Cross-stream Velocity Fluctuations Research Articles

Numerical evaluation of the bubble dynamic influence on the characteristics of multiscale cavitating flow in the bluff body wake

Unsteady cavitating flow around the bluff body widely existed in practical engineering fields. This paper aims to investigate the physics involved in the multi-scale cavitating flow in the wake of a wedge-shaped bluff body, with special emphasis on the bubble effects. Numerical simulation is conducted using the traditional Eulerian-Eulerian (E-E) and newly developed Eulerian-Lagrangian (E-L) methods in OpenFOAM. It is found that, compared with the experimental results, the E-L method can predict more accurate cavitation characteristics than the E-E method due to the contribution of discrete bubbles. Bubbles can significantly influence the behaviors of the multi-scale cavitating flow. Specially, analysis of pressure fluctuations indicates that the bubbles induce a stronger power spectral density in the middle- and high-frequency regions. With the bubble effects considered, the vortex structure is stretched and more extensive in the near wake regions. The strength of time-averaged vorticity distribution decreases in the far wake region. Further analysis of the vortex transport equation shows that bubble dynamics significantly increase the intensity of the stretching term by enhancing the spanwise velocity fluctuations. The intensity of the baroclinic torque term predicted by the E-L method is higher than that by the E-E method near the center line due to the influence of bubbles. On the other hand, the analysis of turbulent kinetic energy distribution indicates that bubbles can induce turbulence by increasing cross-stream velocity fluctuations. The increment of the shear Reynold stress, u′xu′y‾, suggests that the bubbles intensify the coupling between the streamwise and cross-stream velocity fluctuations.

Flow characteristics and passive flow control of circular cylinders with triangular vortex generators: An experimental investigation

Optimizing flow patterns around cylinders is crucial for diverse engineering applications, aiming to mitigate vibrations induced by vortex shedding and minimize drag. Strategies, such as the implementation of vortex generators and modifications in the cylinder's surface geometry, are explored to optimize flow patterns and enhance the performance of structures exposed to fluid dynamics. This study investigated the flow characteristics of a smooth circular cylinder and cylinders equipped with triangular vortex generators placed at different position angles (α=15°, 30°, 60°, and 75°) and yaw angles (β=30°, 45°, 60°, and 75°) at the surface. The particle image velocimetry technique is used to measure the instantaneous and time-averaged velocities of the fluid flow over cylinders at Reynolds Number 8000. It is found that flow characteristics are highly dependent on the position angle of the triangular vortex generators. Specifically, at α=30° and β=60°, a substantial reduction of 17.3, 10.3, and 19.5 % occurred in normalized streamwise and root mean square of cross-stream velocity fluctuations and turbulent kinetic energy values, respectively, in comparison to that of the smooth circular cylinder. Furthermore, a noteworthy drag reduction of 33.3 % was achieved in the case of α=30° and β=60° through the elongation of the recirculation length and the establishment of a low-pressure zone away from the base of the circular cylinder. On the other hand, the lowest Strouhal number is obtained as 0.180 in the case of α=75° and β=30° where it was 0.211 at the smooth CC case. As a result, triangular vortex generators with an area of 1.1% of the total side projection area of the cylinder were found to be very effective in flow control.

Regeneration mechanisms of oblique and streamwise vortices in oblique transition

Oblique transition can be triggered by introducing componentwise optimal perturbations into a plane Couette flow. It is shown that in the middle stage of the transition, oblique vortices are generated again through transient spanwise velocity energy growth in the presence of developed streamwise streaks. In the following stage, streamwise independent kinetic energies of cross-streamwise velocities are intensified due to the nonlinear contribution caused by amplified oblique vortices together with the pressure transport, and the increased cross-stream velocity fluctuations can strengthen the lift-up effect. As a result, the streamwise vortices are formed again, and the streamwise streaks are even enhanced prior to their final breakdown to turbulence.

