Sphere Wake Research Articles

Contribution of rolling resistance to the drag coefficient of spheres freely rolling on a rough inclined surface

The drag coefficient (CD) of a sphere freely rolling without slipping on a rough plane is presented in this study. Increasing panel roughness has been found to increase CD, although lubrication theory predicts that the larger gap imposed by the rougher panel should yield a smaller drag. We propose that this increase in drag is due to the effects of rolling resistance, which increases with panel roughness. The total drag on a sphere is decomposed into fluid drag and drag due to rolling resistance, where the fluid drag is predicted using a combined analytical–numerical approach. It is shown that rolling resistance can be modeled as a resistive torque opposing the sphere motion, generated by the offset contact normal force from the sphere center plane. This coefficient of rolling resistance (μr) can be predicted using the root mean square roughness (Rq) of the panel. Additionally, μr is observed to increase with sphere down-slope velocity and an empirical relationship between μr, Rq, and non-dimensional velocity (U∗) is given. A comparison of the drag predicted by the proposed model with measured data indicates good agreement for all the four panels considered. Consistent with previous literature, a non-linear relationship between μr, Rq, and U∗ is proposed. Although increasing panel roughness leads to a smaller fluid drag due to the larger gap imposed by rougher panels, the drag due to rolling resistance increases more rapidly. This leads to an increase in total drag with increase in the panel roughness. Additionally, increasing panel roughness is observed to have a significant effect on the sphere wake, leading to irregular wake shedding and increase in the Strouhal number.

Dynamics-augmented cluster-based network model

In this study we propose a novel data-driven reduced-order model for complex dynamics, including nonlinear, multi-attractor, multi-frequency and multiscale behaviours. The starting point is a fully automatable cluster-based network model (CNM) (Li et al., J. Fluid Mech., vol. 906, 2021, A21) that kinematically coarse grains the state with clusters and dynamically predicts the transitions in a network model. In the proposed dynamics-augmented CNM (dCNM) the prediction error is reduced with trajectory-based clustering using the same number of centroids. The dCNM is first exemplified for the Lorenz system and then demonstrated for the three-dimensional sphere wake featuring periodic, quasi-periodic and chaotic flow regimes. For both plants, the dCNM significantly outperforms the CNM in resolving the multi-frequency and multiscale dynamics. This increased prediction accuracy is obtained by stratification of the state space aligned with the direction of the trajectories. Thus, the dCNM has numerous potential applications to a large spectrum of shear flows, even for complex dynamics.

Flow past a sphere: Numerical predictions of thixo-viscoelastoplastic wormlike micellar solutions

In this work, we present numerical solutions to the benchmark flow past a sphere problem considering the rheology of semi-diluted and concentrated wormlike micellar viscoelastic solutions using the most recent model-variant of the Bautista–Manero–Puig (BMP) family of fluids, i.e. the BMP+ _ τ p model (López-Aguilar et al., [1] ). This study is performed using our in-house finite volume/element algorithm, stabilised to attain highly-elastic non-linear solutions with two novel constitutive and boundary-condition implementations, i.e. the ABS- f (ABSolute f -functional) and the VGR (Velocity GRadient) corrections (López-Aguilar et al., [2] ). The structuring level of various wormlike micellar fluids with different solute concentrations (from semi-diluted to extremely concentrated solutions) and degrees of strain-hardening is studied with a dynamic dimensionless fluidity that connects the internal structure construction-destruction dynamics with viscoelasticity. In complex flow, a viscoelastic flow-regime unveils the prediction of a flow transition from a stable steady flow (related to a steady sphere settling) to an unsteady quasi-periodic flow regime, in which transient cycles of vortex formation-disappearance behind the sphere are present, resembling experimental findings on negative-wake instabilities and oscillations in the settling velocity for the descend of particles in wormlike micellar solutions. This instability is predicted here for semi-diluted to concentrated wormlike micellar solutions and it is located in flow-rate ranges corresponding to extensional Deborah numbers in the interval of around 26 . 7 ≤ D e e x t ≤ 40 . 3 , as reported experimentally. This behaviour is correlated with a chaotic flow-structure behind the obstacle, for which strong fluctuation in the fluid’s internal-structure is recorded. In the steady viscoelastic flow regime, correlation between the BMP+ _ τ p fluidity structure-variable and its normal-stress response around and downstream of the sphere (in the form of localised extrema) is detected, where interaction between the rheological response of the material and mixed shear-to-extensional deformation provide insights into the forces the fluid elements experiment in the sphere wake. Here, a monotonic decline in the drag-correction factor is observed, this being one of the main experimental manifestations of the steady sphere-settling phase in these materials. These findings may serve to further understand flow transitions and instabilities related to negative wakes in this kind of shear-thinning extensional-hardening fluids (Rothstein and Mohammadigoushki, [3] ). In a plastic regime, complex thixo-viscoelastoplastic BMP+ _ τ p features manifest through the prediction of fore-aft asymmetries promoted by the thixotropic structure construction-destruction and viscoelastic properties of the model under extremely low solute-concentrations. Predicted yield-fronts resemble those reported experimentally by Holenberg et al. [4] for carbopol solutions. • Negative-wake instability predictions in the De -range recorded experimentally. • Correlation between extensional viscosity and normal stresses in the sphere wake. • Localised N 1 and internal-structure f -functional in the sphere wake. • First normal-stress activity in the sphere wake follows extensional viscosity response. • Asymmetric yield-fronts under highly-concentrated gel-like wormlike micellar solutions.

