Time-averaged Velocity Profiles Research Articles

Experimental studies on the frequency selection in flat plate wakes: Mean-flow stability analyses and low-dimensional modeling

The global frequency selection of two-dimensional vortex shedding in the flat plate wake is investigated experimentally. By performing a local stability analysis, and low-dimensional modeling based on the time-averaged mean flow velocity profiles, we have shown that the time-averaged velocity profiles give an accurate estimate of the global shedding frequency. Furthermore, we have analyzed the interaction strengths between the mean flow and the higher harmonics thereby experimentally supporting the theoretical criterion outlined previously by Sipp and Lebedev.

Hydraulics of flow over full-cycle cosine and rectangular sharp-crested weirs

Laboratory experiments were carried out to investigate the hydraulic characteristics of full-cycle cosine and rectangular sharp-crested weirs in free flow condition. The head–discharge relationships of full-cycle cosine weirs were determined theoretically and the discharge coefficients were correlated with weir geometry for both rigid and erodible bed conditions. The proposed model showed higher discharge coefficients in full-cycle cosine weirs in comparison with rectangular sharp-crested weirs. The three-dimensional velocity profiles of full-cycle cosine and rectangular weirs were measured by an acoustic Doppler velocimeter probe. The effects of weir geometry on mean flow and local erosion upstream of the weirs were studied for two sediment sizes of 0.7 mm and 1.2 mm. The time-averaged velocity profiles exhibited strong three-dimensional flow in full-cycle cosine weirs. The strength of the upper eddy in the crest location of full-cycle cosine weirs decreased by increasing the weir gap opening and the jet stream deflected upward due to uneven eddy sizes. The maximum streamwise velocity of cosine weirs increased by four times the average velocity at the crest level of the weir and the transverse velocity profiles captured strong vortices in the vertical plane closest to the weir. The effect of weir gap opening of full-cycle cosine weirs on the total scour volume was studied for two sediment sizes. It was found that the scour volume in the upstream region of cosine weirs was higher than the scour volume formed upstream of a rectangular weir with the same opening.

Flow induced by the single-bubble chain depending on the bubble release frequency

In the present study, we experimentally investigated the flow induced by the in-chain bubbles (2.4–2.8 mm in size) while varying the bubble release frequency (fb=0–31 Hz), using high-speed two-phase particle image velocimetry and shadowgraphy. Along the streamwise (vertical) direction, we identified the transition of in-chain bubble dynamics and associated liquid flow. That is, the released bubbles tend to follow the straight path initially but transition to the oscillating (zigzag or spiral) trajectories. In particular, in the developing regime (near the nozzle), the coherent behavior of bubble chain generates the rib-like oblique jet flow, deviating from the rise path, even after the onset of the path instability of each bubble. As the flow develops, the scattering of rising path along the lateral direction becomes significant, and the liquid flow shows the similarity to the typical single-phase jet flow, which becomes more obvious at fb > 8 Hz. In this regime, with increasing the fb, the oscillating motion of bubbles changes to the flattened spiral one, and the deviation (fluctuation) of the lateral amplitude increases resulting from the enhanced flow-induced wobbling of bubble shape. Finally, we developed an analytical model to predict the evolution of time-averaged vertical velocity profile of the liquid, which considers the contributions of (i) local void fraction and (ii) evolution of upstream flow, and validated successfully with the measurement.

Ultrasound imaging velocimetry in particle-laden flows: counteracting attenuation with correlation averaging

