Cross-flow Component Research Articles

Experimental control of swept-wing transition through base-flow modification by plasma actuators

Control of laminar-to-turbulent transition on a swept-wing is achieved by base-flow modification in an experimental framework, up to a chord Reynolds number of 2.5 million. This technique is based on the control strategy used in the numerical simulation by Dörr & Kloker (J. Phys. D: Appl. Phys., vol. 48, 2015b, 285205). A spanwise uniform body force is introduced using dielectric barrier discharge plasma actuators, to either force against or along the local cross-flow component of the boundary layer. The effect of forcing on the stability of the boundary layer is analysed using a simplified model proposed by Serpieri et al. (J. Fluid Mech., vol. 833, 2017, pp. 164–205). A minimal thickness plasma actuator is fabricated using spray-on techniques and positioned near the leading edge of the swept-wing, while infrared thermography is used to detect and quantify transition location. Results from both the simplified model and experiment indicate that forcing along the local cross-flow component promotes transition while forcing against successfully delays transition. This is the first experimental demonstration of swept-wing transition delay via base-flow modification using plasma actuators.

Numerical simulation of vortex-induced motion of a deep-draft paired-column semi-submersible offshore platform

Understanding and predicting vortex-induced motion (VIM) of offshore systems for deep seawater applications is crucial to improve the system safety and integrity. We report on experimental tow-tank measurements and numerical simulations of VIM of a deep-draft offshore platform, specifically Paired-Column Semisubmersible (PC-Semi). The study is carried out in model scale (1:54), at subcritical flow regime with Re∼104. Motion of the floating structure has three degrees of freedom: in-line, cross-flow, and yaw. Large periodic cross-flow motion is measured for headings 0°, 11.25°, and 22.5°, for reduced velocities (Ur) between 5 and 10. Considerably smaller cross-flow amplitude is recorded at 45° heading across the Ur range considered.An extensive sensitivity study is performed using computational fluid dynamics (CFD) to capture the transient displacement history of VIM (in-line, cross-flow, and yaw motion components). Amplitude and period of cross-flow (transverse) motion are obtained from statistical analysis of VIM time history and subsequently used as the validation criterion between the CFD simulation and the model tests. Satisfactory agreement between the CFD results and tow-tank measurements is achieved with a Delayed Detached Eddy Simulation – Shear Stress Transport (DDES-SST) formulation. This work provides experimental results and serves as a practical starting point to set up a CFD problem to estimate amplitude and period of cross-flow VIM motion for offshore engineering applications.

사봉형 스펙트럼을 이용한 라이저 피로해석 연구

Oil and gas production riser systems need to be designed considering a wide band quarter-modal analysis which contains low-, wave-, VIV(Vortex induced vibration) frequencies. The VIV can be separated into cross-flow(CF) and in-line(IL) components. In this study, the various idealized tri- and quarter-modal spectra are suggested to analyze fatigue damage on the production riser system. In order to evaluate the fatigue damage increment caused by the IL's motion, tri- and quarter-modal spectral fatigue damages are calculated in time domain. And the fatigue damage calculated from two different modal spectra are compared quantitatively. Then the suitability of existent wide band fatigue damage models for quarter modal spectrum was evaluated by comparison of frequency domain calculation and time domain calculation. The result show that although spectral density of IL motion is not remarkable in quantity, the effect on the fatigue damage is significant and existent fatigue damage models are not adequately estimating damage by quarter-modal spectra.

On the stability of von Kármán rotating-disk boundary layers with radial anisotropic surface roughness

We summarise results of a theoretical study investigating the distinct convective instability properties of steady boundary-layer flow over rough rotating disks. A generic roughness pattern of concentric circles with sinusoidal surface undulations in the radial direction is considered. The goal is to compare predictions obtained by means of two alternative, and fundamentally different, modelling approaches for surface roughness for the first time. The motivating rationale is to identify commonalities and isolate results that might potentially represent artefacts associated with the particular methodologies underlying one of the two modelling approaches. The most significant result of practical relevance obtained is that both approaches predict overall stabilising effects on type I instability mode of rotating disk flow. This mode leads to transition of the rotating-disk boundary layer and, more generally, the transition of boundary-layers with a cross-flow profile. Stabilisation of the type 1 mode means that it may be possible to exploit surface roughness for laminar-flow control in boundary layers with a cross-flow component. However, we also find differences between the two sets of model predictions, some subtle and some substantial. These will represent criteria for establishing which of the two alternative approaches is more suitable to correctly describe experimental data when these become available.

