Low-speed Separation Research Articles

Impact of non-axisymmetric tip clearance on the aerodynamic and aeroelastic stabilities of a transonic compressor

The aerodynamic and flutter effects caused by the circumferential non-uniform tip clearances from the non-axisymmetric casing geometry, which are more representative of real operational conditions, should receive more attention. In this paper, sensitivity analyses on clearance and non-uniformity regarding rotor performance are conducted. The isentropic efficiency, total pressure ratio and stability margin of the rotor are reduced by 1.96%, 1.06% and 6.76% for an increase of tip clearance by 1% tip chord, respectively. The non-axisymmetric configuration reconfigures the interaction between leakage flow and mainstream in each rotor passage and alters the strength and influence range of the low-speed separation region generated by shock wave. The phase lag phenomenon observed in the aerodynamics of the non-axisymmetric layout also manifests in the aeroelastic effects but requires a larger phase shift at the maximum clearance sector, owing to the longer timescales for flow adjustment relative to the local tip gap considering the flow-induced vibration. Clearance sensitivity on flutter stability varies with the direction of the traveling wave: under forward traveling conditions, damping values initially decrease and then increase with increasing rotor tip clearance; while under backward traveling condition, blade damping linearly decreases with increasing rotor tip clearance. Additionally, variations in flow conditions and nodal diameters affect flutter stability by altering the position of shock waves and their work on the suction surface of the rotors. The mechanism responsible for unsteady flows induced by non-axisymmetric clearances from the casing ovalization on flutter stability is discussed for the first time in this paper, which can guide the industry to evaluate its impact.

Simulation of flutter suppression for a transonic fan blade based on plasma excitation

Along with the development of advanced high-performance aero-engines to the higher thrust-weight ratio, further improvement of stage load, the adoption of new materials and new lightweight structures, the aeroelasticity of blade structure is becoming more and more prominent. The high cycle fatigue failure of blades significantly reduces the structural reliability during the process of development and using. At the same time, a large number of failure forms of aero-engine experimental and server can be attributed to aeroelastic problems. Therefore, it is urgent to improve the aeroelastic stability of the blade. One of the most important factors is to suppress the airflow separation, but its mechanism is still unclear. Based on this, this paper combines the aerodynamic damping analysis of energy method with the plasma excitation simulation and references low-speed wind tunnel plasma expansion test to consider the effects of different exciter distributions and intensities on flutter. The results show that stall flutter is related to the flow separation, but the flow separation is not a key factor that determinates whether the flutters occurs or not. Flutter suppression is strongly correlated with the shock wave intensity, amplitude of first harmonic aerodynamic force, low-speed separation and aerodynamic work density. In addition, the relative distribution of the excitation field and the positive work zone also has a direct effect on the suppression of flutter.

Reynolds-Averaged Numerical Simulations of Conical Shock-Wave/Boundary-Layer Interactions

We carry out a parametric study of conical shock-wave/turbulent boundary-layer interactions by means of numerical simulation of the Reynolds-averaged Navier–Stokes (RANS) equations, with the eventual goal of establishing the predictive capabilities of standard turbulence models. Preliminary assessment of several linear-eddy-viscosity models for the case of planar interactions shows that the model and its variants as well as the Spalart–Allmaras model yield accurate prediction of the typical interaction length scale. Numerical simulations of conical interactions at high Reynolds numbers under attached and separated flow conditions generally support good capability of RANS to predict the gross flow features, including conditions of incipiently and fully separated flow. Comparison with direct numerical simulation data at low Reynolds numbers suggests that caution should be made because certain turbulence models may yield unrealistic incoming velocity profiles. With this caveat, we again find that RANS models yield satisfactory prediction of the three-dimensional flow organization and associated length scales. The -wave mean wall pressure signature is quantitatively predicted, and accurate information about the fluctuating pressure variance can be obtained from RANS data in a postprocessing stage by extension of correlations developed for low-speed separation bubbles.

Quasi 3D Nacelle Design to Simulate Crosswind Flows: Merits and Challenges

This paper studies the computational modelling of the flow separation over the engine nacelle lips under the off-design condition of significant crosswind. A numerical framework is set up to reproduce the general flow characteristics under crosswinds with increasing engine mass flow rate, which include: low-speed separation, attached flow and high speed shock-induced separation. A quasi-3D (Q3D) duct extraction method from the full 3D (F3D) simulations has been developed. Results obtained from the Q3D simulations are shown to largely reproduce the trends observed (isentropic Mach number variations and high-speed separation behaviour) in the 3D intake, substantially reducing the simulation time by a factor of 50. The agreement between the F3D and Q3D simulations is encouraging when the flow either fully attached or with modest levels of separation but degrades when the flow fully detaches. Results are shown to deviate beyond this limit since the captured streamtube shape (and hence the corresponding Q3D duct shape) changes with the mass flow rate. Interestingly, the drooped intake investigated in the current study is prone to earlier separation under crosswinds when compared to an axisymmetric intake. Implications of these results on the industrial nacelle lip design are also discussed.

