Stall Cell Research Articles

Design of a Compressor Test Rig for Immobilization of the Stall Cell

Capturing internal flow experimentally presents significant challenges due to the asymmetric propagation and strong unsteadiness of a stall cell in the circumferential direction. In this study, a low-speed counter-rotating axial compressor test rig was designed based on a counter-rotating compressor to immobilize a stall cell and measure its internal flow characteristics. Determining an appropriate speed ratio that is capable of stabilizing the circumferential position of the stall cell in the counter-rotating compressor enabled visualization measurements to be conducted successfully. The preliminary results demonstrated the successful immobilization of the stall cell using an appropriate rotating speed ratio. Furthermore, the oil flow visualization measurements confirmed the presence of a distinct stall cell structure on the casing wall. This work represents an innovative approach towards immobilizing stall cells in axial compression systems.

Three-dimensional separation over unswept cantilevered wings at a moderate Reynolds number

We conduct experiments on the structure of the 3D flow fields for unswept cantilevered wings at high angle of attack. Stall cell counter-rotating vortices on the suction surface form for a range of aspect ratios (AR), though AR changes the angle of attack at which the flow separates. Analysis of mean flow volume over the suction surface and near wake shows that the arch vortex forms in the wake and connects to the surface at the stall cell foci. Reynolds stress peaks indicate a strong reliance on the arch vortex location in the mean flow. Spectral analysis of the velocity field at select spanwise planes shows that shedding is coherent but intermittent, and varies strongly along the wingspan.

Flow Pattern of Rotating Stall Cell in Axial Compressor

The results of measurements of the three-dimensional flowfield in a low-speed single-stage compressor operating in deep rotating stall are presented in this paper. The measurement technique includes a specially designed omnidirectional five-hole pressure probe, a specially designed near-wall hot-wire probe, high-frequency pressure transducers, and oil flow visualization. The test results provide a detailed description of the flow in and around the stall cell. The flow pattern of the cell is described by velocity vectors, streamlines, and pressure contours. A definition of the stall cell is proposed by using the streamlines in the frame of reference of the stall cell. Upstream of the rotor, the stall cell is characterized by a reversed jet flow at the rotor’s tip and downstream flow at the midspan and hub. Downstream of the stator, the stall cell is composed of low-speed wake flow with circumferential flow near the shroud from the trailing edge to the leading edge of the stall cell.

Effect of Stall Cells on Pressure Fluctuations Characteristics in a Centrifugal Pump

Rotating stall is an unsteady flow phenomenon, which always leads to instability and efficiency degradation. In order to reveal pressure fluctuations in the impeller of centrifugal pump induced by stall cells, the flow structures in a volute-type centrifugal pump were calculated using Large Eddy Simulation (LES) method. The predicted results of the numerical model were compared with experimental flow-head curve. The simulation results were in good agreement with the experimental results. The stall phenomenon occurred when the flow rate dropped to 70% of design flow rate. Three stall cells located at the entrance of passages could be observed, which remained stationary relative to the rotating impeller. With the decrease of flow rate, the area occupied by stall cells gradually increased. The peak value of pressure fluctuation at 25% of design flow rate is obviously larger than that at 50% of design flow rate. For the unstalled or stalled passage, the impeller-volute interaction played a leading role in the pressure fluctuations of the impeller. For the stalled passage, the amplitude of the low frequency induced by stall cell is relatively insignificant.

