High Subsonic Flow Research Articles

Turbulence Analysis From Long-Exposure-Time Acquisitions With Interferometric Rayleigh Scattering

This study focuses on the analysis of long-exposure-time interferograms obtained with an Interferometric Rayleigh Scattering (IRS) set-up pointing at a high subsonic isothermal jet flow. The objectives are twofold: retrieve from seeding-free optical measurements the mean characteristics of this flow - in particular the mean velocity profile in the jet shear-layer - and determine higher order statistics for the flow velocity in the shear layer. The results are compared to those obtained on the same test bench by use of hot wire anemometry, and conclusions about the relevancy of the approach together with potential improvements are deduced therefrom.

An averaged mass correction scheme for the simulation of high subsonic turbulent internal flows using a lattice Boltzmann method

This paper addresses the simulation of internal high-speed turbulent compressible flows using lattice Boltzmann method (LBM) when it is coupled with the immersed boundary method for non-body-fitted meshes. The focus is made here on the mass leakage issue. The recent LBM pressure-based algorithm [Farag et al. Phys. Fluids 32, 066106 (2020)] has shown its superiority on classical density-based algorithm to simulate high-speed compressible flows. Following our previous theoretical work on incompressible flows [Xu et al. Phys. Fluids 34, 065113 (2022)], we propose an averaged mass correction technique to mitigate mass leakage when simulating high-Mach-number compressible flows. It is adapted to deal here with a density, which is decoupled from the zero-moment definition. The simulations focus on two generic but canonical configurations of more complex industrial devices, the straight channel at different angles of inclination at Mach numbers (Ma) ranging from 0.2 to 0.8, and the National Aeronautics and Space Administration Glenn S-duct at Ma = 0.6. The present results show that mass leakage can be a critical issue for the accuracy of the solution and that the proposed correction technique effectively mitigates it and leads to significant improvements in the prediction of the solution.

Experimental and numerical flutter analysis of a folding fin with multiple asymmetric free-plays

Experimental folding fin models with an adjustable free-play are tested in a wind tunnel. The fin structure is modeled using the free-interface component mode synthesis method, and its free-play is modeled as four independent nonlinear springs with asymmetric stiffness. A nonplanar unsteady vortex-lattice method considering compressibility is employed to address nonlinear deformation and high subsonic flow. Surface spline interpolation is improved through projection and partition. The aeroelastic characteristics of folding fins with different free-play magnitudes, initial conditions and elastic-axis positions are analyzed using an established time-marching method because of its relatively small computation scale and high precision. The results show good consistency among the presented method, the wind tunnel test and the harmonic balance method. There is a negative correlation between the critical speed of divergent motion and the ratio of the initial condition to the free-play magnitude. If either the free-play magnitude or the initial condition is extreme (tiny or vast), the system nonlinearity degenerates to linearity. Generally, the flutter prevention design of a linear model can be applied to a nonlinear model, such as moving the elastic-axis position aftward. The presented fin configuration exhibits an unstable limit cycle oscillation because the orders of coupled flutter modes do not change with variations in equivalent linear stiffness.

Experimental investigation of subsonic and transonic flows through a linear turbine cascade

Experimental investigations of high subsonic and transonic flows through a linear cascade consists of VKI LS-59 blades have been carried out. Emphasis was put on analysis of the flow characteristics at the cascade exit. Experiments reported the static pressure measurements on blade surface, flow field survey downstream the cascade by means of Particle Image Velocimetry (PIV) and visualization by means of the Schlieren method. The turbulent intensity measurements have been performed at the cascade inlet to provide inlet conditions for numerical studies. The measurements have been carried out at two isentropic Mach numbers: M2,is=0.777 (high subsonic) and M2,is=0.975 (transonic). The corresponding Reynolds numbers based on the chord length and isentropic downstream velocity were equal to Re2,is=6.21⋅105 and Re2,is=6.82⋅105, respectively. The numerical simulations with Transition SST model have been performed to complement experiments and to verify applicability of data for testing and development of turbulence models. The RANS-based transition model reproduced correct isentropic Mach number profiles on the blade surface but underpredicted the turbulent mixing in the wake flow.