Characteristics of vortex shedding in the wake of a sphere with a uniaxial through-hole

Wake flows have significant effects on vehicles, buildings, and other objects encountered in daily life. Thus, wake control techniques are of practical importance and have been extensively investigated. One passive wake-control technique is to create a vent on a bluff body, which has been determined to be effective for wake control on a circular cylinder but has not been sufficiently studied for wake control on a sphere. Therefore, this study examined the effect of the vent angle on the characteristics of the wake of a sphere. A sphere with a uniaxial through-hole (vent) was placed in a uniform flow, and the vent angle was varied in the streamwise direction. The Strouhal number in the high-frequency and low-frequency modes was examined. The flow field in the sphere wake varied with respect to the vent angle. Three frequency modes (high, low, and very low), the occurrence of a switching phenomenon, wherein the velocity fluctuated between positive and negative values, and high-frequency energy injection by the vent were observed. In addition, a high dominant frequency was observed only in the positive velocity fluctuation region.

Interaction between a stationary sphere and turbulent flow in a boundary layer

We investigate the interaction of a stationary sphere with turbulent flows in a spatially developing boundary layer over a flat plate by means of fully resolved direct numerical simulations. The diameter of the sphere D is much larger than the Kolmogorov length scale. The sphere Reynolds number ReD is changed by varying the sphere diameter, while the gap ratio is held constant with G/D=0.1, where G is the distance between the bottom of the sphere and the flat plate. The simulation results indicate that there exist complex interactions between the small-scale vortex in the wake of the sphere and the large-scale coherent structures inside the turbulent boundary layer. The jet-like flow through the gap is deflected away from the wall, and the level of deflection increases with ReD. As a result, asymmetrical flow structures are observed in the recirculation zone. In addition, the mean recirculation length is found to decrease with the increase in ReD. Concerning the turbulence modulation, statistics show that the skin-friction drag and boundary layer thickness are decreased in the recirculation region with respect to the undisturbed flow, whereas the displacement thickness is increased due to the presence of reverse flow. These effects become more pronounced as ReD increases. The budget of the turbulent kinetic energy in the sphere wake is also examined. It is revealed that the turbulent production and viscous dissipation rate are significantly enhanced under the effect of the sphere with size of the same order of magnitude as the local boundary layer thickness.

Trajectory-optimized cluster-based network model for the sphere wake

We propose a novel trajectory-optimized cluster-based network model (tCNM) for nonlinear model order reduction from time-resolved data following Li et al. [“Cluster-based network model,” J. Fluid Mech. 906, A21 (2021)] and improving the accuracy for a given number of centroids. The starting point is k-means++ clustering, which minimizes the representation error of the snapshots by their closest centroids. The dynamics is presented by “flights” between the centroids. The proposed trajectory-optimized clustering aims to reduce the kinematic representation error further by shifting the centroids closer to the snapshot trajectory and refining state propagation with trajectory support points. Thus, curved trajectories are better resolved. The resulting tCNM is demonstrated for the sphere wake for three flow regimes, including the periodic, quasi-periodic, and chaotic dynamics. The representation error of tCNM is five times smaller as compared to the approximation by the closest centroid. Thus, the error is at the same level as proper orthogonal decomposition (POD) of same order. Yet, tCNM has distinct advantages over POD modeling: it is human interpretable by representing dynamics by a handful of coherent structures and their transitions; it shows robust dynamics by design, i.e., stable long-time behavior; and its development is fully automatable, i.e., it does not require tunable auxiliary closure and other models.

