Interaction Of Streamwise Vortices Research Articles

Interaction of stream-wise vortices generated by swirler grid

A system of stream-wise vortices has been created using a grid of swirling elements with alternating orientations (like a chessboard). The particle image velocimetry method has been used to map the velocity field in several planes perpendicular to the stream. The mesh-based Reynolds number is 1.35×104 and 2.71×104, respectively. The stream-wise development of turbulent kinetic energy (TKE) shows first an increase in a distance of x≈10M, followed by power-law decay. Individual vortices are detected in each snapshot. The radial profile of TKE transformed to a vortex coordinate system is almost constant, either with maximum as in static frame or zero as observed by previous research. The properties of detected vortices are studied statistically: the meandering amplitude expressed as the standard deviation of vortex positions grows roughly as ∼ex, i.e., faster than expected random-walk growth ∼x. Vortex circulation decays exponentially as predicted by classical Helmholtz theorem. The interaction between neighboring vortices is expressed via correlation of selected quantities. Correlation of energy develops downstream from anticorrelation to a positive correlation. The strongest correlation is observed between the first vortex circulation and the second vortex position perpendicular to their connection line. Other correlations are weak.

Effects of Wavy Leading-Edge Protuberance on Hydrofoil Performance and Its Flow Mechanism

This paper examines the effects on a Clark-y three-dimensional hydrofoil of wavy leading-edge protuberances in a quantitative and qualitative way. The simulation is accompanied by a hybrid RANS-LES model in conjunction with Zwart-Gerber–Belamri model. Detailed discussions of the stable no-cavitating, unsteady cavitating flow fields and the control mechanics are involved. The force characteristics, complicated flow behaviors, cavitation–streamwise vortex interactions, and the cavitating flow instability are all presented. The results demonstrate that protuberances acting as vortex generators produce a continuous influx of boundary-layer vorticity, significantly enhancing the momentum transfer of streamwise vortices and therefore improving the hydrodynamics of the hydrofoil. Significant interactions are described, including the encouragement impact of cavitation evolution on the fragmentation of streamwise vorticities as well as the compartmentation effect of streamwise vorticities binding the cavitation inception inside the troughs. The variations in cavitation pressure are mainly due to the acceleration in steam volume. In summary, it is vital for new hydrofoils or propeller designs to understand in depth the effects of leading-edge protuberances on flow control.

Filtered Rayleigh scattering mixing measurements of merging and non-merging streamwise vortex interactions in supersonic flow

By introducing large-scale streamwise vortices into a supersonic flow it is possible to enhance the rate of mixing between two fluid streams. However, increased vorticity content alone does not explicitly serve as a predictor of mixing enhancement. Additional factors, particularly the mutual interactions occurring between neighboring vortical structures, affect the underlying fundamental physics that influence the rate at which the fluids mix. As part of a larger systematic study on supersonic streamwise vortex interactions, this work experimentally quantifies the average rate of mixing of helium and air in the presence of two separate modes of vortex interaction, the merging and non-merging of a pair of co-rotating vortices. In these experiments vortex-generating expansion ramps are placed on a strut injector. The freestream Mach number is set at 2.5 and helium is injected as a passive scalar. Average injectant mole fractions at selected flow planes downstream of the injector are measured utilizing the filtered Rayleigh scattering technique. The filtered Rayleigh scattering measurements reveal that, in the domain surveyed, the merging vortex interaction strongly displaces the plume from its initial horizontal orientation while the non-merging vortex interaction more rapidly mixes the helium and air. The results of the current experiments are consistent with associated knowledge derived from previous analyses of the two studied configurations which have included the detailed experimental characterization of entrainment, turbulent kinetic energy, and vorticity of both modes of vortex interaction.

