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The importance of experimental setups able to reproduce cardiac functions was well established in the field of clinical innovations. The mock circulatory loops acquired rising relevance, and the possibility to have a complete reproduction of different and specific fluid dynamic conditions within the setup is pivotal. A system with enough versatility to reproduce the physiologic range of both flows and pressures is required. This study describes the design of a versatile setup composed by a custom pulsatile left ventricular pump system and a 3D-printed mock circulatory loop for the in vitro analysis of a patient-specific case of an aortic complex. The performances of the pump were validated first with a set of test flow profiles. It was demonstrated that the system was able to cover a wide range of aortic and mitral flows. Second, the pump system was inserted within the full mock circulatory loop. A patient-specific case was reproduced, both in terms of flow and pressure profiles. A successful validation of the flow and pressure waveforms was obtained by using patient-specific in vivo data from magnetic resonance analysis.

Suitably shaped grooves, placed transverse to the flow, can delay flow separation over curved surfaces. When grooves are fully extruded in the spanwise direction, they may reduce the drag of boat-tailed bodies with vortex shedding, but with the drawback of increasing the spanwise correlation of the vortex shedding. We investigate herein the effect of spanwise-discontinuous grooves through Large Eddy Simulations. A systematic analysis is carried out on the effect of the number, N, of grooves that are present for N equally long portions of the total spanwise length of the boat-tail. Discontinuous grooves further reduce the drag compared with the full-spanwise-extruded groove. Increasing N produces an improvement of the flow-control-device performance, whose maximum value is reached for N=3, corresponding to a spanwise extension of the groove roughly equal to the body crossflow dimension. Above this value, no further improvements are found. The maximum drag reduction is equal to 25.7% of the drag of the boat-tail without grooves and to 17.7% of the one of the boat-tail with the full-spanwise-extruded groove. The lowest drag value occurs for the least correlated vortex-shedding in the spanwise direction. The reduction in the correlation is mainly related to a flow separation line that is less regular in the spanwise direction.

Microreactors are very attractive for intensifying chemical and biochemical reactions, as they enable a large heat transfer with high control of the operating conditions. These features allow improving the achievable reaction yield and selectivity. However, the mixing of reactants should be ensured, especially in simple designs, as the flow regime is laminar. This work aims at characterizing the flow regimes and degree of mixing in a X-shaped micromixer with two opposite inlets and at investigating how these regimes are affected by fluid properties. To this purpose we carry out numerical simulations of two different mixtures, i.e. Water-Water (W-W) and Water-Ethanol (W-E), for Reynolds numbers Re ⩽ 200 , to gain insight into the complex three-dimensional flow structures stemming from the impinging jets at the confluence. Besides, experimental flow visualizations from the W-W case provide a validation of the numerical model. The X-shaped reactor is observed to enable a superior mixing performance compared to T-, Y-, and Arrow-shaped microreactors, which are extensively employed.

Experiments and direct numerical simulations are used jointly to study the asymmetric and symmetric time-periodic regimes occurring in a T-shaped micro mixer for larger Reynolds numbers than those ...

The effectiveness of properly contoured transverse grooves in delaying the flow separation occurring on a two-dimensional boat-tailed bluff body is assessed through numerical simulations. The body has a cross-section with a 3:1 elliptical forebody and a rectangular main part followed by a circular-arc boat tail. Three-dimensional Variational Multiscale Large Eddy Simulations are carried out at R e = D u ∞ ∕ ν = 9 . 6 × 1 0 4 , using a mixed finite-volume/finite-element method. The introduction of one contoured groove on each of the boat-tail lateral surfaces produces a significant delay of flow separation and a consequent increase of the base pressure, with a global drag reduction of the order of 9 . 7 % . The wake dynamical structure remains qualitatively similar to the one typical of blunt-based two-dimensional bodies, with quantitative variations that are consistent with the reduction in wake width caused by boat tailing and by the grooves. The introduction of the grooves leads also to a regularization of the vortex shedding downstream of the body, which is more correlated in the spanwise direction. Finally, a few supplementary simulations show that the effect of the grooves is also robust to the variation of the geometrical parameters defining their location and shape.

