Primary Vortex Research Articles

Topological growth of multi-deck vortex structures above an elevated cross jet

The present numerical study provides a detailed topology-based understanding of the three-dimensional vortical flow interaction process for an elevated square cross jet of low-to-moderate velocity ratio (0.25 ≤R≤ 4.0). For R≤ 0.5, a steady inner-vortex (Iv) formed in the jet pipe owing to the leading-edge jet shear layer roll up as a spiral node. The kinematics of the limiting streamlines and separation-attachment patterns along the jet shear layer confirm the presence of the Iv. For 0.5 < R≤ 1.2, the Iv partly escapes, as its front side remains attached to the pipe hole and the azimuthally extended lee side moved out in a tilted fashion. Moreover, the continuous vortex shedding from the stack created a dominant von Karman-like street that grew along the lower side of the jet wake. Distinctly for R = 4, the intruded convective crossflow pulled up the spiral front node/Iv out of the pipe, which also shifted the onset of the Kelvin–Helmholtz instability upward. Significantly, the pulled-up node/Iv facilitated the above-orifice anti-kidney vortical evolution of the issuing jet shear layer. For the first time, the study displays the triple-deck growth of the kidney and anti-kidney vortices above an elevated square cross jet for 2.5 ≤ R ≤ 4.0 and 1000 ≤ Re ≤ 2000. For 0.5 ≤ R ≤ 1.2, the double-decked kidney vortices involving the primary and the secondary pairs grew above the central jet column. However, the stronger primary pair fast entrained the secondary one close to the orifice edge. With R ≥ 2.5, the lateral jet shear layer experienced an unexpected windward concave warping and restructured to evolve as the anti-kidney third-deck situated above the mid-deck primary kidney vortices. The topological shear layer folding and created kidney/anti-kidney vortices above the elevated square cross jet, for 0.25 ≤R≤ 4.0, appear consistent with the past measurements for a high aspect ratio elliptic flush jet. The anti-kidney vortical growth though was not detected above a square flush jet. The dominating above-orifice topological node in the high-pressure area ensured the anti-kidney vortical growth via the generated local pressure-gradient induced flow acceleration.

Effects of interface diffusion and shock strength on shock-accelerated SF6 cylinder

The effects of interface diffusion and incident shock strength on the evolution of a shock-accelerated SF6 cylinder are numerically instigated. These two effects are manifested by measuring two essential issues that determine the material mixing of a shock–cylinder interaction (SCI), namely, the variations of the cylinder area and the material line length. Three interface types (with one sharp interface and two diffuse interfaces) and five incident shock strengths (Mach number ranges from 1.21 to 2.00) are examined. The numerical results provided in the present study show that both these effects play important roles in deforming the SF6 cylinders and the corresponding cylinder area and material line stretching. The cylinder area variation histories of the present results prove that the one-dimensional theoretical prediction of Giordano and Burtschell [“Richtmyer–Meshkov instability induced by shock–bubble interaction: Numerical and analytical studies with experimental validation,” Phys. Fluids 18, 036102 (2006)] applies not only to those sharp interface cylinders but also to these diffuse interface cylinders. The material line stretching histories prove that the exponential law of Yang et al. [“Applications of shock-induced mixing to supersonic combustion,” AIAA J. 31, 854 (1993)] applies well for diffuse interfaces but not for sharp interfaces. It is found that the exponential growth of the material line for the diffuse cylinders is dominated by the mass transport, and that the secondary instabilities only play their role in the primary vortex region. Finally, a predictive law of the development of material line length on the basis of circulation prediction theory is built. It is interesting that the use of the original circulation prediction model can be extended in such a way.