High-subsonic boundary-layer flows of an organic vapour

Boundary layers of Novec649, a low-global-warming potential fluid of interest for low-grade heat recovery, are investigated numerically by means of linear stability theory, direct numerical simulation (DNS) and large-eddy simulations (LES). This organic vapour is of interest in organic Rankine cycle (ORC) turbines and realistic thermodynamic conditions are selected. Under these conditions, the vapour behaves as a dense gas and, due to its high molecular complexity, real-gas effects occur. In addition, the fluid exhibits large and highly variable heat capacities and density- as well as temperature-dependent transport properties. More specifically we report the first direct and LES of transitional and turbulent boundary layers of Novec649 at high-subsonic conditions$M=0.9$. A controlled transition is performed by using oblique modes determined by linear stability theory extended to dense gases. An oblique-type transition is obtained as in low-speed air flows, where sinuous streaks develop by the lift-up mechanism and break down into turbulence. In the turbulent state, the profiles of dynamic flow properties (velocities, turbulent intensities, turbulent kinetic energy budgets) are little affected by the gas properties and remain very close to incompressible DNS, despite the high-subsonic flow speed. The fluctuations levels for thermodynamic properties have been quantified with respect to air flows. Notwithstanding a drastic reduction, genuine compressibility effects are present. For example, the fluctuating Mach number and the acoustic mode are characteristic of high-speed flows. The influence of forcing frequency and amplitude on the established turbulent state has been investigated using LES. An analysis of integral quantities shows a slow relaxation towards a canonical equilibrium turbulent state for all cases due to the high Reynolds numbers typical of dense gas flows. Overall the present DNS constitutes a valuable reference not only for forthcoming experiments but also for future studies of free-stream transition and loss mechanisms in ORC turbines.

Hybrid Lattice Boltzmann Approach for Simulation of High-Speed Flows

A lattice Boltzmann approach utilizing hybrid lattices for accurate simulation of high-speed transonic (TS) flows is presented in this study. Single-layer lattice models, such as D3Q19, are computationally efficient with low numerical dissipation, but are limited to simulate weakly compressible flows within subsonic Mach regime. Multilayer lattice models, such as D3Q33, are capable of simulating thermal and high-speed compressible flows in TS Mach regime, but suffer from a higher numerical dissipation and higher computational cost. To take full advantages of these two types of lattices, a simulation domain is divided into two regions, high-subsonic (HS) flow regions to be solved by single-layer lattices and TS flow regions to be solved by multilayer lattices. The interface connecting the HS and TS flow regions are represented by double-sided surfels to realize smooth flow transition and ensure satisfaction of fundamental conservation laws. The hybrid lattice Boltzmann approach is stable and robust, and it allows multiple HS/TS flow regions with arbitrary shapes. Extensive validation studies demonstrate that the proposed approach is capable of obtaining accurate aerodynamic prediction of flows with a wide range of Mach numbers, and preserving the low numerical dissipation advantage of single-layer lattices in aeroacoustic computations.

Hot-Wire Anemometry in High Subsonic Organic Vapor Flows

AbstractThe present contribution reports the outcome of an experimental and numerical investigation of the behavior of a constant-temperature hot-wire anemometer in the high-subsonic flow up to a Mach number of 0.7 of the organic vapor Novec™ 649 at pressure and temperature levels of typical organic Rankine cycle (ORC) turbine applications. The experiments were carried out in the calibration section of a closed-loop organic vapor wind tunnel test facility enabling the independent variation of Reynolds numbers, Mach numbers, and total temperature within a certain range. It was found that the calibration and the determination of the sensitivity coefficients can be done in a way as proposed by de Souza and Tavoularis for air. The sensitivity coefficients for velocity and density were essentially equal for higher overheat ratios which support the interpretation of the signals and data reduction for turbulent quantities. Computational fluid dynamics (CFD) provided additional insight into potential real gas effects for hot-wire performance.

