Sensitivity of an asymmetric 3D diffuser to vortex-generator induced inlet condition perturbations

Modifications of the turbulent separated flow in an asymmetric three-dimensional diffuser due to inlet condition perturbations were investigated using conventional static pressure measurements and velocity data acquired using magnetic resonance velocimetry (MRV). Previous experiments and simulations revealed a strong sensitivity of the diffuser performance to weak secondary flows in the inlet. The present, more detailed experiments were conducted to obtain a better understanding of this sensitivity. Pressure data were acquired in an airflow apparatus at an inlet Reynolds number of 10,000. The diffuser pressure recovery was strongly affected by a pair of longitudinal vortices injected along one wall of the inlet channel using either dielectric barrier discharge plasma actuators or conventional half-delta wing vortex generators. MRV measurements were obtained in a water flow apparatus at matched Reynolds number for two different cases with passive vortex generators. The first case had a pair of counter-rotating longitudinal vortices embedded in the boundary layer near the center of the expanding wall of the diffuser such that the flow on the outsides of the vortices was directed toward the wall. The MRV data showed that the three-dimensional separation bubble initially grew much slower causing a rapid early reduction in the core flow velocity and a consequent reduction of total pressure losses due to turbulent mixing. This produced a 13% increase in the overall pressure recovery. For the second case, the vortices rotated in the opposite sense, and the image vortices pushed them into the corners. This led to a very rapid initial growth of the separation bubble and formation of strong swirl at the diffuser exit. These changes resulted in a 17% reduction in the overall pressure recovery for this case. The results emphasize the extreme sensitivity of 3D separated flows to weak perturbations.     

Low frequency sound generated by flow separation in a conical diffuser

The sound field in a circular pipe generated by a concentric jet flow entering the pipe is studied. In the first case the air flow enters the pipe through a convergent nozzle only. In the second case a short diffuser is attached to the nozzle. When the diffuser half angle is small enough to ensure attached flow conditions, the sound pressure level in the duct is reduced over the entire frequency range measured. When the diffuser angle is increased up to the point where flow separation occurs, an increase in the duct sound pressure level is observed. It is shown by means of cross-correlation measurements involving the unsteady wall pressures in the diffuser and the sound pressure in the duct that the increased sound levels are in fact caused by the flow separation in the diffuser.     

Assessment of the organization of a turbulent separated and reattaching flow by measuring wall pressure fluctuations

The effect of local forcing on the organization of a turbulent separated and reattaching flow was assessed by measuring wall pressure fluctuations. Multi-arrayed microphones were installed on the surface to measure the simultaneous spatial and temporal wall pressure fluctuations. Local forcing at the separation edge was applied to the separated flow over a backward-facing step through a thin slit. The organization of the separated and reattaching flow was found to be greatest at the effective forcing frequency. The flow structure was diagnosed by analyzing several characteristics of the wall pressure fluctuations: the wall pressure fluctuation coefficients, wall pressure spectrum, wavenumber-frequency spectrum, coherence, cross-correlation, and multi-resolution autocorrelations of pressure fluctuations using the maximum overlap discrete wavelet transform and continuous wavelet transform. Features indicative of the amalgamation of vortices under the local forcing were observed; this amalgamation process accounted for the observed reduction of the reattachment length. Examination of the wall pressure fluctuations revealed that introduction of local forcing enhanced flapping motion as well as the streamwise and spanwise dispersions of vortical structures.     

Flow in a simple swirl chamber with and without controlled inlet forcing

Results are presented from a swirl chamber with and without controlled inlet forcing. The controlled inlet forcing is induced using arrays of vortex generators placed along one wall of the swirl chamber inlet duct. Flow visualization results are given, along with surveys of circumferential mean velocity, static pressure, and total pressure, at Reynolds numbers (based on inlet duct characteristics) as high as 8000. The controlled inlet forcing provides means to alter and control: (i) the spacing and number of Görtler vortices across the span of the swirl chamber, (ii) the amount of vortex development at a particular Reynolds number and circumferential location, (iii) the circumferential location and Reynolds number of initial Görtler vortex development, and (iv) the circumferential location and Reynolds number of Görtler vortex breakup into more chaotic flow.     

