Transition Position Research Articles

Comparison of Force Measures Between Start Position, Transition Phase, and Midthigh Pull With Weightlifting Performance During Israel National Competition.

Ben-Zeev, T, Sadres, E, and Hoffman, JR. Comparison of force measures between start position, transition phase, and midthigh pull with weightlifting performance during Israel National Competition. J Strength Cond Res 37(11): 2200-2205, 2023-This study compared the relationship between peak force (PKF) and rate of force development (RFD) at isometric start position pull (ISPP), isometric transition position pull (ITPP), and isometric midthigh pull (IMTP) with the snatch (SN), clean and jerk (C&J), and total score (TOT) in 30 national and international Israeli weightlifters (18 men and 12 women). All measures were collected either 1 week before or 1 week after the Israel weightlifting championship. A significant correlation was observed between peak force at all pull positions and performance in the SN, C&J, and TOT for men, women, and both sexes combined ( p < 0.001). The correlations were all very strong (e.g., r values all above 0.90) for both ISPP and ITPP, and all weightlifting performance variables, whereas the correlation between IMTP and weightlifting performance ranged from moderate ( r = 0.69) to strong ( r = 0.89). In addition, significant differences were observed between the correlation coefficient value in ISPP and ITPP with the TOT score compared with the correlations observed in IMTP ( z = 0.7, p = 0.046, and z = 1.7, p = 0.049, respectively). Significant differences were also observed in the women group for ISPP for the SN and TOT score compared with the correlation in IMTP ( z = 1.72, p = 0.043, and z = 1.75, p = 0.040, respectively). Rate of force development at 250 milliseconds significantly ( p values <0.05) correlated with the SN, C&J, and TOT in women ( r 's = 0.65, 0.68, and 0.67, respectively), and when both sexes were combined ( r 's = 0.59, 0.61, and 0.60, respectively), but not for the men. Results of this investigation indicated the importance of isometric force capabilities at ISPP and ITPP for weightlifting performance.

Partially curved-wall polygonal tube with multi-phase plateau force under quasi-static loads

The plateau force is an important factor affecting the performance of energy absorber. In this paper, a partially curved-wall polygonal tube (PCPT) is proposed by studying the relationship between the section shape and the force–displacement response of hexagonal and circular tubes. According to the different contact modes in the deformation process, PCPT can be divided into curved wall phase and straight wall phase. The stiffness, plateau force and the transition position of the curved wall phases can be regulated by the ovality h and the straight wall angle β. The multi-straight wall design makes the straight wall phase of PCPT have stepped plateau force, which improves the energy absorption. The desired plateau force characteristics can be obtained through parameter adjustment, such as stable plateau force. As an energy absorber, PCPT has excellent designability.

Effects of Ribbed Surfaces on Profile Losses of Low-Pressure Turbine Blades

Abstract In this work, streamwise oriented riblets were installed on a flat plate exposed to an adverse pressure gradient typical of low-pressure turbine (LPT) blade and, successively, on the suction side of an LPT cascade operating under unsteady flow. Different riblet dimensions and positions have been tested to quantify their effects on the boundary layer transition and on losses. The flat plate experiments allowed the detailed description of the riblet effects on the coherent structures affecting transition, thus providing a rationale for the identification of the optimal riblet geometry once scaled in wall-units. For riblet heights equal to about 20 wall-units, a maximum loss reduction of 8% was observed. Otherwise, for larger riblet dimensions, earlier transition occurs due to enhanced boundary layer instability and losses increase. Interestingly, the streamwise extension of the ribbed surfaces with respect to the transition region was found to play a minor role compared with the riblet dimension. The riblet configurations providing the highest reduction of viscous losses were then tested in the LPT blade cascade for different Reynolds numbers and with impinging upstream wakes. An overall profile loss reduction comparable to that observed in the flat plate case has been confirmed also in the unsteady operation of the turbine cascade. Low sensitivity of the profile losses to the riblet streamwise extension was also observed in the cascade application. This confirms that positive effects in terms of loss reduction can be obtained even when the exact transition position is not known a priori.