Subchannel analysis of Al2O3 nanofluid as a coolant in VMHWR

AbstractThe main objective of this study is to predict the thermal hydraulic behavior of nanofluids as the coolant in the fuel assembly of variable moderation high performance light water reactor (VMHWR). VMHWR is the new version of high performance light water reactor (HPLWR) conceptual design. Light water reactors at supercritical pressure (VMHWR, HPLWR), being currently under design, are the new generation of nuclear reactors. Water-based nanofluids containing various volume fractions of Al2O3 nanoparticles are analyzed. The conservation equations and conduction heat transfer equation for fuel and clad have been derived and discretized by the finite volume method. The transfer of mass, momentum and energy between adjacent subchannels are split into diversion crossflow and turbulent mixing components. The governed non linear algebraic equations are solved by using analytical iteration methods. Finally the nanofluid analysis results are compared with the pure water results.

Investigation on drag performance of anti-fouling painted flat plates in a cavitation tunnel

The flat plate coated with silicone-type Tin–free self-polishing co-polymer (SPC) or the conventional metal-type Tin-free SPC is prepared to investigate the drag performance of the anti-fouling SPC. The local skin friction of anti-fouling paints is evaluated by a flat plate model test method in the cavitation tunnel. The properties of the boundary layer and the drag performance are investigated by flow and force measurement techniques. The silicone-type SPC paint shows better drag performance than the metal-type paint in the high speed regime. The silicone-type SPC paints also show decreasing roughness function (ΔU+) with the increase of displacement thickness Reynolds number (Reδ⁎) and roughness Reynolds number (ks+). Even in the same silicone-type SPC paints with similar roughness function, drag performance appears differently. The different drag performance in the silicone-type SPC painted surfaces is considered to be affected by different turbulent vortical structures caused by the surface roughness. Y-directional peak position of streamwise turbulence intensity is utilized to estimate the existence of vortical structure. To investigate the reason of the different drag performance in the silicone-type SPC painted surfaces, the POD analysis, extracting the most energetic flow fields, is adopted to find the effects of cross-flow velocity component caused by the turbulent vortical structure.

Microfluidic device with integrated microfilter of conical-shaped holes for high efficiency and high purity capture of circulating tumor cells.

Capture of circulating tumor cells (CTCs) from peripheral blood of cancer patients has major implications for metastatic detection and therapy analyses. Here we demonstrated a microfluidic device for high efficiency and high purity capture of CTCs. The key novelty of this approach lies on the integration of a microfilter with conical-shaped holes and a micro-injector with cross-flow components for size dependent capture of tumor cells without significant retention of non-tumor cells. Under conditions of constant flow rate, tumor cells spiked into phosphate buffered saline could be recovered and then cultured for further analyses. When tumor cells were spiked in blood of healthy donors, they could also be recovered at high efficiency and high clearance efficiency of white blood cells. When the same device was used for clinical validation, CTCs could be detected in blood samples of cancer patients but not in that of healthy donors. Finally, the capture efficiency of tumor cells is cell-type dependent but the hole size of the filter should be more closely correlated to the nuclei size of the tumor cells. Together with the advantage of easy operation, low-cost and high potential of integration, this approach offers unprecedented opportunities for metastatic detection and cancer treatment monitoring.Electronic supplementary materialThe online version of this article (doi:10.1038/srep06052) contains supplementary material, which is available to authorized users.

