Wall Blowing Research Articles

Parametric Optimization of Unsteady End Wall Blowing on a Highly Loaded Low-Pressure Turbine

Efforts to reduce blade count and avoid boundary layer separation have led to low-pressure turbine airfoils with significant increases in loading as well as front-loaded pressure distributions. These features have been independently shown to increase losses within the secondary flow field at the end wall. Compound angle blowing from discrete jets on the blade suction surface near the end wall has been shown to be effective in reducing these increased losses and enabling the efficient use of highly loaded blade designs. In this study, experiments are performed on the front loaded L2F low-pressure turbine airfoil in a linear cascade. The required mass flow is reduced by decreasing the hole count from previous configurations and from the introduction of unsteady blowing. The effects of pulsing frequency and duty cycle are investigated using phase-locked stereo particle image velocimetry to demonstrate the large scale movement and hysteresis behavior of the passage vortex interacting with the pulsed jets. Total pressure loss contours at the cascade outlet demonstrate that the efficiency benefit is maintained with the use of unsteady forcing.

Optimal control of dissimilar heat and momentum transfer in a fully developed turbulent channel flow

AbstractSustained friction drag reduction and heat transfer augmentation are simultaneously achieved in a fully developed channel flow where the averaged transport equations and wall boundary conditions for momentum and heat have identical form. Zero-net-mass-flux wall blowing and suction is assumed as a control input and its spatio-temporal distribution is determined based on optimal control theory. When the root-mean-square value of the control input is 5 % of the bulk mean velocity, the friction drag is decreased by 24 % from the uncontrolled value, whereas the heat transfer is more than doubled. Optimizations with different amplitudes of the control input and different Reynolds numbers reveal that the optimal control inputs commonly exhibit the property of a downstream travelling wave, whose wavelength is ∼250 in wall units and phase velocity is ∼30 % of the bulk mean velocity. Detailed analyses of the controlled velocity and thermal fields show that the travelling wave input contributes to dissimilar heat transfer enhancement through two distinct mechanisms, i.e. direct modification of the coherent velocity and thermal fields and an indirect effect on the random fields. The present results show that the divergence-free velocity vector and the conservative scalar are essentially different, and this is a key to achieving dissimilar heat transfer enhancement in turbulent shear flows.

Adjoint based optimization and control of a separated boundary-layer flow

An adjoint based optimization procedure for the nonlinear Navier–Stokes system is implemented and applied to the dynamics of a separated boundary-layer flow over a bump. For this open flow configuration, boundary conditions which are compatible between the direct and adjoint system are implemented using the influence matrix technique. Beyond a critical Reynolds number, the flow is unstable and subject to instabilities with multiple time-scales. Computing iteratively optimal initial perturbations, the most sensitive regions to forcing are highlighted. Wall blowing–suction actuators are then introduced at these locations and their capability of controlling the unstable linear and nonlinear dynamics is explored, by means of the adjoint based numerical algorithm.

Effects of Chemical Reaction and Soret Effect on Mixed Heat and Mass Transfer for Hiemenz flow through Porous Media with Heat Source

The effects of chemical reaction and thermal-diffusion mixed convection heat and mass transfer for Hiemenz flow through porous media has been studied. The plate is embedded in a uniform porous medium in order to allow for possible fluid wall suction or blowing and has a power-law variation of both the wall temperature and concentration. We used similarity solution to transform the system of partial differential equations, into a boundary value problem of coupled ordinary differential equations. We then solve these ordinary differential equations by a MATLAB routine bvp4c. We conducted a parametric study of all involved parameters and the results represented graphically.

Prediction of Turbulent Heat Transfer in Rotating and Nonrotating Channels With Wall Suction and Blowing

This paper reports on the prediction of heat transfer in a fully developed turbulent flow in a straight rotating channel with blowing and suction through opposite walls. The channel is rotated about its spanwise axis; a mode of rotation that amplifies the turbulent activity on one wall and suppresses it on the opposite wall leading to reverse transition at high rotation rates. The present predictions are based on the solution of the Reynolds-averaged forms of the governing equations using a second-order accurate finite-volume formulation. The effects of turbulence on momentum transport were accounted for by using a differential Reynolds-stress transport closure. A number of alternative formulations for the difficult fluctuating pressure–strain correlations term were assessed. These included a high turbulence Reynolds-number formulation that required a “wall-function” to bridge the near-wall region as well as three alternative low Reynolds-number formulations that permitted integration through the viscous sublayer, directly to the walls. The models were assessed by comparisons with experimental data for flows in channels at Reynolds-numbers spanning the range of laminar, transitional, and turbulent regimes. The turbulent heat fluxes were modeled via two very different approaches: one involved the solution of a modeled differential transport equation for each of the three heat-flux components, while in the other, the heat fluxes were obtained from an explicit algebraic model derived from tensor representation theory. The results for rotating channels with wall suction and blowing show that the algebraic model, when properly extended to incorporate the effects of rotation, yields results that are essentially identically to those obtained with the far more complex and computationally intensive heat-flux transport closure. This outcome argues in favor of incorporation of the algebraic model in industry-standard turbomachinery codes.

