Streamtube Contraction Research Articles

LES prediction of transitional flows in LP turbine cascades: effects of blade loading, flow phenomena and numerical setup

This paper presents detailed validations of a Large Eddy Simulation (LES) methodology for various transitional phenomena in low pressure turbines. The results are discussed to identify key phenomena to be resolved accurately toward future industrial use of LES. Detailed comparisons between experimental and CFD results are made on three different 2D cascades with different blade loading. One is low-lift and fully laminar design, while the others are moderate- and high-lift designs with boundary layer transition. The experimental data are obtained in a low speed linear cascade at Iwate University. All computations are conducted by a carefully-designed overset LES code. For the high-load design with a distinct laminar separation on the suction side, the LES result shows satisfactory agreement with the test. However, although the peak of total pressure loss distribution is predicted quite accurately, integrated cascade losses are over-predicted in the other two cases. For the laminar blade, the LES result implies some differences can exist in the state of wake, while the transitional blade shows delay of transition in the boundary layer. The effects of inflow turbulence intensity, length scale, and stream tube contraction are discussed in detail to improve LES prediction.

Turbulent inflow noise produced by a shrouded propeller

A theoretical model is presented for predicting the noise produced by the interaction of turbulence with a shrouded propeller in an axisymmetric mean-flow. For cases where the free-stream velocity is low, the turbulent eddies well upstream of the propeller are significantly elongated as they pass through the induced streamtube contraction. These elongated turbulent eddies interact multiple times with the rotating propeller blades causing partially coherent loading on the blades. The resulting radiated acoustic pressure spectrum contains peaks which occur at harmonics of the blade passing frequency. The theory is developed in the frequency domain and incorporates rapid distortion theory to obtain the distorted turbulent field at the propeller face, accounting for the potential mean-flow effect of the propeller and the shroud. The unsteady loading on the rotating blades is calculated using isolated blade response functions. The noise radiated to the far-field is then evaluated using a tailored Green's function approach which accounts for the scattering effect of the shroud and is solved using a boundary integral approach. Results are presented for shrouded and unshrouded propellers operating in flows with a range of different free-stream velocities. It is shown that as the free-stream velocity increases, the peak in the noise spectrum at the blade passing frequency reduces in level and the ‘haystack’ surrounding this peak widens. Results showing the effect of the shroud on the radiated noise levels at different frequencies and observer locations are also presented.

A semi-empirical model for streamwise vortex intensification

Vortex intensification plays an important role in a wide range of flows of engineering interest. One scenario of interest is when a streamwise vortex passes through the contracting streamtube of an aircraft intake. There is, however, limited experimental data of flows of this type to reveal the dominant flow physics and to guide the development of vortex models. To this end, the evolution of wing-tip vortices inside a range of streamtube contractions has been measured using stereoscopic particle image velocimetry. A semi-empirical model has been applied to provide new insight on the role of vorticity diffusion during the intensification process. The analysis demonstrates that for mild flow contractions, vorticity diffusion has a negligible influence due to the low rates of diffusion in the vortex flow prior to intensification and the short convective times associated with the streamtube contraction. As the contraction levels increase, there is a substantial increase in the rates of diffusion which is driven by the greater levels of vorticity in the vortex core. A new semi-empirical relationship, as a function of the local streamtube contraction levels and vortex Reynolds number, has been developed. The model comprises a simple correction to vortex filament theory and provides a significant improvement in the estimation of vortex characteristics in contracting flows. For the range of contractions investigated, errors in the estimation of vortex core radius, peak tangential velocity and vorticity are reduced by an order of magnitude. The model can be applied to estimate the change in vortex characteristics for a range of flows with intense axial strain, such as contracting intake streamtubes and swirling flows in turbomachinery.

A 3D analytical model for orthogonal blade-vortex interaction noise

A 3D analytical model of an Orthogonal Blade-Vortex Interaction (OBVI) for Counter-Rotating Open Rotor (CROR) tonal noise is investigated. The specific influence of two parameters taking into account the three-dimensionality of both the vortex velocity and the convection velocity within the rotor-rotor volume is addressed. The first step is to extract the vortex parameters from a recent unsteady Reynolds-Averaged Navier-Stokes computation and validate different vortex models. Lamb-Oseen and Scully vortices reproduce the behavior of the tip-vortex tangential velocity fairly well. Regarding the vortex axial velocity modeling, a Gaussian profile fits well with numerical results. On the one hand, the impact of the stream-tube contraction unbalances the lobes of the unsteady pressure with opposite phases produced by the OBVI event. This effect is larger than that of an equivalent blade sweep. On the other hand, adding the axial velocity deficit to the tangential one also unbalances the pressure lobes. Finally, from an acoustic point of view using Curle's acoustic analogy, both the stream-tube contraction and the axial velocity deficit have the same effect: they turn an acoustically-low efficient quadrupole into a strong dipole making these parameters fundamental for future CROR OBVI investigations.