Ultrasound imaging velocimetry (UIV) refers to the technique wherein ultrasound images are analysed with 2D cross-correlation techniques developed originally in the framework of particle image velocimetry. Applying UIV to opaque, particle-laden multiphase flows have long been considered to be an attractive prospect. In this study, we demonstrate how fundamental differences in acoustical imaging, as compared to optical imaging, manifest themselves in the 2D cross-correlation analysis. A chief point of departure from conventional particle image velocimetry is the strong variation in the intensity profile of the acoustic wavefield, primarily caused by the attenuation of ultrasonic waves in particle-laden flows. Attenuation necessitates using a larger ensemble of correlation planes to obtain satisfactory time-averaged velocity profiles. For a given combination of imaging and flow conditions, attenuation sets upper limits on volume fraction, penetration depth, as well as temporal resolutions that may be accessed confidently. This behaviour is demonstrated in two experimental datasets and is also supported by a modified cross-correlation theory. The modification is brought about by incorporating a lumped model of ultrasonic backscattering in suspensions into existing spatial cross-correlation analysis. The two experimental datasets correspond to two distinct particle-laden pipe flows: (1) a neutrally buoyant non-Brownian suspension in a laboratory-scale flow facility, wherein particle sizes are comparable to the ultrasonic wavelength and (2) a non-Newtonian slurry in an industrial-scale flow facility, wherein particle sizes are much smaller than the ultrasonic wavelength. We illustrate how and to what extent correlation averaging can counteract the adversity caused by attenuation. The work herein offers a template for one to evaluate the performance of UIV in particle-laden flows.Graphical abstract

Role of phase-difference between two adjacent rectangular synthetic jet actuators in active control of flow over a rounded ramp

In the current study, the role of phase-difference between signals of two adjacent synthetic jet actuators (SJAs) in active control of flow over a rounded ramp geometry has been investigated. In order to accurately predict the separation and reattachment locations, wall-resolved large eddy simulation has been utilized to capture the locations of separation and reattachment. The two adjacent SJAs were placed upstream of the separation point. Six phase-differences between the two SJAs were considered, and two momentum coefficients were applied. First, the role of phase-difference in active flow control of a separation bubble behind a ramp-down region was investigated. Furthermore, the impact of an increased momentum ratio on the size and length of the separation zone was investigated to assess the effectiveness of phase-difference with respect to a higher velocity ratio. The effect of increased momentum ratio on the wall pressure fluctuations was also explored. As the second objective of this study, the flow and turbulent features were discussed to unveil the SJA actuation impact on the downstream flow. The time-averaged velocity and turbulent kinetic energy profiles and the turbulent production were examined and compared to the uncontrolled baseline case. It was found that a higher velocity ratio tremendously increased the turbulent energy before the separation point, while further downstream, the level of turbulent energy was uncoupled from the SJA momentum coefficient. Our study showed that by increasing the momentum ratio, the role of phase-difference in reducing the separation thickness lessened. Nevertheless, applying either a positive or a negative phase-difference of pi/2 still postponed the separation point.

Forced and mixed convection experiments in a confined vertical backward facing step at low-Prandtl number

In this paper, we present experimental results for a non-isothermal vertical confined backward facing step conducted with a low-Prandtl number fluid. The eutectic alloy gallium–indium–tin is used as the working fluid. We conducted experiments for different Reynolds and Richardson numbers covering both forced and mixed convection regimes. Time-averaged velocity profiles were measured at six streamwise positions along the test section center-plane with so-called permanent magnet probes. The local Nusselt number was measured in streamwise and spanwise directions along the heating plate mounted right after the step. We further ran RANS simulations of the experiment to study the qualitative influence of assuming a constant specific heat flux thermal boundary condition for the experiment heating plate. The measured velocity profiles show the expected behavior for both studied convection regimes, while the measured streamwise local Nusselt number profiles do not. This is explained by how the heating plate thermal boundary condition is defined. We performed an order of magnitude estimate to estimate the forced- to mixed convection transition onset. The estimate shows good agreement with the experimental data, although further measurements are needed to further validate the estimated transition threshold. The measurement of fluctuating quantities remains an open task to be addressed in future experiments, since the permanent magnet probe measurement equation needs further adjustments.Graphical

Horizontal oil-water two-phase flow characterization and identification with pulse-wave ultrasonic Doppler technique

To characterize and identify the flow patterns of horizontal oil-water two-phase flow, a pulse-wave ultrasonic Doppler technique was adopted to acquire spatiotemporal flowing information non-intrusively. The echogram was firstly constructed from the echo amplitude to analyze the interrelation between the concentration distribution and the echo intensity under different flow patterns. Then the time-averaged velocity profile was calculated from the Doppler frequency shift to investigate the velocity changes at different radial positions with flow patterns. Further, the long-term and dynamic connectivity of time-varying velocity profiles were analyzed through the graph-variate dynamic (GVD) connectivity method to reveal coupling effects in the spatiotemporal velocity field. Based on the above analysis, several features were extracted objectively to characterize flow patterns from different aspects, and their evolutions with different flow conditions were analyzed. The effectiveness of these features was validated through a multi-class support vector machine model with the total identification accuracy of 93.33%.

