Suction Side Boundary Layer Research Articles

Experimental Investigation of Blade Loading Effects at Design Flow in Rotating Passages of Centrifugal Impellers

Laser-Doppler Anemometry (LDA) was used to study the effect of blade loading on the relative velocity field in a rotating passage of a centrifugal-pump impeller. Two variations of the impeller, 8-bladed and 16-bladed, were investigated. The measured primary and secondary velocities and turbulence show that the effect of blade loading is not that previously predicted. The 16-blade impeller with high blade loading has a rapidly thickening suction side boundary layer, suggesting the onset of transient separation near the exit. However, for the 8-blade impeller with even higher blade loading, the onset of separation is not indicated at any measured location in the impeller. At the design flow, it is concluded that the stronger potential eddy and lower solidity associated with the very high blade loading caused a change in the secondary flow pattern, retarding the growth and the likelihood of transitory separation of the suction side boundary layer.

Boundary layer measurements on the pressure and suction sides of a turbine inlet guide vane

Abstract Laser Doppler measurements of shear stress, normal and streamwise mean and fluctuating velocities of the pressure and suction side boundary layers on a turbine inlet guide vane have been made for two turbulence levels Tu. No heat transfer is applied. The transition process is investigated. The blade has a 1.4 m chord. This large chord provides sufficiently thick boundary layers for detailed measurements (down to Y+⩽10, Y+=YUt/ν, Y is the normal to the wall distance, Ut is the friction velocity, ν is the kinematic viscosity). On the pressure side, at the low turbulence level, the onset of the transition occurs at a Gortler number Go=5; while no Gortler vortices have been found. The turbulent state is not completely reached at the trailing edge. At the higher inlet turbulence level, transition occurs at the beginning of the pressure side. The influence of the increase of turbulence is extremely strong on the pressure side compared to that of the suction side. On the suction side, at low Tu, the transition occurs just before a possible separation of the laminar boundary layer. At higher Tu, the beginning of the transition is displaced near the leading edge in a region where the pressure gradient is lower. The transition description is given by the analysis of the development of mean and turbulent quantities.

Unsteady Wake-Induced Boundary Layer Transition in High Lift LP Turbines

The development of the unsteady suction side boundary layer of a highly loaded LP turbine blade has been investigated in a rectilinear cascade experiment. Upstream rotor wakes were simulated with a moving-bar wake generator. A variety of cases with different wake-passing frequencies, different wake strength, and different Reynolds numbers were tested. Boundary layer surveys have been obtained with a single hotwire probe. Wall shear stress has been investigated with surface-mounted hot-film gages. Losses have been measured. The suction surface boundary layer development of a modern highly loaded LP turbine blade is shown to be dominated by effects associated with unsteady wake-passing. Whereas without wakes the boundary layer features a large separation bubble at a typical cruise Reynolds number, the bubble was largely suppressed if subjected to unsteady wake-passing at a typical frequency and wake strength. Transitional patches and becalmed regions, induced by the wake, dominated the boundary layer development. The becalmed regions inhibited transition and separation and are shown to reduce the loss of the wake-affected boundary layer. An optimum wake-passing frequency exists at cruise Reynolds numbers. For a selected wake-passing frequency and wake strength, the profile loss is almost independent of Reynolds number. This demonstrates a potential to design highly loaded LP turbine profiles without suffering large losses at low Reynolds numbers.

Turbulent separated flow over and downstream of a two-element airfoil

Flow characteristics in the vicinity of the flap of a single-slotted airfoil are presented and analysed. The flow remained attached over the model surfaces except in the vicinity of the flap trailing edge where a small region of boundary-layer separation extended over the aft 7% of flap chord. The airfoil configuration was tested at a Mach number of 0.09 and a chord Reynolds number of 1.8 × 106 in the NASA Ames Research Center 7- by 10-Foot Wind Tunnel. The flow was complicated by the presence of a strong, initially inviscid, jet, emanating from the slot between airfoil and flap, and a gradual merging of the main airfoil wake and flap suction-side boundary layer.

Viscous Flow in a Controlled Diffusion Compressor Cascade With Increasing Incidence

A detailed two-component LDV mapping of the flow through a controlled diffusion compressor cascade at low Mach number ( ~ 0.25) and Reynolds number of about 7 × 105, at three inlet air angles from design to near stall, is reported. It was found that the suction-side boundary layer reattached turbulent after a laminar separation bubble, and remained attached to the trailing edge even at the highest incidence, at which losses were 3 to 4 times the minimum value for the geometry. Boundary layer thickness increased to fill 20 percent of the blade passage at the highest incidence. Results for pressure-side boundary layer and near-wake also are summarized. Information sufficient to allow preliminary assessment of viscous codes is tabulated.