On swirl development in a square cross-sectioned, S-shaped duct

The flow in a uniform square cross-sectioned, S-shaped duct was investigated experimentally, at Reynolds number (Re) = 4.73 × 10⁴ and 1.47 × 10⁵, using three S-ducts of different curvature and turning angle. The hydraulic diameter (D) for each S-duct is 150 mm. Besides studying the square cross-sectioned S-duct flow at moderately higher Re than current literature, the S-ducts’ geometry used in this study also have larger curvatures and higher turning angles than those reported in the literature. With surface pressure measurement and smoke wire flow visualization, flow separation at the inside wall of the first bend was detected. Using surface oil flow visualization on the bottom wall of the S-duct and cross-wires measurement at the duct exit, it is shown here that the swirl developed in the first bend was partly attenuated in the second bend due to the formation of swirl of opposite direction. The swirl of an opposite sign results in the formation of a clear dividing or separation line on the bottom wall (and top wall) of the duct. Additional flow features include the formation of streamwise vortices on the outer-wall of the second bend. These streamwise vortices can either be a pair of counter-rotating vortices or a single vortex. The formation mechanism of these streamwise vortices is explained using the Squire and Winter [J Aeronaut Sci 18(4):271–277, 1951] formula and it is shown that the said mechanism is applicable to both Re in the present study.     

Experimental performance analysis of an annular diffuser with and without struts

In this paper, the performance analysis of an annular diffuser is presented. In a typical industrial gas turbine diffuser, a certain number of structural members, called struts, serve both as load bearings support and as passages for cooling air and lubricant oil.

Measurements were made in a 35% scaled down model of a PGT10 gas turbine exhaust diffuser with and without struts in order to determine the total and static pressure development and the effect of struts on both the local phenomena and the overall performance. More realistic flow conditions are made available by a ring of 24 axial guide vanes at inlet, which represent the last turbine rotor. The model has been tested on a wind tunnel facility developed at the University of Perugia with inlet speed around 80 m/s, allowing satisfactory accuracy for flow measurements and similarity with the PGT10 diffuser in terms of Reynolds number. Static pressure taps located at various streamwise positions on the hub and the casing allowed the estimation of pressure recovery development. A Pitot tube and a hot split-film anemometer were used to determine static and total pressure inside the diffuser at different axial positions. The comparison between the two cases, with and without the struts, was made also by the use of global parameters, which correlate static and total pressure.

In a previous paper, a detailed three-dimensional analysis of the flow path inside the diffuser was presented and the detrimental effect of the struts, in terms of flow separation and unsteadiness, was discussed. The stationary flow measurements and the investigation of the diffuser without the struts are presented in this paper. The whole research project represent a complete diffuser investigation available to develop an optimal design and to advance the computational and design tools for gas turbine exhaust diffusers.     

Experimental study of the effects of spanwise rotation on the flow in a low aspect ratio diffuser for turbomachinery applications

Two dimensional time accurate PIV measurements of the flow between pressure and suction side at different spanwise positions of a rotating channel are presented. The Reynolds and Rotation numbers are representative for the flow in radial impellers of micro gas turbines. Superposition of the 2D results at the different spanwise positions provides a quasi-3D view of the flow and illustrates the impact of Coriolis forces on the 3D flow structure. It is shown that the inlet flow is little affected by rotation. An increasing/decreasing boundary layer thickness is reported on the suction/pressure side wall halfway between the channel inlet and outlet. The turbulence intensity moves away from the suction side wall and remains close to the pressure side wall. The instantaneous measurements at mid-height of the rotating channel reveal the presence of hairpin vortices in the pressure side boundary layer and symmetric vortices near the suction side. Hairpin vortices occur in rotation in the pressure and in the suction side, for the measurement plane close to the channel bottom wall.     

Streamwise vortices generated by impinging flows in a confined duct

Particle-tracer technique was employed for visualizing flow structures in a side-inlet square duct. The results obtained indicate that the streamwise vortices developed in the stagnation region of impinging flows are irregularly distributed. As the vortices convect downstream they are first stretched and merged, then squashed due to the non-zero pressure gradient effects caused by the flow separation regions existed along the side walls. The mechanism responsible for generating streamwise vortices in the stagnation region is suggested due to the hydrodynamic instability effect, similar to that previously found for three-dimensional disturbances growing in a two-dimensional stagnation flow.     