Strut Injectors for Scramjets: Total Pressure Losses in Two Streamwise Vortex Interactions

Pressure losses and mixing effectiveness are key parameters in the performance of an injection system suitable for scramjet engines. This work thus focuses on the characterization of the total pressure losses associated with two selected vortex interaction processes in a Mach 2.5 flow in order to better understand the role of vortex dynamics in the operation of scramjet combustors. Specifically, the interaction modes are targeted to obtain merging of two corotating vortices in one case while preventing it in the second. The streamwise vortices are shed from ramps placed on the surface of a strut mounted in a cold-flow supersonic wind tunnel. In-stream pitot and total temperature measurements are conducted at two downstream streamwise stations located within a shock-free region. Stereoscopic particle image velocimetry is used in conjunction with the intrusive measurement techniques to retrieve the local Mach numbers and the total pressure in the flows of interest. The losses are quantified through the pressure loss parameter. It is found that the main contribution to the momentum deficit in the wake is given by the shocks departing from the strut rather than by the vortices, with the merging case yielding slightly higher losses.

Turbulent kinetic energy decay in supersonic streamwise interacting vortices

Only a few fundamental studies on the dynamics and interactions of supersonic streamwise vortices have been conducted so far despite the recognized potential of these structures to enhance supersonic mixing. In an effort to shed light on this largely unexplored field, multiple experimental campaigns were conducted in a Mach 2.5 flow to probe the dynamics of turbulence decay in complex flows originating from selected modes of supersonic streamwise vortex interaction. The first part of the manuscript presents the detailed study of two vortex interaction scenarios: one selected to obtain merging of co-rotating vortices and the other to prevent vorticity amalgamation. In the second part, data from three additional vortex merging cases are used to substantiate the findings of the first part of the study and characterize the decay of turbulence. Stereoscopic particle image velocimetry was employed to probe the resulting flow fields at different downstream stations. It was found that these complex vortex interactions measurably affect both the morphology and the magnitude of the streamwise vorticity and turbulent kinetic energy as well as the associated decays. Particularly, while the turbulent kinetic energy across each vorticity patch undergoes an initial production before decreasing monotonically in both scenarios, its content in the coalesced structure is roughly double that of the isolated vortices. The manuscript also presents the analysis of the turbulence data from 27 supersonic vortical structures differing in shape, strength and modes of interaction, acquired within a range of vortex Reynolds numbers of almost one order of magnitude. Dimensional analysis was then used to correlate the spatial decay of turbulent kinetic energy with the vortex Reynolds number. For all the cases considered here, where the fluctuating Mach number was found to be subsonic, the form of the resulting law was similar to that reported in previous scholarly publications, despite the complexity of the vortex dynamics considered in this work.

Visualization of supersonic turbulent vortical flows with filtered Rayleigh scattering

The interactions of multiple streamwise vortices have the potential to enhance the fuel/air mixing process in the supersonic flowfields intrinsic of scramjet combustors. As part of an ongoing experimental study on the mixing enhancement capabilities of imposed streamwise vortex interactions, high quality visualizations of the resulting turbulent vortical flowfields have been obtained with the filtered Rayleigh scattering technique. By visualizing the structure and organization of the studied flowfields, which are generated by a ramped strut injector in a Mach 2.5 flow, insight into the relevant flow physics governing the vortex interactions is obtained. Filtered Rayleigh scattering is a non-intrusive laser-based diagnostic technique that images the light scattered from the constituent gas molecules of a flow, thus providing a distinct advantage over other flow visualization techniques, in that it is not required to seed the flow with gaseous or particulate flow tracers. However, because the Rayleigh scattered signal is relatively weak, it is easily obscured by light scattered from other sources, necessitating the implementation of a molecular notch filter. This fact, combined with the complex supersonic turbulent vortical flows that are of interest, makes the application of the filtered Rayleigh scattering technique challenging and non-trivial. This work will highlight the experimental considerations necessary to achieve successful visualization of the flows of interest by presenting the methods used to image a single downstream station of a non-merging vortex interaction mode. The resulting flow visualizations illustrate the highly turbulent nature of the probed flowfield while confirming the behavior observed in previous experiments.

