynamics. The power spectral density (PSD) of the centreline pressure exhibits very good agreement with theoretical models and complementary measurements using fast-response pressure transducers, which served to validate the pressure field measurements. Detailed examination of the PSD reveals strong agreement with the literature, which includes the peak Strouhal number of the separation and reattachment shock motions as well as the downward frequency shift along the separation bubble. Furthermore, the pressure fluctuation maps reveal streamwise-elongated structures just downstream of the ramp leading edge that persist well downstream of the reattachment. A time sequence of conditional average pressure fluctuation maps is constructed surrounding isolated pressure excursions in the intermittent region. This sequence, along with two-point cross correlation analysis, provides critical information about the flow processes that drive the separation bubble pulsations in the SBLI units with large separation scales. Overall, the imbalance in the mass within the separation bubble appears to be the critical mechanism that drives the separation bubble pulsations. Furthermore, the pressure perturbations originating at azimuthally offset locations are also observed to influence the separation bubble dynamics.
“…A very similar correlation magnitude was also observed in earlier studies with similar separation scales (e.g. Chandola, Huang & Estruch-Samper (2017) in forward-facing step, Priebe & Martín (2012) in compression ramp and Brusniak & Dolling (1994) in blunt fin interactions). Furthermore, Poggie & Porter (2019) demonstrated that the peak correlation between the incoming boundary layer velocity fluctuation and separation shock location occurs several characteristic boundary layer convection times earlier, consistent with the present measurement.…”
The large-scale pulsations of shock-induced separation with length scale that significantly exceeds the incoming boundary layer thickness are investigated. The shock–boundary layer interaction (SBLI) unit is generated by an inward-turning axisymmetric compression ramp at an inflow Mach number of 2.5. A substantial region surrounding the centre azimuth exhibited mean and dynamic flow features that are consistent with two-dimensional separation. Two-dimensional highly resolved maps of surface pressure field are obtained using fast-response pressure-sensitive paint fluorescence imaging at 40 kHz repetition rate. The measurement domain covered significant regions of the incoming boundary layer through the relaxing boundary layer downstream of the reattachment as well as over 25 boundary layer thicknesses in the azimuthal direction. These measurements provide new insights into the spanwise coupling of the SBLI unit in addition to its inherent dynamics. The power spectral density (PSD) of the centreline pressure exhibits very good agreement with theoretical models and complementary measurements using fast-response pressure transducers, which served to validate the pressure field measurements. Detailed examination of the PSD reveals strong agreement with the literature, which includes the peak Strouhal number of the separation and reattachment shock motions as well as the downward frequency shift along the separation bubble. Furthermore, the pressure fluctuation maps reveal streamwise-elongated structures just downstream of the ramp leading edge that persist well downstream of the reattachment. A time sequence of conditional average pressure fluctuation maps is constructed surrounding isolated pressure excursions in the intermittent region. This sequence, along with two-point cross correlation analysis, provides critical information about the flow processes that drive the separation bubble pulsations in the SBLI units with large separation scales. Overall, the imbalance in the mass within the separation bubble appears to be the critical mechanism that drives the separation bubble pulsations. Furthermore, the pressure perturbations originating at azimuthally offset locations are also observed to influence the separation bubble dynamics.
“…Our present LES results for the turbulent case agree well with the experiments of Chandola et al. (2017) conducted in a turbulent flow at and (see figure 7). Figure 7.Streamwise distribution of the mean wall pressure.…”
Section: Resultssupporting
confidence: 91%
“…It then drops drastically by approximately 75 % of the maximum at the step corner due to the expansion, and then rises again to its initial (freestream) value as the flow reattaches on the upper wall. Our present LES results for the turbulent case agree well with the experiments of Chandola et al (2017) conducted in a turbulent flow at Ma = 2.2 and Re ∞ = 6.5 × 10 7 m −1 (see figure 7).…”
The flow over a forward-facing step (FFS) at
$Ma_\infty =1.7$
and
$Re_{\delta _0}=1.3718\times 10^{4}$
is investigated by well-resolved large-eddy simulation. To investigate effects of upstream flow structures and turbulence on the low-frequency dynamics of the shock wave/boundary layer interaction (SWBLI), two cases are considered: one with a laminar inflow and one with a turbulent inflow. The laminar inflow case shows signs of a rapid transition to turbulence upstream of the step, as inferred from the streamwise variation of
$\langle C_f \rangle$
and the evolution of the coherent vortical structures. Nevertheless, the separation length is more than twice as large for the laminar inflow case, and the coalescence of compression waves into a separation shock is observed only for the fully turbulent inflow case. The dynamics at low and medium frequencies is characterized by a spectral analysis, where the lower frequency range is related to the unsteady separation region, and the intermediate one is associated with the shedding of shear layer vortices. For the turbulent inflow case, we furthermore use a three-dimensional dynamic mode decomposition to analyse the individual contributions of selected modes to the unsteadiness of the SWBLI. The separation shock and Görtler-like vortices, which are induced by the centrifugal forces in the separation region, are strongly correlated with the low-frequency unsteadiness in the current FFS case. Similarly as observed previously for the backward-facing steps, we observe a slightly higher non-dimensional frequency (based on the separation length) of the low-frequency mode than for SWBLI in flat plate and ramp configurations.
“…Contours of pressure PSD in the ( f,x)-plane over axisymmetric step at Mach 3.9 (FFS as per figure 9), together with cross-correlation at selected locations and with respect to separation ρ ox (near plateau start X * U = 0.2, just ahead of step X * U ≈ 0.98 and at downstream reattachment X * D = 1). Detailed cross-correlations may be found in Chandola et al (2017) for the reference large-scale interaction, L/δ o = 30.2.…”
Section: Unsteadiness Of Step-induced Separationmentioning
confidence: 99%
“…It however remains unclear how strong an impact these different factors may bear on the low-frequency unsteadiness and to what extent their influence may vary amongst cases. The present research follows from our recent experimental studies on a highly separated axisymmetric Mach 3.9 STBLI, at high Reynolds number Re e = 6.1 × 10 7 m −1 (Re δ = 2.3 × 10 5 ), where we looked at the unsteady nature of a step-induced separation of length ∼30δ o upstream of the step and extending ∼10δ o behind it (Chandola, Huang & Estruch-Samper 2017). The characteristic flow organisation may be seen in the schlieren image in figure 1.…”
This paper presents an experimental study on shock-wave/turbulent-boundary-layer interaction unsteadiness and delves specifically into the shear layer’s role. A range of axisymmetric step-induced interactions is investigated and the scale of separation is altered by over an order of magnitude – mass in the recirculation by two orders – while subjected to constant separation-shock strength. The effect of the separated shear layer on interaction unsteadiness is thus isolated and its kinematics are characterised. Results point at a mechanism whereby the depletion of separated flow is dictated by the state of the large eddy structures at their departure from the bubble. Low-frequency pulsations are found to adjust in response and sustain a reconciling view of an entrainment–recharge process, with both an inherent effect of the upstream boundary layer on shear layer inception and an increase in the mass locally acquired by eddies as they develop downstream.
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