Direct numerical simulation of slender cones with variable nose bluntness based on graphics processing unit computation

Direct numerical simulation with up to 10×109 scale grid points based on graphics processing unit computation is carried out to investigate the bluntness effect on the hypersonic boundary-layer transition over a slender cone with zero angle of attack at Mach 6. Four cases with the nose radii of 1, 10, 20, and 40 mm are conducted, and the corresponding Reynolds number based on the nose radius varies from 1.0×104 to 4.0×105. Random disturbances with a broad spectrum of frequencies and a wide range of azimuthal wavenumbers were applied to the wall to simulate disturbances caused by wall roughness. The numerical results show that as the nose tip radius increases, the transition position gradually moves downstream with increased transition region. For the case with a nose radius of 1 mm, the flow transition and entropy swallowing occur almost simultaneously, while for other cases, the transition takes place earlier than the entropy swallowing. In consequence, the disturbance amplitude upstream of the transition in the 1 mm case is much larger than that of other cases. To further study the mechanism of the transition, the frequency spectrum analysis is carried out. It is found that all cases exhibit two characteristic frequencies within the transition region, i.e., the high frequency and extremely low frequency. Owing to the influence of the entropy layer, the characteristic high frequency of the 1 mm case is significantly higher than that of other cases. With the increase in the nose radius, the characteristic frequency of the high frequency decreases gradually.

Numerical study of the natural transition of water boundary layers over heated/cooled flat plates

The natural transition of water boundary layers on heated/cooled flat plates is investigated using numerical methods. The influences of wall temperature, oncoming flow velocity, and onco ming flow temperature on the transition are studied. Laminar basic flows are obtained by solving the Blasius equations, while water temperature is taken into consideration. Linear stability analyses of the basic flows are performed, and the transition positions are predicted using the eN method. The spectra of wall pressure fluctuations in the laminar region are also predicted. The results show that the heated wall produces a fuller velocity profile and a thinner boundary layer for the laminar flow and that a cooled wall causes the opposite effects. Consequently, the heated wall delays transition, while the cooled wall advances transition. As wall temperature increases, the effect of transition delay becomes stronger until the maximal delay effect occurs at an optimal wall temperature. Wall temperatures higher than the optimal value weaken the transition delay effect. This phenomenon of the heated wall is called transition reversal. A heated wall suppresses the wall pressure fluctuation, and a cooled wall has the opposite effect. The oncoming flow velocity has little influence on the non-dimensional transition position, while the oncoming flow temperature has a strong influence on it. At the same wall temperature, a higher oncoming flow temperature leads to a thicker boundary layer, stronger instability, and an earlier transition. A lower oncoming flow temperature leads to stronger sensitivity of the transition to the wall temperature.

Facilitation of metastable ice Ⅰ - ice III phase transition of liquid foods at high-pressure sub-zero temperature by perturbation

In this study, the distribution of ice Ⅰ - ice III phase transition positions and its influencing factors on various fruit juices and food model solutions under high pressure (HP) were investigated. In addition, the effect of perturbation on induction of phase change in metastable apple juice was also explored. The phase transition positions of fruit juice samples deviated far from the theoretical value (210 MPa/ –21 °C), with pressure in range of 268 ∼ 305 MPa and temperature between –31.09 °C and –37.21 °C, with the most extreme phase transition position of 305 MPa/ –37.21 °C for apple juice. The phase change position was affected by the type of solution but not by the concentration of solutions. The cooling media namely, sodium chloride (5%, 20%) and glucose solution (5%, 20%) were used as perturbation sources to stimulate the metastable apple juice by using instantaneous volume reduction during the ice Ⅰ - ice III phase transition. They successfully promoted the phase transition of apple juice, where 20% sodium chloride solution significantly (p < 0.05) reduced the phase change pressure of apple juice to 239 MPa. The perturbation effect was influenced by the type and concentration of the perturbation source, the composition of the cavity area and the softness of the sample container.

Measurement and Identification of Supersonic Stationary Crossflow Waves Based on Sublimation Method

Aiming at the crossflow transition problem of supersonic swept wings, an experimental study on the measurement and identification of boundary-layer stationary crossflow waves was carried out at Mach number 2.0. A 65 deg straight-swept-wing model with simple airfoil geometry was designed to capture the crossflow transition phenomenon. The sublimation method for a large-scale cold supersonic wind tunnel was developed to acquire a transition pattern. The stationary crossflow wave identification technology is based on image processing, a vertical and horizontal decomposition method is established, and the quantitative characteristic information (such as the vortex axis spacing and direction) can be identified from high-resolution sublimation images. The experimental results show that the transition position caused by crossflow instability is closed to the leading edge for supersonic wings with large sweep angles at high Reynolds numbers, and the transition position moves upstream with the increase of the unit Reynolds number. The identified vortex axis spacing and direction are basically consistent with the linear stability theory. The stationary disturbance wave information at different Reynolds numbers is compared and analyzed. It is found that the unit Reynolds number plays an important role in the selection and amplification of the unstable stationary modes with different wavelengths in the transition process.