Experiments, i.e. micro-PIV and flow visualization, and direct numerical simulations, are used jointly to investigate how stratification affects mixing and chemical reaction in a T-shaped microreactor fed with two miscible liquids exhibiting a small density difference. The work analyzes the dependence of the degree of mixing on the Reynolds number and correlates the reaction yield with the Damk\"ohler number to help to devise strategies for the practical operation of microreactors with fluids of practical interest.

Despite the very simple geometry and the laminar flow, T-shaped microreactors have been found to be characterized by different and complex steady and unsteady flow regimes, depending on the Reynolds number. In particular, flow unsteadiness modifies strongly the mixing process, however, little is known on how this change may affect the yield of a chemical reaction. In the present work, experiments and 3-dimensional numerical simulations are carried out jointly to analyze mixing and reaction in a T-shaped microreactor with the ultimate goal to investigate how flow unsteadiness affects the reaction yield. The onset of the unsteady asymmetric regime enhances the reaction yield by more than 30%, however, a strong decrease of the yield back to values typical of the vortex regime is observed when the flow undergoes a transition to the unsteady symmetric regime.

Numerical simulations of the blood flow inside a patient-specific thoracic aorta in presence of an aneurysm are considered. We focus on the impact on the numerical predictions of the inlet flow-rate waveform. First, the results obtained by using an idealized and a MRI-measured flow-rate waveform are compared. The measured boundary condition produces significantly higher wall shear stresses than those obtained in the idealized case. Discrepancies are reduced but they are still present even if the idealized inlet waveform is rescaled in order to match the stroke volume. This motivates a systematic sensitivity analysis of numerical predictions to the shape of the inlet flow-rate waveform that is carried out in the second part of the paper. Two parameters are selected to describe the inlet waveform: the stroke volume and the period of the cardiac cycle. A stochastic approach based on the generalized Polynomial Chaos (gPC) approach, in which continuous response surfaces of the quantities of interest in the parameter space can be obtained from a limited number of simulations, is used. For both selected uncertain parameters, we use beta PDFs reproducing clinical data. The two selected input parameters appear to have a significant influence on wall shear stresses as well as on the velocity distribution in vessel regions characterized by large curvature. This confirms the need of using patient-specific inlet conditions to obtain reliable hemodynamic predictions.

Computational Fluid Dynamics (CFD) simulations of blood flow are widely used to compute a variety of hemodynamic indicators such as velocity, time-varying wall shear stress, pressure drop, and energy losses. One of the major advances of this approach is that it is non-invasive. The accuracy of the cardiovascular simulations depends directly on the level of certainty on input parameters due to the modelling assumptions or computational settings. Physiologically suitable boundary conditions at the inlet and outlet of the computational domain are needed to perform a patient-specific CFD analysis. These conditions are often affected by uncertainties, whose impact can be quantified through a stochastic approach. A methodology based on a full propagation of the uncertainty from clinical data to model results is proposed here. It was possible to estimate the confidence associated with model predictions, differently than by deterministic simulations. We evaluated the effect of using three-element Windkessel models as the outflow boundary conditions of a patient-specific aortic coarctation model. A parameter was introduced to calibrate the resistances of the Windkessel model at the outlets. The generalized Polynomial Chaos method was adopted to perform the stochastic analysis, starting from a few deterministic simulations. Our results show that the uncertainty of the input parameter gave a remarkable variability on the volume flow rate waveform at the systolic peak simulating the conditions before the treatment. The same uncertain parameter had a slighter effect on other quantities of interest, such as the pressure gradient. Furthermore, the results highlight that the fine-tuning of Windkessel resistances is not necessary to simulate the post-stenting scenario.

The possibility to obtain in-vitro evaluation of hemodynamic flows and pressures is of capital importance in understanding cardiovascular pathologies and validating new devices. The in-vitro approach brings different advantage including the possibility to evaluate fluid dynamic parameters non invasively and to support in-silico simulations.

In the present paper the effects of the spatial distribution of the inlet velocity in numerical simulations of the thoracic aorta have been investigated. First, the results obtained by considering in-vivo measured inlet velocity distribution are compared with the ones obtained for a simulation having the same flow rate waveform and plug flow condition at the inlet section. The results of the two simulations are consistent in terms of flow rate waveform, but differences are present in the pressure range and in the wall shear stresses, especially in the foremost part of the ascending aorta.