Unsteady RANS Simulations of Strong and Weak 3D Stall Cells on a 2D Pitching Aerofoil

A series of three-dimensional unsteady Reynolds-averaged Navier–Stokes (RANS) simulations are conducted to investigate the formation of stall cells over a pitching NACA 0012 aerofoil. Periodic boundary conditions are applied to the spanwise ends of the computational domain. Several different pitching ranges and frequencies are adopted. The influence of the pitching range and frequency on the lift coefficient (CL) hysteresis loop and the development of leading-edge vortex (LEV) agrees with earlier studies in the literature. Depending on pitching range and frequency, the flow structures on the suction side of the aerofoil can be categorized into three types: (i) strong oscillatory stall cells resembling what are often observed on a static aerofoil; (ii) weak stall cells which are smaller in size and less oscillatory; and (iii) no stall cells at all (i.e., flow remains two-dimensional) or only very weak oval-shaped structures that have little impact on CL. A clear difference in CL during the flow reattachment stage is observed between the cases with strong stall cells and with weak stall cells. For the cases with strong stall cells, arch-shaped flow structures are observed above the aerofoil. They resemble the Π-shaped vortices often observed over a pitching finite aspect ratio wing.

Aerodynamic analysis of a C-class-like catamaran in simplified unsteady wind conditions using LES and URANS modeling

Highlights • A numerical analysis on a C-class-like geometry is performed in unsteady wind conditions at different wind frequencies. • LES and URANS models agree in predicting the global aerodynamic coefficients as well as the flow structures of the wingsail. •Stall cells appear on the wingsail in case of unsteady wind conditions and their features depend on wind frequency. •The wind frequency, affecting the stall cells, leads to a modification of the aerodynamic coefficients of the wingsail.

Stall cell formation over a post-stall airfoil: effects of active perturbations using plasma actuators

Stall cells have been observed over stalled airfoils by many researchers and there have been extensive discussions in the literature on their formation mechanism. They have been known to be unsteady and sensitive to even the smallest upstream perturbations, and therefore, can be made spatially steady by fixing the upstream perturbation location. In the current experimental work, flow with a Reynolds number of 5·105 over a stalled VR-7 airfoil with an aspect ratio 3 was perturbed using a single nanosecond dielectric barrier discharge (NS-DBD) plasma actuator positioned near the leading edge and covering the span of the airfoil. Surface oil flow visualization and stereo particle image velocimetry were employed to investigate spanwise non-uniformities on the airfoil surface as well as in the flow. The results show a gradual appearance of stall cells by increasing the perturbation Strouhal number to approximately 3 times the natural shedding Strouhal number of Stn = 0.6 (Ste ~ 3Stn). The stall cells become well-defined and eventually saturated with no further changes with increasing perturbation Strouhal number beyond 10Stn. The instability responsible for the stall cells is self-sustained as the stall cells persist after the perturbations are stopped. The results of the current work are discussed and interpreted using recent modal and non-modal linear global instability analyses, even though they are carried out in much lower Reynolds number laminar stalled flows over airfoils.

Parametric Investigation of Stall Cell Formation on a NACA 0015 Airfoil

An experimental parametric exploration of stall cells on a NACA 0015 airfoil with aspect ratio was conducted. First, flowfield measurements were performed using stereoscopic particle image velocimetry at conditions where a stall cell was present. This enabled some of the properties associated with stall cell(s) and stall cell transition to be defined. For the parametric study, angles of attack from to 21.5 deg and Reynolds numbers from to were observed using oil-flow visualizations, demonstrating the transition from full span separation to fully formed single or dual stall cells. At and , full span separation with recirculation regions was present, whereas at midrange of Reynolds numbers tested, three-dimensional separation was observed with undulations across the span. At , a single stall cell formed when the Reynolds number exceeded a critical Reynolds number. At the highest angles of attack tested, a dual stall cell configuration was present at high Reynolds numbers. In addition, the evolution of a stall cell was observed to be reminiscent of the mechanism proposed by Weihs and Katz (“Cellular Patterns in Poststall Flow over Unswept Wings,” AIAA Journal, Vol. 21, No. 12, 1983, pp. 1757–1759). It was also shown that, under specific conditions, a bistable condition was present, where two different patterns were obtained for identical flow conditions. Finally, a less-detailed parametric investigation was also conducted for a NACA 0015 airfoil with , which suggested a relationship between the critical Reynolds number and the aspect ratio.