Measurements on the effect of steps on the transition of laminar boundary layers

The effects of steps on the transition of laminar boundary layers were measured on a flat plate for low Reynolds numbers with critical and subcritical step heights. The transition position was measured by determining the intermittency distribution in streamwise direction, including the characteristic length of the transitional region. The results are compared with formulations of a critical step Reynolds number {textrm{Re}_textrm{h}}, i.e., the step height that will instantly trigger transition at the step position, and—for subcritical step heights—with {{Delta N}}-formulations from the literature. For backward-facing steps, the concept of a step Reynolds number can be used to distinguish between subcritical and critical step heights, whereas for forward-facing steps there seems not to be one unique {textrm{Re}_textrm{h}}. Furthermore, for subcritical backward-facing steps the concept of a {{Delta N}}-approximation gives a reasonable description of the experimental observations. Again in contrast, for forward-facing steps a {{Delta N}}-approach scattered a lot and no clear dependency was found between the reduction in the critical N-factor of transition and the relative step height.

Natural transition of the supersonic streamwise corner flow

The laminar-to-turbulence transition of a streamwise corner flow is recognized to occur first at the corner based on the stability analysis, but there is no persuasive experimental evidence to back it up, especially for supersonic flow. In this work, natural transition in a supersonic corner boundary layer has been experimentally studied using a nanoparticle-based planar laser scattering technique. It is inspiring to observe that the natural transition position of the corner boundary layer shows a random behavior among the corner side, flat-plate side, and their combination. Based on an intermittent factor analysis, these stochastic transitions show a dominant preference for transitions occurring near the corner region.

Experimental study on transition of dynamic airfoil in pitching oscillation

The transition characteristics of dynamic airfoil have significant effects on the aerodynamic performance of wind turbines, helicopter rotor blades, jet engine compressor blades, etc. The time domain and time-frequency domain characteristics of transition on a NACA0012 airfoil during its pitching oscillation were experimentally studied using wall pressure measurement technology with high time accuracy in this paper. The variable slip window technology was used to detect the transition position, and the proper orthogonal decomposition (POD) method and wavelet analysis were combined to perform the time-frequency analysis. In the gradual forward movement of the transition, the low-frequency instability is gradually enhanced by the main flow and the inverse pressure gradient, and significantly submerges the high-frequency fluctuated feature. The higher order moments of the wall pressure during dynamic airfoil transition deviate significantly from the Gaussian characteristics, which is caused by the low-frequency instability and high-frequency burst. The POD method is able to distinguish low-frequency instability from the high-frequency feature. The reduced frequency had significant effects on the transition. With the increase of the reduced frequency, the hysteresis effect of the transition became more and more significant, and the frequency component of transition was more concentrated and the energy was stronger.

Effect of a streamwise vortex on supersonic boundary layer transition

Supersonic boundary layer transition induced by a streamwise vortex is experimentally investigated based on the nanoparticle-based planar laser scattering technology, with an inflow Mach number of 2.7 and a unit Reynolds number of 8.79 × 106 m−1. Though the vortex is outside the boundary layer, it results in obvious difference of transition position along spanwise direction. On the up-wash side, transition to turbulence is completed in advance compared to the vortex-free case, and transition position moves more upstream as the vortex approaches the wall. Conversely, the transition process on down-wash side is inhibited. The promotion and inhibition effects on transition are found to be induced by the influence of vortex on streak density.

On-site contactless visualization of the laminar-turbulent flow transition dynamics on wind turbines

Abstract Thermographic flow visualization is already an established imaging method to localize the laminar-turbulent flow transition on the rotor blades of operating wind turbines, while a steady flow state is assumed. To understand the potential of thermographic flow visualization for the investigation of unsteady flow phenomena, its capability to detect the change of the flow transition position due to a wind gust is studied. Previously laminar flow regions become turbulent with the gust, which means a sudden increase of heat transfer between surface and fluid and, thus, a decrease of surface temperature. The latter is detected by evaluating the difference of thermographic images before and during the wind gust. The achievable sensitivity and the temporal resolution are limited by the thermodynamic properties of the rotor blade and the fluid flow, as well as by the natural rotor blade heating with the sun’s radiation. As a result of theory and experiments on real wind turbines, the feasibility to detect flow state changes in the order of seconds is proven. This opens upthe analysis of unsteady flow phenomena on wind turbines by means of thermographic flow visualization.