Effectiveness of active flow control for turbulent skin friction drag reduction

The effectiveness of the opposition control method proposed by Choi et al. [J. Fluid Mech. 262, 75 (1994)] has been studied using direct numerical simulations. In this study, the effects of the amplitude and the phase of wall blowing and suction control input were considered separately. It is found that the amplitude of wall blowing and suction as well as the detection plane location played an important role in active control for skin-friction drag reduction. By changing the amplitude, a substantial drag reduction was achieved for all detection plane locations considered, and the efficiency of the opposition control was also improved. When the control was effective, the drag reduction was proportional to the wall blowing and suction strength. There existed a maximum wall blowing and suction strength, beyond which the opposition control became less effective or even unstable. Turbulence characteristics affected by various wall blowing and suction parameters were analyzed to understand the underlying mechanisms for drag reduction. The wall-normal velocity and vorticity fluctuations showed a strong correlation with drag reduction.

Circulation : Clinical Summaries

<i>Circulation</i> : Clinical Summaries

Marked Variability in Susceptibility to Ventricular Fibrillation in an Experimental Commotio Cordis Model

Precordial blows in sports and daily activities can trigger ventricular fibrillation (VF) (commotio cordis). Whereas chest wall blows are common, commotio cordis is rare. Although factors such as timing, location, orientation, and energy of impact are critically important, we also hypothesize that there is individual susceptibility to commotio cordis. Using our model of commotio cordis, we evaluated individual animal susceptibility to VF induction and assessed animal characteristics that might be involved. This retrospective analysis included 139 juvenile swine (weight, 8 to 54 kg) that were anesthetized and placed prone in a sling to receive chest wall strikes with a ball propelled at 30 to 40 mph. Each animal received a minimum of 4 impacts directly over the cardiac silhouette, all timed to a narrow vulnerable window during cardiac repolarization. Of 1274 total impacts, 360 impacts (28%) resulted in VF. There was wide variability in individual animal susceptibility to VF. In 38 animals, none of the impacts resulted in VF (range, 4 to 18 impacts per animal). The majority of animals (91; 65%) were induced into VF with <30% of the strikes. In fact, only 19 animals (14%) had >50% occurrence of VF with chest wall impacts, and only 7 (5%) had >80% occurrence of chest impacts that induced VF. In the animal-based analysis, individual correlates of VF included animal weight, mean impact velocity, mean left ventricular pressure generated by the blow, mean QRS duration, mean QTc, and QTc variability. In multivariable analysis, mean left ventricular pressure generated by the blow, mean QRS duration, and QTc variability remained significant correlates of risk, and number of impacts gained statistical significance such that animals with more impacts were less susceptible to VF. Swine display a wide range of individual vulnerability to VF triggered by chest wall impact, with a distinct minority being uniquely susceptible. Mild abnormalities in cardiac depolarization and repolarization might underlie this susceptibility. Such individual susceptibility may also be present in humans and contribute to the rarity of commotio cordis.

Reply to: Comments on Alentorn-Geli et al.: Anteromedial portal (AMP) versus transtibial (TT) drilling techniques in ACL reconstruction: a blinded cross-sectional study at two- to five-year follow-up

