Vertical Water Tunnel Research Articles

Experimental investigation on dynamic characteristics of single bubble near wall in shear flow

The bubble near the wall of shear flow will be subjected to both wall-induced lift and shear-induced lift. The two-lift interaction will markedly change the dynamic behavior of the bubble. In this experiment, the lateral motion of a single bubble (deq=2.33−3.42 mm, Reb=470−680) rising near the vertical wall in a linear shear flow was studied. The positive-synergy and negative-synergy between the wall- and shear-induced lift effects on bubble dynamics were compared. The experiment was carried out in a vertical water tunnel with a curved screen used to generate a stable linear shear flow. Using the shadow method and two charge-coupled device cameras, the movement parameters of the bubble were captured, including the movement trajectory of the bubble, length–diameter ratio, and instantaneous velocity. The lift coefficient CL and drag coefficient CD were obtained by quasi-steady-state analysis and calculation. By fitting the steady lift coefficient, the relation of CL with dimensionless wall distance S* and the Reynolds number Reb was obtained. The results show that there is a critical value of the initial dimensionless distance S* about [S*] = 1.6. When S* > [S*], the bubble is subjected to both wall-induced lift and shear-induced lift. The lift coefficient CL decreases with the increase in S*. When S* < [S*], the bubble receives additional induced lift from the wall. The CL of the smaller bubble was smaller and increases with the increase in S*, while the deformation of the larger bubble will interact with the wall to produce deformation lift away from the wall, presenting larger CL, and decrease with the increase in S*.

Scaling high Rayleigh number natural convection boundary layer statistics: A vertical water tunnel experiment

A high Rayleigh number natural convection boundary layer adjacent to the vertical heated wall was investigated at a large-scale facility. The global Rayleigh number (Rax) measured by the temperature difference between the wall and ambient water and the distance from the bottom of the heated wall reached 1013. Experimental results confirm that the global Nusselt number Nux is scaled by power 1/3 of Rax, which is similar to the well-known asymptote of the previously achieved Rax. The velocity field obtained using particle image velocimetry (PIV) indicated that the buoyancy-dominant outer-layer scaling suggested by Wells and Worster [A geophysical-scale model of vertical natural convection boundary layers, J. Fluid Mech. 609, 111–137 (2008)] was not only applicable to scale the lower-order velocity statistics but was also valid as a reasonable measure of the spatial correlation, probability distribution, and quadrant contribution features in the outer layer. The dynamical behavior of fluid motion captured by PIV supported a robust momentum transfer to positive wall-normal direction, which was sustained by the Q1 and Q3 quadrants. In addition, merging the existing literature and current data suggested that near-wall function can be applied at a moderate Rax; the universality of this wall-function model was confirmed around z×∼0.7 (where z× represents the near-wall scaled wall-normal distance, Kiš and Herwig [The near wall physics and wall functions for turbulent natural convection, Int. J. Heat Mass Transfer 55, 2625–2635 (2012)]). At a larger buoyancy regime, it was expected to follow a canonical boundary layer flow.

Towards a model of bubble breakup in turbulence through experimental constraints

Complex bubble breakup in turbulence has been studied and modeled extensively by employing the population balance equation. This equation hinges on two quantities, i.e. daughter bubble size distribution and breakup frequency. Since there is no first-principle equation that can be solved to calculate these two quantities, many phenomenological models based on different physical mechanisms have been proposed. A large number of possible mechanisms at play leads to models with drastically different, and even contradictory, predictions. In contrast, experimental measurements of these two quantities, including several previous works and our own results collected in a vertical water tunnel that features a large homogeneous and isotropic region, seem to be consistent with one another. To resolve the difference between models and experiments, rather than following another physical argument, we approach the problem from a different direction by asking how to constrain a model based on experimental results. The specific constraints extracted from eight experimental results include: (i) direct measurements of daughter bubble size distribution; (ii) Super-Hinze-scale bubble size spectrum for constraining breakup frequency; (iii) Sub-Hinze-scale bubble size spectrum for modeling daughter bubble size distribution; (iv) Convergence time to an equilibrium state. Finally, based on these experimental constraints, a new breakup model that incorporates a corrected formulation for breakup frequency as well as a simplified function for daughter bubble distribution is developed to meet all constraints. Although the new model is deliberately not connected to any specific physical arguments for simplification, it appears to be robust and consistent with all experimental constraints mentioned.