A sensitivity analysis of highly-resolved large-eddy simulations of the flow around a 5:1 rectangular cylinder to the introduction of a small rounding of the upstream edges is presented. Different values of the edge radius of curvature are considered, in a range such that they might reasonably be ascribed to manufacturing tolerances. A stochastic approach is adopted in order to build response curves of the quantities of interest as a function of the radius of curvature. The considered computational set-up, characterized by a fine numerical resolution and a low subgrid-scale (SGS) dissipation, predicts for the body having perfectly sharp edges a short mean recirculation length on the cylinder side, in disagreement with experimental data. On the other hand, even for the smallest considered radius of curvature, the length of the mean recirculation region increases significantly and, hence, the agreement with the experimental data is much improved. It is observed that the sharp edge introduces a higher level of turbulent fluctuations in the shear-layer at separation, which, if not artificially damped by numerical or SGS dissipation, grows faster and leads to a further upstream roll-up of the shear-layers and, hence, to a shorter mean recirculation region than in simulations with rounded edges.

The effect of flow regimes on the reaction yield of a chemical reaction occurring in a T-shaped micro-reactor is investigated both experimentally and numerically by spanning different Reynolds and Damkohler numbers. The experimental set-up is arranged in order to get in-line information on both fluid dynamics and reaction progress along the mixing channel. Numerical simulations are performed using mesh-adaption cycles to well resolve the mixing between reactants; in this manner, the agreement between experiments and numerical simulations is very satisfactory. For the considered reaction involving two reagents different from water, despite flow regimes are coherent with the ones observed in case of water mixing, significant differences arise because stratification occurs due to the different density of the fluid streams. The dependency of reaction yield on both Reynolds and Damkohler numbers is strongly affected by the flow regimes. In the stratified regime, the reaction yield decreases with the Reynolds number, because the reduced residence time hampers the reaction progress, while in the engulfment regime, the Reynolds number has a positive effect on the yield, thanks to the enhanced mixing.

The delay of boundary layer separation over curved solid surfaces is of great importance in many engineering applications. The proposed idea is to introduce small and suitably shaped grooves transverse to the flow (i.e., contoured transverse grooves) to passively generate local steady flow recirculations.

In this work, the numerical simulation of the blood flow inside a patient specific aorta in presence of an aneurysm is considered. A systematic sensitivity analysis of numerical predictions to the shape of the inlet flow rate waveform is carried out. In particular, two parameters are selected to describe the inlet waveform: the stroke volume and the period of the cardiac cycle. In order to limit the number of hemodynamic simulations required, we used a stochastic method based on the generalized polynomial chaos (gPC) approach, in which the selected parameters are considered as random variables with a given probability distribution. The uncertainty is propagated through the numerical model and a continuous response surface of the output quantities of interest in the parameter space can be recovered through a “surrogate” model. For both selected uncertain parameters, we first assumed uniform Probability Density Functions (PDFs) on a given variation range, and then we used clinical data to construct more accurate beta PDFs. In all cases, the two input parameters appeared to have a significant influence on wall shear stresses, confirming the need of using patient-specific inlet conditions.

One of the simplest geometries for micro-mixers has a T-shape, i.e., the two inlets join perpendicularly the mixing channel. The cross-sections of the channels are usually square/rectangular, as straight walls facilitate experimental and modeling analysis. On the contrary, this work investigates through Computational Fluid Dynamics the effect of a cross-section with lateral walls inclined of an angle α as such an inclination may stem from different microfabrication techniques. Considering water as operating fluid, the same mixing performance as square/rectangular cross-sections is obtained for inclinations α ≤ 3 °; this indicates the maximum admissible error on the perpendicularity of the walls in the manufacturing process. Above this value, the presence of inclined walls delays the onset of the engulfment regime at higher Reynolds numbers, and for α ≥ 23 °the mixing is hampered dramatically, as the flow is unable to break the mirror symmetry and enter in the engulfment regime. At low Reynolds numbers, the mixing is moderately improved for α ≥ 10 °, because the vortex regime presents a lower degree of symmetry than that of T-mixers with straight walls.