Improved hybrid model for transitional separated flows over a rough compressor blade

It is known that boundary layer transition and turbulent separation flow after transition can be influenced significantly by surface roughness. Since the traditional hybrid RANS/LES method cannot predict the boundary layer transition, and the RANS-based transition model cannot accurately simulate the massively separated flow, the present study sought to build an effective modeling strategy for the laminar, roughness-induced-transition and attached turbulence/massively separated flows that couples the Very-Large-Eddy-Simulation (VLES) model and a transition model considering roughness effects. Because the hybrid function between the transition model and VLES model constructed in the previous version still has some flaws, there is still a large laminar flow indication area in the turbulent flow region, which will reduce the simulation accuracy of the turbulent flow after transition. In this paper, the hybrid function is improved, which is examined in the simulation of the high-subsonic flow around the V103 compressor blade. Our analysis shows that the new hybrid model operates in the flow around the V103 compressor blade. The predictions of laminar separation bubble-induced transition, separation evolution, and reattachment are in accord with measurements and the LES results over both smooth and rough surfaces, indicating that the present transitional VLES method can achieve the simulation accuracy close to the LES method while reducing the computational cost.

Noise control for high subsonic jet flows by inner wall treatment

Subsonic jet nozzles, commonly used in passenger aircrafts, generate significant noise that travels both downstream and upstream due to large-scale or fine-scale turbulence in the jet plume. To reduce jet noise, a novel wall treatment method, termed the wavy inner wall (WIW), is proposed. With this method, the smooth inner wall near the exit of the nozzle is replaced by treated walls that carry small wavy patterns. Numerical simulations were conducted to investigate the effects of the WIW treatment. Large eddy simulations (LES) were used to predict the unsteady flow field and the far-field noise, followed by the analogy method proposed by Ffowcs Williams and Hawkings. To better understand the mechanism behind the noise reduction achieved by the WIW treatment, the shear-layer instability, radial and azimuthal auto-correlation functions, turbulent kinetic energy, and acoustic source term from the Tam-Auriault (TA) jet-noise model were analyzed. Results indicated that the WIW treatment advances the onset of jet flow instability in the shear-layer, leading to the early breakdown of jet shear-layer and production of different scales of downstream turbulent structures. As a result, the distribution and production of turbulent kinetic energy are affected, and the generation and emission of jet noise are controlled. The WIW treatment enables the control of fine scale turbulence, resulting in the reduction of mid- to high-frequency noise in the far field, while ensuring a low thrust loss. This feature makes the WIW method a promising approach for jet noise control.

Flow Characteristics and Parameter Influence of the Under-Expansion Jet on Circulation Control Airfoil

The enhancement in the jet pressure ratio and jet velocity contributes to expanding the control efficiency and control boundary of circulation control airfoil under high subsonic incoming flow. However, because of an excessive jet pressure ratio, the jet separates prematurely on the Coanda surface, resulting in control failure. In a bid to improve the adhesion capability of the jet under a high pressure ratio, a circulation control airfoil with a converging nozzle and back-facing step structure at the trailing edge was numerically simulated based on the Reynolds averaged Navier−Stokes equation (RANS), and a study was conducted on the complex flow structure of the under-expansion jet on the Coanda surface and the impact of design parameters such as jet pressure ratio, ellipticity, and nozzle height on the jet separation. The results show that the back-facing step provides an expansion space for the under-expansion jet and changes the shock-boundary layer interaction form. As the jet pressure ratio and nozzle height increase, the size of the shock cell increases, the strength of the intercepting shocks on both sides increases, and Mach reflection occurs, resulting in jet stratification and in a decline in the adhesion capability of the jet. The combination design of proper ellipticity and the back-facing step contributes to forming a closed low-pressure vortex area behind the step and promote jet attachment. Reducing the nozzle height can improve the adhesion capability of the jet under a high pressure ratio.