Observations of the Growth and Decay of Stall Cells during Stall and Surge in an Axial Compressor

This research investigated unsteady events such as stall inception, stall-cell development, and surge. Stall is characterized by a decrease in overall pressure rise and nonaxisymmetric throughflow. Compressor stall can lead to surge which is characterized by quasi-axisymmetric fluctuations in mass flow and pressure. Unsteady measurements of the flow field around the compressor rotor are examined. During the stall inception process, initial disturbances were found within the rotor passage near the tip region. As the stall cell develops, blade lift and pressure ratio decrease within the stall cell and increase ahead of the stall cell. The stall inception event, stall-cell development, and stall recovery event were found to be nearly identical for stable rotating stall and surge cases. As the stall cell grows, the leading edge of the cell will rotate at a higher rate than the trailing edge in the rotor frame. The opposite occurs during stall recovery. The trailing edge of the stall cell will rotate at the approximate speed as the fully developed stall cell, while the leading edge decreases in rotational speed in the rotor frame.

Numerical study of airfoil stall cells using a very wide computational domain

The formation of stall cells over a NACA 0012 airfoil at a Reynolds number of one million has been investigated numerically, using unsteady Reynolds-averaged Navier–Stokes (URANS) and delayed detached-eddy simulation (DDES) approaches. The simulations are performed with a very wide computational domain (10 chord length) to minimize the influence of spanwise periodic boundary conditions. For the URANS simulations, four different spanwise mesh resolutions are tested to determine the minimum resolution required to capture the formation of stall cells. Both URANS and DDES results show a sudden decrease in lift and increase in drag between 16° and 17° angle of attack, accompanied by a significant change of separated flow patterns. Stall cell structures are observed clearly in the URANS solutions between 17° and 19° with a spanwise spacing of about 1.4 to 1.8 chord length, which agrees well with a theoretical prediction based on the slope of the lift curve in this angle-of-attack range. The DDES results show much more complex flow patterns over the airfoil at these high angles of attack, although the spectral analysis of wall shear stress suggests the existence of flow structures having a similar spanwise length scale to the stall cells.

Measurement of Three-Dimensional Stall Cells on a Two-Dimensional NACA0015 Airfoil

This work aims to identify and quantify the flowfield associated with stall cells on a two-dimensional NACA0015 airfoil with aspect ratio using oil flow visualization and stereoscopic particle image velocimetry. The three-dimensional structures were explored parametrically using oil flow visualization from to 20 deg and to . A critical Reynolds number was observed, above which stall cells form, and its value increased with angle of attack. Stereoscopic particle image velocimetry measurements were taken to quantify the stall cell and correlate the flowfield over the surface to oil patterns on the surface, including skin friction. Next, partial and full span trips were placed at 10% of the chord to artificially induce stall cells. The flowfields and associated vorticity fields for the various cases were measured. It was found that, for the baseline case, where flow is fully separated over the majority of the span, there are large concentrations of turbulent kinetic energy (mainly streamwise and spanwise normal stresses). When a stall cell was induced, the turbulent kinetic energy magnitude was increased in its vicinity; however, it was significantly reduced outboard of the stall cell where the flow was attached.

Fan Response to Boundary-Layer Ingesting Inlet Distortions

The modeling capabilities of TURBO, an existing turbomachinery analysis code, have been extended to include the ability to solve the external and internal flow fields of a boundary-layer ingesting inlet. Flow solutions are presented and compared with experimental data for several high-Reynolds-number flows to validate the solver modifications. The upstream flowfield was coupled to a hypothetical compressor fan to present a fully developed distortion to the fan. Although the total pressure distortion upstream of the fan was symmetrical for this geometry, the pressure rise generated by the fan blades was not, because of the velocity nonuniformity of the distortion. Total pressure profiles at various axial locations are computed to identify the overall distortion pattern, how the distortion evolves through the blade passages and mixes out downstream of the blades, and where any critical performance concerns might be. Stall cells are identified that are stationary in the absolute frame and are fixed to the inlet distortion. Flow paths around the blades are examined to study the stall mechanism. Rather than a typical airfoil stall, it is observed that the nonuniform pressure loading in the radial direction promotes a three-dimensional dynamic stall. The stall occurs at a point of rapid incidence angle oscillation observed when a blade passes through a distortion and re-attaches when the blade sees more uniform flow outside the distortion.

