Computational and Experimental Study of the Effect of Inlet Swirl on Mixing Mechanisms in an Axisymmetric Lobed Mixer

2014 ◽  
Author(s):  
Joshua R. Brinkerhoff ◽  
Metin I. Yaras
Keyword(s):  
Shear Layer ◽  
Large Scale ◽  
Transient Flow ◽  
Separated Flow ◽  
Suction Side ◽  
Navier Stokes ◽  
Streamwise Vorticity ◽  
Mixing Rate ◽  
The Core ◽  
Axial Stretching

The effect of circumferential inflow swirl on the instability of the shear layer formed between the core and bypass flows discharged from an axisymmetric twelve-lobed mixer is studied through a combined experimental and computational investigation. A series of unsteady Navier-Stokes simulations are performed with 0 and 31 degrees of circumferential swirl specified in the core stream of the lobed mixer. Comparison of the axial- and swirling-inflow cases highlights the effect of swirl on the instability-driven transient flow structures that develop within and downstream of the lobed mixer. Medium- and large-scale unsteady motions are captured by the fine spatial and temporal resolution of the unsteady Navier-Stokes simulations. The simulations are validated against four-wire thermal anemometry measurements in a scaled lobed-mixer wind-tunnel model with turbulent, swirling inflow conditions. The simulation results illustrate that while the axial-inflow case develops layers of streamwise vorticity uniformly along the lobe walls, the core flow in the swirling-inflow case separates from the suction side of the lobe wall near the lobe trough. Roll-up and axial stretching of the separated flow produces Λ-shaped vortical structures upstream of the discharge plane. The Λ-shaped structures interact with the shear layers discharged from the lobe trailing edge and accelerate the breakdown of the shear layer in the swirling-inflow case relative to the axial-inflow case. The extent of this interaction is shown to strongly affect the streamwise mixing rate of the flow downstream of the discharge plane.

Large- and small-scale vortical motions in a shear layer perturbed by tabs

1999 ◽  
Vol 382 ◽  
pp. 307-329 ◽  
Author(s):  
JUDITH K. FOSS ◽  
K. B. M. Q. ZAMAN
Keyword(s):  
Shear Layer ◽  
Pitch Angle ◽  
Large Scale ◽  
Dimensional Space ◽  
Three Dimensional ◽  
Small Scale ◽  
Streamwise Vortices ◽  
Streamwise Vorticity ◽  
Cross Sectional ◽  
Scale Population

The large- and small-scale vortical motions produced by ‘delta tabs’ in a two-stream shear layer have been studied experimentally. An increase in mixing was observed when the base of the triangular shaped tab was affixed to the trailing edge of the splitter plate and the apex was pitched at some angle with respect to the flow axis. Such an arrangement produced a pair of counter-rotating streamwise vortices. Hot-wire measurements detailed the velocity, time-averaged vorticity (Ωx) and small-scale turbulence features in the three-dimensional space downstream of the tabs. The small-scale structures, whose scale corresponds to that of the peak in the dissipation spectrum, were identified and counted using the peak-valley-counting technique. The optimal pitch angle, θ, for a single tab and the optimal spanwise spacing, S, for a multiple tab array were identified. Since the goal was to increase mixing, the optimal tab configuration was determined from two properties of the flow field: (i) the large-scale motions with the maximum Ωx, and (ii) the largest number of small-scale motions in a given time period. The peak streamwise vorticity magnitude [mid ]Ωx−max[mid ] was found to have a unique relationship with the tab pitch angle. Furthermore, for all cases examined, the overall small-scale population was found to correlate directly with [mid ]Ωx−max[mid ]. Both quantities peaked at θ≈±45°. It is interesting to note that the peak magnitude of the corresponding circulation in the cross-sectional plane occurred for θ≈±90°. For an array of tabs, the two quantities also depended on the tab spacing. An array of contiguous tabs acted as a solid deflector producing the weakest streamwise vortices and the least small-scale population. For the measurement range covered, the optimal spacing was found to be S≈1.5 tab widths.