Effective Distance for Vortex Generators in High Subsonic Flows

Vortex generators (VGs) are a passive method by which to alleviate boundary layer separation (BLS). The device-induced streamwise vortices propagate downstream. There is then lift-off from the surface and the vortex decays. The effectiveness of VGs depends on their geometrical configuration, spacing, and flow characteristics. In a high-speed flow regime, the VGs must be properly positioned upstream of the BLS region. Measurements using discrete pressure taps and pressure-sensitive paint (PSP) show that there is an increase in the upstream surface pressure and the downstream favorable pressure gradient. The effective distance for a flat plate in the presence of three VG configurations is determined, as is the height of the device (conventional and micro VGs).

Influence of strut on trapped vortex cavity flameholder: Ignition and flame propagation performances

In order to improve the performance of pilot flame propagation to the core flow, a novel strut design was proposed for the trapped vortex cavity flameholder. Experiments were conducted under high subsonic flow conditions to investigate the influences of strut on the ignition performance and flame propagation performance of trapped vortex cavity combustor. Furthermore, the flame development was acquired using a high-speed camera and the flame information and the flow field were combined to explicate the flame stabilization mechanisms of two flameholder layouts. The results showed that, both flameholder layouts achieved reliable ignition and flame stability under inlet flow velocity lower than Ma = 0.42. Cavity combined with strut weakened the ignition performance of the trapped vortex cavity-only flameholder based combustor, but improved the blowout performance and enhanced the flame stability. Compared with the cavity-only flameholder layout, the lean ignition equivalence ratio of the cavity-strut flameholder layout was increased of 14.6%, and the lean blowout equivalence ratio was decreased by 56.2%, under the inlet flow at 600 K. Strut changed the original flame propagation path of the cavity-only layout, and caused the flame stabilization mechanism change from cavity flame stabilization to strut flame stabilization. Although the change of flame stabilization mechanism prolongs the ignition delay time, it enhances the flame stability of the combustor. Moreover, the flame propagation ability to the radial direction behind the cavity combined with strut is enhanced, which is more suitable to the engineering combustor with higher requirements on circumferential flame propagation.

The Profile Loss of Additive Manufactured Blades for Organic Rankine Cycle Turbines

Results from an experimental profile loss study are presented of an additive manufactured linear turbine cascade placed in the test section of a closed-loop organic vapor wind tunnel. This test facility at Muenster University of Applied Sciences allows the investigation of high subsonic and transonic organic vapor flows under ORC turbine flow conditions at elevated pressure and temperature levels. An airfoil from the open literature was chosen for the cascade, and the organic vapor was Novec 649TM. Pitot probes measured the flow field upstream and downstream of the cascade. The inflow turbulence level was 0.5%. The roughness parameters of the metal-printed blades were determined, and the first set of flow measurements was performed. Then, the blade surfaces were further finished, and the impact of roughness on profile losses was assessed in the second flow measurement set. Although the Reynolds number level was relatively high, further surface treatment reduces the profile loss noticeably in organic vapor flows through the printed cascade.

On boundary-layer receptivity to entropy waves

In this paper, we consider the generation of the Tollmien–Schlichting waves in the boundary layer on the surface of a wing exposed to entropy waves. It is well known that the free-stream turbulence is composed of two perturbation modes: the vorticity waves and the entropy waves. The receptivity of the boundary layer to the vorticity waves has been studied extensively by various authors. The entropy waves have not attracted such attention. We show that, in high speed subsonic flows, the entropy waves are as important for the receptivity as the vorticity waves. Methodologically, our work relies on the asymptotic analysis of the Navier–Stokes equations at large values of the Reynolds number, which results in the formulation of a suitably modified triple-deck theory. The entropy waves produce oscillations of the gas temperature and density, but the velocity and the pressure remain unperturbed to the leading order. This precludes the entropy waves from penetrating the boundary layer, as happens, for example, with the acoustic waves. Our analysis reveals that the entropy waves decay rapidly in the transition layer that forms near the outer edge of the boundary layer. We find that an entropy wave alone cannot generate the Tollmien–Schlichting waves. However, when the boundary layer encounters a wall roughness, the flow near the roughness appears to be perturbed not only inside the boundary layer but also in the inviscid region outside the boundary layer. The latter comes into the interaction with the density perturbations in the entropy wave. As a result, a localised ‘forcing’ is created that produces the Tollmien–Schlichting waves. In this paper we present the results of a linear and nonlinear receptivity analysis. We find that the nonlinearity enhances the receptivity significantly, especially when a local separation region forms on the roughness.