Despite the very simple geometry, T-shaped micro-mixers are characterized by different flow regimes when operated for liquid mixing. In this work, experiments and direct numerical simulations are employed jointly to investigate such flow regimes. First, a novel methodology is proposed to provide a quantitative comparison with numerical predictions starting from simple flow visualization experiments that use non-collimated light. This method is applied and validated in the steady flow regimes. Moreover, flow visualizations provide for the first time an experimental support to the flow dynamics scenarios for the periodic unsteady flow regimes previously proposed in the literature on the basis of numerical simulations. In particular, at a Reynolds number, based on the bulk velocity and on the hydraulic diameter of the mixing channel, of approximately 225, the flow passes from a steady engulfment regime to an unsteady asymmetric regime, which is characterized by a periodic dynamics of the three-dimensional structures present in the mixer. In this regime, the flow in the mixing channel always remains asymmetric promoting, thus, the mixing between the two streams. By further increasing the Reynolds number, another change in flow dynamics is observed. In the new regime, called periodic symmetric regime, the flow always maintains a double mirror symmetry in the mixing channel and this leads to a dramatic decrease of mixing. The flow unsteadiness is mainly due to a periodic shift of the vortical structures in the top part of the mixer along the direction parallel to the incoming flow.

The fluctuations of the base pressure and of the near-wake velocity are experimentally characterized for a constant-diameter axisymmetric blunt-based body with different boundary-layer thicknesses and for boat-tailed bodies with various extents of flow separation. The tests are carried out at R e = u ∞ D / ν = 9.6 × 10 4 . The analysis is performed through time-frequency techniques based on the wavelet and Hilbert transforms. Two frequencies are detected: a higher frequency, which corresponds to the shedding of hairpin vortices, and a lower one, associated with the pulsation of the recirculation region. The shedding occurs with a preferential vertical symmetry plane due to the presence of the support and the corresponding frequency becomes dominant downstream of the recirculation region and over the base contour. The lower frequency is present in the pressure signals from the central region of the base and in the adjacent velocity field. The two icdominant frequencies decrease with increasing boundary-layer thickness and significantly increase with decreasing boat-tail diameter ratio. Despite the differences in geometry and flow conditions, the lower frequency is always found equal to 1 / 3 of the higher one. Furthermore, a Strouhal number based on the wake width and the velocity defect at a suitable cross section downstream of the recirculation region remains practically constant.

We describe the results of a numerical and experimental investigation aimed at assessing the performance of a control method to delay boundary layer separation consisting of the introduction on the surface of contoured transverse grooves, i.e. of small cavities with an appropriate shape orientated transverse to the incoming flow. The shape of the grooves and their depth – which must be significantly smaller than the thickness of the incoming boundary layer – are chosen so that the flow recirculations present within the grooves are steady and stable. This passive control strategy is applied to an axisymmetric bluff body with various rear boat tails, which are characterized by different degrees of flow separation. Variational multiscale large eddy simulations and wind tunnel tests are carried out. The Reynolds number, for both experiments and simulations, is $Re=u_{\infty }D/\unicode[STIX]{x1D708}=9.6\times 10^{4}$; due to the different incoming flow turbulence level, the boundary layer conditions before the boat tails are fully developed turbulent in the experiments and transitional in the simulations. In all cases, the introduction of one single axisymmetric groove in the lateral surface of the boat tails produces significant delay of the boundary layer separation, with consequent reduction of the pressure drag. Nonetheless, the wake dynamical structure remains qualitatively similar to the one typical of a blunt-based axisymmetric body, with quantitative variations that are consistent with the reduction in wake width caused by boat tailing and by the grooves. A few supplementary simulations show that the effect of the grooves is also robust to the variation of the geometrical parameters defining their shape. All the obtained data support the interpretation that the relaxation of the no-slip boundary condition for the flow surrounding the recirculation regions, with an appreciable velocity along their borders, is the physical mechanism responsible for the effectiveness of the present separation-control method.

The results of several investigations aimed at assessing the performance of contoured transverse grooves as a method to delay flow separation are described. The physical mechanism at the basis of this passive technique is the local relaxation of the no-slip boundary condition, with a consequent reduction of the viscous losses and an increase of the downstream near-wall momentum. Numerical simulations and experiments showed that the application of one groove transverse to the flow direction may delay boundary layer separation both in laminar and turbulent conditions. As a consequence, significant increases of the pressure recovery in plane diffusers and decreases of the drag of boat-tailed axisymmetric and two-dimensional bluff bodies were obtained. It is shown that robust configurations may be devised provided the shape and dimension of the grooves are suitably chosen in order to assure the formation of steady and stable local flow recirculations.