Experimental investigation of the flow past passive vortex generators on an airfoil experiencing three-dimensional separation

The use of passive vortex generators (VGs) as a simple and effective way to delay or suppress separation on an airfoil optimized for wind turbine blades is examined experimentally. The profile experiences three-dimensional separation of the Stall Cell type. Pressure, Flow Visualization and Stereo Particle Image Velocimetry experiments are discussed for the case of triangular counter rotating VGs with common flow up. For a Reynolds number of 0.87×106, Stall Cell formation is delayed for 5°, and lift increases up to α=15°. In total, maximum lift increases by 44%, while drag increases by 0.002 at pre-stall angles of attack. At α=16° the flow bifurcates between separated and attached flow conditions. At α=10° the flow is examined in detail and an investigation on the turbulence characteristics is carried out by correlating Reynolds stresses production to time averaged flow gradients. Strong turbulent interaction is observed between the two vortices and the underlying flow up to 37.2 VG heights downstream of the VGs, while further downstream (up to 47.2 VG heights) diffusion governs the flow. A wandering motion of the VG vortices leads to increased v`v`¯ normal stress values between the two vortices.

An experimental and numerical investigation on the formation of stall-cells on airfoils

Stall Cells (SCs) are large scale three-dimensional structures of separated flow that have been observed on the suction side of airfoils designed for or used on wind turbine blades. SCs are unstable in nature but can be stabilised by means of a localized disturbance; here in the form of a zigzag tape covering 10% of the wing span. Based on extensive tuft flow visualisations, the resulting flow was found macroscopically similar to the undisturbed flow. Next a combined investigation was carried out including pressure recordings, Stereo-PIV measurements and CFD simulations. The investigation parameters were the aspect ratio, the angle of attack and the Re number. Tuft and pressure data were found in good agreement. The 3D CFD simulations reproduced the structure of the SCs in qualitative agreement with the experimental data but had a delay of ~3deg in capturing the first appearance of a SC. The error in Cl max prediction was 7% compared to 19% for the 2D cases. Tests show that SCs grow with Re number and angle of attack. Also analysis of the time averaged computational results indicated the presence of three types of vortices: (a) the trailing edge line vortex (TELV) in the wake, (b) the separation line vortex (SLV) over the wing and (c) the SC vortices. The TELV and SLV run parallel to the trailing edge and are of opposite sign, while the SC vortices start normal to the wing suction surface, then bend towards the SC centre and later extend downstream, with their vorticity parallel to the free stream.

Study of a stall cell using stereo particle image velocimetry

The structure of Stall Cells (SCs) on wings is analyzed on the basis of stereo particle image velocimetry measurements. All experiments regard a Reynolds number 0.87 × 106 flow around a rectangular wing with endplates and an aspect ratio of 2.0. The inherently unstable stall cell is stabilized by means of a localized spanwise disturbance. Velocity, vorticity, and Reynolds stress data above the wing and in the wake are presented and discussed, also in combination with Computational Fluid Dynamics data. The present study completes and clarifies the previously suggested models regarding the SC structure. The SC emerges in between the separation and trailing edge shear layer where three different types of vortices are identified: (a) the stall cell vortices that start normal to the wing surface and continue downstream aligned with the free stream, (b) the separation line vortex, and (c) the trailing edge line vortex that both run parallel to the wing trailing edge and grow significantly at the center of the stall cell. Analysis of the Reynolds stress data reveals high anisotropy. Concentration of high streamwise shear stress values is connected to the two shear layers and high cross shear Reynolds stresses are connected to vortex stretching. High normal Reynolds stress values are observed (a) in the separation but not in the trailing edge shear layer indicating the flapping of the former and (b) along the stall cell vortices indicating their wandering motion. The eddy viscosity based Reynolds averaged Navier-Stokes simulations are found in good qualitative agreement with the experiments in terms of the type and position of the identified vortex structures, an agreement which is linked to the correct trend in the predicted shear Reynolds stresses distributions. Quantitative deviations of the numerical results from the measurements are attributed to the isotropic definition of the turbulence model. Therefore, use of large eddy simulation is suggested for better prediction of the flow.