Dear Editor, We read with great interest the comments on our article entitled “Anteromedial portal (AMP) versus transtibial (TT) drilling techniques in ACL reconstruction: a blinded crosssectional study at twoto five-year follow-up” [1] by Sunil Gurpur Kini. In his first concern, Dr. Kini states that the TT group in our study may have more knee laxity because of a too vertical tibial starting point considering that we drilled at 20° in the coronal plane. Studies referenced by Dr. Kini to support his first concern reported a tibial tunnel orientation of 60–70° in the coronal plane, but these angles were considered from the horizontal line, whereas the 20° in our study was considered from the vertical line [4, 5, 7]. Other authors perform the tibial tunnel with a mean angulation of 60.6°, ranging up to 74.2° [9]. Thus, we can not assume our tibial tunnel significantly differs from the existing literature. Dr. Kini highlighted the excellent cadaveric study by Bedi et al. [3] comparing the AMP and TT drilling. The TT drilling was performed with oblique positioning of the tibial guide at 60° in the coronal plane, reaching a mean femoral angulation of 54.81° ± 7.17°, corresponding approximately to 10:45 in a clock position. The TT group in our study had the femoral tunnel in the 11 (or 13) o’clock position, which we believe is comparable with the interesting study by Bedi et al. Although both TT and AMP drilling may produce a femoral tunnel close to ACL footprints, the tibial starting point to achieve this position in the TT drilling must be more medial and could be too close to the tibial joint line. This may result in a short tibial tunnel and enlarged tibial tunnel aperture, which may in turn compromise the graft fixation and incorporation [6]. In addition, a too medialised tibial tunnel starting point during the TT technique may result in an increased risk of injury to the MCL, and may produce a more lateral and posterior tibial tunnel intraarticular entry point resulting in a less effective reconstruction from a biomechanical point of view [8]. In his other concern, Dr. Kini discusses the risk of a short femoral tunnel and the increased risk of posterior wall blow out with the AMP technique. In our experience with the AMP technique, the posterior wall blow out is extremely rare. Using the Acufex Endoscopic femoral drill guide (Smith and Nephew®), a security distance can be systematically left in the posterior wall of the femoral condyle, thus preventing its rupture. The posterior wall Eduard Alentorn-Geli and Gonzalo Samitier contributed equally to this work. E. Alentorn-Geli : P. Alvarez :R. Cugat Artroscopia Garcia-Cugat, Hospital Quiron, Barcelona, Spain

Medial wall blow out fractures in children

Background Fractures of the facial skeleton in children are relatively uncommon in comparison with adults. Within the orbital region fractures are recognised to be rare due the timing of development of the maxillary and frontal sinuses which are not fully developed until 12 years of age. We have recently seen several cases of medial wall fractures in children, which according to the existing literature should be exceptional. This prompted a review of the unit database to clarify the incidence and identify common aetiological factors for these injuries. Method Retrospective case note review using the ACFU database. Results Six cases were identified with a most cases occurring in males. The aetiology was found to be variable in each case with no identifiable pattern and the subsequent management reviewed. Conclusion These six cases highlight that this pattern of fracture can occur more commonly than the existing literature suggests and this should be remembered by those who treat facial trauma in children.

Use of active control for elimination of flow separation with estimation of energy costs

Problems were posed and solved concerning the aerodynamic computation of the flow past an airfoil with an active boundary layer control device used to prevent flow separation. A moving wall, suction, or tangential blowing in the boundary layer was used as a flow control device. The turbulent boundary layer was computed by directly solving the boundary layer equations using an implicit difference scheme with adaptive grid generation and the determination of the computational domain size. A software code was developed, and numerical simulations were performed taking into account the energy costs related to the flow control device. The numerical results showed that the active flow control devices can be used to prevent flow separation.

0305 ポリマー溶液の壁面滲出による乱流摩擦抵抗低減(OS3-1 流れの抵抗低減,オーガナイズドセッション)

Experimental investigation on the turbulent drag reduction with blowing polymer solution from the permeable channel wall was performed. Measurement of the drag reduction has been carried out with changing condition of blowing polymer solution (e.g. blowing rate and concentration). As a result, obvious drag reduction was obtained with blowing polymer solution and the maximum drag reduction rate achieved about 20 %. In addition, it seems that there is close relationship between the drag reduction rate and the mass flux of polymer. Because it is the merit of wall blowing that polymer can exist in the whole near-wall region, we discussed the difference between our results (wall blowing) and the effect of slot injection. Based on these results, the effect of wall blowing polymer solution on the heterogeneous drag reduction was discussed.

채널유동에서 질량분사에 의한 표면유동의 진동 특성

The interaction between wall blowing and oxidizer flow can generate a very complicated flow characteristics in combustion chamber of hybrid rockets. LES analysis was conducted with an in-house CFD code to investigate the features of turbulent flow without chemical reactions. The numerical results reveal that the flow oscillations at a certain frequency exists on the fuel surface, which is analogous to those observed in the solid propellant combustion. However, the observation of oscillating flow at a certain frequency is only limited to a very thin layer adjacent to wall surface and the strength of the oscillation is not strong enough to induce the drastic change in temperature gradient on the surface. The visualization of fluctuating pressure components shows the periodic appearance of relatively high and low pressure regions along the axial direction. This subsequently results in the oscillation of flow at a certain fixed frequency. This implies that the resonance phenomenon would be possible if the external disturbances such as acoustic excitation could be imposed to the oscillating flow in the combustion chamber.