The present work describes a strategy for the aerodynamic drag reduction of elongated axisymmetric bluff bodies, which can be viewed as simplified models of road vehicles. One well-known method to reduce the drag of this type of body is a geometrical modification denoted as boat-tailing, which consists in a gradual reduction of the body cross-section before a sharp-edged base (Maull et al, J R Aeronaut Soc 71, 854–858, 1967, [1], Mair, Aeronaut Q 20, 307–320, 1969, [2], Wong and Mair, J Wind Eng Ind Aerodyn, 12, 229–235, 1983, [3]). We combine herein boat-tailing with properly contoured transverse grooves to further delay boundary-layer separation and to reduce drag.

Two arrow-shaped micro-mixers, obtained from the classical T-shaped geometry by tilting downward the inlet channels, are considered herein. The two configurations, having different tilting angle values, have been chosen since they show significantly different flow topologies and mixing performances at low Reynolds numbers. In the present paper, we use both experimental flow visualizations and direct numerical simulations to shed light on the mixing behavior of the two configurations for larger Reynolds numbers, for which the mixers present unsteady periodic flows, although in laminar flow conditions. The tilting angle influences the flow dynamics also in the unsteady regimes and has a significant impact on mixing. The configuration characterized by the lower tilting angle, i.e., α = 10°, ensures a better global mixing performance than the one with the larger angle, i.e., α = 20°.

Pulsating Heat Pipes are two phase passive heat transfer devices characterized by a thermally induced two phase oscillating flow. The correct detection of the dominant frequencies of such oscillations is fundamental to fully characterize the device thermofluidic operation but the studies available in the literature are very heterogenous and results are often discordant. In this work, the concept of dominant frequency in Pulsating Heat Pipes is thoroughly discussed and defined analytically. The wavelet transform is used to characterize the fluid pressure signal in the frequency domain varying the heat power input at the evaporator and in the condenser zone of a full-scale Pulsating Heat Pipe tested in microgravity conditions. During the slug-plug flow regime, the dominant frequencies falls in the range 0.6–0.9 Hz, showing an increasing trend with the heat load input. The Cross-Correlation reveals that the two signals at the evaporator and at the condenser are very similar. Finally, the instantaneous angle of phase is calculated and lies between 310 and 360 deg. This value can be physically interpreted as a repeatable time shift between the two signals that can be used to evaluate the flow local mean velocity (0.09–0.13 m/s) constituting a valuable alternative to the visualization techniques.

The present work focuses on a passive strategy consisting in the introduction of properly contoured transverse grooves to delay the flow separation occurring on a boat-tailed bluff body before its sharp-edged base. We consider a two-dimensional body having a cross-section with a 3:1 elliptical forebody and a rectangular main part followed by a circular-arc boat tail. We carry out Variational Multiscale Large Eddy Simulations at Re = Du∞/v = 9.6 × 104. A boat-tail drag reduction of the order of 9.7% is produced by the significant delay of the flow separation caused by the groove and by the consequent increase of the base pressure. This effect is mainly due to the relaxation of the no-slip condition over the small and steady recirculation region inside the groove, which reduces the momentum losses near the wall and thus delays boundary layer separation. The flow control device is also robust to small variations of the groove location and depth.Copyright © 2018 by ASME