Geometrical characterization of stall cells on rectangular wings

ABSTRACTThe onset of stall cells (SCs) is experimentally investigated on a flattop loaded 18% thick airfoil optimized for use on wind turbine blades, exhibiting trailing edge separation. SCs are dynamic coherent vortical structures that appear on wings under separated flow conditions. Although SCs have been known for long, neither are their characteristics completely documented nor their generating mechanisms fully understood. The present investigation aims at providing additional information on the geometric characteristics in terms of width, length and occupied area. The relevant data are presented as functions of Reynolds (Re) number, angle of attack and aspect ratio (AR) of the model. In the tests reported, the dynamic character of SCs is suppressed by imposing a localized flow disturbance. For the specific airfoil and for the Re and AR range tested, it is found that: the angle of attack at which SCs are initially formed decreases linearly with Re number and independently of the AR; unlike two‐dimensional separation, their chordwise length increases with Re; the SC area relative to the wing planform area (defined as the relative SC area) grows asymptotically with angle of attack and Re number reaching an upper bound, which is independent of the AR; at intermediate angles of attack, the SC relative area is higher for the lower AR wing; for a fixed increment in Re number, the growth of the SC relative area is independent of the initial Re number; at lower angles of attack, the actual SC area is independent of the wing span. Copyright © 2013 John Wiley & Sons, Ltd.

Experimental and computational analysis of stall cells on rectangular wings

ABSTRACTThe present paper is the second part of a combined (experimental and computational) study on stall cells (SCs) on a rectangular wing. In the first part, tuft data were used in order to geometrically characterize a stabilized SC resulting from a localized spanwise disturbance introduced by a zigzag tape. Here, pressure measurements on the model and in the wake and aerodynamic polars at midspan are reported. The wing model had an aspect ratio value of 2, the Reynolds number was 106 and the range of angles of attack (α) was from −6° to 16°. Experimental results confirm previous findings. Furthermore, two‐dimensional and three‐dimensional Reynolds Averaged Navier‐Stokes RANS simulations are used in order to better understand the structure of SCs. 3D simulations reproduce the experimental data with a 3° delay in α and permit a qualitative analysis. It is found that the SC vortices start normal to the wing surface and extend downstream in the wake; the evolution of the SC vortices in the wake is in strong interaction with the separation line vortex and the trailing edge line vortex; as the SC vortex develops downstream in the wake, its centreline is contracted towards the SC centre; the wing wake is pushed upstream at the centre of the SC and downstream at the sides by the SC vortices; spanwise lift and drag distributions always attain their minimum at the SC centre. Copyright © 2013 John Wiley & Sons, Ltd.

Influence of Rotating Stall Cell on Aerodynamic Characteristics and Noise of a Multiblade Fan

Influence of Rotating Stall Cell on Aerodynamic Characteristics and Noise of a Multiblade Fan

Unsteady Flow with Rotating Stall Cell in a Diagonal Flow Fan

Unsteady Flow with Rotating Stall Cell in a Diagonal Flow Fan

Closure to “Discussion of ‘Comparative Studies on Short and Long Length-Scale Stall Cell Propagating in an Axial Compressor Rotor’ ” (2001, ASME J. Turbomach., 123, p. 31)

Closure to “Discussion of ‘Comparative Studies on Short and Long Length-Scale Stall Cell Propagating in an Axial Compressor Rotor’ ” (2001, ASME J. Turbomach., 123, p. 31)