On the influence of co-flow on the shocks and vortex rings in the starting phases of under-expanded jets

In the starting phases of continuously blowing under-expanded jets, this numerical study investigates the effect of co-flow (UaUj) (a) on the circulation and evolution of primary vortex ring (PVR) and (b) on the occurrence of Mach reflection, slipstream generation, and subsequent formation of counter rotating vortex rings (CRVRs). With increase in co-flow (UaUj), the PVR circulation gradually decreases. The size of supersonic PVR gradually decreases with increase in co-flow (UaUj), and at high magnitudes of co-flow (UaUj≳0.3), the supersonic PVR attains a circular shape. The strengths of embedded shock (ES) and vortex-induced shock are found to decrease with increase in co-flow (UaUj), and at high magnitudes of co-flow (UaUj≳0.3), these shocks may even cease to form inside the supersonic PVR. An increase in co-flow (UaUj) causes the expansion fan to become more and more narrow. This reduces the acceleration of the supersonic flow inside the inviscid core, thereby weakening the incident oblique shock (IOS), which in turn increases the pressure prevailing downstream of this shock inside the inviscid core. The increase in co-flow (UaUj) also leads to a simultaneous decrease in the pressures prevailing in front of the downstream marching PVR and Mach disk (MD) of the inviscid core due to the reduction in the strength of precursor shock. As the magnitudes of pressures prevailing in the upstream and downstream of Mach disk approach each other, hence, MD also weakens. This shows that with the increase in co-flow (UaUj), there is weakening of the different shocks (i.e., ES, IOS, and MD) involved in Mach reflection. This causes a reduction in the strength of the resulting slipstream, thereby affecting the formation of CRVR patterns.

Development of Vortex Asymmetry on a Generic Projectile Configuration

A combined experimental and numerical study was carried out to understand the development of vortices on the U.S. Army/Navy basic finner configuration at high angles of incidence. The experiments were carried out at the Florida A&M University and Florida State University low-speed wind tunnel, and numerical simulations were performed at U.S. Army Research Laboratory using the U.S. Department of Defense Kestrel solver. The results show a good agreement of basic aerodynamic characteristics between the experimental data and numerical simulations at low angles of incidence over the range of flow conditions; however, at high angles of incidence, the predicted aerodynamic coefficients from simulations were relatively lower. Both experimental and numerical simulation results show that asymmetric vortices develop on the cone cylinder and interact with the fins downstream at high angles of incidence. The primary vortices eventually lift off from the body, resulting in multiple secondary shear-layer vortices. The vortex liftoff location moves upstream with an increase in angles of incidence. The liftoff location of the two primary vortices can result in a change of side-force magnitude and direction. At supersonic speeds, the characteristics of the two primary vortices in terms of their size, strength, and the location on the body are very different as compared to those at subsonic speeds.

Dynamics of the interaction of synthetic jet vortex rings with a stratified interface

The vortex dynamics in a stratified environment is of fundamental importance in various flow systems, however, the mechanism of successive vortex–interface interactions remains not fully understood. This study investigates synthetic jet vortex rings impinging on a water–oil interface by experiment and numerical simulation. Two cases in the laminar and transitional regimes are concerned with the Froude number (Fr) of 0.35 and 0.82, respectively. The vortex rings for the two cases follow a similar evolution with three distinct stages, including vortex free-travel, vortex–interface interaction and vortex stretching. As products of the vortex–interface interaction, the secondary vortex rings are baroclinically generated in both water and oil followed by the formation of hairpin vortices through azimuthal deformation. Distinguished from an isolated vortex–interface interaction, complex interactions occur between the primary vortex ring and hairpin vortices, which can enhance the vortex ring instability in existence of the stratification for the laminar case. However, the vortex ring instability for the transitional case can develop earlier due to nonlinear effects with increasing Reynolds number. The spatio-temporal properties of the interface deformation and interfacial waves are analysed. The maximum penetration height of vortex rings scales with Fr2 as that for the isolated situation. Particularly, the transitional case is found to produce destabilization of the interfacial waves at a further radial location. The mechanism is revealed by connecting the dynamics of the vortex and the interface, which can be attributed to the radial development of perturbances stemming from the vortex ring instability under strong vortex–interface interaction.