Study of Flow Patterns in a Moving Bed Reactor for Chemical Looping Combustion Based on Machine Learning Methods

Abstract A tower-type moving bed can be used as the air reactor in a chemical looping combustion system because of its low-pressure drop and smooth operation. In our previous simulation, a quasi-two-dimensional numerical model was established using discrete element method (DEM) approach to investigate the velocity and solid residence time distributions in the moving bed. In this work, the flow patterns under different operating and structural parameters are studied and optimized via machine learning methods. The random Forest regression model is applied to evaluate the importance of each variable to the solid flow pattern, while the feed forward neural network is applied to buildup a high-accuracy model to predict the solid axial velocity in the moving bed without the requirement to understand the physical mechanisms. Results show that the solid mass flux has the least impact on the mass flow index, while the axial position has the dominant influence and what comes next is the wedge angle, reactor angle, and ratio of down-comer diameter to reactor diameter. Further, based on the established feed forward neural network model, relation between the effective transition position and structural parameters of the moving bed is built, which provides valuable guidance for optimization of the reactor configuration.

EXPERIMENTAL INVESTIGATION OF THE TRANSITION BETWEEN LINEAR TO CHAOTIC REGIMES OF SLOSHING

The aim of this paper is to characterize the sloshing regimes in a cylinder and to statistically study the chaotic transition close to the natural frequency. The study was motivated by the lack of experimental data close to the chaotic transition. Using a dedicated shaking table called SHAKESPEARE, a statistical study was performed to exhibit a specific trend close to the transition. The performed tests were compared to the literature and show a slight difference which could be due to a different position of the probes. Close to the transition, a "plateau" in the amplitude of the free surface was observed and characterized using a phase portrait. It shows an elliptical limit cycle dependent on the test conditions. It is especially dependent on the starting point (still or moving water) which was studied in this paper. A later transition position was then observed when applying a frequency at the beginning of the test to break the "still water" assumption.

Numerical Investigations on Unsteady Features of Rounded Trailing Edge Airfoils

As a practical rounded trailing edge airfoil for coaxial rotors, DBLN-526 is a fore and aft symmetrical airfoil with two steps on its lower side. This airfoil has been used at the inboard section of the coaxial rotor system. As there are always two eddies behind the airfoil because of its rounded trailing edge, the interaction between the separation and transition should be considered. Thus, the unsteady Reynolds-averaged Navier-Stokes- (RANS-) based γ − Re ¯ θ t model was used to analyze the unsteady transition and separation features. Three different rounded trailing-edge airfoils were compared with DBLN-526. Power spectrum density analysis and Δ Cl calculations demonstrated that the lift coefficient fluctuation of DBLN-526 was smaller than that of other rounded airfoils with different angles. Further investigation indicated that the locations of transition for DBLN-526 can be fixed at a wide range of angles by the unique design on its lower side. Because of this settled transition location, the size of separation is decreased, and the position of separation is settled as well, which leads to a lower lift coefficient fluctuation. The turbulent kinetic energy after the transition was higher, which injected a lot of energy into the boundary layer, and the separation zone near the transition position was relatively smaller. This study provides an indication for controlling separation and reducing unsteady fluctuations for rounded trailing edge airfoils.

Quantification of wind turbine energy loss due to leading‐edge erosion through infrared‐camera imaging, numerical simulations, and assessment against SCADA and meteorological data

AbstractQuantification of the performance degradation on the annual energy production (AEP) of a wind farm due to leading‐edge (LE) erosion of wind turbine blades is important to design cost‐effective maintenance plans and timely blade retrofit. In this work, the effects of LE erosion on horizontal axis wind turbines are quantified using infrared (IR) thermographic imaging of turbine blades, as well as meteorological and SCADA data. The average AEP loss of turbines with LE erosion is estimated from SCADA and meteorological data to be between 3% and 8% of the expected power capture. The impact of LE erosion on the average power capture of the turbines is found to be higher at lower hub‐height wind speeds (peak around 50% of the turbine rated wind speed) and at lower turbulence intensity of the incoming wind associated with stable atmospheric conditions. The effect of LE erosion is investigated with IR thermography to identify the laminar to turbulent transition (LTT) position over the airfoils of the turbine blades. Reduction in the laminar flow region of about 85% and 87% on average in the suction and pressure sides, respectively, is observed for the airfoils of the investigated turbines with LE erosion. Using the observed LTT locations over the airfoils and the geometry of the blade, an average AEP loss of about 3.7% is calculated with blade element momentum simulations, which is found to be comparable with the magnitude of AEP loss estimated through the SCADA data.