Flow over a confined mounted fence at low and moderate Reynolds number: A numerical study

Direct numerical simulations of a flow over a wall mounted fence confined by two parallel walls were conducted at low and transitional Reynolds number (50⩽ReD⩽2000) for blockage ratios ranging from 0.25 (low blockage) to 0.9 (high blockage). A primary recirculation bubble forms downstream of the fence and the length of this recirculation region increased with Reynolds numbers. A secondary recirculation bubble appeared on the opposite wall to the mounted fence for the laminar cases with higher blockage ratios at sufficiently large Reynolds number. The separation and reattachment locations were documented and compared with numerical and experimental data. The length of the primary recirculation bubble in current simulations are consistent with the literature while the comparison of the secondary recirculation bubble with experimental data had limited agreement. This secondary recirculation bubble disappeared for the cases at transitional Reynolds number. Instead, the strong primary recirculation bubble induced a corner eddy at the rear of the fence. ‘Packets’ of cross stream velocity fluctuations were observed as the initially 2D, steady, laminar shear layer breakdowns to a 3D turbulent flow. The space–time diagram also showed ripple-like fluctuations of the shear layer further downstream of the fence with a single distinct peak observed in the streamwise and cross flow velocity fluctuations spectra. This peak in the spectra can be predicted by linear stability analysis of a mixing layer (Yasuo et al., 1986). In the near wake, scale-like features were observed while patches of fluctuations of a longer timescale were noted in the recirculation region.

Experimental Study of Pressure and Velocity Fluctuations Induced by Cavitation in a Small Venturi Channel

The purpose of this paper is to investigate experimentally the influence of the cavitation extent on the pressure and velocity fluctuations in a small convergent–divergent channel. The mean cavity length is determined from high-speed photography images. The mean pressure and the intensity of the pressure fluctuations are obtained from the transient pressure signals recorded by two pressure transducers at the inlet and outlet of the test section. The statistical turbulence quantities are derived from the instantaneous velocity fields measured by the laser-induced fluorescent particle image velocimetry (PIV-LIF) technique. The experimental results show that the decrease of the cavitation number (the increase in the extent of cavitation) leads to a rise in the turbulent fluctuations in the wake region due to the impact of vapour clouds collapsing, while the presence of a vapour phase is found to reduce the streamwise and cross-stream velocity fluctuations in the attached cavity. It might be attributed to two mechanisms: the presence of a vapour phase modifies the vortex-stretching process, and the cavitation compressibility damps out the turbulent fluctuations. Similar effects of cavitation are also observed in the pressure fluctuations.

Bubble image velocimetry with a field-deployable acoustic camera

This study explores performing bubble image velocimetry (BIV) analysis on images acquired from an acoustic camera (AC) to obtain measurements of the flow in the wake of a circular cylinder at two flowrates. The AC observes the returns from air micro-bubbles suspended in the flow which are used as tracer particles in PIV-style analysis. The size distribution of the micro-bubbles is measured with an acoustic bubble spectrometer (ABS). The AC BIV measurements of the wake agree well with the comparison measurements made by cross-stream arrays of acoustic Doppler velocimeters (ADVs) for the lower of the two measured flowrates. The AC BIV measurements of cross-stream profiles of the mean downstream velocities and the profiles of characteristics of the velocity fluctuations, such as the standard deviation of the downstream and cross-stream velocity fluctuations, the covariance of the horizontal velocity fluctuations, and the kinetic energy of the horizontal velocity fluctuations, all matched the ADV measurements well. However, the measurement technique is sensitive to bubble size distribution, as the higher flowrate AC BIV measurements struggled to replicate the ADV measurements, particularly the measurements of the velocity fluctuations. The higher flowrate experiment is revealed to have an order of magnitude more bubbles with size exceeding 15 present in the flow than the lower flowrate experiment. The increased bubble density caused increased noise in the AC images and a ‘flickering’ effect, with bubbles further away from the camera being intermittently obscured by those closer to the camera. The ability to make PIV-style measurements with an AC has ramifications for flow measurements in the field. ACs operate in turbid environments, do not require a laser and extensive optics, and do not require calibration with a grid plate to convert pixel space to physical space.