Hairpin vortex shedding from axisymmetric bodies

Characteristics of hair-pin vortex shedding from axisymmetric bodies, such as a sphere and a circular disk, were investigated by flow visualization. The wakes behind a sphere and a circular disk at Re = 650, 800 were visualized by water tunnel experiment with fluorescence dye. The hair-pin vortex was continuously separated from the axisymmetric bodies. However, the orientation of hair-pin vortex shedding in various mode was depended on the Reynolds number. In this study, the azimuthal angle of the head of hair-pin vortex were observed from downstream direction. In the case of the sphere wake, the azimuthal angle of the hair-pin vortex was almost constant direction of 270 degree at Re = 650. The angle was gradually scattered at Re = 650. For the circular disk, it was found that the shedding angle of the hairpin vortex was more unstable as the Reynolds number increased than the case of sphere.

Mode transformations of vortex shedding behind a sphere with the effect of Lorentz force

In this paper, the modes of vortex shedding in the wake of a stationary sphere with the Lorentz force, which can be generated by the actuators in weakly conductive fluids and is parallel to the sphere surface, are numerically investigated at Re = 300. The relations among the wake structures, the vorticity distribution, and the motions of the rear stagnation point and the separation point are discussed before and after the application of Lorentz force. From this, the mechanism of mode transformations in the sphere wake is revealed. The results indicate that the fluid near the sphere surface is accelerated with the application of Lorentz force. The rear stagnation point and the separation point move rearward and become steady. Therefore, the vorticity on the rear surface of the sphere gradually decays and becomes steady, which leads to the periodic shedding mode of hairpin vortex replaced by the steady double-thread wake structure. Moreover, the wake structure varies with the interaction parameter N of the Lorentz force. When the Lorentz force is relatively small (N = 0.04), the vibration amplitude of hairpin vortex is weakened, and the vortex heads disappear with vortex legs and necks left. The wake is still periodic due to the hairpin vortex shedding. As N increases to 0.08, the hairpin vortex is suppressed completely, while the double-thread wake appears. The wake is steady due to the suppression of the hairpin vortex shedding. As N further increases to 0.20, a spanwise vortex ring is formed around the double-thread wake.

Flow and heat transfer in the wake of a triangular arrangement of spheres

This research work seeks to investigate the influence of spacing and heat transfer on the wake behavior of a triangular arrangement of spheres. Four experimental configurations have been investigated at three Reynolds numbers, Re1 = 350, Re2 = 700, and Re3 = 1050. Two isothermal cases were investigated with spacing between the spheres of zero and one sphere diameter, and two cases were investigated with an applied heat flux at the same spacing conditions. The time resolved particle image velocimetry results revealed various flow phenomena including flow separations, von Karman vortex shedding, and Kelvin–Helmholtz instabilities. The turbulent statistics reveal the effect of proximity and heat transfer on the time averaged values of the wake size, turbulent strengths, and Reynolds shear stress in the wake of each sphere, namely, the laminarization effects from the addition of heat and the suppression of the lead sphere wake from the proximity of the trailing spheres. These results are complemented by the application of proper orthogonal decomposition (POD) to the flow fields, which extracts the coherent structures from the flow. The modes that describe the coherent structures are extracted and described in detail, which provide further insight into effects of the experimental conditions on the temporal behavior of the flow. Many of the low order modes are found to be associated in pairs, corresponding to asymmetric structures or advection of a given structure downstream. The capability of POD to produce reduced order models of the flow is then utilized to facilitate vortex identification analysis. A turbulent kinetic energy based mode truncation criteria, which has been found to enhance vortex identification capability, is applied to select the POD modes and temporal coefficients to be used in the reduced order modeling. The reconstructed velocity fields are then analyzed with vortex identification algorithms to extract the vortex cores and boundaries. The combination of these approaches allows the study of the effect of proximity and heat transfer on the vortex characteristics, such as size, strength, and distribution.