Vortex-induced vibration (VIV) of a circular cylinder in combined steady and oscillatory flow

Vortex-induced vibration (VIV) of a circular cylinder in the combined steady and oscillatory flow is investigated numerically by solving the two-dimensional Reynolds-Averaged Navier-Stokes equations. The focus of the study is to investigate the effects of flow ratio, a, on the response of the cylinder. The flow ratio is defined as the percentage of the steady flow velocity component in the total fluid velocity. Simulations are carried out for a constant Keulegan–Carpenter (KC) number of 10 and flow ratios ranging from 0 to 1 with an increment of 0.2. The reduced velocities for each flow ratio range from 2 to 25 to ensure that the whole lock-in regime is covered. In the resonance regime, the frequency of the cross-flow vibration component may lock onto the oscillatory flow frequency or the natural frequency of the system, depending on the flow ratio. It locks onto twice the oscillatory flow frequency if the flow ratio a≤0.2, and locks onto the natural frequency of the system if a≥0.6. It is found that the lock-in regime in the combined steady and oscillatory flow is wider than both the one in the pure steady flow and the one in the pure oscillatory flow. The widest lock-in regime occurs as the flow ratio is 0.4 and 0.6 and it is about twice as wide as that in the pure oscillatory or pure steady flow. The response at a=0.2 is very similar to that in the pure oscillatory flow case (a=0), while at a=0.8 the response is very similar to that in the pure steady flow case. At a=0.8, the amplitude of the cylinder in the cross-flow direction reaches as high as 1.5 diameters in the “Super upper” branch because of the slowly increasing fluid velocity. It is also found that the vortex shedding goes through 2S, 2P and 2T modes in one period of oscillatory flow at a=0.8 and Vr=7, where the cross-flow amplitude is the maximum.

Spectral analysis of turbulent viscoelastic and Newtonian channel flows

The one-dimensional spectra in the streamwise direction of the velocity and vorticity fields in turbulent channel flows of Newtonian and non-Newtonian viscoelastic fluids are presented for friction Reynolds numbers up to Reτ0=1000.The most striking feature induced by viscoelasticity is a marked drop, as rapid as k−5, in the energy level of the streamwise velocity spectra at high wave-numbers, and in agreement with experimental data by Warholic et al. (1999) [15]. The scaling of the streamwise velocity spectra for viscoelastic flow share some characteristics with the Newtonian spectra, but also exhibit unique properties. In particular, the logarithmic correction to the usual k−1 law at the intermediate scales (eddies with size one to ten times the distance from the wall), found by del Álamo et al. (2004) [7] in the case of Newtonian turbulence, still holds in viscoelastic flows; although, with different scaling coefficients. In contrast, the longest modes of the spectra of the streamwise velocity component are found to behave differently. These modes are longer in viscoelastic flows and their scaling with the channel centerline velocity, here confirmed for Newtonian flow, fails for high drag reduction viscoelastic turbulent flows.As for vorticity, it is found that the spectra of its cross-flow component in viscoelastic flows exhibit a significantly higher energy level at large scales, with a tendency towards a k−1 law for high drag reduction and high Reynolds number.

Subchannel analysis of nanofluids application to VVER-1000 reactor

In this study a thermal-hydraulic analysis is performed of nanofluids as a coolants in subchannels in hot fuel assembly of VVER-1000 reactor by subchannel method. Water-based nanofluids containing various volume fractions of Al2O3 nanoparticles are analyzed. The conservation equations and conduction heat transfer equation for fuel and clad have been derived and discretized by finite volume method. The transfer of mass, momentum and energy between adjacent subchannels are split into diversion crossflow and turbulent mixing components. The governing non linear algebraic equations are solved by numerical iteration methods. Finally the nanofluids analysis results are compared with the pure water results. To validate the applied approach, the model and COBRA-EN code results are compared for the case of pure water. Results show that the heat transfer increases with nanoparticles volume concentration in the subchannel geometry.