Comparison of Flow Characteristics of Different Sphere Geometries Under the Free Surface Effect

Comparison of the experimental results of turbulent flow structures between a smooth sphere and a sphere with a vent hole, roughened, and o-ring is presented in the presence of a free-surface. Dye visualization and particle image velocimetry (PIV) techniques were performed to examine effects of passive control methods on the sphere wake for Reynolds number Re = 5000 based on the sphere diameter with a 42.5mm in an open water channel. Instantaneous and time-averaged flow patterns in the wake region of the sphere were examined from point of flow physics for the different sphere locations in the range of 0≤h/D ≤2.0 where h was the space between the top point of the sphere and the free surface. The ratio of ventilation hole to sphere diameter was 0.15, o-ring was located at 55 o with a 2 mm from front stagnation point of the sphere and roughened surface was formed by means of totally 410 circular holes with a 3 mm diameter and around 2 mm depth in an equilateral triangle arrangement. The flow characteristics of instantaneous velocity vectors, vorticity contours, time-averaged streamline patterns, Reynolds stress correlations and streamwise and cross-stream velocity fluctuations for both the smooth and passively controlled sphere were interpreted.

Passive control of flow structure interaction between a sphere and free-surface

Flow characteristics for both a smooth and a vented sphere such as velocity vectors, patterns of streamlines, vorticity contours, stream-wise fluctuations, cross-stream velocity fluctuations and Reynolds stress correlations between a sphere and free-surface for various submerged ratio at Re =5,000 are studied by using dye visualization and the particle image velocimetry technique. Passive control of flow structure interaction between sphere and free surface was examined by using a modified geometry which has a 15% sphere diameter hole passing through the sphere equator. Both of the spheres were separately placed beneath the free surface with different positions from touching to the free surface to two sphere diameters below the free surface. It is demonstrated that reattachment point of the separated flow to the free surface varies for both of the sphere cases as the sphere position alters vertically through the water flow while the flow structure for the vented sphere occurs considerably symmetrical due to forming of a pair of counter-rotating ring vortices.

The influence of developed cavitation on the flow of a turbulent shear layer

Developed cavitation in a shear layer was studied experimentally in order to determine the effect that the growth and collapse of cavitation have on the dynamics of shear flows. Planar particle imaging velocimetry (PIV) was used to measure the velocity field, the vorticity, strain rates, and Reynolds stresses of the flow downstream of the cavitating and noncavitating shear layer; the flow pressures and void fraction were also measured. The flow downstream of a cavitating shear flow was compared to the noncavitating shear flow. For cavitating shear layers with void fractions of up to 1.5%, the growth rate of the shear layer and the mean flow downstream of the shear layer were modified by the growth and collapse of cavitation bubbles. The cross-stream velocity fluctuations and the Reynolds stresses measured downstream of the cavitating shear layer were reduced compared to the entirely noncavitating flow. This result is inconsistent with a scaling of the shear stress within the shear flow based on the mean flow. The decrease in the cross-stream fluctuations and Reynolds stresses suggests that the cavitation within the cores of strong streamwise vortices has decreased the coupling between the streamwise and cross-stream velocity fluctuations.

Liquid breakup at the surface of turbulent round liquid jets in still gases

An experimental study of liquid column breakup lengths and turbulent primary breakup properties at the surface of turbulent round liquid jets in still air at standard temperature and pressure is described. Jet exit conditions were limited to non-cavitating water and ethanol flows, long length/diameter ratio (greater than 40:1) constant-diameter round injector passages, jet exit Reynolds numbers of 5000–200,000, jet exit Weber numbers of 235–270,000 and liquid/gas density ratios of 690 and 860, at conditions where direct effects of viscosity were small (e.g., liquid jet Ohnesorge numbers were smaller than 0.0053). Three liquid column breakup modes were observed, as follows: a weakly turbulent Rayleigh-like breakup mode observed at small jet exit Weber and Reynolds numbers, a turbulent breakup mode observed at moderate jet exit Weber numbers, and an aerodynamic bag/shear breakup mode observed at large jet Weber numbers. The turbulent primary liquid column breakup mode was associated with conditions where drop diameters resulting from turbulent primary breakup along the liquid surface were comparable to the diameter of the liquid column itself. The bag/shear liquid column breakup mode was observed when liquid turbulence caused large deformations of the liquid column so that portions of it were in a gaseous cross flow; this resulted in bag or shear liquid column breakup, very similar to the breakup of non-turbulent liquid jets in gaseous cross flow. Mean streamwise drop velocities after breakup were comparable to mean streamwise velocities within the jet whereas mean cross stream drop velocities after breakup were comparable to cross stream velocity fluctuations within the liquid. Rates of primary breakup at the liquid surface are reported as surface efficiency factors (the fraction of the maximum possible cross stream drop liquid flux at the surface based on the mean relative cross stream velocity of the drops at the surface and the liquid density). The resulting surface efficiency factors varied from small values near the onset of liquid surface breakup to values of the order of unity as conditions near breakup of the liquid column as a whole were approached.