Initially generated pure mode A in the three-dimensional wake transition of a circular cylinder

The initially generated pure mode A, as a transitional stage, is investigated in the three-dimensional wake transition of a circular cylinder. Direct numerical simulations are carried out over a range of Reynolds numbers from 100 to 210. According to the different dynamic behaviors of hydrodynamic parameters and similar features in the spatiotemporal evolution of vorticity in the near wake, two stages are identified. The first, investigated here, is the initial generation of pure mode A at Reynolds numbers less than 195, while the second, already reported, is the full development of pure mode A at Reynolds numbers greater than 195. The relationship between the volume-RMS (root-mean-square) vorticity and Reynolds number indicates two critical Reynolds numbers, 145 and 195 (at most). The first critical Reynolds number denotes the initial appearance of three-dimensional instability. The second critical Reynolds number indicates the transition of pure mode A from the initially generated state to the fully developed state in the near wake. After the first critical Reynolds number, the evolution of the vorticity in the near wake and on the rear surface of the cylinder clearly shows that the appearance of pure mode A is a gradual process, rather than a sudden process accompanied by a jump in vortex shedding frequency. In particular, as the Reynolds number increases, the streamwise vorticity first appears on and near the cylinder surface, then in the shear layers, and finally in the shedding primary vortices, instead of appearing instantaneously in the shedding vortices after the instability of primary vortex cores.

Experimental investigation of the near-surface flow dynamics in downburst-like impinging jets

Downbursts are strong downdrafts that originate from thunderstorm clouds and create vigorous radial outflows upon hitting the ground. This study is part of the comprehensive experimental research on downburst outflows produced as large-scale impinging jets in the WindEEE Dome simulator at Western University, Canada. The 2800 tests carried out form the largest database of experimental measurements on downburst winds developed thus far, which is made available to the public in its whole and described in detail in a complementary study. Therefore, the current manuscript merely focuses on the data post-processing outcomes and interpretation of results from a selected subset of measurements. Impinging jets are here simulated as transient phenomena in which velocity time series are characterized by a sudden ramp-up of velocity, followed by the velocity peak, a short statistically stationary region, and the final velocity slowdown, as it is expected to occur in the actual downbursts. A dominant velocity peak that was systematically observed in all velocity records is associated with the radial advection of the primary vortex in the outflow. Depending on the radial distance from the downdraft, the primary vortex was sometimes preceded by a secondary, much smaller, vortex close to the surface. Vertical profiles of mean velocity and turbulence intensity are for the first time characterized through the extent of a downburst-like event in the spatiotemporal domain. Particularly, these profiles rapidly change in relation to the passage of the primary vortex and consequent variation of the surface layer thickness. This study lays out a foundation for an experimental model of non-stationary downburst outflows to come.

Dense velocity reconstruction with VIC-based time-segment assimilation

The vortex-in-cell time-segment assimilation (VIC-TSA) method is introduced. A particle track is obtained from a finite number of successive time samples of the tracer’s position and velocity can be used for reconstruction on a Cartesian grid. Similar to the VIC + technique, the method makes use of the vortex-in-cell paradigm to produce estimates of the flow state at locations and times other than the measured ones. The working principle requires time-resolved measurements of the particles’ velocity during a finite time interval. The work investigates the effects of the assimilated length on the spatial resolution of the velocity field reconstruction. The working hypotheses of the VIC-TSA method are presented here along with the numerical algorithm for its application to particle tracks datasets. The novel parameter governing the reconstruction is the length of the time-segment chosen for the data assimilation. Three regimes of operation are identified, based on the track length and the geometrical distance between neighbouring tracks. The regime of adjacent tracks arguably provides the optimal trade-off between spatial resolution and computational effort. The VIC-TSA spatial resolution is evaluated first by a numerical exercise; a 3D sine wave lattice is reconstructed at different values of the particles concentration. The modulation appears to reduce (cut-off delay) when the time-segment length is increased. Large-scale PIV experiments in the wake of a circular cylinder at Red = 27,000 are used to evaluate the method’s suitability to real data, including noise and data outliers. Both primary vortex structures in the Kármán wake as well as interconnecting ribs are present in this complex flow field, with a typical diameter close to the average inter-particle distance. When the time-segment is increased to adjacent tracks and beyond, a more regular time dependence of local and Lagrangian properties is observed, confirming the suitability of the time-segment assimilation for accurate reconstruction of sparse velocity data.Graphical abstract