Measurement and prediction of bottom boundary layer hydrodynamics under modulated oscillatory flows

Velocities are measured using LDA within the bottom boundary layer for 10 oscillatory flow tunnel experiments involving regular and amplitude modulated oscillatory flows over a gravel-rough bed and a sand-rough bed. Corresponding regular and modulated flows were equivalent in terms of rms velocity, oscillatory flow period, skewness and asymmetry. The experimental results are compared with predictions based on a 1DV RANS model with k−ϵ turbulence closure. The effects of modulation on the hydrodynamics of individual flow half-cycles and on the time-averaged hydrodynamics are analysed. Turbulence is carried over from one half-cycle to the following half-cycle but the main hydrodynamic properties within a half-cycle (boundary layer thickness, peak turbulent kinetic energy, peak turbulent stress) show little or no dependence on prior half-cycle flow conditions. Turbulence propagation from the bed and vertical profiles of skewness, asymmetry, time-averaged turbulent kinetic energy and time-averaged turbulent stress show remarkably little effect of modulation. Modulation does not affect the shape of the time-averaged velocity profile generated by the non-symmetric flows, but it does reduce the magnitude. The RANS model shows generally good agreement with the measured hydrodynamics above about one grain diameter from the bed, but, because of its assumption of rough turbulent flow, poorly predicts the hydrodynamics of the low-velocity half-cycles of the modulated flows over the sand-rough bed.

Multi-phase modelling of surf-zone sediment transport and bed evolution under plunging breakers

In this numerical study, sediment transport and bed evolution in the surf zone under plunging breakers are investigated; this is a challenging topic that involves complex hydrodynamics coupled with sediment dynamics. Here, a Eulerian–Eulerian multi-phase model for sediment transport, with the capability to capture the free surface, is used. Waves are generated using an open-source toolbox ‘waves2foam’, based on a relaxation method. The model is validated with published experimental results in three cases: (i) surf-zone hydrodynamics over a sloped-rigid bed, (ii) suspended sediment concentration over a sloped-sediment pit, and (iii) bed profile change of a sloped-mobile bed. The profiles of time-averaged velocity, concentration, sediment fluxes, and the time series of concentration are presented. The numerical results suggest that the undertow plays a key role in offshore sediment transport. The instantaneous flow fields show that the vortices under plunging breakers roll up the sediment layers originally near the bed; in such an event, the buoyancy provides a centripetal force to sediment particles. The vortices further transport the suspended sediment onshore and cause strong mixing in water column. This is marked as the underlying mechanism for significant wave-driven flux in the surf zone.

Heat transfer and flow structure around a heated cylinder by upstream installation of a grooved cylinder

The aim of this study was to investigate the effects of installing the grooved and smooth control cylinder upstream of a circular bare cylinder with a focus on the heat transfer and the flow structure behind the bare cylinder. The Reynolds number based on the bare cylinder diameter (D = 50 mm) is Re = 5 000. Control cylinder has a constant diameter as d = 25 mm. Flow characteristics and heat transfer were determined the varying gap space L (center-to-center distance between the bare cylinder and the control cylinder) and surface geometries. The experimental study was conducted in a water channel to collect the temperature and velocity data. Heat transfer coefficient and Nusselt number were determined by measuring the surface temperature distributions. A digital particle image velocimetry (PIV) system was also used to quantify the flow characteristics in order to assess the effectiveness of the flow control with different grooved surface (smooth, rectangular, triangular and circular grooved), in comparison with a bare cylinder case. As to the temperature measurement results, the optimum gap space was defined as L/D = 2.0. It was found that when the control cylinder is introduced, the separation point shifted downstream of the bare cylinder. The cylinders with groove geometry, however, proved to improve heat transfer in comparison to a single main cylinder. The velocity profiles in the time-averaged streamwise direction caught the peak of the single cylinder at L/D = 2.0, where the time-averaged velocity profiles of the control cylinders were minimum. The vorticity components showed that grooved cylinders were more effective in flow control, especially at L/D = 2.0, compared to a single cylinder.