Particle response in a planar sudden expansion flow

Particle velocity and concentration statistics were measured in a vertically downward planar sudden expansion flow for large-eddy particle Stokes numbers ( τ p U o/5 H) ranging from 0.5 to 7.4. Particles with Stokes numbers greater than 3 did not enter the recirculation zone, exhibited substantial attenuation of cross-stream velocity fluctuations, and had large streamwise velocity fluctuations in regions of strong velocity gradient. The smallest particles filled the recirculation zone and showed strong response to the large eddies in the flow. Phase-locked particle concentration measurements showed that these particles were centrifuged away from vortex cores and concentrated between vortices. Intermediate-size particles with Stokes numbers of 1.4 were injected intermittently into the recirculation zone as tongues of particles moving down between vortices. Particle Reynolds number was found to have negligible effect on the particle velocity statistics.

96/05685 Quantitative electron spectroscopic analysis of the surface chemistry of bituminous coal

The combined effects of elevated free-stream turbulence intensity (FSTI) and streamwise acceleration on flow and thermal structures in transitional boundary layers were investigated experimentally on a heated flat plate with elevated FSTI levels ranging from 3 to 7%. The acceleration strength, based on the parameter, K [≡ (ν/U∞2)dU∞/dx], rangedfrom 0.39 × 10−6 to 4.1 × 10−6. A three-wire probe was used to measure detailed momentum and thermal boundary layer structures, including the streamwise and cross-stream velocity fluctuations, temperature fluctuation, Reynolds stresses, Reynolds heat fluxes, eddy viscosity, turbulent thermal diffusivity, and turbulent Prandtl number. The results show that FSTI has a greater effect than streamwise acceleration on flow and thermal structures in transitional boundary layers. However, the effect of streamwise acceleration on increasing the transition length is significant. Furthermore, streamwise acceleration suppresses the magnitude of uv and slows down the uv transition process relative to u′ even when elevated FSTI seems to have a dominant effect. Acceleration is also shown to suppress ϵH more than ϵM in the transitional region.

Combined effects of elevated free-stream turbulence and streamwise acceleration on flow and thermal structures in transitional boundary layers

The combined effects of elevated free-stream turbulence intensity (FSTI) and streamwise acceleration on flow and thermal structures in transitional boundary layers were investigated experimentally on a heated flat plate with elevated FSTI levels ranging from 3 to 7%. The acceleration strength, based on the parameter, K [≡ (ν/ U ∞ 2)d U ∞/dx] , rangedfrom 0.39 × 10 −6 to 4.1 × 10 −6. A three-wire probe was used to measure detailed momentum and thermal boundary layer structures, including the streamwise and cross-stream velocity fluctuations, temperature fluctuation, Reynolds stresses, Reynolds heat fluxes, eddy viscosity, turbulent thermal diffusivity, and turbulent Prandtl number. The results show that FSTI has a greater effect than streamwise acceleration on flow and thermal structures in transitional boundary layers. However, the effect of streamwise acceleration on increasing the transition length is significant. Furthermore, streamwise acceleration suppresses the magnitude of uv and slows down the uv transition process relative to u′ even when elevated FSTI seems to have a dominant effect. Acceleration is also shown to suppress ϵ H more than ϵ M in the transitional region.