Stability of a channel flow subject to wall blowing and suction in the form of a traveling wave

Inspired by the recent finding by Min et al. [J. Fluid Mech. 558, 309 (2006)], the stability of a channel flow subject to wall blowing and suction in the form of a traveling wave is investigated by combined use of the Floquet analysis, direct numerical simulation, and singular value decomposition analysis. Results show that stability highly depends on the phase speed of the traveling wave; most disturbances become highly unstable when the phase speed is around 40% of the centerline velocity, while streamwise streak-type three-dimensional disturbances become stabilized with transient growth suppressed when the phase speed exceeds the centerline velocity for both subcritical and supercritical Reynolds numbers. This destabilization is interpreted by investigation of wave interactions. An upstream-traveling wave, which reduces mean drag, does not stabilize the flow.

Influence of Knee Flexion Angle on Femoral Tunnel Characteristics When Drilled Through the Anteromedial Portal During Anterior Cruciate Ligament Reconstruction

The purpose of this study was to determine the influence of knee flexion angle for drilling the femoral tunnel during anterior cruciate ligament (ACL) reconstruction via the anteromedial (AM) portal on resulting tunnel orientation and length. In 8 fresh cadaveric knees, the ACL was excised and 2.4-mm guidewires were drilled through the AM bundle footprint using a 5-mm endofemoral aimer via the AM portal. We compared knee flexion angles of 90 degrees , 110 degrees , 130 degrees , and maximum flexion. Anteroposterior-, lateral-, and tunnel-view radiographs were measured to determine tunnel orientation, o'clock position, and direct measurement to determine intra-osseous tunnel length. With regard to tunnel orientation, each increase in knee flexion angle resulted in significantly more horizontal tunnel both on the anteroposterior view and on the lateral view. While on the tunnel view, the pin became more vertical with knee flexion. At 90 degrees , tunnel length was significantly less (27 +/- 9 mm) than at greater angles, and the guidewires were either resting against the posterior cortex or breaching it. The results of this study show the knee flexion angle influences the position of the femoral drilling. It appears in the current study that 110 degrees is optimum, while the 90 degrees pin leads to short tunnel and is so close to the posterior wall there are high risks of posterior wall blow out when drilling the tunnel at its final diameter. Also, 130 degrees of knee flexion is responsible for high tunnel acuity and, finally, maximum flexion being quite variable from one specimen to another cannot be recommended. Tunnels drilled through the AM portal at 90 degrees are at risk of back wall blow out.

Abstract 2970: Mechanism of Increased Susceptibility of Youth to Sudden Death with Chest Wall Impact (Commotio Cordis)

Background. Commotio cordis (CC), sudden cardiac death secondary to blunt, innocent-appearing chest wall blows in sports and other daily activities, is being reported with increasing frequency. Children and adolescents are most at-risk for this phenomenon. In an experimental animal model, it has been shown that ventricular fibrillation (VF) can be induced with a baseball blow to the chest wall during the vulnerable period of repolarization. We hypothesized that in this model, larger subjects would require higher impact velocities to induce VF. Methods. Under general anesthesia, 41 juvenile swine (16 to 55 kg) were placed prone in a sling and received chest wall blows at 30 to 60 mph with a lacrosse ball aimed directly over the cardiac silhouette. Impacts were timed to occur during the vulnerable period for VF (10 –30 ms prior to the T-wave peak). Animals were divided into 4 groups based on weight (16 to 25kg, 26 to 35kg, 36 to 45kg, and 46 to 55kg) and the frequency of VF was analyzed. Results. In 419 impacts, the smallest animals (16 to 25kg) were most vulnerable to VF and incidence of VF decreased incrementally as animal size increased. Impacts at 30 mph only induced VF in the smallest animals. In the largest animals (46 to 55kg), VF was only induced at impacts of 60 mph (figure ). Conclusion. Impact velocity and subject size are important variables in the generation of CC. In this animal model, smaller animals are more vulnerable to generation of VF across a wider range of impact velocities. These data might underlie the higher vulnerability to CC that has been observed in children and young adults and has implications for the prevention of CC with chest wall protectors.