An investigation of wake flows produced by asymmetrically structured porous coated cylinders

The vortex shedding tone of a cylinder in uniform flow can be reduced by applying a porous coating yet this mechanism is not fully understood. An experimental investigation of asymmetric structured porous coated cylinders (SPCCs) was conducted in a small anechoic wind tunnel using a hot-wire anemometry probe placed in the boundary layer, separated shear layer and wake, in conjunction with a microphone in the far-field. Tests were conducted at Reynolds numbers 105, 1.53×105, and 1.66×105. Each SPCC revealed a widened and deeper wake and reduced turbulent kinetic energy levels in the separated shear layer than the bare baseline cylinder. Furthermore, each SPCC revealed two tones that were a multiple of two apart in both the velocity and acoustic power spectral densities. It was shown that the higher frequency tone is generated by localized flow behaviors in the separated shear layer, independent of the vortex shedding tone and its magnitude is inversely related to the SPCC windward surface porosity. Applying a more densely spaced porous region on the cylinder windward side resulted in higher frequency broadband contributions that were shown to be independent of the velocity fluctuations in the wake region. Time-averaged velocity profiles in the wake revealed that the leeward side porosity strongly influences the drag coefficient. Linear stability analysis revealed that the SPCCs develop absolute instabilities in the near wake.

Flow-Field Measurements in a Metal Additively Manufactured Offset Strip Fin Array Using Laser Doppler Velocimetry

Abstract Metal additive manufacturing (AM) of heat exchanger enables custom and conformal designs for a wide range of applications. However, one challenge with metal AM is the resultant surface roughness formed when using this process, which is nonexistent during traditional manufacturing processes. The goal in this study is to explore how this roughness impacts the pressure drop and flow field of a commonly used heat exchanger surface called an offset strip fin (OSF). Two OSFs of the same geometry are tested: one with an average fin roughness of 34 μm from metal AM and the other with an average fin roughness 2.5 μm, used as a baseline. The roughness from the metal AM process increased pressure losses and transitioned the flow to turbulent-like behavior at lower Reynolds numbers when compared with the smooth fin. Laser Doppler velocimetry (LDV) measurements captured the row number in the fin array where transition from laminar to turbulent-like flow occurred. The location of transition from low to high turbulence levels occurred earlier in the fin array as the Reynolds number was increased for the smooth and rough fins. Wake profiles of time-averaged axial velocity were similar between the rough and smooth fins, with the rough fins having higher levels of turbulence intensity (TI) and less symmetric wake profiles. Overall, this study indicates that a pressure loss penalty is associated with using metal AM OSF due to the resultant surface roughness and an earlier transition to turbulent-like flow.

Combining application of wavelet analysis and genetic algorithm in wind tunnel simulation of unidirectional natural wind field near a sand ground surface.

This study proposes a method for the approximate simulation of field gust in an atmospheric boundary layer wind tunnel driven by an axial fan. Wavelet analysis and genetic algorithm have been adopted to convert actual field gust data into a series of simplified and smooth control signals. One frequency converter is used to receive the signals via a programmable logic controller and change the fan speed correspondingly. Results show that the low-frequency energetic part of field gust on top of a saltation layer has been almost reproduced in the wind tunnel with energy fluctuations ±3.26% from the mean compared with the field data in terms of energy. The appropriate filter frequency for field gust velocity history is 1/32Hz. A unidirectional natural wind field near a sand bed surface can be approximately simulated in the tunnel by adjusting carefully the roughness element arrangement. The time-averaged velocity profile of the simulated wind, which satisfies the logarithmic law, is nearly equivalent to the steady flow with the same average velocity.