Effects of Elevated Free-Stream Turbulence on Flow and Thermal Structures in Transitional Boundary Layers

The effects of elevated free-stream turbulence on flow and thermal structures in transitional boundary layers were investigated experimentally on a heated flat plate. Detailed boundary layer measurements using a three-wire probe and wall heat transfer were made with free-stream turbulence intensities of 0.5, 3.8, 5.5, and 6.4 percent, respectively. The onset of transition, transition length, and the turbulent spot formation rate were determined. The statistical results of the streamwise and cross-stream velocity fluctuations, temperature fluctuation, Reynolds stresses, and Reynolds heat fluxes were presented. The eddy viscosity, turbulent thermal diffusivity, and the turbulent Prandtl number were calculated and related physical mechanisms are discussed.

Probe systems for static pressure and cross-stream turbulence intensity

A recent study of total-pressure probes for use in highly turbulent streams is extended herein by developing probe systems that measure time-averaged static or ambient pressure and turbulence intensity. Arrangements of tubular probes of circular and elliptical cross section are described that measure the pressure at orifices on the sides of the probes to obtain different responses to the cross-stream velocity fluctuations. When the measured data are combined to remove the effect of the presence of the probes on the local pressure, the time-averaged static pressure and the cross-stream components of turbulence intensity can be determined. If a system of total pressure tubes, as described in an accompanying paper, is added to the static pressure group to form a single cluster, redundant measurements are obtained that permit accuracy and consistency checks.

Turbulence modulation in homogeneous dilute particle-laden flows

Continuous-phase properties were studied for homogeneous dilute particle-laden flows caused by nearly monodisperse glass particles falling in a stagnant water bath. Test conditions included 0.5, 1.0 and 2.0 mm diameter particles (yielding particle Reynolds numbers based on terminal velocities of 38, 156 and 545) with particle volume fractions less than 0.01%. Measurements included mean and fluctuating velocities, as well as temporal spectra and spatial correlations of velocity fluctuations in the streamwise and cross-stream directions, using a two-point phase-discriminating laser velocimeter. Flow properties were also analysed using a stochastic method involving linear superposition of randomly-arriving particle velocity fields.For present test conditions, liquid velocity fluctuations varied solely as a function of the rate of dissipation of particle energy in the liquid. The flows were highly anisotropic with streamwise velocity fluctuations being roughly twice cross-stream velocity fluctuations. Correlation coefficients and temporal spectra were independent of both particle size and the rate of dissipation of particle energy in the liquid. The temporal spectra indicated a large range of frequencies even though particle Reynolds numbers were relatively low, since both mean and fluctuating velocities in the particle wakes contributed to the spectra because particle arrivals were random. The theory predicted many of the features of the flows reasonably well but additional information concerning the mean and turbulent structure of the wakes of freely moving particles having moderate Reynolds numbers in turbulent environments is needed to address deficiencies in predictions of integral scales and streamwise spatial correlations.

The influence of particles on the spatial stability of two-phase mixing layers

A modified Rayleigh equation has been derived for the study of spatial instability of gas–particle two-phase mixing layers. The particles are assumed to have a material density much greater than the carrier fluid. The mathematical model is based on the assumptions that the mean flow profile can be approximated as that of the particle-free single-phase mixing layer and that the two phases are in dynamic equilibrium at the start of the perturbation. The resulting eigenvalue problem was solved numerically. The major finding from the analysis is that the presence of the particles enhances the stability of the two-phase flow and decreases the amplification rate of perturbations in the flow. The results show that the stability of the flow is enhanced with increased particle loading and decreased free-stream velocity ratio. For a given free-stream velocity ratio, the most amplified growth rates decrease almost linearly with the particle loading. The most amplified growth rate, however, occurs near the same angular frequency as that of the particle-free flow. The effect of particle on the perturbation phase velocity is most significant for smaller values of the angular frequency. However, the presence of particles does not change the phase velocity at the angular frequency corresponding to the most amplified rate for the single-phase flow. The phase velocity of the perturbations is slower compared to particle-free flows for modes with frequencies below the maximum amplified frequency, while it is the opposite for modes at frequencies above the maximum amplified frequency. The profiles of streamwise and cross-stream velocity fluctuations are also modified by the existence of particles. The magnitude of effects is related closely to the particle loading. For temporal instability of a two-phase mixing layer, the growth rate in time decreases linearly with increasing particle loading, which is consistent with the spatial instability results.