Wake Flow Measurements Behind Rotating Smooth Spheres and Baseballs Near Critical Reynolds Numbers

Abstract The flow field behind spinning baseballs at two different seam orientations was investigated, and compared with a smooth sphere, to isolate effects of seams on the Magnus effect at Reynolds numbers of 5×104 and 1×105. The rotational speed of the three spheres varied from 0 to 2400 rpm, which are typical of spin rates imparted to a thrown baseball. These spin rates are represented nondimensionally as a relative spin rate relating the surface tangential velocity to the freestream velocity, and varied between 0 and 0.94. Mean velocity profiles, streamline patterns, and power spectral density of the velocity signals were taken using hot-wire anemometry and/or stereoscopic particle image velocimetry in the wake region. The sphere wake orientation changed over a range of relative spin rates, indicating an inverse Magnus effect. Vortex shedding at a Strouhal number of 0.25 was present on the sphere at low relative spin rates. However, the seams on the baseball prevented any consequential change in wake orientation and, at most spin rates, suppressed the shedding frequency exhibited by the sphere. Instead, frequencies corresponding to the seam rotation rates were observed in the wake flow. It was concluded that the so-called inverse Magnus effect recorded by previous investigators at specific combinations of Reynolds number and relative spin rate on a sphere exists for a smooth sphere or an axisymmetrically dimpled sphere but not for a baseball near critical Reynolds numbers, where the wake flow pattern is strongly influenced by the raised seams.

Computational Analysis of Sphere Wakes in a Linearly Stratified Fluid

Computational Analysis of Sphere Wakes in a Linearly Stratified Fluid

The Downstream of a Density-Stratified Sphere Wake

The Downstream of a Density-Stratified Sphere Wake

On the onset of instability in the wake of super-hydrophobic spheres

We report an experimental investigation of free falling super-hydrophobic (SH) spheres in glycerine-water mixtures over a wide range of Reynolds number. SH coatings have the ability to reduce the contact area between the surrounding liquid and the solid surface by entrapping an air layer in the surface roughness. We investigate the effect of this air plastron on the hydrodynamic performance of spheres, focusing our attention on the onset of wake instabilities. Our results emphasise the key role of the surface roughness properties on the triggering of wake instabilities. It is shown that, unlike what was reported in previous numerical studies on SH bluff-bodies, local deformation of the interface may act as a by-pass mechanism promoting earlier transition, yielding a decrease of the critical Reynolds number at which the wake becomes unstable. A scenario coupling the hydrodynamic instabilities (scaling with the Reynolds number) and the interface deformation (scaling with the roughness-based Weber number) is proposed to describe the different transition mechanisms in presence of SH surfaces. It is found that the promotion of wake instabilities over SH surfaces occurs when the roughness-based Weber number is larger than a critical threshold. These findings are of primary importance for guiding the design of resilient and efficient SH surfaces.

LES investigation of cavitating flows around a sphere with special emphasis on the cavitation–vortex interactions

Large eddy simulation (LES) was coupled with a homogeneous cavitation model to study turbulent cavitating flows around a sphere. The simulations are in good agreement with available experimental data and the simulated accuracy has been evaluated using the LES verification and validation method. Various cavitation numbers are simulated to study important flow characteristics in the sphere wake, e.g. periodic cavity growth/contraction, interactions between the cloud and sheet cavitations and the vortex structure evolution. The spectral characteristics of the wake for typical cloud cavitation conditions were classified as the periodic cavitation mode, high Strouhal number mode and low Strouhal number mode. Main frequency distributions in the wake were analyzed and different dominant flow structures were identified for each of the three modes. Further, the cavitation and vortex relationship was also studied, which is an important issue associated with complex cavitating sphere wakes. Three types of cavitating vortex structures alternate, which indicates that three different cavity shedding regimes may exist in the wake. Analysis of vorticity transport equation shows a significant vorticity increase at the cavitation closure region and in the vortex cavitation region. This study provides a physical perspective to further understand the flow mechanisms in cavitating sphere wakes. Three types of cavitating vortex structures alternate, i.e. the sub-scale vortex, fine vortex structure and large-scale vortex, are clearly discernible. The cavity shedding process produces the streamwise vortex cavitation, horseshoe-like shaped vortex cavitation and other complex vortex structures. This study provides a physical perspective to further understand the flow mechanisms in cavitating sphere wakes.