Study of two supersonic streamwise vortex interactions in a Mach 2.5 flow: Merging and no merging configurations

This work presents the detailed analysis of the flow fields resulting from two supersonic vortex interaction modes in a Mach 2.5 cold flow. The vortex interactions were selected beforehand by means of a reduced order modeling tool to obtain merging of the co-rotating structures in two counter-rotating vortex pairs in one case and to prevent their merging in the second. To experimentally target the flow physics of interest, expansion ramps of the same height but different width and mutual distance were placed on the surface of a strut injector. The resulting flow fields have been characterized using stereoscopic particle image velocimetry surveys conducted at selected streamwise planes. Accurate measurements of the velocity and vorticity fields, strain rates, circulation, and the rate of change of the area of each vorticity patch in the surveyed flow highlight the differences between merging and non-merging scenarios as well as the role of viscosity and turbulent diffusion in the resulting vortex dynamics. In particular, it is observed that, for the high Reynolds numbers typical of these supersonic flows and the associated time scales, an inviscid, non-diffusive formulation can be used for a reduced-order description that allows to capture the relevant dynamics related to the morphological distortion of the interacting streamwise vorticity patches introduced into the supersonic flow. The distribution of the strain rates in the flows resulting from the selected vortex configurations suggests that a faster mixing process may be expected in the merging case. However, the volumetric entrainment measurements show that the configuration in which merging was not attained yields a much larger entrainment of surrounding fluid compared to the merging case.

Effects of residual riblets of impeller's hub surface on aerodynamic performance of centrifugal compressors

In this article, the effects of residual riblets of impeller's hub surface on the aerodynamic performance of a compressor are studied in detail. Several accurate geometrical models of impellers after their hub surface are machined, with various tool passes are built. The number of tool passes for the hub surface finish of a single passage of the impeller is denoted by N. Meanwhile, a dimensionless parameter Bmax is introduced to characterize the size of the ribs of the arbitrary impeller, which is the percentage ratio rate of the maximum cusp height of the hub surface to the exit blade width. The geometrical models are numerically studied by solving the Reynolds averaged Navier–Stokes equations using the SST turbulent model. The predictions show that a group of secondary cross-stream vortices are generated by the interaction of streamwise vortices above the hub surface with riblets’ tips. For the impeller researched in this article, with the decline of Bmax, the average viscous shear stress on the hub surface decreases gradually at first and then starts increasing and leveling off; accordingly, the polytropic efficiency of the compressor, on the contrary, increases gradually and then starts leveling off later. When Bmax comes to 0.58, the average viscous shear stress of the fluids on the hub surface reaches its minimum, and it is found that the polytropic efficiency of the compressor reaches a peak and stable value. The simulation results are validated against experimental data at the design Mu, and the maximum discrepancies are less than 3.8%.

Vortex dynamics studies in supersonic flow: Merging of co-rotating streamwise vortices

For air-breathing propulsion systems intended for flight at very high Mach numbers, combustion is carried out at supersonic velocities and the process is mixing limited. Substantial increase in mixing rates can be obtained by fuel injection strategies centered on generating selected modes of supersonic, streamwise vortex interactions. Despite the recognized importance, and potential of the role of streamwise vortices for supersonic mixing enhancement, only few fundamental studies on their dynamics and interactions have been conducted, leaving the field largely unexplored. A reduced order model that allows the dynamics of complex, interacting, supersonic vortical structures to be investigated, is presented in this work. The prediction of the evolution of mutually interacting streamwise vortices represents an enabling element for the initiation of an effective, systematic experimental study of selected cases of interest, and is an important step toward the design of new fuel injection strategies for supersonic combustors. The case presented in this work is centered on a merging process of co-rotating vortices, and the subsequent evolution of a system composed of two counter-rotating vortex pairs. This interaction was studied, initially, with the proposed model, and was chosen for the peculiarity of the resulting morphology of the vorticity field. These results were used to design an experimental investigation with the intent to target the same specific complex flow physics. The experiment revealed the same peculiar features encountered in the simulation.

Interaction of a streamwise vortex with the normal shock

Interaction of a supersonic streamwise vortex with the normal shock is considered. Two interaction modes (weak and strong interaction) are identified. It is demonstrated numerically that strong interaction can lead to vortex breakdown.