Improved detached-eddy simulation of the turbulent unsteady flow past a square cylinder

The separated turbulent flow past a square cylinder mounted centrally inside a plane channel has been numerically investigated in this work. A three-dimensional paradigmatic calculation was carried out to capture the essence of flow parameters through different detached-eddy simulations (DESs). The predictions were performed based on the original DES, delayed DES, and one modified DES (MDES). First, MDES closure was validated by comparing the simulation results with the experimental data of separated flow through a U-turn duct, thereafter commencing the predictions of the flow past a square cylinder. The comparisons between the numerical and available experimental results show that the MDES has better capability than the DES to capture the characteristics of global variables including the Strouhal number, mean drag and lift coefficients, and their standard deviations for the square cylinder case. Furthermore, a detailed quantitative comparison in terms of the time-averaged velocity profiles at specified locations reveals that MDES could be more reasonable and has better fidelity than the other two models. A good match among the contours of eddy viscosity, Q-criterion, and turbulent kinetic energy shows that MDES has correctly reproduced the turbulent production in the wake flow. The main significance of this study is that it demonstrates the potential of a modified DES closure to investigate the unsteady dynamics in the wake flow past a square cylinder and expected to make sense for other wake flows of engineering interest.

Large Eddy Simulation of Film Cooling with Bulk Flow Pulsation: Comparative Study of LES and RANS

The effects of bulk flow pulsations on film cooling in gas turbine blades were investigated by conducting large eddy simulation (LES) and Reynolds-averaged Navier–Stokes simulation (RANS). The film cooling flow fields under 32 Hz pulsation in the mainstream from a cylindrical hole inclined 35° to a flat plate at the average blowing ratio of M = 0.5 were numerically simulated. The LES results were compared to the experimental data of Seo, Lee, and Ligrani (1998) and Jung, Lee, and Ligrani (2001). The credibility of the LES results relative to the experimental data was demonstrated through a comparison of the time-averaged adiabatic film cooling effectiveness, time- and phase-averaged temperature contours, Q-criterion contours, time-averaged velocity profiles, and time- and phase-averaged Urms profiles with the corresponding RANS results. The adiabatic film cooling effectiveness predicted using LES agreed well with the experimental data, whereas RANS highly overpredicted the centerline effectiveness. RANS could not properly predict the injectant topology change in the streamwise normal plane, but LES reproduced it properly. In the case of the injectant trajectory, which greatly influences film cooling effectiveness, RANS could not properly predict the changes in the streamwise velocity peak due to flow pulsation, but they were predicted well with LES. RANS greatly underpredicted the streamwise velocity fluctuations, which determine the mixing of main flow and injectant, whereas LES prediction was close to the experimental data.

Surface periarterial spaces of the mouse brain are open, not porous.

Fluid-dynamic models of the flow of cerebrospinal fluid in the brain have treated the perivascular spaces either as open (without internal solid obstacles) or as porous. Here, we present experimental evidence that pial (surface) periarterial spaces in mice are essentially open. (1) Paths of particles in the perivascular spaces are smooth, as expected for viscous flow in an open vessel, not diffusive, as expected for flow in a porous medium. (2) Time-averaged velocity profiles in periarterial spaces agree closely with theoretical profiles for viscous flow in realistic models, but not with the nearly uniform profiles expected for porous medium. Because these spaces are open, they have much lower hydraulic resistance than if they were porous. To demonstrate, we compute hydraulic resistance for realistic periarterial spaces, both open and porous, and show that the resistance of the porous spaces are greater, typically by a factor of a hundred or more. The open nature of these periarterial spaces allows significantly greater flow rates and more efficient removal of metabolic waste products.

Subsurface flow and associated oxygen transfer induced by turbulence in a gravel bed river

A model for the subsurface flow and associated oxygen transfer induced by surface flow and slip velocity is presented in a gravel bed river. The shear velocity (U *) and hydraulic conductivity (K) were used to characterize the surface flow and subsurface flow, respectively. The subsurface flow field was described by using the Darcy and the Brinkman equations. The interaction between surface and subsurface flow can be represented by the slip velocity via the shear velocity (U *) and hydraulic conductivity (K). The effect of turbulence above the permeable surface on the subsurface flow can also be described by the Reynolds stress in terms of eddy viscosity (νt ). The model was found to substantially reproduce the experimental time-averaged subsurface velocity profiles. It was shown that turbulence penetrating through permeable sediments is limited to a thinner layer within a few centimetres from the sediment/water interface. This result can be useful to evaluate the effect of turbulence over the permeable surface on oxygen penetration into the sediment.