Control of stationary cross-flow modes in a Mach 3.5 boundary layer using patterned passive and active roughness

AbstractSpanwise-periodic roughness designed to excite selected wavelengths of stationary cross-flow modes was investigated in a three-dimensional boundary layer at Mach 3.5. The test model was a sharp-tipped$1{4}^{\circ } $right-circular cone. The model and integrated sensor traversing system were placed in the Mach 3.5 supersonic low disturbance tunnel (SLDT) equipped with an axisymmetric ‘quiet design’ nozzle at NASA Langley Research Center. The model was oriented at a$4. {2}^{\circ } $angle of attack to produce a mean cross-flow velocity component in the boundary layer over the cone. The research examined both passive and active surface roughness. The passive roughness consisted of indentations (dimples) that were evenly spaced around the cone at an axial location that was just upstream of the first linear stability neutral growth branch for cross-flow modes. The active roughness consisted of an azimuthal array of micrometre-sized plasma actuators that were designed to produce the effect of passive surface bumps. Two azimuthal mode numbers of the passive and active patterned roughness were examined. One had an azimuthal mode number that was in the band of initially amplified stationary cross-flow modes. This was intended to represent a controlled baseline condition. The other azimuthal mode number was designed to suppress the growth of the initially amplified stationary cross-flow modes and thereby increase the transition Reynolds number. The results showed that the stationary cross-flow modes were receptive to both the passive and active patterned roughness. Only the passive roughness was investigated at a unit Reynolds number where transition would occur on the cone. Transition front measurements using the Preston tube approach indicated that the transition Reynolds number had increased by 35 % with the subcritical wavenumber roughness compared with the baseline smooth tip cone, and by 40 % compared with the critical wavenumber roughness. Based on the similarities in the response of the stationary cross-flow modes with the active roughness, we expect it would produce a similar transition delay.

Phasing mechanisms between the in-line and cross-flow vortex-induced vibrations of a long tensioned beam in shear flow

The mechanisms of phasing between the in-line and cross-flow vortex-induced vibrations of a cylindrical tensioned beam in non-uniform flow are studied by direct numerical simulation. Three types of responses are considered, mono-frequency, narrowband, and broadband multi-frequency vibrations; in all cases, in-line and cross-flow vibration components occurring with a frequency ratio of 2 are phase-locked within regions of wake-body synchronization. The in-line/cross-flow phase difference exhibits a persistent spanwise drift when vibration components present significant traveling-wave behavior; this drift depends linearly on the in-line/cross-flow wavenumber difference, controlled by the beam non-linear dispersion relation and also impacted by the effective added mass variability.

Numerical analysis of cavitating propeller and pressure fluctuation on ship stern using a simple surface panel method “SQCM”

This paper presents a calculation method for the pressure fluctuation induced by a cavitating propeller. This method consists of two steps: the first step is the calculation of propeller sheet cavitation, and the second step is the calculation of pressure fluctuation on the ship stern. It is for practicality that we divide the method into two steps but do not calculate these steps simultaneously. This method is based on a simple surface panel method “SQCM” which satisfies the Kutta condition easily. The SQCM consists of Hess and Smith type source panels on the propeller or cavity surface and discrete vortices on the camber surface according to Lan’s QCM (quasi-continuous vortex lattice method). In the first step, the cavity shape is solved by the boundary condition based on the free streamline theory. In order to get the accurate cavity shape near the tip of the propeller blade, the cross flow component is taken into consideration on the boundary condition. In the second step, we calculate the cavitating propeller and the hull surface flow simultaneously so as to calculate the pressure fluctuation including the interaction between the propeller and the hull. At that time, the cavity shape is changed at each time step using the calculated cavity shape gotten by the first step. Qualitative agreements are obtained between the calculated results and the experimental data regarding cavity shape, cavity volume and low order frequency components of the pressure fluctuation induced by the cavitating propeller.