Simulation of aerodynamic performance affected by vortex generators on blunt trailing-edge airfoils

An investigation was carried out by numerical simulation on a wind turbine airfoil and a blunt trailing-edge airfoil with and without vortex generators (VGs), and the performance of the airfoils was analyzed. By the simulation of airfoil DU 91-W2-250 it was verified that the numerical method and model were credible. Based on this airfoil, a new one with a blunt trailing edge of 6% chord was blended by symmetrically adding thickness, and its characteristics curves were obtained through computing at key angles of attack. Additionally, the pressure distribution on blended airfoil was analyzed by comparing to the airfoil without blend. The interaction of streamwise vortices induced by VGs with trailing vortex or separation vortex was considered, followed by the uncovery of how VGs can suppress the boundary layer separation.

Streamwise vortex interaction with a horseshoe vortex

Flow control in turbomachinery is very difficult because of the complexity of its fully 3-D flow structure. The authors propose to introduce streamwise vortices into the control of internal flows. A simple configuration of vortices was investigated in order to better understand the flow control methods by means of streamwise vortices. The research presented here concerns streamwise vortex interaction with a horseshoe vortex. The effects of such an interaction are significantly dependent on the relative location of the streamwise vortex in respect to the leading edge of the profile. The streamwise vortex is induced by an air jet. The horseshoe vortex is generated by the leading edge of a symmetric profile. Such a configuration gives possibility to investigate the interaction of these two vortices alone. The presented analysis is based on numerical simulations by means of N-S compressible solver with a two-equation turbulence model.

Interaction of a Streamwise Vortex with an Oblique Shock Wave

Interaction of a supersonic streamwise vortex with an oblique shock wave is considered. A mathematical model of the streamwise vortex is constructed. Three interaction regimes (weak, moderate, and strong) are found. It is shown numerically that vortex breakdown is possible in the case of strong interaction. The influence of the governing parameters on the interaction type is studied. It is shown that the main effect on the interaction type is exerted by the streamwise velocity and angle of the wedge forming the shock wave. The effect of splitting of the primary vortex on the shock wave in the case of moderate and strong interaction regimes is found.

Coherent vortex model for surface pressure fluctuations induced by the wall region of a turbulent boundary layer

Exact solutions of the Navier–Stokes equations describing the interaction of streamwise vortices with a rigid surface are utilized to develop a conceptual model for the surface pressure spectrum associated with the wall region of a turbulent boundary layer. The evolution of single as well as pairs of coherent streamwise vortices, which principally govern the production of turbulence in the wall region, is considered in the presence of local straining flow induced by larger, outer-layer eddies. The surface pressure signatures of the coherent vortex motion and the associated power spectrum of the pressure are examined. Based on the results of the exact solutions, the surface pressure spectrum of an ensemble of independent coherent structures is modeled using the assumption of ergodicity in the manner described by Townsend and Lundgren for homogeneous turbulence. The free parameters in the model are estimated through comparison with available results from experiments and numerical simulations. The model, especially the one involving pairs of streamwise vortices, predicts the high frequency and high spanwise wave number range of the surface pressure spectrum quite well. Further, the probability density function of surface pressure associated with the model compares well with experimental results. Interestingly, the model also suggests that the contribution of the viscous interaction to low wave number spectral elements accounts for the discrepancy between experimental observations at such wave numbers and the prediction of the Kraichnan–Phillips theorem.

Streamwise vortices near a density interface

Abstract The interaction of streamwise vortices with a density interface is investigated by experiments and by computations of the stratified cavity flow. The flow pattern of the primary circulation is different from that of homogeneous fluid. The results show that the deepening of a mixed layer into a region of constant density gradient proceeds with three-dimensional deformation of the interface and that pairs of counter-rotating streamwise vortices appear in the strong shear layer near the interface. The comparison between the three-dimensional and two-dimensional computations indicates that this streamwise vortical structure contributes to mixing across the interface.