Computational fluid dynamics (CFD) and 4D-flow magnetic resonance imaging (MRI) are synergically used for the simulation and the analysis of the flow in a patient-specific geometry of a healthy thoracic aorta. CFD simulations are carried out through the open-source code SimVascular. The MRI data are used, first, to provide patient-specific boundary conditions. In particular, the experimentally acquired flow rate waveform is imposed at the inlet, while at the outlets the RCR parameters of the Windkessel model are tuned in order to match the experimentally measured fractions of flow rate exiting each domain outlet during an entire cardiac cycle. Then, the MRI data are used to validate the results of the hemodynamic simulations. As expected, with a rigid-wall model the computed flow rate waveforms at the outlets do not show the time lag respect to the inlet waveform conversely found in MRI data. We therefore evaluate the effect of wall compliance by using a linear elastic model with homogeneous and isotropic properties and changing the value of the Young’s modulus. A stochastic analysis based on the polynomial chaos approach is adopted, which allows continuous response surfaces to be obtained in the parameter space starting from a few deterministic simulations. The flow rate waveform can be accurately reproduced by the compliant simulations in the ascending aorta; on the other hand, in the aortic arch and in the descending aorta, the experimental time delay can be matched with low values of the Young’s modulus, close to the average value estimated from experiments. However, by decreasing the Young’s modulus the underestimation of the peak flow rate becomes more significant. As for the velocity maps, we found a generally good qualitative agreement of simulations with MRI data. The main difference is that the simulations overestimate the extent of reverse flow regions or predict reverse flow when it is absent in the experimental data. Finally, a significant sensitivity to wall compliance of instantaneous shear stresses during large part of the cardiac cycle period is observed; the variability of the time-averaged wall shear stresses remains however very low. In summary, a successful integration of hemodynamic simulations and of MRI data for a patient-specific simulation has been shown. The wall compliance seems to have a significant impact on the numerical predictions; a larger wall elasticity generally improves the agreement with experimental data.

Background: Flow stimuli in the natural world are varied and contain a wide variety of directional information. Nature has developed morphological polarity and bidirectional arrangements for flow sensing to filter the incoming stimuli. Inspired by the neuromasts found in the lateral line of fish, we present a novel flow sensor design based on two curved cantilevers with bending orientation antiparallel to each other. Antiparallel cantilever pairs were designed, fabricated and compared to a single cantilever based hair sensor in terms of sensitivity to temperature changes and their response to changes in relative air flow direction.Results: In bidirectional air flow, antiparallel cantilever pairs exhibit an axially symmetrical sensitivity between 40 μV/(m s−1) for the lower air flow velocity range (between ±10–20 m s−1) and 80 μV/(m s−1) for a higher air flow velocity range (between ±20–32 m s−1). The antiparallel cantilever design improves directional sensitivity and provides a sinusoidal response to flow angle. In forward flow, the single sensor reaches its saturation limitation, flattening at 67% of the ideal sinusoidal curve which is earlier than the antiparallel cantilevers at 75%. The antiparallel artificial hair sensor better compensates for temperature changes than the single sensor.Conclusion: This work demonstrated the successive improvement of the bidirectional sensitivity, that is, improved temperature compensation, decreased noise generation and symmetrical response behaviour. In the antiparallel configuration, one of the two cantilevers always extends out into the free stream flow, remaining sensitive to directional flow and preserving a sensitivity to further flow stimuli.

Despite the very simple geometry, T-shaped micro-mixers are characterized by different and complex laminar flow regimes. In the present work, experiments and direct numerical simulations are employed jointly to investigate unsteady periodic flow regimes, viz. the asymmetric and symmetric regimes. The first is characterized by a periodic dynamics of the three-dimensional structures and by a high degree of mixing, while in the second the flow always maintains a double mirror symmetry in the mixing channel, which causes a dramatic decrease of the mixing performance. A methodology, allowing us to quantitatively compare the numerical predictions with experimental flow visualizations, is used to investigate these unsteady regimes and to evaluate the relevant frequencies and the degree of mixing. In both regimes the characteristic non-dimensional frequency, based on the bulk velocity and hydraulic diameter of the mixing channel, increases with the Reynolds number, but a significant discontinuity is found at the transition from the first to the second regime.

The impact of outflow boundary conditions on the results of numerical simulations of the flow inside ascending thoracic aortic aneurysms is investigated. The adopted outflow conditions are based on the lumped three-element Windkessel model, in which it is assumed that the pressure at the outflow is related to the flow rate and to a terminal constant pressure, through an electric circuit analogue that has a proximal resistance in series with a parallel arrangement of a capacitance and of a distal resistance. The values of the Windkessel model parameters must be a-priori specified. The impact on the numerical simulation results of uncertainties in the values of the Windkessel model parameters is quantified here through a stochastic approach. The propagation of the considered uncertainties is evaluated, in particular, for the instantaneous and time-averaged wall shear stress. The generalized Polynomial Chaos is used as a surrogate model to obtain continuous response surfaces of the quantities of interest in the parameter space starting from a few deterministic simulations. A patient-specific geometry, obtained through in-vivo imaging, is considered. The analysis is carried out for both rigid and compliant vessel walls. Our results show that, although the uncertainties in the selected outflow parameters may give significant variability of the instantaneous shear stresses in regions characterized by flow recirculation or large streamline curvature, the impact on cycle-averaged shear stresses is moderate. Taking into account the wall compliance seems to reduce the impact of the uncertainties in the outflow parameters compared to the rigid case.