A new approach to two-phase flow analysis in a rod bundle

This paper deals with the development of a mathematical and numerical technique for the steady-state subchannel analysis. In view of the importance of the drift-flux model (DFM) in two phase flow analysis, the conservation equations are based on this model. The conservation equations are expressed in terms of five field equations: mixture and momentum continuity; liquid energy equations; gas continuity; and radial cross flow equation for the adjacent subchannels. The numerical algorithm of the single-phase subchannel analysis used in the DIYANA code are extended for the present two phase flow subchannel analysis. The transfer of mass, momentum and energy between adjacent subchannels are split into diversion, turbulent mixing and void drift cross-flow components. The transfer of mass by turbulent mixing is assumed to occur in a volume-for-volume scheme reflecting experimental observations. The phenomenon of lateral vapor drift and turbulent mixing enhancement with flow regime are included in the model. In order to validate the prediction, the GE 3×3 rod bundle experiments with both uniform and non-uniform radial power distribution are used. Good agreement has been obtained between the present numerical prediction and the available experimental data. Simulation results showed that the turbulent mixing and void drift phenomena flatten the void fraction and equilibrium vapor quality profile in the rod bundle cross section. The mathematical formulation is considered to be a major step toward a more basic understanding of two-phase flow analysis in fuel rod bundles. Therefore, a fast, efficient and stable numerical technique is presented in this paper for thermal-hydraulic analysis of nuclear reactors.

A mathematical approach to predict dryout in a rod bundle

In this paper, we present the development of a mathematical model to predict dry-out in a rod bundle using film thickness model and subchannel analysis. In two-phase annular flow regime, liquid phase is assumed to divide into liquid film attached to the rod surface and droplets entrained into the gas (vapor) phase existing in the subchannels. Conservation equations in two-phase annular flow regime for each subchannel consist of two mixture equations for the continuity and the momentum, gas phase continuity, liquid film continuity and liquid phase energy. Also, at the interface of adjacent subchannels, radial diversion cross flow equation is considered. It is assumed that dryout occurs when the local film thickness in the annular flow approaches zero. The transfer of mass, momentum and energy between adjacent subchannels are split into diversion cross-flow, turbulent mixing and void drift components. Rod-centered approach which is compatible with the annular flow regime is used for subchannel analysis. It was observed that turbulent mixing and void drift phenomena have significant effect on the prediction of dryout power. The model is verified against several available experimental data and proved viable for prediction of dryout heat flux in rod bundle geometry especially for mass flux greater than 400kg/m2s.

대각도 받음각을 갖는 무인잠수정에 작용하는 동유체력 특성에 관한 실험적 연구

회류수조에서의 대각도 정적(static) 모형실험을 통해 Manta형 무인잠수체에 작용하는 동유체력을 측정하였으며, 동유체력에 미치는 Reynolds수의 영향을 고찰하였다. 이를 위해 동유체력을 cross-flow drag과 양력(lift force)으로 성분 분석을 하였으며, 양력 성분에는 Reynolds수의 영향을 무시하고, cross-flow drag 성분에만 Reynolds수의 영향을 고려하였다. 그 후 이들 두 성분을 다시 합성함으로써 실물 무인잠수정에 작용하는 동유체력의 추정 기법을 제시하였다. The authors adopt the Unmanned Undersea Vehicle(UUV), the shape of which is like a manta. They call here it Manta UUV. Manta UUV has been designed from the similar concept of the UUV called Manta Test Vehicle(MTV), which was originally built by the Naval Undersea Warfare Center of USA(Lisiewicz and French, 2000; Simalis et al., 2001; U.S. Navy, 2004). The present study deals with the effect of Reynolds numbers on hydrodynamic forces acting on Manta UUV at large angles of attack. The large angles of attack cover the whole range of 0 to <TEX>${\pm}$</TEX> 180 degrees in horizontal plane and in vertical plane respectively. Static test at large attack angles has been carried out with two Manta UUV models in circulating water channel. The authors assume that the experimental results of hydrodynamic forces (lateral force, yaw moment, vertical force and pitch moment) are analyzed into two components, which are lift force component and cross-flow drag component. First of all, Based on two dimensional cross-flow drag coefficient at 90 degrees of attack angle, the cross-flow drag component at whole range of attack angles is calculated. Then the remainder is assumed to be the lift force component. The only cross-flow drag component is assumed to be subject to Reynolds number.entstly the authors suggest the methodology to predict hydrodynamic derivertives acting on the full-scale Manta UUV.