Using digital holographic microscopy for simultaneous measurements of 3D near wall velocity and wall shear stress in a turbulent boundary layer

A digital holographic microscope is used to simultaneously measure the instantaneous 3D flow structure in the inner part of a turbulent boundary layer over a smooth wall, and the spatial distribution of wall shear stresses. The measurements are performed in a fully developed turbulent channel flow within square duct, at a moderately high Reynolds number. The sample volume size is 90 × 145 × 90 wall units, and the spatial resolution of the measurements is 3–8 wall units in streamwise and spanwise directions and one wall unit in the wall-normal direction. The paper describes the data acquisition and analysis procedures, including the particle tracking method and associated method for matching of particle pairs. The uncertainty in velocity is estimated to be better than 1 mm/s, less than 0.05% of the free stream velocity, by comparing the statistics of the normalized velocity divergence to divergence obtained by randomly adding an error of 1 mm/s to the data. Spatial distributions of wall shear stresses are approximated with the least square fit of velocity measurements in the viscous sublayer. Mean flow profiles and statistics of velocity fluctuations agree very well with expectations. Joint probability density distributions of instantaneous spanwise and streamwise wall shear stresses demonstrate the significance of near-wall coherent structures. The near wall 3D flow structures are classified into three groups, the first containing a pair of counter-rotating, quasi streamwise vortices and high streak-like shear stresses; the second group is characterized by multiple streamwise vortices and little variations in wall stress; and the third group has no buffer layer structures.     

Separation control in a conical diffuser with an annular inlet: center body wake separation

In many practical applications of conical diffusers, the flow is fed by an annular flow passage formed by a center body. Flow separation, which occurs if the center body ends abruptly, is undesirable because it degrades the diffuser performance. The present experiment utilizes magnetic resonance velocimetry to acquire three-component mean velocity measurements for a set of conical diffusers with an annular inlet. The results show strong coupling between the diffuser wall boundary layer development and the wake of the center body. Coanda blowing is used to mitigate the center body wake separation. The diffuser wall boundary layer is thick in the absence of the central separation bubble and separates when Coanda blowing is too strong.     

Experimental and computational study of the flow induced by a plasma actuator

A complementary experimental and computational study of the flow field evoked by a plasma actuator mounted on a flat plate was in focus of the present work. The main objective of the experimental investigation was the determination of the vector force imparted by the plasma actuator to the fluid flow. The force distribution was presently extracted from the Navier–Stokes equations directly by feeding them with the velocity field measured by a PIV technique. Assuming a steady-in-mean, two-dimensional flow with zero-pressure gradient, the imbalance between the convective term and the momentum equation’s right-hand-side terms reveals the desired resulting force. This force-distribution database was used afterwards as the source term in the momentum equation. Furthermore, an empirical model formulation for the volume-force determination parameterized by the underlying PIV-based model is derived. The model is tested within the RANS framework in order to predict a wall jet-like flow induced by a plasma actuator. The Reynolds equations are closed by a near-wall second-moment closure model based on the homogeneous dissipation rate of the kinetic energy of turbulence. The computationally obtained velocity field is analysed along with the experimental data focussing on the wall jet flow region in proximity of the plasma actuator. For comparison purposes, different existing phenomenological models were applied to evaluate the new model’s accuracy. The comparative analysis of all applied models demonstrates the strength of the new empirical model, particularly within the plasma domain. In addition, the presently formulated empirical model was applied to the flow in a three-dimensional diffuser whose inflow was modulated by a pair of streamwise vortices generated by the present plasma actuator. The direct comparison with existing experimental data of Grundmann et al. (2011) demonstrated that the specific decrease of the diffuser pressure corresponding to the continuous forcing was predicted correctly.     

Control of an axisymmetric subsonic air jet by plasma actuator

It is known that surface non-thermal plasma actuators have proved their efficiency for aerodynamics flow control. In this study, a dielectric barrier discharge (DBD) is mounted on the diffuser of an axisymmetric turbulent air jet in order to control the flow separation along a 12-degree diffuser bevel. The momentum created by the actuator is applied to separate an air flow naturally attached to the diffuser for air flow velocity up to 40 m s⁻¹. Laser sheet visualizations and LDV measurements are achieved to characterize the unforced and forced air jet. The flow separation, the induced velocity fluctuations, the jet mixing improvement and vectoring are investigated. The main results of this study demonstrate that DBD actuators are suitable to fully detach the air flow along the bevel for a velocity of 20 m s⁻¹ and that a jet vectoring between 13.5° and 5.5° could be achieved for velocity ranging between 20 and 40 m s⁻¹. Considerations about a potential improvement of the jet mixing are also introduced and the laser sheet visualization attests that induced flow perturbations are highly 3D.     