분류유동이 있는 채널 난류유동의 LES 해석

Recent experimental data shows that the noticeable feature of irregular roughened spots on the fuel surface occurs during the combustion test with PMMA/GOX in the hybrid rocket motor. The generation of these unexpected patterns is likely to be resulted from the disturbed boundary layer due caused by wall blowing which is intented to simulate the process of fuel vaporization. LES technique was implemented to investigate both the flow characteristics near fuel surface and the subsequent evolution of turbulence modified by the wall blowing. Simple channel geometry instead of circular grain configuration was used for the investigation without chemical reactions in order to allow for a focused examination on the near-wall behavior at the Reynolds number of 22,500. It was shown that the wall blowing pushed turbulent structures upwards making them tilted and this skewed displacement, in effect, left the foot prints of the structures on the surface. This change of kinematics may explain the formation of irregular isolated spots on the fuel surface observed in the experiment.

Natural Convection from a Permeable Sphere Embedded in a Variable Porosity Porous Medium Due to Thermal Dispersion

The laminar natural convection boundary-layer flow of an electricallyconducting fluid from a permeable sphere embedded in a porous medium with variable porosity is considered. The non-Darcy effects including convective, boundary, inertial and thermal dispersion effects are included in this analysis. The sphere surface is maintained at a constant heat flux and is permeable to allow for possible fluid wall suction or blowing. The resulting governing equations are nondimensionalized and transformed into a nonsimilar form and then solved numerically by using the secondlevel local non-similarity method that is used to convert the non-similar equations into a system of ordinary differential equations. Comparisons with previously published work are performed and excellent agreement is obtained. A parametric study of the physical parameters is conducted and a representative set of numerical results for the velocity and temperature profiles as well as the local skin-friction coefficient and the Nusselt number are illustrated graphically to show interesting features of Darcy number, inertia coefficient, the magnetic parameter, dimensionless coordinate, dispersion parameter, the Prantdl number and suction/blowing parameter.

Blowing/Suction Effects on Hydromagnetic Simultaneous Heat and Mass Transfer by Natural Convection from a Vertical Cylinder Embedded in a Thermally Stratified Porous Medium

The problem of steady, laminar, hydromagnetic, combined heat and mass transfer in non-Darcy natural convection flow along a permeable vertical cylinder embedded in a thermally stratified, fluid-saturated porous medium with a stratified free stream in the presence of thermal and solutal dispersion is investigated. The surface of the cylinder is assumed to be permeable to allow for possible fluid wall blowing or suction. A generalized flow model is used to include the effects of macroscopic viscous term and microscopic inertial force. The nonsimilar form of the transformed equations is solved numerically. A parametric study illustrating the influence of the magnetic field, the local thermal stratification parameter ξ, the thermal dispersion parameter Dsγ, the solutal dispersion parameter Dsζ, and suction/blowing parameter fω is made on the profiles of velocity, temperature, and concentration. Numerical data for the skin friction factor, the local Nusselt number, and the local Sherwood number have been presented in tabular form for various parametric conditions.

Commotio Cordis—Sudden Cardiac Death with Chest Wall Impact

Commotio cordis (CC), sudden death as a result of a blunt, often innocent-appearing chest wall blow, is being reported with increasing frequency. The clinical spectrum is diverse; however, a substantial number of cases occur in youth athletics. In events that occur during sport, victims are struck by projectiles regarded as standard implements of the game. Sudden death is instantaneous and victims are most often found in ventricular fibrillation (VF). Overall survival is poor; however, successful resuscitation can be achieved with early defibrillation. Autopsy is notable for the absence of any significant cardiac or thoracic injury. Development of an experimental model has allowed for substantial insights into the underlying mechanisms of sudden death. In anesthetized juvenile swine, induction of VF is instantaneous following chest wall blows occurring during a vulnerable window before the T wave peak. Crucial variables including the velocity of impact, impact location, and hardness of the impact object have been identified. Rapid left ventricular (LV) pressure rise following chest impact likely results in activation of ion channels via mechano-electric coupling. The generation of inward current via mechano-sensitive ion channels likely results in augmentation of repolarization and nonuniform myocardial activation, and is the cause of premature ventricular depolarizations that are triggers of VF in CC. While softer-than-standard safety baseballs reduce the risk of CC, commercially available chest protectors are ineffective in preventing CC. The development of more effective chest protectors and more widespread use of automated external defibrillators at youth sporting events are needed.