Flow structure of the ridge integrated submerged inlet

Control of the boundary layer upstream and in the region of the engine inlet is an important consideration for airplane designers. One important aspect of this problem can be seen in the performance analysis of submerged inlets for which the design is based on boundary layer ingestion. With a large offset between the flush entrance and the compressor face, these low drag concepts are suitable for stealth applications, but when the wetted area upstream of the inlet is long, the ingestion of thickened boundary layer results in flow separation at the bends, pressure recovery losses and finally reduction of thrust. In this paper, a ridge surface is used as a passive integration solution to prevent the upstream boundary layer from entering into the inlet. To investigate the new integration method, a submerged inlet with a triangular entrance is designed and its flow pattern is simulated numerically by ANSYS Fluent solver with and without the ridge configuration. The vortex structures, breakdowns, boundary layer transition and efficiency factors are determined by second order accuracy and use of two different structural mesh domains and flow solvers. Results indicate that the ridge surface improves the inlet efficiency significantly from low to high-subsonic flow regimes and a variety of sideslip angles.

Effect of leakage flow on the endwall cooling performance in high subsonic turbine cascade

Gas Turbine is widely used in many fields, for the purpose of achieving higher efficiency, the main flow temperature needs to be improved. In the combustion chamber, the flame profile is designed to be a flat pattern distribution to reduce NOx, as well as to decrease thermal load. Thermal protection for the hot-end components has become an important issue.Within the present study, experimental and numerical methods are used to investigate the effects of leakage flow on cascade endwall cooling performance for high subsonic flow condition. Test variable including blowing ratio and upstream slot inclination angle. Blowing ratio(M) ranged from 0.3 to 2, upstream slot inclination angles are 0°, 45°,60°.The results stated that when the blowing ratio increases, the magnitudes of adiabatic cooling effectiveness increases, but the growing slope decreases. It is showed that the optimal performance is observed at a test condition of blowing ratio around 1.7. Under low blowing ratio (0.3–0.6) condition, the adiabatic cooling effectiveness of suction side is significantly better than the pressure side. As the blowing ratio increases, the coverage of coolant on endwall becomes more uniform, and the endawall thermal loads decreases.When compared at a particular value of blowing ratio, a higher adiabatic cooling effectiveness zone approaches to the suction side with a 45° slot inclination angle compared to 30 and 60° inclination angle. Changing the upper stream slot inclination angle, the local distribution of adiabatic cooling effectiveness is varied, but the spanwise averaged values of η do not show discernible variation.

Aeroacoustic Coupling Effect During the Ascent of Space Transportation Systems

After the failure of maiden flight 157 of Ariane 5 Evolution Cryotechnique type A (ECA), the inquiry board responsible for the investigation reported as one of the probable causes the “non-exhaustive definition of the loads to which the Vulcain 2 engine is subjected during flight.” As a result, many research activities were set up with the objective to isolate the driving mechanisms. Driving mechanisms such as pumping, flapping, or swinging were found to excite oscillations in the base region. Despite this finding, the question of why the pressure fluctuations and base flow excitations are especially pronounced in the subsonic to transonic flow regime remained unclear. This question is addressed in the study at hand. Results from the literature are combined with recent wind-tunnel measurements on the base model of a space transportation system. The measurement data were acquired by means of particle image velocimetry, pressure transducers, and high-speed schlieren imaging. The results suggest that an aeroacoustic coupling takes place between a jet noise generation mechanism called screeching and the near-wake dynamics. Due to the nature of both effects, they are likely to reach a resonating frequency during the ascent. The resonant frequency appears to increase unfavorable fluctuations in the base region, which are held responsible for the amplified unsteady loads. A schematic model concept is proposed, which describes the underlying governing mechanism. It provides an explanation for question why buffeting effects are especially amplified in the high subsonic ambient flow regime.