Motion of finite-size spheres released in a turbulent boundary layer

Individual magnetic wax spheres with specific gravities of 1.006, 1.054 and 1.152 were released from rest on a smooth wall in water at friction Reynolds numbers, Reτ=680 and 1320 (sphere diameters d+=58 and 122 viscous units, respectively). Three-dimensional tracking was conducted to understand the effects of turbulence and wall friction on sphere motions. Spheres subjected to sufficient mean shear initially lifted off of the wall before descending back towards it. These lifting spheres translated with the fluid above the wall, undergoing saltation or resuspension, with minimal rotations about any axis. By contrast, spheres that did not lift off upon release mainly slid along the wall. These denser spheres lagged the fluid more significantly due to greater wall friction. As they slid downstream, they began to roll forward after which small repeated lift-off events occurred. These spheres also rotated about both the streamwise and wall-normal axes. In all cases, the sphere trajectories were limited to the buffer and logarithmic regions, and all wall collisions were completely inelastic. In the plane parallel to the wall, the spheres migrated in the spanwise direction about 12% of the streamwise distance traveled suggesting that spanwise forces are important. Variations in sphere kinematics in individual runs were likely induced by high and low momentum zones in the boundary layer, vortex shedding in the sphere wakes, and wall friction. The repeated lift-offs of the forward rolling denser spheres were attributed to a Magnus lift.

Flow measurements in the near wake of a smooth sphere and one mimicking a pine cone

The flow field in the sphere wake may be affected by roughness patterns such as those found on pine cones. These may enhance pollination by generating favorable flow patterns that capture conspecific pollen. A combination of time-resolved particle image velocimetry (PIV) and tomographic PIV is used to study the instantaneous three-dimensional vortex shedding, as well as mean velocities and turbulent stresses, in the near wake of a smooth and a rough (mimicking a pine cone) sphere, at intermediate Reynolds numbers (from 200 to 5000), where little quantitative flow field data are available.

Flow past a single stationary sphere, 1. Experimental and numerical techniques

Flow separation across bluff bodies has been a fascinating area of fluid mechanics since 1851. A large number of computational and experimental studies have been performed in the last several decades which have enabled the understanding and hence helped in distinguishing the flow regimes based on the characteristics exhibited by different flow parameters. These signify some physical phenomena resulting due to flow separation under different flow conditions. A lot of research from the 70's to 90's was conducted using hot wire techniques for investigating the flow features. These techniques provided three-dimensional insights into the flow and helped in quantitative, reliable and robust corroboration of earlier qualitative findings which were based on inadequate visualization techniques. Moreover, the rapid growth in computing resources in the last couple of decades, have made it possible to use highly efficient numerical techniques to investigate separated flows with much higher accuracy and control. Also, the introduction of sophisticated measurement techniques such as PIV/ PLIF has aided in gaining deeper insights into the physics of sphere wakes. This work aims to present the developments and findings in the study of flow past a stationary sphere with respect to experimental and computational investigations. The statistical and signal processing techniques which form the chief medium to characterize the turbulence in such adverse pressure gradient wake flows, have also been discussed briefly. All the previous studies have been critically analysed presented as a coherent theme. This paper also brings out the knowledge gaps and accordingly recommendations have been made for the future research.

The effect of imposed rotary oscillation on the flow-induced vibration of a sphere