Laser-Doppler measurements of laminar and turbulent flow in a pipe bend

Laser-Doppler measurements are reported for laminar and turbulent flow through a 90° bend of circular cross-section with mean radius of curvature equal to 2.8 times the diameter. The measurements were made in cross-stream planes 0.58 diameters upstream of the bend inlet plane, in 30, 60 and 75° planes in the bend and in planes one and six diameters downstream of the exit plane. Three sets of data were obtained: for laminar flow at Reynolds numbers of 500 and 1093 and for turbulent flow at the maximum obtainable Reynolds number of 43 000. The results show the development of strong pressure-driven secondary flows in the form of a pair of counter-rotating vortices in the streamwise direction. The strength and character of the secondary flows were found to depend on the thickness and nature of the inlet boundary layers, inlet conditions which could not be varied independently of Reynolds number. The quantitative anemometer measurements are supported by flow visualization studies. Refractive index matching at the fluid-wall interface was not used; the measurements consist, therefore, of streamwise components of mean and fluctuating velocities only, supplemented by wall pressure measurements for the turbulent flow. The displacement of the laser measurement volume due to refraction is allowed for in simple geometrical calculations. The results are intenden for use as benchmark data for calibrating flow calculation methods.     

Manipulation of the reverse-flow region downstream of a fence by spanwise vortices

The experimental investigation of a turbulent separated flow over a fence is presented. By introducing a periodic disturbance upstream of the separation region in front of the fence, the time averaged length of the separation region downstream of the fence was reduced by as much as 40%. Two types of flow manipulation were applied: an oscillating cross-flow with zero net mass-flux through a spanwise slot in the floor of the test section and a spanwise oriented, oscillating spoiler. The cross-flow was generated by a loudspeaker system connected to a chamber underneath the spanwise slot. Both types of flow manipulation generate spanwise vortices at the fence that convect into the region downstream of the fence where they enhance the mixing in the shear layer and reduce the time mean length of the reverse-flow region downstream of the fence. Velocity profiles phase averaged with respect to the forcing frequency and phase triggered flow visualisations show that the spanwise vortices cause the long reverse-flow region of the unmanipulated flow to break up into separate smaller regions. While the time mean length of the reverse-flow region is reduced in the manipulated case, the length of the region where instantaneous reverse-flow occurs is not changed. The data presented include wall pulsed-wire measurements of the wall shear-stress and its turbulent fluctuations, and LDA measurements of the streamwise and the wall-normal velocity components and turbulent stresses.     

Dielectric-barrier-discharge vortex generators: characterisation and optimisation for flow separation control

We investigated the use of dielectric-barrier-discharge plasma actuators as vortex generators for flow separation control applications. Plasma actuators were placed at a yaw angle to the oncoming flow, so that they produced a spanwise wall jet. Through interaction with the oncoming boundary layer, this created a streamwise longitudinal vortex. In this experimental investigation, the effect of yaw angle, actuator length and plasma-induced velocity ratio was studied. Particular attention was given to the vortex formation mechanism and its development downstream. The DBD plasma actuators were then applied in the form of co-rotating and counter-rotating vortex arrays to control flow separation over a trailing-edge ramp. It was found that the vortex generators were successful in reducing the separation region, even at plasma-to-free-stream velocity ratios of less than 10%.     

The stability and development of tip and root vortices behind a model wind turbine

An inclined turbulent jet discharging a passive scalar into a turbulent crossflow is investigated under conditions of favorable, zero and adverse streamwise pressure gradient. Experiments are conducted in water by means of magnetic resonance velocimetry and magnetic resonance concentration measurements. The velocity ratio and density ratio are equal to one for all cases. The flow configuration is relevant to film cooling technology, the molecular properties of the fluid being immaterial in the fully turbulent regime. Under favorable pressure gradient (FPG), the streamwise acceleration tilts the jet trajectory toward the wall, which would be beneficial for the film cooling performance. However, the counter-rotating vortex pair is strengthened in the accelerating flow by streamwise stretching. Also, the crossflow boundary layer is significantly thickened by increasingly adverse pressure gradient, which affects the mass transfer from the jet. Overall, the more intense counter-rotating vortices and the thinner boundary layer associated with increasingly FPG enhance the scalar dispersion into the main flow, hampering the film cooling performance in terms of surface effectiveness.     

Experimental and numerical investigation on the flow field within a compact inlet duct

A combined experimental and numerical investigation of the flow field in a short, rectangular, diffusing S-shape inlet duct was conducted. The inlet duct had a length-to-hydraulic diameter ratio of 1.5 and an inflow Mach number of 0.44. The flow field was diagnosed utilizing stereoscopic particle image velocimetry, surface static pressure measurements, and high frequency total pressure measurements both on the lower surface and at the duct’s aerodynamic interface plane. To complement the experimental investigation and to aid in understanding the flow field associated with this complex geometry, a numerical flow simulation was undertaken. The flow field exhibited massive flow separations and shear layer formations at both turns of the compact inlet. Moreover, secondary flow structures along the duct’s lower surface and along the duct’s side walls were identified. It was shown that the two counter-rotating flow structures along the duct’s lower surface resulted in high levels of total pressure loss at the aerodynamic interface plane. A high fidelity spectral analysis of the pressure signals at the aerodynamic interface plane and along the lower surface was conducted and demonstrated that a high frequency surface static pressure sensor could identify flow separation in a non-intrusive fashion, allowing for future use in a closed-loop control scheme for active flow control. This work was part of a more comprehensive study which was to utilize active flow control to improve performance metrics of such compact inlets.     