scholarly journals Scaling of separated shear layers: an investigation of mass entrainment

2017 ◽  
Vol 826 ◽  
pp. 851-887 ◽  
Author(s):  
Francesco Stella ◽  
Nicolas Mazellier ◽  
Azeddine Kourta
Keyword(s):  
Shear Layer ◽  
Large Scale ◽  
Separated Flow ◽  
Layer Growth ◽  
Upper And Lower Bounds ◽  
Stream Velocity ◽  
Physical Parameters ◽  
Separated Shear Layer ◽  
Mass Entrainment ◽  
Recirculation Length

We report an experimental investigation of the separating/reattaching flow over a descending ramp with a $25^{\circ }$ expansion angle. Emphasis is given to mass entrainment through the boundaries of the separated shear layer emanating from the upper edge of the ramp. For this purpose, the turbulent/non-turbulent interface and the separation line inferred from image-based analysis are used respectively to mark the upper and lower bounds of the separated shear layer. The main objective of this study is to identify the physical parameters that scale the development of the separated shear layer, by giving a specific emphasis to the investigation of mass entrainment. Our results emphasise the multiscale nature of mass entrainment through the separated shear layer. The recirculation length $L_{R}$, step height $h$ and free-stream velocity $U_{\infty }$ are the dominant scales that organise the separated flow (and related large-scale quantities as pressure distribution or shear layer growth rate) and set mean mass fluxes. However, local viscous mechanisms seem to be responsible for most of local mass entrainment. Furthermore, it is shown that large-scale mass entrainment is driven by incoming boundary layer properties, since $L_{R}$ scales with $Re_{\unicode[STIX]{x1D703}}$, and in particular by its turbulent state. Surprisingly, the relationships evidenced in this study suggest that these dependencies are established over a large distance upstream of separation and that they might also extend to small scales, at which viscous entrainment is dominant. If confirmed by additional studies, our findings would open new perspectives for designing effective separation control systems.

Experimental study of physical mechanisms in the control of supersonic impinging jets using microjets

2008 ◽  
Vol 613 ◽  
pp. 55-83 ◽  
Author(s):  
FARRUKH S. ALVI ◽  
HUADONG LOU ◽  
CHIANG SHIH ◽  
RAJAN KUMAR
Keyword(s):  
Shear Layer ◽  
Nozzle Exit ◽  
Feedback Loop ◽  
Large Scale ◽  
Impinging Jet ◽  
Impinging Jets ◽  
Operating Conditions ◽  
Jet Flows ◽  
Streamwise Vorticity ◽  
Control Approach

Supersonic impinging jet(s) inherently produce a highly unsteady flow field. The occurrence of such flows leads to many adverse effects for short take-off and vertical landing (STOVL) aircraft such as: a significant increase in the noise level, very high unsteady loads on nearby structures and an appreciable loss in lift during hover. In prior studies, we have demonstrated that arrays of microjets, appropriately placed near the nozzle exit, effectively disrupt the feedback loop inherent in impinging jet flows. In these studies, the effectiveness of the control was found to be strongly dependent on a number of geometric and flow parameters, such as the impingement plane distance, microjet orientation and jet operating conditions. In this paper, the effects of some of these parameters that appear to determine control efficiency are examined and some of the fundamental mechanisms behind this control approach are explored. Through comprehensive two- and three-component velocity (and vorticity) field measurements it has been clearly demonstrated that the activation of microjets leads to a local thickening of the jet shear layer, near the nozzle exit, making it more stable and less receptive to disturbances. Furthermore, microjets generate strong streamwise vorticity in the form of well-organized, counter-rotating vortex pairs. This increase in streamwise vorticity is concomitant with a reduction in the azimuthal vorticity of the primary jet. Based on these results and a simplified analysis of vorticity transport, it is suggested that the generation of these streamwise vortices is mainly a result of the redirection of the azimuthal vorticity by vorticity tilting and stretching mechanisms. The emergence of these longitudinal structures weakens the large-scale axisymmetric structures in the jet shear layer while introducing substantial three-dimensionality into the flow. Together, these factors lead to the attenuation of the feedback loop and a significant reduction of flow unsteadiness.