Separation Control for Aero Engine Intakes, Part 2: High-Speed Investigations

This paper documents the second part of an investigation into how fluidic control can be used to reduce separationinduced total pressure nonuniformity in an aeroengine intake. Flow conditions typical for an intake operating in a pure 90 deg crosswind are recreated in a laboratory-scale experiment. The lip rig concept, discussed in Part 1 (Wakelam, C. T., Hynes, T. P., Hodson, H. P., Evans, S. W., and Chanez, P., “Separation Control for Aeroengine Intakes Part 1: Low-Speed Investigation ofControl Strategies,” Journal of Propulsion andPower, Vol. 28,No. 4, 2012, pp. 758–765), has been developed for application in a high-speed study. The sector rig exhibits the same behaviors as a three-dimensional intake: low-speed separation hysteresis, attachedflowbucket atmoderate fan-faceMach numbers, and shock-induced separation. The flow control method of vortex generator jets is applied to reduce the negative effects of this shock-induced separation.Measurements of static pressure distribution show that separation is delayed to a higher fan-faceMach number when fluidic control is applied. Total pressure measurements in the fan-face plane show that distortion can be reduced over the full range of separated cases.A nonintrusivemethodof inferring the state of the flow in the intake, which could be used in a fully active control system, is also presented.

On the wake with and without vortex shedding suppression behind a two-dimensional square cylinder in proximity to a plane wall

A comparative study of the wake characteristics behind a two-dimensional square cylinder in proximity to a plane wall was made on two systems, i.e. G/ D=0.25 and 0.5, which corresponds to the wakes with and without suppression of the vortex shedding, respectively. Here, G is the gap distance and D is the width of the square cylinder. Synchronized measurements of wall-pressure fluctuations and velocity fluctuations were made using a microphone array and a split-fiber film, respectively. For the system G/ D=0.5, a regular Karman-like vortex shedding at fD/ U 0=0.13 was found by inspection of the streamwise velocity spectra. When the gap width is reduced to G/ D=0.25, a low-speed separation bubble was formed in the region 4.25< x/ D<9.25, while the intermittent existence of large-scale vortices at the reduced frequency fD/ U 0=0.116 was identified in the upper shear layer. The convective vortex dynamics were analyzed in terms of the cross-correlation of the wall-pressure fluctuation, coherence and cross-correlation of wall-pressure fluctuations with velocity fluctuations at various positions located at the lower edge of the cylinder ( y/ D= G/ D) and the outer edge of the upper shear layer. The convection velocities of the Karman-like vortices in the system G/ D=0.5 was determined to be U c / U 0=0.64. However, in the system G/ D=0.25 the convection velocities before and behind x/ D=3.25 were determined to be U c / U 0=0.32 and 0.64, respectively.

Laser streak velocimetry for two-dimensional flows in gases

Abstract : A new velocity measuring technique, laser streak velocimetry (LSV), was developed for two-dimensional flows in gases. A high power argon ion laser beam was formed into a thin sheet of light to illuminate a two-dimensional plane in the flow around a body. Ten micron sized particles were introduced into the flow, and photographs were taken of the particles as they traversed the light sheet. The length of the streak made by each particle on the photograph was a measure of its velocity since the camera shutter speed was known. The velocity profile in the boundary layer of a flat plate was measured and found to agree within 4% of the theoretical value. Low Reynolds number flow around a circular cylinder was measured, and velocities in the wake were found in close agreement with theory and with previous measurements. A velocity mapping of the non-steady vortex shedding of a Karman vortex street was also made. The flow around a delta wing was analyzed due to current interest in low speed separation. The velocities above the surface of a 60 deg delta wing were measured by LSV, and results were compared to theory. The flow was found to be non-conical, contrary to the assumption of most analytical treatments. Future developments of the laser streak velocimeter were discussed. (Author)

A reorienting gradient zonal rotor for low-speed separation of cell components

A low-speed zonal rotor of large sample capacity is described. The rotor does not require a rotating seal; instead, separations are carried out in density gradients which are loaded and unloaded at rest and which reorient in the rotor during acceleration and deceleration. The rotor was employed to affect the quantitative separation of liver tissue into subcellular components and to fractionate polydisperse liver glycogen rate-zonally.