VR helicity density and its application in turbomachinery tip leakage flows

Helicity is an important quantity that represents the topological interpretation of vortices; however, helicity is not a Galilean invariant. In this study, VR helicity density (HVR) is derived via taking the dot product of vorticity with the unit real eigen vector of the velocity gradient tensor when the complex eigenvalues exist. The analytical solution of HVR is derived to resolve it in a local pointwise manner, and the Galilean invariance of HVR is proved. Tip leakage flow structures in a direct numerical simulation of a tip leakage flow model and a delayed detached eddy simulation of a low-speed large-scale axial compressor rotor are extracted using helicity, eigen helicity density and HVR methods. Results show that the utilization of HVR permits the identification and accentuation of concentrated vortices. Vortices identified by HVR appear in more connective states. As in the case of helicity, the sign of HVR distinguishes between primary and secondary vortices, while eigen helicity density fails. The normalized HVR is superior to the normalized helicity density in locating the vortex axis, especially for the induced vortex structures. Hence, HVR is a strong candidate to replace the helicity density, especially when Galilean invariance is required.

Wake flow modification of vehicle external door mirror with slots

A passive flow control method is applied to a vehicle external door mirror by introducing flow from a slot drilling from the front surface to four slots around the rear surface. The wake flow fields behind the door mirror and slotted door mirror are experimentally investigated via particle image velocimetry measurements at a Reynolds number of 12,736. The time-averaged flow fields suggest that the reverse-flow region of the slotted door mirror is reduced and pushed downstream; correspondingly, the turbulent kinetic energy of the slotted door mirror is reduced, implying the suppression of unsteadiness and a reduction in the drag force acting on the door mirror. As a result of the jet flow, the wake flow of the slotted door mirror exhibits a weaker spatial correlation and a smaller length scale, indicating the suppression of large-scale vortex shedding and the enhancement of small-scale flow motion by the slots. The jet flows from the left and right slots of the slotted door mirror prevent the upper and lower shear layers from interacting with each other, which promotes symmetric vortex shedding in the near-wake region. The power spectral density and modal energy distributions suggest that the strength of the primary vortex shedding is reduced by the slotted door mirror. The spatial patterns of the first two proper orthogonal decomposition modes suggest that the jet flow distorts the dominant large-scale structure, reducing the strength of the flow oscillation behind the slotted door mirror. The spatial distributions of modes 3 and 4 suggest that the symmetric vortex shedding is enhanced by the jet flow from the slotted door mirror.

On the links between unsteady separation and Nusselt number deterioration during vortex-wall interaction

A CFD study of an axisymmetric vortex ring interacting with a flat, constant-temperature, heated wall is conducted to study the physical nature of the mechanisms leading to deterioration in wall heat transfer due to the unsteady boundary layer separation resulting from vortex-wall interaction. The study utilizes a supplemental computation of a hypothetical flow configuration in which slip is allowed at the heated wall to eliminate separation of the boundary layer during the interaction. Comparison between the physical and hypothetical cases enables “isolation”, and hence understanding of the role of separation in the deterioration of Nusselt number (Nu). It is found that the nature of the near-wall upwash velocity leading to Nu deterioration is fundamentally altered by the presence of spearation. Without separation, the upwash is driven directly by the primary vortex ring. In contrast, with separation, the near-wall upwash is modified significantly by blockage from boundary layer separation and interaction with the secondary vortex ring. The modification is associated with the development of a near-wall “sublayer” that is responsible for the deterioration of Nu independent of the behavior of the overall thermal boundary layer thickness.