Large eddy simulation of propeller wake instabilities

The wake of a five-bladed marine propeller at design operating condition is studied using large eddy simulation (LES). The mean loads and phase-averaged flow field show good agreement with experiments. Phase-averaged and azimuthal-averaged flow fields are analysed in detail to examine the mechanisms of wake instability. The propeller wake consisting of tip and hub vortices undergoes streamtube contraction, which is followed by the onset of instabilities as evident from the oscillations of the tip vortices. Simulation results reveal a mutual-induction mechanism of instability where, instead of the tip vortices interacting among themselves, they interact with the smaller vortices generated by the roll-up of the blade trailing edge wake in the near wake. It is argued that although the mutual-inductance mode is the dominant mode of instability in propellers, the actual mechanism depends on the propeller geometry and the operating conditions. The axial evolution of the propeller wake from near to far field is discussed. Once the propeller wake becomes unstable, the coherent vortical structures break up and evolve into the far wake, composed of a fluid mass swirling around an oscillating hub vortex. The hub vortex remains coherent over the length of the computational domain.

The Effect of Streamtube Contraction on the Characteristics of a Streamwise Vortex

The ingestion of a vortex by an aero-engine is potentially an area of concern for current and future aircraft-engine configurations. However, there are very little experimental data on the characteristics of a streamwise vortex undergoing ingestion through a contracting streamtube. To address this dearth of knowledge, the ingestion of a streamwise vortex has been studied experimentally using stereoscopic particle image velocimetry (stereo PIV). A subscale model of an aircraft intake has been used to generate a contracting capture streamtube, and an isolated streamwise vortex has been generated upstream of the intake using semispan NACA 0012 and delta wing vortex generators (VGs). A range of contraction ratios, vortex Reynolds numbers, and vortex initial conditions have been examined. Measurements on planes perpendicular to the freestream flow show that the vortex undergoes notable levels of intensification through the contraction streamtube. The characteristics of the vortex are dependent on the streamtube contraction level, the initial aerodynamic characteristics of the vortex, and the trajectory that the vortex follows inside the capture streamtube. Results from inviscid, incompressible vortex filament theory have been compared with the experimental data. At relatively low streamtube contraction ratios this theory provides a good estimate of the vortex characteristics. However, at higher contraction levels, there are notable levels of diffusion, which render the vortex less intense than that anticipated from vortex filament theory.

Noise generation by turbulence–propeller interaction in asymmetric flow

AbstractThis paper is concerned with a particular source of both broadband and tonal aeroengine noise, termed unsteady distortion noise. This noise arises from the interaction between turbulent eddies, which occur naturally in the atmosphere or are shed from the fuselage, and the rotor. This interaction produces broadband noise across a broad frequency spectrum. In cases in which there is strong streamtube contraction, which is especially true for open rotors at low-speed conditions (such as at take-off or for static testing), tonal noise at frequencies equal to multiples of the blade passing frequency are also produced, owing to the enhanced axial coherence caused by eddy stretching. In a previous paper (Majumdar & Peake, J. Fluid Mech., vol. 359, 1998, pp. 181–216), a model for unsteady distortion noise was developed in axisymmetric flow. However, asymmetric situations are also of much interest, and in this paper we consider two cases of asymmetric distortion: firstly that induced by the proximity of a second rotor, and secondly that caused by non-zero inclination to the flight direction, as found at take-off. This requires significant extension of the previous axisymmetric analysis. We find that the introduction of asymmetric flow features can have a significant decibel effect on the radiated sound power. For instance, in low-speed conditions we find that the tonal level is reduced significantly by the proximity of a second rotor, compared to the axisymmetric case, while the effect on the broadband levels is rather modest.