scholarly journals A numerical model for the time-dependent wake of a pedalling cyclist

2019 ◽  
Vol 233 (4) ◽  
pp. 514-525
Author(s):  
Martin D Griffith ◽  
Timothy N Crouch ◽  
David Burton ◽  
John Sheridan ◽  
Nicholas AT Brown ◽  
...  
Keyword(s):  
Drag Force ◽  
Large Scale ◽  
Aerodynamic Drag ◽  
Experimental Studies ◽  
Wake Flow ◽  
Time Dependent ◽  
Navier Stokes ◽  
Streamwise Vorticity ◽  
Leg Cycling ◽  
The Impact

A method for computing the wake of a pedalling cyclist is detailed and assessed through comparison with experimental studies. The large-scale time-dependent turbulent flow is simulated using the Scale Adaptive Simulation approach based on the Shear Stress Transport Reynolds-averaged Navier–Stokes model. Importantly, the motion of the legs is modelled by joining the model at the hips and knees and imposing solid body rotation and translation to the lower and upper legs. Rapid distortion of the cyclist geometry during pedalling requires frequent interpolation of the flow solution onto new meshes. The impact of numerical errors, that are inherent to this remeshing technique, on the computed aerodynamic drag force is assessed. The dynamic leg simulation was successful in reproducing the oscillation in the drag force experienced by a rider over the pedalling cycle that results from variations in the large-scale wake flow structure. Aerodynamic drag and streamwise vorticity fields obtained for both static and dynamic leg simulations are compared with similar experimental results across the crank cycle. The new technique presented here for simulating pedalling leg cycling flows offers one pathway for improving the assessment of cycling aerodynamic performance compared to using isolated static leg simulations alone, a practice common in optimising the aerodynamics of cyclists through computational fluid dynamics.

RANS Analyses of Turbofan Nozzles With Internal Wedge Deflectors for Noise Reduction

2009 ◽  
Vol 131 (4) ◽  
Author(s):  
James R. DeBonis
Keyword(s):  
Kinetic Energy ◽  
Flow Field ◽  
Noise Reduction ◽  
Turbulent Kinetic Energy ◽  
Large Scale ◽  
Navier Stokes ◽  
Core Flow ◽  
The Core ◽  
Computational Fluid Dynamics Cfd ◽  
Performance Results

Computational fluid dynamics (CFD) was used to evaluate the flow field and thrust performance of a promising concept for reducing the noise at take-off of dual-stream turbofan nozzles. The concept, offset stream technology, reduces the jet noise observed on the ground by diverting (offsetting) a portion of the fan flow below the core flow, thickening and lengthening this layer between the high-velocity core flow and the ground observers. In this study a wedge placed in the internal fan stream is used as the diverter. Wind, a Reynolds averaged Navier–Stokes (RANS) code, was used to analyze the flow field of the exhaust plume and to calculate nozzle performance. Results showed that the wedge diverts all of the fan flow to the lower side of the nozzle, and the turbulent kinetic energy on the observer side of the nozzle is reduced. This reduction in turbulent kinetic energy should correspond to a reduction in noise. However, because all of the fan flow is diverted, the upper portion of the core flow is exposed to the freestream, and the turbulent kinetic energy on the upper side of the nozzle is increased, creating an unintended noise source. The blockage due to the wedge reduces the fan mass flow proportional to its blockage, and the overall thrust is consequently reduced. The CFD predictions are in very good agreement with experimental flow field data, demonstrating that RANS CFD can accurately predict the velocity and turbulent kinetic energy fields. While this initial design of a large scale wedge nozzle did not meet noise reduction or thrust goals, this study identified areas for improvement and demonstrated that RANS CFD can be used to improve the concept.