Experimental Investigation of the Near-Surface Flow Dynamics in Downburst-like Impinging Jets Immersed in ABL-like Winds

Downburst winds are strong downdrafts of cold air that embed into the atmospheric boundary layer (ABL) and produce intense horizontal outflow upon impingement on the ground. They are highly transient and three-dimensional extreme wind phenomena with a limited spatiotemporal structure that often makes the anemometric measurements in nature inadequate for reconstructing their complex flow fields. In the framework of the project THUNDERR, an experimental campaign on downburst outflows has been carried out at the WindEEE Dome at Western University, Canada. The present study analyzes the three-dimensional interaction between downburst (DB) outflows produced as large-scale impinging jets and ABL winds. Most experimental, numerical and analytical models in the literature neglect this flow interplay or treat it in an oversimplistic manner through a vector superposition. We found that the generated near-surface outflow is asymmetric, and a high-intensity wind zone develops at the interface between DB and ABL winds. The time variability of the leading edge of the outflow was investigated by synchronizing all wind measurements across the testing chamber. The three-dimensional flow structure was studied using a refined grid of Cobra probes that sampled the flow at high frequencies. The passage of the primary vortex produced a significant decrease in the height of maximum radial wind speed, predominantly in the ABL-streamwise direction. The turbulence intensity was the highest in the region where DB propagates into oppositely directed ABL winds.

The effects of caudal fin's bending stiffness on a self-propelled carangiform swimmer

The hydrodynamic performance of a self-propelled carangiform swimmer with a flexible caudal fin in the absence of a free stream is numerically investigated, where the fin's dimensionless bending stiffness varies from 10−3 to 150. It reveals that large flexibility of the caudal fin has a negative impact on the propulsion and moderate rigidity is found to increase the hydrodynamic performance. Two different vortex configurations are observed at low and high bending stiffnesses: (i) reverse Bénard–von Kármán (rBvK) vortex configuration and (ii) deflected reverse Bénard–von Kármán wake with the secondary vortex street, respectively. With the increase in bending stiffness, the thrust-producing part switches from the swimmer body to the caudal fin corresponding to the switch of the vortex configuration. Furthermore, the thrust and drag productions are examined. As the bending stiffness increases, the “active portion” of the caudal fin provides more kinetic energy to the wake flow. It is found that the deflected rBvK is induced by the vortical strength imbalance of two adjacent vortices, and the secondary vortex street is formed by the large strain between the primary vortex and the secondary vortex street. Meanwhile, the dynamic mode decomposition analysis indicates that the dominant mode of the dynamic flow field is the excited frequency resonant mode and the inherent frequency of the secondary vortex street is the same as the undulatory frequency. These results shed new light onto the role of the flexible caudal fin in self-propelled biological systems and may provide some inspirations to autonomous underwater vehicle design.

Efficient Optimization Design of Vortex Generators in a Highly Loaded Compressor Stator

Abstract Secondary flow transfers the boundary layer from the endwall to suction side corner region, which deteriorates the performance of compressors. Vortex generators (VGs) can attenuate the secondary flow when implemented at the endwall of the blade passage. In order to make an efficient optimization design of the VGs layout, mathematical models are introduced to replace the actual VGs. However, the model is only effective at the small incidence. In this paper, first, a mathematical model is presented to replace the actual VGs facing a lager incidence, and validated by computational fluid dynamics results with actual VGs. Second, an efficient design method for optimizing VGs layout is presented based on VG model and genetic algorithm (GA)-support vector machine (SVM) method. The method is illustrated in a highly loaded compressor stator, and the VGs layout is optimized and applied in the blade passage. The results show that the VGs can prevent the secondary flow from the suction corner, and reduce the total pressure loss coefficient at the trailing edge of the stator by 15%. Finally, the flow mechanism of the VGs is investigated using numerical method. The results show that the VGs can induce a primary vortex at the tip and prevent the circumferential migration of boundary layers. Moreover, the secondary flow can produce a large incidence at the hub of the VGs, thus increasing the aerodynamic losses of the VGs secondary vortex.