Contrarotating Open Rotor Operation for Improved Aerodynamics and Noise at Takeoff

The contrarotating open rotor is, once again, being considered as an alternative to the advanced turbofan to address the growing pressure to cut aviation fuel consumption and carbon dioxide emissions. One of the key challenges is meeting community noise targets at takeoff. Previous open rotor designs are subject to poor efficiency at takeoff due to the presence of large regions of separated flow on the blades as a result of the high incidence needed to achieve the required thrust. This is a consequence of the fixed rotor rotational speed constraint typical of variable pitch propellers. Within the study described in this paper, an improved operation is proposed to improve performance and reduce rotor-rotor interaction noise at takeoff. Three-dimensional computational fluid dynamics (CFD) calculations have been performed on an open rotor rig at a range of takeoff operating conditions. These have been complemented by analytical tone noise predictions to quantify the noise benefits of the approach. The results presented show that for a given thrust, a combination of reduced rotor pitch and increased rotor rotational speed can be used to reduce the incidence onto the front rotor blades. This is shown to eliminate regions of flow separation, reduce the front rotor tip loss and reduce the downstream stream tube contraction. The wakes from the front rotor are also made wider with lower velocity defect, which is found to lead to reduced interaction tone noise. Unfortunately, the necessary increase in blade speed leads to higher relative Mach numbers, which can increase rotor alone noise. In summary, the combined CFD and aeroacoustic analysis in this paper shows how careful operation of an open rotor at takeoff, with moderate levels of repitch and speed increase, can lead to improved front rotor efficiency as well as appreciably lower overall noise across all directivities.

Measurements of Intake Separation Hysteresis in a Model Fan and Nacelle Rig

A 1/20-scale, low speed model rig representing the fan and nacelle of a high bypass ratio jet engine has been tested under crosswind conditions. The flow conditions under which the intake flow separates and reattaches have been found to exhibit considerable hysteresis. This phenomenon has been examined by a careful test procedure in which the crosswind angle has been slowly increased and then decreased. Measurements of the hysteresis associated with separation and reattachment are presented for independent variations in stream-tube contraction ratio, ground clearance, fan operating point and Reynolds number. The results reveal that particular care must be taken to allow for any hysteresis when testing intakes under crosswind conditions. They also indicate that separation hysteresis is particularly sensitive to fan operating point and the position of the ground plane. These findings suggest that it is important for high Reynolds number intake tests and calculations to include a ground plane and a model of the downstream turbomachinery. © 2002 by the author(s).

A Numerical Study of the Stabilisation of a Lean Laminar Premixed V-Shaped Flame

The stabilisation of the base of a numerically computed lean methane/air V-shaped flame is investigated by calculating the different contributions to the stretch rate, the burning velocities in the burnt and unburnt mixtures and the flame temperature for various mixture velocities. The investigation shows that fluid dynamic phenomena are more imporant for the stabilisation of the flame base than flame stretch effects. On the one hand, flow straining in combination with thermal expansion leading to stream tube contraction is responsible for the increase of the mass flow rate above the flame holder. On the other hand, the stand-off distance in relation with the length of the recirculation zone above the flame holder is important for the mass flow rate towards the flame base. Furthermore, changes in flame stretch play a less dominant role: it is observed that the mass burning rate of the flame base is nearly independent of the inlet velocity and the effect of preferential diffusion on the flame temperature and the burning velocity is found to be small for the lean methane/air flame considered.

Noise generation by the interaction between ingested turbulence and a rotating fan

An important criterion in the development of modern aeroengines is the identification of the dominant noise sources under typical aircraft take-off and approach conditions, and also in ground-based tests in which the engine is stationary. In this paper, we develop a theoretical model for unsteady distortion noise, which results from the interaction of ingested atmospheric turbulence with the rotating fan, with a view to providing a better understanding of the important physical mechanisms in this particular aspect of sound generation. The theory, developed in the frequency domain, is applicable for any arbitrary spectral form of atmospheric turbulence upstream of the fan, and as a simple model we take the von Kármán spectra for isotropic turbulence. The key fluid dynamical process in unsteady distortion is the deformation of turbulent eddies into long, narrow filaments as they enter the engine, due to the strong streamtube contraction experienced by the steady, non-uniform mean flow generated by the fan. Simple models of the steady flow fields are provided for both open and ducted rotor geometries. The distorted turbulent field at the fan face can be obtained using rapid distortion theory, and considerable simplification is made here by noting that the number of blades in typical aeroengine fans is large, allowing the application of asymptotic analysis and the derivation of closed-form expressions for those parts of the turbulence spectrum at the fan face which dominate the radiation. The unsteady forces exerted on the rotating fan blades are then calculated via a strip-theory approach. The resulting sound scattered to the far field is then evaluated using asymptotic theory for open and ducted rotors. Results are presented in the form of frequency spectra for the turbulent field at the fan face, the blade forces and the radiated sound for typical testing and aircraft operating conditions. High tonal noise levels are obtained under static conditions, whereas the sound is generally broadband in flight. The dependence on turbulence parameters such as the integral lengthscale is highlighted.