The Role of Streamwise Vorticity in the Control of Impinging Jets

2003 ◽  
Author(s):  
F. S. Alvi ◽  
H. Lou ◽  
C. Shih
Keyword(s):  
Velocity Field ◽  
Shear Layer ◽  
Acoustic Waves ◽  
High Speed ◽  
Feedback Loop ◽  
Large Scale ◽  
Field Measurements ◽  
Impinging Jets ◽  
Streamwise Vorticity

Supersonic impinging jets produce a highly unsteady flowfield leading to very high dynamic pressure loads on nearby surfaces. In earlier studies, we conclusively demonstrated that arrays of supersonic microjet, 400 μm in diameter, effectively disrupted the feedback loop inherent in high-speed impinging jet flows. This feedback disruption results in significant reductions in the adverse effects associated with such flows. In this paper, by primarily using detailed velocity field measurements, we examine the role of streamwise vorticity in order to better understand the mechanisms behind this control scheme. The velocity field measurements clearly reveal the presence of well-organized, streamwise vortices with the activation of microjets. This increase in streamwise vorticity is concomitant with a reduction in the azimuthal vorticity of the primary jet. We propose that the streamwise vorticity is mainly a result of the redirection of the azimuthal vorticity, which leads to a weakening of the large-scale structures in the primary jet. The appearance of strong vortices in the shear layer near the nozzle exit due to microjets further weakens the spatial coherence of the coupling between the acoustic waves and shear layer instability, while thickening the jet shear layer. All these effects are thought to be collectively responsible for the efficient disruption of the feedback loop using microjets.

Fundamental Analysis of the Secondary Flows and Jet-Wake in a Torque Converter Pump: Part 2 — Flow in a Curved Stationary Passage and Combined Flows

2003 ◽  
Author(s):  
R. Flack ◽  
K. Brun
Keyword(s):  
Secondary Flow ◽  
Suction Side ◽  
Torque Converter ◽  
Secondary Flows ◽  
Wake Flow ◽  
Streamwise Vorticity ◽  
Shell Side ◽  
The Core ◽  
Flow Circulation ◽  
Vorticity Component

Previously, experimental results for the velocity field in a torque converter pump showed strong jet/wake characteristics including backflows and circulatory secondary flows. Navier-Stokes flow models were developed herein to independently analyze the pump pressure-to-suction side jet/wake flow, the core-to-shell side jet/wake flow, and the secondary flows. Two relatively simple models were employed: (i) a rotating 2-D straight-walled duct and (ii) a 180° flow bend. Parametric studies were undertaken to evaluate the effect that operating conditions and geometry had on the characteristics. Using the modified equations for the generation of streamwise vorticity and the results from the two-dimensional jet/wake model for the normal and binormal vorticity components, trends for the secondary flows in the torque converter pump were predicted. Predicted secondary flows in the torque converter pump circulated in the counter-clockwise direction (positive streamwise vorticity) in the pump mid-plane and in the clockwise direction (negative streamwise vorticity) in the pump exit plane. These trends agreed with experimental observations. Both the Reynolds number and the modified Rossby number were seen to have a significant influence on the streamwise vorticity and, thus, on the magnitude of the secondary flow velocities. The pump mid-plane counter-clockwise secondary flow circulation was primarily caused by the interaction of the pressure-to-suction side jet/wake non-uniform flow (and the associated normal vorticity component) with the high radial/axial flow turning angle the flow underwent while passing through blade passage. Similarly, the pump exit plane clockwise secondary flow circulation was caused by the core-to-shell side jet/wake non-uniform flow (and the associated binormal vorticity component) being rotated about a fixed centerline (pump shaft). Thus, the pump streamwise vorticity, which was responsible for the generation circulatory secondary flows, was directly related to the pump jet/wake phenomena.