The variation of the base drag of an axisymmetric bluff body caused by modifications of the boundary-layer separating at the sharp-edged contour of its base is analysed through different numerical simulations, and the results are compared with those of a previous experimental investigation. Variational MultiScale Large-Eddy Simulations (VMS-LES) are first carried out on the same nominal geometry and at the same Reynolds number of the experiments. Subsequently, Direct Numerical Simulations (DNS) are performed at Reynolds numbers that are roughly two orders of magnitude lower, in order to investigate on the sensitivity of the main findings to the Reynolds number. The results of experiments, VMS-LES and DNS simulations show that an increase of the base pressure – and thus a decrease of the base drag – may be obtained by increasing the boundary layer thickness before separation, which causes a proportional increase of the length of the mean recirculation region behind the body. In spite of the different setups, Reynolds numbers and turbulence levels in the experiments and numerical simulations, in all cases the base pressure is found to be directly proportional to the length of the mean recirculation region, which is thus a key index of the base drag value. In turn, the recirculation length seems to be connected with the location of the incipient instability of the detaching shear layers, which can be moved downstream by an increase of the thickness of the separating boundary layer and upstream by an increase of the turbulence level.

A flow control strategy to improve the efficiency of plane diffusers, based on the introduction of local flow recirculations along the diffuser diverging walls, is investigated. Three reference diffuser configurations, all with laminar flow and with the same area ratio ( A R = 2 ) but with different divergence half-angles ( α = 2 ∘ , α = 3 . 5 ∘ and α = 5 ∘ ), are considered. The variation of the divergence angle produces different flow patterns: in the diffuser with α = 2 ∘ the flow is attached along the diverging walls, while the diffusers with α = 3 . 5 ∘ and α = 5 ∘ are characterized by asymmetric zones of separated flow with different extents. Local recirculations are obtained by means of properly contoured cavities, whose geometries are optimized in order to maximize the pressure recovery in the diffusers. The introduction of the optimal cavities leads to an increase in pressure recovery for all the considered configurations, even when no separation is present without the introduction of the cavities. The success of the control is due both to a virtual geometry modification of the diffuser and to a reduction of the momentum losses in the small recirculation regions inside the cavities. Classical shape optimizations of the diverging walls are also carried out by using Bezier curves. If an adequate number of degrees of freedom is used in the optimization, the optimal geometry corresponds to the presence of one or more localized recirculation regions along the diffuser optimized diverging walls, i.e. to a flow configuration that is very similar to the one characterizing the diffusers with optimized cavities. The efficiency increases provided by the shape optimization are comparable to those given by the contoured cavities.

A passive control strategy, which consists in introducing contoured cavities in solid walls, is applied to a plane asymmetric diffuser at a Reynolds number that implies fully-turbulent flow upstream of the diffuser divergent part. The analysed reference configuration, for which experimental and numerical data were available, is characterized by an area ratio of 4.7 and a divergence angle of 10°. A large zone of steady flow separation is present in the diffuser without the introduction of the control. One and two subsequent contoured cavities are introduced in the divergent wall of the diffuser and a numerical optimization procedure is carried out to obtain the cavity geometry that maximizes the pressure recovery in the diffuser and minimizes the flow separation extent. The introduction of one optimized cavity leads to an increase in pressure recovery of the order of 6.9% and to a significant reduction of the separation extent, and further improvement (9.6%) is obtained by introducing two subsequent cavities in the divergent wall. The most important geometrical parameters are also identified, and the robustness of the solution to small changes in their values and in the Reynolds number is assessed. The present results show that the proposed control strategy, previously tested in the laminar regime, is effective also for turbulent flows at higher Reynolds numbers. As already found for laminar flow, the success of the control is due both to a virtual geometry modification of the diffuser and to a favourable effect of the cavities in reducing the momentum losses near the wall.