Effect of shock strength on oblique shock-wave/vortex interaction

An experimental study of the interaction between streamwise vortices and two-dimensional oblique shock waves has been conducted at Mach 2.5. The experiments involved positioning an instrumented two-dimensional wedge downstream of a semispan wing so that the trailing tip vortex from the wing interacted with the oblique shock wave formed over the wedge surface. The experiments were designed to simulate interaction of streamwise vortices with shock waves formed over aerodynamic surfaces or in supersonic inlets. The influence of oblique shock wave intensity on this inherently three-dimensional interaction was examined for vortices of variable strength. Results indicate that the interaction of a moderate strength vortex with an oblique shock wave can lead to the formation of a steady separated shock structure upstream of the oblique shock front. A significant expansion of the vortex core is observed in these cases, and the scale of the structure increases with shock wave intensity. In some instances the separated shock structure continues through the oblique shock front to strike the shock-generating wedge forming a three-dimensional shock-wave/boundary-layer interaction. The experiments indicate that significant distortion of streamwise vortices can be precipitated by oblique shock fronts with supersonic downstream conditions.

Airfoil pressure measurements during oblique shock-wave/vortex interaction in a Mach 3 stream

An experimental investigation of the interaction between streamwise discrete vortices and oblique shocks formed over a two-dimensional wedge surface was conducted in a Mach 3 blowdown wind tunnel. An instrumented two-dimensional wedge was placed downstream of a semiispan wing so that the trailing tip vortex would arbitrarily interact with the shock wave formed over the wedge surface. The experiments were designed to simulate interaction of streamwise vortices from upstream bodies with shock waves formed over aft surfaces as might be encountered in supersonic flight of aircraft and missiles. The influence of vortex strength and vortex-airfoil vertical separation distance on the interaction was examined.

Temporal development of turbulent boundary layers with embedded streamwise vortices

The interaction of streamwise vortices with turbulent boundary layer has been investigated using large-eddy simulation. The initial conditions are a pair of counterrotating Oseen vortices with flow between them directed toward the wall (common-flow-down), superimposed on various instantaneous realizations of a turbulent boundary layer. The time development of the vortices and their interaction with the boundary layer are studied by integrating the filtered Navier-Stokes equations in time. The most important effects of the vortices on the boundary layer are the thinning of the boundary layer between vortices (downwash region) and the thickening of the boundary layer in the upwash region. The vortices first move toward the wall as a result of the self-induced velocity, and then apart from each other because of the image vortices due to the solid wall. The Reynolds stress profiles highlight the highly three-dimensional structure of the turbulent boundary layer modified by the vortices. The presence of significant turbulent activity near the vortex center and in the upwash region suggests that localized instability mechanisms in addition to the convection of turbulent energy by the secondary flow are responsible for this effect. High levels of turbulent kinetic energy and secondary stresses in the vicinity of the vortex center are also observed. The numerical results show good agreement with experimental results.

Interaction of a streamwise vortex with a turbulent mixing layer

An experimental study has been conducted on the interaction of a single streamwise vortex with a plane turbulent mixing layer. The vortex was generated by a half-delta wing mounted in the settling chamber of a blower-driven wind tunnel. The single-stream mixing layer originated from the top lip of the extended contraction exit, with a tripped initial boundary layer. Initially, the vortex rode below the mixing layer and its effect was to locally distort the mean velocity and Reynolds stress distributions in the lower (high-speed) side of the mixing layer. Once the vortex became embedded within the mixing layer, the distortion, which took the form of a wrinkle, spread through the whole width of the layer. The induced extra strain rates in the mixing layer resulted in the generation of additional, relatively large, Reynolds stresses in the regions around the vortex. In particular, the secondary shear stress (∼(u′w′)) reached a maximum absolute value equivalent to about 40% of the maximum primary shear stress (∼(u′v′)), thus indicating significantly increased three-dimensionality in the mixing layer. The position and behavior of ∼(u′w′) was found to be strongly correlated with that of the streamwise vorticity. The vortex appeared to decay rapidly once it was embedded in the mixing layer; the maximum mean streamwise vorticity was observed to decay as approximately 1/X2 in this region. By the last measurement station, almost all the effects in the mixing layer resulting from the vortex, and the identity of the vortex itself, had subsided.