Fundamental Analysis of the Secondary Flows and Jet-Wake in a Torque Converter Pump—Part II: Flow in a Curved Stationary Passage and Combined Flows

2005 ◽  
Vol 127 (1) ◽  
pp. 75-82 ◽  
Author(s):  
R. Flack ◽  
K. Brun
Keyword(s):  
Reynolds Number ◽  
Secondary Flow ◽  
Suction Side ◽  
Torque Converter ◽  
Secondary Flows ◽  
Wake Flow ◽  
Streamwise Vorticity ◽  
Shell Side ◽  
The Core ◽  
Vorticity Component

Previously, experimental results for the velocity field in a torque converter pump showed strong jet/wake characteristics including backflows and circulatory secondary flows. Navier-Stokes flow models were developed herein to independently analyze the pump pressure-to-suction side jet/wake flow, the core-to-shell side jet/wake flow, and the secondary flows. Two relatively simple models were employed: (i) a rotating two-dimensional straight-walled duct and (ii) a 180 deg flow bend. Parametric studies were undertaken to evaluate the effect that operating conditions and geometry had on the characteristics. Results from the model showed that the core side wake, which was due to flow separation caused by rapid radial flow turning, was primarily a function of the Reynolds number; increasing the Reynolds number increased the core-to-shell side jet/wake flow. The passage length (or curvature) strongly affected the core-to-shell jet/wake. Using the modified equations for the generation of streamwise vorticity and the results from the two-dimensional jet/wake model for the normal and binormal vorticity components, trends for the secondary flows in the torque converter pump were predicted. Predicted secondary flows in the torque converter pump circulated in the counterclockwise direction (positive streamwise vorticity) in the pump midplane and in the clockwise direction (negative streamwise vorticity) in the pump exit plane. These trends agreed with experimental observations. Both the Reynolds number and the modified Rossby number were seen to have a significant influence on the streamwise vorticity and, thus, on the magnitude of the secondary flow velocities. The pump midplane counter-clockwise secondary flow circulation was primarily caused by the interaction of the pressure-to-suction side jet/wake nonuniform flow (and the associated normal vorticity component) with the high radial/axial flow turning angle the flow underwent while passing through blade passage. Similarly, the pump exit plane clockwise secondary flow circulation was caused by the core-to-shell side jet/wake nonuniform flow (and the associated binormal vorticity component) being rotated about a fixed centerline (pump shaft). Thus, the pump streamwise vorticity, which was responsible for the generation circulatory secondary flows, was directly related to the pump jet/wake phenomena.

scholarly journals Numerical analysis of high-lift configurations with oscillating flaps

2021 ◽  
Author(s):  
Johannes Ruhland ◽  
Christian Breitsamter
Keyword(s):  
Reynolds Number ◽  
Flow Separation ◽  
Separated Flow ◽  
Navier Stokes ◽  
Rotational Axis ◽  
High Lift ◽  
Hinge Point ◽  
Reduced Frequency ◽  
Flap Deflection ◽  
The Impact

AbstractThis study presents two-dimensional aerodynamic investigations of various high-lift configuration settings concerning the deflection angles of droop nose, spoiler and flap in the context of enhancing the high-lift performance by dynamic flap movement. The investigations highlight the impact of a periodically oscillating trailing edge flap on lift, drag and flow separation of the high-lift configuration by numerical simulations. The computations are conducted with regard to the variation of the parameters reduced frequency and the position of the rotational axis. The numerical flow simulations are conducted on a block-structured grid using Reynolds Averaged Navier Stokes simulations employing the shear stress transport $$k-\omega $$ k - ω turbulence model. The feature Dynamic Mesh Motion implements the motion of the oscillating flap. Regarding low-speed wind tunnel testing for a Reynolds number of $$0.5 \times 10^{6}$$ 0.5 × 10 6 the flap movement around a dropped hinge point, which is located outside the flap, offers benefits with regard to additional lift and delayed flow separation at the flap compared to a flap movement around a hinge point, which is located at 15 % of the flap chord length. Flow separation can be suppressed beyond the maximum static flap deflection angle. By means of an oscillating flap around the dropped hinge point, it is possible to reattach a separated flow at the flap and to keep it attached further on. For a Reynolds number of $$20 \times 10^6$$ 20 × 10 6 , reflecting full scale flight conditions, additional lift is generated for both rotational axis positions.

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