Transition and Turbulence Modeling for the Prediction of Cavitating Tip Vortices

Abstract This study evaluates the influence of transition and turbulence modeling on the prediction of wetted and cavitating tip vortices for an elliptical wing, while investigating the numerical errors. Transition modeling increases the quality of numerical predictions since the assumption of a fully turbulent boundary layer, commonly found in literature, contributes to underprediction of the tip vortex cavity size. By applying the local correlation-based transition model (LCTM) and controlling the boundary layer thickness using different turbulent inflow conditions, the pressure in the vortex was found to reduce by 20% for an Angle of Attack (AoA) of 5 deg. The consequent increase in cavity size was found to be of a similar order of magnitude. At 9 deg AoA, transition always occurs just downstream of the leading edge, making this AoA more suitable to investigate the effect of different turbulence modeling approaches. Azimuthal and axial velocity fields are validated against stereographic-particle image velocimetry (S-PIV) measurements. The time-averaged velocity profiles predicted by delayed detached-eddy simulation (DDES) and improved delayed detached-eddy simulation (IDDES) are close to the experiments; however, no velocity fluctuations and vortex dynamics are observed around the vortex. A comparison of wetted and cavitating simulations shows that the cavity leads to a change in the balance between production and destruction of turbulence kinetic energy, which reduces the turbulent diffusion in and around the cavity compared to wetted flow conditions. Consequently, the vapor flow exhibits the characteristics of a potential flow. Whether this is physically plausible remains to be investigated.

Hemodynamics alteration in patient-specific dilated ascending thoracic aortas with tricuspid and bicuspid aortic valves

In this paper, we evaluate computationally the influence of blood flow eccentricity and valve phenotype (bicuspid (BAV) and tricuspid (TAV) aortic valve) on hemodynamics in ascending thoracic aortic aneurysm (ATAA) patients. 5 TAV ATAA, 5 BAV ATAA (ascending aorta diameter >35 mm) and 2 healthy subjects underwent 4D flow MRI. The 3D velocity profiles obtained from 4D flow MRI were given as input boundary conditions to a computational fluid dynamics analysis (CFD) model. After performing the CFD analyses, we verified that the obtained time-averaged velocity profiles and flow eccentricity were in good agreement with 4D flow MRI. Then we used the CFD analyses to evaluate the time-averaged wall shear stress (TAWSS) and the local normalized helicity (LNH). We found that the flow eccentricities at the aortic root were not significantly different (p > 0.05) between TAV and BAV phenotypes. TAWSS (R2 = 0.697, p = 0.025) and absolute LNH (R2 = 0.964, p < 0.001) are in good correlation with flow eccentricity. We conclude that eccentricity at the aortic root is a major determinant of hemodynamics patterns in ATAA patients regardless of the aortic valve phenotype.

Numerical Analysis of Heat Transfer Enhancement in a Micro-Channel Due to Mechanical Stirrers

Abstract Using two-dimensional numerical simulations of the momentum, mass, and energy conservation equations, we investigate the enhancement of heat transfer in a rectangular micro-fluidic channel. The fluid inside the channel is assumed to be stationary initially and actuated by the motion imparted by mechanical stirrers, which are attached to the bottom of the channel. Based on the direction of the oscillation of the stirrers, the boundary conditions can be classified as either no-slip (when the oscillation is perpendicular to the length of the channel) or periodic (when the oscillation is along the length of the channel). The heat transfer enhancement due to the motion of the stirrers (with respect to the stationary stirrer situation) is analyzed in terms of the Reynolds number (ranging from 0.7 to 1000) and the Peclet number (ranging from 10 to 100). We find that the heat transfer first increases and then decreases with an increase in the Reynolds number for any given Peclet number. The heat transferred is maximum at a Reynolds number of 20 for the no-slip case and at a Reynolds number of 40 for the periodic case. For a given Peclet and Reynolds number, the heat flux for the periodic case is always larger than the no-slip case. We explain the reason for these trends using time-averaged flow velocity profiles induced by the oscillation of the mechanical stirrers.