Using deformable particles for single-particle measurements of velocity gradient tensors

We measure the deformation of particles made of several slender arms in a two-dimensional (2D) linear shear and a three-dimensional (3D) turbulent flow. We show how these measurements of arm deformations along with the rotation rate of the particle allow us to extract the velocity gradient tensor of the flow. The particles used in the experiments have three symmetric arms in a plane (triads) and are fabricated using 3D printing of a flexible polymeric material. Deformation measurements of a particle free to rotate about a fixed axis in a 2D simple shear flow are used to validate our model relating particle deformations to the fluid strain. We then examine deformable particles in a 3D turbulent flow created by a jet array in a vertical water tunnel. Particle orientations and deformations are measured with high precision using four high speed cameras and have an uncertainty on the order of $10^{-4}$ radians. Measured deformations in 3D turbulence are small and only slightly larger than our orientation measurement uncertainty. Simulation results for triads in turbulence show deformations similar to the experimental observations. Deformable particles offer a promising method for measuring the full local velocity gradient tensor from measurements of a single particle where traditionally a high concentration of tracer particles would be required.

V-ONSET (Vertical Octagonal Noncorrosive Stirred Energetic Turbulence): A vertical water tunnel with a large energy dissipation rate to study bubble/droplet deformation and breakup in strong turbulence.

A vertical water tunnel facility has been constructed to study the dynamics of turbulent multiphase flow. The new system features several unique designs that allow us to study bubble deformation and breakup in strong turbulence: (i) The mean flow can be adjusted to balance the rising velocity of buoyant bubbles/droplets so that they can stay in the view area for an extended period of time. (ii) Turbulence is generated and controlled using a 3D-printed jet array that can fire 88 random high-speed momentum jets with the individual jet velocity of up to 12 m/s. This component allows us to attain turbulence with a high energy dissipation rate (≥0.1 m2/s3), which is orders-of-magnitude higher than most of the existing turbulent multiphase flow facilities. (iii) Turbulence generated in the test section is nearly homogeneous and isotropic, and the turbulent fluctuations are also decoupled from the mean flow. The resulting turbulence intensity can be varied between 20% and 80% with the speed of the mean flow at around 0.2 m/s. (iv) This system has an octagonal test section that allows six cameras to image and reconstruct the 3D shape of deforming bubbles/droplets in turbulence. The same set of cameras was also used for tracking tracers in the surrounding turbulent flow. Both the reconstruction and particle tracking were completed using our in-house codes that were parallelized to run on high-performance computing clusters efficiently.

Twente mass and heat transfer water tunnel: Temperature controlled turbulent multiphase channel flow with heat and mass transfer.

A new vertical water tunnel with global temperature control and the possibility for bubble and local heat and mass injection has been designed and constructed. The new facility offers the possibility to accurately study heat and mass transfer in turbulent multiphase flow (gas volume fraction up to 8%) with a Reynolds-number range from 1.5 × 104 to 3 × 105 in the case of water at room temperature. The tunnel is made of high-grade stainless steel permitting the use of salt solutions in excess of 15% mass fraction. The tunnel has a volume of 300 l. The tunnel has three interchangeable measurement sections of 1 m height but with different cross sections (0.3 × 0.04 m2, 0.3 × 0.06 m2, and 0.3 × 0.08 m2). The glass vertical measurement sections allow for optical access to the flow, enabling techniques such as laser Doppler anemometry, particle image velocimetry, particle tracking velocimetry, and laser-induced fluorescent imaging. Local sensors can be introduced from the top and can be traversed using a built-in traverse system, allowing, for example, local temperature, hot-wire, or local phase measurements. Combined with simultaneous velocity measurements, the local heat flux in single phase and two phase turbulent flows can thus be studied quantitatively and precisely.

An Experimental Study on the Vertical Flow Past a Finite-Length Horizontal Cylinder at Low Reynolds Numbers

The research studies the characteristics of the vertical flow past a finite-length horizontal cylinder at low Reynolds numbers (ReD) from 250 to 1080. The experiments were performed in a vertical closed-loop water tunnel. Flow fields were observed by the particle tracer approach for flow visualization and measured by the Particle Image Velocimetry (P.I.V.) approach for velocity fields. The characteristics of vortex formation in the wake of the finite-length cylinder change at different regions from the tip to the base of it. Near the tip, a pair of vortices in the wake was observed and the size of the vortex increased as the observed section was away from the tip. Around a distance of 3 diameters of the cylinder from its tip, the vortex street in the wake was observed. The characteristics of vortex formation also change with increasing Reynolds numbers. At X/D = -3, a pair of vortices was observed in the wake for ReD = 250, but as the ReD increases the vortex street was observed at the same section. The vortex shedding frequency is analyzed by Fast Fourier Transform (FFT). Experimental results show that the downwash flow affects the vortex shedding frequency even to 5 diameters of the cylinder from its tip. The interaction between the downwash flow and the Von Kármán vortex street in the wake of the cylinder is presented in this paper.