Experimental investigation on flow characteristics and unsteady heat transfer of noncircular impinging synthetic jets

This study investigates the flow characteristics and unsteady heat transfer of noncircular synthetic jets impinging on a heated plate. Thermographic phosphor thermometry and time-resolved particle image velocimetry are used to acquire quantitative wall temperature and flow field information, respectively. At a constant orifice-to-wall distance (H/De = 7.5), five orifice configurations (circular, square, elliptic with aspect ratio (AR) of 3, and rectangular with ARs of 3 and 5) are examined at a wall temperature of 80 °C. The orifice configuration has a significant effect on the heat transfer and vortex behavior of the impinging synthetic jets. The square orifice displays better stagnation cooling performance with 42% increase in the maximum stagnation cooling coefficient relative to the circular case due to the larger impinging velocity and deeper penetration into the wall shear layer. Axis-switching of the vortex rings is detected for both the elliptic and rectangular orifices before they impinge onto the wall. The axis-switching enhances the near-wall mixing and turbulent kinetic energy in the elliptic and rectangular orifices, and thus heat transfer compared with the circular case. Additionally, stronger vortices and deeper penetration into the wall shear layer are observed in the major-axis plane than in the minor-axis plane, leading to enhanced heat transfer in the major-axis direction. In particular, the rectangular orifice with an AR of 5 induces strong secondary structures in the trailing jet following the primary vortex ring, and these are responsible for the double-peak distribution of wall temperature in the major-axis direction. This is the first time a double-peak temperature distribution has been reported for noncircular impinging synthetic jets. The rectangular orifice with an AR of 5 also yields a larger cross-stream velocity in the minor-axis direction, indicating a stronger wall jet. Thus, among the cases considered in this study, the rectangular orifice with AR = 5 achieves the optimal cooling performance and the maximum time-averaged cooling area reaches more than 14 times the jet orifice area.

Influence of aspect ratio on the wake dynamics of a pitching wing

The three-dimensional flow around a custom wing performing pitching oscillation is analyzed numerically using large-eddy simulation with the wall adapting local eddy-viscosity subgrid-scale model. The wake characteristics behind the pitching wing with three different aspect ratios, AR=2/3, 1, 4/3, is investigated in an upstream flow with Reynolds number, Re=2640. The Q−function is employed to identify the swirling structures in the flow field, and four primary vortices, namely, primary leading edge vortex (PLEV), secondary leading edge vortex, wing tip vortices, and trailing edge vortex, are identified. The evolution of vortices over the wing surface is presented in detail, and the arrangement of vortex rings in the wake is compared for three different aspect ratios. The PLEV develops into an arch vortex using the self-induction phenomenon, and the interlinking between the subsequent vortex rings depends on the arch vortex leaving the trailing edge. The force coefficient computed over the wing shows a positive thrust for AR=4/3. The mean velocity field contours indicate a spanwise loading on the wing, which is unaccountable in two-dimensional studies. The Reynolds stresses generated due to large-scale periodic velocity fluctuations in the wake are presented. The vorticity field in the midplane of the wing appears to be diffusive at higher aspect ratios. The comparison of vorticity from the three-dimensional and two-dimensional models suggests that a two-dimensional model will over predict the vorticity distribution and force coefficients in the wake.

Investigation of vortex motion mechanism of synthetic jet in a cross flow

To investigate the mechanism of vortex motion, a pitched synthetic jet is arranged on the bottom of a cross flow and simulated by incompressible Naiver–Stokes equations with large eddy simulation. The Liutex vector identification method is utilized to quantitatively study the rotation strength and the core size of the vortex structures according to various driving frequencies (St = 0.25, 0.5, 0.75, 1.0) and amplitudes (A0 = 1.0, 1.5, 2.0, 2.5) of the synthetic jet during 21 periods. The results show that both the frequency and the amplitude play a significant role in the vortex generation mechanism of the synthetic jet. However, the amplitude makes a positive contribution to the rotation strength and the vortex core size, while the frequency makes a negative contribution. The vortex motion mechanism of a synthetic jet in a cross flow can be summarized as follows: low frequency and small amplitude favor the primary clockwise vortex, while high frequency and large amplitude motivate the anticlockwise vortex generation.