Computational evaluation of geometric effects on aerodynamic performance of circulation control airfoils

The geometric effects of Coanda trailing edges on the aerodynamic performance of an airfoil are numerically evaluated for a range of different freestream Mach numbers and momentum coefficients. A Circulation control (CC) airfoil with a circular trailing edge (ACTE) proves to have better control effectiveness at low subsonic freestream speeds (Mach = 0.1). A CC airfoil having an elliptic trailing edge (AETE) outperforms the ACTE at high subsonic flow conditions. The occurrence of C μ-stall for the AETE is greatly postponed, and meanwhile the maximum net lift coefficient increment achieved for the AETE (Δ C L = 0.51) is slightly higher than that of the ACTE (Δ C L = 0.50) at Mach 0.6. Compared to the ACTE, the AETE is found to have better control consistency at different operating velocities and better control stability when the Coanda jet is supersonic. Through careful consideration of the aerodynamic performance and the control effects, the most appropriate axial ratio for an AETE ellipse is within the interval from 1.5 to 2. Finally, the flow field instability phenomenon and the jet detachment induced by the supersonic Coanda jet are investigated. A self-sustained shock-wave instability phenomenon without jet detachment is first observed in this paper.

High subsonic flow of an organic vapor past a circular cylinder

A circular cylinder was tested in the cross-flow of an organic vapor (Novec™ 649) and of air over the subsonic (M < 0.4) and high subsonic (0.4 < M < 0.8) speed range in a continuously running pressurized closed-loop wind tunnel test facility. Time-averaged pressure measurements gave information on surface pressure distributions, and the corresponding drag and base pressure drag coefficients were obtained. Due to the charging of the wind tunnel, different values of the compressibility factor (0.876 < Z < 0.999) could be achieved for the organic vapor flow. This enabled in combination with the results for air an assessment of the impact of non-ideal gas dynamics on the form drag of a cylinder in the considered highly subsonic flow regime. The new experimental data were compared with available literature results. Changes in surface pressure distribution at higher subsonic velocities were identified and discussed. It was found that non-ideal gas effects did not strongly affect the overall drag. The variation of drag coefficient over the Mach number range was comparable with literature data for ideal-gas compressible flow, including shock-less and intermittent shock wave, and permanent shock wave flows regimes. At Mach 0.4, the flow of Novec™ 649 was in the shock-less regime and exhibited a pronounced dependency on the Reynolds number. An increase in drag was observed at Mach 0.6 which was attributed to the commencement of vortex shedding. Non-ideal thermodynamics only affected the flow locally and a reduction of the critical pressure coefficient in the high subsonic flow regime was observed in the surface pressure distribution. However, this mechanism did not alter significantly the overall drag behavior.Graphic abstractDrag coefficient CD against Re for several Mach numbers M and comparison with available literature results obtained for air (colored symbols indicate different Mach number clusters)

Effect of air-assistant on ignition and flame-holding characteristics in a cavity-strut based combustor

For promoting the expansion of pilot flame in high-subsonic flows, radial flame-holders are commonly introduced to combine with the pilot flame-holder, which may inversely bring about issues to the pilot ignition and flame stability. To simultaneously obtain satisfactory ignition and flame-propagation performance, a novel air-assistant design is proposed for the cavity-strut based combustor. Furthermore, comparison experiments are conducted under ambient pressure to investigate the influence of air-assistant on combustor characteristics. The discrepancies are experimentally explored in terms of lean ignition, blowout, and combustion characteristics in three cavity-based combustors accompanied with numerical flow fields. The results indicate that the air-assistant design can widen the ignitable space for ignition, extend the flammable Mach number, and decrease the lean ignition and blowout fuel/air ratio by 18.6% and 17.9% respectively. The air-assistant method can also accelerate the kernel generation and flamelet growth and suppress the flame quenching in the cavity-strut structure, which enhances the flame stability in either strut-stabilizing or cavity-stabilizing mechanism. The introduction of air-assistant regulates the flame structure and eventually increases the temperature rise of combustor by 250–350 K within the full range of operating fuel/air ratio. Moreover, the temperature distribution altered by air-assisting is more suitable for engineering combustors with higher requirements on circumferential flame propagation.