Particle Entrainment in Spherical-Cap Wakes

In this work we study the preferential concentration of small particles in the turbulent wake behind a spherical-cap object. We present a model predicting the mean particle concentration in the near-wake as a function of the characteristic Stokes number of the problem, the turbulence level and the Froude number. We compare the model with our experimental results on this flow, measured in a vertical water tunnel.

Gap-flow patterns behind twin-cylinders at low Reynolds number

The flow structures, drag coefficients (Cd) and vortex shedding characteristics around a single square cylinder and twin side-by-side square cylinders were experimentally investigated with various Reynolds numbers (Re) and gap ratios (g*) in a vertical water tunnel. The Reynolds number (Re) and gap ratio (g*) were 178 < Re < 892 and 0 ≤ g* ≤ 2.5, respectively. The flow patterns and vortex shedding frequency were determined using the particle tracking flow visualization (PTFV). The flow structures, velocity properties, and drag coefficients were calculated using the particle image velocimetry (PIV). The topological flow patterns of vortex evolution processes were plotted and analyzed based on critical point theory. Furthermore, the flow structures behind twin side-by-side square cylinders were classified into three modes — single vortex-street mode, gap-flow mode and couple vortex-streets mode. The maximum Cd occurred in the single vortex-street mode, and the minimum Cd occurred in the gap-flow mode. The highest Strouhal number (St) occurred in the single vortex-street mode, and the lowest St occurred in the gap-flow mode.

243 鉛直水路中に懸垂支持された球の高レイノルズ数における流力振動

243 鉛直水路中に懸垂支持された球の高レイノルズ数における流力振動

Distinctive Features of Accelerated Motion of Variously Shaped Bodies in Water

The results of an experimental investigation of accelerated motion of a combined body of revolution, a sphere and rectangular flat plates in water are presented in the paper. The experiments were conducted in a vertical water tunnel. The objects of investigation started from the state of rest and moved under the action of gravity. The nature of the accelerated motion is shown to be individual for each type of objects, and yet in all the instances the stage of motion with a velocity and an acceleration monotonically varying gives way to the stage of jerking motion with sharp oscillation of values of these quantities.

1540 界面活性剤水溶液中の円柱の抗力特性に関する研究

The flow around a circular cylinder in surfactant solution was investigated experimentally by measurement of the drag in the Reynolds number range 500 < Re < 6,000. The experiments were performed in a vertical re-circulating water tunnel. The drag coefficient was measured by the apparatus, which could measure the drag acting on the circular cylinder directly. Three cylinders of diameter d=10,19 and 25mm were tested, the ratio of length to diameter was (l/d)=11.2. Test surfactant solution was aqueous solution of Ethoquad O/12 (Lion Co.) at concentrations of 200ppm, and sodium salicylate was added as a counterion. It was cleared that the drag coefficient of a circular cylinder in surfactant solution larger than that in tap water at the Reynolds number range 500 < Re < 4,000. The value of the drag coefficient in surfactant solution was dependent on not only (l/d) but also cylinder diameter. According to the increase of the Reynolds number, the drag coefficient decreased by (Re)^<-3/2>. On the other hand, the value of the drag coefficient in surfactant solution became smaller than that in tap water at Re&cong;5,000.

Overview of A Large-Scale Vertical Water Tunnel Constructed in CREPI

Overview of A Large-Scale Vertical Water Tunnel Constructed in CREPI

Bubble- and spiral-type breakdown of slender vortices

Bubble- and spiral-type breakdown and periodic transition between the two were visualizedby varying the volume flux in a transparent, low-speed, vertical water tunnel with a diffuser inset. A feedback model for the initiation and development of bubble-type breakdown is proposed, and the transition from bubble- to spiral-type breakdown is explained by an asymmetry of the circumferential vorticity distribution. Both are qualitatively confirmed by experimental flow visualizations. The initiation of the bubble-type breakdown and its transition to the spiral-type were successfully simulated by solving the Navier-Stokes equations for three-dimensional, unsteady, and incompressible flow with boundary conditions similar to those of an experimental investigation. Hence, for the first time it was possible to compare the results of numerical and experimental flow visualizations and therefore to draw a conclusion about the reliability of simulating vortex breakdown numerically.

A flow visualization study of the flow in a 2 D array of fins

A study of the flow field in a 2 D arrangement of fins was carried out by means of flow visualization in a vertical water tunnel. A similar arrangement of fins had been tested as a conceptual heat sink, and the heat transfer measurement showed a dependence of the heat transfer qualities on the geometrical parameters of the fin's array. The current study examines the complex flow field structure in order to obtain a better understanding of the convection process. The model is built of several series of fins, generating a multi-cell structure. The investigation included a systematic variation of the fins' chord and inclination angle, as well as the flow velocity. Two main flow structures were observed. In the first one, the flow separates from the leading edge of each fin. Due to the influence of its neighbouring fins, the flow re-attaches to the fin, creating a closed separation zone. A vortex fills this separation zone. The main flow accelerates in the passage that is created between the separation bubble and the neighbouring fin. In the second flow structure, the flow separates from both leading and trailing edges of each fin. The separation is in alternating order and generates a nonsteady pair of vortices. This separation pattern is similar to that of a single plate at high angle of attack, and the effect of the neighbouring fins is only to narrow the wake. The pressure drop across the model was measured and correlated to a single nondimensional variable on a single curve.