Aerothermal Computations for Laminar–Turbulent Transition Onset Measurement Using Infrared Imaging Technique

This paper deals with the computation of heat exchange in transitional boundary layers for the prediction of the laminar–turbulent transition detection using infrared thermography. An aerothermal coupling between a heat equation solver and a boundary-layer solver is presented here. This coupling is first used to provide guidelines in the academic framework of a flat plate, to achieve an enhanced temperature gradient at the transition onset in order to improve infrared imaging, and thus the transition position detection. The method is then successfully applied on a real case, by comparing the numerical predictions to measurements obtained during transonic wind-tunnel tests on a two-dimensional wing model.

Experimental and modeling investigation of freezing characteristic curve of silty clay using TDR

The soil freezing characteristic curve was investigated using TDR, and the freezing point model was established based on measured temperature-time curve. A modified model was established based on developed relationships between model parameters and total water content ωtotal. The freezing degree was defined to describe the freezing process. The freezing point increased hyperbolically with increasing ωtotal and decreased linearly with increasing dry density. A critical water content ωcr was proposed, the growth rate of unfrozen water content gradually decreased with increasing ωtotal when ωtotal < ωcr, and it was almost unaffected by ωtotal and dry density when ωtotal > ωcr. The three parameters that were related to initial phase transition position, residual unfrozen water content and phase transition intensity in the modified model changed with ωtotal rather than fixed constants, thereby overcoming the limitation that unfrozen water content continuously increases with increasing ωtotal in the original model, which is suitable and important for changing total water content in multi-field coupling analysis of freezing soil. Comparison between predicted and measured results showed a satisfactory agreement.

Experimental study of the influence of structural parameters on pressure fluctuations of cone–cylinder–hemisphere models

When a viscous fluid flows over the surface of an object, different regions of the wall form different boundary layers. Fluctuating pressure in this boundary layer acts on the surface of the structure, causing it to vibrate and radiate noise; simultaneously, structural deformations will also have an impact on the flow field, and boundary layer pressure fluctuation is the most important component of flow noise. The characteristic parameters of a model, such as its wall thickness and the length of its parallel body section, will affect the pressure fluctuations it experiences. However, most studies treat the structure of the model as a rigid body. Therefore, this paper conducted experiments to examine the influences of the wall thickness and the parallel body length of a model on the pressure fluctuations it experiences. It was found that the fluctuating pressure at a given measuring position increases with decreasing wall thickness, and it decreases with increasing parallel body length. Then, this study demonstrated through comparative experiments that elastic and scale effects are important factors that cannot be ignored in calculations and experiments relating to pressure fluctuations. In addition, according to the characteristics of pressure fluctuation test values in different regions, the pressure-fluctuation prediction empirical formulas for different regions of the boundary layer were established or improved on the basis of previous research on pressure fluctuation in different regions of the boundary layer. Finally, by pasting a flow exciter at the transition position in the boundary layer of the model can keep its flow noise down, the experimental results show that rough particles can split the large vortex into smaller vortices and reduce flow noise by more than 5 dB. These results and empirical formulas provide references for numerical and experimental research examining pressure fluctuations.

Characterization of metastable high pressure phase transition positions and its influence on the behavior of microbial destruction

Effect of perturbation factors on phase transition metastable positions of whole milk (4% fat content) and their influence on microbial destruction characteristics of non-pathogenic Escherichia coli inoculated in milk subjected to high pressure low temperature treatment were evaluated using a specially developed high pressure (HP) cooling system. Initially, the phase transition data of milk transitioning through the metastable phases were obtained and fitted successfully using Simon-like models as done in previous studies and polynomial formulas with R2 of 0.997 & 0.996 for ice I, and 0.989 & 0.989 for ice III, respectively. The phase transition position of milk was explored with 5% and 10% sodium chloride solution as perturbation sources, respectively. Results showed that the 5% sodium chloride solution can reduce the transition pressure of milk by 43 MPa and increase the transition temperature by 4.1 °C, so that the milk can achieve phase transition at lower pressure and higher temperature. Phase transition microbial destruction was characterized by discontinuity, mutation and segmentation when the phase transition pressure interval 250– 300 MPa was carefully refined. The inactivation amount of E. coli before the phase transition (250 MPa) was 1.11 log and the phase transition process itself brought an additional 1.26 log destruction of E. coli population in milk. Industrial relevanceHigh pressure low temperature (HPLT) phase change kinetics were employed to enhance microbial destruction. HPLT was established based on a self-cooling unit positioned inside conventional HP chamber offering opportunities for scale up and commercialization. The effectiveness of HPLT phase transition for Escherichia coli destruction was demonstrated. The related research in metastable state provides a reference point for commercial application of high-pressure-low-temperature technology for microbial destruction and quality enhancement.