This experimental study investigates the effect of imposed rotary oscillation on the flow-induced vibration of a sphere that is elastically mounted in the cross-flow direction, employing simultaneous displacement, force and vorticity measurements. The response is studied over a wide range of forcing parameters, including the frequency ratio $f_{R}$ and velocity ratio $\unicode[STIX]{x1D6FC}_{R}$ of the oscillatory forcing, which vary between $0\leqslant f_{R}\leqslant 5$ and $0\leqslant \unicode[STIX]{x1D6FC}_{R}\leqslant 2$ . The effect of another important flow parameter, the reduced velocity, $U^{\ast }$ , is also investigated by varying it in small increments between $0\leqslant U^{\ast }\leqslant 20$ , corresponding to the Reynolds number range of $5000\lesssim Re\lesssim 30\,000$ . It has been found that when the forcing frequency of the imposed rotary oscillations, $f_{r}$ , is close to the natural frequency of the system, $f_{nw}$ , (so that $f_{R}=f_{r}/f_{nw}\sim 1$ ), the sphere vibrations lock on to $f_{r}$ instead of $f_{nw}$ . This inhibits the normal resonance or lock-in leading to a highly reduced vibration response amplitude. This phenomenon has been termed ‘rotary lock-on’, and occurs for only a narrow range of $f_{R}$ in the vicinity of $f_{R}=1$ . When rotary lock-on occurs, the phase difference between the total transverse force coefficient and the sphere displacement, $\unicode[STIX]{x1D719}_{total}$ , jumps from $0^{\circ }$ (in phase) to $180^{\circ }$ (out of phase). A corresponding dip in the total transverse force coefficient $C_{y\,(rms)}$ is also observed. Outside the lock-on boundaries, a highly modulated amplitude response is observed. Higher velocity ratios ( $\unicode[STIX]{x1D6FC}_{R}\geqslant 0.5$ ) are more effective in reducing the vibration response of a sphere to much lower values. The mode I sphere vortex-induced vibration (VIV) response is found to resist suppression, requiring very high velocity ratios ( $\unicode[STIX]{x1D6FC}_{R}>1.5$ ) to significantly suppress vibrations for the entire range of $f_{R}$ tested. On the other hand, mode II and mode III are suppressed for $\unicode[STIX]{x1D6FC}_{R}\geqslant 1$ . The width of the lock-on region increases with an increase in $\unicode[STIX]{x1D6FC}_{R}$ . Interestingly, a reduction of VIV is also observed in non-lock-on regions for high $f_{R}$ and $\unicode[STIX]{x1D6FC}_{R}$ values. For a fixed $\unicode[STIX]{x1D6FC}_{R}$ , when $U^{\ast }$ is progressively increased, the response of the sphere is very rich, exhibiting characteristically different vibration responses for different $f_{R}$ values. The phase difference between the imposed rotary oscillation and the sphere displacement $\unicode[STIX]{x1D719}_{rot}$ is found to be crucial in determining the response. For selected $f_{R}$ values, the vibration amplitude increases monotonically with an increase in flow velocity, reaching magnitudes much higher than the peak VIV response for a non-rotating sphere. For these cases, the vibrations are always locked to the forcing frequency, and there is a linear decrease in $\unicode[STIX]{x1D719}_{rot}$ . Such vibrations have been termed ‘rotary-induced vibrations’. The wake measurements in the cross-plane $1.5D$ downstream of the sphere position reveal that the sphere wake consists of vortex loops, similar to the wake of a sphere without any imposed rotation; however, there is a change in the timing of vortex formation. On the other hand, for high $f_{R}$ values, there is a reduction in the streamwise vorticity, presumably leading to a decreased total transverse force acting on the sphere and resulting in a reduced response.

Flow around in-line sphere array at moderate Reynolds number

In the present study, we perform a series of water-tunnel experiments to investigate the flow around an in-line sphere array at the Reynolds number of 1000, based on the free-stream velocity and sphere diameter, focusing on how the sphere wake changes by the additional spheres located behind while the spacing between them being varied. We try to capture the qualitative picture of the wake behind each sphere and to measure quantitative distribution of flow statistics using dye visualization and particle image velocimetry techniques. When the spacing is less than one sphere diameter, the gap flow is steady and axisymmetric, while the last wake is planar symmetric with a weaker turbulence level. When the spacing is twice larger than the sphere diameter, on the other hand, the flows in the gap and final wake tend to recover the axisymmetric feature, which is similar to that of a single sphere wake at the same Reynolds number. Between these two regimes, there is a transitional flow regime (the spacing is comparable to the sphere diameter) where the flow is still planar symmetric, but the turbulence level is enhanced. Depending on the flow topology, it is found that the characteristic frequencies (i.e., Strouhal number) in the shear layers and the gap are also affected. Finally, based on our analysis, we propose a flow regime map for each wake in the considered sphere arrays and also estimate the pressure distribution from the velocity measurements, by which the forces acting on each sphere can be assessed.

Flutter to tumble transition of buoyant spheres triggered by rotational inertia changes

Heavy particles sink straight in water, while buoyant bubbles and spheres may zigzag or spiral as they rise. The precise conditions that trigger such path-instabilities are still not completely understood. For a buoyant rising sphere, two parameters are believed to govern the development of unsteady dynamics: the particle’s density relative to the fluid, and its Galileo number. Consequently, with these parameters specified, the opportunities for variation in particle dynamics appear limited. In contrast to this picture, here we demonstrate that vigorous path-oscillations can be triggered by modulating a spherical particle’s moment of inertia (MoI). For a buoyant sphere rising in a turbulent flow, MoI reduction triggers a tumble–flutter transition, while in quiescent liquid, it induces a modification of the sphere wake resulting in large-amplitude path-oscillations. The present finding opens the door for control of particle path- and wake-instabilities, with potential for enhanced mixing and heat transfer in particle-laden and dispersed multiphase environments.