Solution-adaptive structured-unstructured grid method for unsteady turbomachinery analysis. II - Results

A solution-adapt ive method for the time-accurate analysis of two-dimension al flows in multistage turboma- chinery is presented. The method employs a hybrid structured-unstructured zonal grid topology in conjunction with appropriate modeling equations and solution techniques in each zone, thus combining the advantages of both structured and unstructured grid methods. An efficient and robust grid adaptation strategy is also used for the unstructured grid regions. The numerical methodology is presented in detail in Part I of this article. Results obtained using this method for different turbomachine flow configurations are presented in this article. The numerical results compare well with available experimental data and other structured grid based simulations. ART I of this article describes a hybrid-grid procedure for the analysis of unsteady turbomachinery flows that combines the advantages of both unstructured and structured grid methodologies. The method is implemented within the zonal framework of Ref. 1, which is generalized to include both structured and unstructured grid domains. The region in the immediate vicinity of the airfoils (inner region) is dis- cretized using structured grids, while the rest of the domain (outer region) is discretized using an unstructured triangular grid. In the viscous inner regions, the Navier-Stokes equations are solved using an implicit, third-order accurate, upwind- biased scheme. In the inviscid outer region, the Euler equa- tions are solved using either a central difference scheme or an upwind scheme that incorporates a linear reconstruction procedure. The solution in the outer unstructured region is advanced in time explicitly using the same time-step values as for the structured regions which are time-advanced in an implicit manner. An efficient and robust grid adaptation strat- egy with both grid refinement and coarsening capabilities is also used for the unstructured grid. For generality, three- dimensional effects of stream-tube contraction are also mod- eled. The present method is capable of treating multistage turbomachinery configurations. Part I of this article described the domain decomposition, grid generation, inner and outer grid solution procedures, grid adaptation strategy, and the various boundary conditions used. Results obtained using this method for different turbomachine flow configurations are presented in this article.

A Study of Stall in a Low Hub–Tip Ratio Fan

An investigation has been carried out in order to define the process of rotating stall inception in a low-speed, low hub–tip ratio fan. Based on elementary cascade analysis, the fan would be expected to stall from the root. However, considerable experimental evidence indicates that tip stall is more frequently incurred. Although the analysis has been undertaken for a specific fan, it is considered to be representative of a broad range of machines. The analysis has involved two primary considerations, first the effect of streamtube contraction, which has been studied theoretically, and second, real flow effects (those not contained in the theoretical model), which have been studied experimentally. The study of streamtube contraction indicates that the root rematches to a more stable operating point, thus alleviating some of the problems in that region. The experimental investigation was undertaken on an isolated rotor, with successive build modifications to increase the likelihood of rotating stall inception at the root. It was apparent that real fluid effects tended to steepen the root pressure rise characteristic, thus enhancing the stability in that region. The performance of the fan at the tip tended to be poor, providing a pressure characteristic with a lower negative gradient than anticipated, indicating less stability than simple flow models would suggest. Hot-wire flow mapping at the rotor exit supported the overall conclusion that the rotor showed a strong reluctance to stall at the root apparently due to “centrifuging” of the boundary layer toward the tip.

Application of an unsteady Navier-Stokes solver to transonic turbinedesign

This study presents a numerical evaluation of the performance of the first stage of a new-generation turbine design. The numerical method solves the two-dimensional Navier-Stokes equations using a system of patched grids. Three-dimensional effects of stream-tube contraction are also modeled. The study focuses on the effects of axial gap variation on the unsteady rotor-stator interactions and on stage performance. Results are presented for three different axial gaps. The results indicate that the unsteady interactions can be very large in this design. These interactions affect not only the stage efficiency but also substantially alter the time-averaged features of the flow. In particular, for the case of the smallest axial gap, it was found that there was an unsteady shock on the stator suction surface which spanned the gap region and impinged upon the moving rotor airfoils.

The Use of Surface Static Pressure Data as a Diagnostic Tool in Multistage Compressor Development

A technique has been developed to analyze static pressure distributions obtained from the surfaces of stators in a multistage compressor to determine incident Mach number and flow angle and the turning and streamtube contraction for the individual stator sections. This data analysis technique permits this nonintrusive type of pressure measurement to be used to optimize the design of stator airfoils during the development phase of a multistage compressor and, in the development of analytic compressor representations, to segregate rotor and stator performance and to improve the modeling of endwall blockage. The value of this technique has been demonstrated in cascade testing of compressor airfoils, in single-stage and three-stage rig testing, and in engine testing.