Turbulent separated flow over and downstream of a two-element airfoil

Flow characteristics in the vicinity of the flap of a single-slotted airfoil are presented and analysed. The flow remained attached over the model surfaces except in the vicinity of the flap trailing edge where a small region of boundary-layer separation extended over the aft 7% of flap chord. The airfoil configuration was tested at a Mach number of 0.09 and a chord Reynolds number of 1.8 × 106 in the NASA Ames Research Center 7- by 10-Foot Wind Tunnel. The flow was complicated by the presence of a strong, initially inviscid, jet, emanating from the slot between airfoil and flap, and a gradual merging of the main airfoil wake and flap suction-side boundary layer.

Investigations on the vorticity sheets of a close-coupled delta-canard configuration

Comprehensive aerodynamic investigations have been carried out on a close-coupled, A = 2.31 delta-canard configuration at low speed. Results of three-component, surface-pressure, and flowfield measurements, as well as oil-flow patterns, are presented for the canard-off and canard-on configurations. The main interference effects take place above the wing; the formation of the wing vortices is delayed considerably to positions downstream of the apex. The canard vortices pass the wing leading edge relatively high, and they are moved downward and inward above the wing. During this process, a fusion between the canard's vorticity sheet and the suction-side boundary layer of the wing takes place in the inner portion of the wing. The canard vortex system is maintained up to stations downstream of the wing trailing edge.

Tip Leakage in a Centrifugal Impeller

The effects of tip leakage have been studied using a 1-m-dia shrouded impeller where a leakage gap is left between the inside of the shroud and the impeller blades. A comparison is made with results for the same impeller where the leakage gap is closed. The static pressure distribution is found to be almost unaltered by the tip leakage, but significant changes in the secondary velocities alter the size and position of the passage wake. Low-momentum fluid from the suction-side boundary layer of the measurement passage and tip leakage fluid from the neighboring passage contribute to the formation of a wake in the suction-side shroud corner region. The inertia of the tip leakage flow then moves this wake to a position close to the center of the shroud at the impeller outlet.

Computation of Three-Dimensional Turbulent Turbomachinery Flows Using a Coupled Parabolic-Marching Method

A new coupled parabolic-marching method was developed to compute the three-dimensional turbulent flow in a turbine endwall cascade, a compressor cascade wake, and an axial flow compressor rotor passage. The method solves the partially parabolized incompressible Navier–Stokes equation and continuity in a coupled fashion. The continuity equation was manipulated using pseudocompressibility theory to give a convergent algorithm for complex geometries. The computed end-wall boundary layers and secondary flow compared well with the experimental data for the turbine cascade as did the wake profiles for the compressor cascade using a k–ε turbulence model. Suction side boundary layers, pressure distributions, and exit stagnation pressure losses compared reasonably well with the data for the compressor rotor.

Effects of channel rotation upon turbulent boundary layer on a concave surface.

Effects of channel rotation and wall curvature on the flow in a turbulent boundary layer developing on a concave wall were examined experimentally using a curved channel of a constant cross section. In the case where the Coriolis force acted in the direction opposite to the centrifugal one due to the wall curvature, the turbulence intensities and the Reynolds stress were decreased with the channel rotation, but they were almost left unchanged when the two forces acted in the same direction. The Monin-Oboukov formula modified for the curved and rotating channel wall was ascertained to be almost valid to the flow in the suction side boundary layer.

A Blade-to-Blade Solution of the Flow in a Centrifugal Compressor Impeller with Splitters

The matrix through flow analysis is used to predict the blade-to-blade flow in a centrifugal compressor impeller which contains splitters (half vanes). The presence of solid boundaries in the flow field and the stagnation points associated with it are treated by assuming that the flow around the stagnation point is isentropic. This removes the instability which the matrix through flow method suffers when stagnation points are encountered within the flow field. The splitters in the present work have the same profile with the full blades. Three different geometries are tested. In the first, the leading edge is 3 mm from the eye, the next is 6 mm, and the third has the leading edge at the exit of the inducer. The calculations show that the differences shown between the three cases are linked with different circumferential components of velocity. The geometry, therefore, of the leading edge is of basic importance in the development of the velocity profiles inside the different channels of the impeller. It is also shown that the distance between the eye of the impeller and the leading edge of the splitter does not affect the flow greatly as long as the leading edge is within the axial part of the impeller. The presence of splitters in general is shown to reduce the width of the suction side boundary layer.

Effects of System Rotation on the Performance of Two-Dimensional Diffusers

Experiments with incompressible flow are reported concerning the effects of Coriolis acceleration on flow separation and on separated flow in plane-wall diffusers of rectangular cross section. The diffusers were rotated about an axis perpendicular to the plane of the nearly two-dimensional flow in order to simulate some features of the blade-to-blade flow distribution in the radial portion of the centrifugal impeller. Various stall regimes are mapped on coordinates of rotation number and diffuser area ratio (at fixed wall length). Diffuser pressure-recovery coefficient is reported as a function of area ratio and rotation number. These data demonstrate that, by suppressing turbulent mixing and shear stress in the suction-side boundary layers, the Coriolis acceleration field greatly enhances the tendency for stall to appear in a diffuser. This effect causes a corresponding reduction in the throat-to-exit pressure recovery as compared to that of nonrotating diffusers of the same geometry and inlet flow blockage.