The effect of streamwise vortices on the turbulence structure of a separating boundary layer

A high Reynolds number flat plate turbulent boundary layer is investigated in a wind-tunnel experiment. The flow is subjected to an adverse pressure gradient which is strong enough to generate a weak separation bubble. This experimental study attempts to shed some new light on separation control by means of streamwise vortices with emphasize on the change in the boundary layer turbulence structure. In the present case, counter-rotating and initially non-equidistant streamwise vortices become and remain equidistant and confined within the boundary layer, contradictory to the prediction by inviscid theory. The viscous diffusion cause the vortices to grow, the swirling velocity component to decrease and the boundary layer to develop towards a two-dimensional state. At the position of the eliminated separation bubble the following changes in the turbulence structure were observed. The anisotropy state in the near-wall region is unchanged, which indicates that it is determined by the presence of the wall rather than the large scale vortices. However, the turbulence in the outer part of the boundary layer becomes overall more isotropic due to an increased wall-normal mixing and a significantly decreased production of streamwise fluctuations. The turbulent kinetic energy is decreased as a consequence of the latter. Despite the complete change in mean flow, the spatial turbulence structure and the anisotropy state, the process of transfer of turbulent kinetic energy to the spanwise fluctuating component seems to be unchanged. Local regions of anisotropy are strongly connected to maxima in the turbulent production. For example, at spanwise positions in between those of symmetry, the spanwise gradient of the streamwise velocity cause significant production of turbulent fluctuations. Transport of turbulence in the spanwise direction occurs in the same direction as the rotation of the vortices.     

Laser velocimetry measurements in a laboratory vaned diffuser

Laser velocimetry measurements were made within a laboratory radial vaned diffuser with three different blade configurations. Measurements were made through passages with four, six and eight blades installed at off design conditions. Also, in the eight blade diffuser measurements were made between the blade passage exit and diffuser exit so that the complete secondary flow could be defined. The flow was found to separate from the blades and form large separation zones. The separation zones consisted primarily of two vortices rotating in opposite directions. At the passage exit the separation region encompassed 23% of the circumferential area for the four blade diffuser, 45% for the six blade and 40% in the eight blade diffuser. Separation occurred at 23%, 27% and 50% from the leading edge of the blades for the 4, 6 and 8 bladed diffusers, indicating that more blades better controlled the separation. Turbulence intensities ranged from approximately 5% to 15% in the primary flow and reached a few hundred percent in the secondary flow within the separation regions.     

Control of diffuser jet flow: turbulent kinetic energy and jet spreading enhancements assisted by a non-thermal plasma discharge

An axisymmetric air jet exhausting from a 22-degree-angle diffuser is investigated experimentally by particle image velocimetry (PIV) and stereo-PIV measurements. Two opposite dielectric barrier discharge (DBD) actuators are placed along the lips of the diffuser in order to force the mixing by a co-flow actuation. The electrohydrodynamic forces generated by both actuators modify and excite the turbulent shear layer at the diffuser jet exit. Primary air jet velocities from 10 to 40 m/s are studied (Reynolds numbers ranging from 3.2 to 12.8 × 10⁴), and baseline and forced flows are compared by analysing streamwise and cross-stream PIV fields. The mixing enhancement in the near field region is characterized by the potential core length, the centreline turbulent kinetic energy (TKE), the integrated value of the TKE over various slices along the jet, the turbulent Reynolds stresses and the vorticity fields. The time-averaged fields demonstrate that an effective increase in mixing is achieved by a forced flow reattachment along the wall of the diffuser at 10 m/s, whereas mixing enhancement is realized by excitation of the coherent structures for a primary velocity of 20 and 30 m/s. The actuation introduces two pairs of contra-rotating vortices above each actuator. These structures entrain the higher speed core fluid toward the ambient air. Unsteady actuations over Strouhal numbers ranging from 0.08 to 1 are also studied. The results suggest that the excitation at a Strouhal number around 0.3 is more effective to enhance the turbulence kinetic energy in the near-field region for primary jet velocity up to 30 m/s.