An investigation of bluff body flow structures in variable velocity flows

The present study explores three-dimensional vortex-dynamics past a wall-attached bluff body kept in a variable velocity field with numerical simulations. A trapezoidal pulse of mean velocity, consisting of acceleration phase from rest followed by constant velocity phase and deceleration phase to rest, is imposed at the inlet of the computational domain similar to the experimental study of Das et al. [“Unsteady separation and vortex shedding from a laminar separation bubble over a bluff body,” J. Fluids Struct. 40, 233–245 (2013)]. For a wide range of Reynolds numbers (96≤Reb≤2390), acceleration Reynolds numbers (196≤Rea≤978), and deceleration Reynolds numbers (310≤Red≤1522), different stages of flow evolution are systematically analyzed. The flow evolution starts with the formation of a primary vortex followed by a two-dimensional circular array of spanwise vortex tubes by inflectional shear-layer instability. At a sufficiently high Reynolds number, the shear layer vortices originated from two-dimensional fluctuations deformed by three-dimensional instabilities, giving fragmented streamwise vorticity. In addition, long-wavelength “tongue-like structures” and short-wavelength “rib-like structures” are evident near the top wall and the bluff body, respectively. The streamwise vorticity generation equation indicates that the spanwise vortex tubes initially tilt, resulting in streamwise vorticity, further amplified by the vortex stretching process. The distinct flow features, including mode shape, frequency, and growth rate associated with the shear-layer instability, are identified using the dynamic mode decomposition (DMD) algorithm. Using the maximum growth rate criteria, the DMD technique successfully separates the coherent shear layer modes associated with two-dimensional shear layer instability from the flow field.

Uncertainty and sensitivity analysis of heat transfer in hypersonic three-dimensional shock waves/turbulent boundary layer interaction flows

Hypersonic shock waves/turbulent boundary layer interaction flow can cause severe localized thermal loads, while the fluctuations in freestream conditions and inevitable mechanical vibrations can strongly influence the flow structures and heat transfer. This study aims to quantify the uncertainties in heat transfer and pressure of fin-plate configurations and find their main contributors. Freestream velocity, freestream static pressure, freestream static temperature, wall temperature, and fin angles are chosen as uncertainty sources. The point-collocation non-intrusive polynomial chaos method is employed to propagate the uncertainties. Subsequently, Sobol indices are utilized to denote the contribution of each input parameter. The uncertainty and sensitivity results are plotted along representative lines and on the entire bottom surface to obtain the key parameters of uncertainties. As a consequence, the uncertainty in heat transfer can exceed 30%, which is higher than that in pressure. Freestream velocity, freestream static pressure, and fin angles are the key parameters of heat transfer. Freestream velocity is the biggest contributor to the uncertainty in heat flux near the heat flux peak by influencing flow reattachment. Fin angles dominate the developments of primary vortexes and are the source of the uncertainties near the leading edge of fins. The Sobol index of freestream static pressure is relatively high at the separation line, which reveals that the boundary of flow separation is sensitive to static pressure.

The vortex-induced vibration of an elliptic cylinder with different aspect ratios

This study investigates the hydrodynamic characteristics of an elastically-mounted elliptic cylinder at low mass ratio m*=2 and low damping ratio ζ=0.007. The aspect ratio (AR) defined by the characteristic length in the streamwise and transverse direction varies in the range 0.5≤AR≤2.0. The dynamic response is characterized as a function of reduced velocity. The streamlined profile of the elliptic cylinder results in a delay of the vortex shedding and a diminished strength of the averaged vorticity. However, the hydrodynamic characteristics become more complicated for large ARs. Both the response amplitude and the hydrodynamic forces acting on the cylinder are enhanced as the afterbody is reduced. The PSD behaviors show the existence of four different flow regimes. Several new pertinent modes, such as 2 Po*, 2T*, etc., are captured through flow visualizations. A comprehensive analysis of the vortex shedding indicates the sub-vortices generated at the rear of the cylinder plays a key role in the formation and evolution wake modes. Besides, the time-averaged vorticity magnitude at the vortex core elucidate that despite secondary vortices account for a large proportion of the primary vortices, they only make a small contribution to the suppression of the vibration amplitude.