Measurement of wind noise levels in streamlined probes

This paper investigates the wind noise pressure spectra measured by aerodynamically designed devices in turbulent flow. Such measurement probes are often used in acoustic measurements in wind tunnels to reduce the pressure fluctuations generated by the interaction of the devices with the incident flow. When placed in an outdoor turbulent environment however, their performance declines noticeably. It is hypothesized that these devices are measuring the stagnation pressures generated by the cross flow components of the turbulence. Predictions for the cross flow contribution to the stagnation pressure spectra based on measured velocity spectra are developed, and are then compared to the measured pressure spectra in four different probe type devices in windy conditions outdoors. The predictions agree well with the measurements and show that the cross flow contamination coefficient is on the order of 0.5 in outdoor turbulent flows in contrast to the published value of 0.15 for measurements in a turbulent jet indoors.

On the mean characteristics of the juncture vortex in cross-flow planes

Juncture flow of wings with leading edge sweep angles of 0°, ±15°, ±30°, and ±45° mounted on a flat plate for a turbulent approach boundary layer was investigated using laser Doppler velocimetry (LDV) in a subsonic wind tunnel. Time-averaged measurements in the cross-flow planes indicated incompletely formed vortical structures. Additionally, in the cross-flow plane at the airfoil maximum thickness location, measured velocity directions were inconsistent with vortex rotation. Complex motion of the juncture vortex system in the cross-flow direction was observed during flow visualization of the configuration in a water tunnel, suggesting masking of the mean vortex structure by the global motion of the structures. The mean vortex signature with correct rotational direction was ‘frozen’ by subtracting a constant cross-flow velocity component from the LDV mean measurements. Vorticity contours in the cross-flow planes were calculated using a finite differencing scheme on the measured data. Good comparison of vortex core estimated locations between the ‘frozen’ vortex and from vorticity contours was observed. Another artefact of the measurements was asymmetry in the vertical velocities, which is attributed to motion of the vortex core towards the flat plate in the maximum thickness plane and away from it in the 80 per cent chord plane. The horizontal location of the vortex core was observed to be influenced by wing sweep; however, the vertical location was insensitive to this parameter.

SI1-1: The effect of early inlet valve closing on combustion within a direct injection spark ignition engine(SI: Spark-Ignition Engine Combustion,General Session Papers)

To develop an understanding of the combustion produced by two different valve strategies the techniques of combustion imaging, for flame front propagation, and laser Doppler anemometry, LDA, for the air flow and turbulence characteristics in the vicinity of the spark plug were applied in the optical engine configured to replicate the 2000 rpm 2.7 bar IMEP load point of a conventional thermodynamic engine for both valve strategies. A sequence of combustion images were taken at 4^0 CA intervals to span the entire combustion event with fifty images taken at each crankangle to produce a mean image for each crankangle in the sequence. A Matlab code, utilising the "jet" colormap, identified the minimum and maximum visible flame intensity levels and the intensity contour corresponding to the flame front overlaid with the combustion chamber outline. A radial coordinate system was applied which allowed the flame front propagation and velocity to be quantified in both the vertical and horizontal planes. Laser Doppler anemometry was applied to quantify the time resolved mean and fluctuating flow characteristics of the axial and cross flow components directly below the spark plug in the pent roof. The engine was motored, without fuel injection, to establish the flow field likely to exist at the time of spark and the appearance of the flame kernel.

Cross-flow effect on combustion of a solid propellant with harmonically varying pressure

The response function of the burning rate of a solid propellant to periodically varying pressure is found with allowance for the cross-flow (erosion) effect in the linear approximation of the phenomenological theory of nonsteady combustion. Numerical results are obtained for the simplest propellant model with the minimum number of parameters. An elementary acoustic perturbation (planar monochromatic traveling acoustic wave) is considered. The role of steady-state and nonsteady cross-flow components at low and high values of the erosion ratio is elucidated.