Numerical simulation of unsteady flow about cambered plates

Reported herein is a theoretical and experimental investigation of a nonimpulsively-started steady ambient flow, subsequently decelerating to rest at prescribed rates, about two-dimensional cambered plates (circular arcs with included angles of 120,180, and 240 deg) for the purpose of understanding the consequences of wake return and secondary separation on impermeable rigid surfaces. The numerical simulations are based on computational methods with vortices. Experiments are carried out in a vertical water tunnel. The results of the physical and numerical experiments are found to be in good agreement.

Flow pattern and turbulence intensity in stacks of interrupted parallel-plate surfaces

An experimental study is performed to investigate the pressure drop distribution and turbulence intensity for airflow through a stack of parallel plates with periodic interruptions. The interrupted surfaces are constructed in an offset-strip form and a perforated form. Hot-wire measurements of the flow stream in the test core are made to determine turbulence intensity variations as a function of the Reynolds number. Results on the pressure drop and turbulence intensity are compared with those of the uninterrupted flow case. The offset-strip surface yields higher flow with higher turbulence intensity than the perforated and uninterrupted surfaces, an indication of superior heat transfer performance. Flow visualization by the dye injection method is performed on flow through a stack of offset-strip surfaces in a vertical water tunnel. Steady laminar flow, vortex shedding, oscillating flow, and turbulent flow are observed in sequence along the flow passage. The effects of Reynolds number on the location of the transition from steady to unsteady (oscillating and turbulent) flow regimes are determined. The study provides evidence to support the theory of secondary laminar-flow enhancement and transition-turbulent flow enhancement of heat transfer in compact heat exchangers [1–3].

Instabilities of Tubular Beams Simultaneously Subjected to Internal and External Axial Flows

This paper examines the dynamics and stability of cylindrical tubular beams conveying fluid and simultaneously subjected to axial external flow. In deriving the equation of small motions, inviscid hydrodynamic forces are obtained by slender-body theory, modified to account for the boundary-layer thickness of the external flow; internal dissipation and gravity effects are also taken into account. Solutions are obtained by means of a method similar to Galerkin’s, with the eigenfunctions approximated by Fourier series. Calculations are presented for tubular beams either clamped at both ends or cantilevered. It is shown that for sufficiently high flow velocities, either internal or external, the system, is subject to divergence and/or flutter. In the case of clamped-clamped tubular beams the effect of the two flows (internal and external) on stability is additive, so that if either flow is just below the corresponding critical value for instability, an increase in the other flow precipitates instability. This is not always the case for cantilevered beams; if the system is just below the threshold of instability due to either flow, instability may be eliminated if the other flow is increased. Experiments conducted with moulded rubber tubular beams in a vertical water tunnel corroborate the theoretically predicted behavior.

Forces on Cylinders Near a Plane Boundary in a Sinusoidally Oscillating Fluid

The in-line and transverse forces acting on circular cylinders placed near a plane boundary in a sinusoidally oscillating fluid in a U-shaped vertical water tunnel have been measured. The period parameter UmT/D was varied from about 2 to 40, the Reynolds number from 4000 to 25,000, and the gap between the cylinder and the plane boundary from 0.01 D to 1.0D. The drag and inertia coefficients for the in-line force have been determined through the use of the Morison’s equation and the Fourier analysis, least squares method, and a modified least squares method. The transverse force coefficients have been obtained for the forces toward the wall and away from the wall. The results show that the in-line and transverse forces could acquire very large magnitudes and give rise to serious oscillations. For very small values of the period parameter, effects of flow separation become negligible and the inertia coefficient for the in-line force and the lift coefficient for the transverse force approach those predicted by the potential theory.

Supercavitating Flat Plate With an Oscillating Flap at Zero Cavitation Number

The results of experimental and theoretical investigations on a supercavitating flat plate with an oscillating flap at zero cavitation number are presented. The experiment was carried out in a vertical free-jet water tunnel using 3-in. chord and 2-in. chord flat-plate hydrofoils, both having flap-chord ratios of 0.29. Various relative locations of the free surfaces were used and reduced frequency range of zero to four was covered. Amplitude and phase angle of lift, drag, and moment as well as the surface wave speed were measured. The problem was also solved analytically by means of a first-order perturbation theory and the results were compared with the experimental data.